WEAR AND CONGRUENCE OF TOTAL KNEE ARTHROPLASTIES: COMPARISON OF TWO SIMILAR PROSTHESES WITH DIFFERENT CONGRUENCIES

Abstract

Purpose: The purpose of this work was to compare polyethylene (PE) wear between prostheses with similar function but different congruency of the femoral implant / PE insert and, as a corollary, the mobility of the plateau.

Material and methods: We studied two groups of knee prostheses: prostheses preserving both cruciate ligaments (n = 20), and stabilised prostheses without preservation of the cruciate ligaments (n = 20). Four representative samples of ten patients by type of total knee arthroplasty (TKA) were selected at random among a cohort of 105 patients operated on between 1994 and 1996 with a mean follow-up of five years. All patients were reviewed with AP and lateral radiographs, a view in the plateau plane, and goniometry. Using this random selection, patients in the two groups were comparable for operative age (69 years), diagnosis (degenerative disease), sex ratio, IKS score (> 80/100), and follow-up. The only difference between the two groups was the postoperative goniometry: 180±2° for fixed plateau; 178±3° for mobile plateau (p< 0.05).

Result: Penetration of the femur in the PE insert (U) (after correction for radiographic magnification) was: TKA two cruciates fixed plateau: U=3.5±1.5mm; TKA two cruciates mobile plateau: U=0±1mm (p< 0.001); TKA posterior stabilisation fixed plateau: U=2.5±1mm; TKA posterior stabilisation mobile plateau: U=0 mm (p< 0.001). A difference of more than 3° in the mechanical axis did not show detectable wear in the group of congruent prostheses while for fixed plateau prostheses, wear appeared when the mechanical axis was 180°.

Discussion: Few studies have compared PE wear of TKAs with identical form and function. The random selection allowed us to compare homogeneous groups of patients eliminating selection bias of the retrospective analysis and of the effect of patients lost to follow-up. The highly significant difference between the groups compared avoided potential ß risk. However the quality of the PE and its mode of sterilisation were not known with certainty for the tested implants.

Conclusion: This study demonstrates the importance of congruency as a factor reducing PE wear in TKA. This parameter is more favourable when the postoperative mechanical axis is perfectly corrected.