Biomedical subjects

V Saikko

Publications and source records attributed to V Saikko.

13 recordsLinked to original sources

PubMed2006

Effect of contact pressure on wear and friction of ultra-high molecular weight polyethylene in multidirectional sliding.

Computational wear models need input data from valid tribological tests. For the wear model of a total hip prosthesis, the contact pressure dependence of wear and friction of ultra-high molecular weight polyethylene (UHMWPE) against polished CoCr in diluted calf serum lubricant was studied, and useful input data produced. Two test devices were designed and built: a heavy load circularly translating pin-on-disc (HL-CTPOD) wear test device and an HL-CTPOD friction measurement device. Both can be used with a wide range of loads. The wear surface diameter of the test pin was kept constant at 9 mm, whereas the load was varied so that the nominal contact pressure ranged from 0.1 to 20 MPa. The wear factor decreased with increasing contact pressure, whereas the coefficient of friction first increased with increasing contact pressure with low pressure values and then decreased. Up to the pressure of 2.0 MPa, the wear mechanisms and wear factors were in good agreement with clinical findings. In the critical range of 2.0-3.5 MPa, the wear mechanisms and wear factors started to differ from clinical ones, and the decrease of the wear factor steepened. The discrepancy became more and more evident as the pressure was gradually increased beyond 3.5 MPa. It appears that the pressure value of 2.0 MPa should not be exceeded in pin-on-disc wear tests that are to reproduce the clinical wear of UHMWPE acetabular cups.

Acetabulum↗

PubMed2005

A hip wear simulator with 100 test stations.

A novel high-capacity hip wear simulator of the pin-on-disc type was designed, built, and validated. This so-called Super-CTPOD (circularly translating pin-on-disc) device has as many as 100 separate test stations, being an advanced version of the previously validated 12-station CTPOD. A validity test was done so that in all stations the specimens and the test conditions were as similar as possible. Hence, for the first time in this field, an adequate number of similar tests was done for a proper statistical analysis of wear data. The pins were conventional, gamma-sterilized ultra-high molecular weight polyethylene, and the discs were polished CoCr. The lubricant was diluted calf serum and the test length 3 million cycles. In the course of the test, the pins became highly polished, whereas the discs remained practically unchanged. The majority of the polyethylene wear particles were rounded, with a mean diameter of 0.25 microm. The 100 wear factor values computed from the 100 steady state wear rate values of the pins were normally distributed, the mean +/- 95 per cent confidence interval being 1.63 +/- 0.017 x 10(-6) mm3 /N m. The standard deviation was 5.4 per cent of the mean. There were no outliers. The wear mechanisms and the wear factor agreed well with clinical findings. Altogether, the Super-CTPOD test system was shown to be a unique combination of validity, low variation, capacity, efficiency, reliability, productivity, economy, ease of operation, and compact size.

Equipment Design↗

PubMed2005

A 12-station anatomic hip joint simulator.

A novel 12-station hip joint simulator with an anatomic position of the prosthesis was designed and built. The motion of the simulator consists of flexion-extension and abduction-adduction. The load is of the double-peak type. The validation test was done with three similar 28 mm CoCr-polyethylene joints in diluted calf serum lubricant for 3.3 x 10(6) cycles. The bearing surfaces of the polyethylene cups were burnished, the CoCr heads were undamaged, the wear particles were in the 0.1-1 microm size range, and the mean wear factor of the polyethylene cups was 5.7 x 10(-7) mm(3)/N m. These essential observations were in good agreement with clinical findings. In addition, three similar 50 mm CoCr/CoCr joints, representing the contemporary large-diameter metal-on-metal articulation were tested. The wear of the CoCr/CoCr joints was calculated from the Co and Cr concentrations of the used lubricant quantified with atomic absorption spectroscopy. The bearing surfaces of the CoCr/CoCr joints showed mild criss-cross scratching only. The average wear factor of polyethylene cups was 275 times that of the CoCr/CoCr joints. The tribological behaviour of the large-dia. CoCr/CoCr appeared to be dominated by fluid film lubrication, as indicated by very low frictional heating and wear, making it tribologically superior to the conventional CoCr/polyethylene, and therefore very interesting clinically. In conclusion, the simulator proved to be a valid, reliable, practical, economical, and easy-to-operate tool for wear studies of various hip replacement designs.

Computer Simulation↗

PubMed2003

Increased volumetric wear of polyethylene liners with more than 3 years of shelf-life time.

We analyzed 20 retrieved gamma-sterilized polyethylene liners (Biomet Hexloc). The long-term durability varied significantly depending on shelf-life time before implantation. Liners with a shelf-life time of 3 years or more evinced significantly ( P 0.002) higher volumetric wear than those with a shelf life time less than 3 years. Infrared spectroscopy and scanning calorimetry showed that all explanted implants underwent substantial in vivo oxidation and crystallization. The oxidative ageing of polyethylene renders the polyethylene liner susceptible to severe wear. Scanning electron microscopy of the bearing surface of the liner revealed abrasive wear as a dominant mechanism. Moreover, poor acetabular design produces excessively thin liners, substandard locking mechanism, and backside wear of the liner. The primary reason for severe wear in the Hexloc liner was poor modular design and oxidative degradation of the polyethylene.

