Underloading, not overloading, of the patellofemoral joint increases the risk of early osteoarthritis after ACL reconstruction

      Background: Approximately 50% of adolescents and young adults who rupture their anterior cruciate ligament (ACL) develop osteoarthritis within 10 years. While most reports of osteoarthritis after ACL injury focus on the tibiofemoral joint, patellofemoral osteoarthritis is common and strongly associated with symptoms and impaired function. The mechanisms underpinning the development of patellofemoral osteoarthritis after ACL injury are unknown but may relate to altered joint loading. This study aimed to determine if altered patellofemoral loading is associated with the presence and/or worsening of early patellofemoral osteoarthritis following ACL reconstruction (ACLR).
      Methods: Forty-six participants (30 men, age 27±5 years, BMI 24.5±3.1 kg.m-2) were randomly selected from a cohort of 111 consecutively recruited patients with primary hamstring-tendon ACLR and completed magnetic resonance imaging (MRI) and biomechanics testing of their index knee 1-year post-ACLR. Biomechanics testing involved recording trunk and lower-limb movement with a 12-camera 3D motion analysis system and ground reaction force data during the landing phase of a standardised forward hop. These data were input into a musculoskeletal model to calculate patellofemoral joint contact forces normalised to body weight. Follow-up MRI was completed on 32 participants 5-years post-ACLR. Early patellofemoral osteoarthritis was defined as the presence of a patellofemoral cartilage lesion by an experienced musculoskeletal radiologist. Generalised linear models (Poisson) assessed the relationship between patellofemoral joint loading and prevalent early patellofemoral osteoarthritis at 1-year and worsening patellofemoral osteoarthritis between 1- and 5-years adjusting for sex and age.
      Results: At 1-year following ACLR, 14 (30%) participants had early patellofemoral osteoarthritis. Those with a lower peak patellofemoral joint contact force were more likely to have early patellofemoral osteoarthritis (prevalence ratio: 1.37, 95%CI 1.02-1.85). Of the 32 participants with 5-year follow-up data, 9 (28%) displayed worsening patellofemoral osteoarthritis. A lower peak patellofemoral joint contact force at 1-year increased the risk of worsening patellofemoral osteoarthritis (risk ratio: 1.54, 95%CI 1.13-2.11).
      Discussion: Young adults following ACLR who underload their patellofemoral joint during a hopping task are at high risk of early patellofemoral osteoarthritis onset and progression within the first 5-years after ACLR. For every one unit (i.e., body weight) decrease in peak patellofemoral joint contact force, the risk of incident or progressive early patellofemoral osteoarthritis increases ~50%. These findings challenge traditional thinking that joint overloading drives post-traumatic osteoarthritis and provides new targets for osteoarthritis prevention.
      Conflict of interest statement: My co-authors and I acknowledge that we have no conflict of interest of relevance to this abstract.