DOES COMPUTER-ASSISTED MEASUREMENT OF VARUS AND VALGUS KNEE JOINT LAXITY REFLECT THE PROPERTIES OF THE COLLATERAL LIGAMENTS? AN IN VITRO STUDY

Abstract

As well as improved component alignment, recent publications have shown that navigation systems can assess knee kinematics and provide a quantitative measurement of soft tissue characteristics. In particular, navigation-based measures of varus and valgus stress angles have been used to define of the extent of soft-tissue release required at the time of the placement of the prosthesis. However, the extent to which such navigation-derived stress angles reflect the restraining properties of the collateral ligaments of the knee remain unknown. The aim of this cadaveric study was to investigate correlations between the structural properties of the collateral ligaments of the knee and stress angles measured with an optically-based navigation system.

Nine fresh-frozen cadaveric knees (age 81 ± 11 years) were resected 10-cm proximal and distal to the knee joint and dissected to leave the menisci, cruciate ligaments, posterior joint capsule and collateral ligaments. The resected femoral and tibial were rigidly secured within a test system which replicated the lower limb and permitted kinematic registration of the knee using the standard workflow of a commercially available image free navigation system. Frontal plane knee alignment and varus-valgus stress angles in extension were acquired. The manual force required to produce varus-valgus stress angles during clinical testing was quantified with a dynamometer attached to the distal tibial segment. Following assessment of knee laxity, bone–ligament–bone specimens were prepared and mounted within a uniaxial materials testing machine. Following 10 preconditioning cycles specimens were extended to failure. Force and crosshead displacement were used to calculate principal structural properties of the ligaments including ultimate tensile strength and stiffness as well as the instantaneous stiffness at loads corresponding to those applied during varus-valgus stress testing. Differences in the structural properties of the collateral ligaments and the varus and valgus laxity of the knee were evaluated using paired t tests, while potential relationships were investigated with scatter plots and Pearson’s product moment correlations.

There was no significant difference in the mean varus (4.3 ± 0.6°) and valgus laxity measured (4.3 ± 2.1°) for the nine knees or the corresponding distal force application required during stress testing (9.9 ± 2.5N and 11.1 ± 4.2N, respectively). Six of the nine knees had a larger varus stress angle compared to the valgus angle. There was no significant difference in the stiffness of the medial (63 ± 15 N/mm) and lateral (57 ± 13 N/mm) collateral ligaments during failure testing. The medial ligament, however, was approximately two fold stronger than its lateral counterpart (780 ± 214N verse 376 ± 104N, p< 0.001). While the laxity measures of the knee were independent of the ultimate tensile strength and stiffness of the collateral ligaments, there was a significant correlation between the force applied during stress testing and the instantaneous stiffness of the medial (r = 0.91, p = 0.001) and lateral (r = 0.68, p = 0.04) collateral ligaments.

The findings of the current study suggest that computer-assisted measures of passive knee laxity are largely independent of the ultimate strength and stiffness of the collateral ligaments. The force applied during manual stress testing of the knee, however, was strongly correlated with the instantaneous stiffness of the collateral ligaments suggesting users may attend to the low-stress behaviour of the ligaments. Nonetheless the force applied during stress testing varied between knees, as did the resultant angular deviation. Therefore to make use of the quantified data given by navigation systems, further work to understand the relationships between applied force, resultant stress angles and clinical outcomes for knee arthroplasty is required.