Q: My father-in-law, mother-in-law, and my own mother have all fallen and broken their hips. Of course, we never really knew if the hip broke and they fell or they fell and the hip broke. In any case, I got to wondering why is it always the hip that breaks? Why don't older adults who fall break a leg or an ankle instead?

A: It's a good question and one that may not have a single best answer. First, it should be noted that osteoporosis (brittle bones from loss of bone density) is a common problem in the elderly. Women usually start to lose bone density in their 50s and 60s after menopause.

Men lose bone mass, too, but it's delayed by 10 years so occurs more in their late 60s and early 70s. The risk of bone fracture increases with any amount of osteoporosis but the more brittle the bone, the greater the risk. Inactivity and a sedentary lifestyle also leads to general deconditioning and loss of muscle mass called sarcopenia and muscle strength. Along with that comes a loss of flexibility and balance. Visual impairment, the use of multiple medications, and the presence of diabetes or other health problems head the list of additional risk factors for falls.

Once a fall happens, what determines which bones are broken? Body mass, direction of the fall, and reflexive reactions to right the body in an attempt to regain balance during the fall have something to do with it. The force of the landing and the angle of impact create vectors that help determine where the force is translated along the bone.

But a new finding might also help explain fracture locations. MRIs and arthroscopic exams of joint cartilage have shown that the biomechanical properties of joint cartilage might have something to do with it. It turns out that the cartilage in the hip weakens with age and is less resistant to stress compared to the ankle, for example.

The tensile stiffness of hip and knee articular cartilage also decreases with age much faster compared to the stiffness of the ankle. The thinner ankle cartilage just doesn't seem to respond to the aging process in the same way hip or knee cartilage does. Scientists think that might help explain why people are more likely to develop arthritis in the knee compared to the ankle as well.

Reference: Matthew E. Mitchell, MD, et al. Cartilage Transplantation Techniques for Talar Cartilage Lesions. In Journal of the American Academy of Orthopaedic Surgeons. July 2009. Vol. 17. No. 7. Pp. 407-414.