Revision Acetabulum Treatment Options: The Use of Porous Tantalum in Complex Acetabular Revisions

Charles Secretan, M.D., FRCSC
Guy Lavoie, M.D., FRCSC

Despite the success of primary total hip arthroplasty (THA), the absolute number of patients requiring revision surgery is increasing due to the broadening indications and the increasing number of THAs being performed. During revision surgery, deficits in host bone are common and extensive acetabular bone loss can be challenging and technically demanding. Several treatment options have been employed as possible solutions to acetabular bone deficits where there is a high hip centre. Some of these include a dual diameter cup, a structural bone allograft, the use of cages, the use of jumbo cups and more recently, the use of tantalum acetabular components with tantalum augments. Tantalum is a material that demonstrates particular promise in this regard because of its inherent mechanical properties, overall structure, biocompatibility and bioactivity.

Porous tantalum has an open-cell structure comprised of repeating dodecahedrons with a microstructural appearance similar to cancellous bone (Figure 1)1,2. The modulus of elasticity for porous tantalum is similar to that of subchondral bone but the yield and ultimate strength is ten times greater2. This creates an environment which limits the stress shielding observed in more rigid implants but at the same time bears physiologic loads greater than either cancellous or cortical allografts1,2. Additionally, porous tantalum has a relatively high coefficient of friction compared to both conventional porous coatings and natural bone grafts, providing improved initial stability while bone ingrowth occurs1,2. Tantalum has been used in a variety of medical implants since the mid-1900's and is known to be relatively immunologically inert in vivo3. However, porous tantalum is biologically active and animal studies have demonstrated bone ingrowth within porous tantalum structures4,5. These studies have demonstrated histologically that there is increased bone-implant contact with the passage of time, as well evidence of Haversian remodeling within the pores of the implants. Mechanical testing revealed that the shear fixation strength of the tantalum implants at four weeks is much higher than CoCr sintered beads and several other porous materials4. Additionally, retrieval studies of tantalum acetabular components have confirmed the ingrowth potential of human bone within the tantalum pores1.
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Figure 1. Scanning electron micrograph demonstrating the microstructural appearance of porous tantalum. (Image complements of Zimmer)

In the revision setting, acetabular defects with minimal bone loss (Paprosky type I and type II) have been treated successfully with the use of beaded hemispherical acetabular components secured with screws, with or without cancellous allograft. In contrast, revisions involving greater than 50% host bone loss (Paprosky type IIIa and type IIIb) have proven much more difficult resulting in failure rates of up to 19%6. It is in these situations, and particularly in those involving a pelvic discontinuity, that the use of porous tantalum shows promise. Briefly, the surgical technique involves first identifying the desired cup position, followed by progressive reaming to engage the host bone. The deficient regions are identified and augments are placed into the defects and secured with screws. Morselized bone graft is placed in the contained regions. A small amount of cement is then placed on the side of the augment facing the cup and the acetabular component is impacted into place. Multiple screws are used to supplement the fixation (Figure 2). Utilizing techniques analogous to these, Paprosky et al. recently reported on 23 revision cases with major acetabular bone loss (type IIIa and IIIb) demonstrating no mechanical failures with an average follow-up of 35 months7. Also, in 2007, Weeden et al. described 43 revision cases with type IIIa and IIIb pelvic defects treated in the above manner. With a mean of 2.8 years follow-up there was a 98% success rate observed6. Indeed, the use of porous tantalum in the reconstruction of difficult Paprosky type IIIa and IIIb acetabular defects does demonstrate short-term results which are encouraging. Other developments include the use of a porous tantalum acetabular component supported by a protective cage in complex revision cases, as described by Gross et al8. It is speculated that the porous nature of the tantalum may provide improved remodeling of morselized allograft resulting in improved biologic fixation and decreased stresses on the cages. Work is also being done utilizing tantalum shells as "internal plates" with some systems employing the use modular titanium cage flanges (Figure 3).

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Figure 2. A) AP and B) Lateral of a Paprosky grade IIIa acetabular defect treated with a posterior-superior tantalum augment and tantalum acetabular component.

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Figure 3. A) AP and B) Lateral of a Paprosky grade IIIb acetabular defect with extensive bone loss treated with a tantalum wedge, a modular titanium cage flange, and a large tantalum shell.

Poor outcomes in patients with difficult Paprosky type III defects have prompted surgeons to seek out more reliable methods of fixation in these technically demanding situations. Porous tantalum is a viable treatment option because it generates an environment with increased mechanical stability, bioactivity, and a more physiologic transfer of load to host bone. All of these characteristics lead to improved early bone ingrowth and a more durable construct. The use of tantalum components with augments is also less technically demanding and requires less surgical dissection than many of the techniques which employ cages or large structural allografts. Indeed, the use of tantalum appears to answer many of the questions faced in these difficult cases but this optimism must be tempered by the fact that the results reported to date are only short-term. Further work is still required to confirm that the theoretic advantages of porous tantalum will lead to improved long-term outcomes in complex acetabular reconstructions.

References

  1. Levine, B., Della Valle, C.J. & Jacobs, J.J. Applications of porous tantalum in total hip arthroplasty. J. Am. Acad. Orthop. Surg. 14, 646-655 (2006).
  2. Levine, B.R., Sporer, S., Poggie, R.A., Della Valle, C.J. & Jacobs, J.J. Experimental and clinical performance of porous tantalum in orthopedic surgery. Biomaterials 27, 4671-4681 (2006).
  3. Black, J. Biological performance of tantalum. Clin. Mater. 16, 167-173 (1994).
  4. Bobyn, J.D., Stackpool, G.J., Hacking, S.A., Tanzer, M. & Krygier, J.J. Characteristics of bone ingrowth and interface mechanics of a new porous tantalum biomaterial. J. Bone Joint Surg. Br. 81, 907-914 (1999).
  5. Bobyn, J.D., Toh, K.K., Hacking, S.A., Tanzer, M. & Krygier, J.J. Tissue response to porous tantalum acetabular cups: a canine model. J. Arthroplasty 14, 347-354 (1999).
  6. Weeden, S.H. & Schmidt, R.H. The use of tantalum porous metal implants for Paprosky 3A and 3B defects. J. Arthroplasty 22, 151-155 (2007).
  7. Flecher, X., Sporer, S. & Paprosky, W. Management of Severe Bone Loss in Acetabular Revision Using a Trabecular Metal Shell. J. Arthroplasty (2008).
  8. Gross, A.E. & Goodman, S.B. Rebuilding the skeleton: the intraoperative use of trabecular metal in revision total hip arthroplasty. J. Arthroplasty 20, 91-93 (2005).

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