bluuebunny/crossref_metadata_2025 huggingface dataset:
An anti-wind-up controller design for rocket control systems with control input saturation
Authors: Kim J , Choi H D , Lee J H , Lee D S
Published: October 2002

An anti-wind-up controller for the attitude control of a two-stage sounding rocket has been designed. Once the actuator comes up to saturation limits, a feedback signal is generated in this controller from the difference between the compensator output and the actuator control input, and is used to make the actuator work in linear ranges. When this type of controller was applied to the two-stage sounding rocket attitude control system, a remarkable improvement was made in the control performance as compared with the PID (proportional, integral, plus derivative) controller case. Since the performance of this anti-wind-up controller was sensitive to the variation of control gains, a modified anti-wind-up controller has been also proposed to compensate for this.
Identification of gyroplane lateral/directional stability and control characteristics from flight test
Authors: Houston S S
Published: October 2002

This paper presents an analysis of test data recorded during flight trials of a gyroplane. This class of rotary-wing aircraft has found limited application in areas other than sport or recreational flying. However, the accident rate is such that a study of the configuration's stability and control characteristics is timely, and in addition substantive data are required for a new airworthiness and design standard that is under development. The paper complements previous work on the longitudinal degrees of freedom and, as a consequence, serves to consolidate the understanding of gyroplane stability and control. The identified derivatives are related to specific aspects of the layout of the gyroplane, and hence the influence of design on the static and dynamic behaviour is quantified. It is concluded that robust estimates of the lateral and directional stability and control derivatives have been identified. This analysis has focused on ‘high-speed’ flight, and the identified derivatives highlight benign and ‘conventional’ characteristics in this part of the flight envelope.
A novel shearography system for aerospace non-destructive testing
Authors: Parker S C J , Salter P L
Published: October 2002

Non-destructive testing (NDT) is a vital process in aerospace engineering to ensure the structural integrity of components. Shearography offers numerous advantages over established inspection techniques. In particular, it has the potential to reduce inspection times by an order of magnitude, can be applied to complex geometries and does not require immersion of the component. These advantages are likely to become increasingly important for next-generation aircraft designs, which necessitate the use of highly complex structures and materials. Despite the inherent advantages of optical inspection, commercial systems have failed to address many of the practical limitations.

This paper identifies the problems that have inhibited technology transfer of shearography from the laboratory to the harsh production environment. Innovative solutions are presented and a production demonstrator is described which has been built at British Aerospace to evaluate these concepts. The system is currently under evaluation by both optical experts and NDT specialists. Early results, validated by destructive testing, have already been valuable in inspecting components that have been notoriously difficult to examine using existing techniques.
Failure analysis of a frangible laminated composite canister cover
Authors: Wu J H , Wang W T , Kam T Y
Published: October 2002

The static burst strength of a frangible laminated composite canister cover subjected to uniform pressure is studied via both theoretical and experimental approaches. The frangible canister cover, which is fabricated with four plate-like laminated composite parts, is designed to fail in a predetermined pattern. The stress distribution in the canister cover is determined using the finite element method and the failure of the cover is identified on the basis of a failure criterion. A number of laminated composite canister covers were fabricated and subjected to static burst pressure testing. The failure modes of the frangible covers are studied and the experimental results are used to verify the theoretical predictions. Close agreements between the experimental and theoretical results have been observed. The canister covers that had been tested did fail in accordance with the predetermined pattern.
Biological reactions to wear debris in total joint replacement
Authors: Ingham E , Fisher J
Published: October 2002

The vast majority of total hip prostheses currently implanted consist of a hard metal or ceramic femoral head articulating against an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup. Over the last 10 years, evidence has accumulated to show that these prostheses are prone to failure due to late aseptic loosening and few survive beyond 25 years. With an increasing need to implant hip prostheses in the younger, more active patient the need to understand the mechanisms of failure and to develop artificial hip joints using alternative materials have become major issues in the orthopaedic community. This review focuses initially on our current understanding of the biological reactions to UHMWPE prosthetic wear debris in vivo and in vitro since this is believed to be the main cause of late aseptic loosening. While the precise mechanisms of osteolysis induced by UHMWPE wear debris have not been elucidated, the major message to emerge is that it is not the wear volume that determines the biological response to the debris, but the concentration of the wear volume that is within the critical size range (0.2-0.8 μm) for macrophage activation.

