Acetabular cartilage thickness: accuracy of three-dimensional reconstructions from multidetector CT arthrograms in a cadaver study

Abstract

Purpose: To prospectively quantify the accuracy of hip cartilage thickness estimated from three-dimensional (3D) surfaces, generated by segmenting multidetector computed tomographic (CT) arthrograms by using direct physical measurements of cartilage thickness as the reference standard.

Materials and methods: Four fresh-frozen cadaver hip joints from two male donors, ages 43 and 46 years, were obtained; institutional review board approval for cadaver research was also obtained. Sixteen holes were drilled perpendicular to the cartilage of four cadaveric acetabula (two specimens). Hip capsules were surgically closed, injected with contrast material, and scanned by using multidetector CT. After scanning, 5.3-mmcores were harvested concentrically at each drill hole and cartilage thickness was measured with a microscope. Cartilage was reconstructed in 3D by using commercial software. Segmentations were repeated by two authors. Reconstructed cartilage thickness was determined by using a published algorithm. Bland-Altman plots and linear regression were used to assess accuracy. Repeatability was quantified by using the coefficient of variation, intraclass correlation coefficient (ICC), repeatability coefficient, and percentage variability.

Results: Cartilage was reconstructed to a bias of -0.13 mm and a repeatability coefficient of + or - 0.46 mm. Regression of the scatterplots indicated a tendency for multidetector CT to overestimate thickness. Intra- and interobserver repeatability were very good. For intraobserver correlation, the coefficient of variation was 14.80%, the ICC was 0.88, the repeatability coefficient was 0.55 mm, and the percentage variability was 11.77%. For interobserver correlation, the coefficient of variation was 13.47%, the ICC was 0.90, the repeatability coefficient was 0.52 mm, and the percentage variability was 11.63%.

Conclusion: Assuming that an accuracy of approximately + or - 0.5 mm is sufficient, reconstructions of cartilage geometry from multidetector CT arthrographic data could be used as a preoperative surgical planning tool.

Figure 1:

Photograph of disarticulated hip joint shows femoral head, capsule, labrum, and acetabulum. Four drill holes were created perpendicular to cartilage surface in a diamond pattern in four quadrants of acetabulum. Arrows = drill holes in posterosuperior quadrant (prior to harvest of cores).

Figure 2:

Digital microscopic image (magnification, 40×) of bisected acetabular osteochondral core shows interface of subchondral bone and cartilage (dashed line); 1.5-mm diameter hole drilled prior to multidetector CT arthrography served as a fiducial in image data. After imaging, 5.3-mm osteochondral cores were harvested around drill hole center. Physical measurements of core cartilage thickness were acquired at locations (black arrows) adjacent to drill hole. Optical measuring grid, with 0.10-mm markings, is visible. White arrows = dimensions of hole and core.

Figure 3a:

Representative right acetabulum. (a) Sagittal multidetector CT arthrogram shows 1.5-mm drill hole, cartilage, contrast agent, and subchondral bone (arrows). Note that drill hole was not filled with contrast agent, presumably owing to surface tension and high viscosity of the solution. (b) Lateral oblique 3D reconstruction of bone (dark gray) and cartilage (light gray) after semiautomatic segmentation.

Figure 3b:

Representative right acetabulum. (a) Sagittal multidetector CT arthrogram shows 1.5-mm drill hole, cartilage, contrast agent, and subchondral bone (arrows). Note that drill hole was not filled with contrast agent, presumably owing to surface tension and high viscosity of the solution. (b) Lateral oblique 3D reconstruction of bone (dark gray) and cartilage (light gray) after semiautomatic segmentation.

Figure 4:

Fringe plots of acetabular cartilage thickness for left and right hips of one specimen. Thicker cartilage dominated anterosuperior region; thinner cartilage was predominately confined to posterior lunate surface. * = approximate locations of physical measurements.

Figure 5:

Bland-Altman plot shows data for mean cartilage thickness plotted against difference. Data points for both trials of observer 1 (Ob. #1) are plotted. Best-fit lines for observers 1 (solid line) and 2 (Ob. #2, dashed line) were nearly identical. Dotted lines = 95% tolerance interval for observer 1. Equations for both best-fit lines indicated tendency for multidetector CT (MDCT) to respectively over- and underestimate cartilage thicker and thinner than approximately 1.25 mm.

Figure 6:

Scatterplot of physical measurements (Phy. Thick.) of cartilage thickness plotted against multidetector CT measurements (MDCT. Thick.). Data points for both trials of observer 1 (Ob. #1) are plotted. Linear regressions of observers 1 (solid line) and 2 (Ob. #2, dashed line) indicated tendency for multidetector CT to overestimate thickness measurements. Strong coefficients of determination (R ²) demonstrated that 3D reconstructions of multidetector CT images are well suited to measure acetabular cartilage thickness.