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Modular femoral implants are primarily used for femoral component revision in total hip arthroplasty.
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Modular femoral implants facilitate optimum diaphyseal and metaphyseal fixation with the appropriate size stem and proximal body.
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Several stem and proximal body sizes are available to help achieve the desired offset and leg length.
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Preoperative planning is essential to determine the type and length of the stem and proximal body necessary to reach the desired goal.
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The type of stem used—cylindric, tapered, or fluted—is dependent on the location and quality of the best bone available for fixation.
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Clinical results with modular femoral stems at the present time are similar to those with conventional, extensively coated stems.
Femoral component revision can be a challenging problem for the orthopedic surgeon. Bone loss and poor-quality host bone are the major problems encountered at the time of femoral component revision. The primary goals of revision total hip arthroplasty are to alleviate pain, restore hip mechanics, and provide a stable and durable implant. Revision total hip arthroplasty can be time-consuming, with prolonged anesthesia time, and can result in significant blood loss and fluid shifts leading to medical complications. Therefore it is prudent to achieve the desired end result in an expeditious manner. The surgeon should be well prepared for the operative experience with a thorough understanding of the pathomechanics leading to failure and should develop a comprehensive preoperative plan.
Several treatment options are currently available for the reconstructive surgeon at the time of femoral component revision. Treatment options are primarily based on the extent of bone loss, the quality of available host bone and soft tissues, and the experience of the treating surgeon. Implant options include long-stem cemented implants, porous, extensively coated implants, modular extensively coated or fluted stems, impaction grafting, allograft-prosthetic composites, and tumor-type mega prostheses. The purpose of this chapter is to discuss the use of modular femoral stems in revision total hip arthroplasty.
The concept of modular implants has been used in total hip arthroplasty for over three decades. The concept of modular femoral heads and acetabular liners is well known to the orthopedic surgeon. McBride initially used the modular femoral stems in 1948. Bousquet and Bornard developed a proximal modular stem that featured a proximal body attached to a stem with a conical mounting post. The S-ROM femoral implant is the prototype of modular stems. The current S-ROM system is the fourth generation in the evolution of the Sivash stem initially introduced into the United States in 1972. The S-ROM consisted of a titanium alloy with distal flute fixation and a modular proximal sleeve providing rotational freedom.
In 1987 Wagner introduced a tapered, fluted, uncemented femoral stem design for revision total hip arthroplasty. He reported bone regeneration after the use of a cementless, tapered revision stem that was fixed in the diaphysis. The stem consisted of titanium alloy with a 2-degree taper with eight longitudinal ridges. This fluted stem design provided a high degree of rotational stability. Current design modular stems include both the Sivash and Wagner stem design concepts for femoral component revision, providing both a conical, tapered, fluted stem design and a cylindric design that can be either straight or curved. The cylindric stems are either smooth and polished with flutes or rough with porous or hydroxyapatite coating. The proximal bodies come in varying diameters and lengths to accommodate the diaphyseal-metaphyseal mismatch encountered at the time of femoral component revision ( Fig. 43-1 ). Proximal bodies also come in various offsets and designs in order to maximize the proximal ingrowth and restore leg length.
BIOMECHANICS OF MODULAR STEMS
Modular fixation involves choosing proximal and distal parts independently based on the “fit and fill” concept. The geometry of the femoral canal demonstrates significant variability such that modularity can facilitate both distal and proximal fixation independently. This type of intraoperative customization has led to the popularity of modular femoral implants over monolithic stem designs.
One of the primary advantages of modular stems for femoral component revision is independent fixation of the diaphysis with the appropriate diameter stem and independent fixation of the metaphysis with the adequate canal-filling proximal body. From a biomechanical perspective, three sets of factors have to be considered during the use of modular femoral stems. These include (1) the geometry, length, and surface finish of the modular stem; (2) the length, shape, and surface finish of the proximal body; and (3) the strength of the taper connecting the proximal body to the stem. Current design stems used today vary in both geometry and surface finish. Highly polished, smooth cylindric stems with flutes are designed for maximal fit without distal ingrowth in order to promote proximal stability and proximal ingrowth. Extensively coated cylindric types of stems are designed for true distal fixation. Tapered, fluted stems with splines have a grit-blasted surface with corundum to promote greater bony ingrowth in the proximal regions of the femur. All the three stem designs have certain indications and uses based on the quality and location of the host bone available for fixation during femoral component revision.
