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Histologic Analysis of a Retrieved Microporous-coated Femoral Prosthesis A Seven-Year Case Report
Abstract
A microporous-coated femoral prosthesis with an average pore size of 80 to 100 microns was retrieved after seven years in a revision operation on a failed cemented acetabular component. The proximal anterior and lateral aspects of the implant were encrusted with bone. The bone was attached to the porous coating in regions of ingrowth. The implant was stable within the femur although most of the porous coating was not bone ingrown. There was progressive bone resorption in the proximal femur, suggestive of a reactive biologic mechanism. There was also a sizable posterior pocket of granulation tissue containing chronic inflammatory round cells associated with numerous birefringent polymeric particles. Polyethylene wear debris and acrylic cement were engulfed by multinucleated giant cells enveloping the loose cup.
... As such, there have been a large number of clinical studies aiming to investigate bone remodelling patterns around the implant. These studies have used various imaging modalities such as DEXA or quantitative computed tomography (QCT) (1,2). However, bone density alone cannot be used as a major indicator for bone quality (3). ...
... Once segmentation was completed, QCT image stack of each follow-up is aligned in 3D according to the image position information recorded in the respective header file. Bone density value at each point of the QCT image is extracted according to the Hounsfield equation as shown in Eq. [1]. ...
... Hounsfield value pixel value* rescale slope rescale intercept = + [1] Rescale slope and rescale intercepts are designed parameters, obtained from CT machine. Then, Hounsfield values are converted to bone density values in mg * mL −1 by using the calibration Eq. [2] given below (12). ...
Background:
Computational models in the form of finite element analysis technique that incorporates bone remodeling theories along with DEXA scans has been extensively used in predicting bone remodeling patterns around the implant. However, majority of such studies used generic models. Therefore, the aim of this study is to develop patient-specific finite element models of total hip replacement patients using their quantitative computed tomography (QCT) scans and accurately analyse bone remodelling patterns after total hip arthroplasty (THA).
Methods:
Patient-specific finite element models have been generated using the patients' QCT scans from a previous clinical follow-up study. The femur was divided into five regions in proximal-distal direction and then further divided into four quadrants for detailed analysis of bone remodeling patterns. Two types of analysis were performed-inter-patient and intra patient to compare them and then the resulting bone remodeling patterns were quantitatively analyzed.
Results:
Our results show that cortical bone density decrease is higher in diaphyseal region over time and the cancellous bone density decreases significantly in metaphyseal region over time. In metaphyseal region, posterior-medial (P-M) quadrant showed high bone loss while diaphyseal regions show high bone loss in anterior-lateral (A-L) quadrant.
Conclusions:
Our study demonstrated that combining QCT with 3D patient-specific models has the ability of monitoring bone density change patterns after THA in much finer details. Future studies include using these findings for the development of a bone remodelling algorithm capable of predicting surgical outcomes for THA patients.
... 108 The porous coating itself can promote biological fixation between the implant and the bone, but many clinical retrieval studies of porous implants have revealed fibrous fixation rather than bone ingrowth. 109,110 Some strict operative techniques (such as the use of autogeneous-graft bone chips at the time of operation) and implant design could promote consistent bone growth into a porous-coated implant. However, a layer of fibrous tissue still could be seen in the regions where the gaps were not filled with autogeneous-graft bone chips. ...
... HA-Coated vs. Porous Fixation Before the adoption of HA coatings, the femoral stem with a porous surface was once the most preferred in cementless fixation, with the aim being to increase bone growth into the pore structure of the implant. However, some clinical retrieval studies have exhibited fibrous, rather than bony, ingrowth into porous surfaces, 109 so the addition of HA may improve osseointegration. Animal models have supported the belief that, unlike uncoated porous titanium implants, HA-coated ones may limit the extent of the fibrous membrane formed and can even overcome a 1-mm gap between the implant and bone. ...
The clinical use of plasma-sprayed hydroxyapatite (HA) coatings on metal implants has aroused as many controversies as interests over the last decade. Although faster and stronger fixation and more bone growth have been revealed, the performance of HA-coated implants has been doubted. This article will initially address the fundamentals of the material selection, design, and processing of the HA coating and show how the coating microstructure and properties can be a good predictor of the expected behavior in the body. Further discussion will clarify the major concerns with the clinical use of HA coatings and introduce a comprehensive review concerning the outcomes experienced with respect to clinical practice over the past 5 years. A reflection on the results indicates that HA coatings can promote earlier and stronger fixation but exhibit a durability that can be related to the coating quality. Specific relationships between coating quality and clinical performance are being established as characterization methods disclose more information about the coating. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 58: 570–592, 2001
... Several different types and designs of non-cemented prostheses are available but so far there is no evidence suggesting that non-cemented joint replacements perform better than cemented ones. A significant percentage of patients with non-cemented hip replacement have persistent thigh pain and limbing (106) and retrieval studies of non-cemented metal porous coated hip and knee prostheses have revealed that many of the components had become fixed to the skeleton by fibrous tissue ingrowth instead of bony ingrowth (44,80,81,86,107). For these reasons great efforts have concentrated upon enhancement of bony ingrowth into the prosthetic surface. ...