The review then considers whether the problem of wear-debris-induced osteolysis may be overcome with the use of new generation metal-on-metal or ceramic-on-ceramic prostheses. For metal-on-metal prostheses, the prospects for increasing the osteolysis free life of the implant are good but additional biological problems associated with the nanometre size and reactivity of the wear particles in vivo may emerge. For the ceramic-on-ceramic prostheses, although initial prospects are encouraging, more data are needed on the characteristics of the wear particles generated in vivo before predictions can be made. It is concluded that the pre-clinical testing of any new materials for joint replacement must include an analysis of the wear particle characteristics and their biological reactivity in addition to the usual assessment of wear.
A new method for quantitative evaluation of perceived sounds from mechanical heart valve prostheses
Authors: Johansen P , Riis C , Hasenkam J M , Paulse P K , Nygaard H
Published: October 2002

Closing clicks from mechanical heart valve prostheses are transmitted to the patient's inner ear mainly in two different ways: as acoustically transmitted sound waves, and as vibrations transmitted through bones and vessels. The purpose of this study was to develop a method for quantifying what patients perceive as sound from their mechanical heart valve prostheses via these two routes.

In this study, 34 patients with implanted mechanical bileaflet aortic and mitral valves (St Jude Medical and On-X) were included. Measurements were performed in a specially designed sound insulated chamber equipped with microphones, accelerometers, preamplifiers and a loudspeaker. The closing sounds measured with an accelerometer on the patient's chest were delayed 400 ms, amplified and played back to the patient through the loudspeaker. The patient adjusted the feedback sound to the same level as the ‘real-time’ clicks he or she perceived directly from his or her valve. In this way the feedback sound energy includes both the air- and the bone-transmitted energies. Sound pressure levels (SPLs) were quantified both in dB(A) and in the loudness unit sone according to ISO 532B (the Zwicker method).

The mean air-transmitted SPL measured close to the patient's ear was 23 ± 4dB(A). The mean air- and bone-transmitted sounds and vibrations were perceived by the patients as an SPL of 34 ± 5dB(A). There was no statistically significant difference in the perceived sound from the two investigated bileaflet valves, and no difference between aortic and mitral valves.

The study showed that the presented feedback method is capable of quantifying the perceived sounds and vibrations from mechanical heart valves, if the patient's hearing is not too impaired. Patients with implanted mechanical heart valve prostheses seem to perceive the sound from their valve two to three times higher than nearby persons, because of the additional bone-transmitted vibrations.
A general axisymmetric contact mechanics model for layered surfaces, with particular reference to artificial hip joint replacements
Authors: Jin Z M
Published: October 2002

A general axisymmetric contact mechanics model for layered surfaces is considered in this study, with particular reference to artificial hip joint replacements. The indenting surface, which represents the femoral head, was modelled as an elastic solid with or without coating, while the other contacting surface, which represents the acetabular cup, was modelled as a two-layered solid. It is shown that this model is applicable to current total hip joint prostheses employing ultra-high molecular weight polyethylene (UHMWPE) acetabular cups against metallic, metallic with coating or ceramic femoral heads as well as metal-on-metal combinations. The effect of cement is also investigated for these prostheses using this model. The use of a metallic bearing surface bonded to a UHMWPE substrate for acetabular cups is particularly examined for metal-on-metal hip joint replacements. Both the contact radius and the contact pressure distribution are predicted for examples of these total hip joint replacements, under typical conditions. Application of contact mechanics to the design of artificial hip joint replacements employing various material combinations is discussed.
A device for improved reduction of tibial fractures treated with external fixation
Authors: Moorcroft C I , Thomas P B M , Ogrodnik P J , Verborg S A
Published: October 2002

A widely used method of treatment for unstable tibial shaft fractures is unilateral external fixation. The majority of fixators act as three distinct devices: an intra-operative reduction device, a device to maintain fracture alignment during healing and an aid to healing by allowing movement at the fracture site. Conventional operative techniques require the surgeon to manipulate a number of degrees of freedom at once, making reduction of the fracture difficult, and results in the fixator being out of alignment with the long axis of the bone. An operative method has been developed that separates reduction and fixation. A dedicated device has been designed to improve the per-operative control of fracture fragments during fracture reduction. The device has been used in clinical trials for the reduction of 22 diaphyseal tibial fractures. Compared with previous operative techniques there has been a saving of 53 per cent in fracture reduction time and an overall saving of 10 per cent in operating time. Fracture alignment has been improved compared with reductions achieved with a fixator which potentially improves healing and lowers the rate of malunion. In each case the fixator has been applied in alignment with the bone, improving dynamization and reducing the likelihood of malunion due to fixator cam slippage.
Parametric optimization for tumour identification: Bioheat equation using ANOVA and the Taguchi method
Authors: Sudharsan N M , Ng E Y K
Published: October 2002

Breast cancer is the number one killer disease among women. It is known that early detection of a tumour ensures better prognosis and higher survival rate. In this paper an intelligent, inexpensive and non-invasive diagnostic tool is developed for aiding breast cancer detection objectively. This tool is based on thermographic scanning of the breast surface in conjunction with numerical simulation of the breast using the bioheat equation. The medical applications of thermographic scanning make use of the skin temperature as an indication of an underlying pathological process. The thermal pattern over a breast tumour reflects the vascular reaction to the abnormality. Hence an abnormal temperature pattern may be an indicator of an underlying tumour.

