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.

Numerical simulation of the insertion process of an uncemented hip prosthesis in order to evaluate the influence of residual stress and contact distribution on the stem initial stability.

The long-term success of a cementless total hip arthroplasty depends on the implant geometry and interface bonding characteristics (fit, coating and ingrowth) and on stem stiffness. This study evaluates the influence of stem geometry and fitting conditions on the evolution and distribution of the bone-stem contact, stress and strain during and after the hip stem insertion, by means of dynamic finite element techniques. Next, the influence of the mechanical state (bone-stem contact, stress and strain) resulted from the insertion process on the stem initial resistance to subsidence is investigated. In addition, a study on the influence of bone-stem interface conditions (friction) on the insertion process and on the initial stem stability under physiological loading is performed. The results indicate that for a stem with tapered shape the contact in the proximal part of the stem was improved, but contact in the calcar region was achieved only when extra press-fit conditions were considered. Changes in stem geometry towards a more tapered shape and extra press fit and variation in the bone-stem interface conditions (contact amount and high friction) led to a raise in the total insertion force. A direct positive relationship was found between the stem resistance to subsidence and stem geometry (tapering and press fit), bone-stem interface conditions (bone-stem contact and friction interface) and the mechanical status at the end of the insertion (residual stress and strain). Therefore, further studies on evaluating the initial performance of different stem types should consider the parameters describing the bone-stem interface conditions and the mechanical state resulted from the insertion process.

Assessment of relative brain-skull motion in quasistatic circumstances by magnetic resonance imaging.

Brain-skull relative motion plays a pivotal role in the etiology of traumatic brain injury (TBI). The present study aims to assess brain-skull relative motion in quasistatic circumstances, and to correlate cortical regions with high motion amplitudes with sites prone to cerebral contusions. The study includes 30 healthy volunteers scanned using a clinical 3-T MR scanner in four different head positions. Through image processing and 3D model registration, pairwise comparisons were performed to calculate the brain shift between sagittal and coronal head positional change. Next, local brain deformation was evaluated by comparison between cortical and ventricular amplitudes. Finally, the influence of age, sex, and skull geometry on the cortical and ventricular motion was investigated. The results describe complex brain shift patterns, with high regional and inter-individual variations, outweighing age and sex patterns. Regions with maximum motion amplitudes were identified at the inferolateral aspects of the frontal and temporal lobes, congruent with predilection sites for contusions. No significant influences of age and sex on the cortical shift amplitudes were detected. The 3D cortical deviations varied from -7.86 mm to +5.71 mm for the sagittal head movement, and from -11.46 mm to +7.30 mm for head movement in the coronal plane, for a 95% confidence interval. The present study contributes to a better understanding of the mechanopathogenesis of frontotemporal contusions, and is useful for the optimization of finite-element head models and neurosurgical navigation procedures. Moreover, our results prove that in vivo MRI allows for accurate assessment of brain-skull relative motion in quasistatic conditions.

Fast and accurate specimen-specific simulation of trabecular bone elastic modulus using novel beam-shell finite element models.

Elastic modulus and strength of trabecular bone are negatively affected by osteoporosis and other metabolic bone diseases. Micro-computed tomography-based beam models have been presented as a fast and accurate way to determine bone competence. However, these models are not accurate for trabecular bone specimens with a high number of plate-like trabeculae. Therefore, the aim of this study was to improve this promising methodology by representing plate-like trabeculae in a way that better reflects their mechanical behavior. Using an optimized skeletonization and meshing algorithm, voxel-based models of trabecular bone samples were simplified into a complex structure of rods and plates. Rod-like and plate-like trabeculae were modeled as beam and shell elements, respectively, using local histomorphometric characteristics. To validate our model, apparent elastic modulus was determined from simulated uniaxial elastic compression of 257 cubic samples of trabecular bone (4mm×4mm×4mm; 30µm voxel size; BIOMED I project) in three orthogonal directions using the beam-shell models and using large-scale voxel models that served as the gold standard. Excellent agreement (R(2)=0.97) was found between the two, with an average CPU-time reduction factor of 49 for the beam-shell models. In contrast to earlier skeleton-based beam models, the novel beam-shell models predicted elastic modulus values equally well for structures from different skeletal sites. It allows performing detailed parametric analyses that cover the entire spectrum of trabecular bone microstructures.

