Male-Female Differences in Knee Laxity and Stiffness

Abstract

Background: It has been reported that over 70% of anterior cruciate ligament (ACL) injuries occur in noncontact situations and that females are at 2 to 8 times greater risk of ACL injury than males. Increased joint laxity and reduced knee stiffness in female knees have been suggested as possible explanations for the higher ACL injury rates in females. Hypothesis: Compared with male knees, female knees will demonstrate increased laxity and reduced stiffness along the anterior-posterior (AP), internal-external (IE), and varus-valgus (VV) directions. Study Design: Controlled laboratory study. Methods: Forty-seven fresh-frozen human cadaveric knees were tested (22 male and 25 female) by use of a robotic system. Mean ages were 34.6 years (range, 19-45 years) for males and 28.4 years (range, 16-42 years) for females. Joint laxity and stiffness were measured from force-vs-displacement or torque-vs-rotation curves recorded for 3 modes of testing: ±134 N AP force, ±5 N·m IE torque, and ±10 N·m VV moment. Results: Compared with male knees, female knees had greater internal laxity from 0° to 50° flexion (P < .01; maximum difference of 8.3° at 50° of flexion) and greater valgus laxity from 0° to 50° of flexion (P < .05; maximum difference of 1.6° at 50° of flexion). However, female knees exhibited greater anterior laxity only at 50° of flexion (P < .03; difference of 1.3 mm). No significant male-female differences in anterior or posterior stiffness were found. Male knees had 42% greater internal stiffness from 0° to 30° of flexion (P < .03), 35% greater valgus stiffness at 10° of flexion (P < .03), and 19% greater varus stiffness at 50° of flexion (P < .03). Conclusion: Female knees demonstrated significantly increased laxity and reduced stiffness compared with males. This finding was not uniform but was dependent on the direction tested and the knee flexion angle. Clinical Relevance: Understanding the risk factors for noncontact ACL injury is important for injury prevention. In combination with other female-specific risk factors, increased knee laxity may be a contributing factor associated with the higher rate of female ACL injuries.