Seven important parameters are identified and analysis of variance (ANOVA) is performed using a 2n design (n = number of parameters, 7). The effect and importance of the various parameters are analysed. Based on the above 27 design, the Taguchi method is used to optimize the parameters in order to ensure the signal from the tumour maximized compared with the noise from the other factors. The model predicts that the ideal setting for capturing the signal from the tumour is when the patient is at basal metabolic activity with a correspondingly lower subcutaneous perfusion in a low temperature environment.
Stress distribution in the dento-alveolar system using digital photoelasticity
Authors: Asundi A , Kishen A
Published: October 2002

Past research has confirmed that the governing factors in cellular modelling and remodelling adhere to sound principles of engineering mechanics. Hence studies of stress distributions would provide better understanding of the functional adaptation of dental supporting structures. Photoelasticity is an established experimental tool to study whole-field stress distribution in structures subjected to forces. However, it has certain limitations that make its application in biological specimens tedious. In this investigation an advanced digital photoelastic system is used to visualize and study the nature of the stress distribution in dental supporting structures. These digital fringe patterns are analysed using a phase-shift technique. The present biomechanical study shows that dental supporting structures exhibit a characteristic stress distribution, promoting structural adaptation based on needs. Furthermore, the advantage of using a digital image processing system along with the circular polariscope is discussed.
Comparison of gas plasma and gamma irradiation in air sterilization on the delamination wear of the ultra-high molecular weight polyethylene used in knee replacements
Authors: Reeves E A , Barton D C , FitzPatrick D P , Fisher J
Published: October 2002

Early failure of knee replacements is thought to be due to the combination of sterilization by gamma irradiation in air and the high cyclic stresses that they endure during use. Such failures are shown through delamination and permanent deformation of the ultra-high molecular weight polyethylene (UHMWPE) component.

This study investigated whether gas plasma sterilization, as an alternative to gamma irradiation in air, would give better performance after ageing in a knee replacement using a metal pin on polymer plate wear test. Fourier transform infrared (FTIR) analysis was performed on the components to assess oxidation levels and a finite element stress analysis model is presented to estimate strain at failure in the UHMWPE.

Delamination occurred in the majority of the gamma-irradiated plates but did not occur in any of the gas-plasma-sterilized plates. The FTIR analysis showed that the plates gamma irradiated in air were highly oxidized when compared with the gas-plasma-sterilized plates. Plastic strain at failure was determined for the gamma-irradiated plates and found to be less than 2.4-14 per cent.
Development of surgical instruments for implanting a flexible fixation device for the lumbar spine
Authors: Leahy J C , Stevenson E C , Hukins D W L , Mathias K J , Shepherd D E T , Wardlaw D
Published: October 2002

A new flexible fixation device for the lumbar spine has been developed. This paper describes the development and evaluation of two surgical instruments required for implanting this device. Prototypes were designed, manufactured and then evaluated for use in surgery. Further evaluation was performed, if necessary, and the design finalized, in accordance with BS EN 12011. This process involved close collaboration between engineers and surgeons.
Influence of simulator kinematics on the wear of metal-on-metal hip prostheses
Authors: Firkins P J , Tipper J L , Ingham E , Stone M H , Farrar R , Fisher J
Published: October 2002

There is now considerable interest in metal-on-metal bearings for hip prostheses. Extremely low wear rates (0.1 mm3/106 cycles) have been reported in some simulator studies, while in vivo studies, although still very low, have shown wear rates of the order of 1 mm3/106 cycles. The aim of this study was to compare wear rates of metal-on-metal bearings in two hip simulators with different kinematic inputs. In the simulator with three independent input motions which produced an open elliptical wear path with a low level of eccentricity, the wear rates were very low as recorded previously in other simulators. In the simulator with two input motions which produced an open elliptical wear path with greater eccentricity the wear rate was at least ten times higher and closer to clinical values. The motion and kinematic conditions in the contact are critical determinants of wear in metal-on-metal bearings.
A comparison between automatically generated linear and parabolic tetrahedra when used to mesh a human femur
Authors: Polgar K , Viceconti M , Connor J J
Published: October 2002

