Javad Parvizi
Kang-Il Kim
Currently over 20,000 revision hip arthroplasties are performed in North America per year. Longer life expectancy, implantation of prosthetic hips in younger and active patients, and the increase in the number of patients with hip arthroplasty in place for decades are some of the reasons for the rise in the incidence of revision THA. Hip arthroplasty may fail and necessitate revision for many reasons (Table 15-1). The goal of revision THA, as for primary surgery, is to relieve the patients' symptoms and restore function. The main challenge of revision surgery, however, is to accomplish these objectives in the setting of compromised bone stock, poor soft tissue, and the possible presence of infection. Hence, both planning and execution of revision hip arthroplasty can be very different and in many occasions much more difficult than primary arthroplasty. Extensile surgical approaches, more sophisticated prosthetic devices, and more restricted postoperative protocols are common in revision arthroplasty. Because of the above noted challenges, the outcome of revision arthroplasty in terms of improvement in function (as measured by validated instruments), complication rate, and longevity of the prosthesis are inferior compared with primary THA.
Management of Failed Hip Arthroplasty
Pain is usually the main symptom of patients presenting with failed hip arthroplasty. Other modes of presentation can include mechanical symptoms (such as subluxation) and dysfunction owing to hip stiffness or limp. Wear and osteolysis may be asymptomatic. Regardless of the mode of presentation, all patients with presumed failure of hip arthroplasty need a detailed clinical and radiographic evaluation.
Clinical Evaluation
The duration of symptoms, intensity, and the degree of patient disability imparted by the symptoms may be elicited from detailed history. The cause of pain that has been present since primary THA is likely to be different from symptoms that commenced many years after the initial arthroplasty. Evaluation of patient comorbidities such as diabetes predisposing to infection, history of spinal disease that can masquerade as hip pain, medications that may cause muscular pain, and previous surgical history in the affected hip as well as other joints is also critical. Detailed examination to confirm suspected cause, measurement of limb length, assessment of abductor strength and gait, and detection of neurovascular insufficiency are also necessary.
Diagnostic Evaluation
Routine anteroposterior and lateral radiographs can be very informative and are likely to demonstrate the cause of failure in most cases. Gross component malpositioning, severe osteolysis, wear, radiolucent lines indicative of loosening, fractures, limb-length discrepancy, and occasionally signs of infection can be discerned from the initial radiographs. A full-length femur radiograph is valuable if long-stem femoral fixation is anticipated. On occasion other imaging modalities such as long-leg standing radiographs to assess limb length, computerized tomography (CT) to better assess component positioning or the degree of osteolysis, or MRI to evaluate coexistence of spinal conditions may need to be performed. Furthermore, nuclear imaging and aspiration of the joint to confirm or rule out periprosthetic infection may also need to be considered. On very rare occasions specialized tests such as intravenous pyelography or angiography may be performed in patients with intrapelvic components or intrapelvic cement that may need to be removed.
Preoperative Planning
Revision THA can vary from a relatively simple procedure (such as the change of acetabular liner) to a very complex surgery (such as revision of well-fixed acetabular and femoral components). Preoperative planning is paramount to ensure appropriate provisions are in place during revision surgery. Although the decision to
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perform revision of one or both components can be made in most cases prior to surgery, sometimes unrecognized loosening, malpositioning, or damage to the bearing surface of a nonmodular femoral stem necessitates revision of a component that was not anticipated preoperatively. Furthermore, removal of well-fixed and well-positioned monolithic femoral components, to allow better visualization of the acetabulum, may be necessary during revision surgery. Hence, it is essential for the reconstructive surgeon to have studied the previous operative records of the patient to ensure that appropriate components, instruments, and support teams will be available during revision THA.
