Effects of age and rate of twist on torsional fracture patterns in infant porcine femora

Abstract

Objective: Long bone fractures are a common injury in the pediatric population. Differentiation between abusive, or non-accidental trauma, and accidental trauma in children remains challenging for forensic practitioners. A recent clinical-based study was able to separate pediatric abusive from accidental trauma based on femoral fracture pattern using the ratio of fracture length over bone diameter (fracture ratio), as determined from radiographic analysis of this fractured bone. The forensic literature indicates more cases of abuse in younger pediatric victims than accidental cases. While this was the case in the clinical study, the effect was not shown to be statistically significant. Furthermore, while speed of trauma was not considered in the clinical study, a laboratory study with an immature bovine model indicates rotational speed influences fracture pattern, but specimen age was not varied in that study. Therefore, the objective of the current study was to use immature porcine femora to investigate the effects of age and rate of twist on a modified version of this fracture ratio parameter.

Methods: Fifteen pairs of porcine femora with various ages were twisted until observable failure using a custom-built torsional fixture. The left femur of each pair was twisted to failure at a rate of 3 deg/s, while the right femur was twisted at a rate of 90 deg/s. The torque and angle of rotation were recorded at a sampling rate of 10,000 Hz. Fracture ratio was defined as total fracture length divided by bone diameter.

Results: Fracture ratio increased with specimen age, with specimens under the low rate of twist yielding a consistently lower fracture ratio than those from specimens under the high rate of twist. The results showed that both specimen age and rate of twist were significant factors influencing fracture ratio.

Conclusion: The determination of abusive from accidental trauma in criminal cases, based on the pattern of long bone fracture alone, may need to include additional data on the specific age of the pediatric victim and the potential speed of the traumatic event.

Keywords: Animal model; Forensic biomechanics; Fracture ratio; Long bone; Pediatric abuse; Spiral fracture.

Fig. 1

Photograph of the custom-built laser alignment fixture with a laser beam on the center of the bone.

Fig. 2

Schematic example of measuring the total fracture length using fracture segments (summation of segments 1, 2 and 3) in a single 2-dimensional plane of view. A fracture ratio was defined as total fracture length divided by bone diameter.

Fig. 3

Anteroposterior (AP) fracture ratio (measured on anterior side of bone, n = 15) with respect to specimen age for different rotational rates (logarithmic regressions).

Fig. 4

Mediolateral (ML) fracture ratio (measured on medial side of bone, n = 11) with respect to specimen age for different rotational rates (logarithmic regressions).

Fig. 5

Photographs of low-rate tests, showing the typical fracture patterns with an increase in specimen age (2, 4, and 9 days) top-bottom, and its effect on AP fracture ratio (left column). Photographs showing the increase in the longitudinal fracture component (segment 2) in the more aged test specimens (13, 18, and 26 days) top-bottom, and its effect on AP fracture ratio (right column).