Effect of Patella Alta on the Native Anatomometricity of the Medial Patellofemoral Complex: A Cadaveric Study

Objectives: Patella alta has been identified as an important risk factor for lateral patellar instability and medial patellofemoral complex (MPFC) reconstruction failure. To our knowledge, no prior study has analyzed the anisometry of the MPFC in the setting of patella alta as this may play a role in the postoperative outcomes and could affect surgical technique. Methods: Eight (n=8) fresh frozen cadaveric knees were used in this study. No IRB approval was needed at our institution due to the use of cadaveric specimen. The MPFC was identified and dissected along with the patellar tendon, and quadriceps tendon. A custom-made jig was utilized to evaluate MPFC lengths from 0-90° of flexion. Length was measured at four possible reconstruction locations (midpoint patella [MP], MPFC osseous center [FC], superior medial pole of the patella [SM], and quadriceps tendon [Q]) along the extensor mechanism using a 3D robotic arm. These measurements were repeated at 0, 20, 40, 60 and 90° degrees of flexion. Degrees of increasing severity of patella alta at Caton-Deschamps index (CDI) ratios of 1.0, 1.2, 1.4, and 1.6, were then investigated. Results: CDI and attachment sites significantly affect changes in MPFC length from 0-90° of flexion (p < 0.0005). Proximal attachment points had more robust length changes than distal ones and increases in CDI enhanced these differences. Point Q at CDI 1 was similar to SM at 1.2 (p = 0.234), SM and FC at 1.4 (p = 0.89 and p = 0.073) and FC at 1.6 (p = 0.928). SM at CDI of 1 was similar to FC at 1.2 and 1.4 (p = 0.414 and p = 0.503) and MP at 1.6 (p = 0.473), while FC at CDI 1 was similar to the FC at 1.2 (p = 0.157) and MP at 1.4 and 1.6 (p = 0.068 and p = 0.519). Finally, the MP was similar at CDI 1.2 (p = 0.888) and 1.4 (p = 0.385) compared to at CDI of 1. At the quadriceps points, MPFC length and flexion degrees showed a moderate negative linear correlation at a CDI of 1 (r = -0.484, p = 0.002) and 1.6 (r = -0.692, p < 0.0005) (Fig 1). At the MP location at a CDI ratio of 1.6, significant differences were seen at 0° vs 90° (p = 0.027), 0° vs 60° (p = 0.044), 0° vs 40° (p = 0.016), and 0° vs 20° (p = 0.044), suggesting differing length change properties between the Q and MP attachment sites. Conclusions: Anisometry varies with location of the patellar attachment and with patellar height within the MPFC. The superior aspect of the MPFC demonstrated the most isometric behavior, increasing linearly with increasing flexion. The inferior aspect of the MPFC retains a relatively constant length at 20 to 90° of flexion. Increasing CDI amplified these results. These findings demonstrate that proximal based grafts loosen significantly before engaging in the trochlea and this effect is exaggerated in the setting of patella alta. Furthermore, this data suggests that soft tissue surgical reconstructions may be modified to correct for patella alta in place of performing distalization, which carries significant morbidity.

Differential anatomy within the femoral origin of the medial patellofemoral complex: Implications or reconstruction

