Megaprosthesis for Severe Femoral Bone Loss: Heavy Metal

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CHAPTER SYNOPSIS

This chapter outlines the role and technique of proximal femoral replacement in the armamentarium of the reconstructive hip surgeon. Simply stated, when the proximal 25% to 50% of the femur is absent, limited options are available. These options consist of allograft-prosthesis composite or megaprosthesis (heavy metal). The latter has been used more widely in tumor reconstruction but has recently been accepted in nontumor applications. The expansion of this technology has been facilitated by the development of modular systems from a number of manufacturers.

IMPORTANT POINTS

1

Indications include neoplastic tumor resection and nonneoplastic proximal femoral deficiency, such as particulate debris–induced osteolysis, proximal stress shielding, periprosthetic fracture, prosthetic infection, and multiply revised femur.
2

Contraindications include sepsis and absence of adequate distal femur for fixation.

CLINICAL/SURGICAL PEARLS

History
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  Previous infection
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  Dislocation
Physical examination
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  Leg length determination
Optimization of patient’s preoperative medical status
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  Preoperative medical clearance
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  Nutritional status
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  Smoking cessation
Radiology
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  Anteroposterior and lateral views of the femur
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  Anteroposterior view of contralateral femur with measuring device
Preoperative planning
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  Review of patient’s previous operative reports
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  Assessment of acetabular component
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    Determination for retention or revision of the acetabular component
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    Provision for the appropriate liners
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    Consideration for constrained acetabular components or liners
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  Selection of appropriate proximal femoral replacement system

CLINICAL/SURGICAL PITFALLS

1

Incorporate previous incisions into the surgical approach.
2

Select the most familiar surgical approach.
3

Be extensile in surgical approach to avoid any difficulty visualizing the acetabulum or distal femur.
4

Wide exposure and careful removal of femoral and acetabular components minimize bone loss.
5

Careful and meticulous preparation of the distal femur is needed to accept the proximal femoral replacement.
6

Appropriate assessment of the acetabular component determines component fixation and orientation.
7

Consider constrained liners to avoid postoperative dislocation.
8

Reconstruction of appropriate length of the femur facilitates soft tissue balance.
9

Efficient surgery minimizes operative time and therefore potential exposure and subsequent infection.
10

Slow and deliberate postoperative physiotherapy and rehabilitation allow appropriate wound healing and recovery of function.

INTRODUCTION

Having emerged as one of the most successful operative interventions in the past several decades, total hip arthroplasty has improved the quality of life for numerous individuals. Patients electing to undergo arthroplasty are increasingly younger and returning to more active lifestyles. Although materials and techniques for arthroplasty have improved, implant durability remains finite. These factors, coupled with increased life expectancy, have led to complex revision scenarios. Bone loss, either acetabular or femoral, is one of the more challenging problems facing the reconstructive surgeon. The etiology of bone loss on the femoral side may be attributed to infection, requiring radical debridement with two-staged exchange, periprosthetic femoral fracture, osteolysis, or stress shielding.

Regardless of the cause, massive femoral bone loss represents one of the most complex problems encountered in hip reconstructive surgery, and the armamentarium available to achieve optimal reconstruction is limited. Proximal femoral megaprosthesis, allograft-prosthesis composite (APC), total femoral replacement, and resection arthroplasty are all considered viable options for these cases.

Megaprostheses ( Figs. 38-1 and 38-2 ), also known as tumor prostheses, initially were created with the aim of reconstructing the proximal femur after tumor resections. Encouraging results expanded its indications to challenging hip arthroplasty revisions in which the remaining femoral bone stock precludes the use of more conventional methods. First-generation megaprostheses consisted of nonmodular, custom-made implants with a fixed stem size, neck length, and offset. Newer generation megaprostheses consist of modular implants that provide several alternatives to achieve the desired restoration demanded by each individual case. Cemented or press-fit stems with different configurations, incorporated degrees of version, and a wide variety of extension segment sizes are available to facilitate the precise reconstruction of the proximal femur.

The technical difficulty of APCs combined with the questionable long-term survivorship of these large allograft constructs and the increasing technology in proximal femoral replacements has lead to an increased use of megaprostheses. The largest disadvantage of megaprostheses is the limited ability to reconstruct the abductor mechanism. Nevertheless, a significant difference in outcomes or complication rates between APC and megaprosthesis has not been clearly established.

Increased number of comorbidities, critical alteration of the normal femoral anatomy, disruption of the abductor mechanism, and the presence of abundant scar tissue from multiple previous procedures are commonly seen in these patients and represent an increased risk of perioperative complications. Despite the fact that this is a complex surgical procedure associated with high rates of complications, several studies report a substantial improvement in terms of functional recovery and pain control in the majority of patients.

