Long-term results after implantation of McKee-Farrar total hip prostheses

Summary From 1973 to 1976, McKee-Farrar total hip arthroplasties were performed in the Orthopaedics Department at the Centre of Pulmology. In the present study, only patients with a complete record of radiological changes at the bone/implant or bone/cement interface were evaluated. A series of 81 patients with 100 total hip prostheses in situ according to the Mayo Clinic hip score and 36 patients who had undergone 36 revision operations were studied. The design of the metal cup with metal studs at the outer surface has a favourable effect on the stability of the implant/cement combination: loosening at the implant/cement interface did not occur in any of these cases and there were few cement fractures. In spite of the metal-to-metal combination, abrasion is slight and there is no danger of metallosis.With the kind support of the Ludwig Boltzmann Institute of Rheumatology, A-2500 Baden, Austria     

Effect of bone porosity on the mechanical integrity of the bone-cement interface

BACKGROUND: Osteopenia is one factor that may influence the decision about the type of implant fixation to use in total hip arthroplasty. However, clinical studies generally do not associate the outcome of an arthroplasty with the degree of osteopenia. The mechanical integrity of the cement fixation of an implant may be affected by the relative degree of osteopenia, which could account for some of the variable long-term results after total hip arthroplasty performed with cement. The purpose of this study was to determine the effects of bone porosity, trabecular orientation, cement pressure, and cement penetration depth on fracture toughness at the bone-cement interface. METHODS: Trabecular bone from the proximal part of bovine femora was used with a single brand of commercial acrylic bone cement to form compact-tension interface specimens representing a range of bone porosities, orientations, and cement pressures within a clinically achievable range. All specimens were loaded to failure with use of a servohydraulic testing machine, and fracture toughness at the interface was calculated. After testing, images of a representative sample of specimens were made with use of computed tomography to measure the penetration depth of the cement into the bone. RESULTS: Significant correlations were found between fracture toughness and bone porosity, trabecular orientation, and cement pressure, with bone porosity having the strongest effect (p < 0.000015). Examination of the computed tomographic images also showed a significant correlation between fracture toughness and maximum cement penetration depth (p < 0.033), as well as significant partial correlations between maximum and mean penetration depth and bone porosity (p < 0.0037 and p < 0.0028). CONCLUSION: The fracture resistance of the bone-cement interface is greatly improved when the ability of the cement to flow into the intertrabecular spaces is enhanced.     

In vivo loss of cement–bone interlock reduces fixation strength in total knee arthroplasties

Prevention of aseptic loosening of total knee arthroplasties (TKAs) remains a clinical challenge. Understanding how changes in morphology at the implant–bone interface with in vivo service affect implant stability and strength could lead to new approaches to mitigate loosening. Enbloc TKA retrievals and freshly‐cemented TKA tibial components were used to determine if the mechanical strength of the interface depended on the amount of cement–bone interlock and the morphology of the supporting bone under the cement layer. Implants were sectioned into small specimens of the cement–interface–bone from under the tibial tray. Micro‐CT scans were used to document interlock morphology and architecture of the supporting trabecular bone. Axial compression tests were used to assess mechanical behavior. Postmortem retrievals had lower contact fraction (42 ± 55%) compared to freshly‐cemented constructs (121 ± 61%) (p = 0.0008). Supporting bone architecture parameters were not different for the two groups. Increased interface contact fraction and supporting bone volume fraction (BV/TV) were positive predictors of interface strength (r² = 0.72, p = 0.0001). For the same supporting bone BV/TV, postmortem specimens had weaker interfaces; they were also more compliant. Cemented TKAs with in vivo service experience a loss of fixation strength and increased micro‐motion due to the loss of cement–bone interlock. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1052–1060, 2014.     

