Biological processes at bone – porous tantalum interface. A review article

Stefanos D Koutsostathis¹, George A Tsakotos², Ioannis Papakostas³, George A Macheras⁴.

4^th Orthopaedic Department, KAT Hospital, Athens Greece

Address for Correspondence:

Stefanos D Koutsostathis
4^th Orthopaedic Department, KAT Hospital, Athens Greece.
8 Areos str, Maroussi Attiki, 151 22.

Phone:  +302108051949
E-mail: skoutsostathis@gmail.com

Abstract:

The use of biomaterials is continuously increasing in daily orthopaedic practice. New materials are introduced in order to combine as many as possible favourable properties in one and only material. Porous tantalum has unique characteristics. Its elasticity is close to subchondral bone, while at the same time it is highly adherent. It has a great porosity of up to 80% of its total volume which promotes osteointegration. It can be constructed in a variety of sizes and shapes that allows its application in many areas of orthopaedic surgery.

J.Orthopaedics 2009;6(4)e3

Keywords:

porous tantalum; interface; acetabular implant

Introduction:

During the last 25 years various biomaterials and porous surfaces have been used in the area of reconstructive orthopaedic surgery, in order to achieve osteo-integration of the implants. Clinical and histological data from removed parts of arthroplasties advocate that porous surfaces reinforce the implant’s stability via biological integration procedures. However, the most common used materials, titanium and cobalt-chromium alloy, can’t respond to the demanding mechanical status of orthopaedic implants because of their mechanical features. This is the reason why they are used only as modes of superficial coating, while the implant’s supporting structure is made from another compact metal. Also, the rate of porous texture of these coatings is about 30%-50% of their total volume and this restricts their ability of bony penetration. The latter is proportional to the amount of pores, expressed as percentage of the total volume of the material^1-3. In the area of structural grafts at the same time, a large number of synthetic biomaterials have been introduced and significant improvements in use of bone allografts have been made. The purpose of all these is the restoration of bone deficit areas with simultaneous bone penetration and integration. The biomaterial’s ability to succeed this, depends mainly on the porous texture, the mechanical resistance, both short and long term, the elasticity (which ideally should approach the elasticity of cancellous bone), the resistance to absorption, the immune inactivity and the ease of manufacturing and use. It is obvious that the combination of a high ratio of porous texture, with the appropriate mechanical properties for use in orthopaedic surgery in one and only biomaterial is particularly desirable.

Properties of pure tantalum

Pure tantalum (ASTM F-560) is a highly biocompatible material which does not provoke cellular reactions as other materials like nickel, cobalt and chromium^4,5 do. It is a hard metal which resists in oxidation, corrosion and concomitant ion production⁶. Tantalum is being used for over 50 years as an implant in humans, in a variety of applications: electrodes for pacemakers⁷, plates for cranioplasty⁸ , clips for ligation⁹ , femoral stems¹⁰, wires, nets or plates  in nerve surgery¹¹ and as a medium of marking in radiological studies for implant migration¹². The property of tantalum to adhere to bone, possibly via a not well elucidated chemical connection procedure, is known for many years, mainly through the application of tantalum in the area of dental implantation¹³. Historically, only the pure titanium and tantalum have this property that also newer biomaterials (CaHAP, crystalloids, hyaloids) have. According to Kokubo et al¹⁴, after the exposition of tantalum in body moist, a superficial layer of sodium tantalum is created, ions of Na⁺ are released from this and connections of tantalum with hydroxyl roots are made (Ta-OH). The latter are connected with free Ca ions (Ca⁺⁺) and thus amorphous calcium tantalum is created (Calcium Tantalate). The combination of calcium tantalum with phosphorus ions leads to amorphous phosphoric calcium, which is converted to hydroxyapatite. In this way, a solid connection between bone and metal is achieved. Recently, Findlay and Whelldon^15,16 tested the ability of polished tantalum to support the growth and function of normal human osteoblastic cells, in comparison with other substrates and specifically titanium and chromium-cobalt (that have proved in clinical practice their ability for bony integration). They studied the number and morphology of adherent osteoblasts with electronic microscopy and their rate of proliferation and biological activity, expressed via mRNA concentrations. They concluded that pure tantalum is at least the same effective with the above mentioned biomaterials as a substrate for adhesion, growth, differentiation and function of human osteoblasts.

