Summary

Variability in the axial distribution of Vastus Lateralis may influence Patellofemoral Instability in patients with Femoral Torsion

Abstract

Background

Patellofemoral joint (PFJ) instability is a common problem which occurs when the bony and soft tissue restraints that hold the patella enlocated, are overcome by the lateral force vector of the quadricep. The pathology is multifactorial and involves limb alignment, and intrinsic morphology of the knee and soft tissues. The force vector exerted by the quadriceps has been previously described by the Q angle in the coronal plane, and the Quadriceps Torsion Angle (QTA) in the axial plane, which are simplified expressions of a complex anatomical structure. This study aims to assess the 3-dimensional relationship between the femur, Vastus Medialis and Lateralis (VM and VL), by creating a subject-specific femoral coordinate system to determine the position of the quadriceps relative to the femur and to evaluate muscle distributions in control and dislocator subjects.

Methods

Seventeen knees were assessed in seventeen participants with a mean age of 14.7yrs. They were grouped into: controls with no PFJ dislocation (n=5), those with femoral torsion <15° & QTA <50° (n=6), and those with femoral torsion >25° (n=6) and QTA >50. MRI imaging was conducted for each participant, followed by 3D reconstruction of the vastii using Mimics 26.0 (Materialise, Leuven) to create subject-specific models.
A subject-specific, cartesian reference system was defined using the posterior femoral condylar axis (PCA), the mechanical axis (MA) and the perpendicular bisector of the PCA at the level of the MA intersection as the “x”, “z” and “y” axes, respectively. The muscles for each participant were divided into 50 slices along the MA and the centroid lines of the vastii were defined as the centre of the muscle area for each slice. The positional relationship of the vastii to the femur and their distribution in the axial plane was compared between the groups using a quadrant based reference system in the x-y plane.

Results

In the coronal plane, a statistically significant difference (p=0.022) was observed in the position of VM relative to the femur between controls (8.46 ±0.53°) and dislocators with femoral torsion >25° (7.19 ±0.90°). Regarding VL distribution in the axial plane, there was a statically significant difference (p=0.042) between the dislocator groups in the anterior-lateral quadrant; those without torsion had 63.67 ±13.18%, while those with torsion had 44.23 ±15.72%. No significant differences were found in vastus medialis bulk distribution between the groups.

Conclusions

These preliminary results help in understanding the quadriceps anatomy, and its variability in PFJ instability, potentially identifying patients needing more aggressive surgical management. Results demonstrate that FT <15° and QTA <50° may impact axial distribution of VL. They also support the use of QTA as a basic indicator of muscle position which may in turn help determine if derotational osteotomies should be performed proximally or distally. These results provide a foundation for further investigation into the complex relationship between bony and soft tissue pathoanatomy with implications for surgical planning in Patellofemoral Instability.