... Numerous clinical studies have investigated bone remodelling patterns around the femoral component in total hip arthroplasty (THA) [1][2][3][4]. These studies have used various imaging modalities such as radiographs, DEXA or quantitative CT (qCT) scans. ...
We used quantitative CT in conjunction with finite element analysis to provide a new tool for assessment of bone quality after total hip arthroplasty in vivo. The hypothesis of this prospective five-year study is that the combination of the two modalities allows 3D patient-specific imaging of cortical and cancellous bone changes and stress shielding.
We tested quantitative CT in conjunction with finite elements on a cohort of 29 patients (31 hips) who have been scanned postoperatively and at one year, two years and five years follow-up. The method uses cubic Hermite finite element interpolation for efficient mesh generation directly from qCT datasets. The element Gauss points that are used for the geometric interpolation functions are also used for interpolation of osteodensitometry data.
The study showed changes of bone density suggestive of proximal femur diaphysis load transfer with osteointegration and moderate metaphyseal stress shielding. Our model revealed that cortical bone initially became porous in the greater trochanter, but this phenomenon progressed to the cortex of the lesser trochanter and the posterior aspect of the metaphysis. The diaphyseal area did not experience major change in bone density for either cortical or cancellous bone.
The combination of quantitative CT with finite element analysis allows visualization of changes to bone density and architecture. It also provides correlation of bone density/architectural changes with stress patterns enabling the study of the effects of stress shielding on bone remodelling in vivo. This technology can be useful in predicting bone remodeling and the quality of implant fixation using prostheses with different design and/or biomaterials.
... Secondly, if osseointegration is slow, too much fibrous tissue forms. This leads to a fixation of the prosthesis to fibrous tissue rather than to bone, which in turn leads to early loosening through continued micromotion of the prosthesis (Bobyn et al., 1987; Engh et al., 1987; Søballe et al., 1993; Hauptfleish et al., 2006). ...
Micro-textured biomaterials might enhance cytocompatibility of silicon-based micro-electro-mechanical system (bio-MEMS) dummies. Photolithography-physical vapour deposition was used to produce diamond-like carbon (DLC) or Ti squares and circles on silicon, and also their inverse replicas; then DLC and Ti were compared for their guiding potential, using a SaOS-2 cell model. Scanning electron microscopy at 48 hours indicated cells were well-spread on large-sized patterns (several cells on one pattern) and assumed the geometrical architecture of underlying features. Medium-sized patterns (slightly smaller than solitary indicator cells) were inhabited by singular cells, which stretched from one island to another, assuming longitudinal or branching morphologies. On small-sized patterns (much smaller than individual cells;rpar; cells covered large micro-textured areas, but cellular filopodia bypassed the bare silicon. Immunofluorescence and confocal laser scanning microscopy indicated that the actin cytoskeleton and vinculin-containing adhesion junctions were present on the patterned areas, but not on the bare silicon. Cell density/coverage disclosed a 3.4-3.7-fold preference for the biomaterial patterns over silicon substrate (p 0.001). Differences in the cellular response between materials were lost at 120 hours when cells were confluent. The working hypothesis was proven; enhancement by micro-patterning depends on the pattern size, shape and material and can be used to improve biocompatibility during the initial integration phase of the device.
... Several different types and designs of non-cemented prostheses are available but so far there is no evidence suggesting that non-cemented joint replacements perform better than cemented ones. A significant percentage of patients with non-cemented hip replacement have persistent thigh pain and limbing (106) and retrieval studies of non-cemented metal porous coated hip and knee prostheses have revealed that many of the components had become fixed to the skeleton by fibrous tissue ingrowth instead of bony ingrowth (44,80,81,86,107). For these reasons great efforts have concentrated upon enhancement of bony ingrowth into the prosthetic surface. ...
