bluuebunny/crossref_metadata_2025 huggingface dataset:
The design of a finger wear simulator and preliminary results
Authors: Joyce T J , Unsworth A
Published: October 2002

A dual-cycle finger wear simulator has been designed, manufactured and commissioned. The simulator interspersed dynamic flexion-extension motion under light load with a heavier static ‘pinch’ load to a test prosthesis immersed in a lubricant heated to 37°C. A validation test was undertaken on a size 2 Swanson prosthesis, leading to prosthesis failure in less than 1 million cycles. A second test was carried out on a Durham metacarpophalangeal prosthesis. After 4.8 million cycles a total wear factor for the joint of 0.60 × 10−6 mm3/N m was calculated, with no cracks or damage visible. Both test results compare well with earlier tests undertaken on the Stokoe finger wear simulator.
Review of arm motion analyses
Authors: Anglin C , Wyss U P
Published: October 2002

Interest in arm movements has increased tremendously in recent years. This interest has been motivated by different goals: the desire for a more scientific approach to replacement or support of the joints of the upper limb, the need for input to biomechanical computer models, and the clinical interest in comparing normal movements with pathological movements. The availability of commercial marker-tracking systems has facilitated achieving these goals. However, the complex nature of arm movements and the lack of standardized movements raises many challenges. In comparison with gait analysis, few arm motion analyses have been conducted. The purpose of this review is to aid researchers and clinicians interested in conducting an arm motion study in choosing the appropriate methodology. This is accomplished both by describing the methods used in past investigations and by highlighting important findings. Due to the variety of research goals, there is sometimes more than one appropriate method and the choice is left to the reader. Nevertheless, since it is extremely desirable to record and express the data in a standardized way, standardization proposals are described. This review, which focuses on methodology rather than results, addresses the following topics: motivations and tasks studied, tracking methods, the shoulder complex, joint centres and rotation axes, marker positions, coordinate system definitions, terminology and rotations, accuracy, and presentation methods.
A method for the evaluation of the change in volume of retrieved acetabular cups
Authors: Raimondi M T , Sassi R , Pietrabissa R
Published: October 2002

Studies on retrieved hip prostheses are currently performed in order to assess the wear mechanisms and the overall wear rate of such artificial joints. Several reported studies on the survival of artificial hips have been based on the measurement of the amount of worn material directly on retrieved acetabular cups. The estimation of the change in volume, V, of the cup cavity is particularly difficult in the case of slight wear due to several factors of which the most critical is the lack of information on the unworn geometry of the cup.

This paper presents a new measuring technique, which is described in detail and has been applied to estimate the wear of 65 acetabular cups harvested from revised hip arthroplasty. The coordinate data of several points on the articular surface are sampled using a coordinate measuring machine (CMM). The value of V is calculated mathematically from the measurements by using the hypothesis that the actual shape of both slightly worn and highly worn surfaces has a small departure from a truly spherical shape. The wear volume is estimated with reasonable accuracy mainly on those cups showing penetration depths greater than 0.2 mm, corresponding to an amount of wear greater than 100 mm3 for a 32 mm cup. The uncertainty in the results is estimated for each cup. The repeatability of the technique is studied for a case showing very slight wear. The advantages, disadvantages and limitations of the method are presented and discussed.
The design and testing of a composite lower limb prosthesis
Authors: Strike S , Hillery M
Published: October 2002

In unilateral lower limb prosthetic gait, the intact limb, together with the hip on the affected side, strive to compensate for the loss of the missing below-knee musculature. The resultant abnormal gait patterns can eventually lead to pathologies of the spine and other joints. The aim of this research was to design a trans-tibial dynamic elastic response (DER) prosthesis which could simulate the power generation and absorption properties of an intact foot and shank segments. A carbon fibre sickle-shaped prosthesis was developed in which the strain energy from the early part of the stride was stored, and was released again at the point of take-off, to simulate the action of the missing musculature. Stress analysis techniques were used in the design of the prosthesis, for the purposes of computing and maximizing the strain energy of the elements. A force transducer was designed into the prosthesis to verify the analysis. Video motion analyses of the prosthesis were carried out in order to determine the most appropriate shape that would reduce gait asymmetries. The research shows that biomechanical techniques, together with good engineering design and the selection of modern materials, can lead to a prosthesis which approaches the function of a natural foot.
Pliable polylactide plates for guided bone regeneration: Manufacturing and in vitro
Authors: Kellomäki M , Paasimaa S , Törmälä P
Published: October 2002

