An evaluation of methods for the assessment of healing of open wounds in the dog

Assessment of re-epithelialization is commonly used to evaluate rates of cutaneous wound healing in experimental studies with treated and control wounds; results may be expressed as percentage of re-epithelialization over the wound surface. This is difficult with crusted lesions where wound margins are obscured. However, even when measured histologically, this approach does not allow accurate assessment of dermal repair. This study aimed to evaluate and compare the use of clinical assessment, high-resolution ultrasound (HRU) and histological morphometry in assessment of mode and rate of healing in open wounds of canine skin. The dorsal thoracolumbar area of 10 beagle dogs was clipped and two rows of six full-thickness cylinders of skin removed (5-mm punch biopsies) on either side of the midline under general anaesthesia. Wounds were allocated randomly on left and right sides to treatment or control groups and wound gel (active formulation or base) applied twice daily; wounds were allowed to form a scab and heal by second intention. Concentric 8-mm punch biopsy samples of the healing wounds were taken on days 1, 2, 4, 8 and 14 for histopathological assessment, leaving two wounds per dog to be followed by HRU throughout the study. Wounds were imaged longitudinally and transversely with 20 MHz HRU daily for 28 days using the wound gel as the transmission medium. Wound morphometry was performed on the digital HRU images (diameter at top, middle and bottom, and depth of wound) and on sections from paraffin-embedded biopsy specimens. The 8-mm biopsies were not uniformly concentric in relation to the original wounds, owing to concealment of wound margins by crust formation. This confirmed the inaccuracy of visual assessment of healing in the crusted lesions. To allow for asymmetrical positioning of the 8-mm biopsies, trigonometric extrapolation was used to calculate the real extent of the ulcer and re-epithelialization; only specimens including both left and right wound margins were used. Measurements at days 4, 8 and 14 were log transformed and analysed using a split-plot model and the LSD Tukey test. Tongues of new epithelium were 0.20 ± 0.07 mm (SE) at day 4 (11.1%), 0.37 ± 0.08 mm at day 8 (19.3%), and 0.73 ± 0.07 mm at day 14 (35.7%) (P < 0.05). High-resolution ultrasound provided images throughout the skin allowing differentiation of the wound and surrounding collagen. Longitudinal and transverse measurements of wound depth and width were analysed using a mixed model with post hoc Bonferroni adjustment. Depth reduction was linear whereas width was not. Depth reduced most consistently and rapidly, followed by width at the bottom and then at the middle and top (P < 0.001); wounds narrowed more rapidly transversely (P < 0.001). Dermal healing was still incomplete at 28 days. Scanning with HRU enables frequent, noninvasive, quantitative assessment of structural changes deep within wounds in canine skin. The method shows great promise. Histopathological assessment allows more precision, but does not allow serial examination of wound sites. Funding: The Royal Veterinary College, Innovet.