Collateral ligament laxity in knees: what is normal?

BACKGROUND: Proper alignment and balancing of soft tissues of the knee are important goals for TKA. Despite standardized techniques, there is no consensus regarding the optimum amount of collateral ligament laxity one should leave at the end of the TKA. QUESTIONS/PURPOSES: I asked (1) what is the collateral laxity in young healthy volunteers, and (2) is there a difference in collateral laxity between males and females.
METHODS: The femorotibial mechanical angle (FTMA) was measured in 314 knees in healthy volunteers aged 19 to 35 years. Subjects with a history of pain, malalignment, dysplasia, or trauma were excluded. Twenty-five knees were excluded because the hip center could not be acquired, and 22 were excluded because of a history of pain and trauma, leaving 267 knees for inclusion in the study. Of these, 155 were from men and 112 were from women. A validated method using a computer navigation system was used to obtain the measurements. A 10-Nm torque was used to stress the knee in varus and valgus at 0° extension and 15° flexion. An independent t-test and ANOVA were applied to the data to calculate any significant difference between groups (p<0.05).
RESULTS: The mean (SD) unstressed supine FTMA was varus of 1.2° (SD, 4°) in 0° extension and varus of 1.2° (SD, 4.4°) in 15° flexion (p=0.88). On varus torque of 10 Nm, the supine FTMA changed by a mean of 3.1° (SD, 2°) (95% CI, 2.4°-3.8°; p<0.001) in 0° extension and 6.9° (SD, 2.6°) (95% CI, 6.2°-7.7°; p<0.001) in 15° flexion. On valgus torque of 10 Nm, the FTMA changed by a mean of 4.6° (SD, 2.2°) (95% CI, 3.9°-5.3°; p<0.001) in 0° extension and 7.9° (SD, 3.4°) (95% CI, 7.1°-8.7°; p<0.001) in 15° flexion. The mean unstressed FTMA in 0° extension was varus of 1.7° (SD, 4°) in men and 0.4° (SD, 3.9°) in women (p=0.01). Differences in collateral ligament laxity were seen between men and women (p<0.001 for valgus torque and 0.035 for varus torque in 15° flexion). With valgus torque at 0° flexion, the supine FTMA change was valgus of 4.2° (SD, 2.0°) for men and 5.0° (SD, 2.4°) for women, while at 15° flexion the FTMA change was valgus 7.6° (SD, 3.6°) for men and 8.3° (SD, 3.2°) for women With varus torque at 0° flexion, additional varus was -3.0° (SD, 1.8°) for men and -3.3° (SD, 2.2°) for women, while at 15° flexion, varus was -7.0° SD, (2.5°) for men and -6.9° (SD, 2.8°) for women.
CONCLUSIONS: The collateral laxity in young healthy volunteers was quantified in this study. The collateral ligament laxity is variable in different persons. In addition, ligaments in women are more lax than in men in valgus stress. CLINICAL RELEVANCE: This study was conducted on young, healthy knees. Whether the findings are applicable to arthritic knees and replaced knees needs additional evaluation. However the findings provide a baseline from which to work in the evaluation of arthritic knees and in the case of TKA.