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Biomedical subjects

W H Harris

Publications and source records attributed to W H Harris.

At least 73 records · Page 4Linked to original sources
PubMed1997

Factors affecting cement strains near the tip of a cemented femoral component.

A generic three-dimensional finite-element model of the upper half of the femur containing a cemented femoral stem of a total hip arthroplasty was developed to study those factors influencing cement strains near the tip of a cemented femoral component. This generic model was verified through another three-dimensional finite-element model that had been created based on the precise geometry of a cadaver femur implanted with a contemporary cemented femoral component. This cadaveric femoral reconstruction had been created with strain gauges embedded in the cement mantle and was then loaded under conditions simulating single leg stance and stairclimbing. By use of the cement strains measured experimentally in the cadaver femur, and comparison of them with those obtained from the finite-element model of that cadaver femur, it was possible to establish proper material properties, boundary conditions, and loading conditions for the generic model. The generic model was then modified parametrically to determine those factors that influence the strains occurring within the cement mantle near the tip of a cemented femoral component. These models suggest that the single factor that most adversely influenced peak strains at or near the tip of the prosthesis was a thin cement mantle. This effect was present both when the cement mantle was reduced in thickness and when a similar effect occurred by virtue of a varus or valgus placement of the stem. Factors that decreased the peak cement strains near the tip of the femoral stem included a more flexible stem and thicker cement mantles. This effect of a more flexible stem could be obtained by changing the modulus of the metal implant by uniformly reducing the thickness of the stem, or by tapering the stem within the same bone geometry. Thicker cement mantles reduced both the axial and the shear strains occurring at the tip of the prosthesis. The presence or absence of a hole in the tip of the prosthesis per se, as for a centralizer, had no significant effect on the peak cement strains seen around the tip of the prosthesis; however, truncating the tip of the prosthesis from a hemisphere to a flat profile, which resulted in a sharp corner at the tip of the prosthesis, produced a 35% increase in cement strains at the tip as a result of a stress concentration effect. Thus, the common way of modifying the tip to have a hole for a centralizer, which involved truncating the tip, increased the cement strains occurring near the tip of the prosthesis.

Bone Cements↗
PubMed1997

Enhanced stability of uncemented canine femoral components by bone ingrowth into the porous coatings.

The following questions were answered in this study: (1) What is the initial stability of proximally porous-coated canine femoral components? (2) Does bone ingrowth occur under these conditions? (3) Is the stability enhanced by tissue ingrowth in vivo? The stability of proximally porous-coated femoral components of canine total hip arthroplasties after 6 months to 2 years of in vivo service in dogs was measured in vitro using displacement transducers under loads simulating canine midstance. This was compared with the stability of identical components under the same loading conditions immediately after implantation in vitro in the contralateral femurs. The femurs were then sectioned and bone ingrowth into the porous coatings was quantified. The results showed that immediately after implantation the implants can move as much as 50 microns, but that the bone ingrowth into porous coatings of canine femoral components can occur even under such conditions. These data also suggested that the relative motion existing at the time of insertion can be reduced to very small amounts (< 10 microns) by bone ingrowth.

Alloys↗
PubMed1997

Wear of retrieved cemented polyethylene acetabula with alumina femoral heads.

Four yttrium-stabilized alumina ceramic-on-polyethylene articulations obtained from patients who were undergoing revision surgery for sepsis (3) or recurrent dislocation (I) between 34 and 73 months were evaluated to assess their in vivo wear performance. The annual volumetric wear of the acetabular components determined directly by a fluid displacement method ranged from 58 to 140 mm3/y. Scanning electron microscope examination of these four ceramic heads revealed similar surface damage in all cases from a variety of causes. These included differential granular wear (alumina grains and yttrium-stabilized alumina grains at different depths), multidirectional scratches with heaped up boundaries, and incompletely sintered grains, as well as the formation of craters and separation of grain boundaries. The femoral heads in this small series of revision cases show that yttrium-stabilized alumina ceramic heads may develop surface irregularities from either manufacturing processes or in vivo use. The wear rates of this type of alumina-on-polyethylene articulation up to the time of revision were not substantially different from those found in other metal-on-polyethylene articulations retrieved at revision surgery.

