Background

When reversing the hard-soft articulation in inverse shoulder replacement, i.e. hard inlay and soft glenosphere, the tribological behaviour of such a pairing has to be tested thoroughly. Therefore, two hard materials for the inlay, CoCr alloy and alumina toughened zirconia ceramic (ATZ) articulating on two soft materials, conventional UHMWPE and vitamin E stabilised, highly cross-linked PE (E-XLPE) were tested.

The morphology of the proximal part of the humerus varies largely. Morphometric features characterizing the three-dimensional geometry of the proximal humerus have revealed a wide difference within individuals. These parameters include head size, radius of curvature, inclination angle, retroversion angle, offsets and neck-shaft angle. Different implant designs have been adapted so as to make provision for these anatomical variations. However, the optimal design criteria are yet to be established. Implant design is one of the main factors determining the success of Total Shoulder Arthroplasty (TSA) since slight modifications in the implant anatomy could have significant biomechanical effects. Therefore, this study investigates the three-dimensional morphometric parameters of the South African proximal humerus which will serve as a basis for designing a new Total Shoulder Prosthesis for the South African population. Sixteen South African (SA) fresh cadaveric humeri (8 left, 8 right; 8 paired) were used in this study. The data consisted of 6 men and 2 women with ages ranging from 32 to 55 years (43.13 ±8.51). The humeri were scanned using a Computer Tomography (CT) scanner. The Digital Imaging and Communications in Medicine (DICOM) files from the CT data were imported into medical modelling software, MIMICS for reconstruction. The 3D reconstructed model of the humeri as an STL file was used for further processing. The STL data were generated as a cloud of points in a CAD software, SolidWorks. These were then remodeled by defining the detailed Referential Geometric Entities (RGEs) describing the anatomical characteristics. Anatomical reference points were defined for the anatomical neck plane, the epiphyseal sphere and the metaphyseal cylinder. Also, axes were defined which comprises of the humeral head axis and the metaphyseal axis. Thereafter, the posterior offsets medial offsets and the inclination angles were measured based on the RGEs. The posterior offset varied from 0.07 mm to 2.87 mm (mean 1.20 mm), the medial offset varied from 4.40 mm to 8.45 mm (mean 6.50 mm) while the inclination angle varied from 114.00º to 133.87º (mean 121.05º). The outcome of the study showed that the shape and dimensions of the proximal humerus varies distinctively. The articular surface is not a perfect sphere and differs independently with respect to the inclination angles. In addition, variations were noticeable in the medial and lateral offsets. The morphometric data on the African shoulder is very limited and this study will significantly contribute to the shoulder data repository for the SA population. The morphometric parameters measured in this study will be useful in designing a South African shoulder prosthesis that mimics the native shoulder hence eliminating post-surgical complications

