Introduction

Reverse Shoulder Arthroplasty (RSA) is recognized to be an effective solution for rotator cuff deficient arthritic shoulders, but there are still concerns about impingement and range of motion (ROM). Several RSA biomechanical studies have shown that humeral lateralization can increase ROM in planar motions (e.g. abduction). However, there is still a debate whether humeral lateralization should be achieved with a larger sphere diameter or by lateralizing the center of rotation (COR). The latter has shown to decrease the deltoid moment arm and increase shear forces, where the former may pose challenges in implanting the device in small patients. The aim of this study was to evaluate how humeral lateralization achieved by varying COR lateral offset and glenosphere diameter in a reverse implant can affect impingement during activities of daily living (ADLs).

Lateralizing the center of rotation in reverse shoulder arthroplasty has been the subject of renewed interest due to complications associated with medialized center of rotation implants. Benefits of lateralization include: increased joint stability, decreased incidence of scapular notching, increased range of motion, and cosmetic appeal. However, lateralization may be associated with increased risk of glenoid loosening, which may result from the increased shear forces and the bending stresses that manifest at the bone-implant interface. To address glenoid loosening in reverse implants with lateralized joint centers, recent studies have focused on testing and improving implant fixation. However, these studies use loads derived from literature specific to subjects with normal anatomy. The aim of this study is to characterize how joint center lateralization affects the loading in reverse shoulder arthroplasty.

Using an established computational shoulder model that describes the geometry of a commercial reverse prosthesis (DELTA® III, DePuy), motion in abduction, scapular plane elevation, and forward flexion was simulated. The simulations were run for five progressively lateralized centers of rotation: −5, 0, +5, +10, and +15 mm (Figure 1). The model was modified to simulate a full thickness rotator cuff tear, where all cuff musculature except Teres Minor were excluded, to reflect the clinical indication for reverse shoulder arthroplasty on cuff tear arthropathy patients. To analyze the joint contact forces, the resultant glenohumeral force was decomposed into compression, anterior-posterior shear, and superior-inferior shear on the glenoid.

Joint center lateralization was found to affect the glenohumeral joint contact forces and glenoid loads increased by up to 18% when the center was lateralized from −5 mm to +15 mm. Compressive forces were found to be more sensitive to lateralization in abduction, while changes in shear forces were more affected in forward flexion and scapular plane abduction. On average, the superior shear component showed the largest increases due to lateralization (up to a 21% increase), while the anterior-posterior shear component showed larger changes than those of compression, except in the most lateralized center position (Figure 2).

The higher joint loads in the lateralized joint centers reflect a shortening of the Deltoid muscle moment arms (Figure 3), since the muscle needs to exert more force to provide the desired motions. The additional shear forces generated by the lateralization may increase the risk of the ‘rocking-horse’ effect. Together with the lateralized joint center, this creates an additional bending stress at the bone-implant interface that puts the implant at further risk of loosening (Figure 1). Current studies on implant fixation tend to use loads in compression and superior shear that exceed the forces seen in this study but have not investigated anterior-posterior shear loads. Our data support that loading in anterior-posterior direction can be significant. Using inappropriate loads to design fixation may result in excessive loss of bone stock and/or unforeseen implant loosening. The implication is that future studies may be performed using this more relevant data set to navigate the tradeoff between fixation and bone conservation.

The normal shoulder requires the basic mechanical characteristics of range of motion, stability and strength. However, each of these characteristics can be compromised by arthritis or rotator cuff tear and are often associated with strong pain. Shoulder arthoplasty is one of the most common solutions for pain relief and to restore shoulder functionality. There are many available designs of prosthesis trying to address different shoulder pathologies. Despite this, there are relatively few studies investigating the biomechanics of a total joint replacement and suggest advantages, disadvantages and possible solutions.

The Newcastle shoulder model has been used to investigate the biomechanical properties of a total shoulder replacement having a reverse anatomy design. This model allows the simulation of implantation of the prosthesis and the prediction of muscle and joint forces. To address the requirement of accurate insertion of the prosthesis, the standard surgical procedure has been simulated. The current model was modified to represent the bones, muscles and implant alignment after surgery.

