Accuracy of Preoperative Magnetic Resonance Imaging in Predicting Intra-articular Pathology Associated With Patellar Instability Diagnosed by Arthroscopy: A High Rate of Discordant Findings Altering Surgical Treatment.

Background: Acute traumatic patellar dislocations may cause intra-articular derangements of the knee in the setting of acute and chronic instability which may not always be appropriately identified on magnetic resonance imaging (MRI). Purpose: To determine the rates of coexisting intra-articular pathology in patients with patellar instability requiring medial patellofemoral ligament (MPFL) reconstruction and to report discrepancies between preoperative magnetic resonance imaging (MRI) findings and diagnostic arthroscopy results. Study Design: Cohort study (Diagnosis); Level of evidence, 3. Methods: Patients who underwent MPFL reconstruction at a single institution between 2010 and 2023 were identified. Operative reports and preoperative MRIs were reviewed. Records without full MRI reports as read by a radiologist or cases in which a diagnostic arthroscopy was not conducted were excluded. Intra-articular pathologies noted on MRI and confirmed by arthroscopy were documented. MRI findings were considered to be discordant from arthroscopic findings if a lesion was identified on diagnostic arthroscopy that was not present in the full MRI report and that lesion resulted in additional surgical procedures. Results: There were 331 arthroscopic procedures conducted within the study population of 508 patients who underwent diagnostic arthroscopy at the time of their patellar stabilization procedures. These procedures included 180 loose body removals, 95 osteochondral lesions treated via a variety of interventions including open reduction and internal fixation (26 cases), particulated allograft chondral grafting (30 cases), osteochondral allograft transplantation (9 cases), and microfracture (30 cases), and 39 meniscal tears resulting in 33 partial meniscectomies and 6 meniscal repairs. Among these 508 patients, 105 (21%) demonstrated discordance of preoperative MRI and arthroscopic findings that necessitated additional procedures. This discordance was responsible for a total of 109 (33%) of the 331 arthroscopic procedures. These included 75 loose body removals, 2 osteochondral fracture fixation procedures, 2 microfractures for full-thickness cartilage loss, 16 partial meniscectomies, and 2 meniscal repairs, among others. Conclusion: Among patients undergoing patellar stabilization, 21% had intra-articular lesions not identified on preoperative MRI detected at the time of diagnostic arthroscopy that resulted in surgical intervention. This study, representing the largest cohort to date of patellar stabilization surgeries with both MRI and arthroscopy findings, underscores the importance of diagnostic arthroscopy for improving diagnostic accuracy and addressing associated intra-articular pathology.