Structural And Morphologic Maladaptation Underlies Patellar Tendinopathy Following ACL Injury

Abstract

Anterior cruciate ligament (ACL) rupture is a prevalent sports injury associated with prolonged muscle weakness and atrophy. Patellofemoral pain is also common following ACL injury, but the direct impact of ACL injury on knee tendons has not been defined. Tendons are crucial for force transmission from muscle to bone, and the highly structured collagen network within tendon supports optimal load bearing and force transduction. With development of tendinopathy—a common cause of knee pain and restricted sports participation—normal tendon morphology shifts to a pathologic phenotype with altered collagen content and organization, which may contribute to impaired sports performance. PURPOSE: Determine if altered morphology representative of tendinopathy exists in patellar tendon after ACL injury. METHODS: Surgical ACL transection (ACL-T) was performed unilaterally on healthy adult mice (n=7). The patellar tendon was collected from injured and uninjured knees 7d after ACL-T and processed for morphological analysis. Immunohistochemical and histological techniques were used to assess collagen structure and abundance and tendon morphology. RESULTS: Collagen 1 content in the patellar tendon of the injured knee was significantly elevated by 78% at 7d after ACL-T (30.7 ± 3.3 pixels/μm2 vs. 17.2 ± 5.7 pixels/μm2; p=0.03). The collagen 3 to collagen 1 ratio was similarly higher in patellar tendon from the injured knee (0.09 ± 0.05 vs. 0.05 ± 0.04) but did not reach statistical significance. Densely packed collagen was 18% lower in the patellar tendon of the ACL-T injured knee (1.5 ± 0.3 pixels/μm2 vs. 1.8 ± 0.2 pixels/μm2; p=0.08). CONCLUSIONS: Collagen 1 content is starkly elevated in patellar tendon 7d after ACL-T in mice, consistent with tendinopathy. ACL injury was associated with reduced abundance of densely organized collagen, despite the elevated abundance of collagen 1. These findings are consistent with tendon pathology and may impair tendon loading capacity and force transduction. These data are the first to identify patellar tendon structural and morphologic maladaptation following ACL injury. Support: R01 AR072061