A biomechanical investigation of torsion and classic metaphyseal lesions

The classic metaphyseal lesion (CML) is considered to be a strong indicator of physical abuse in infants. These fractures extend through the metaphysis of a long bone near the growth plate, disrupting the trabecular bone structure. The mechanism(s) behind this fracture type are not entirely understood. The present study investigated experimentally the possible loading conditions required to generate CMLs in pig models. Fifty hindlegs from stillborn pigs were tested in torsion using a mechanical testing machine and a digital torque wrench. Fractures to the metaphysis that resemble CMLs were produced during a combined loading event which applied torsion to the knee joint (distal femur, proximal tibia, and proximal fibula). A visual assessment of the physeal surface of each specimen was performed after testing, and classic metaphyseal lesions were identified by comparison with findings from Love et al. (2011). The only fractures seen in both the femora and tibiae were CMLs, with both bone types having 60% (30 of 50) total fractures. The frequency of total fractures on fibulae was less, with 42% (21 of 50). The frequency of CMLs in the fibulae, however, is lower than the frequency of total fractures (38%, n=50), as physeal fractures also presented in some of those bones. Thus, the CML was the most common type of fracture produced due to the torque applied to the bones during testing. When assessing the number of CMLs that occurred on each limb, nine limbs had only one CML present, while 28 limbs had one or more CMLs. Additionally, complete CMLs made up 51.9% (41 of 79) of the total number, while partial made up 48.1% (38 of 79). Out of the 150 bones tested, 79 (52.7%) showed fractures resembling CMLs in human infants. A fracture was 35% more likely to occur in a femur or a tibia than in a fibula, and multiple CMLs per specimen were more common than single CMLs. The physeal surfaces of the specimens recorded as having CMLs showed damage to the trabecular bone. Three physeal fractures were noted; one Type IV physeal fracture on a proximal tibia and one on a proximal fibula, and a Type II physeal fracture on a proximal fibula. No diaphyseal fractures of any kind were produced. Further investigation of these conditions is encouraged to gain more insight into the injury mechanism