Effects of phenylbutazone on bone activity and formation in horses

Objective-To determine the effects of phenylbutazone (PBZ) on bone activity and bone formation in horses. Animals-12 healthy 1- to 2-year-old horses. Procedures-Biopsy was performed to obtain unicortical bone specimens from 1 tibia on day 0 and from the contralateral tibia on day 14. Fluorochromic m arkers were administered IV 2 days prior to and on days 0, 10, 15, and 25 a fter biopsy was performed. Six horses received PBZ (4.4 mg/kg of body weigh t, PO, q 12 h) and 6 horses were used as controls. All horses were euthanat ized on day 30 and tissues from biopsy sites, with adjacent cortical bone, were collected. Osteonal density and activity, mineral apposition rate (MAR ), and percentage of mineralized tissue filling the biopsy-induced defects in cortical bone were assessed. Serum samples from all horses were analyzed for bone-specific alkaline phosphatase activity and concentration of PBZ. Results-MAR was significantly decreased in horses treated with PBZ. Regiona l acceleratory phenomenon was observed in cortical bone in both groups but was significantly decreased in horses treated with PBZ. Osteonal activity w as similar at all time points in all horses. In control horses, percentage of mineralized tissue filling the cortical defects was significantly greate r in defects present for 30 days, compared with defects present for 14 days . Differences in percentage of mineralized tissue were not detected in hors es treated with PBZ. Conclusions and Clinical Relevance-PBZ decreased MAR in cortical bone and a ppeared to decrease healing rate of cortical defects in horses.