The role of subchondral bone, and its histomorphology, on the dynamic viscoelasticity of cartilage, bone and osteochondral cores.

OBJECTIVE: Viscoelastic properties of articular cartilage have been characterised at physiological frequencies. However, studies investigating the interaction between cartilage and subchondral bone and the influence of underlying bone histomorphometry on the viscoelasticity of cartilage are lacking. METHOD: Dynamic Mechanical Analysis (DMA) has been used to quantify the dynamic viscoelasticity of bovine tibial plateau osteochondral cores, over a frequency sweep from 1 to 88 Hz. Specimens (approximately aged between 18 and 30 months) were neither osteoarthritic nor otherwise compromised. A maximum nominal stress of 1.7 MPa was induced. Viscoelastic properties of cores have been compared with that of its components (cartilage and bone) in terms of the elastic and viscous components of both structural stiffness and material modulus. Micro-computed tomography scans were used to quantify the histomorphological properties of the subchondral bone. RESULTS: Opposing frequency-dependent loss stiffness, and modulus, trends were witnessed for osteochondral tissues: for cartilage it increased logarithmically (P < 0.05); for bone it decreased logarithmically (P < 0.05). The storage stiffness of osteochondral cores was logarithmically frequency-dependent (P < 0.05), however, the loss stiffness was typically frequency-independent (P > 0.05). A linear relationship between the subchondral bone plate (SBP) thickness and cartilage thickness (P < 0.001) was identified. Cartilage loss modulus was linearly correlated to bone mineral density (BMD) (P < 0.05) and bone volume (P < 0.05). CONCLUSION: The relationship between the subchondral bone histomorphometry and cartilage viscoelasticity (namely loss modulus) and thickness, have implications for the initiation and progression of osteoarthritis (OA) through an altered ability of cartilage to dissipate energy.

Lumbar model generator: a tool for the automated generation of a parametric scalable model of the lumbar spine.

Low back pain is a major cause of disability and requires the development of new devices to treat pathologies and improve prognosis following surgery. Understanding the effects of new devices on the biomechanics of the spine is crucial in the development of new effective and functional devices. The aim of this study was to develop a preliminary parametric, scalable and anatomically accurate finite-element model of the lumbar spine allowing for the evaluation of the performance of spinal devices. The principal anatomical surfaces of the lumbar spine were first identified, and then accurately fitted from a previous model supplied by S14 Implants (Bordeaux, France). Finally, the reconstructed model was defined according to 17 parameters which are used to scale the model according to patient dimensions. The developed model, available as a toolbox named the lumbar model generator, enables generating a population of models using subject-specific dimensions obtained from data scans or averaged dimensions evaluated from the correlation analysis. This toolbox allows patient-specific assessment, taking into account individual morphological variation. The models have applications in the design process of new devices, evaluating the biomechanics of the spine and helping clinicians when deciding on treatment strategies.

Effect of frequency on crack growth in articular cartilage.

Cracks can occur in the articular cartilage surface due to the mechanical loading of the synovial joint, trauma or wear and tear. However, the propagation of such cracks under different frequencies of loading is unknown. The objective of this study was to determine the effect of frequency of loading on the growth of a pre-existing crack in cartilage specimens subjected to cyclic tensile strain. A 2.26mm crack was introduced into cartilage specimens and crack growth was achieved by applying a sinusoidally varying tensile strain at frequencies of 1, 10 and 100Hz (i.e. corresponding to normal, above normal and up to rapid heel-strike rise times, respectively). These frequencies were applied with a strain of between 10-20% and the crack length was measured at 0, 20, 50, 100, 500, 1000, 5000 and 10,000 cycles of strain. Crack growth increased with increasing number of cycles. The maximum crack growth was 0.6 ± 0.3 (mean ± standard deviation), 0.8 ± 0.2 and 1.1 ± 0.4mm at frequencies of 1, 10 and 100Hz, respectively following 10,000 cycles. Mean crack growth were 0.3 ± 0.2 and 0.4 ± 0.2 at frequencies of 1 and 10Hz, respectively. However, this value increased up to 0.6 ± 0.4mm at a frequency of 100Hz. This study demonstrates that crack growth was greater at higher frequencies. The findings of this study may have implications in the early onset of osteoarthritis. This is because rapid heel-strike rise times have been implicated in the early onset of osteoarthritis.