Total Shoulder Arthroplasty (TSA) is a solution to fixing shoulder complications and restoring normal shoulder functionality. Shoulder arthritis is one of the common indicators of TSA. Studies suggest that 15% and 7% of the total Rheumatoid Arthritis (RA) and Osteoarthritis (OA) patients respectively, in sub-Saharan Africa, have degenerated shoulders. These patients are implanted with a Total Shoulder Prosthesis (TSP). There are limited literature available on the morphometric features of African shoulders. Previous studies have indicated that differences in shoulder surface geometry of the European and African populations, exists. This study aims at identifying the structural differences of the humeral articulating surfaces between South African and Swiss data sets. The South African data set included the Computerised Tomography (CT) scans of cadavers sourced from the University of Cape Town and the Swiss data set included the cadaver CT scans obtained from the SICAS Medical Image Repository. Sixty reconstructed models of humerus were generated from these scans of 30 (bilateral) healthy cadavers (15 South African and 15 Swiss) using Mimics®. The humeral articulating surfaces were separated from the shaft by performing in-silico surgery using SOLIDWORKS®, according to the guidelines provided orthopaedic surgeons. A Matlab code was generated to determine the superior-inferior (S-I) and the anterior-posterior (A-P) circular diameter and the peak points (PPs) of the articulating surfaces. The PPs were defined as the highest point on the articulating surface, which is most likely to be in contact with the glenoid. The S-I diameter was found to be significantly greater (p<0.01) than the A-P diameter for both the data sets (average difference = 5.02mm). Both the average A-P and S-I diameter for the Swiss data set were significantly larger (p = 0.02 and p = 0.03) than the South African data set by 2.36 mm and 2.70 mm respectively. The PPs were found to lie at an off-set from the origin. in case of the Swiss data set the average PP lie on the superior-posterior (S-P) quadrant and for the South African data set the average PP was found to lie on the anterior-inferior (A-I) quadrant. The A-P variation on the position of PP was highly significant (p = 0.003). The results obtained in this study sheds light on the observed morphological variations between the South African and Swiss data sets. The observed circular diameter values are similar to the literature. The observed results suggest that the average TSP needed for the Swiss data set would have been larger than the ones needed for the South African data set. PP is a novel feature which has not been studied extensively. The fact that the average Swiss data set PP lie in the S-P quadrant might suggest that these humeral heads are more retroverted and superiorly tilted when compared to the South African data set. These morphometric variations can play a major role in post-TSA kinematics. The future scope of this study is to highlight other morphometric variations, if any, for the gleno-humeral articulating surfaces

Wear of polymeric glenoid components has been identified as a cause of loosening and failure of shoulder implants1,2 in vivo. A small number of shoulder joint simulators have been built for in vitro wear testing, however none have been capable of testing with physiological motion patterns in three axes and with physiological loading. The Newcastle Shoulder Wear Simulator was designed with three axes of motion, which are programmable so that different activities of daily living might be replicated. The simulator uses three pneumatic cylinders with integral position encoders to move five shoulder prostheses simultaneously in the flexion-extension, abduction-adduction, and internal-external rotation axes. Axial loading is applied with pneumatic cylinders supplied from a pneumatic proportional valve via a manifold, which also supplies a sixth static control station. In order to establish if that the machine can actually perform as intended, commissioning trials were conducted replicating lifting a 0.5 Kg weight to head height as a daily living activity. During the commissioning trials JRI Orthopaedics Reverse VAIOS shoulder prostheses were tested in 50% bovine serum at ambient temperature. The results show that the shoulder joint wear simulator can satisfactorily reproduce a daily living activity deliberately selected for having a large range of motion and loading. Other daily activities, such as drinking from a mug, are less demanding in the ranges of motion and loading and represent no difficulty in being reproduced on the simulator. Now successfully commissioned, this new multi-station shoulder wear simulator can wear test current and new designs of shoulder prosthesis in vitro

Arthritis of the glenohumeral joint accompanied by an irreparable tear of the rotator cuff can cause severe pain, disability and loss of function, particularly in the elderly population. Anatomical shoulder arthroplasty requires a functioning rotator cuff, however, reverse shoulder arthroplasty is capable of addressing both rotator cuff disorders and glenohumeral deficiencies. The Aequalis Reversed Shoulder Prosthesis design is based on two bio-mechanical principles by Grammont; a medialized center of rotation located inside the glenoid bone surface and second, a 155 degree angle of inclination. Combined, they increase the deltoid lever arm by distalizing the humerus and make the prosthesis inherently stable. 24 consecutive primary reverse total shoulder arthroplasties were performed by a single surgeon for arthritis with rotator cuff compromise and 1 as a revision for a failed primary total shoulder replacement between December 2009 and October 2012. Patients were assessed postoperatively with the use of the DASH score, Oxford shoulder score, range of shoulder motion and plain radiography with Sirveaux score for scapular notching. Mean age at the time of surgery was 72.5 years (range 59 to 86). Average follow up time was 19.4 months (range 4 to 38). Functional outcome scores from our series were comparable with patients from other follow up studies of similar prosthesis design. All patients showed improvement in range of shoulder movement postoperatively. Complications included one dislocation, one acromion fracture and one humeral shaft fracture. No cases of deep infection were recorded. Overall, the short-term clinical results were promising for this series of patients and indicate reverse shoulder arthroplasty as an appropriate treatment for this group of patients

