In Vivo Length Changes Between the Attachments of the Medial Patellofemoral Complex Fibers in Knees With Anatomic Risk Factors for Patellar Instability.

BACKGROUND: Medial patellofemoral complex (MPFC) reconstruction plays an important role in the surgical treatment of patellar instability. Anatomic reconstruction is critical in re-creating the native function of the ligament, which includes minimizing length changes that occur in early flexion. Anatomic risk factors for patellar instability such as trochlear dysplasia, patella alta, and increased tibial tuberosity to trochlear groove (TT-TG) distance have been shown to influence the function of the MPFC graft in cadaveric studies, but the native length change patterns of the MPFC fibers in knees with anatomic risk factors have not been described. PURPOSE: To describe the in vivo length changes of the MPFC fibers in knees with anatomic risk factors for patellar instability and identify the optimal attachment sites for MPFC reconstruction. STUDY DESIGN: Controlled laboratory study. METHODS: Dynamic computed tomography imaging was performed on the asymptomatic knee in patients with contralateral patellar instability. Three-dimensional digital knee models were created to assess knees between 0° and 50° of flexion in 10° increments. MPFC fiber lengths were calculated at each flexion angle between known anatomic attachment points on the extensor mechanism (quadriceps tendon, MPFC midpoint [M], and patella) and femur (1, 2, and 3, representing the proximal to distal femoral footprint). Changes in MPFC fiber length were compared for each condition and assessed for their relationships to morphologic risk factors (trochlear depth, Caton Deschamps Index [CDI], and TT-TG distance). RESULTS: In 22 knees, native MPFC fibers were found to be longer at 0° than at 20° to 50° of flexion. Length changes observed between 0° and 50° increased with the number of risk factors present. In the central fibers of the MPFC (M-2), 1.7% ± 3.1% length change was noted in knees with no anatomic risk factors, which increased to 5.6% ± 4.6%, 17.0% ± 6.4%, and 26.7% ± 6.8% in the setting of 1, 2, and 3 risk factors, respectively. Nonanatomic patella-based attachments were more likely to demonstrate unfavorable length change patterns, in which length was greater at 50° than 0°. In patellar attachments, an independent relationship was found between increasing length changes and TT-TG distance, while in quadriceps tendon attachments, a trend toward a negative relationship between length changes and CDI was noted. All configurations demonstrated a strong relationship between percentage change in length and number of morphologic risk factors present, with the greatest influence found in patella-based attachments. CONCLUSION: The MPFC fibers demonstrated increased length changes in knees when a greater number of morphological risk factors for patellar instability were present, which worsened in the setting of nonanatomic configurations. This suggests that the function of the intact MPFC in patients with anatomic risk factors may not reflect previously described findings in anatomically normal knees. Further studies are needed to understand the pathoanatomy related to these changes, as well as the implications for graft placement and assessment of length changes during MPFC reconstruction techniques. CLINICAL RELEVANCE: MPFC length change patterns vary based on the number of morphologic risk factors for patellar instability present and should be considered during reconstructive procedures.

Redefining Trochlear Dysplasia: Normal Thresholds Vary by Measurement Technique, Landmarks, and Sex.

BACKGROUND: Trochlear dysplasia is a known risk factor for patellar instability. Multiple radiographic measurements exist to assess trochlear morphology, but the optimal measurement technique and threshold for instability are unknown. PURPOSE: To describe the optimal measurements and thresholds for trochlear dysplasia on magnetic resonance imaging (MRI) that can identify knees with patellar instability in male and female patients. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 3. METHODS: Knee MRI scans of patients with patellar instability were compared with those of age- and sex-matched controls. Measurements of the sulcus angle, lateral trochlear inclination (LTI), and trochlear depth were performed on axial images using bony and cartilaginous landmarks. Receiver operating characteristic curve analysis was performed, with the area under the curve (AUC) describing the accuracy of each diagnostic test. Optimal cutoff values were calculated to distinguish between knees with and without patellar instability. AUC and cutoff values were reported for each measurement as well as for male and female subgroups. RESULTS: A total of 238 knee MRI scans were included in this study (138 female, 100 male; age range, 18-39 years). Trochlear depth measurements had the greatest diagnostic value, with AUCs of 0.79 and 0.82 on bone and cartilage, respectively. All measurements (sulcus angle, LTI, trochlear depth) on bone and cartilage had an AUC ≥0.7 (range, 0.70-0.86), with optimal cutoff values of 145° (bone) and 154° (cartilage) for the sulcus angle, 17° (bone) and 13° (cartilage) for LTI, and 4 mm (bone) and 3 mm (cartilage) for trochlear depth. Optimal cutoff values in female patients varied from those in male patients for all measurements except for cartilaginous trochlear depth. CONCLUSION: Normal thresholds for trochlear dysplasia varied based on the use of bony versus cartilaginous landmarks. Cartilaginous trochlear depth measurements had the greatest ability to identify knees with patellar instability. Furthermore, optimal cutoff values for all measurements except for cartilaginous trochlear depth differed between female and male patients. These findings suggest that sex-specific parameters of normal values may be needed in the assessment of risk factors for patellofemoral instability.