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Biomedical subjects

P S Walker

Publications and source records attributed to P S Walker.

At least 19 recordsLinked to original sources
PubMed1992

A comparison of cortical strain after cemented and press-fit proximal and distal femoral replacement.

Proximal and distal femoral replacements with intramedullary stems are usually cemented in place but frequently show severe bone remodeling changes in the long term, which can contribute to a loosening process. The remodeling is likely to be associated with stress and strain distribution. This study compared the strain patterns, particularly the maximum principal strains, between cemented and press-fit components in five cadaveric femurs by using a photoelastic coating technique. The specimens were loaded with 2,000 N in the sequences of intact femur and press-fit and cemented stems. On the medial side of the bone for proximal femoral replacements, the strain values for the press-fit stems on the proximal, middle, and distal regions were 73 +/- 11%, 78 +/- 15%, and 80 +/- 15% of normal, respectively, but for the cemented stems they were 53 +/- 15%, 57 +/- 19%, and 60 +/- 20%. The differences were statistically significant (p less than 0.05). On the lateral side, the overall strain values for the press-fit stem were higher than those for the cemented stem, but the differences were not statistically significant (p greater than 0.05). On the medial side of the distal femoral replacement, the strain values for the press-fit stems in the proximal and middle regions were significantly closer to normal than for the cemented stems. Because the press-fit femoral stems (both the proximal and distal replacement) transferred closer to normal strains than the cemented stems, less adverse bone remodeling may be expected, which could be reflected in increased longevity.

Analysis of Variance↗
PubMed1992

A simplified radiographic method for measuring bone-component motion in total knees.

Measurement of the relative motion between an implant and the surrounding bone over different time periods is valuable for assessing and comparing the component stability and predicting the potential future outcome. The RSA method, where small beads are implanted in the bone adjacent to the component, can measure implant-bone position to an accuracy of about 0.1 mm. However, the method involves special radiographic views and analytical software, not readily available. For purposes of component assessment on a more routine basis and for multi-centre trials, a method was developed where standard A-P and M-L radiographs were used. Computer software was written which estimated the out-of-plane rotations of the component relative to the plane of the film, and then carried out corrections to enhance the accuracy of calculation of the bead heights relative to the component. The theoretical errors were shown to be less than 0.04 mm for the expected range of out-of-plane rotations. When radiographs of components in simulated bones were taken at a range of rotations, the 95% confidence limits for axial displacement were found to be less than +0.3 mm, and for rotation in the plane to be 0.6 degrees. This indicated that the method was useful for studying knee components where the sinkage could reach a range of 0.5-2 mm in a 2 year period.

Biomechanical Phenomena↗
PubMed1992

The CADCAM contribution to customized orthopaedic implants.

CADCAM (computer aided design/manufacture) production methods are often associated with mass production; working in the medical field at the Department of Biomedical Engineering, the requirement is for one-off, individualized implants. Using a knowledge-based system, implant designs are produced from X-ray data. Assembly from modular components has greatly reduced the production time of implants for bone tumour cases. CADCAM techniques are also used in the production of custom-made hip replacements using digitized data gathered from radiographs. Femoral canal shape is calculated and the optimal implant designed and manufactured from titanium alloy on the Department's CNC (computer numerically controlled) machines.

Computer-Aided Design↗
PubMed1992

Polyethylene wear in unicondylar knee prostheses. 106 retrieved Marmor, PCA, and St Georg tibial components compared.

106 unicondylar knee replacement tibial components were retrieved and analyzed for the amount and type of polyethylene wear. Three different designs were retrieved which had essentially the same femorotibial conformity. Each design showed a characteristic failure pattern. The polyethylene of PCA tibial components showed serious delamination after only short durations, as a result of heat pressing. St Georg sledge prostheses showed some delamination after 4 years' duration due to sub-surface cracks which were initiated by fusion defects in the polyethylene; metal backing of the components did not affect delamination of this prosthesis. The Marmor designs showed the least wear, with shiny depressions and surface pitting; no delamination was observed in the Marmor prosthesis. Molecular weight determination by gel permeation chromatography and analysis of crystallinity using Fourier transformation infra-red spectroscopy demonstrated that St George polyethylene had higher molecular weight and crystallinity than Marmor polyethylene. In some of the components investigated, crystallinity and molecular weight of the polyethylene were reduced under the wear track when compared with the unworn polyethylene. Since fusion defects may cause delamination of polyethylene we urge manufacturers to reduce the number of such defects.

Corrosion↗
PubMed1992

Uncemented intramedullary fixation of implants using polyethylene sleeves. A roentgenographic study.

