Larger diameter femoral heads used in conjunction with a highly cross-linked ultra-high molecular weight polyethylene: a new concept.

The design of femoral and acetabular components of metal-on-polyethylene total hip arthroplasty implants has been dominated by the limitations of the wear properties of ultra-high molecular weight polyethylene (UHMWPE). As a result, the commonest femoral head diameters used range from 22 to 32 mm, the latter producing maximal volumetric wear. Cross-linking has been shown to improve significantly the wear resistance of acetabular components when tested in vitro against conventional femoral head sizes (22-32 mm). We expanded the study of the wear behavior of 1 type of electron-beam cross-linked UHMWPE with femoral head diameters ranging from 22 to 46 mm. The simulated gait studies showed that wear was independent of head size for the range of femoral head sizes studied. Even for the 46-mm femoral head, wear was reduced significantly using criteria of gravimetric and geometric measurements and morphologic appearance of the machining marks out to 11 million cycles of simulated gait.

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.

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.

Bone grafts and bone substitutes in hip and knee surgery.

Bone grafts and bone substitutes are an essential part of the armamentarium of orthopedic surgeons. This article presents the current knowledge about bone replacements and reviews the available sources, techniques, and indications of the various types of autograft, allograft, and synthetic agents that are used in contemporary hip and knee replacement surgery.

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.

Polyethylene wear in cases using femoral stems of similar geometry, but different metals, porous layer, and modularity.

In this report, 83 total hip arthroplasties in 75 patients with femoral stems of similar geometry but different metals, porous surfaces, and femoral head-neck design were compared at a mean follow-up of 66 months (range, 40-104 months). One type of acetabular component and polyethylene were implanted in all hips. The femoral stem was monoblock in 25 hips, and in 58 it had a modular head-neck piece; 70 stems had chrome-cobalt heads, and 13 heads were titanium. Equally satisfactory clinical results were obtained with either type of femoral implant (i.e., modular and monoblock). The calculated average annual linear polyethylene wear was significantly higher for the titanium stems with a plasma-spray porous surface and chrome-cobalt head on a Morse taper than the chrome-cobalt, beaded, monoblock stems (0.22 mm/year vs 0.07 mm/year, P < .0001). The prevalence of periprosthetic osteolysis was higher for these modular stems ( 15.7% vs 0%), but this difference was not statistically significant (P = .09). Gross corrosion was present on the taper surfaces of an autopsy-retrieved femoral implant with a modular cobalt-chrome head on a titanium stem. Particles of chromium 3-orthophosphate were present at the taper rim and in the periarticular tissues.

Analysis of the kinematics of different hip simulators used to study wear of candidate materials for the articulation of total hip arthroplasties.

We developed an analytical technique to determine the paths traced by specific points on the femoral head against the acetabulum in the human hip joint during gait. The purpose of the study was to apply this technique to the mechanical hip simulators chosen to conduct wear tests on polymeric acetabular liners used in total hip replacements. These simulators differ from one another in the type of motion produced, apart from other variables such as type of lubricant and head position. Due to the variation in the kinematics between the machines, the paths traced by the points on the femoral head against the acetabular liner ranged from simple linear traces to figure-8 loops and quasi-elliptical paths during a single simulator cycle. The distances traveled by these points during the same period also varied appreciably among the different hip simulator designs. These results are important when combined with other studies that have shown that kinematics can play an important role in the outcome of in vitro wear experiments. The kinematic differences quantified in this study can partially explain the substantial differences in wear data reported from different simulator designs and also underscore the usefulness of the technique described in this study in judging the results from different hip simulator experiments.

Jumbo cups and morsalized graft.

Revision of failed acetabular components presents a formidable problem due to associated loss of bone and sclerosis of the remaining bone. Uncemented acetabular components with porous surfaces have revolutionized acetabular revision surgery. They can be stabilized into the existing host bone with supplemental screws even in the face of major bone loss. Nonstructural particulate bone grafts can then be used to supplement the bone stock. With large defects, jumbo acetabular components ranging in sizes from 70 to 80 millimeter outer diameters can be stabilized on the acetabular rim while the defects can be grafted with morsalized bone. Nineteen of such revisions performed for major bone loss without pelvic discontinuity between February 1986 and December 1988 were evaluated at a mean follow-up period of ten years (range eight to eleven years). One component had been revised for sepsis. None of the others had been revised. Definite radiographic failure of fixation of the acetabular component was not seen on any of the other hips. These results strongly support the use of jumbo uncemented acetabular components with morsalized bone grafts even in the face of major acetabular bone loss.

Importance of a thin cement mantle. Autopsy studies of eight hips.

The question whether thin cement mantles around cemented femoral components led to an increased frequency of cracks in the cement was asked. Microscopically, multiple cross sections of eight femurs retrieved at autopsy from clinically successful total hip replacements after prolonged in vivo service containing well fixed Harris Design 2 cemented femoral components were studied. None of the components were loose by radiographic criteria. All were fixed solidly when loaded in vitro in simulated stair climbing and gait, as assessed by high resolution micromotion sensors. The specimens were sectioned transversely at 5-mm increments. The cross sections were examined under a dissecting microscope at x 100. A thin mantle arbitrarily was defined as a mantle of less than 1 mm in thickness. The analysis of the contact radiographs showed that the routine anteroposterior and lateral radiographs underestimated the prevalence of thin cement mantles and mantle defects. Although overall on all the cross sections 9% of the aggregated cement mantles was classified as having thin cement, 92 of the 101 cement cracks occurred in areas of the mantles that were less than 1 mm thick.