Acetabulum↗
PubMed1997

Assessment of the symmetry of bone strains in the proximal femoral medial cortex under load in bilateral pairs of cadaver femurs.

In past studies, it has been assumed that contralateral femurs from the same patient have sufficiently symmetric mechanical properties that one femur could reliably serve as representative of the other in mechanical testing experiments dealing with surface bone strains. To assess the accuracy of this assumption, 10 pairs of cadaveric femurs without evidence of osseous abnormalities were instrumented with strain gauges on the periosteal and endosteal surfaces of the proximal medial cortex and strain measurements were made under simulated conditions of single-leg stance and stairclimbing. Although the strains in femurs from different individuals varied greatly, the degree of symmetry in the strains of contralateral femurs was high. These findings add support to the use of contralateral femurs without discernible abnormalities as suitable controls for each other in such situations.

Adult↗
PubMed1997

Vastus-Psoas release for acetabular exposure in revision hip surgery.

A technique is presented for wide exposure of the acetabulum for revision total hip arthroplasty surgery in the presence of a solidly fixed, modular, or monoblock femoral component without the need for trochanteric osteotomy. The technique involves release of the proximal portion of the vastus lateralis, vastus intermedius, and vastus medialis muscles and the iliopsoas tendon form the femur and placement of the femoral head/neck posterior to the acetabulum. The exposure afforded by this release usually precludes the need for trochanteric osteotomy and/or removal of a well-fixed femoral component in revision surgery that is being done for isolated loosening of acetabular components, thereby decreasing operative time, morbidity, and the risks of complication of trochanteric osteotomy.

Acetabulum↗
PubMed1997

Acetabular and femoral fixation 15 years after cemented total hip surgery.

Total hip arthroplasty in patients younger than 51 years of age remains controversial. The authors report results in 47 hips in 40 patients at an average followup of 15 years. Second generation femoral cementing techniques were used. Special attention was given to studying the differences between the outcome of acetabular versus femoral fixation. The patient group included 11 in whom custom components were used and 13 who had structural autografts for severe acetabular dysplasia. One femoral component and 10 acetabular components were revised for aseptic loosening. After 15 years, improved cementing techniques produced a marked reduction in the rate of femoral aseptic loosening. Loss of fixation of cemented acetabular components was common.

Acetabulum↗
PubMed1997

Bulk structural autogenous grafts and allografts for reconstruction of the acetabulum in total hip arthroplasty. Sixteen-year-average follow-up.