Trabecular resorption patterns of cement–bone interlock regions in total knee replacements

With in vivo service, there is loss of mechanical interlock between trabeculae and PMMA cement in total knee replacements. The mechanisms responsible for the loss of interlock are not known, but loss of interlock results in weaker cement–bone interfaces. The goal of this study was to determine the pattern of resorption of interdigitated bone using a series of 20 postmortem retrieved knee replacements with a wide range of time in service (3–22 years). MicroCT scans were obtained of a segment of the cement–bone interface below the tibial tray for each implant. Image processing methods were used to determine interface morphology and to identify supporting, interdigitated, resorbed, and isolated bone as a function of axial position. Overall, the amount of remaining interdigitated bone decreased with time in service (p = 0.0114). The distance from the cement border (at the extent of cement penetration into the bone bed) to 50% of the interdigitated volume decreased with time in service (p = 0.039). Isolated bone, when present, was located deep in the cement layer. Overall, resorption appears to start at the cement border and progresses into the cement layer. Initiation of trabecular resorption near the cement border may be a consequence of proximity to osteoclastic cells in the adjacent marrow space. Clinical Significance: Aseptic loosening of joint replacements remains an important clinical problem. This work explores the process and pattern of trabecular bone resorption responsible for loss of interface fixation. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2773–2780, 2017.

     

Femoral bone loss in total knee arthroplasty. A review.

Femoral bone loss in total knee arthroplasty (TKA) is a common feature and is mainly attributed to three etiological factors. Stress shielding causes an "osteopenia" type of bone loss behind the anterior flange and adjacent to the distal aspects of the femoral component. Using dual-energy x-ray absorptiometry, decrease in bone mineral density up to 44% has been measured in these areas. Secondly, polyethylene, cement and metal particles are released by implant wear and may cause the less common "osteolysis" type of bone loss located directly at the anterior and posterior implant-cement-bone interfaces. This type of bone loss occurs mainly in young, male, overweight patients with osteoarthritis. Finally, implant loosening leads to bone loss at the bone cement-implant interface and results in "hollowing out" of the distal femur in a stemmed TKA. Femoral bone loss may be reduced by diminishing the stress-shielding effect, by improving the quality of the polyethylene insert, and by decreasing the rate of implant loosening still further. In revision TKA, femoral bone loss is often underestimated in the preoperative radiographs. Classification of bone loss should be made during surgery, and should be based upon the size of the defect generated and the grade of containment. The choice among cement filling, metal augmentation, custom-made TKA, solid bone grafts, and morsellized bone grafts in reconstruction of bone defects will depend upon the type of bone loss, the bone quality, the surgeon's preference and philosophy, and the availability of grafts and implants.     

Surface finish mechanics explain different clinical survivorship of cemented femoral stems for total hip arthroplasty

The ability of bone cement to adhere to the implant surface is dependent on the surface finish. Stems with a rough surface finish require greater force to disrupt their interface with the cement than do stems with a smooth or polished surface. However, if micromotion occurs at the cement-metal interface, the fretting of a smoother surface implant results in less cement and metallic abrasion than an implant with a rough surface finish. Today, surgeons implant femoral stems with a wide variety of surface finish and textures that are supported by the previously mentioned contrasting philosophy of fixation. This article presents the micro and macro surface finish mechanics, history, and rationale for changes in surface finish, the clinical and operative implications of changes in surface finish, the retrieval analysis, and the clinical evidence that examine the consequences of changes in surface finish in the outcome of cemented femoral stems for total hip arthroplasty. Current data and our own experience support the use of cemented femoral stems with a smooth or polished surface finish.     