CLASSIFICATION SYSTEMS

Several classification systems have been proposed to describe the status of the femur at the time of revision. The authors have adopted the Mallory classification ( Fig. 38-3 ). This classification essentially has three types of femurs that are addressed at the time of surgery. In type I, the cortex and the corticocancellous junction is intact. These revision femurs can be dealt with using routine primary implants, either cemented or cementless. In type II, the cortex is intact but the corticocancellous junction is deficient. The authors believe these femora are best treated with a platform-loading calcar prosthesis with or without supplemental strut allografting. In type III, both the cortex and the corticocancellous junction are deficient. The system also has subtypes A, B, and C. Type IIIA and IIIB refer to more proximal deficiency of the cortex, which can be dealt with by varying lengths of calcar-replacing implants of modular design. These are generally used in conjunction with strut allografting. Type IIIC represents the most difficult challenge. These cases have a loss of the proximal third to half of the femur. These femora usually are treated with an APC or a proximal femoral replacement ( Fig. 38-4 ).

INDICATIONS AND CONTRAINDICATIONS

Proximal femur megaprostheses are reserved for cases in which the reconstruction of the proximal femur cannot be achieved by conventional methods because of the presence of a severely deteriorated femoral bone stock that precludes adequate fixation of the majority of the revision stems. This includes failed total hip arthroplasties associated with massive osteolysis and stress shielding, failed resection arthroplasties, and failed internal fixation or nonunion after complex comminuted femoral fractures.

Encouraging results after the reconstruction of severe periprosthetic fractures, especially those classified as type IIIB (around the stem; loose stem, poor proximal bone stock) based on the Vancouver system described by Duncan and Masri have been previously reported. Older patients with lower physical demands are considered better candidates for this procedure than younger and more active patients because of the significant incidence of implant failure in highly active patients. For patients with higher physical demands, preference should be given to techniques that restore the femoral bone stock and preserve reconstructive options for the future.

Ongoing infection and any type of neurologic impairment are considered absolute contraindications for the insertion of megaprostheses as well as the majority of reconstructive techniques. The use of megaprostheses is not recommended when less than 10 cm of distal femur remains; this would not allow a secure fixation of the prosthesis and potentially lead to early failure. Because of the complexity and high physiologic demands related to this surgery, the presence of significant comorbidities that may preclude the use of anesthesia is considered a relative contraindication.

PREOPERATIVE CONSIDERATIONS

The surgical technique for proximal femoral replacement is technically demanding. It must commence with thorough preoperative planning. Perhaps the most important part of this preoperative planning is an accurate history and physical examination. All patients undergoing proximal femoral replacement require a baseline C-reactive protein level, erythrocyte sedimentation rate, and routine complete blood count to evaluate for an underlying infection. If suspicion of infection is high, CT-guided aspiration of the hip joint as well as intraoperative tissue samples and frozen section should be performed. Culture and sensitivity as well as cell count should be performed with a white blood cell count of less than 2000 white blood cells/mL, indicating the absence of infection.

Optimization of health parameters and adequate control of comorbidities should be attempted before surgery by a multidisciplinary team. Patient counseling about the extensive nature of the procedure, leg-length discrepancy, and reasonable functional expectations is important before undergoing this type of procedure. The requirement for an extended convalescent period also should be emphasized. Functional outcomes will be predetermined by baseline abductor function and preoperative functional status. Patients should be informed about the risk of complications that are most common with this complex procedure, including infection and dislocation. In addition, because the surgery will be associated with significant blood loss, the patient should be advised regarding autodonation, and patients with low hemoglobin and hematocrit levels should be considered for preoperative erythropoietin. The nutritional status of the patient should be optimized preoperatively, and smoking cessation should be advised for at least 2 to 4 weeks preoperatively.

The next step generally is an assessment of leg-length discrepancy. Clinical and radiographic evaluation in the form of a scanogram can be extremely helpful. Assessment of bony anatomy and bone loss is routinely performed on standardized radiographic views of the femur, hip, and pelvis (anteroposterior pelvis, anteroposterior cross-table lateral of the hip and femur series). Assessment of bone loss is useful to develop a plan for cement removal and the selection of the appropriate type and size of the stem that will reconstruct the remaining femur. Regarding the size of the prosthesis, the potential osteotomy site should be identified, looking at the most proximal portion of adequate femoral cortex. In addition, prosthesis templates may be used to determine correct size of modular pieces and neck length that would most properly restore leg length. In cases in which the femoral architecture is substantially altered, the contralateral femur may be used for this purpose. The radiographic evaluation also identifies diaphyseal deformities from previous femoral fractures to prevent an unexpected violation of the femoral cortex during the insertion of the stem.

