Interestingly, relative motion has been shown to increase with increasing flexion in a biomechanical study2 using the same Triathlon TS system. The authors of that study found they were unable to complete testing at flexion greater than 30º because, absent the stabilizing influence of surrounding ligament and muscle, the sample deformation was so significant that it caused fracture.2 In the case of our patient, the incompetence of her extensor mechanism likely resulted in increased forces transmitted through the implant than might be expected in more physiologic circumstances. This higher stress may account in part for the failure of the implant at the known weakest point, the stem-condyle modular junction.
Modular implants are routinely used, given the variability of scenarios encountered in revision surgery and the need for customization to provide the best approximation of physiologic functioning of the joint. However, modular components introduce junctional points, which are potential points of failure. Stresses on the femoral component occur in multiple dimensions besides the axial loading and medial-lateral, anterior-posterior rocking seen with the tibial component. The maximum stress is observed at the distal-most aspect of the stiffest or most well-fixed components, in this case, the articulation between the cemented stem and the cemented condylar component. Poor distal femoral fixation compounds the problem.
Numerous case reports have documented such failures in other knee systems. Issack and colleagues5 described 2 cases of fracture through the taper lock between the femoral component and the stem extension in the Optetrak stemmed-constrained condylar knee prosthesis (Exactech). Westrich and colleagues6 reported disengagement of the locking bolt of the Insall-Burstein II Constrained Condylar Knee (Zimmer) leading to failure. Lim and colleagues4 reported stem-condyle junctional failure of the Total Condylar III (DePuy, Johnson & Johnson) due to locking-screw failure. Butt and colleagues7 reported a case of failure at the femoral component–stem junction caused by screw breakage. All of these cases involved failure at the condylar-stem junction that was readily apparent on routine preoperative imaging.
Our case is noteworthy because there was no preoperative radiographic evidence that the components were loose or the junction had failed. As with many revision systems observed by Fehring and colleagues,8 determination of fixation is often based on the appearance of the stem because the distal femoral interfaces may be obscured by the intercondylar box. This suggests that a loose component at the stem-condylar junction could easily be overlooked and not appropriately revised based on imaging alone. A solution for achieving stability at the time of revision surgery is to obtain good distal bone apposition and fixation. In this case, a cemented stem with a metaphyseal cone was used for femoral fixation (Figures 3A, 3B).
While long-term, abnormally high stress transmitted through the modular junction may account for the implant’s failure, to our knowledge, this is the first report of its kind related to this particular implant. If quadriceps weakness contributed to this failure, it is worth considering that quadriceps weakness is common after TKA and may persist without appropriate rehabilitation and activity. Furthermore, the lack of evidence on plain radiographs makes this particular form of failure very difficult to screen. A high degree of suspicion for loosening should be maintained in patients with pain and instability after revision TKA with this implant as well as with other modular revision knee systems.