Matthew S. Austin, James J. Purtill, and Brian A. Klatt
DEFINITION
Acetabular bone deficiency may occur primarily (eg, dysplasia, inflammatory arthritis, seronegative arthropathy) or secondarily (eg, aseptic or septic loosening of acetabular components, osteolysis, trauma, iatrogenic loss during removal of well-fixed components).
The use of antiprotrusio cages is indicated in situations where an uncemented porous-coated acetabular component will not gain reliable initial stability.
ANATOMY
The confluence of the ilium, the ischium, and the pubis forms the hemispherically-shaped acetabulum, each contributing to the anterior and posterior walls and columns.
Surgical landmarks include the anterior and posterior walls, dome, and medial wall “teardrop.”
The acetabulum is normally oriented with 45 degrees of inclination and 15 degrees of anteversion relative to the pelvic plane.
PATHOGENESIS
Acetabular bone deficiency may occur primarily due to dysplasia. This type of deficiency usually does not require the use of an antiprotrusio cage.
Certain conditions (eg, rheumatoid arthritis, juvenile rheumatoid arthritis, anklylosing spondylitis, Paget disease) may predispose to acetabular protrusio but usually do not require the use of an antiprotrusio cage.
The antiprotrusio cage is used most often in cases of secondary bone stock deficiency so massive that the use of a cementless press-fit acetabular component is precluded.
NATURAL HISTORY
The natural history of massive acetabular bone defects that would require an antiprotrusio cage is not known. Patients usually require revision surgery to return to functional activities.
PATIENT HISTORY AND PHYSICAL FINDINGS
The history should be directed to determine whether the source of pain is extrinsic or intrinsic.
The patient's pain may be extrinsic (eg, lumbar radiculopathy, intrapelvic pathology), in which case revision surgery may fail to relieve pain completely.
Pain usually, but not always, is located in the groin.
Infection always must be assessed with careful questioning about previous infections, fevers, chills, wound drainage, and pain at rest.
Start-up pain is a characteristic indication of loosening.
Medical comorbidities must be assessed to determine the presence of any that may compromise the outcome of the surgery or place the patient at increased risk of complications.
The skin should be inspected visually for placement of prior incisions and signs of infection.
The appropriate incision for the surgical approach must be used with an adequate (6 cm) skin bridge.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Plain radiographs, including AP of the pelvis and AP and true lateral of the hip should be obtained to classify bone loss and to adequately preoperatively plan (FIG 1).
CT scans may be used to assess the remaining bone stock.
This is especially important in cases of superior or posterior bone loss that may require allograft reconstruction.
The CT scan can help determine the need to have allograft bone available to reconstruct the defect and may help to dictate the approach if a posterior column reconstruction or plating is necessary.
CT scans with intravascular contrast are useful in situations in which a prior implant is medial to Kohler's line and proximity of the implant to the vessels and intra-abdominal contents is unknown.
Erythrocyte sedimentation rate and C-reactive protein are useful screening tools to detect infection.
Aspiration of the hip to assess for infection is valuable if either the erythrocyte sedimentation rate, C-reactive protein, or clinical suspicion is elevated.
Nuclear medicine studies such as technetium Tc 99 methylene diphosphonate in combination with gallium citrate, indium 111–labeled leukocyte scans, positron emission tomographic scans with 18Ffluorodeoxyglucose (PET-FDG), and sulfur colloid scans may help differentiate aseptic from septic loosening.
DIFFERENTIAL DIAGNOSIS
The following conditions may contribute to pain and may be the cause of continued pain after the surgery:
Lumbar radiculopathy
Spinal stenosis
Sacroiliac degenerative joint disease
Intra-abdominal pathology
Intrapelvic pathology
Neuropathy
Meralgia paresthetica
Complex regional pain syndrome
Vascular claudication
Primary bone tumors
Metastasis
NONOPERATIVE MANAGEMENT
Nonoperative management of severe acetabular bone loss is reserved for those patients for whom surgical management is contraindicated. These include patients with substantial medical comorbidities and patients with active infection.
