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

Background and purpose - Studies of fracture healing have mainly dealt with shaft fractures, both experimentally and clinically. In contrast, most patients have metaphyseal fractures. There is an increasing awareness that metaphyseal fractures heal partly through mechanisms specific 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 specific 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-inflammatory treatment than shaft fractures. Interpretation - The unique characteristics of inter-trabecular bone formation in metaphyseal fractures can lead to differences from shaft healing regarding the effects of age, loading, or drug treatment. This casts doubt on generalizations about fracture healing based solely on shaft fracture models.