Hydroxyapatite-coated titanium for orthopedic implant applications

The interface mechanical characteristics and histology of commercially pure (CP) titanium- and hydroxyapatite- (HA) coated Ti-6Al-4V alloy were investigated. Interface shear strength was determined using a transcortical push-out model in dogs after periods of three, five, six, ten, and 32 weeks. Undecalcified histologic techniques with implants in situ were used to interpret differences in mechanical response. The HA-coated titanium alloy implants developed five to seven times the mean interface strength of the uncoated, beadblasted CP titanium implants. The mean values for interface shear strength increased up to 7.27 megaPascals (MPa) for the HA-coated implants after ten weeks of implantation, and the maximum mean value of interface shear strength for the uncoated CP titanium implants was 1.54 MPa. For both implant types there was a slight decrease in mean shear strength from the maximum value to that obtained after the longest implantation period (32 weeks). Histologic evaluations in all cases revealed mineralization of interface bone directly onto the HA-coated implant surface, with no fibrous tissue layer interposed between the bone and HA visible at the light microscopic level. The uncoated titanium implants had projections of bone to the implant surface with apparent direct bone-implant apposition observed in some locations. Measurements of the HA coating material made from histologic sections showed no evidence of significant HA resorption in vivo after periods of up to 32 weeks.     

Bone structure changes in hip joint endoprosthesis implantation over the course of many years. A quantitative study

The stability of implanted artificial joints is limited. One main factor for this is the change of the bone tissue near the implant. Therefore we studied femura with stem endoprostheses derived from autopsy. All specimens were cut in horizontal and longitudinal sections. X-rays were made of all sections. Then the specimens were embedded undecalcified and ground to 10 microns. The wellknown phenomenons at the bone cement border were found too. Direct bone cement contact is about 5%. The 4 studied femura show a higher porosity of cortical bone as controls. The loss of cortical bone is up to 40% after 55 months. There is a correlation between new load situation and the localisation of bone loss.     

The rationale of cementless revision of cemented arthroplasty failures

Patients with failed cemented total hip replacements develop minor to massive bone loss. In nearly all cases, the quality of the endosteal surface has been converted to a sclerotic tube. Because of the bone loss and the change in character of the remaining bone stock with which to form an interface with polymethylmethacrylate (PMMA), the microinterlock necessary for long-term interface stability cannot be achieved. Moreover, the first failure leads to the production of cement particles that incite a histiocytic osteolytic response. The difficulty in achieving interfacial stability in the revision setting is borne out in the published clinical results of cemented revision total hip arthroplasties showing failure rates ranging from 17% to 60.3%. Porous-surfaced prostheses have been introduced to permit biologic fixation as an alternative to cement fixation of each implant component. The good midterm clinical results and the lack of catastrophic bone loss when failure does occur have given impetus to the application of this technology to the revision situation. The preliminary clinical experience with cementless revision arthroplasty has yielded favorable results. The authors have fully developed the rationale against the use of PMMA in revision total hip replacements and for the cementless approach with porous-surfaced metal prostheses and bone grafts. The latter appear as a viable approach to a growing clinical problem.     

Osseous integration of calcium phosphate in osteoporotic vertebral fractures after kyphoplasty: initial results from a clinical and experimental pilot study

Introduction This study evaluated the radiological changes at the bone–cement interface of calcium phosphate cement (CPC) and polymethylmethacrylate (PMMA) 12 months after kyphoplasty. In a pilot experiment, we additionally performed a histomorphometric analysis in osteopenic foxhounds to analyze the process of osseous integration of CPC and PMMA.Methods Twenty postmenopausal female patients with 46 vertebral compression fractures (VCF) were treated by kyphoplasty, utilizing CPC (N=28) or PMMA (N=18) for intravertebral stabilization. After a 12-month follow-up, we measured the density changes of border voxels at the bone–cement interface by computed tomography (CT) using dedicated software algorithms. We defined the border-voxel density (BVD) as a parameter of cement resorption at the interface. We also investigated the bone–implant interface in three osteopenic foxhounds by histomorphometry 3, 6, and 12 months after cement implantation.Results Twelve months after kyphoplasty, only CPC showed a significant decrease of the BVD compared to PMMA (p<0.01), indicating a slow progress of resorption at the interface. Histomorphometry of the dog vertebrae showed near total bone coverage of CPC implants, whereas the PMMA surface exhibited only 30% direct bone contact (p<0.01). We also observed a time-dependent increase in the number of discernable osteons close to the interface of CPC, but no bone tissue within PMMA (p<0.01).Conclusions The decrease of the BVD 12 months after kyphoplasty may indicate osseous integration of CPC by: (1) the ingrowth of bone tissue and (2) osteonal penetration close to the interface.     

