Thomas Joyce | Newcastle University (original) (raw)

Papers by Thomas Joyce

The mechanical response of skin to external loads is influenced by anisotropy and viscoelasticity... more The mechanical response of skin to external loads is influenced by anisotropy and viscoelasticity of the tissue, but the underlying mechanisms remain unclear. Here, we report a study of the main effects of tissue orientation (TO, which is linked to anisotropy) and strain rate (SR, a measure of viscoelasticity), as well as the interaction effects between the two factors, on the tensile properties of skin from a porcine model. Tensile testing to rupture of porcine skin tissue was conducted to evaluate the sensitivity of the tissue modulus of elasticity (E) and fracture-related properties, namely maximum stress (σ U) and strain (ε U) at σ U , to varying SR and TO. Specimens were excised from the abdominal skin in two orientations, namely parallel (P) and right angle (R) to the torso midline. Each TO was investigated at three SR levels, namely 0.007-0.015 s −1 (low), 0.040 s −1 (mid) and 0.065 s −1 (high). Two-factor analysis of variance revealed that the respective parameters responded differently to varying SR and TO. Significant changes in the σ U were observed with different TOs but not with SR. The ε U decreased significantly with increasing SR, but no significant variation was observed for different TOs. Significant changes in E were observed with different TOs; E increased significantly with increasing SR. More importantly, the respective mechanical parameters were not significantly influenced by interactions between SR and TO. These findings suggest that the trends associated with the changes in the skin mechanical properties may be attributed partly to differences in the anisotropy and viscoelasticity but not through any interaction between viscoelasticity and anisotropy.

Wear, 2001

A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To fur... more A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To further the knowledge of the wear of XLPE against itself, a number of 'pin-on-plate' wear tests were undertaken, under different conditions of lubrication. These were distilled water, bovine serum and dry conditions. A second group of tests were then carried out, in which multi-directional motion was applied to the test pins. All tests had XLPE pins loaded at 40 N rubbing against XLPE plates. All the XLPE came from the same batch.

Journal of the Mechanical Behavior of Biomedical Materials, 2015

Cover Pictures: The micrograph shows the transition from a parallel to an inclined growth of colu... more Cover Pictures: The micrograph shows the transition from a parallel to an inclined growth of columnar dendrites in a Sn-15wt.% Pb alloy. The morphology transition is caused by the initiation of convection in the liquid phase driven by a rotating magnetic field (see contribution page 53ff).

Journal of Functional Biomaterials, 2015

A recognized issue related to metal-on-metal hip resurfacings is early fracture of the femur. Mos... more A recognized issue related to metal-on-metal hip resurfacings is early fracture of the femur. Most theories regarding the cause of fracture relate to clinical factors but an engineering analysis of failed hip resurfacings has not previously been reported. The objective of this work was to determine the wear volumes and surface roughness values of a cohort of retrieved hip resurfacings which were removed due to early femoral fracture, infection and avascular necrosis (AVN). Nine resurfacing femoral heads were obtained following early fracture of the femur, a further five were retrieved due to infection and AVN. All fourteen were measured for volumetric wear using a co-ordinate measuring machine. Wear rates were then calculated and regions of the articulating surface were divided into "worn" and "unworn". Roughness values in these regions were measured using a non-contacting profilometer. The mean time to fracture was 3.7 months compared with 44.4 months for retrieval due to infection and AVN. Average wear rates in the early fracture heads were 64 times greater than those in the infection and AVN retrievals. Given the high wear rates of the early fracture components, such wear may be linked to an increased risk of femoral neck fracture.

Engineering design is a core subject in many Engineering departments and is greatly valued by fut... more Engineering design is a core subject in many Engineering departments and is greatly valued by future employers. Over five years a second-year engineering design course has been nurtured to focus on developmental projects associated with sustainability which emphasise hands-on learning. Students work in groups, with recent projects focusing on the design of domestic scale wind turbines. The design is taken from concept through manufacture to final testing using a wind tunnel. The commercial and societal relevance of the project is emphasised, as is the need for team working. Each team is given a budget of £100 (€110). The majority of the timetable is devoted to laboratory sessions where hand and power tools are available. Assessment methods include presentations, formal reports, 'weekly updates' and individual logbooks. Learning outcomes are based on UK-SPEC. Creativity and innovation are encouraged during the design process. Regular assessment occurs throughout with an emphasis on rapid, formal feedback. Student feedback improves year on year with typical student attendance above 90%. The number of students taking the Design degree stream doubled last year. The key features which have led to success are allowing students to take a paper design through to manufacture (encompassing the hands-on and 'realistic' aspects of engineering) and trusting in students' creativity. Both areas led to students having a sense of ownership of their learning and developing into professional engineers. Assessment workload for staff is high but is seen by students as evidence of staff commitment to their learning.

Replacement joints, or prostheses, are an important tool for reducing pain and restoring function... more Replacement joints, or prostheses, are an important tool for reducing pain and restoring functionality to patients with diseases such as arthritis. However, even the most modern prostheses have a limited life span and a major cause of failure is wear.

