Radiography of the feet is commonly indicated for lameness diagnosis or follow-up, balance assessment in conjunction with a farrier, or for pre-purchase examinations. The purpose of this critically appraised topic was to identify recent scientific advances in foot radiography in order to direct the practice of evidence-based radiography.
The search terms (horse* OR equine*) AND (foot OR feet OR digit* OR hoof OR hooves OR phalan* OR navicular) AND (radiograph* OR radiolog*) were generated and input into the PubMed search engine. Following exclusion of studies more than 5 years old and those determined not to relate directly to the question, six useful results were yielded. Three studies pertained to the acquisition of views,1-3 and three to patient preparation or positioning.4-6
A prospective study assessed the diagnostic accuracy of one ‘standard’ palmaro55-65°proximal-palmarodistal oblique (‘skyline’) view versus the combination of one ‘standard’ and one ‘shallow’ view (35-45°) of the navicular bone for palmar compact bone (flexor cortex) erosions.1 Thirty-one horses, of which 30 were Quarter horses, were included, totalling 36 limbs, of which 18 were cases and 18 were controls. The gold-standard by which horses were grouped as cases (with erosive lesions) versus controls (without erosive lesions) was a radiographic consensus agreement by six radiologists, of which five were observers in the study. Other observer groups were imaging residents, experienced equine vets, and 4th year veterinary students (all five observers). Observers were asked to determine whether erosive lesions were present or not, and to grade severity on a five-point scale, based on one versus two radiographic projections, presented in a random order. The sensitivity for erosions significantly increased with two views versus one view in all observer groups, and accuracy increased for all observers combined. However, this study has several major scientific flaws. The gold standard for diagnosis of erosions was not appropriate; the radiologists constituting the test observers contributed to the determination of case versus control status, based on the test radiographs. Histology or MRI would have been more appropriate. Further, because the use of a shallow view was not assessed independently, conclusions for the usefulness of two as opposed to one view cannot be made. The angle for the ‘standard’ view (55-65°) was steeper than that widely considered as standard in the UK (45°). However, the study population was mostly composed of Quarter horses, which tend to have more upright feet.
With advances in digital radiography and post-processing technology, one group considered whether there was still a need to acquire a separate dorsoproximal-palmarodistal oblique view (‘upright navicular’ view) as well as an ‘upright pedal’ view.2 Upright navicular and upright pedal (manipulated for the navicular bone) radiographic images of 30 front feet of 15 horses were acquired and subjectively assessed for image quality by four observers. There was no significant difference in combined quality scores between the two views, and agreement between the observers was good. Although the measured image quality for the upright navicular view was no better than the manipulated upright pedal view, the ability to detect pathology was not assessed. Further research would definitely be required before recommendations for the use of one versus two views could be made, and the authors acknowledged this. The benefit of acquiring an upright navicular view may be in the use of a different radiographic angle.
As part of a retrospective study which aimed to comprehensively characterise the radiographic appearance of ossified ungular cartilages (‘side bone’), the usefulness of a flexed oblique view for the detection of fractures of ossified ungular cartilages was assessed.3 Of 36 horses with fractures, 100% were identified in the respective flexed oblique view, compared with 50.0% in the dorsopalmar, 45.2% in the ‘upright pedal’, and 38.1% in the lateromedial. Approximately one quarter of fractures were only seen in the respective flexed oblique view, and so otherwise would have been missed.
Patient preparation or positioning
Two studies evaluated the effect of horse stance on measurements of the hoof capsule or phalanges.4-5 In the first, lateromedial radiographs of the front feet of five horses were acquired with the metacarpal region positioned vertically, and at various degrees angled cranially and caudally, as well as with the forelimbs only positioned on raised blocks.4 The angle of the solear border of the distal phalanx was not significantly altered by stance. The hoof-pastern axis (HPA) and angle of the distal interphalangeal (DIP) joint were both significantly linearly correlated with stance. The effect of mediolateral limb position (neutral versus base-wide or base-narrow stance) on measurements in the dorsopalmar view was evaluated in only one horse. In that one horse, the width of the medial aspect of the DIP joint was narrowed when the horse was positioned base-wide, and stance had no effect on mediolateral balance of the distal phalanx. Limitations of this study include the small population, which only included Standardbreds, and the use of only one horse for the investigation into the effect of stance on measurements in the dorsopalmar view. As a result, the data provides extremely weak evidence that cannot reliably be applied to the wider population.
More recently, one group investigated whether a unipodal (i.e. raising the contralateral limb as a mode of restraint) versus bipodal stance affected measurements of mediolateral or sagittal balance of the distal phalanx.5 Fourteen feet of seven horses were radiographed (lateromedial and dorsopalmar views) with the horse weightbearing through both forelimbs and with the contralateral limb raised, and one observer made the predefined measurements. Compared with a bipodal stance, a unipodal stance significantly increased the degree of flexion of the DIP joint and the angle of the deep digital flexor tendon (DDFT) at the level of the navicular bone and decreased the width of the lateral aspect of the DIP joint (thereby significantly altering the mediolateral balance of the DIP joint).
In the final paper we reviewed, the benefit of packing the sulcus of the frog for upright views of the feet was assessed.6 Image quality in dorsoproximal-palmarodistal oblique views of 24 cadaver limbs with and without packing was subjectively assessed by five observers. There was an overall preference for non-packed images, and artefacts were more frequently observed in packed than non-packed images. However, the quality of packing was graded as poor in approximately 25% of radiographs and good in only 42%. Image quality scores were inversely correlated with packing quality, which appeared to be excessive (over-packing) in the figures provided. This was a major limitation. Additionally, the study did not investigate the diagnostic accuracy of packed versus non-packed images in limbs with pathology.
In brief summary, the routine use of two skyline views of the navicular bone at two different angles is currently not justified. However, the choice of angle is likely to be important. Compared with steeper angles, shallower views would skyline the distal aspect of the navicular bone better, which is the predilection site for erosions. Digital upright pedal images can be post-processed to optimise visualisation of the navicular bone however, a separate upright navicular view should still be acquired routinely. Flexed oblique views are essential in order to fully evaluate ossified ungular cartilages and may be the only view in which a radiolucent line consistent with a fracture can be seen.
The positioning of the forelimbs of the horse (whether vertical and/or raised on blocks relative to the hindlimbs) may affect the HPA and should be considered when assessing or reassessing the HPA. Lifting the contralateral forelimb as restraint can also affect the HPA as well as the angle of the DDFT and the mediolateral balance of the DIP joint. There is no strong evidence to support the cessation of routine packing of the sulci of the frogs for foot radiography, however the quality of packing appears to be important, and overpacking may create artefacts.
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