The significance of wear and material fatigue in loosening of hip prostheses

Particles created by wear and disintegration of implant materials give rise to foreign body reactions in the tissue surrounding joint endoprostheses. Histiocytes and foreign body giant cells phagocytize the particles released and form granulomas, which lead in turn to remodelling and resorption of the bone at the interface between implant and bone. As a consequence of this, osteolysis develops, which may lead to loosening and complete failure of fixation of the implant. Radiographically, the areas of osteolysis appear as localized, round, oval or oblong scalloping defects or as radiolucent lines in the endosteal sections of the bone immediately adjacent to the implants. This paper reports on 21 hip joint endoprostheses in which polyethylene and bone cement particles induced large areas of osteolysis at the bone/cement interface. In 8 cases the polyethylene particles originated from the convex joint surfaces of ball heads in "soft-top" endoprostheses (with or without simultaneous replacement of the acetabulum by a metal cup), and in 5 cases they originated from the anchoring surfaces of non-cemented cone-shaped screw-in sockets (Endler type); osteolysis and loosening of these endoprostheses appeared on average 48.2 and 76.6 months after implantation, respectively. The bone cement fragments came from the bone cement mantle of the femur, which had become fractured, disrupted and crushed, in 8 cases of total hip replacement with cemented prostheses; osteolysis appeared on average 87 months after primary implantation in these cases. Tissue samples taken at revision surgery from the joint capsule, the bone/cement interface and the osteolytic areas were processed into histological sections for microscopy and examined in the usual way. The type and amount of phagocytized material were subjected to semiquantitative analysis. We were able to show that osteolysis at the bone/cement interface can be induced by foreign body reactions to abraded polyethylene particles alone as well as by reactions to fragmented bone cement. The morphology of the tissue reaction to particles of the different materials is quite similar. The effect of the foreign body granulomas depends less on the type of the polymer than on the amount of abrasion and fragmentation products released into the surrounding tissue. This again proves that the life-time of joint endoprostheses depends essentially on factors influencing the wear rate. Polymer materials, with low wear resistance, are unsuitable for convex joint surfaces and for direct fixation to bone.(ABSTRACT TRUNCATED AT 400 WORDS)     

Morphometric examination of straight, tapered titanium stems: a retrieval study.

Cementless straight, tapered rectangular cross section titanium stems have been used by the senior author since 1979. Thirty-four stems retrieved postmortem, after between 10 days and 15.2 years in situ, were studied morphometrically and histologically. Nineteen stems were first generation (1979-1986), with an incomplete sagittal taper and a mean surface roughness of 1 microm (Ra 1.23 +/- 0.3 microm); 15 stems were second generation after 1986, with a full biplane taper and a mean surface roughness of 4 microm (Ra 4.14 +/- 0.36 microm). Implant surface bony coverage was determined morphometrically in 10 segments of the stems, and expressed as a bone implant contact index. Histologically, there were no differences between implants with different levels of roughness. Morphometrically, the first-generation stems showed significant differences in coverage (distal > proximal); second-generation stems had a more uniform pattern. Stems retrieved early after arthroplasty had a mean bone implant contact index of 10%. The mean bone implant contact index showed attainment of maximum coverage by 5 years after arthroplasty, without additional apposition or loss thereafter. Patients younger than 65 years at arthroplasty had similar bone implant contact indices to patients 65 years or older; coverage in the six patients 80 years of age at retrieval did not differ from the rates in the other patients. Morphometry was able to provide objective evidence of design change effects. No differences in coverage were found in terms of times in situ, patient age at arthroplasty and at retrieval, and degree of stem surface roughness.     

