Publications by authors named "Gerd Huber"

59 Publications

Densification of cancellous bone with autologous particles can enhance the primary stability of uncemented implants by increasing the interface friction coefficient.

J Biomech 2022 Jun 20;139:111149. Epub 2022 May 20.

Hamburg University of Technology, Hamburg, Germany.

Sufficient primary stability is one of the most important prerequisites for successful osseointegration of cementless implants. Bone grafts, densification and compaction methods have proven clinically successful, but the related effects and causes have not been systematically investigated. Postoperatively, the frictional properties of the bone-implant interface determine the amount of tolerable shear stress. Frictional properties of different implant surfaces have been widely studied. Less attention has been paid to the influence of host bone modifications. The purpose of this study was to investigate the influence of densification of cancellous bone with bone particles on the interface friction coefficient. Cancellous bone samples from femoral heads were densified with bone particles obtained during sample preparation. The densification was quantified using micro-Ct. Friction coefficients of the densified and paired native samples were determined. Densification increased the BV/TV in the first two millimeters of the bone samples by 10.5 ± 2.7% to 30.5 ± 2.7% (p < 0.001). The static friction coefficient was increased by 10.5 ± 6.1% to 0.43 ± 0.03. The static friction coefficient increased with higher BV/TV of the bone interface, which is represented by the top 2 mm of the bone. The increase in contact area, intertrabecular anchorage and particle bracing could be responsible for the increase in friction. Optimization of particle shape and size based on the patient's individual bone microstructure could further increase frictional resistance. Bone densification has the potential to improve the primary stability of uncemented implants.
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http://dx.doi.org/10.1016/j.jbiomech.2022.111149DOI Listing
June 2022

Impact of Screw Diameter on Pedicle Screw Fatigue Strength-A Biomechanical Evaluation.

World Neurosurg 2021 08 1;152:e369-e376. Epub 2021 Jun 1.

Department of Trauma Surgery, Orthopaedic Surgery and Plastic Surgery, University Medical Center Göttingen, Göttingen, Germany. Electronic address:

Objective: Loosening of pedicle screws is a frequently observed complication in spinal surgery. Because additional stabilization procedures such as cement augmentation or lengthening of the instrumentation involve relevant risks, optimal stability of the primarily implanted pedicle screw is of essential importance. The aim of the present study was to investigate the effect of increasing the screw diameter on pedicle screw stability.

Methods: A total of 10 human cadaveric vertebral bodies (L4) were included in the present study. The bone mineral density was evaluated using quantitative computed tomography and the pedicle diameter using computed tomography. The vertebrae underwent instrumentation using 6.0-mm × 45-mm pedicle screws on 1 side and screws with the largest possible diameter (8-10-mm × 45-mm) on the other side. Fatigue testing was performed by applying a cyclic loading (craniocaudal sinusoidal 0.5 Hz) with increasing peak force (100 N + 0.1 N/cycle) until screw head displacement of 5.4 mm was reached.

Results: The mean fatigue load was 334 N for the 6-mm diameter screws and was increased significantly to 454 N (+36%) for the largest possible diameter screws (P < 0.001). With an increase in the fatigue load by 52%, this effect was even more pronounced in vertebrae with reduced bone density (bone mineral density <120 mg/cm; n = 7; P < 0.001). The stiffness of the construct was significantly greater in the largest diameter screw group compared with the standard screw group during the entire testing period (start, P < 0.001; middle, P < 0.001; end, P = 0.009).

Conclusions: Increasing the pedicle screw diameter from a standard 6-mm screw to the largest possible diameter (8-10 mm) led to a significantly greater fatigue load.
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http://dx.doi.org/10.1016/j.wneu.2021.05.108DOI Listing
August 2021

Which length should the neck segment of modular revision stems have?

Clin Biomech (Bristol, Avon) 2022 Apr 4;94:105286. Epub 2021 Feb 4.

Institute of Biomechanics, TUHH Hamburg University of Technology, Denickestraße 15, 21073 Hamburg, Germany.

Background: Fractures of modular revision stems at the taper junction are rare but severe clinical problems. The purpose of this study was the estimation of taper loading to identify configurations which are less prone to failure.

Methods: A parametrical analytical 3-D model was developed to determine the influence of neck segment length, offset and anteversion on the loading at the modular taper junction between neck segment and stem. Published in-vivo hip joint forces were used to simulate different activities.

Findings: No unique ideal neck segment length can be specified due to the differences in loading magnitude and direction between activities. The best neck segment length for walking is longer than for high loading activities as stair climbing and jogging. A medium length between 70 mm and 90 mm appears to be a good compromise. A shorter offset (37 mm vs. 47 mm) reduces the stress by about 25% for walking and jogging. Retroverted implantation by 5° increases the loading whereas an anteverted implantation by 5° reduces it. A high offset (47 mm) combined with a short neck segment length (50 mm) reaches about 80% of the taper yield strength for jogging (taper diameter 13 mm).

Interpretation: Simplified 2-D modelling falsely predicts no bending at the taper junction for a long neck segment, whereas the 3-D model shows substantial stress load along the whole stem length. Stem tapers of short as well as very long neck segments are higher risk for failure. Neck segment length should lie in the range between 70 mm and 90 mm.
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http://dx.doi.org/10.1016/j.clinbiomech.2021.105286DOI Listing
April 2022

Taper corrosion: a complication of total hip arthroplasty.

EFORT Open Rev 2020 Nov 13;5(11):776-784. Epub 2020 Nov 13.

Center for Musculoskeletal Surgery, Orthopedic Department, Charité - Universitätsmedizin Berlin, Berlin, Germany.

The focus on taper corrosion in modular hip arthroplasty increased around 2007 as a result of clinical problems with large-head metal-on-metal (MoM) bearings on standard stems. Corrosion problems with bi-modular primary hip stems focused attention on this issue even more.Factors increasing the risk of taper corrosion were identified in laboratory and retrieval studies: stiffness of the stem neck, taper diameter and design, head diameter, offset, assembly force, head and stem material and loading.The high variability of the occurrence of corrosion in the clinical application highlights its multi-factorial nature, identifying the implantation procedure and patient-related factors as important additional factors for taper corrosion.Discontinuing the use of MoM has reduced the revisions due to metal-related pathologies dramatically from 49.7% (MoM > 32 mm), over 9.2% (MoM ⩽ 32 mm) to 0.8% (excluding all MoM).Further reduction can be achieved by omitting less stiff Ti-alloys and large metal heads (36 mm and above) against polyethylene (PE).Standardized taper assembly of smaller and ceramic heads will reduce the clinical occurrence of taper corrosion even further. If 36 mm heads are clinically indicated, only ceramic heads should be used.Taper-related problems will not comprise a major clinical problem anymore if the mentioned factors are respected. Cite this article: 2020;5:776-784. DOI: 10.1302/2058-5241.5.200013.
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http://dx.doi.org/10.1302/2058-5241.5.200013DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7722945PMC
November 2020

Cortical threaded pedicle screw improves fatigue strength in decreased bone quality.