FIG 1 • AP radiograph of the hip demonstrates medial migration of the acetabular cup with significant loss of acetabular bone stock.
SURGICAL MANAGEMENT
Surgical management begins with preoperative planning.
The radiographs are assessed and it is determined whether the defect can be reconstructed with a cementless acetabular component or will require an antiprotrusio cage.
The surgical approach is planned.
If posterior column plating is anticipated, a posterior approach is indicated.
If this is not required, a direct lateral or posterior approach may be used, according to the surgeon's preference.
The acetabulum is exposed, bone loss is assessed, and the determination as to the appropriate reconstructive choice is made.
Preoperative Planning
Planning for the antiprotrusio cage begins with appropriate radiographs.
The radiographs allow for classification of the defect and aid in planning for the reconstruction.
We have found the Paprosky classification to be helpful in defining bone deficiency and in predicting the method of reconstruction3 (FIG 2).
Paprosky type 1 acetabular defects have minimal bone loss and usually can be reconstructed using just a cementless component.
Type 2A defects have an intact superior rim, and the acetabulum is oval in shape. The anterior and posterior columns are intact. The implant has migrated less than 2 cm. These defects may be reconstructed with so-called “jumbo” cups or cementless reconstruction with additional bone grafting or trabecular metal augments. The socket also may be placed in a more superior position to attain greater contact with host bone.
Type 2B defects are similar to type 2A defects, with the exception of loss of the superior rim. The implant has migrated less than 2 cm. The superior rim can be reconstructed with an uncemented socket in association with bone grafting or trabecular metal augments.
Type 2C defects involve medial bone loss with intact anterior and posterior columns. The medial bone loss may be reconstructed with bone graft or trabecular metal augments.
Type 3A defects generally migrate more than 2 cm superolaterally. The medial wall and ischium usually are still present, but damaged. These defects can be reconstructed with bone grafting or trabecular metal augments and an uncemented socket.
Type 3B defects generally migrate more than 2 cm superomedially. There is loss of the teardrop and severe damage to the ischium. Pelvic discontinuity should be suspected. These defects can be recontructed with bulk allograft, trabecular metal, antiprotrusio cage, or a combination. Posterior column plating may be needed in cases of acute discontinuity. If this is a possibility, then a posterior approach should be selected.
Pelvic discontinuity should be suspected if a fracture is noted that involves both the anterior and posterior columns, the inferior hemipelvis has migrated medial to the superior hemipelvis, or the inferior hemipelvis is rotated in relation to the superior hemipelvis.
Large posterior column defects may predispose to cage failure, and reconstruction of the defect should be included in preoperative planning.
An appropriate device should be selected.
FIG 2 • Preoperative radiographs of a patient with severe acetabular bone loss due to superior and medial migration of construct as well as pelvic discontinuity (Paprosky type 3B defect). An AP radiograph of the pelvis (A), an AP view of the hip (B), and a frog-leg lateral view of the hip (C) are obtained. Occasionally more specialized views of the pelvis, such as the Judet view, may be necessary to assess integrity of the acetabular columns.
The flanges should be malleable to allow the cage to be shaped to the bone.
The implant must have sufficient strength.
We believe that an inferior flange that gains fixation in or on the ischium is superior to a hook that gains fixation on the teardrop.
Positioning
The patient is positioned according to surgeon preference, as dictated by the surgical approach. The hip should be draped in such a fashion as to allow wide surgical exposure.
Approach
The surgeon should select a posterior approach if he or she anticipates the need for posterior column plating or reconstruction. Otherwise, the surgeon should select the approach that is most familiar and comfortable.
A trochanteric ostetomy, either conventional or extended, may improve exposure of the acetabulum and pelvis.
Exposure of the sciatic nerve (posterior approach) is left to the judgment of the surgeon.
TECHNIQUES
ACETABULAR EXPOSURE
The acetabulum is exposed, and the failed construct and any soft tissue that interferes with visualization should be retracted or removed (TECH FIG 1A–E).