Forty-three prostheses with noncemented, high-molecular-weight, polyethylene-sleeved components were used in the treatment of bone tumors around the knee in growing children. The average age of the patients was 11 years. There were 27 boys and 16 girls. There were 17 sleeved components in the distal femur and 26 in the proximal tibia. The average follow-up time was 27 months. The roentgenograms were nominally obtained at three, six, 12, 18, and 24 months, and irregularly thereafter, and were assessed using a zonal evaluation scheme. A sclerotic interface around the polyethylene sleeves invariably developed and progressed in density up to 28 months postoperatively. There were significant zonal differences in sclerosis, particularly between the plateau and the mid-sleeve zone. Only in one patient did a sleeve become loose and require revision.

Bone Neoplasms↗
PubMed1991

Motion of the knee after condylar resurfacing--an in vivo study.

The purpose of this study was determine the amounts of rotation and displacement occurring in a relatively unconstrained condylar replacement knee, and to compare the motion with a matched group of normal knees. The motion was measured using an electromagnetic device, the 3-Space Tracker, for various common activities, for 25 normal volunteers and 25 total knee patients. The main variables studied were internal-external rotation of the tibia about its long axis and anterior-posterior displacement of the femoral origin with respect to the tibia, as a function of flexion angle or per cent of gait cycle. The motion of the total knees was very similar to that of the normals. For sitting, standing and free-swing, the knee rotated internally by 5-10 degrees and the femur displaced posteriorly by 9-14 mm, as the knee was flexed from 0 to 90 degrees. For walking and going upstairs and downstairs the absolute values of the rotations and displacements were similar to the above. However, the mean values of rotation were less due to greater variation in the rotation patterns, due probably to the increased laxity of the knee during the swing phases. From these data, taking the mean motions and one standard deviation (S.D.), it is suggested that a knee prosthesis should allow a rotation from minus to plus 12 degrees (a total range of 24 degrees) and an anterior-posterior displacement of 13 mm. It is suggested that these motions be subject to progressive restraint from the neutral position in the manner of the natural knee.

Activities of Daily Living↗
PubMed1991

Fetal development and neuronal/glial cell specificity of cAMP-dependent protein kinase subunit mRNAs in rat brain.

All known actions of cAMP in the brain require cAMP-dependent protein kinase (cAMPdPK), which consists of regulatory (R) and catalytic (C) subunits (R2C2). Using homologous rat cDNAs for all known cAMPdPK subunit isoforms found in the brain (RI alpha, RI beta, RII alpha, RII beta, C alpha, C beta) we observe that, in the fetal rat brain from 12 days of gestation to birth, while alpha subunit (RI alpha, RII alpha, C alpha) mRNA levels are abundant, beta subunit (RI beta, RII beta, C beta) mRNA levels increase from undetectable or very low levels to abundant levels. Furthermore, while alpha subunit mRNA levels are abundant in both primary neuronal and primary glial cultures, beta subunit mRNA levels are very low (C beta) or undetectable (RI beta, RII beta) in primary glial cultures, but are abundant in primary neuronal cultures. Thus, prior to about 12 days of gestation, cAMP in the brain may act only via the alpha cAMPdPK subunits in neuronal and glial precursor cells. After 12 days of gestation, coincident with the onset of final cell division in neurons, beta cAMPdPK subunits may also mediate the effects of cAMP predominantly in neurons.

Animals↗
PubMed1991

Revision of failed knee replacements using fixed-axis hinges.

Forty failed hinged arthroplasties of the knee were revised by the insertion of another hinged implant. In 14 cases the prosthesis used at the revision operation was similar to the primary implant; in 26, a hinge with an elongated femoral stem was used, usually replacing part of the femoral shaft. In seven of these knees an elongated tibial stem was also required, though the tibial shaft was replaced in only two of them. There were many complications. Fracture of the femur at the tip of the femoral stem was the most frequent. Sixteen first revisions failed and were revised a second time; 12 required replacement of the distal femoral shaft and three required replacement of the proximal tibia. The incidence of complications in knees requiring a second revision was even higher. Four required a third revision after an average interval of three years. Failure of a hinged prosthesis results in bone loss mainly in the femur. Revision of a failed hinged prosthesis with another of the same design is unlikely to be successful and may cause fracture of the femur.

Bone Cements↗
PubMed1991

The dominance of cyclic sliding in producing wear in total knee replacements.