Eur Spine J 2021 01 17;30(1):128-135. Epub 2020 Sep 17.

Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany.

Purpose: Inadequate anchoring of pedicle screws in vertebrae with poor bone quality is a major problem in spine surgery. The aim was to evaluate whether a modified thread in the area of the pedicle could significantly improve the pedicle screw fatigue strength.

Methods: Fourteen human cadaveric vertebral bodies (L2 and L3) were used for in vitro testing. Bone density (BMD) was determined by quantitative computed tomography. Vertebral bodies were instrumented by standard pedicle screws with a constant double thread on the right pedicle and a partial doubling of the threads-quad thread-(cortical thread) in the area of the pedicle on the left pedicle. Pulsating sinusoidal, cyclic load (0.5 Hz) with increasing peak force (100 N + 0.1 N/cycles) was applied orthogonal to the screw axis. The baseline force remained constant (50 N). Fatigue test was terminated after exceeding 5.4-mm head displacement (~ 20° screw tilting).

Results: The mean fatigue load at failure was 264.9 N (1682 cycles) for the standard screws and was increased significantly to 324.7 N (2285 cycles) by the use of cortical threaded screws (p = 0.014). This effect is particularly evident in reduced BMD (standard thread 241.2 N vs. cortical thread 328.4 N; p = 0.016), whereas in the group of vertebrae with normal BMD no significant difference could be detected (standard thread 296.5 N vs. cortical thread 319.8 N; p = 0.463).

Conclusions: Compared to a conventional pedicle screw, the use of a cortical threaded pedicle screw promises superior fatigue load in vertebrae with reduced bone quality.
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http://dx.doi.org/10.1007/s00586-020-06593-3DOI Listing
January 2021

Rescue Augmentation: Increased Stability in Augmentation After Initial Loosening of Pedicle Screws.

Global Spine J 2021 Jun 21;11(5):679-685. Epub 2020 Apr 21.

Department of Trauma Surgery, Orthopaedic Surgery and Plastic Surgery, University Medical Center Göttingen, Göttingen, Germany.

Study Design: Biomechanical study.

Objectives: Failure of pedicle screws is a major problem in spinal surgery not only postoperatively, but also intraoperatively. The aim of this study was to evaluate whether cement augmentation may restore mounting of initially loosened pedicle screws.

Methods: A total of 14 osteoporotic or osteopenic human cadaveric vertebral bodies (L2)-according to quantitative computed tomography (QCT)-were instrumented on both sides by conventional pedicle screws and cement augmented on 1 side. In vitro fatigue loading (cranial-caudal sinusoidal, 0.5 Hz) with increasing peak force (100 N + 0.1 N/cycles) was applied until a screw head displacement of 5.4 mm (∼20°) was reached. After loosening, the nonaugmented screw was rescue augmented, and fatigue testing was repeated.

Results: The fatigue load reached 207.3 N for the nonaugmented screws and was significantly ( = .009) exceeded because of initial cement augmentation (300.6 N). The rescue augmentation after screw loosening showed a fatigue load of 370.1 N which was significantly higher ( < .001) compared with the nonaugmented screws. The impact of bone density on fatigue strength decreased from the nonaugmented to the augmented to the rescue-augmented screws and shows the greatest effect of cement augmentation on fatigue strength at low bone density.

Conclusions: Rescue augmentation leads to similar or higher fatigue strengths compared with those of the initially augmented screws. Therefore, the cement augmentation of initially loosened pedicle screws is a promising option to restore adequate screw stability.
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http://dx.doi.org/10.1177/2192568220919123DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8165920PMC
June 2021

Biomechanical comparison of titanium miniplates versus a variety of CAD/CAM plates in mandibular reconstruction.

J Mech Behav Biomed Mater 2020 11 7;111:104007. Epub 2020 Aug 7.

Department of Oral and Maxillofacial Surgery, Charité - Universätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin and Berlin Institute of Health, Augustenburger Platz 1, 13353, Berlin, Germany; Berlin Institute of Health (BIH), Anna-Louisa-Karsch-Straße 2, 10178, Berlin, Germany.

Background: Titanium plate fixation of free flaps in mandibular reconstruction involves complications such as osseous non-union or imaging artefacts. Interosteotomy movement (IOM) is known to affect bone healing. This study aimed to compare IOM and mechanical integrity of four different fixation systems in a mandible reconstruction model.

Methods: Two polyurethane (PU) fibula segments were fixed in right-sided defects of PU mandibles. Laser-melted patient-specific titanium plates were fixed with non-locking-screws (Ti-NL) or locking-screws (Ti-L). The third group consisted of locking-screws for patient-specific polyetheretherketone (PEEK-L) plates. The last group used titanium miniplates and monocortical screw fixation (Ti-MP). All models were loaded unilaterally via cyclic dynamic loading with increasing loads to simulate mastication. IOM was registered using a 3D optical tracking system.

Findings: PEEK-L showed highest vertical displacement (p = 0.010), lowest stiffness (p = 0.004) and highest IOM (p = 0.001). All specimen in PEEK-L demonstrated abnormal bending (n = 5) or plate fracture (n = 1). Vertical displacement or stiffness did not differ between any of Ti-MP, Ti-L and Ti-NL. IOM in Ti-MP was higher than in Ti-L and Ti-NL (p = 0.001).

Interpretation: Mechanical integrity of all titanium plates complies with established standards. In this model, the screw system did not influence IOM. In the tested composition and shape, PEEK plates do not seem to guarantee sufficient mechanical integrity for a use in mandibular reconstruction. Thus modifications are needed. Future clinical studies are needed to clarify optimal IOM after mandible reconstruction.
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http://dx.doi.org/10.1016/j.jmbbm.2020.104007DOI Listing
November 2020

Lag-Screw Osteosynthesis in Thoracolumbar Pincer Fractures.

Global Spine J 2021 Sep 3;11(7):1089-1098. Epub 2020 Aug 3.

38987TUHH Hamburg University of Technology, Hamburg, Germany.

Study Design: Biomechanical.

Objective: This study evaluates the biomechanical properties of lag-screws used in vertebral pincer fractures at the thoracolumbar junction.

Methods: Pincer fractures were created in 18 bisegmental human specimens. The specimens were assigned to three groups depending on their treatment perspective, either bolted, with the thread positioned in the cortical or cancellous bone, or control. The specimens were mounted in a servo-hydraulic testing machine and loaded with a 500 N follower load. They were consecutively tested in 3 different conditions: intact, fractured, and bolted/control. For each condition 10 cycles in extension/flexion, torsion, and lateral bending were applied. After each tested condition, a computed tomography (CT) scan was performed. Finally, an extension/flexion fatigue loading was applied to all specimens.