Journal of applied biomaterials & biomechanics (JABB)

Wear tests were undertaken in which ultra high molecular weight polyethylene (UHMWPE) was rubbed ... more Wear tests were undertaken in which ultra high molecular weight polyethylene (UHMWPE) was rubbed against itself. Tests primarily employed a pin-on-plate wear test machine, with distilled water, Ringer solution and dilute bovine serum being used as the lubricants. Loads of 10N and 40N were employed, and some test pins had a rotational motion added. In all cases wear was high, with mean wear factors of up to 91 ϫ 10 -6 mm 3 /Nm being measured, but the addition of rotation reduced the amount of material worn from the test plates. In the presence of bovine serum and under reciprocation only, pin wear was relatively low. With bovine serum as the lubricant, total mean wear factors for the UHMWPE couples were calculated to be in the range of 35 to 58 ϫ 10 -6 mm 3 /Nm. Therefore the pin-on-plate tests showed that the choice of lubricant as well as the motion applied to the test pin had a significant influence on the wear volumes measured. A two-piece UHMWPE 'prosthesis' with matching hemispherical faces was fabricated and tested on a finger simulator. Distilled water was used as the lubricant and wear factors were found to be greater for the metacarpal component, 21 ϫ 10 -6 mm 3 /Nm, than the phalangeal component, 3 ϫ 10 -6 mm 3 /Nm, after ten million cycles of testing. This result paralleled the greater wear seen by the plate than by the pin in the pin-on-plate tests under reciprocating motion

Human anatomy is a complex subject but how can engineering students, with little formal backgroun... more Human anatomy is a complex subject but how can engineering students, with little formal background knowledge, be quickly taught this intricate and new topic? Innovative teaching materials in the form of web-based anatomical software and human joint models were introduced into a Masters level bioengineering course two years ago. In the current year a visit to a dissection lab was added and, based on student feedback, dynamic models of human joints were purchased and introduced. For the software and the models bespoke teaching documents, which students worked through in pairs or small groups, were produced by the author. Anatomy and the associated language can be seen as a dry, difficult and peripheral subject by engineering students. Therefore an aim was to use multiple, hands-on and interactive methods to help students to learn anatomy quickly. Student feedback has been collected over two academic years and has been equally positive about learning from the software and the models of human joints. In addition the author noted that students appeared to interact well with the teaching materials and the small group working encouraged discussion and sharing of opinion. At approximately £4,400 (€5,000) for one year"s access for ten seats, the software is expensive. Combining the positive learning benefits of non-hierarchal, small group and peer-to-peer learning with innovative and novel teaching aids quickly and effectively enhanced the student learning experience of atypical material by a cohort of engineering students.

Journal of applied biomaterials & biomechanics (JABB)

A two-piece finger prosthesis has been proposed, manufactured from silane cross-linked polyethyle... more A two-piece finger prosthesis has been proposed, manufactured from silane cross-linked polyethylene. Using a finger wear simulator the prosthesis was tested in a dilute bovine serum lubricant. Five tests were undertaken, totalling over 45 million cycles of wear testing. In each test, a statically loaded control prosthesis was included. In all tests it was found that the lubricant uptake of the test components exceeded that of the control components, consequently no gravimetric wear was measured. To investigate this result further, six cross-linked polyethylene prostheses were taken and soaked in the serum lubricant at 37 degrees C. Three of the prostheses were statically loaded and the other three were unloaded. These tests lasted for over one hundred and sixty days. It was found that the lubricant uptake of the unloaded control components was greater than that of the loaded com-ponents. The test with statically loaded prostheses was repeated, firstly with distilled water and then in lubricant heated to 30 degrees C and 40 degrees C. No significant difference in weight increase due to lubricant uptake at these two temperatures was found. The weight increase due to soaking in dilute bovine serum was several times that due to soaking in distilled water. (Journal of Ap-plied Biomaterials & Biomechanics 2004; 2: 136-42).

The Journal of Arthroplasty

Cross-linked polyethylenes are being introduced widely in acetabular cups in hip prostheses as a ... more Cross-linked polyethylenes are being introduced widely in acetabular cups in hip prostheses as a strategy to reduce the incidence of wear debris-induced osteolysis. It will be many years before substantial clinical data can be collected on the wear of these new materials. Silane cross-linked polyethylene (XLPE) was introduced into clinical practice in a limited series of acetabular cups in 1986 articulating against 22.225-mm alumina ceramic femoral heads and showed reduced wear rates compared with conventionally sterilized (gamma irradiation in air) ultra-high molecular weight polyethylene (UHMWPE). We compared the wear of XLPE manufactured in 1986 with the wear of UHMWPE manufactured in 1986 in nonirradiated and irradiated forms. In the nonirradiated forms, the wear of XLPE was 3 times less than UHWMPE when articulating against smooth counterfaces. The nonirradiated materials did not show signs of oxidation. In the irradiated forms, only UHMWPE showed high levels of oxidation, and this caused a substantial increase in wear. Antioxidants added to XLPE during processing gave resistance to oxidative degradation. When sliding against scratched counterfaces, the wear of UHMWPE increased by a factor of 2 to 3 times. Against the same scratched counterfaces, the wear of XLPE increased dramatically by 30 to 200 times. This difference may be attributed to the reduction in toughness of XLPE. Clinically, XLPE has been articulated against damage-resistant ceramic heads, and this probably has been an important factor in contributing to reduced wear. New cross-linked polyethylenes differ considerably from XLPE. This study indicates that it is prudent to examine the wear of new polyethylenes under a range of conditions that may occur in vivo.

