RECONSTRUCTION 2023, Jan, 08 | 12:00 am

Metaphyseal Porous Metal Cones

Introduction

  • Highly porous metals, particularly porous tantalum, are relatively new biomaterials that offer several advantages over other current conventional materials used for implants due to their uniformity, structural continuity, strength, low stiffness, high porosity, and high coefficient of friction.

  • Clinical validation of the excellent potential for biologic fixation with porous tantalum and titanium in a wide variety of adult reconstructive applications has been recently reported.

  • The design intent for these porous tantalum metaphyseal (TM) cones was to develop a set of implants that would address the variable patterns of severe tibial bone loss encountered during revision total knee arthroplasty.

     

    P ITFALLS

    • Contained defects with a substantial supportive cortical rim may be more appropriate for impaction grafting, particularly in younger patients.

       

    • Small uncontained defects that are less than 5–10 mm in depth and isolated to one tibial plateau will likely be more amenable to standard metal blocks.

     

    Indications

  • Large contained or uncontained tibial or femoral bony defects in a failed total knee replacement due to instability, osteolysis, infection, or aseptic loosening.

  • The size of the defect is typically larger than is appropriately reconstructed with traditional modular blocks or wedges.

     

    Controversies

    • Reconstruction of large tibial or femoral defects in young patients may be more appropriately performed with bulk allograft in an attempt to reconstitute bone stock for future revision surgery. Alternatively, large defects may be amenable to reconstruction with custom prostheses or tumor “megaprostheses” in patients with insufficient bone support or potential for osseointegration.

    • The porous metal metaphyseal cones were developed to provide mechanical support with biologic integration and avoid the graft nonunion and resorption that can occur with bulk allograft.

    • Removal of the porous metal metaphyseal augments has not been required to date and may be technically challenging. Therefore, the surgeon should consider this potential limitation when deciding whether these implants are indicated for

    a particular patient and revision total knee arthroplasty.

     

     

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    • The defects can be classified with the Anderson Orthopaedic Research Institute Bone Defect classification. Porous metaphyseal cones are typically indicated for types 2 and 3 defects, which are characterized by moderate to severe cancellous and/ or cortical defects.

      Treatment Options

      • Smaller bone defects can be effectively treated with limited amounts of morselized cancellous bone graft, small-fragment structural allograft, thicker polyethylene inserts, or use of modular augments attached to revision prosthetic implants.

      • Large or massive bone defects require more heroic reconstructive efforts such as the use of large structural allografts, impaction bone grafting techniques with or without mesh augmentation, fabrication of custom prosthetic components, or the use of specialized hinged knee components.

      • The ideal reconstruction method for large and massive bone defects has yet to be determined. The porous metal metaphyseal cones are indicated for these large tibial and

      femoral defects and were designed to avoid the incidence of nonunion and resorption associated with bulk allograft reconstructions. Short-term evidence now exists that supports the use of these novel implants in the reconstruction of large tibial defects in revision total knee arthroplasty.

       

      Examination/Imaging

    • A thorough physical examination should be performed, as before any revision knee arthroplasty.

      • Skin and soft tissues

      • Ligament stability

      • Extensor mechanism and knee motion

      • Neurovascular status of the limb

    • Radiographs should include anteroposterior (AP), lateral, and patellofemoral views of the affected knee.

      • The AP and lateral views should be sufficiently proximal and distal to evaluate the entire extent of osteolysis or bone loss in the femur and/or tibia.

      • In AP (Fig. 1A) and lateral (Fig. 1B) radiographs of a patient with severe medial tibial plateau

deficiency secondary to osteolysis and polyethylene wear, the osteolysis can be seen extending past the tibial stem.

 

 

 

A B

FIGURE 1

 

 

 

Metaphyseal Porous Metal Cones

 

A B

FIGURE 2

 

 

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  • Plain film radiographs may underestimate the true extent of bone loss, particularly with respect to osteolysis secondary to polyethylene wear. A computed tomography (CT) scan or magnetic resonance imaging (MRI) may be helpful for preoperative planning in cases of severe osteolysis or polyethylene wear to delineate the amount and location of deficiency and remaining bone stock available for the reconstruction.

    • Coronal (Fig. 2A) and sagittal (Fig. 2B) CT scan images of the patient in Figure 1 demonstrate complete cortical disruption due to the large osteolytic defect in the medial tibial plateau. The osteolysis and cortical wall absence are well delineated on the CT images.

    • CT scans and MRIs should be performed utilizing techniques that minimize metal artifact and maximize visualization of the osseous structures.

    Positioning

  • The surgical positioning of the extremity and the patient is at the discretion of the surgeon and does not differ from the proper positioning required to adequately perform standard revision total knee arthroplasty procedures.

