Characteristics of bone ingrowth and interface mechanics of a new porous tantalum biomaterial

Analysis of bone ingrowth on a tantalum cup

BACKGROUND

Trabecular Metal (TM) is a new highly porous material made of tantalum (Zimmer, Warsaw, Indiana, USA). Its three-dimensional structure is composed of a series of interconnected dodecahedron pores that are on average 550 microm in diameter. This size is considered optimal for bone ingrowth and is similar to trabecular bone. The elastic modulus of TM (3 GPa) is more similar to that of cancellous (0,1-1,5 GPa) or cortical (112-18 GPa) bone and is significantly less similar to that of Titanium (110 GPa) and Co-Cr alloys (220 GPa). These features enable bone apposition and remodeling. The purpose of the present study was to evaluate the histology of the bone-implant interface in a human specimen.

MATERIALS AND METHODS

A highly porous tantalum cup (Zimmer, Warsaw, Indiana, USA) was removed for recurrent dislocations three years after implantation. In order to obtain a slice of the cup, two cuts were made on the centre using an Exakt cutting machine. Then the slice was embedded in a Technovit resin and a Hematoxylin-eosin stain was used to study the bone tissue. Bone ingrowth was calculated using a method based on simple calculations of planar geometry.

RESULTS

The histological evaluation of the periprosthetic tissues revealed a typical chronic inflammation with few particles of polyethylene that were birefringent using polarized light. The quantitative evaluation of bone ingrowth revealed that more than 95% of voids were filled with bone.

DISCUSSION

In the literature, a lot of studies focused on tantalum were carried on animal model. Up to now little information is available about the histology of the bone-tantalum interface in a human artificial joint. We had an opportunity to remove a well integrated cup hence this study. The histology confirmed the strong relationship between the structure of this material and bone. The morphometric analysis revealed a high percentage of bone ingrowth.

The use of tantalum porous metal implants for Paprosky 3A and 3B defects

Complex acetabular defects are difficult to reconstruct. For severe pelvic defects, tantalum metal acetabular implants may provide a viable solution over traditional implants. This short-term follow-up reviews 43 acetabular revisions treated with tantalum acetabular implants. These revisions included 33 Paprosky type 3A defects, involving 30% to 50% of host acetabulum loss, and 10 type 3B defects, having similar or greater bone loss with a pelvic discontinuity. Tantulum acetabular modular augments were used in 26 cases to support the shell. At a mean of 2.8 years' follow-up, 42 components were stable, and 1 failed because of septic loosening. The overall success rate was 98%. One revision was performed because of loosening secondary to sepsis; none were performed for aseptic loosening. Implants made from highly porous tantalum metal provide a surface that is highly conducive to bone ingrowth. Combined with the ability to use modular augments for added support and stability, this technology may change the way major defects are reconstructed.

Mechanical properties of open-pore titanium foam

Open-pore titanium foams are produced using the so-called space holder method. The mechanical properties of titanium foams with porosities of 50-80% are studied. The stiffness and yield strength of the foams are found to encompass the property range between cancellous bone and cortical bone. The analyzed foams are found to be anisotropic due to the use of nonspherical space holder particles which rearrange during the compaction of the powder mixture. The titanium foams are stronger perpendicular to the compaction direction and weaker along the compaction axis. In view of the application as an implant material in the lumbar spine, an intermediate porosity of 60-65% is analyzed more in detail. The typical yield strength of titanium foam with 62.5% porosity is above 60 MPa in compression, bending, and tension. Stiffness values vary with the testing method from 7-14 GPa.

Direct tendon attachment and healing to porous tantalum: an experimental animal study

BACKGROUND

The ability to directly attach soft-tissue to metal would have broad clinical application. Previous attempts to obtain normal tendon-to-bone attachment strength have been unsuccessful. In the present study, we hypothesized that when the initial interface mechanical environment is carefully controlled, a highly porous form of tantalum metal would allow the ingrowth of tendon tissue with clinically relevant tendon-to-implant fixation strength approaching that of an intact tendon-to-bone insertion.

METHODS

Supraspinatus tendons from forty skeletally mature dogs were reattached to the greater tuberosity between two custom-designed porous tantalum washers. Clinical function as judged on the basis of gait analysis, reattachment fixation strength and stiffness, and tendon function as seen through muscle volume were evaluated preoperatively, immediately postoperatively, and at three, six, and twelve weeks after surgery. Qualitative and quantitative histomorphologic evaluation was performed at three, six, and twelve weeks after surgery.

RESULTS

Gait analysis with use of force-plate measurements demonstrated return to a normal gait pattern by three weeks after surgery. Tendon-implant strength as a percentage of normal, contralateral controls increased significantly, from 39% at the time of surgery to 67% at three weeks, 99% at six weeks, and 140% at twelve weeks (p < 0.0014). The stiffness of the construct also increased and approached that of normal tendon, measuring 47% at the time of surgery, 62% at three weeks, 94% at six weeks, and 130% at twelve weeks (p < 0.0299). Supraspinatus muscle volume initially decreased by 33% but recovered to 92% of normal by twelve weeks (p < 0.01). Histomorphologic evaluation showed Sharpey-like fibers inserting onto the surface of the porous tantalum. Quantitative histomorphometric analysis revealed a time-dependent increase in the density of the collagen tissue filling the metal voids below the implant surface of first the bottom washer and then the top washer.

CONCLUSIONS

Robust biologic ingrowth of tendon into a porous tantalum implant surface can be achieved under conditions of secure initial mechanical fixation. The strength and stiffness of the tendon-implant construct reached normal levels by six to twelve weeks in this animal model.

