Bone stress injuries: RISK FACTORS (part 1)

BSIs can be viewed as occurring when the mechanical stimulus at a specific bone site repeatedly exceeds the threshold for microdamage formation. As the bone mechanical stimulus is dependent on the interaction between the applied load and the ability of the bone to resist deformation, risk factors for BSIs can be grouped into 2 categories:

1. Factors modifying the load applied to the bone

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

1) The load applied to a bone during athletic activities represents the summation of external and internal forces. It has magnitude, rate, frequency, duration and direction components. Factors modifying the load applied to a bone include biomechanical, training and muscle factors, playing surface and shoes and inserts (orthoses and insoles).

BIOMECHANICAL FACTORS

Faulty biomechanics can be dichotomised into those related to abnormal forces and abnormal motions. Increased forces on a normally aligned skeleton can result in abnormal bone loading, with athletes exhibiting high ground reaction force (GRF) magnitudes and rates, and high accelerations during the early stance phase of running gait being proposed to be at heightened BSI risk. Normal forces applied to a malaligned skeleton can also abnormally load bone. Altered movement patterns can potentially alter magnitude and/or rate of bone loading, as well as altering the direction in which a bone is loaded and the subsequent distribution of strain within the bone.

The net result may be increased loading of a less accustomed bone site. Statically assessed biomechanical variables that have been implicated in BSI development include external rotation range of motion of the hip, leg length discrepancy, and both pes planus and cavus. Dynamically assessed variables implicated in BSI development include greater peak hip adduction, knee internal rotation and peak rearfoot eversion in the frontal plane during running, and less knee flexion in the sagittal plane. Having the combination of abnormal forces coupled with a malaligned lower limb is thought to further amplify BSI risk.

REFERENCES

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