Position of Impact

■ Notice that owing to the difference in the body's response to a fall, the effect of the position of impact is such that a hip fracture is much more likely to occur after a fall in a patient in their 80 s rather than one in their 60 s

19.5.2.1.1 Distribution of Contact Force During Impact to the Hip

■ Recent research has found that during a fall on the hip, two pathways exist for energy absorption and force generation at contact:

- A compressive load path directly in line with the hip

- A flexural load path due to deformation of muscles and ligaments peripheral to the hip

■ The result suggested that only 15% of total impact force is distributed to structures peripheral to the hip and that peak forces directly applied to the hip are well within the fracture range of the elderly femur. It was also found that impacting with the trunk upright significantly increases peak force applied to the hip (Robinovitch et al., Ann Biomed Eng 1997)

■ Another study found a 38% reduction in the trunk angle at impact, and a 7% reduction in hip impact velocity for relaxed vs muscle-active falls; thus, presence of protective muscle reflex activation during a fall may have a bearing on the final results (Van den Kroonenberg et al., J Bio-mech 1996). Another study (also in J Biomech) showed that active responses reduce the impact forces experienced at the hip and shoulder in falls to the side. Decreased effectiveness of protective responses, due to increases in reaction time and decreases in strength with age, may help explain why so many hip fractures occur in the elderly, but so few occur in younger people (Sabick et al., J Biomech 1999)

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