Pneumorrhachis following cervical spine surgery.

A 66-year-old man experienced symptomatic pneumorrhachis (air within the spinal canal) following a cervical laminoplasty for the excision of meningioma. Following an uneventful intraoperative course, he suffered a fluctuating hemiparesis of varying severity. Urgent imaging demonstrated extradural and intradural air in the spinal canal. Treatment with supplemental oxygen and dexamethasone was commenced, and the patient's symptoms improved over a period of three days with full resolution at six weeks. Pneumorrhachis can be avoided by allowing air to escape from the spinal canal through positioning, and displacement with irrigation fluid at the time of wound closure. However, if pneumorrhachis does occur, oxygen therapy, positioning of the patient to mitigate the gravitational effect of the air bubbles and supportive treatment are the central elements of management. Other possible causes of neurological deficit should be ruled out. This is particularly important as treatment options for some differential diagnoses can potentially cause harm if started based on clinical impression alone, for example, re-exploration for suspected haematoma. Only a small number of previous reports have described symptomatic pneumorrhachis as a complication of spinal surgery. This patient was successfully managed with conservative measures following the exclusion of other spinal cord pathologies.

Modelling dielectric elastomer circuit networks for soft biomimetics.

In order to obtain entirely soft bio-inspired robots, fully soft electronic circuits are needed. Dielectric elastomers (DEs) are electroactive polymers that have demonstrated multifunctionality. The same material can achieve different tasks like actuation, sensing, or energy harvesting. It has been shown that basic logic and memory functions can be realized with similar soft structures by combining multiple DE actuators and DE switches. Thus it would be possible to build, with the same materials and processes, a soft structure that mimics a biological being with all these capabilities. This contribution is focused on the modelling of the aforementioned soft electro-mechanical circuit networks. It is here reported the building process of a comprehensive SIMULINK model including the electro-mechanical behaviour of DE logic units and their interconnections. Conventional models deal with a single aspect of DEs, generating complex finite-element simulations. This contribution, based on a former model for an inverter-based DEO, shows how to integrate these various mathematical models and, with the help of direct measurements, create a software representation of DE circuit networks. This work is intended to demonstrate the validity of a recently introduced model and apply it to more complex circuit networks that have a higher number of components. Since, at the present state, the building processes are by hand, adding components generates more variability due to sample-to-sample variation and human error. Despite this, the model still shows a qualitatively good prediction of the devices' behaviour. Furthermore, the introduction of new materials and automatic processes will help to reduce this variability, allowing the model to reach even better performance.

Modeling of dielectric elastomer oscillators for soft biomimetic applications.

Biomimetic, entirely soft robots with animal-like behavior and integrated artificial nervous systems will open up totally new perspectives and applications. However, until now, most presented studies on soft robots were limited to only partly soft designs, since all solutions at least needed conventional, stiff electronics to sense, process signals and activate actuators. We present a novel approach for a set up and the experimental validation of an artificial pace maker that is able to drive basic robotic structures and act as artificial central pattern generator. The structure is based on multi-functional dielectric elastomers (DEs). DE actuators, DE switches and DE resistors are combined to create complex DE oscillators (DEOs). Supplied with only one external DC voltage, the DEO autonomously generates oscillating signals that can be used to clock a robotic structure, control the cyclic motion of artificial muscles in bionic robots or make a whole robotic structure move. We present the basic functionality, derive a mathematical model for predicting the generated signal waveform and verify the model experimentally.

An efficient and accurate prediction of the stability of percutaneous fixation of acetabular fractures with finite element simulation.

Posterior wall fracture is one of the most common fracture types of the acetabulum and a conventional approach is to perform open reduction and internal fixation with a plate and screws. Percutaneous screw fixations, on the other hand, have recently gained attention due to their benefits such as less exposure and minimization of blood loss. However their biomechanical stability, especially in terms interfragmentary movement, has not been investigated thoroughly. The aims of this study are twofold: (1) to measure the interfragmentary movements in the conventional open approach with plate fixations and the percutaneous screw fixations in the acetabular fractures and compare them; and (2) to develop and validate a fast and efficient way of predicting the interfragmentary movement in percutaneous fixation of posterior wall fractures of the acetabulum using a 3D finite element (FE) model of the pelvis. Our results indicate that in single fragment fractures of the posterior wall of the acetabulum, plate fixations give superior stability to screw fixations. However screw fixations also give reasonable stability as the average gap between fragment and the bone remained less than 1 mm when the maximum load was applied. Our finite element model predicted the stability of screw fixation with good accuracy. Moreover, when the screw positions were optimized, the stability predicted by our FE model was comparable to the stability obtained by plate fixations. Our study has shown that FE modeling can be useful in examining biomechanical stability of osteosynthesis and can potentially be used in surgical planning of osteosynthesis.

