bluuebunny/crossref_metadata_2025 huggingface dataset:
Life assessment of a critical aeroengine part using finite element crack modelling
Authors: Karnezis P , Jesson J R
Published: October 2002

During the design of a Rolls-Royce military gas turbine aeroengine several years ago, the rear flange was identified as the life-limiting part of the outer casing of the combustion chamber (CCOC). An early finite element (FE) analysis identified the dowel pin hole in the flange as a potentially critical area and predicted a low cycle fatigue (LCF) safe life much lower than the design target. However, whole engine testing since then has proved those predictions to be too pessimistic, but LCF testpieces failed to deliver a better life estimate, possibly because of the difficulty of achieving representative testing conditions. In order to predict a more accurate life for the component, an alternative procedure incorporating FE static crack analysis, LCF testing, crack propagation tests and analytical calculations was used to life the component. FE submodelling techniques were applied to study the behaviour of a growing crack, with boundary conditions taken from FE global models of the component and the LCF testpiece. The FE analysis explained the discrepancies between earlier modelling attempts and experimental results, and provided the necessary information to declare an increased component life.
Robotic tacking of high-quality composite preforms
Authors: Dewing A , Sarhadi M , Mitchell T A , Davis R I , Eastham J
Published: October 2002

A new manufacturing methodology, termed shape-inclusive lay-up, has been developed for manufacture of dry-fibre preforms using non-crimp fabric (NCF). Shape-inclusive lay-up enables preforms for three-dimensional components to be produced and stored flat, introducing the shape only when the preform is loaded into the mould. The lay-up process is jigless, and complex components can be produced as a single preform, reducing both preform production and moulding times. Aerospace components with varying degrees of complexity can be manufactured using this technique, including skin components and reinforcement structures. A range of technologies has been developed for automating the lay-up process. This paper focuses on the requirements of the joining process. The various methods of ply attachment investigated are described. The method that both satisfies the process requirements and is suitable for automation is single-sided stitching using a loose chain stitch. The stitching process has been studied and implemented for the production of dry carbon preforms. A stitching device in the form of a robotic end-effector is described. Prototype testing is detailed and the results of these tests are discussed.
Comparison of friction and lubrication of different hip prostheses
Authors: Scholes S C , Unsworth A
Published: October 2002

It is well documented that an important cause of osteolysis and subsequent loosening of replacement hip joints is polyethylene wear debris. To avoid this, interest has been renewed in metal-on-metal and ceramic-on-ceramic prostheses. Various workers have assessed the lubrication modes of different joints by measuring the friction at the bearing surfaces, using different lubricants. Measurements of friction factors of a series of hip prostheses were undertaken using carboxymethyl cellulose (CMC) fluids, silicone fluids, synovial fluid and different concentrations of bovine serum as the lubricant. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing CMC fluids or silicone fluids as the lubricant. Mixed lubrication was found to occur in the metal-on-metal (CoCrMo/CoCrMo) joints with all lubricants at a viscosity within the physiological range. This was also the case for the metal-on-plastic (CoCrMo/ultra-high molecular weight polyethylene) joints. The ceramic-on-ceramic (Al2O3/Al2O3) joints, however, exhibited full fluid film lubrication with the synthetic lubricants but mixed lubrication with the biological lubricants. Employing a biological fluid as the lubricant affected the friction to varying degrees when compared with the synthetic lubricants. In the case of the ceramic-on-ceramic joints it acted to increase the friction factor tenfold; however, for the metal-on-metal joints, biological fluids gave slightly lower friction than the synthetic lubricants did. This suggests that, when measuring friction and wear of artificial joints, a standard lubricant should be used.
Tissue engineering: Confronting the transplantation crisis
Authors: Nerem R M
Published: October 2002

Tissue engineering is the development of biological substitutes and/or the fostering of tissue regeneration/remodelling. It is emerging as a technology which has the potential to confront the crisis in transplantation caused by the shortage of donor tissues and organs. With the development of this technology, there is emerging a new industry which is at the interface of biotechnology and the traditional medical implant field. For this technology and the associated industry to realize their full potential, there are core, enabling technologies that need to be developed. This is the focus of the Georgia Tech/Emory Center for the Engineering of Living Tissues, newly established in the United States, with an Engineering Research Center Award from the National Science Foundation. With the development of these core technologies, tissue engineering will evolve from an art form to a technology based on science and engineering.
A review of robotics in surgery
Authors: Davies B
Published: October 2002

A brief introduction is given to the definitions and history of surgical robotics. The capabilities and merits of surgical robots are then contrasted with the related field of computer assisted surgery. A classification is then given of the various types of robot system currently being investigated internationally, together with a number of examples of different applications in both soft-tissue and orthopaedic surgery. The paper finishes with a discussion of the main difficulties facing robotic surgery and a prediction of future progress.
Design of a surface replacement prosthesis for the proximal interphalangeal joint
Authors: Ash H E , Unsworth A
Published: October 2002

A surface replacement finger joint prosthesis was designed specifically for the proximal interphalangeal joint (PIPJ). The two-piece design consisted of a bi-condylar proximal phalangeal head and a conforming bi-concave middle phalangeal base. The bearing surfaces were designed as close to the original anatomy of the PIPJs as possible, using detailed information obtained from a previous anatomical study of 83 PIPJs by the present authors.

