Treatment of Bone Losses in Revision Total Hip and Knee Arthroplasty Using Trabecular Metal : Current Literature

Revision Total Knee or Hip Arthroplasty is challenging procedures for surgeons usually characterized by bone loss. There are different options available to treat those bone losses. However, there is still a concern on the stability of bone-implant interface, which is mandatory to achieve good long-term results in prosthetic implants. Recently, porous tantalum has been introduced, with the aim of improving the bone-implant interface fixation and implant primary stability. Different solutions for the treatment of bone defects in both revision Total Knee and Hip Arthroplasty have been proposed. In revision Total Hip Arthroplasty (THA) tantalum shells can be used to treat Paprosky type III defects also, because of their mechanical properties. Similarly, trabecular metal has been proposed in revision Total Knee Arthroplasty (TKA), being considered a viable option to treat severe type 2 or 3 defects. The aim of this paper is to review the mechanical properties and characteristics of tantalum. Furthermore, we will discuss its role in treating bone defects in both revision THA and TKA, as well as the outcome reported in literature.


Introduction
Osteointegration in orthopedic implants can be compared to bone healing in fractures.Stability at the bone-implant interface is mandatory for implant's success.Excessive micromovements result in fibrous attachment fixation with a collagen structure throughout the porous network [1], while bigger movements can even produce a pseudocapsule [2] surrounding the implant that can lead to early failure.The limits for implant integration have been established in some studies [2,3] showing bone ingrowth for movements smaller than 50 microns, pseudoligamentous healing from 50 to 150 microns, and pseudocapsule formation for movements bigger than 150 microns.However, a small mechanical stimulation at the interface can enhance implant healing in the first phase after surgery.
Another important aspect in implant integration is pore size; recent studies by Bobyn and associates [3] showed the relationship between pore size and bone ingrowth rate.This may be related to different microenvironments present within different sizes pores and their effect on osteogenesis [4].
Traditionally, orthopedic implants have been produced with titanium, stainless steel, and cobalt-chromium (Co-Cr).However, the limits of these materials in porosity, high elastic modulus, and low frictional characteristics have pushed researchers to develop new materials with better performances.One of the answers to this search is tantalum, firstly introduced by Zimmer (Warsaw, IN).The excellent resistance to erosion-corrosion and high frictional characteristics associated with its bioactivity make tantalum a promising material for orthopedic implants.

Basic Science
Tantalum is an elemental metal with great characteristics of biocompatibility, corrosion resistance, and a porous geometry.Tantalum has a uniform and continuous structure that allows for greater strength, lower stiffness, higher volumetry porosity, and a higher coefficient of friction compared to other porous metals [5].
Porous tantalum structure is produced by pyrolysis of thermosetting polymer foam; it creates a low-density vitreous carbon skeleton characterized by a repeating dodecahedron that produces a regular structure with interconnecting pores [6].Tantalum is then deposited on this structure by vaporization or infiltration [7][8][9].The production process usually produces coverage that ranges between 40 and 60 m in thickness.However, designers and engineers can vary all parameters of implants regarding shape and pores dimension acting on the skeleton production and metal apposition.For these reasons, tantalum mechanical properties can be changed varying the thickness covering the polymer skeleton (Figure 1).
In common orthopedic application, tantalum pore dimension ranges from 400 to 600 m resulting in a 75% to 85% porosity.At the end of the process, 99% of weight is due to tantalum and 1% to the skeleton [6].The porous structure has higher porosity than other materials used in orthopedic implants like sintered beads (30-35%) and fiber metal (40-50%) [8], providing a great increase in elasticity and making porous tantalum mechanical properties very close to those of the subchondral bone.
The great number of pores and their dimension positively affects stability of the implant, increasing friction coefficient, up to even three times higher than sintered beads materials [10].
In the long period, primary stability is enhanced by tantalum bioactivity that produces a stable oxide component (Ta 2 O 5 ) on the surface forming a bone-like apatite layer [11].These characteristics represent a great improvement compared to conventional materials.Classically, implants' bioactive coatings applied to the surface of the implant grant bone integration.The effect is to bond bone; however, they showed to degrade over time and debond from the implant surface with possible loosening.
Tantalum morphology provides a scaffold for bone growth and osteoblast interaction [2,7,12,13], producing a bone ingrowth that has been measured to be 0.2 to 2 mm in 4 weeks [12].For these reasons, pullout tests demonstrated that tantalum could resist doubled shears forces, compared to conventional materials (18.5 MPa versus 9.3 MPa of the Co-Cr sintered beads) [3,7,12].Complete incorporation into the bone is usually achieved around 16 weeks after implant and no major variations can be observed at one year [7].
The association of great friction rate and the subsequent osteointegration makes tantalum the perfect material for bone loss managing and defect filling.
In addition, tantalum is relatively inert in vivo and can be compared to titanium; by the way, tantalum can produce occasional macrophage reaction at the implant-tissue interface [14].At our knowledge, there are no studies reporting inflammatory reaction to tantalum.