Development and characterization of a porous poly(methyl methacrylate) scaffold with controllable modulus and permeability

Functional restoration following extensive bone injury often requires bone grafting. The primary source of graft material is either autograft or allograft. The use of both material sources is well established, however both suffer limitations. In response, grafting alternatives are being investigated. This manuscript presents the development of a highly porous scaffold with controllable elastic modulus and permeability for use in tissue grafting and tissue engineering applications that is manufactured from FDA approved poly(methyl methacrylate) (PMMA). Fifteen protocol variations based on the commonly used porogen leaching technique for porous scaffold fabrication were employed to control scaffold pore size, pore interconnectivity, and structural strength. Scaffolds were tested for porosity, permeability, elastic modulus, cell culture compatibility, and fatigue tested in compression. Scaffold permeability ranged from 6.6 x 10(-16) m(2) to 1.4 x 10(-10) m(2), and elastic modulus was adjustable between 14 and 322 MPa; data similar to cancellous bone specimens from a variety of species and anatomic locations. Fatigue evaluations revealed 65% strength maintenance after 80,000 loading cycles, and in vitro culture with marrow-derived stromal cells show no cytotoxic effects based on Live/Dead assay. The scaffolds detailed herein will help broaden the spectrum of available orthopaedic tissue scaffolds for research in this evolving field. , 2007.

In vitro and in vivo characteristics of PCL scaffolds with pore size gradient fabricated by a centrifugation method

Polycaprolactone (PCL) cylindrical scaffolds with gradually increasing pore size along the longitudinal direction were fabricated by a novel centrifugation method to investigate pore size effect on cell and tissue interactions. The scaffold was fabricated by the centrifugation of a cylindrical mold containing fibril-like PCL and the following fibril bonding by heat treatment. The scaffold showed gradually increasing pore size (from approximately 88 to approximately 405 microm) and porosity (from approximately 80% to approximately 94%) along the cylindrical axis by applying the centrifugal speed, 3000 rpm. The scaffold sections were examined for their in vitro cell interactions using different kinds of cells (chondrocytes, osteoblasts, and fibroblasts) and in vivo tissue interactions using a rabbit model (skull bone defects) in terms of scaffold pore sizes. It was observed that different kinds of cells and bone tissue were shown to have different pore size ranges in the scaffold for effective cell growth and tissue regeneration. The scaffold section with 380-405 microm pore size showed better cell growth for chondrocytes and osteoblasts, while the scaffold section with 186-200 microm pore size was better for fibroblasts growth. Also the scaffold section with 290-310 microm pore size showed faster new bone formation than those of other pore sizes. The pore size gradient scaffolds fabricated by the centrifugation method can be a good tool for the systematic studies of the interactions between cells or tissues and scaffolds with different pore size.

Porous tantalum implant in early osteonecrosis of the hip: preliminary report on operative, survival, and outcomes results

Porous tantalum implants are used in early osteonecrosis of the hip. Device evaluation included surgical time, blood loss, hospitalization, patient-controlled analgesia use, transfusions, implant survival, and outcomes. Mean blood loss was 70 mL. Mean operative time was 36 minutes. Average hospitalization was less than 1 day. No patient required patient-controlled analgesia use or transfusions. Results were compared to a historical vascularized fibular graft population. All parameters were less than the fibular graft groups (P < .00001). All surviving implants (86%) resulted in good to excellent outcomes. Kaplan-Meier analysis at 39 months was 86% and 67% for the implant and fibular graft, respectively (P = .21). Early outcomes demonstrate that porous tantalum implants are a safe option for femoral head salvage. Continued follow-up is necessary to determine the long-term success.

Repairing defect and preventing collapse of canine femoral head using titanium implant enhanced by autogenous bone graft and rhBMP-2

There is a direct relationship between mechanical stress and the progressive collapse of the necrotic region in osteonecrosis of the femoral head. The titanium implant combined with autogenous bone graft and recombinant human bone morphogenetic protein (rhBMP)-2 to repair the defect and prevent collapse of the femoral head was investigated. The femoral head defects were made by the trapdoor procedure and then the defects were filled, respectively, with the titanium implant combined with autogenous bone graft and rhBMP-2, with autogenous bone graft and rhBMP-2, and with autogenous bone graft alone. Roentgenographic and histological examinations were performed at various times postoperatively. The defects were repaired completely and the titanium implant was integrated with the surrounding bone tissues. The defects healed faster than did without rhBMP-2. No trapdoor cartilage collapsed and joint space narrowed. The titanium implant combined with autogenous bone graft and rhBMP-2 can enhance the repairing procedure and prevent the collapse of the femoral head.

Structural and mechanical evaluations of a topology optimized titanium interbody fusion cage fabricated by selective laser melting process

A topology optimized lumbar interbody fusion cage was made of Ti-Al6-V4 alloy by the rapid prototyping process of selective laser melting (SLM) to reproduce designed microstructure features. Radiographic characterizations and the mechanical properties were investigated to determine how the structural characteristics of the fabricated cage were reproduced from design characteristics using micro-computed tomography scanning. The mechanical modulus of the designed cage was also measured to compare with tantalum, a widely used porous metal. The designed microstructures can be clearly seen in the micrographs of the micro-CT and scanning electron microscopy examinations, showing the SLM process can reproduce intricate microscopic features from the original designs. No imaging artifacts from micro-CT were found. The average compressive modulus of the tested caged was 2.97+/-0.90 GPa, which is comparable with the reported porous tantalum modulus of 3 GPa and falls between that of cortical bone (15 GPa) and trabecular bone (0.1-0.5 GPa). The new porous Ti-6Al-4V optimal-structure cage fabricated by SLM process gave consistent mechanical properties without artifactual distortion in the imaging modalities and thus it can be a promising alternative as a porous implant for spine fusion.

A physical vapor deposition method for controlled evaluation of biological response to biomaterial chemistry and topography

The purpose of this study was to characterize a technique to effectively mask surface chemistry without modifying surface topography. A thin layer of titanium was deposited by physical vapor deposition (PVD) onto different biomaterial surfaces. Commercially pure titanium disks were equally divided into three groups. Disks were either polished to a mirror finish, grit blasted with alumina particles, or grit blasted and subsequently plasma sprayed with a commercial grade of hydroxyapatite (HA). A subgroup of each of these treatment types was further treated by masking the entire disk surface with a thin layer of commercially pure titanium deposited by PVD. A comparison of surface topography and chemical composition was carried out between disks within each treatment group. Canine marrow cells were seeded on all disk surfaces to determine the stability of the PVD Ti mask under culture conditions. The PVD process did not significantly alter the surface topography of any samples. The thin titanium layer completely masked the underlying chemistry of the plasma sprayed HA surface and the chemistry of the plasma vapor deposited titanium layer did not differ from that of the commercially pure titanium disks. Aliquots obtained from the media during culture did not indicate any significant differences in Ti concentration amongst the Ti and Ti-masked surfaces. The PVD application of a Ti layer on HA coatings formed a stable, durable, and homogenous layer that effectively masked the underlying surface chemistry without altering the surface topography.