Three different types of pliable and bioabsorbable plates, 0.4 mm thick, were developed for guided bone regeneration to cover cranial defects. The processing (extrusion, melt-spinning, knitting and heat pressing) and in vitro degradation of the materials were studied. Materials used were poly-L, DL-lactide with an L/DL ratio of 70/30 (PLA70) and poly-L, D-lactide with an L/D ratio of 96/4 (PLA96). The initial tensile strengths of γ-sterilized PLA96, PLA70 and the PLA70-PLA96 composite plates were 45.7 ± 3.8, 51.2 ± 3.6 and 24.7 ± 5.1 MPa respectively. The composite plates were the stiffest and lasted for more than 24 weeks. The glass transition temperature ( Tg) of both polymers decreased in vitro. The crystallinity of PLA96 increased tenfold within 18 weeks. For initially amorphous PLA70 the highest melting enthalpy was 89 J/g at 60 weeks. PLA70 became partially crystalline and the plates changed from transparent to white and swollen. Extrusion and sterilization decreased the initially different molecular weight ( Mw) values to the same level. After 18 weeks of hydrolysis, Mw was 15 000 Da for PLA96 and 12 000 Da for PLA70. For the components of the composite plate Mw was 15 000 Da for the PLA70 plate and 27 000 Da for the PLA96 mesh. Morphologically, all the hydrolysed plates retained, for a long period, a solid surface layer under which a porous structure formed. Crystalline branches and some single crystals were seen. The composite plates had the slowest degradation rate and they remained intact the longest.
Glenohumeral contact forces
Authors: Anglin C , Wyss U P , Pichora D R
Published: October 2002

Glenohumeral contact forces have only been calculated previously either for simple abduction or for athletic activities. The objective of this study was to determine the glenohumeral contact forces for tasks which are demanding of the shoulder but which would commonly be performed by older people. The functional tasks chosen were using the arms to stand up from and sit down into a chair, walking with a cane, lifting a 5 kg box to shoulder height with both hands, and lifting a 10 kg suitcase. The trunk angles, arm angles and hand loads of six healthy subjects, average age 55 years, were recorded. This information was input into a biomechanical computer model which optimized the muscle force distribution by minimizing the sum of squared muscle stresses subject to constraints on the maximum muscle forces and maintaining the direction of the resultant force within the glenoid fossa. Average contact forces ranged from 1.3 to 2.4 times body weight (930-1720 N), the highest force being for lifting a suitcase. This latter value would be even higher if lifting either a greater load or to a greater height. Thus, contact forces at the shoulder should not be underestimated. This study provides functionally relevant contact forces which can be used for mechanical testing or finite element modelling of shoulder prostheses.
Characterization of vortices using pulsed-wave Doppler ultrasound
Authors: Gaupp S , Wang Y , How T V , Fish P J
Published: October 2002

The detection and characterization of vortices from a Kaman vortex generator by means of a 20 MHz pulsed-wave Doppler ultrasound system were assessed. Measurements were made at different steady flowrates in a 10 mm internal diameter polyurethane tube, 14 mm distal to a circular cylinder of diameter 2 mm, placed across the tube inlet. The results were compared with those obtained with a two-component laser Doppler anemometer system. There was generally good agreement between the two techniques in the measurement of convective flow velocity, frequency of vortex shedding and the circulation velocity of the vortices. It is concluded that pulsed-wave Doppler ultrasound is a suitable technique for investigating vortical flow structures.
Friction in orthopaedic zirconia taper assemblies
Authors: Macdonald W , Aspenberg A , Jacobsson C M , Carlsson L V
Published: October 2002

The torque resistance of zirconia ceramic heads/titanium taper trunnion junctions was tested in accordance with ISO 7206-9:1994(E); using twelve modified heads of 32 mm diameter under representative physiological conditions. Test parameters studied included assembly force, vertical load during test (test load) and head length. Mean torque resistances measured were 8.9 N m for a 1 kN test load and 15 N m at 4 kN test load. Coefficients of friction calculated for the torsional stability ranged from 0.06 to greater than 1.0.

Multiple regression analysis confirmed that the failure torques measured were significantly dependent on test load (β = 0.77; P < 0.001) whereas assembly force and head length played a lesser, insignificant, part in the variation. Data from push-on/pull-off tests were used to calculate coefficients of friction under axial loading, which were significantly correlated with taper angle and material. Torque testing shows greater variability than push-on/pull-off tests for similar combinations, and for zirconia heads on other tapers. The coefficients of friction measured (0.16-0.31) are significantly different from values typically used in stress analyses.
Simplified motion and loading compared to physiological motion and loading in a hip joint simulator
Authors: Smith S L , Unsworth A
Published: October 2002

Two wear tests were conducted using the Durham Hip Joint Wear Simulator to investigate the effects of simplified motion and loading on ultra-high molecular weight polyethylene (UHMWPE) acetabular cup wear rates. Bovine serum was used as a lubricant and a gravimetric technique was used to measure wear.

The first wear test duration was 7.1 × 106 cycles and investigated the effect of simplified loading. This was achieved by using full physiological motion and loading for the first 5 × 106 cycles of the test, then physiological motion with simplified loading for the final 2.1 × 106 cycles of the wear test. The UHMWPE acetabular cup wear rates using full physiological motion and loading were 32.2 and 51.7 mm3/106 cycles against zirconia and CoCrMo femoral heads respectively. Using simplified loading the cup wear rates were 30.1 and 49.2 mm3/106 cycles against zirconia and CoCrMo respectively which was not significantly different from wear rates with physiological loading.

