Instability is a major cause for revision surgery in total knee replacement (TKR). With a balanced gap technique, the ligaments are theoretically balanced. However, there is concern that ligament releases needed to align the leg may cause instability. Furthermore, no information is available about the relationship between the amount of varus-valgus laxity directly after implantation and at a later postoperative interval. This prospective clinical study investigated whether ligament releases necessary during total knee replacement (TKR) led to a higher varus-valgus laxity during peroperative examination and after 6 months.

In this prospective cohort study, in 49 patients a primary TKR was implanted using a balanced gap technique. Varus and valgus laxity of the knee was assessed in extension and flexion (70 degrees) per-operative (before and after implant) with a navigation system and post-operative with standardised stress radiographs (both methods 15 Nm stress applied).

Knees were catalogued according to ligament releases performed during surgery: no releases, lateral releases, medial releases with posteromedial condyle (PMC), and medial releases with superficial medial collateral ligament (SMCL). ANOVA was used to test between release groups.

At surgery, before and after implantation of the prosthesis, there was no difference in varus or valgus laxity in extension and flexion between knees that did not need a ligament release (n=22), knees with lateral release (n=5), knees with medial SMCL releases (n=15) and knees with medial PMC releases (n=7). Six months after TKR, varus or valgus laxity in extension and flexion was not significantly different between the release categories.

In conclusion, ligament releases of the SMCL, PMC, and lateral structures performed during a balanced gap technique in TKR do not lead to an increased varus-valgus laxity in extension and flexion at 6 months after surgery. Therefore, routine releases of these structures to achieve neutral leg alignment can safely be performed without causing increased varus-valgus laxity. The results of this study suggest that the reported high incidence of revisions for ligament instability after TKR is not likely to be caused by routine ligament releases when a balanced gap technique is used. Apparently, there is not a ligament instability problem as long as the gaps are properly filled with prosthesis components. We believe that the conclusion of this study would also be valid when bone referenced techniques are applied instead of tensors, as long as the gaps created are balanced.

Balancing the PCL in a PCL-retaining total knee replacement (TKR) is important, but sometimes difficult to execute in an optimal manner. Due to the orientation of the PCL it is conceivable that flexion gap distraction will lead to anterior movement of the tibia relative to the femur. This tibio-femoral repositioning influences the tibio-femoral contact point, which on its turn affects the kinematics of the TKR. So far, the amount of tibiofemoral repositioning during flexion gap distraction is unknown which leads to uncertain kinematic effects after surgery. The goal of this study was to quantitatively describe the parameters of the flexion gap (gap height, anterior tibial translation and femoral rotation) and their relationship while the knee is distracted during implantation of a PCL-retaining TKR with the use of computer navigation. Furthermore, the effect of PCL elevation angle on the flexion gap parameters was determined.

In 50 knees, during a ligament-guided TKR procedure, the flexion gap was distracted with a double-spring tensor with 100 and 200 N after the tibia had been cut. The flexion gap height, anterior tibial translation and femoral rotation were measured intra-operatively using a CT-free navigation system. PCL elevation was calculated based on the femoral and tibial insertion sites as indicated by the surgeon with the pointer of the navigation system.

To identify a relationship between flexion gap height increase and anterior tibial translation, the ratio between anterior translation and gap height increase was determined for each patient between 100 and 200 N.

The mean gap height increased 2.2 mm (SD 0.96) and mean increase in anterior tibial translation was 4.2 mm (SD 1.6). Hence, on average, for each mm increase in gap height, the tibia moved 1.9 mm (SD 0.96) in anterior direction. Knees with a steep PCL showed significantly more AP translation for each mm gap height increase (gap/AP-ratio was 1 : 2.31 (SD 0.63)) compared to knees with a flat PCL (gap/AP-ratio was 1 : 1.73 (SD 0.50)).

The increase in femur (exo)rotation was on average 0.60° (SD 1.4).

With a tensioned PCL the tibia will move anteriorly on average 1.9 mm for every extra mm that the flexion gap is increased. The flexion gap dynamics can be explained in part by the orientation of the PCL: the greater the elevation angle, the more anterior tibial displacement during distraction of the flexion gap. The surgeon must be aware that distraction of the flexion gap influences the tibiofemoral contact point. The tibio-femoral contact point will move posteriorly and stresses in the PCL will rise and produce limited flexion and pain. In case of a conforming insert AP-movement will be limited but high PE stresses may be introduced that can lead to wear. This information may be helpful in selecting the optimal soft tissue balancing procedure and the optimal PE insert thickness in PCL retaining TKR.