Porous tantalum preparation and properties

Porous tantalum is a new biomaterial, which was constructed a decade ago^17,18. Preparation of porous tantalum starts with polyurethane’s foam pyrolysis (thermal conversion). This foam turns to a skeleton of low density hyaloid carbon, with a characteristic repeated duodenofundament structure that leads to pores connected with smaller holes (Fig 1). By the method of chemical vapor deposition-infiltration (CVD/CVI), metal tantalum deposition is taking place on carbon skeleton and a porous metal construction is formed. Due to crystallographic processes and the tantalum’s orientation during the deposition, this procedure leads to the creation of a surface with special texture, similar to the cancellus bone. The mean pore’s diameter is 547+/- 52μm. Pores are connected with smaller holes. The pore’s two-dimensional size is 430 +/- 270 μm. The porosity is about 75%-80% of the total material’s volume. Pore’s size is ideal for vascular tissue ingrowth. The large amount of empty space in this material allows deep and extensive tissue penetration (Fig 2) and this results to stronger adhesion and biological fixation¹⁹. This geometry also allows the penetration of non-biological materials, as polyethylene, that can be press - molded onto shell leading to a strong monoblock construction. Moreover, this allows, potentially, the deposition of osteoinductive and osteoconductive factors²⁰ which promote the material’s bone integration. The thin coating layer from tantalum at the initial scaffold, with thickness 10μm-100μm, can give high mechanical properties, because this deposition has 100% density, the granule’s metal size is less than 1-5μm and other’s material mixing is less than 0,05%. The typical mean thickness of tantalum coating is 50 μm. This microarchitecture possibly participates in the overall osteogenic response, as has been proven in the past from studies in cell cultures with materials²¹ of similar microarchitecture. Porous tantalum has also favorable elasticity and friction ratio²¹. Biomaterial’s elasticity is 3GPa, very close to subchondral bone, and this permits the normal load transferring from implant to the surrounding cancellous bone. This is an important factor for implant’s long-lasting survival. Tantalum’s resistance at compression is 50-80 Mpa, almost like cancellus bone, and the resistance at rotational deformity is 40-60Mpa. The resistance threshold in traction is 18-20Mpa. As a result, porous tantalum has a high degree of plasticity in compression without failure, and can afford the usual loads of total hip arthroplasty or while supporting the femoral head as a rod in cases of femoral avascular necrosis. The same happens in the area of bone grafts, where resistance at compression-traction-bending appears in ex vivo studies to be larger than cancellous bone’s and others metal scaffold’s resistance. This supports the hypothesis of a more stable mechanical environment after implantation, where we expect integration and bone penetration at scaffold. Also, tantalum has higher friction ratio with bone, in comparison with other porous materials. This factor increases the initial stability after implantation. The preparation method of porous tantalum and the mechanical properties mentioned above, allow its formation in a variety of simple or complicated shapes, sometimes even individual patterns, either as coating surfaces, or as a structural bulky material. As a conclusion, in comparison with the conventional metallic materials used in orthopaedic surgery, porous tantalum has a significantly greater rate of porous texture, lower rigity -similar to cancellous bone- and higher friction ratio, which is important in the initial stability after implantation. In comparison with bone grafts, it has similar geometry, more predictable quality and mechanical properties, which are constant and not downgraded after material’s implantation. 

Fig 1. Porous tantalum’s characteristic duodenofundament structure.