The effect of simplified motion was investigated in a second wear test of 5.0 × 106 cycles duration. Physiological loading was applied across the prosthesis with physiological motion in the flexion/extension plane only. Mean wear of the acetabular component dropped to 0.197 mm3/106 cycles.

The surfaces of all the actabular cups were subject to gross examination, optical microscopy and scanning electron microscopy. No notable difference was observed between the cups subjected to physiological motion and loading and those subjected to simplified loading. The cups worn with a single plane of motion had a much smaller worn area and a notable difference in surface features to the other cups. Simplifed loading is therefore an acceptable simplification in simulator testing but simplifying motion to the flexion/extension plane axis only is unacceptable.
Determination of the major dimensions of femoral implants using morphometrical data and principal component analysis
Authors: Low F H , Khoo L P , Chua C K , Lo N N
Published: October 2002

This paper describes the work that leads to the establishment of a set of major parameters for the design of symmetrical prosthetic implants for the Asian population. In the study, 62 sets of femurs harvested from cadavers were used. The morphometrical data obtained are compared with known results and found to be in good agreement with Asian knees. Subsequently, the data are treated and analysed using the principal component analysis, a statistical technique for analysing multivariate data. The analysis has resulted in the establishment of the major design parameters for six different sizes of femoral implants. Details of the analysis are presented. The major parameters obtained in this work are compared with those of existing implants. Results of the comparison are presented. The relationship between the anterio-posterior and medio-lateral dimensions is also examined and reported.
Numerical investigation of fixed orthodontic brackets
Authors: Middleton J , Hübsch P , Kralj B , Knox J , Jones M L
Published: October 2002

This paper presents the results of investigations into the mechanical behaviour of fixed orthodontic brackets. Special attention has been paid to the detailed modelling of the interface between the bracket base and an adhesive by which attachment is made to the tooth surface. Properties of this interface were taken as design parameters with the aim of investigating their influence on the stress field induced during bracket removal at the end of the treatment. Stress-strain analysis was performed using a finite element program developed in-house.
Temperature and ageing condition effects on the characterization of acrylic bone cement
Authors: Baleani M , Cristofolini L , Toni A
Published: October 2002

This study investigates the effect of the environmental temperature and ageing condition on the characterization of acrylic bone cement. The tests were performed according to ISO 5833. The testing parameters were allowed to vary within the limits defined by the standard, in order to assess their effect on the results of the test. In certain cases the tests were also performed under conditions which the standard does not provide for but which are likely to occur clinically.

This investigation showed that the cement behaviour may also change in the temperature range specified in the standard. Therefore, it is deemed appropriate to correlate the curing parameters of the bone cement to the environmental temperature, performing the test at different temperatures. In this way the effect of temperature on the duration of the phases in the cement curing could be assessed. The resultant graphical representation of the effect of temperature on the duration of the phases in cement curing has direct clinical relevance. Furthermore, this study showed that the ageing conditions of the mechanically tested specimens affected the results. Hence, it is deemed advisable to modify the ageing conditions of the specimens, fixing them closer to the in vivo conditions.
Suprapubic track pressure and force-deformation measurements in a (live) human subject and in animal models post-mortem
Authors: Coveney V A , Gepi-Attee S , Grover D , Painter D
Published: October 2002

Tests have been performed on animal models shortly post-mortem and on a healthy human subject in order to obtain estimates of the forces which act on suprapubic urinary catheters and similar devices and to develop an abdominal wall simulator. Such data and test methods are required for the systematic design of suprapubic devices because of the dual need to maintain the functionality of devices and to avoid excessive pressure on soft body tissue which could lead to ischaemia and in turn necrosis. In the post-mortem animal models, electrical excitation was applied to the abdominal wall in order to stimulate muscle activity. Two types of transducers were used: a soft membrane transducer (SMT) for pressure measurement and novel instrumented ‘tongs’ to determine indentation stiffness characteristics in the suprapubic track or artificial pathway created for a device. The SMT has been extensively used in the urethras and bladders of human subjects while the tongs were built speci. cally for these tests. Only the well-established SMT was used with the human subject; a peak pressure of 22 kPa was obtained. In the animal models the pressure profile given by the SMT had a peak whose position corresponded well with the estimated location of the rectus muscle measured on the fixed tissue section. The peak value was 5.5 kPa, comparable with values likely to cause necrosis if maintained for more than 1 day. Remarkably consistent indentation stiVness values wereobtained with the instrumented tongs; all values were close to 0.45 N/mm (33 kPa/mm).
Magnetic tracker calibration for an augmented reality system for therapy
Authors: Tian X , Ling K V , Ng W S
Published: October 2002

