The resistance of cortical bone tissue to failure under cyclic loading is reduced with alendronate

•Mechanical fatigue cycles of cortical bone beams were reduced by 3-year, high-dose alendronate treatment of female dogs.•High-dose alendronate treatment resulted in 14% reduced osteon cross-sectional size.•High- and low-dose treatments resulted in 20% reduction of osteocyte lacuna density.

Bisphosphonates are the most prescribed preventative treatment for osteoporosis. However, their long-term use has recently been associated with atypical fractures of cortical bone in patients who present with low-energy induced breaks of unclear pathophysiology. The effects of bisphosphonates on the mechanical properties of cortical bone have been exclusively studied under simple, monotonic, quasi-static loading. This study examined the cyclic fatigue properties of bisphosphonate-treated cortical bone at a level in which tissue damage initiates and is accumulated prior to frank fracture in low-energy situations. Physiologically relevant, dynamic, 4-point bending applied to beams (1.5 mm × 0.5 mm × 10 mm) machined from dog rib (n = 12/group) demonstrated mechanical failure and micro-architectural features that were dependent on drug dose (3 groups: 0, 0.2, 1.0 mg/kg/day; alendronate [ALN] for 3 years) with cortical bone tissue elastic modulus (initial cycles of loading) reduced by 21% (p < 0.001) and fatigue life (number of cycles to failure) reduced in a stress-life approach by greater than 3-fold with ALN1.0 (p < 0.05). While not affecting the number of osteons, ALN treatment reduced other features associated with bone remodeling, such as the size of osteons (− 14%; ALN1.0: 10.5 ± 1.8, VEH: 12.2 ± 1.6, × 103 μm2; p < 0.01) and the density of osteocyte lacunae (− 20%; ALN1.0: 11.4 ± 3.3, VEH: 14.3 ± 3.6, × 102 #/mm2; p < 0.05). Furthermore, the osteocyte lacunar density was directly proportional to initial elastic modulus when the groups were pooled (R = 0.54, p < 0.01). These findings suggest that the structural components normally contributing to healthy cortical bone tissue are altered by high-dose ALN treatment and contribute to reduced mechanical properties under cyclic loading conditions.