Stimulation of implant fixation by parathyroid hormone (1–34)-A histomorphometric comparison of PMMA cement and stainless steel

Whereas continuous exposure to PTH results in bone resorption, PTH administration at intermittent doses results in bone formation by increasing osteoblast number and activity. PTH leads to faster fracture repair and better fixation of orthopaedic implants in animal models. The present study evaluates if PTH is able to increase the contact surface between bone and implant and whether the effect of PTH is dependent on implant material characteristics. The implants were made as rods, either of stainless steel or Palacos R bone-cement. The steel rods had a surface roughness of R(a) 0.1 microm and the cement rods R(a) 2.2 microm. In 40 adult male rats, one cement rod was inserted in the left tibia and one steel rod in the right tibia. After implantation, the rats were divided into groups by random. One group was injected three times a week with human PTH (1-34) at a dose of 60 microg/kg BW/injection. The second group was injected with the vehicle only. Both groups were then divided into groups for 2 and 4 weeks time till tibial harvest. The tibial segments around the hole of the rods were then prepared by standard histological techniques. The linear tissue surfaces, that had been in contact with the surface of the rod, were analyzed in a blind fashion. PTH increased the bone contact fraction compared with the vehicle in the steel group from 7.4 (SD 7.6) to 21.1 (SD 10.7) % after 2 weeks and from 9.8 (SD 8.1) to 47.1 (SD 13.3) % after 4 weeks. In the cement group PTH increased the contact index again compared with the vehicle from 7.8 (SD 10.2) to 53.6 (SD 26.3) % already after 2 weeks and from 14.3 (SD 15) to 65.6 (SD 15.7) % after 4 weeks. The bone trabeculae adjacent to the implant had become fewer and thicker after the treatment with PTH (1-34), with an increase of the bone mass in the area next to the bone-implant-interface. The earlier onset of PTH effects in the rougher cement group suggests that intermittent PTH treatment might lead to an increased micro-interlock between implant and bone, and might therefore be considered as a possible drug to enhance incorporation of orthopedic implants.     

Cadaveric study validating in vitro monitoring techniques to measure the failure mechanism of glenoid implants against clinical CT

Definite glenoid implant loosening is identifiable on radiographs, however, identifying early loosening still eludes clinicians. Methods to monitor glenoid loosening in vitro have not been validated to clinical imaging. This study investigates the correlation between in vitro measures and CT images. Ten cadaveric scapulae were implanted with a pegged glenoid implant and fatigue tested to failure. Each scapulae were cyclically loaded superiorly and CT scanned every 20,000 cycles until failure to monitor progressive radiolucent lines. Superior and inferior rim displacements were also measured. A finite element (FE) model of one scapula was used to analyze the interfacial stresses at the implant/cement and cement/bone interfaces. All ten implants failed inferiorly at the implant‐cement interface, two also failed at the cement‐bone interface inferiorly, and three showed superior failure. Failure occurred at of 80,966 ± 53,729 (mean ± SD) cycles. CT scans confirmed failure of the fixation, and in most cases, was observed either before or with visual failure. Significant correlations were found between inferior rim displacement, vertical head displacement and failure of the glenoid implant. The FE model showed peak tensile stresses inferiorly and high compressive stresses superiorly, corroborating experimental findings. In vitro monitoring methods correlated to failure progression in clinical CT images possibly indicating its capacity to detect loosening earlier for earlier clinical intervention if needed. Its use in detecting failure non‐destructively for implant development and testing is also valuable. The study highlights failure at the implant‐cement interface and early signs of failure are identifiable in CT images. © 2018 The Authors. Journal of Orthopaedic Research^® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:2524–2532, 2018.     