Treatment of Bone Deficiency in Revision Total Hip Arthroplasty
The number of Total Hip Arthroplasties (THA) is increasing, as well as life expectancy.Consequently, the number and complexity of revision THA continue to increase, with a further increase expected in 2030 of 137% compared to 2005 [15].Revision THA are usually challenging for surgeons, particularly on the acetabular side, which is more commonly characterized by bone loss.In socket revisions, the surgeon has to manage bone defects and provide a stable construct; in these cases an accurate preoperative planning is mandatory to achieve good results [16].Different classification systems have been used for acetabular defects to describe the severity of bone losses.Paprosky et al. developed a classification evaluating the acetabular defect type, size, and location, pointing at the appropriate reconstructive options' selection.The classification is based on four radiographic measures obtained through an anteroposterior radiograph of the pelvis: (1) superior hip center migration, (2) ischial osteolysis, (3) position of the implant relative to the Kohler (ilioischial) line, and (4) teardrop osteolysis (Table 1) [17].
Bone losses are consequently classified depending on the support for the new acetabular cup of the acetabulum: completely supportive (type I), partially supportive (type II), or unsupportive (type III).The reconstruction options for acetabular revision include hemispherical cementless cup, jumbo cup, structural or morselized allograft, custom or triflanged implants, and antiprotrusio cage [18].Trabecular metal implant or augments can be useful to treat major bone defects, allowing for increased biological fixation, primary stabilization, and easier surgical technique, also when compared to structural allograft.

Reconstructive Options Using Trabecular Metal.
Highly porous metal components are popular options for both primary and revision THA.Conversely to titanium components, which require at least 50% of supporting bone stock, these cups can be used also in Paprosky type III defects, because of their mechanical properties [18,19].Using trabecular metal cups allows for good osteointegration and primary stability of the system [20].There are two types of tantalum cups: with incorporated locking mechanism for the polyethylene insert or predisposed for cementation of the polyethylene insert.The second option allows a greater malleability of implant by positioning it in the area of the defect not considering its position and orientation that can be compensated by reorienting the insert at the time of cementation [18].When less than 50% of bone stock is available, structural allograft or metal augments may be required to allow stability of the cup.Porous metal augments can be assembled intraoperatively acting as a structural allograft.The surgical technique includes securing the augments to the pelvis with multiple screws and then securing the revision shell to the augment with bone cement.
The shell is then secured to the pelvis with multiple screws [6,21,22].When the augment is secured to the acetabular shell, the surface area of the cup available for bone ingrowth can be raised up to 30 or 40% of the surface more than the implant with no augments [16].However, care should be used implanting trabecular metal cups: Springer et al. detected 7 transverse acetabular fractures in a series of 37 trabecular metal cup revisions; all fractures likely occurred intraoperatively [23].
Trabecular metal augments can be also useful for type III defect; in presence of massive contained or uncontained defects involving more than 50% of the acetabulum, a protective cage may be necessary.Particularly, if morselized or structural grafts are used and the necessary amount of contact is not achievable, the bone graft should be protected by a cage [24].These cages have different advantages: they can be used as a template for bone stock restoration and they allow placing the hip center in an anatomic position independently by the cage position.However, the surgical technique is demanding and the cages do not allow for bone integration [24].Different authors reported poor results using cages, with a high failure rate, particularly in those cases with insufficient column or superior dome bone stock to support the cage and the graft or in case of pelvic discontinuity [16,25,26].For this reason, different authors proposed using trabecular metal in these cases as well.Some authors proposed to use a "cup-cage" system.The trabecular metal cup, because of its properties, allows for a better healing environment for the graft.If an adequate amount of bone is not available, a cage can protect the cup.The cage protects the cup until bone graft remodeling is complete, and then the stress leaves off the cage and is transferred on the trabecular metal cup [16,24,27,28].
When the anterior and posterior walls are insufficient in a pelvic discontinuity, trabecular metal cups and augments can be used to restore acetabular ring integrity.The augments are placed in the inferior and superior aspect of the acetabular defect, secured to the bone with screws, and the cup is then inserted press-fit into the contained bone defect of host bone.To provide rigid fixation, multiple screws can be placed in both the superior and inferior hemipelvis.This construct can be used as an internal plate to stabilize discontinuity [25,28,29].