Four sizes of prosthesis were designed with maximum head diameters of 7, 8, 9 and 10 mm. Fixation of the joint prosthesis was achieved by an interference fit between the stems of semicircular crosssection and the phalangeal bone shafts. The main considerations for the stem designs were the offset from the centre of rotation, angle of inclination, length, and cross-sectional shape and size. It is proposed that the two components will be made from cross-linked polyethylene (XLPE) because it can be injection moulded to produce the complex shapes of the joint prosthesis. In addition, XLPE against itself has shown comparable wear rates with stainless steel against ultra-high molecular weight polyethylene from previous work by Joyce et al.
Robotic-assisted internal fixation of hip fractures: A fluoroscopy-based intraoperative registration technique
Authors: Browbank I , Bouazza-Marouf K , Schnabler J
Published: October 2002

The internal fixation of proximal femoral (hip) fractures is the most frequently performed orthopaedic surgery procedure. When using a sliding compression hip screw, a commonly used fixation device, accurate positioning of the device within the femoral neck-head is achieved by initially drilling a pilot hole. A cannulated component of the hip screw is then inserted over the guide wire (surgical drill bit), which is used to drill the pilot hole. However, in practice, this fluoroscopically controlled drilling process is severely complicated by a depth perception problem and, as such, a surgeon can require several attempts to achieve a satisfactory guide wire placement. A prototype robotic-assisted orthopaedic surgery system has therefore been developed, with a view to achieving accurate right-first-time guide wire insertions. This paper describes the non-invasive digital X-ray photogrammetry-based registration technique which supports the proposed robotic-assisted drilling scenario. Results from preliminary laboratory (in vitro) trials employing this registration technique indicate that the cumulative error associated with the entire X-ray guided robotic system is within acceptable limits for the guide wire insertion process.
Adsorption control of synovia constituents on artificial joint materials by means of an electric field: Evaluation of tribological characteristics
Authors: Nakanishi Y , Murakami T , Higaki H
Published: October 2002

The supplemental lubrication mechanism by means of an electric field was proposed to reduce friction and wear for the advanced joint prosthesis with the low elastic modulus bearing surface. The possibility of application of this mechanism to the prosthesis was investigated by the fundamental and experimental procedures in simplified sliding conditions. Conductive silicon rubber was used as the low elastic modulus surface. The counterfaces were a titanium alloy and a stainless steel. Protein (y-globulin) in lubricant appeared to cause the tribological characteristic to deteriorate in the mixed lubrication regime. High friction seemed to be brought about by the obstruction against the entraining of the fluid flow and the high shear stress due to the microbonding between the asperities of bearing surfaces, which were derived from the adsorbed protein on the hydrophobic surfaces. The repulsive force between the adsorbed film and the bearing materials by means of the d.c. electric field, and the continuous change in polarity on the surface by means of the a.c. electric field appeared to affect the adsorbed film adjacent to the bearing material, so that friction and wear were varied.
The development of a long, dual-platform triaxial walkway for the measurement of forces and temporal-spatial data in the clinical assessment of gait
Authors: Hynd D , Hughes S C , Ewins D J
Published: October 2002

Force-plate measurement of the ground reaction force (GRF) has, for many years, been considered a vital component of the comprehensive assessment of human gait in the clinical context. For example, the data can be used in the adjustment of prostheses and orthoses and in identifying the mechanisms underlying a gait dysfunction. However, commercial force plates are usually only capable of measuring GRF data from one step in a single traverse. That can lead to problems of ‘targeting’ and, with less able subjects, fatigue before the necessary data have been collected.

Previous work at the University of Surrey resulted in a prototype dual-platform force walkway capable of measuring the vertical component of the GRF and estimating the position of application of that force for multiple foot contacts in a single traverse. In addition, temporal-spatial information, e.g. speed and step length, could also be determined.

This paper describes the development of a longer walkway that can measure the three orthogonal components of the GRF and provide a more accurate estimate of the position of application of that force. Software to allow the rapid reduction of gait data to useful clinical information has also been developed.
Long-term results for Kinemax and Kinematic knee bearings on a six-station knee wear simulator
Authors: Ash H E , Burgess I C , Unsworth A
Published: October 2002

A long-term wear test was performed on Kinemax and Kinematic (Howmedica Inc.) knee bearings on the Durham six-station knee wear simulator. The bearings were subjected to flexion/extension of 65–0°, anterior-posterior translation of between 4.5 and 8.5 mm and a maximum axial load of 3 kN. Passive abduction/adduction and internal/external rotation were also permitted, however, two of the stations had a linkage system which produced ± 5° active internal/external rotation. The bearings were tested at 37 °C in a 30 per cent bovine serum solution and the test was run to 5.6 × 106 cycles. The bearings from stations 2 and 3, and stations 4 and 5 were swapped during the test to investigate the effects of interstation variability.

