Abstract
Introduction
Measuring the step off during total knee replacement (TKR) is a newly developed operative strategy (“spacer technique”; Heesterbeek et al, KSSTA 2014;22(3):650–9) to determine the optimal contact point (CP) of the femur with the tibia postoperative and to balance the posterior cruciate ligament (PCL) in cruciate-retaining TKR. Engineers have calculated the ideal step off for every size of the TKR, for which the tibiofemoral contact point in 90° will be at the designed position. With this study we determined the postoperative CP in CR-TKA and investigated whether (adverse) clinical outcome was correlated with the CP.
Methods
23 patients presenting with non-inflammatory osteoarthritis, a good functioning PCL, and indication for surgery with a PCL-retaining TKR were selected. Intraoperative PCL balancing was performed with the spacer technique. At 3 months postoperative, a pair of mediolateral radiographs was made using a set-up used for radiostereometric analysis (RSA). The patient was positioned standing with the operated leg in 90 degrees, 50% weight-bearing, knee flexion on a 30 cm-step. Model-based RSA software (RSAcore) was used to determine the 3D positions of the femur and tibia component, that were exported to custom-written software for determining the CP. The CP was defined as the point with the smallest distance between both the medial and lateral femur condyles and tibia plateau. It is expressed as the ratio of the anterior-posterior CP distance and the maximum anterior-posterior tibia plateau size, with 0 being anterior, 1 being posterior. Patients with reduced flexion capacity at follow-up, leading to manipulation under anaesthesia and/or scopic releases, were categorized as COMP, the other patients as no-COMP. CP was compared between these groups.
Results
Preliminary data show that the mean medial CP of the total group was 0.51 (sd 0.05), mean lateral CP was 0.61 (sd 0.03) (p<0.001). Six out of 23 patients had flexion-related complications and for this reason further patient inclusion was stopped. The medial CP of the COMP-group (n=6) was at 0.54 (sd 0.01), which was significantly more posterior than the medial CP of the no-COMP group (n=17) (0.50 (sd 0.05)) (p=0.004). (Figure 1) The lateral CP was similar for both groups (p=0.76).
Discussion
The medial CP relates to clinical outcome; patients with reduced flexion capacity had a more posterior CP. This might be an indication for a too tightly balanced PCL, but we need to investigate this further. None of the patients had a medial CP at the theoretically optimal position.
