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The most important factor that determines the technique and the outcome of revision hip arthroplasty is bone stock.
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The allograft-prosthesis composite is a viable solution that provides pain relief, function, and a stable implant and does not compromise the distal host canal, facilitating further revision surgery.
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Initial stability is obtained with apposition of the allograft on the host bone through a step-cut or oblique osteotomy supplemented with wires and strut grafts.
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Long-term stability is achieved by union at the allograft-host junction; using the host proximal bone with its soft-tissue attachments as a biologic envelope around the junction promotes union.
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Union is usually achieved in 6 to 10 weeks.
Bone stock is the paramount factor that determines the appropriate technique in addressing a failing total hip arthroplasty, provides reference to the complexity of the revision procedure, and gives an indication of the expected outcome. The degree of bone loss is associated with the number of revision hip operations, and because younger patients are having hip arthroplasty surgery, this problem is expected to become more prevalent.
Several classification systems are used to evaluate femoral bone loss. The system that we use has five types. We have modified this system since its original publication ( Table 46-1 ) and use proximal femoral allografts in types 4 and 5. With the current implant designs, we use proximal femoral allografts in femoral defects extending 8 cm or more into the femoral diaphysis. Tumor prosthesis can be used as an alternative in selected patients.
| Gross Classification | Type of Defect | Treatment Alternatives |
|---|---|---|
| Type 1 | No significant bone loss | Conventional cemented or uncemented femoral component |
| Type 2 | Contained (cavitary) bone loss | Proximally porous-coated implant |
| Extensively porous-coated implants for ingrowth | ||
| Extensively grit-blasted titanium implants for ongrowth | ||
| Impaction grafting with a cemented component | ||
| Modular implants for proximal or extensive ingrowth or ongrowth | ||
| Long-stemmed cemented implants | ||
| Type 3 | Segmental (full circumferential) bone loss from the proximal femur that is less than 5 cm in length and involves the calcar and the lesser trochanter but does not extend into the diaphysis | As in type 2, with a calcar replacement option |
| Type 4 | Segmental (full circumferential) bone loss of greater than 8 cm in length extending into the diaphysis | Allograft-prosthesis composite or tumor prosthesis |
| Type 5 | As in type 4, with the addition of a periprosthetic fracture | Allograft-prosthesis composite or tumor prosthesis |
The tumor or megaprosthesis has the advantages of being modular, being available off the shelf, and carrying no possibility of disease transmission. Operating time for implanting such components is usually less than for proximal femoral allograft reconstruction. The disadvantages of tumor prostheses are that host bone or muscle cannot be effectively reattached, they do not restore bone stock, and they violate the distal host canal with cement or a porous-coated stem, rendering further revision surgery even more difficult.
A proximal femoral allograft, on the other hand, allows bone and muscle attachment. The reattachment of the greater trochanter in particular reduces the risk of dislocation and improves function. With an appropriate technique the distal canal is not violated, and this facilitates further revision surgery. A solid union at the graft-host junction augments the existing bone stock, which is important in young patients. The allograft-prosthesis composite disadvantages are the potential for disease transmission and the possibility of a poor result because of biologic complications including resorption, fracture, and host graft nonunion.
INDICATIONS, CONTRAINDICATIONS, AND PITFALLS
Proximal femoral allografts in the form of allograft-prosthesis composites are indicated in hip joint reconstruction surgery either in revision total hip replacement or after tumor resection. In revision hip arthroplasty performed at our institution, full circumferential structural femoral allograft-prosthesis composites have been used in uncontained segmental femoral defects that extend for more than 8 cm into the femoral diaphysis, especially if adequate distal stabilization cannot be achieved. Another indication is the presence of Vancouver type B3 periprosthetic femoral fractures with significant loss of bone stock rendering the distal fixation of a long, uncemented femoral stem at least difficult, if not impossible. Proximal femoral allograft is a useful technique, particularly in young patients, because it preserves distal bone stock and potentially improves the proximal bone stock to facilitate future reconstruction.
Active infection is a contraindication to the use of proximal femoral allografts in a single-stage revision surgery. However, we have successfully used allograft-prosthesis composites in a second-stage reconstruction surgery in patients with negative infection markers and frozen sections.