Journal of applied biomaterials & biomechanics (JABB)

The potential for all-polymer prostheses has not been widely investigated. It might be expected t... more The potential for all-polymer prostheses has not been widely investigated. It might be expected that the wear of such biomaterial combinations would be excessive, but an in vivo study of all polymer knee prostheses reported that there were no failures due to wear, even after ten years of clinical use. This design of knee prosthesis used polyacetal and ultra high molecular weight polyethylene (UHMWPE) as the biopolymers. Similarly, an earlier in vitro study of polyacetal and UHMWPE hip prostheses indicated lower wear than for a cobalt chrome and UHMWPE combination. Therefore this study set out to test the polyacetal and UHMWPE combination in a wear screening rig which had previously been validated against clinical data for artificial hip joints. Two different motion conditions were applied to the test samples and each biopolymer was tested as both pin and plate. Interestingly it was found that, whatever the contribution from pin or plate, the total mean wear factors were 1.5 ϫ 10 -6 mm 3 /Nm under reciprocation-only, and 4.1 ϫ 10 -6 mm 3 /Nm under multi-directional motion. These wear factors were greater than those found when a conventional metal-on-UHMWPE couple was tested under the same loading, motion and lubricant conditions. A comparison was also undertaken with the wear of other orthopaedic biopolymer combinations, namely cross-linked polyethylene (XLPE) against itself, and UHMWPE against itself. The XLPE pairing showed somewhat lower wear than the polyacetal and UHMWPE couple, while the UHMWPE pairing showed the highest wear of all, approximately an order of magnitude greater than the polyacetal and UHMWPE combination.

Tribology - Materials Surfaces & Interfaces

ABSTRACT

Bio-medical materials and engineering

Six metacarpophalangeal prostheses were each wear tested to five million cycles. Each prosthesis ... more Six metacarpophalangeal prostheses were each wear tested to five million cycles. Each prosthesis consisted of a metacarpal component with an approximately hemispherical shell on a titanium body, articulating against a titanium phalangeal component. Four prostheses had a shell made from ultra-high molecular weight polyethylene (UHMWPE) and two had a shell made from polyether ether ketone (PEEK). The tests were undertaken using a finger wear simulator. Despite pre-soaking and the use of control components, lubricant uptake by the metacarpal components was significant. Gravimetrically, the UHMWPE test components showed a greater weight gain than the UHMWPE control components. Therefore there was no apparent wear of any of the UHMWPE test metacarpal components. The original concentric machining marks of the UHMWPE components could still be seen after five million cycles of testing. For the metacarpal components with PEEK shells, gravimetric wear could be measured. Gravimetrically, all of the titanium phalangeal components showed little or no wear. Light scratches in the direction of sliding appeared on the articulating faces of all metacarpal and phalangeal test components, indicating slight abrasive wear.

Bio-medical materials and engineering

The key joint of the forefoot during gait is the first metatarsophalangeal joint. It plays an imp... more The key joint of the forefoot during gait is the first metatarsophalangeal joint. It plays an important role in propelling the human form but can be subject to a number of diseases which can lead to its replacement with an artificial joint. Some of these designs of prosthesis employ a two-piece ball and socket arrangement and are available with a range of biomaterial couples including ceramic-on-ceramic, metal-on-metal and metal-on-polymer. Calculation of predicted lubrication regimes applicable to these implant designs was undertaken. Modelling the ball and socket implant as an equivalent ball-on-plane model and employing elastohydrodynamic theory allowed the minimum film thickness to be calculated and in turn the lambda ratio to indicate the lubrication regime. The calculations were undertaken for a 50 to 1500 N range of loading values, a 0 to 30 mm/s range of entraining velocities, and a 3 to 15 mm radius range of sizes. Calculations showed that the ceramic-on-ceramic and metal-on-metal implants could operate under fluid film lubrication, whereas the metal-on-polymer combination operated in the boundary lubrication regime. It was also recognized that manufacturing capabilities are critical to the radial clearances and values of surface roughness that can be achieved, and thus the predicted lubrication regime.

Rheumatoid arthritis is a common and debilitating disease which often affliets the small joints o... more Rheumatoid arthritis is a common and debilitating disease which often affliets the small joints of the hands, especially the metacarpophalangeal joints. Although there have been recent improvements in drug treatments, for a number of patients the final clinical intervention entails the replacement of the natural joint with a prosthesis. The vast majority of such implants are single-piece, flexible, silicone designs. This group includes the Swanson, Sutter and NeuFlex metacarpophalangeal prostheses. The most common mode of failure of these implants is fracture, often at the junction of the distal stem and the hinge. Although such fracture does not always mean that a revision operation is required, it is self-evident that if the fracture rate could be reduced, such implants should provide enhanced results in the hands of patients. A clear way to reduce the fracture rate is to understand how fracture has come about. A cohort of 12 failed and explanted Sutter metacarpophalangeal prosthe...