The anterior and posterior walls/columns, superior dome/rim, medial wall, teardrop, and ischium are identified, and bone loss is noted (TECH FIG 1F–H).
TECH FIG 1 • Acetabular exposure. A. The acetabulum after exposure with a direct lateral (modified Hardinge) approach in the supine position. B. The polyethylene cup is removed. C. Unstable cage construct is removed in one piece with cement. D,E. Failed construct after removal. Once all material is removed, the acetabular landmarks are clearly exposed and evaluated: teardrop (F), posterior column (G), superior dome/rim (H).
INTRAOPERATIVE DETERMINATION OF RECONSTRUCTIVE TECHNIQUE
The remaining bone stock is assessed and corroborated with the preoperative assessment of bone loss.
Paprosky types 1, 2A, 2B, 2C, and 3A defects usually can be reconstructed with an uncemented acetabular component with or without bone grafting or trabecular metal augments.
All acetabuli, especially Paprosky type 3B defects, should be tested for pelvic discontinuity.
If a pelvic discontinuity exists (TECH FIG 2), the surgeon may then elect to use an antiprotrusio cage with or without posterior column plating or additional bone grafting.
Alternatively, the surgeon may elect to distract the discontinuity with an uncemented socket and bridge the defect in this manner.
The acetabulum should be assessed for the remaining bone's ability to support an uncemented component. To support an uncemented cup, the remaining bone stock should allow for at least partial inherent stability of the reamer or trial.
Reamers or trials that are inherently unstable may not be appropriate for cementless reconstruction.
The reamers are not used initally to shape the acetabulum but, rather, are used to assess the ability of the remaining bone to support a socket.
If it is determined that the remaining bone stock cannot support a socket, then an antiprotrusio cage should be used.
TECH FIG 2 • Intraoperative determination of reconstructive technique. A large central defect is visible where the cage was removed. Pelvic discontinuity is demonstrated (arrow) with a fracture of the posterior column.
BONE PREPARATION
The ilium must be exposed with care to avoid the superior gluteal neurovascular bundle (TECH FIG 3A).
The size of the defect can be assessed with a standard acetabular reamer. The reamer can then be used to remove small amounts of bone that may prevent complete seating of the cage (TECH FIG 3B).
It should not be necessary to remove significant bone, because the indication for use of this device is severe bone loss.
The outer diameter of the reamer that best fits the acetabulum determines the outer diameter of the cage.
The trial cage or the actual implant can be used to determine the removal of small amounts of bone that interfere with complete seating of the cage (TECH FIG 3C).
TECH FIG 3 • Bone preparation. A. The ilium is exposed. B. Minimal reaming is performed to allow the cage to seat. C. A trial cage is seated to determine whether the actual cage will seat. Small amounts of bone that interfere are removed.
BONE GRAFT AND TRABECULAR METAL AUGMENTATION
The cage may not be stable in situations with severe superior dome or posterior bone loss.
It may be necessary to augment the acetabulum with either structural or particulate bone graft (TECH FIG 4) or trabecular metal to provide the cage with support.
Severe superior dome or posterior wall or column bone loss may be reconstructed with structural allograft fashioned from a distal femur, proximal tibia, or acetabular allograft.
Trabecular metal acetabular components can be used in the cup–cage construct.
It may be necessary to support the trabecular metal acetabular component with a cage if native host–bone contact is less than 50%. This technique is beyond the scope of this chapter.
TECH FIG 4 • Bone graft and trabecular metal augmentation. A. Allograft cancellous bone is placed into the prepared acetabulum. B. Large reamer is used in reverse to crush and distribute the graft.
CAGE IMPLANTATION
The ischial flange can be placed on or in the ischium (TECH FIG 5A,B).
The advantage of placing the flange on the ischium is that screws can be used to fix the cage to the ischium (TECH FIG 5C,D).
The advantage of blade-plating the flange in the ischium is avoidance of the sciatic nerve (TECH FIG 5E).
Both methods allow for stable fixation.
