Andre M. Loyd

Use of a three-dimensional, normative database of pediatric craniofacial morphology for modern anthropometric analysis.

Background: Surgical correction of cranial abnormalities, including craniosynostosis, requires knowledge of normal skull shape to appreciate dysmorphic variations. However, the inability of current anthropometric techniques to adequately characterize three-dimensional cranial shape severely limits morphologic study. The authors previously introduced three-dimensional vector analysis, a quantitative method that maps cranial form from computed tomography data. In this article, the authors report its role in the development and validation of a normative database of pediatric cranial morphology and in clinical analysis of craniosynostosis. Methods: Normal pediatric craniofacial computed tomography data sets were acquired retrospectively from the Duke University Picture Archive and Communications System. Age increments ranging from 1 to 72 months were predetermined for scan acquisition. Three-dimensional vector analysis was performed on individual data sets, generating a set of point clouds. Averages and standard deviations for the age and gender bins of point clouds were used to create normative three-dimensional models. Anthropometric measurements from three-dimensional vector analysis models were compared with published matched data. Preoperative and postoperative morphologies of a sagittal synostosis case were analyzed using three-dimensional vector analysis and the normative database. Results: Three- and two-dimensional representations were created to define age-incremental normative models. Length and width dimensions agreed with previously published data. Detailed morphologic analysis is provided for a case of sagittal synostosis by applying age- and gender-matched data. Conclusions: Three-dimensional vector analysis provides accurate, comprehensive description of cranial morphology with quantitative graphic output. The method enables development of an extensive pediatric normative craniofacial database. Future application of these data will facilitate analysis of cranial anomalies and assist with clinical assessment.

Tensile failure properties of the perinatal, neonatal, and pediatric cadaveric cervical spine.

Study Design. Biomechanical tensile testing of perinatal, neonatal, and pediatric cadaveric cervical spines to failure. Objective. To assess the tensile failure properties of the cervical spine from birth to adulthood. Summary of Background Data. Pediatric cervical spine biomechanical studies have been few due to the limited availability of pediatric cadavers. Therefore, scaled data based on human adult and juvenile animal studies have been used to augment the limited pediatric cadaver data. Despite these efforts, substantial uncertainty remains in our understanding of pediatric cervical spine biomechanics. Methods. A total of 24 cadaveric osteoligamentous head-neck complexes, 20 weeks gestation to 18 years, were sectioned into segments (occiput-C2 [O-C2], C4–C5, and C6–C7) and tested in tension to determine axial stiffness, displacement at failure, and load-to-failure. Results. Tensile stiffness-to-failure (N/mm) increased by age (O-C2: 23-fold, neonate: 22 ± 7, 18 yr: 504; C4–C5: 7-fold, neonate: 71 ± 14, 18 yr: 509; C6–C7: 7-fold, neonate: 64 ± 17, 18 yr: 456). Load-to-failure (N) increased by age (O-C2: 13-fold, neonate: 228 ± 40, 18 yr: 2888; C4–C5: 9-fold, neonate: 207 ± 63, 18 yr: 1831; C6–C7: 10-fold, neonate: 174 ± 41, 18 yr: 1720). Normalized displacement at failure (mm/mm) decreased by age (O-C2: 6-fold, neonate: 0.34 ± 0.076, 18 yr: 0.059; C4–C5: 3-fold, neonate: 0.092 ± 0.015, 18 yr: 0.035; C6–C7: 2-fold, neonate: 0.088 ± 0.019, 18 yr: 0.037). Conclusion. Cervical spine tensile stiffness-to-failure and load-to-failure increased nonlinearly, whereas normalized displacement at failure decreased nonlinearly, from birth to adulthood. Pronounced ligamentous laxity observed at younger ages in the O-C2 segment quantitatively supports the prevalence of spinal cord injury without radiographic abnormality in the pediatric population. This study provides important and previously unavailable data for validating pediatric cervical spine models, for evaluating current scaling techniques and animal surrogate models, and for the development of more biofidelic pediatric crash test dummies.