The success of bone ingrowth into porous coated implants depends on several factors which can be separated into five main groups: implant related factors, such as design of implant, surface structure and pore characteristics. status of host bone bed, such as underlying disease (rheumatoid arthritis, osteoporosis), available bone stock, use of drugs and surgical technique. mechanical stabilization and loading conditions applied on the implant. adjuvant therapies such as bone grafting and HA coating which might enhance the amount of bone ingrowth. remodeling of periprosthetic bone. Once bone ingrowth has occurred, maintenance of bony anchorage depends on bone remodeling at the interface. The present series of studies were performed in order to investigate the effect of some of these factors on bone ingrowth in relation to hydroxyapatite (HA) and titanium alloy (Ti) coating when subjected to pathological and mechanical conditions mimicking the clinical situation. HA- and Ti-coated implants were inserted into the femoral condyles of mature dogs. The observation period ranged from 4 to 16 weeks, and the results were evaluated by mechanical push-out testing, histomorphometric analysis, polarized light microscopy, UV fluorescence microscopy, collagen analysis and transmission electron microscopy (microanalysis). There were no complications related to the operative procedures and all dogs were terminated according to the original time schedule. Host bone related factors were studied in the initial experiments. First, the effect of a gap between bone and implant was studied and compared with press-fit insertion. The HA-coating yielded superior effect on bone ingrowth compared to Ti in situations where the implant was surrounded by a gap and also where the implants were inserted in press-fit. Gaps of 1 mm and 2 mm around the implant were bridged by bone around HA implants whereas significantly less amounts of bone filled the gap around Ti implants. The gap-healing capacity of bone was increased even at a relatively great distance (400 microns) from the HA surface. This finding indicates that the osteoconductive effect of HA is not limited to the bone forming capacity on the surface of the implant. A positive gradient of newly formed bone was found towards the HA-coating, this gradient not being found towards the Ti-coating. In order to investigate the significance of arthritic bone changes (osteopenia) on fixation of porous coated implants we adopted the Carragheenin-induced gonarthritis model resulting in substantial bone loss as determined by CT-scanning.(ABSTRACT TRUNCATED AT 400 WORDS)
A total of 102 uncemented Link Rippen System (RS) total hip prostheses were implanted in 100 patients whose mean age was 57 years. The Merle d'Aubigné hip score was 9.3 preoperatively and 15.3 at the latest follow-up. The thigh was painful in 56 per cent; 5 per cent had moderate or severe pain. Forty-five per cent of patients had a slight limp and 9 per cent a severe limp at the most recent follow-up. Fifteen per cent still needed a cane.
A Kaplan-Meier survivorship analysis revealed a 74 per cent probability of survival at 6 years.
Serial radiographic evaluations revealed that 16 per cent of patients (non-revised hips) had acetabular component migration, 10 per cent had continuous femoral radiolucen-cies, 14 per cent had subsidence and 31 per cent progressive varus migration.
### History of the Use of Hydroxyapatite in Orthopaedics
The term apatite was first applied to minerals by Werner45, in 1788. It now denotes a family of crystals with the formula M10(RO4)6X2, where M is usually calcium, R is usually phosphorus, and X is hydroxide or a halogen such as fluorine. The relationship to bone mineral was first suggested by Proust and Klaproth45, also in 1788. Only after the development and use of x-ray diffraction did Dejong confirm, in 1926, that the inorganic phase of bone was an apatite49. Bone mineral was found to be quite complex and included various types of hydrated calcium phosphates, the most common being calcium hydroxyapatite (Ca10[PO4]6[OH]2).
To the best of our knowledge, the earliest use of calcium-phosphate materials in humans was as a powder of varying crystalline composition to improve bone-healing. Albee and Morrison, in 1920, reported accelerated formation of callus3, but others later observed no major advantage with use of the hydroxyapatite powder80,147. As a bulk implant, calcium-phosphate materials were first used for dental applications133, as reported in 1971. More recent reports in the dental literature have attested to the success of bulk calcium-phosphate materials composed of pure hydroxyapatite and used as a bone-graft substitute12,35,57,68,135,137. Patients who had hydroxyapatite grafts were followed for a maximum of seven years57 and were evaluated clinically12,35,57,137, radiographically12,35,57, and with computer-assisted densitometry68. In two studies135,137, biopsy specimens were obtained for histological analysis. In addition to impressive evidence of osseointegration, no adverse effects of hydroxyapatite …
Young active patients will usually outlive the fixation of a total hip or knee arthroplasty. There is widespread concern about the considerable risk of failure in these patients, especially because of the subsequent need for even more difficult operative treatment with fewer beneficial results (1–4). As a result, there is an increased interest in other principles for implant fixation, such as biological fixation to the bone without the use of bone cement (5).
The use of hydroxyapatite-coated prosthesis for total hip arthroplasty began in 1985 and has gained favor with reports of prompt biological fixation and clinical success. The quality and quantity of the coating is sensitive to application techniques, metal substrate, and manufacturing controls—all important in creating a clinically successful coating. Bioactive coatings other than hydroxyapatite have had mixed success and are generally not available for clinical use at this time. Clinical experience with hydroxyapatite-coated hip implants have focused on the femoral component, where results approach the standard of cemented components. Acetabular components have been less impressive when hydroxyapatite is the primary source of fixation without macrostructured substrate or augmentation with screws. Analysis of sequential follow-up radiographs in patients with hydroxyapatite-coated hips consistently reveals the changes associated with prompt and tenacious biologic fixation of the femoral component. These changes are apparent at 6 months and persist beyond 8 years. While the optimal design of the components, the surface structure of the metal substrate, and the ideal application method are still evolving, the use of hydroxyapatite-coated prostheses for hip arthroplasty has been proven safe and successful for clinical use where biologically fixed components are indicated.
