Abstract
Purpose: In contemporary modular knee designs tibial insert locking mechanisms are intended to fix between the tibial insert and the tibial base-plate for preventing backside wear to limit interface micro motion. We describe two cases of failure of the locking mechanism of the tibial insert and tibial base-plate in one cruciate retaining (CR) type knee prosthesis (Maxim® CR knee system, Biomet Orthopaedics, Inc., Warsaw, IN, USA) over 10 years after primary total knee arthroplasty.
Cases: At 11 and 15 years postoperatively, both patients developed pain and a sudden instability with walking without any traumatic events. After the appearance of symptoms, there was locking mechanism failure evident on radiographs of both patients. Each patient underwent revision surgery, and the inserts were removed. The removed tibial inserts demonstrated significant wear in the anterior medial quarter and fracture of the locking bar was observed in each case. Clinical function for each patient after revision surgery was satisfactory.
Discussion: Some factors are described for failure of the tibial insert as follows. At first, during full knee flexion stress is concentrated on the anteriomedial side of the tibial insert which promotes wear, because the acceptable rotation range of the femorotibial component of the Maxim® total knee arthroplasty is markedly restricted. Additionally, wear may be accelerated by patient factors, such as a high activity level and high body mass index. It has been assumed that wear finally reaches the region around the deep groove for locking, which is a structural weak point, resulting in rupture of the locking system.
Conclusion: The failures in our two cases suggest that an alternative locking mechanism should be considered for this prosthesis.
