Intramedullary fixation of artificial hip joints with bone cement-precoated implants. I. Interfacial strengths

The use of a bioactive bone cement containing apatite-wollastonite glass-ceramic filler and bisphenol-a-glycidyl methacrylate resin for acetabular fixation in total hip arthroplasty

Aims

In the 1990s, a bioactive bone cement (BABC) containing apatite-wollastonite glass-ceramic (AW-GC) powder and bisphenol-a-glycidyl methacrylate resin was developed at our hospital. In 1996, we used BABC to fix the acetabular component in primary total hip arthroplasty (THA) in 20 patients as part of a clinical trial. The purpose of this study was to investigate the long-term results of primary THA using BABC.

Patients and Methods

A total of 20 patients (three men and 17 women) with a mean age of 57.4 years (40 to 71), a mean body weight of 52.3 kg (39 to 64), and a mean body mass index (BMI) of 23.0 kg/m2 (19.8 to 28.6) were evaluated clinically and radiologically. Survival analyses were undertaken, and wear analyses were carried out using a computer-aided method.

Results

The mean follow-up was 17.6 years (1.5 to 21.1). Radiological loosening occurred in four sockets with aseptic loosening at a mean of 7.8 years (1.5 to 20.7). Kaplan–Meier survival analyses using revision of the acetabular component, radiological loosening of the acetabular component, and the worst-case scenario with revision of the acetabular component to include the two patients lost to follow-up as endpoints yielded survival rates of 94.7%, 84.4%, and 85.0% at ten years, and 70.0%, 84.4%, and 62.8% at 20 years, respectively. Wear analysis revealed a mean linear wear rate of 0.068 mm per year.

Conclusion

The long-term results of primary THAs using BABC were unsatisfactory. Its brittle nature and poor handling properties need to be improved before it becomes an alternative method of fixing the acetabular component in cemented THA. Cite this article: Bone Joint J 2019;101-B:787–792.

Random damage and characteristics of debris particles are two important and yet ignored factors in the mechanical integrity of the stem-cement interface of a total hip replacement: influence of the surface finish of the metal stem

The importance of the conditions at the stem-cement interface in cemented total joint replacements (THRs) with regard to the in vivo longevity of the implant is well recognized. In the present study, we used a simplified model of one part of a cemented THR (alloy rectangular beam bonded to rectangular cement plate) to study the influence of surface finish of the alloy beam (stem) on two measures of the evolution of random damage at the alloy beam-cement plate interface (stem-cement interface), under quasi-static direct shear load. Three surface finishes of the beams were used: satin-finish, grit-blasted, and plasma-sprayed. The random damage events were monitored from the emitted acoustic signals, with the two measures computed from these signals being the intensity of random damage events (IRDE) and the mean damage event energy (MDEE). Large number of random damage events (higher values of IRDE and low value of MDEE) occurred with grit blasted specimens, suggesting a high probability for the generation of debris particles at the interface. These findings, in conjunction with details on the size and shape of the debris particles, obtained using scanning electron microscopy, lead to the suggestion that satin-finish stems are desirable for use in cemented THRs.

Pull-out strength of a polished tapered stem is improved by placing bone cement over the shoulder of the implant

Displacement of a polished stem while attempting closed reduction of a dislocated total hip or during dislocation itself is a rare but significant complication. Our aim was to assess whether applying bone cement over the shoulder of the implant can help to prevent this. We conducted an in vitro mechanical study with tensile testing machine. We cemented 7 sawbones with a standard cementing technique and another 7 with additional cement over the shoulder of the implant. The mean pull-out force in the routine cementing technique was 2066 N (SD, 256.65), and it was 3220 N (SD, 312.22) for the group with the cement on the shoulder. There is a statistically significant difference of 1154 N. We recommend that when a polished stem is used, bone cement should be applied over the shoulder of the implant.