The average wear rate and standard error was 3.00 ± 0.98 mg/106 cycles (range 1.33-4.72 mg/106 cycles) for the Kinemax bearings and 3.78 ± 1.04 mg/106 cycles (range 1.87-4.89 mg/106 cycles) for the Kinematic bearings. There were no significant differences in wear rates between the different bearing designs, the addition of active internal/external rotation or a change of stations. However, the wear tracks were different for the two types of bearings and with active internal/external rotation. The wear rates and factors were generally lower than previously published in vitro wear results; however, this may have been due to a difference in the axial loads and lubricants used. The appearance of the wear tracks with active internal/external rotation was comparable with those seen on explanted knee bearings.
Radiographic assessment of the cement mantle thickness of the femoral stem in total hip replacement: A case study of 112 consecutive implants
Authors: Isaac G H , Busch C A , Shetty V , Drabu K J
Published: October 2002

The results are reported of a radiographic study of cement mantle thickness in 112 consecutive primary hip replacements. Measurements were made by three observers of the apparent cement thickness medially and laterally using standard anterior-posterior radiographs. The average cement thickness was 3.2 mm, which is 1.2 mm greater than the size difference between the broach and the prosthesis, and was in the range 2–5 mm in 67 per cent of all measurement points. This has significance for the design of instrumentation to prepare the femoral cavity to give a defined cement mantle thickness.

There was a greater cement mantle thickness proximally than distally. In 95 cases it was possible to determine the orientation of the stem within the cement mantle, which showed an even distribution between varus and valgus orientation; 49 per cent were within 1 ° of neutral and only one case was more than 5° from neutral.
Time-dependent mechanical behaviour of the periodontal ligament
Authors: van Driel W D , van Leeuwen E J , Von den Hoff J W , Maltha J C , Kuijpers-Jagtman A M
Published: October 2002

The process of tooth displacement in response to orthodontic forces is thought to be induced by the stresses and strains in the periodontium. The mechanical force on the tooth is transmitted to the alveolar bone through a layer of soft connective tissue, the periodontal ligament. Stress and/or strain distribution in this layer must be derived from mathematical models, such as the finite element method, because it cannot be measured directly in a non-destructive way. The material behaviour of the constituent tissues is required as an input for such a model. The purpose of this study was to determine the time-dependent mechanical behaviour of the periodontal ligament due to orthodontic loading of a tooth. Therefore, in vivo experiments were performed on beagle dogs. The experimental configuration was simulated in a finite element model to estimate the poroelastic material properties for the periodontal ligament. The experiments showed a two-step response: an instantaneous displacement of 14.10 ± 3.21 μm within 4 s and a more gradual (creep) displacement reaching a maximum of 60.00 ± 9.92 μm after 5 h. This response fitted excellently in the finite element model when 21 per cent of the ligament volume was assigned a permeability of 1.0 × 10−14m4/Ns, the remaining 97 per cent was assigned a permeability of 2.5 × 10−17 m4/N s. A tissue elastic modulus of 0.015 ± 0.001 MPa was estimated. Our results indicate that fluid compartments within the periodontal ligament play an important role in the transmission and damping of forces acting on teeth.
A hip joint simulator study using new and physiologically scratched femoral heads with ultra-high molecular weight polyethylene acetabular cups
Authors: Barbour P S M , Stone M H , Fisher J
Published: October 2002

This study validates a hip joint simulator configuration as compared with other machines and clinical wear rates using smooth metal and ceramic femoral heads and ultra-high molecular weight polyethylene (UHMWPE) acetabular cups. Secondly the wear rate of UHMWPE cups is measured in the simulator with deliberately scratched cobalt-chrome heads to represent the type of mild and severe scratch damage found on retrieved heads. Finally, the scratching processes are described and the resulting scratches compared with those found in retrieved cobalt-chrome heads. For smooth cobalt-chrome and zirconia heads the wear rates were found to be statistically similar to other simulator machines and within the normal range found from clinical studies. An increased wear rate was found with cobalt-chrome heads scratched using either the diamond stylus or the bead cobalt-chrome but the greatest increase was with the diamond scratched heads which generated scratches of similar dimensions to those on retrieved heads. A greater than twofold increase in wear rate is reported for these heads when compared with smooth heads. This increased wear rate is, however, still within the limits of data from clinical wear studies.
Anisotropic response of the human knee joint meniscus to unconfined compression
Authors: Leslie B W , Gardner D L , McGeough J A , Moran R S
Published: October 2002

