Aziza Mahomed

Accelerated aging for testing polymeric biomaterials and medical devices

Elevated temperature is frequently used to accelerate the aging process in polymers that are associated with medical devices and other applications. A common approach is to assume that the rate of aging is increased by a factor of 2(DeltaT/10), where DeltaT is the temperature increase. This result is a mathematical expression of the empirical observation that increasing the temperature by about 10 degrees C roughly doubles the rate of many polymer reactions. It is equivalent to assuming that the aging process is a first order chemical reaction with an activation energy of 10R/log(e)2, where R is the universal gas constant. A better approach would be to determine the activation energy for the process being considered but this is not always practicable. The simple approach does not depend on the temperature increase, provided that it is not so great that it initiates any physical or chemical process that is unlikely to be involved in normal aging. If a temperature increment theta were to increase a given polymer reaction rate n times, then an elevated temperature would increase the rate of aging by a factor of n(DeltaT/theta).

Swelling of medical grade silicones in liquids and calculation of their cross-link densities

Four medical grade silicones were swollen, until they reached equilibrium (i.e. constant mass) in eight liquids at 25 degrees C. The greatest swelling was obtained with n-heptane but the volume fraction, varphi, of the silicones in their swollen state was not significantly different (p<0.05) in this liquid than in cyclohexane. For each grade of silicone, varphi was plotted against delta(l), the liquid solubility parameter, for each liquid in which it was swollen. A second-order polynomial was plotted through the results; the minimum in this polynomial provided a value for the polymer solubility parameter, delta(p). The Flory polymer-liquid interaction parameter, chi, was calculated for the four best liquids, using Hildebrands solubility parameter theory. An alternative method for calculating chi, directly from swelling measurements, was shown to produce physically unreasonable results. The cross-link density, upsilon, was calculated, from varphi and chi, for each grade of silicone, using the Flory-Rehner equation. Since the values of two parameters involved in Hildebrands theory cannot be determined reliably and because the Flory-Rehner equation is an approximation, absolute values of upsilon cannot be obtained. However, the relative values of upsilon obtained were higher for the harder grades then for the softer grades and similarly, the grades with the higher Youngs modulus had higher upsilon values.

Friction in metal-on-metal total disc arthroplasty: effect of ball radius.

Total disc arthroplasty (TDA) can be used to replace a degenerated intervertebral disc in the spine. There are different designs of prosthetic discs, but one of the most common is a ball-and-socket combination. Contact between the bearing surfaces can result in high frictional torque, which can then result in wear and implant loosening. This study was designed to determine the effects of ball radius on friction. Generic models of metal-on-metal TDA were manufactured with ball radii of 10, 12, 14 and 16 mm, with a radial clearance of 0.015 mm. A simulator was used to test each sample in flexion-extension, lateral bending and axial rotation at frequencies of 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75 and 2 Hz under loads of 50, 600, 1200 and 2000 N, in new born calf serum. Frictional torque was measured and Stribeck curves were plotted to illustrate the lubrication regime in each case. It was observed that implants with a smaller ball radius showed lower friction and showed boundary and mixed lubrication regimes, whereas implants with larger ball radius showed boundary lubrication only. This study suggests designing metal-on-metal TDAs with ball radius of 10 or 12 mm, in order to reduce wear and implant loosening.

Effect of axial load on the flexural properties of an elastomeric total disc replacement.

Study Design. Twelve Cadisc-L devices were subjected to flexion (0°–6°) and extension (0° to −3°) motions at compressive loads between 500 N and 2000 N at a flexural rate between 0.25°/s and 3.0°/s. Objective. To quantify the change in flexural properties of the Cadisc-L (elastomeric device), when subjected to increasing magnitudes of axial load and at different flexural rates. Summary of Background Data. The design of motion preservation devices, used to replace degenerated intervertebral discs, is commonly based on a low-friction, ball-and-socket-articulating joint. Recently, elastomeric implants have been developed that attempt to provide mechanical and motion properties that resemble those of the natural disc more closely. Methods. Twelve Cadisc-L devices (MC-10 mm-9° and MC-10 mm-12° size) were supplied by Ranier Technology Ltd (Cambridge, United Kingdom). The devices were hydrated and tested using a Bose spinal disc-testing machine (Bose Corporation, ElectroForce Systems Group, Eden Prairie, MN) in Ringers solution at 37°C. A static load of 500 N was applied to a device and it was then subjected to motions of 0° to 6° to 0° (flexion) and 0° to −3° to 0° (extension) at a flexural rate of 0.25°/s, 0.5°/s, 1.0°/s, 1.5°/s, 2.0°/s, and 3.0°/s. Tests were repeated at 1000 N, 1500 N, and 2000 N. Results. Regression analyses showed a significant (R2 > 0.99, P < 0.05) linear increase in bending moment and flexural stiffness with flexion and extension angles (at 1000 N and higher loads)—a significant (R2 > 0.994, P < 0.05) linear decrease in flexural stiffness in flexion and extension as flexural rate increased. Conclusion. The bending moment of the Cadisc-L increased linearly with flexion and extension angles at 1000 N and higher loads. Flexural stiffness increased with compressive load but decreased with flexural rate.