Results: Biomechanical results revealed a nonsignificant increase in stiffness in extension/flexion of the fractured specimens compared with the intact ones. For lateral bending and torsion, the stiffness was significantly lower. Compared with the fractured specimens, no changes in stiffness due to bolting were discovered. CT scans showed an increasing fracture gap during axial loading both in extension/flexion, torsion, and lateral bending in the control specimens. In bolted specimens, the anterior fragment was approximated, and the fracture gap nullified. This refers to both the cortical and the cancellous thread positions.

Conclusion: The results of this study concerning the effect of lag-screws on pincer fractures appear promising. Though there was little effect on stiffness, CT scans reveal a bony contact in the bolted specimens, which is a requirement for bony healing.
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http://dx.doi.org/10.1177/2192568220941443DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8351070PMC
September 2021

Comparative Biomechanical In Vitro Study of Different Modular Total Knee Arthroplasty Revision Stems With Bone Defects.

J Arthroplasty 2020 11 17;35(11):3318-3325. Epub 2020 Jun 17.

Department for Trauma Surgery and Orthopaedics, UKE University Medical Center Hamburg-Eppendorf, Hamburg, Germany.

Background: The aim of this study is to investigate the effects of different stem lengths and types including cones on primary stability in revision total knee arthroplasty with different femoral bone defects and fixation methods in order to maximize bone preservation. It is hypothesized that longer stems provide little additional mechanical stability.

Methods: Thirty-five human femurs were investigated. A distal bone defect, Anderson Orthopedic Research Institute classification (s. 33) type-F2a, was created in group 1-3 and type-F3 in group 4-6. A cemented, rotating hinge femoral component was combined with different stems (100 and 160 mm total or hybrid cemented cones, or a 100-mm custom-made anatomical cone stem). The femora were loaded according to in vivo loading during gait. Relative movements were measured to investigate primary stability. Pull-out testing was used to obtain a parameter for the primary stability of the construct.

Results: Relative movements were small and similar in all groups (<40 μm). For small defect, the pull-out forces of cemented long (4583 N) and short stems (4650 N) were similar and about twice as high as those of uncemented stems (2221 N). For large defects, short cemented stems with cones showed the highest pull-out forces (5500 N). Long uncemented stems (3324 N) and anatomical cone stems (3990 N) showed similar pull-out forces.

Conclusion: All tested stems showed small relative movements. Long cemented stems show no advantages to short cemented stems in small bone defects. The use of cones or an anatomical cone stem with hybrid cementation seems to offer good stability even for larger bone defects. The use of a short cemented stem (with or without cone) may be a suitable choice with a high potential for bone preservation in total knee arthroplasty revision with respective bone defects.
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http://dx.doi.org/10.1016/j.arth.2020.06.035DOI Listing
November 2020

Fatigue strength reduction of Ti-6Al-4V titanium alloy after contact with high-frequency cauterising instruments.

Med Eng Phys 2020 07 18;81:58-67. Epub 2020 May 18.

Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany. Electronic address:

Contact of implants with high-frequency cauterising instruments has serious implications for patient safety. Studies have reported a possible direct connection of fatigue failure of Ti-6Al-4V implants with electrocautery contact. Such contacts were observed at the polished neck of titanium hip stems, which are subjected to high-tension loads. Evidence of electrocautery contact has also been found on a retrieved spinal fixator with a rough surface; however, no fatigue failure related to electrocautery contact has been reported thus far. The influence of the heat-affected zone caused by flashover on the mechanical behaviour of the Ti-6Al-4V titanium alloy is not yet fully understood. Then, the aim of this study was to investigate whether the polished areas of Ti-6Al-4V implants are especially susceptible to fatigue failure after electrocautery contact. Flashovers caused by electrocautery contact were induced on titanium specimens with different surface roughnesses. These specimens were subjected to cyclic loading in a four-point-bending test setup, which represented the stress resulting from physiological loading activities (~861 MPa). In this test setup, electrocautery contact was found to reduce the fatigue strength of the titanium alloy significantly-by up to 96%-as revealed from the median value of the cycles to failure. Cycles to failure showed a dependence on the flashover duration, with a flashover for 40 ms leading to fatigue fracture. Despite the lower fatigue strength of a rough polished surface in the undamaged state, it is less prone to the damaging effect of flashover than a smooth polished surface.
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http://dx.doi.org/10.1016/j.medengphy.2020.05.016DOI Listing
July 2020

Do SiNx coatings bear the potential to reduce the risk of micromotion in modular taper junctions?

Proc Inst Mech Eng H 2020 Sep 7;234(9):897-908. Epub 2020 Jun 7.

Institute of Biomechanics, Hamburg University of Technology (TUHH), Hamburg, Germany.

Fretting corrosion is one contributor to the clinical failure of modular joint arthroplasty. It is initiated by micromotion in metal junctions exposed to fluids. Omitting metal-on-metal contacts could help to reduce the corrosion risk. The coating of one metal taper partner with a ceramic-based silicon nitride (SiNx) coating might provide this separation. The aim of the study was to identify whether a SiNx coating of the male taper component influences the micromotion within a taper junction. Hip prosthesis heads made of CoCr29Mo6 (Aesculap) and Ti6Al4V (Peter Brehm) were assembled (2000 N) to SiNx-coated and uncoated stem tapers made of Ti6Al4V and CoCr29Mo6 (2×2×2 combinations, each n = 4). Consecutive sinusoidal loading representing three daily activities was applied. Contactless relative motion in six degrees of freedom was measured using six eddy-current sensors. Micromotion in the junction was determined by compensating for the elastic deformation derived from additional monoblock measurements. After pull-off, the taper surfaces were microscopically inspected. Micromotion magnitude reached up to 8.4 ± 0.8 µm during loading that represented stumbling. Ti6Al4V stems showed significantly higher micromotion than those made of CoCr29Mo6, while taper coating had no influence. Statistical differences in pull-off forces were found for none of the taper junctions. Microscopy revealed CoCr29Mo6 abrasion from the head taper surface if combined with coated stem tapers. Higher micromotion of Ti6Al4V tapers was probably caused by the lower Young's modulus. Even in the contact areas, the coating was not damaged during loading. The mechanics of coated tapers was similar to uncoated prostheses. Thus, the separation of the two metal surfaces with the objective to reduce in vivo corrosion appears to be achievable if the coating is able to withstand in vivo conditions. However, the hard ceramic-based stem coating lead to undesirable debris from the CoCr29Mo6 heads during loading.
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http://dx.doi.org/10.1177/0954411920930616DOI Listing
September 2020

Reduced cement volume does not affect screw stability in augmented pedicle screws.