Adult↗

PubMed2001

Effect of counterface roughness on the wear of conventional and crosslinked ultrahigh molecular weight polyethylene studied with a multi-directional motion pin-on-disk device.

The effect of counterface roughness on the wear of conventional gamma-sterilized, and electron-beam-crosslinked ultrahigh molecular weight polyethylene was studied with a circularly translating pin-on-disk device. The counterfaces, CoCr disks, were either polished, or roughened so that they represented the type of roughening and the range of surface roughness values (R(a) = 0.014-0.24 microm) observed in explanted femoral heads of total hip prostheses. The lubricant was diluted calf serum, and the test length 3 million cycles. A total of 24 tests were done. With both types of polyethylene, there was a strong correlation between R(a) and wear factor k. The power equations were k = 5.87 x 10(-5)(R(a))(0.91) for conventional polyethylene (R(2) = 0.94), and k = 7.87 x 10(-5)(R(a))(2.49) for crosslinked polyethylene (R(2) = 0.82). Crosslinking improved wear resistance significantly. The wear of crosslinked polyethylene against the roughest counterfaces was lower than the wear of conventional polyethylene against the polished counterfaces. Against rough counterfaces, the wear of crosslinked polyethylene was an order of magnitude lower than that of conventional polyethylene. On the crosslinked polyethylene pins that were tested against polished counterfaces, remains of original machining marks were still visible after the test. The average size of wear particles produced by both types of polyethylene against rough counterfaces was similar, 0.4 microm, whereas that produced by conventional and crosslinked polyethylene against polished counterfaces was significantly smaller, 0.2 and 0.1 microm, respectively.

Biocompatible Materials↗

PubMed2001

Wear simulation of total hip prostheses with polyethylene against CoCr, alumina and diamond-like carbon.

The wear of ultra-high molecular weight polyethylene acetabular cups was studied with a new biaxial hip wear simulator using diluted calf serum as a lubricant. The cups had been packed and gamma-irradiated in argon. The cups were articulated against two established types of femoral head, alumina and CoCr, and one experimental type, CoCr coated with diamond-like carbon (DLC). The diameter of the heads was 28 mm. Polyethylene against alumina and against CoCr were studied because their clinical wear behaviour is relatively well known. The new simulator was validated with these established materials. The wear mechanisms, including the size and shape of the wear particles, agreed well with those seen in clinical studies. The average wear rates of the cups against alumina and CoCr heads were 48 and 56 mg per 1 million cycles, respectively. The order is in agreement with clinical observations. The average wear rate against DLC was 58 mg per 1 million cycles. As a counterface for polyethylene, DLC did not markedly differ from alumina and CoCr.

Aluminum Oxide↗

PubMed2000

Wear simulation of UHMWPE for total hip replacement with a multidirectional motion pin-on-disk device: effects of counterface material, contact area, and lubricant.

The wear of nonirradiated ultra-high molecular weight polyethylene (UHMWPE) was studied with a circularly translating pin-on-disk (CTPOD) device. With this simple device, the wear of the acetabular cup is simulated. Two shapes of a polyethylene pin were used: flat wear faces of 3.0 and 8.9 mm in diameter. The load was always 70.7 N. Tests were done against stainless steel plates in serum, albumin, and gamma-globulin lubricants and against alumina plates in serum. With 3.0-mm-in-diameter wear faces, the wear factors were an order of magnitude lower than the average clinical wear factors of polyethylene acetabular cups. With 8.9-mm-in-diameter wear faces, the wear factors and wear mechanisms were consistent with clinical ones. The mean wear factor against stainless steel in serum was 1.46 times higher than that against alumina, and the difference was statistically significant. The mean wear factors in albumin and gamma globulin were close to that in serum; the differences were not statistically significant. The results indicate that albumin and gamma globulin are the fractions of synovia that influence the wear of polyethylene. A practical test parameter combination for the CTPOD device has now been established for future studies of modified and new materials and of lubricants.

Arthroplasty, Replacement, Hip↗

PubMed1999

Type of motion and lubricant in wear simulation of polyethylene acetabular cup.

The wear of ultra-high molecular weight polyethylene, the most commonly used bearing material in prosthetic joints, is often substantial, posing a significant clinical problem. For a long time, there has been a need for simple but still realistic wear test devices for prosthetic joint materials. The wear factors produced by earlier reciprocating and unidirectionally rotating wear test devices for polyethylene are typically two orders of magnitude too low, both in water and in serum lubrication. Wear is negligible even under multidirectional motion in water. A twelve-station, circularly translating pin-on-disc (CTPOD) device and a modification of the established biaxial rocking motion hip joint simulator were built. With these simple and inexpensive devices, and with the established three-axis hip joint simulator, realistic wear simulation was achieved. This was due to serum lubrication and to the fact that the direction of sliding constantly changed relative to the polyethylene specimen. The type and magnitude of load was found to be less important. The CTPOD tests showed that the subsurface brittle region, which results from gamma irradiation sterilization of polyethylene in air, has poor wear resistance. Phospholipid and soy protein lubrication resulted in unrealistic wear. The introduction of devices like CTPOD may boost wear studies, rendering them feasible without heavy investment.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1998

A multidirectional motion pin-on-disk wear test method for prosthetic joint materials.

