Newer modes of characterising equine tendons: ultrasound elastography and ultrasound tissue characterisation
Ultrasound Elastography (UE) and Ultrasound Tissue Characterisation (UTC) are both relatively new modes of ultrasonographic assessment which have recently begun to attract both scientific and clinical interest as potential ways to improve tendon characterisation. In this journal club we reviewed the available equine-specific literature in order to determine the current potential clinical use of these modalities.
Ultrasound Elastography has been used in human medical and oncological fields for around 30 years and the technology has more recently been applied to musculoskeletal injuries. The basic principle of UE is that repeated/rhythmical probe pressure is applied by the user which results in deformation of the structure being evaluated. The stiffness of the structure is estimated by the application of Young’s modulus, allowing tissue within the region of interest to be categorised in terms of softness versus hardness. Two studies by the same research group were discussed.
The first aimed to use UE to characterise the metacarpal tendons in 19 limbs of 17 control horses, and to determine inter- and intra-observer repeatability.1 The inclusion and exclusion criteria for recruitment of ‘clinically normal’ control horses was not rigorous. Tendons showed predominantly blue and cyan pixels, indicating ‘hard tissue’ (e.g. mean 70.9-80.2% blue, 13.3-21.1% cyan for the superficial digital flexor tendon [SDFT]), a low proportion of green pixels (intermediate hardness; e.g. mean 5.8-7.6% for SDFT) and minimal yellow and red pixels (soft tissue; mean 0.1-0.4% yellow, mean 0.1-0.3% red). The confidence intervals were large, which may be explained by the poor selection of ‘normal’ horses, and/or individual variability as a result of other factors (e.g. breed, age, sex etc). The authors report no significant difference within and between operators and concluded that the inter- and intra-observer repeatabilities were good, however a kappa of 0.46 for the interobserver repeatability indicated only moderate agreement. In addition, the two operators did not acquire measures completely independently, therefore there was a potential for bias. Although the authors reported that limb position had no significant effect on measures this was only assessed in three horses which is too few for statistical analysis and meaningful conclusions cannot be drawn.
The second study used UE to characterise acute (<2 weeks duration) and chronic (>2 weeks duration) naturally occurring lesions in 57 horses using the same technique as in the first study.2 Definitive diagnosis was made by greyscale ultrasound or magnetic resonance imaging (MRI). Multiple structures and lesion types of the distal limb were included rather than having strict inclusion criteria. Acute lesions were significantly ‘softer’ and showed more green, yellow and red pixels (p<0.0001 for all). Subjectively there was good correlation of UE grades with both greyscale ultrasound and MRI grades. We noted that there was overlap in the reported values for the proportion of blue and red pixels associated with chronic lesions with those reported in normal horses in the first study. A small number of horses were re-evaluated at various time intervals, however the data presented is of limited value.
Developed in order to quantify tendon integrity3, UTC involves the motorised translation of an ultrasound transducer over the length of a tendon, during which transverse images are acquired every 0.2mm. Algorithms applied to the data volume blocks categorise echoes based upon their stability i.e. the fibre alignment as type 1-4, with 1 being parallel fibres and 4 being amorphous.
Docking et al. 4 used UTC to assess SDFT structure changes in 13 Thoroughbred racehorses after maximal exercise (a race) compared with five matched control horses. There were significant differences (P<0.05) in the proportion of echo types 1-3 at day one and two following the race. Echo type returned to baseline on day three. In agreement with studies using UTC in footballers, UTC may have the ability to detect short term changes in tendon structure following exercise, however we currently do not know the implications of these changes. More research is warranted in order to determine whether UTC can ultimately differentiate post-exercise responses predictive of injury from normal responses.
In a prospective longitudinal cohort study of the SDFTs of 32 juvenile racehorses at on facility, UTC was performed at 3 time points (q.60-90 days).5 Region of interest analysis at five predetermined specific sites showed predominantly type 1 and 2 echoes (>85%) and minimal type 3 and 4 echoes (<15%). The echo types showed a reciprocal relationship over the sampling points. However, the clinical meaningfulness of this data is currently unknown because of the small number of horses, the short time period and the lack of horse outcomes as a result of this only being a pilot investigation. Results of a larger cohort followed over a longer time period could be valuable.
For UE, the two existing studies do not act to support the use of UE in clinical practice. The added diagnostic value of UE over greyscale ultrasound or MRI is unclear, as is its role in monitoring lesions. Better quality research would be needed to support its use in clinical equine practice. The use of shear-wave elastography is progressing in human medical fields and provides a more objective and repeatable results in comparison with compression elastography. To our knowledge the application of this technique in equine musculoskeletal imaging has not been reported. Ultrasound tissue characterisation appears to have the potential to offer diagnostic advantages over greyscale ultrasound, with the potential to quantify subtle changes. There may be value in its use in elite horses in order to detect early sub-clinical damage and predict injuries, and to help direct training regimens. However, this is entirely speculative and a lot more research using this relatively new tool is needed.
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