Finite element models of bone segments generated from computed tomography data using automatic mesh generation algorithms are becoming common not only in research but also in clinical applications such as computer aided orthopaedic surgery. Especially in the case of the latter application, the models cannot be verified against an experimental measurement, therefore their inherent accuracy should be well known before drawing conclusions based on the calculated results. This study was carried out to assess the performance of tetrahedral solid finite elements with linear and quadratic displacement functions when they are used to mesh the human femur in conjunction with automatic mesh generator methods. Ten-node quadratic tetrahedra (T10) having parabolic displacement functions were compared with four-node linear tetrahedron elements (T4) on the basis of accuracy and central processing unit (CPU) time. From the analyses of 11 finite element meshes, it was concluded that linear tetrahedral elements should be avoided and quadratic tetrahedral elements ought to be chosen for the purposes of finite element analysis of the human femur. When incremental loading and iterative solution is necessary, the coarsest possible T10 mesh compatible with accuracy is needed to minimize computer capacity and CPU time.
Comparative two- and three-dimensional finite element modelling techniques for tibial fractures
Authors: Mishra S , Gardner T N
Published: October 2002

Often the choice of a two-dimensional modelling approach over a three-dimensional approach is made on the basis of available resources, and not on task appropriateness. In the case of simulating the mechanical behaviour of irregular anatomical structures in biomedical engineering, the authenticity of two-dimensional model behaviour and the interpretation of model solutions is of particular concern since little comparable two-dimensional and three-dimensional data have been published. As part of a research programme, a comparison was made between two-dimensional and three-dimensional finite element models (FEMs) that examine the stress-strain environment of a clinical bone fracture and callus. In comparison with the three-dimensional model, the two-dimensional model substantially underestimated peak compressive principal stresses in the callus tissue and peak equivalent strains. This was a consequence of geometrical and structural asymmetry in a plane perpendicular to the two-dimensional model. However, the two-dimensional model predicted similar patterns of stress and strain distribution to the corresponding mid-longitudinal plane of the three-dimensional model, and underestimates of peak stress and strain were much reduced. This confirmed that despite the irregular geometry and structure of the subject, the two-dimensional model provided a valid mechanical simulation in the plane of the fracture that it represented.
In vitro comparison of the two hard-hard articulations for total hip replacements
Authors: Rieker C , Konrad R , Schoun R
Published: October 2002

Polyethylene particle disease is one of the major causes of late aseptic loosening of total hip replacement. Two hard-hard articulations (alumina-on-alumina and metal-on-metal) have been developed in Europe as an alternative to the ultra-high molecular weight polyethylene (UHMWPE) articulations. Even though these hard-hard articulations are on the market and numerous reports have been published about them, only a very limited number of studies allowing a direct in vitro comparison of the two articulations have been published so far.

This paper compares in vitro these two types of articulation (alumina-on-alumina and metal-on-metal), which have been tested with a hip simulator for their tribological behaviour using exactly the same experimental methodology. This comparison shows that these two types of hard-hard articulation have very similar abrasive wear behaviour with four main features:

1. A running-in wear period (1 × 106 cycles) gives a cumulative wear of about 20 μm with head diameters of 28 mm. 2. After the running-in wear, there is a stabilization of the linear wear behaviour with a low linear wear rate/106 cycles for both types of articulation. 3. The volumetric wear rate of both articulations (<2.0 mm3/year for head diameters of 28mm) is significantly lower than that observed for metal-on-polyethylene or ceramic-on-polyethylene articulations having the same head diameter. 4. Abrasive wear is readily apparent (indicating a mixed lubrication regime) with both types of articulation.

The extremely low wear performance of these articulations is confirmed and they constitute a lowwear alternative to the UHMWPE articulations currently used.
New joints for the Millennium: Wear control in total replacement hip joints
Authors: Dowson D
Published: October 2002

Hip joint replacement is described as the greatest achievement in orthopaedic surgery in the twentieth century. The field has been dominated for some forty years by implants based upon metallic femoral heads and stems and polymeric acetabular cups. At the dawn of the new Millennium, many alternative materials and designs are now being proposed or evaluated. The reasons for these developments and the current contributions of engineering science and tribology to advances in hip replacement are discussed. Illustrations are presented of the significant changes being proposed or introduced. While the new designs of total hip replacements offer exciting engineering contributions to the future of joint replacement, the long-term benefits to patients will depend upon the biological response to the new devices.
Effect of blood flow, tumour and cold stress in a female breast: A novel time-accurate computer simulation
Authors: Ng EY-K , Sudharsan N M
Published: October 2002