Investigation of interfacial strength and its structure on the development of a new design of UHMWPE acetabular component

Previous studies associated with the development of a new ultra-high-molecular-weight polyethylene (UHMWPE) acetabular component have shown high interfacial tensile strengths through chemical and mechanical bonds between virgin UHMWPE or polymethylmethacrylate (PMMA) and PMMA/methylmethacrylate (MMA) monomer treated UHMWPE. Along with the interfacial strength, the mechanism of interfacial strength development has been investigated, correlating the interfacial strength to its structure, with the different molding temperatures or amount of PMMA in the treated UHMWPE. Of three different fracture patterns-adhesive, mixed, and cohesive-most fractures occurred in the mixed or cohesive mode, indicating either a strong interface or a weak bulk phase. Load-displacement plots from the interfacial tensile tests represented two distinct fracture patterns, suggesting the nature of interfacial structure. Comparison of theoretical and real interfacial strength showed a close match between the two strengths for the interface between PMMA and treated UHMWPE, but a large difference for the interface between UHMWPE and treated UHMWPE. This result hints that although the PMMA/treated UHMWPE interface develops its interfacial strength in a relatively simple mechanism of direct chemical bonds, the UHMWPE/treated-UHMWPE interface builds its strength in a complex way.

Stem surface roughness alters creep induced subsidence and 'taper-lock' in a cemented femoral hip prosthesis

The clinical success of polished tapered stems has been widely reported in numerous long term studies. The mechanical environment that exists for polished tapered stems, however, is not fully understood. In this investigation, a collarless, tapered femoral total hip stem with an unsupported distal tip was evaluated using a 'physiological' three-dimensional (3D) finite element analysis. It was hypothesized that stem-cement interface friction, which alters the magnitude and orientation of the cement mantle stress, would subsequently influence stem 'taper-lock' and viscoelastic relaxation of bone cement stresses. The hypothesis that creep-induced subsidence would result in increases to stem-cement normal (radial) interface stresses was also examined. Utilizing a viscoelastic material model for the bone cement in the analysis, three different stem-cement interface conditions were considered: debonded stem with zero friction coefficient (mu=0) (frictionless), debonded stem with stem-cement interface friction (mu=0.22) ('smooth' or polished) and a completely bonded stem ('rough'). Stem roughness had a profound influence on cement mantle stress, stem subsidence and cement mantle stress relaxation over the 24-h test period. The frictionless and smooth tapered stems generated compressive normal stress at the stem-cement interface creating a mechanical environment indicative of 'taper-lock'. The normal stress increased with decreasing stem-cement interface friction but decreased proximally with time and stem subsidence. Stem subsidence also increased with decreasing stem-cement interface friction. We conclude that polished stems have a greater potential to develop 'taper-lock' fixation than do rough stems. However, subsidence is not an important determinant of the maintenance of 'taper-lock'. Rather subsidence is a function of stem-cement interface friction and bone cement creep.

Interfacial strength of compression-molded specimens between PMMA powder and PMMA/MMA monomer solution-treated ultra-high molecular weight polyethylene (UHMWPE) powder

The interface between bone cement and ultra-high molecular weight polyethylene (UHMWPE) has been considered a weak link of cemented UHMWPE acetabular cup in total hip replacement (THR). For the improvement of this weak interface, adhesion between the UHMWPE acetabular cup and bone cement made of polymethylmethacrylate (PMMA) has been investigated in our laboratory. Virgin UHMWPE powders were treated with methyl methacrylate (MMA) monomer and PMMA/MMA solution. The treated UHMWPE powders were then compression-molded with virgin UHMWPE powders or PMMA powders, creating two different interfaces, i. e., treated/virgin UHMWPE powder and treated UHMWPE/PMMA powder. For the present study, the interfacial strengths between PMMA powder and the treated UHMWPE power were investigated following the same protocol previously set. The maximum interfacial strength was 17.0 +/- 0.25MPa with the same molding condition of 166.5 degrees C, 38.7 MPa and l h. In addition to the molding condition, we tested the strengths for the treated UHMWPE powders, which have different ratios between PMMA/MMA solution and MMA-treated UHMWPE powders. Significant differences on the interfacial strengths resulted due to the ratio change; more PMMA in the PMMA/MMA solution-treated UHMWPE powder exhibited higher interfacial strength. Scanning electron microscopic (SEM) pictures showed that the interface is composed of three major portions: PMMA powder, UHMWPE, and coated PMMA, indicating strong mechanical interlocking of UHMWPE and PMMA powder matrix and chemical bonding between PMMA powder and the precoated PMMA onto the UHMWPE. In addition, another interfacial strength between PMMA powder, which is equivalent to the outermost part of the cup, and bone cement was investigated. The average strength reached up to 42.4 +/- 3.6 MPa, close to the tensile strength of bone cement itself.