Magnetic trackers are widely used position/orientation measurement devices in interactive computer systems. These trackers output six-degree-of-freedom position and orientation of their receiver coordinates with respect to their transmitter coordinates. It is a potential solution for realtime position/orientation measurement in an augmented reality system for therapy (ART). However, an obstruction to be overcome is that a magnetic tracker is vulnerable to measurement inaccuracy caused by magnetic field distortions (including ferric and non-ferric distortions) when it is used in metallic environments. The purpose of this paper is to discuss calibration methods used to compensate distortions produced by metallic objects which are moving during the measurement procedure, but fixed with a receiver. Past work on the calibration of magnetic trackers has concentrated on static distortions, which are produced by stationary metallic objects in the environment. Compared with static distortions, in this paper a relatively dynamic situation is considered where the distortion is not constant with time. However, the location of the metallic objects is reflected in the outputs of the receiver so that the distortion can be compensated. This type of distortions, which is called ‘quasi-static distortion’ in this paper, is common in an ART. Compensation of quasi-static distortion is more complicated than that of static distortion and has not been mentioned in past publications. Some new compensation methods for quasi-static distortion will be introduced here. The results demonstrate that the methods are capable of significantly reducing quasi-static distortions.
A five-station hip joint simulator
Authors: Smith S L , Unsworth A
Published: October 2002

A five-station hip joint wear simulator was designed and built which featured simplified motion and loading. An elliptical wear path was produced using approximately sinusoidal motion in the flexion/extension and internal/external rotation axes and the dynamic loading approximated to a square wave. Five 28 mm diameter zirconia femoral heads articulated against ultra-high molecular weight polyethylene acetabular cups in 25 per cent bovine serum for 5 × 106 cycles. Gravimetric wear measurement was used with moisture absorption compensation using a dynamically loaded soak control. With motion of physiological magnitude, the mean acetabular cup wear rate was 52.2 mm3/106 cycles which is comparable with a number of clinical studies.
Discussion: A technique for measuring the compressive modulus of articular cartilage under physiological loading rates with preliminary results
Authors: Mann R W
Published: October 2002

On the issue of ultrasound measurement of cartilage thickness, in his Reply to my initial Communication [1] Dr Shepherd asserts that we ‘assume’ a velocity for the speed of sound in cartilage in our ultrasonic measurement of acetabula cartilage thickness and topography [2]. Page 254 of our 1981 Journal of Biomechanics article [2] states: ‘Sound velocity in the respective media was experimentally determined using samples of known geometry.’ A prior publication of the Leeds group in 1989 [3] cited reference [2] and stated ‘Efforts by the present authors to measure cartilage thickness using ultrasound indicate… Thickness measurements were difficult to obtain and even with immaculate cartilage, signal attenuation was pronounced. The unreliability in obtaining measurements prevented even a rudimentary repeatability study from being performed.’ Dr Shepherd also cites in his Reply to my Communication [1] a brief abstract [4] by his group which also dismisses ultrasound measurement with ‘ultrasonic measurement … its accuracy in measuring the thickness of human articular cartilage in situ has not been rigorously tested, nor has the potential of ultrasonic measurements for measuring mechanical properties of cartilage been evaluated’; abstract [4] cites no references.
A new medical microrobot for minimal invasive surgery
Authors: Zhou Y S , Quan Y X , Yoshinaka K , Ikeuchi K
Published: October 2002

A new medical microrobot to drive endoscopes into endocoeles is presented. The structure of the microrobot is very simple; it consists of a right spirally grooved micromotor, a left spirally grooved cylinder and a flexible coupling. When the micromotor rotates, a hydrodynamic mucus film is formed because highly viscous mucus exists in endocoeles. The mucus film can prevent direct contact between the microrobot and the endocoele, and the injury to organic tissues may be avoided. The locomotion speed and the hydrodynamic mucus film thickness formed when the microrobot drives endoscopes into endocoeles have been calculated according to hydrodynamic lubrication theory. The results indicate that the microrobot can be suspended to drive endoscopes quickly into endocoeles. This has been confirmed by experimests.
Unsteady viscous flow model on moving the domain through a stenotic artery
Authors: Ng E Y K , Siauw W L
Published: October 2002

An unsteady Navier-Stokes (N-S) solver based on the method of operator splitting and artificial compressibility has been studied for the moving boundary problem to simulate blood flow through a compliant vessel. Galerkin finite element analysis is used to discretize the governing equations. The model has been applied to a time-varying computational domain (two-dimensional tube) as a test case for validation. Consideration has been given to retaining the space conservation property. The same code is then applied to a hypothetical critical high-pressure gradient over a short length of blood vessel based on the spring and dashpot model. The governing equation for the blood vessel is based on two-dimensional dynamic thin-shell theory that takes into account the curvature of the stenotic portion of the vessel. Progressing the solution towards steady state is considered, as the main objective is to show the viability of the current technique for fluid/structure interactions. Preliminary results of the wall velocity and displacement based on steady state prediction agree well with data in the literature. Results, such as the streamlines, wall pressures and wall shear stress depict the possible progression of arterial disease.
The influence of phospholipid concentration in protein-containing lubricants on the wear of ultra-high molecular weight polyethylene in artificial hip joints
Authors: Bell J , Tipper J L , Ingham E , Stone M H , Fisher J
Published: October 2002