Canal-filling modular cylindric stems are used in a similar fashion as monolithic extensively coated stems that provide distal fixation. The primary disadvantage to the use of distal fixation with extensively coated stems is the inevitable proximal stress shielding that occurs over time. The use of tapered, conical, fluted stems is also popular. Owing to the conical, tapered design, these stems are loaded more proximally than are extensively coated, cylindric stems with distal fixation. Rotational stability using tapered, conical fluted stems is achieved by splines or flutes measuring 1 to 2 mm ( Fig. 43-2 ).
The proximal body is a metaphyseal sleeve available as a taper-fit, cylindric, conical, or calcar bearing. The sleeve may be porous or hydroxyapatite coated. Proximal fixation with host bone contact is attractive because it could provide long-term biologic ingrowth which would minimize proximal stress shielding and unload some of the stress placed at the Morse taper junction.
Initially designed modular femoral implants failed primarily at the Morse taper junction. The stress placed at the Morse taper with cyclic loading over time led to failure with fracture of the taper. The strength of the taper junction has been a significant concern because of the high stress concentration that occurs at this region. The Morse taper junction needs to adequately address cyclic loading to avoid fatigue failure and withstand fretting and corrosion. In a revision situation in which there is a lack of proximal host bone available for ingrowth, the Morse taper junction may bear significant loads, leading to failure. Improvement in the biomechanical properties of the Morse taper junction has been developed through the use of nitride impregnation, burnishing, and shot peening. Shot peening is a surface-hardening process through which small spheres of material such as steel or ceramic are used to bombard the taper junction. These spheres impart small indentations on the surface of the taper junction, which packs the surface molecules tighter, resulting in their greater compression. The shot peening process increases the fatigue strength by 33%. The current accepted guidelines established by the International Standards Organization (ISO) recommend that the Morse taper junction of modular femoral implants be able to tolerate 2300 newtons (N) or 517 pounds of cyclic load. Current design junctions exceed these guidelines and are able to tolerate 4450 N (1000 pounds). Ideally, the stresses placed at these taper junctions may be diminished over time as gradual proximal ingrowth occurs between the host bone and the proximal body of the implant.
Two types of locking mechanisms are commonly employed to secure the proximal body to the stem. One is the Morse taper, and the other consists of cylindric locks with teeth that are held together by compression screws. Taper junctions are an effective means of independently securing distal and proximal components together in modular hip stem implants. The Morse taper works in compression and flexion but is not reliable until positively locked, which is difficult to judge when the taper is assembled inside the femoral canal. The disadvantage of the cylindric locks is that the screws can loosen, which can lead to increased motion at the interface and possible disassembly. To overcome these problems some manufacturers have combined both locking mechanisms. The addition of a proximal locking screw forces the Morse taper in compression. Locking of the components is best achieved on the back table outside the femur, thus essentially assembling the prosthesis before introducing the prosthesis into the femur. The disadvantage with this is that modularity is lost. If the taper is not locked, the implant junction will fail. It is absolutely essential that the Morse taper junction be locked appropriately.
Another design concern is that if the proximal body does not fill the metaphysis and provide proximal ingrowth, then it will not provide a seal or gasket. The upper part of the femoral canal is included in the effective joint space, raising the possibility of femoral osteolysis migrating to the distal area where there is ingrowth. Cameron, in a retrospective study of proximally modular femoral stem fixation, concluded that the stem-sleeve junction provides an adequate seal or gasket for at least the first two decades of service life and that distal osteolysis is rare.
INDICATIONS AND PREOPERATIVE PLANNING
Modular femoral stem usage has steadily increased over the last two decades. The indication for these stems is primarily in revision total hip arthroplasty in patients with bone deficiency or distortion resulting from prior reconstruction or trauma. Modular femoral implants are ideal for periprosthetic fractures. In these situations a cylindric stem for distal fixation is used to achieve rigid fixation distal to the fracture pattern. The stem provides intramedullary stabilization of the fracture similar to an intramedullary nail. The fracture can also be secured with cerclage cables after insertion of the stem followed by insertion of the proximal body to restore leg length and offset.
A thorough preoperative plan is absolutely essential before surgical intervention. The differential diagnosis in the evaluation of the painful hip for which arthroplasty is being considered must include infection in addition to aseptic loosening. Radiographs should be reviewed to determine the extent of bone loss, osteolysis, deformity, or impending fracture. If there is any clinical suspicion of infection, adequate laboratory studies should be performed in addition to hip aspiration.
