Histological evaluation of the incorporation and remodeling of structural allografts in critical size metaphyseal femur defects in rats of different ages.

Insufficient bone regeneration is a common issue for patients with extensive bone damage, therefore the use of allografts is required. With increasing life expectancy, there is a higher risk of bone repair issues after fractures or orthopedic surgical intervention. We studied incorporation and remodeling of structural allografts in critical size metaphyseal femur defects in 52 rats aged 3-month-old and 12-month-old who underwent surgeries creating a bone defect, which was either filled with a structural allograft (3-month-old - 3moAllo; 12-month-old - 12moAllo) or left empty (3-month-old - 3moE; 12-month-old - 12moE). Histological analyses were performed 14, 28 and 90 days after the surgery. The percentage of bone and fibrous tissues, and allograft relative to the defect area was evaluated. The transmission electron microscopy was carried out 14 days after allograft implantation. When the defect was empty, slower bone regeneration was observed in 12moE rats versus 3moE, leading to sufficient irregularities in the anatomic structure of the femur 90 days after the surgery. When a structural allograft was used, the area of the fibrous tissue was larger in the defects of 12moAllo compared with 3moAllo rats 90 days after surgery. No age-related differences were found in the allograft remodeling and structures of the osteocytes, osteoblasts, and osteoclasts over the observation period. Evident issues with bone regeneration were found in critical size defects both of 12moE and 12moAllo rats. However, the allograft use allowed the bone maintaining anatomic structure 90 days after the surgery.