Evaluation of machining methods for trabecular metal implants in a rabbit intramedullary osseointegration model

Implant success is dependent in part on the interaction of the implant with the surrounding tissues. Porous tantalum implants (Trabecular Metal, TM) have been shown to have excellent osseointegration. Machining this material to complex shapes with close tolerances is difficult because of its open structure and the ductile nature of metallic tantalum. Conventional machining results in occlusion of most of the surface porosity by the smearing of soft metal. This study compared TM samples finished by three processing techniques: conventional machining, electrical discharge machining, and nonmachined, "as-prepared." The TM samples were studied in a rabbit distal femoral intramedullary osseointegration model and in cell culture. We assessed the effects of these machining methods at 4, 8, and 12 weeks after implant placement. The finishing technique had a profound effect on the physical presentation of the implant interface: conventional machining reduced surface porosity to 30% compared to bulk porosities in the 70% range. Bone ongrowth was similar in all groups, while bone ingrowth was significantly greater in the nonmachined samples. The resulting mechanical properties of the bone implant-interface were similar in all three groups, with only interface stiffness and interface shear modulus being significantly higher in the machined samples.

Generation of a tissue-engineered thymic organoid

The thymic microenvironment provides essential support for the generation of a functional and diverse population of human T cells. In particular, the three-dimensional (3D) thymic architecture contributes to critical cell-cell interactions. We report that thymic stroma, arrayed on a synthetic 3D matrix, supports the development of functional human T cells from hematopoietic precursor cells. Newly generated T cells contain T-cell receptor excision circles and are both fully mature and functional. The coculture of T-cell progenitors with thymic stroma can thus be used to generate de novo functional and diverse T-cell populations. This novel tissue engineered thymic system has biological applications for the study of T-lymphopoiesis and self-tolerance as well as potential therapeutic applications including the immune reconstitution of immunocompromised patients and the induction of tolerance in individuals receiving tissue or organ transplants.

Porous tantalum patellar augmentation: the importance of residual bone stock

Trabecular metal augmentation of bone defects has been associated with favorable bone ingrowth. Animal studies also suggest fibrous tissue attachment to trabecular metal can be achieved. We treated 16 patients with total knee arthroplasty (18 knees) with severe patellar bone loss using trabecular metal patellar reconstruction. The patients were divided into two groups based on the amount of residual patellar bone stock present at the time of surgery: Group 1 (six patients, seven knees) with no patellar bone stock and Group 2 (10 patients, 11 knees) in whom at least 50% of the patellar component surface was covered by host bone. All seven patellar components in Group 1 loosened within 1 year. Two of these developed necrosis of the extensor mechanism leading to extensor mechanism discontinuity. One component in Group 2 became infected and loosened, whereas the remaining 10 components remained stable at minimum 12-month followup. Our results suggest stable fixation of a trabecular metal patellar component can be achieved when residual bone is present for implant fixation, but early loosening is likely to occur when soft tissue is used for fixation to the implant.

Applications of porous tantalum in total hip arthroplasty

Porous tantalum is an alternative metal for total joint arthroplasty components that offers several unique properties. Its high volumetric porosity (70% to 80%), low modulus of elasticity (3 MPa), and high frictional characteristics make it conducive to biologic fixation. Tantalum has excellent biocompatibility and is safe to use in vivo. The low modulus of elasticity allows for more physiologic load transfer and relative preservation of bone stock. Because of its bioactive nature and ingrowth properties, tantalum is used in primary as well as revision total hip arthroplasty components, with good to excellent early clinical results. In revision arthroplasty, standard and custom augments may serve as a structural bone graft substitute. Formation of a bone-like apatite coating in vivo affords strong fibrous ingrowth properties and allows for substantial soft-tissue attachment, indicating potential for use in cases requiring reattachment of muscles and tendons to a prosthesis. Development of modular components and femoral stems also is being evaluated. The initial clinical data and basic science studies support further investigation of porous tantalum as an alternative to traditional implant materials.

Effects of electromagnetic stimulation on calcified matrix production by SAOS-2 cells over a polyurethane porous scaffold

There is increasing interest in designing new biomaterials that could potentially be used in the form of scaffolds as bone substitutes. In this study we used a hydrophobic crosslinked polyurethane in a typical tissue-engineering approach, that is, the seeding and in vitro culturing of cells using a porous scaffold. Using an electromagnetic bioreactor (magnetic field intensity, 2 mT; frequency, 75 Hz), we investigated the effect of the electromagnetic stimulation on SAOS-2 human osteoblast proliferation and calcified matrix production. Cell proliferation was twice as high; expression of decorin, osteocalcin, osteopontin, type I collagen, and type III collagen was greater (1.3, 12.2, 12.1, 10.0, and 10.5 times as great, respectively); and calcium deposition was 5 times as great as under static conditions without electromagnetic stimulation. RT-PCR analysis revealed the electromagnetically upregulated transcription specific for decorin, fibronectin, osteocalcin, osteopontin, transforming growth factor-beta, type I collagen, and type III collagen. The immunolocalization of the extracellular matrix constituents showed their colocalization in the cell-rich areas. The bioreactor and the polyurethane foam were designed to obtain cell colonization and calcified matrix deposition. This cultured biomaterial could be used, in clinical applications, as an osteoinductive implant for bone repair.

The use of a trabecular metal acetabular component and trabecular metal augment for severe acetabular defects

Stable acetabular fixation cannot be reliably achieved with the use of a hemispherical porous coated component alone in patients with a Paprosky type IIIa defect. The purpose of the present study was to determine the short-term results of a tantalum porous coated hemispherical acetabular component supported with a modular tantalum augment in Paprosky type IIIa defects. A total of 28 patients (28 hips) were treated for a type IIIa acetabular defect between 2001 and 2003 with the use of a trabecular metal acetabular component supported with a modular, superiorly placed trabecular metal augment. At an average of 3.1 years follow-up, 1 patient required rerevision for recurrent instability. The remaining hips remain radiographically stable. Clinically, the patients' modified Postel Merle d'Aubigne score improved from 6.8 preoperatively to 10.6 postoperatively. The use of a trabecular metal acetabular component with a superiorly placed trabecular metal augment demonstrates encouraging short-term results for Paprosky type IIIa acetabular defects.

