Biomedical subjects

D O O'Connor

Publications and source records attributed to D O O'Connor.

At least 19 recordsLinked to original sources

PubMed2001

A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. Recipient of the 1999 HAP Paul Award.

Increasing cross-linking has been shown in vitro and in vivo to improve markedly the wear resistance of ultra-high-molecular-weight polyethylene (UHMWPE). The reduction in the mechanical properties of polyethylene under certain methods used to produce cross-linking has been a concern, however. These reductions are known to result from the processes used to increase the cross-link density and could affect the device performance in vivo. We present a novel method of increasing the cross-link density of UHMWPE in which UHMWPE is irradiated in air at an elevated temperature with a high-dose-rate electron beam and subsequently is melt-annealed. This treatment improves markedly the wear resistance of the polymer as tested in a hip simulator, while maintaining the mechanical properties of the material within national and international standards. This method leads to the absence of detectable free radicals in the polymer and, as a result, excellent resistance to oxidation of the polymer.

Biocompatible Materials↗

PubMed2001

A new pin-on-disk wear testing method for simulating wear of polyethylene on cobalt-chrome alloy in total hip arthroplasty.

Hip simulator studies show that the wear of ultra-high molecular weight polyethylene against a cobalt alloy head depends on the wear path, especially the combination of a predominantly linear wear direction on which is superimposed motions in different directions. We postulated that multidirectional motion was necessary to generate realistic wear rates in pin-on-disk testing. To assess this hypothesis, a new pin-on-disk tester was developed, capable of unidirectional and bidirectional motion. Unidirectional motion produced no detectable wear. The rectangular motion produced wear rates, surface morphologies, and wear particles consistent with human acetabular specimens. The results for 1 Hz and 2 Hz were similar.

Chromium Alloys↗

PubMed1999

Unified wear model for highly crosslinked ultra-high molecular weight polyethylenes (UHMWPE).

Crosslinking has been shown to improve the wear resistance of ultra-high molecular weight polyethylene in both in vitro and clinical in vivo studies. The molecular mechanisms and material properties that are responsible for this marked improvement in wear resistance are still not well understood. In fact, following crosslinking a number of mechanical properties of UHMWPE are decreased including toughness, modulus, ultimate tensile strength, yield strength, and hardness. In general, these changes would be expected to constitute a precursor for lower wear resistance, presenting a paradox in that wear resistance increases with crosslinking. In order to understand better and to analyze this paradoxical behaviour of crosslinked UHMWPE, we investigated the wear behavior of (i) radiation-crosslinked GUR 1050 resin, (ii) peroxide-crosslinked GUR 1050 resin and (iii) peroxide-crosslinked Himont 1900 resin using a bi-directional pin-on-disk (POD) machine. Wear behavior was analyzed as a function of crystallinity, ultimate tensile strength (UTS), yield strength (YS), and molecular weight between crosslinks (Mc). The crosslink density increased with increasing radiation dose level and initial peroxide content. The UTS, YS, and crystallinity decreased with increasing crosslink density. While these variations followed the same trend, the absolute changes as a function of crosslink density were different for the three types of crosslinked UHMWPE studied. There was no unified correlation for the wear behavior of the three types of crosslinked UHMWPE with the crystallinity, UTS and YS. However, the POD wear rate showed the identical linear dependence on Mc with all three types of crosslinked UHMWPEs studied. Therefore, we have strong evidence to propose that Mc or crosslink density is a fundamental material property that governs the lubricated adhesive and abrasive wear mechanisms of crosslinked UHMWPEs, overriding the possible effects of other material properties such as UTS, YS and crystallinity on the wear behavior.

Biocompatible Materials↗

PubMed1997

Comparison of cerclage techniques using a hose clamp versus monofilament cerclage wire or cable.

To assess the compressive force generated by different techniques of cerclage wiring, the compression produced by 6 different cerclage techniques, including monofilament wires, Dall-Miles cables (Howmedica, Rutherford, NJ), and hose clamps, was measured. Tests were performed on both a polysulfone cylinder model and a cadaver bone model. The hose clamp generated significantly higher compression force than any other cerclage device tested. Doubled wires produced similar compressive force as the cables. As the hose clamp is the least expensive and produces the maximum compression, it may be advantageous in instances in which maximum compression is desirable as a temporary device, but because it is not implantable, it must be replaced by other techniques prior to wound closure. The Dall-Miles cable is expensive and its compressive force can be equaled at lower cost by a doubled wire. The use of a single strand of monofilament wire was the least effective.

Bone Wires↗

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↗

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

Differences in stiffness of the interface between a cementless porous implant and cancellous bone in vivo in dogs due to varying amounts of implant motion.

