The ability of bone tissue to differentiate between axial and torsional loading was determined with use of a functionally isolated turkey-ulna model of bone adaptation. Surface modeling and intracortical remodeling were quantified after four weeks of 5000 cycles per day of axial loading sufficient to cause 1000 microstrain normal to the long axis of the bone (five ulnae), 5000 cycles per day of torsional loading sufficient to cause 1000 microstrain of shear strain (five ulnae), or disuse (six ulnae). Of these three distinct regimens, only disuse caused a significant change in gross areal properties (12 per cent loss of bone;​ p < 0.05) as compared with those in the contralateral, intact control ulnae (sixteen ulnae). This finding suggested that both axial and torsional loading conditions were suitable substitutes for functional signals normally responsible for bone homeostasis. However, the intracortical 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 ± 40.5 compared with 36 ± 8.5 pores);​ it also increased the area lost because of porosis as compared with that in the controls (1.39 ± 0.252 compared with 0.202 ± 0.062 square millimeter);​ however, the mean size of the individual pores was similar to that in the controls (0.00565 ± 0.0019 compared with 0.00561 ± 0.0029 square millimeter). Conversely, torsional loading failed to increase substantially the number of pores (67 ± 22.6 pores), the area of bone lost because of porosis (0.352 ± 0.114 square millimeter), or the size of the pores (0.00525 ± 0.0035 square millimeter) as compared with those in the controls. Although disuse failed to increase substantially the number of intracortical 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 square millimeter).

It appears that bone tissue can readily differentiate between distinct components of the strain environment, with strain per se necessary to retain coupled formation and resorption, shear strain 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 clear that specific parameters within the strain environment have distinct strategic roles in defining this architecture.

CLINICAL RELEVANCE:​ These data demonstrate that the processes that control the modeling and remodeling of bone tissue are capable of distinguishing between specific parameters of the functional strain and stress milieu. Shear strain minimizes bone turnover, suggesting that exercise or rehabilitation programs, or both, for the inhibition of osteoporosis should maximize diverse activities. If a high degree of bone turnover is warranted, as in the promotion of fracture-healing or bone ingrowth, axial conditions should prevail. With an improved understanding of the mechanisms that permit the cell to differentiate between changes in its volume and shape, the ability to treat many musculoskeletal disorders as well as to regulate the osseous response to procedures such as joint reconstruction, distraction osteogenesis, and fracture-healing will improve.