Uncemented Acetabular Revision with Metal Augments or Cup-Cage Combinations
Uncemented Acetabular Revision with Metal Augments or Cup-Cage Combinations
INDICATIONS/CONTRAINDICATIONS
The last several decades have seen the number of complex revisions with severe acetabular bone loss increase dramatically. Fortunately, better uncemented acetabular technology has allowed many of these revisions to be treated with uncemented acetabular components alone. As such, the role for traditional antiprotrusio acetabular cages has diminished over the last decade. With the advent of highly porous metals; however, the role for uncemented acetabular components with acetabular augments and/or cup-cage constructs has expanded and now offers surgeons new options for dealing with massive acetabular bone loss in the revision setting.
The use of acetabular augments and/or cup-cage constructs is indicated when the use of an uncemented acetabular component alone can be predicted to have a high likelihood of failure. The role of antiprotrusio cages in particular has evolved considerably over the last several decades. Traditional use of the antiprotrusio cage involved placement of the cage construct against host bone with multiple screw fixation into the ileum and pelvis and cementation of a polyethylene cup into the cage. The limited potential for osseointegration of the cage into host bone, however, led to substantial failure rates of these constructs. As uncemented acetabular technology has improved, the use of isolated cage constructs has diminished.
Nevertheless, innovative ways to implement the use of antiprotrusio cages in conjunction with an uncemented acetabular cup (the cup-cage construct) led to continued interest in their use. These constructs have been of particular interest in the setting of pelvic discontinuity and to enhance initial cup stability in other complex acetabular reconstructions.
Structural augments made from porous metals were initially made to replace the need for structural allografts, and their development has provided new alternatives for acetabular reconstruction. These augments not only provide structural support and stability of the uncemented acetabular component but also provide a surface that is highly conducive to bone ingrowth, which hopefully will enhance the durability of the reconstruction.
Uncemented acetabular components are undeniably technically easier to implant alone than in combination with augments or antiprotrusio cages and thus are the preferred method of acetabular reconstruction when possible. The exact amount of host bone necessary to achieve implant stability and, ultimately, bone ingrowth into an uncemented acetabular component alone is unknown, but the traditional teaching that 50% of host bone contact is needed is no longer applicable in all scenarios since the advent of porous metals.
The quality and location of the host bone are integral factors to consider when contemplating uncemented acetabular reconstruction alone because successful reconstruction with modest host bone available is possible. This is especially true if the host bone available is at the acetabular rim and a good peripheral fit of the acetabular component can be achieved. Nevertheless, there are circumstances in which there is so little host bone available or the geometry of the acetabular defect dictates that a stable, uncemented hemispherical cup alone is unobtainable and ingrowth into the acetabular component unlikely without augmented fixation. In these circumstances, the use of porous metal augments alone or in combination with
a cup-cage construct (1) has provided a valuable tool to gain initial implant stability and allows ingrowth of bone into the reconstruction. Additionally, in the setting of a pelvic discontinuity (2), the cup-cage construct (with or without posterior plating) is one option for acetabular reconstruction.
The main contraindication to uncemented acetabular reconstruction with the use of augments or a cup-cage construct is active infection in the hip joint.
PREOPERATIVE PREPARATION
History and physical examination are crucial elements to understand the reason for implant failure. Attention to details of the history helps the surgeon avoid repeating techniques that may have led to implant failure.
It is essential in the setting of revision total hip arthroplasty to rule out infection before proceeding with surgery. We recommend routine evaluation with an erythrocyte sedimentation rate and C-reactive protein preoperatively. If the history or physical examination are concerning for infection, preoperative hip aspiration with a cell count and culture is indicated. Careful examination and documentation of the neurovascular status of the limb and the function of the abductor musculature are essential. It is also prudent to obtain previous operative reports with implant records prior to surgery so that appropriate implants are available, especially if isolated acetabular revision is being considered.
