CARTILAGE AND SUBCHONDRAL BONE THICKNESS DISTRIBUTION WITH MR-IMAGING

Rationale and Objectives. The authors mapped articular cartilage (AC) and subchondral bone (SB) thicknesses in human acetabula in vitro by u sing magnetic resonance (MR) imaging and validated AC measurements by using light microscopy. Materials and Methods. Left and right acetabul a from a deceased patient who had undergone left hemiarthroplasty were image with fat-suppressed spoiled gradient-recalled acquisition in th e steady state (repetition time = 55 msec, echo time = 15 msec, flip a ngle = 50 degrees, matrix = 256 x 256, field of view = 8 cm). AC and S B thickness maps were generated from image data by using analytic geom etry, which enabled correction for thickness overestimation due to obl ique sectioning. Cartilage bone plugs were extracted from the acetabul a, and light microscopy was used to validate the thickness measurement s obtained with MR imaging. Results. Standard errors between thickness measurements obtained with MR imaging and light microscopy were 0.37 and 0.33 mm for the left and right AC, respectively, which is consiste nt with the voxel resolution of the MR imaging sequence (0.31 x 0.31 x 0.8 mm). SB thickness of the cartilage plugs could not be reliably me asured with light microscopy and, therefore, could not be validated. C ontour maps showed that SB thickness gradients were rapid and focal co mpared with the rather smooth gradients in AC thickness; however, thic ker AC was accompanied by thicker SB for left (r(2) = .261, P = 0001) and right (r(2) = .308, P = .0001) acetabula. Average thickness differ ences between left and right acetabular AC and SB were 0.13 mm (P = .0 15) and 0.11 mm (P = .026), respectively. Although it was the operated hip that had thicker articular tissues, the differences ware within t he pixel resolution (<0.31 mm). Conclusion. AC and SB thickness distri bution can be accurately determined by combining noninvasive MR imagin g and analytic geometry, which may also provide a means for quantitati ve, longitudinal assessment of focal AC defects.