Publications by authors named "Michael Morlock"

157 Publications

Modified acetabular component liner designs are not superior to standard liners at reducing the risk of revision : an analysis of 151,096 cementless total hip arthroplasties from the German Arthroplasty Registry.

Bone Joint J 2022 Jul;104-B(7):801-810

Charite - Center for Musculosceletal Surgery, Berlin, Germany.

Aims: Registry studies on modified acetabular polyethylene (PE) liner designs are limited. We investigated the influence of standard and modified PE acetabular liner designs on the revision rate for mechanical complications in primary cementless total hip arthroplasty (THA).

Methods: We analyzed 151,096 primary cementless THAs from the German Arthroplasty Registry (EPRD) between November 2012 and November 2020. Cumulative incidence of revision for mechanical complications for standard and four modified PE liners (lipped, offset, angulated/offset, and angulated) was determined using competing risk analysis at one and seven years. Confounders were investigated with a Cox proportional-hazards model.

Results: Median follow-up was 868 days (interquartile range 418 to 1,364). The offset liner design reduced the risk of revision (hazard ratio (HR) 0.68 (95% confidence interval (CI) 0.50 to 0.92)), while the angulated/offset liner increased the risk of revision for mechanical failure (HR 1.81 (95% CI 1.38 to 2.36)). The cumulative incidence of revision was lowest for the offset liner at one and seven years (1.0% (95% CI 0.7 to 1.3) and 1.8% (95% CI 1.0 to 3.0)). No difference was found between standard, lipped, and angulated liner designs. Higher age at index primary THA and an Elixhauser Comorbidity Index greater than 0 increased the revision risk in the first year after surgery. Implantation of a higher proportion of a single design of liner in a hospital reduced revision risk slightly but significantly (p = 0.001).

Conclusion: The use of standard acetabular component liners remains a good choice in primary uncemented THA, as most modified liner designs were not associated with a reduced risk of revision for mechanical failure. Offset liner designs were found to be beneficial and angulated/offset liner designs were associated with higher risks of revision. Cite this article:  2022;104-B(7):801-810.
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http://dx.doi.org/10.1302/0301-620X.104B7.BJJ-2021-1791.R1DOI Listing
July 2022

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

Variability in Femoral Preparation and Implantation Between Surgeons Using Manual and Powered Impaction in Total Hip Arthroplasty.

Arthroplast Today 2022 Apr 20;14:14-21. Epub 2022 Jan 20.

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

Background: The influence of the surgical process on implant loosening and periprosthetic fractures (PPF) as major complications in uncemented total hip arthroplasty (THA) has rarely been studied because of the difficulty in quantification. Meanwhile, registry analyses have clearly shown a decrease in complications with increasing experience. The goal of this study was to determine the extent of variability in THA stem implantation between highly experienced surgeons with respect to implant size, position, press-fit, contact area, primary stability, and the effect of using a powered impaction tool.

Methods: Primary hip stems were implanted in 16 cadaveric femur pairs by three experienced surgeons using manual and powered impaction. Quantitative CTs were taken before and after each process step, and stem tilt, canal-fill-ratio, press-fit, and contact determined. Eleven femur pairs were additionally tested for primary stability under cyclic loading conditions.

Results: Manual impactions led to higher variations in press-fit and contact area between the surgeons than powered impactions. Stem tilt and implant sizing varied between surgeons but not between impaction methods. Larger stems exhibited less micromotion than smaller stems.

Conclusions: Larger implants may increase PPF risk, while smaller implants reduce primary stability. The reduced variation for powered impactions indicates that appropriate measures may promote a more standardized process. The variations between these experienced surgeons may represent an acceptable range for this specific stem design. Variability in the implantation process warrants further investigations since certain deviations, for example, a stem tilt toward varus, might increase bone stresses and PPF risk.
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http://dx.doi.org/10.1016/j.artd.2021.10.005DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8789517PMC
April 2022

Influence of acetabular cup thickness on seating and primary stability in total hip arthroplasty.

