[Implant anchorage: In vivo and in vitro analyses : Clusters for implant anchorage and safety]. / Implantatverankerung: In-vivo- und In-vitro-Analysen : Cluster Implantatverankerung und -sicherheit.

Aseptic implant loosening is the primary cause of revisions in arthroplasty. Various in vitro and in vivo methods are available for assessing implant fixation and stability. The aim of the Musculoskeletal Biomechanics Research Network (MSB-NET) is to continuously improve or develop these methods. In vitro analyses are often conducted using static and dynamic ISO and ASTM standards, while RSA, DXA, and EBRA analyses are established in vivo methods for evaluating implant fixation. Primary stability analyses, as well as acoustical methods, provide additional opportunities to detect loosening early and precisely evaluate implant stability. The cluster serves as a link between basic research, clinical practice, and end users to promote in vitro and in vivo methods to improve implant safety.

Does glenoid inclination affect the anterior stability of reverse total shoulder arthroplasty? A biomechanical study.

PURPOSE: The anterior stability of reverse total shoulder arthroplasty is affected by multiple factors. However, the effect of glenosphere inclination on stability has rarely been investigated, which is what this study aims to look into. METHODS: Reverse shoulder arthroplasty was performed on 15 cadaveric human shoulders. The anterior dislocation forces and range of motion in internal rotation in the glenohumeral joint (primary measured parameters) were tested in a shoulder simulator in different arm positions and implant configurations, as well as with a custom-made 10° inferiorly inclined glenosphere. The inclination and retroversion of the baseplate as well as the distance between the glenoid and coracoid tip in two planes (secondary measured parameters) were evaluated on CT scans. RESULTS: In biomechanical testing, the custom-made inclined glenosphere showed no significant influence on anterior stability other than glenoid lateralisation over all arm positions as well as the neck-shaft angle in two arm positions. The 6 mm lateralised glenosphere reduced internal rotation at 30° and 60° of glenohumeral abduction. In 30° of glenohumeral abduction, joint stability was increased using the 155° epiphysis compared with the 145° epiphysis. The mean inclination was 16.1°. The inclination was positively, and the distance between the glenoid and coracoid tip in the anterior-to-posterior direction was negatively correlated with anterior dislocation forces. CONCLUSIONS: The custom-made inferiorly inclined glenosphere did not influence anterior stability, but baseplate inclination itself had a significant effect on stability.

Radiopacity Enhancements in Polymeric Implant Biomaterials: A Comprehensive Literature Review.

Polymers as biomaterials possess favorable properties, which include corrosion resistance, light weight, biocompatibility, ease of processing, low cost, and an ability to be easily tailored to meet specific applications. However, their inherent low X-ray attenuation, resulting from the low atomic numbers of their constituent elements, i.e., hydrogen (1), carbon (6), nitrogen (7), and oxygen (8), makes them difficult to visualize radiographically. Imparting radiopacity to radiolucent polymeric implants is necessary to enable noninvasive evaluation of implantable medical devices using conventional imaging methods. Numerous studies have undertaken this by blending various polymers with contrast agents consisting of heavy elements. The selection of an appropriate contrast agent is important, primarily to ensure that it does not cause detrimental effects to the relevant mechanical and physical properties of the polymer depending upon the intended application. Furthermore, its biocompatibility with adjacent tissues and its excretion from the body require thorough evaluation. We aimed to summarize the current knowledge on contrast agents incorporated into synthetic polymers in the context of implantable medical devices. While a single review was found that discussed radiopacity in polymeric biomaterials, the publication is outdated and does not address contemporary polymers employed in implant applications. Our review provides an up-to-date overview of contrast agents incorporated into synthetic medical polymers, encompassing both temporary and permanent implants. We expect that our results will significantly inform and guide the strategic selection of contrast agents, considering the specific requirements of implantable polymeric medical devices.

Very early migration of a calcar-guided short stem: a randomized study of early mobilization and the influence of a calcium phosphate coating with 60 patients.