Experimental and clinical performance of porous tantalum in orthopedic surgery

Porous tantalum, a new low modulus metal with a characteristic appearance similar to cancellous bone, is currently available for use in several orthopedic applications (hip and knee arthroplasty, spine surgery, and bone graft substitute). The open-cell structure of repeating dodecahedrons is produced via carbon vapor deposition/infiltration of commercially pure tantalum onto a vitreous carbon scaffolding. This transition metal maintains several interesting biomaterial properties, including: a high volumetric porosity (70-80%), low modulus of elasticity (3MPa), and high frictional characteristics. Tantalum has excellent biocompatibility and is safe to use in vivo as evidenced by its historical and current use in pacemaker electrodes, cranioplasty plates and as radiopaque markers. The bioactivity and biocompatibility of porous tantalum stems from its ability to form a self-passivating surface oxide layer. This surface layer leads to the formation of a bone-like apatite coating in vivo and affords excellent bone and fibrous in-growth properties allowing for rapid and substantial bone and soft tissue attachment. Tantalum-chondrocyte composites have yielded successful early results in vitro and may afford an option for joint resurfacing in the future. The development of porous tantalum is in its early stages of evolution and the following represents a review of its biomaterial properties and applications in orthopedic surgery.

Clinical and histologic results related to a low-modulus composite total hip replacement stem

BACKGROUND

Osteolysis secondary to stress shielding in patients with a total hip arthroplasty has been attributed to greater stiffness of the prosthetic femoral stem compared with the stiffness of the femur. This concern led to the development of a composite femoral stem implant with a structural stiffness similar to that of the native femur. The stem consists of a cobalt-chromium-alloy core surrounded by polyaryletherketone and titanium mesh for bone ingrowth. The purpose of this study was to determine the intermediate-term clinical, radiographic, and histologic results of the use of this stem.

METHODS

Twenty-eight patients (nineteen men and nine women) with an average age of 51.3 years underwent primary total hip arthroplasty with the Epoch stem and were followed for an average of 6.2 years. Harris hip scores were determined and radiographic studies were performed preoperatively, postoperatively, and at two-year intervals thereafter. In addition, dual x-ray absorptiometry scans were made up to two years postoperatively to evaluate osseous resorption. Two femora obtained at autopsy thirteen and forty-eight months after surgery were analyzed for bone ingrowth and ongrowth.

RESULTS

The Harris hip scores averaged 56 points preoperatively and improved to 97 points at the time of the last follow-up. Dual x-ray absorptiometry scans demonstrated the greatest decrease in mean bone density (27.5%) in Gruen zone 7 at two years. Radiographs demonstrated no instances of migration, and only one hip had osteolysis. All stems had stable osseous fixation. Histologic evaluation of the two femora that had been retrieved at autopsy at thirteen and forty-eight months showed the mean bone ingrowth (and standard deviation) along the entire length of the stem to be 49.62% +/- 13.04% and 73.57% +/- 8.48%, respectively, and the mean bone ongrowth to be 54.18% +/- 7.68% and 80.92% +/- 6.06%, respectively.

CONCLUSIONS

Intermediate-term follow-up of hips treated with the Epoch stem indicated excellent clinical success, radiographic evidence of osseous integration, and histologic findings of osseous ingrowth and ongrowth. Although the implant has been associated with excellent results in both the short and the intermediate term, longer follow-up will be necessary to assess the long-term function of the implant.

Restoration of acetabular bone loss 2005

The surgical strategy for revision of the acetabular component is determined by available host bone stock. Contained (cavitary) bone loss is the most common pattern of bone loss and can be addressed by morsellized bone graft. If contact can be made with at least 50% host bone, conventional uncemented cups can be used. If, however, contact with 50% host bone cannot be achieved, a protective cage and a cemented cup is an acceptable option but with a significant complication rate. The use of trabecular metal, which provides a more favorable environment for bone graft remodeling and host bone ingrowth, has allowed us to address larger contained defects without resorting to a cage. For massive contained defects, a combination of a trabecular metal cup protected by a cage has been used.

3-D Analysis of Pore Structure of Porous Biomaterials Using Micro Focus X-Ray Computed Tomography

Generally, characterizations of pore structures of porous biomaterials are mainly based on 2-dimensional (2-D) analysis using cross sectional micrographs. However, interconnectivity of each pore may be more important factor, when tissue ingrowth into deeper pores is considered. In this paper, using micro-CT imaging with 3-D image processing software, analyses of porous material based on 3-demensional (3-D) geometrical considerations were successfully performed. Plasmasprayed porous titanium implant (PT) and four types of sintered porous titanium implants (ST50- 200, ST50-500, ST70-200, and ST70-500) that possess different porosities (50% and 70%) and pore sizes (200-500+m and 500-1500+m) were analyzed in this study. A micro focus X-ray computed tomography system was employed to acquire microstructural information from the porous implants. Using 3-D image processing software, we performed three types of 3-D analysis including detection of the dead space (% dead pore), analysis of interconnectivity by blocking the narrow pore throat with caliber less than 52 +m (% pore with narrow throat) and analysis of material construct by contracting thin strut with thickness less than 52 +m (% construct with thin strut). ST50S and ST50L possessed interconnected porous structure with thicker strut; however, pore throat was considered to be relatively narrow. On the other hand, PT implant possesses favorable interconnectivity despite its’ low porosity; however, relatively thin strut indicate the structural disadvantage for mechanical property. These results suggest that the 3-D analysis of pore and strut structure using micro focus X-ray computed tomography and 3-D image processing software will provide effective information to develop porous implant.