The response of meniscal tissue to axial, radial and circumferential compressive forces was measured at physiologically relevant levels of load in eight pairs of human knee joint menisci. Compression was unconfined and uniaxial. Stress-strain data were fitted to a two-parameter exponential model. The tissue was found to be significantly stiffer to axial compressive forces than to radial and circumferential forces. No significant difference was found between the responses to circumferential and radial forces.
The effects of different wire and screw combinations on the stiffness of a hybrid external fixator
Authors: Yang L , Saleh M , Nayagam S
Published: October 2002

Hybrid external fixators that use tensioned wires in the metaphysis and screws in the diaphysis combine the advantages of both unilateral and circular fixators and provide considerable flexibility in frame construction. This study aims to determine the effects of wire and screw combinations on the mechanical characteristics of such systems. Acrylic rods were used to simulate a long bone with a transverse fracture gap. A standard hybrid fixator design was used to stabilize the fracture in accordance with modern concepts with wires being used on one side of the fracture and screws on the other side. Twelve different wire and screw combinations were tested in axial compression and four-point bending in two planes. All the fixators tested demonstrated stiffening under increasing axial load, the extent of which appeared to correlate inversely with the number of wires used. Increasing the number of wires by one increased the axial stiffness by 7-16 N/mm (average 11 N/mm). The bending stiffness was largely controlled by the number of wires and increased by 0.42-1.03 N m/mm (average 0.74 N m/mm) for each additional wire. The axial stiffness of fixators with three screws was 4-7 N/mm (average 6 N/mm) higher than those with two screws if the offset screw was not in use. When the offset screw was used no stiffness advantages were found for three screws over two screws. The use of an offset screw substantially increased axial stiffness (by 12-26 N/mm, average 20 N/mm) and is recommended for unstable fractures. The two-ring hybrid fixators with four wires in one ring and three screws in the other had stiffness characteristics similar to the conventional four-ring Ilizarov fixator. Knowledge gained regarding the effects of wire and screw combinations on overall stiffness will be helpful both in the construction of frames and in destabilization during fracture healing.
A viscoelastic analysis of the tensile weakening of deep femoral head articular cartilage
Authors: Egan J M
Published: October 2002

Articular cartilage from below the surface of the femoral head of the hip joint shows a profound age-dependent weakening in its tensile mechanical properties. This ageing is also associated with a reduced viscoelastic response in the older tissue. A constitutive model of the viscoelastic behaviour of deep articular cartilage (as discussed by Egan in 1988) is used to generate a graphical pattern which represents the mechanical behaviour. This constitutive approach suggests that the tensile weakening of the older cartilage is due to an age-related reduction in the recruitment of load-carrying structures as the tissue is deformed.

The viscoelastic constitutive model also predicts a reduction in the tensile strength of deep articular cartilage with rate of deformation. This prediction is supported by experimental fracture stress data.

A weakening of the tensile integrity of the microstructure of articular cartilage could make the tissue less able to sustain normal compressive physiological loading without damage and thus make the tissue more susceptible to osteoarthritic degeneration. The constitutive approach indicates that the weakening of the older tissue may be related to changes within the microstructure which determine how applied mechanical energy is stored and dissipated.
Biomechanical simulation of an amputated forearm with and without a prosthesis
Authors: Meimoun Y , Mizrahi J
Published: October 2002

In this study a computer simulation was developed for analysing the performance of a below-elbow amputated forearm, with and without a prosthesis. The upper extremity was represented in terms of two rigid bodies, the arm and the forearm. Five muscles, three elbow flexors and two elbow extensors, were included in the model. The muscle model used was the five-component model, including the contractile, parallel, series and viscous elements and the muscle mass. Dynamic and static simulations were conducted, with and without prosthesis, to study parametrically the effects of stump length, tendon distal transfer, tendon or muscle shortening and muscle physiological cross-sectional area. The performance measures which were the most affected included flexion moment of the forearm about the elbow, muscle moment, force in the joint, flexion rate and mechanical energy. The simulation presented an interesting case when the amputation site is more proximal than the anatomical insertion point of a muscle, necessitating shortening of the muscle to avoid the situation where it exerts no force. It was also found that, of the changeable parameters, the most beneficial changes in the forearm parameters for improved dynamic performance were: (a) tendon distal transfer and (b) increase of the muscle cross-sectional area, the latter achievable by means of physical training.
Advanced textile composite ring for Ilizarov external fixator system
Authors: Baidya K P , Ramakrishna S , Rahman M , Ritchie A
Published: October 2002

The use of a radiolucent composite material permits easier and more accurate radiographic evaluation of the bone healing process, and results in a much lighter system. The finite element method (FEM) is employed to determine the worst possible in-service loading condition and the ring dimensions are modified accordingly. Half-ring prototypes are produced using two types of composite materials: knitted aramid fibre fabric reinforced epoxy and random short carbon (RSC) fibre reinforced epoxy. The in-plane compressive strength and axial stiffness of the complete frame are tested according to ASTM specifications. The performance is evaluated, and compared with an existing system in simulated in-service conditions.
An improved three-dimensional direct numerical modelling and thermal analysis of a female breast with tumour
Authors: Ng E Y K , Sudharsan N M
Published: October 2002