To determine the mechanical properties of the interface between the tissue ingrowth into porous coatings and the implant, porous-coated cylindrical implants were inserted into the distal femur in 20 mature dogs and oscillated in vivo 8 hours per day for 6 weeks at fixed amounts of micromotion (0, 20, 40 and 150 microns). Applied torques and resulting displacements were recorded. The torsional resistance per unit angular displacement (TR/AD), reflecting the stiffness of the bone-porous coating interface, was 0.88 +/- 0.25 N-M/deg immediately after implantation in the 20-micron displacement group. It increased with time after surgery, reaching a maximum of 1.25 +/- 0.60 N-M/deg at 6 weeks. The TR/AD was lower initially (0.77 +/- 0.43 N-M/deg) in the 40-micron group and gradually decreased with time after surgery, reaching a maximum of 0.54 +/- 0.13 N-M/deg at 6 weeks. The TR/AD was even lower (0.24 +/- 0.10 N-M/deg) in the 150-micron group initially and remained the same (0.16 +/- 0.09 N-M/deg) with time after surgery. Histologic evaluation showed bone ingrowth in continuity with the surrounding bone in the 20-micron group consistent with the high stiffness values at sacrifice. In contrast, a mixture of fibrocallus and bone were found at the bone-porous coating interface in the 40-micron group, consistent with the intermediate stiffness values. In contrast, despite the fact that bone was found in the depth of the porous coating in the dogs in the 150-micron group, the low stiffness values were a reflection of fibrous tissue formation at the interface in that group, because of the large motion disrupting bony ingrowth at the bone-porous coating interface. By monitoring the torsional resistance per unit of angular displacement dynamically in vivo, it was possible to evaluate the mechanical properties of the bone-porous coating interface as tissue ingrowth proceeded. Twenty microns of oscillating displacement was compatible with stable bone ingrowth with high interface stiffness, whereas 40 and 150 microns of motion was not.

Animals↗

PubMed1996

Loci of movement of selected points on the femoral head during normal gait. Three-dimensional computer simulation.

Wear of ultrahigh-molecular-weight polyethylene and the subsequent lytic response to the particulate wear debris are the dominant problems in total joint arthroplasty surgery. Wear testing apparatus can play a vital role in the in vitro evaluation of the many factors involved in wear, such as head size, surface roughness, materials for the head, and new materials for the socket. Wear of ultrahigh-molecular-weight polyethylene may be influenced by the wear path. For the related polymer, high-density polyethylene, the wear path is critical to wear magnitude. What is the actual path taken by a single point (or by multiple representative points) on the femoral head of a total hip arthroplasty as it passes through the gait cycle? The goal of this computer simulation study was to trace the paths of specific points on the femoral head as they moved against the polyethylene cup during a single cycle of normal gait to illustrate the motions occurring at the intraarticular surface of the hip joint. This study also yielded unusual data on the "distance traversed" by these points during a single gait cycle. It was found that there was not one path, but rather there were many, and the paths varied widely in both shape and length depending on the location on the femoral head. Moreover, the differences in excursion and direction at different sites during the loaded phase were great. In addition, distances traveled by different points on the femoral head of any given size varied by a factor greater than 2. Most of the points traced quasielliptical paths. This automatically means that the paths of neighboring points cross each other, creating multidirectional shear forces on the acetabular cup surface which may be important in the localization and extent of wear. The plots of traces of the points derived from this study can serve as benchmarks for the ability of hip simulators to reproduce the actual distances and paths of travel of individual points on the femoral head.

Computer Simulation↗

PubMed1996

The importance of multidirectional motion on the wear of polyethylene.

The development of a new hip simulator for the study of bearing materials used in total hip replacements has led to several findings which add important new information to the understanding of wear process of ultra-high molecular weight polyethylene, the most commonly used bearing material today for total joint replacements. Using this hip simulator which is capable of applying the physiological motion pathways occurring during gait to total hip components which are held in the correct anatomical position under the complex loading conditions of the hip in gait, the authors have shown that physiological motion pathways produce very different wear rates and morphology of the wear surface than unidirectional reciprocating pathways. Scanning electron microscopy studies show striking differences in the morphology of the wear surfaces of the polyethylene depending upon the relative motions of the components. Wear rates, surface morphology and particle debris generation consistent with clinical and retrieved studies are achieved when physiological conditions are simulated.

Hip Joint↗

PubMed1995

Influence of intraoperative femoral fractures and cerclage wiring on bone ingrowth into canine porous-coated femoral components.

