Abstract
Introduction
Shoulder arthoplasty has increased in the last years and its main goal is to relieve pain and restore function. Shoulder prosthesis enters in the market without any type of pre-clinical tests. Within this paper we present study experimental and computational tests as pre-clinical testing to evaluate total shoulder arthoplasty performance.
Materials and methods
An in vitro experimental simulator was designed to characterize experimentally the intact and implanted shoulder glenoid articulation. Fourth generation Sawbones® composite left humerus and scapula were used and the cartilage was replicated with silicone for the intact articulation (figure 1). In the intact experimental articulation we considered the inferior glenohumeral ligament as an elastic band with equivalent mechanical properties. For the implanted shoulder, the Comprehensive® Total Shoulder System (Biomet®) with a modular Hybrid® glenoid base and Regenerex® central post was considered (figure 2). The prostheses were implanted by an experienced surgeon and clinical results from orthopedic registers were collected.
The system structures were placed to simulate 90º in abduction, including the following muscle forces: Deltoideus 300N, Infraspinatus 120N, Supraspinatus 90N and Subscapularis 225N. The finite element model was created with tetrahedral linear elements with linear elastic and isotropic material for the humerus in figure 3, (Young's modulus for cortical bone − 16.5 GPa; trabecular bone − 124 MPa). Anisotropic behavior was considered for the scapula model (E11 = 342.1 MPa, E22 = 212.8 MPa, E33 = 194.4 MPa). The shoulder prosthesis was of polyethylene with 1GPa and titanium with 110 GPa. The Poisson's ratio was 0.3 in all material, except for polyethylene where we assumed a value of 0.4. A long-term post-operative condition was simulated.
Results
The experimental results were compared with numerical ones for model validation. The strains measured evidence the effect of the implant presence, manly in the scapula. In the anterior region presents an increase of strains (+26%) was observed for the anterior region and decrease (−52%) in the posterior region, suggesting strain shielding in figure 4.
At the glenoid cavity, the numerical principal strains present safety values of strains (200 to 2500) µε in both axial and coronal planes. This indicates that on the long-term the glenoid prosthesis is well fixed to the surrounding bone tissue and bone integrity is maintained despite the presence of the implant. However there are some peak values (2500, 25 000 µε) that were observed in some small areas in the posterior and distal regions. Results were compared with clinical ones.
Discussion and Conclusions
The proposed pre-clinical test with the articulation at 90º in abduction can predict bone behavior when total shoulder prosthesis is implanted and in the long term post-operative condition. The results obtained evidence some critical regions around the glenoid component. This pre-clinical test can be implemented to improve the concepts before market.