In vivo experimental data

Bobyn et al²² studied biological processes at bone-porous tantalum interface in skeletal mature dogs, after implantation of a tantalum rod at the femoral cortical bone. The progress in bone penetration was recorded at 4, 16, and 52 weeks, after excision of the area of implantation and expression of the bone penetration as the rate of available pores that have been filled with new bone. Two different sizes of pores were studied, big (650μm) and small (430μm) diameter and the rates of filling at 4-16-52 weeks were: at 4 weeks 52,9% / 41,5%, at 16 weeks 69,2% / 63,1% and at 52 weeks 70,6% / 79,7% respectively. These differences are not important in clinical practice. A material with smaller pore’s diameter is preferable, because it is mechanically more stable due to its thicker scaffold rods. New bone formation at porous tantalum - implant interface followed fixed pattern: at 2 weeks it appeared intramedullary and at the limits of the cortical drilled hole. New bone at scaffold rods was observed in small amounts at 2 weeks, with small increase at 3 weeks. However, at 4 weeks the new bone’s penetration at porous tantalum was a constant finding in a big rate of surface and very often at the whole sectioning diameter. At 16 and 52 weeks bone penetration was dense and extensive, while the new bone in the scaffold and surrounding cortical bone had histological findings of bone remodelling. Haver’s system formation was present in the first case and increased blood flow and porous texture in the second. At 4 weeks, the mean mechanical power of fixation was 18,5Mpa (shearing resistance), which is obviously greater in comparison with other studies of similar protocol involving other metallic porous materials (1,2-13,1 Mpa). This difference is due to the higher rate of porous texture, which results, for the same volume of metallic porous scaffold and the same rate of available empty space, in a greater amount of bone penetration (in absolute prices). This fact theoretically leads to higher mechanical tolerance in a shorter period of time. The same group studied the porous tantalum’s behavior in a completely functional, normally loaded model of total hip replacement in dogs¹⁹. The material was used as a metallic shell in a hemispherical acetabular component where the polyethylene was integrated in the shell with a special procedure (compression molding) in a depth of 1 mm. In this way the distal 2mm of the porous tantalum’s diameter were left available for bone penetration. The implant was removed after 26 weeks, en block with the surrounding acetabular bone. After sectioning in frontal level, in pieces of 2mm thickness, which were separated in 5 consecutive zones with equal angle, radiological examination revealed stable adhesion in the circumference (zone A and E) where the acetabular bone had greater radiological density. In a line with the dome of the hemispherical component (zone C), a radiological lucent sign appeared, until 3mm, as it was expected according to the technique of implantation (press fit, based on the periphery of the implant). However, in most cases there was a radiologic impression of filling of the lucent sign with new bone. This impression was proved after histological examination. The analysis of bone penetration in electronic microscope was as following: at zones A, E the greater amount of new bone formation was measured, approximately 25,1%. At these zones, who are in the periphery of the acetabulum, the contact between component and bone was obviously more uniform and the bone more thick. The total amount of filling was 16,8%, and 17,3% if the zone C was excluded. At this zone, as expected, the bone’s penetration ranged from nothing to very little. These rates are comparable with those for other porous materials. However taking into account the absolute bone mass per volume of material, the porous tantalum is superior, thanks to the greater amount of porous texture. This is equivalent to greater mechanical resistance of the interface. Signs of new bone formation were observed outside of the implant’s zone and acetabular limits, rarely with histological continuity with the new bone at zones A and E. This finding is a strong sign of osteoinductive properties. Histologically the new formed bone had normal structure, normal cellularity and vascularity, without cysts or osteoclastic activity. Pores not occupied from bone were occupied with dense fibrous tissue. This is desirable because the lack of free pores acts as a barrier to the synovial and wear products. We must note here, that porous tantalum’s ability for adhesion to fibrous tissue and regeneration of the latter has been studied experimentally by placing it into dog’s subcutaneous tissue²³. Older studies²⁴ showed that fibrous tissue appears histologically a healing ability in contact to porous structures with pore’s diameter bigger than 50μ. In this study a satisfactory penetration of dense, vascular fibrous tissue was observed, with similar histological and mechanical resistance features as a normal healing fibrous tissue. Andreykiv et al²⁵ interpret the bone penetration in porous tantalum as a procedure proportional to bone healing. Under the condition of initial mechanical stability, the initial filling with granuloma tissue is followed by the procedure of endomembranous ossification. The meaning of stable mechanical environment has to do with the limitation of micro-movements under limits which are different, depending on the interface. This critical limit has not been elucidated about the porous tantalum and this is why the above scientists take into account in their studies the number of 20μm, which is very low, but offers security to accept the results. The bone in the interface is a source of vessels and osteoblasts. After implantation, mesenchymal cells migrate from the surface to the porous tantalum. Mesenchymatic cells differentiate according to the model of Prendergast et al²⁶ depending on the stimulation from 2 different biophysical factors, the maximum tension of shearing deformity γ and the relevant velocity liquid/solid ν. High levels of these stimulate the differentiation of mesenchymatic cells to fibroblasts, intermediate levels  to chondral cells, while low levels to osteoblasts. The differentiation to mixed row cells is possibly based on the above model. So the initial mechanical stability is important for the penetration from mesenchymatic cells and the differentiation to bone.