Clinical Outcomes.
Different studies focused on clinical outcomes of trabecular metal constructs in revision THA (Table 2).Given that most of these studies are characterized by short-term follow-up or small population, good clinical and radiological outcomes are reported, despite a considerable complication rate related to the complexity of the surgery [19-21, 25, 28-41].
Van Kleunen et al. in 2009 [42] reported 90 patients with Paprosky type 2 or more acetabular bone defects, treated with revision shell associated with metal augments.There were 8 cases of infection and one revision for dislocation.No revisions for aseptic loosening were detected.The authors concluded that revision shells associated with metal augments are a viable option to treat moderate to severe bone losses in acetabular revision.Jafari et al. [43] compared the outcomes of titanium and tantalum cup in 283 hip revisions with mixed acetabular bone defects; the authors concluded that there was a higher failure rate in massive bone deficiency for titanium cups compared to tantalum cups, which showed radiographical better fixation.Similarly, Fernández-Fairen et al. [44] evaluated 263 patients treated with monoblock or revision tantalum shell, with 85.9% subjective satisfaction, and no aseptic loosening at 73-month follow-up.Skyttä et al. [45] evaluated 827 patients treated with trabecular metal shell in revision hip arthroplasty with patterns of bone loss, representing the biggest population described in literature.The authors observed a 92% overall survivorship at 3-year follow-up, with 2% of aseptic loosening.Furthermore, each additional year in age decreased the risk of revision by 2.4%.Recently, Long et al. [46] evaluated 599 revision shells at a minimum of 24 months of follow-up reporting 7.8% of reoperation, with 2.3% of removed cup but without any case of aseptic loosening.
In the recent years, some authors described their experience using cup-cage constructs.Abolghasemian et al. [47] evaluated 26 patients affected by pelvic discontinuity and treated with cup-cage construct and 16 patients treated using standard cages.The authors reported a reduced complication and revision rate in the cup-cage group, with a higher pelvic discontinuity healing-rate compared to the standard cages.Recently, Amenabar et al. [48] described their results on 64 patients affected by Paprosky types 3 and 4 defects with 61% of pelvic discontinuity and treated using cup-cage constructs.The 5-and 10-year cumulative survivorship was, respectively, 93% and 85%.The authors concluded that cupcage constructs are a viable option in the treatment of major acetabular bone losses.

Treatment of Bone Deficiency in Revision Total Knee Arthroplasty
Similar to THA, also the number of revision Total Knee be a demanding procedure for both the patients and the surgeons.Usually, the surgeons have to deal with bone losses, ligamentous deficiencies, and loss of adequate fixation.Engh and Ammeen [49] proposed the "Anderson Orthopaedic Research Institute" (AORI) classification of bone defects.Femoral and tibial bone losses are classified into three types, according to the extent and severity of the defect.Type 1 defects are characterized by intact metaphyseal bone stock, without subsidence of the components nor osteolysis.In type 2 defects the metaphyseal bone is damaged in one (A) or both (B) condyles.On the femoral side these defects are usually distal to the epicondyles, and on the tibial side they are up to or below the fibular head.Type 3 defects include major bone losses, generally associated with ligamentous insufficiency, proximal to the epicondyle in the femur and distal to the tibial tuberosity at the tibia.Furthermore, bone defects can be divided into cavitary or segmental, depending on the status of the cortical ring surrounding the defect [50].The bone defects should be reclassified after removal of the components, as their amount usually increases much during surgery.A minimum defect theoretically involves less than 50% of a single condyle, with a depth of less than 5 mm.A moderate defect involves 50% to 70% of a single condyle with a depth of 5 to 10 mm.An extensive defect involves more than 70% of a condyle with a depth over 10 mm.Finally, a massive cavitary defect is characterized by the total disruption of one or both femoral condyles and can be either associated with an intact cortical rim or not [50].
In this scenario, obtaining a stable fixation of the revision implant is mandatory.Morgan-Jones et al. [52] recently defined the "zonal fixation" concept.The femur and tibia can be divided into three zones: the joint surface or epiphysis, the metaphysis, and the diaphysis.The authors suggest that solid fixation should be achieved in at least two out of the three zones.