This study analyzed the migration of a calcar-guided short stem to determine the course of very early migration, as well as evaluated the effect of an additional calcium phosphate (CP) coating on a titanium plasma spray (TPS) coating, which has not been analyzed previously. Sixty patients were enrolled in this study and were treated with the A2 calcar-guided short stem. The implant coating was randomized with either the TPS or an additional CP coating, and radiostereometric analysis was performed with the baseline measurement before initial weight-bearing, along with follow-up examinations at 1 week, 6 weeks, 3 months, and 6 months. Implant migrations were 0.27 mm (standard deviation [SD], 0.13 mm) and 0.74 mm (SD, 1.11 mm) at 1 week and 6 months post-surgery, respectively, and 65% and 87% of the implants reached their final position 1 week and 6 weeks after surgery, respectively. After 6 weeks, 3 months, and 6 months, a significant increase was noted in the migration of the CP coating group vs. that of the TPS coating group. Upon the final observation at 6 months, the groups displayed on average a 0.74-mm migration. Most of the analyzed implants ceased migration within the first week post-surgery, but the CP coating demonstrated a higher and more prolonged migration compared to the TPS coating.

Contribution of various forefoot areas to push-off peak at different speeds and slopes during walking.

BACKGROUND: Push-off during the terminal stance phase has a major impact on forward progression during walking. During this phase, the ground reaction force is applied to a small area under the forefoot. A better understanding of how single forefoot areas contribute to push-off peak in healthy subjects is needed to develop biomimetic orthopedic devices for forefoot amputees. RESEARCH QUESTION: What is the contribution of different forefoot sole areas to push-off peak as a function of speed and slope? METHODS: In this analytical study, 15 healthy subjects walked on a treadmill at different speeds (0.8 m/s; 1.2 m/s; 1.6 m/s; max. gait speed) without de-/inclination and on different slopes (-10°; -5°; 0°; 5°; 10°) with normal walking speed. The Novel Pedar-X System was used to measure vertical sole force. Push-off peak of the entire sole was determined and relative contributions of the areas under the hallux, first ray, and toes (I-V) were calculated and analyzed using separate repeated-measures ANOVA (α = 0.05). RESULTS: Push-off peak increases with faster walking speeds as well as with 10° inclination. Downhill walking is associated with a reduced push-off peak. The contribution of all forefoot areas increases with faster walking speeds and at a declination of -10°. Push-off contribution of the area under the hallux increases by about 64.6% at fast walking compared to slow walking and this increase is higher than that of the area under the first ray and toes (p < 0.05). SIGNIFICANCE: These findings indicate the major role of the hallux in speed generation and the importance of the forefoot during downhill walking. The results show the need for an adequate assistive device even in hallux amputation cases to compensate for deficits in the push-off phase.

What do users and their aiding professionals want from future devices in upper limb prosthetics? A focus group study.

BACKGROUND: High rejection rates of upper limb prosthetics indicate that current prosthetic devices only partially meet user demands. This study therefore investigated the benefits and challenges with current prostheses, associated services and potential areas for improvement from the perspective of upper limb prosthesis users and various professionals working in the field of upper limb and hand prosthetics. METHODS AND FINDINGS: Seven different focus group discussions were conducted with 32 participants. Participants were grouped by prosthesis type, if they were prosthesis users, or professionals. All focus group discussions were transcribed verbatim, and a summarizing content analysis was performed. Three main topic areas to be addressed emerged from the interviews: 1. a properly functioning prosthesis, 2. the infrastructure, and 3. users' psychological and physical prerequisites. The interaction between a well-functioning prosthesis and a well-developed infrastructure was shown to be important for successful use. CONCLUSIONS: Our study raises many of the same issues that have been reported in previous qualitative studies, dating back over several decades. This study underlines the need to include users and professionals in the future development of prosthetic devices.

Comparison of conventional socket attachment and bone-anchored prosthesis for persons living with transfemoral amputation - mobility and quality of life.