Breast cancer is a dreadful disease among women and early detection helps in achieving a cure. The mammogram is presently the standard tool for detecting breast abnormality, but its sensitivity is lower for women with dense breasts. It has been found that women with an abnormal thermogram are at a higher risk and have a poorer prognosis. However, performing and interpreting thermograms requires meticulous training. Computer simulations can be an additional tool to help the clinician in the interpretation. In this paper, a novel and flexible finite element model of a female breast is developed. Both steady state and time-dependent solutions are obtained. Steady state solutions globally match experimental thermographic results with the proper choice of blood perfusion source terms, tissue thickness and geometric scaling factor. Although the simulations may not be useful in providing a unique solution (i.e. exact size and location of the tumour owing to the complex physiological relationship between the tumour and the breast surface temperature), it would nevertheless help in the ‘analysis by elimination’. An example of this type of analysis is also presented.
Chest and abdominal surface motion measurement for continuous monitoring of respiratory function
Authors: Earthrowl-Gould T , Jones B , Miller M R
Published: October 2002

The spirometric devices currently available are not well suited for continuous or ambulatory monitoring of subjects. The proposed solution details a method of assessing lung function that avoids the need for flow measurement at the mouth and provides more detailed information of the dynamics of chest motion while allowing the subject to move freely. The solution to this problem comprises a distributed sensor system that allows regional motions during breathing manoeuvres to be investigated as well as a measurement of tidal respiratory volume. Preliminary studies demonstrate performance benefits of an array of sensors over two-compartment respiratory measurements.
Design revision of a partially cemented hip stem
Authors: Viceconti M , Cristofolini L , Toni A
Published: October 2002

In a previous preclinical study the prototype version of a partially cemented hip stem, cement-locked uncemented (CLU) prosthesis, showed optimal primary stability and moderate stress shielding. However, numerical analysis suggested that the prototype design would induce relatively high stresses in the cement and a significant relative motion between cement and metal. The present study aimed to verify if these problems could be eliminated once the CLU design is improved. The revised design was analysed using a complete finite element model of an implanted human femur. To further strengthen the predictions of the finite element analysis, the cement damage induced by a severe load history was assessed experimentally in synthetic femurs implanted with the improved CLU stem or with a clinically successful fully cemented stem. The modifications made to the CLU stem design did not reduce its good primary stability but decreased the metal-cement relative micro-motion. The same load induced stresses in the cement mantle of the improved CLU stem that were significantly lower than those predicted for the prototype design. Although the presence of modelling artefacts produced a highly localized stress peak of 13 MPa, 99 per cent of the cement volume was subjected to a principal tensile stress lower then 4 MPa. These levels of stress compare favourably with the tensile fatigue limit of the acrylic cement used in this study (9.7 MPa). The experimental results further supported these findings. The cemented stem showed a number of cracks per volume unit approximately ten times higher than the partially cemented stem under investigation.
Flow instabilities in a graft anastomosis: A study of the instantaneous velocity fields
Authors: Bates C J , O'Doherty D M , Williams D
Published: October 2002

The major cause of arterial bypass graft failure is intimal hyperplasia. Fluctuating wall shear stresses in the graft, which are associated with disturbed flow, are believed to be important factors in the development and localization of intimal hyperplasia. This study, based upon water as the working fluid, has investigated the flow structure inside a 30° Y-junction with different fillet radii at the intersection between the graft and the host artery at various Reynolds numbers and distal outlet segment (DOS) to proximal outlet segment (POS) flow ratios. The structure of the flow has been investigated experimentally using particle image velocimetry (PIV). The two-dimensional instantaneous velocity fields confirm the existence of a very complex flow, especially in the toe and heel regions for the different fillet radii and clearly identify features such as sinks, sources, vortices and strong time dependency.
Medical image based, preformed titanium membranes for bone reconstructions: Design study and first clinical evaluation
Authors: Pattijn V , Samson I , Sloten JVander , Van Audekercke R , Swaelens B , De Buck V
Published: October 2002

The currently used intralesional or marginal surgical treatment of a bone tumour in the extremities shows some shortcomings in providing a restoration of the mechanical strength of the bone and the containment of the used filling materials. The use of a medical image based, preformed and custom-made titanium membrane screwed onto the periosteal side of the bone is introduced. This study looks in detail into the design process and the biomechanical evaluation of such a membrane. The buckling strength of the membrane, the strength at the perforation holes and the strength of the screw-bone fixation are tested experimentally. The two latter experiments are performed with different screw types. From the performed tests it appears that a titanium membrane without a wave pattern, of 0.3 mm thickness, fixed to the bone with seven trabecular bone screws (4 mm diameter and 28 mm length) is capable of carrying the anticipated mechanical loads on the reconstructed tibia. The medical image based design methodology and the first clinical application of such a preformed and custom-made titanium membrane are reported and discussed. The feasibility of preformed titanium membranes for bone reconstruction in tumour surgery is demonstrated.
Measurement of the bony anatomy of the humerus using magnetic resonance imaging
Authors: Murdoch A H , Mathias K J , Smith F W
Published: October 2002