Subject-specific modelling of lower limb muscles in children with cerebral palsy.

BACKGROUND: Recent studies suggest that the architecture of spastic muscles in children with cerebral palsy is considerably altered; however, only little is known about the structural changes that occur other than in the gastrocnemius muscle. In the present study, Magnetic Resonance Imaging (MRI) and subject-specific modelling techniques were used to compare the lengths and volumes of six lower limb muscles between children with cerebral palsy and typically developing children. METHODS: MRI scans of the lower limbs of two children with spastic hemiplegia cerebral palsy, four children with spastic diplegia cerebral palsy (mean age 9.6 years) and a group of typically developing children (mean age 10.2 years) were acquired. Subject-specific models of six lower limb muscles were developed from the MRI data using a technique called Face Fitting. Muscle volumes and muscle lengths were derived from the models and normalised to body mass and segmental lengths, respectively. FINDINGS: Normalised muscle volumes in the children with cerebral palsy were smaller than in the control group with the difference being 22% in the calf muscles, 26% in the hamstrings and 22% in the quadriceps, respectively. Only the differences in the hamstrings and the quadriceps were statistically significant (P=0.036, P=0.038). Normalised muscle lengths in the children with cerebral palsy were significantly shorter (P<0.05), except for soleus and biceps femoris. No significant relationship was found between normalised lengths and volumes of any muscle in either group. INTERPRETATION: The present results show that lower limb muscles in ambulatory children with cerebral palsy are significantly altered, suggesting an overall mechanical deficit due to predominant muscle atrophy. Further investigations of the underlying causes of the muscle atrophy are required to better define management and treatment strategies for children with cerebral palsy.

Accuracy of a computer-assisted navigation system in resurfacing hip arthroplasty.

We developed a method to assess the accuracy of an image-free resurfacing hip arthroplasty navigation system in a proximal femur with normal and abnormal anatomy. A phantom lower limb allowed deformation in varus/valgus and ante-retroversion. At specific points during the simulated surgical procedure, information was compared between a digital caliper and the computer navigation system angular measurements. Repeated serial tests were undertaken. In the setting of normal anatomical alignment of the proximal femur, the mean error of the system characterised as the difference between the measured computer navigation and caliper angles was 0.6 degrees in the frontal plane and 3.4 degrees in the lateral plane. In the setting of abnormal anatomical alignment, the mean error was 0.4 degrees in the frontal plane and 2.1 degrees in the lateral plane. This is the first study designed to assess the accuracy of a femoral navigation system for resurfacing hip arthroplasty in normal settings and in the presence of angular deformity. The study demonstrates satisfactory in-vitro accuracy.

Error propagation from kinematic data to modeled muscle-tendon lengths during walking.

Kinematic data from 3D gait analysis together with musculoskeletal modeling techniques allow the derivation of muscle-tendon lengths during walking. However, kinematic data are subject to soft tissue artifacts (STA), referring to skin marker displacements during movement. STA are known to significantly affect the computation of joint kinematics, and would therefore also have an effect on muscle-tendon lengths which are derived from the segmental positions. The present study aimed to introduce an analytical approach to calculate the error propagation from STA to modeled muscle-tendon lengths. Skin marker coordinates were assigned uncorrelated, isotropic error functions with given standard deviations accounting for STA. Two different musculoskeletal models were specified; one with the joints moving freely in all directions, and one with the joints constrained to rotation but no translation. Using reference kinematic data from two healthy boys (mean age 9y 5m), the propagation of STA to muscle-tendon lengths was quantified for semimembranosus, gastrocnemius and soleus. The resulting average SD ranged from 6% to 50% of the normalized muscle-tendon lengths during gait depending on the muscle, the STA magnitudes and the musculoskeletal model. These results highlight the potential impact STA has on the biomechanical analysis of modeled muscle-tendon lengths during walking, and suggest the need for caution in the clinical interpretation of muscle-tendon lengths derived from joint kinematics.