Fatigue strength of bovine articular cartilage-on-bone under three-point bending: the effect of loading frequency.

BACKGROUND: The objective of this study was to determine the influence of loading frequency on the failure of articular cartilage-on-bone specimens under three-point bending. METHODS: In this study, cyclic three-point bending was used to introduce failure into cartilage-on-bone specimens at varying loading frequencies. Sinusiodally varying maximum compressive loads in the range 40-130 N were applied to beam-shaped cartilage-on-bone specimens at frequencies of 1, 10, 50 and 100 Hz. RESULTS: The number of cycles to failure decreased when loading frequency increased from normal and above gait (1 and 10 Hz) to impulsive loading frequencies (50 and 100 Hz). It was found that 67 and 27% of the specimens reached run-out at loading of 10,000 cycles at frequencies of 1 and 10 Hz, respectively. However, 0% of the specimens reached run-out at loading frequencies of 50 and 100 Hz. CONCLUSION: The results indicate that increasing the loading frequency reduces the ability of specimens to resist fracture during bending. The findings underline the importance of the loading frequency concerning the failure of articular cartilage-on-bone and it may have implications in the early onset of osteoarthritis.

Viscoelastic properties of human bladder tumours.

The urinary bladder is an organ which facilitates the storage and release of urine. The bladder can develop tumours and bladder cancer is a common malignancy throughout the world. There is a consensus that there are differences in the mechanical properties of normal and malignant tissues. However, the viscoelastic properties of human bladder tumours at the macro-scale have not been previously studied. This study investigated the viscoelastic properties of ten bladder tumours, which were tested using dynamic mechanical analysis at frequencies up to 30Hz. The storage modulus ranged between 0.052MPa and 0.085MPa while the loss modulus ranged between 0.019MPa and 0.043MPa. Both storage and loss moduli showed frequency dependent behaviour and the storage modulus was higher than the loss modulus for every frequency tested. Viscoelastic properties may be useful for the development of surgical trainers, surgical devices, computational models and diagnostic equipment.

Effect of the variation of loading frequency on surface failure of bovine articular cartilage.

BACKGROUND: Mechanical loading of synovial joints can damage the articular cartilage surface and may lead to osteoarthritis. It is unknown if, independent of load, frequency alone can cause failure in cartilage. This study investigated the variation of articular cartilage surface damage under frequencies associated with normal, above normal and traumatic loading frequencies. METHOD: Cartilage on bone, obtained from bovine shoulder joints, was tested. Damage was created on the cartilage surface through an indenter being sinusoidally loaded against it at loading frequencies of 1, 10 and 100 Hz (i.e., relevant to normal, above normal and up to rapid heel-strike rise times, respectively). The frequencies were applied with a maximum load in the range 60-160 N. Surface cracks were marked with India ink, photographed and their length measured using image analysis software. RESULTS: Surface damage increased significantly (P < 0.0001) with frequency throughout all load ranges investigated. The dependence of crack length, c, on frequency, f, could be represented by, c=A(log10(f))2+B(log10(f))+Dc=A(log10(f))2+B(log10(f))+D where A = 0.006 ± 0.23, B = 0.62 ± 0.23 and D = 0.38 ± 0.51 mm (mean ± standard deviation). CONCLUSION: The increase in crack length with loading frequency indicated that, increased loading frequency can result in cartilage becoming damaged. The results of this study have implications in the early stages of osteoarthritis.

Frequency dependent viscoelastic properties of porcine bladder.

The aim of this study was to measure the viscoelastic properties of bladder tissue. Porcine bladders were dissected into rectangular strips and loops. Dynamic Mechanical Analysis was used to measure the viscoelastic properties of the bladder tissue (storage and loss stiffness) tested in a frequency range of up to 10 Hz. Storage stiffness was found to be consistently higher than loss stiffness. Average storage stiffness was found to be 1.89 N/mm and 0.74 N/mm for looped and rectangular samples, respectively. Average loss stiffness was found to be 0.24 N/mm and 0.11 N/mm for looped and rectangular samples, respectively. The results of this study are important for computational modelling of the bladder and for ensuring that engineered bladder tissues have physiological viscoelastic properties.

