I Have a Patient With Distal Osteolysis and a Loose Stem. What Type of Implant Should I Use and What Bone Stock Can I Anticipate Finding at the Time of Surgery?
Mark F.
Schinsky,
MD
When preparing for a revision total hip arthroplasty, bony deficiencies must be accurately assessed and classified to ensure proper treatment choices. Several classification systems have been described for femoral defects, including the system developed by the American Academy of Orthopaedic Surgeons Committee on the Hip, the Endo-Klinik classification, the Mallory classification, the Saleh system, and the Paprosky femoral defect classification.
The Paprosky classification system provides an algorithmic approach to femoral assessment based on the location and extent of bone loss and helps guide the surgeon in selecting the appropriate method for reconstruction. The surgeon must use both the preoperative radiographs as well as the intraoperative findings to properly assess the femoral bone quality. Once the existing femoral component has been removed, a reverse hook curette can be used to sound the femoral canal, remove any existing pseudomembrane, and ensure previously used cement has been entirely removed. This intraoperative technique may reveal bone loss or defects in the patient’s host bone that was unsuspected on preoperative radiographs alone. The Paprosky femoral defect classification system can then be applied to these assessments, arranging the deficiencies into 5 basic types as follows1,2:
1. Type I: In Type I defects, bone loss is minimal and primarily of the cancellous metaphyseal bone with only partial loss of the calcar and anterior posterior (AP) bone. The diaphysis remains intact. This type of bone loss can be seen inresurfacings or after removal of a cementless femoral component without a biologic ingrowth surface. Femora with these defects are not significantly different from those seen during primary total hip arthroplasty.
2. Type II: In Type II defects, the diaphysis remains intact; however, the calcar is deficient with extensive metaphyseal cancellous bone loss. These defects are often seen after severe femoral component subsidence, metaphyseal osteolysis, or removal of a cemented component.
3. Type IIIA: In Type IIIA defects, the metaphysis is severely damaged and the defect involves the junction with the diaphysis. In this defect type, a minimum of 4 cm of scratch-fit can be obtained near the isthmus. This defect type can be encountered after removal of a grossly loose cemented femoral component that was put in with first-generation cement techniques.
4. Type IIIB: In Type IIIB defects, the metaphysis is severely damaged and the defect extends further into the diaphysis with less than 4 cm of diaphyseal bone available for distal fixation. Distal osteolysis seen with cementless femoral prostheses or failed cemented components inserted using cement restrictors can lead to this type of defect.
5. Type IV: In Type IV defects, both the metaphysis and the diaphysis are extensively damaged, the cortices are thinned with a widened femoral canal, and the isthmus is nonsupportive. These defects are rare but represent a significant challenge to reconstruct.
After the femoral bone loss is thoroughly assessed and the appropriate Paprosky femoral defect type determined, reconstruction can proceed. Reconstruction of a Type I defect can proceed similar to a primary total hip arthroplasty (THA). Implant choices include proximally porous-coated stems, 6-inch extensively coated stems, and cemented stems. Intraoperative axial and rotational stability must be achieved.
As Type II defects lack supportive proximal bone and often have minimal remaining cancellous bone, proximally porous-coated and cemented stems are generally not appropriate for reconstructing these femurs. Fully porous-coated stems that achieve distal fixation and bypass the proximal defect are preferred. A standard 6-inch stem can be used in most patients, though a calcar replacement may be required.
Since Type IIIA defects retain at least 4 cm of diaphysis near the isthmus available for a scratch-fit, an extensively porous-coated stem is an appropriate choice for reconstruction. These femora generally require the stem to be 8 or 10 inches in length to achieve fixation. The anterior bow of the femur must be taken into consideration during reaming and implant positioning. A bowed stem may be necessary to avoid perforation. Other choices for reconstruction include impaction grafting or modular, cementless, distally tapered stems.
Type IIIB defects often lack the diaphyseal bone necessary for successful placement of cylindrical, extensively porous-coated implants. This implant design had a 50% failure rate in one series when used for Type IIIB defects.2 Even with a short isthmus, modular, cementless, distally tapered stems with flutes can achieve excellent initial axial and rotational stability and are the implant of choice for these defects. The proximal modularity allows for alterations in limb length, offset, and femoral anteversion. Should the cortical tube remain intact and the canal width exceed 18 mm, impaction grafting can be undertaken. Impaction grafting involves cementing in a polished, tapered stem into a bed of firmly packed allograft. Potential advantages include improving the cancellous bed to accept cement in order to create a mechanically stable construct and restoring bone stock.
Options for reconstructing Type IV defects include impaction grafting, allograft prosthetic composites (APCs), and proximal femoral-replacing endoprostheses. The patient’s age, activity level, and comorbidities help the surgeon choose the appropriate reconstruction. APCs can potentially restore bone stock, provide proximal support for the prosthesis, and allow for soft tissue and greater trochanter reattachment. APCs are generally reserved for younger patients. In elderly patients with lower activity levels or those patients who can not tolerate a more extensive procedure and have a nonreconstructable proximal femur, a femoral replacing endoprosthesis is an acceptable alternative.
Not every case can be precisely classified by any system, and multiple reconstruction options exist. However, the Paprosky femoral defect classification allows for an algorithmic, step-wise approach to the assessment of bony deficiencies and their ultimate reconstruction.
References
1. Paprosky WG, Burnett RS. Assessment and classification of bone stock deficiency in revision total hip arthroplasty. Am J Orthop. 2002;31(8):459.
2. Della Valle CJ, Paprosky WG. Classification and an algorithmic approach to the reconstruction of femoral deficiency in revision total hip arthroplasty. J Bone Joint Surg Am. 2003;85-A(Supp 4):1.