Modular femoral implants rely primarily on distal fixation. Various stem geometry, surface finishes, and sizes are available. Cylindric stems can be smooth and polished with flutes or can have a rough coated surface. Fluted, polished cylindric stems may be used when there is adequate proximal bone available for ingrowth. An advantage of polished cylindric stems is the ease of extraction in case of future revision. The disadvantages are that there are enormous hoop stresses on introduction, with the possibility of fracture and thigh pain. When the proximal bone is compromised by loss of cancellous and cortical bone, then distal fixation with a porous-coated cylindric stem may be the best option. The disadvantages to distal fixation are the inevitable stress shielding that occurs and the possibility of thigh pain and difficulty in future revisions. Both smooth and extensively coated cylindric stems are available in straight and bowed options to minimize anterior cortical fracture when long stems are necessary. The tips of cylindric stems are either bullet shaped, coronally split, or triflanged for ease of insertion and to minimize thigh pain.
A tapered, fluted stem design is used if there is adequate cortical bone thickness available at the proximal femur below the level of the lesser trochanter where there is maximum contact with the tapered stems. Tapered stems will subside if the proximal bone is inadequate and if the implant is undersized. Tapered stems ensure high axial stability and transmit extremely high forces. In cases of an extended trochanteric osteotomy, tapered stems may have a higher incidence of subsidence owing to loss of circumferential cortical contact. In our experience with patients undergoing extended trochanteric osteotomy to remove either a well-fixed implant or distal cement, distal fixation with a cylindric porous-coated stem may provide more predictable results. Distal conical fixation involves reaming the distal canal to a cone. The stem must be straight regardless of length because it is not possible to ream a cone for a bowed stem.
The patient must be assessed clinically before surgery to evaluate neurovascular status, any prior surgical scars, presence of associated leg length discrepancy, and abductor strength. Operative notes from previous surgery can provide information about the type and size of previous implants used. Preoperative evaluation must include an assessment of the bone loss with adequate radiographs ( Fig. 43-3 A-C ). The tapered stems and cylindric stems come in varying sizes. Preoperative templating using x-ray films to determine the type of stem including its length and diameter is essential before the actual procedure ( Fig. 43-3 D and E ).
Templating must be performed separately for the proximal and distal aspects of the femur. The widest stem diameter that fills the femoral canal distal to femoral defects is selected when cylindric stems are used. The length of the revision stem should be at least two cortical diameters beyond the most severe distal defect or the distal extent of the extended trochanteric osteotomy. This gives a minimum of 5 to 7 cm of contact between the stem and femoral cortex for rigid, immediate distal fixation. If the projected length extends beyond the isthmus midpoint, a curved stem often is necessary to avoid anterior cortical perforation. Preoperative planning will allow the surgeon to predict any offset or leg length concerns that may arise and will minimize intraoperative frustrations ( Fig. 43-3 F and G ).
The surgeon must plan for the use of intraoperative radiographs, blood conservation techniques such as cell-saver devices, preoperative antibiotics, and availability of bone grafts. Surgical equipment needed for implant and cement removal such as high-speed burs, flexible osteotomes, and Moreland instruments must be available. Medullary preparation instruments should include flexible reamers with ball-tipped guidewires. For repair of extended trochanteric osteotomy, one should anticipate use of bone clamps, circumferential wires, trochanteric plates, cortical strut grafts, and cancellous bone. A comprehensive plan can minimize the operative time and decrease the incidence of complications associated with prolonged surgery.
Most implant companies offer some type of modular femoral fixation. Some examples of the modular femoral fixations available are S-ROM (DePuy, Warsaw, IN); ZMR Hip System (Zimmer, Warsaw, IN), based on the original Wagner design; Restoration Modular Revision Hip System (Stryker, Kalamazoo, MI); Mallory-Head Modular Calcar Revision System (Biomet, Warsaw, IN); Margron (Portland Orthopaedics, Atlanta, GA); ProFemur (Wright Medical Technology, Arlington, TN); Link MP (Link America, Denville, NJ); AccuMatch M-Series (Exactech, Gainesville, FL); and MRP-Titan (Peter Brehm, Weisendorf, Germany) ( Table 43-1 ).
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