This study was undertaken during the development of a new humeral intramedullary nail. To determine the overall size of the nail it was necessary to have the dimensions of the humerus and intramedullary canal. The dimensions of the bony anatomy of the humerus were obtained using an open magnetic resonance imager. The right arm of 20 volunteers was scanned and the length and the dimensions of both the intramedullary canal and the cortical bone were measured. The diameter of the canal was found to be 12.1 ± 2.6 mm (mean ± standard deviation) with the middle 50 per cent of measurements between 10 and 14 mm. The overall diameter of the bone was 19.3 ± 2.3 mm, with the middle 50 per cent of measurements between 18 and 21 mm. This study has shown that magnetic resonance imaging (MRI) is an effective method of collecting dimensional data from any part of the skeleton for the development of medical devices.
A spatial model of the knee for the preoperative planning of knee surgery
Authors: Olanlokun K F T , Wills D P M
Published: October 2002

A model on the spatial mechanical behaviour of the passive knee is presented. The femoral articular surfaces were represented by generalized, sagittally elliptical, toroidal surfaces. The medial and lateral tibial articular surfaces were represented by a dished spherical surface and the lower hemihyperbolic region of a torus respectively. Anatomical articular cartilage, knee ligaments and the posterior capsule were represented by spring-like deformable elements with non-linear load versus deflection characteristics. All the forces that act on the femur relative to the tibia were represented by three orthogonal forces and three associated moments. Spatial, articulation-dependent femoro-tibial kinematic constraint equations of the passive knee were formulated in an analytically explicit manner, based on the natural coordinates of the articular surfaces. The constraint equations were solved algebraically in closed form. Equations were derived that describe spatial femoro-tibial motion, ligament length, ligament strain, ligament-based elastic potential energy and the quasi-static equilibrium of the passive knee. Software was written, simulations on the motion characteristics and load versus deflection characteristics of the knee were carried out and graphical results were presented. The simulation of planar flexion/extension was almost spontaneous. The time taken to simulate spatial six-degree-of-freedom femoro-tibial motion was less than 2.5 min. The models were found to be capable of representing real-life passive knees to a high degree of satisfaction. It has been demonstrated that the models can provide knee surgeons with additional information on major aspects of the preoperative planning of knee surgery. The models can be used to enhance the preoperative planning of ligament reconstruction, articular surfaces related surgery, osteotomy and patellar tendon transfer surgery.
Prediction of skin burn injury. Part 1: Numerical modelling
Authors: Ng E Y-K , Chua L T
Published: October 2002

Skin burn injury and heat tolerance in man is a complex interaction of physical heat exchange processes and the potential for physiological adjustments. In recently developed one-dimensional finite difference and two-dimensional finite element bioheat transfer based numerical models of the skin, the prediction of the thermal efficacy of cooling treatment was evaluated. With the use of a standard Arrhenius model for damage rate, the extent of burn injury was computed from the transient temperature solution. Simulated isotherms results for current models were found to be consistent in both one- (experimental) and two-dimensional (analytical) data. The two-dimensional code provided some additional precision to previously published models. This was due to the mesh-independent grid which enabled more accurate numerical prediction of thermal and injury patterns as they developed during the injury process. Numerical data obtained in the current study suggest that non-thermal factors must be considered in the calculation of the efficacy of postburn cooling therapy. However, when dealing with living tissue, any numerical model can provide only an approximation to conditions in real exposures. Together with analysis of variance, the models were used for investigating the precedence of the various parameters and the effects of varying these parameters on assessment of burn injury resulting from exposure of skin surface to heat sources (Part 2).
Structural design and performance of a rear support walking frame
Authors: Woollam J , Miller K , McLeod N , Batty D , Stallard J
Published: October 2002

Rear support walking frames provide predetermined vertical support for patients with dysfunctional lower limbs that have limited active control; the support is provided through a spring-loaded boom hinged on an upright stanchion mounted at the rear of a wheeled frame within which the patient ambulates. The application of these frames for total-body-involved cerebral palsy patients, in combination with a walking orthosis, has highlighted a number of practical problems that need to be addressed for the system to become fully viable. A composite material prototype walking frame has been developed that permits the patient to be transferred by a single carer without the need to use inappropriate manual handling techniques. The frame has improved structural properties, with stiffness in the sagittal and coronal planes increasing by between 50 and 100 per cent. Evaluation with patients showed that the greater structural stiffness permitted the objective of improved continuity of walking to be achieved. The strength of the frame is such that it can accommodate patients of up to 80 kg, more than twice that possible in the earlier system. Since the structural yield point is approximately twice the maximum working load, the device should not be prone to unacceptable fatigue characteristics. Despite the use of carbon composite materials (which have brittle failure characteristics), the mode of failure is of progressive collapse and is therefore inherently safe. The successful outcome of prototype testing has justified production development. Work is now proceeding on a design that incorporates further improvements in structural performance and ease of manufacture.
Technological aspects of minimal access surgery
Authors: Frank T G , Hanna G B , Cuschieri A
Published: October 2002

