Laboratory and clinical scientists and practicing clinicians need definitions of union, delayed union, and nonunion. Fracture union is a gradual process, so quantitative measures are the most meaningful. However, end point definitions also are useful, but they need empirical validation. The measure that has received the best validation in human fractures is bending stiffness. Quantitative radiologic assessment of healing is difficult because varying patterns of bone bridging can occur, including periosteal, endosteal, and intercortical patterns. Natural fracture healing was studied in 43 cases of isolated, closed, conservatively treated tibial shaft fracture with serial measurements of bending stiffness and standard radiographs. Three healing groups were defined on the basis of stiffness recovery patterns. Four cases showed delayed union, defined as failure to reach a stiffness of 7 N-m per degree by 20 weeks from fracture. The remaining cases had normal union, but at differing rates. Callus index was used as a measure of periosteal new bone formation. Stiffness measurements correlated more strongly than callus index with injury severity and functional outcome at 6 months. However, the callus index predicted behavior in those fractures that showed no tendency to heal at the 10-week stage. That is, absence of periosteal new bone in these cases presaged delayed union. These delayed union cases all eventually healed, still without producing periosteal callus, but other fractures in the series healed very rapidly, also without periosteal callus. The implication is that endosteal healing is capable of very rapid fracture bridging if conditions are right, but it also can occur late, after the periosteal healing response has ceased. These observations suggest a more rational approach to the definition of union, delayed union, and nonunion than that provided by the selection of arbitrary times. For conservatively treated fractures at least, delayed union can be defined as the cessation of the periosteal response before the fracture successfully has been bridged. Nonunion is the cessation of both the periosteal and endosteal healing responses without bridging.

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