Utility of double-contrast multi-detector CT scans to assess cartilage thickness after tibial plafond fracture

Abstract

The pathophysiology of post-traumatic osteoarthritis (PTOA) after intra-articular fractures is poorly understood. Pursuit of a better understanding of this disease is complicated by inability to accurately monitor its onset, progression and severity. Common radiographic methods used to assess PTOA do not provide sufficient image quality for precise cartilage measurements. Double-contrast MDCT is an alternative method that may be useful, since it produces high-quality images in normal ankles. The purpose of this study was to assess this technique's performance in assessing cartilage maintenance in ankles with an intra-articular fracture. Thirty-six tibial plafond fractures were followed over two years, with thirty-one MDCTs being obtained four months after injury, and twenty-two MDCTs after two years. Unfortunately, clinical results with this technique were unreliable due to pathology (presumed arthrofibrosis) and technical problems (pooling of contrast). The arthrofibrosis that developed in many patients inhibited proper joint access and contrast infiltration, although high-quality images were obtained in eleven patients. In this patient subset, in which focal regions of cartilage degeneration could be visualized, thickness could be measured with a high degree of fidelity. While thus useful in selected instances, double-contrast MDCT was too unreliable to be recommended to assess these particular types of injuries.

Figure 1

The subchondral bone and cartilage surfaces were traced slice-by-slice in a series of 2D coronal CT images. Bone and cartilage 3D surfaces were generated by stacking these 2D curves along the whole series.

Figure 2

Sagittal views of representative double-contrast MDCTs are shown. In the top row, cartilage thinning is apparent between (A) early and (B) later scans, particularly in the central region. The middle row illustrates a case that preserved its cartilage between time points (C:4months, and D:24months). Image (E) illustrates a joint with severe thinning and exposed bone near a step-off (arrow). The CT slice in (F) is an example of a failed study. Note the pooling of contrast in the anterior region (left side of image).

Figure 3

Thickness measurement repeatability was tested by reprocessing a CT study 6 months after the initial analysis. A Bland-Altman plot illustrates the results.

Figure 4

(a) This representative histogram of the observed cartilage thickness in an ankle joint demonstrates global loss of cartilage thickness from 4 to 24 months. Average cartilage thickness decreased in this particular case from 1.82mm to 0.97mm. This ankle was graded KL 3. (b) The histogram from a second case demonstrates the re-distribution in cartilage thickness from 4 to 24 months. Average cartilage loss in this case was only from 0.80mm to 0.74mm. Cartilage which was globally thin appeared to become more uniform over time. This case was also graded at KL=3.

Figure 5

(A) This finite element plot depicts the habitual contact stress exposure (MPa-s) computed over the joint surface. (B) This contour plot shows the variation in cartilage thickness over the joint surface. Beginning from a situation of assumed uniform thickness (1.7mm), the cartilage showed thinning areas of substantially elevated contact stress.