PERFORMANCE CHARACTERISTICS OF CUSTOM-DESIGNED POSTERIOR-STEP GLENOID PROSTHESES IN TOTAL SHOULDER ARTHROPLASTY

Open Access
Author:
Kirane, Yatin
Graduate Program:
Kinesiology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 14, 2009
Committee Members:
  • Neil Sharkey, Dissertation Advisor
  • Neil Sharkey, Committee Chair
  • April D Armstrong, Committee Chair
  • Stephen Jacob Piazza, Committee Member
  • Andris Freivalds, Committee Member
  • Kevin Paul Black, Committee Member
Keywords:
  • posterior glenoid defect
  • glenoid prosthesis
  • shoulder arthroplasty
Abstract:
Total shoulder arthroplasty (TSA) is the treatment of choice for advanced gleno-humeral osteoarthritis. Among different complications of the procedure, loosening of the glenoid component (the socket) is most common. The risk for glenoid component loosening is greatly increased with pre-existing bone erosion in the posterior glenoid, for which the treatment options are currently limited. The goal of the present project was to employ a cadaver model to evaluate the mechanical efficacy of two Posterior-step glenoid prostheses (Poly-step and Ti-step), two custom implants designed to compensate for a typical posterior glenoid defect. Our first hypothesis was that TSA using a standard (STD) glenoid prosthesis, in the absence of a defect, will not significantly alter joint stability, as measured by gleno-humeral translations, but will increase loading of the glenoid, as measured by the peri-glenoid principal bone strains. The second hypothesis was that TSA using custom Poly-step and Ti-step prostheses, to compensate for a biconcave posterior glenoid defect, will reverse the alterations in peri-glenoid strains and joint translations as caused by the defect. Furthermore, we also hypothesized that consequences of implantation of custom Poly-step and Ti-step prostheses in the presence of a defect will be similar to those following STD prosthesis implantation in the absence of such a defect. Fifteen non-embalmed, fresh frozen human shoulder specimens were tested in a custom-built loading apparatus after fixing three triaxial strain gauge rosettes around the glenoid, and two marker-clusters on the acromion and the humerus. The arm was placed in 90° of abduction in neutral, in 30° of horizontal flexion and 30° of horizontal extension, while applying static forces to the tendons of the rotator cuff muscles, and axial forces to the distal humerus using a linear actuator. Peri-glenoid bone strains and marker-motion were recorded using analogue-to-digital converter and custom software, and a four-camera motion capture system respectively. Principal strains and joint translations were derived from the strains and marker-motion respectively, and were compared across conditions, arm positions and implant types. In five out of fifteen specimens, TSA was performed using a standard glenoid prosthesis while in the remaining ten specimens, a defect (20° biconcave type) was surgically created in the posterior glenoid, and custom Polyethylene-step and Titanium-step glenoid prostheses were implanted in five specimens each. We observed that implantation of a STD glenoid prosthesis, in the absence of posterior glenoid defect, had no significant effect on glenoid bone loading or joint stability (p > 0.05). Creation of a 20° biconcave defect in the posterior glenoid caused some significant alterations in glenoid strains but not in joint translations, as compared to those in the native joints (p < 0.05). Implantation of a Polyethylene-step glenoid prosthesis in the presence of a defect, successfully reversed the mechanical alterations caused by the defect, while implantation of a Titanium-step prosthesis did not. Significantly greater anterior compressive strains and posterior tensile strains were observed following TSA using Ti-step prosthesis, and humeral head translations were also more anteriorly directed, as compared to the STD and Poly-step prostheses. Therefore, we concluded that Polyethyelene-step glenoid prosthesis may be a viable option for treating posterior glenoid defects, and warrants further mechanical evaluation in the form of fatigue loading and testing for loosening etc. If successful, this prosthesis may provide an important treatment avenue for surgeons performing TSA on arthritic shoulders with posterior glenoid deficiency, and potentially reduce the incidence of glenoid component loosening and failure.