Posterior cruciate ligament recruitment affects antero-posterior translation during flexion gap distraction in total knee replacement. An intraoperative study involving 50 patients

Abstract

Background and purpose: Because of the oblique orientation of the posterior cruciate ligament (PCL), flexion gap distraction could lead to anterior movement of the tibia, which would influence the tibiofemoral contact point. This would affect the kinematics of the TKR. We assessed the flexion gap parameters when the knee is distracted during implantation of a PCL-retaining TKR. Furthermore, the effects of PCL elevation (steep or flat) and collateral ligament releases on the flexion gap parameters were determined.

Methods: During a ligament-guided TKR procedure in 50 knees, the flexion gap was distracted with a double-spring tensor with 200N after the tibia had been cut. The flexion gap height, anterior tibial translation, and femoral rotation were measured intraoperatively using a CT-free navigation system.

Results: During flexion gap distraction, the greatest displacement was seen in anterior-posterior direction. Mean ratio between increase in gap height and tibial translation was 1 to 1.9, and was highest for knees with a steep PCL (1 to 2.3). Knees with a flat PCL and knees with a ligament release had a larger increase in PCL elevation when the gap was distracted.

Interpretation: When the PCL is tensioned, every extra mm that the flexion gap is distracted can be expected to move the tibia anteriorly by at least 1.7 mm (flat PCL), or more if there is a steep PCL. This changes the tibiofemoral contact point, which may have consequences for polyethylene wear.

Figure 1.

Additional points selected with pointer as input to determine the location of the femoral and tibial PCL insertion sites.

Figure 2.

3D view of the knee with the parameters of interest: gap height (GAP), anterior-posterior translation (AP), medio-lateral displacement (ML), and rotation of the femur (ROT.).

Figure 3.

Schematic illustration of PCL elevation, which is the angle (α) between the (cut) tibial surface (not illustrated) and the calculated PCL (black line).

Figure 4.

3D illustration of 2 knees in sagittal view (no fibula). The situation with 100 N of tension is shown in red, and that with 200 N is shown in blue. The amounts of gap height increase and anterior tibial translation are visible and different for the knee with a flat PCL (left) and the knee with a steep PCL (right). Note the increase in PCL elevation after 200 N has been applied (blue PCL).

Figure 5.

Scatter plot of change in gap height against change in anterior tibial translation for knees with a flat or a steep PCL after the flexion gap tension has increased from 100 N to 200 N. Each entry represents one patient (n = 50).

Figure 6.

Scatter plot of change in gap height against change in anterior tibial translation for knees with or without a release after the flexion gap tension has increased from 100 N to 200 N. Each entry represents one patient (n = 50).