Acetabular reconstruction with bulk structural autogenous grafts and allografts from the femoral head in complex total hip arthroplasty was highly successful at an average of five years postoperatively but was much less so by 11.8 years. To assess the longer-term fate of such grafts, we reviewed the results of eighty-one consecutive total hip arthroplasties performed by the senior one of us with use of these grafts. Nine hips in nine patients who had died and two hips that were infected in two patients were excluded. Therefore seventy hips (sixty-two patients) were included in this study. The average duration of follow-up was 16.5 years (range, 14.1 to 21.4 years). Sixty-one arthroplasties were performed to treat various forms of congenital dysplasia. Fifteen arthroplasties, ten of which were revision operations, were performed with allograft, and fifty-five were performed with autogenous graft. The average age of the patients at the time of the index operation was 45.2 years (range, sixteen to sixty-nine years). All of the sockets, which had an average outer diameter of forty millimeters (range, thirty-four to fifty millimeters), were inserted with cement. The average coverage of the acetabular component by the bulk graft was 49 per cent (range, 15 to 100 per cent). All of the grafts united. At the latest follow-up examination, twenty-five acetabular components (36 per cent) had been revised for aseptic loosening, eighteen (26 per cent) had radiographic evidence of loosening, and twenty-seven (39 per cent) were rigidly fixed and in place. The average Harris hip score for the hips in which the implant remained rigidly fixed was 74 points, while that for the hips in which the implant was loose but had not been revised was 69 points. Nine of the fifteen acetabular components supported by allograft and sixteen (29 per cent) of the fifty-five supported by autogenous graft were revised (p = 0.03). However, the total rate of acetabular components that were either loose or revised was ten of fifteen and thirty-three (60 per cent) of fifty-five, respectively. This difference was not significant (p = 0.4), with the numbers available. Regression analysis revealed that a younger age at the time of the operation and the extent of coverage of the acetabular component by the graft were associated with the need for revision. Twenty-one (78 per cent) of the twenty-seven acetabular components that remained rigidly fixed were supported by graft over less than 50 per cent of the contact area, while only nine (36 per cent) of the twenty-five that were revised were so supported (p < 0.05). None of the nine acetabular components with 30 per cent of the contact area or less covered by graft were revised. In nineteen of the twenty-two revisions of the acetabular component performed after the index operation, the socket was inserted without cement; the average outer diameter of the socket was fifty-three millimeters (range, forty to fifty-eight millimeters). Both the structural autogenous grafts and the structural allografts used in acetabular reconstruction in total hip replacement functioned well for the initial five to ten years. By an average of 16.5 years, nine of the fifteen hips treated with allograft and sixteen (29 per cent) of the fifty-five treated with autogenous graft had been revised. The greater the extent of the coverage of the acetabular component by the graft, the greater the rate of late failure.

Acetabulum↗
PubMed1997

In vivo skeletal responses to porous-surfaced implants subjected to small induced motions.

Cylindrical porous-coated implants were placed in the distal femoral metaphyses of twenty dogs and were subjected to zero, twenty, forty, or 150 micrometers of oscillatory motion for eight hours each day for six weeks with use of a specially designed loading apparatus. The in vivo skeletal responses to the different magnitudes of relative motion were evaluated. Histological analysis demonstrated growth of bone into the porous coatings of all of the implants, including those that had been subjected to 150 micrometers of motion. However, the ingrown bone was in continuity with the surrounding bone only in the groups of implants that had not been subjected to motion or that had been subjected to twenty micrometers of motion; in contrast, the implants that had been subjected to forty micrometers of motion were surrounded in part by trabecular bone but also in part by fibrocartilage and fibrous tissue, and those that had been subjected to 150 micrometers of motion were surrounded by dense fibrous tissue. Trabecular microfractures were identified around three of the five implants that had been subjected to forty micrometers of motion and around four of the five that had been subjected to 150 micrometers of motion, suggesting that the ingrown bone had failed at the interface because of the large movements. The architecture of the surrounding trabecular bone also was altered by the micromotion of the implant. The implants that had stable ingrowth of bone were surrounded by a zone of trabecular atrophy, whereas those that had unstable ingrowth of bone were surrounded by a zone of trabecular hypertrophy. The trabeculae surrounding the fibrocartilage or fibrous tissue that had formed around the implants that had been subjected to forty or 150 micrometers of motion had been organized into a shell of dense bone tangential to the implant (that is, a neocortex outside the non-osseous tissue).

Animals↗
PubMed1997

Total hip arthroplasty performed with insertion of the femoral component with cement and the acetabular component without cement. Ten to thirteen-year results.