Bio-medical materials and engineering

This paper describes the design of a pin on plate rig which has been modified to give multi-direc... more This paper describes the design of a pin on plate rig which has been modified to give multi-directional motion to the test pins, resulting in elliptical and quasi-elliptical wear paths. Such paths are closer to those seen in vivo by a femoral head articulating against an acetabular cup. The description of the rig is augmented by the results of a test of ultra high molecular weight polyethylene pins rubbing against stainless steel plates. with dilute bovine serum at 37 degrees C being used as the lubricant. With standard reciprocating motion, a mean wear factor of 0.085 x 10(-6) mm3/N m was found, but with multi-directional motion, the mean wear factor increased to 1.10 x 10(-6) mm3/N m, which is very similar to in vivo wear factors.

Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine, 2015

Clinical results of the PyroCarbon proximal interphalangeal joint replacement are inconsistent wi... more Clinical results of the PyroCarbon proximal interphalangeal joint replacement are inconsistent with various complications reported. To address this, in vitro testing was conducted using finger joint simulators. Two PyroCarbon proximal interphalangeal prostheses were tested in a lubricant of dilute bovine serum to 5 × 10(6) cycles of flexion-extension (90°-30°) with dynamic forces of 10 N applied. At intervals of 3000 cycles testing ceased and a static load of 100 N was applied to simulate gripping. In addition, two 'control' prostheses were immersed alongside the test prostheses to account for lubricant absorption. Wear and roughness averages (Ra) were measured every 1 × 10(6) cycles. Minimal wear for all of the components was measured with a negligible increase in Ra for most of the components. One condyle of one component increased in Ra over the 5 × 10(6) cycles with a value above the recommended 50 nm. Unidirectional marks were visible on the condyle from micrographs, co...

Lubricants, 2015

A finger prosthesis consisting of a Cobalt-chromium (CoCr) proximal component and an Ultra-high-m... more A finger prosthesis consisting of a Cobalt-chromium (CoCr) proximal component and an Ultra-high-molecular-weight-polyethylene (UHMWPE) medial component (both mounted on hydroxyapatite coated stems) was evaluated to 5,000,000 cycles in an in vitro finger simulator. One "test" prosthesis was cycled through flexion-extension (90°-30°) with a dynamic load of 10 N, whilst immersed in a lubricant of dilute bovine serum. Additionally, a static load of 100 N was applied for 45 s every 3000 cycles to simulate a static gripping force. A second "control" prosthesis was immersed in the same lubricant to account for absorption. Gravimetric and Sa (3D roughness) measurements were taken at 1,000,000 cycle intervals. Micrographs and Sa values revealed negligible change to the CoCr surfaces after 5,000,000 cycles. The UHMWPE also exhibited no distinctive Sa trend, however the micrographs indicate that polishing occurred. Both the CoCr and UHMWPE test components progressively decreased in weight. The CoCr control component did not change in weight, whilst the UHMWPE component gained weight through absorption. To account for the disparity between surface and gravimetric results, the hydroxyapatite coatings were examined. Micrographs of the test stems revealed that the hydroxyapatite coating was partially removed, whilst the micrographs of the control stems exhibited a uniform coating. OPEN ACCESS Lubricants 2015, 3 245

The bone & joint journal, 2015

Concerns have been raised that deformation of acetabular shells may disrupt the assembly process ... more Concerns have been raised that deformation of acetabular shells may disrupt the assembly process of modular prostheses. In this study we aimed to examine the effect that the strength of bone has on the amount of deformation of the acetabular shell. The hypothesis was that stronger bone would result in greater deformation. A total of 17 acetabular shells were inserted into the acetabula of eight cadavers, and deformation was measured using an optical measuring system. Cores of bone from the femoral head were taken from each cadaver and compressed using a materials testing machine. The highest peak modulus and yield stress for each cadaver were used to represent the strength of the bone and compared with the values for the deformation and the surgeon's subjective assessment of the hardness of the bone. The mean deformation of the shell was 129 µm (3 to 340). No correlation was found between deformation and either the maximum peak modulus (r² = 0.011, t = 0.426, p = 0.676) or the y...

Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine, 2013

Bovine serum is the lubricant recommended by several international standards for the wear testing... more Bovine serum is the lubricant recommended by several international standards for the wear testing of orthopaedic biomaterials; however, there are issues over its use due to batch variation, degradation, cost and safety. For these reasons, alternative lubricants were investigated. A 50-station Super-CTPOD (circularly translating pin-on-disc) wear test rig was used, which applied multidirectional motion to ultra-high-molecular-weight polyethylene test pins rubbing against cobalt chromium discs. Thirteen possible alternative lubricants were tested. The use of soy protein as a lubricant gave statistically higher wear, while soya oil, olive oil, Channel Island milk, whole milk, whey, wheatgerm oil, 11 mg/mL egg white, albumin/globulin mix and albumin/globulin/chondroitin sulphate mix all gave statistically lower wear than bovine serum. The lubricants giving the closest wear results to bovine serum were 20 and 40 mg/mL egg white solutions. A light absorbance assay found that these egg whi...

