Abstract
Passive knee stability is provided by the soft tissue envelope which resists abnormal motion. There is a consensus amongst orthopedic surgeons that a good outcome in TKA requires equal tension in the medial and the lateral compartment of the knee joint, as well as equal tension in the flexion and extension gap. The purpose of this study was to quantify the ligament laxity in the normal non-arthritic knee before and after standard posterior-stabilized total knee arthroplasty (PS-TKA). We hypothesized that the medial collateral ligament (MCL) and the lateral collateral ligament (LCL) will show minimal changes in length when measured directly by extensometers in the native human knee during varus/valgus laxity testing. We also hypothesized that due to differences in material properties and surface geometry, native laxity is difficult to be completely reconstructed using contemporary types of PS-TKA.
Methods:
A total of 6 specimens were used to perform this in vitro cadaver test using extensometers to provide numerical values for laxity and varus-valgus tilting in the frontal plane. See Fig. 1 The test set-up.
Findings:
This study enabled a very precise measurement of varus and valgus laxity as compared with the clinical assessment which is a subjective measure. The strains in both ligaments in the replaced knee were different from those in the native knee. Both ligaments were stretched in extension, in flexion the MCL tends to relax and the LCL remains tight. Fig. 2 Initial and maximal strain values in the MCL during valgus and varus laxity testing in different flexion angles. a: intact knee, b: replaced knee. and Fig. 3 Initial and maximal strain values in the LCL during valgus and varus laxity testing indifferent flexion angles. a: intact knee, b: replaced knee.
Interpretation:
As material properties and surface geometry of the replaced knee add stiffness to the joint, we recommend when using a this type of PS-TKA to avoid overstuffing the joint in order to obtain varus/valgus laxity close to the native joint.
