Patellofemoral joint kinematics in individuals six-month after anterior cruciate ligament reconstruction

Purpose: Osteoarthritis (OA) of the patellofemoral joint (PFJ) is prevalent among individuals undergoing anterior cruciate ligament reconstruction (ACLr). Postoperative radiographic OA in the PFJ has been reported in 11-90% (median 36%) of patients 2-15 years after the surgery. Even though PFJ OA after ACLr is highly prevalent, most attention has been focused on tibiofemoral joint. Various studies have examined tibiofemoral biomechanics in individuals after ACLr, and their associations with longitudinal changes in tibiofemoral OA, yet it remains unclear whether individuals after ACLr exhibit altered PFJ kinematics. Therefore, the purpose of the current study was to examine whether PFJ kinematics in six degrees of freedom differed among the ACLr knees, uninjured contralateral knees, and healthy control knees. Methods: Twenty-two patients with unilateral ACLr were recruited at 6-month after surgery (14 males, age 27.2 ± 8.1 years, body mass index 24.1 ± 2.9 kg/m2). All patients underwent anatomic single-bundle ACLr by board-certificated, fellowship-trained orthopedic surgeons and underwent standard postoperative rehabilitation protocol. Additionally, 10 healthy individuals with no history of knee injuries were recruited as controls (7 males, age 32.1 ± 5.1 years, body mass index 23.6 ± 2.6 kg/m2). All subjects underwent bilateral knee magnetic resonance imaging (MRI) using a 3T MRI scanner (GE Milwaukee, WI) with an eight-channel knee coil (Invivo, Inc., Gainesville, FL). Two sets of T2-weighted, fat-saturated fast spin-echo images were acquired in full knee extension and 30° of knee flexion for both knees. During the scan, the subjects were positioned supine with an in-house built loading apparatus applying 25% of the subjects’ weight at the bottom of the feet. Femoral and patella bone regions of interest were automatically segmented on the extended and flexed images to provide the quantification of PFJ kinematics. A V-net convolutional neural network was used for the bone automatic segmentation for both the extended and flexed scans. Forty images from the baseline time point were manually segmented and used for model development. The data was split into 25 training, 5 validation and 10 holdout scans, after algorithm validation, six-month bone segmentation were inferred from the trained model and kinematics were quantified using in-house algorithms written in MATLAB (Mathworks Inc, CA). PFJ kinematics in six degrees of freedom included patella flexion, tilt, and rotation, as well as anterior-posterior, medio-lateral, and superior-inferior translations were quantified. Independent t test and Chi-square analysis were used to compare the demographics between ACLr subjects and controls. One-way ANOVA was used to compare PFJ kinematics among the ACLr knees (n=22), uninjured contralateral knees (n=22), and healthy control knees (n=20). Results: For the automated segmentation, the mean DICE coefficients for the holdout sets of the extended and flexed data were 0.93 and 0.92, respectively. No significant group differences were observed for the demographics. Significant main effect for group was observed for the patella medio-lateral translation in both extended (p=0.038) and flexed (p=0.008) conditions. In the extended condition, post-hoc analysis revealed a more laterally displaced patella in the ACLr knees compared to healthy control knees (53.9 ± 7.4 mm vs 47.9 ± 8.1 mm, p=0.033). No significant difference was found between the ACLr knees and the uninjured contralateral knees (53.9 ± 7.4 mm vs 50.9 ± 6.7 mm, p=0.538). The results were similar for the flexed condition, with significant difference between ACLr knees and healthy control (54.1 ± 6.7 mm vs 46.9 ± 9.0 mm, p=0.006), but not ACLr knees and uninjured contralateral knees (54.1 ± 6.7 mm vs 51.3 ± 5.6 mm, p=0.620) (Figure 1). No significant main effects were found for other variables (Table 1). Conclusions: Our findings suggest altered PFJ kinematics in individuals six-month after ACLr. Specifically, ALCr knees exhibit a more laterally displaced patella when compared to healthy controls. Previous studies have shown that a laterally displaced patella is associated with patellofemoral pain and subchondral bone lesions. This is likely due to the fact that a laterally displaced patella decreases the utilized contact area of the PFJ and further elevates stress. Due to the cross-sectional design of the study, we were not able to determine whether the displaced patella is the result of the ACLr or a natural characteristic in individuals who sustained ACL tears. However, it is possible that a laterally displaced patella is a risk factor to the development of PFJ OA after ACLr. Longer follow-up study is needed to elucidate this relationship. Identifying the biomechanical risk factors associated with PFJ OA in individuals with ALCr provides us with a better understanding of the disease mechanisms. More importantly, these risk factors may provide information that helps to design better rehabilitation programs that defer the incidence and progression of OA post ACLr.View Large Image Figure ViewerDownload Hi-res image Download (PPT)