The mechanical response of skin to external loads is influenced by anisotropy and viscoelasticity... more The mechanical response of skin to external loads is influenced by anisotropy and viscoelasticity of the tissue, but the underlying mechanisms remain unclear. Here, we report a study of the main effects of tissue orientation (TO, which is linked to anisotropy) and strain rate (SR, a measure of viscoelasticity), as well as the interaction effects between the two factors, on the tensile properties of skin from a porcine model. Tensile testing to rupture of porcine skin tissue was conducted to evaluate the sensitivity of the tissue modulus of elasticity (E) and fracture-related properties, namely maximum stress (σ U) and strain (ε U) at σ U , to varying SR and TO. Specimens were excised from the abdominal skin in two orientations, namely parallel (P) and right angle (R) to the torso midline. Each TO was investigated at three SR levels, namely 0.007-0.015 s −1 (low), 0.040 s −1 (mid) and 0.065 s −1 (high). Two-factor analysis of variance revealed that the respective parameters responded differently to varying SR and TO. Significant changes in the σ U were observed with different TOs but not with SR. The ε U decreased significantly with increasing SR, but no significant variation was observed for different TOs. Significant changes in E were observed with different TOs; E increased significantly with increasing SR. More importantly, the respective mechanical parameters were not significantly influenced by interactions between SR and TO. These findings suggest that the trends associated with the changes in the skin mechanical properties may be attributed partly to differences in the anisotropy and viscoelasticity but not through any interaction between viscoelasticity and anisotropy.

Wear, 2001

A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To fur... more A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To further the knowledge of the wear of XLPE against itself, a number of 'pin-on-plate' wear tests were undertaken, under different conditions of lubrication. These were distilled water, bovine serum and dry conditions. A second group of tests were then carried out, in which multi-directional motion was applied to the test pins. All tests had XLPE pins loaded at 40 N rubbing against XLPE plates. All the XLPE came from the same batch.

Journal of the Mechanical Behavior of Biomedical Materials, 2015

Cover Pictures: The micrograph shows the transition from a parallel to an inclined growth of colu... more Cover Pictures: The micrograph shows the transition from a parallel to an inclined growth of columnar dendrites in a Sn-15wt.% Pb alloy. The morphology transition is caused by the initiation of convection in the liquid phase driven by a rotating magnetic field (see contribution page 53ff).

Journal of Functional Biomaterials, 2015

A recognized issue related to metal-on-metal hip resurfacings is early fracture of the femur. Mos... more A recognized issue related to metal-on-metal hip resurfacings is early fracture of the femur. Most theories regarding the cause of fracture relate to clinical factors but an engineering analysis of failed hip resurfacings has not previously been reported. The objective of this work was to determine the wear volumes and surface roughness values of a cohort of retrieved hip resurfacings which were removed due to early femoral fracture, infection and avascular necrosis (AVN). Nine resurfacing femoral heads were obtained following early fracture of the femur, a further five were retrieved due to infection and AVN. All fourteen were measured for volumetric wear using a co-ordinate measuring machine. Wear rates were then calculated and regions of the articulating surface were divided into "worn" and "unworn". Roughness values in these regions were measured using a non-contacting profilometer. The mean time to fracture was 3.7 months compared with 44.4 months for retrieval due to infection and AVN. Average wear rates in the early fracture heads were 64 times greater than those in the infection and AVN retrievals. Given the high wear rates of the early fracture components, such wear may be linked to an increased risk of femoral neck fracture.

Engineering design is a core subject in many Engineering departments and is greatly valued by fut... more Engineering design is a core subject in many Engineering departments and is greatly valued by future employers. Over five years a second-year engineering design course has been nurtured to focus on developmental projects associated with sustainability which emphasise hands-on learning. Students work in groups, with recent projects focusing on the design of domestic scale wind turbines. The design is taken from concept through manufacture to final testing using a wind tunnel. The commercial and societal relevance of the project is emphasised, as is the need for team working. Each team is given a budget of £100 (€110). The majority of the timetable is devoted to laboratory sessions where hand and power tools are available. Assessment methods include presentations, formal reports, 'weekly updates' and individual logbooks. Learning outcomes are based on UK-SPEC. Creativity and innovation are encouraged during the design process. Regular assessment occurs throughout with an emphasis on rapid, formal feedback. Student feedback improves year on year with typical student attendance above 90%. The number of students taking the Design degree stream doubled last year. The key features which have led to success are allowing students to take a paper design through to manufacture (encompassing the hands-on and 'realistic' aspects of engineering) and trusting in students' creativity. Both areas led to students having a sense of ownership of their learning and developing into professional engineers. Assessment workload for staff is high but is seen by students as evidence of staff commitment to their learning.

Replacement joints, or prostheses, are an important tool for reducing pain and restoring function... more Replacement joints, or prostheses, are an important tool for reducing pain and restoring functionality to patients with diseases such as arthritis. However, even the most modern prostheses have a limited life span and a major cause of failure is wear.