From a literature survey, it was evident that a wide variety of kinematic conditions occur at the femoral-tibial bearing surfaces, including various degrees of rolling and sliding. A test machine was constructed to reproduce these conditions, applied to spherical-ended metal 'femoral' components acting on a flat polyethylene 'tibial' plateau. The load was cyclic at 2.2 kN for 10 million cycles with distilled water lubricant. For cyclic load only, a shiny depression was formed. With oscillating and sliding superimposed, there was severe surface and subsurface cracking resulting in high wear. When rolling motion was applied, a shiny wear track was formed with minimal cracking and wear. Such surface phenomena were observed in retrieved knee specimens, probably reflecting the kinematics associated with the knee. Low-conformity components inserted with high ligamentous laxity are susceptible to anteroposterior sliding and hence high wear. More-conforming components are less susceptible to wear because they limit sliding as well as reduce contact stresses.

Corrosion↗
PubMed1991

Range of motion in total knee arthroplasty. A computer analysis.

A three-dimensional computer model of the knee was formulated based on sectional and coordinate data from knee specimens. The model was consistent with published data in terms of contact points and ligament length patterns. Prosthetic components were designed, and surgical placement was simulated. Maximum flexion was limited by tension in the posterior cruciate ligament. Increased dishing of the tibial surface reduced flexion, but some dishing was considered necessary for reduction of contact stresses. Anteroposterior translation of the tibial component had little effect on flexion. Femoral translation had some offset, and posterior positioning reduced flexion. The most important surgical variable was tibial component tilt in the sagittal plane. Posterior tilt increased motion, while anterior tilt decreased motion. The results apply to the choice of total knee system, instrument design, and surgical technique.

Computer Simulation↗
PubMed1990

Glucose transport activity in L6 muscle cells is regulated by the coordinate control of subcellular glucose transporter distribution, biosynthesis, and mRNA transcription.

Chronic (24 h) insulin treatment and/or glucose deprivation of differentiated rat L6 skeletal muscle cells resulted in an increase in glucose transport activity and a 2-3-fold increase in the number of plasma membrane-associated cytochalasin B binding sites and immunoreactive glucose transporters. In contrast to the acute effect of insulin, chronic treatment did not decrease the number of cytochalasin B binding sites or immunoreactive glucose transporter proteins present in intracellular low density microsomes. Although acute insulin stimulation of glucose transport activity was not affected by cycloheximide, chronic insulin stimulation of glucose transport activity and glucose transporter protein were decreased. In contrast, the stimulation of glucose transport activity by both acute and chronic glucose deprivation were cycloheximide-insensitive. Previously we have reported that chronic insulin treatment transiently induces the rat brain/HepG2 glucose transporter subtype (GLUT-1) mRNA, whereas glucose deprivation induces a substained increase (Walker, P. S., Ramlal, T., Donovan, J. A., Doering, T. P., Sandra, A., Klip, A., and Pessin, J. E. (1989) J. Biol. Chem. 264, 6587-6595). Consistent with these data, nuclear run-on analysis demonstrated a transient 3-fold increase in the rate of GLUT-1 glucose transporter mRNA transcription induced by either chronic insulin treatment or glucose deprivation. The combination of chronic insulin treatment with glucose deprivation resulted in a more persistent 3-4-fold increase in transcription rate than either treatment alone. These data demonstrate that prolonged insulin- and glucose-dependent regulation of glucose transporter function occurs by a complex mechanism which includes enhanced GLUT-1 mRNA transcription and glucose transporter synthesis, as well as changes in the subcellular distribution of glucose transporter proteins.

Animals↗
PubMed1990

An investigation of a compliant interface for press-fit joint replacement.

Earlier in vitro studies showed that a compliant layer between a metal surface and trabecular bone improved the load distribution. In this study, the behavior of a compliant layer of Dacron velour was investigated in vivo using a patella resurfacing in a sheep as a model. Bilateral cases were used to compare the velour interface with a direct metal-to-bone interface. For the metal patellas, a fibrous layer developed adjacent to the metal while the underlying bone formed a new subchondral-like layer. With the velour interface, fibrous tissue invaded the velour, followed later by bone, which sometimes reached the metal surface. For follow-ups of 8 months or more, the load across the interface was transferred over localised patches, for both the press-fit and velour interfaces. There was no significant difference in the areas of contact. There was evidence that this was due to the irregularity of the bony surface beneath the fibrous layer, or to bone nodules actually growing up to the metal. It was concluded that in this in vivo model, the velour layer did not retain a more uniform load distribution compared with the press-fit joint, due to the nature of the bone and fibrous tissue that formed at the interfaces.

Animals↗
PubMed1990

Prediction of total knee motion using a three-dimensional computer-graphics model.