Studies of biologic fixation have traditionally ignored or poorly documented the effect of osteoporosis on the quality and quantity of bone formed around orthopaedic implants. In the clinical setting, most orthopaedic implants are placed in aged, osteoporotic skeletons. It is a paradox that most studies of biologic fixation traditionally have used young, non-osteoporotic animals to demonstrate the osseointegration of an implant. The argument may be raised that biologic fixation has been reserved for young individuals, therefore justifying testing in young, non-osteoporotic bone. However, the clinical indications for biologic fixation have expanded to include old as well as young candidates for cementless implants. Despite the broadening indications to include old, osteoporotic candidates, the effect of osteoporosis on the quality and quantity of biologic fixation of an implant has, with few exceptions, been ignored. Furthermore, studies of adjunctive coatings have been conducted with little concern for the confounding effect of bone characteristics on the quality and quantity of bone formed around the implant. The purpose of this chapter is to review the limited information available on the impact of osteoporosis on implant fixation and speculate on the role further study may play on refining this topic in the future.
The goal of osseointegration of orthopedic and dental implants is the rapid achievement of a mechanically stable and long lasting fixation between living bone and the implant surface. In total joint replacements of cementless designs, coatings of calcium phosphates were introduced as a means of improving the fixation of implants. Of these, hydroxyapatite (HA) is the most widely used and most extensively investigated. HA is highly osseoconductive, and the positive effect is well documented in both basic and long-term clinical research [1-6]. This chapter describes experimental and clinical studies evaluating bone-implant fixation with HA coatings.
Bone-interfacing surgical implants used in orthopedics and dentistry must bear the forces of normal patient activity with minimal risk of mechanical failure of the implant. This requires using appropriate materials and designs for implant fabrication. Additionally, reliable long-term implant attachment to host bone must be assured so that effective force transfer between implant and bone occurs for the patient's lifetime without the implant loosening. With recent advances in implant designs and techniques for their placement, effective implant fixation to bone can last for years (decades) either directly or through an acceptable intermediate fibrous tissue layer at the bone-implant interface. With approximately 500,000 artificial hips implanted annually worldwide and the demand for other joint replacements approaching the same order of magnitude, as well as the recent major growth in the use of dental implants (300,300 projected for insertion in North America alone in 1991), the assurance of effective implant-to-bone fixation is extremely important.
Studies of implant biocompatibility have resulted from concerns over the cumulative effects of foreign element release through implant corrosion and wear. Accumulation of this debris in tissues both local and remote to implant sites over the long term is a concern. Of equal importance, for load-bearing implants, are studies to determine the important factors for successful long-term implant fixation. Current trends in design and use of both dental and orthopedic implants reflect the trial-and-error approach that has characterized this field for decades.
A cell culture system for biocompatibility testing of hip implant materials is described. Human bone marrow cells have been chosen because these cells are in direct contact with the biomaterial after implantation in situ. The sensitivity of this method is evaluated for materials which are already being used as implants in humans and animal, e.g., hydroxyapatite (HA) ceramic, pure titanium, and ultra-high-molecular-weight polyethylene (UHMWPE). As indicative parameters of biocompatibility primary cell adherence, cell number, cell proliferation, production of extracellular matrix, cell vitality, and cell differentiation are described. After 2 weeks in culture, obvious differences between the biomaterials with respect to the indicative parameters could be observed. Cell numbers were greatest on the HA specimens. In the case of titanium alloys, we observed a decreased number of cells. The production of extracellular matrix was high for the HA ceramics but reduced for titanium specimens. The polymers allowed only a few adherent cells and showed no signs of extracellular matrix production. The results can be correlated astonishingly well to animal experiments and clinical experiences. Therefore, we suggest that this cell culture system seems to be a useful tool for biocompatibility testing of bone implantation materials. It also helps reduce animal experiments. With the help of flow cytophotometry, we analyzed the influence of biomaterials on large numbers of cells with respect to differentiation. There were similar populations of T cells and monocytes on all specimens tested. Extended B-cell and granulocyte populations, however, were observed with titanium and UHMWPE. Most osteocalcin-containing cells adhered to the HA ceramics.
Vergleich von biomedizinischen Werkstoffen aus Titan, Polyäthylen Keramik unter Anwendung einer humanen Knochenmarkszellkultur. Camparasion between different biomaterials.
Uncemented total hip arthroplasty has proven to be an acceptable alternative to cemented total hip arthroplasty with good short-term results. With the elimination of the use of polymethyl methacrylate for component fixation, failure at the bone-cement interface, with resultant osteolysis and progressive loosening, was thought to be preventable. Unfortunately the ultra-high-molecular-weight polyethylene acetabular insert can wear and produce particulate debris. This debris can stimulate an osteolytic reaction and lead to late aseptic loosening in a cementless total hip arthroplasty.
