RATIONALE FOR A BONE DEFECT CLASSIFICATIONA bone defect classification -is needed to categorize the extent of bone loss that will be encountered at the time of revision knee surgery. On the basis of this categorization, the surgeon can anticipate the difficulty of the revision procedure and properly prepare for the reconstruction by obtaining any required special equipment prior to surgery, thereby avoiding unpleasant intraoperative surprises.

-preoperatively helps the surgeon select an appropriate implant for the revision surgery.

-it may be useful for analyzing clinical results of revision knee arthroplasty surgery.

-provides a method of comparison.

GUIDELINE FOR CLASSIFYING DEFECTS

RATIONALE OF THE ANDERSON ORTHOPAEDIC RESEARCH INSTITUTE (AORI) BONE DEFECT CLASSIFICATION (ENGH)

To be practical in the clinical setting, a bone defect classification must be easy to understand and remember.

The classification should provide a rationale for selecting a specific treatment option.

To address these issues, we established three categories of bone defects that are applicable to both the femur and the tibia:

• Type 1 defect (intact metaphyseal bone): Minor bone defects that will not compromise the stability of a revision component

• Type 2 defect (damaged metaphyseal bone): • Loss of cancellous bone in the metaphyseal

segment, requiring an area of cement fill, augments, or bone graft with the revision surgery to restore a reasonable joint line level.

• Type 2 bone defect scan occur in one femoral condyle or tibial plateau (2A), or in both condyles or plateaus (2B).

• Type 3 defect (deficient metaphyseal segment)• Bone loss that comprises a major portion of

either condyle or plateau. • These defects are occasionally associated with

collateral or patellar ligament detachment and usually require long stemmed revision implants plus bone grafting or a custom or hinged component for the revision surgery

• A defect is classified only when a component is removed. When both the femoral and tibial components are revised, a defect classification is assigned to the femur and also to the tibia. Intraoperatively, the defect classification is either confirmed or changed.

• We selected the terms "intact" (Type l), "damaged'‘ (Type 2), and "deficient" (Type 3) to describe the categories of bone defects.

• Generally, the defect type should correspond with the type of reconstruction selected for the revision procedure.

• A Type 1 defect with intact metaphyseal bone present at a relatively normal

joint line level would be revised with a primary, non stemmed implant.• A Type 2 defect with damaged cancellous bone is best managed with a

revision component that has augments (either modular or nonmodular) and a stem. The stem and augment( s) reduce stress on a fixation interface that is less than optimal because of the damaged metaphyseal bone

• A Type 3 defect with deficient bone for implant fixation requires either an allograft or large augment (greater than 1 cm) to replace the deficient bone in concert with a stemmed revision component, or a custom or hinged implant that substitutes for the lost bone and associated ligament instability.

RADIOGRAPHIC LANDMARKS FOR CLASSIFYING BONE DEFECTS

• The femoral epicondyles, the posterior femoral condyles, and the location of the patella relative to the joint line are radiographic landmarks used to differentiate complex femoral defects.

• The fibular head and the tibial tubercle should be used as landmarks to classify tibial defects.

• The relationship of the component relative to these landmarks provides information essential in classifying bone defects.

ASSESSING PERIPROSTHETIC BONE LOSS OF THE FEMUR AND TIBIA PREOPERATIVELY

• Femoral bone destruction from osteolysis may be difficult to identify on the prerevision AP radiographs because bone loss is often obscured by the femoral component

• On prerevision AP radiographs, tibial osteolysis is easier to visualize than is femoral osteolysis because the metaphyseal segment of bone is nothidden by the component

BONE GRAFT• When a bone graft is needed in revision surgery, the surgeon should

consider a variety of options. • Because autograft bone has osteoinductive properties, when placed in a

vascularized cancellous bed it rapidly unites with host bone• The autograft bone should retain the capability of remodeling in response to

its new environment, thus avoiding the osteoclastic and reparative response that might weaken an allograft that has not revascularized.

Iliac crest graft is used rarely because of the morbidity associated with obtaining the graft, the limited amount of graft that can be harvested, and the availability of alternative sources of graft material from bone banks.

• Allograft bone is available from regional bone banks. The most readily available source of large structural pieces of cancellous bone is from femoral heads harvested at the time of elective total hip arthroplasty for degenerative arthritis. Massive bone loss with multiple failed revisions may require full segment allografts. In this situation, whole distal femurs and proximal tibiae can be obtained from cadaveric retrievals. These freshly frozen grafts, occasionally sterilized with low-dose gamma radiation, maintain substantial strength to be used for segmental reconstruction.