It is well known that malignant tumour tissue generally has higher metabolic and blood perfusion rates than most normal tissues. The authors aim to show that the tissue temperature profile within the breast and the surface temperature profile can be quantified to develop an expert system or diagnostic tool for breast cancer detection. The surface temperature and tissue temperature profiles are analysed for a three-dimensional numerical model of a normal breast and a breast with a tumour. Tumours of different sizes are placed at various locations. In the model, the tissue temperature profile is distorted at the tumour location and was found to compare well with in vivo tests. It was also found that as the tumour was moved to deeper locations its effect on surface temperature was lower. It was observed that small tumours in deeper regions do not have a significant isolated impact on the surface. The numerical results could also capture a shift in the position of the tumour. For tumours greater than 10mm in the superficial regions and of significant size in deeper regions, it could be seen that the surface temperature distribution of the breast is directly related to the position and size of the tumour embedded in it. The feasibility of providing a diagnostic tool in conjunction with numerical modelling and high-resolution thermograms is also discussed.
A comparative evaluation of dental luting cements by fracture toughness tests and fractography
Authors: Ryan A K , Orr J F , Mitchell C A
Published: October 2002

In recent years there has been a shift from traditional methods of investigating dental materials to a fracture mechanics approach. Fracture toughness (KIC) is an intrinsic material property which can be considered to be a measure of a material's resistance to crack propagation. Glassionomer cements are biocompatible and bioactive dental restorative materials, but they suffer from poor fracture toughness and are extremely susceptible to dehydration.

The main objective of this study was to evaluate the fracture toughness of three types of commercially available dental cements (polyacid-modified composite resin, resin-modified and conventional glass ionomer) using a short-rod chevron-notch test and to investigate and interpret the results by means of fractography using scanning electron microscopy. Ten specimens of each cement were fabricated according to manufacturers' instructions, coated in varnish, and stored at ambient laboratory humidity, 100 per cent relative humidity, or in water at 37 °C for 7 days prior to preparation for testing. Results indicated that significant differences existed between each group of materials and that the fracture toughness ranged from 0.27 to 0.72 MN/m3/2. It was concluded that the resin-modified glass-ionomer cement demonstrated the highest resistance to crack propagation. Fractographs clearly showed areas of stable and unstable crack growth along the fractured surfaces for the three materials examined.
In vitro testing of the primary stability of the VerSys enhanced taper stem: A comparative study in intact and intraoperatively cracked femora
Authors: Monti L , Cristofolini L , Toni A , Ceroni R G
Published: October 2002

The Zimmer VerSys enhanced taper uncemented stem was tested in vitro for primary stability. Six stems were implanted in six composite femora. Three femora were unintentionally damaged by cracking of the bone during stem insertion and press-fit. A previously validated protocol was used to load the specimens cyclically and to record locally the rotational and axial bone/implant relative motions in terms of elastic motion and permanent migration.

For the undamaged femora, the VerSys stem showed excellent primary stability, in terms of both elastic motions (less than 9 μm) and permanent migrations (less than 6 μm), and in both axial and rotational directions, comparing favorably with the literature. Intraoperatively-induced proximal cracks did influence the primary stability of the stem in terms of permanent migration. As cracks may easily be produced in the diaphyseal canal during insertion of cementless stems, which rely for primary stability on conical fitting into the canal, great care must be taken in preparation of the canal and insertion of the stem.
A novel method for the prediction of functional biological activity of polyethylene wear debris
Authors: Fisher J , Bell J , Barbour P S M , Tipper J L , Mattews J B , Besong A A , Stone M H , Ingham E
Published: October 2002

The comparative performance of artificial hip joints has been extensively investigated in vitro through measurements of wear volumes. in vivo a major cause of long-term failure is wear-debris-induced osteolysis. These adverse biological reactions are not simply dependent on wear volume, but are also controlled by the size and volumetric concentration of the debris. A novel model is presented which predicts functional biological activity; this is determined by integrating the product of the biological activity function and the volumetric concentration function with the wear volume over the whole particle size range. This model combines conventional wear volume measurements with particle analysis and the output from in vitro cell culture studies to provide a new indicator of osteolytic potential. The application of the model is demonstrated through comparison of the functional biological activity of wear debris from polyethylene acetabular cups articulating under three different conditions in a hip joint simulator.
Modelling femoral curvature in the sagittal plane: A cadaveric study
Authors: Stephenson P , Seedhom B B
Published: October 2002