Ten asymmetric porous-coated tibial knee components were consecutively retrieved from nine patients with implantation times ranging from 1 week to 48 months. Morselized autograft bone chips were applied to the resected surface of the tibia prior to component implantation. Microradiographic analysis showed bone in contact with 64 +/- 10% of the porous-coated interface. Backscattered electron imaging of the bone-implant interface revealed bone within 8-22% of the porous coating. Although bone in the form of autograft bone chips was observed in the porous coating of the 1 and 3 week specimens, it was not connected to the host bone. By 3 and 6 months, the bone chips were integrated and connected to the host bone providing osseous continuity from the porous coating to the skeleton. This study shows that reproducible bone ingrowth into porous-coated tibial components is achievable using autograft bone chips to promote bone fixation.
Retrieval studies of porous-coated prostheses have demonstrated deficient bony ingrowth in high percentages. Possible reasons for this are lack of initial mechanical stability and the presence of osteopenia. The authors studied ingrowth of osteopenic bone into titanium alloy (Ti) porous-coated implants with and without hydroxyapatite (HA) coating in an experimental dog model. Unilateral osteopenia of the knee with a 20% reduced bone density as judged by computed tomography (CT) scanning (P less than .001) was induced in 12 mature dogs by weekly intraarticular injections of Carragheenin into the right knee for 12 weeks, with the left knee serving as control. Ti porous-coated cylinders were inserted in press-fit bilaterally in the lateral femoral condyles in six dogs. HA-coated titanium plugs were implanted similarly in another sex-, age-, and weight-matched group of six dogs. Bony ingrowth after 4 weeks was significantly reduced for Ti implants in osteopenic bone compared to control bone, but HA-coated implants were covered by equal amounts of bone tissue. Bone-implant shear strength of Ti implants also was reduced in osteopenic bone compared to control bone. In control bone, the anchorage of Ti implants was stronger than HA-coated implants, whereas the fixation of Ti and HA-coated implants was equal in the osteopenic bone. The results demonstrate that the bony fixation of Ti porous-coated implants is weakened by the presence of experimentally induced osteopenia. However, the fixation of HA-coated implants was not affected by the osteopenic condition in the surrounding bone. The fixation of Ti and HA-coated implants was equal in osteopenic bone, whereas the fixation of Ti porous-coated implants was superior to that of HA-coated implants in control bone.
A noncemented and clinically stable porous-coated patellar component (PCA) was removed from a patient after 11 months because of infection. It was sectioned and examined histologically in undecalcified, thin-ground sections. The bone ingrowth into the porous space was measured at eight levels. Each histologic section was quantified by a conventional point-counting method using a square grid. There was inhomogeneous, but extensive, bone ingrowth, often extending to the core of the patellar component, with direct contact between bone and porous coating without any interstitial fibrous membrane.
In uncemented total hip arthroplasty, a complete filling of the gap between femoral prosthesis and the host bone is difficult and defects would remain, because the anatomy of the reamed intramedullary canal cannot fit the prosthesis. Therefore, it seems practical to fill the gap with a clay containing hydroxyapatite (HA), which has an osteoconductive character. The clay (HA clay) is made by mixing HA granules (size 0.1 mm or more) having a homogeneous pore distribution and a porosity of 35-48 vol%, and a viscous substance such as a saline solution of sodium alginate (SSSA). In the first experiment, the ratio of HA granules and sodium alginate in SSSA is set for the same handling properties of HA clay and polymethylmethacrylate bone cement (standard viscosity) before hardening. As a result, the ratio is set for 55 wt% of HA in the clay and 12.5 wt% of sodium alginate in SSSA (i.e., HA:sodium alginate:saline solution = 9.8:1:7). In the second study, the gap between the femoral stem and bone model is completely filled with HA clay. However, the gap is not filled only with HA granules or HA granules mixed with saline solution. In the third animal experiment, using an unloaded model, histology shows that HA clay has an osteoconductive property bridging the gap between the implant and the cortical bone without any adverse reaction. HA clay is considered a useful biomaterial to fill the gap with adequate bone ingrowth.
A clay containing hydroxyapatite (HA) was developed for use as a filling material between an uncemented implant and bone. It consists of 55% HA granules greater than 0.1 mm in size with a homogeneous pore distribution and a porosity of 35% to 48% in a saline solution of sodium alginate (6%). Ti-6A1-4V alloy rods with smooth surfaces were implanted into the distal medullary canal of one osteotomised tibia of 32 Japanese white rabbits. Sixteen rods were inserted with HA clay and 16 without the clay to act as a control group. Six of each group were killed at one week and ten at 12 weeks postoperatively. The pull-out strength of the implants with HA clay was significantly greater at 12 weeks (p < 0.05), as was the percentage of the area of the new bone (p < 0.05). The study suggests that HA clay has an osteoconductive property, allowing adequate bone fixation across a gap at an early stage. The use of HA clay to enhance the early stability of uncemented components may help to improve the functional outcome of total joint arthroplasty.