Summary Statement. The current biomecahnical study demonstrated that the stemless peripheral leg humeral component prototype and central screw humeral component prototype achieved similar initial fixation as stemmed Global Advantage humeral component in terms of resultant micromotion in total shoulder arthroplasty. Introduction. A stemless humeral component may offer a variety of advantages over its stemmed counterpart, e.g. easier implantation, preservation of humeral bone stock, fewer humeral complications, etc. However, the initial fixation of a stemless humeral component typically depends on cementless metaphyseal press-fit, which could pose some challenges to the initial stability. Long-term success of cementless implants is highly related to osseous integration, which is affected by initial implant-bone interface motion. 1. The purpose of the study was to biomechanically compare micromotion at the implant-bone interface of three humeral components in total shoulder arthroplasty. Patients & Methods. Three humeral components were evaluated: Global Advantage, a central screw prototype, and a peripheral leg prototype. All components were the smallest sizes available. Global Advantage is a stemmed design. Both central screw prototype and peripheral leg prototype are stemless designs. Five specimens were tested for each design. Composite analogue humeral models were utilized to simulate the humeral bone. The cortical wall had a thickness of 3 mm and a density of 481 kg/m. 3. , while the cancellous density was 80 kg/m. 3. The model was custom fabricated to accommodate 40 mm humeral component and had a 45° resected surface and a square base to facilitate test setup. Each humeral component was implanted per its surgical technique. The construct was clamped in a vise with the humeral shaft angled at 27°. A MTS test system was employed to conduct the test. A sinusoidal compressive load from 157 N to 1566 N (2BW) was applied to the humeral component at 1 Hz for 100 cycles. The implant-bone interface micromotion was measured with a digital image correlation system which had a resolution of less than 1 micron. The micromotion measurement was transformed to 2 components: 1 was parallel and the other perpendicular to the humeral resection surface. Peak-valley micromotion from the last 10 cycles were averaged and utilised for data analyses. A one-way ANOVA and post-hoc Tukey tests were performed to compare the micromotion of different designs (α=0.05). Results. Micromotion of Global Advantage parallel to the resection (X-Axis) was significantly less than that of central screw prototype and peripheral leg prototype. Micromotion of peripheral leg prototype perpendicular to the resection (Y-Axis) was significantly less than Global Advantage and central screw prototype. There was no significant difference between different designs in resultant micromotion. Discussion/Conclusion. Clinical studies have shown that current stemless shoulder prosthesis yielded encouraging results in mid-term follow-ups. Particularly, the stemless Arthrex Eclipse humeral component, a central screw design, has been reported to have a secure bony fixation and ingrowth at an average of 23 months postoperatively. 4. The current study demonstrated that the stemless peripheral leg prototype and central screw prototype achieved similar initial fixation as stemmed Global Advantage in terms of resultant micromotion, and provided biomechanical evidence that stemless humeral components could have comparable initial stability to stemmed counterparts

Wear of polyethylene is associated with aseptic loosening of orthopaedic implants and has been observed in hip and knee prostheses and anatomical implants for the shoulder. The reversed shoulder prostheses have not been assessed as yet. We investigated the volumetric polyethylene wear of the reversed and anatomical Aequalis shoulder prostheses using a mathematical musculoskeletal model. Movement and joint stability were achieved by EMG-controlled activation of the muscles. A non-constant wear factor was considered. Simulated activities of daily living were estimated from in vivo recorded data.

After one year of use, the volumetric wear was 8.4 mm³ for the anatomical prosthesis, but 44.6 mm³ for the reversed version. For the anatomical prosthesis the predictions for contact pressure and wear were consistent with biomechanical and clinical data. The abrasive wear of the polyethylene in reversed prostheses should not be underestimated, and further analysis, both experimental and clinical, is required.