J Orthop Res 2021 Dec 2. Epub 2021 Dec 2.

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

Insufficient primary stability of acetabular hip cups is a complication resulting in early cup loosening. Available cup designs vary in terms of wall thickness, potentially affecting implant fixation. This study investigated the influence of different wall thicknesses on the implantation process and the resulting primary stability using excised human acetabula. Implantations were performed using a powered impaction device providing consistent energy with each stroke. Two different wall thicknesses were compared in terms of seating progress, polar gap remaining after implantation, bone-to-implant contact area, cup deflection, and lever out moment. Thin-walled cups showed higher lever out resistance (p < 0.001) and smaller polar gaps (p < 0.001) with larger bone contact toward the dome of the cup (p < 0.001) compared to thick-walled cups. Small seating steps at the end of the impaction process were observed if a high number of strokes were needed to seat the cup (p = 0.045). A high number of strokes led to a strain release of the cup during the final strokes (p = 0.003). This strain release is indicative for over-impaction of the cup associated with bone damage and reduced primary stability. Adequate cup seating can be achieved with thin-walled cups with lower energy input in comparison to thicker ones. Thin-walled cups showed improved primary stability and enable implantation with lower energy input, reducing the risk of over-impaction and bone damage. Additional strokes should be avoided as soon as no further seating progress has been observed.
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http://dx.doi.org/10.1002/jor.25232DOI Listing
December 2021

Osteolysis following PE Wear of a Hastings Head on a Monoblock Hip Stem.

Case Rep Orthop 2021 7;2021:9989395. Epub 2021 Oct 7.

Institute of Biomechanics, Hamburg University of Technology, Denickestraße 15 (K) 21073 Hamburg, Germany.

We report a case of extended osteolysis, requiring a third revision of the left hip in an 85-year-old man 46 years after index operation. Major polyethylene (PE) wear occurred due to a missmatched combination of a bipolar Hastings head with a PE liner and head damage of the originally maintained stem. This case demonstrates that bipolar heads should not be used with PE cup liners since the respective bearing diameters cannot be guaranteed to match due to missing specifications. Furthermore, putting a Hastings head on an already damaged head of the stem should be omitted and rather the stem should initially be revised.
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http://dx.doi.org/10.1155/2021/9989395DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8516556PMC
October 2021

Experimental validation of the abrasive wear stage of the gross taper failure mechanism in total hip arthroplasty.

Med Eng Phys 2021 09 9;95:25-29. Epub 2021 Jul 9.

Endolab Mechanical Engineering GmbH, Rosenheim, Germany.

Background: Gross taper failure (GTF) is a rare but catastrophic failure mode of the head-stem-taper junction of hip prostheses, facilitated by massive material loss. GTF is a two stage process initiated by corrosion leading to head bottoming out, followed by abrasive wear due to the head rotating on the stem. The purpose of this study was to reproduce the clinical failure patterns and to determine the material loss during simulated gait.

Methods: Six cobalt-chromium alloy heads (36 mm, 12/14 taper) with three different head lengths (short / medium / extra long) were combined with stem taper replicas made from titanium alloy sized to achieve bottoming out. A hip simulator was used to simulate gait loading after (ISO 14242-1 for 2 million cycles).

Results: Wear patterns from in-vitro testing match the clinical failure patterns. Stem taper wear increased linearly with time (p< 0.001). After two million cycles the material loss of short / medium / extra long heads was (M+-STD) 1168±242 mg / 400±23 mg / 94±12 mg on the stem side and 46±36 mg / 46±24 mg / 70±8 mg on the head side. Stem taper wear decreased with increasing head length (p=0.01), whereas clinical failures are mostly seen for long and extra long heads.
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http://dx.doi.org/10.1016/j.medengphy.2021.07.003DOI Listing
September 2021

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

Factors influencing periprosthetic femoral fracture risk.