RADIOGRAPHY
Plain radiographs should be obtained to assess the entire prosthetic hip joint. Specifically, the quality and quantity of the surrounding femoral, acetabular, and pelvic bone should be evaluated. Routine use of an anteroposterior pelvis, anteroposterior femur, and lateral radiograph of the hip is recommended. The surgeon should look for areas of acetabular bone loss, osteolysis, and any evidence of pelvic discontinuity. Radiographic signs of pelvic discontinuity on an AP pelvis view include a visible fracture line that traverses both the anterior and posterior columns, medial translation of the inferior hemipelvis, or rotation of the inferior hemipelvis (as evidenced by asymmetry of the obturator foramina). Judet views of the pelvis can be helpful to identify a true pelvic discontinuity. Computed tomography (CT) scan of the pelvis is not routinely utilized by the authors but can be helpful to further delineate and define any bony defects. Multiple classifications to define the extent of bone loss exist, but Paprosky's classification of acetabular defects (3) is the most commonly used in North America and provides a practical classification scheme that can help guide treatment.
TECHNIQUE
The authors prefer to operate with the patient in the lateral decubitus position, though revision hip surgery can be performed with the patient supine as well. Wide preparation and draping of the extremity is essential to allow access to the entire pelvis and ileum, especially if a cup-cage construct is being considered. Intravenous antibiotics should be administered prior to incision unless
there is a high suspicion for infection, in which case, antibiotics may be held until good intraoperative cultures can be obtained.
Adequate exposure of the acetabulum and pelvis can be accomplished through a multitude of operative approaches. Surgeon preference, philosophy, and experience are the major determinants of the approach chosen. Whatever approach is chosen, however, it is important to understand that if a cup-cage construct is to be utilized, exposure of the ileum is necessary and places the superior gluteal nerve at risk. This risk is increased with the use of traditional posterior or anterior exposures. The risk of injury to the superior gluteal nerve can be minimized by the use of a transtrochanteric approach, though the authors commonly use an anterolateral approach incorporating a small osteotomy of the anterior greater trochanter.
To perform a traditional anterolateral approach, a straight lateral incision is made, incorporating any previous incisions, if possible. Dissection is carried down to the level of the iliotibial band fascia, and the fascia is incised in line with its fibers. Often, the anterior aspect of the gluteus maximus must be split in line with its fibers as well. Next, a straight osteotome is used to osteotomize a small portion of the anterior aspect of the greater trochanter. The anterior portion of the gluteus medius and the vastus lateralis are left in continuity with this segment of the greater trochanter. In most cases, the osteotomy is only a small wafer of bone, usually no more than 0.5 cm thick. This segment is reflected anteriorly, and the anterior and superior portions of the hip pseudocapsule are excised. When adequate exposure of the femoral component, femoral head, and acetabular rim are achieved, the hip is dislocated anteriorly. If revision of the femoral component is to be undertaken, removal at this point will facilitate acetabular exposure. If the femoral component is to be left in place, mobilization of the femur with further removal of the pseudocapsule may be necessary to achieve adequate acetabular exposure.
It is important to identify and protect the sciatic nerve, when possible, to minimize the risk of injury, especially when contemplating a cup-cage construct or posterior acetabular plating in the setting of a pelvic discontinuity.
Adequate exposure of the entire acetabular rim is critical to assess any and all acetabular defects. This is achieved by removing scar and pseudocapsule from the entire periphery of the acetabulum until the entire rim is visible.
Once the acetabular component has been removed, careful assessment of the residual acetabulum and any bony defects is essential. Typically, the extent and pattern of bone loss encountered at the time of surgery dictates the type of reconstruction. Meticulous inspection of both the anterior and posterior columns is critical to determine if there is a pelvic discontinuity as this will have reconstructive implications. The authors' preferred technique to check for a pelvic discontinuity is to use a Cobb elevator and push on the inferior portion of the hemipelvis and assess whether any motion occurs between the superior and inferior portions of the acetabulum. Motion between these two segments confirms the presence of a pelvic discontinuity.
The modern considerations for acetabular reconstruction in the setting of massive bone loss (with or without a pelvic discontinuity) are an uncemented acetabular component alone, an uncemented acetabular component in conjunction with structural augments, utilization of a cup-cage construct (with or without posterior plating or structural augmentation), or a custom triflange component. The use of an uncemented acetabular component with augments and the cup-cage construct is discussed in this chapter.