Load sharing results for standardised tasks (Abduction, Forward Flexion) showed great differences between anatomical and prosthetic models. In the latter the shear forces on the glenoid site were reversed, the compression stresses were reduced and the joint contact vectors were always within the humeral cup providing joint stability. This is an important effect of the reverse design, which reverses the envelope of the joint forces increasing also the muscle moment arms crossing the GH joint. The most affected group is the m.deltoid that becomes able to compensate for the dysfunctional rotator cuff muscles. The biomechanical model suggests that a reverse anatomy design can restore GH joint stability for patients with severe RC damage. Increased muscle moment arms also compensate for the lost contribution of the RC muscles to elevation.

Introduction The complex movement of scapula is significant for the support of the arm and the stability of the shoulder joint. Recent investigations showed an adaptation in scapula rhythm after total shoulder replacement with a big variability within subjects. The latter can change the loading pattern in the glenohumeral contact forces and affect the performance of shoulder prosthesis.

Methods In this study, Newcastle shoulder model was used to simulate a total shoulder arthroplasty and investigate joint stability. The model describes the DELTA ^® prosthesis; a reverse anatomy design with a socket component attached to the humeral head and a hemi-ball to the glenoid. Scapula kinematics data of 6 shoulders were recorded using a palpating technique. The subjects had a total shoulder replacement after severe rotator cuff damage. Standard and daily activities were then analysed.

Results and Discussion Scapula kinematics data show an increased scapular lateral rotation, which influences the joint contact forces. Comparing contact forces on the Glenohumeral joint, results indicate that the scapula rhythm adaptation reduces the compressive forces and shifts the shear component more superiorly to the glenoid. The scapula rhythm data used in this study show a large variability, which also affect the loading results. This effect is more significant in “reaching tasks”, where high humeral elevation is required and joint contact loads are maximum. The anterior shear forces in these tasks can be as great as 19% of body weight

Conclusions The adaptation in scapulohumeral rhythm after a shoulder joint replacement has already been reported. The reason for this adaptation cannot be explained yet and may be pain related or due to muscle adaptation that takes place after the arthroplasty. This change in kinematics influences the loading pattern of the glenohumeral joint. In particular the increased shear forces must be taken into considered in prosthetic design.

Background: The Delta Inverse Shoulder Arthroplasty, has gained considerable popularity in France and Europe for Rotator Cuff (RC) deficient patients. However complications related to surgical difficulties with insertion, mechanical loosening, and polyethylene (PE) wear are now being reported. The increasing numbers of complications are alarming.

Methods: A computerised analysis of the kinematics associated with inverse shoulder replacements has been developed. Different glenoid dome sizes and humeral cup dimensions have been tested. This has highlighted the deficiencies associated with the design of the inverse shoulder prostheses currently available on the market. Modifications of the prosthesis geometry and the inclusion of a mobile bearing have also been subjected to computer analysis and have demonstrated improved kinematics and a reduction in the risk of bone/prosthesis contact. A medium term follow-up of 9 patients with a mobile bearing Inverse shoulder design has been carried out. Compression tests have been carried out on the PE bearing to identify the risk of plastic deformity of the PE.

Results: The results of the computerised analysis and the clinical results from the first 9 patients will be reported. One patient suffered an operative complication with failure of reduction of the prosthesis which was rectified at an immediate re-operation. The early clinical results are similar to the good results obtained with the Delta prosthesis. However polyethylene bearing failure has occurred in one case, highlighting the importance of optimising the geometry of the inverse shoulder. Both Radio-Stereometric Analyses (RSA) and prospective randomised studies comparing this Inverse prosthesis with either a Cuff Tear Arthropathy (CTA) hemiarthroplasty designes (when Gleno-Humeral(GH) OA is present) or with cuff reconstruction using augmentation (when no GH OA is present) have been designed and will be discussed.