Bone Joint J 2021 Apr;103-B(4):650-658

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

Aims: Periprosthetic femoral fractures (PPF) are a serious complication of total hip arthroplasty (THA) and are becoming an increasingly common indication for revision arthroplasty with the ageing population. This study aimed to identify potential risk factors for PPF based on an analysis of registry data.

Methods: Cases recorded with PPF as the primary indication for revision arthroplasty in the German Arthroplasty Registry (Endoprothesenregister Deutschland (EPRD)), as well as those classified as having a PPF according to the International Classification of Diseases (ICD) codes in patients' insurance records were identified from the complete datasets of 249,639 registered primary hip arthroplasties in the EPRD and included in the analysis.

Results: The incidence of PPFs was higher (24.6%; 1,483) than reported in EPRD annual reports listing PPF as the main reason for revision (10.9%; 654). The majority of fractures occurred intraoperatively and were directly related to the implantation process. Patients who were elderly, female, or had comorbidities were at higher risk of PPFs (p < 0.001). German hospitals with a surgical volume of < 300 primary procedures per year had a higher rate of PPFs (p < 0.001). The use of cemented and collared prostheses had a lower fracture risk PPF compared to uncemented and collarless components, respectively (both p < 0.001). Collared prostheses reduced the risk of PPF irrespective of the fixation method and hospital's surgical volume.

Conclusion: The high proportion of intraoperative fractures emphasises the need to improve surgeon training and surgical technique. Registry data should be interpreted with caution because of potential differences in coding standards between institutions. Cite this article:  2021;103-B(4):650-658.
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http://dx.doi.org/10.1302/0301-620X.103B4.BJJ-2020-1046.R2DOI Listing
April 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

Automated age estimation of young individuals based on 3D knee MRI using deep learning.

Int J Legal Med 2021 Mar 17;135(2):649-663. Epub 2020 Dec 17.

Medical and Industrial Image Processing, University of Applied Sciences of Wedel, Feldstraße 143, 22880, Wedel, Germany.

Age estimation is a crucial element of forensic medicine to assess the chronological age of living individuals without or lacking valid legal documentation. Methods used in practice are labor-intensive, subjective, and frequently comprise radiation exposure. Recently, also non-invasive methods using magnetic resonance imaging (MRI) have evaluated and confirmed a correlation between growth plate ossification in long bones and the chronological age of young subjects. However, automated and user-independent approaches are required to perform reliable assessments on large datasets. The aim of this study was to develop a fully automated and computer-based method for age estimation based on 3D knee MRIs using machine learning. The proposed solution is based on three parts: image-preprocessing, bone segmentation, and age estimation. A total of 185 coronal and 404 sagittal MR volumes from Caucasian male subjects in the age range of 13 and 21 years were available. The best result of the fivefold cross-validation was a mean absolute error of 0.67 ± 0.49 years in age regression and an accuracy of 90.9%, a sensitivity of 88.6%, and a specificity of 94.2% in classification (18-year age limit) using a combination of convolutional neural networks and tree-based machine learning algorithms. The potential of deep learning for age estimation is reflected in the results and can be further improved if it is trained on even larger and more diverse datasets.
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http://dx.doi.org/10.1007/s00414-020-02465-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7870623PMC
March 2021

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

Gross Stem Taper Failure with Head Dissociation in a Very Active Patient with an Uncemented Femoral Stem.

JBJS Case Connect 2020 Jul-Sep;10(3):e1900662

1Institute of Biomechanics, Hamburg University of Technology, Hamburg, Germany 2Orthopaedische Chirurgie Muenchen, OCM-Clinic Munich, Munich, Germany 3Charité-University Medicine Berlin, Berlin, Germany.

Case: We report a case of gross taper failure (GTF) in a very active 81-year-old man 14 years after index operation. The patient presented with acute hip pain and shortening of the left leg. X-rays showed the dissociation of the head. The stem had to be revised because of the massive stem taper damage.