Porous-coated Ti-6A1-4V has a fatigue strength approximately one-third that of the uncoated alloy. The interfacial geometry between the porous coating and the implant substrate is notchlike, leading to stress concentrations that have been shown to be the main cause for the reduction in fatigue strength. In this study, the effect of interfacial geometry on fatigue strength of porous-coated Ti-6A1-4V is quantified. The interface between porous coating and implant is modeled using linear elastic, plane strain finite element analysis. Integrated with the numerical analysis is an experimental verification of enhanced fatigue behavior. Changes in interfacial geometry are conceived, and their effectiveness in reducing stress concentrations are determined. A doubling of fatigue strength can be achieved for newly conceived geometries over conventional porous coating geometries.
Osteointegration was defined as a "direct structural and functional connection between ordered living bone and the surface of a load-carrying implant." Although osteointegration was meant originally to describe a biologic fixation of the titanium dental implants, it is now used to describe the attachment of other materials used for dental and orthopedic applications as well. Analyses of material-bone interface showed that osteointegrated implants can have an intervening fibrous layer or direct bone apposition characterized by bone-bonding depending on the composition and surface properties of the biomaterial. This article reviews biologic (host tissue properties and response), biomechanical, and biomaterial factors affecting osteointegration. Biologic factors include the quality of bone. Biomaterial factors include the effect of material composition on the bone-material interface. Suggested areas for future research include determining the correlation between oral bone status and osteoporosis, the effect of gender, age, and endocrine status (e.g., osteoporosis) on implant success or failure, the effect of calcium phosphate coating composition and crystallinity on in vivo performance of implants, the factors contributing to accelerated osteointegration, and development of osteoinductive implants.
The significance of artificial joint pathology has become clear in view of the approximately 600 000 joint prosthesis implantations performed worldwide each year. Today the chief indications for endoprosthetic joint replacement are serious degenerative joint disease and femoral neck fracture in combination with osteoarthrosis and rheumatoid arthritis; the procedures are less liberally applied in younger patients because of their greater life expectancy and the risk of long-term complications.
In order to study bone growth conducting capacities of new biomaterials under standardized conditions, a goat model was developed based on a canine model by Soballe. Titanium alloy implants with and without a hydroxyapatite coating were used as positive and negative controls, and these were implanted with a circumferential gap of one millimeter in the spongious bone of the knee condyles of two groups of four goats. These goats were sacrificed at 6 and 25 weeks. A second experiment was done on two groups of four goats with the same type of titanium alloy and hydroxyapatite-coated implants as controls and with Polyactive 55-45 coated titanium alloy implants for testing. These goats were sacrificed at 9 and 25 weeks, respectively. Qualitative and quantitative differences in gap healing were evaluated through light microscopy, and initiation and direction of bone apposition were determined with fluorescence microscopy. Apposition of bone was seen directly on all hydroxyapatite surfaces and on some of the noncoated titanium alloy surfaces. The difference between the percentage of bone growth on the titanium alloy implants and the hydroxyapatite-coated implants appeared to be divergent in time: the bone growth on the noncoated implants declined after 9 weeks in contrast to the steady increase of bone growth on the hydroxyapatite-coated implants towards the 25 week follow-up time (p = 0.02). No significant difference was found between the first and the second experiment: apposition of bone on the implants differed only 6.6% on a scale of 0% to 100%. Only scarce bone growth was seen on the polyactive-coated implants in this model. The newly tested Polyactive 55-45 coating apparently needs initial bone contact for bone-bonding and therefore showed hardly any direct bone formation on its surface. The clear differences in the reaction of bone to the coated and noncoated implants in this goat study and the reproducibility of these reactions of bone to the different controls indicate the sensitivity of the currently used animal model and its suitability for use as a bioactivity assay.
The hypothesis in this study is that the stem stiffness-to-bone stiffness ratio influences the incidence and type of bone remodeling and fixation with cemented total hip arthroplasty. Ninety-one patients with 99 hips had cemented stems using 3 different anatomic porous replacement designs. The APR I and APR II titanium stems with proximal porous coating on the proximal one fourth of the stem were cemented into 49 and 35 patients. The APR II-C stem, which is a cobalt-chrome stem only for cemented fixation, was cemented into 15 patients. These 3 different stem designs were used to study different metals as well as different stem shapes. The average follow-up was 4.3 years (range, 2-10 years) with all hips having 2 years' follow-up and 42 hips at least 5 years' follow-up. Bone remodeling was measured as stress shielding, calcar resorption, and distal hypertrophy on anteroposterior and lateral radiographs of the hip. Stress shielding was measured by the 4 grades described by Engh. A stem stiffness-to-femoral bone stiffness ratio was calculated from the plain radiographs with the stem stiffness known from the manufacturer and the bone stiffness calculated using measurements of the outer and inner diameters of the femur. There was no statistical difference for bone remodeling and fixation between the 3 stem shapes or 2 metal types used in these hips. No stem was loose, and only 10 had radiolucent lines. Stress shielding was statistically related to stem stiffness but was more strongly related to the axial stiffness ratio, mediolateral bending stiffness ratio, anteroposterior stiffness ratio, and torsional stiffness ratio. Stress shielding grade 3 and 4 was present in 20% of hips with a torsional stiffness ratio < 0.33, in 38% of hips with a torsional stiffness ratio of 0.34 to 0.5, and in 70% of hips with a torsional stiffness ratio > 0.5. Five-year results showed no statistical change in stress shielding, calcar resorption, and distal hypertrophy from the 2-year observations. The stem stiffness-to-bone stiffness ratio influenced bone remodeling but not fixation of these cemented stems.
