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Chapter Synopsis
This chapter reviews classification systems for femoral bone loss in total hip arthroplasty.
Important Points
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Preoperative radiographic evaluation of bone loss helps to evaluate intraoperative findings.
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Extended trochanteric osteotomy should be used with little hesitation if needed to aid in implant extraction and safe, accurate femoral preparation.
Clinical/Surgical Pearls
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Appropriate treatment of femoral bone loss can be determined by intraoperative classification.
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Needed instrumentation and structural bone grafts should be determined preoperatively.
Introduction
Revision total hip arthroplasty (THA) can often be difficult and requires significant preoperative planning before the procedure. Availability of the appropriate hip instrumentation, implants, and bone grafts requires knowledge of the amount of bone loss on both sides of the hip joint. This chapter reviews the preoperative and intraoperative classification of femoral bone loss associated with revision THA.
Imaging and Templating
Appropriate imaging for determining the classification of femoral bone loss in revision THA includes radiographs and computed tomography (CT) scans. CT scans can be used to create three-dimensional models that will illustrate the femoral bone loss. They can also be used to fabricate custom implants for use in the operating room when standard implants will not fit in the femur during revision surgery. Less often, other imaging techniques such as magnetic resonance imaging and ultrasonography may aid in determining bone loss, cement loosening, and soft tissue changes.
Imaging
Preoperative examination of the patient should focus on actual or perceived limb length inequality. The imaging should include full-length femur films as well as dedicated views of the pelvis and the affected hip. The full length of the femur should be evaluated, including the proximal metaphyseal bone stock and the diaphyseal integrity. A cross-table lateral radiograph of the femur will help the clinician evaluate perforations of the femoral cortex. The lateral view should also be evaluated for the bow of the femur and possible remodeling of bone in response to aseptic loosening.
Gruen and colleagues established a simple method for describing the location of bone loss due to osteolysis or stress shielding. The Gruen zones help to locate the bone loss in detail before revision surgery. Zone 1 ( Fig. 46.1 ) involves the greater trochanter, and zone 7 affects the lesser trochanter. The location of bone loss helps to determine the need for supplemental support in the form of allograft or revision implant. Johnston and associates modified the use of the zones of Gruen to include lateral radiographs in determining the extent of endosteal cavitation and resorption of shaft bone.
Templating
Digital or conventional hard-copy templates can be used preoperatively to assess the placement of the implant to prevent complications such as cortical perforation. Appropriate templating can serve as a guide for the upcoming surgery (e.g., sizing, location). Templates may also reveal relationships to anatomic landmarks, such as the greater trochanter and lesser trochanter, that may improve intraoperative placement of the implants. Although templating is beneficial, it is not a precise indication of actual bone loss found during surgery. Furthermore, the process of removing a well-fixed femoral stem can increase the loss of bone available for implantation.
Classification Systems
Chandler and Penenberg
The Chandler and Penenberg classification, published in 1989, attempted to stratify bone loss based on location and damage to the femur. The six main categories of this system were calcar deficiency, trochanteric deficiency, cortical thinning, cortical perforation, femoral fracture, and circumferential deficiency ( Table 46.1 ). This classification was intended to help with intraoperative consideration and treatment options including the use of allografts.
| Category | Description |
|---|---|
| 1A | Calcar deficiency: intramedullary deficiency |
| 1B | Calcar deficiency: total deficiency |
| 2 | Trochanteric deficiency |
| 3 | Cortical thinning |
| 4 | Cortical perforation |
| 5A | Fracture about or distal to stem of femoral component: host bone |
| 5B | Fracture about or distal to stem of femoral component: allograft bone |
| 6A | Circumferential deficiency of metaphysis and proximal diaphysis: remaining proximal diaphyseal bone |
| 6B | Circumferential deficiency of metaphysis and proximal diaphysis: complete loss of proximal femur |
The first category is deficiency and destruction of the calcar region of the femur. It is divided into two sections: intramedullary deficiency and total deficiency. The pathologic cause of this first category is often stress shielding of the proximal femur, including the calcar, with a retained cortical shell (seen in THAs with distal fixation). Polyethylene wear leading to loss of cancellous bone may also fit this pattern. The second type of calcar deficiency appears in loose femoral components caused by motion leading to complete proximal calcar bone loss. The loose femoral component often drifts into varus, causing the collapse of the medial supporting structures.
The second category in this classification system relates to the state of the greater trochanter bone stock. Bone loss may be caused by osteolysis, motion, or fracture. Bone deficiency of the greater trochanter may not signify stem loosening but may significantly alter the soft tissues of the hip. If there is loss of the abductors in patients undergoing revision THA, stability and function of the hip may be affected. The operative approach during revision surgery may be altered in patients with significant trochanteric bone deficiency. The next two categories are determined by the state of the cortical bone in the femur. The third category of the Chandler and Penenberg classification is cortical thinning. Thinning is an important consideration during surgery to prevent undesired fracture or perforation during implant removal and placement of the revision implant. The fourth category involves a cortical perforation caused by a loose stem drifting into varus, by osteolysis, or by iatrogenic injury during the revision surgical procedure. The cortical perforations are treated based on location and should be bypassed with the revision femoral stem or reinforced with strut allograft.
Fractures about or distal to the stem of the femoral component comprise the fifth category of this classification. Type A refers to fractures in host bone, whereas type B fractures are in a previous alloprosthetic composite.
The last category of the Chandler and Penenberg classification is deficiency of the metaphysis and proximal diaphysis. Type 6A femurs exhibit a loss of the proximal metaphysis and the trochanter with some remaining diaphysis. This diaphyseal segment can be a thin shell. Type 6B femurs have complete loss of the proximal femur, including the proximal diaphysis. These patients often need distal fixation with a tapered stem or a proximal femoral replacement. Care should be taken to protect the diaphyseal bone with cerclage cables to prevent fracture during implantation.
Endo-Klinik
The Endo-Klinik classification system is based on cemented stems and is often used in Europe. The first grade indicates a clinically loose stem with radiolucent lines in the proximal half of the cement mantle. This type of cement loss is usually caused by poor cementing, debonding, or early osteolysis. On grade 2 radiographs, the medullary canal of the proximal femur has expanded as a result of endosteal erosion with circumferential radiolucent lines. These stems may start to show subsidence of the implant. In grade 3, the femur shows continued bone loss with a widened medullary canal. Cortical defects are present especially in those stems that collapse into varus with stress on the lateral cortical surface. The final grade involves complete loss of the proximal third of the femur with extension to the diaphysis, jeopardizing the use of a long-stem prosthesis ( Table 46.2 ).
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