The mechanical and morphological properties of 6 year-old cranial bone

Traumatic Brain Injury (TBI) is a leading cause of mortality and morbidity for children in the United States. The unavailability of pediatric cadavers makes it difficult to study and characterize the mechanical behavior of the pediatric skull. Computer based finite element modeling could provide valuable insights, but the utility of these models depends upon the accuracy of cranial material property inputs. In this study, 47 samples from one six year-old human cranium were tested to failure via four point bending to study the effects of strain rate and the structure of skull bone on modulus of elasticity and failure properties for both cranial bone and suture. The results show that strain rate does not have a statistically meaningful effect on the mechanical properties of the six year-old skull over the range of strain rates studied (average low rate of 0.045 s(-1), average medium rate of 0.44 s(-1), and an average high rate of 2.2 s(-1)), but that these properties do depend on the growth patterns and morphology of the skull. The thickness of the bone was found to vary with structure. The bending stiffness (per unit width) for tri-layer bone (12.32±5.18 Nm(2)/m) was significantly higher than that of cortical bone and sutures (5.58±1.46 Nm(2)/m and 3.70±1.88 Nm(2)/m respectively). The modulus of elasticity was 9.87±1.24 GPa for cranial cortical bone and 1.10±0.53 GPa for sutures. The effective elastic modulus of tri-layer bone was 3.69±0.92 GPa. Accurate models of the pediatric skull should account for the differences amongst these three distinct tissues in the six year-old skull.

Evaluation of pediatric skull fracture imaging techniques

Radiologic imaging is crucial in the diagnosis of skull fracture, but there is some doubt as to whether different imaging modalities can accurately identify fractures present on a human skull. While studies have been performed to evaluate the efficacy of radiologic imaging at other anatomical locations, there have been no systematic studies comparing various CT techniques, including high resolution imaging with and without 3D reconstructions to conventional radiologic imaging in children, we investigated which imaging modalities: high-resolution CT scan with 3D projections, clinical-resolution CT scans or X-rays, best showed fracture occurrence in a pediatric human cadaver skull by having an expert pediatric radiologist examine radiologic images from fractured skulls. The skulls used were taken from pediatric cadavers ranging in age from 5 months to 16 years. We evaluated the sensitivity and specificity for the imaging modalities using dissection findings as the gold standard. We found that high-resolution CT scans with 3D projections and conventional CT provided the most accurate fracture diagnosis (single-fracture sensitivity of 71%) followed by X-rays (single-fracture sensitivity of 63%). Linear fractures outsider the region of the sutures were more identifiable than diastatic fractures, though the incidence of false positives was greater for linear fractures. In the two cases where multiple fractures were present on the same anatomical skull location, the radiologist was less likely to identify the presence of additional fractures than a single fracture. Overall, the high-resolution and clinical-resolution CT scans had the similar accuracy for detecting skull fractures while the use of the X-ray was both less accurate and had a lower specificity.

The compressive stiffness of human pediatric heads

Head injury is a persistent and costly problem for both children and adults. Globally, approximately 10 million people are hospitalized each year for head injuries. Knowing the structural properties of the head is important for modeling the response of the head in impact, and for providing insights into mechanisms of head injury. Hence, the goal of this study was to measure the sub-injurious structural stiffness of whole pediatric heads. 12 cadaveric pediatric (20-week-gestation to 16 years old) heads were tested in a battery of viscoelastic compression tests. The heads were compressed in both the lateral and anterior-posterior directions to 5% of gauge length at normalized deformation rates of 0.0005/s, 0.01/s, 0.1/s, and 0.3/s. Because of the non-linear nature of the response, linear regression models were used to calculate toe region (<2.5%) and elastic region (>2.5%) stiffness separately so that meaningful comparisons could be made across rate, age, and direction. The results showed that age was the dominant factor in predicting the structural stiffness of the human head. A large and statistically significant increase in the stiffness of both the toe region and the elastic region was observed with increasing age (p<0.0001), but no significant difference was seen across direction or normalized deformation rate. The stiffness of the elastic region increased from as low as 5 N/mm in the neonate to >4500 N/mm in the 16 year old. The changes in stiffness with age may be attributed to the disappearance of soft sutures and the thickening of skull bones with age.