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TABLE 15-1 Indications for Revision HIP Arthroplasty |
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Figure 15-1 Anteroposterior radiograph (A) of a proximally coated stem in a patient with severe thigh pain. Aseptic loosening of the femoral stem was suspected but obscure to confirm based on the radiographs. Note the reactive lines around the proximally coated region of the stem and the lack of osteointegration. Anteroposterior radiograph of the hip in a patient with gross loosening and subsidence of the femoral stem (B). Revision in which modular femoral stem was used (C). |
Indication for Revision Arthroplasty
Aseptic Loosening
Prosthesis loosening is one of the most common indications for revision THA. Aseptic loosening can present with radiolucent lines at the prosthesis/bone, cement/bone, or cement/prosthesis (debonding) interfaces. Radiolucent lines indicative of definite loosening are usually progressive (i.e., developed over time) or circumferential (covering the entire prosthesis surface) and usually >2 mm wide. Diagnosis of aseptic loosening of an uncemented femoral stem can be challenging (Fig. 15-1A). Engh et al. have described major and minor radiographic signs of stem loosening: The presence of reactive lines (white lines around the stem), stem subsidence (Fig. 15-1B), and distal pedestal not in contact with the tip of the stem are some of those radiographic signs. Component subsidence or change in position, if subtle, can be ascertained only by evaluation of serial radiographs. Further imaging studies such as oblique radiographs may be required to confirm aseptic loosening that is suspected clinically but cannot be confirmed on conventional radiographs. A clinical history of start-up pain usually is present with a loose femoral stem. Revision of loose femoral components often can be done without the need for extensile approaches (Fig. 15-1C) unless the component subsides under the greater trochanter, making extraction without a fracture difficult. Revision of a loose acetabular component can also be performed with minimal bone loss if careful exposure of the acetabulum is performed. Exposure of the
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acetabulum in the presence of a well-fixed femoral stem can be challenging.
Periprosthetic Osteolysis
Osteolysis denotes loss of focal bone mass over time (Fig. 15-2). Around hip implants, the process is usually the result of activation of macrophages and osteoclasts that can occur with generation of wear particles or with infection. Osteolysis without component loosening can be asymptomatic and forms the main rationale for periodic evaluation of patients with joint arthroplasty, particularly those at specific risk for this problem. Active patients with high demand on their prosthetic hip are in this category. Conventional radiography underestimates the degree of osteolysis. In recent years the use of CT scans to assess the extent and location of osteolysis has been described.
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Figure 15-2 Anteroposterior radiograph of the hip in a 69-year-old patient with polyethylene wear and extensive osteolysis (A). The revision of both the acetabular and femoral component was performed (B). Trabecular metal augments were used to fill the defect in the acetabulum (C). |
The indications for intervention, and the optimal type of surgical procedure for patients with asymptomatic osteolysis is currently in evolution and not universally agreed on. There is, however, no dispute in that surgical intervention should be considered for patients with extensive osteolysis and impending periprosthetic fracture, particularly in younger patients in whom bone mass is critical, and for symptomatic patients. Options available for treatment of osteolysis around well-fixed acetabular components include isolated periacetabular bone grafting through a trapdoor or through access points around the cups or through screw holes, replacement of the bearing surface with or without bone grafting of the lesion (done through screw holes), or revision of the acetabular component. Osteolysis associated with loose acetabular or femoral components is addressed by revision arthroplasty. Frequently particulate or bulk allograft bone is used in an effort to restore the bone mass and allow mechanical fixation of the components.
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Instability
Instability following THA is not an uncommon complication. The incidence of dislocation following THA varies between 0.3% and 10% after primary and up to 28% after revision arthroplasty. It is well accepted that dislocation is more common after THA in patients with impaired cognition, soft tissue laxity or deficiency, underlying diagnosis of hip fracture, and female gender. Furthermore, technical factors such as the use of a small-diameter femoral head and performing the surgery through a posterolateral approach without capsular repair seem to adversely influence the risk of instability. Other technical factors such as the use of elevated acetabular liners, femoral component neck geometry, and femoral component offset may also influence the risk of instability. As our understanding of the causes of dislocation has evolved over the last decade, refinements in surgical techniques have been introduced that probably will result in a decline in this complication.
Dislocation following THA can occur at any time. However, most reported dislocations occur within the first few months after the index surgery. About two thirds of dislocations can be treated by closed reduction without a need for further intervention. Recurrent instability, which often necessitates reoperation, can occur owing to many reasons. The most common include component malpositioning and soft tissue laxity. However, two important points need to be mentioned. First is that the cause of recurrent instability in many cases may be multifactorial. Second, the exact cause for recurrent instability in some cases cannot be discerned with certainty. The surgical treatment options available to address recurrent instability consist of revision of malpositioned components, use of a larger-diameter femoral head, bipolar arthroplasty, greater trochanteric advancement, soft tissue re-enforcement, and the use of constrained liners.