  • It is mandatory the surgeon position the patient in a manner that allows sufficient tibial exposure and

    visualization necessary to address these large defects.

     

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    This is obtained through knee hyperflexion and tibial external rotation. This can be achieved by keeping the foot free and utilizing a footrest placed at the position of maximal knee flexion.

    • Alternatively, many commercial leg holders are available that provide added assistance during the surgical procedure; however, the surgeon must ensure that use of the leg holder does not preclude the necessary exposure discussed above.

      Portals/Exposures

    • The surgical exposure required for the use of the porous metal metaphyseal cones is no different than that required for typical revision total knee arthroplasty procedures and is discussed in Chapter 17.

       

      P EARLS

      • An accurate level of the final tibial and femoral cortical supporting surfaces will be determined after the “freshening” cuts and may create smaller deficiencies than originally anticipated upon initial removal of the failed implants.

         

      • The porous TM cones may be utilized with any revision total knee system; however, the unique features and dimensions of the metaphyseal regions of the tibial and femoral components of each system should be assessed for compatibility prior to the surgical procedure.

         

        P ITFALLS

      • Failure to adequately remove all fibrous and osteolytic tissue from the bone surfaces of the tibial and femoral metaphyseal cavities will create difficulty in accurate determination of the correct porous metal cone size and shape.

       

    • The existing and failed tibial and femoral components are carefully removed with a variety of available techniques that are well described in Chapter 19. The residual fibrous tissue is débrided and removed to ensure an accurate assessment of the bony anatomy.

      Procedure

      Step 1: Defect Assessment

    • The surgeon must perform an accurate assessment of the tibial and femoral defects. The quantity and location of remaining cortical and cancellous

      bone must be noted and considered in the final assessment of whether porous metal metaphyseal cones are indicated to augment the reconstruction.

    • The most common tibial scenario encountered is typically a severe contained or uncontained medial tibial plateau bony defect with varying amounts of lateral tibial plateau remaining for structural support (see Fig. 1). The most common femoral defect appropriate for porous metal cones is a severe medial and lateral condyle cancellous bone deficiency with an intact, yet minimally supportive, cortical rim.

    • The assessment should include a determination of

      what size and shape of porous metaphyseal cone might be appropriate, with respect to its fit within the tibial or femoral metaphysis as well as the tentative location and placement required to reconstitute the proximal tibial or distal femoral supporting surface.

       

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      P EARLS

      • Familiarity with the various TM cones and cone trials is helpful. The surgeon must be familiar with all the sizes and shapes prior to surgical implantation and should have the industry sales representative present with all available implants well

        in advance of the surgical procedure.

         

      • Do not be overly concerned with stability of the smooth plastic trials, as the actual porous tantalum cone will have a greater interference fit and stability due to the coefficient

        of friction.

         

        P ITFALLS

      • Avoid excessive removal of vital cancellous and/or cortical bone, as this may be a sign the TM cone is not necessary.

       

      Metaphyseal Porous Metal Cones

       

      Step 2: Alignment and Defect Preparation

  • Once it has been determined that a porous metaphyseal cone is appropriate and required for reconstitution of the metaphyseal defect, an intramedullary stem trial or reamer may be used to create an appropriately aligned finishing cut,

    reconstitute a tibial or femoral surface perpendicular to the mechanical axis of the tibia/femur, and assess the correct alignment and position of the metaphyseal cone (Fig. 3).

  • Visual inspection of the metaphyseal region and associated defect is performed with respect to the fit of the TM cone trial.

  • A high-speed burr is used to contour the metaphyseal bone to accommodate the TM cone trial with the maximal bone contact and stability possible.

  • The TM cone trials can be used with the trial tibial and femoral components and stems at this point to assess joint alignment, stability, and motion for final implant determination as routinely performed in revision total knee replacement.

     

    Instrumentation/ Implantation

    • Intramedullary reamer or stem trial.

    • High-speed round burr.

    • Porous metal tibial cones (Zimmer, Warsaw, IN). These are available in a number of different shapes and sizes to accommodate the variety of bone defects encountered as well as to match the size of the patients’ proximal tibial metaphyseal region. The basic tibial shapes include full and stepped designs with four different widths and two different heights. A variety of femoral shapes and sizes are available as well.

     

     

    FIGURE 3

     

     

    P EARLS

    • The rotation of the final implant is not dependent on the final rotation of the femoral or tibial components, as the porous metal cones are designed to fit within the defect to reconstitute the metaphyseal platform. There is sufficient room within the porous metal cone to allow rotation

      of the tibial and femoral components into correct position to optimize stability and patellofemoral mechanics.

       

    • The morselized allograft and/or putty can also be placed along the internal and most inferior aspect of the porous metal to prevent intrusion of cement from the undersurface of the implant.