STRUCTURAL AUGMENTATION
Structural augmentation of acetabular bone deficiency with the use of porous metal augments is an effective alternative to the use of allograft bone. Acetabular augments have several design features that make them reliable for reconstruction. First, they are made of porous material that has shown excellent bone ingrowth capability (4). They also allow for screw fixation and come in multiple sizes to accommodate a multitude of different defects. There are fenestrations in the augments that allows for adjunctive morselized bone graft. Lastly, they can be unitized to the acetabular shell by both screw and cement fixation providing excellent initial stability of the construct.
In our experience, there are three types of acetabular defects and corresponding augment configurations that account for the majority of defects encountered at the time of acetabular reconstruction. Type I defects are peripheral, segmental defects for which a “flying buttress” configuration is often optimal (see Fig. 24-1A-E). Type II defects include large combined cavitary and peripheral segmental
rim defects that cannot be managed by reaming up to a larger-sized acetabular component. For type II defects,
augments can be placed into the defect itself for added stability (see Fig. 24-2A, B). Lastly, large combined cavitary and segmental defects of the medial acetabulum (type III defects) occasionally benefit from augmentation to provide initial stability of the cup in the setting of good peripheral rim fit (see Fig. 24-3A-C).
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FIGURE 24-1 A: The image depicts an augment placed in a “flying buttress” configuration for a peripheral defect in the acetabulum. B: The image shows the acetabular component being implanted after placement of the augment. C: A close-up of an augment utilized for a type I acetabular defect in the “flying buttress” configuration. D: The acetabular component and augment after final fixation of the construct. E: This is the 2-year postoperative x-ray of an acetabular reconstruction for a type I defect.
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FIGURE 24-2 A: Illustration depicting a type II acetabular defect with augment and acetabular component in place. B: Four-year postoperative x-ray of an acetabular reconstruction for a type II acetabular defect.
If an uncemented acetabular component with structural augmentation is deemed appropriate, the acetabulum should be reamed moderately to remove any wall prominences and smooth the acetabulum while minimizing the amount of bone removed from the residual supportive healthy bone of the acetabulum. Sizing of the acetabular component can be accomplished using the hemispherical reamers as a guide or by using trial acetabular components. Once the appropriate size acetabular component is chosen, the size and location of any augment/s should be determined.
Trial augments should be placed and a trial acetabular shell impacted to determine optimal positioning and optimal fit of the components. Occasionally, to optimize the fit of an augment, and ultimately, the uncemented acetabular component, a small amount of host bone must be removed. This can be achieved with incremental use of a burr (for type I defects) or an acetabular reamer (for type II and III defects) taking care to remove only the bone needed for proper fit of the construct. Once host bone contact with the trial augments and trial acetabular component has been maximized, the real components should be opened and prepared for insertion. Typically, the augments are secured first (see Fig. 24-1C). The trial component can be used to temporarily hold the real augments in place until adequate screw fixation of the augments has been achieved. This also ensures that the augments do not move in such a way as to prevent optimal host bone contact with the acetabular component.
Once the augments have been secured to the host bone, antibiotic laden cement should be placed on the exposed metal of the augments and the real acetabular component impacted (before the cement hardens) in an effort to unitize the augments to the acetabular shell. Care should be taken not to place excessive cement when unitizing the components as extra cement can extrude into the host bone/implant interface, which is not ideal.
Multiple screw fixation of the acetabular component to the host bone then is commenced. If possible, screw placement through the acetabular component and augment is ideal (see Fig. 24-4). Liberal use of screw fixation through the acetabular component
into host bone is preferred by the authors. Some designs of highly porous metal shells facilitate these difficult cases as new screw holes can be placed through the cup with a high-speed metal cutting burr in locations where the host bone is of the best quality and greatest quantity (see Fig. 24-5). Once screw fixation is complete, the acetabular liner is inserted into the cup, by snap fit fixation or cementation depending on the cup design.
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FIGURE 24-3 A: Illustration of a type III acetabular defect with augments and acetabular component in place. B: Intraoperative photograph of augmentation of a type III acetabular defect. Note the bone graft placed in the fenestrations of the augments. C: Two-year postoperative radiograph of an acetabular reconstruction for a type III defect.
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FIGURE 24-4 Screw placement through the acetabular component and augment will improve the initial stability of the construct.
