INFLUENCE OF HUMERAL IMPLANT-GLENOID INCONGRUENCY ON PERIGLENOID BONY DEFORMATION AND ON DISPLACEMENT OF CEMENTED GLENOID IMPLANTS: AN IN VITRO EXPERIMENTAL STUDY OF PROSTHESIS-BEARING CADAVERIC SCAPULAE

Abstract

Purpose: There is no consensus concerning the ideal incongruency of the prosthetic head and the glenoid implant in total shoulder arthroplasty. Certain recent publications suggest the rate of periglenoid lucency is lower if the incongruency is greater than 5.5 mm. The purpose of this experimental in vitro work was to study the influence of changing humeral head-glenoid congruency on periglenoid bony malformations of prosthesis-bearing cadaveric scapulae and on the motion of the glenoid implants.

Material and methods: Five scapulae from subjects aged 76 to 91 years at death were harvested and implanted with five stem cemented glenoid implants with an identical curvature. Five metallic balls with different radii were used to simulate incongruency of the humeral head-glenoid implant varying from 0 (perfect congruency) to 6 mm (0.2, 4.5, and 6 mm). The protocol involved preloading at 400 N following a normal axis for the glenoid implant and then posteroanterior translation and inferosuperior translation of 2.5 mm. The force necessary to impose the translation displacement, periglenoid bony deformations, and implant displacement compared with the bony glenoid were measured with a traction-compression device using deformation gauges and two CCD cameras in compliance with a published protocol.

Results: Increasing incongruency decreased the force necessary to displace the metallic balls, decreased periglenoid bony deformations around the loaded zones and decreased the degree of prosthetic displacement facing the loaded zone.

Discussion: The limitations of this experimentation are the small number of implants tested and the subsequent lack of statistical analysis concerning the reality of the differences observed. Besides, the experimental protocol cannot reproduce the normal conditions of the prosthesis articulation. Nevertheless, these results appear to favour the idea of greater bone and prosthetic tolerance with lesser humeral head-glenoid implant congruency. This might provide an explanation for the fewer glenoid lucent lines found in vivo in similar congruency situations.

Conclusion: These results suggest that a certain degree of incongruency of total shoulder prostheses could reduce the risk of periprosthetic lucency. Ideal incongruency remains to be determined with further in vitro and in vivo studies.