Conclusion: This case demonstrates that GTF at the head-stem taper junction following mechanically assisted crevice corrosion is not limited to a specific taper design and material. Other risk factors such as high activity level can lead to this failure pattern also in established stem and taper designs.
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http://dx.doi.org/10.2106/JBJS.CC.19.00662DOI Listing
April 2021

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

Time-dependent Viscoelastic Response of Acetabular Bone and Implant Seating during Dynamic Implantation of Press-fit Cups.

Med Eng Phys 2020 07 19;81:68-76. Epub 2020 May 19.

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

Deformation of an acetabular cup implant during cementless implantation is indicative of the radial compressive forces, and such of the initial implant fixation strength. Stress relaxation in the surrounding bone tissue following implantation could reduce the deformation of the cup and thus primary implant fixation. The aim of this study was therefore to determine the early shape change of the implanted cup immediately after implantation with different press-fit levels and whether recording the force during cup impaction can be used to estimate initial cup fixation. Cup implantations into porcine acetabulae (n=10) were performed using a drop tower. The force induced by the drop weight and cup seating after each impact was recorded. Deformation of the implanted cup was determined with strain gauges over a period of 10min. Lever-out torques were measured to assess the initial fixation strength. Stress relaxation in the bone caused a reduction in cup deformation of 13.52±4.06% after 1min and 29.34±5.11% after 10min. The fixation strength increased with a higher force magnitude during impaction (R=0.810, p=0.037). Reduction of the radial compressive forces due to stress relaxation of the surrounding bone should be considered during press-fit cup implantation in order to compensate for the reduced fixation strength over time. In addition, recording the implantation force could help to estimate initial fixation strength.
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http://dx.doi.org/10.1016/j.medengphy.2020.05.012DOI 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

Osteoidosis leads to altered differentiation and function of osteoclasts.

J Cell Mol Med 2020 05 13;24(10):5665-5674. Epub 2020 Apr 13.

Experimental Plastic Surgery, Clinic for Plastic and Hand Surgery, Technische Universität München, Munich, Germany.

In patients with osteomalacia, a defect in bone mineralization leads to changed characteristics of the bone surface. Considering that the properties of the surrounding matrix influence function and differentiation of cells, we aimed to investigate the effect of osteoidosis on differentiation and function of osteoclasts. Based on osteomalacic bone biopsies, a model for osteoidosis in vitro (OIV) was established. Peripheral blood mononuclear cells were differentiated to osteoclasts on mineralized surfaces (MS) as internal control and on OIV. We observed a significantly reduced number of osteoclasts and surface resorption on OIV. Atomic force microscopy revealed a significant effect of the altered degree of mineralization on surface mechanics and an unmasking of collagen fibres on the surface. Indeed, coating of MS with RGD peptides mimicked the resorption phenotype observed in OIV, suggesting that the altered differentiation of osteoclasts on OIV might be associated with an interaction of the cells with amino acid sequences of unmasked extracellular matrix proteins containing RGD sequences. Transcriptome analysis uncovered a strong significant up-regulation of transmembrane glycoprotein TROP2 in osteoclastic cultures on OIV. TROP2 expression on OIV was also confirmed on the protein level and found on the bone surface of patients with osteomalacia. Taken together, our results show a direct influence of the mineralization state of the extracellular matrix surface on differentiation and function of osteoclasts on this surface which may be important for the pathophysiology of osteomalacia and other bone disorders with changed ratio of osteoid to bone.
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http://dx.doi.org/10.1111/jcmm.15227DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7214153PMC
May 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

Contact conditions for total hip head-neck modular taper junctions with microgrooved stem tapers.

J Biomech 2020 04 25;103:109689. Epub 2020 Feb 25.