This study examined the possibility of representing the mid third of the human femur with two straight sections. This portion of the femur visually has a distinct curvature, which can potentially present problems when considering implant stem designs to be introduced in this region. Sixteen femora were sectioned at 10mm intervals along the femoral shaft in the mid third region (35-65 per cent of femoral length). Photographic records were obtained of each section against a consistent axis system to which all coordinates were referenced. The position of the centre of the medullary canal cross-sectional area along the femur, in relation to fixed orthogonal planes, has been analysed; the outer anterior cortex was also modelled. The results showed that the medullary centre of area plots and the anterior cortex coordinates are suitably modelled as two straight lines. For each bone it was possible to define the intersection point between the two straight sections (point of angulation), and the subtended angle between these sections (angle of incidence). The average point of angulation for the medullary plots occurred at 57 per cent along the femur, while the mean angle of incidence was 6.5°. The anterior surface had an average point of angulation at 58 per cent along the femur with the mean angle of incidence being 22.2°. The centre-line of the medulla was also found to be almost parallel to the outer anterior surface for sections distal to the point of angulation. It is proposed therefore, that this difference in angulation is the result of medullary expansion/cortical thinning towards the proximal extremity of the femur, causing the straight-line model of the medulla to angulate less than the outer anterior cortex.
The development of a modular humeral prosthesis locking mechanism
Authors: Fowler N K , Nicol A C , Marsden D C
Published: October 2002

A new modular humeral prosthesis has been developed, incorporating a novel locking mechanism to prevent head-stem disassembly. A series of test protocols was designed to evaluate the integrity of the new locking mechanism and a dedicated test rig was manufactured. Preliminary test results were disappointing and the locking mechanism failed at lower applied loads than predicted. Microscopy studies were conducted to establish the cause of this poor performance and with the aid of statistical analysis, these studies assisted in the development process. Development of the design was directed at the control of the position and angle of contact between bearing surfaces of the locking mechanism. The optimized design was tested under the maximum anticipated loading conditions and performed satisfactorily.
The effect of femoral head diameter upon lubrication and wear of metal-on-metal total hip replacements
Authors: Smith S L , Dowson D , Goldsmith A A J
Published: October 2002

It has been found that a remarkable reduction in the wear of metal-on-metal hip joints can be achieved by simply increasing the diameter of the joint. A tribological evaluation of metal-on-metal joints of 16, 22,225, 28 and 36 mm diameter was conducted in 25 per cent bovine serum using a hip joint simulator. The joints were subject to dynamic motion and loading cycles simulating walking for both lubrication and wear studies. For each size of joint in the lubrication study, an electrical resistivity technique was used to detect the extent of surface separation through a complete walking cycle. Wear of each size of joint was measured gravimetrically in wear tests of at least 2 × 106 cycles duration.

Joints of 16 and 22.225mm diameter showed no surface separation in the lubrication study. This suggested that wear would be proportional to the sliding distance and hence joint size in this boundary lubrication regime. A 28 mm diameter joint showed only limited evidence of surface separation suggesting that these joints were operating in a mixed lubrication regime. A 36 mm diameter joint showed surface separation for considerable parts of each walking cycle and hence evidence of the formation of a protective lubricating film.

Wear testing of 16 and 22.225mm diameter metal-on-metal joints gave mean wear rates of 4.85 and 6.30mm3/106 cycles respectively. The ratio of these wear rates, 0.77, is approximately the same as the joint diameters ratio, 16/22.225 or 0.72, as expected from simple wear theory for dry or boundary lubrication conditions. No bedding-in was observed with these smaller diameter joints. For the 28 mm diameter joint, from 0 to 2 × 106 cycles, the mean wear rate was 1.62 mm3/106 cycles as the joints bedded-in. Following bedding-in, from 2.0 × 106 to 4.7 × 106 cycles, the wear rate was 0.54mm3/106 cycles. As reported previously by Goldsmith in 2000 [1], the mean steady state wear rate of the 36 mm diameter joints was lower than those of all the other diameters at 0.07 mm3/106 cycles. For a range of joints of various diameters, subjected to identical test conditions, mean wear rates differed by almost two orders of magnitude. This study has demonstrated that the application of sound tribological principles to prosthetic design can reduce the wear of metal-on-metal joints, using currently available materials, to a negligible level.
Computational studies of ‘whiplashg’ injuries
Authors: Gentle C R , Golinski W Z , Heitplatz F
Published: October 2002

The term ‘whiplash’ was initially used to describe injuries to the neck caused by the head being forced backwards during a rear-end collision in cars without head restraints. The addition of head restraints in the 1970s was expected to solve this problem by preventing excessive extension of the neck but experience suggests the problem still exists. This paper reviews available experimental studies of whiplash and uses the data to construct a finite element model which is capable of dynamically simulating whiplash collisions and predicting the forces in all the relevant neck ligaments. For the first time, it is shown that trauma occurs long before the head hits the head restraint as a result of displacement between the head and the torso caused by the head's inertia leading to markedly different acceleration histories. It is concluded that experimental and computational studies must be used together to produce progress in biomechanical studies.
Some failure modes of four clinical bone cements
Authors: Liu C , Green S M , Watkins N D , Gregg P J , McCaskie A W
Published: October 2002