The type of surgical strategy elected to address recurrent instability depends on the cause. Revision arthroplasty for recurrent instability in general is much more likely to be successful when a discernible cause for instability can be identified.
Revision of the malpositioned component is one of the most common of surgical interventions for dislocation (Fig. 15-3). Although definitions of malpositioning vary, the optimal positioning for acetabular component is thought to be 10 to 30 degrees of anteversion and 35 to 50 degrees of inclination or abduction. When components are positioned outside this zone, instability is more likely to result. The position of the acetabular component in relation to the anatomic landmarks (the teardrop on the radiographs) is also important. Cross-sectional studies such as CT may be needed to accurately assess component position (Fig. 15-3B).
If soft tissue laxity or deficiency is deemed to be the main cause of instability, then the use of a larger femoral head, bipolar arthroplasty, or more commonly a constrained liner is advocated (Fig. 15-4). One of the main attractions of constrained liners is that they may be used without the need to revise a well-fixed and well-positioned acetabular components.
Periprosthetic Infection
Despite all the recent advances, periprosthetic infection (PPI) continues to occur following THA. The incidence of PPI is reported to be between 0.5% and 3%. Several factors adversely influence the incidence of PPI, including revision surgery and compromised immune status of the patient.
Total joint arthroplasty infections are sometimes categorized based on the presumed mechanism and timing of infection. Acute postoperative infections are thought to result from infecting organisms that gained access to the joint during surgery or soon after from overlying skin or a draining wound. Infections of this type generally become symptomatic within a few days or weeks of the arthroplasty. Late chronic infections may result from proliferation of organisms inoculated during surgery, either from the air, surgical instruments, or the implant itself. The lag period is the time taken for the organisms to proliferate and declare deep infection. Hematogenous infections represent seeding of an arthroplasty site by organisms carried by the blood stream from a different site (e.g., urinary tract infection, cutaneous or mucosal ulcer, and so on). The distinction between these types may be difficult and is somewhat arbitrary.
Although diagnosis of florid PPI can be reached without much difficulty, the detection of occult infection using the current methods for diagnosis can be challenging. A high degree of suspicion for PPI should be entertained in patients with early loosening of components. Serologic tests such as C-reactive protein, erythrocyte sedimentation rate, and white cell count may be used to screen for infection. Aspiration of the joint is the most definitive test for ongoing PPI. A few nonspecific changes suggestive of infection may be apparent on plain radiographs (Fig. 15-5A). These include periosteal reaction, scattered foci of osteolysis, and generalized bone resorption in the absence of wear. However, most patients with PPI, especially those presenting acutely, do not have obvious radiographic findings suggestive of infection, or have radiographic features indistinguishable from those seen in aseptic loosening. The use of additional imaging such as the bone scan (particularly labeled white blood cell scans), and recently, positron emission tomography (PET) may occasionally be useful (Fig. 15-5B).
The possible treatment of PPI includes irrigation and debridement of the hip with retention of the components, one-stage exchange arthroplasty, or resection arthroplasty with delayed reimplantation. PPI occurring early (within 4 weeks) after index arthroplasty or hematogenous infection presenting acutely may be treated with irrigation and debridement without resection of the components as long as the components are well positioned and fixed. The reported success rate of this method varies but probably is about 50%. Two-stage exchange is the most common method of treating prosthetic infection in North America. An antibiotic-impregnated bone cement spacer frequently is placed in the hip after the initial resection, and the patient usually is treated with 4 to 8 weeks of intravenous antibiotics. On rare occasions one-stage exchange arthroplasty may be considered for patients infected with a low-virulence organism that is sensitive to most antibiotics. Resection arthroplasty may be the treatment of choice for a select group of patients
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with inadequate bone stock or soft tissues precluding reimplantation or recalcitrant infection.
