The scientists surgically treated the osteonecrotic lesions using bone grafts from healthy young and old (above 12 months) mice. Histology and immunostaining revealed high levels of cell proliferation, an early indication of bone healing, by day 5 after surgery. However, by day 14, only bone grafts from young mice were able to form bony bridges, as observed by histology and micro-CT imaging. Bone grafts from old mice were unable to produce new bone during the healing process, indicating that they were less osteogenic (capable of forming new bone).

The WNT pathway (a crucial pathway in developmental biology) is emerging as a therapeutic target in bone disease, because amplified WNT signalling causes an increase in bone mineral density. Thus, the authors hypothesized that an amplification in WNT signalling could rescue the age-related decline in osteogenic capacity observed in bone grafts from older mice. They tested this by incubating bone grafts in WNT3A, a bone-promoting protein, for 1 hr before surgically replacing osteonecrotic lesions.

WNT3A-treated bone grafts from old mice were able to form bony bridges by day 14 after grafting; these were absent in untreated controls. Immunostaining for osteogenic markers (Runx2 and Osterix) revealed significantly higher levels in WNT3A-treated bone grafts compared with untreated controls. Furthermore, treating bone grafts from old mice with WNT3A restored expression levels to those observed in bone grafts from young mice.

WNT3A-treated bone grafts from old mice performed equally well to bone grafts from young mice. By day 14, both groups generated new bone volume at a similar rate and by day 30, all defects showed evidence of bony bridging. WNT3A treatment not only enhanced bone formation in bone grafts from old mice, but also produced high-quality bone similar to that seen in young mice.

Osteonecrosis is a bone disease that usually affects the aged population. And if left untreated, the bone can collapse, which occurs in about 50% of patients. For these reasons, a WNT-based therapeutic approach may prove useful in the treatment of osteonecrosis, especially in older patients.