Factors influencing stability at the interface between a porous surface and cancellous bone: a finite element analysis of a canine in vivo micromotion experiment.

Several factors contribute to the success of stable bony ingrowth into the porous coated surfaces of orthopaedic implants used in hip arthroplasty. Despite having good bony apposition, bony ingrowth might not occur if the relative motion between bone and implant is large. Therefore, determining the limiting micromotion value that inhibits stable bony ingrowth is important. From a previous canine in vivo micromotion study performed at our laboratory, this limiting value was found to be 20 microns. Initially, cementless orthopaedic implants are stabilized only by frictional forces at the bone-implant interface. Therefore, other parameters such as the coefficient of friction and the compressive force normal to the interface should be considered as important factors which stabilize the interface along with micromotion. The purpose of this analytical study was to elucidate how the stability at the bone-implant interface is influenced by various factors, namely, motion of the implant, the coefficient of friction, the degree of pres fit, and the modulus of the surrounding cancellous bone in determining the stability of the bone-implant interface. Nonlinear and linear finite element models which simulated the immediate postsurgical condition and the end point of the canine in vivo micromotion experiment, respectively, were used to this end. From the results of the finite element models it was possible to identify the displacement magnitude for which the implant slipped relative to the bone as the motion of the implant was increased incrementally. This was done for combinations of the coefficient of friction, press fit, and Young's modulus of cancellous bone. This was used as an indicator of the limiting implant motion value beyond which bony ingrowth will be inhibited. The stress distribution in the surrounding cancellous bone bed was also obtained from the results of the finite element analyses for different press-fit conditions. The results of the study indicated that under slight press-fit conditions, the implant slipped relative to bone for implant motions as low as 20 microns. For higher degrees of press fit and reasonable values for the coefficient of friction, no slip occurred for implant motions as much as 100 microns. Although higher degrees of press fit were theoretically conducive to better implant stability, the concomitant high stresses in the adjacent cancellous bone will tend to compromise the integrity of the press fit. This was also evident when the results of an analytical model with a lower degree of press fit correlated well with those of the canine in vivo experiment in which a higher press fit was used, suggesting a possibility of achieving a less than desired press fit during the process of implantation. Through this study the importance of factors other than implant motion was emphasized. The results of the study suggest that the limiting value of implant motion that inhibits bone ingrowth might vary with the degree of press fit for reasonable coefficients of friction.

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).

Periprosthetic femoral osteolysis around an uncemented nonmodular Moore prosthesis.

Periprosthetic osteolysis is a major problem in total joint arthroplasty surgery today. The particulate debris from ultrahigh-molecular-weight polyethylene, polymethyl methacrylate, and corrosion products from modular connections have been implicated in this process. A case of femoral osteolysis at the tip of a nonmodular Moore prosthesis, in the absence of polyethylene, polymethyl methacrylate, and modular connections, is reported. In the area of osteolysis, histochemical and in situ hybridization techniques established the expression of messenger ribonucleic acid encoding for certain cytokines implicated in bone resorption, preferentially in the area of osteolysis. This case illustrates that the etiology of periprosthetic osteolysis is multifactorial and can occur in the absence of polyethylene, methacrylate, or modular components. All joint implants should be monitored for the development of this complication.

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.

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.

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.

The Otto Aufranc Award. Skeletal response to well fixed femoral components inserted with and without cement.

Previous studies evaluating femoral remodeling after total hip arthroplasty have used clinical radiographs and dual energy xray absorptiometry. Limitation of these techniques make it impossible to quantify the magnitude of bone loss in terms of cortical thinning and cortical bone area and bone mineral density changes. Femoral cortical bone remodeling after cemented and cementless replacement was quantified and possible determinants of bone remodeling in terms of clinical and radiographic variables were evaluated. Forty-eight anatomic specimen femora from 24 patients with unilateral cemented and cementless hip replacements were analyzed. Cortical thickness, cortical bone area, and bone mineral density was assessed in 4 quadrants at 5 discrete levels. The maximum cortical bone loss by level was at the middle section for the cemented femurs and at the midproximal and middle sections for the cementless femurs. However, if one examines individual quadrants, the proximal medial cortex still represents the specific region of maximal bone loss for both types of implant fixation. The posterior cortex had substantially more bone loss, even in the diaphyseal levels, than had been previously appreciated. A strong correlation was noted between the bone mineral density of the control femur and the percentage decrease of bone mineral density in the remodeled femur. Based on this data, it seems that the less dense the bone is before hip replacement surgery, the greater the extent of bone loss after total hip arthroplasty regardless of the fixation type.

Management of limb length inequality during total hip replacement.

Significant limb length inequality is not an uncommon problem after total hip replacement. Preoperative measurement of limb length inequality, preoperative planning with radiographic templates, and intraoperative correction with measurements of limb lengths before and after the insertion of the trial components using special calipers can reduce the incidence and magnitude of this problem. A review of 85 consecutive patients who had primary total hip arthroplasty in which these techniques were used by a single surgeon, showed that 43 had limb inequality preoperatively ranging from 0.5 to 7.25 cm, but only 14 (16%) had limb length inequality after surgery. Eleven limbs (13%) had been lengthened 0.5 to 1 cm compared with the contralateral limb. Of the 42 patients with equal limb lengths preoperatively, 3 had a lengthened limb postoperatively compared with their contralateral limb. Four patients were using lifts on the same side because the limb was too short, and 2 were using lifts on the other side because the limb was too long. None of the other patients complained about limb length inequality. The techniques described above are helpful in minimizing limb length inequality during total hip replacements.

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.

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.

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.