Successful preliminary walking experiments on a transtibial amputee fitted with a powered prosthesis.

This paper presents the results of preliminary walking experiments on a transtibial amputee wearing a powered prosthesis. The prosthesis prototype serves as a proof-of-concept implementation for investigating the potential of pleated pneumatic artificial muscles to power a transtibial prosthesis. The device is equipped with pleated pneumatic artificial muscles, and tethered to a laboratory pressure source. The prosthesis is capable of providing the amputee with 100% of the required push-off torque and it can adapt its joint stiffness to the walking speed. This study supports the hypothesis that a powered transtibial prosthesis with adaptable stiffness might be beneficial to the amputee.

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.

A fast convolution-based methodology to simulate 2-D/3-D cardiac ultrasound images.

This paper describes a fast convolution-based methodology for simulating ultrasound images in a 2-D/3-D sector format as typically used in cardiac ultrasound. The conventional convolution model is based on the assumption of a space-invariant point spread function (PSF) and typically results in linear images. These characteristics are not representative for cardiac data sets. The spatial impulse response method (IRM) has excellent accuracy in the linear domain; however, calculation time can become an issue when scatterer numbers become significant and when 3-D volumetric data sets need to be computed. As a solution to these problems, the current manuscript proposes a new convolution-based methodology in which the data sets are produced by reducing the conventional 2-D/3-D convolution model to multiple 1-D convolutions (one for each image line). As an example, simulated 2-D/3-D phantom images are presented along with their gray scale histogram statistics. In addition, the computation time is recorded and contrasted to a commonly used implementation of IRM (Field II). It is shown that COLE can produce anatomically plausible images with local Rayleigh statistics but at improved calculation time (1200 times faster than the reference method).

Relation between subject-specific hip joint loading, stress distribution in the proximal femur and bone mineral density changes after total hip replacement.

In the prediction of bone remodelling processes after total hip replacement (THR), modelling of the subject-specific geometry is now state-of-the-art. In this study, we demonstrate that inclusion of subject-specific loading conditions drastically influences the calculated stress distribution, and hence influences the correlation between calculated stress distributions and changes in bone mineral density (BMD) after THR. For two patients who received cementless THR, personalized finite element (FE) models of the proximal femur were generated representing the pre- and post-operative geometry. FE analyses were performed by imposing subject-specific three-dimensional hip joint contact forces as well as muscle forces calculated based on gait analysis data. Average values of the von Mises stress were calculated for relevant zones of the proximal femur. Subsequently, the load cases were interchanged and the effect on the stress distribution was evaluated. Finally, the subject-specific stress distribution was correlated to the changes in BMD at 3 and 6 months after THR. We found subject-specific differences in the stress distribution induced by specific loading conditions, as interchanging of the loading also interchanged the patterns of the stress distribution. The correlation between the calculated stress distribution and the changes in BMD were affected by the two-dimensional nature of the BMD measurement. Our results confirm the hypothesis that inclusion of subject-specific hip contact forces and muscle forces drastically influences the stress distribution in the proximal femur. In addition to patient-specific geometry, inclusion of patient-specific loading is, therefore, essential to obtain accurate input for the analysis of stress distribution after THR.

Assessment of the primary rotational stability of uncemented hip stems using an analytical model: comparison with finite element analyses.