Long-term follow-up study of bioactive bone cement for repairing a segmental defect in a canine femur

We report on a 7-year long-term follow-up study of a bioactive bone cement (BA cement) that was used to repair a segmental defect in a canine femur. Bilateral femoral segmental defects were repaired with metallic implants that were fixed to the femur using two kinds of bone cement. The BA cement used in this study consists of an apatite- and wollastonite-containing glass ceramic (AW-GC) with a bis-phenol-alpha-glycidyl methacrylate (bis-GMA)-based resin. The bone-cement interface was examined histologically. Previous short-term studies have shown that using BA cement for segmental replacement of the canine femur produced excellent biomechanical and histological results. The BA cement maintained the fixation of a metallic implant to the femur very well. In contrast, the PMMA cement did not maintain alignment under long-term weight-bearing conditions. The results of histological examinations showed direct bonding between the BA cement and bone, while an intervening soft tissue layer was observed at the bone-cement interface with the PMMA cement. The BA cement bonded to the bone through a Ca-P-rich reactive layer, which was twice as thick after 7 years than it was at 26 weeks. No adverse effects of BA cement were observed during the 7-year observation period.     

Mechanical characteristics of the bone-graft-cement interface after impaction allografting.

Impaction allografting is an attractive procedure for the treatment of failed total hip replacements. The graft-cement-host bone interface after impaction allografting has not been characterized, although it is a potential site of subsidence for this type of revision total hip reconstruction. In six human cadaveric femurs, the cancellous bone was removed proximally and local diaphyseal lytic defects were simulated. After the impaction grafting procedure, the specimens were sectioned in 6 mm transverse sections and push-out tests were performed. From the adjacent sections the percentage cement contact of the PMMA cement with the endosteal bone surface was determined. The host bone interface mechanical properties varied significantly along the femur largely due to different interface morphologies. The apparent host bone interface shear strength was highest around the lesser trochanter and lowest around the tip of the stem. A significant positive correlation was found between the percentage cement contact and the apparent host bone interface shear strength (r2 = 0.52). The sections failed in 69% of the cases through a pure host bone interface failure without cement or allograft failure, 19% failed with local cement failure, and 12% with a local allograft failure. The apparent host bone interface strength was on average 89% lower than values reported for primary total hip replacements and were similar to cemented revisions proximally and lower distally. This study showed that cement penetration to the endosteal surface enhanced the host bone-graft interface.     

Construct damage and loosening around glenoid implants: A longitudinal micro‐CT study of five cadaver specimens

The evolution of failure of bone and cement leading to loosening of glenoid components following shoulder arthroplasty is not well understood. The purpose of this study was to identify and visualize potential mechanisms of mechanical failure within cadavers, cemented with two types of components, and subject to cyclic loading. Five glenoid cadaver bones were implanted with either a three‐pegged polyethylene component, or prototype posteriorly augmented component which addresses posterior bone loss. Specimens were loaded by constant glenohumeral compression combined with cyclic anterior–posterior displacement of the humeral head relative to the glenoid. At six time points across 100,000 cycles, implant loosening micromotions were optically measured, and specimens were imaged by micro‐computed tomography. Scans were 3D registered and inspected for crack initiation and progression, and micro‐CT based time‐lapse movies were created. Cement cracking initiated at stress concentrations and progressed with additional cyclic loading. Failure planes within trabecular bone and the bone–cement interface were identified in four of the five specimens. Implant subsidence increased to greater than 1.0 mm in two specimens. Cemented glenoid structural failure can occur within the cement, along planes of trabecular bone, or at the bone cement interface. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1053–1060, 2016.     