BACKGROUND: For patients with transfemoral amputation experiencing issues with their sockets, bone-anchored prosthesis systems are an alternative and sometimes the only way to be mobile and independent. The present cross-sectional study aimed to investigate the gait performance and quality of life of a group of patients treated with bone-anchored systems compared to those of participants treated with a conventional socket-suspended prosthesis. METHODS: A total of 17 participants with a socket-suspended and 20 with a bone-anchored prosthesis were included. Gait patterns were examined for symmetry, and performance was assessed using the six-minute walk test and the timed "Up & Go" test. Magnetic resonance imaging was performed to detect signs of osteoarthritis in both hips. Mobility in everyday life and quality of life were assessed using questionnaires. FINDINGS: There were no differences between the groups regarding the quality of life, daily mobility, and gait performance. The step width was significantly higher for the patients using socket-suspended prosthesis. The socket-suspended group showed a significant asymmetry regarding the step length. In the socket-suspended group, the prosthetic leg showed significantly higher cartilage abrasion than the contralateral leg did. INTERPRETATION: Large differences in the measured outcomes in both groups illustrate the very different capabilities of the individual participants, which is apparently not primarily determined by the type of treatment. For patients who are satisfied with the socket treatment and perform well, bone-anchored prosthesis systems may not necessarily improve their functional capabilities and perceived quality of life.

The development and evaluation of an in-vitro shoulder simulator with active muscle simulation.

The purpose of the present study was to develop a novel active in-vitro shoulder simulator to emulate all forms of planar and non-planar glenohumeral motions with active muscle simulation on cadaver specimens or shoulder models and to critically evaluate its performance. A physiologic shoulder simulator, driven using simulated muscle force, was developed to dynamically realize accurate kinematic control in all three rotational degrees of freedom (DOF) under physiological kinetic boundaries. The control algorithm of the simulator was implemented using three parallel running independent control loops, which regulate the forces of individual muscles in the respect DOF and work asynchronously in disparate sequences adapted to specific motions (abduction, flexion/extension and rotation). Three cadaveric specimens were used to evaluate the kinematic and kinetic performance of the simulator during simulated motions. High kinematic accuracy (maximum mean deviation ≤ 2.35° and RMSE 1.13°) and repeatability (maximum and average SD of ≤ 1.21° and 0.67°) were observed in all three rotational DOF investigated. The reliabilities of all individual muscle forces actuated in the simulator during planar and non-planar motions were generally excellent, with the 95% CIs of ICC estimates of > 0.90 for most instances (30/36). A novel shoulder simulator with active muscle simulation was developed and evaluated. Its capability to reproduce kinematics and kinetics in a physiological range for all DOF was systematically evaluated for multiple kinetic and kinematic outcome variables. The presented simulator is a powerful tool for investigating the biomechanics of physiological and pathological shoulder joints and to evaluate various surgical interventions. Acquisition of reliable data in joint kinetics and translational kinematics during active motions is critical to assess shoulder pathologies and appropriate treatments. We provide a unique muscle activated physiologic shoulder simulator, which allows the comprehensive acquisition of joint kinematic and kinetic data during repeated realistic planar and non-planar motions.

Predictive simulation of post-stroke gait with functional electrical stimulation.

Post-stroke patients present various gait abnormalities such as drop foot, stiff-knee gait (SKG), and knee hyperextension. Functional electrical stimulation (FES) improves drop foot gait although the mechanistic basis for this effect is not well understood. To answer this question, we evaluated the gait of a post-stroke patient walking with and without FES by inverse dynamics analysis and compared the results to an optimal control framework. The effect of FES and cause-effect relationship of changes in knee and ankle muscle strength were investigated; personalized muscle-tendon parameters allowed the prediction of pathologic gait. We also predicted healthy gait patterns at different speeds to simulate the subject walking without impairment. The passive moment of the knee played an important role in the estimation of muscle force with knee hyperextension, which was decreased during FES and knee extensor strengthening. Weakening the knee extensors and strengthening the flexors improved SKG. During FES, weak ankle plantarflexors and strong ankle dorsiflexors resulted in increased ankle dorsiflexion, which reduced drop foot. FES also improved gait speed and reduced circumduction. These findings provide insight into compensatory strategies adopted by post-stroke patients that can guide the design of individualized rehabilitation and treatment programs.