Management of bone loss: augments, cones, offset stems

Trabecular metal augmentation has added new treatment options for severe proximal tibial bone defects in revision knee arthroplasty. Porous tantalum tibial cones provide mechanical support for the tibial component and have the potential for long-term biologic fixation. These cones facilitate restoration of the proximal tibia metaphysis in Type 2 and 3 defects. Ten tantalum tibial cones were press-fit into the prepared cavitary defect of a series of revision knee arthroplasties. Voids between the cone and host bone were filled with morselized grafting material. The core tibial component was cemented into the implanted tibial cone; fixation was enhanced with stem extensions, which were press-fit in four knees and cemented in six knees. Extensions ranged from 75-200 mm with length dependent upon the residual bone quality. Offset stems were used in 3 tibias. At follow-up (average 10 months), radiographic evaluation revealed no evidence of loosening or change in position. Strength, range of motion, and stability were comparable to previously reported series of revision arthroplasties. Trabecular metal cones can help reconstruct large cavitary defects and, along with stem extensions and offset stems, may eliminate the need for extensive bone grafting or structural allograft in revision knee arthroplasty.

LEVEL OF EVIDENCE

Therapeutic study, level V (expert opinion). See the Guidelines for Authors for a complete description of levels of evidence.

Development of a biologic prosthetic composite for cartilage repair

At present there is no satisfactory treatment for deep osteochondral defects. Here we report the development of a biologic prosthetic composite containing periosteum from 2-month-old rabbits and a porous tantalum scaffold. When cultured under chondrogenic conditions, the composites form a robust hyaline-like cartilage outgrowth that is attached to the porous scaffold by fibrous tissue ingrowth. The mechanical properties of these composites are similar to those of normal osteochondral plugs after only 6 weeks in culture. Thus, porous tantalum scaffolds are compatible with the chondrogenic capacity of periosteum. We hypothesize that these periosteum-porous tantalum composites will be useful for the repair of major osteochondral defects. However, in vivo experiments using biological resurfacing of large osteochondral defects with a porous tantalum scaffold and autologous periosteal graft in animal models are necessary to further explore this possibility. The implications of a successful method for cartilage regeneration would be great in terms of the number of patients affected and the quality of life for each of those patients.

Three-dimensional growth of differentiating MC3T3-E1 pre-osteoblasts on porous titanium scaffolds

The present work assesses the potential of three-dimensional porous titanium scaffolds produced by a novel powder metallurgy process for applications in bone engineering through in vitro experimentation. Mouse MC3T3-E1 pre-osteoblasts were used to investigate the proliferation (DNA content), differentiation (alkaline phosphatase activity and osteocalcin release) and mineralisation (calcium content) processes of cells on titanium scaffolds with average pore sizes ranging from 336 to 557 microm, using mirror-polished titanium as reference material. Scanning electron microscopy was employed to qualitatively corroborate the results. Cells proliferate on all materials before reaching a plateau at day 9, with proliferation rates being significantly higher on foams (ranging from 123 to 163 percent per day) than on the reference material (80% per day). Alkaline phosphatase activity is also significantly elevated on porous scaffolds following the proliferation stage. However, cells on polished titanium exhibit greater osteocalcin release toward the end of the differentiation process, resulting in earlier mineralisation of the extracellular matrix. Nevertheless, the calcium content is similar on all materials at the end of the experimental period. Average pore size of the porous structures does not have a major effect on cells as determined by the various analyses, affecting only the proliferation stage. Thus, the microstructured titanium scaffolds direct the behaviour of pre-osteoblasts toward a mature state capable of mineralising the extracellular matrix.

Canine carpal joint fusion: a model for four-corner arthrodesis using a porous tantalum implant

PURPOSE

Interest has focused on porous materials that promote bony ingrowth. In this study a porous tantalum implant was used as an adjunct to intercarpal stabilization in a canine model of wrist arthrodesis.

METHODS

A defect was created at the junction of the radiocarpal, ulnocarpal, and fourth carpal bones, analogous to a four-corner fusion site in humans. A tantalum cylinder was press-fit and stabilized with K-wires. Controls were represented by creating the defect without implant placement. Animals were killed at 4, 8, and 12 weeks.

RESULTS

Histology showed bony ingrowth as early as 4 weeks and mechanical testing showed a statistically significant increase in strength of the construct over time. Controls failed to achieve union at any time point.

CONCLUSIONS

The implant served as an adjunct to stabilization of the carpus in this model of four-corner fusion, suggesting a novel application of this material in conditions in which bone graft has been required previously. This study represents a preliminary investigation of the use of a tantalum device for intercarpal stabilization; it does not compare this technique with conventional methods.

Particle migration and gap healing around trabecular metal implants

Bone on-growth and peri-implant migration of polyethylene particles were studied in an experimental setting using trabecular metal and solid metal implants. Cylindrical implants of trabecular tantalum metal and solid titanium alloy implants with a glass bead blasted surface were inserted either in an exact surgical fit or with a peri-implant gap into a canine knee joint. We used a randomised paired design. Polyethylene particles were injected into the knee joint. In both types of surgical fit we found that the trabecular metal implants had superior bone ongrowth in comparison with solid metal implants (exact fit: 23% vs. 7% [p=0.02], peri-implant gap: 13% vs. 0% [p=0.02]. The number of peri-implant polyethylene particles was significantly reduced around the trabecular metal implants with a peri-implant gap compared with solid implants.

Pedicle screw fixation enhances anterior lumbar interbody fusion with porous tantalum cages: an experimental study in pigs

STUDY DESIGN

A porous tantalum implant (Hedrocel, Implex Corp., Allendale, NJ), designed to assist interbody lumbar fusion, was tested biologically in an experimental model. A total of 11 female Danish landrace pigs received 3 levels of anterior lumbar interbody fusion at L2-L3, L4-L5, and L6-L7. Each level was randomly assigned one of the procedures: (1) implantation of PT-ring with pedicle screw fixation (PSF), (2) implantation of a porous tantalum ring (PT-ring) stabilized anteriorly with staples, or (3) implantation of carbon fiber cage (CF-cage) stabilized anteriorly with 2 staples. Each implant was filled with autogenous iliac crest bone graft.

OBJECTIVES

To evaluate the effects of PSF on the incorporation of autologous bone in a porous tantalum interbody device and to compare healing in PT-rings to that in CF-cages of autologous bone.

