Abstract
Introduction
The Exeter RimFit™ flanged cemented cup features ultra-highly cross-linked polyethylene and was introduced onto the market in the UK in 2010. We aimed to examine the rates of radiolucent lines observed when the Rimfit™ cup was implanted using a ‘rimcutter’ technique with the cup sitting on a prepared acetabular rim, and a ‘trimmed flange’ technique were the flange is cut so that it sits inside the prepared acetabular rim.
Patients/Materials & Methods
The radiographs of 150 (75 ‘rimcutter’, 75 ‘trimmed flange’) Rimfit™ hip replacements were critically evaluated to assess for radiolucency at the cement bone interface. This group was then compared to a historic pre-Rimfit™ cohort of 76 patients.
Results
Rimfit™ Cups inserted using the rimcutter technique exhibited significantly higher rates of radiolucency than cups inserted using the trimmed flange technique at 1 year post surgery (1 Delee and Charnley zone: rimcutter=73/77 (95%) versus trimmed flange=14/75 (19%); 2 zones: 43/75 (57%) versus 0/75 (0%); all 3 zones: 13/75 (17%) versus 0/75 (0%):(p<0.01)). There were significantly more radiolucencies for the ‘rimcutter’/Rimfit™ group when compared to a historic group of non-‘rimcutter’/non-Rimfit™ cups implanted by the same surgeon (p<0.01).
Discussion
95% of Rimfit™ cups inserted using the rimcutter technique exhibited radiolucent lines at 12 months. This may be due to increased flange stiffness related to the ultra-highly cross-linked polyethylene and the closer seal due to the narrow circumferential ridge created by the rimcutter. Insertion of the cup may lead to lower initial cement penetration which further deteriorates with weight bearing due to the flange offloading the socket. This may be exacerbated by hydraulic pumping of fluid trapped under the flange at the time of implantation that further degrades the cement bone interface.
Conclusion
On the basis of these results we have stopped using the Rimfit™ cup in combination with the rimcutter device and technique.