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Figure 15-3 Anteroposterior radiograph of a patient with component malpositioning and instability (A). The computed tomography clearly showed malposition of the acetabular component as retroversion (B). The retroverted cup was repositioned during revision surgery (C). |
Polyethylene Wear
Wear of the bearing surfaces is the most important factor limiting the longevity of the hip arthroplasty. Age and activity level are among the most important predictors of bearing wear rate. Polyethylene wear per se is usually not an indication for revision THA unless it results in polythene fracture, wear-through, osteolysis, or pain (owing to synovitis from wear debris). In recent years great strides in the design of articulation materials have been made with promising prospects. The introduction of highly cross-linked polyethylene is one such improvement that has resulted in reduction of wear both in vitro and in vivo. Other alternative bearing surfaces that are currently in use include alumina ceramic-on-ceramic and metal-on-metal articulations.
Periprosthetic Fracture
Various classifications for periprosthetic fractures around the hip have been proposed. The Vancouver periprosthetic fracture classification system is widely used and lends itself to devising a treatment strategy. This classification system takes into account the site of fracture, the status of the femoral component, and the proximal femoral bone quality. Type A fractures are around the proximal femur involving the greater trochanter or the lesser trochanter. The stability of the stem is not usually compromised. These fractures often are associated with osteolysis of the proximal femur. Treatment is directed toward management of underlying osteolysis. The fracture itself does not usually require fixation
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unless the greater trochanter is displaced to compromise abductor function. Type B fractures are those occurring around the femoral stem. In type B1 fractures the femoral stem is stable. Most can be treated with internal fixation of the fracture using plates, screws, cerclage cables, and strut grafts while retaining the well-fixed stem. Fractures associated with loosening of the femoral stem (types B2 and B3) are treated with revision of the femoral component and simultaneous fracture fixation (Fig. 15-6).
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Figure 15-4 Anteroposterior radiograph of a patient with recurrent instability that was deemed secondary to inadequate soft tissue (abductor) envelope (A). The acetabular component, although slightly vertical, was found to be well fixed during surgery. A constrained liner with a larger femoral head was used to address the problem in this patient (B). |
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Figure 15-5 Plain radiograph showing area of focal osteolysis (scalloping) around the midportion of a well-fixed uncemented stem (A). This appearance is highly suggestive of periprosthetic infection. Increased uptake in the corresponding area of infection in the right femur was noted on the PET scan (B). |
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Figure 15-6 Anteroposterior radiograph of a patient with Vancouver type B3 periprosthetic fracture (A). Proximal femoral replacement was used for reconstruction of this fracture (B). |
Leg-Length Discrepancy
Limb-length discrepancy (LLD) following total hip replacement is not uncommon and can be bothersome to the patient. Patients frequently complain of LLD in the early postoperative period, and in most instances the LLD is functional and the symptoms resolve with time. However, there remains a small subset of patients who continue to be symptomatic. Most can be treated with shoe lifts to compensate for the problem; further surgery is performed only in rare instances (Fig. 15-7).
Material Failure
Fracture of the femoral stem was not infrequent with early prostheses made of stainless steel or cast cobalt chromium that were susceptible to fatigue fracture. Improvements in engineering and metallurgy have enabled manufacturers to produce arthroplasty components that are extremely strong and resilient. Fractures of modern monolithic femoral stems are exceedingly rare. Fracture of the femoral stem, in the rare occasions that may occur, usually involves modular components (Fig. 15-8). Poor proximal bone support in a stem that is well fixed distally can lead to cantilever loading of the stem and subsequent fatigue fracture.
With increasing use of modular acetabular components, dissociation or dislodging of the acetabular liner has also been reported. This problem was more common particularly with some designs of acetabular components that did not have sophisticated locking mechanisms.
Classification of Bone Deficiency
One of the major challenges of revision THA is the management of bone loss. Depending on the extent and location of bone loss, different surgical strategies are used.
There are two commonly used acetabular bone deficiency classification systems. The American Academy of Orthopaedic Surgeons (AAOS) system can be used in both primary and revision arthroplasty. It categorizes bone loss as segmental or cavitary defects along with pelvic discontinuity and arthrodesis. Segmental defect describes full-thickness
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loss of bone in the supporting rim of acetabulum or the medial wall. Cavitary defect describes volumetric loss of bone within the acetabular cavity. Complete transverse disruption of the supporting anterior and posterior columns of acetabulum constitutes pelvic discontinuity. Another classification system proposed by Paprosky is based on the severity of bone loss and helps predict the surgeon's ability to achieve implant stability with a hemispheric cup. The system describes four radiographic landmarks to evaluate the extent of bone loss, which are the following: proximal migration of acetabular component (migration of joint center), integrity of the teardrop, ischial bone loss, and violation of the Köhler line by prosthetic migration.