BACKGROUND: Sufficient primary stability is a prerequisite for the clinical success of cementless implants. Therefore, it is important to have an estimation of the primary stability that can be achieved with new stem designs in a pre-clinical trial. Fast assessment of the primary stability is also useful in the preoperative planning of total hip replacements, and to an even larger extent in intraoperatively custom-made prosthesis systems, which result in a wide variety of stem geometries. METHODS: An analytical model is proposed to numerically predict the relative primary stability of cementless hip stems. This analytical approach is based upon the principle of virtual work and a straightforward mechanical model. For five custom-made implant designs, the resistance against axial rotation was assessed through the analytical model as well as through finite element modelling (FEM). RESULTS: The analytical approach can be considered as a first attempt to theoretically evaluate the primary stability of hip stems without using FEM, which makes it fast and inexpensive compared to other methods. A reasonable agreement was found in the stability ranking of the stems obtained with both methods. However, due to the simplifying assumptions underlying the analytical model it predicts very rigid stability behaviour: estimated stem rotation was two to three orders of magnitude smaller, compared with the FEM results. CONCLUSION: Based on the results of this study, the analytical model might be useful as a comparative tool for the assessment of the primary stability of cementless hip stems.

The correlation between the SOS in trabecular bone and stiffness and density studied by finite-element analysis.

For the clinical assessment of osteoporosis (i.e., a degenerative bone disease associated with increased fracture risk), ultrasound has been proposed as an alternative or supplement to the dual-energy X-ray absorptiometry (DEXA) technique. However, the interaction of ultrasound waves with (trabecular) bone remains relatively poorly understood. The present study aimed to improve this understanding by simulating ultrasound wave propagation in 15 trabecular bone samples from the human lumbar spine, using microcomputed tomography-based finite-element modeling. The model included only the solid bone, without the bone marrow. Two structural parameters were calculated: the bone volume fraction (BV/TV) and the structural (apparent) elastic modulus (E(s)), and the ultrasound propagation parameter speed of sound (SOS). Relations between BV/TV and E(s) were similar to published experimental relations. At 1 MHz, correlations between SOS and the structural parameters BV/TV and Es were rather weak, but the results can be explained from the specific features of the trabecular structure and the intrinsic material elastic modulus E(i). In particular, the systematic differences between the three main directions provide information on the trabecular structure. In addition, at 1 MHz the correlation found between the simulated SOS values and those calculated from the simple bar equation was poor when the three directions are considered separately. Hence, under these conditions, the homogenization approach-including the bar equation-is not valid. However, at lower frequencies (50-300 kHz) this correlation significantly improved. It is concluded that detailed analysis of ultrasound wave propagation through the solid structure in various directions and with various frequencies, can yield much information on the structural and mechanical properties of trabecular bone.

Spatial differences in sensible and latent heat losses under a bicycle helmet.

This research aims at quantifying spatial gradients in skin temperature and sweat production under a bicycle helmet. Distribution of sweat production, skin temperature and air temperature was measured at different positions under a bicycle helmet on five male and four female test persons. Effort level was 100 and 150 watt for men (low and high effort level) and 80 and 120 W for women (low and high effort level). Skin temperatures were found to be spatially different (P < 0.05): frontal and lateral region varied 4.6 degrees C at low effort level and 5.3 degrees C at high effort level. Sweat production was found to be not significantly different (P > 0.05). Finally, air temperature variations were found to be spatially different (P < 0.05). Average air temperature differed 2.3 degrees C between lateral and frontal region at high effort level and 2.7 degrees C at low effort level. The results of this research can be used to help designing helmets with better thermal comfort.

Per-operative vibration analysis: a valuable tool for defining correct stem insertion: preliminary report.

BACKGROUND: Defining the stem insertion end point during total hip replacement still relies on the surgeon's feeling. When a custom-made stem prosthesis with an optimal fit into the femoral canal is used, the risk of per-operative fractures is even greater than with standard prostheses. Vibration analysis is used in other clinical settings and has been tested as a means to detect optimal stem insertion in the laboratory. The first per-operative use of vibration analysis during non-cemented custom-made stem insertion in 30 patients is reported here. MATERIALS AND METHODS: Thirty patients eligible for total hip replacement with uncemented stem prosthesis were included. The neck of the stem was connected with a shaker that emitted white noise as excitation signal and an impedance head that measured the frequency response. The response signal was sent to a computer that analyzed the frequency response function after each insertion phase. A technician present in the operating theatre but outside the laminated airflow provided feed-back to the surgeon. RESULTS: The correlation index between the frequency response function measured during the last two insertion hammering sessions was >0.99 in 86.7% of the cases. In four cases the surgeon stopped the insertion procedure because of a perceived risk of fracture. Two special cases illustrating the potential benefit of per-operative vibration analysis are described. CONCLUSIONS: The results of intra-operative vibration analysis indicate that this technique may be a useful tool assisting the orthopaedic surgeon in defining the insertion endpoint of the stem. The development of a more user-friendly device is therefore warranted.