SUMMARY OF BACKGROUND DATA

Despite the promising results that early clinical trials have shown, interbody fusion cage technology is still under debate because of uncertainties that include indications for surgery, criteria for fusion, material, cage design, cage subsidence, and the effect of immediate stabilization.

METHODS

Pigs were euthanized 6 months after surgery. Fusion segments were evaluated by plain radiography, conventional radiograph tomography, and histology.

RESULTS

Fusion segments with PSF had significantly fewer radiolucencies than the other 2 levels (P = 0.002). Improved interface healing and fusion rate were observed in PT-rings when supplemented with PSF (P = 0.03). His tomorphometric results showed that the percentage of bone and bone marrow space in the center of a PT-ring was not significantly different from that of its adjacent vertebral body, but PSF increased bone marrow and decreased fibrous tissue formation in a tantalum cage. However, a CF-cage had higher bone volume and lower bone marrow space inside the cage compared with its adjacent vertebral body (P < 0.001). Fibrous tissue formation inside and around a CF-cage was more than that of a PT-ring (P < 0.05).

CONCLUSION

Interbody fusion using a PT-ring cage packed with autologous bone achieved higher interface healing and more reliable fusion when fixated with supplementary pedicle screws than did fixated anteriorly with 2 staples. A lesser amount of bone graft was required, and bone remodeling was enhanced in the PT-ring when compared to the CF-cage.

Bone ingrowth simulation for a concept glenoid component design

Glenoid component loosening is the major problem of total shoulder arthroplasty. It is possible that uncemented component may be able to achieve superior fixation relative to cemented component. One option for uncemented glenoid is to use porous tantalum backing. Bone ingrowth into the porous backing requires a degree of stability to be achieved directly post-operatively. This paper investigates the feasibility of bone ingrowth with respect to the influence of primary fixation, elastic properties of the backing and friction at the bone prosthesis interface. Finite element models of three glenoid components with different primary fixation configurations are created. Bone ingrowth into the porous backing is modelled based on the magnitude of the relative interface micromotions and mechanoregulation of the mesenchymal stem cells that migrated via the bonded part of the interface. Primary fixation had the most influence on bone ingrowth. The simulation showed that its major role was not to firmly interlock the prosthesis, but rather provide such a distribution of load, that would result in reduction of the peak interface micromotions. Should primary fixation be provided, friction has a secondary importance with respect to bone ingrowth while the influence of stiffness was counter intuitive: a less stiff backing material inhibits bone ingrowth by higher interface micromotions and stimulation of fibrous tissue formation within the backing.

Radiographic evaluation of a monoblock acetabular component: a multicenter study with 2- to 5-year results

Serial radiographs of a porous tantalum monoblock acetabular cup design were evaluated for cup stability and signs of successful osteointegration. Of 574 primary consecutive total hip replacements in 542 patients performed by 9 surgeons at 7 hospitals, 414 cases were available for minimum 2-year follow-up. Follow-up averaged 33 months and ranged from 24 to 58 months. Postoperative radiographs revealed acetabular gaps in 100 zones in 80 (19%) hips: 29 in zone I, 67 in zone II, and 4 in zone III. At last follow-up, 84 (84%) of the zones with gaps completely filled in, and all 4- and 5-mm gaps filled in. There was no progression of any postoperative gap, no evidence of continuous periacetabular interface radiolucencies, no evidence of lysis, and no revisions for loosening. Although these short-term results are encouraging, further follow-up will be required to assess whether the monoblock design and the low modulus of elasticity of porous tantalum will reduce the incidence of periacetabular stress shielding and occurrence of osteolysis.

Biological performance of uncoated and octacalcium phosphate-coated Ti6Al4V

The in vivo behavior of a porous Ti6Al4V material that was produced by a positive replica technique, with and without an octacalcium phosphate (OCP) coating, has been studied both in the back muscle and femur of goats. Macro- and microporous biphasic calcium phosphate (BCP) ceramic, known to be both osteoconductive and able to induce ectopic bone formation, was used for comparison purpose. The three groups of materials (Ti6Al4V, OCP Ti6Al4V and BCP) were implanted transcortically and intramuscularly for 6 and 12 weeks in 10 adult Dutch milk goats in order to study their osteointegration and osteoinductive potential. In femoral defects, both OCP Ti6Al4V and BCP were performing better than the uncoated Ti6Al4V, at both time points. BCP showed a higher bone amount than OCP Ti6Al4V after 6 weeks of implantation, while after 12 weeks, this difference was no longer significant. Ectopic bone formation was found in both OCP Ti6Al4V and BCP implants after 6 and 12 weeks. The quantity of ectopically formed bone was limited as was the amount of animals in which the bone was observed. Ectopic bone formation was not found in uncoated titanium alloy implants, suggesting that the presence of calcium phosphate (CaP) is important for bone induction. This study showed that CaPs in the form of coating on metal implants or in the form of bulk ceramic have a significantly positive effect on the bone healing process.

Effect of porosity on the fluid flow characteristics and mechanical properties of tantalum scaffolds

In many cases of traumatic bone injury, bone grafting is required. The primary source of graft material is either autograft or allograft. The use of both material sources are well established, however, both suffer limitations. In response, many grafting alternatives are being explored. This article specifically focuses on a porous tantalum metal grafting material (Trabecular Metaltrade mark) marketed by Zimmer. Twenty-one cylindrical scaffolds were manufactured (66% to 88% porous) and tested for porosity, intrinsic permeability, tangent elastic modulus, and for yield stress and strain behavior. Scaffold microstructural geometries were also measured. Tantalum scaffold intrinsic permeability ranged from 2.1 x 10(-10) to 4.8 x 10(-10) m(2) and tangent elastic modulus ranged from 373 MPa to 2.2 GPa. Both intrinsic permeability and tangent elastic modulus closely matched porosity-matched cancellous bone specimens from a variety of species and anatomic locations. Scaffold yield stress ranged from 4 to 12.7 MPa and was comparable to bovine and human cancellous bone. Yield strain was unaffected by scaffold porosity (average = 0.010 mm/mm). Understanding these structure-function relationships will help complete the basic physical characterization of this new material and will aid in the development of realistic mathematical models, ultimately enhancing future implant designs utilizing this material.