In a prospective randomized trial, 62 consecutive primary cementless total hip arthroplasties in 55 patients were performed by one surgeon using either hydroxyapatite coated (35 hips) or nonhydroxyapatite coated femoral prostheses (27 hips). The dual tapered femoral stem had a Ti-6AI-4V plasma sprayed circumferential proximal porous coat applied to the proximal 1/3 of the stem. The middle 1/3 had a roughened blasted textured surface, and the distal 1/3 had a smooth surface. The hydroxyapatite coated femoral stems had an additional hydroxyapatite coating applied to the proximal porous coat with use of an air plasma process. The patients in the two groups were not significantly different regarding age (48.2 +/- 9.0 years hydroxyapatite group, 50.4 +/- 8.7 years control group), gender, Charnley class, or length of followup (4.4 +/- 0.7 years hydroxyapatite group, 4.9 +/- 1.0 years control group). Forty-nine patients (54 hips) were available for clinical followup, and 45 patients (50 hips) had radiographic followup. A minimum 3-year followup was recorded. To date, there have been no femoral prostheses failures. No femoral implant has migrated or subsided. Radiographically, the hydroxyapatite coated stems showed trends toward increased distal stem related cortical hypertrophy, increased cancellous condensation and less endosteal cavitation. Two nonhydroxyapatite coated stems had distal endosteal cavitation, whereas no hydroxyapatite coated stems did. There were two cases of acetabular osteolysis (revision in one) and two cases of acetabular cup migration (nonrevised), all occurring in the control group. The overall revision rate was 4%. There was no difference in Harris hip scores at 6 months (80.6 +/- 13.0 points hydroxyapatite group, 83.8 +/- 12.4 points control group) or at last followup (85.6 +/- 15.4 points hydroxyapatite group, 89.7 +/- 13.4 control group). The Harris hip pain scores also were not significantly different at 6 months or at last followup. Multiple linear regression analysis controlling for age, gender, and length of followup revealed no significant predictors of Harris hip or pain scores. The results of this study at an average of 4.6 years do not indicate a significant clinical advantage to the use of hydroxyapatite coated femoral prostheses in primary cementless total hip arthroplasty when judged by the criteria of Harris hip scores and femoral stem survivorship.
The clinical use of plasma-sprayed hydroxyapatite (HA) coatings on metal implants has aroused as many controversies as interests over the last decade. Although faster and stronger fixation and more bone growth have been revealed, the performance of HA-coated implants has been doubted. This article will initially address the fundamentals of the material selection, design, and processing of the HA coating and show how the coating microstructure and properties can be a good predictor of the expected behavior in the body. Further discussion will clarify the major concerns with the clinical use of HA coatings and introduce a comprehensive review concerning the outcomes experienced with respect to clinical practice over the past 5 years. A reflection on the results indicates that HA coatings can promote earlier and stronger fixation but exhibit a durability that can be related to the coating quality. Specific relationships between coating quality and clinical performance are being established as characterization methods disclose more information about the coating.
A tight fixation between bone and implant materials is of great importance for a successful outcome of procedures such as total knee arthroplasty (TKA) and total hip arthroplasty (THA). Surface modification of titanium metal and titanium alloy is one of the attractive methods to improve the biological affinity of orthopedic prostheses. Recent studies reported that titanium substrates were provided with bone-bonding ability, that is, osteoconductivity, through a chemical treatment with hydrogen peroxide solution containing tantalum chloride. The present study investigated the histological and mechanical effects of such treatment of the surface of titanium fiber mesh. Titanium alloy rods of 7.6 x 7.6 x 20 mm that had a titanium fiber mesh of 250 microm were implanted bilaterally into the distal aspect of the femur of adult beagle dogs. At 3, 5, and 8 weeks after implantation, the rods were removed to examine their bonding strength and histological compatibility with bone. Bonding strength was evaluated by the pull-out test. The bonding strength of the treated specimen with bone increased with time, and was faster than that of untreated specimens. At 8 weeks postoperative, the bonding strength of both the treated and untreated specimens became almost equal. The amount of newly formed bone on and in the titanium fiber mesh was significantly increased by the chemical treatment (p<0.05). The rate of bone formation in the fiber mesh was accelerated after chemical treatment. These results indicated that the surface treatment enhanced bone formation in the initial stage thank to the osteoconductive property of the titanium fiber mesh conferred by the chemical treatment so that faster bonding strength was achieved. Surface treatment providing titanium mesh with osteoconductivity has the advantages of the fast healing and tight bonding for prostheses in TKA and THA.