Fifty-two consecutive primary total hip arthroplasties were performed in forty-seven unselected patients by one surgeon. The prosthesis included a hemispherical porous-coated acetabular component, inserted without cement and with the use of screws through three peripheral flanges, and a femoral component, inserted with a so-called second-generation cementing technique. No patient was lost to radiographic follow-up, and the clinical result was known for all patients. The average age at the time of the index operation was fifty-seven years (range, twenty-nine to seventy-nine years). Four patients (four hips) who died were last examined less than ten years postoperatively (the minimum follow-up period for this study) and one hip was revised, leaving forty-seven non-revised hips in forty-two surviving patients who were followed for at least ten years. The duration of clinical follow-up of these forty-two patients averaged 12.3 years (range, 10.8 to 13.3 years), and the duration of radiographic follow-up averaged 12.1 years (range, 10.0 to 13.0 years). One (2 per cent) of the original fifty-two hips was revised for late recurrent dislocation, without loosening, 9.7 years after the index arthroplasty. The rate of dislocation was relatively high (13 per cent; seven hips), and we believed it to be related to the shallow-chamfer acetabular design combined with the small femoral head. At the time of the latest follow-up, no femoral component was loose. One (2 per cent) of the fifty-two acetabular components was loose according to radiographic criteria, but the hip functioned well (Harris hip score, 94 points) 12.4 years after the index arthroplasty. Pelvic osteolysis developed in one hip (2 per cent); femoral osteolysis, in eight (15 per cent); and distal femoral osteolysis, in three (6 per cent). The average Harris hip score for the forty-seven non-revised hips increased from 48 points (range, 26 to 63 points) preoperatively to 89 points (range, 67 to 100 points) at the time of the most recent follow-up. Forty (85 per cent) of the forty-seven hips had a good or excellent result, whereas five (11 per cent) had a fair result (score, 74 to 79 points) and two (4 per cent) had a poor result (score, 67 and 69 points). The hybrid primary total hip arthroplasty resulted in very good clinical function at ten to thirteen years, although the rate of dislocation was high.

Acetabulum↗
PubMed1997

Wear of polyethylene acetabular components in total hip arthroplasty. An analysis of one hundred and twenty-eight components retrieved at autopsy or revision operations.

We evaluated the rates of volumetric wear and the patterns of wear of 128 acetabular components retrieved during an autopsy or a revision operation between one and twenty-one years after total hip arthroplasty. Twenty-two all-polyethylene components were retrieved at autopsy from hips that had been functioning well at the time of death (Group A). The remaining 106 components--eighty-four all-polyethylene components (Group B) and twenty-two metal-backed components (Group C)--were retrieved during revision operations. All 128 components had been inserted with cement. The mean rate of volumetric wear, determined directly with a fluid-displacement method, was thirty-five cubic millimeters per year (range, eight to 116 cubic millimeters per year) for Group A, sixty-two cubic millimeters per year (range, eight to 256 cubic millimeters per year) for Group B, and ninety-four cubic millimeters per year (range, twelve to 284 cubic millimeters per year) for Group C. Multivariate regression analysis showed a significant relationship (p < 0.05) between the size of the femoral head and the calculated mean annual rate of volumetric wear. The rate of volumetric wear was highest in association with thirty-two-millimeter femoral heads and lowest in association with twenty-two-millimeter heads; according to linear regression analysis, this represented a 7.5 per cent increase (Group A) or a 10 per cent increase (Group B) in the rate of wear for every one-millimeter increase in the size of the head. Linear regression analysis also showed a significant relationship between the duration that the implant had been in situ and the rate of wear (p < 0.05), with the rate being highest initially after the operation and decreasing with an increasing duration in situ. With the numbers available, the patient's age and gender and the side of the arthroplasty did not have a significant relationship to the annual rate of volumetric wear. Increased thickness of the polyethylene was related to a decreased rate of wear (p < 0.05) in the group of metal-backed components, which had a 25 per cent increase in the rate of wear for every one-millimeter decrease in thickness, but not in the other groups. The estimated median annual rates of wear, after adjustment of confounding variables to a hypothetical constant set of median values for the parameters (duration in situ, 132 months; diameter of the femoral head, twenty-six millimeters; and thickness of the polyethylene, eight millimeters), were significantly different among the three groups of components (p < 0.05). Histological evaluation of the worn surfaces showed the predominant mechanisms of wear to be abrasion and adhesion rather than fatigue-cracking or delamination. The highly worn areas were polished to a glassy finish on gross examination, but scanning electron microscopy showed numerous multidirectional scratches along with fine, drawn-out fibrils with a diameter of one micrometer or less oriented parallel to each other. These fibrils are the most likely source of submicrometer wear particles. Thus, wear appeared to occur mostly at the surface of the components and to be due to large-strain plastic deformation and orientation of the surface layers into fibrils that subsequently ruptured during multidirectional motion.