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FIGURE 24-5 A metal cutting burr can be used to create holes in porous metal acetabular components to ensure screw fixation into areas where the best bone is present.
CUP-CAGE CONSTRUCT
Historically, antiprotrusio cages were placed directly against host bone (5,6), and a polyethylene liner was then cemented into the cage. These implants provided good initial stability but provided for minimal bone ingrowth potential that led to substantial rates of failure with longer rates of follow-up (7,8,9,10,11). The development of highly porous metals has allowed for innovative methods of using the antiprotrusio cages and has led to the authors preferred technique of antiprotrusio cage usage: the cup-cage construct (Fig. 24-3A). The cup-cage construct, initially described by Gross and Goodman (1), marries the initial stability provided by traditional cages with the ingrowth capabilities of porous metals. Currently, the authors rarely rely on traditional antiprotrusio acetabular cages alone.
If it is determined that an uncemented acetabular component with or without augments will not provide adequate implant fixation and stability to allow bone ingrowth into the components or if the surgeon simply wishes to enhance the initial stability of the construct, a cup-cage construct may be considered. The exposure described above is carried out with a few modifications. Exposure of the ilium above the acetabular rim must be adequate to accept the flange of the antiprotrusio cage. Typically, this requires exposure of 3 to 4 cm of ileum above the acetabular rim. Careful exposure is required to minimize risk to the superior gluteal neurovascular bundle.
Similarly, the proximal aspect of the ischium must be exposed for insertion of the inferior flange of the cage. Dissection typically is carried down the ischium to the level of the proximal hamstring tendon origins taking great care to stay on bone and minimize risk to the sciatic nerve. Direct visualization of the sciatic nerve to keep it protected is ideal, if possible. In addition, keeping the hip extended and the knee flexed is a useful adjunct to protect the sciatic nerve during this portion of the procedure. Utilizing trial antiprotrusio cage trial implants can help the surgeon determine whether enough of the ileum and ischium have been exposed and to judge the approximate size and contour of the cage needed for implantation. Next, a slot is made in the ischium to accept the ischial flange of the antiprotrusio cage. Occasionally, pelvic contour precludes insertion of the ischial flange into the ischium. In these rare circumstances, consideration of removal of the inferior flange or placement of the flange on the outside of the ischium can be considered, though neither of these choices provides much initial stability of the cage.
After an adequate exposure has been achieved and the ischial slot has been created, the acetabular component and any augments needed are implanted as described above. The real antiprotrusio cage is then brought onto the operative field, and final contouring is carried out to optimize ileal flange position and ischial flange insertion. Contouring should commence in accordance with the ideal fit of the trial cage.
Cage insertion should be carried out from superior to inferior (see Fig. 24-6) ensuring the ischial flange rests inside the created slot in the ischium. The cage should be impacted into place ensuring it rests on the acetabular component and the ileal flange rests nicely against the ileum. Once seated, the cage should be secured to the pelvis with the use of multiple fully threaded, 6.5-mm cancellous screws. If possible, screws should be placed through the dome of the cage and through the acetabular component into host bone (see Fig. 24-7). As many screws as can be placed safely into the ileum should be utilized.
Next, place trial liners into the cage taking care to position the liner in a position that will maximize hip stability postoperatively. Once an appropriate liner and position have been chosen, the real polyethylene liner should be opened and the back surface roughened with use of a burr to ensure
excellent cement interdigitation into the polyethylene. Methyl methacrylate should then be mixed with antibiotics until it achieves a doughy state at which point the real liner can be cemented into the cup-cage construct (see Fig. 24-8). It is important to recognize that the position of the liner can be adjusted during cementation, and every effort should be made to place the liner in the optimal position to maximize hip stability postoperatively. The liner should be held in place until the cement hardens. If femoral reconstruction is to take place, it may be completed at this point in the operation. The hip is then reduced, and hip stability and leg lengths are assessed. Closure is routine and should commence according to surgeon preference.
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FIGURE 24-6 Insertion of the cage should commence from superior to inferior so that the ischial flange can be placed into the ischial slot.
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FIGURE 24-7 A: Illustration of a cup cage construct with cage placement on top of the acetabular component. B:
Cage placement over the top of an acetabular component.