Three-dimensional adult male head and skull contours

Objective: Traumatic brain injury (TBI) is a major public health issue, affecting millions of people annually. Anthropomorphic test devices (ATDs) and finite element models (FEMs) provide a means of understanding factors leading to TBI, potentially reducing the occurrence. Thus, there is a need to ensure that these tools accurately model humans. For example, the Hybrid III was not based on 3-dimensional human head shape data. The objective of this study is to produce average head and skull contours for an average U.S. male that can be used for ATDs and FEMs. Methods: Computed tomography (CT) scans of adult male heads were obtained from a database provided by the University of Virginia Center for Applied Biomechanics. An orthographic viewer was used to extract head and skull contours from the CT scans. Landmarks were measured graphically using HyperMesh (Altair, HyperWorks). To determine the head occipital condyle (OC) centroid, surface meshes of the OCs were made and the centroid of the surfaces was calculated. The Hybrid III contour was obtained using a MicroScribe Digitizer (Solution Technologies, Inc., Oella, MD). Comparisons of the average male and ATD contours were performed using 2 methods: (1) the midsagittal and midcoronal ATD contours relative to the OC centroid were compared to the corresponding 1 SD range of the average male contours; (2) the ATD sagittal contour was translated relative to the average male sagittal contour to minimize the area between the 2 contours. Results: Average male head and skull contours were created. Landmark measurements were made for the dorsum sellae, nasion skin, nasion bone, infraorbital foramen, and external auditory meatus, all relative to the OC centroid. The Hybrid III midsagittal contour was outside the 1 SD range for 15.2 percent of the average male head contour but only by a maximum distance of 1.5 mm, whereas the Hybrid III midcoronal head contour was outside the 1 SD range for 12.2 percent of the average male head contour by a maximum distance of 2 mm. Minimization of the area between the midsagittal contours resulted in only 2.3 mm of translation, corroborating the good correlation between the contours established by initial comparison. Conclusions: Three-dimensional average male head and skull contours were created and measurements of landmark locations were made. It was found that the 50th percentile male Hybrid III corresponds well to the average male head contour and validated its 3D shape. Average adult head and skull contours and landmark data are available for public research use at http://biomechanics.pratt.duke.edu/data.

The response of the adult and ATD heads to impacts onto a rigid surface

Given the high incidence of TBI, head injury has been studied extensively using both cadavers and anthropomorphic test devices (ATDs). However, few studies have benchmarked the response of ATD heads against human data. Hence, the objective of this study is to investigate the response of adult and ATD heads in impact, and to compare adult Hybrid III head responses to the adult head responses. In this study, six adult human heads and seven ATD heads were used to obtain impact properties. The heads were dropped from both 15cm and 30cm onto five impact locations: right and left parietal, forehead, occiput and vertex. One set of drops were performed on the human heads and up to four sets were carried out on the ATD heads. For each drop, the head was placed into a fine net and positioned to achieve the desired drop height and impact location. The head was then released to allow free fall without rotation onto a flat aluminum 34 -inch thick platen. The platen was attached to a three-axis piezoelectric load cell to measure the impact force. The peak resultant acceleration, head impact criterion (HIC) and impact stiffness were calculated using the force/time curve and drop mass. No statistical differences were found between the adult human heads and the adult Hybrid III head for 15cm and 30cm impacts (p>0.05). For the human heads, the mid-sagittal impact locations produced the highest HIC and peak acceleration values. The parietal impacts produced HICs and peak accelerations that were 26-48% lower than those from the mid-sagittal impacts. For the ATD heads, the acceleration and HIC values generally increased with represented age, except for the Q3, which produced HIC values up to higher than the other ATD heads. The impact responses of the adult Hybrid III onto different impact locations were found to adequately represent the impact stiffness of human adult head impacts from 30cm and below onto a rigid surface. The Q3 dummy consistently produced the highest HIC values of the ATD heads, and produced higher acceleration and HIC values than the adult human heads as well, which is contrary to neonatal data demonstrating that the head acceleration increases with age.