Biomechanics of skull fracture and intracranial injury in young children as a consequence of a low height fall

A challenge for clinicians when presented with a significant head injury in a young child and a postulated fall height is to determine the plausibility of such an injury. Previous authors have aimed to determine the head injuries that can result from a low height fall, however due to a lack of clarity it is difficult to determine a fall height at which certain head injuries including skull fracture and intra cranial injury (ICI) becomes more likely. Biomechanical thresholds aimed at young children exist for skull fracture and adult thresholds for subdural haemorrhage, however they have not been assessed against the injuries seen in a clinical setting. Consequently this study investigated low height falls in a paediatric clinical setting to determine differentiating variables and characteristics in the mechanism of head injury between children with a minor head injury and those with a skull fracture and / or ICI. The primary aim of which was to determine a fall height threshold for skull fracture and / ICI in young children. Following this, biomechanical methods were used to include, the development of an accurate anthropomorphic testing device (ATD) and a finite element model of an infant head, to investigate the differentiating variables and ultimately the clinical fall height threshold. Method A case control study of children ≤ 48 months of age who had a minor head injury and those with a skull fracture and / or ICI, to identify variables and characteristics of falls that influenced injury severity. Children were ascertained from those who attended the University Hospital of Wales Cardiff from a low height fall. The clinical characteristics and biomechanical variables evaluated included the mechanism of injury, surface of impact, site of impact and fall height (taking into consideration height of object and centre of gravity of the child’s body and head mass). Categorical variables were assessed using a Chi Square test and continuous variables using Student t-test or the non parametric equivalent. A modified logistic regression was used to evaluate the likelihood of sustaining a skull fracture and /or ICI based on fall height. Initially to investigate the differentiating variables a biofidelic infant headform was designed via image processing and segmentation of computed tomography (CT) datasets and manufactured using materials with similar properties to the bone and soft tissues of the head. The headform impact response was initially validated against infant cadaver data and then it was subject to tests classed as sub-injurious based on the clinical data collected from the hospital. The headform was dropped at impact angles of 90o, 75o and 60o at three velocities (2.4m/s, 3m/s, 3.4m/s) corresponding to three heights (0.3m, 0.45m, 0.6m), onto four domestic surfaces (carpet, carpet & underlay, laminate and wood) using two skin friction surrogates (latex, polyamide). A Student t-test was used to measure the affect of the coefficient of static friction and a three factorial ANOVA to measure the affect of impact velocity, surface type and angle of impact had on kinematic variables (peak g, HIC, rotational acceleration, change in rotational velocity and duration of impact). Finally to investigate the differentiating variables a finite element (FE) model of an infant head was developed, again through image processing of infant head CT datasets. The FE model consisted of the scalp, sutures, cranial bones, dura membranes, cerebral spinal fluid, bridging veins and the brain and the impact response was also initially validated against infant cadaver data. Post validation a parametric test across four different scenarios (0.3m impact onto the occipital, frontal, vertex and parietal areas of the head) was conducted to assess the affect material properties have on impacted response of the model. Finally the FE was used to assess the affect height (0.3m, 0.6m, 1.2m) and anatomical site of impact have on the impact response of the head, including kinematic variables and material response variables. Results Identified cases included 416 children with a minor head injury and 47 with a skull fracture and / or ICI. The mean fall height for minor head injuries was significantly lower than for a fall causing skull fracture and / or ICI (P<0.001). Utilising the height of centre of gravity of the head, no skull fracture and / or ICI was sustained in children who fell <0.6m (2ft). Skull fractures and / or ICI were more likely in children ≤12 months (P<0.001), following impacts to the temporal/parietal or occipital region of the head (P<0.01), and impacts onto wood (P<0.05). All tests using the biofidelic headform were conducted with impact velocities corresponding to fall heights ≤0.6m, where an increase in impact velocity, increase in surface stiffness and a decrease in impact angle significantly affected both rotational and translation kinematic variables (P<0.05). Peak rotational accelerations at 90 degrees were 11, 363 rad/s2 on wood at an impact velocity corresponding to a height of 0.6m and significantly increased to 16,980 rad/s2 with a 30 degree decrease in impact angle (P<0.001). However head injury criterion (HIC) decreased for wood at impact velocity corresponding to 0.6m from 245 to 121 for a 30degree decrease in impact angle (P