Journal of applied biomaterials & biomechanics (JABB)

Wear tests were undertaken in which ultra high molecular weight polyethylene (UHMWPE) was rubbed ... more Wear tests were undertaken in which ultra high molecular weight polyethylene (UHMWPE) was rubbed against itself. Tests primarily employed a pin-on-plate wear test machine, with distilled water, Ringer solution and dilute bovine serum being used as the lubricants. Loads of 10N and 40N were employed, and some test pins had a rotational motion added. In all cases wear was high, with mean wear factors of up to 91 ϫ 10 -6 mm 3 /Nm being measured, but the addition of rotation reduced the amount of material worn from the test plates. In the presence of bovine serum and under reciprocation only, pin wear was relatively low. With bovine serum as the lubricant, total mean wear factors for the UHMWPE couples were calculated to be in the range of 35 to 58 ϫ 10 -6 mm 3 /Nm. Therefore the pin-on-plate tests showed that the choice of lubricant as well as the motion applied to the test pin had a significant influence on the wear volumes measured. A two-piece UHMWPE 'prosthesis' with matching hemispherical faces was fabricated and tested on a finger simulator. Distilled water was used as the lubricant and wear factors were found to be greater for the metacarpal component, 21 ϫ 10 -6 mm 3 /Nm, than the phalangeal component, 3 ϫ 10 -6 mm 3 /Nm, after ten million cycles of testing. This result paralleled the greater wear seen by the plate than by the pin in the pin-on-plate tests under reciprocating motion

Human anatomy is a complex subject but how can engineering students, with little formal backgroun... more Human anatomy is a complex subject but how can engineering students, with little formal background knowledge, be quickly taught this intricate and new topic? Innovative teaching materials in the form of web-based anatomical software and human joint models were introduced into a Masters level bioengineering course two years ago. In the current year a visit to a dissection lab was added and, based on student feedback, dynamic models of human joints were purchased and introduced. For the software and the models bespoke teaching documents, which students worked through in pairs or small groups, were produced by the author. Anatomy and the associated language can be seen as a dry, difficult and peripheral subject by engineering students. Therefore an aim was to use multiple, hands-on and interactive methods to help students to learn anatomy quickly. Student feedback has been collected over two academic years and has been equally positive about learning from the software and the models of human joints. In addition the author noted that students appeared to interact well with the teaching materials and the small group working encouraged discussion and sharing of opinion. At approximately £4,400 (€5,000) for one year"s access for ten seats, the software is expensive. Combining the positive learning benefits of non-hierarchal, small group and peer-to-peer learning with innovative and novel teaching aids quickly and effectively enhanced the student learning experience of atypical material by a cohort of engineering students.

Journal of applied biomaterials & biomechanics (JABB)

A two-piece finger prosthesis has been proposed, manufactured from silane cross-linked polyethyle... more A two-piece finger prosthesis has been proposed, manufactured from silane cross-linked polyethylene. Using a finger wear simulator the prosthesis was tested in a dilute bovine serum lubricant. Five tests were undertaken, totalling over 45 million cycles of wear testing. In each test, a statically loaded control prosthesis was included. In all tests it was found that the lubricant uptake of the test components exceeded that of the control components, consequently no gravimetric wear was measured. To investigate this result further, six cross-linked polyethylene prostheses were taken and soaked in the serum lubricant at 37 degrees C. Three of the prostheses were statically loaded and the other three were unloaded. These tests lasted for over one hundred and sixty days. It was found that the lubricant uptake of the unloaded control components was greater than that of the loaded com-ponents. The test with statically loaded prostheses was repeated, firstly with distilled water and then in lubricant heated to 30 degrees C and 40 degrees C. No significant difference in weight increase due to lubricant uptake at these two temperatures was found. The weight increase due to soaking in dilute bovine serum was several times that due to soaking in distilled water. (Journal of Ap-plied Biomaterials & Biomechanics 2004; 2: 136-42).

The Journal of Arthroplasty

Cross-linked polyethylenes are being introduced widely in acetabular cups in hip prostheses as a ... more Cross-linked polyethylenes are being introduced widely in acetabular cups in hip prostheses as a strategy to reduce the incidence of wear debris-induced osteolysis. It will be many years before substantial clinical data can be collected on the wear of these new materials. Silane cross-linked polyethylene (XLPE) was introduced into clinical practice in a limited series of acetabular cups in 1986 articulating against 22.225-mm alumina ceramic femoral heads and showed reduced wear rates compared with conventionally sterilized (gamma irradiation in air) ultra-high molecular weight polyethylene (UHMWPE). We compared the wear of XLPE manufactured in 1986 with the wear of UHMWPE manufactured in 1986 in nonirradiated and irradiated forms. In the nonirradiated forms, the wear of XLPE was 3 times less than UHWMPE when articulating against smooth counterfaces. The nonirradiated materials did not show signs of oxidation. In the irradiated forms, only UHMWPE showed high levels of oxidation, and this caused a substantial increase in wear. Antioxidants added to XLPE during processing gave resistance to oxidative degradation. When sliding against scratched counterfaces, the wear of UHMWPE increased by a factor of 2 to 3 times. Against the same scratched counterfaces, the wear of XLPE increased dramatically by 30 to 200 times. This difference may be attributed to the reduction in toughness of XLPE. Clinically, XLPE has been articulated against damage-resistant ceramic heads, and this probably has been an important factor in contributing to reduced wear. New cross-linked polyethylenes differ considerably from XLPE. This study indicates that it is prudent to examine the wear of new polyethylenes under a range of conditions that may occur in vivo.