Intraoperative femoral fractures occur more frequently with cementless than with cemented components. In this study, the influence of controlled femoral fractures fixed with cerclage wires on rotational stability and bone ingrowth into porous-coated canine femoral components was evaluated. These data were compared with results of previous studies on unrecognized femoral fractures (not stabilized) and on the intact canine femur. Micromotion analysis revealed a significant increase in rotational instability in fractures not stabilized with cerclage wires (P < .05) compared with the intact femur. Experimentally created femoral fractures had a significantly deleterious effect on bone ingrowth even after cerclage wiring. This appears to be caused by a lack of bone ingrowth deep to the fracture and an increase in femoral component micromotion. In clinical practice, femoral fractures occurring during cementless total hip arthroplasty are a serious problem, and use of a cemented prosthesis is recommended if rotational stability of the stem cannot be ensured.

Animals↗

PubMed1994

Fit of the uncemented femoral component and the use of cement influence the strain transfer the femoral cortex.

To determine whether the strain patterns produced in the femoral cortex after uncemented femoral arthroplasty are influenced by the fit of the component and whether these patterns are different from those of cemented components, cortical surface strains of cadaveric femurs subjected to loads simulating single-limb stance were measured before and after the insertion of uncemented, collared, straight-stemmed femoral components. The effects of press fit, loose fit, and precise fit of the components were evaluated and were contrasted to the strain patterns occurring after insertion of cemented femoral components. Strains varied markedly, depending on the fit of the stem of the uncemented femoral component within the isthmus. Nearly normal patterns of femoral strain were produced when the femoral stem was fit precisely at the isthmus, and the proximal femoral strains were similar to those of the intact state. In contrast, press fit and loose fit at the isthmus altered the strain patterns. The proximal medial axial strains were significantly reduced with press fit, to a mean of 39% of normal (p < 0.05), and increased with loose fit, to a mean of 141% of normal (p < 0.05). The prostheses fixed with cement showed a mean reduction in proximal medial axial strains to 33% of normal, which was comparable with press fit uncemented components even though the collar was well seated. Thus, our findings indicated that, in the immediate postoperative period, femoral strain patterns can be influenced by the fit of an uncemented component within the isthmus and by the use of cement.

Aged↗

PubMed1994

A quantitative in vitro assessment of fit and screw fixation on the stability of a cementless hemispherical acetabular component.

This investigation quantifies in vitro the effect of component fit, as well as the effect of adjuvant screw fixation, on the initial stability of cementless hemispherical titanium acetabular total hip arthroplasty components and assesses apposition of the acetabular components to bone. Six, fresh human hemipelvi (3 matched pairs) were harvested at autopsy. Titanium alloy acetabular components with a porous surface of commercially pure titanium fiber mesh (Harris Galante Porous acetabular components, Zimmer, Warsaw, IN) were used for implantation. Initially, each acetabulum was underreamed to achieve a 2 mm press-fit with the acetabular component. Pressure-sensitive film had been placed along the dome and medial wall at the bone-implant interface to assess the completeness of seating. After the implant was impacted into the acetabular cavity, relative motion between the implant and bone was measured during simulated single leg stance. Adjuvant fixation of the implant was then obtained with the insertion of four 6.5 mm cancellous screws. High-contrast roentgenograms of the specimens in multiple views were obtained after initial cup insertion and again after screw insertion. The stability of each implant under load was measured with four, three, two, one, and no screws in place. Further reaming of the bone was done to create a 1 mm press-fit. The sequence was then repeated. Further reaming was done to create an exact-fit and the sequence was repeated again. Under these conditions, 1 mm press-fit with or without screws provided the optimum combination of fit stability.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗

PubMed1993

High assembly strains and femoral fractures produced during insertion of uncemented femoral components. A cadaver study.

The assembly strains produced in cadaver femurs during uncemented femoral arthroplasty were measured using strain gages and photoelastic coatings. Resecting the femoral neck, reaming the canal with power drills, and rasping with an optimal size rasp, as determined by preoperative radiographic templating, produced small assembly strains, up to 300 microstrain. Insertion of an optimal-size prosthesis after preparing the femoral canal with instruments the same size as the prosthesis produced moderate assembly strains, up to 1,000 microstrain. Half a millimeter press-fit of optimal prostheses produced larger assembly strains, up to 2,000 microstrain. Half a millimeter press-fit of a prosthesis that was also one size (1.0 mm) larger than that determined to be optimum produced even larger assembly strains (2,000-6,000 microstrain) and longitudinal linear fractures in the femoral cortex. Insertion of prostheses that were smaller than the rasps produced minimal strains in the femoral cortex. The magnitude of peak strains produced by press-fitting the femoral components and the small amounts of disparity between the size of the recess and the prosthesis necessary to produce these strains show the narrow range of tolerances available to the surgeon. Cementless femoral arthroplasty requires great care in preparing the femoral canal to the appropriate size as determined from preoperative templating, using accurate and precisely toleranced instrumentation and prosthetic components in order to avoid femoral fractures, yet obtain a stable fit.