Osteolysis in alloarthroplasty of the hip. The role of bone cement fragmentation

Movement at the interface between bone and cement and fractures of the cement can cause fragmentation of the polymethylmethacrylate (PMMA) bone cement implant. In order to obtain further information about the effect of PMMA fragments on the surrounding tissue and the role of such particles in the development of bone resorption, the authors investigated 17 patients with cemented total hip endoprostheses showing osteolysis and implant loosening in the femoral shaft with (Group B) and without (Group A) involvement of the acetabulum. The roentgenographic follow-up examinations revealed an initially slow and later more rapid extension of the endosteal bone erosions, with a predilection for the tip of the stem, the lesser trochanter, and laterally for the middle of the stem. At revision surgery, tissue samples were taken from the joint capsule and the bone-cement interface, in particular from the osteolysis in the femoral shaft and the acetabulum. The tissue samples were processed for histology, microscopically examined, and semiquantitatively evaluated. The retrieved devices were also carefully inspected. Large foreign-body granulomas were found at the bone-cement interface and in the joint capsule. Histiocytes and foreign-body giant cells stored particles of PMMA and polyethylene, of which fragmented bone cement predominated. Granulomatous tissue invaded bone canals and marrow spaces and induced resorption of the surrounding bone. In four cases in Group A, tissue from the osteolysis contained only fragmented bone cement, demonstrating that PMMA particles alone may be responsible for triggering focal bone resorption. Osteolysis seems to begin at the site where disintegration of bone cement begins. In cases in which polyethylene particles were found in the tissue in addition to fragmented bone cement, wear from the ultrahigh molecular weight polyethylene socket has been increased by entrapment of PMMA particles between the joint surfaces. Thus, fragmentation of bone cement and abrasion of polyethylene enhance each other. Bone cement particles promote polyethylene wear, which in turn promotes granuloma formation, bone resorption, and subsequent bone cement disintegration.     

Effect of cementing technique and cement type on thermal necrosis in hip resurfacing arthroplasty--a numerical study

Femoral fractures within resurfacing implants have been associated with bone necrosis, possibly resulting from heat generated by cement polymerization. The amount of heat generated depends on cement mantle volume and type of cement. Using finite element analysis, the effect of cement type and volume on thermal necrosis was analyzed. Based on CT-data of earlier implantations, two different models were created: a thick mantle model, representing a low-viscosity "cement filling" technique, and a thin mantle model, representing a high viscosity "cement packing" technique. Six cement types were analyzed. The polymerization heat generation and its effect on bone necrosis were predicted. In the thin cement mantle models, no thermal necrosis was predicted. Thick cement mantle models produced thermal necrosis at the cement-bone interface depending on cement type. In the worst case, 6% of the bone at the cement-bone interface became necrotic, covering almost the entire cross-sectional area. The current findings suggest a potential thermal drawback of thick cement mantles, although it is unclear whether thermal bone necrosis significantly affects implant fixation or increases the fracture risk. Furthermore, our study showed distinct differences between the heat generated and resulting thermal damage caused by the various cement types.     

Micromechanics of postmortem‐retrieved cement–bone interfaces

The cement–bone interface plays an important role in load transfer between cemented implant systems and adjacent bone, but little is known about the micromechanical behavior of this interface following in vivo service. Small samples of postmortem‐retrieved cement–bone specimens from cemented total hip replacements were prepared and mechanically loaded to determine the response to tensile and compressive loading. The morphology of the cement–bone interface was quantified using a CT‐based stereology approach. Laboratory‐prepared specimens were used to represent immediate postoperative conditions for comparison. The stiffness and strength of the cement–bone interface from postmortem retrievals was much lower than that measured from laboratory‐prepared specimens. The cement–bone interfaces from postmortem retrievals were very compliant (under tension and compression) and had a very low tensile strength (0.21 ± 0.32 MPa). A linear regression model, including interface contact fraction and intersection fraction between cement and bone, could explain 71% (p < 0.0001) of the variability in experimental response. Bony remodeling following an arthroplasty procedure may contribute to reduced contact between cement and bone, resulting in weaker, more compliant interfaces. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:170–177, 2010