Objectives: The medial patellofemoral complex (MPFC) includes the medial patellofemoral ligament (MPFL) and medial quadriceps tendon femoral ligament (MQTFL). Recent reports have described reconstruction of this fan-shaped ligament to treat patellar instability using a double stranded technique to recreate both components of the complex, with a common origin on the medial femur. Much effort has been placed on accurately identifying the “point” of femoral origin during reconstruction due to the influence of femoral tunnel position on MPFC graft function, however, the MPFC origin is elongated in nature. Therefore, the purpose of this study was to describe the shape and orientation of the MPFC origin and identify the difference between the most proximal and distal margins of the elongated femoral footprint. Methods: 20 paired fresh frozen cadaveric knees were dissected. From an intraarticular approach, the MPFC was exposed and followed to its footprint on the medial femur. All other soft tissue was removed from the distal femur, and the footprint of the MPFC, the adductor tubercle and medial epicondyle were marked. Images of the medial femur were analyzed using Image J software. The length and width of the MPFC footprint was described to the nearest 0.1mm, as well as the angle of the long axis of the footprint relative to the axis of the femoral shaft (0.1 degrees). The position of the footprint’s most proximal and distal margins were identified and described in relation to the adductor tubercle and medial epicondyle. The positions for each were compared using paired t tests. Results: 17 knees from 10 cadavers (7M, 3F, mean age 73.1) were included in this study. The MPFC femoral footprint had a length of 11.7mm+/-1.8mm (Range 9.6,15.7) and a width of 1.7mm+/-0.4mm (Range, 0.9, 2.2). The long axis of the footprint was found to lie at an angle 14.6+/-16.6 degrees anterior to the axis of the femoral shaft. The most proximal fibers originated 7.4mm+/-3.8mm anterior and 1.8mm+/-4.7mm distal to the adductor tubercle, and 4.1mm+/-2.6mm posterior and 8.4mm+/-5.6mm proximal to the medial epicondyle. The most distal fibers originated 4.9mm+/-4.2mm anterior and 1.3mm+/-4.3mm and distal to the adductor tubercle, as well as 7.1mm+/-2.4mm posterior and 0.5mm+/-5.6mm distal to the medial epicondyle. Overall, the distal margin of the footprint was 10.9mm+/-1.7mm distal (p<0.001) and 2.6mm+/-3.2mm more posterior (p=0.005) than the proximal margin of the MPFC origin. Conclusions: The femoral footprint of the MPFC is ribbon shaped, with the distal margin being 10.9 mm distal and 2.6 mm posterior to the proximal margin. This differential anatomy of the femoral origin suggest that MPFL and MQTFL reconstruction may require separate placements of the femoral tunnels to anatomically recreate these fibers. Further biomechanical studies are needed to determine the optimal femoral tunnel placement in the setting of double-limbed MPFC reconstruction, as well as the long term benefit of this technique in the treatment of patellar instability.

Effect of Patella Alta on the Native Anatomometricity of the Medial Patellofemoral Complex: A Cadaveric Study

Background: Patella alta has been identified as an important risk factor for lateral patellar instability and medial patellofemoral complex (MPFC) reconstruction failure. Purpose: To evaluate the length changes of the MPFC at multiple possible reconstruction locations along the extensor mechanism in varying degrees of patella alta throughout knee motion. Study Design: Controlled laboratory study. Methods: Eight fresh-frozen cadaveric knees were used in this study. The MPFC was identified and dissected with the patellar tendon and quadriceps tendon. A custom-made jig was utilized to evaluate lengths from 0° to 90° of flexion with physiological quadriceps loading. Length was measured with a 3-dimensional robotic arm at 4 possible reconstruction locations along the extensor mechanism: the midpoint patella (MP), the MPFC osseous center (FC), the superior medial pole of the patella (SM) at the level of the quadriceps insertion, and 1 cm proximal to the SM point along the quadriceps tendon (QT). These measurements were repeated at 0°, 20°, 40°, 60° and 90° of flexion. Degrees of increasing severity of patella alta at Caton-Deschamps index (CDI) ratios of 1.0, 1.2, 1.4, and 1.6 were then investigated. Results: Patella alta and MPFC attachment site location significantly affected changes in MPFC length from 0° to 90° of flexion ( P< .0005). Length changes at attachment MP showed no difference when CDI 1.0 was compared with all patella alta values (CDI 1.2, 1.4, 1.6; P > .05). Similarly, FC showed no difference in length change from 0° to 90° until CDI 1.6, in contrast to proximal attachments (SM, QT), which demonstrated significant changes at CDI 1.4 and 1.6. When length changes were analyzed at each degree of flexion (0°, 20°, 40°, 60°, 90°), Spearman correlation analysis showed a moderate negative linear correlation for QT at CDI 1.0 ( r= −0.484; P = .002) and 1.6 ( r = −0.692; P < .0005), demonstrating constant loosening at the QT point at normal and elevated patellar height. In contrast, no differences in length were observed for MP at CDI 1.0 throughout flexion, and at CDI 1.6, there was a difference only at 0° ( P < .05). Points FC and MP at CDI 1.6 had similar length change properties to points SM and QT at CDI 1.0 ( P > .05), suggesting that distal attachments in the setting of patella alta may provide similar length changes to proximal attachmentswith normal height. Conclusion: Anisometry of the MPFC varies not only with attachment location on the extensor mechanism but also with patellar height. Increased patellar height leads to more significant changes in anisometry in the proximal MPFC attachment point as compared with the distal component. In the setting of patella alta, including a CD ratio of 1.6, the osseous attachments of the MPFC remain nearly isometric wheras the proximal half length changes increase significantly. Clinical Significance: The results of this study support the idea that the MPFC should be considered as 2 separate entities (proximal medial quadriceps tendon femoral ligament and distal medial patellofemoral ligament) owing to their unique length change properties.