Acetabulum↗
PubMed1997

Options for primary femoral fixation in total hip arthroplasty. Cemented stems for all.

In considering the subject of choices for the planning of femoral fixation in total hip arthroplasty, the strength behind the argument that cementing should be preferred in all ages and all conditions comes from three major positions, all supported by excellent data. First, the long term data (15-20 years) for well performed cemented stems are excellent and unmatched. Second, with good cementing techniques, the long term results in patients younger than 50 years of age are equal to the results in patients older than 50 years of age. No longer is there validity to the prior arguments that cementing does not work well in younger patients or that younger patients should have cementless femoral stems. Third, improvements in cementing technique have been shown in nationwide studies to produce statistically significantly better results than the original techniques.

Arthroplasty, Replacement, Hip↗
PubMed1996

Differential effects of different forms of hydroxyapatite and hydroxyapatite/tricalcium phosphate particulates on human monocyte/macrophages in vitro.

A possible complication associated with the use of hydroxyapatite (HA) or HA/tricalcium phosphate (HA/TCP) coating on the surfaces of prosthetic devices used for dental and orthopedic implants is their potential to fragment and thus exist as wear debris. In contrast to the so-called osteoconductive properties of HA or HA/TCP coatings, in particulate form these materials may lead to an adverse pattern of cellular and tissue responses at the bone-implant interface. We have established an in vitro cell culture system to characterize the biologic and biochemical effects of various particulate materials. The present study demonstrates that the HA/TCP particles derived from different sintering temperatures exhibit differential effects on cultured human monocyte/macrophages (M/M). The HA/TCP particles dried at 110 degrees C were the most biologically active, stimulating significant release of interleukin 1 beta (IL-1 beta), IL-6, tumor necrosis factor alpha, and prostaglandin E2 (PGE2), products implicated as important mediators of inflammation in diverse pathologic conditions. Other particles, sintered at either 900 or 1200 degrees C, did not stimulate production of cytokines or PGE2. HA/TCP particles from plasma-spray coatings also failed to release proinflammatory products. These results suggest that the biochemical and crystalline structural properties of particles markedly affects their capacity to modulate M/M function. This in vitro culture system should be useful in characterizing the specific physical and chemical properties of HA or HA/TCP particulates that are responsible for stimulating proinflammatory cell responses.

Biocompatible Materials↗
PubMed1996

In vitro measurement of strain in the bone cement surrounding the femoral component of total hip replacements during simulated gait and stair-climbing.