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Figure 15-7 The anteroposterior radiograph (A) of the hip in a 54-year-old patient in whom the acetabular component has been placed inferior to the anatomic position (teardrop), with the revised hip radiograph (B) demonstrating proper positioning of the cup that addressed the limb lengthening. |
Femoral Bone Loss
Several classification methods for periprosthetic femoral bone loss have been proposed. Most define the severity of proximal bone loss by the amount of cancellous bone loss, the amount of cortical bone loss, and the distal extent of this bone loss. These classification methods can be used to choose necessary revision implants.
Surgical Exposure
An essential element of successful revision arthroplasty is obtaining adequate exposure of the hip. If possible, the old incision should be used or incorporated into the new incision. In many revisions a conventional anterolateral or posterolateral approach may be used. The advantages of anterolateral or direct lateral approaches include excellent acetabular exposure, decreased rate of sciatic nerve palsy, and reduced incidence of postoperative dislocations. Disadvantages of the anterolateral approach include difficulty of accessing the posterior column for cage implantation or bone grafting, increased risk of heterotopic ossification, reduced visualization of the proximal femur, and the higher incidence of postoperative limp. Advantages of the posterolateral approach are good visualization of the posterior column and entire acetabulum, preservation of the abductor mechanisms, and lower incidence of heterotopic ossification. Disadvantages include difficult acetabular exposure when the femoral component is in situ, and a higher dislocation rate compared with anterior or lateral approaches.
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Figure 15-8 Anteroposterior radiograph of the pelvis demonstrating fatigue fracture of the modular conical femoral stem. |
Extensile exposures not infrequently are required during revision arthroplasty. One of the most common extensile exposures is the extended trochanteric osteotomy (ETO). ETO is considered the exposure of choice for removal of well-fixed stems. It is also particularly useful for removal of a well-fixed cement mantle, particularly in infected cases. Another extensile exposure is the Wagner osteotomy in which the proximal femur is split in the coronal plane. Finally, conventional trochanteric osteotomy, or so-called trochanteric
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slide (in which the abductors, greater trochanter, and vastus lateralis are kept in continuity), also are useful in selected complex revisions. Extensile exposures allow better visualization of both the femur and the acetabulum while minimizing soft tissue damage. Fixation of extended osteotomies can be performed with the use of cables or wires. The osteotomy fragment can sometimes be advanced to modulate soft tissue tension and enhance hip stability.
Removal of Acetabular Component
Good exposure of the acetabulum is key for removal of the acetabular component while avoiding bone loss. Removal of a loose acetabular component usually can be performed with relative ease. In recent years acetabular extraction systems have been introduced; these consist of thin curved osteotomes that can be inserted behind the cup and rotated around a ball placed inside the liner. It is crucial that the osteotome be placed at the interface between the cup and the cement mantle or the bone as opposed to being inserted into the substance of retroacetabular bone.
Removal of Femoral Component
Extraction of a loose femoral component usually can also be performed after obtaining good exposure of the femur. Extended exposures may be needed for removal of a well-fixed femoral stem or cement mantle. In cases where a portion of cement mantle remains, particularly in the distal femur, the use of an anterolateral window in the cortex may be most appropriate for complete removal of the remaining cement. It is important to remember that any such defect in the cortex will need to be bypassed by a stem spanning at least two cortical diameters distal to the defect. In most cases the cement mantle can be removed with the use of a combination of ultrasonic or mechanical extraction systems.
Acetabular Revision
An acetabular component may be revised for loosening, infection, malposition, or wear. The objective of the procedure should be restoration of the hip center of rotation, remedy of deficient bone, and secure component fixation in optimal orientation.
Cementless Acetabular Revision
A cementless hemispheric cup can be used in acetabular revisions. Several large series report excellent mid- to long-term survivorship of their techniques. The outcome is good even with limited acetabular bone grafting including morselized grafting of cavitary defects. The prerequisite for a successful outcome is obtaining appropriate press fit and adequate surface area of contact between the cementless cup and the host bone. The use of newer high-friction, highly porous metals such as tantulum may allow successful reconstruction using this method even when the bone loss is severe. It is important to note that the location of the contact area is a critical factor. Deficiencies of the superior weight bearing dome at the socket may be filled with structural allograft or metallic component augments. Many defects may be obliterated by reaming of the cavity to allow insertion of a jumbo acetabular cup. The advantage of using a jumbo cup is that it allows restoration of the hip center of rotation and improved cup contact against the host bone.
