Radiographic assessment of the ratio of the hoof wall distal phalanx distance to palmar length of the distal phalanx in 415 front feet of 279 horses

Determination of the ratio of the hoof distal phalanx distance (HDPD) to the length of the palmar aspect of the distal phalanx (HDPD ratio) may be helpful for the diagnosis of laminitis. Estimates for normal values have varied (<= 0.25 to <= 0.30) and been based on small numbers of horses. No study has compared external characteristics of the hoof wall and HDPD ratio. The objective was to document the HDPD ratio in a large number of horses/ponies of various breeds; and to assess relationships between signalment and hoof level factors and the HDPD ratio and height:bodyweight ratio. This retrospective study included 415 feet from 279 horses with foot pain and no known history of laminitis, or clinical signs of acute laminitis. Lateromedial radiographs were assessed; the HDPD and palmar length of the distal phalanx were measured digitally and the ratio was calculated. The presence of divergent growth rings was determined from lateromedial photographs. Factors associated with HDPD ratio were assessed using linear mixed effects models, built using a step-wise backward elimination procedure with horse included as a random effect, and the final model was selected based on Akaike information criterion. The mean HDPD ratio was 0.25 +/- 0.03 (median = 0.25; IQR 0.23-0.26; range 0.19-0.36). 42.2% of feet had a HDPD ratio >0.25. There was no significant difference in median HDPD ratio for feet with divergent growth rings compared with those without (P = 0.16). In the final mixed effects model, HDPD ratio decreased with increasing age (coefficient -0.0014; P = 0.008) and with increasing height:bodyweight ratio (coefficient -0.13; P = 0.02) and was greater in Cob breeds (coefficient 0.02; P = 0.001) compared with Warmbloods. However, this model only explained approximately 8.4% of HDPD ratio variability. A limitation of the study was the absence of a control nonlame group. The conclusion of the study was that variation in the HDPD ratio was not adequately explained by the factors investigated and large values may reflect lamellar pathology.