USE OF TRABECULAR METAL IN ACETABULAR DEFECTS IN REVISION TOTAL HIP ARTHROPLASTY

Abstract

Revision arthroplasty of the acetabulum is one of the most challenging problems in orthopaedic surgery today. The goals of acetabular revision are to restore anatomy and provide stable fixation for the new acetabular component.

The most important parameter affecting the surgeon's ability to achieve this is bone stock. Bone stock deficiency must be identified and classified to plan appropriately for the surgery. The American academy of orthopedic surgeons classification of acetabular defects or the system described by Paporsky et. al offers a useful categorization of defects and development of revision strategy. Bone stock deficiency can be classified as contained (cavitary) or un-contained (segmental). A contained defect is cavitary in that the acetabulum is ballooned out and weakened but the columns are intact. An uncontained defect is segmental in that there is full-thickness loss of bone involving the acetabular rim and the adjacent anterior or posterior column.

Most bone defects can be defined by routine radiographs but on occasion Judet views are helpful to define the anterior and posterior columns. The final definition of the defect only can be made intraoperatively and may be more extensive than was anticipated. The surgeon must allow for this in planning for resource needs.

For treating these challenging cases, options include the use of large cementless hemispherical cups, reconstruction cages or rings, structural allograft, bi-lobed or “double bubble” cups, impaction grafting with cemented components, and custom triflanged components. For the most severe deficiencies, relatively inconsistent results with these options have been presented in the literature. As presented in the literature, the results of the above techniques are inconsistent. To address the multivariable nature of revising a failed acetabular component, the use of a porous tantalum system of acetabular components and augments was proposed and used as a solution for such cases. Tantalum has excellent mechanical and biologic compatibility with host bone and can be manufactured with a high-friction surface for optimizing the primary stability of the component. The characteristics of the porous structure, in conjunction with the bioactivity of material surface, are shown to induce bone ingrowth with complete osseointegration of the scaffold at 4 to 6 months.

Currently TM acetabular components exist in three versions : a monoblock acetabular component, a monoblock acetabular component with peripheral screw fixation and a modular acetabular component coated with an open cell tantalum framework. The latter allows for additional dome and ischial screw fixation and accepts a highly cross-linked and thermally stabilized polethylene liner. The cup moreover combines with modular 10°, 20° and constrained liners offering the surgeon a wide choice to address most of the instability problems that may be encountered during revision surgery. The three components have a similar elliptical shape with a flattened dome and share the advantage of the convex high-friction trabecular surface that comes into contact with bone. Owing to its high porosity (80%) and optimal 3-dimensional structure mimicking cancellous bone, with an average pore size of 550 μm. Standard acetabular shell coatings are 30% porous. This increase in porosity has the theoretical advantage of increasing bone in-growth, which has been demonstrated in the canine model. The last unique property is a low modulus of elasticity. Trabecular metal has a modulus just above cortical bone. This theoretically should improve bone remodeling of both compromised host bone and allograft. (105)

The three versions of the TM cups can be combined with porous tantalum augments, if necessary when used in the presence of structural bone defects. These augments act like structural bone graft substitutes. The cup may also be supplemented with a cage fixed into the ilium (the so-called “cup-cage” construct) to offload the porous metal cup to allow time for bony ingrowth and cup stabilization in case of severe acetabular defect.

Despite the relatively short follow-up period, these modular components appear to provide a promising choice, with the added advantage that they accept a variety of polyethylene liners. Porous tantalum showed excellent bone ingrowth, and no change in position was noted. The healing and incorporation of allograft bone nearby the porous tantalum is remarkable. Clinical results were good and radiological assessment showed excellent gap filling, bone apposition and incorporation of bone grafts. Similar results have been reported in other studies using TM cups. (136)

There are some limitations of use of TM revision system; such as lack of longer clinical follow-up period to determine the efficacy of porous tantalum in RTHA, the high cost of manufacture of these implants, inability to easily replace the bearing surface if a revision for polyethylene wear or osteolysis is required in monoblock component, undesirable use of porous tantalum augments for the younger patient, and incidence of a transverse acetabular fracture associated with the use of the TM revision acetabular component.

Good short-term clinical and radiological results have been reported by many authors in type II, IIIa and type IIIb defects using trabecular metal revision shells and augments. Longer-term follow-up of these cases remains mandatory to provide further insight into the long-term performance of this material.