Wear in metal-on-metal total disc arthroplasty

The wear of a model metal-on-metal ball-and-socket total disc arthroplasty was measured in a simulator. The ball had a radius of 10 mm, and there was a radial clearance between ball and socket of 0.015 mm. The model was subjected to simultaneous flexion–extension, lateral bending, axial rotation (frequency: 1 Hz) and compression (frequency: 2 Hz, maximum load: 2 kN). Throughout the tests, the models were immersed in calf serum diluted to a concentration of 15 g protein per litre, at a controlled temperature of 37 °C. Tests were performed on three models. At regular intervals (0, 0.5, 1, 2, 3, 4 and 5 million cycles), mass and surface roughness were determined; mass measurements were converted into the volume lost as a result of wear. All measurements were repeated six times. Wear occurred in two stages. In the first stage (duration about 1 million cycles), there was a linear wear rate of 2.01 ± 0.04 mm3 per million cycles; in the second stage, there was a linear wear rate of 0.76 ± 0.02 mm3 per million cycles. Surface roughness increased linearly in the first million cycles and then continued to increase linearly but more slowly.

Polymer-on-Metal or Metal-on-Polymer Total Disc Arthroplasty: Does it Make a Difference?

Study Design. Mechanical testing of total disc arthroplasty (TDA). Objective. To compare the friction between a polymer socket-on-metal ball and metal socket-on-polymer ball TDA. Summary of Background Data. A degenerate intervertebral disc can be replaced by TDA. The most common designs have a ball and socket articulation; the contact between the surfaces leads to friction. Friction needs to be minimized to prevent loosening and wear. One of the common material combinations in disc arthroplasty devices is the articulation of a metal socket on polymer ball. However, the combination of a polymer socket on metal ball (which is used in hip arthroplasty) has not been investigated for TDA. Methods. TDA models with either a polymer socket/metal ball or a metal socket/polymer ball were manufactured with ball radii of 10 and 14 mm, each with a radial clearance of 0.35 mm. Samples were tested using a spine simulator with a lubricant of diluted newborn calf serum. Each sample was subjected to an axial load of 1200 N; motions of flexion-extension, lateral bending, and axial rotation were then applied at frequencies of 0.25 to 2 Hz. Frictional torque was measured to compare the performance of the TDAs. Results. The frictional torque was found to be significantly higher for a disc with a metal socket/polymer ball than for a disc with a polymer socket/metal ball for both 10 and 14 mm radii in axial rotation, lateral bend, and extension. The frictional torque in flexion (0°–6°) was not found to be significantly different between the 2 different material combinations. However, when the flexion motion was reduced to 0° to 2°, frictional torque in the metal socket/polymer ball was found to be significantly higher than the polymer socket/metal ball. Conclusion. TDA with a combination of a polymer socket/metal ball has lower friction than the conventional TDA with metal socket/polymer ball. This conclusion has implications in the design of TDA.

Wear of the Charité® lumbar intervertebral disc replacement investigated using an electro-mechanical spine simulator

The Charité® lumbar intervertebral disc replacement was subjected to wear testing in an electro-mechanical spine simulator. Sinusoidally varying compression (0.6–2 kN, frequency 2 Hz), rotation (±2°, frequency 1 Hz), flexion–extension (6° to −3°, frequency 1 Hz) and lateral bending (±2°, frequency 1 Hz) were applied out of phase to specimens immersed in diluted calf serum at 37 °C. The mass of the ultra-high-molecular weight polyethylene component of the device was measured at intervals of 0.5, 1, 2, 3, 4 and 5 million cycles; its volume was also measured by micro-computed tomography. Total mass and volume losses were 60.3 ± 4.6 mg (mean ± standard deviation) and 64.6 ± 6.0 mm3. Corresponding wear rates were 12.0 ± 1.4 mg per million cycles and 12.8 ± 1.2 mm3 per million cycles; the rate of loss of volume corresponds to a mass loss of 11.9 ± 1.1 mg per million cycles, that is, the two sets of measurements of wear agree closely. Wear rates also agree closely with measurements made in another laboratory using the same protocol but using a conventional mechanical spine simulator.