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.

Lateral head impacts and protection of the temporal area by bicycle safety helmets.

BACKGROUND: The protective effectiveness of bicycle helmets has been demonstrated in several epidemiologic studies. However, the temple region is only minimally covered by most helmet models. Impact tests were performed on human cadavers to investigate whether current bicycle helmets are capable of preventing direct contact on the temporal area in side impacts. METHODS: Lateral head impacts, corresponding to a force load of 15,000 N on an nonhelmeted head, were applied on 11 helmeted cadavers by a steel pendulum with a flat impact surface, and the contact between the impactor plate and the temporal and zygomatic area was investigated by means of paint transfer. In eight tests, a common design bicycle helmet was used, whereas in three tests the helmets provided larger temporal coverage (temporal helmet edge <10 mm above Frankfort plane). The skulls were inspected for fractures. RESULTS: In seven of the eight tests with common design bicycle helmets, contact had occurred and in one of these a skull fracture was seen. The helmets with a larger temporal coverage consistently prevented such contact loading. CONCLUSIONS: The common designs of commercially available bicycle helmets do not prevent direct contact loading on the temporal and zygomatic arch region and this contact loading is potentially harmful. The present preliminary study strongly questions the effectiveness of these helmets in providing accurate protection of the temporal and zygomatic area.

Mechanics of acute subdural hematomas resulting from bridging vein rupture.

OBJECT: Based on data from primate experiments it is known that rotational acceleration in the sagittal plane and in a forward direction is most likely to produce acute subdural hematomas due to bridging vein rupture. For protection against these lesions, knowledge of rotational acceleration tolerance levels in humans is required. In the present study the authors analyze human tolerance levels for bridging vein rupture by performing head impact tests in cadavers. METHODS: Ten unembalmed cadavers were subjected to 18 occipital impacts producing head rotation in the sagittal plane with varying rotational acceleration magnitudes and pulse durations. Rotational acceleration was calculated from the linear acceleration histories recorded by three uniaxial accelerometers mounted on the side of the head. Bridging vein ruptures were detected by injecting contrast dye into the superior sagittal sinus under fluoroscopy and by autopsy procedures. Bridging vein ruptures were produced in six head impact tests: one test with a pulse duration of 5.2 msec and a peak rotational acceleration of 13,411 rad/second2; three tests with a pulse duration between 7 and 8 msec and a peak rotational acceleration of 12,558, 10,607, and 8567 rad/second2; and two tests with a pulse duration longer than 10 msec and a peak rotational acceleration as low as 5267 rad/second2. CONCLUSIONS: This is the only cadaveric study of bridging vein rupture focused on short pulse durations, which are usually associated with falls. The data suggest a tolerance level of approximately 10,000 rad/second2 for pulse durations shorter than 10 msec, which seems to decrease for longer pulse durations.

Biomechanical properties of the superior sagittal sinus-bridging vein complex.

Finite element models (FEM) of the head are frequently used to simulate traumatic brain injury, leading to a better understanding of brain injury tolerance. The strength of a FEM of the head is dependent on the use of correct material characteristics, experimentally derived for each intracranial tissue, including parasagittal bridging veins (BV). These veins are prone to rupture in their subdural portion upon head impact, giving rise to an acute subdural hematoma (ASDH). The junction of these veins to the superior sagittal sinus (SSS) has been described as an area with distinct vein wall architecture. To understand the biomechanical characteristics of acute subdural hematoma, we studied the SSS-BV complex by loading it to failure in a tensile test. 37 BVs from 9 fresh cadavers were dissected, leaving small strips of SSS attached to the veins. The units were clamped on the SSS and the cortical end of the BV. Strain rates ranged from 0.1-3.8 s(-1). From force-time and strain-time histories, we calculated ultimate strain (epsilon(U)), ultimate stress (sigma(U)), yield strain (epsilon(Y)), yield stress(sigma(Y)) and Young's modulus (E). A mixed-model multivariate analysis of variance (MANOVA) was used to study correlations and strain rate sensitivity of these parameters. We found no strain rate sensitivity. The biomechanical response of the SSS-BV unit in this study was found to be stiffer than reported biomechanical behavior of bridging veins. We conclude that the SSS-BV junction plays an important role in bridging vein rupture, and warrants further investigation to provide FEM with correct material properties for bridging veins.