Journal of applied biomaterials & biomechanics (JABB)

The potential for all-polymer prostheses has not been widely investigated. It might be expected t... more The potential for all-polymer prostheses has not been widely investigated. It might be expected that the wear of such biomaterial combinations would be excessive, but an in vivo study of all polymer knee prostheses reported that there were no failures due to wear, even after ten years of clinical use. This design of knee prosthesis used polyacetal and ultra high molecular weight polyethylene (UHMWPE) as the biopolymers. Similarly, an earlier in vitro study of polyacetal and UHMWPE hip prostheses indicated lower wear than for a cobalt chrome and UHMWPE combination. Therefore this study set out to test the polyacetal and UHMWPE combination in a wear screening rig which had previously been validated against clinical data for artificial hip joints. Two different motion conditions were applied to the test samples and each biopolymer was tested as both pin and plate. Interestingly it was found that, whatever the contribution from pin or plate, the total mean wear factors were 1.5 ϫ 10 -6 mm 3 /Nm under reciprocation-only, and 4.1 ϫ 10 -6 mm 3 /Nm under multi-directional motion. These wear factors were greater than those found when a conventional metal-on-UHMWPE couple was tested under the same loading, motion and lubricant conditions. A comparison was also undertaken with the wear of other orthopaedic biopolymer combinations, namely cross-linked polyethylene (XLPE) against itself, and UHMWPE against itself. The XLPE pairing showed somewhat lower wear than the polyacetal and UHMWPE couple, while the UHMWPE pairing showed the highest wear of all, approximately an order of magnitude greater than the polyacetal and UHMWPE combination.

Tribology - Materials Surfaces & Interfaces

ABSTRACT

Bio-medical materials and engineering

Six metacarpophalangeal prostheses were each wear tested to five million cycles. Each prosthesis ... more Six metacarpophalangeal prostheses were each wear tested to five million cycles. Each prosthesis consisted of a metacarpal component with an approximately hemispherical shell on a titanium body, articulating against a titanium phalangeal component. Four prostheses had a shell made from ultra-high molecular weight polyethylene (UHMWPE) and two had a shell made from polyether ether ketone (PEEK). The tests were undertaken using a finger wear simulator. Despite pre-soaking and the use of control components, lubricant uptake by the metacarpal components was significant. Gravimetrically, the UHMWPE test components showed a greater weight gain than the UHMWPE control components. Therefore there was no apparent wear of any of the UHMWPE test metacarpal components. The original concentric machining marks of the UHMWPE components could still be seen after five million cycles of testing. For the metacarpal components with PEEK shells, gravimetric wear could be measured. Gravimetrically, all of the titanium phalangeal components showed little or no wear. Light scratches in the direction of sliding appeared on the articulating faces of all metacarpal and phalangeal test components, indicating slight abrasive wear.

Bio-medical materials and engineering

The key joint of the forefoot during gait is the first metatarsophalangeal joint. It plays an imp... more The key joint of the forefoot during gait is the first metatarsophalangeal joint. It plays an important role in propelling the human form but can be subject to a number of diseases which can lead to its replacement with an artificial joint. Some of these designs of prosthesis employ a two-piece ball and socket arrangement and are available with a range of biomaterial couples including ceramic-on-ceramic, metal-on-metal and metal-on-polymer. Calculation of predicted lubrication regimes applicable to these implant designs was undertaken. Modelling the ball and socket implant as an equivalent ball-on-plane model and employing elastohydrodynamic theory allowed the minimum film thickness to be calculated and in turn the lambda ratio to indicate the lubrication regime. The calculations were undertaken for a 50 to 1500 N range of loading values, a 0 to 30 mm/s range of entraining velocities, and a 3 to 15 mm radius range of sizes. Calculations showed that the ceramic-on-ceramic and metal-on-metal implants could operate under fluid film lubrication, whereas the metal-on-polymer combination operated in the boundary lubrication regime. It was also recognized that manufacturing capabilities are critical to the radial clearances and values of surface roughness that can be achieved, and thus the predicted lubrication regime.

Rheumatoid arthritis is a common and debilitating disease which often affliets the small joints o... more Rheumatoid arthritis is a common and debilitating disease which often affliets the small joints of the hands, especially the metacarpophalangeal joints. Although there have been recent improvements in drug treatments, for a number of patients the final clinical intervention entails the replacement of the natural joint with a prosthesis. The vast majority of such implants are single-piece, flexible, silicone designs. This group includes the Swanson, Sutter and NeuFlex metacarpophalangeal prostheses. The most common mode of failure of these implants is fracture, often at the junction of the distal stem and the hinge. Although such fracture does not always mean that a revision operation is required, it is self-evident that if the fracture rate could be reduced, such implants should provide enhanced results in the hands of patients. A clear way to reduce the fracture rate is to understand how fracture has come about. A cohort of 12 failed and explanted Sutter metacarpophalangeal prosthe...