Minimal access surgery (MAS) is bringing about a revolution in surgical practice with certain operations being almost wholly carried out using MAS techniques in some countries. This paper describes the development, current status and future prospects of MAS from a technological perspective.
Quality framework for force plate testing
Authors: Fleming H E , Hall M G , Dolan M J , Paul J P
Published: October 2002

Following work done in C.E.C. project AIM 2002 CAMARC II, a framework of the relevant standards for testing equipment used in laboratories analysing human movement is presented. In particular, gait analysis laboratories may provide a service for a clinical team considering treatment such as surgical intervention or provision of orthoses. In this case the analysis and equipment used in the analysis must have assured quality so that the correct clinical interpretation of the results is possible. The operation of a gait analysis laboratory may be described in quality standards terminology as the operation of a ‘special process’. The operation of a special process requires that particular attention be paid to procedures including confirmation of the equipment and regular testing to ensure correct operation of the equipment. In general this is difficult to achieve since equipment manufacturers still have to address this technical requirement. Although difficult, these procedures must be accomplished in the operation of a special process. Passages of the most pertinent standards are presented and a methodology of periodic confirmation and regular spot checking of equipment suggested to ensure its correct operation. Although force plates are specifically addressed, the framework could be applied to other dynamometers and measurement equipment.
Clinical wear behaviour of ultra-high molecular weight polyethylene cups paired with metal and ceramic ball heads in comparison to metal-on-metal pairings of hip joint replacements
Authors: Semlitsch M , Willert H G
Published: October 2002

In the course of 30 years of hip endoprosthetics, a number of material combinations for the cups and balls of total hip prostheses have proven successful under clinical conditions. Favourably priced hip prostheses with polyethylene cups and metal balls are available for older patients with a moderate range of activity. Polyethylene wear of 100-300 μm/year is to be expected with these models. Ceramic balls (aluminium oxide for diameters 32 and 28 mm and zirconium oxide for 22 mm) paired with polyethylene cups are recommended for patients with a life expectancy of 10 to 20 years, because the expected polyethylene wear rate with this material combination is only 50-150 μm/year. In other words, the life cycle of the polyethylene cup is doubled, when it is paired with a ceramic ball. A similar polyethylene wear rate is also to be expected with oxygen-deep-hardened TiAlNb metal balls, which are currently the subject of a clinical field study. Last but not least, CoCrMoC metal-metal and Al2O3 ceramic-ceramic pairings, which have the lowest wear rate of 2-20 μm/year, are available for highly active patients with a life expectancy of more than 20 years. As far as the cup-ball pairing is concerned and under the current pressure of costs, the surgeon should be able to select the optimum hip prosthesis model for every patient from these three categories.
Discrepancy between penetration depths derived from radiographic and direct measurement of acetabular components
Authors: Hall R M , Siney P , Craig P S , Unsworth A , Wroblewski B M
Published: October 2002

The most common technique for assessing penetration due to wear in acetabular components is with the aid of the most recent serial radiograph. This approach, which is often termed the uni-radiographic method, has been shown to underestimate the more reliable value of the penetration depth deduced from direct measurement of explanted sockets. In this article the causes of the discrepancies between the two data sets are explored. Ninety-six sockets were available from revision surgery for which both the penetration depth and angle could be measured using the shadowgraphic technique in both the coronal and wear planes. Further, the penetration depth for each of the sockets was also assessed from pre-revision X-rays. A significant discrepancy was observed between the penetration depths measured in the wear plane of the replica Δ P w and that measured from the radiograph, Δ Px-ray. The discrepancy was greatest for loose sockets as opposed to those that were still fixed at revision surgery. Using the corresponding data from the shadowgraph measurements, it was possible to deduce that the errors have arisen from the radiographic measurement of wear in the coronal plane and the formula used in calculating Δ Px-ray. If these errors (which cannot be calculated from the X-ray data alone) were taken into consideration, then the systematic bias between radiographic and shadowgraphic measurement was greatly reduced. The largest portion of the discrepancy was accounted for by wear occurring out of the plane of the radiograph, and this, in general, coincides with the coronal plane. Overall, these results indicate that the accurate measurement of wear from serial radiographs is not possible and that improved performance in terms of accuracy can only be achieved when a three-dimensional system is used. An alternative method for deducing the radio-graphic penetration depth is proposed which, theoretically, negates the error arising from the inaccuracy of the formula.
Glass-ionomers in medicine and dentistry
Authors: Nicholson J W
Published: October 2002