Rigid intramedullary nail fixation of femoral fractures in adolescents: what evidence is available?

BACKGROUND: Femoral fracture in adolescents is a significant injury. It is generally agreed that operative fixation is the treatment of choice, and rigid intramedullary nail fixation is a treatment option. However, numerous types of rigid nails to fix adolescent femoral fractures have been described. Hence, the aim of this paper was to collate and evaluate the available evidence for managing diaphyseal femoral fractures in adolescents using rigid intramedullary nails. MATERIALS AND METHODS: A literature search was undertaken using the healthcare database website ( http://www.library.nhs.uk/hdas ). Medline, CINAHL, Embase, and the Cochrane Library databases were searched to identify prospective and retrospective studies of rigid intramedullary nail fixation in the adolescent population. RESULTS: The literature search returned 1,849 articles, among which 51 relevant articles were identified. Of these 51 articles, 23 duplicates were excluded, so a total of 28 articles were reviewed. First-generation nails had a high incidence of limb length discrepancy (Küntscher 5.8 %, Grosse-Kempf 9 %), whilst second-generation nails had a lower incidence (Russell-Taylor 1.7 %, AO 2.6 %). Avascular necrosis was noted with solid Ti nails (2.6 %), AO femoral nails (1.3 %) and Russell-Taylor nails (0.85 %). These complications have not been reported with the current generation of nails. CONCLUSIONS: Rigid intramedullary nail fixation of femoral fractures in adolescents is a useful procedure with good clinical results. A multiplanar design and lateral trochanteric entry are key to a successful outcome of titanium alloy nail fixation.

Design of a surgical instrument for removing bone to provide screw access to a spinal fusion cage.

A surgical instrument to aid implantation of a range of lumbar spinal fusion cages has been developed. Once the cage is in position, the entrance to screw holes is partially blocked by the edge of the vertebral body. In order to insert fixation screws to secure the cage between the vertebrae, some part of the blocking edge has to be removed. Rongeurs are currently being used, but they can be time consuming and have the disadvantage that they may remove more bone than is necessary and may cause damage to the fusion cage if not used with care. In addition, access around some of the screw holes may be difficult. The aim of this instrument was to overcome these shortcomings. This paper describes the design of a surgical instrument for cutting edges from vertebral bodies. The development and evaluation of concept designs are presented and discussed. Potential risks were considered and modifications were performed on the selected concept. Functional prototypes were manufactured and tested on sheep lumbar vertebrae. The results showed that the newly designed cutting instrument functions as required and removes the required amount of bone from the vertebral body edge.

Range of motion of the metacarpophalangeal joint in rheumatoid patients, with and without a flexible joint replacement prosthesis, compared with normal subjects.

BACKGROUND: The metacarpophalangeal is commonly affected by rheumatoid arthritis. This may lead to joint replacement with a flexible prosthesis. The aims of this study were to determine the effects of rheumatoid arthritis on joint motion and to determine whether joint replacement needs to restore the full range of motion. METHODS: Three-dimensional motion analysis was used to measure the range of motion of the metacarpophalangeal joint in rheumatoid patients with and without a flexible silicone arthroplasty, when performing pinch and key grips, when making a fist and when spreading the fingers. The results were compared with those from younger and older normal subjects. FINDINGS: There appeared to be a trend for a decrease in range of motion from younger normal to older normal to rheumatoid (no prosthesis) to rheumatoid (with prosthesis) subject groups. However, statistically different (p<0.05) results were only observed for some movements (mostly involved in making a fist), in some fingers and between some subject groups. The only exception to this apparent trend was in flexion/extension when spreading the fingers into abduction. INTERPRETATION: Making a fist is the most sensitive simple measure of range of motion in the metacarpophalangeal joint. Successful replacement of the metacarpophalangeal joint in patients with rheumatoid arthritis need not restore the normal range of motion.

Effect of side holes in cervical fusion cages: a finite element analysis study.