The Frank Stinchfield Award. Influence of cement technique on the interface strength of femoral components

The optimal surface finish for polymethylmethacrylate cemented femoral components remains controversial. Concerns about early debonding of the prosthesis-cement interface have led surgeons to use roughened surfaces to enhance the cement-prosthesis bond. However, loosening of roughened stems is associated with the generation of excessive wear debris. The purpose of the current study was to determine whether the time to cementation influenced the cement-prosthesis bond of four roughened cobalt chrome surfaces (60 grit-blasted, 10 grit-blasted, 10 grit-blasted with polymethylmethacrylate precoating, glass bead-blasted) and one polished cobalt chrome surface. Fixation strength was assessed using mechanical pushout and tensile testing. Roughened and polymethylmethacrylate precoated surfaces had significantly greater tensile and shear strengths at early cementation times compared with polished surfaces. However, roughened components had significant decreases in tensile and shear strengths as cementation time increased from 2 to 4 minutes and 2 to 6 minutes. In contrast, tensile and shear strengths for the polished surface were significantly lower than for the roughened surfaces and did not change with longer cementation times. When using a roughened or precoated cemented femoral component, the surgeon should consider cementing earlier with wetter cement to maximize the cement-prosthesis bond. When implanting a polished femoral component, it is preferable that the cement is doughy, because the cement-prosthesis bond is not influenced by the wetness of the cement and it is easier to maintain the orientation of the femoral component.

Long-term survival of the T-28 versus the TR-28 cemented total hip arthroplasties

Between 1974 and 1980, 550 total hip arthroplasties (THAs) (479 patients) were performed using T-28 and TR-28 cemented prostheses (TR-28 is shot-blast chrome and T-28 is polished stainless steel). There were 379 cemented THAs in 321 patients in the T-28 group and 171 cemented THAs in 158 patients in the TR-28 group. Average follow-up of the patients still alive at the end of the study was 20.96 years in the T-28 group and 17.54 years in the TR-28 group. When considering failure as revision of the hip for aseptic acetabular loosening, there were 36 (9.5%) failures in the T-28 group and 12 (7%) failures in the TR-28 group. This difference was statistically significant (P = .0132). When considering failure as radiographic acetabular loosening with or without radiographic femoral loosening, there were 52 failed acetabula (13.7%) in the T-28 group and 18 failed acetabula (10.5%) in the TR-28 group. These differences were not statistically significant. When considering failure as revision for aseptic femoral loosening with or without acetabular component loosening, there were 42 failures (11.1%) in the T-28 group and 22 failures (12.8%) in the TR-28 group. This difference was not statistically significant. When considering failure as radiographic femoral loosening with or without acetabular component loosening, there were 42 failures (11.1%) in the T-28 group and 27 failures (15.8%) in the TR-28 group. This difference was statistically significant for log-rank test (P = .0318) and Wilcoxon's test (P = .0083). Surface finish may be an important contributor to the survival of cemented femoral stems.

Precoating of ultrahigh molecular weight polyethylene with polymethylmethacrylate: interfacial strength

Fixation of polymeric implants, especially an ultrahigh molecular weight polyethylene (UHMWPE) acetabular cup, to a host bone site has been a challenge since its first conception from the Charnley low friction total hip arthroplasty. Destabilization of the acetabular cup, similar to the well-documented cases of femoral stems, is caused mainly by aseptic loosening; the mobile loosened particles further contribute to the progression of aseptic loosening. Although the obvious fixation problems lie in the bone-bone cement interface, little work has been done to reduce the loosening by improving the acetabular components as a whole in cemented procedures. Most of the grooved outer surface, external fixation devices, and metal backings have been introduced to avoid problematic fixation of the cup to bone cement; nevertheless, the designs themselves to some degree became the source of the loosening problems. One possible way to improve the adhesion of acrylic bone cement to the UHMWPE acetabular cup is precoating the surface with polymethylmethacrylate (PMMA). This study successfully precoated the UHMWPE surface with PMMA, showing good chemical and mechanical stability, and suggests the optimal conditions of variables involved in the newly developed precoating process. The highest interfacial tensile strength was 11.51 +/- 0.65 MPa, which is stronger than those of UHMWPE and metal in metal-backed cups (6.3 MPa) and bone-bone cement (8.5 MPa). Further chemical analysis and mechanical testing are in progress, yet the present result of the mechanical tensile strength test showed that the precoating process for the UHMWPE surface could be a viable means toward stable fixation of the polymeric implants by using PMMA bone cement.

