Consider this passage from the late Robert Heaney: "Although bone mass is certainly the most extensively studied of the fragility factors, low bone mass is not the whole of the osteoporosis story and may not even be its most important component (despite frequent assertions to the contrary). If one could magically normalize bone mass in everyone, would one eliminate osteoporotic fractures? The best answer that can be given today is ‘no.’" https://pubmed.ncbi.nlm.nih.gov/9471936/
Assume (as you should) that Heaney is correct. What else, besides intrinsic bone conditions, might lead to fragility fractures of the hip?
(A fragility fracture is one sustained by a low energy mechanism, such as falling from a standing height.)
Lotz and Hayes [https://pubmed.ncbi.nlm.nih.gov/2355030/] demonstrated in a cadaver hip fracture model that the energy required to fracture the proximal femur was nearly one order of magnitude less than then the estimated energy imparted during a typical “low energy” fall from standing height.
Because a fall imparts far more than enough energy to break the hip, yet most falls do not cause a fracture, it is clear that energy absorption – e.g., catching yourself when you fall, and having adequate soft tissue to pad the femur – is an important factor preventing injury to the bone.
Further, the population-based prospective cohort study reported by Albertson et al. [https://pubmed.ncbi.nlm.nih.gov/20334634/ ] revealed that other independent risk factors including age ≥80 years, weight <60 kg, prior fragility fracture and impaired ability to rise increase the risk of fragility fracture risks for females.
Another issue is that geriatric patients may have problems with gait, balance, and mobility which increases their propensity to fall. In a case-control study of older females (median age: 80 years) admitted to the hospital with a first hip fracture, Grisso et al. [https://pubmed.ncbi.nlm.nih.gov/2017229/ ] found lower-limb dysfunction, a history of a previous stroke, Parkinson's disease, barbiturate use, and visual impairment all increased the risk of hip fracture significantly. They conclude that preventative measures should aim to reduce the falls among patients with these risk factors, in addition to optimizing bone mineral density.
Clearly, the cause of fragility hip fractures is multifactorial. While bone mineral density remains an important risk factor for fragility fractures, it may garner a disproportionate amount of attention as it easily quantifiable. Moreover, it offers a target for pharmacologic intervention. Still, prevention of fragility hip fractures should be targeted at other risks factors in addition to optimizing bone mineral density. Collectively, these factors are at least equally important as bone mass.
Additional Points to Consider
If other factors outside the bone are at least as important as bone mass, is osteoporosis (etymology: “porous bone”) really the best name for the condition? (See: In the Beginning was the Word JBJS: February 2006 - Volume 88 - Issue 2 - p 442-445 https://pubmed.ncbi.nlm.nih.gov/16452759/)