Effects of ACL interference screws on articular cartilage volume and thickness measurements with 1.5 T and 3 T MRI

Abstract

Objective: To assess the effects of interference screws, which are commonly used to surgically fix an anterior cruciate ligament (ACL) graft in the ACL-deficient knee, and magnetic field strength on cartilage volume and thickness measurements with quantitative magnetic resonance imaging (qMRI).

Methods: Five cadaver knees were imaged using a cartilage-sensitive sequence (T1-weighted water-excitation, three-dimensional (3D) fast low-angle shot) on 1.5T and 3T scanners with and without interference screws implanted. The tibiofemoral articular cartilage was segmented and reconstructed from the magnetic resonance images, and volume and thickness measurements were made on the resulting 3D models.

Results: Although several load-bearing regions showed significant differences in volume and thickness between magnet strengths, most showed no significant difference between screw conditions. The medial tibial cartilage showed a mean decrease in volume of 5.9% and 8.0% in the presence of interference screws at 3T and 1.5T, respectively. At 3T and 1.5T, the medial tibial cartilage showed a mean decrease in thickness of 7.0% and 12.0%, respectively, in the presence of interference screws.

Conclusions: Caution should be used when interpreting thickness and volume of cartilage at 3T in the presence of interference screws, particularly in the medial tibial compartment. Additionally, 3T and 1.5T qMRI should not be used interchangeably to assess structural changes in tibiofemoral articular cartilage during longitudinal studies.

Conflict of interest statement

Conflict of Interest None of the authors have any personal or financial relationships that could bias this work. However, the interference screws that were used for this study were donated by Arthrex, Inc.

Figures

Figure 1

Representative sagittal-plane images of a single cadaver specimen imaged using the T1-weighted WE-3D FLASH sequence (a) at 3T without interference screws, (b) at 3T with interference screws, (c) at 1.5T without interference screws, and (d) at 1.5T with interference screws. These slices capture the lateral femoral condyle, so only the femoral interference screw is visible. It can be seen that the distortion is relatively localized around the femoral screw, and did not affect the overall cartilage segmentations of the tibiofemoral joint. Image (b) shows an air artifact near the anterior portion of the meniscus. Segmentation was interpolated over air artifacts.

Figure 2

(A) The “notch” is marked by crosshairs on the lateral side of the TF joint, seen in the sagittal plane. (B) A cylinder was fit to the femoral condyles. (C) A line was drawn from the notch to the cylinder axis. The location of each ROI in the sagittal plane of the femur is shown.

Figure 3

The ROIs for the tibial compartments are shown. Axes with the same orientation as those from the medial compartment were centered about the centroid of the lateral compartment to establish the coordinate system in that ROI. The orthogonal was oriented out of the page.

Figure 4

The mean femoral cartilage thicknesses for both screw condition and magnetic field strengths (error bars represent 1 standard deviation). (A) On the medial femoral condyle, there were no significant differences in thickness between magnet strengths or screw conditions for any ROI. (B) On the lateral femoral condyle, the 70-100° ROI showed a strong trend for interaction between magnet strengths (p=0.053). No other ROI on the lateral femoral condyle showed any significant differences in thickness between magnet strengths or screw conditions.

Figure 5

Average medial and lateral tibial cartilage thicknesses for both screw conditions and magnetic field strengths (error bars represent 1 standard deviation). For the medial tibial ROI, there was no significant difference in thickness between magnet strengths (p=0.35), but there was a significant difference in thickness between screw conditions (p=0.03). For the lateral tibial ROI, however, there was a significant difference in thickness between magnet strengths (p=0.004), but not screw conditions (p=0.57). No significant interaction was found between screw condition and magnetic field strength for any tibial ROI (p>0.11).