Aim Post-traumatic osteoarthritis and osteochondral injuries can cause significant pain and morbidity. Appropriate MRI sequences combined with image analysis techniques can be used to reproducibly measure quantitative cartilage parameters, hence offering a tool for monitoring and detecting degenerative change earlier than previously possible. We demonstrate the performance of a directional gradient vector flow (dGVF) snake segmentation algorithm on an isotropic MR sequence, which allows segmentation of the full articular surfaces (including malleoli) of the ankle.

Method Eight ankles were imaged using a 1.5T MRI scanner with an isotropic 3D T1 weighted FLASH sequence with water excitation, resolution 0.3 x 0.3 x 0.3 mm. A subset of five ankles were imaged four times with repositioning and re-shimming of the magnet between acquisitions. Images were interpolated to 0.15 mm3 and segmented using a dGVF snake. Following 3D reconstruction of the cartilage layers normal thickness from cartilage to bone was measured at each voxel on the cartilage surface.

Results The mean cartilage thickness (±S.D) was 1.80 mm (±0.05 mm); 1.83 mm (±0.07 mm) and 1.81 mm (±0.07 mm) for the talus, tibia and cumulative ankle cartilage respectively. To measure the technical precision of the segmentation method we determined the coefficient of variation of the four repeated measurements in five ankles. The mean coefficients of variation (min-max) from the repeated measurements were 1.74% (0.69%–3.57%); 1.20% (0.26%–3.06%) and 1.52% (0.26%–3.57%) for the talus, tibia and cumulative ankle cartilage respectively.

Conclusion We believe that the reported isotropic image sequence and segmentation algorithm is a valid tool for quantitative assessment of the entire ankle joint. A possible application is the early detection of cartilage injury and degenerative change due to injury or illness.

Aim To characterise the mechanical properties of the ankle, it is essential to have accurate joint morphology and measurements of the cartilage thickness and its variation across the joint. Thickness and volume measurements are also useful tools for detecting and monitoring degenerative change, however baseline measurements are required, to act as a ‘gold standard’. We present details of ankle cartilage thickness and distribution over the entire ankle joint, using a high precision stereophotogrammetry system.

Method Twelve cadaveric ankles surfaces with photo targets, rigidly attached, were imaged using a stereo-photographic system, which generates a dense 3D point cloud of co-ordinates on the surface (typically 70,000 points per surface, accuracy ±2 μm). After imaging the surface, the cartilage was dissolved using 5% sodium hypochlorite to reveal the subchondral bone and the process was repeated. The two surfaces were combined and the normal distance from cartilage surface to bone was calculated at every point on the cartilage surface.

Results The mean cumulative cartilage thickness of the ankle joint was 1.18±0.23 mm, the mean maximum cumulative cartilage thickness of the entire ankle joint was 2.17±0.46 mm. When considering the cartilage layers of the talus and the tibia-fibula complex separately, the mean and mean maximum thickness for the talus was 1.17±0.18 mm and 2.12±0.54 mm respectively. For the tibia-fibula complex, the mean and mean maximum thickness was 1.18±0.28 mm and 2.3±0.57 mm respectively. 3D cartilage thickness maps were also produced

Conclusion The cartilage maps show that the thickest cartilage occurs at the shoulders of the talus, as opposed to the talar dome, as reported in earlier studies, which were unable to assess the highly curved regions of the ankle. This method also provides a gold standard for validating MRI cartilage measurements.