Eur Spine J 2020 06 23;29(6):1297-1303. Epub 2020 Mar 23.

Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany.

Purpose: Cement augmentation of pedicle screws is able to improve screw anchorage in osteoporotic vertebrae but is associated with a high complication rate. The goal of this study was to evaluate the impact of different cement volumes on pedicle screw fatigue strength.

Methods: Twenty-five human vertebral bodies (T12-L4) were collected from donors between 73 and 97 years of age. Bone density (BMD) was determined by quantitative computed tomography. Vertebral bodies were instrumented by conventional pedicle screws, and unilateral cement augmentation was performed. Thirteen vertebrae were augmented with a volume of 1 ml and twelve with a volume of 3 ml bone cement. A fatigue test was performed using a cranial-caudal sinusoidal, cyclic load (0.5 Hz) with increasing compression force (100 N + 0.1 N/cycles).

Results: The load to failure was 183.8 N for the non-augmented screws and was increased significantly to 268.1 N (p < 0.001) by cement augmentation. Augmentation with 1 ml bone cement increased the fatigue load by 41% while augmentation with 3 ml increased the failure load by 51% compared to the non-augmented screws, but there was no significant difference in fatigue loads between the specimens with screws augmented with 1 ml and screws augmented with 3 ml of bone cement (p = 0.504).

Conclusion: Cement augmentation significantly increases pedicle screw stability. The benefit of augmentation on screw anchorage was not significantly affected by reducing the applied volume of cement from 3 ml to 1 ml. Considering the high risk of cement leakage during augmentation, we recommend the usage of a reduced volume of 1 ml bone cement for each pedicle screw. These slides can be retrieved under Electronic Supplementary Material .
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http://dx.doi.org/10.1007/s00586-020-06376-wDOI Listing
June 2020

Conventional rotator cuff versus all-suture anchors-A biomechanical study focusing on the insertion angle in an unlimited cyclic model.

PLoS One 2019 27;14(11):e0225648. Epub 2019 Nov 27.

Department of Trauma-, Hand-, and Reconstructive Surgery, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.

Purpose: The purpose of this study was to compare the biomechanical properties of an all-suture anchor to a conventional anchor used commonly in rotator cuff repairs. Furthermore, the biomechanical influence of various implantation angles was evaluated in both anchor types in a human cadaveric model.

Methods: 30 humeri were allocated into three groups with a similar bone density. The two different anchor types were inserted at a predefined angle of 45°, 90° or 110°. Biomechanical testing included an initial preload of 20N followed by a cyclic protocol with a stepwise increasing force of 0,05N for each cycle at a rate of 1Hz until system failure. Number of cycles, maximum load to failure, stiffness, displacement and failure mode were determined.

Results: 27 anchors failed by pullout. There was no significant difference between the conventional and the all-suture anchor regarding mean pullout strength. No considerable discrepancy in stiffness or displacement could be perceived. Comparing the three implantation angles no significant difference could be observed for the all-suture or the conventional anchor.

Conclusion: All-suture anchors show similar biomechanical properties to conventional screw shaped anchors in an unlimited cyclic model. The exact insertion angle is not a significant predictor of failure.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0225648PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6880995PMC
March 2020

Micromotion at the head-stem taper junction of total hip prostheses is influenced by prosthesis design-, patient- and surgeon-related factors.

J Biomech 2020 01 15;98:109424. Epub 2019 Oct 15.

Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany.

Taper junctions of modular hip prostheses are susceptible to fretting and crevice corrosion. Prevalence and significance increase for cobalt-chromium heads assembled on titanium-alloy stems. Retrieval and in-vitro studies have identified micromotion between the taper components to accelerate the corrosion process. The aim of this study was to identify the most critical factors contributing to increased micromotion, which is most likely influenced by design-, patient- and surgeon-related aspects. Micromotion between head and stem taper surfaces was measured for different taper surface topographies and load orientations. Consecutive visual images were recorded through windows in the head component. By image matching analysis the local micromotions at the taper junction between head and stem tapers were determined. To extend the findings to taper regions not visible through the windows, finite element models were generated. The models were further utilized to investigate the influence of head length, taper angle difference and assembly force on micromotion. Significantly higher micromotion (+20%) was found under varus loading (7.1 µm) in comparison to valgus loading (5.9 µm). Smooth and microgrooved stem tapers exhibited equal amounts of micromotion. The numerical model revealed head tilting and recurring taper contact changes in terms of cyclic engagement/disengagement during the loading sequences. Especially long heads (+240%) and low assembly forces (+53%) were found to substantially increase micromotion (from 2.7 µm to 9.3 µm and from 4.1 µm to 8.8 µm, respectively). This study accentuates the susceptibility of taper junctions to a variety of factors, which need to be appreciated in preoperative planning and surgical procedure to reduce the amount of micromotion and such minimize the risk of critical corrosion.
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http://dx.doi.org/10.1016/j.jbiomech.2019.109424DOI Listing
January 2020

Biomechanical analysis of conventional anchor revision after all-suture anchor pullout: a human cadaveric shoulder model.

J Shoulder Elbow Surg 2019 Dec 13;28(12):2433-2437. Epub 2019 Jul 13.

Department of Trauma-, Hand-, and Reconstructive Surgery, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.

Hypothesis And Background: The possibility of implanting a conventional anchor at the pullout site following all-suture anchor failure was evaluated in a biomechanical cadaveric model. The hypothesis of the study was that anchor revision would yield equal biomechanical properties.

Methods: Ten human humeri were obtained, and bone density was determined via computed tomography. After all-suture anchor (n = 5) and conventional 4.5-mm anchor (n = 5) insertion, biomechanical testing was conducted. Following all-suture anchor pullout, a conventional 5.5-mm anchor was inserted at the exact site of pullout (n = 5) and biomechanical testing was reinitiated. Testing was conducted using an initial preload of 20 N, followed by an unlimited cyclic protocol, with a stepwise increasing force of 0.05 N for each cycle at a rate of 1 Hz until system failure. The number of cycles, maximum load to failure, stiffness, displacement, and failure mode, as well as macroscopic observation at the failure site including diameter, shape, and cortical destruction, were registered.

Results: The defect following all-suture pullout showed a mean diameter of 4 mm, and conventional revision was possible in each sample. There was no significant difference between the initial all-suture anchor implantation and the conventional anchor implantation or the conventional revision following all-suture failure regarding mean pullout strength, stiffness, displacement, or total number of cycles until failure.

Conclusion: Conventional anchor revision at the exact same site where all-suture anchor pullout occurred is possible and exhibits similar biomechanical properties.
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http://dx.doi.org/10.1016/j.jse.2019.04.053DOI Listing
December 2019

Evaluation of the capability of the simulated dual energy X-ray absorptiometry-based two-dimensional finite element models for predicting vertebral failure loads.

