Objectives: 1) To prospectively determine the incidence of post-traumatic osteoarthritis (PTOA) at 10 years after anterior cruciate ligament reconstruction (ACLR) in young athletic patients on clinical radiographs: and 2) to determine the average difference in clinical radiographic osteoarthritis changes (joint space narrowing [JSN] and osteophyte formation) between the ACLR and contralateral ACL-intact knees. Methods: The first 146 patients in an ongoing prospective nested cohort study within the Multicenter Orthopaedic Outcomes Network (MOON) cohort returned onsite for minimum 10-year follow-up. Inclusion criteria were that patients had a sports-related ACL injury, no prior history of knee surgery, no contralateral ACL injury, and were less than 33 years of age at the time of their ACLR. Bilateral knee standing metatarsophalangeal (MTP) view radiographs were obtained and graded by International Knee Documentation Committee (IKDC), Osteoarthritis Research Society International (OARSI), and modified Kellgren-Lawrence (KL) criteria by two blinded reviewers. Inter-rater reliability was determined for all clinical radiographic OA grading criteria. The incidence and severity of ipsilateral and contralateral knee osteoarthritis were determined among patients without a contralateral ACL injury before 10-year follow-up (n=133). Results: Inter-rater reliability was substantial for IKDC (Gwet’s AC1 = 0.71), moderate for KL (0.48) and almost perfect for OARSI (0.84) grading systems. The 10-year incidence of PTOA on clinical radiographs in the ACLR knee was 43% as defined by osteophytes and 27% as defined by JSN (Table 1). In the contralateral ACL-intact knee, the incidence of osteophyte-defined OA was 10% and JSN-defined OA was 5%. The maximum side-to side difference in medial or lateral compartment OARSI osteophyte grade was 0 in 65% of patients, 1 in 20%, and 2+ in 15% (Figure 1) (Table 2). The maximum difference in OARSI JSN grade was 0 in 77% of patients, 1 in 19%, and 2+ in 4% (Figure 2) (Table 2). Conclusions: In young active patients, the 10-year incidence on clinical radiographs of osteophyte-defined PTOA after ACLR is 43% and JSN-defined PTOA is 27%. The average difference in degree of osteophyte formation (≤1 grade in 85%) and JSN (≤1 grade in 96%) between the ACLR knee and contralateral ACL-intact knee is small.

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