Pressure distribution over the palm region during forward falls on the outstretched hands

Abstract

Falls on the outstretched hands are the cause of over 90% of wrist fractures, yet little is known about bone loading during this event. We tested how the magnitude and distribution of pressure over the palm region during a forward fall is affected by foam padding (simulating a glove) and arm configuration, and by the faller's body mass index (BMI) and thickness of soft tissues over the palm region. Thirteen young women with high (n=7) or low (n=6) BMI participated in a "torso release experiment" that simulated falling on both outstretched hands with the arm inclined either at 20° or 40° from the vertical. Trials were acquired with and without a 5 mm thick foam pad secured to the palm. Outcome variables were the magnitude and location of peak pressure (d, θ) with respect to the scaphoid, total impact force, and integrated force applied to three concentric areas, including "danger zone" of 2.5 cm radius centered at the scaphoid. Soft tissue thickness over the palm was measured by ultrasound. The 5mm foam pad reduced peak pressure, and peak force to the danger zone, by 83% and 13%, respectively. Peak pressure was 77% higher in high BMI when compared with low BMI participants. Soft tissue thickness over the palm correlated positively with distance (d) (R=0.79, p=0.001) and force applied outside the danger zone (R=0.76, p=0.002), but did not correlate with BMI (R=0.43, p=0.14). The location of peak pressure was shunted 4 mm further from the scaphoid at 20° than that of 40° falls (d=25 mm (SD 8), θ=-9° (SD 17) in the 20° falls versus d=21 mm (SD 8), θ=-5° (SD 24) in the 40° falls). Peak force to the entire palm was 11% greater in 20° compared with 40° falls. These results indicate that even a 5 mm thick foam layer protects against wrist injury, by attenuating peak pressure over the palm during forward falls. Increased soft tissue thickness shunts force away from the scaphoid. However, soft tissue thickness is not predicted by BMI, and peak pressures are greater in high individuals than that of low BMI individuals. These results contribute to our understanding of the mechanics and prevention of wrist and hand injuries during falls.

Fig. 1

Schematic of the “torso release experiment”. In some trials, we placed a 5 mm thick layer of EVA foam (13 × 21 cm, density of 46.6 kg/m³) over the palmar surface, as shown in the inset. Surgical positioning mats (Vac-Pac, Olympic Medical, Seattle, WA, USA) were placed under the knee, shin, and foot to ensure consistent positioning of the participant between successive trials in a given impact configuration.

Fig. 2

Techniques for analyzing pressure data. Raw force traces (shown in (a) from a low BMI participant and in (b) from a high BMI participant) tended to exhibit two successive peaks in force (F_max1 and F_max2). (c) The locations of peak pressure corresponding to the instants of F_max1 and F_max2 from the same sample trials. (d) The location of peak pressure was expressed as the distance (d) from the scaphoid and angle (θ) from the line between scaphoid and hamate. Three different areas were defined over the palm region: area A (danger zone, dark gray) comprised a circle of 5 cm diameter centered at the scaphoid; area B (light gray) was an adjacent donut shape of 9 cm outer diameter and 5 cm inner diameter centered at the scaphiod; and area C (white) consisted of the remainder of the palm region.

Fig. 3

Typical pressure profiles from trials in each condition. In general, the peak pressure was located between the scaphoid and hamate, with the contact area increasing, and the peak pressure decreasing, in the padded (versus unpadded) condition. (a) Low BMI, unpadded 20° fall, (b) low BMI, unpadded 40° fall, (c) low BMI, padded 20° fall, (d) low BMI, padded 40° fall, (e) high BMI, unpadded 20° fall, (f) high BMI, unpadded 40° fall, (g) high BMI, padded 20° fall, (h) high BMI, padded 40° fall. In (a)–(h), the magnitude of pressure is indicated by a color scale, which can be interpreted by referring to the color bar legend at the right of each figure, showing units of N/cm² (1 N/cm²=10 kPa). Note the differences between the figures in the displayed pressure range.

Fig. 4

Effect of the 5 mm thick EVA foam pad, BMI and impact configuration on the magnitude of peak pressure over the palm region. (a) The pad significantly reduced peak pressure (p=0.001). (b) Peak pressure was greater in high BMI than in low BMI participants (p=0.02). (c) There was no significant effect of impact configuration on peak pressure (p=0.131).

Fig. 5

Location of peak pressure. Each data point represents an individual trial in each condition across all subjects. The average location of peak pressure was 25 (SD 8) mm from the scaphoid and −9 (SD 17) deg. from the line between scaphoid and hamate in the 20° fall, and 21 mm (SD 8) mm from the scaphoid and −5 (SD 24) deg. in the 40° fall.

Fig. 6

Force distribution over the three defined areas of the palm. (a) Unpadded versus padded. (b) 20° versus 40° fall configuration. (c) Low versus high BMI. (d) Percent force applied to each area. For example, 45% of total force was applied to the danger zone, and 43% and 12% of total force were applied to area B and C, respectively, in the unpadded condition. Note that data were normalized by body weight (N) before conducting statistical analysis, and an asterisk indicates statistical significance (p<0.05).