Keaveny Tony M, Bouxsein Mary L
Department of Mechanical Engineering, University of California, Berkeley, California 94720-1740, USA.
J Bone Miner Res. 2008 Oct;23(10):1541-7. doi: 10.1359/jbmr.080406.
Because of the dichotomous nature of a bone fracture, when Phi, the ratio of the applied impact force to the bone strength, is greater than a critical value--the biomechanical fracture threshold--fracture should occur. We sought to elucidate the conceptual implications of this biomechanical fracture threshold with application to hip fracture. We used data from the PaTH study, a 2-yr clinical trial in postmenopausal women treated with alendronate, PTH, or their combination. Outcomes included the force applied to the hip in a sideways fall as estimated from subject height and weight; femoral strength as determined by QCT-based finite element analysis; the load-to-strength ratio Phi; and total hip areal BMD from DXA. Results indicated that those with "very low" femoral strength (<2000 N) invariably had load-to-strength ratio Phi values well above the theoretical biomechanical fracture threshold (Phi = 1), but those with "moderately low" femoral strength (2000-4000 N) displayed Phi values both above and below the theoretical biomechanical fracture threshold. This finding implies that the risk of a hip fracture can be high in those with only moderately low BMD because femoral strength can be low relative to fall impact forces. The observed weak correlation between areal BMD and the load-to-strength ratio Phi (r2 = 0.14) suggests that consideration of the biomechanical fracture threshold may improve fracture risk assessment, particularly for those in the osteopenic range. Regarding treatment effects, only those subjects having load-to-strength ratio Phi values within a relatively narrow "transition zone" of +/- 20% of the assumed biomechanical fracture threshold at baseline were predicted to change fracture status during the trial. In theory, outcomes of fracture trials may be dominated by the responses of those within the "transition zone" at baseline, and treatment benefits in terms of fracture efficacy may depend the patient's baseline status with respect to the biomechanical fracture threshold. We conclude that consideration of the theoretical implications of the biomechanical fracture threshold may lead to new insights and advances in the assessment and treatment of osteoporosis.
由于骨折具有二分性，当施加的冲击力与骨骼强度之比Phi大于临界值（生物力学骨折阈值）时，骨折就会发生。我们试图阐明这一生物力学骨折阈值在髋部骨折中的概念意义。我们使用了来自PaTH研究的数据，这是一项针对接受阿仑膦酸盐、甲状旁腺激素或其联合治疗的绝经后女性进行的为期2年的临床试验。结果包括根据受试者身高和体重估算的侧方跌倒时施加在髋部的力；通过基于定量计算机断层扫描（QCT）的有限元分析确定的股骨强度；负荷与强度之比Phi；以及通过双能X线吸收法（DXA）测得的全髋部面积骨密度。结果表明，股骨强度“非常低”（<2000 N）的患者，其负荷与强度之比Phi值始终远高于理论生物力学骨折阈值（Phi = 1），但股骨强度“中度低”（2000 - 4000 N）的患者，其Phi值既有高于也有低于理论生物力学骨折阈值的情况。这一发现意味着，骨密度仅中度低的人群髋部骨折风险可能很高，因为相对于跌倒冲击力而言，股骨强度可能较低。观察到的面积骨密度与负荷与强度之比Phi之间的弱相关性（r2 = 0.14）表明，考虑生物力学骨折阈值可能会改善骨折风险评估，特别是对于骨量减少范围内的人群。关于治疗效果，只有那些在基线时负荷与强度之比Phi值处于假定生物力学骨折阈值的±20%相对较窄“过渡区”内的受试者，预计在试验期间骨折状态会发生变化。理论上，骨折试验的结果可能由基线时处于“过渡区”的受试者的反应主导，而骨折疗效方面的治疗益处可能取决于患者相对于生物力学骨折阈值的基线状态。我们得出结论，考虑生物力学骨折阈值的理论意义可能会为骨质疏松症的评估和治疗带来新的见解和进展。