Adult↗

PubMed1993

Femoral component offset. Its effect on strain in bone-cement.

The magnitude of the offset of the femoral prosthesis strongly influences the mechanics of the hip following a total hip arthroplasty. An increased offset increases the moment arm of the abductor muscles. This reduces the abductor force required for normal gait and, consequently, reduces the resultant force across the hip joint. These factors are advantageous. However, increased offset also increases the bending moment on the implant, which could adversely increase the strain in the medial cement mantle. To evaluate the relative advantages and disadvantages of these conflicting results of increasing the offset of the femoral component the authors measured in vitro in cadaver femora the effect of differing offsets of the femoral component on strain in the cement mantle. After testing the intact femora, the authors cemented femoral prostheses in place and quantified the abductor force, resultant force, and strain in the cement mantle under loading conditions simulating single limb stance at different femoral offset levels. The reduction in both abductor and resultant force was substantial with increased femoral component offset, but the strain in the cement of the proximal medial portion of the cement mantle was not significantly increased.

Bone Cements↗

PubMed1992

A finite element study of the initiation of failure of fixation in cemented femoral total hip components.

In order to study initial mechanisms of failure in cemented femoral total hip components, an anatomically accurate three-dimensional linear finite element model was constructed and verified against experimental strain measurements in the cement mantle. Good agreement was found between predicted and measured strains. The likelihood of failure initiation due to cement-prosthesis debonding and crack initiation at voids was studied for loading conditions simulating both one-legged stance and stair climbing. The "out of plane" forces involved in stair climbing appear to be the greatest threat to the fixation of total hip replacements. In stair climbing, cement-prosthesis debonding and pore crack initiation were probable in the proximal anteromedial region of the cement mantle, and near the distal tip of the implant. The proximal stresses in stair climbing were higher than the distal stresses in either stair climbing or one-legged stance.

Bone Cements↗

PubMed1992

The mechanism of loosening of cemented acetabular components in total hip arthroplasty. Analysis of specimens retrieved at autopsy.

Late aseptic loosening of cemented acetabular components is governed by the progressive, three-dimensional resorption of the bone immediately adjacent to the cement mantle. This process begins circumferentially at the intraarticular margin and progresses toward the dome of the implant. Evidence of bone resorption at the cement-bone interface was present even in the most well-fixed implants before the appearance of lucent lines on standard roentgenographic views. The mechanical stability of the implant was determined by the three-dimensional extent of bone resorption and membrane formation at the cement-bone interface. The leading edge of the membrane is a transition zone from regions of membrane interposition between the cement and the bone to regions of intimate cement-bone contact. Histologic analysis revealed that progressive bone resorption is fueled by small particles of high density polyethylene (HDP) migrating along the cement-bone interface and bone resorption occurs as a result of the macrophage inflammatory response to the particulate HDP. Evidence in support of a mechanical basis for failure of fixation was lacking. The mechanism of late aseptic loosening of a cemented acetabular component is therefore biologic in nature, not mechanical. This is exactly opposite to the mechanism of loosening on the femoral side of a cemented total hip replacement, which is mechanical in nature.

Acetabulum↗

PubMed1991

Technique to detect migration of femoral components of total hip arthroplasties on conventional radiographs.

Critical to the evaluation of patients with painful total hip arthroplasties and to the assessment of the efficacy of fixation of new prosthetic designs and/or new insertion techniques is the ability to detect migration of the femoral components. The authors present a simple and effective technique using plain radiography that can detect migration of 1 mm with a high degree of accuracy.

Equipment Failure↗

PubMed1991

Micromotion of cemented and uncemented femoral components.

We evaluated the initial stability of cemented and uncemented femoral components within the femoral canals of cadaver femurs during simulated single limb stance and stair climbing. Both types were very stable in simulated single limb stance (maximum micromotion of 42 microns for cemented and 30 microns for uncemented components). However, in simulated stair climbing, the cemented components were much more stable than the uncemented components (76 microns as against 280 microns). There was also greater variation in the stability of uncemented components in simulated stair climbing, with two of the seven components moving 200 microns or more. Future implant designs should aim to improve the initial stability of cementless femoral components under torsional loads; this should improve the chances of bony ingrowth.

Adult↗