A realistic pin-on-disk wear test method for prosthetic joint materials has been developed. The new method, called circularly translating pin-on-disk (CTPOD), yields wear rates and wear mechanisms similar to those observed in retrieved polyethylene acetabular cups. In the established methods, where a polyethylene specimen slides against a unidirectionally rotating, or reciprocating, metallic or ceramic counterface, the wear rate typically is two orders of magnitude too low. In the present study, also, the reciprocator gave negligible wear. In the CTPOD method, considerable wear occurs because the direction of sliding rotates relative to the polyethylene pin, hence, the molecular orientation effect of polyethylene is avoided.

Biocompatible Materials↗

PubMed1998

Low wear and friction in alumina/alumina total hip joints: a hip simulator study.

We studied the wear of state-of-the-art alumina/alumina total hip joints with the uniaxial five-station, and the three-axis single-station hip joint simulators of Helsinki University of Technology. The diameters of the joints were 26, 28 and 32 mm. Visual examination and weighing of the components showed no wear. Scanning electron microscopy showed that (a) the wear marks were slight, consisting of removal of grains, multidirectional grooving and mild abrasion, (b) the wear mechanisms were similar to those seen in components removed from patients, and (c) there was no marked difference in the specimens worn in the uniaxial vs. three-axis simulator. The coefficient of friction was measured throughout the 5 million cycle wear test with the three-axis simulator. The average value was 0.007. We conclude that alumina/alumina joints had low wear and friction under the present test conditions. The results are promising, since minimal wear is one of the prerequisites for long-term success in prosthetic joints.

Aluminum Oxide↗

PubMed1998

Metal release from total hip articulations in vitro: substantial from CoCr/CoCr, negligible from CoCr/PE and alumina/PE.

We used a hip joint simulator to compare the metal release from CoCr/CoCr, CoCr/PE, and alumina/PE total hip articulations. The metal release was quantified by analyzing the Co, Cr, and Ni contents of the bovine serum lubricant used with atomic absorption spectroscopy. CoCr/CoCr articulations released substantial amounts of metal, whereas CoCr/PE was equal to the control, alumina/PE, in that metal release was negligible. The metal release was in accordance with the known clinical wear rates of CoCr/CoCr articulations. The largest dimensional changes occurred in polyethylene cups, the penetrations of CoCr heads to the polyethylene cups being twice that of the alumina head, which is consistent with clinical experience. The research on the wear behavior of different materials, aiming to find a prosthesis with negligible wear, needs to be continued. Due to the substantial metal release, the CoCr/CoCr articulation is hardly the final solution of the wear problem in total hip arthroplasty.

Aluminum Oxide↗

PubMed1992

A five-station hip joint simulator for wear rate studies.

The aim of the work has been the development of a hip joint simulator for comparative wear rate studies of long duration. A five-station apparatus has been designed, constructed and tested. Five total hip joints can be tested at the same time in identical conditions. The flexion-extension motion and the superior-inferior component of the joint contact force are incorporated. The motion is electromechanical and the loading pneumatic. The angle and load waveforms are fixed and simulate level walking. For accurate wear measurements each station employs a control prosthesis. The conditions of the control prosthesis in regard to loading, exposure to lubricant and environment temperature (37 +/- 1 degree C) are identical to those of the test prosthesis. The acetabular cups can be readily removed for periodic wear measurements and reassembled in exactly the original position. Extensive tests have shown that the simulator is a practical and reliable instrument in the wear rate studies of various designs of total hip joint.

Equipment Design↗

PubMed1992

A simulator study of friction in total replacement hip joints.

Frictional behaviour of 22 different femoral head-acetabular cup combinations was studied on a new servo-hydraulic microcomputer-controlled hip joint simulator using various flexion-extension angle and superior-inferior load set value waveforms and using distilled water at 37 +/- 1 degrees C as lubricant. Six different head materials were included in the study, whereas all cups were ultra-high molecular weight polyethylene (UHMWPE). Most head-cup combinations studied are commercially available. No distinctly superior joint design can be pointed out, but the frictional behaviour of alumina ceramic against UHMWPE proved overall most favourable (mu min was 0.02), whereas that of non-ion-implanted titanium alloy Ti-6Al-4V against UHMWPE proved strikingly poor (mu max was 0.15). The lowest frictional torque was in 22 mm joints, but frictional torque did not always increase straightforwardly with increasing diameter of the femoral head. The measurements form an extensive comparison between a wide variety of head-cup combinations. The simulator is apparently a useful instrument in the study of frictional behaviour of new designs, materials, surface treatments and coatings that are frequently introduced.

Friction↗