The Matrix Approach to Patellar Instability

Abstract

Patellar instability is a challenging orthopedic condition affecting both pediatric and adult populations. The diagnosis and treatment of this condition present challenges for surgeons because of the multitude of classifications and treatment options available in the literature, leading to potential confusion in treatment strategies. Nonoperative treatments often prove ineffective, with reported recurrence rates nearing. Consequently, numerous surgical interventions have been developed in pursuit of improved outcomes. However, the results of these early interventions have not been universally successful, resulting in over 100 surgical interventions being recommended for patellofemoral instability, and none of which have achieved universal success. This hesitancy among surgeons to recommend surgery can leave patients inadequately treated. This article aims to share our matrix approach to patellar instability, developed over the past decade. By providing insights into the condition, we hope to stimulate interest among aspiring surgeons and facilitate a comprehensive understanding of the diagnosis and management of patellofemoral instability.

Keywords: dislocation; matrix approach; patella; patellofemoral instability; subluxation.

Figure 1. The matrix approach traffic light coding system to assess the severity of the risk factors.

Red color denotes severity necessitating treatment, orange indicates borderline cases possibly requiring intervention, and green signifies normality where intervention is unnecessary. A white code is assigned when information is insufficient to determine whether a factor should be classified as green, orange, or red. Figures are provided by the authors.

Figure 2. Femoral and tibial torsion are recognized risk factors for patellar instability.

Figure 2a illustrates the typical range of femoral internal rotation, while Figure 2b depicts a child exhibiting excessive femoral internal rotation. Figure 2c displays the standard range for the foot-thigh angle, serving as an indicator of tibial external torsion, while Figure 2d presents a child with excessive tibial torsion. Figures are provided by the authors.

Figure 3. A CT scan of the lower limb’s rotation profile.

A CT scan is employed to validate both femoral (as shown in Figure 3a) and tibial torsion (as depicted in Figure 3b). Figure 3a indicates a greater degree of femoral torsion on the left side (measuring 39°) compared to 25° on the right side. Conversely, the tibial torsion of both tibias appears to be nearly symmetrical. Figures are provided by the authors.

Figure 4. The Salenius curve.

The Salenius curve demonstrates the alignment of the lower limbs, indicating whether there is a valgus or varus alignment based on the age of the patients. Figures are provided by the authors.

Figure 5. Zones of mechanical axes.

Zone 1 encompasses the area around the knee center and femoral notch. Zone 2 covers the femoral condyle, while Zone 3 extends beyond the femoral condyle. Figures are provided by the authors.

Figure 6. Trochlear dysplasia grading according to the matrix approach.

A trochlea exhibiting a sulcus angle of less than 144° is categorized as green, indicating normalcy. De Jour's type A and type B are designated as orange, whereas types C and D are labeled as red. Figures are provided by the authors.

Figure 7. The Insall-Salvati index.

Patella alta is characterized by the abnormal and elevated placement of the patella relative to the femur. It is diagnosed when the Insall-Salvati index exceeds 1. Figures are provided by the authors.

Figure 8. TTTG distance.

The TTTG distance is assessed by drawing a line originating from the posterior femoral condyle (marked with red line number 1) and perpendicular lines drawn from the deepest part of the trochlea (indicated by blue line number 2) and the most prominent part of the tibial tuberosity (denoted by yellow line number 3). A typical TTTG distance falls within the range of 9-15 millimeters. Figures are provided by the authors.

Figure 9. The Beighton score functions as a method for assessing generalized ligament hyperlaxity.

A Beighton score of 7 out of 9 or higher is categorized as red, 4-6 out of 9 is classified as orange, and 4 or fewer out of 9 is denoted as green. The above child has a Beighton score of 9 out of 9, categorizing her into the red category of the matrix score. Figures are provided by the authors.