LAUSR

SUMMARY. Routine and quantitative histologic studies on femoral head separation (FHS) associated with coxofemoral joint disarticulation at necropsy were conducted on 125 femoral heads collected from 21- to 50-day-old clinically normal broilers. The study compared groups demonstrating grossly detached femoral heads (DFHs) with those having attached femoral heads (AFHs). Marked microscopic lesions compatible with osteochondrosis (OCD) consistently occurred along the separation surface in the DFH population. The histologic changes consisted of cartilage degeneration and necrosis sometimes forming small clefts or microfractures. Hemorrhage and less frequent inflammatory cells were often present along the separation surfaces. Small foci of OCD in the femur occurred in the AFH group with lesser frequency and severity. The histologic changes were mainly found within the proximal proliferative zone of the physis near the epiphyseal junction. Histomorphometry disclosed significant quantitative reductions in chondrocyte density with increased pyknosis occurring adjacent to the separation site and to a lesser extent in deeper regions of the growth plate for the DFH compared with AFH. Measurements made along the separation surface of the percentage length occupied by osteochondrotic defects and actual separated cartilage disclosed significant differences between evaluation groups. However, determinations of vascular canal areas present within two or more regions of the growth plate revealed a slight and significant increased area for DFH compared with AFH. Severity scores for the occurrence of microthrombi within the growth plate showed no difference between the groups. The pathogenesis of FHS in broilers is related to defective cartilage production or degeneration resulting in increased fragility. This contrasts with the proposed pathogenesis of OCD in mammals, which involves ischemic necrosis due to underlying vascular defects. The results for the FHS-disarticulation model also differ from those reported for glucorticoid-induced femoral head necrosis in broilers. The FHS-associated lesions occurred without histologic evidence of bacterial chondritis or osteomyelitis.