Evaluation of accuracy of an electromagnetic computer-assisted navigation system in total knee arthroplasty.

The objective of this study was to develop a method to assess the accuracy of an electromagnetic technology image-free navigation system for total knee arthroplasty in a leg with normal or abnormal mechanical alignment. An acrylic phantom leg was constructed to simulate tibia and femur deformation. Determination of actual leg alignment was achieved using a digital caliper unit. In the setting of normal alignment, the mean error of the system characterised as the difference between the measured computer navigation and digital caliper angles ranged between 0.8 degrees (frontal plane) and 1.5 degrees (lateral plane). In the setting of abnormal alignment, the mean error ranged between 0.4 degrees (frontal plane) and 1.6 degrees (lateral plane). Deformity had no demonstrable effect on accuracy. The study demonstrates satisfactory in vitro system accuracy in both normal and abnormal leg mechanical alignment settings.

Virtual reality assessment of technical skill using the Bonedoc DHS simulator.

The Bonedoc DHS simulator is a virtual reality simulator of screw and plate fixation of hip fractures which runs on a standard PC. We hypothesised that the simulator would be able to discriminate between subjects with different levels of operative experience. Three groups (medical students (MSs), basic trainees (BTs), and advanced trainees (ATs)) performed six virtual operations. Measurements included: reduction position, incision length, misplaced drill-holes, final screw placement, X-rays taken, surgical time as well as computer and operative experience. The accuracy, number of X-rays and speed were significantly different between novices and trainee surgeons (p<0.01, p<0.05, p<0.05). Intra-articular screw penetration by the medical students occurred 12 times, basic trainees 6 times and advanced trainees twice (p<0.01, MS vs. trainees). Amongst trainees, the advanced trainees placed the lag screw more accurately and took less X-rays (ns). The basic trainees performed the complete procedure fastest at 6 min compared to ATs at 9 min (p<0.05) but were not as accurate. The Bonedoc DHS simulator provides a means to discriminate between novices and trainee surgeons.

A simulation-based training system for hip fracture fixation for use within the hospital environment.

We report development of a PC-based virtual reality training system for hip fracture fixation that comprises a surgical simulator and an assessment component. The simulator allows hip fracture fixation to be performed on a virtual hip model using two-dimensional radiographic images to guide fracture reduction and implant placement. Ten operative scenarios with increasing complexities of fracture type are available. The face validity of the simulator was tested using a 26 item feedback questionnaire, with answers on a 5 cm visual analogue scale from 'disagree strongly' to 'agree strongly'. Ten study participants, aged 20-50, and with variable levels of surgical skills, each performed six operative scenarios on the simulator before completing the questionnaire. The results showed that the simulator had good face validity, with the majority of subjects stating it provided a realistic view of the operating environment (median score 8.2/10) and that the three-dimensional view provided was all that was required (median score 7.8/10). The subjects considered the simulator was able to test problem solving ability (median score 8.0/10). These results confirm that this simulator achieves good face validity without computationally intensive touch feedback (haptics). Overall, this study demonstrates that non-haptic simulators have a larger role to play in virtual simulation than is currently recognised.

Accuracy of a computer-assisted navigation system for total knee replacement.

The object of this study was to develop a method to assess the accuracy of an image-free total knee replacement navigation system in legs with normal or abnormal mechanical axes. A phantom leg was constructed with simulated hip and knee joints and provided a means to locate the centre of the ankle joint. Additional joints located at the midshaft of the tibia and femur allowed deformation in the flexion/extension, varus/valgus and rotational planes. Using a digital caliper unit to measure the coordinates precisely, a software program was developed to convert these local coordinates into a determination of actual leg alignment. At specific points in the procedure, information was compared between the digital caliper measurements and the image-free navigation system. Repeated serial measurements were undertaken. In the setting of normal alignment the mean error of the system was within 0.5 degrees . In the setting of abnormal plane alignment in both the femur and the tibia, the error was within 1 degrees . This is the first study designed to assess the accuracy of a clinically-validated navigation system. It demonstrates in vitro accuracy of the image-free navigation system in both normal and abnormal leg alignment settings.

Anatomically based modelling of the human skull and jaw.