The proliferation and phenotypic expression of human osteoblasts on tantalum metal

Tantalum (Ta) is increasingly used in orthopaedics, although there is a paucity of information on the interaction of human osteoblasts with this material. We investigated the ability of Ta to support the growth and function of normal human osteoblast-like cells (NHBC). Cell responses to polished and textured Ta discs were compared with responses to other common orthopaedic metals, titanium and cobalt-chromium alloy, and tissue culture plastic. No consistent differences, that could be attributed to the different metal substrates or to the surface texture, were found in several measured parameters. Attachment of NHBC to each substrate was similar, as was cell morphology, as determined by confocal microscopy. Cell proliferation was slightly faster on plastic than on Ta at 3 days, but by 7 days neither the absolute cell numbers, nor the number of cell divisions, was different between Ta and the other substrates. No consistent, substrate-dependent differences were seen in the expression of a number of mRNA species corresponding to the pro-osteoclastic or the osteogenic activity of osteoblasts. No substrate-dependent differences were seen in the extent of in vitro mineralisation by NHBC. These results indicate that Ta is a good substrate for the attachment, growth and differentiated function of human osteoblasts.

Revision and salvage patellar arthroplasty using a porous tantalum implant

The purpose of this study was to assess the clinical results of a new surgical technique and novel porous tantalum implant for augmentation or arthroplasty of the patella for 11 patients who underwent revision total knee arthroplasty. Preoperative average knee function and pain scores were 24 and 20, respectively, and average range of motion (ROM) was 62 degrees. The low knee scores reflect the immobility, trauma, and/or pain associated with the patients' presenting conditions. At the most recent follow-up (average, 32 months), the average knee function and pain scores were 69 and 53, respectively, and the average ROM was 103 degrees. Radiographically, all implants were stable, and patient satisfaction has been excellent. These results indicate that this surgical technique and porous tantalum prosthesis can substantially improve function and reduce pain for patients with severe patellar bone loss and other complicating factors.

Bone ingrowth characteristics of porous tantalum and carbon fiber interbody devices: an experimental study in pigs

BACKGROUND CONTEXT

In preclinical and clinical joint replacement applications, porous tantalum has been shown to be osteoconductive and effective for biological fixation. Relatively little research has been undertaken to investigate the porous tantalum implants for potential application in intervertebral spinal fusion.

PURPOSE

The current study was designed to assess the radiographic and histological performance of porous tantalum and carbon fiber devices in the porcine anterior lumbar interbody fusion (ALIF) model.

STUDY DESIGN

A total of 10 Danish Landrace pigs underwent a three-level anterior intervertebral lumbar arthrodeses at L2-L3, L4-L5 and L6-L7. Each level was randomly allocated to one of three implants: a solid piece of porous tantalum, a porous tantalum ring packed with autograft or a carbon fiber cage, likewise packed with autograft. Two staples for fixation were supplemented in front of implant.

METHODS

Pigs were sacrificed 3 months after operation. Specimens were evaluated by plain radiography, conventional tomography and histology.

RESULTS

Bone graft filled into the central hole of the porous tantalum ring was less than that of the carbon fiber cage (p<.001). Radiolucencies around the porous tantalum solid were significantly higher than the carbon fiber cage (p=.02) and were not different between the porous tantalum ring and the carbon fiber cage. The bone volume in the hole of implants, within the pores of the porous tantalum and in the implant interface did not differ between implants. Bone volume in the hole of the porous tantalum ring did not differ from that of the adjacent vertebral bone; however, it was significantly different in the carbon fiber cage and the adjacent vertebral bone (p=.005).

CONCLUSIONS

In this porcine ALIF model, the radiographic and histological appearances of the porous tantalum ring were equivalent to those of the carbon fiber cage. The high presence of radiolucencies and fibrous tissue layer at the vertebrae-implant interface suggests that an initial stabilizing biomechanical environment is important in order to achieve bone ingrowth in the interbody fusion devices in this ALIF model.

Extracortical bone bridging in tumor endoprostheses. Radiographic and histologic analysis

BACKGROUND

Aseptic loosening remains a major problem following prosthetic replacement after resection of periarticular tumors. Attempts to decrease the rate of loosening led to the introduction of a composite segmental prosthesis in which the shoulder of the intramedullary stem is porous-coated to allow extracortical bone bridging and bone ingrowth. The purposes of this study were to determine the extent of extracortical bone bridging around, and the amount of bone growth into, the porous-coated shoulder of endoprostheses implanted following the resection of periarticular bone tumors and to correlate the radiographic and histologic findings.

METHODS

Twenty tumor endoprostheses implanted with use of the extracortical bone-bridging technique were evaluated radiographically to determine the extent of extracortical bone and the amount of bone ingrowth. Five of these endoprostheses were retrieved and subjected to histologic analysis with backscattered electron microscopy and transmitted light microscopy to determine the extent of bone ingrowth.

RESULTS

At a mean of twenty-eight months postoperatively, varying amounts of extracortical bone formation were seen radiographically in all patients. Radiographs also appeared to show bone growth into the porous-coated segment of all implants. However, histologic analysis of the five retrieved prostheses revealed that none of the extracortical bone had actually grown into the porous-coated segment of the implant.

CONCLUSIONS

This study confirmed that autogenous bone-grafting of the bone-implant junction of a tumor endoprosthesis consistently results in the formation of extracortical bone. Although radiographs seemed to indicate that this bone grows into the porous coating, this was not confirmed histologically. Growth of extracortical bone into the extramedullary, porous-coated portion of tumor endoprostheses in humans may not be attainable with the current prosthetic design and surgical technique.

