Date Published: January 28, 2019
Publisher: Public Library of Science
Author(s): Laurence Evrard, Fabrice Audigié, Lélia Bertoni, Sandrine Jacquet, Jean-Marie Denoix, Valeria Busoni, Ann Carstens.
This descriptive study aimed to compare the magnetic resonance appearance of the distal interphalangeal joint articular cartilage between standing weight-bearing and non-weight-bearing conditions. Ten forefeet of live horses were scanned in a standing low-field magnetic resonance system (0.27 T). After euthanasia for reasons unrelated to the study, the non-weight-bearing isolated feet were scanned in a vertical positioning reproducing limb orientation in live horses. The same acquisition settings as during the weight-bearing examination were used. Thickness and cross-sectional area of the distal interphalangeal articular cartilage and joint space were measured on tridimensional T1-weighted gradient echo high resolution frontal and sagittal images at predetermined landmarks in both conditions and were compared using a linear mixed-effects model. Frontal images were randomized and submitted to 9 blinded readers with 3 different experience levels for identification of weight-bearing versus non-weight-bearing acquisitions based on cartilage appearance. Weight-bearing limbs had significantly thinner distal interphalangeal cartilage (p = 0.0001) than non-weight-bearing limbs. This change was greater in the distal phalanx cartilage than that of the middle phalanx. Blinded readers correctly identified 83% (range 65 to 95%) of the images as weight-bearing or non-weight-bearing acquisitions, with significantly different results observed among the different readers (p < 0.001) and groups (p < 0.001). These results indicate that distal interphalangeal articular cartilage and particularly cartilage of the distal phalanx thins when weight-bearing compared to the non-weight-bearing standing postmortem conditions and suggest that cartilage abnormalities may be more difficult to identify on weight-bearing standing magnetic resonance imaging.
With the emergence of a low-field standing system, magnetic resonance (MR) imaging is increasingly used in the diagnosis of foot pain in horses. [1–4] Recent equine literature has addressed the possibility of evaluating articular cartilage using MR imaging, [5–11] and many MR studies have assessed articular cartilage on equine cadaver limbs. [5–8, 10–16] In recent publications on equine MR imaging, the articular cartilage thickness has been assessed on distal interphalangeal,  metacarpophalangeal [6, 10, 11, 17] and carpal joints.  Some studies demonstrated that MR imaging is reliable for cartilage thickness measurement when compared to histologic measurements of the carpal and metacarpophalangeal articular cartilage on isolated limbs when using high field magnets. [11, 12] However, other studies have failed to consistently correlate high field MR imaging measurement with gross cartilage thickness, particularly in the metacarpophalangeal joint.  Furthermore, some concerns have been raised about limitations associated with spatial resolution of the modality for assessing thin articular cartilage.  Moreover, whereas the limb is imaged in a non-weight-bearing position during high field MR imaging, it is weight-bearing during low field acquisition in a standing MR imaging unit, except in the few cases where the magnet is used in patients under general anaesthesia. The effect of loading on in vivo or ex vivo cartilage thickness has been well demonstrated in human orthopedics, e.g. in the knee [19–24], hip , and ankle,  and articular cartilage deformation is routinely assessed in human patients in the diagnosis of early degenerative joint disease. [20, 27–29] On the contrary, no report discusses the potential difference in cartilage appearance between weight-bearing and non-weight-bearing conditions in the equine patient. This study aimed to compare the appearance of the distal interphalangeal joint (DIPJ) articular cartilage between standing weight-bearing and non-weight-bearing conditions using a low-field MR system. We hypothesized that articular cartilage would have a different appearance in terms of thickness, delineation, homogeneity and signal intensity between standing weight-bearing and standing non-weight-bearing postmortem acquisitions. More particularly, we hypothesized that cartilage thickness would be less in images acquired in a weight-bearing position compared with non-weight-bearing limbs.
Difference in cartilage thickness due to load has been demonstrated in human studies [19–21, 25, 26]. It explains the findings of this study where a significantly thinner articular cartilage was observed in the weight-bearing limb compared to the same non-weight-bearing limb. A difference in articular cartilage thickness has been observed in the equine carpus, between antebrachiocarpal and middle carpal joints, [32, 33] but no report discusses in detail different thickness of two opposing articular cartilage surfaces in horses. Differences in biochemical composition and thickness may lead to different cartilage behaviors under load. [33–37] In ex-vivo studies of the human femorotibial joint, tibial cartilage deforms and strains markedly more in both compression and shear, than femoral cartilage as a result of femoral cartilage being stiffer.  This greater deformation in human tibial cartilage as compared to that of the femoral condyle is also observed with MR imaging following impact loading.  The difference between the cartilage thicknesses of the middle and distal phalanges may have similar causes and be related to strain direction and magnitudes varying with joint location and tissue depth, and to difference in biomechanical properties of the cartilage. [40, 41]
The results of the present study demonstrate that the articular cartilage of the DIPJ thins when loaded compared to post mortem isolated decreased weight-bearing distal limbs and differences exist between cartilage deformation in the middle and distal phalanges. Some cartilage abnormalities easily visualized on non-weight-bearing limbs may therefore be more difficult to identify on standing foot MR imaging when asymmetrical joint space thinning due to positioning is a common feature and DIPJ cartilage is compressed and sheared. Caution should therefore be recommended in interpreting articular cartilage on weight-bearing limbs and in using cartilage thickness values obtained on non-weight-bearing limbs as a reference for standing MR imaging on clinical cases.