Paper 05: Patellar Dislocation in Adolescent Patients: Influence on Patellofemoral and Tibiofemoral Cartilage based on T1ρ Relaxation Times

OBJECTIVES: Patellar dislocations are associated with a high risk of patellofemoral osteoarthritis (OA). The risk is further elevated for adolescent patients and patients who experience multiple dislocations. Quantitative MRI using T1ρ relaxation time mapping characterizes cartilage properties based on concentration of proteoglycans. Elevated T1ρ values identify cartilage degradation that could develop into post-traumatic OA. The current study compares T1ρ relaxation times throughout the knee between adolescents being treated for a single dislocation, multiple dislocations, and healthy controls. The hypothesis of the study is that patellar dislocation will increase T1ρ relaxation times for patellofemoral cartilage, with higher T1ρ relaxation times for multiple dislocations than single dislocations. METHODS: The IRB-approved study included 13 patients (16 ± 2 years; 5 females; BMI: 23 ± 3 kg/m(2), 47 ± 38 days since dislocation) being treated for an initial, unilateral traumatic lateral patellar dislocation, 10 being treated for multiple unilateral dislocations (17 ± 2 years, 2 females; 23 ± 4 kg/m(2), 55 ± 24 days since most recent dislocation), and 10 healthy controls (16 ± 2 years, 5 females; 22 ± 2 kg/m(2)). MRI scans included a 3D water excitation dual energy steady state (DESS) scan of the knee for segmentation of cartilage surfaces (slice thickness of 0.7 mm) and a T1ρ relaxation time scan (fat saturated scan, slice thickness = 4 mm, spin lock times = 0, 10, 40, and 80 ms, spin-lock frequency = 500 Hz). Automated algorithms based on machine learning were used to automatically segment cartilage into six compartments: patella, trochlear groove, lateral/medial femur, lateral/medial tibia. The cartilage on the patella and within the trochlear groove were further divided into medial, lateral and central regions. The central region was centered on the patellar ridge for the patella and the deepest points throughout the trochlear groove and extended one-third of the distance to the medial and lateral edges. Additional automated algorithms mapped T1ρ relaxation times to the reconstructed cartilage surfaces (Fig. 1). Mean T1ρ relaxation times were quantified within each region for each knee. The mean T1ρ relaxation times within the regions were compared between the three groups with ANOVA’s and Student-Newman-Keuls post-hoc tests. Statistical significance was set at p < 0.05. RESULTS: T1ρ relaxation times were significantly longer for injured knees (both single and multiple dislocations) than controls at the medial and central patella, central trochlear groove, and lateral tibia (Fig. 2). For regions on the patella, the significant differences represented a difference between injured and control knees of 20% or larger. For the other significant regions, the difference was approximately 10%. No significant differences were identified between single and multiple dislocations. CONCLUSIONS: For adolescent patients, a patellar dislocation is associated with elevated T1ρ relaxation times for cartilage throughout the knee. Elevated T1ρ values indicate loss of proteoglycan content that reduces cartilage integrity and could be an initiation point for progression to OA. Elevated relaxation times from quantitative MRI is consistent with previous studies focused on patellar dislocations. The current study uniquely focuses on an adolescent population and uniquely compares single to multiple dislocations in the acute phase following injury. The results set a baseline for characterization of progressive cartilage degradation to OA following patellar dislocation for adolescents. The elevated T1ρ relaxation times within the patellofemoral joint are consistent with high risk of long term patellofemoral OA for adolescents who experience patellar dislocations. The elevated T1ρ relaxation times at the lateral tibia indicate an additional risk of tibiofemoral OA. The similar T1ρ relaxation times for single and multiple dislocations was not expected due to the higher risk of patellofemoral OA following multiple dislocations. Traumatic impact of the medial patella against the lateral femoral condyle during a dislocation is a primary contributor to cartilage degradation in the acute phase following a patellar dislocation, as demonstrated by the largest difference between injured knees and controls occurring at the medial patella for first time dislocators. Additional dislocations did not further increase T1ρ relaxation times at the medial patella. Recurrent dislocations could be less traumatic due to previous injury to medial soft tissue restraints, decreasing the influence on cartilage properties. Inflammation and pathologic anatomy can also contribute to cartilage degradation in adolescents experiencing patellar dislocations. Traumatic injury induces an inflammatory response that can be detrimental to cartilage. Pathologic anatomy associated with patellar dislocations includes patella alta, trochlear dysplasia and a lateral position of the tibial tuberosity. Pathologic anatomy can create loading conditions during daily function that adversely influence cartilage. Inflammation and anatomy likely contribute to the significantly elevated T1ρ relaxation times away from the medial patella. Over the long term, inflammation and anatomy may play a greater role in the risk of OA for multiple dislocators than repeated impact injuries. To continue to characterize the continuum from patellar dislocation to OA, additional recruitment is warranted to determine if significant differences can be identified in additional regions. Additional characterization of the influence of inflammation and pathologic anatomy on cartilage degradation is also warranted to identify patients at highest risk of OA and develop treatment strategies to preserve cartilage following patellar dislocations.