If records of the previous operative intervention are available, a review of the surgical report will facilitate an understanding of which devices are present. Questions to be answered include: Is the acetabular component in satisfactory position and alignment? Is it well fixed? Are modular replacement polyethylene liners available? Is the locking mechanism satisfactory? What is the maximum femoral head size that can be placed within the current acetabulum, and is a constrained option available? The surgeon must be cognizant that dislocation is a significant concern in proximal femoral replacement; therefore a constrained option may be a requirement. If the current acetabular component does not offer a constrained option and if a constrained liner cannot be cemented into the existing socket, the surgeon must be prepared to revise this acetabular component. Therefore the appropriate tools required to remove the acetabular component, such as the Innomed Universal Hip Cup Removal System (Innomed, Inc., Savannah, Ga.), should be available. With the current availability of porous metal technology, the authors now routinely use such devices in acetabular revision. Two acetabular components of porous metal have satisfactory constrained liners: the Regenerex RingLoc acetabular component with Freedom constrained liner (Biomet Inc., Warsaw, Ind.) and the Trident Tritanium acetabular component with bipolar constrained liner (Stryker, Kalamazoo, Mich.). On the femoral side, what tools, if any, will be required to remove the component? At what level is the bone stock adequate? What length of proximal femoral replacement will be required? What is the optimal fixation mode? Is it cement ( Fig. 38-5 ), press fit ( Fig. 38-6 ), or the newer Compliant Pre-Stress System (Compress, Biomet) ( Fig. 38-7 )? Compress technology involves novel spring-loaded implants developed to address the problem of loosening caused by stress shielding. Compress implants are modular titanium alloy cementless devices that change the way forces transfer through bone. They have been used in primary and revision oncology, primary and revision arthroplasty, and posttrauma cases in sites that include the proximal, intercalary, and distal femur; proximal tibia; and proximal and distal humerus. Compress devices have a porous coated surface that is compressed against the host cortical bone surface with a series of Belleville washers tightened over an intramedullary traction bar. An anchor plug is fixed inside the host bone with five transverse pins. When the implant is tightened against the bone, the washers deform and act as a spring, generating stress to the bone. The spring washers direct forces to load the bone rather than the prosthetic device.

SURGICAL TECHNIQUE

On the day of the surgical intervention, the patient is evaluated in the preoperative holding by the anesthesia team. At the authors’ institution, patients undergoing revision total hip arthroplasty generally undergo a spinal with a long-acting narcotic coupled with endotracheal intubation to maintain the airway and provide adequate relaxation during the entire procedure. Large-bore intravenous access is established and preoperative antibiotics are administered as the patient is taken from the preoperative holding to the operating room. The authors have routinely administered 1 to 2 gm of first-generation cephalosporin (penicillin-allergic patients are given 900 mg of clindamycin) and 80 mg of gentamycin. After suitable and adequate induction of anesthesia and while the patient is in the supine position, leg length determination is noted.

Before starting the procedure, the surgical table should be carefully prepared with the surgical revision equipment and the proximal femoral megaprosthesis that will be used, including modular segments of different sizes to allow the satisfactory reconstruction of the component intraoperatively. If bone allografts are included in the reconstruction, they may be available to the complete satisfaction of the surgeon. Because of the high incidence of wound complications and infections, strict control of all the variables is vital, including operating room environment, antibiotic prophylaxis, skin lavage, meticulous prepping and occlusive draping, and careful tissue handling.

The patient is placed in the lateral decubitus position with the appropriate hip facing the operative field. Before draping the patient, the peripheral nerve sites are evaluated for adequate padding. The extremity is prepped and draped with exposure available from the iliac crest to the knee. A direct lateral approach is used ( Fig. 38-8 ) . The incision generally starts 5 to 10 cm proximal to the tip of the greater trochanter and extends distally along the lateral thigh. Previous incisions are marked, and every attempt to incorporate these in the current incision is made. In addition, old scars are ellipticated whenever possible. The incision is then carried down through the skin and subcutaneous tissues to the level of fascia lata, which is then incised along the line of the skin incision. The tissue planes should be recognized and properly dissected for future restoration during closure, even though this can sometimes be difficult because of the extensive amount of scar tissue. The sciatic nerve should be identified and protected to avoid excessive tension or injury during the procedure. Special care is taken during the manipulation of the soft tissue because this has been associated with better functional outcomes and fewer postoperative complications. A continuous soft tissue sleeve, including the abductors and the vastus lateralis, should be maintained to prevent trochanteric migration or escape. This can be done with or without a trochanteric fragment. Elevating the vastus lateralis in continuity with the anterior one third of the gluteus medius and minimus off the femur facilitates an anterior dislocation with a combination of flexion, external rotation, and adduction.