Bicycle-related head injury: a study of 86 cases.

Within the framework of a bicycle helmet research program, we have set up a database of bicycle accident victims, containing both accident and clinical data. The database consists of a consecutive series of 86 victims of bicycle accidents who underwent a neurosurgical intervention in our hospital between 1990 and 2000. Data were obtained from police files, medical records, computed tomography head scans and a patient questionnaire. In only three victims, the wearing of a helmet was documented. In this study, the head injuries are analysed and the relation between the different types of head injuries and outcome is assessed. Forty-four accidents were collisions with a motor vehicle and 42 accidents were falls. Most impacts occurred at the side (57%) or at the front (27%) of the head. The most frequent injuries were skull fractures (86%) and cerebral contusions (73%). Age was negatively correlated with outcome (P = 0.0002 ) and positively correlated with the number (P = 0.00002) and volume (P = 0.00005) of contusions and the presence of subdural haematomas (P = 0.000001). The injuries with the strongest negative effect on outcome were: subarachnoid haemorrhage (P = 0.000001), multiple (P = 0.000005) or large ( P 0.0007) contusions, subdural haematoma (P = 0.001) and brain swelling (P = 0.002). A significant coexistence of these four injuries was found. We hypothesise that in many patients the contusions may have been the primary injuries of this complex and should therefore be considered as a main injury determining outcome in this study. We believe that such findings may support a rational approach to optimising pedal cyclist head protection.

Peri-implant bone tissue strains in cases of dehiscence: a finite element study.

When patients with a narrow alveolar bone ridge are treated with oral implants, a dehiscence can occur. The lack of bony support at the buccal or lingual side of the implant may present an unfavourable situation from a biomechanical point of view. The hypothesis as to whether the presence of dehiscence leads to an increased risk of marginal bone overload was tested by means of the finite element method. Three different situations for a cylindrical oral implant, which was placed in a mandible, were modelled: i) no dehiscence, ii) a dehiscence at the buccal side and iii) dehiscences at the buccal and lingual sides. It was found that the presence of buccal and/or lingual dehiscences led to a marked increase in marginal bone strains at the mesial and distal sides of the implant, thus increasing the risk of bone tissue overload at these locations. Marginal bone strains at the buccal and/or lingual sides, however, did not increase.

How correctly does an intramedullary rod represent the longitudinal tibial axes?

In a robot-assisted procedure for preparing the tibia in total knee arthroplasty, developed in the authors' laboratory, an intramedullary rod is used to register the tibia. In 18 formalin-fixed tibias, the difference in orientation was calculated between the intramedullary rod and several longitudinal tibial axes used in clinical practice. This was done using roentgenstereophotogrammetric analysis. Three tibial axes and two insertion techniques were considered. In three-dimensional space, small differences between the axes are observed. The results showed a high standard deviation, indicating the importance of anatomic differences. In the frontal plane, the difference in orientation between rod and tibial axes never exceeded +/- 2 degrees. In the sagittal plane, the observed differences were larger. Significant differences between the considered axes appeared. The results of the two insertion techniques were not significantly different. Because an intramedullary rod frequently is used for alignment of the tibia in conventional surgery, these results also are valuable for conventional surgery. In the current study, the accuracy of the intramedullary alignment is examined, without influences of the sawing procedure. Moreover, the study is not limited to the frontal plane; the total accuracy in three-dimensional space, and the accuracy in the frontal and the sagittal planes were studied.