Tapered, fluted revision stems are most useful for proximally deficient femora (Paprosky types IIIA, IIIB, and IV).
Such stems also are useful for periprosthetic fractures associated with nonsupportive proximal femoral bone (Vancouver types B2 and B3).
Most modular stems require proximal bony support to protect the modular junction.
Determining the highest level of supportive femoral bone is important to place the taper into this bone. An extended trochanteric osteotomy is useful for removal of the old stem and facilitates placement of a relatively straight, tapered, fluted stem down a bowed femur, thus preventing anterior perforation of the stem through the anterior cortex. The intact diaphysis often will benefit from a cable around the highest level of intact bone to prevent fracture and provide a better end point during stem insertion. The proximal, nonsupportive metaphyseal bone should be reconstructed around the upper stem for support.
Axial stability is difficult to achieve when the bone is osteoporotic or when the length of intact diaphysis is less than 4 cm. A cable around the uppermost part of the intact diaphysis, with or without strut grafts, will help provide axial stability and discourage subsidence. Reconstruction of proximal bone stock provides support to the upper stem and averts stem fracture at the modular junction.
The number of revision total hip replacements is predicted to more than double over the next 25 years from an estimated 40,800 in 2005 to approximately 96,700 in 2030. Total hips are now being implanted in much younger patients, and the risk of multiple revisions demands better means of reconstruction. Although revision arthroplasty is amenable to many options when bone stock is good, poor bone stock often jeopardizes reconstruction with more customary implants.
Before the use of cementless implants, cemented components served as the workhorse for revision arthroplasties. However, cemented revision components present several problems, including difficulty obtaining adequate interdigitation and fixation of cement into the smooth, sclerotic, endosteal bone of the femur; extrusion of cement through femoral defects; and the need for a large volume of cement that impedes subsequent revisions and can threaten the patient’s cardiopulmonary system during introduction.
Cementless fixation provides solutions to many of these problems. However, proximal femoral bone loss continues to challenge adequate fixation and osseointegration with proximally fixed components. Long, extensively porous coated femoral components allow more distal fixation. However, with severe proximal bone loss or large, ectatic canals, even these stems can fail. In addition, fitting the stem to the canal often proves difficult, resulting in undersizing of the femoral component and failure of bony ingrowth in up to 18% of cases or fracture of the femur in as many as 30% of cases in some studies.
Wagner first described the use of a distally tapered, fluted, revision femoral component in 1987 ( Fig. 36-1 Axial stability was achieved by driving the tapered stem into the intact femoral diaphysis, which had been milled into a conical shape to accept the distally tapered femoral component. Longitudinal flutes provided rotational control of the prosthesis. However, subsidence of the stem, particularly in cases of weak or osteoporotic bone, continued to be an obstacle to complete success.
Because the final height of the stem at which axial stability was attained was somewhat unpredictable, a modular proximal body was introduced in subsequent design modifications. Modularity in total hip components initially raised concern about corrosion and breakage at the modular junctions. However, although some modular designs have seen fatigue failures, the ability to adjust the height and anteversion of the proximal stem after achieving solid primary fixation of the distal stem proved extremely valuable in correcting leg length and achieving hip stability. At least one manufacturer has redesigned the modular junction in its stems of 17 mm or larger to improve the strength in this high-stress area. Modular, tapered, fluted stems have now become one of the mainstays for reconstructing femurs with significant proximal bone loss.
Revision Femoral Components
Femoral reconstruction can be accomplished in numerous ways. No standard classification system has been adopted, but the most commonly used methods for femoral fixation and reconstruction are shown in Table 36-1 . Revision femoral components can be cemented or cementless, although cemented revision components have largely fallen out of favor.