Surgical Options Using Trabecular Metal and Techniques.
Available options to treat bone defects are bone cement, autograft, morselized or structural allograft, metal augments, cones, and sleeves [53].The chosen technique depends on patient age, life expectancy, bone loss classification, need for diaphyseal fixation, and ligamentous insufficiency.Radnay and Scuderi [51] developed an algorithm to deal with bone losses (Table 3).If the defects are small and cavitary and involve less than 1/4 of the cortical rim, they can be filled using cement or bone grafts.Conversely, if the defect involves more than 1/4 of the cortical rim or it is segmental, metal augments must be considered.Metal augments should also be considered if more than 40% of the implant interface is not supported by host bone.Metal augments resulted to be particularly useful in 5 to 10 mm type 2 defects.Major bone defects, particularly type 3 defects, are the most demanding to treat.Allograft, metal augment, hinged implant, and long stems may turn out to be useful in obtaining adequate fixation and stability in these cases.Trabecular metal is indicated in treating major bone loss, providing adequate fixation and stability.Trabecular metal has been proposed in revision TKA  because of its properties, such as increased osteointegration and structural stability.Tantalum cones can be a viable option to treat severe type 2 or 3 bone defects and can be used in association with tantalum augments to increase primary fixation [50].Stepped cones are also available when the bone defect is greater on one side of the tibial plateau compared to the other [51].
4.1.1.Tantalum Cones' Surgical Technique.Bony surfaces have to be completely debrided from necrotic bone and residual cement, and the bone defect is sized.The residual bone has to be prepared for the tantalum cones.Dedicated instrumentation with trial is available.Once the surgeon has chosen the size of the cone, the defect is prepared using high speed burr, to reach the shape of the cone (Figure 2).On the femoral side, dedicated rasp can be used, but carefully in order to avoid fracturing the impacted bone (Figure 3).Both on the tibial and on femoral side, care should be taken not to overresect the bone, because the cones have to be inserted with a tight press-fit.If there is still a defect between the cone and the host bone, it can be filled using morselized grafting (Figure 4).Once the cone is impacted into the host bone, the final tibial or femoral component is cemented into the cone.Association of a tibial or femoral stem is useful to guarantee better stability and fixation of the construct (Figure 5) [50,51,54,55].Trabecular metal has also been proposed to treat severe patellar bone defect in revision TKA.Porous tantalum patellar component allows for implantation of a polyethylene patellar component.However, using a patellar tantalum component as an alternative to bone grafting requires sufficient    4) [54,[57][58][59][60][61][62][63].
Most of these studies reported good clinical and radiological outcomes with low revision rate for aseptic loosening (0.9%), despite an increased revision rate for infection (2.2%) [55].Howard et al., in 2011 [66], reported 24 femoral cones used to fill type 2 or 3 femoral bone defects.The authors reported a high reoperation rate, but without any aseptic loosening or mobilization of the tantalum cone at short-term  of the tibial cone component higher than 95% at the latest follow-up.Few papers described the result of porous tantalum patellar component in revision TKA [56,64,65].Nasser and Poggie described their results on 11 patients affected by severe patellar bone losses and treated using porous tantalum patellar component.The authors concluded about good clinical and radiological short-term outcomes, with high patients' satisfaction [65].Kamath et al. in 2012 [56] described their results in 23 patients in which a porous tantalum patellar component was used due to severe patellar bone defects.All patellae had less than 10 mm residual bone thickness.The survivorship rate was 83% at the final follow-up.

Conclusion
The use of tantalum in revision total hip replacement provides a higher integration potential in a shorter time: this helped reducing surgeons' fears when addressing a pelvic discontinuity or big acetabular bone losses.Tantalum provides a good and reliable substitute of bone, even in the very first period after implantation.
Big bone losses can be reduced to normal and easier to handle dimensions, decreasing the surgical challenge; however, in pelvic discontinuity, the problem still can be worrisome.In these cases, tantalum osteoinductive characteristics enhance acetabular ring healing potential, representing a very effective structure and scaffold and providing a good longterm outcome.
In revision total knee replacement, as well as for hips, the osteoinductive potential of tantalum is of paramount importance in the long period, associated with the excellent structural support and defect filling characteristics provided in the first phase after surgery.
In conclusion, tantalum showed to be a very effective material in orthopedic surgery, especially in revision surgery, thanks to the great ductility of the material and its intrinsic characteristics.High porosity, elasticity, bioactivity, biocompatibility, and osteoinductivity are of paramount importance in orthopedic implants and tantalum is actually the state of the art for all these aspects.

Figure 1 :
Figure 1: Porous structure of the trabecular metal.

Figure 2 :
Figure 2: Tibial bone defect at the end of the preparation for implanting a tantalum cone.

Figure 3 :
Figure 3: Preparation with rasp for a femoral tantalum cone.

Figure 4 :
Figure 4: Tibial tantalum cone implanted.The blue arrow shows the filling of the remaining defect with morselized bone.

4. 2 .
Clinical Outcomes.Considering the recent introduction of tantalum materials, only short-term follow-up studies are available (Table

Figure 5 :
Figure 5: Postoperative X-ray showing (a) lateral view of a revision TKA with a tibial tantalum cone and (b) anteroposterior view of a revision TKA with a tibial tantalum cone.

Table 2 :
Results for revision THA using porous tantalum acetabular components (THA: Total Hip Arthroplasty, N/A: not reported).

Table 4 :
Results for revision TKA or patellar replacement using porous tantalum components (TKA: Total Knee Arthroplasty, N/A: not reported).