This paper describes the current uses and future prospects for glass-ionomer cements in dentistry and medicine. Glass-ionomers divide into two chemical types, one is known as self-hardening and sets entirely by a neutralization reaction to give relatively brittle materials, the other is known as resin-modified and sets partly by polymerization and partly by neutralization to give slightly tougher materials. Compared with the self-hardening cements, these latter materials have improved aesthetics and easier clinical handling. Both types bond well to enamel and dentine, and release clinically useful amounts of fluoride. They have been used in a variety of applications in dentistry, including as liners/bases, luting cements for stainless steel crowns, and in various restorative procedures for both permanent and primary teeth. The resin-modified glass-ionomers are particularly promising for these latter uses, though it is too soon to be sure how durable they will prove to be over the longer term. Self-hardened glass-ionomers have been shown to have much better biocompati-bility than resin-modified glass-ionomers in a variety of situations and consequently they have been used for various non-dental applications, such as ear, nose and throat surgery and craniofacial reconstruction.
The design of a vibration transducer to monitor the integrity of dental implants
Authors: Cawley P , Pavlakovic B , Alleyne D N , George R , Back T , Meredith N
Published: October 2002

Bone-anchored titanium implants are being used increasingly to provide support for prostheses replacing missing teeth in edentulous and partially dentate patients. A technique is required to monitor bone formation at the implant-tissue interface during healing, and also to check whether there has been bone loss from around the top of the implant. One possible method is to screw a beam into the implanted fixture and to measure the first flexural resonance frequency of the resulting system. This resonance frequency is affected by both the exposed length of fixture and the stiffness of the interface between the implant and the bone. This paper describes the design of a beam-like transducer for clinical trials of the technique. The sensitivity of the transducer resonance frequency to the changes of interest is dependent on the thickness and length of the beam element. However, the choice of these dimensions is constrained by the need to avoid closely spaced resonances. The performance of different transducer shapes and the influence of the thickness and length of the beam element in the transducer has been studied. The results have been used to finalize a transducer design for the clinical trials.
The estimation of acetabular cup wear volume from two-dimensional measurements: A comprehensive analysis
Authors: Derbyshire B
Published: October 2002

This paper describes a mathematical investigation of the relationship between wear volume, wear depth and wear direction in acetabular components. The analysis takes into account the cylindrical and conical portions at the mouth of certain types of socket and also incorporates the effect of an initial radial discrepancy between the femoral head and socket. Published formulae for converting linear wear measurements to wear volumes are shown to be incorrect. Wear volume is shown to be highly dependent on the wear direction, increasing by more than 90 per cent over a 60° range. Cylindrical and conical portions of a 22.225 mm cup may contribute up to about a third as much wear as the hemispherical socket. At low wear depths, the neglect of a radial discrepancy between the components can result in an overestimation of wear volume in excess of 100 per cent.
Friction of composite cushion bearings for total knee joint replacements under adverse lubrication conditions
Authors: Stewart T , Jin Z M , Fisher J
Published: October 2002

Conventional joint replacements consist of a polished metallic or ceramic component articulating against a layer of polyethylene. Although the friction in the contact between these articulating surfaces is low, polyethylene wear is produced as a result of a boundary/mixed lubrication regime. Wear debris is generated by direct asperity contact, abrasion, adhesion and fatigue, and has been shown to cause adverse tissue reactions which can lead to joint failure. The introduction of soft compliant materials, similar in stiffness to articular cartilage, has shown that with cyclic loading and relative motion between the articulating surfaces typical of normal walking, a fluid film can be maintained through combined entraining and squeeze-film actions, and hence wear can be minimized.

For 95 per cent of the time, however, we are not walking but standing still or moving slowly. A pendulum simulator has been used in the present study to investigate the effect of adverse tribological conditions which may lead to fluid film breakdown, such as severe cyclic loading, particularly in the swing phase, reduced sliding velocity, reduced stroke length and start-up after a period of constant loading. Friction of a model composite cushion knee bearing, manufactured from a graded modulus (20–1000 MPa) layer of polyurethane, sliding against a polished metal cylinder has been measured for various lubricants and the results have been analysed using a Stribeck assessment. Severe cyclic loading, decreased sliding velocity and decreased stroke length have been found to limit the degree of fluid entrainment previously allowed during the swing phase of normal walking, thus allowing breakdown of fluid films and elevated levels of friction and surface damage. Soft layer joint replacements must therefore be designed to operate with thick elastohydrodynamic fluid films to provide some degree of protection when tribological conditions become severe, or alternatively incorporate alternative boundary or mixed lubrication mechanisms. This study quantifies a potential limitation of the cushion bearing concept.