Damages of the tibial post in constrained total knee prostheses in the early postoperative course - a scanning electron microscopic study of polyethylene inlays.(Research article)

Authors: Adrian Skwara (corresponding author) [1]; Carsten O Tibesku [2]; Rudolf Reichelt [3]; Susanne Fuchs-Winkelmann [1]

Background

Due to the growing number of revision total knee arthroplasties, posterior stabilized and constrained total knee prostheses have become more and more popular in recent years. They allow intrinsic stabilization in knees with ligamentous instability. So far, clinical results of constrained total knee arthroplasty (TKA) have been reported only in medium-term follow-up [1]. Nevertheless, hinged prostheses are still being discussed for salvage total knee arthroplasty [2]. However, if stability can not be obtained with an unconstrained implant progressive levels of constraint, but as little constraint as possible should be used [3].

Posterior stabilized total knee prostheses that are similar in design to constrained prostheses also tend to increasing axis deviations and inlay breakage after a few years, especially in patients with severe preoperative axis deviation of the leg axis of more than 10[degrees] in the coronal plane [4]. Several case reports described a fracture of the polyethylene tibial post in different posterior stabilized knee prostheses even if the tibial post was reinforced by a metal rod [4, 5, 6, 7]. Studies about retrieved posterior stabilized knee prostheses showed that especially the backside of the post can be a source of polyethylene wear [4, 8]. Li et al. demonstrated in a cadaver study after TKA a higher contact force at the tibial post and less posterior femoral translation at low flexion and hyperextension resulting an anterior post impingement and additional polyethylene wear [9]. In unconstrained flat-on-flat total knee prostheses a correlation between the patients' activity and the creep reaction and deformation of the polyethylene was reported [10]. Other parameters, such as the kind of sterilization, manufacturing and thickness of the inlay have been pointed out repeatedly and have been optimized by many manufacturers worldwide.

Due to the increasing deviation of the mechanical leg axis and breakage of the post, the post has to be regarded as the weak point of the constrained total knee joint arthroplasty, where the tibial post is not reinforced with a metal rod. From the mechanical point of view, however, the transition between the post and the femoral cam cause extensive loads on the post and occurs as a weak point of this design. This proved to be a serious problem with constraint prostheses, which are expected to provide a higher stability and tolerate these acting forces. The following investigation of unloaded and loaded constraint polyethylene inlays was performed to elucidate initially this problem in polyethylene inlays without metal rod reinforcement.

Methods

Ten polyethylene inlays were investigated using scanning electron microscopy. All inlays were of the same type and size. The model used was an 11 mm thick constrained inlay of the Genesis II total knee (Genesis II constraint, Smith & Nephew, Schenefeld, Germany). The size is called "5-6" which is identical for sizes 5 and 6 of the tibial component. The inlays consist exclusively of ultrahigh-molecular weight polyethylene (UHMWPE; ASTM F 648) without any metal reinforcement and were formed by milling to its final shape. All inlays were sterilized by gas sterilization using ethylene oxide.

Three polyethylene inlays were unused and acted as controls. The samples were subsequently prepared according to a standardized preparation protocol, mentioned below.

Seven inlays had been retrieved from patients, four men and three women, with a constrained prosthesis during revision surgery. The mean age of the patients was 66.4 years (min 48.6 years, max 80.0 years). The patients had a body mass index at the time of surgery of 31 …