The objective of this study was to investigate the effect of side holes on the predicted von Mises stress levels in cervical spinal fusion cages subjected to compressive loading. Models with between zero and ten side holes were developed. Finite element analysis (FEA) was used to simulate compression of the cage, made from the polymer PEEK (polyetheretherketone), between two adjacent vertebrae. The analyses were validated by experimental tests. In all of the models, the von Mises stress was highest at the cage-vertebrae interface with peak stresses of between 14 and 18 MPa. Increasing the Young's modulus of the vertebrae from 12 to 30 GPa increased the peak stress on average by 29 per cent. The stresses in the models were lower than the compressive strength of PEEK (118 MPa), and are well within the PEEK fatigue strength reported (60 MPa at 10 million cycles). This study suggests that the number of side holes had a negligible effect on the stress distribution within the cage; the stress magnitudes were fairly constant across all of the models and did not change substantially with the number of holes. Hence, a cervical cage with side holes is unlikely to fail in compression.

Pedicle Screw Surgery in the UK and Ireland: A Questionnaire Study.

Pedicle screw (PS) malpositioning rates are high in spine surgery. This has resulted in the use of computed navigational aids to reduce the rate of malposition; but these are often expensive and limited in availability. A simple mechanical device to aid PS insertion might overcome some of these disadvantages. The purpose of this study was to determine the demand and design criteria for a simple device to aid PS placement, as well as to collect opinions and experiences on PS surgery in the UK and Ireland. A postal questionnaire was sent to 422 spinal surgeons in the UK and Ireland. 101 questionnaires were received; 67 of these (16% of total sent) contained useful information. 78% of surgeons experienced problems with PS placement. The need for a simple mechanical device to aid PS placement was expressed by 59% of respondent surgeons. The proportion of respondents that inserted PSs in the cervical spine was 14%; PSs are mainly inserted in the thoracic, lumbar and sacral spine, but potential exists for a PS placement aid for the cervical and thoracic spine. From the experiences of these 67 surgeons, there is evidence to suggest that surgeons would prefer a pedicle aid that is multiple use, one-piece, hand-held, radiolucent, unilateral and uses the line of sight principle in traditional open surgery. Based on the experiences of 67 surgeons, there is evidence to suggest that computed navigational aids are not readily used in PS surgery and that a simple mechanical device could be a better option. This paper provides useful data for improving the outcomes of spinal surgery.

A review of the design process for implantable orthopedic medical devices.

The design process for medical devices is highly regulated to ensure the safety of patients. This paper will present a review of the design process for implantable orthopedic medical devices. It will cover the main stages of feasibility, design reviews, design, design verification, manufacture, design validation, design transfer and design changes.

Frequency dependence of viscoelastic properties of medical grade silicones.

Cylinders of medical grade silicone elastomers, (29 mm in diameter and 13 mm thick), immersed in physiological saline solution at 37 degrees C, were investigated by dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 +/- 5 N was applied over a frequency range, f, of 0.02-100 Hz. Values of the storage, E', and loss, E'', moduli for the cylinders were found to depend on f; the dependence of E' or E'' on the logarithm (base 10) of f was represented by a third-order polynomial. Above about 0.3 Hz, the cylindrical specimens appeared to be undergoing the onset of a transition from the rubbery to the glassy state. There was no significant difference between results obtained at 37 and 23 degrees C; pretreatment of specimens in physiological saline at 37 degrees C for 24 h and 29 days had no appreciable effect on the results.

Crack growth of medical-grade silicone using pure shear tests.

Silicone elastomers are commonly used in the manufacture of single-piece joint replacement implants for the finger joints. However, the survivorship of these implants can be poor, with failure typically occurring from fracture of the stems. The aim of this paper was to investigate the crack growth of medical-grade silicone using pure shear tests. Two medical-grade silicones (C6-180 and Med82-5010-80) were tested. Each sample had a 20 mm crack introduced and was subjected to a sinusoidally varying tensile strain, with a minimum of 0 per cent and a maximum in the range 10 to 77 per cent. Testing was undertaken at a frequency of 10 Hz. At various times during testing, the testing machine was stopped, the number of cycles completed was noted, and the crack length measured. Graphs of crack length against number of cycles were plotted, as well as the crack growth rate against tearing energy. The results show that Med82-5010-80 is more crack resistant than C6-180. Graphs of crack growth rate against tearing energy can be used to predict the failure of these medical-grade elastomers.

Soft layered concept in the design of metacarpophalangeal joint replacement implants.

