What Is the “Perfect” Lateral Radiograph? Effects of Beam Directionality and Condylar Alignment on the Perceived Location of the Medial Patellofemoral Ligament

Abstract

Background:

The identification of the medial patellofemoral ligament (MPFL) insertion site relies heavily on the use of radiographic landmarks such as the Schöttle point. However, little is known on how the variation in condylar alignment and beam directionality affects this perceived location.

Purpose:

To quantify changes in the perceived MPFL insertion site when posterior proximal or posterior distal condylar alignment is optimized and when fluoroscopic beam directionality alters.

Study Design:

Controlled laboratory study.

Methods:

A total of 16 cadaveric knees were included. The femoral insertion of the MPFL was identified and fixed with a radiopaque metal marker after dissection. Fluoroscopic images in the medial-to-lateral (ML) direction were taken with the posterior proximal condyles aligned and then with the posterior distal condyles aligned. Beam directionality was flipped to the lateral-to-medial (LM) direction, and images with posterior proximal and posterior distal condylar alignment were taken. Images were then superimposed onto the reference image, with discrepancies between marker locations quantified in the x- and y-axis. The 2-tailed Student t test was performed on all obtained variations.

Results:

In the ML direction, proximal versus distal alignment resulted in a mean proximal displacement of 0.43 ± 1.86 mm (P = .37) and a mean posterior displacement of −0.11 ± 0.55 mm (P = .42). In the LM direction, proximal versus distal alignment resulted in a mean proximal displacement of 1.95 ± 2.37 mm (P = .005) and a mean anterior displacement of 0.22 ± 1.95 mm (P = .67). When comparing ML to LM beam directionality with optimized proximal condylar alignment, there was a mean distal displacement of −0.26 ± 1.73 mm (P = .56) and a mean anterior displacement of 2.22 ± 1.87 mm (P < .001). When comparing ML to LM beam directionality with optimized distal condylar alignment, there was a mean proximal displacement of 1.26 ± 2.53 mm (P = .07) and a mean anterior displacement of 2.55 ± 1.71 mm (P < .001).

Conclusion:

Beam directionality and condylar alignment affected the perceived location of the MPFL insertion. However, shooting in the ML direction and aligning the posterior proximal femoral condyles resulted in the least amount of total variation. Future studies are needed to determine the amount of variation in perceived MPFL insertion sites that are clinically significant.

Clinical Relevance:

This information provides surgeons with the insight that beam directionality and condylar alignment significantly affect the perceived location of the MPFL insertion site and that surgeons can use fluoroscopic imaging in the ML direction with posterior proximal condylar alignment to minimize image variation.