Force attenuation in trochanteric soft tissues during impact from a fall

Abstract

Summary:The risk for hip fracture from a fall is known to decrease with increased body mass index (weight/height*), a relative measure of obesity. To explore whether this reduced risk is due to the protective effect of increased soft-tissue cushioning in obese individuals, we used an impact pendulum and surrogate human pelvis to conduct simulated fall impact experiments on trochanteric soft tissues harvested from the cadavers of nine elderly individuals. For each impact, the total applied energy was 140 J. Peak forces ranged from 4,050 to 6,420 N, and tissue energy absorption ranged from 8.4 to 81.6 J. Increased tissue thickness correlated strongly with both decreased peak force (r2 = 0.91) and increased tissue energy absorption (r2 = 0.76). However, peak forces in all cases were within 1 SD of previously reported average fracture forces for elderly cadaveric femora. This suggests that force attenuation in trochanteric soft tissues alone is insufficient to prevent hip fracture in falls in which an elderly person lands directly on the hip. In such falls, additional energy-absorbing mechanisms, such as breaking the fall with an outstretched hand and eccentric contraction of the quadriceps during descent, are likely to be involved if fracture does not occur.Several studies have shown that high body mass index (weight/height2) decreases the risk for hip fracture in a fall (4,7,11,24). However, the reason for this association is unknown. One possibility lies in the observed correlation between high body mass index and high regional bone density (2), which correlates with increased femoral fracture strength (3,12). A n alternative explanation is that high body mass indices may reflect decreased body height and thus lower potential energy and height of descent in a fall from standing. However, the above possibilities do not appear to explain fully the link between body mass index and fracture risk given that a recent study by Greenspan et al. (7) found that body mass index, femoral bone mineral density, and potential energy of the fall (calculated from body mass and fall height) all were significant and independent predictors of fracture risk. A final possibility is that individuals with high body mass indices are likely to possess a greater thickness of fat tissue over the greater trochanter (13), and this fat cushions the impact and reduces the energy that must be absorbed by the femur. It is this last possibility that we are interested in exploring here.Several previous studies have shown that considerable energy is absorbed by skin and subcutaneous tissues during impact to the human body. Nikolic et al. (15) conducted an in vitro study with a dual pendulum system to determine the energy required to cause fracture of the radius from an impact to the palmar side of the hand. By comparing fracture energies from specimens having various degrees of intact skin, adipose tissue, muscle, and ligament, they concluded that 30-55% of the total impact energy is absorbed by the skin and subcutaneous tissue, 25-40% is absorbed...

Factors that influence soft tissue thickness over the greater trochanter: Application to understanding hip fractures

“…Trochanteric soft tissue has been identified as a mechanical protective factor against hip fracture (Robinovitch et al, 1995) and a starting point for engineering-based interventions to prevent fallrelated injuries (Robinovitch et al, 2009;Cameron et al, 2010). From an epidemiological perspective, women who suffer hip fractures have significantly less soft tissue overlying the greater trochanter of the femur (Lauritzen et al, 1997;Cummings and Melton, 2002;Dufour et al, 2012), and hip fracture risk can be predicted directly from measurements of trochanteric soft tissue (Dufour et al, 2012).…”

Comparison of Hip Geometry, Strength, and Estimated Fracture Risk in Women With Anorexia Nervosa and Overweight/Obese Women

“…Attenuated force accounted for the cushioning effects of soft tissue overlying the greater trochanter and was calculated as peak force minus 71 N/mm of trochanteric soft tissue (14,15,27). Trochanteric soft tissue thickness was directly measured from whole-body DXA by calculating the horizontal distance from the lateral aspect of the greater trochanter to the lateral aspect of the air-soft tissue boundary (14,16).…”

Biomechanics of Musculoskeletal Injury

“…Substantial energy can also be absorbed by skin and fat overlying the hip region (Robinovitch et al, 2000). Peak femoral impact force actually www.intechopen.com decreases in a linear manner with increasing soft-tissue thickness at a rate of approximately 79 N per 1mm change in thickness (Robinovitch et al, 1995), and peak pressure over the greater trochanter averaged 266% higher in low BMI participants than in high BMI participants in another study (Choi & Robinovitch, 2010). Additionally, there are actions that fallers can take to moderate the force applied directly to the femur.…”