The cage is shaped to the contour of the ilium and ischium while allowing for seating of the socket portion of the cage into the remaining acetabulum. Usually the ischial flange must be bent laterally to follow the contour of the ischium.
The ischial flange is fixed to or in the ischium using the surgeon's preferred method.
TECH FIG 5 • Cage implantation. A. The cage is shown in the orientation in which it will be inserted. B. Cage being inserted. C,D. Screw fixation to ischium. E. Blade plate fixation of the ishium flange. F,G. Screw fixation to the ileum. H. Cement pressurization. I. Cement after pressurization. J,K. Cup cemented in place. L. Polyethylene liner placed into cup.
The socket portion of the cage must be fully seated within the acetabulum to maximize the stability of the construct.
The flange must be contoured to the ilium to minimize cage motion. Usually this requires the flange to be bent medially with some rotation.
Once the cage is appropriately contoured to the ischium, ileum, and acetabulum and fixed to the ischium, it is fixed with screws through the dome of the cage.
Fixation usually can be obtained in the posterior and anterior columns, with care to stay within the recognized safe zones.
Additional screws should then be passed through the superior flange into the ilium (TECH FIG 5F,G). The number of screws is limited by the amount of bone that can safely provide fixation.
A polyethylene liner is then cemented into the cage construct.
The cement is placed in the socket and pressurized (TECH FIG 5H–K).
A liner designed for cementation or a liner appropriately modified for cementation is then cemented into place in the appropriate position (TECH FIG 5L).
A 2-mm cement mantle is desirable.
Care must be taken not to leave large areas of the liner uncovered.
FIG 3 • Postoperative radiographs are taken to verify the position of the cage.
POSTOPERATIVE CARE
Radiographs are taken to confirm appropriate cage positioning (FIG 3). Intraoperative radiographs may be beneficial to confirm cage positioning prior to final fixation of the device.
The patient is allowed protected weight bearing as tolerated if bone augmentation was not necessary.
The patient is restricted to toe-touch weight bearing for 6 to 12 weeks if bone augmentation was necessary. He or she is then allowed to increase weight bearing progressively on a schedule that is individualized to each patient.
OUTCOMES
The survivorship of antiprotrusio cages has been acceptable in shortto mid-term follow-up.
However, because most of these devices cannot achieve biologic fixation, it is assumed that they will loosen over time, although one series reported 92% survivorship at 21 years.4
Other series have shown 100% survivorship at 7.3 years5 and 93.4% survivorship at 10.9 years.2
The success of these devices also will depend on the environment in which they are placed.
Most surgeons in North America are selectively using these devices in cases of severe acetabular deficiency in which a press-fit uncemented socket is not appropriate. This may predispose these devices to failure.
A recent report on the use of the cage in the setting of discontinuity revealed a 31% revision rate at 46 months.1
COMPLICATIONS
The use of antiprotrusio cages is accompanied by the complications inherent to significant revision surgery:
Blood loss
Infection
Neurovascular injury
Construct failure
Anesthetic and medical complications
REFERENCES
1. Paprosky WG, Sporer S, O'Rourke MR. The treatment of pelvic discontinuity with acetabular cages. Clin Orthop Relat Res 2006;453:183–187.
2. Pieringer H, Auersperg V, Bohler N. Reconstruction of severe acetabular bone-deficiency: the Burch-Schneider antiprotrusio cage in primary and revision total hip arthroplasty. J Arthroplasty 2006; 21:489–496.
3. Sporer SM, Paprosky WG, O'Rourke M. Managing bone loss in acetabular revision. J Bone Joint Surg Am 2005;87A:1620–1630.
4. Wachtl SW, Jung M, Jakob RP, et al. The Burch-Schneider antiprotrusio cage in acetabular revision surgery: a mean follow-up of 12 years. J Arthroplasty 2000;15(8):959–963.
5. Winter E, Piert M, Volkmann R, et al. Allogeneic cancellous bone graft and a Burch-Schneider ring for acetabular reconstruction in revision hip arthroplasty. J Bone Joint Surg Am 2001;83A:862–867.