Department of Orthopedic Surgery, Rush University Medical Center, 1611 W Harrison St Suite 201, Chicago, IL 60612, United States. Electronic address:

Implant failure due to fretting-corrosion of head-neck modular junctions is a rising problem in total hip arthroplasty. Fretting-corrosion initiates when micromotion leads to metal release; however, factors leading to micromotion, such as microgrooves on the stem taper, are not fully understood. The purpose of this study is to describe a finite element analysis technique to determine head-neck contact mechanics and investigate the effect of stem taper microgroove height during head-neck assembly. Two-dimensional axisymmetric finite element models were created. Models were created for a ceramic femoral head and a CoCrMo femoral head against Ti6Al4V stem tapers and compared to available data from prior experiments. Stem taper microgroove height was investigated with a generic 12/14 model. Head-neck assembly was performed to four maximum loads (500 N, 2000 N, 4000 N, 8000 N). For the stem taper coupled with the ceramic head, the number of microgrooves in contact and plastically deformed differed by 2.5 microgrooves (4%) and 6.5 microgrooves (11%), respectively, between the finite element models and experiment. For the stem taper coupled with the CoCrMo head, all microgrooves were in contact after all assembly loads in the finite element model due to an almost identical conical angle between the taper surfaces. In the experiments, all grooves were only in contact for the 8000 N assembly load. Contact area, plastic (permanent) deformation, and contact pressure increased with increasing assembly loads and deeper microgrooves. The described modeling technique can be used to investigate the relationship between implant design factors, allowing for optimal microgroove design within material couples.
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http://dx.doi.org/10.1016/j.jbiomech.2020.109689DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7187651PMC
April 2020

How to proceed with asymptomatic modular dual taper hip stems in the case of acetabular revision.

Materials (Basel) 2020 Mar 2;13(5). Epub 2020 Mar 2.

Roechling SE & Co. KG, 68165 Mannheim, Germany.

How to proceed with a clinically asymptomatic modular Metha Ti alloy stem with dual taper CoCr neck adapter in case of acetabular revision? To systematically answer this question the status of research and appropriate diagnostic methods in context to clinically symptomatic and asymptomatic dual taper stem-neck couplings has been evaluated based on a systematic literature review. A retrieval analysis of thirteen Metha modular dual taper CoCr/Ti alloy hip stems has been performed and a rational decision making model as basis for a clinical recommendation was developed. From our observations we propose that in cases of acetabular revision, that for patients with a serum cobalt level of > 4 µg/L and a Co/Cr ratio > 3.6, the revision of the modular dual taper stem may be considered. Prior to acetabular revision surgery a systematic diagnostic evaluation should be executed, using specific tests such as serum metal (Co, Cr) ion analysis, plain antero-posterior and lateral radiographs and cross-sectional imaging modalities (Metal Artefact Reduction Sequence Magnetic Resonance Imaging). For an asymptomatic Metha dual taper Ti alloy/CoCr stem-neck coupling at the stage of acetabular revision careful clinical decision making according to the proposed model should be followed and overreliance on any single examination should be avoided, considering the complete individual differential diagnosis and patient situation.
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http://dx.doi.org/10.3390/ma13051098DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7084958PMC
March 2020

The influence of broach design on bone friction and osseodensification in total hip arthroplasty.

Clin Biomech (Bristol, Avon) 2020 03 18;73:234-240. Epub 2019 Dec 18.

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

Background: The process of cavity preparation by broaching has an impact on the primary stability of uncemented hip stems and on the periprosthetic fracture risk. Osseodensifying broaches may increase primary stability, but have the potential to raise cortex strains and facilitate fracture. The aim of this study was to determine the influence of broach design on the forces acting during broaching, on the microstructure of the broached bone bed and the amount and depth of osseodensification.

Methods: Broach models representing compaction, blunt extraction and sharp extraction broaches, were used for quasi-static simulation of femoral cavity preparation on bovine trabecular bone cuboids. Broaching forces were measured and micro-computed tomography scans performed prior and after testing. Friction coefficients during broaching, bone densification parameters and size of the debris particles pushed into the bone were determined.