Med Eng Phys 2019 07 28;69:43-49. Epub 2019 May 28.

Department of Design Engineering and Mathematics, School of Science and Technology, Middlesex University, The Burroughs, Hendon, NW4 4BT London, UK.

Prediction of the vertebral failure load is of great importance for the prevention and early treatment of bone fracture. However, an efficient and effective method for accurately predicting the failure load of vertebral bones is still lacking. The aim of the present study was to evaluate the capability of the simulated dual energy X-ray absorptiometry (DXA)-based finite element (FE) model for predicting vertebral failure loads. Thirteen dissected spinal segments (T11/T12/L1) were scanned using a HR-pQCT scanner and then were mechanically tested until failure. The subject-specific three-dimensional (3D) and two-dimensional (2D) FE models of T12 were generated from the HR-pQCT scanner and the simulated DXA images, respectively. Additionally, the areal bone mineral density (aBMD) and areal bone mineral content (aBMC) of T12 were calculated. The failure loads predicted by the simulated DXA-based 2D FE models were more moderately correlated with the experimental failure loads (R = 0.66) than the aBMC (R = 0.61) and aBMD (R = 0.56). The 2D FE models were slightly outperformed by the HR-pQCT-based 3D FE models (R = 0.71). The present study demonstrated that the simulated DXA-based 2D FE model has better capability for predicting the vertebral failure loads than the densitometric measurements but is outperformed by the 3D FE model. The 2D FE model is more suitable for clinical use due to the low radiation dose and low cost, but it remains to be validated by further in vitro and in vivo studies.
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http://dx.doi.org/10.1016/j.medengphy.2019.05.007DOI Listing
July 2019

Influence of flexural rigidity on micromotion at the head-stem taper interface of modular hip prostheses.

Med Eng Phys 2019 06 10;68:1-10. Epub 2019 Apr 10.

Institute of Biomechanics, TUHH Hamburg University of Technology, Denickestraße 15, 21073 Hamburg Germany.

Fretting corrosion as one reason for failure of modular hip prostheses has been associated with micromotion at the head taper junction. Historically the taper diameter was reduced to improve the range of motion of the hip joint. In combination with other developments, this was accompanied by increased observations of taper fretting, possibly due to the reduced flexural rigidity of smaller tapers. The aim of the study was to investigate how the flexural rigidity of tapers influences the amount of micromotion at the head taper junction. Three different stem and two different taper designs were manufactured. Experimental testing was performed using three different activity levels with peak loads representing walking, stair climbing and stumbling. The relative motion at the head-stem taper was measured in six degrees of freedom. Micromotion was obtained by subtraction of the elastic deformation derived from monoblock and finite element analysis. Less rigid tapers lead to increased micromotion between the head and stem, enlarging the risk of fretting corrosion. The influence of the stem design on micromotion is secondary to taper design. Manufacturers should consider stiffer taper designs to reduce micromotion within the head taper junction.
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http://dx.doi.org/10.1016/j.medengphy.2019.03.020DOI Listing
June 2019

Determination of local micromotion at the stem-neck taper junction of a bi-modular total hip prosthesis design.

Med Eng Phys 2019 03 22;65:31-38. Epub 2019 Jan 22.

Institute of Biomechanics, Hamburg University of Technology (TUHH), Denickestrasse 15, Hamburg 21073, Germany.

High rates of clinical complications with bi-modular hip prostheses are attributed to failure of the stem-neck taper junction. Taper wear analyses have shown extensive material loss as a result of corrosion, potentially initiated by micromotion. The purpose of the study was to determine the amount of micromotion at this junction for different loading, assembly and material conditions. Micromotion between the neck adapter (CoCr29Mo6-alloy) and the stem (TiMo12Zr6Fe2-alloy; both Rejuvenate, Stryker) within the taper junction of a bi-modular hip stem were determined by image matching analysis of consecutively recorded images through windows in the stem component. A finite element model was used to determine the micromotion in the taper regions outside the windows and validated with the measured micromotion. With the model, the influence of the load amplitude, assembly force and component materials were then investigated. Determined micromotion (14-79 µm) by far exceeded critical values (5 µm) associated with the onset of fretting corrosion. Increasing assembly forces achieved a significant reduction in micromotion. The numerical model revealed insufficient assembly to cause the neck to perform rocking motions under load, repetitively changing taper contact in combination with gap opening, which facilitates fluid ingress into the junction. Changing the stem material to a stiffer Ti-alloy achieved a reduction of the micromotion of about 30%. This study emphasises the high importance of material selection, assembly force and loading on the susceptibility of bi-modular hip stems to fretting and crevice corrosion. These findings can serve to explain the increased rate of clinically reported problems with this particular prosthesis design.
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http://dx.doi.org/10.1016/j.medengphy.2019.01.003DOI Listing
March 2019

Patient specific glass fiber reinforced composite versus titanium plate: A comparative biomechanical analysis under cyclic dynamic loading.

J Mech Behav Biomed Mater 2019 03 18;91:212-219. Epub 2018 Dec 18.

Institute for Biomechanics, TUHH Hamburg University of Technology, Denickestraße 15, 21703 Hamburg, Germany.

Objectives: Free flap fixation with patient specific titanium (TI) plates is commonly performed after oncologic mandible resection, but plate exposure, osseous nonunion and imaging artefacts are associated complications. The aim of this study was to analyze interfragmentary movements and fatigue behaviour of patient specific titanium plates in comparison to a novel glass fiber reinforced composite (GFRC) plate in vitro.

Methods: Two polyurethane fibula segments were fixed to a corresponding mandible (Synbone AG, Malans, CH) with a patient specific 2.0 mm titanium plate (DePuy Synthes, Umkirch, Germany and Materialise, Leuven, Belgium) or one of two patient specific GFRC plates with different glass fiber orientation. Plate fixation to the fibula segments was performed with monocortical non-locking screws in all groups. Plate fixation to the mandible was performed with bicortical locking screws in the titanium group and with bicortical non-locking screws in the GFRC groups. Mastication was simulated via cyclic dynamic loading on the left side at a rate of 1 Hz with increasing peak loading (+0.15 N/cycle, Bionix, MTS, Eden Prairie, USA). A three-dimensional optical measuring system (PONTOS 5 M, GOM, Braunschweig, Germany) was used to determine interfragmentary movements between mandible and fibula segments.

Results: Mean plate stiffness of GFRC plates was 431 ± 64 N/mm and 453 ± 70 N/mm versus 560 ± 112 N/mm in the titanium group. No significant differences were found for the number of loading cycles until a vertical displacement of 1.0 mm (p = 0.637) and for vertical displacement over time (p = 0.490). Interosteotomy gap movement differed significantly between titanium and GFRC plates in the right distal (p = 0.001), intermediate (p = 0.006) and left distal gap (p = 0.025).