Fig 2. Deep and extensive bone penetration in an acetabular cup implanted for 52 weks in a full loading canine model

Histological data from retrieved implant’s analysis

Histological studies, with electronic microscope, of an acetabular porous tantalum implant, which was removed after recurrent hip dislocation two years after surgery²⁷, revealed bone penetration and bone adhesion in the entire surface of the implant. (Fig 3) Electronic microscopic study depicted deep bone penetration, particularly in the periphery of the shell, without fibrous tissue interposition in bone-tantalum interface. Histological depiction was compatible with the radiological depiction of complete implant’s integration.

Fig 3. Retrieved TMT acetabular component. Excision was performed with the use of a special cutting instrument due to stable bony fixation.

Clinical data from the use of TMT cups and other applications

The TMT acetabular component consists of a porous tantalum shell with an elliptical flat shape geometry and a solid attached polyethylene after specific preparation (compression molding). Polyethylene’s penetration into the metallic scaffold is about 1-2mm and available depth left for bone penetration about 2-3mm. Patients with sufficient bone substrate for peripheral support of the acetabular component were included in a multicenter prospective study²⁸, where specific interest was given in the presence, in the AP pelvis radiograph, of bone gap and it’s progression. Acetabular-component interface was separated in 3 contiguous zones 60^ο each, in a variant of De Lee and Charnley zones. Among 574 total hip arthroplasties, 414 were observed for a minimum of 2 years. 20% presented in the initial radiograph with a “gap” located in the central zone II, as expected. The width of half of these gaps was more than 1mm, ranging 1mm-5mm. At the last examination (2-5 years) the gap had disappeared in 85% of the cases, including all of the 5mm gaps, while the same happened with 10 co-existing osteoarthritic cysts. At arthroplasties without initial gap, the last examination revealed new lucent line in 9%, smaller than 1mm, limited in one only zone, with equal distribution between the three zones. In total, with radiographic criteria, 412 of 414 arthroplasties were stable, via bone integration, without migration. The fact that bone gap filing is taking place with new bone formation  and without acetabular component migration, has been proven by a study of migration of this acetabular implant with the EBRA software²⁹. This specific software can estimate the bone gaps with accuracy rate of 0,1mm and after reduction to scale 1:1,15 based on the distance between the radiology source and the pelvis, so the measures are in agreement with the real dimensions. It also allows the analysis of the radiographs after scanning in a high analysis electronical scanner and the estimation of possible implant’s migration in relation to stable parts of the pelvis, eliminating systematic errors from possible pelvis inclination. In this study at the equatorial of the implant, “gaps” were found of  4mm max, at 25 from a total of 180 total hip arthroplasties during 1998-2001. After 12-24 weeks all the gaps were restored (Fig 4 and 5), without component migration in any axis. In specific, implant’s migration was 0,01mm at X axis and 0,02mm at Ψ axis, which practically means absence of migration. These hips were subsequently followed³⁰ for a time ranged from 8 to 10 years. For the purpose of this study, 151 hips were available for final evaluation. The average pre-operative total Harris Hip Score was 44.0 ± 13.8 (4 to 86.75), increased at one-year to 95.2 ± 4.8 (81 to 100) (p < 0.05)  and remained constant through the latest follow-up at 97.0 ± 6.2, (58.85 to 100) (p < 0.05). The average Oxford Hip Score improved from a preoperative score of 43.3 ±6.5 to 15.2 ±2.3 at one-year postoperatively and 13.9 ±2.3 at the latest follow up. There was no radiographic evidence of gross polyethylene wear, progressive radiolucencies, osteolytic lesions, acetabular fracture or component subsidence. There were 7 (4.5%) postoperative complications all unrelated to the acetabular component. This study reveals both clinical