Porous Tantalum in Orthopedic Surgery: An Answer or a Question?

Aless; ro Geraci

doi:10.4172/2153-0777.1000e108

ISSN: 2153-0777

Journal of Bioengineering and Bioelectronics

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Porous Tantalum in Orthopedic Surgery: An Answer or a Question?

Alessandro Geraci^*
Orthopaedic and Traumatology Department, Santa Maria del Prato Hospital, Feltre, Italy
Corresponding Author :	Alessandro Geraci Orthopaedic and Traumatology Department Santa Maria del Prato Hospital Via Bagnolis Sur Ceze 332032, Feltre, (BL), Feltre, Italy Tel: 00393284527728 E-mail: geracialessandro@libero.it
Received February 23, 2012; Accepted February 25, 2012; Published February 27, 2012
Citation: Geraci A (2012) Porous Tantalum in Orthopedic Surgery: An Answer or a Question? J Biochip Tissue chip 2:e108. doi:10.4172/2153-0777.1000e108
Copyright: © 2012 Geraci A. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Tantalum (TA) was discovered in Sweden in 1802 by Anders Ekeberg and isolated in 1820 by Berzelius Jöns. The TA is a transition hard and ductile metal, very resistant to corrosion, especially to acids, and it is a good conductor of heat and electricity. It is rather rare in nature and is found in the mineral tantalite. The TA is used in surgical instruments and systems of the prosthesis, because it does not react with the body fluids [1]. In industrial engineering, the primary use for the TA, in the form of metal powder, is in the production of electronic components, in the construction of chemical plants and in the production of armaments and aerospace devices.

For its extreme resistance to acids and compatibility with the tissues of the body, will be used in surgical instruments, optical and dental, and steel grades for bone plates. One of the most important problems of orthopedic surgeons, is to find the metals that are well tolerated by bone tissue.

The potentially best bone replacement materials are grafts derived from the patient himself. They are biocompatible, osteoconductive and osteoinductive, and there is no danger of immuno-rejection. There is, however, only a limited amount of autograft available for each patient and the extraction induces additional trauma [2,3]. With allografts, derived from donators, or xenografts from animal tissue, there is an additional risk of immuno-rejection and disease transmission [4]. In addition to these biogenic materials, metallic implants [5] as well as ceramic [6], polymer and composite [7] biomaterials have been developed for bone replacement applications. These materials should be well integrated into the remaining bone, which implies not only full biocompatibility (to avoid immunoreactions) but also osteoconductive properties in order to ensure a tight connection with bone.

In orthopedic surgery, the metal must have an elastic and mechanical quality bone-like, a capacity for resistance to dynamic forces imposed in the bone, a resistance to the human environment in which it is implanted [8]. In short, the metal must adapt to the bone to integrate with it without causing rejection, even inducing the formation of new bone. These characteristics depend not only on the type of metal selected, but above all by the porous three-dimensional geometric shape that it must have to imitate the bone [9].