Bio-medical materials and engineering

This paper describes the design of a pin on plate rig which has been modified to give multi-direc... more This paper describes the design of a pin on plate rig which has been modified to give multi-directional motion to the test pins, resulting in elliptical and quasi-elliptical wear paths. Such paths are closer to those seen in vivo by a femoral head articulating against an acetabular cup. The description of the rig is augmented by the results of a test of ultra high molecular weight polyethylene pins rubbing against stainless steel plates. with dilute bovine serum at 37 degrees C being used as the lubricant. With standard reciprocating motion, a mean wear factor of 0.085 x 10(-6) mm3/N m was found, but with multi-directional motion, the mean wear factor increased to 1.10 x 10(-6) mm3/N m, which is very similar to in vivo wear factors.

Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine, 2015

Clinical results of the PyroCarbon proximal interphalangeal joint replacement are inconsistent wi... more Clinical results of the PyroCarbon proximal interphalangeal joint replacement are inconsistent with various complications reported. To address this, in vitro testing was conducted using finger joint simulators. Two PyroCarbon proximal interphalangeal prostheses were tested in a lubricant of dilute bovine serum to 5 × 10(6) cycles of flexion-extension (90°-30°) with dynamic forces of 10 N applied. At intervals of 3000 cycles testing ceased and a static load of 100 N was applied to simulate gripping. In addition, two 'control' prostheses were immersed alongside the test prostheses to account for lubricant absorption. Wear and roughness averages (Ra) were measured every 1 × 10(6) cycles. Minimal wear for all of the components was measured with a negligible increase in Ra for most of the components. One condyle of one component increased in Ra over the 5 × 10(6) cycles with a value above the recommended 50 nm. Unidirectional marks were visible on the condyle from micrographs, co...

Lubricants, 2015

A finger prosthesis consisting of a Cobalt-chromium (CoCr) proximal component and an Ultra-high-m... more A finger prosthesis consisting of a Cobalt-chromium (CoCr) proximal component and an Ultra-high-molecular-weight-polyethylene (UHMWPE) medial component (both mounted on hydroxyapatite coated stems) was evaluated to 5,000,000 cycles in an in vitro finger simulator. One "test" prosthesis was cycled through flexion-extension (90°-30°) with a dynamic load of 10 N, whilst immersed in a lubricant of dilute bovine serum. Additionally, a static load of 100 N was applied for 45 s every 3000 cycles to simulate a static gripping force. A second "control" prosthesis was immersed in the same lubricant to account for absorption. Gravimetric and Sa (3D roughness) measurements were taken at 1,000,000 cycle intervals. Micrographs and Sa values revealed negligible change to the CoCr surfaces after 5,000,000 cycles. The UHMWPE also exhibited no distinctive Sa trend, however the micrographs indicate that polishing occurred. Both the CoCr and UHMWPE test components progressively decreased in weight. The CoCr control component did not change in weight, whilst the UHMWPE component gained weight through absorption. To account for the disparity between surface and gravimetric results, the hydroxyapatite coatings were examined. Micrographs of the test stems revealed that the hydroxyapatite coating was partially removed, whilst the micrographs of the control stems exhibited a uniform coating. OPEN ACCESS Lubricants 2015, 3 245

The bone & joint journal, 2015

Concerns have been raised that deformation of acetabular shells may disrupt the assembly process ... more Concerns have been raised that deformation of acetabular shells may disrupt the assembly process of modular prostheses. In this study we aimed to examine the effect that the strength of bone has on the amount of deformation of the acetabular shell. The hypothesis was that stronger bone would result in greater deformation. A total of 17 acetabular shells were inserted into the acetabula of eight cadavers, and deformation was measured using an optical measuring system. Cores of bone from the femoral head were taken from each cadaver and compressed using a materials testing machine. The highest peak modulus and yield stress for each cadaver were used to represent the strength of the bone and compared with the values for the deformation and the surgeon's subjective assessment of the hardness of the bone. The mean deformation of the shell was 129 µm (3 to 340). No correlation was found between deformation and either the maximum peak modulus (r² = 0.011, t = 0.426, p = 0.676) or the y...

Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine, 2013

Bovine serum is the lubricant recommended by several international standards for the wear testing... more Bovine serum is the lubricant recommended by several international standards for the wear testing of orthopaedic biomaterials; however, there are issues over its use due to batch variation, degradation, cost and safety. For these reasons, alternative lubricants were investigated. A 50-station Super-CTPOD (circularly translating pin-on-disc) wear test rig was used, which applied multidirectional motion to ultra-high-molecular-weight polyethylene test pins rubbing against cobalt chromium discs. Thirteen possible alternative lubricants were tested. The use of soy protein as a lubricant gave statistically higher wear, while soya oil, olive oil, Channel Island milk, whole milk, whey, wheatgerm oil, 11 mg/mL egg white, albumin/globulin mix and albumin/globulin/chondroitin sulphate mix all gave statistically lower wear than bovine serum. The lubricants giving the closest wear results to bovine serum were 20 and 40 mg/mL egg white solutions. A light absorbance assay found that these egg whi...