Kinematically aligned (KA) total knee arthroplasty (TKA) aims to restore normal function of the knee by aligning the distal and posterior femoral joint line along the functional transverse axes and the tibialis joint line to that of its normal or pre arthritis [1]. The medial pivot (MP) design in the TKA is designed to provide near normal knee kinematics with a stable point of contact on both the medial and lateral side to allow posterior translation of the femoral condyle during knee flexion [2]. Potential changes in stresses across the joint with this new design and technique have yet to be investigated.

We report an 81-year-old man (BMI 28.32) who underwent cemented primary TKA using the GMK sphere (Medacta International AG, Castel San Pietro, Switzerland) (femur 3, tibia 3 and posterior stabilized polyethylene ( PE) 11 , and a resurfaced patella) via a medial parapatellar approach. The PE was inserted according to the technique guide and securely engaged in the locking mechanism. Intraoperatively, the knee was very well balanced, with a range of motion (ROM) of 0 to 130 degrees (Figure 1).

At two weeks, her postoperative visit was uneventful with a well-healed surgical incision and ROM of 0-110 degrees. At routine six-week follow-up, posterior dissociation of the PE was observed on radiographs. There was no trauma or unusual activity reported by the patient. His ROM was painless from 0 to 125 degrees (Figure 2).

Figure
2: Frontal (left) and lateral (right) X-rays obtained at the postoperative visit at 6 weeks showing a posterior dislocation of the tibial polyethylene with an asymmetric joint space.

Plans were made for revision surgery and intraoperatively it was noted that the posterior lateral portion of the EP was clearing the peripheral locking mechanism. No damage to the tibial base locking mechanism was observed. A PE of similar size (11mm) was replaced and secured with an optional center screw. At the most recent routine visit (12-month follow-up), the patient was painless with a ROM of 0 to 120 degrees and had no radiographic evidence of PE dissociation (Figure 3).

Figure
3: Frontal (left) and lateral (right) X-rays obtained following revision surgery with polyethylene swap and placement of the central tibial screw.

Dissociation of the PE from a fixed-bearing tibial plateau is uncommon. Undiagnosed, it can lead to spin-out and potentially damage ligamentous and neurovascular structures, the patellar tendon and the tibial locking mechanism [3,4]. After reviewing the literature, about twenty cases of fixed plateau PE dissociation after primary TKA have been documented. No cases of dissociated fixed-platform poly have been reported with the MP design in KA TKA. Most dissociations occurred after stabilized posterior TKA (PS) [5,6] and a few after TKA preserving the cruciate ligaments [7,8]. After careful review of the existing literature, we can categorize the causes of dissociation as technique-related, implant-related, and patient-related.

Technical causes

First, incorrect placement of the PE during insertion due to inadequate exposure is considered a common cause of failure. [4]. Second, inadequate soft tissue balance in the sagittal and coronal planes can lead to abnormal stresses on locking mechanisms. Tight flex space can cause PE to fail at the anterior miters [9]. Additionally, if the flexion gap is loose, abnormal anterior translation of the femoral component may cause the posterior legs to break from the locking grooves. Third, unusual varus-valgus laxity causes abnormal loading with flexion and adduction, which can cause the locking mechanism to fail [10]. Fourth, unresected posterior osteophytes of the femur may impinge on the posterior aspect of the PE and cause anterior poly uplift. [11].

Causes related to implants

In the PS TGA, with increasing flexion angle, the cam mechanism engages driving the femoro-tibial contact further back. This can lead to abnormal contact pressure in the posterior half of the EP and can dissociate the locking mechanism forward. [5,12]. In the CR TKA, a tight PCL or improper slope can cause abnormal pressure in the posterior half of the poly and cause the anterior miters to fail [13].

Here, we describe the first case in the literature with an MP design with a posterolaterally failing locking mechanism. The MP design has a ball joint configuration on the medial side and a convex configuration on the lateral side in an effort to replicate native kinematics. However, with this design, it is believed that there is more anterior to posterior translation in the lateral compartment compared to traditional designs (PS and CR). The increased translation in the side compartment can increase the forces in the peripheral locking mechanisms. In this case, the abnormally increased pressure on the anterior portion of the side compartment poly component resulted in the failure of the posterior lateral dovetail locking mechanism, leading to posterior dislocation of the PE implant.

A locking mechanism between the PE and the tibial plateau can be classified as linear, peripheral or central locking. Linear locking mechanisms use the tongue in a groove (Stryker Traithalon, Depuy Attune) with tracks that run front to back and medial to lateral. They may also sometimes have an additional locking pin to prevent anterior lift off (Biomet Vanguard, Stryker Triathlon). Peripheral capture mechanisms (Smith and Nephew Genesis I and II and Depuy Sigma, Arthrex) have a snap fit with beveled edges on the entire periphery or on a segment of the periphery. Finally, central mechanisms use a mushroom pin with a peripheral collar for rotational stability or a central locking screw (Medacta). The Medacta Tibial Tray has a peripheral locking mechanism and a central screw that locks the PE to the Tibial Tray (Figure 4). This central screw is described in the surgical technique as an optional step for added security but is not mandatory as the peripheral capture locking mechanism may suffice.

Figure
4: Photo of the Medacta Tibial Base showing the peripheral locking mechanism with the optional central screw hole.

Patient factors

Soft tissue laxity with atypical translations [7]high BMI [12]repetitive hammering activities [14]faulty extension mechanism [15] are some of the causes described for the dissociated fixed bearing PE.

This is the first reported case with a dissociated EP in a fixed deck MP design in KA TKA. Although the reason for the failure of the locking mechanism is unclear, we believe that the abnormal anterior translation of the lateral compartment and the first-generation peripheral locking mechanism are the reasons for the dissociation. However, technique-related causes such as poor seating and poorly balanced ligaments are very subjective and cannot be ruled out. This case also highlights the importance of regular follow-up and careful clinical and radiological evaluation, especially when adapting to new techniques and technologies, to avoid devastating complications.