|Reconstruction Technique||Cement||Modularity||Fixation Location||Mandatory Bone Graft||Example|
|Cemented component||Yes||No||Proximal and Distal||No||Any long-stem cemented component|
|Cementless proximal fixation||No||Yes or no||Proximal (metaphyseal)||No||S-Rom (DePuy, Warsaw, Ind.); Mallory/Head (Biomet Inc., Warsaw, Ind.)|
|Cementless distal fixation||No||Yes or no||Distal (diaphyseal)||No||Wagner SL (Protek AG); Link MP (Exactech)|
|Cementless proximal and distal fixation||No||Yes or no||Metaphyseal and diaphyseal||No||Solution (DePuy); Echelon, extensively porous coated (Smith & Nephew, Memphis, Tenn.)|
|Proximal structural allograft prosthesis composite||Yes (to allograft); also may cement to host bone||Yes or no||Proximal–to allograft; distal–to host bone||Yes (structural)||Any long-stem component|
|Morselized (impaction) grafting||Yes||No||Proximal and distal||Yes (morselized)||Exeter (Exeter, UK)|
Cementless fixation method includes proximal fixation, distal fixation, or a combination of the two. Extensively porous coated stems are designed to achieve both, although in practice the fixation more commonly is distal. Cementless stems can be modular or nonmodular; straight or bowed; porous coated, grit blasted, or hydroxyapatite coated; and fluted or nonfluted.
A third option for femoral fixation is the requisite use of bone graft to obtain primary stability. Bone grafting may be either structural or morselized. Although structural allograft-prosthesis composites can successfully rebuild bone stock—particularly important in younger patients—they present the potential complications of nonunion, graft resorption, component loosening, infection, and fracture. Morselized impaction grafting has met with reasonably good success in the hands of experienced surgeons. It provides primary structural support of the femoral component, even in extremely deficient femurs.
Femoral Bone Deficiencies
Numerous classification systems for femoral bone loss have been described. Although these systems have many similarities, the author believes a classification system that helps guide the type of fixation and predicts the results of intervention is most useful. For those reasons, this author prefers the Paprosky system ( Table 36-2 ).
|I||Minimal loss of metaphyseal cancellous bone and an intact diaphysis|
|II||Extensive loss of metaphyseal cancellous bone but an intact diaphysis|
|IIIA||Severe nonsupportive metaphyseal damage with >4 cm intact diaphyseal bone at isthmus|
|IIIB||Severe nonsupportive metaphyseal damage with <4 cm intact diaphyseal bone at isthmus|
|IV||Severe bone metaphyseal and diaphyseal bone loss with no supportive bone in diaphysis|
The Paprosky system breaks femoral bone loss into types I, II, III, and IV. Type I femurs have only minimal loss of metaphyseal cancellous bone and an intact diaphyseal isthmus. Type II femurs have more extensive loss of proximal metaphyseal bone but an intact femoral diaphysis. Types I and II femoral metaphyses are supportive. Type III femurs are subdivided into IIIA and IIIB. Type IIIA femurs have extensive, nonsupportive metaphyseal damage but more than 4 cm of intact diaphyseal isthmus, whereas type IIIB femurs have similar extensive metaphyseal loss with some intact diaphyseal femur of less than 4 cm in length. Type IV femurs have global bone loss without supportive bone in either the metaphysis or diaphysis.
Periprosthetic Femoral Fractures
The most accepted classification system for periprosthetic femoral fractures is the Vancouver classification system described by Duncan and Masri ( Table 36-3 ). The Vancouver system divides periprosthetic femoral fractures into types A, B, and C. Type A fractures involve the trochanteric region only and spare the femoral shaft. Type B fractures occur about the femoral component. Type B fractures are further subdivided into types B1, B2, and B3. In B1 fractures the femoral component remains stable within the proximal femur. In B2 fractures the femoral component is loose, but the bone stock surrounding the implant is supportive. B3 fractures occur about the femoral component, and the surrounding bone is deficient and nonsupportive. Type C fractures occur well distal to the femoral component.
|A G||Greater trochanter|
|A L||Lesser trochanter|
|B||Around the femoral component|
|B2||Prosthesis unstable but surrounding bone supportive|
|B3||Prosthesis unstable and surrounding bone deficient and nonsupportive|
|C||Distal to prosthesis|
INDICATIONS AND CONTRAINDICATIONS
Wagner’s revision stem is a straight stem tapered 2 degrees along its entire length. It uses 8 longitudinal flutes for rotational control, is grit blasted on its surface, and ranges from 190 to 305 mm in length. Cementless diaphyseal fixation allows treatment of proximally deficient femora without the use of structural allograft or cement. Although Wagner met with reasonable success, the prosthesis of his design is not without complications, most notably axial subsidence, leg shortening, and hip instability ( Fig. 36-2 ).