There is considerable interest in the wear of polyethylene and the resulting wear-debrisinduced osteolysis in artificial hip joints. Proteins play an important role as boundary lubricants in vivo in the pseudosynovial fluid, and these are reproduced in in vitro tests through the use of bovine serum. Little is known, however, about the effect of phospholipid concentrations within proteinaceous solutions on the wear of ultra-high molecular weight polyethylene (UHMWPE). The effects of protein-containing lubricants with 0.05, 0.5 and 5 per cent (w/v) phosphatidyl choline concentrations on the wear of ultra-high molecular weight polyethylene (UHMWPE) were compared with 25 per cent (v/v) bovine serum which had 0.01 per cent (w/v) lipid; the effects were compared in a hip joint simulator with smooth (n = 4) and scratched (n = 3) femoral heads. The control bovine serum lubricant produced UHWMPE wear of 55 and 115mm3/106 cycles on the smooth and rough heads respectively. The increased phospholipid concentration significantly reduced the wear rate. At the higher concentration (5% w/v phosphatidyl choline) the average wear was reduced to less than 2 mm3/106 cycles. Even with the relatively low concentrations of 0.05% w/v phosphatidyl choline the wear was reduced by at least threefold compared with the bovine serum tests for both the smooth and rough femoral heads. There may be considerable differences in the phospholipid concentrations in patients' synovial fluid and this is highly likely to produce considerable variation in wear rates. In vitro, differences in the phospholipid concentration of lubricants may also cause variation in wear rates between different simulator tests.
The design of a five-degree-of-freedom powered orthosis for the upper limb
Authors: Johnson G R , Carus D A , Parrini G , Marchese SScattareggia , Valeggi R
Published: October 2002

In response to the need for a sophisticated powered upper-limb orthosis for use by people with disabilities and/or limb weakness or injury, the MULOS (motorized upper-limb orthotic system) has been developed. This is a five-degree-of-freedom electrically powered device having three degrees of freedom at the shoulder, one at the elbow and one to provide pronation/supination. The shoulder mechanism consists of a serial linkage having an equivalent centre of rotation close to that of the anatomical shoulder; this is a self-contained module in which power transmission is provided by tensioned cables. The elbow and pronation/supination modules are also self-contained.

The system has been designed to operate under three modes of control:

1. As an assistive robot attached directly to the arm to provide controlled movements for people with severe disability. In this case, it can be operated by a variety of control interfaces, including a specially designed five-degree-of-freedom joystick. 2. Continuous passive motion for the therapy of joints after injury. The trajectory of the joints is selected by ‘walk-through’ programming and can be replayed for a given number of cycles at a chosen speed. 3. As an exercise device to provide strengthening exercises for elderly people or those recovering from injury or surgery. This mode has not been fully implemented at this stage.

In assistive mode, prototype testing has demonstrated that the system can provide the movements required for a range of simple tasks and, in continuous passive motion (CPM) mode, the programming system has been successfully implemented. Great attention has been paid to all aspects of safety. Future work is required to identify problems of operation, and to develop new control interfaces.
MANUS—a wheelchair-mounted rehabilitation robot
Authors: Driessen B J F , Evers H G , v Woerden J A
Published: October 2002

Rehabilitation robots are assistive devices designed for use by people with severe disability in order to gain independence in tasks of daily living. MANUS is a wheelchair-mounted generalpurpose manipulator now in use with over 100 people in their homes in the Netherlands, in France and in other countries. MANUS has six main degrees of freedom, excluding the gripper. To keep the arm slim, the motors are placed in the main base. The resulting low mass of the arm ensures a reduced impulse in case of collision. A complex system of toothed belts, hollow shafts and geared wheels drives the end-effector. MANUS is equipped with a two-fingered gripper. The fingers have passive canting mechanisms which ensure a three-point grasping action for most objects. A modular control system is used for steering the manipulator to its desired position. For this, a simple but effective user interface has been developed which is configurable by the end-user as well as by the therapist.
The development of the Edinburgh modular arm system
Authors: Gow D J , Douglas W , Geggie C , Monteith E , Stewart D
Published: October 2002