We present here an anatomically based model of the human masticatory system that provides a framework for simulating the complex chewing process. The initial motivation for creating this model was the desire to have a computational model of the human jaw that can be used to simulate the action of simple bites, and to calculate the stresses and forces on the teeth that are involved. The model created also provides a platform that can be used to investigate other features of the masticatory system. To construct this global model, individual models of the bones of the skull and jaw were created from generic data sets. Geometric models of the muscles of mastication were also created and attached to the appropriate bones. To complete this initial model, representations of the crowns of the teeth were created and a basic model of the temporomandibular joint (TMJ) was included. The finite element method was used to solve for the stresses and strains created by the loading conditions during a clenching simulation involving the mandible bone. The model presented here is also discussed in relation to a model of the entire musculo-skeletal system being developed as part of the Physiome Project.

A cerebral palsy assessment tool using anatomically based geometries and free-form deformation.

A geometrical analysis tool for investigating muscle length change in cerebral palsy (CP) patients is presented. A subset of anatomically based geometries from the International Union of Physiological Sciences (IUPS) Physiome Project is used, which is derived from the visible human (VH) data set with muscle attachment information, and customised using volume-preserving free-form deformation (FFD), the 'host-mesh' technique. The model's intended use is to provide pre- and post-surgery assessment for muscle lengthening, a surgery performed to help slacken tight muscles and improve gait. The model is illustrated using healthy patient data from motion capture as a validation followed by three CP case studies to highlight its use. The methodology is presented in three stages, (1) a FFD of the complete lower limb, (2) a focused geometric study on the semimembranosus (SM) and gastrocnemius (GT) muscles, and (3) an improved hybrid mechanics-FFD approach as an improvement for future analysis, with differentiation between muscle and tendon lengthening, and contact detection between sliding muscles. Finally, the issues, limitations, in particular with the marker system, and model improvements are discussed.

Stress analysis of hemispherical ceramic hip prosthesis bearings.

We present design criteria and test data for ceramic hemispherical 'resurfacing' prosthesis bearings that are attached by placement on the conical trunnion of the prosthesis stem. These large diameter bearings will be subjected to higher torques than conventional 28 mm bearings. Prototypes were fabricated and tested. The pattern of failure and subsequent analysis suggests that fracture initiated on the surface of the bore and that this was preceded by the development of large circumferential hoop stresses. Widening the bore will improve stability under torque loads but this may be at the expense of bearing strength. A finite element analysis has shown that the optimum taper bore to bearing diameter ratio is 0.43 and that designs with large taper bore to bearing diameter ratios (over 0.6) should be avoided due to the development of excessively high hoop stresses on the inner bore surface. High meridional tensile stresses can appear where the internal roof surface joins the conical bore surface as the load is removed. A smooth radius between the conical bore and the roof will help to overcome this. Further improvements include a thinner roof that is smoothly arched internally. Thus, the design of the component can be optimised, making it less susceptible to fracture without compromising the stability of the ceramic head on its trunnion.

How do changes to plate thickness, length, and face-connectivity affect femoral cancellous bone's density and surface area? An investigation using regular cellular models.

Models of regular cellular-solids representing femoral head 'medial group' bone were used to (1) compare thickness data for plate-like and beam-like structures at realistic surface areas and densities; (2) test the validity of a standard formula for trabecular thickness (Tb.Th); and (3) study how systematic changes in cancellous bone thicknesses, spacing, and face-connectivity affect relative density and surface area. Models of different face-connectivities, produced by plate removal from the unit cell, were fitted to bone density and surface area data. The medial group bone was anisotropic: the supero-inferior (SI) direction was the principal direction for bone plate alignment and the plane normal to this had the largest number of bone/void intersections per unit line length (P(I)). A comparison of boundary perimeter per unit area data, in planes normal to SI, with surface area data placed the medial group bone between prismatic structures in which walls are parallel to one principal direction and isotropic structures. Selective removal of plates from a closed-cell model produced a similar result. For the same relative density and surface-area, plate-like models had significantly thinner cross-sections than beam-like models. The formula for Tb.Th produced overestimates of model plate thickness by up to 20% at realistic femoral cancellous densities. Trends in data on surface area to volume ratio and density observed on sampled medial group bone could be simulated by plate thickness changes on models of intermediate face-connectivity (approximately 1.5) or by plate removal from models with relatively thick and short (low aspect-ratio) plates. The latter mechanism is unrealistic for it resulted in beam-like structures at low 'medial group' densities, an architecture unlike the predominantly plate-like bone in the sample.

