Background: Traditional radiographic measurements of distal phalanx (Pd) displacement are not useful for diagnosis and monitoring of acute laminitis.
Objectives: To compare LLZ between healthy horses and those with acute or subacute laminitis. We hypothesized that the distance between the inner hoof wall and Pd (“lamellar lucent zone”; LLZ) measured on lateromedial digital radiographs would be increased in acute and subacute laminitis.
Study design: Retrospective and in vivo experimental study. Methods: Forelimb radiographs from 32 healthy and 18 laminitic mixed-breed horses were analyzed retrospectively. Laminitis was defined by at least 2 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°), remodeling, or distal displacement of the Pd. 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), sub-acute 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 created for select measurements. Results: The marginal mean [95% confidence interval] LLZ (mm) was increased in acute and subacute laminitis compared to control in the proximal (acute=8.8 [8.4-9.2]; subacute=9 [8.4-9.5]; control=7.3 [6.9-7.7]), middle (acute=8.9 [8.5-9.3] ; subacute=9.1 [8.6-9.6]; control=6.9 [6.5-7.2]), and distal (acute=9.2 [8.6-9.7]; subacute=10.2 [9.5-10.9]; control=7.5 [7-8]) dorsal lamellar regions (P<0.001). At a cut-off of >7.5 mm for middle LLZ, sensitivity was 87% and specificity 91% for diagnosis of acute laminitis. Using the ratio of middle LLZ to distal phalanx cortical length, sensitivity and specificity were 95% (cutoff >0.11).
Main Limitations: Lack of histological confirmation of acute laminitis. Conclusions and clinical importance: LLZ measurements are potentially useful for radiographic diagnosis of acute and subacute laminitis.