The novel arthroscopic subscapular quadriceps tendon-bone sling procedure provides increased stability in shoulder cadavers with severe glenoid bone loss.

PURPOSE: Treatment of anterior glenoid bone loss in patients with recurrent anterior shoulder instability is a challenge. The subscapular sling method with quadriceps tendon bone (QTB) graft is a modification of the subscapular sling with a semitendinosus (ST) graft. The aim of the study was to test the biomechanical stability of the QTB sling procedure in human shoulder cadavers with severe anterior glenoid bone loss. METHODS: Fourteen cadaveric shoulders were tested with a force-moment-guided robot in three conditions: physiologically intact, anterior glenoid bone resection, and the subscapular sling procedure with a QTB graft. Joint stability was measured in anterior, anterior inferior and inferior directions in four glenohumeral joint positions: 0° and 60° of glenohumeral abduction, with each at 0° and 60° of external rotation. Maximum external rotation was measured at 0° and 60° glenohumeral abduction. Computer tomography scans were obtained preoperatively to plan the glenoid bone resection, as well as postoperatively to calculate the proportion of the glenoid bone actually resected. RESULTS: Significantly decreased translations were observed in the shoulders with the QTB sling compared to the intact joint and the glenoid bone loss model. No significant differences in maximum external rotation were observed between the three different conditions. CONCLUSION: This biomechanical study revealed a significant stabilizing effect of the arthroscopic subscapular QTB graft sling procedure in human shoulder cadavers without compromising external rotation. Clinical trials may reveal the usefulness of this experimental method.

Experimental evaluation of precision and accuracy of RSA in the lumbar spine.

PURPOSE: Roentgen stereophotogrammetric analysis is a technique to make accurate assessments of the relative position and orientation of bone structures and implants in vivo. While the precision and accuracy of stereophotogrammetry for hip and knee arthroplasty is well documented, there is insufficient knowledge of the technique's precision and, especially accuracy when applied to rotational movements in the spinal region. METHODS: The motion of one cadaver lumbar spine segment (L3/L4) was analyzed in flexion-extension, lateral bending and internal rotation. The specific aim of this study was to examine the precision and accuracy of stereophotogrammetry in a controlled in vitro setting, taking the surrounding soft tissue into account. The second objective of this study was to investigate the effect of different focal spot values of X-ray tubes. RESULTS: Overall, the precision of flexion-extension measurements was found to be better when using a 0.6 mm focal spot value rather than 1.2 mm (± 0.056° and ± 0.153°; respectively), and accuracy was also slightly better for the 0.6 mm focal spot value compared to 1.2 mm (- 0.137° and - 0.170°; respectively). The best values for precision and accuracy were obtained in lateral bending for both 0.6 mm and 1.2 mm focal spot values (precision: ± 0.019° and ± 0.015°, respectively; accuracy: - 0.041° and - 0.035°). CONCLUSION: In summary, the results suggest stereophotogrammetry to be a highly precise method to analyze motion of the lumbar spine. Since precision and accuracy are better than 0.2° for both focal spot values, the choice between these is of minor clinical relevance.

Is the strain pattern of conventional stems negatively affected by a previously short stem THA? An experimental study in cadavaric bone.