Interfacial properties of self-reinforced composite poly(methyl methacrylate)

Total joint prostheses are often fixed in the bone using bone cement. The cement mantle, however, is prone to fatigue fracture that can lead to failure of the mantle, evolution of bone cement particles, and eventual loosening and failure of the prosthesis. A new material, self-reinforced composite poly(methyl methacrylate) (SRC-PMMA) was developed previously by the authors. This material has a similar chemical composition to bone cement, with the matrix and reinforcing fibers both fabricated from PMMA. One potential use for this material is as a precoat for hip prostheses or other stemmed prostheses. This study sought to examine the strength of the bonds that SRC-PMMA forms with simulated prostheses and bone cement. SRC-PMMA was woven about Co-Cr rods and push out tests were performed. Samples were tested in air as processed or after immersion in saline for 30 days at 37 degrees C. Three different weaves were investigated and compared to bone cement. Bone cement and SRC-PMMA formed interfacial bonds with Co-Cr rods that failed at an average load (stress) of 980 N (2.0 MPa). After saline immersion, the bone cement's interfacial bond strength was 642 N (1.23 MPa) and the tight weave SRC-PMMA was statistically stronger at 973 N (1.86 MPa). The shear strength within bone cement alone as measured by push out tests was an order of magnitude higher at 9210 N (15.2 MPa) in air and 9900 N (15.7 MPa) after saline immersion. The bond between SRC-PMMA and bone cement was 10,900 N (17.9 MPa) in air and 9610 N (15.8 MPa) after immersion in saline. Woven SRC-PMMA performed as well or better than bone cement in these push out tests.

The effect of a silane coupling agent on the bond strength of bone cement and cobalt-chrome alloy

Debonding of the cement-implant interface has been hypothesized to be the leading initial indicator of failed total hip prostheses. Many attempts have been made to increase the bond strength of this interface by precoating the implant, increasing the implant's surface roughness, and creating macro-grooves or channels on the implant. However, each of these approaches introduces new complications. This study introduces a unique silane coupling agent used to chemically bond the bone cement to the implant. Cylindrical cobalt-chrome samples were treated with the silane coupling agent, bonded to polymethylmethacrylate, and pushed out to failure. The mean shear strengths were compared to the failure strengths of untreated samples. Half of the specimens were tested immediately following cement curing, and the other half were tested after immersion in saline solution for 60 days. The mean shear strength of the silane-coated samples ranged from 18.2 to 24.1 MPa, and the mean shear strength of the uncoated samples ranged from 7.6 to 15.0 MPa. The increase in strength following silane coating noted in this study may increase the longevity of the implant by decreasing debonding at the interface and, therefore, subsequent failure due to loosening.

Failure of total hip arthroplasty with a precoated prosthesis. 4- to 11-year results

A series of early femoral component failures prompted a detailed retrospective clinical and radiographic review of 176 hybrid cemented total hip arthroplasties using a polymethyl-methacrylate coated femoral prosthesis. All surgeries were performed using third generation cement techniques. Average length of followup was 6.3 years (range, 3-12 years). Twenty-one patients died, and one underwent revision surgery because of sepsis. Of the remaining 154 total hip arthroplasties, 23 (15%) of the femoral components failed (21 revised, two definitely loose). The average time to revision was 3.9 years. None of the acetabular components failed. Comparison between the failure and nonfailure groups revealed that poor cement mantles (Grades C or D) with distal cement mantle deficiencies were statistically significant predictors of femoral failure. The most common mechanism of failure was progressive, circumferential cement-bone interface osteolysis with relative preservation of the cement-metal interface. Debonding of the cement column from the prosthesis was a late finding and occurred in only 45% of failed cases. Incorporating the techniques of centralization and centrifugation significantly improved clinical results. Strengthening of the cement-prosthesis interface may magnify the deleterious effects of a poor cement mantle and predisposes the cement-bone interface to failure.

