REGIONAL ANALYSIS OF LATERAL MENISCUS MECHANICS IN CADAVERIC KNEES USING A NOVEL ULTRA-HIGH FIELD MRI METHOD

The lateral meniscus is an important load distributor in the knee. Injuries or degradation of the meniscus can disrupt knee mechanics, which can lead to increased stress on the cartilage and eventual development of OA. Clinically, meniscal extrusion can be measured with MRI and is often used as a surrogate for disrupted meniscal mechanics. It is used to help diagnose meniscal pathology; however, these assessments are often done only in the coronal plane with the knee in a supine and unloaded posture. Furthermore, clinical 3T MRI scanners lack the image resolution to accurately measure strains in the meniscus and cartilage. Thus, the link between extrusion and loaded knee mechanics is unclear. As a result, surgeons are uncertain how important it is to restore extrusion when repairing the meniscus with procedures like centralization. Ultra-high field MRI provides sufficient resolution to measure both cartilage and meniscal mechanics during loading, without disruption or simulation of the articulating joint surfaces.

Determine where in the meniscus extrusion changes the most under load, and how this relates to meniscal strain. We hypothesize that meniscal extrusion will increase the most in the body region, and that the region with the greatest extrusion will have the largest strains.

Six human knee lateral compartments (mean age 70 yrs) with ACL and meniscus preserved were tested. Specimens were placed in a novel pneumatic compression apparatus customized for use in a 9.4T MRI scanner. Morphologic FLASH and RARE scans (axial, sagittal, and coronal resolution of 0.06 × 0.12 × 0.4 mm) were acquired before loading and after 2 hours of 48% body weight loading. The change in meniscal extrusion from unloaded to loaded states was calculated in the radial direction, at every degree around the meniscus circumference. For the strain analysis, the MR images were resampled to 0.2 mm isotropic resolution. The Advanced Normalization Tools module in 3D Slicer was used to register the resampled loaded image to the resampled unloaded image. Gradients of the resulting displacement field from the image registration were then used to calculate meniscal strains in the radial, circumferential, and axial directions. Average strains were determined for six different regions of the meniscus: internal and external anterior horn, internal and external body, and internal and external posterior horn. Repeated one-way ANOVA tests with Tukey's multiple comparisons were used to compare the strains in different regions, and to compare extrusions at different 15° increments around the circumference of the meniscus.

Loading produced the greatest extrusion changes in the anterior horn and the smallest extrusion changes in the meniscus body (Figure 1). Change in meniscal extrusion after loading was greater at 60° than at 15° (p = 0.011) or 30° (p = 0.0059). Radial strain was lowest in the internal posterior horn region (0.1%) and greatest in the external anterior horn region (4.6%) (Figure 2). Circumferential strain was lowest in the internal anterior horn region (1.1%) and greatest in the internal body region (1.8%). Compressive axial strain was lowest in the external body region (8.2%) and greatest in the internal anterior horn region (15%). Radial strain was greater in the internal posterior horn than in exterior body region (p = 0.045), and axial strain was greater in the internal anterior horn than the external anterior horn (p =0.012).

Contrary to our hypothesis, the anterior horn had the greatest increase in extrusion after loading. The greatest amount of radial and compressive axial strains were also in the anterior horn. Circumferential strains remained consistent across meniscal regions. Our findings suggest that current surgical approaches to fix coronal plane extrusion may be insufficient for restoring mechanics in the knee, given the limited change in extrusion and strain in the meniscal body relative to other meniscal regions.