The fracture or failure behaviours of four commercial acrylic-based bone cements have been examined in tensile, bending and compression modes, and their mechanical properties are reviewed. It was found that Palacos R-40 bone cement had high radiopaque agent concentration, with high surface hardness. It exhibited a much lower bending strength and bending modulus compared with the other three bone cements (CMW1, CMW2000 and Simplex P). The textures of tensile fracture surfaces produced were similar for the four bone cements studied. The fracture surface was fragmented by crevices, which developed through the matrix and around large undissolved polyme-thylmethacrylate (PMMA) beads. Three bands with different features existed on the bending fracture surfaces, with an abrupt transition between them. It appears that the agglomerates of zirconium dioxide particles are implicated in Palacos R-40 bone cement fracture surface. The examination of compressive failed specimens revealed that a ‘yielded crack band’ existed across the transverse section. Plastic deformation resulted in the PMMA beads being squashed in the longitudinal direction and dilated in the transverse direction.
A novel instrumented ring for the measurement of grip force adjustments during precision grip tasks
Authors: van der Kamp M , Conway B A , Nicol A C
Published: October 2002

A wide range of scientific and clinical research studies use the measurement of grip force to quantify hand function and activities during daily living. Many applications of instrumented objects can be found in the biomechanical and neurophysiological literature. However, these were found not to be suitable for the measurement of grip force and force modulations during precision grip independently from the hand orientation.

The low-cost precision grip force transducer described here is capable of recording magnitude, direction and modulation of the force exerted on a closed ring. The design is based on a standard proving ring, onto which a second set of strain gauges is applied. The outputs of both Wheatstone bridges yield a unique signature for every position under a two-point load.

The tested aluminium ring had an outer diameter of 83 mm, a wall thickness of 3 mm and a height of 12mm. With eight single bending strain gauges used, the maximum load was 100N. During a grip task, tremor components from d.c. to 45 Hz could be detected. The newly developed ring might therefore find a use in many biomechanical and neurophysiological studies as a tool for measuring grip force and its fine modulations.
Hip simulator wear testing according to the newly introduced standard ISO 14242
Authors: Kaddick C , Wimmer M A
Published: October 2002

Ultra-high molecular weight polyethylene (UHMWPE) acetabular cups were tested against alumina-ceramic femoral heads using a new type of hip joint simulator according to ISO/FDIS 14242-1. Bovine serum as well as newborn calf serum were used as test fluids. Total polyethylene wear was determined by weight loss of the cups. In addition, wear depth and its distribution were recorded by means of a coordinate measurement system. Wear particle analysis and inspection of the worn polyethylene surfaces using light and scanning electron microscopy (SEM) were performed to analyse damage and identify the acting wear mechanisms.

The total wear rate was determined to be 22.07 ± 1.75 mg/106 cycles for the bovine serum group and 26.57 ± 3.55 mg/106 cycles for the calf serum group. Unexpectedly, the formation of two wear vectors corresponding to recent clinical findings was detected. Retrieved polyethylene wear debris was comparable in size and shape with clinical findings. The test method described by ISO/FDIS 14242-1 produced reliable and reproducible wear data using UHMWPE acetabular cups articulating against alumina-ceramic heads. In the authors' opinion, the lubricant composition should be described in more detail, since the protein and additive content seem to have a high impact on the wear results. It needs to be emphasized that the findings of this study cannot be regarded as a general validation of hip wear tests according to ISO/FDIS 14242-1 but are limited to the material combinations investigated herein. Further testing of other clinically relevant materials and interlaboratory ring tests must follow.
Predictive rheological models for the consolidation behaviour of articular cartilage under static loading
Authors: Nguyen T , Oloyede A
Published: October 2002

This paper evaluates an existing rheological model of articular cartilage and explores the representative capacity of other phenomenological models of the tissue's matrix within the framework of mechanical consolidation. A unique feature is the introduction of a swelling element in tandem with ‘fluid-filled’ hyperelastic rheological elements to predict the transient load-induced behaviour of the tissue and evaluate the role of swelling in determining its load-carriage mechanism. The rheological models proposed have been used to predict the dependence of the one-dimensional consolidation response of the articular cartilage matrix, and the results obtained have been compared with published experimental results. This comparison demonstrates that the hydrostatic excess pore pressure, especially in the initial stages of deformation cannot be predicted without an adequate representation of swelling and its non-linear interaction with mechanical governing parameters such as permeability and stiffness.
Risk analysis for a radio-carpal joint replacement
Authors: Shepherd D E T
Published: October 2002