BACKGROUND: A short stem hip arthroplasty can be revised in many cases using a conventional stem. Furthermore, in some cases the implantation of a short stem is intended, but intraoperatively reasons may lead to the decision to implant a conventional stem after previous preparation of a short stem. OBJECTIVE: In both cases it is questionable if the anchorage of a conventional stem is negatively affected by the previous preparation of a short stem. In clinical practice mid- or long-term follow up for these special cases hardly exist. METHODS: The strain patterns for the conventional Bicontact stem in primary implantation and after preparation of the proximal femur for a METHA short stem were tested biomechanically in three pairs of cadaveric femora. RESULTS: The strain patterns for the conventional Bicontact after preparation of the METHA short stem were similar to conditions after testing the conventional stem in primary conditions. CONCLUSIONS: These data lead to the consequence that in clinical practise the implantation of a conventional stem after preparation of a short stem and even after revision of a short stem is possible without increased risk of loosening or long-term stress-shielding.

Biomechanical Assessment of Three Osteosynthesis Constructs by Periprosthetic Humerus Fractures.

BACKGROUND: Biomechanical stability assessment of 3 different constructs for proximal fixation of a locking compression plate (LCP) in treating a Worland type C periprosthetic fracture after total shoulder arthroplasty. METHODS: 27 Worland type C fractures after shoulder arthroplasty in synthetic humeri were treated with 14-hole LCP that is proximally fixed using the following: (1) 1 × 1.5 mm cerclage wires and 2x unicortical-locking screws, (2) 3 × 1.5 mm cerclage wires, or (3) 2x bicortical-locking attachment plates. Torsional stiffness was assessed by applying an internal rotation moment of 5 Nm and then after unloading the specimen, an external rotation moment of 5 Nm at the same rate was applied. Axial stiffness was assessed by applying a 50 N preload, and then applying a cyclic load of 250 N, then increasing the load by 50 N each time, until a maximum axial load of 2500 N was reached or specimen failure occurred. RESULTS: With regard to internal as well as external rotational stiffness, group 1 showed a mean stiffness of 0.37 Nm/deg and 0.57 Nm/deg, respectively, group 2 had a mean stiffness of 0.51 Nm/deg and 0.39 Nm/deg, respectively, while group 3 had a mean stiffness of 1.34 Nm/deg and 1.31 Nm/deg, respectively. Concerning axial stiffness, group 1 showed an average stiffness of 451.0 N/mm, group 2 had a mean stiffness of 737.5 N/mm, whereas group 3 had a mean stiffness of 715.8 N/mm. CONCLUSION: Group 3 displayed a significantly higher torsional stiffness while a comparable axial stiffness to group 2.

Anatomic factors influencing the anterior stability of reverse total shoulder arthroplasty.

BACKGROUND: Several factors affect the stability of the reverse shoulder arthroplasty. The influence of bony anatomy on anterior stability remains unclear. This study aimed to identify the correlations between bony anatomy and anterior dislocation forces. METHODS: The differences in anterior dislocation force in reverse total shoulder arthroplasty reported in a previous biomechanical study were used to analyze the anatomic factors influencing anterior stability. The critical shoulder angle, glenocoracoid distance in 2 planes, and glenoid inclination were measured in the tested specimens using 3-dimensional computed tomographic scans and radiographs. Anatomic parameters were then correlated with the anterior dislocation forces. RESULTS: The critical shoulder angle had no correlation with anterior stability. The glenocoracoid distance in anteroposterior direction showed a negative correlation with the stability of a reverse shoulder arthroplasty with a 9-mm lateralized glenosphere and 155° humeral inclination in 30° and 60° glenohumeral abduction with the arm in 30° external rotation (r = -0.662, P = .004; r = -0.794, P = .011) and 30° glenohumeral abduction with neutral rotation (r = -0.614, P = .009). Using the same hardware configuration, the anterior stability had a negative correlation with the glenocoracoid distance in the mediolateral direction in 30° of glenohumeral abduction with the arm in 0° and 30° of external rotation (r = -0.542, P = .025; r = -0.497, P = .042). CONCLUSION: The distance between the coracoid tip and glenoid in 2 planes had a significant negative correlation with the anterior stability of the reverse shoulder arthroplasty with a lateralized glenosphere and 155° humeral inclination. The findings suggest that only glenoid lateralization is influenced by the bony anatomy.

