INFLUENCE OF THE PCL AND PATELLA POSITION ON THE GAP KINEMATICS IN TKA

Abstract

Background: Correct ligament balance is a critical factor in both cruciate retaining and substituting total knee arthroplasty (TKA). Due to a lack in current tools, however, little data exists on gap kinematics with the patella is in its anatomical position and with the ligaments tensed. The objective of this study was to quantify the effects of the patellar position and PCL resection on gap kinematics when constant tension is applied to the medial and lateral compartments.

Methods: A novel computer-controlled tensioner was used to measure the medial and lateral gaps in 10 normal knee specimens throughout a full range of motion. Gaps were measured medially and laterally using constant applied forces of 50N, 75N and 100N per side. Gap data were acquired at 0°, 30°, 60°, 90°, 120° of flexion. The test was performed with the patella everted and reduced, and with the PCL intact and resected.

Results: At 90° of flexion:

• the mean medial gap was 1.5–2.5mm smaller than the mean lateral gap for all scenarios and forces tested (p< 0.05);

• everting the patella decreased the medial and lateral gaps by 1mm and 1.3mm with an intact PCL, and by 1mm and 2.7mm with the PCL resected, respectively;

• PCL resection resulted in increased flexion gap heights of ~1–2mm for both sides.

During knee flexion from 30° to 90°, the PCL tended to squeeze the medial compartment by 1–2mm (p< 0.05). Increasing the force from 50N to 100N per side resulted in a mean gap increase of 0.5mm throughout the range of flexion.

Conclusions: Measurement of gap kinematics with a computer-controlled tensioner and a completely reduced patella is feasible. Everting the patella and resecting the PCL both have significant effects on flexion gap balance and symmetry. Knees which are balanced with the patella everted may be post-operatively 1–3mm more lax in flexion than planned. Retaining the PCL may result in asymmetric tightening of the medial gap from 30° to 90°.