Bone stress injuries: RISK FACTORS (part 4)

2. Factors influencing the ability of the bone to resist load without damage accumulation

The amount and rate of strain engendered when a load is applied to a bone is dependent upon the ability of the bone to resist deformation in the direction of loading. For a given applied load, less rigid bones experience greater strain and at a faster rate than more rigid bones. This means they are more susceptible to microdamage and BSI formation. Skeletal features than influence bone rigidity include the amount of bone material present (mass) and its distribution (structure). There is strong evidence that both contribute to BSI risk and so it is important to consider modifiable factors that may contribute to these characteristics. 3 modifiable factors in athletes that may impact the ability of bone to resist loading and contribute to BSI risk are physical activity history, energy availability and calcium and vitamin D status.

Physical activity history

A longer history of physical activity appears to be protective against BSI development. The skeleton responds and adapts to mechanical loading in a site-specific manner to increase its rigidity in the direction of loading. It does this principally during the growing years by depositing small amounts of new bone on the outer periosteal surface at a distance from the bending axes. As the rigidity of a unit area of bone is proportional to the fourth power of its distance from a bending axis, the addition of a small amount of mass to the outer surface of a bone results in a disproportionate increase in bone strength. The net result is a decrease in the bone strain engendered in response to a given load and an increase in bone fatigue life.

Energy availability

Females are at greater risk of BSI susceptibility. This appears to relate to interrelationships between energy availability, menstrual function and bone mass. While athletes can have one or more of these components, low energy availability appears to be the central factor. This results from insufficient dietary intake to meet exercise energy expenditure. It can result from low dietary intake (whether inadvertent, intentional or psychopathological) and/or excessive exercise energy expenditure. The menstrual and skeletal changes associated with low energy availability reduce the ability of bone to resist load and/or repair microdamage. The net result is heightened BSI risk, especially in elite female long distance runners where the difference between dietary energy intake and exercise energy expenditure is often small and the incidence of menstrual dysfunction is often elevated.

Calcium and vitamin D status

Calcium combines with phosphate to form hydroxyapatite crystals to endow bone with rigidity., while vitamin D contributes by promoting calcium absorption in the gut and reabsorption in the kidneys. Prospective studies provide evidence for roles of both low calcium and vitamin D in BSIs. A large randomised controlled trial demonstrated a 20% reduction in BSI incidence in female Navy recruits with suboptimal baseline daily calcium intake when they were supplemented with daily calcium and vitamin D. Overall, the data suggest that athletes should ensure sufficient calcium and vitamin D intakes to meet or exceed the current recommended dietary allowances of 1000-1300mg and 600 IU (for individuals aged 14 - 50), respectively.

REFERENCES

Brukner, P., 2012. Brukner & Khan's clinical sports medicine. North Ryde: McGraw-Hill