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FIGURE 24-8 The cup-cage construct after cementation of the polyethylene liner.
PEARLS AND PITFALLS
Protection of the Sciatic Nerve
Injury to the sciatic nerve can occur in these large reconstructions. Care should be taken to identify the nerve at the time of exposure, if possible, and the position of the nerve should be checked regularly throughout the reconstruction. Keeping the hip in an extended position and the knee in a flexed position during the case can minimize tension on the nerve and minimize the risk of injury.
Placement of the Ischial Flange
Placing the ischial flange during cage insertion can be challenging. In order to maximize correct positioning, care should be taken to place the slot in the ischium in enough anteversion to allow the cage to sit down against the acetabular component and ileum properly. Trial cages are an invaluable tool in ensuring the proper orientation and positioning of the component and should be utilized liberally prior to insertion of the real implant. Cages typically are thin and made of titanium. Care must be taken not to repetitively bend the flanges on the real implant as they will weaken with time and are at risk for fracture. It is ideal that the trial fits properly and that manipulations are undertaken using the trial component so that the final component undergoes only the necessary modifications.
Maximizing Host Bone/Implant Contact
Obstruction of host bone and the acetabular implant interface by metal augments is possible if care is not taken to ensure a proper fit of the entire construct before implantation. Trial augments and acetabular components are vital to maximizing the host bone contact with the real implants. Similarly, when unitizing the augment/s to the acetabular component with cement, care must be taken not to
place excessive cement that may escape into the host bone-implant interface. This can interfere with the ingrowth capability of the construct.
POSTOPERATIVE MANAGEMENT
Patients are typically mobilized the day after surgery, and weight-bearing status is determined based on intraoperative events and bone quality. For the majority of reconstructions in which metal augmentation or a cup-cage construct are used, patients are kept touch weight bearing for the first 2 months postoperatively with progressive weight bearing thereafter. Most patients are full weight bearing by 3 to 4 months postoperatively.
Consideration may be given to using a brace postoperatively to protect the soft tissues, allow the greater trochanteric osteotomy to heal (if performed), and reduce the risk of early dislocation.
COMPLICATIONS
Infection, dislocation, and nerve injury are the most common early complications encountered in these massive reconstructions. Every effort to avoid these complications intraoperatively and postoperatively should be undertaken as previously described.
Ultimately, the success of the reconstruction is dependent on the construct providing enough stability for a long enough period of time to allow bone ingrowth into the uncemented acetabular component. If construct stability fails prior to ingrowth, loosening of the acetabular component will occur.
RESULTS
Though, the cup-cage reconstruction is a relatively new technique, midrange follow-up suggests this technique is a very reasonable option in the setting of Paprosky IIIA and IIIB acetabular defects and in the management of pelvic discontinuity. Our early experience with these constructs has shown excellent results at midterm follow-up (see Fig. 24-9). In our review of 32 cup-cage constructs, 31/32 cup-cages were intact at a mean follow-up of 31 months. No cup-cages showed evidence of aseptic loosening, migration, or screw breakage. None of these hips had undergone revision for loosening. These cases did have a high complication rate as 5/32 (16%) hips experienced a complication including 3 reoperations for instability, 1 reoperation for chronic infection, and 1 thigh seroma treated conservatively.
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FIGURE 24-9 The 2-year postoperative radiograph of a cup-cage construct.
Ballester Alfaro and Sueiro Fernandez (12) reviewed 19 patients with Paprosky IIIA and IIIB acetabular defects that were managed with porous metal acetabular shells and buttress augments or cup-cage constructs in conjunction with modular augments. At a mean of 26-month follow-up (18 to 43 months), there were no failures and no clinical or radiographic evidence of aseptic loosening. The authors concluded that these reconstructions are reasonable options in this difficult patient population.
Kosashvili et al. (13) reviewed 26 hips reconstructed with cup-cage constructs for pelvic discontinuity at a mean follow-up of 44.6 months (range 24 to 68). They found that 23/26 cup-cage constructs showed no evidence of loosening (defined as migration greater than 5 mm) at the time of last follow-up. All patients showed improvements in their Harris hip score, and the authors concluded that the cup-cage construct was a reliable option in patients with a pelvic discontinuity.
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