RELATIONSHIP BETWEEN IN VIVO BEARING MOBILITY CONSERVATION AND PE WEAR IN A POSTERO-STABILISED ROTATING PLATFORM TOTAL KNEE PROSTHESIS: RETROSPECTIVE STUDY OF 120 LCS-RP® AT MINIMUM 10- YEAR FOLLOW-UP.

Abstract

The New Jersey Low Contact Stress prosthesis was introduced in 1977. LCS mobile bearing prostheses have been implanted in patients in the United States since 1985. The PCL-sacrificing rotating platform design of the LCS® prosthesis was designed to minimally constrain knee kinematics while minimising bone-cement-prosthesis interface stresses and polyethylene wear.

The purpose of the current study was to assess the in vivo rotating platform PE wear of the LCS-RP® and to check if the PE tray mobility is preserved over time.

We investigated 120 total knee replacements in 108 patients (38 men, 70 women) treated by the LCS-RP® prosthesis for osteoarthritis or rheumatoid arthritis. Average age of the patients was 70.4 years (range: 36–81). Average follow-up was 15.3 years (range: 10–18).

PE wear assessment was performed using a specific analysis model created in the Imagika® software on AP radiographs.

Rotating platform mobility was assessed thanks to another analysis model. We used prosthetic landmarks (tip of the prosthetic stem, tibial tray and tantalum markers embedded into the PE) visible on lateral radiographs.

Linear wear rates were 0.012+/−0.007 mm (tibial plate/ PE interface), 0.026+/−0.034 mm (PE/internal condyle interface) and 0.017+/−0.041 mm (PE/external condyle interface). There were no significant differences between wear values of both internal and external sides of the upper surface of the PE insert (P = 0.000054) and of tibial plate PE/external condyle PE interfaces (P = 0.0012), but a P value of 0.063 was observed between the tibial plate PE interface and the internal condyle PE interface.

The PE range of motion was 5.3 +/− 1.2° (range: −2.5 (external) to +3.2 (internal)) at last follow-up as compared to the first radiograph (3 months after operation). All trays preserved there mobility in rotation.

Annual PE wear rates were extremely low as well as at the lower and the upper surfaces of the mobile bearing. Our results suggest that the long term mobility conservation contributed to low PE wear rates despite a high anatomic congruency due to the prosthetic design.