The metacarpophalangeal (MCP) joint is crucial for hand function, but the joints are frequently affected by arthritis, leading to pain and disability. Joint replacement implants are used to replace the diseased MCP joint. This paper presents an investigation of applying the soft layered concept in the design of a new MCP joint replacement implant. Analytical methods were used to investigate the minimum film thickness for a novel MCP joint with a soft layer. The effect of load, entraining velocity, radial clearance, radius of the metacarpal head, elastic modulus and thickness of the soft layer were investigated. The soft layered joints show an enhanced predicted film thickness and some evidence of fluid film lubrication that should help to reduce wear rates. It may be beneficial for future MCP joint implant designs to utilise the soft layered joint concept.

The effect of the environment on the mechanical properties of medical grade silicones.

Silicone spacers have been in use as replacement joints in the human hand for over 30 years. Since they were first used there has been a number of designs all of which have had problems with fracture. This may be due to a defect in the material caused during implantation, or by bony intrusions within the arthritic hand after implantation. The aim of this research was to investigate the effect of the environment on the mechanical properties of medical grade silicones used for human implantation. The materials were subjected to static tensile testing after various forms of ageing. The environmental conditions included temperatures of 37 and 80 degrees C and the environments of Ringer's solution, distilled water, and air. The environmental conditions employed resulted in reduced mechanical strength with ageing time of the silicones. This research supports the view that failure of silicone implants in the hand could be partly attributed to the effects of environmental ageing of the material.

Viscoelastic properties of composites of calcium alginate and hydroxyapatite.

Calcium alginate was reinforced with hydroxyapatite (HA) particles, whose dimensions were a few micrometres, with mass fraction, mf, values in the range 0-0.8. Cylindrical samples of these composite materials were subjected to cyclic compression in the frequency range f = 0.001-20 Hz; in these tests a sinusoidal load of amplitude 1 N was applied either side of a static compression of 2 N. Storage and loss moduli, E' and E'', respectively, were found to be independent of particle size; however, E' increased with frequency consistent with the materials undergoing a glass transition. Above frequencies of about 0.05 Hz, E' > E'' for all materials. For each frequency, the dependence of the moduli on log10f could be represented by a third order polynomial; these equations can be used to calculate E' and E'' for a range of compositions. Approximate values of (E*) = square root of E'2 + E''2 are predicted by a Reuss model.

Wear of medical grade silicone rubber against titanium and ultrahigh molecular weight polyethylene.

The replacement of arthritic small joints, such as the fingers and wrist, has typically involved the use of one-piece silicone rubber implants. Newer designs have involved the silicone moving against either a titanium or ultrahigh molecular weight polyethylene (UHMWPE) component. The aim of this study was to investigate the wear of medical grade silicone rubber against titanium and UHMWPE. A pin-on-disc apparatus was used to slide a titanium and UHMWPE pin against a silicone disc, in the presence of either a Ringer's solution or bovine serum lubricant. Testing was undertaken at a sliding speed of 0.079 m/s and was continued for 10 km. Wear factors for titanium against silicone were 40.0 x 10(-6) mm(3)/N m and 66.5 x 10(-6) mm(3)/N m for bovine serum and Ringer's solution, respectively. The wear factors for UHMWPE against silicone were higher with values of 84.4 x 10(-6) mm(3)/N m and 88.3 x 10(-6) mm(3)/N m for bovine serum and Ringer's solution, respectively. The results of this study will be useful in future designs of finger and wrist implants that combine silicone rubber with either titanium or UHMWPE.

Finite element modelling of the Ilizarov external fixation system.

This study describes a computational method for predicting the mechanical response of any configuration of the Ilizarov external fixation system. Mechanical testing of each of the individual components (ring, threaded rod, and wire) of the Ilizarov system was used to determine the stiffness of each component. Finite element (FE) analysis was then used to model each of the individual components. Each model was tuned to match the mechanical testing. A modular FE modelling system, using a master input file, was then developed where the tuned FE models of the individual components could be generated, positioned, and interconnected to replicate a range of fixator configurations. The results showed that the stiffness predications from the FE modelling of the fixator configurations were consistently 10 per cent higher than the stiffness values obtained from the mechanical testing. The FE modelling system can be used to predict the characteristic response of the fixator configurations and clearly shows the relative changes in that response for variations in the number of components used.