Effect of lubricants on friction in laboratory tests of a total disc replacement device

Some designs of total disc replacement devices have articulating bearing surfaces, and these devices are tested in vitro with a lubricant of diluted calf serum. It is believed that the lubricant found in total disc replacement devices in vivo is interstitial fluid that may have properties between that in Ringer’s solution and diluted calf serum. To investigate the effect of lubricants, a set of friction tests were performed on a generic model of a metal against metal ball-and-socket total disc replacement device. Two devices were tested: one with a ball radius of 10 mm and other with a ball radius of 16 mm; each device had a radial clearance of 0.015 mm. A spine simulator was used to measure frictional torque for each device in axial rotation, flexion–extension and lateral bending at frequencies of 0.25–2 Hz, under 1200 N axial load. Each device was tested with two different lubricants: a solution of new born calf serum diluted with deionised water and Ringer’s solution. The results showed that the frictional torque generated between the bearing surfaces was significantly higher in Ringer’s solution than in diluted calf serum. The use of Ringer’s solution as a lubricant provides a stringent test condition to detect possible problems. Diluted calf serum is more likely to provide an environment closer to that in vivo. However, the precise properties of the fluid lubricating a total disc replacement device are not known; hence, tests using diluted calf serum may not necessarily give the same results as those obtained in vivo.

Viscoelastic Properties of Elastomers for Small Joint Replacements

In the first part of this paper, the E’ (storage modulus) and E” (loss modulus) values of six cylindrical specimens of Silastic® Q7-4780 medical grade silicone (suitable as a 90 day implant material), immersed in containers with physiological saline solution at 37°C, were measured using dynamic mechanical analysis. The results were compared with previous published data of a medical grade silicone, suitable as a 29 day implant material. The moduli of Q7-4780 were frequency-dependent as expected and were not significantly different from the shorter-term implant material. In the second part of this paper, the E’ and E” values of Elast-Eon™ 3, a polyurethane with poly(dimethylsiloxane) (PDMS) and poly(hexamethylamine oxide) (PHMO) segments, were measured before and after accelerated aging in an oven at 70°C for 38 days. The moduli of six cylindrical specimens of Nagor® medium hardness medical grade silicone were also investigated using the same aging conditions. The results showed, that both the E’ and E” of the Elast-Eon™3 were affected by accelerated aging. However the properties of Nagor® silicone were not affected.

S-30 Friction in Ball-and-Socket Total Disc Arthroplasty

School of Mechanical Engineering, University of Birmingham, Birmingham, UK This study investigated the effects on friction of changing the dimensions of a ball-and-socket Total Disc Arthroplasty (TDA). A generic ball-and-socket model was designed and manufactured based on the dimensions and geometry of a metal-on-metal Maverick (Medtronic, Minneapolis, USA) device. Keeping the radial clearance similar to the Maverick, the ball and socket dimensions varied between 10 to 16 mm and 10.015 to 16.015 mm, respectively, in order to enable the comparison between different dimensions. The implants were made out of Cobalt Chrome Molybdenum alloy, with a surface roughness of 0.05 μm. A Bose spine simulator (Bose Corporation, ElectroForce Systems Group, Minnesota, USA) was used to apply an axial compressive force to the TDA. Axial rotation of ±2° was then applied at various frequencies and the resulting frictional torque measured. The tests were performed under an axial load of 50, 600 and 1200 N and frequencies of 0.5, 1.0, 1.5 and 2.0 Hz, for four different samples of radii 10, 12, 14 and 16 mm (48 combinations in total). The results showed variation of frictional torque in different frequencies for all four samples under constant axial load. It was observed that the frictional torque had the lowest value for the implant with ball radius of 16mm. It might be concluded that the implant with larger ball radius may create less friction and hence offer a longer life.