Nano-apatite coating closely mimicking bone mineral was grown directly on titanium soaked in an aqueous solution containing all major inorganic components present in the body, mainly, HCO3(-), Ca(2+), HPO4(2-), and Mg(2+) ions. The removal of HCO3(-) ions from the solution in the form of CO2 resulted in the increase of solution pH. As a consequence of this reaction, the nano-apatite coating was formed on the surface of titanium with composition and structure equivalent to those of bone mineral. The biomimetic nano-apatite was demonstrated to be capable of conducting bone formation and promoting direct bone apposition. This bioactive coating also affected the behavior of human osteoblasts as indicated by their morphologies observed in cell culture study.
Biomimetic calcium phosphate (Ca-P) coatings were applied onto dense titanium alloy (Ti6Al4V) and porous tantalum (Ta) cylinders by immersion into simulated body fluid at 37 degrees C and then at 50 degrees C for 24 h. As a result, a homogeneous bone-like carbonated apatitic (BCA) coating, 30 microm thick was deposited on the entire surface of the dense and porous implants. Noncoated and BCA-coated implants were press-fit implanted in the femoral diaphysis of 14 adult female goats. Bone contact was measured after implantation for 6, 12, and 24 weeks, and investigated by histology and backscattered electron microscopy (BSEM). After 6 weeks, bone contact of the BCA-coated Ti6Al4V implants was about 50%. After 12 and 24 weeks, bone contact was lower in comparison with the 6-week implantations at, respectively 24 and 39%. Regarding the BCA-coated porous Ta implants, bone contacts were 17, 30, and 18% after 6, 12, and 24 weeks, respectively. However, bone contact was always found significantly higher for BCA-coated dense Ti6Al4V and porous Ta cylinders than the corresponding noncoated implants. The results of this study show that the BCA coating enhances the bone integration as compared to the noncoated implants.
We designed an in vivo study to determine if the superimposition of a microtexture on the surface of sintered titanium beads affected the extent of bone ingrowth. Cylindrical titanium intramedullary implants were coated with titanium beads to form a porous finish using commercial sintering techniques. A control group of implants was left in the as-sintered condition. The test group was etched in a boiling acidic solution to create an irregular surface over the entire porous coating. Six experimental dogs underwent simultaneous bilateral femoral intramedullary implantation of a control implant and an acid etched implant. At 12 weeks, the implants were harvested in situ and the femora processed for undecalcified, histological examination. Eight transverse serial sections for each implant were analysed by backscattered electron microscopy and the extent of bone ingrowth was quantified by computer-aided image analysis. The extent of bone ingrowth into the control implants was 15.8% while the extent of bone ingrowth into the etched implants was 25.3%, a difference of 60% that was statistically significant.
These results are consistent with other research that documents the positive effect of microtextured surfaces on bone formation at an implant surface. The acid etching process developed for this study represents a simple method for enhancing the potential of commonly available porous coatings for biological fixation.
Uncemented acetabular components are associated with a significant incidence of polyethylene wear and secondary osteolysis. The new tantalum/polyethylene composite (Hedrocel) acetabular component is designed to reduce the polyethylene wear and to increase the longevity of the acetabular cups. We report our short- term clinical outcome and patient satisfaction following use of an uncemented tantalum acetabular component in a single centre. During 1999 to 2002, 113 uncemented tantalum acetabular cups were implanted in 105 patients in our institution. The average age at operation was 56.8 years. All patients were assessed pre- and post-operatively with the Oxford 12 item hip questionnaire and standard radiographs of the pelvis. At a mean follow-up of 32 months (range: 18 to 48), 112 Hedrocel cups were assessed in 104 patients. Subjective patient's satisfaction was also assessed. At the time of evaluation, one patient had died due to an unrelated cause. Eight patients had bilateral acetabular cups implanted. The mean Oxford hip score improved from 45 preoperatively to 14 post-operatively. Subjectively 99% were very satisfied or satisfied. Only one patient expressed dissatisfaction about the outcome of this surgery. Radiologically, there were no signs of cup loosening or wear. This study shows that at short-term the new uncemented tantalum/polyethylene composite (Hedrocel) acetabular component can yield a satisfactory clinical and radiological outcome and has a high patient satisfaction. Although the short-term result from our centre is very encouraging, similar results from other centres and longer follow-up are required.
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