The strains in the cement mantle surrounding the cemented femoral component of a total hip replacement were measured in vitro, using strain gauges embedded within the cement mantle adjacent to the femoral component in femurs from cadavers under physiologic loads simulating both single-limb stance and stair-climbing. Cement strains in the most proximal portion of the cement mantle were measured with and without full contact of the collar of the femoral stem on the cortex of the medial portion of the femoral neck during both loading conditions. To our knowledge, these are the first studies to contrast by direct measurement the strain profile in the cement mantle of a cemented femoral component under simulated stair-climbing with that occurring under simulated single-limb stance. They extend the findings from finite element analyses and from clinical specimens retrieved at autopsy in identifying those regions of the cement mantle most likely to fail. At two specific foci, the magnitude of the strain in the cement mantle approaches values that could lead to early fatigue failure of the cement. The two regions in which the strains were highest (greater than 1,000 microstrain) were the most proximal portions of the cement mantle and near the tip of the femoral component. Although these two regions are recognized areas of high strain and also common sites of cement debonding and cement mantle failure, the strain-gauge studies showed that the magnitude of cement strains in the proximal portion of the cement mantle were highest during stair-climbing; in contrast, high strains at the tip region occurred in both gait and stair-climbing. Contact between the collar and the medial portion of the femoral neck reduced the strain in the proximal portion of the cement mantle not only in single-limb stance but in stair-climbing as well. The level of strain recorded in these studies for a simulated person weighing 115 pounds (52 kg) could lead to cement fracture during extended in vivo service life of a cemented femoral component, from either single-limb stance or stair-climbing. This risk would be increased if a void or defect existed in the cement mantle at these sites. Moreover, the increase in strain in the cement mantle was linear with increases in body weight between 100 and 200 pounds (45 and 91 kg) of spinal load, indicating that strains in a heavy patient could readily exceed the fatigue limit of the cement, particularly if a stress riser such as a pore in the cement or a sharp corner of the prosthesis were present. These data reemphasize the need to continue efforts to develop methods to strengthen bone cement and to reduce those factors that increase the strain in the cement mantle of cemented femoral components of total hip arthroplasty, particularly proximally and near the tip.

Adult↗
PubMed1996

Gamma sterilization of UHMWPE articular implants: an analysis of the oxidation problem. Ultra High Molecular Weight Poly Ethylene.

Gamma irradiation of Ultrahigh Molecular Weight Polyethylene (UHMWPE) leads to long-lived free radicals which react with oxygen. Diffusion of oxygen, occurring over months or years, controlled by the permeability characteristics of the polymer, results in progressive oxidation, breaking of polymer chains, alteration of the crystalline portion of the polymer, and deterioration of the mechanical properties of the polymer. This paper reviews the observations in the literature on this issue and then presents a conceptual model concerning the interplay of radical diffusion, oxygen diffusion, non-uniform permeability, and free-radically driven chain reactions in order to explain these observations. The suggested model is based on literature that is available on the oxidation of linear polyethylenes during and after irradiation. The model directs the attention of researchers in the field of orthopaedic implants to the complexity of the process and the variety of issues and parameters to be considered while studying the long-term effects of radiation sterilization on UHMWPE.

Biopolymers↗
PubMed1996

Long-duration metal-on-metal total hip arthroplasties with low wear of the articulating surfaces.

The 20-year performance of metal-on-metal hip articulations has not been reported. Five McKee-Farrar total hip prostheses and one Sivash prosthesis were obtained at revision surgery after a mean implantation time of 21.3 years. A radiographic, histologic, implant, and wear analysis was performed on these total hip implants with cobalt-chrome metal-on-metal articulations. All cases were associated with femoral component loosening, but the bearing surfaces performed remarkably well. The worst case estimate of combined femoral and acetabular linear wear was 4.2 microns per year, about 25 times less than that typically seen with polyethylene. Metal particles and foreign-body inflammation were seen in all cases, but the volume of reactive tissue was small compared with what is generally seen at revision of hips with a polyethylene acetabular bearing. This may be due to a reduced particle burden or a decreased inflammatory reaction to particulate metal, or both. In addition to articular wear, other sources of metal particles included femoral neck impingement on the acetabular rim, stem burnishing, and corrosion. Prosthetic hip reconstructions can fail for many reasons, including suboptimal femoral stem and/or acetabular cup design and/or fixation. By today's standards, the McKee-Farrar and Sivash stem and acetabular component designs are suboptimal; however, after more than 20 years of use, the metal-on-metal bearing surfaces in these cases demonstrated low wear and do not appear to be the cause of failure. Recent advances in total hip arthroplasty, which include improved implant design, materials, manufacturing, and fixation, combined with a better understanding of the mechanisms of implant loosening and failure, suggest that the cobalt-chrome metal-on-metal bearing be reexamined as an alternative to polyethylene when exceptional durability is required.

Adult↗