Greater early migration of a short-stem total hip arthroplasty is not associated with an increased risk of osseointegration failure: 5th-year results from a prospective RSA study with 39 patients, a follow-up study.

Background and purpose - Short-stem hip arthroplasty has been a viable alternative to standard stems for the treatment of hip osteoarthritis for over 10 years. This study assessed whether a correlation existed between a greater initial increase in implant migration and inferior clinical outcomes at 5 years postoperatively. Results on these patients after 2 years have been published previously.Patients and methods - Radiostereometry and clinical scoring were undertaken after surgery and at 3, 6, 12, and 24 months, and 5 years postoperatively. The migration and the clinical outcomes data from the patients with initial migrations at 3 months above the 75th percentile (≥ 75% group) were compared with those with migrations at 3 months of less than the 75th percentile (< 75% group).Results - Between 3 months and 5 years after surgery, the mean resultant implant migrations were 0.40 mm (SD 0.32) in the ≥ 75% group and 0.39 mm (SD 0.25) in the < 75% group. The mean Harris Hip Scores and SF-36 physical scores at 5 years postoperatively were 100 (SD 0.4) and 44 (SD 12), respectively, for the ≥ 75% group and 99 (SD 2) and 50 (SD 10), respectively, for the < 75% group. The differences between the patient groups were not statistically significant.Interpretation - There was no correlation between a greater initial migration and inferior clinical outcomes at 5 years postoperatively. Despite a greater initial migration, there were no risks of early aseptic loosening and inferior midterm clinical outcomes associated with a short-stem implant with a primary metaphyseal anchorage.

A short stem with metaphyseal anchorage reveals a more physiological strain pattern compared to a standard stem - an experimental study in cadavaric bone.

PURPOSE: The proposed advantages of short stem hip arthroplasties are bone preserving strategies and less soft tissue damage. Bone preserving strategies do not only include a more proximal resection of the femoral neck, but especially for short stem hip arthroplasties with predominantly metaphyseal fixation a presumed more physiologic load transfer and thus a reduction of stress-shielding. However, the hypothesized metaphyseal anchorage associated with the aforementioned benefits still needs to be verified. Unfortunately, mid- to long-term clinical studies are missing. METHODS: Therefore, the METHA short stem as a short stem with proposed metaphyseal anchorage and the Bicontact® standard stem were tested biomechanically in three pairs of cadaveric femora while strain gauges monitored their corresponding strain patterns. RESULTS: For the METHA stem, the strains in all tested locations including the region of the calcar were similar to conditions of cadaver without implanted stem. The Bicontact stem showed approximately half of strain of the non-implanted cadaveric femura with slightly increasing strain from proximal to distal. CONCLUSIONS: Summarizing, the current study revealed primary metaphyseal anchorage of the METHA short stem and a metaphyseal-diaphyseal anchorage of the Bicontact stem.

Force, impulse and energy during falling with and without knee protection: an in-vitro study.

The mechanics of protective knee padding mitigating injury from a high-force fall have not been investigated in real-life scenarios to date. This study compares the effect of wearing knee pads to unprotected impact on a hard surface. We hypothesized that knee pads reduce the force and energy transmitted to the bony structures of the knee cap compared with unprotected conditions. Eight human knee cadaver specimens were embedded and fixed with a flexion angle of 100 degrees in a custom-made drop testing device (75 kg including the knee). The usage of a knee pad led to an average peak force attenuation on impact of 15% (no pad: 5932 N SD: 2472 N; pad: 4210 N SD: 2199 N; p < 0.001). Contact time on the plate was higher with a knee pad (no pad: 0.015 s SD: 0.009 s; pad: 0.028 s SD: 0.014 s; p < 0.001). Therefore, the observed impulse was also increased (no pad: 62.2 Ns SD: 17.8 Ns; pad: 74.6 Ns SD: 18.6 Ns; p < 0.001). This effect diminished as drop height was increased. Energy dissipation, defined as the difference between kinetic energy pre-impact and peak potential energy post-impact, was higher without a knee pad (no pad: 10.5 J SD: 6.2 J; pad: 4.2 J SD: 5.0 J; p < 0.001). The results from this study illustrate the magnitude of influence that knee pads have on peak forces, transmitted impulse, and energy transfer from a high-force impact in real-life scenarios. Contrary to expectations, the knee pad did not act as a mechanical damper. The mechanical behavior more closely resembled a spring that temporarily stores energy and consequentially reduces peak forces upon impact. Based on this study, future developments in padding might benefit from focusing on the aspect of energy storage and temporarily delayed energy dissipation.