Modularity has not been investigated in any significant way in the development of upperlimb prostheses. The components currently available are the results of different research programmes conducted at different times by a variety of academic and commercial teams. The result is a historical hodge-podge of systems which are largely mutually exclusive in terms of compatibility. The Edinburgh work seeks to solve this problem by designing components which are neutral in structural terms so that left- and right-handed prostheses can be configured from the same basic parts. Currently, child and adult components are separate items; this does not need to be the case and the design presented here allows adult elbow parts to be used as children's shoulders. This paper will cover the rationale behind the design values and the technical aspects of the development.
Three-dimensional metacarpophalangeal joint kinematics using two markers on the phalanx
Authors: Speirs A D , Small C F , Bryant J T , Pichora D R , Zee B Y
Published: October 2002

A protocol for analysing three-dimensional metacarpophalangeal (MCP) joint motion in vivo using two markers on the proximal phalanx is described. The analysis uses an assumption that the rotation of the phalanx about its own long axis is zero. In an experimental study 24 volunteers had surface markers applied to the dorsal surfaces of their hands and index and long finger proximal phalanges, with three-dimensional marker positions recorded in two hand and finger postures in an incomplete box design using a test-retest protocol. Kinematic parameters from the optoelectronic system were compared with those obtained from three-dimensional reconstruction of bone landmarks and of the marker positions identified on stereoradiographs. Pronation/supination angles obtained from bone landmarks showed high test-retest variability, reflecting the difficulty in obtaining reliable pronation/supination data in small bones without the use of implanted markers. Changes in MCP joint extension and deviation angles determined using two surface markers agree with those obtained from bone landmarks. The results indicate a reproducible protocol for tracking MCP joint motion using only two phalangeal markers, suggesting that the ‘no-rotation assumption’ can be applied without affecting measures of extension and deviation motion in the normal joint.
Finite element biphasic indentation of cartilage: A comparison of experimental indenter and physiological contact geometries
Authors: Warner M D , Taylor W R , Clift S E
Published: October 2002

In experimental cartilage indentation studies, the indenter is typically a plane-ended or hemispherically ended cylinder and can be either porous or non-porous. Joints such as the hip and knee, however, have much higher radii of curvature than those used in experimental indentation testing. In order to interpret the results from such testing in a physiological context it is therefore useful to explore the effect of contact geometry on the pore pressure and strain distribution generated in the cartilage layer. Articular cartilage can be described as a saturated porous medium, and can be considered a biphasic material consisting of a porous, permeable solid matrix, and an interstitial fluid phase. This behaviour has been predicted in this study using the ABAQUS soils consolidation procedure. Four contact geometries were modelled: two typical experimental configurations (5 mm radii cylindrical indenter and hemispherical indenters) and two effective radii representative of the hip and knee (20 and 100mm). A 10 per cent deformation, or a load of 0.9kN, was applied over a ramp time of 2 s, which was then maintained for a further 100 s. The porous indenter generated less pore pressure compared with the equivalent non-porous indenter and produced higher values of compress-ive strain in the solid matrix. The predictions made using the cylindrical indenters, porous and non-porous, were dominated by the concentrations at the edge of the indenter and overestimated the peak compressive strain in the tissue by a factor of 21 and a factor of 14 respectively when compared with the hip model. The hemispherical indenter predicted peak strains in similar positions to those predicted using physiological radii, however, the magnitude was overestimated by a factor of 2.3 when compared with the knee and by 5.7 when compared with the hip. The pore pressure throughout the cartilage layer reduced significantly as the radius of the indenter was increased.
Scaling laws for wall shear stress through stenoses under steady and pulsatile flow conditions
Authors: Chua LP , Yu S C M , Xue Q
Published: October 2002

Most patients with atherosclerosis exhibit isolated stenoses of one or more epicardial coronary arteries. The wall shear stresses produced in high-grade stenosis are important in the understanding of atheromatous plaque rupture and thrombosis. This study is designed to establish a method which can be used to scale the different wall shear stresses obtained under different flow conditions to be normalized and subsequently collapsed on to a single general curve. The simulations include both steady and pulsatile flow. The reduced area percentages of the stenoses studied are 50, 75 and 90 per cent. Scaling laws for steady and pulsatile flow conditions are proposed and presented. It can be found from the results that the scaling analysis for pulsatile flow conditions is more complicated than for steady flow conditions and is restricted to, and only valid at, certain time intervals.
Importance of pin geometry on pin-on-plate wear testing of hard-on-hard bearing materials for artificial hip joints
Authors: Besong A A , Jin Z M , Fisher J
Published: October 2002

The contact mechanics between the pin and the plate used in simple wear screening tests were investigated in this study. Both soft-on-hard, such as ultra-high molecular weight polyethylene (UHMWPE)-on-metal or UHMWPE-on-ceramic, and hard-on-hard, such as metal-on-metal, bearing couples were considered. The effect of the pin geometry and the misalignment between the pin and the plate were investigated on the predicted contact pressure distribution at the bearing surfaces using the finite element method. It was demonstrated that in the case of soft-on-hard bearing couples, neither the geometrical discontinuity of the pin surface nor the misalignment could cause a significant increase in the contact stress. However, for hard-on-hard combinations, even with a very small misalignment of 0.5° between the pin and the plate, the geometrical discontinuity could lead to a more than tenfold increase in the predicted contact stress. This elevated contact stress may lead to a large scatter in the wear data and, even more importantly, structural damage of the bearing surfaces.
Contact mechanics of a novel metal-on-metal total hip replacement
Authors: Besong A A , Lee R , Farrar R , Jin Z M
Published: October 2002