Twenty-three knees were sectioned, digitized, and standardized to determine the 'average' three-dimensional bony geometry and ligamentous attachments. Data on normal knee motion were obtained from a cadaveric study. An algorithm was written to simulate three-dimensional patella motion. Verification of the knee model was achieved by determining femoro-tibial and patello-femoral contact locations, as well as ligament length patterns, and comparing the results with published data. The criterion for maximum predicted knee motion with a prosthesis in place was the length of the posterior cruciate ligament. Three total knee replacement surfaces were mathematically generated: flat, laxity and conforming. A greater flexion angle was obtained with a flat tibial surface than for the laxity or conforming. Posterior tibial component displacement increased the range of motion, but only slightly. For all tibial surfaces, increased range of motion was achieved with a 10 degrees posterior tilt of the tibial tray. Anterior femoral component displacement increased motion due to reduction in posterior cruciate tension during flexion. The results are applicable to the design and surgical technique of total knee replacement.

Algorithms↗
PubMed1990

Effect of press-fit femoral stems on strains in the femur. A photoelastic coating study.

A photoelastic coating method was used to study the strain patterns on the surface of the femur, before and after insertion of femoral stems. The anatomically shaped stems were press-fit without a collar, press-fit with a collar, and press-fit with proximal cementing to approximate a bone ingrowth situation. The strain patterns of the intact femurs were consistent with bending. The pattern changed considerably after stem insertion, probably due to the stiffening effect of the stems. Nevertheless, the collarless press-fit stem preserved 64% of the magnitude of the proximal medial strain. The collar restored the average to normal, but the surface strains varied because of the localized regions of contact between the collar and the bone. Proximal cementing gave results similar to those of the collarless press-fit. The press-fit stems often showed local patches of high strain, probably reflecting local endosteal contact points. Local high stresses at the level of the distal tip were seen in only a few instances.

Cadaver↗
PubMed1990

A comparative study of uncemented tibial components.

The effect of tibial component stem and post design on load distribution and on displacements at the component-bone interface was investigated. Four different configurations were tested with a flat component used as a control. Loading conditions included vertical central, vertical offset (eg, varus/valgus), shear and torque. Both artificial and cadaveric bones were used in the study. Pressure-sensitive film was used to obtain pressure patterns at the interface. Image processing was then used to quantify the load distribution. For shear and torque, relative motion was seen as a smeared pressure pattern. This was calibrated in terms of microns of displacement as a function of image density. The central stemmed and bladed designs performed better than short-pegged designs, in resistance to offset loading. In shear and torque, short pegs close to the component periphery, or a central stem with blades, produced the least interface displacement. The application of this work to the design of components for both press-fit and cemented application is discussed.

Cementation↗
PubMed1990

Uncemented press-fit total knee arthroplasty.

Nineteen total knee arthroplasties in 16 patients were performed using a new prosthesis designed specifically for uncemented, press-fit fixation without any provision for tissue ingrowth. This design may be suited for use in patients with juvenile rheumatoid arthritis, postseptic knees, failed total knee arthroplasties with large bone stock deficiencies, and young, active, overweight individuals. Preservation of bone stock is possible because the fixation stems are nails rather than larger cement pegs, removal for revision is easier without cement, sequestration of microbes within cement cannot occur, and bone graft may be continually loaded with this type of prosthesis. Pain relief, discarding crutch support, operative blood loss, and corrective alignment were equivalent to results obtained with cemented arthroplasty. Seventeen of the 18 knees available for review (average follow-up period, 3 years, 7 months; range of follow-up period, 2 years, 10 months to 4 years, 11 months) were pain-free. Range of motion averaged -2 degrees of extension to 101 degrees of flexion. Ambulation was unrestricted and unsupported in the majority of patients. Radiographic evaluation showed the development of increased bone density under the tibial plateau as well as a sclerotic line at the ends of the femoral and tibial stems. This observation gives support to the theory that multiple microtrabecular fractures with healing form a new supportive "subchondral bone plate." There was one incidence of tibial subsidence. Prosthesis migration and loosening were not observed.

Adult↗
PubMed1990

Total knee arthroplasty with the kinematic prosthesis. Results after five to nine years: a follow-up note.

A review of the results of 192 kinematic total knee replacements five to nine years after the operation showed that the results were still satisfactory. At the time of the review, the ages of the patients ranged from twenty-two to eighty-seven years. About half of the patients had rheumatoid arthritis and the other half, osteoarthrosis. About 90 per cent of the results were rated good or excellent, and the average range of flexion was 109 degrees. Radiolucency was present around 40 per cent of the tibial components, 30 per cent of the femoral components, and 60 per cent of the patellar components, but the lines were thin and not progressive. The complications included loosening of the patellar components in five knees, one fracture of the tibial tray with loosening of the patellar component, one fracture of the patellar component, and one dislocation of the patellar component.

Adult↗