The impact of total joint replacement in rheumatoid arthritis

Management of the patient with rheumatoid arthritis (RA) requires a multidisciplinary approach, the role of the surgeon being to improve functional ability for the patient by reconstructing a deteriorated joint by total joint arthroplasty (TJA). An advantage of prosthetic evaluation over pharmacological medication evaluations is that the 'compliance' of the patient with the treatment (i.e. the TJA) is 100%, even at long-term follow-up. However, long-term follow-up of prosthesis evaluation is as difficult as the evaluation of any other intervention. Although the goal of any intervention on an RA patient is to improve functional ability, and thus self-support, of the patient, objective evaluation of the surgical procedure, and of its impact on the patient, can be difficult. The potential chronic course of RA makes evaluation of a specific surgical procedure and its effect on the patient difficult to interpret. The success of the TJA is generally judged on a survivorship analysis at 10 or 15 years in national registries (i.e. >40000 implants); revision surgery is used as an end-point for survival of the TJA. With a mean 90% survival at 10-year follow-up, total hip arthroplasty and total knee arthroplasty may be considered gold-standard TJA procedures for the patient. While revision is the end-point, the course to this end-point starts with progressive micromotion of the prosthesis. The effect of prosthetic changes, and of medication on prosthesis migration, can be measured very accurately by radiostereometry. The latter measures the actual performance of the TJA in the bone. Next to these more procedure-oriented evaluations, patient-oriented evaluations (e.g. quality of life, patient expectations) are of importance in judging the impact of the TJA on the RA patient. These evaluations provide evidence that the pre-operative status of the joint/extremity determines the extent of post-operative functional gain. Thus, postponing TJA for too long will give less functional benefit.

Tantalum as a buffer layer in diamond-like carbon coated artificial hip joints

The acid resistance of tantalum coated and uncoated human hip joint prostheses was studied with commercial CrCoMo acetabular cups. The samples were exposed to 10% HCl solution and the quantities of dissolved Cr, Co, and Mo were measured with proton-induced X-ray emission (PIXE). The absolute quantities were obtained with the use of Cr and Se solution standards. Tantalum coatings (thicknesses 4-6 microm) were prepared in vacuum with magnetron sputtering. Tantalum coating decreased the corrosion rate by a factor of 10(6). As a spinoff from recent wear tests on artificial hip joints it was shown that tantalum has excellent mechanical properties as an intermediate layer of diamond-like carbon (DLC) coatings. When tantalum was tested together with DLC on three metal-on-metal hip joint pairs in a hip simulator, no observable defects occurred during 15 million walking cycles with a periodic 50-300-kg load (Paul curve).

Porous titanium-nickel for intervertebral fusion in a sheep model: part 1. Histomorphometric and radiological analysis

Porous titanium-nickel (PTN) implants represent an alternative to traditional intervertebral fusion cages. Indeed, PTN materials possess interconnecting pores with cell capillarity properties that may promote bone ingrowth and intervertebral fusion without the need for bone grafting. In this study, a PTN intervertebral fusion device was compared to a conventional TiAlV cage packed with autologous bone in a sheep model. The two devices were implanted at two noncontiguous intervertebral lumbar sites for 3, 6, and 12 months. PTN osseointegration showed a time-dependent trend increasing from 21.4% to 37.6% (3-12 months), whereas TiAlV cages remained at the same level of bone ingrowth (22.7%-25.4%; 3-12 months). Furthermore, PTN bone apposition (10.9%-24.2%; 3-12 months) was significantly higher than that of TiAlV implants (1.1%-5.1%; 3-12 months; p < 0.001, ANOVA). Radiological fusion scores increased with postsurgery time regardless of material type, but were consistently superior for PTN (12.5-18.5; 3-12 months) than for TiAlV cages (2.0-15.0; 3-12 months; p < 0.001, ANOVA). Implant materials were not significantly different according to the radiological interbody index based on preoperative disc height: Interbody index began at 132.6% (PTN) and 123.5% (TiAlV) immediately after surgery, then declined to 80.8% (PTN) and 91.0% (TiAlV) after 12 months. Nevertheless, ungrafted PTN constituted an excellent substrate for osteogenic cell integration and represents a new osteoconductive biomaterial with improved fusion characteristics in comparison to conventional TiAlV cages.

Bone grafting, implants, and plating options for anterior cervical fusions

The basic principles of cervical spine surgery continue to include adequate decompression, provision of a structurally competent, biologically functional bone graft, and creation of a stable construct to allow for solid fusion. In recent years, the options to achieve these goals have expanded significantly. Bone banking and bone graft substitutes yield increasingly viable alternatives to autogenous bone graft. New prosthetic implants and cages are currently under investigation. The science of bone growth factors seems to be promising and is expected to revolutionize the approach to spinal arthrodesis. Various plating systems are available to provide internal stability to cervical spine constructs. It is important to understand the biomechanics of plating systems so that the optimal system may be used in a given situation. Long constructs place significant loads on SGs and anterior plates. It is important to consider the use of additional fixation, such as posterior segmental fixation in long constructs, which may be prone to failure using only anterior plate fixation. Anterior cervical plating for single-level ACDF remains controversial, whereas plating has been shown to improve the results of multilevel ACDF. Plating may provide a useful salvage option for a cervical nonunion, especially if deformity or neurologic compression dictates an anterior approach. Hardware failures may occur with anterior cervical plating, but most remain asymptomatic and do not require operative intervention.

The effect of substrate roughness and hydroxyapatite coating thickness on implant shear strength

This study examined the role of substrate preparation and hydroxyapatite (HA) coating thickness on bone ongrowth and shear strength in a bilateral bicortical sheep model. Plasma-sprayed and grit-blasted titanium implants with different thickness HA coatings were examined at 4, 8, 12, and 26 weeks after implantation. Shear strength increased with time for all implants. Plasma-sprayed implants were superior to grit-blasted implants at all time points. The 100-microm-thick HA layer used in the present study provided greater fixation and ongrowth and less resorption compared with the 50-microm-thick layer. We did not observe any advantage in using a thicker HA coating for the titanium substrates examined.

Enhancement of bone growth into porous intramedullary implants using non-invasive low intensity ultrasound

An in vivo study was designed to determine if non-invasive low intensity ultrasound could enhance bone growth into porous intramedullary implants. Fully porous intramedullary rods were implanted bilaterally into the ulnae of six dogs. In each dog, one ulna served as a control and the other was treated with 20 min of daily ultrasound stimulation for 6 consecutive weeks. Analysis of serial transverse sections indicated an average of 119% more bone growth into the ultrasound-treated implants compared with the contralateral controls (P < 0.001). In each of the 6 dogs, there was a significantly greater amount of bone ingrowth on the ultrasound-stimulated side. These data indicate a clear potential for externally applied ultrasound therapy to augment biological fixation.