Cement debonding process of total hip arthroplasty stems

Retrieval studies have indicated that debonding of the stem cement interface in total hip arthroplasty precedes clinical failure of femoral components. This study addressed the mechanisms that play a role in the debonding process by analyzing how debonding is likely to proceed in the course of time. It was investigated whether debonding is an immediate process or if it is likely to develop slowly with time, which interface stress components contribute particularly to its progression, and whether the mechanical integrity of the cement mantle is likely to be compromised by the debonding process. To answer these questions, a 3-dimensional finite element model of a femoral total hip arthroplasty reconstruction was developed and used to simulate the debonding process. The results showed that debonding was governed by the shear stress component at the interface. Debonding started in the tip region and the proximal, medial anterior region. These debonded regions expanded until the whole interface was de bonded. Cement stresses slowly increased at the end of the debonding process to a level twice as high as the initial one. The probability of debonding, as measured by an interface failure index, remained constant as debonding progressed. This indicates that, for this particular design, much less surface area is required for load transfer than is provided by the stem, and the debonding process does not necessarily accelerate quickly once debonding is initiated.

Enhanced strength in cemented stem fixation using adhesive acrylic cement as a metal coating material

One cause of aseptic loosening of cemented total hip arthroplasty is mechanical weakness at the interface between the metal stem and the cured bone cement. Adhesive acrylic bone cement containing 4-methacryloyloxyethyl trimellitate anhydride (4-META) was applied as a metal coating material to increase the strength of the cemented fixation. The 4-META cement has 2-3 times greater tensile bond strength to metals than does commercial acrylic bone cement. The shear strength of the coated metals fixed with bone cement was approximately 4 times greater in SUS-304 and 3 times greater in titanium (Ti) alloy than those of uncoated metals, and this strength did not decrease after 1 week's immersion in saline. The coating process using the 4-META cement can be performed at normal room temperature, so that metal stems for bone cement fixation could be coated during the course of an operation resulting in potentially improved results of total hip arthroplasty.

Strength of cement-metal interfaces in fatigue: comparison of smooth, porous and precoated specimens

Radiographic follow-up studies of cemented total hip arthroplasty have shown that failure of the cement-metal interface of the femoral component is as high as 25% at 10 years. Recent analyses of clinically successful cemented femoral components obtained in toto with the surrounding cement and femurs after many years of in-vivo service have suggested that the mechanism of the initiation of failure of fixation of cemented femoral components is debonding at the cement-metal interface. Since this critical interface is subjected primarily to cyclic loading, the evaluation of different surface preparations should be studied in fatigue, not static testing. In the current study, several contemporary methods for increasing the strength of the cement-metal interface were evaluated by testing the interfacial fatigue pushout strength under varying conditions of cyclic loading. The effect of a smooth 'implant finish' surface, a surface coated with polymethylmethacrylate (PMMA precoated surface), a combination of a textured surface with PMMA precoat, and a porous titanium mesh coated surface were examined. Precoating the metal with a thin film of PMMA significantly increased the number of compressive fatigue loading cycles required for failure of the cement-metal interface under cyclic loading compared to a smooth, uncoated surface. Adding indentations to the surface and then precoating with PMMA further significantly increased the fatigue life of the cement-metal interface. The strongest interface in fatigue was the titanium fibermesh-cement interface.

Orthopedic prosthesis fixation

The fixation of orthopedic implants has been one of the most difficult and challenging problems. The fixation can be achieved via: (a) direct mechanical fixation using screws, pins, wires, etc.; (b) passive or interference mechanical fixation where the implants are allowed to move or merely positioned onto the tissue surfaces; (c) bone cement fixation which is actually a grouting material; (d) biological fixation by allowing tissues to grow into the interstices of pores or textured surfaces of implants; (e) direct chemical bonding between implant and tissues; or (f) any combination of the above techniques. This article is concerned with various fixation techniques including the potential use of electrical, pulsed electromagnetic field, chemical stimulation using calcium phosphates for the enhancement of tissue ingrowth, direct bonding with bone by glass-ceramics and resorbable particle impregnated bone cement to take advantages of both the immediate fixation offered by the bone cement and long term fixation due to tissue ingrowth.

