Pronounced And Persistent Underloading Of The Operated Knee During Common Exercises Used In ACLR Rehabilitation

PURPOSE: Underloading of knee contact forces during gait early after anterior cruciate ligament reconstruction (ACLR) has recently been linked to poor long-term cartilage health. Potential underloading of knee contact forces during exercises used in ACLR rehab remains unknown. We posit there will be pronounced and persistent ACLR knee underloading during common strengthening exercises despite large improvements in patient reported outcomes. METHODS: Thirty athletes were tested at 3 and 5 months after ACLR (12 males, BMI = 24 ± 3, age = 22 ± 6 yr, Tegner = 7/10 ± 2). Testing included the ACL-RSI scale and the IKDC subjective knee form. 3D kinematic (200 Hz) and kinetic (1000 Hz) data were collected during squats, leading leg of a step up, and trailing leg of a step down from a 20 cm box. A validated musculoskeletal model estimated peak contact forces of the patellofemoral (PF) and tibiofemoral (TF) joints. Multiple leg x time repeated measures ANOVAs were conducted (α = 0.05) with effect sizes (d) calculated to assess magnitude of change between factors. RESULTS: ACL-RSI (3mo = 31 ± 24%, 5mo = 51 ± 22%; p < 0.01) and IKDC scores (3mo = 63 ± 12, 5mo = 76 ± 12; p < 0.01) made large improvements over time (d ≥ 0.87). Large reductions in joint contact forces were observed in the ACLR versus the uninvolved knee during every exercise comparison (p < 0.01, d = 1.1-3.0). The uninvolved forces reduced in all exercises except the step up, but ACLR knee forces made small to no change with all exercises over time. - Peak Joint Contact Forces (all data in Body weight) Exercise task 3 months 5 months p-value over time Effect size (d) of change over time ACL Knee PF joint Squats 2.83 ± 0.61* 2.91 ± 0.68* 0.31 0.11 Uninvolved PF joint Squats 4.13 ± 0.78 3.75 ± 0.89 <0.01 0.46 ACL Knee TF joint Squats 3.48 ± 0.56* 3.53 ± 0.57* 0.34 0.10 Uninvolved TF joint Squats 4.67 ± 0.70 4.30 ± 0.79 <0.01 0.50 ACL Knee PF joint Step Up 1.53 ± 0.63* 1.72 ± 0.68* 0.07 0.28 Uninvolved PF joint Step Up 3.26 ± 0.53 2.77 ± 0.75 <0.01 0.76 ACL Knee TF joint Step Up 3.36 ± 0.81* 3.47 ± 0.75* 0.40 0.14 Uninvolved TF joint Step Up 4.89 ± 0.55 4.62 ± 0.77 0.07 0.41 ACL Knee PF joint Step Down 2.63 ± 1.05* 2.87 ± 1.15* 0.09 0.22 Uninvolved PF joint Step Down 5.48 ± 0.95 4.69 ± 1.08 <0.01 0.78 ACL Knee TF joint Step Down 4.81 ± 1.27* 5.24 ± 1.45* 0.03 0.32 Uninvolved TF joint Step Down 7.51 ± 0.99 6.69 ± 1.13 <0.01 0.77 CONCLUSIONS: Marked underloading of knee contact forces of the ACLR limb were observed during squats, step ups, and step downs. Underloading failed to normalize from 3 to 5 months after ACLR. The severe and persistent underloading could contribute to the high post-ACLR risk of knee osteoarthritis. Underloading is likely underappreciated by clinicians and their patients who concurrently report large improvements in physical function and sports readiness. Funded by a research grant from the Academy of Orthopaedic Physical Therapy of the APTA