DIFFERENTIATION OF THE BONE-TISSUE REMODELING RESPONSE TO AXIAL AND TORSIONAL LOADING IN THE TURKEY ULNA

The ability of bone tissue to differentiate between axial and torsiona l loading was determined with use of a functionally isolated turkey-ul na model of bone adaptation. Surface modeling and intracortical remode ling mere quantified after four weeks of 5000 cycles per day of axial loading sufficient to cause 1000 microstrain normal to the long axis o f the bone (five ulnae), 5000 cycles per day of torsional loading suff icient to cause 1000 microstrain of shear strain (five ulnae), or disu se (six ulnae). Of these three distinct regimens, only disuse caused a significant change in gross areal properties (12 per cent loss of bon e; p < 0.05) as compared with those in the contralateral, intact contr ol ulnae (sixteen ulnae). This finding suggested that both axial and t orsional loading conditions were suitable substitutes for functional s ignals normally responsible for bone homeostasis. However, the intraco rtical response was strongly dependent on the manner in which the bone was loaded. Axial loading increased the number of intracortical pores by a factor of seven as compared with that in the controls (246 +/- 4 0.5 compared with 36 +/- 8.5 pores); it also increased the area lost b ecause of porosis as compared with that in the controls (1.39 +/- 0.25 2 compared with 0.202 +/- 0.062 square millimeter); however, the mean size of the individual pores was similar to that in the controls (0.00 565 +/- 0.0019 compared with 0.00561 +/- 0.0029 square millimeter). Co nversely, torsional loading failed to increase substantially the numbe r of pores (67 +/- 22.6 pores), the area of bone lost because of poros is (0.352 +/- 0.114 square millimeter), or the size of the pores (0.00 525 +/- 0.0035 square millimeter) as compared with those in the contro ls. Although disuse failed to increase substantially the number of int racortical pores (59 +/- 22.4 pores), significant area (1.05 +/- 0.35 square millimeters; p < 0.05) was lost within the cortex because of a threefold increase in the mean size of each pore (0.0178 +/- 0.0126 sq uare millimeter). It appears that bone tissue can readily differentiat e between distinct components of the strain environment, with strain p er se necessary to retain coupled formation and resorption, shear stra in achieving this goal by maintaining the status quo, and axial strain increasing intracortical turnover but retaining coupling. While it is clear that load influences bone mass and morphology, it is also, clea r that specific parameters within the strain environment have distinct strategic roles in defining this architecture. CLINICAL RELEVANCE: Th ese data demonstrate that the processes that control the modeling and remodeling of bone tissue are capable of distinguishing between specif ic parameters of the functional strain and stress milieu. Shear strain minimizes bone turnover, suggesting that exercise or rehabilitation p rograms, or both, for the inhibition of osteoporosis should maximize d iverse activities. If a high degree of bone turnover is warranted, as in the promotion of fracture-healing or bone ingrowth, axial condition s should prevail. With an improved understanding of the mechanisms tha t permit the cell to differentiate between changes in its volume and s hape, the ability to treat many musculoskeletal disorders as well as t o regulate the osseous response to procedures such as joint reconstruc tion, distraction osteogenesis, and fracture-healing will improve.