Patients with malignancies may benefit more from tumor implants because of the detrimental effects of chemotherapy and radiotherapy on allograft host healing. Also, extensive resection of muscle and bone, including the greater trochanter, makes reattachment of muscle and bone irrelevant. In addition, patients with multiple comorbidities, limited life expectancy, or need for fast mobilization benefit from a tumor prosthesis, as it does not require an extended non–weight-bearing period.
Problems with the proximal femoral allograft may occur if the allograft is cut too short. The final allograft length should be determined intraoperatively after multiple trials. Reaming of the allograft or placement of metal plates and screws may weaken the allograft and may result in fractures or in late allograft-prosthesis composite failures. Allowing cement to flow distal to the allograft-host junction has a number of deleterious effects: adequate pressurization and interdigitation at the allograft-cement interface is not achieved; the interface at the allograft-host junction is compromised and nonunion is probable; and the distal femur is violated, rendering possible further revision surgery more complicated. We recommend the preparation of the allograft-prosthesis composite on a separate table. A relatively small-diameter stem has to be chosen, as with this technique there is no need for distal press fit. A large-diameter stem may require reaming of the allograft in order to achieve an adequately thick cement mantle. Reaming the allograft weakens its mechanical properties and thus is not recommended. Finally, preserving the shell of the proximal femur with its soft-tissue attachments and wrapping it around the host-graft junction are expected to enhance union.
PREOPERATIVE PLANNING
Preoperative planning will determine the level of deficient proximal femur, the approximate length and diameter of allograft required, and the correct size of the femoral component. For this purpose an anteroposterior (AP) pelvic radiograph, AP and lateral radiographs of the involved femur, and a lateral view of the involved hip are necessary. Further imaging may be required to address possible acetabulum revision issues ( Fig. 46-1 ).
It is prudent to order an allograft that is longer than estimated. The diameter of the host femur and allograft should be approximately equal. This would ensure a good fit at the level of the allograft-host junction. It is best not to have an allograft with a significantly wider diameter than that of the host as this may prevent or delay union or may result in poor stability of the construct. Usually the allograft’s diameter is smaller than that of the host femur because of lysis and cavitation. This is not disadvantageous, as the allograft can be telescoped into the host femur for 1 cm or 2 cm, which enhances union and stability. It is important for the allograft canal to accommodate the implant to be used and also to allow at least a 2-mm thick cement mantle after reaming and broaching. Radiographs of the allograft with a known magnification rate are therefore important for preoperative planning, as is a template of the femoral implant. Allografts are usually imaged in their sterile packaging. Any preoperative leg length discrepancy should be measured and taken into account during templating.
TECHNIQUE
Preparation of the Allograft
The allograft used in this procedure is stored at −70° C and in our institution is irradiated with 2.5 Mrad. Graft preparation is performed on a separate surgical table by a second surgical team while the surgical exposure is being performed by the operating surgeon. We recommend using fresh frozen allograft from an American Association of Tissue Banks–accredited tissue bank. The larger banks process the bone in bactericidal and virucidal solutions before the bone is deep frozen. Some banks provide irradiated bone, which is 10% to 20% weaker, depending on the dose. To replace a proximal femur we prefer to use a proximal femoral allograft, but a distal femur will accept a larger implant and is used by some surgeons. The allograft is thawed in 5% povidone-iodine solution after cultures have been taken. After the bone has thawed and been stripped of soft tissue, it is prepared for the femoral implant. The femoral head is excised about 1 cm above the lesser trochanter or even at the base of the lesser trochanter, facilitating insertion of the implant and allowing room for adjusting the version. Lengthening of the leg is not carried out via the neck cut but rather by the length of the allograft below the lesser trochanter. The allograft should be cut long at first. A stable graft-host junction is necessary; either a step-cut or oblique graft-host osteotomy can be used to obtain stability. An oblique osteotomy is easier and allows adjustment of the version without having to make major changes to the osteotomy. An oblique osteotomy should be as long as possible—at least 2 cm in length. Occasionally there is enough of a canal-diameter discrepancy between the graft and the host that the graft can be telescoped into the host canal for a couple of centimeters, making the step-cut or oblique osteotomy unnecessary ( Fig. 46-2 ).
