Immunophenotyping of Synovial Tissue in Adolescents Undergoing ACL Reconstruction: What Is the Role of Synovial Inflammation in Arthrofibrosis?

BACKGROUND: Loss of motion and arthrofibrosis after anterior cruciate ligament (ACL) reconstruction (ACLR) can be devastating complications for athletes. The cellular and molecular pathogenesis of arthrofibrosis is poorly understood, limiting prevention and treatment options. Synovial inflammation may contribute to post-ACLR arthrofibrosis. HYPOTHESIS: Higher synovial immune cell infiltration and inflammatory/catabolic gene expression patterns at the time of ACLR would correlate with poorer motion-related outcomes. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: Patients aged 10 to 18 years undergoing primary ACLR were enrolled in a prospective pilot study, and synovial tissue biopsy specimens were obtained during ACLR. Flow cytometry and single-cell RNA sequencing explored synovial cell types/frequencies and gene expression. Principal component analysis was performed, followed by clustering which grouped patients into distinct immunophenotypes based on their synovial cell composition. Clinical follow-up data with knee range of motion (ROM), need for lysis of adhesions, and patient-reported outcome measures were collected and compared between immunophenotypes. RESULTS: Enrolled patients (n = 17) underwent ACLR at a median of 37 days after injury. Analysis revealed 3 distinct immunophenotypes. Type 1 consisted of patients with the longest time between injury and surgery and the lowest hematopoietic and T-cell infiltration. Types 2 and 3 had similar times between injury and surgery; type 2 had intermediate while type 3 had the highest hematopoietic and T-cell percentages. Type 3 was associated with worse ROM at 2 and 6 weeks postoperatively; T-cell prevalence and ROM were inversely correlated at those time points. The only patient requiring lysis of adhesions for arthrofibrosis had a type 3 immunophenotype. CONCLUSION: Synovial immune infiltration after ACL injury shows variability between patients that clusters into 3 immunophenotypes correlating with early ROM and the risk of arthrofibrosis. T-cell recruitment and infiltration were the strongest factors correlated with ROM outcomes and present an exciting venue for future research on post-ACLR arthrofibrosis.