Quantification of ovine bone adaptation to altered load: morphometry, density, and surface strain.

Mechanical loading in the sheep proximal radius was increased by ulnar osteotomy (Group O), decreased by Steinmann pinning (Group P) and unaltered in sham operated controls (Group C). A series of intravenous fluochromes was given to label bone formation and the adaptive response was measured at intervals up to 24 weeks. Surface strains, measured in an in vitro rig, were reduced in Group P and increased significantly in Group O. Cross-sectional area (CSA) peaked in all groups at 6 weeks consistent with a regional acceleratory phenomenon (RAP) and the formation of fibrolamellar bone of low density. In Groups C and P the extent of the RAP was related to the degree of operative trauma and the newly-formed bone was subsequently resorbed. In Group P, CSA fell below control levels suggesting negative adaptation. The increase in area was sustained in Group O and bone density increased due to mineralisation and infilling. Periosteal labelling of the scaffold of parallel-fibred bone peaked at 10 weeks, followed by labelling of lamellar bone filling the gaps in the structure. This adaptive response reduced surface strains to near normal levels by 24 weeks in Group O. Positive adaptation was distinguished from the non-specific RAP due to surgical trauma. The adaptive response is both 'intelligent', in that it utilises and adds to the RAP, and accurate, as no further increase in cross-sectional area was required after 6 weeks.