Mechanical characteristics of the stem-cement interface

The mechanical characteristics of the interface between a metallic stem and the surrounding poly(methyl methacrylate) bone cement were determined from experimental tests and finite element analyses. Push-through-stem tests of straight and tapered titanium alloy stems, surrounded by cement columns, were performed and the resulting load-displacement behavior and strain distribution on the surface of the cement column were measured for loading, unloading, and reloading. Test geometries were modelled using nonlinear, axisymmetric, finite element analyses, which incorporated Coulomb friction elements at the titanium alloy-cement interface. Initial residual stresses, due to curing of the cement column, were modeled by thermal contraction of the cement. Good agreement was obtained between load-displacement curves and surface strains predicted from the nonlinear analysis and those obtained from experiments, when a coefficient of friction of 0.3 was assumed for the stem-cement interface. These results show that, in the absence of chemical adhesion, the load-displacement behavior of a stem-cement composite can be described completely in terms of the friction at the interface and the residual stresses normal to the interface.

Bone-particle-impregnated bone cement: an in vivo weight-bearing study

To evaluate an experimental inorganic-bone-particle-impregnated bone cement, canine hip prostheses were implanted in dogs using a regular bone cement on one side and the experimental bone cement on the other. In a preliminary feasibility study, bone ingrowth into the resorbed bone-particle spaces was established 3 months after implantation in three dogs. In a more detailed study, twenty-eight (28) dogs were divided in four groups to delineate the effects of time on the phenomena of bony ingrowth. One month after implantation, active bone ingrowth into the bone cement was obvious. By 3 months postimplantation, the ingrowth appeared to have traversed the thickness of the bone-particle-impregnated cement. By the fifth month, most of the interconnected inorganic bone particles were replaced by new bone. At the end of a year, the ingrown bone was mature and negligible new bone activity was present. Biomechanical pushout tests closely corroborated the histologic observations. The maximum shear strength of the cement/bone interface of the experimental side reached 3.6 times that of the control side at 5 months postimplantation. No further improvements were seen at 12 months postimplantation. A viable bone/cement interface may result in a better orthopedic implant fixation system by combining the advantages of both cement for immediate rigidity and biological ingrowth for longterm stability.

The effect of surface preparation on metal/bone cement interfacial strength

This study is concerned with finding practical ways for strengthening metal/bone cement (M/BC) interfaces via surface alterations and identifying fundamental mechanisms underlying M/BC adherence. Shear strengths have been inferred from torsion tests using shear-lag analysis. The variables examined with regard to their effects on interfacial strength are substrate material, surface roughness, interface porosity, passivation and sterilization, surface cleaning procedures, and use of bone cement precoated metals. M/BC interfaces can be substantially strengthened by applying the bone cement to the metal with high pressure. This would be a practical way to strengthen interfaces for precoated implants. The acrylic polymerized in vivo would employ the usual low pressure method. Otherwise, the main method for improving M/BC interfaces is through changing surface topography. Cleaning or chemical treatments have relatively minor effects. Roughened surfaces, as expected, produce stronger interfaces. Dramatic strength improvements occurred with a porous arc plasma sprayed layer on the substrate. Surprisingly, highly polished surfaces also improve interface strength (compared to less polished surfaces). The hypothesis is advanced that M/BC adherence depends upon superposition of mechanical interlocking and atomic interaction effects, with the latter predominating for finer finishes and vice versa. Differences exist between materials which are independent of roughness.

Bone-particle-impregnated bone cement: an in vitro study

Bone-particle-impregnated bone cement specimens (up to 30% by weight) were characterized by various test methods. The experimental bone cement showed decreased crack propagation rates and increased Young's modulus, while the ultimate tensile strength and impact strength were decreased. The viscosity could be adjusted by adding initiators lost when substituting the PMMA powder with bone particles. The present study warranted further in vivo experiments on the possibility of tissue ingrowth for which the new bone cement was developed.

