Patellofemoral Cartilage Degradation Based on T1ρ Relaxation Times Varies Inversely With BMI After Patellar Dislocation

Background: Elevated body mass index (BMI) is a primary factor contributing to osteoarthritis of the knee, but data are lacking on the relationship between BMI and cartilage properties shortly after a traumatic injury. Purpose/Hypothesis: The purpose of the study was to relate BMI to patellofemoral cartilage properties after a patellar dislocation, using magnetic resonance imaging–based T1ρ relaxation times to assess cartilage. It was hypothesized that high BMI is correlated with long patellofemoral cartilage T1ρ relaxation times, indicating cartilage degradation, in both control knees and knees after patellar dislocation. Study Design: Cross-sectional study; Level of evidence, 3. Methods: T1ρ relaxation time mapping was performed for underweight to obese (BMI < 40 kg/m 2 ) patients after a first-time patellar dislocation (n = 26; 19.8 ± 6.7 years old; 46 ± 36 days since dislocation), after multiple dislocations (n = 15; 18.4 ± 5.1 years old; 63 ± 33 days since most recent dislocation), and for healthy controls (n = 20; 20.1 ± 6.6 years old). For medial, lateral, and central regions of patellofemoral cartilage, mean T1ρ relaxation times were correlated against BMI. Statistical significance was set at P < .05. Results: For the control group, long T1ρ relaxation times were significantly correlated with high BMI for every region of patellofemoral cartilage except the lateral patella ( P < .025; r 2 = 0.26-0.63). On the contrary, for the first-time and multiple dislocations groups, long T1ρ relaxation times were significantly correlated with low BMI ( P < .05; r 2 = 0.24-0.50) for the medial patella (multiple dislocations only), central patella, lateral patella, medial trochlear groove (first-time dislocation only), central trochlear groove (first-time dislocation only), and lateral trochlear groove (first-time dislocation only). Conclusion: Long patellofemoral cartilage T1ρ relaxation times were related to high BMI for healthy knees, but unexpectedly related to low BMI after patellar dislocation. The results do not indicate a heightened risk of progressive cartilage degradation to osteoarthritis for overweight and obese patients immediately after patellar dislocation. For patients at a healthy weight and underweight, the traumatic injury is associated with early cartilage degradation, which could potentially initiate progressive degradation to posttraumatic osteoarthritis.