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Cone augment techniques restore bone, help revision TKA implants work optimally
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Significant bone defects associated with component loosening, subsidence and osteolysis are commonly encountered during revision total knee arthroplasty. The severity of bone loss may pose a challenge for establishing a stable construct during the procedure.
Management of these severe defects with significant bone loss of either the proximal tibia or the distal femur includes using modular augmentation, structural allografts or megaprostheses. The choice of reconstruction technique depends upon the severity of the defect and the available technology. Experience has shown that preoperative radiographs typically underestimate the actual amount of bone loss in a failing total knee arthroplasty (TKA). In fact, CT scans are a better imaging technology for assessing the bone status of the tibia and femur adjacent to the implants prior to revision TKA. Accurate preoperative assessment of the juxta-articular bone is important in the preoperative planning so that the appropriate revision implants are available during the procedure.
Figure 1. The Trabecular Metal tibial cone aids in
revision total knee arthroplasty (TKA) cases
(Zimmer; Warsaw, Ind.).
Images: Scuderi GR
The recent addition of Trabecular Metal metaphyseal cones (Zimmer; Warsaw, Ind.) for either the proximal tibia or the distal femur has provided an effective option that can be used in a range of revision cases in conjunction with most available revision TKA systems. These cones are available in multiple shapes and dimensions (Figure 1), and fit many of the defects that are encountered.
Enhanced structural support
The porous tantalum Trabecular Metal material provides structural support for the core implants and eliminates the concerns often associated with traditional autogenous bone grafting, such as graft resorption, disease transmission, improper graft size, allograft fracture and the occasional graft shape mismatch. The porous structure of the cones provides for bone ingrowth by the host bone, creating stable metaphyseal fixation for either the femoral or tibial component.
Giles R. Scuderi
Those cases with significant bone loss, such as Andersen Orthopaedic Research Institute (AORI) type 2B or type 3 defects, present greater challenges. Thus, porous tantalum cones for the proximal tibia and distal femur provide a modular alternative for metaphyseal reconstruction. They are used in the proximal tibia and implanted in a press-fit cementless manner to recreate the cortical rim and provide a stable platform for the final component. The variety of shapes in which they are available can help address both cancellous bone loss and cortical defects.
The distal femoral cones help re-establish the metaphyseal–diaphyseal junction and create a stable base for the femoral component. These modular constructs absorb compressive loads and provide both structural and mechanical support. The material properties of porous tantalum allow it to achieve rapid bone in-growth and osteointegration with the potential for long-term biologic fixation and restoration of bone stock.
Surgical technique
Following removal of the failed knee components, the severity of the bone defects is assessed.
Figure 2. Following removal of the tibial component, a large osteolytic bone defect was encountered in the tibial metaphysis.
On the tibial side, it is important to restore the metaphyseal cortical rim. In many cases, especially those with osteolysis, there is a cavitary defect of the proximal tibia and a thinning of the cortical rim (Figure 2). To re-establish the tibial metaphyseal cortical rim for subsequent stability of the revision tibial implant, a porous tantalum tibial cone is selected that will fill the defect and provide a base for the tibial component.
The surgeon contours the cavitary defect with a high-speed burr, just enough to accept the cone in a tight press-fit manner. He or she then places a provisional template in the defect to assess the final preparation needed. Then, the porous tantalum cone is impacted into place in a tight press-it manner (Figure 3).
Figure 3. The final porous tantalum device is impacted in place.
Next, any small defects between the cone and the host bone are filled with either a bone substitute material or allogenic cancellous bone chips to avoid having any bone cement creep in between the host bone and the cone.
Figure 4. This patient’s radiograph shows the
final components for a revision TKA case.
The final tibial component is then implanted onto the restored tibial metaphysis by means of all the traditional techniques, with the goal of restoring the mechanical axis and achieving stable fixation (Figure 4). Additional augments can be added to the tibial tray, if necessary, to restore the mechanical axis.
After that, the core tibial component is cemented to the tibial metaphyseal trabecular cone and the decision is made to use either a press-fit cementless stem or a cemented diaphyseal stems, which is dependent upon the quality of the residual bone and stability of fixation.
Severe distal femoral bone loss
In cases with severe distal femoral bone loss, porous tantalum cones are effective for restoring the distal metaphysis and they provide a foundation for rebuilding the distal femur. A femoral tantalum cone is selected that will fill the defect, and the bone is contoured with a high-speed bur to accept the implant. A provisional implant helps guide the preparation of the host bone. In this particular case (Figure 5), a trumpet-like defect was encountered in the distal femoral metaphysis so, following bone preparation, a femoral cone was implanted (Figure 6). This was done to restore the femoral metaphysis because this approach then provides the base for reconstruction of the femur.
Figure 5. A trumpet-like defect of the distal femur was seen with severe metaphyseal bone loss.
Similar to the tibial cones, the femoral cones are impacted in place with cementless fixation and any residual voids are filled with bone graft substitute or allogenic cancellous bone graft to avoid cement penetration between the cone and host bone. Additional augments used in the distal femur and posterior condyles can be placed on the femoral component to establish the distal joint line and proper posterior condylar offset. The core femoral component is then cemented to the metaphyseal cone and the choice of whether to use press-fit or cemented diaphyseal stems should be based upon the quality of the bone and the stability of fixation (Figure 7).
Figure 6. The femoral trabecular cone is depicted in place during surgery.
Clinical results
Early reports demonstrate excellent short-term follow-up of 12 months to 47 months with evidence of osseointegration and no mechanical failures in a group of 25 patients treated with the technique. In a 2009 study, Long and I reported the results of 16 revision TKAs in which tibial cones were used to manage severe type 2 and type 3 tibial bone defects. At a minimum and average follow-up of 24 months and 31 months, respectively, no mechanical failures occurred and all radiographs demonstrated stable osseointegration into the cones.
Figure 7. This radiographic image shows the
final component that was used.
These clinical results are similar to those reported in 2008 by Meneghini and colleagues at the Mayo Clinic.
In research published in 2011, Howard and colleagues reported that tantalum femoral cones also provided effective structural support with biologic fixation for femoral components in revision TKA.
Larger studies with longer follow-up are needed to study this area further but, to date, these augments appear to provide a viable alternative to structural allograft and impaction grafting in the management of severe tibial and femoral bone defects in revision TKA.
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Disclosure: Scuderi is a consultant to Zimmer, Salient Surgical, Medtronic and ConvaTec, and receives royalties from Zimmer.