and radiographic evidence of well fixed and stable acetabular components through 8-10 years, thus confirming the initial hypothesis of increased early stability, safe osteoindegration and more “physiologic” load transfer of the tantalum acetabular component. During the last decade porous tantalum has been used in several orthopaedic applications. Its use in hip arthroplasty has been extended from primary to revision cases. Cementless acetabular revision with the tantalum acetabular shell demonstrated excellent clinical and radiographic results, even with severe (Paprosky 3A and 3B) acetabular bone defects^31,32. In the area of knee arthroplasty, porous tantalum metaphyseal cones³³ effectively provide structural support for the tibial implants and is a good treatment method for large tibial bone defects during revision knee replacement. Porous tantalum cone has also been used³⁴ as an “internal plate” for reconstruction of a combined segmental/cavitary defect of the proximal tibia in difficult cases sush as Charcot Arthropathy.  Support of femoral head^35,36 in avascular necrosis- Ficat stages I and II- with porous tantalum rods has shown good results, comparable with those using vascular fibular autograft. Other applications include salvage patella replacement^37,38, oncological implants³⁹ for bone integration and soft tissues adhesion, carpal-cubitocarpal joint fusion⁴⁰, arthrodesis in cervical and lumbar spine^41,42, subchondral replacement scaffolds⁴³ in chondral lesions repair. Recent experimental studies in a model of hip arthroplasty in dogs⁴⁴ have shown that combining a tantalum’s shell with an intravenous dose 0,1 mg/kg of biphosphonate zoledronic acid immediately after surgery, led to a 85% greater bone formation into the porous material, 6 weeks after implantation. This finding means that porous tantalum in combination with biphosphonates can increase the biological fixation into the bone’s interface. This combination, as proved by recent experimental studies⁴⁵, can be performed and with local use of biphosphonate after physical and chemical connection with hydroxyapatite coated porous tantalum implant: intramedullary implantation in ulna bone at a dog showed statistically important greater area of surface been bone integrated and in total, greater volume of new bone formation in contact and around the implant. 

Fig 4. Polar gap at the initial AP radiograph. The gap was filled with new bone formation at 24 weeks. Despite the fact that the periphery  of the implant was left uncovered, the arthroplasty functions well, without signs of loosening at the latest follow up

Fig 5. New bone formation onto uncovered shell. This is a sign of implant’s osteoinductive properties.

Conclusions:

Porous tantalum is a relatively new biomaterial with unique mechanical properties. It has a great porosity, up to 80% of its volume, that favors bone penetration and osteoindegration. Its elasticity is close to subchondral bone, offering more normal patterns of load transmission, while at the same time its adherent nature offers initial stability after implantation; this ensures a mechanically stable environment which is important for favorable long term results. The capability of different shapes and patterns in manufacturing allows the consideration of porous tantalum in a variety of orthopaedic surgical applications. For any implant, where we require good long-lasting functionality, the proof in practice with continuous observation is essential. This is established by long-lasting clinical prospective studies, and laboratory-histological examination of well-functioning implants which are removed for some reason. Experimental data, however, and the first to mid-term clinical results, indicate that the unique physical and chemical properties of porous tantalum provide new opportunities in construction and application of orthopaedic implants. 

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This is a peer reviewed paper 

Please cite as: Stefanos D Koutsostathis: Biological processes at bone – porous tantalum interface. A review article.

J.Orthopaedics 2009;6(4)e3

URL: http://www.jortho.org/2009/6/4/e3

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