Conclusions: CAD/CAM titanium plates with locking screws provide increased stiffness and reduced interosteotomy movements in comparison to CAD/CAM glass fiber reinforced composite plates with non-locking titanium screws. Future studies should evaluate the influence of mechanobiologically optimized fixation systems on bone healing in free flap surgery.
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http://dx.doi.org/10.1016/j.jmbbm.2018.12.014DOI Listing
March 2019

Variogram-based evaluations of DXA correlate with vertebral strength, but do not enhance the prediction compared to aBMD alone.

J Biomech 2018 08 25;77:223-227. Epub 2018 Jul 25.

Institute for Surgical Technology and Biomechanics, University of Bern, Bern, Switzerland.

Ancillary evaluation of spinal Dual-energy X-ray Absorptiometry (DXA) via variogram-based texture evaluation (e.g., Trabecular Bone Score) is used for improving the fracture risk assessment, despite no proven relationship with vertebral strength. The purpose of this study was thus to determine whether classical variogram-based parameters (sill variance and correlation length) evaluated from simulated DXA scans could help predicting the in vitro vertebral strength. Experimental data of thirteen human full vertebrae (i.e., with posterior elements) and twelve vertebral bodies were obtained from two existing studies. Areal bone mineral density (aBMD) was calculated from 2D projection images of the 3D HR-pQCT scan of the specimens mimicking clinical DXA scans. Stochastic predictors, sill variance and correlation length, were calculated from their experimental variogram. Vertebral strength was measured as the maximum failure load of human vertebrae and vertebral bodies from mechanical tests. Vertebral strength correlated significantly with sill variance (r = 0.727) and correlation length (r = 0.727) for the vertebral bodies, and with correlation length (r = 0.593) for full vertebrae. However, the stochastic predictors improved the strength prediction made by aBMD alone by only 11% for the vertebral bodies while no improvement was observed for the full vertebrae. Despite a correlation, classical variogram parameters such as sill variance and correlation length do not enhance the prediction of in vitro vertebral strength beyond aBMD. It remains unclear why some variogram-based evaluations of DXA improve fracture prediction without a proven relationship with vertebral strength.
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http://dx.doi.org/10.1016/j.jbiomech.2018.07.009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6091628PMC
August 2018

Time to augment?! Impact of cement augmentation on pedicle screw fixation strength depending on bone mineral density.

Eur Spine J 2018 08 9;27(8):1964-1971. Epub 2018 Jun 9.

Department of Trauma Surgery, Orthopaedic Surgery and Plastic Surgery, University Medical Center Göttingen, Robert-Koch-Str. 40, 37099, Göttingen, Germany.

Purpose: Cement augmentation of pedicle screws is known to increase their mechanical strength. Aim was to evaluate the impact of cement augmentation on pedicle screw fatigue strength in dependence of the bone mineral density (BMD).

Methods: Twenty-one human L2 vertebral bodies from donors between 19 and 96 years of age were used for in vitro experiments. BMD was measured using quantitative computed tomography (QCT). Two pedicle screws were inserted in each specimen and unilaterally augmented with bone cement. Fatigue testing was performed using a cranio-caudal sinusoidal, cyclic load (0.5 Hz) with increasing compression force (100 N + 0.1 N/cycles). Results were evaluated for the BMD groups: normal: BMD > 120 mg/cm, osteopenic: BMD 80-120 mg/cm, and osteoporotic: BMD < 80 mg/cm bone mass.

Results: There was a significant correlation between fatigue force and BMD for the non-augmented and augmented screws (non-augmented R = 0.839, p < 0.001; augmented R = 0.551, p < 0.001). There was a significantly increased fatigue strength of the augmented screws over the non-augmented screws in the osteoporotic group (p = 0.001), while the differences in the other groups were not significant (normal p = 0.818/osteopenic p = 0.132).

Conclusions: The benefit of pedicle screw cement augmentation significantly depends on the bone mineral density and has the greatest extent of increased fatigue strength in osteoporotic vertebrae. Preoperative measurement of the BMD is strongly recommended to predict the benefit of augmentation and reinforce the decision for cement augmentation. These slides can be retrieved under Electronic Supplementary Material.
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http://dx.doi.org/10.1007/s00586-018-5660-7DOI Listing
August 2018

CAD-CAM plates versus conventional fixation plates for primary mandibular reconstruction: A biomechanical in vitro analysis.

J Craniomaxillofac Surg 2017 Nov 1;45(11):1878-1883. Epub 2017 Sep 1.

Department of Oral and Maxillofacial Surgery (Head: Priv. -Doz. Dr. med. Dr. med. dent. Henning Hanken), University Medical Center Hamburg-Eppendorf, Martinistraße 52, 20246 Hamburg, Germany.

Background: CAD/CAM reconstruction plates have become a viable option for mandible reconstruction. The aim of this study was to determine whether CAD/CAM plates provide higher fatigue strength compared with conventional fixation systems.

Material And Methods: 1.0 mm miniplates, 2.0 mm conventional locking plates (DePuy Synthes, Umkirch, Germany), and 2.0 mm CAD/CAM plates (Materialise, Leuven, Belgium/DePuy Synthes) were used to reconstruct a polyurethane mandible model (Synbone, Malans, CH) with cortical and cancellous bone equivalents. Mastication was simulated via cyclic dynamic testing using a universal testing machine (MTS, Bionix, Eden Prairie, MN, USA) until material failure reached a rate of 1 Hz with increasing loads on the left side.

Results: No significant difference was found between the groups until a load of 300 N. At higher loads, vertical displacement differed increasingly, with a poorer performance of miniplates (p = 0.04). Plate breakage occurred in miniplates and conventional locking plates. Screw breakage was recorded as the primary failure mechanism in CAD/CAM plates. Stiffness was significantly higher with the CAD/CAM plates (p = 0.04).

Conclusion: CAD/CAM plates and reconstruction plates provide higher fatigue strength than miniplates, and stiffness is highest in CAD/CAM systems. All tested fixation methods seem sufficiently stable for mandible reconstruction.
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http://dx.doi.org/10.1016/j.jcms.2017.08.024DOI Listing
November 2017

The taper corrosion pattern observed for one bi-modular stem design is related to geometry-determined taper mechanics.

Med Eng Phys 2017 08 21;46:79-88. Epub 2017 Jun 21.

