Evaluation of digital radiographic measurements for the diagnosis of acute laminitis.

Background: Traditional radiographic measurements of distal phalanx (Pd) displacement based on the outer hoof wall are not useful for diagnosis of acute laminitis.

Objectives: We hypothesised that the distance between the inner hoof wall and Pd ('lamellar lucent zone'; LLZ) measured on lateromedial digital radiographs would be increased in horses with acute and subacute laminitis compared with healthy horses.

Study design: Retrospective cohort study and in vivo experimental study.

Methods: Forelimb radiographs from 32 healthy and 18 laminitic mixed-breed horses were analysed retrospectively. Laminitis was defined by at least two clinical signs (acute multi-limb lameness, increased digital pulse amplitude and/or persistently warm hooves) for ≤3 days (acute), or >3 and ≤14 days (subacute) duration, without overt palmar rotation (≥3°), remodelling, or distal displacement of the Pd. Some laminitic limbs initially classified as acute were subsequently radiographed later in hospitalisation and also included as subacute. Twelve measurements including LLZ at proximal, middle, and distal locations were performed on 126 forelimb radiographs that met the criteria for acute laminitis (n = 39), subacute laminitis (n = 23) or healthy control (n = 64). A mixed effects linear regression model was used for comparisons and a receiver-operator characteristic (ROC) curve was created for select measurements. Three horses were serially radiographed to study the impact of magnification and obliquity.

Results: The mean [95% confidence interval] LLZ (mm) was increased in acute and subacute laminitis compared with control in the proximal (acute = 8.8 [8.4-9.2]; subacute = 9 [8-9.9]; control = 7.3 [7-7.7]), middle (acute = 8.9 [8.5-9.3]; subacute = 9.1 [8.2-10]; control = 6.9 [6.5-7.2]), and distal (acute = 9.1 [8.5-9.7]; subacute = 10.2 [9.1-11.3]; control = 7.5 [7.2-7.8]) dorsal lamellar regions (p < 0.001). At a cut-off of >7.5 mm for middle LLZ, sensitivity was 87% [73%-94%] and specificity 91% [81%-96%] for diagnosis of acute laminitis. Using the ratio of middle LLZ to distal phalanx cortical length, sensitivity was 95% [81%-99%] and specificity was 95% [87%-99%] (cut-off >0.11). Magnification and obliquity affected absolute LLZ measurements but did not alter LLZ ratios.

Main limitations: No histological confirmation of laminitis.

Conclusions: LLZ measurements are potentially useful for radiographic diagnosis of acute and subacute laminitis.