The load carrying and fatigue properties of the stem-cement interface with smooth and porous coated femoral components

Porous coated surfaces for fixation of total hip replacement are a current trend in clinical orthopedics. Such devices are designed to be fixed by ingrowth of bony tissue, although in the absence of FDA approval for biologic fixation, fixation with PMMA cement is recommended by the implant manufacturers. In order to characterize the mechanical properties of the micro-interlocked stem-cement interface, we tested both porous coated and smooth femoral components in cement mantles of consistent overall geometry. Under conditions of increasing load the smooth stems demonstrated stepwise irreversible subsidence into the mantle. Axial and circumferential strains measured in the cement containment vessels with the smooth stems showed that stepwise increases in tensile hoop strain occurred concomitantly with the stepwise incidents of stem subsidence. When subjected to the same loading conditions, the porous coated stems did not undergo stepwise incidents of subsidence, and hoop strain generation was reduced. In addition, a twofold increase in the failure load of the stem-cement interface was measured with the porous coated stems. Fatigue loading for 10(7) loading cycles did not result in gross failure of either the micro-interlocked or smooth interfaces. However, the data showed that during fatigue loading, stepwise subsidence of the smooth stems again occurred. The final subsidence magnitude of the smooth stem-cement interface at 10(7) loading cycles was six times greater than the value associated with the porous coated stem. Thus the porous coating of femoral stems was shown to dramatically improve the load carrying capability and fatigue characteristics of the stem-cement interface.

Metal/cement interface strength in cemented stem fixation

To characterize the strength of the interface between stem-type metal implants and bone cements, a fracture mechanics parameter was used. This parameter, the critical strain energy release rate (Gc), was determined from "push-out" tests of cylindrical specimens. The specimens, formed using molds of bone, were maintained and tested at body temperature. The strength of interfaces formed with cancellous bone surrounding the cement mantle was significantly less than the strength of those formed in apposition to cortical bone. A marked degradation of strength was found with saline immersion for SS316LVM/cement interfaces formed with Zimmer regular, Simplex-P, and Zimmer LVC cements. After 60 days of immersion the interface Gc was only 10-20% of the value for bulk cement. Interfaces formed with thin-film polymethylmethacrylate-precoated metals (SS316LVM, Co-Cr-Mo, and Ti-6A1-4V) yielded "dry" Gc values one order of magnitude greater than those measured with interfaces formed with uncoated metals. Moreover, the strength of precoated SS316LVM/cement interfaces formed with all three brands of cement did not change after saline immersion for 60 days.

Cement strain measurement surrounding loose and well-fixed femoral component stems

Strain measurement within the cement surrounding stemmed total hip femoral components was accomplished using PMMA encapsulated and embedded strain gauges. Cement strain measurement associated with a well-bonded stem-cement interface and an unbonded stem-cement interface (i.e., loose prosthesis) was performed. The presence of a stem-cement bond was found to reduce proximal cement strain magnitudes while having little effect on distal cement strain magnitudes. The assurance of a stem-cement bond on only the proximal third of the interface was found to have an effect similar to that of a complete stem-cement bond. The results of this experimental investigation confirm the theoretical prediction that the stem-cement bond is important in maintaining the integrity of the cement mantle surrounding a stemmed femoral component.

Acrylic bone cement: in vitro and in vivo property-structure relationship--a selective review

The nature and curing characteristics of acrylic bone cement are presented to give some basic understanding of the key to improving its performance in in vivo clinical use. The use of electron paramagnetic resonance (EPR) spectroscopy in the (polymerization) setting and curing/aging of bone cement under in vitro and in vivo conditions is presented. The current research effort to improve the implant fixation by precoating the prosthesis with acrylic polymers is also discussed.

Intramedullary fixation of artificial hip joints with bone cement-precoated implants. II. Density and histological study

Bilateral coxofemoral hemiarthroplasties were performed in dogs using experimental and control implants, which were fixed with bone cement. The stem of the experimental implant was precoated with bone cement, about 2 mm thick. After 1, 3, and 6 months the femora with implant specimens were harvested and sectioned for mechanical and histological evaluation. Histological observations on the implant-bone interface and density measurements of the bone cement are reported. The density of the precoated bone cement was higher than the same cement used for implant fixation at the time of implantation (1.202 vs. 1.188 g/mL). The precoating also resulted in milder histological reactions, including thinner fibrous tissue capsule and smaller gap between bone and cement. The present results and the previously reported mechanical findings strongly support our hypothesis that a better and longer lasting prosthesis fixation can be achieved using cement-precoated prosthesis combined with the customary cement fixation technique.