The processing and characterization of animal-derived bone to yield materials with biomedical applications. Part III: material and mechanical properties of fresh and processed bovine cancellous bone.

Conversion of bovine cancellous bone to a useful biomedical xenograft material involves several processing steps which include boiling, defatting and deproteination (i.e. bleaching). This study has shown how these processes can influence cancellous bone modulus and strength. It was found that prolonged boiling in water for six hours followed by NaOCl bleaching had a deleterious effect on the overall strength of the bovine bone. In contrast, bone samples subjected to only moderate boiling (1.5 hours) exhibited a 22% stiffness increase due mainly to the effects of drying. The same stiffened samples, when subjected to the bleaching procedure, retained some strength with only a small reduction in moduli values. It can be concluded that careful control of defatting and bleaching procedures on bovine bone is able to give a strong, albeit, brittle material with preservation of the original bone architecture. The bone xenograft materials are worthy of further investigation in in vivo clinical trials to assess their performance in contact with biological fluids.

Haematology and histopathology of seven-week-old broilers after early food restriction.

The haematology and histopathology of seven-week-old broilers were examined after periods of early food restriction, for six, 10 or 14 days from six days old. After several weeks on an ad libitum diet the birds failed to compensate for the weight lost during early food restriction. Immediately after the periods of food restriction, the birds demonstrated significantly increased heterophil/lymphocyte ratios, reduced eosinophils and slightly raised basophil counts. At seven weeks old, a significant reduction was seen in red and white blood cells and thrombocyte numbers together with significant increases in mean cell haemoglobin and mean cell volume. The haematological profile demonstrated a macrocytic normochromic anaemia caused possibly by a folic acid deficiency as a result of the food restriction. Histopathological lesions were seen in the heart, lungs and liver from birds on all diets but there were more lesions the longer the food had been restricted. Lung disease was more marked where there was inadequate ventilation. Cartilaginous and osseous lung nodules were significantly fewer after food restriction. It was postulated that the increase in pathological lesions in the food-restricted birds may be associated with a stress response.

Ectopic cartilaginous and osseous lung nodules induced in the young broiler by inadequate ventilation.

The study attempted to induce cartilaginous and osseous nodules in the lungs of young broilers by providing them with inadequate ventilation. In experimental birds there were more nodules than in two groups of control birds, with significantly more occurring in the left lung than in the right. A majority of the nodules were fibrous. Histologically, the lungs of the experimental birds had lesions similar to those seen in an ascites syndrome or with induced hypoxia. In another experiment, when the lungs of healthy adult broiler breeders were examined, more osseous-type nodules were present. Very few nodules were seen in the lungs of wild birds. It was suggested that poor ventilation, similar to that likely to be experienced in some bird accommodation, may increase the development of cartilaginous and osseous nodules in the lungs of broiler chickens.

The incidence of ectopic cartilaginous and osseous lung nodules in young broiler fowls with ascites and various other diseases.

The incidence of ectopic cartilaginous and osseous lung nodules in young broiler fowls with ascites and various other diseases has been determined. In all groups of diseased birds there were more nodules in the lungs compared with control birds. The birds that died with an ascitic syndrome had a significantly greater number of nodules than the other groups of diseased birds, with significantly more occurring in the left lung than in the right (P<0.001). Histologically, the fibrous cartilaginous type of nodule was the most numerous and the osseous type the least numerous. This is the first detailed study in fowls in which a relationship has been demonstrated between the presence of cartilaginous and osseous lung nodules and various disease states.

The gastrocnemius tendon of domestic fowl: histological findings in different strains.

Vascular and histological studies were performed on normal gastrocnemius tendons from male birds considered susceptible (broiler breeding) and insusceptible (laying strain) to noninfectious tendon rupture. Structural differences between strains were noted at the site where rupture usually occurs. This site contains a hypovascular region which develops at an earlier age in broiler type fowls. Histological sections revealed that vessel occlusion had occurred in birds with hypovascular tendon segments. Chondrocytes and chondroid cells adjacent to occluded vessels accumulated lipid and died. This finding was particularly apparent in broiler fowls where cartilaginous plaques were larger and thicker than in laying strain fowls. The presence of occluded vessels, cartilage death and chondroid infiltration of collagen bundles in broiler type fowls are interpreted as structural changes which may predispose to noninfectious tendon rupture. Indeed they may also predispose to tendon lesions associated with pathogens.