Influence of tibial slope asymmetry on femoral rotation in patients with lateral patellar instability

Abstract

Purpose: The geometry of the tibial plateau and its influence on the biomechanics of the tibiofemoral joint has gained increased significance. However, no quantitative data are available regarding the inclination of the medial and lateral tibial slope in patients with patellar instability. It was therefore the purpose of this study to evaluate tibial slope characteristics in patients with patellar dislocations and to assess the biomechanical effect of medial-to-lateral tibial slope asymmetry on lateral patellar instability.

Methods: Medial and lateral tibial slope was measured on knee magnetic resonance images in 107 patients and in 83 controls. The medial-to-lateral tibial slope asymmetry was assessed as the intra-individual difference between the medial and lateral tibial plateau inclination considering severity of trochlear dysplasia. The effect of tibial slope asymmetry on femoral rotation was calculated by means of radian measure.

Results: Severity of trochlear dysplasia was significantly associated with an asymmetric inclination of the tibial plateau. Whereas the medial tibial slope showed identical values between controls and study patients (n.s.), lateral tibial plateau inclination becomes flatter with increasing severity of trochlear dysplasia (p < 0.01). Consequently, the intra-individual tibial slope asymmetry increased steadily (p < 0.01) and increased internal femoral rotation in 20° and 90° of knee flexion angles in patients with severe trochlear dysplasia (p < 0.01). In addition, the extreme values of internal femoral rotation were more pronounced in patients with patellar instability, whereas the extreme values of external femoral rotation were more pronounced in control subjects (p = 0.024).

Conclusion: Data of this study indicate an association between tibial plateau configuration and internal femoral rotation in patients with lateral patellar instability and underlying trochlear dysplasia. Thereby, medial-to-lateral tibial slope asymmetry increased internal femoral rotation during knee flexion and therefore might aggravate the effect of femoral antetorsion in patients with patellar instability.

Level of evidence: III.

Fig. 1

First transverse MR image in craniocaudal direction that shows the entire tibial head. In this transverse image, the corresponding sagittal slices located most closely to the tibial head centre (solid line) and in the centre of the medial and lateral tibial plateau (dashed lines) were determined. Similarly, the corresponding coronal section located most closely to the tibial head centre (dotted line) was established

Fig. 2

The sagittal plane that represents the corresponding image to the solid line in Fig. 1 is shown in (a). This image was used to identify the longitudinal axis of the tibial diaphysis. The longitudinal axis was defined as the midpoint of the anterior–posterior width of the tibia at two points located 4–5 cm distally to the joint line and as distally as possible. To measure the medial and lateral tibial slope, the longitudinal axis was assigned to the corresponding planes in the centre of the lateral tibial plateau (b) and in the centre of the medial tibia plateau (c) as shown as dashed lines in Fig. 1. The conjugation line between the peak anterior and posterior points of the tibia plateau measured the inclination of the tibial slope perpendicular to the longitudinal axis of the tibia

Fig. 3

The longitudinal axis of the femur and the tibia was established in the frontal plane referencing most closely to the centre of the tibial head in the corresponding transverse image (dotted line in Fig. 1). The longitudinal axis was defined as the midpoint of the medial-to-lateral width of the tibia as distally as possible and at the midpoint of the tibial head. A similar approach was used to establish the diaphyseal axis of the femur in the coronal plane. The aDLFA and mPMTA was then measured as the angle between the longitudinal axis of the femur and tibia and the joint line represented by the most distally located points of the femoral condyles, and the peak points of the medial and lateral tibial plateau. aDLFA anatomical distal lateral femur angle, mPMTA mechanical proximal medial tibial angle

Fig. 4

Illustrated is the effect of tibial slope asymmetry on femoral rotation by means of a difference in height between the medial and lateral tibial plateau in 20° and 90° of knee flexion. In the frontal view, the maximal observed effect on internal and on external femoral rotation is shown in comparison with the neutral position of the femoral condyles in both knee flexion angles

Fig. 5

Shown are the mean values and SD of the medial and the lateral tibial slope in controls and in patients considering different grades of trochlear dysplasia

Fig. 6

Shown are the mean values and SD of internal femoral rotation in controls and in patients considering different grades of trochlear dysplasia with the knee positioned in 20° and 90° of knee flexion