     

    Metaphyseal Porous Metal Cones

     

    Step 3: Final Metaphyseal Cone Implantation

    • The appropriate porous tantalum cone size and shape are chosen.

    • The final implant is impacted in the tibial (Fig. 4A)

      |or femoral (Fig. 4B) metaphysis carefully with size-specific impactors, and positioned with intimate contact against peripheral bone to obtain a stable frictional fit.

       

       

       

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      A

       

       

       

      B

      FIGURE 4

       

       

       

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      P ITFALLS

      • Be careful of overly aggressive impaction of the final implant. Tibial and femoral metaphyseal bone in the revision setting is typically sclerotic, damaged, mechanically weak, and prone to inadvertent fracture. The

        frictional coefficient of the actual porous tantalum implant will create greater resistance to insertion and subsequent stability.

         

      • Tibial components with wider keels will have less rotational freedom within the porous metal metaphyseal cone. This may limit the ability to enact the proper tibial component rotation independently from the metaphyseal defect.

       

      Metaphyseal Porous Metal Cones

       

      FIGURE 5

       

  • Once the porous metal cone is in its final and stable position, any areas or voids between the periphery of the porous tantalum cone and the adjacent bone of the proximal tibia are filled with morselized cancellous bone or putty (Fig. 5).

    • This will prevent any egress of bone cement between the cone and host bone during cementation of the stemmed component (Fig. 6A and 6B).

    • The morselized allograft and/or putty also facilitates osseointegration and restoration of bone stock.

       

       

      Instrumentation/ Implantation

      • Size-specific porous metal metaphyseal cone impactors.

       

       

       

       

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      Metaphyseal Porous Metal Cones

       

      A

       

       

       

      B

      FIGURE 6

       

      Step 4: Cementation of Final Components

      • The internal surface of the cones essentially reconstitutes the proximal metaphyseal surface to provide a receptive surface for cementation of the final tibial implant to the porous cone.

      • The tibial and/or femoral revision prosthetic component is inserted through the cone using either cementless or cemented stem extensions. With either type of stem fixation, polymethylmethacrylate is placed between the porous cone and the tray and

 

 

 

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P EARLS

  • It is advantageous to contour and smooth the curing cement around the exterior of any exposed porous tantalum material, such as occurs in the area of uncontained defects, particularly in the vicinity of the medial collateral ligament (MCL). This helps minimize the postoperative medial knee pain that can occur due to local irritation of soft tissues that are intended to be mobile, such as the MCL, against the high frictional surface of porous tantalum.

     

  • Whether to utilize a cemented or cementless stem extension is at the discretion of the performing surgeon. However, regardless

of stem extension fixation, cementation must encompass the entire metaphyseal region to extend past the distal extent of the tibial porous tantalum cone

and/or the proximal extent of the femoral cone.

 

Metaphyseal Porous Metal Cones

 

A B

FIGURE 7

the proximal keel of the tibial component and/or between the box and augments of the femoral component. This is analogous to metaphyseal cementation, to unitize the prosthetic stemmed implants to the porous cone.

  • Once the cement has hardened, the remainder of the surgical procedure is carried out in standard fashion with insertion of the appropriate polyethylene insert and meticulous wound closure.

  • Postoperative AP (Fig. 7A) and lateral (Fig. 7B) radiographs demonstrate a stable reconstruction of the patient in Figure 1 with a porous TM tibial cone and cemented stems to reconstitute the severe medial tibial osteolytic defect and cortical disruption.

     

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    Postoperative Care and Expected Outcomes

    • The postoperative care of revision knee arthroplasty patients who have reconstructions utilizing porous TM cones is not different from that for those undergoing a standard revision total knee arthroplasty.

    • The patients are allowed unrestricted resumption of knee flexion at the discretion of the operating surgeon based on the quality of the soft tissues, extensor mechanism integrity, and wound closure. No motion restrictions are needed based solely on the use of the metaphyseal cones.

    • Patients are allowed to bear weight as tolerated based on the implant stability and quality of reconstruction.

      • If the surgeon achieves an inherently stable porous metaphyseal cone and final implant construct, the patient is allowed to bear weight as tolerated.

      • If the surgeon feels that, due to very severe bone loss, the mechanical stability of the construct is tenuous, the patient is restricted to partial weight bearing for 6 weeks and radiographs are obtained at that follow-up interval. If there is no evidence of implant or construct migration, the patient is then allowed to progress to weight bearing as tolerated.

    • Although difficult to see due to the extent of metal material, osseointegration at the points of osseous contact are typically evident during the first postoperative year as increased density and streaming trabeculae adjacent to the porous tantalum surface.

    • Excellent short-term outcomes can be expected, equivalent to those obtained with bulk allograft, custom implants, or large modular metal augments. Long-term outcomes are unavailable with this novel reconstruction material and technique.