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

Abstract

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.

Length Changes of the Medial Patellofemoral Ligament During In Vivo Knee Motion: An Evaluation Using Dynamic Computed Tomography

“…Cadaveric studies have shown that patella alta and a TT-TG distance >20 mm increase MPFL anisometry during knee flexion. 24,35 Recently, Tanaka et al 33 demonstrated a similar relationship in vivo in asymptomatic knees with anatomic risk factors for patellar instability, including patella alta, tuberosity lateralization, and trochlear dysplasia. In our study with healthy participants, we did not find any profound effects of the TT-TG distance, CDI, or LTI on MPFL length changes, as these were in the normal range (CDI <1.2; TT-TG distance <20 mm; LTI <11° 4 ).…”

“…After reconstruction, this could potentially result in the inability to attain full extension because of excessive graft tension and failure as a consequence. 24,35 Recently, Tanaka et al 33 investigated in vivo MPFL length changes from 0° to 50° of flexion in knees with anatomic risk factors for patellar instability, including patella alta, tuberosity lateralization, and trochlear dysplasia. In accordance with cadaveric studies, they showed increased MPFL slackening during knee flexion in the presence of anatomic risk factors, especially when multiple risk factors were present simultaneously.…”

“…In accordance with cadaveric studies, they showed increased MPFL slackening during knee flexion in the presence of anatomic risk factors, especially when multiple risk factors were present simultaneously. 33 In practice, patients with anatomic risk factors for patellar instability usually undergo additional procedures to correct their abnormalities, and the optimal surgical treatment method for patellar instability is still a matter of debate. 7 Therefore, it would be interesting to investigate whether a similar relationship exists between these anatomic parameters and in vivo MPFL length changes in healthy knees.…”

Derotational distal femoral osteotomy improves subjective function and patellar tracking after medial patellofemoral ligament reconstruction in recurrent patellar dislocation patients with increased femoral anteversion: A systematic review and meta‐analysis

“…Following osteotomy, the lower limb alignment is restored, leading to a more normal knee joint biomechanical environment. This significantly reduces the length change of the MPFL graft during knee flexion and extension, thereby reducing graft tension and postoperative symptoms [38]. It also facilitates early postoperative rehabilitation exercises, contributing to a decreased risk of knee stiffness.…”