The Laxity of the Native Knee: A Meta-Analysis of in Vitro Studies.

BACKGROUND: Although soft-tissue balancing plays an important role in knee arthroplasty, we are aware of no objective target parameters describing the soft-tissue tension of the native knee. In the present study, we aimed to meta-analyze data from studies investigating native knee laxity to create a guide for creating a naturally balanced knee joint. METHODS: PubMed and Web of Science were searched for studies with laxity data published from 1996 through 2016. Graphs were digitally segmented in cases in which numerical data were not available in text or table form. Three-level random-effects meta-analyses were conducted. RESULTS: Seventy-six studies evaluating knee laxity at various flexion angles (0° to 90°) were included. Knee laxity was significantly different between 0° and 90° of flexion (p < 0.001) in all 6 testing directions, with mean differences of 0.94 mm and -0.35 mm for anterior and posterior translation, 1.61° and 4.25° for varus and valgus rotation, and 1.62° and 6.42° for internal and external rotation, respectively. CONCLUSIONS: Knee laxity was dependent on the flexion angle of the knee joint in all degrees of freedom investigated. Furthermore, asymmetry between anterior-posterior, varus-valgus, and internal-external rotation was substantial and depended on the joint flexion angle. CLINICAL RELEVANCE: If the goal of knee arthroplasty is to restore the kinematics of the knee as well as possible, pooled laxity data of the intact soft tissue envelope could be useful as a general guide for soft-tissue balancing in total knee arthroplasty.

Effect of the humeral neck-shaft angle and glenosphere lateralization on stability of reverse shoulder arthroplasty: a cadaveric study.

BACKGROUND: Lateralizing the glenosphere and decreasing the humeral neck-shaft angles are implant design parameters that reduce the risk of scapular impingement. The effects of these parameters on joint stability remain unclear. This study evaluated the effect of glenosphere lateralization and humeral neck-shaft angle on joint stability by quantifying the anterior dislocation force in different arm positions. METHODS: Reverse shoulder arthroplasty was performed on 19 human shoulder specimens. Anterior dislocation force and maximum external rotation were evaluated using a robot-based shoulder simulator. By varying the neck-shaft angle and magnitudes of glenosphere lateralization, 12 configurations were analyzed with the glenohumeral joint in 30° and 60° of abduction, in neutral, and in 30° of external rotation. RESULTS: At 30° of abduction, measurements showed significantly higher dislocation forces for the 9-mm and 6-mm lateralized glenosphere than for the 0-mm (P < .0001, P = .007) nonlateralized glenosphere. At 60° of abduction, measurements showed significantly higher dislocation forces for the 9-mm and 6-mm lateralized glenosphere than for the 0-mm (P < .0001, P = .0007) and 3-mm (P = .0003, P = .04) glenosphere. Configurations with a neck-shaft angle of 135° showed significantly higher dislocation forces than configurations with a neck-shaft angle of 145° (P = .02) or 155° (P = .02) at 30° of abduction in 30° of external rotation. Neck-shaft angle and glenosphere lateralization had no influence on maximum external rotation capability. CONCLUSION: Glenosphere lateralization significantly increased anterior stability of the glenohumeral joint without influencing the range of passive external rotation. The humeral neck-shaft angle only had a minor effect on anterior stability.