The contact mechanics of a novel metal-on-metal total hip replacement (THR) were investigated in this study. The metal-on-metal prosthesis considered consists of a cobalt-chrome acetabular insert connected to a titanium shell through a taper contact, articulating against a cobalt-chrome femoral head. Both the experimental measurement of the displacement of the acetabular insert and the contact area between the two bearing surfaces, and the corresponding numerical predictions using the finite element method have been conducted. Excellent agreement has been demonstrated between the experimental measurement and the finite element prediction under various loads up to 3 kN. The maximum contact pressure at the articulating surfaces has been predicted to be about 31 MPa from a simple axisymmetric finite element model, significantly lower than that of a similar cup but with a monoblock construct. This has been mainly attributed to the flexibility of the insert, leading to an increase in the conformity between the femoral head and the acetabular insert. In addition, the predicted maximum contact pressure is only slightly increased to 37 MPa, from a more realistic three-dimensional anatomical finite element model. The design features on metal-on-metal THRs have been shown to reduce contact stresses and may improve tribological performances of these hard-on-hard bearing couples.
Long-term measurement of metacarpophalangeal joint motion in the normal and rheumatoid hand
Authors: Fowler N K , Nicol A C
Published: October 2002

Low-profile flexible goniometers were used in an instrumented glove to monitor metacarpophalangeal (MCP) joint usage in patients with rheumatoid arthritis and control subjects over 3 days. Statistical analysis of the results provided descriptors of total joint motion per hour, the number of movements per hour and their duration, amplitude and angular velocity. The results show the patient group to have less overall joint usage than the control group, with movements being slower and smaller and with a restricted range. Yearly rates of joint usage were derived; these values are considerably higher than predicted in prosthetic joint testing protocols.
An in vitro assessment of tissue compression damage during circular stapler approximation tests, measuring expulsion of intracellular fluid and force
Authors: McGuire J , Wright I C , Leverment J N
Published: October 2002

Circular staplers are among the many instruments used during minimally invasive or open surgery that should approximate soft tissue within safe compression limits. Previous in vivo sutureline blood flow measurement has suggested a safe thickness reduction limit of 25 per cent during circular stapling procedures. The present work investigates in vitro assessment of the maximum safe compression of large and small porcine intestines, measuring the required compressive force and the expelled intracellular fluid (measured as a potassium solution). A test-rig mounted on a materials testing machine allowed staplers of three sizes to compress tissue samples to thicknesses ranging from 90 to 45 per cent of original thickness. The expelled fluid was collected in 40 ml of 0.9 per cent NaCl and analysed in a flame photometer. The force measurements indicated that the small intestinal tissue samples underwent a sharp increase in stiffness over a strain range of 0.19-0.34. The large bowel tissue underwent a similar increase in stiffness over a planar compressive strain range of 0.19-0.40. The regularity of the potassium output results was limited, making it difficult to draw firm conclusions, although there was some indication that the potassium output from small intestinal tissue may change from erratic to approximately linear at a strain of approximately 0.19, while that of the large bowel tissue appeared to be approximately linear over the entire strain range tested. From the force measurements, it is concluded that strain-induced structural change may help provide a useful definition of safe tissue approximation. The possible implications for reduced dehiscence (wound disintegration) and stricture incidence in stapled anastomoses are discussed.
The vibration of an artery-like tube conveying pulsatile fluid flow
Authors: Zhang Y L , Reese J M , Gorman D G , Madhok R
Published: October 2002

A hybrid method for investigating pulsatile fluid flow in a long, thin, artery-like tube subjected to external excitations is presented. The non-linear partial differential equations governing the motion of the system, which incorporate the influence of circumferential strains, are solved by a combination of a finite element method, a finite difference method and a method of characteristics with interpolation. An initially axially stretched elastic tube conveying pulsating fluid, simply supported at both ends, is modelled to assess the effect of external harmonic excitation on the dynamic responses of the tube and the fluid flow. The results agree well with new experimental data. Comparison of the predicted results with those of a decoupled model demonstrates that it is necessary to consider the mechanism of fluid-structure interaction fully in the study of initially stretched cylindrical tubes conveying pulsatile fluid flow. An analysis of these coupling effects is presented for Womersley numbers α = 2.81 and 3.97 and a mean flow Reynolds number Re = 875.
Analysis of mixed lubrication mechanism in metal-on-metal hip joint replacements
Authors: Jin Z M
Published: October 2002