Options for Reconstruction of Bone Defects
Type I: Segmental Deficiency
Most segmental defects that involve a small area of acetabular rim can be treated using a cementless hemispheric cup. The defect is usually reamed away prior to insertion of the uncemented cup. Defects remaining after reaming can be treated with particulate bone grafting. The bone graft is packed into the defect using the reamer in a reverse direction. For larger defects with intact columns, structural graft or metal augments may be used.
Type II: Cavitary Deficiency
Most cavitary defects also can be managed with cementless hemispheric components. In most cases the rim area is intact allowing press-fit insertion of the cementless component. Larger defects can be filled with particulate bone grafting. For larger defects, especially in the medial wall region, impaction bone grafting with wire mesh can be considered.
Type III: Combined Segmental and Cavitary Deficiency
There are several different options for treating these defects. Cementless jumbo cups, with bone grafting, can be used in most cases as long as adequate surface area of contact between the porous acetabular cup and the host bone can be achieved. For cases with a small surface area of contact and a large defect, proper fixation of an uncemented component may not be possible. For these cases cemented cups with impaction grafting or reconstruction cages may be more appropriate.
Type IV: Pelvic Discontinuity
One of the most challenging problems in revision THA is acetabular reconstruction for pelvic discontinuity with complete separation of inferior and superior regions as a result of transverse defects and/or fractures. In spite of all preoperative radiographic assessments, pelvic discontinuity may be recognized or detected only during surgery. Hence, it is crucial for reconstruction surgeons to ensure that appropriate acetabular components are in place to deal with this problem during complex revisions. Pelvic discontinuity should be suspected in patients with massive bone loss. Any defects leading to disruption of the posterior column should also lead the surgeon to suspect pelvic discontinuity. When detected, appropriate reduction and fixation of the “fracture,” usually by plating of the posterior column, typically is performed prior to insertion of an acetabular component.
Femoral Revision
The main principles for femoral reconstruction are to obtain rigid fixation for the femoral component, to restore limb
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length and hip stability, and when possible to restore bone stock.
Cementless Femoral Revision
Proximally Coated Stems
Although proximally coated femoral stems are used commonly during primary THA, the results of revision THA using this stem design are disappointing. The main reason is that after removal of a previous femoral stem, little to no bone may exist to allow proper fixation of this stem design and subsequent biologic integration in the metaphyseal region of the proximal femur. Hence, proximally coated femoral stems are not commonly used during revision surgery.
Extensively Coated Stems
Fully porous-coated femoral stems are commonly used for revision hip arthroplasty. The extensively coated stems bypass the sclerotic and often weakened proximal femur to achieve secure fixation in the relatively healthy femoral diaphysis. The implant's initial rotational stability and subsequent biologic fixation are excellent in most cases. To optimize the outcome, these stems should be implanted in femoral canals with adequate (>5 cm) diaphyseal length for scratch fit. The stem should bypass any defects in the cortex by at least two canal diameters. An onlay cortical allograft may be used for larger defects (more than one third of the canal diameter) of the cortex. All cement in the femoral canal should be removed before reaming the canal. A curved stem, conforming to the anatomy of the canal, should be used when longer components are being implanted. Depending on the design, reaming of the femoral canal to an appropriate diameter to allow good press fit without causing fracture should be carried out. In cases with high likelihood for fracture of the femur or in cases in which longitudinal fracture of the femur occurs, cerclage cables or wires may be used. Although extensively coated stems are reported to have excellent outcomes in revision surgery, some problems are associated with this femoral component design, one of which is stress shielding.
Modular Femoral Stems
Fluted, tapered modular cementless femoral stems also can be used to gain diaphyseal fixation. Modular femoral stems provide intraoperative versatility to allow adjustment of limb length, offset, and version.
Proximal Femoral Replacement
After the initial success of the megaprosthesis in patients with neoplastic conditions, the indications for this type of reconstructive procedure were expanded to include patients presenting with failed THA and massive proximal femoral bone loss. With refinements in design, namely introduction of modular prostheses, megaprostheses have been used in cases of proximal femoral bone deficiency.