Risk analysis is required by the medical device directives to provide evidence that manufacturers have eliminated or reduced risks as far as possible so that a medical device does not compromise the safety of patients or health workers. This paper presents a risk analysis for the Swanson wrist implant, which is made from an implantable-grade silicone elastomer and used to replace the radio-carpal joint in the rheumatoid wrist. The main hazards identified were that the implant fractures and that silicone synovitis occurs in patients. The results of this risk analysis will be used to aid the design of a new wrist implant.
Investigation of wear of knee prostheses in a new displacement/force-controlled simulator
Authors: Barnett P I , McEwen H M J , Auger D D , Stone M H , Ingham E , Fisher J
Published: October 2002

The performance of two knee simulators designed by ProSim (Manchester, UK) was evaluated by comparison of the wear seen in the press-fit condylar (PFC) Sigma (DePuy) knee prosthesis. Twelve specimens of the same design and manufacturing specification, were subjected to a wear test of 2 ×106 cycles duration using bovine serum as a lubricant. The anterior/posterior displacement and internal/external rotation inputs were based on the kinematics of the natural knee. International Standards Organization (ISO) standards were used for the flexion and axial load. The wear rates and wear scar areas were compared across all stations. The mean wear rates found were 17.6 ± 5 mm3/106 cycles for stations 1 to 6 and 19.6 ± 4 mm3/106 cycles for stations 7 to 12, resulting in an overall mean wear rate of 18.1 ± 3 mm3/106 cycles. The differences between the two simulators were not significant. The average wear scar area seen on inserts from stations 1 to 6 was calculated at 32.4 ± 1 per cent of the intended articulating surface. Similarly on stations 7 to 12 the average wear scar area was 30.7 ± 3 per cent. The wear scars seen were a good physiological representation of those found from clinical explant data. This study has shown good repeatability from the simulator, both within and between the simulators.
Growth and remodelling of the autologous bone transplant used in a pediatric femoral reconstruction
Authors: Taddei F , Viceconti M , Manfrini M , Toni A
Published: October 2002

The aim of the present work was to assess how growth and remodelling changed the morphology of the transplanted fibula used to reconstruct the proximal femur of a 5 year old child affected by a Ewing's sarcoma during the first 3 years of follow-up. The morphological evolution of the transplant was quantitatively assessed on diagnostic images. Special software was developed to perform three-dimensional measurements on computed tomography (CT) datasets, while state-of-the-art image processing software was used for conventional radiography. The measurements were then correlated with the loads expected to act on the hip during the various stages of the rehabilitation protocol. A simple cantilever beam model was used for a gross estimate of the risk of fracture of the transplant. The results of the analysis showed that there is no clear correlation between the morphological changes of the autograft and the hip loading conditions experienced. A part from a drastic increase in the periosteal radius in the frontal plane, occurring in the first 10 months after the operation, the growth of the transplanted fibula seems well within the ranges of the normal fibular growth. The cantilever beam model suggested that, although the autograft is currently subjected to subcritical stresses, morphological evolution could increase the risk of fracture in the next few years if a normal level of loading were allowed.
Computational and experimental simulations of the haemodynamics at cuffed arterial bypass graft anastomoses
Authors: Cole J S , Wijesinghe L D , Watterson J K , Scott D J A
Published: October 2002

The development of intimal hyperplasia at arterial bypass graft anastomoses is a major factor responsible for graft failure. A revised surgical technique, involving the incorporation of a small section of vein (vein cuff) into the distal anastomosis of polytetrafluoroethylene (PTFE) grafts, alters the distribution of intimal hyperplasia and improves graft performance. Numerical and in vitro flow visualization experiments have been conducted to identify the flow behaviour in the cuffed bypass model and to determine whether the improved performance of the cuffed system can be accounted for by haemodynamic factors. The flowfield at the cuffed anastomosis is characterized by an expansive recirculation. Separation occurs at the graft heel, and at the cuff toe as the blood enters the recipient artery. Wall shear stresses in the vicinity of the cuff heel are low, but high shear stresses and large spatial gradients in the shearing force act for a time on the artery floor. In the conventional model, a less disturbed flow prevails while the gradients of shear stress on the floor are smaller. Aspects of the anastomotic haemodynamics are worsened when the cuff is employed. The superior patency rates of cuffed bypasses may not be explained purely on the basis of local haemodynamic factors.
Stress relaxation modelling of polymethylmethacrylate bone cement
Authors: Eden O R , Lee A J C , Hooper R M
Published: October 2002

This paper describes tests that were carried out to model the stress relaxation behaviour of polymethylmethacrylate (PMMA) bone cement. Stress relaxation of bone cement is believed to be a significant factor in the mechanism of load transfer in the femoral stem of a polished, collarless taper-fit replacement hip joints. It is therefore important that this condition and its implications are understood. Stress relaxation was carried out on PMMA samples of varying age in four-point bending configuration. It was shown that the samples stiffened with age and that the amount of stress relaxation reduced as the samples aged. The experimental results of the stress relaxation were accurately modelled on the double exponential of the Maxwell model so that long-term predictions of the stress condition could be made from short-term mechanical tests.