A simple mixed lubrication model has been developed to predict the asperity contact and wear for the metal-on-metal bearing couple for total hip joint replacements. It has been shown that the femoral head radius has a large effect on the predicted asperity contact and wear depending on the lubrication regime. An increase in the femoral head radius can lead to an increase in wear under a predominantly boundary lubrication regime, but this trend can be reversed under a mixed lubrication regime towards fluid film lubrication. These observations are consistent with the recent experimental findings from hip simulator studies by Smith and co-workers.
Development of a model for occipital fixation—validation of an analogue bone material
Authors: Mullett H , O'Donnell T , Felle P , O'Rourke K , FitzPatrick D
Published: October 2002

Several implant systems may be used to fuse the skull to the upper cervical spine (occipito-cervical fusion). Current biomechanical evaluation is restricted by the limitations of human cadaveric specimens. This paper describes the design and validation of a synthetic testing model of the occipital bone. Data from thickness measurement and pull-out strength testing of a series of human cadaveric skulls was used in the design of a high-density rigid polyurethane foam model. The synthetic occipital model demonstrated repeatable and consistent morphological and biomechanical properties. The model provides a standardized enviroment for evaluation of occipital implants.
Selection of elastomeric materials for compliant-layered total hip arthroplasty
Authors: Quigley F P , Buggy M , Birkinshaw C
Published: October 2002

A selection procedure has been developed to identify suitable commercial materials for use in compliant-layer artificial hip joints. Mechanical requirements, notably hardness and strength, as well as biocompatibility, constituted the specification for the compliant layer. Applying these constraints, candidate materials were identified in a broad range of polymeric material classes. Detailed sourcing and literature searching helped to identify materials appropriate to the application, with suitable mechanical and physical properties, as well as a history of successful clinical use. Some likely materials were identified but were prohibited from further consideration by limited commercial availability. Physical and mechanical characterization together with literature data were used to determine the relative ranking of the candidate materials and through a weighted materials property selection procedure the materials of choice were identified. The linear segmented aromatic polyurethanes, Tecothane 1085 and Estane 5714F1, emerged as the preferred materials.
Smoothing of pixelated finite element models of cancellous bone structures and the effect on the predicted structural properties of the bone
Authors: Dobson C A , Sisias G , Phillips R , Langton C M , Fagan M J
Published: October 2002

Many areas of biomedical engineering involve the modelling of biological systems, often using data from medical scanning techniques such as computed microtomography (μCT), and the prediction of the mechanical properties of these systems via finite element models. These models, and also those produced from remodelling simulations on idealized bone structures, are inherently highly pixelated and therefore have a high degree of surface roughness. The purpose of this paper is to demonstrate that this surface roughness need not necessarily have an influence on the predicted properties of the object under examination. To demonstrate this, two-dimensional idealized models of cancellous bone structures were used that were initially depleted and then rebuilt stochastically. A hysteresis effect was observed such that a significant amount of rebuilding beyond the original density was required to regain the initial intact stiffness. To ensure that this effect was not an artefact of the high degree of surface roughness of the rebuilt structures, a two-stage smoothing procedure was applied to assess if this had any effect on the stiffness of the structures. The superpixelation of the structures appeared to have a more profound effect than the smoothing procedures, although the smoothed structures still had stiffness and density values similar to those of the original structures, with a hysteresis effect still evident. This proves that the pixelization of the structures does not have a significant effect on the predicted mechanical properties of the structures. This work has important implications for other models that exhibit a high degree of surface roughness.
Prediction of skin burn injury. Part 2: Parametric and sensitivity analysis
Authors: Ng E Y-K , Chua L T
Published: October 2002

Part 2 of this paper presents an analysis of variance (ANOVA) for investigating the precedence of the various parameters, and the effects of varying these parameters, in assessment of burn injury resulting from the exposure of skin surface to heat sources. A one-dimensional model based on the finite difference method (FDM), as implemented in a spreadsheet software application, is applied to the assessment of burn injury. Henriques' theory of skin burns is used for determining the spatial and temporal extent of tissue damage. The ranks of the effects of various factors were obtained. It was found that the highest ranked factor is the initial tissue temperature followed by the thermal conductivity of the epidermal layer. The effect of blood perfusion rate is ranked much below the combinations of other factors. The results from the present numerical experiment agree well with the results obtained by Palla. Sensitivity analysis of the critical exposure levels was also carried out and results are discussed. In this study, the effects of the various parameters on injury threshold were investigated. Again, the results indicate that the four parameters: thermal conductivity of the epidermis and dermis, convective heat transfer coefficient and initial tissue temperature, have a pronounced influence on assessing the burn injury threshold. It was also found that fat thermal conductivity and blood perfusion rate have no obvious effect on injury threshold. A two-dimensional analysis was further conducted to determine the sensitivity of the predicted injury to the values of frequency factor, P, and apparent activation energy, Δ E, used in the models. Part 1 of this study details the development of the computer models based on the one- and two-dimensional bioheat equations.