The greater trochanter is excised, allowing for reattachment of the host trochanter. If the patient does not have a greater trochanter, then the allograft greater trochanter should be left in place with a cuff of abductor muscle insertion for attachment of the patient’s abductors. Reaming is then carried out with straight rigid reamers for a straight implant or with flexible reamers for a bowed implant. The calcar region is milled until the implant can be seated. We ream only enough cortex to allow insertion of the implant with a 2-mm–thick cement mantle. It is important not to ream excessively in an attempt to insert a larger-diameter implant for a press fit into the host bone distally, as the allograft will be weakened. The host canal is almost always larger than the allograft canal, and if the surgeon attempts to use an implant large enough to obtain a press fit distally, then the allograft will have to be excessively reamed and weakened. We commonly use a 13.5- or 14-mm–diameter stem, which does not usually provide a press fit distally. This is not necessary, because the implant is cemented into the allograft and once the allograft-host junction unites, the entire construct is stable.
Because the implant is cemented into the allograft but not the host, stability is achieved at the graft-host junction. Cementing proximally into the allograft and distally into the host would interfere with graft-host union (by stress shielding the graft-host junction, leading to graft resorption). Also, cementing distally would compromise future revision surgery.
Before the implant is cemented into the allograft, the graft is triple-washed in 5% povidone-iodine solution and 1% hydrogen peroxide and bacitracin (50,000 units per liter of 0.9% normal saline). Finally, we use the hydrogen peroxide again as a drying agent. The graft is then dried with sponges passed through the canal. The femoral component is cemented into the allograft on a separate table. This ensures that no cement enters the graft-host junction. A cement gun is used to insert the cement into the allograft, and the cement is pressurized by plugging the canal distally with a finger. The implant is then inserted in the correct version, which has been determined, along with the length, by a trial reduction. After the implant is seated, the cement is cleaned off the distal stem and also off the surface of the osteotomy, using damp sponges. The graft-implant composite is then ready for insertion into the host. Additional fine-tuning of the length of the graft and version of the osteotomy may be necessary, depending on the final trial reductions.
Revision Surgical Technique
The revision is carried out with the patient positioned in the lateral decubitus position on the nonoperative side. A straight lateral incision is used, incorporating old scars if possible. We prefer to use a trochanteric slide approach for improved exposure ( Fig. 46-3 ). Often the proximal femur is so deficient that the trochanteric fragment is very thin, but it is important to keep it in continuity with the abductors and vastus lateralis muscles. We have modified the trochanteric slide to reduce the incidence of posterior dislocation. We leave the posterior capsule and external rotators intact by leaving about 1 cm of posterior greater trochanter attached to the femur. After the trochanteric osteotomy has been completed, the vastus lateralis muscle is reflected off the septum down to the level to which the coronal femoral split is to be performed. This is determined by preoperative planning and intraoperative visualization of the junction of the deficient and healthy host femur ( Fig. 46-4 ). The vastus lateralis muscle is reflected anteriorly only enough to do the split, about 1 or 2 cm. The trochanter is retracted anteriorly, and the femur is then split in the coronal plane down to the level of femur that is considered healthy enough to not require replacement by allograft. The femoral split is easily done with a saw or osteotomes because the proximal femur is so deficient. At the level of healthy femur, transverse cuts are made anteriorly and posteriorly, each extending about a quarter of the way around the femur, leaving the medial half of the femur intact. The deficient femur is then pried open with multiple osteotomes. At the level of the horizontal cut, the medial half of the femur stays intact and can be used as the step-cut or oblique osteotomy. Before the old femoral implant is removed or dislocated, a pin is inserted into the iliac crest and a fixed point on the host femur is identified and marked with a drill hole. This point must be in healthy host femur distal to the allograft because it is a reference point for measuring the leg length. The distance between the pin in the iliac crest and the fixed point on the host distal femur is measured and noted. The preoperative leg length discrepancy can then be compared in order to adjust the leg length if appropriate during the surgery. The hip is then dislocated and the femoral component removed.
