Peter Verschueren

The mechanical properties of cranial bone: The effect of loading rate and cranial sampling position

Linear and depressed skull fractures are frequent mechanisms of head injury and are often associated with traumatic brain injury. Accurate knowledge of the fracture of cranial bone can provide insight into the prevention of skull fracture injuries and help aid the design of energy absorbing head protection systems and safety helmets. Cranial bone is a complex material comprising of a three-layered structure: external layers consist of compact, high-density cortical bone and the central layer consists of a low-density, irregularly porous bone structure. In this study, cranial bone specimens were extracted from 8 fresh-frozen cadavers (F=4, M=4; 81+/-11 years old). 63 specimens were obtained from the parietal and frontal cranial bones. Prior to testing, all specimens were scanned using a microCT scanner at a resolution of 56.9 microm. The specimens were tested in a three-point bend set-up at different dynamic speeds (0.5, 1 and 2.5 m/s). The associated mechanical properties that were calculated for each specimen include the 2nd moment of inertia, the sectional elastic modulus, the maximum force at failure, the energy absorbed until failure and the maximum bending stress. Additionally, the morphological parameters of each specimen and their correlation with the resulting mechanical parameters were examined. It was found that testing speed, strain rate, cranial sampling position and intercranial variation all have a significant effect on some or all of the computed mechanical parameters. A modest correlation was also found between percent bone volume and both the elastic modulus and the maximum bending stress.

Biomechanics of frontal skull fracture.

The purpose of the present study was to investigate whether an energy failure level applies to the skull fracture mechanics in unembalmed post-mortem human heads under dynamic frontal loading conditions. A double-pendulum model was used to conduct frontal impact tests on specimens from 18 unembalmed post-mortem human subjects. The specimens were isolated at the occipital condyle level, and pre-test computed tomography images were obtained. The specimens were rigidly attached to an aluminum pendulum in an upside down position and obtained a single degree of freedom, allowing motion in the plane of impact. A steel pendulum delivered the impact and was fitted with a flat-surfaced, cylindrical aluminum impactor, which distributed the load to a force sensor. The relative displacement between the two pendulums was used as a measure for the deformation of the specimen in the plane of impact. Three impact velocity conditions were created: low (3.60+/-0.23 m/sec), intermediate (5.21+/-0.04 m/sec), and high (6.95+/-0.04 m/sec) velocity. Computed tomography and dissection techniques were used to detect pathology. If no fracture was detected, repeated tests on the same specimen were performed with higher impact energy until fracture occurred. Peak force, displacement and energy variables were used to describe the biomechanics. Our data suggests the existence of an energy failure level in the range of 22-24 J for dynamic frontal loading of an intact unembalmed head, allowed to move with one degree of freedom. Further experiments, however, are necessary to confirm that this is a definitive energy criterion for skull fracture following impact.

The relation between mechanical impact parameters and most frequent bicycle related head injuries

The most frequent head injuries resulting from bicycle accidents include skull fracture acute subdural hematoma (ASDH), cerebral contusions, and diffuse axonal injury (DAI). This review includes epidemiological studies, cadaver experiments, in vivo imaging, image processing techniques, and computer reconstructions of cycling accidents used to estimate the mechanical parameters leading to specific head injuries. The results of the head impact tests suggest the existence of an energy failure level for the skull fracture, specific for different impact regions (22-24J for the frontal site and 5-15J for temporal site). Typical linear patterns were described for frontal, parietal and occipital skull fracture. Temporal skull fracture described considerably higher variability. In term of contusion mechanogenesis, the experiments proved that relative brain-skull motion will not be prevented if the maximum frequency of the impact frequency spectrum stays below 150Hz or below the frequency corresponding to the impedance peak of the head under investigation. The brain shift patterns in humans, both in dynamic and quasistatic situations were shown to be very complex, with maximum amplitudes localized at the level of the inferolateral aspects of the frontal and temporal lobes. The resulting brain maximum amplitudes differed when the head was subjected to a sagittal or lateral motion. Finally, the presented data support the existence of a critical elongation/stretch criterion for the occurrence of ASDH due to BV rupture, located around 5mm elongation or 25% stretch limit. In addition, a tolerance level lying around 10,000rad/s(2) for pulse durations below 10ms was established for BV rupture, which seems to decrease with increasing pulse duration. The described research indicates that injury specific tolerance criteria can provide a more accurate prediction for head injuries than the currently used HIC. Internal brain lesions are strongly related to rotational effects which are not appropriately accounted by the commonly accepted head injury criterion (HIC). The research summarized in this paper adds significantly to the creation of a fundamental knowledge for the improvement of bicycle helmets as well as other head protective measures. The described investigations and experimental results are of crucial importance also for forensic research.

Statistical Shape Modeling and Population Analysis of the Aortic Root of TAVI Patients

The new transcatheter technique to implant synthetic aortic valves offers a treatment to patients previously considered untreatable. However, the majority of patients suffer from leakage alongside the implant. Using a statistical shape model of the anatomy, a correlation was discovered between leakage and the shape of the sinuses of Valsalva.

The Mechanical Properties of Cranial Bone

Linear and depressed skull fractures are frequent mechanisms of head injury and are often associated with traumatic brain injury. Accurate knowledge of the fracture of cranial bone can provide insight into the prevention of skull fracture injuries and associated lesions of soft neural tissue and help aid the design of energy absorbing head protection systems. Cranial bone is a complex material comprising of a three-layered structure: external layers consisting of compact, high-density cortical bone and a central layer consisting of a low-density, irregularly porous structure. In the current study, a significantly large set of cranial bone specimens (parietal and frontal bones) were extracted from 8 crania and, after μCT imaging, the specimens were tested in a three-point bend set-up at dynamic speeds. Important mechanical and morphological properties were calculated for each specimen. The mechanical properties were consistent with those previously reported in the literature. Potential correlations between the calculated parameters were examined statistically. Testing speed, strain rate, cranial sampling position and intercranial variation were found to have a significant effect on some or all of the computed mechanical parameters.

Clinical and Biomechanical Research for Bicycle Helmet Optimisation

Epidemiological studies on bicycle accidents show that a substantial fraction of the cyclists that call for medical aid, are suffering from skull and brain damage. The aim of the research performed at the K.U.Leuven is to reduce the risk of serious head injuries by the creation of a new type of bicycle helmet. To achieve this goal, a clinical review of pedal cyclist head lesions has been performed, a 3D accident simulation program has been developed, skull behaviour during impact testing has been studied and the current protective effect of a typical series of bicycle helmet has been evaluated. The absence of a temple cover allows a potentially dangerous contact between the temple of the head and the impact surface [1].

Cerebral Bridging Vein Rupture in Humans

A well known cause of death and disability after head trauma is the occurrence of an acute subdural haematoma (ASDH) due to bridging vein rupture. In the past, damage to the bridging veins and ASDH has been shown to be related to angular acceleration of the head in the sagittal plane. The objective of the present study was to establish critical peak angular accelerations in short duration impacts on the head (<15ms), typical for falls and collisions. 18 tests were performed. The results complement impact data from Lowenhielm. Moreover, the tolerance criteria for bridging vein disruption suggested by Lowenhielm are confirmed for short pulse durations. The first states that the peak angular acceleration cannot exceed 4500 rad/s?. The second constitutes a limitation of the change in angular velocity to 40 rad/s. If both of these conditions are fulfilled, the integrity of the bridging veins is said to be assured.