Chronic inflammation of the articular capsule is recognized as one of the primary mechanisms of cartilage damage, change in tissue elasticity and cause of fibrosis [4]. Since elastography determines tissue elasticity by measuring the degree of strain that tissues experience in response to external stress [3], we aimed to prove if SE was a feasible technique for evaluating the DJC in sound horses (Group S) and horses with osteoarthrosis (Group P).
No significant difference was found between measures taken on the same structure, by two different operators or between repeated measures from the same operator (P > 0.05). But looking at IRR and ICC, sound horses had better reproducibility and repeatability compared to those of Group P, with higher values of both parameters; horses with OA had a higher variability of the elastographic pattern of the DJC, probably due to different stages of the pathologic process in these joints.
The qualitative appearance of the DJC was slightly softer in the longitudinal scans than in the transverse scans (4.95 vs 4.79 in Group S and 3.78 vs 3.68 in Group P). The longitudinal scans also showed more reliability, potentially due to the anisotropy of musculoskeletal tissues or the increased number of artefacts on the lateral and medial sides of the transverse ultrasonographic image [10–12]. To reduce this variability, the probe was held perpendicularly to the tissues and the lateral and medial aspects of the US images were avoided when selecting the ROI. Finally, as previously described in studies of tendon elastography in humans, it was determined that the EIs and SRs were lower for longitudinal scans than in transverse scans [11].
For the qualitative analysis a significant difference was detected between Group S and Group P when these were considered as a whole sample (P < 0.05), and when Group S was compared to the affected limbs and to the not affected limbs of Group P. We suppose that the difference between the Group S and the not affected limb of the Group P can be ascribed to subclinical degenerative changes in the contralateral “sound” joint of this group. However, the lack of further information (MRI, arthroscopy, histology) makes difficult to confirm this theory. Most of all a significative difference was detected between the affected and not affected limb of Group P (P < 0.05), suggesting different elastic features between affected and not affected capsule on the same horse.
Concerning the semi-quantitative analysis, EI and SR values were significantly lower in Group S than in Group P. This difference was significative in both scans when Group S was compared to the affected limb of Group P, but only in longitudinal view when Group S was compared to the not affected limb of Group P.
The DDET was chosen as the reference tissue because it was expected to experience the same degree of strain. The ROI was consistently placed at the level of the fetlock articular space in order to facilitate similar comparison of the target region with the same area of the DDET. However, the lack of elastographic data about the DDET could be responsible for the high standard deviation that was observed.
The choice of the ROI is critical for successful SE, especially when large structures are examined or when no clear lesions are visible on B-mode ultrasonography. In this study, only a small ROI was required. In cases where larger structures are examined, care should be taken when analysing results and a computerized analysis of the percentage of pixel distribution may be needed [13].
Even though mild sedation for research purposes was provided to all patients, this may not be necessary in clinical practice when horses are calm and standing still. Standoff pads were not used because they may modify the elastographic appearance of examined tissues and increase artefact occurrence [8].
We successfully evaluated the distal insertion of the fetlock joint capsule in mature horses of different breeds. The technique was feasible in both groups of horses, sound and suffering from fetlock OA, although higher variability of data was found in patients with radiographic and ultrasonographic signs of OA. We also demonstrated that sound horses had an elastogram suggestive of softer DJC compared to the other group.
Although the procedures were not overly technical, reproducibility of the techniques for scanning tissues, reading elastograms, and storing data should be evaluated in less skilled operators. Further studies should investigate the effects of age and breed on the appearance of this anatomical landmark.