TUHH Hamburg University of Technology, Institute of Biomechanics, Denickestrasse 15, 21073 Hamburg, Germany. Electronic address:

Bi-modular primary hip stems exhibit high revision rates owing to corrosion at the stem-neck taper, and are associated with local adverse tissue reactions. The aim of this study was to relate the wear patterns observed for one bi-modular design to its design-specific stem-neck taper geometry. Wear patterns and initial geometry of the taper junctions were determined for 27 retrieved bi-modular primary hip arthroplasty stems (Rejuvenate, Stryker Orthopaedics) using a tactile coordinate-measuring device. Regions of high-gradient wear patterns were additionally analyzed via optical and electron microscopy. The determined geometry of the taper junction revealed design-related engagement at its opening (angle mismatch), concentrated at the medial and lateral apexes (axes mismatch). A patch of retained topography on the proximal medial neck-piece taper apex was observed, surrounded by regions of high wear. On the patch, a deposit from the opposing female stem taper-containing Ti, Mo, Zr, and O-was observed. High stress concentrations were focused at the taper apexes owing to the specific geometry. A medial canting of the components may have augmented the inhomogeneous stress distributions in vivo. In the regions with high normal loads interfacial slip and consequently fretting was inhibited, which explains the observed pattern of wear.
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http://dx.doi.org/10.1016/j.medengphy.2017.06.003DOI Listing
August 2017

Hybrid Instrumentation in Lumbar Spinal Fusion: A Biomechanical Evaluation of Three Different Instrumentation Techniques.

Global Spine J 2017 Feb 1;7(1):47-53. Epub 2017 Feb 1.

Department of Spine and Scoliosis Surgery, Asklepios Klinik St. Georg, Hamburg, Germany.

Study Design: Ex vivo human cadaveric study.

Objective: The development or progression of adjacent segment disease (ASD) after spine stabilization and fusion is a major problem in spine surgery. Apart from optimal balancing of the sagittal profile, dynamic instrumentation is often suggested to prevent or impede ASD. Hybrid instrumentation is used to gain stabilization while allowing motion to avoid hypermobility in the adjacent segment. In this biomechanical study, the effects of two different hybrid instrumentations on human cadaver spines were evaluated and compared with a rigid instrumentation.

Methods: Eighteen human cadaver spines (T11-L5) were subdivided into three groups: rigid, dynamic, and hook comprising six spines each. Clinical parameters and initial mechanical characteristics were consistent among groups. All specimens received rigid fixation from L3-L5 followed by application of a free bending load of extension and flexion. The range of motion (ROM) for every segment was evaluated. For the rigid group, further rigid fixation from L1-L5 was applied. A dynamic Elaspine system (Spinelab AG, Winterthur, Switzerland) was applied from L1 to L3 for the dynamic group, and the hook group was instrumented with additional laminar hooks at L1-L3. ROM was then evaluated again.

Results: There was no significant difference in ROM among the three instrumentation techniques.

Conclusion: Based on this data, the intended advantage of a hybrid or dynamic instrumentation might not be achieved.
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http://dx.doi.org/10.1055/s-0036-1583945DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5400169PMC
February 2017

Biomechanical Effects of a Dynamic Topping off Instrumentation in a Long Rigid Pedicle Screw Construct.

Clin Spine Surg 2017 05;30(4):E440-E447

*Spine Center, Asklepios Klinik St. Georg †Institute of Biomechanics, TUHH Hamburg University of Technology ‡Department of Legal Medicine, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.

Study Design: Biomechanical ex vivo study.

Objective: To determine if topping off instrumentation can reduce the hypermobility in the adjacent segments when compared with the classic rigid spinal instrumentation.

Summary Of The Background Data: Long rigid instrumentation might increase the mechanical load in the adjacent segments, the resulting hypermobility, and the risk for adjacent segment disease. Topping off instrumentation intends to reduce the hypermobility at the adjacent level by more evenly distributing segmental motion and, thereby, potentially mitigating adjacent level disease.

Materials And Methods: Eight human spines (Th12-L5) were divided into 2 groups. In the rigid group, a 3-segment metal rod instrumentation (L2-L5) was performed. The hybrid group included a 2-segment metal rod instrumentation (L3-L5) with a dynamic topping off instrumentation (L2-L3). Each specimen was tested consecutively in 3 different configurations: native (N=8), 2-segment rod instrumentation (L3-L5, N=8), 3-segment instrumentation (rigid: N=4, hybrid: N=4). For each configuration the range of motion (ROM) of the whole spine and each level was measured by a motion capture system during 5 cycles of extension-flexion (angle controlled to ±5 degrees, 0.1 Hz frequency, no preload).

Results: In comparison with the intact spine, both the rigid 3-segment instrumentation and the hybrid instrumentation significantly reduced the ROM in the instrumented segments (L2-L5) while increasing the movement in the adjacent segment L1-L2 (P=0.002, η=0.82) and in Th12-L1 (P<0.001, η=0.90). There were no ROM differences between the rigid and hybrid instrumentation in all segments.

Conclusions: Introducing the dynamic topping off did not impart any significant difference in the segmental motion when compared with the rigid instrumentation. Therefore, the current biomechanical study could not show a benefit of using this specific topping off instrumentation to solve the problem of adjacent segment disease.
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http://dx.doi.org/10.1097/BSD.0000000000000244DOI Listing
May 2017

Insufficient stability of pedicle screws in osteoporotic vertebrae: biomechanical correlation of bone mineral density and pedicle screw fixation strength.

Eur Spine J 2017 11 8;26(11):2891-2897. Epub 2017 Apr 8.

Department of Trauma-, Orthopaedic- and Plastic Surgery, University Medical Center Göttingen, Robert-Koch-Str. 40, 37099, Göttingen, Germany.

Purpose: Loosening of pedicle screws is one major complication of posterior spinal stabilisation, especially in the patients with osteoporosis. Augmentation of pedicle screws with cement or lengthening of the instrumentation is widely used to improve implant stability in these patients. However, it is still unclear from which value of bone mineral density (BMD) the stability of pedicle screws is insufficient and an additional stabilisation should be performed. The aim of this study was to investigate the correlation of bone mineral density and pedicle screw fatigue strength as well as to define a threshold value for BMD below which an additional stabilisation is recommended.

Methods: Twenty-one human T12 vertebral bodies were collected from donors between 19 and 96 years of age and the BMD was measured using quantitative computed tomography. Each vertebral body was instrumented with one pedicle screw and mounted in a servo-hydraulic testing machine. Fatigue testing was performed by implementing a cranio-caudal sinusoidal, cyclic (0.5 Hz) load with stepwise increasing peak force.

Results: A significant correlation between BMD and cycles to failure (r = 0.862, r  = 0.743, p < 0.001) as well as for the linearly related fatigue load was found. Specimens with BMD below 80 mg/cm only reached 45% of the cycles to failure and only 60% of the fatigue load compared to the specimens with adequate bone quality (BMD > 120 mg/cm).

Conclusions: There is a close correlation between BMD and pedicle screw stability. If the BMD of the thoracolumbar spine is less than 80 mg/cm, stability of pedicle screws might be insufficient and an additional stabilisation should be considered.
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http://dx.doi.org/10.1007/s00586-017-5091-xDOI Listing
November 2017

A new algorithm for estimating the rod volume fraction and the trabecular thickness from in vivo computed tomography.