Porous TA is reportedly a good substitute for structural bone graft in several applications. The most important characteristics of the TA to the bone implant are chemical and electrical neutrality, the tensile strength, pressure, torsion, to the effort of curvature, to corrosion [10]. Clinical and histological evidence from retrieved implants clearly demonstrate that these porous surfaces enhance bone tissue ingrowth and are effective in supplementing the stability of the implant by biological fixation [11]. Through the formation of a bone-like apatite layer in simulated body fluid, TA has been shown to be bioactive and biologically bond to bone. In several in vitro and animal studies, the porous TA metal has provided a scaffold for bone ingrowth and mechanical attachment. Based on animal models, clinical studies and evidence from retrieved implants, it is clear that porous surfaces support tissue ingrowth or ongrowth and are generally effective for supplementing the primary mechanic fixation by osteointegration [12]. The porosity of the TA is given by its processing in the plots as trabecular human bone. From attempts to model an bone architecture to the TA is born the “trabecular metal” (TMT) [13]. TMT has an unusually large and interconnecting porous surface which corresponds to between 75 and 80% of its total volume and an overall geometry, shape and size similar to cancellous bone. The high-volume porosity enables extensive tissue infiltration and strong attachment. The microtexture of trabecular metal is osteoconductive. Several studies have been published on this new material, taking into consideration animal models or clinical series. Bobyn et al. [14] studied the characteristics of bone ingrowth of TA in a canine model using first, cylinder implants and then a fully functional total hip arthroplasty model demonstrating high extent of bone filling. In a multicentric study Gruen et al. [15] evaluated a monoblock acetabular component reporting encouraging results despite the short follow-up (two to five years). Macheras et al. [16] undertook a radiological study to evaluate the osteoconductive properties of the TA bone interface and the possibility to fill an initial gap. The increased rate of development of the interfacial shear strength with porous TA can best be attributed to the higher volume fraction available for ingrowth. The bone ingrowth, defined as the percentage of pores filled by bone, is very high, only comparable to the one observed by Bobyn et al. [17] in the simple transcortical canine model (90% vs. 80%). But in a fully functional canine total hip arthroplasty model the same authors reported a much lower value (16.8% ± 5.7%). Engh et al. [18] in a post-mortem study of porous coated acetabular components observed a mean extent of bone ingrowth of 32%. Pidhorz et al. [19] in a post-mortem study of titanium fiber-metal porous coated acetabular components observed a mean extent of bone ingrowth of 12.1 ± 8.2%.

The TA, therefore, has a supporting function of the bone and the filling of gaps of tissue. It is used, in fact, promoting the anchoring of hip and knee implants and being used as important as augments in bone defects in failed prostheses.

But the TA is still a metal and a metal always remains as a foreign substance and inert that fits into a living material: the bone tissue. The osteoconductivity is an element of merit, which interconnects bone and metal. But the metal is not absorbed by bone, is only inhabited by it. TA remains the bone and does not evolve with it. Bone is the element which has to adapt to the host material. So why not bet on finding materials or ceramic timber with trabecular features that interact with the bone, bone substitute temporarily, to be later absorbed by bone?

Polymeric and ceramic materials can also be resorbable, ceramic materials having the advantage of higher strength and stiffness compared to the polymeric materials [20], although the intrinsic brittleness of ceramics limits their applicability. Among the ceramic materials, the calcium phosphates are known to have promising biological properties [21], in that they can be biocompatible, resorbable, osteoconductive and even osteoinductive under appropriate.

The mechanical properties of cellular solids such as porous ceramics depend mainly on three parameters: the apparent density, the properties of the base material and the architecture of the structure [22]. The possibilities of optimizing the properties of the base material are limited by the requirements of biocompatibility and bioresorbability. In previous experiments we built several periodic three-dimensional cellular solids with constant apparent density and showed by compression testing that a simple change in the architecture of the unit cell can account for variations by almost a factor of three in strength and, independently, in defect tolerance [23]. The research is focusing on ceramic hydroxyapatite, but their tribological and mechanical quality is not yet valid augment support to become important. In the Institute of Science and Technology for Ceramics-National Research Council (Faenza -Italy) it is developing the project “bone-aid” [24]. Bone-Aid is a project aimed at creating a new generation of bone substitutes with regenerative properties, based on the mineral component of human bone obtained by chemical modification of wood. At the base of this study is the chemical transformation of the wood in hydroxyapatite (a phase which makes up 75% by weight natural bone) without changing the structure of origin. These plants, according to the concepts of “tensegrity” trigger in vivo processes responsible for the remodeling and regeneration of new bone organized and bio-mechanically efficient. The application of these new design solutions will allow full functional recovery of organs used for the support and movement, in a much shorter time than current solutions using minimally invasive surgery.

The possibilities are many, many ideas are present, the proposals are still developing, deepening, improvement. Only engineering will give a real answer to the true use of TA and its eventual replacement with identical material quality but fully absorbable by the bone tissue.

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