Inter-trabecular bone formation: a specifi c mechanism for healing of cancellous bone

Background and purpose — Studies of fracture healing have mainly dealt with shaft fractures, both experimentally and clini- cally. In contrast, most patients have metaphyseal fractures. There is an increasing awareness that metaphyseal fractures heal partly through mechanisms specifi c to cancellous bone. Several new models for the study of cancellous bone healing have recently been presented. This review summarizes our current knowledge of cancellous fracture healing. Methods — We performed a review of the literature after doing a systematic literature search. Results — Cancellous bone appears to heal mainly via direct, membranous bone formation that occurs freely in the marrow, probably mostly arising from local stem cells. This mechanism appears to be specifi c for cancellous bone, and could be named inter-trabecular bone formation. This kind of bone formation is spatially restricted and does not extend more than a few mm outside the injured region. Usually no cartilage is seen, although external callus and cartilage formation can be induced in meta- physeal fractures by mechanical instability. Inter-trabecular bone formation seems to be less sensitive to anti-infl ammatory treat- ment than shaft fractures. Interpretation — The unique characteristics of inter-trabecu- lar bone formation in metaphyseal fractures can lead to differ- ences from shaft healing regarding the effects of age, loading, or drug treatment. This casts doubt on generalizations about frac- ture healing based solely on shaft fracture models.