Med Phys 2016 Dec;43(12):6598

National Scientific and Technical Research Council (CONICET) and Department of Electrical Engineering and Computers, National University of the South, Bahía Blanca 8000, Argentina.

Purpose: Existing microstructure parameters are able to predict vertebral in vitro failure load, but for noisy in vivo data more complex algorithms are needed for a robust assessment.

Methods: A new algorithm is proposed for the microstructural analysis of trabecular bone under in vivo quantitative computed tomography (QCT). Five fractal parameters are computed: (1) the average local fractal dimension FD, (2) its standard deviation FD.SD, (3) the fractal rod volume ratio fRV/BV, (4) the average fractal trabecular thickness fTb.Th, and (5) its coefficient of variation fTb.Th.CV. The algorithm requires neither an explicit skeletonization of the trabecular bone, nor a well-defined transition between bone and marrow phases. Two experiments were conducted to compare the fractal with established microstructural parameters. In the first, 20 volumes-of-interest of embedded vertebrae phantoms were scanned five times under QCT and high-resolution (HR-)QCT and once under peripheral HRQCT (HRpQCT), to derive accuracy and precision. In the second experiment, correlations between in vitro HRQCT structural parameters were obtained from 76 human T, T, or L vertebrae. In vitro fracture data were available for a subset of 17 human T vertebrae so that linear regression models between failure load and microstructural HRQCT parameters could be analyzed.

Results: The results showed correlations of fTb.Th and fRV/BV with their nonfractal pendants trabecular thickness (Tb.Th) and respective structure model index (SMI) while higher precision and accuracy was observed on the fractal measures. Linear models of bone mineral density with two and three fractal microstructural HRQCT parameters explained 86% and 90% (adjusted R) of the failure load and significantly improved the linear models based only on BMD and established standard microstructural parameters (68%-77% adjusted R).

Conclusions: The application of fractal methods may grant further insight into the study of bone quality in vivo when image resolution and quality are less than optimal for current standard methods.
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http://dx.doi.org/10.1118/1.4967479DOI Listing
December 2016

Assembly force and taper angle difference influence the relative motion at the stem-neck interface of bi-modular hip prostheses.

Proc Inst Mech Eng H 2016 Jul 10;230(7):690-9. Epub 2016 May 10.

Institute of Biomechanics, Hamburg University of Technology (TUHH), Hamburg, Germany.

Bi-modular hip arthroplasty prostheses allow adaptation to the individual patient anatomy and the combination of different materials but introduce an additional interface, which was related lately to current clinical issues. Relative motion at the additional taper interface might increase the overall risk of fretting, corrosion, metallic debris and early failure. The aim of this study was to investigate whether the assembly force influences the relative motion and seating behaviour at the stem-neck interface of a bi-modular hip prosthesis (Metha(®); Aesculap AG, Tuttlingen, Germany) and whether this relation is influenced by the taper angle difference between male and female taper angles. Neck adapters made of titanium (Ti6Al4V) and CoCr (CoCr29Mo) were assembled with a titanium stem using varying assembly forces and mechanically loaded. A contactless eddy current measurement system was used to record the relative motion between prosthesis stem and neck adapter. Higher relative motion was observed for Ti neck adapters compared to the CoCr ones (p < 0.001). Higher assembly forces caused increased seating distances (p < 0.001) and led to significantly reduced relative motion (p = 0.019). Independent of neck material type, prostheses with larger taper angle difference between male and female taper angles exhibited decreased relative motion (p < 0.001). Surgeons should carefully use assembly forces above 4 kN to decrease the amount of relative motion within the taper interface. Maximum assembly forces, however, should be limited to prevent periprosthetic fractures. Manufacturers should optimize taper angle differences to increase the resistance against relative motion.
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http://dx.doi.org/10.1177/0954411916648717DOI Listing
July 2016

A description of spinal fatigue strength.

J Biomech 2016 Apr 11;49(6):875-880. Epub 2016 Feb 11.

Institute of Biomechanics, TUHH Hamburg University of Technology, Germany.

Understanding fatigue failure of the spine is important to establish dynamic loading limits for occupational health and safety. In this study experimental data were combined with published data to develop a description of the predictive parameters for spinal fatigue failure. 41 lumbar functional spinal units (FSUs) from cadaveric spines (age 49.0 ± 11.9 yr) where cyclically loaded. Three different levels of sinusoidal axial compression (0-3 kN, 0-2kN or 1-3kN) were applied for 300,000 cycles. Further, published data consisted of 70 thoracic and lumbar FSUs loaded in axial compression for 5000 cycles. Cyclic forces ranged from lower peaks (Fmin) of 0.7-1kN to upper peaks (Fmax) of 1.2-7.1 kN. Based on Wöhler analysis, a fatigue model was developed accounting for three parameters: I) specimen-specific scaling based on the endplate area, II) specimen-specific strength dependency on age or bone mineral density, III) load-specific correction factors based on Fmax and Fmin. The most predictive model was achieved for a combination of Fmax, endplate area and bone mineral density; this model explained 61% of variation (p<0.001). A model including Fmax, endplate area and age explained only 28% of variation (p<0.001). Inclusion of a load-specific correction factor did not significantly improve model prediction of fatigue failure. This analysis presents the basis for the prediction of specimen-specific fatigue failure of the lumbar spine, provided the endplate area and bone mineral density can be derived.
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http://dx.doi.org/10.1016/j.jbiomech.2016.01.041DOI Listing
April 2016

Failure of the human lumbar motion-segments resulting from anterior shear fatigue loading.

Ind Health 2016 Aug 30;54(4):308-14. Epub 2016 Jan 30.

Leeds Institute of Rheumatic and Musculoskeletal Medicine, University of Leeds, U.K.

An in-vitro experiment was designed to investigate the mode of failure following shear fatigue loading of lumbar motion-segments. Human male lumbar motion-segments (age 32-42 years, n=6) were immersed in Ringer solution at 37°C and repeatedly loaded, using a modified materials testing machine. Fatigue loading consisted of a sinusoidal shear load from 0 N to 1,500 N (750 N±750 N) applied to the upper vertebra of the motion-segment, at a frequency of 5 Hz. During fatigue experiments, several failure events were observed in the dynamic creep curves. Post-test x-ray, CT and dissection revealed that all specimens had delamination of the intervertebral disc. Anterior shear fatigue predominantly resulted in fracture of the apophyseal processes of the upper vertebrae (n=4). Exposure to the anterior shear fatigue loading caused motion-segment instability and resulted in vertebral slip corresponding to grade I and 'mild' grade II spondylolisthesis, as observed clinically.
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http://dx.doi.org/10.2486/indhealth.2015-0162DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4963543PMC
August 2016
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