Megaprotheses currently are reserved for use mostly in elderly or sedentary patients with massive proximal bone loss. In younger patients in whom bone loss of high magnitude is encountered that cannot be reconstructed by conventional means, an allograft-prosthetic composite would be preferred over femoral prosthetic replacement.
Allograft-Prosthetic Composite
Allograft bone used may be in the form of strut grafts placed against defects in the femoral cortex or an allograft-prosthetic composite (APC). The type and length of allograft used depend on various factors, particularly the extent of bone loss and the status of the soft tissues. An important prerequisite for the use of APC is the availability of sufficient distal femoral length for secure fixation of the femoral stem. The advantages of using an APC (besides its ability to restore bone mass) are that it allows transmission of normal load to the distal host bone and prevents further distal bone loss. The soft tissue in the proximal region of the femur also can be attached to the allograft bone with some potential for healing and integration. The major disadvantages of using an APC are the higher incidence of infection, graft fracture, nonunion of the allograft with the host bone, technical difficulty of fashioning the composite, and the relatively long operation time.
Cemented Femoral Revision
Although cementless femoral stems are the most commonly used revision femoral components, cemented stems also may be used in selected cases. Cemented femoral revision generally is indicated in patients with good bone stock and available cancellous bone for interdigitation. Following removal of a previous femoral component, usually little if any cancellous bone remains; thus the mechanical bond of cement to bone in revision typically is reduced, particularly if the previous component was cemented. Hence, the use of cemented femoral components during revision surgery is limited. Femoral impaction allografting and revision with allograft-prosthesis composite are two scenarios in which cemented femoral stems need to be used. Another occasion is when a well-fixed prior cement mantle is left in place and another femoral component is either impacted into the mantle or cemented into the previous cement mantle, the so-called cement-within-cement technique. If this strategy is to be used, some important steps needs to be taken. The mantle is dried, then multiply scratched and roughened to increase the contact surface area and improve interdigitation for the new cement mantle. The cement is injected during a relatively liquid stage.
Impaction Allograft with Cemented Stem Revision
This difficult technique is most commonly considered for femora with cavitary metaphyseal or diaphyseal deficiencies and an intact cortical envelope. Small segmental deficiencies also can be treated with the use of a wire mesh or cortical strut graft. The technique involves compression of particulate cancellous bone allograft into the cavitary defects to reconstruct the osseous architecture of the femur and concomitant use of a polished cemented femoral stem. Before insertion of the grafts, the inner surface of the femur should be cleaned thoroughly. A cement plug is inserted to restrict the bone graft. The recommended graft size is generally as small as 4 to 6 mm3. Special instruments are used to obtain optimal graft impaction. The stem
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is cemented into the graft mantle. The major advantage of using femoral impaction allograft is the restoration of bone stock. This technique, however, is challenging. Subsidence of the stem, either with or without the graft mantle, have been reported in a few series, emphasizing that the results are dependent on technique and patient selection. A high incidence of periprosthetic fracture also has been reported at the tip of the femoral stem when short-stemmed implants were used routinely.
Selection of Fixation Method
Although there are no hard-and-fast rules with regard to which stem design or type should be used during revision surgery, some general rules apply. The first goal should be a good long-term clinical outcome, and a secondary goal should be restoration or maintenance of bone mass. The first objective usually precludes the use of conventional monoblock proximally coated uncemented stems in almost all cases and also the use of cemented stems in most cases. The use of both aforementioned stem designs necessitate good quality and volume of bone in the metaphysis and the canal, which is rarely the case during revision surgery. Hence, uncemented distally fixed stems are used more commonly. When extensively coated stems are being used, it is crucial to ensure that adequate diaphyseal fixation is achieved. This minimizes the possibility of stem subsidence, pain, instability, and limb shortening. For patients with extensive bone loss, proximal femoral replacement in older and less active patients and allograft-prosthesis composite in younger patients are preferred. Regardless of which stem design and type are used, revision surgery can be a challenging experience and should be undertaken only by those familiar with all the intricacies. Attention to detail and delivery of optimal surgical care are essential for a predictable and good outcome of any surgical procedure, particularly revision arthroplasty.
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