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

Abstract

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.

Why Determining the Native Length Change Pattern of Medial Patellofemoral Ligament Is Still a Challenge: State-of-the-Art Review of Potential Sources of Heterogeneity within Studies Evaluating Isometry of MPFL

“…From 10 original articles included in this review, nine studies identified multiple fibers of MPFL for assessment of length change [7,14,21,22,[24][25][26][27][28] and one study assessed only a single fiber [23]. Various femoral and patellar attachments were analyzed, as specified in Table 2.…”

“…Nine studies performed measurements in 3D [14,[21][22][23][24][25][26][27][28]; however, only in four was MPFL measured with respect to its convexity [7,14,24,25]. CT was utilized in four studies [14,22,24,25], caliper in two studies [21,27], MRI in one study [7], suture in one study [23], and other measurement methods in two studies [26,28]. Only five out of 10 studies reported precision of measurements [14,21,23,24,27].…”

“…While it is not clear whether demographics influence biomechanics of MPFL, it was reported to influence the biomechanics of other ligaments, potentially making it important to precisely report number, age, and sex of patients/cadavers [16,20]. Within original studies included in this review, all 10 articles reported number of patients/cadavers [7,14,[21][22][23][24][25][26][27][28]; however, only seven studies specified the age of patients/cadavers [7,14,[22][23][24][25][26], and also seven studies reported the sex of patients/cadavers [7,14,[21][22][23][24][25]. Within included review articles, only one of them investigated number of patients/cadavers and their age as potential factors influencing MPFL length change pattern [16].…”

“…Hamstrings tension may also introduce heterogeneity due to anterior translation of the tibia in case of inaccurate simulation. Such anterior translation could influence patellar kinematics through patellar ligament tension, similar to the patellar height [27,28].…”

“…Out of 10 original articles included in this review, six of them assessed cadavers [21][22][23][26][27][28] and four assessed patients [7,14,24,25]. In cadaveric studies, two out of six simulated high tension of quadriceps muscle [22,23], two simulated resting tension [27,28], one simulated quadriceps tension with unspecified load [21], and one did not simulate quadriceps tension at all [26]. Only one cadaveric study simulated high load activity of ITB [23] or hamstrings [22], with all other cadaveric studies not simulating any tension.…”