Findings: Friction coefficients during sharp extraction exceeded those during compaction and blunt extraction broaching (by 38% and 37%, P < .001). Total bone densification was enhanced for compaction and blunt extraction compared to sharp extraction broaching (increase of 121% and 117%, P = .005), resulting from higher densification depths for compaction (P = .001) and higher maximum densification for blunt extraction broaching (P = .008), with the latter producing fewer large particles than compaction broaching (P = .005).

Interpretation: Higher friction coefficients indicate a decreased periprosthetic fracture risk with sharp extraction broaches for equal implantation forces. The blunt extraction and compaction designs investigated densified the bone to a similar extent. Blunt extraction broaching may support better osseointegration due to smaller bone debris particles.
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http://dx.doi.org/10.1016/j.clinbiomech.2019.12.012DOI Listing
March 2020

The hammer: instrument of Thor (and orthopaedic surgeons).

BMJ 2019 Dec 16;367:l6648. Epub 2019 Dec 16.

Centre for Hip Surgery, Wrightington Hospital, Wigan, UK.

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http://dx.doi.org/10.1136/bmj.l6648DOI Listing
December 2019

Metal Ion Release after Hip and Knee Arthroplasty - Causes, Biological Effects and Diagnostics.

Z Orthop Unfall 2020 Aug 9;158(4):369-382. Epub 2019 Dec 9.

Institute of Biomechanics, Hamburg University of Technology.

All metal implants in human bodies corrode which results in metal ions release. This is not necessarily a problem and represents for most patients no hazard. However, if a critical metal ion concentration is exceeded, local or rarely systemic problems can occur. This article summarizes the mechanisms of metal ion release and its clinical consequences. Several situations can result in increased metal ion release: metal-on-metal hip arthroplasties with increased wear, increased micromotion at taper interfaces, direct metal-metal contact (polyethylene wear, impingement), erroneously used metal heads after ceramic head fracture. Possible problems are in most cases located close to the concerned joint. Furthermore, there are reports about toxic damage to several organs. Most of these reports refer to erroneously used metal heads in revisions after a broken ceramic head. There is currently no evidence of carcinogenic or teratogenic effects of implants but data is not sufficient to exclude possible effects. Cobalt and chromium blood levels (favorably in whole blood) should be measured in patients with suspected elevated metal ions. According to current knowledge levels below 2 µg/l seem to be uncritical, levels between 2 and 7 µg/l are considered borderline with unknown biological consequences and levels above 7 µg/l indicate a local problem which should be further diagnosed. Metal ion levels always need to be interpreted together with clinical symptoms and imaging results.
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http://dx.doi.org/10.1055/a-0929-8121DOI Listing
August 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

Adapter sleeves are essential for ceramic heads in hip revision surgery.

Clin Biomech (Bristol, Avon) 2020 01 20;71:1-4. Epub 2019 Oct 20.

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

Background: Removing a head during isolated acetabular revision surgery can cause damage to the stem taper surface from extraction tool contact. Implanting a ceramic head on the damaged stem taper might elevate the fracture risk, which can be mitigated with the use of titanium adapter sleeves. The aim of this study was to investigate whether the improved fracture strength of modern generation ceramic heads allows the direct implantation on damaged stem tapers without an adapter sleeve.

Methods: Finite element models of taper junctions with and without adapter sleeve were generated. Different stem taper damages were modelled to investigate the influence on the ceramic head fracture load under axial compression.

Findings: Heads without adapter sleeves exhibited slightly higher or equal fracture strengths compared with sleeved heads for most scenarios. However, a small metal elevation on the stem taper caused a drastic decrease of the fracture strength if no adapter sleeve was used (-96%). The sleeved head was not influenced by the metal elevation damage.

Interpretation: Adapter sleeves are essential to ensure patient safety and prosthesis longevity whenever implanting ceramic heads on used stem tapers.
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http://dx.doi.org/10.1016/j.clinbiomech.2019.10.018DOI Listing
January 2020
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