SELECTING THE BEST BONE DEFECTMANAGEMENT OPTION

• The bone defect classification should be helpful in preoperatively selecting the best option for managing perioprosthetic bone defects.

• Type 1 defects should not require special equipment or implants. • At the surgeon’s discretion, either a primary or revision implant system is selected. • Augments to restore joint line position should not be necessary. • Small cancellous defects that result from removing a failed implant should be filled with cement, particulate graft, or

synthetic graft material. With Type 1 defects, structural bone graft is not needed.

The most common causes of Type 2 defects are aseptic loosening of an implant with component migration or iatrogenic bone loss with previous surgeries.

The bone defect may include some granuloma and either soft osteopenic bone or dense sclerotic bone at the base of the defect.

The granuloma must be thoroughly debrided to viable cancellous bone. Sclerotic and necrotic bone that may be present should be resected or removed with a high speed burr or oscillating saw. • A cancellous structure at the base of the bone defect is the appropriate texture for cement interdigitation or graft

union. Viable cancellous bone should not be sacrificed to create close contact between the implant and host bone. However, the augmented component should rest against at least 50% viable host bone to avoid the possibility of implant subsidence. Trial implants with augments are used to evaluate flexion-extension stability, knee range of motionjoint line level, and the size of augments required. Small defects still present beneath an augment can be filled with cement or particulate graft.

• Type 3 defects most often are caused by severe osteolysis, multiple revisions with iatrogenic bone loss, failure of hinged knee implants, and malunion or nonunion of supracondylar fractures. Augments are not large enough to span the gap from viable host bone to the implant at a proper joint line position. Consequently, the defect should be filled with a solid material that contributes support for the component. Unless at least 50% of therevision component is supported by viable stronghost bone, morselized graft is contraindicated. Aallograft provides excellent support and unites rapidly to host bone at the base of a bone defect. Bulk allograft, which is the appropriate composition for filling large metaphyseal defects, provides an excellent cancellous structure to bond with methylmethacrylate. An additional advantage of allograft is that it can be shaped to fill any size defect.I0 Custom implants and hinged components should be reserved for cases of severe bone destruction, ligament loss, and associated knee instability. As a rule, the least-constrained implant thatprovides satisfactory knee stability should be selected. Varus-valgus constrained components provide adequate stability for most Type 3 bone defectreconstructions, but should be used only if satisfactory intraoperative stability is not achieved with posterior-stabilized trial components. Varus-valgus constrained or rotating-hinge implants increase stress on the fixation interface that is already compromised by the presence of damaged bone and ununited bone grafts.

SELECTING AN IMPLANT• Whenever one component of a total knee arthroplasty has failed but its counterpart is

stable, undamaged, and correctly aligned, a partial revision should be considered. Most modular revision implant systems permit the use of a revision component with a primary component of the same design. Contemporary revision components are designed to properly articulate with primary components manufactured by the same company. .Once the failed implant is identifiedidentified from prerevision radiographs, the surgeon should contact the manufacturer’s representative to determine if a revision component is available for use in conjunction with a retained component. If there is no bone damage (Fl/Tl), a primary component can be used as the revision component in a partial revision surgery. However, primary components of older implant designs that have not been manufactured for many years may not be available. In addition, revision components were never developed for use with these early knee implants. Therefore, it is impossible to retain femoral or tibial component from the primary arthroplasty in conjunction with a partial revision that, because of bone damage, necessitates using a stemmed and augmented implant. In this instance, a full revision is the only option. Serial radiographs are particularly useful in evaluating the reason for component failure and the extent of bone damage. When radiographs reveal a correctly aligned femoral component witha tibial component that failed either by aseptic loosening or component malalignment, the surgeon should consider revising only the tibial component. The most important aspect of the revision surgery will be to avoid undercorrection of the deformity. A primary component should not be used because damaged tibial bone does not provide optimal fixation stability with a shortstemmed component. A long-stemmed revisioncomponent is essential when augments or bone graft are used.

REVISION IMPLANT OPTIONS• Options for augmenting the femoral or tibial components differ with each revision system. • The augments may be a fixed part of the implant that simply replaces anticipated bone loss with

a thicker metal component. • Fixed augments eliminate potential wear debris from a modular augment that is not rigidly

attached to an implant. However, fixed augments also eliminate the intraoperativeflexibility available with modular augments that come in a variety of thicknesses and can be used in various locations. When a revision component with fixed augments is used for revision surgery, the preoperative planning differs from that required when an implant with a modular augment(s) is involved. A greater degree of precision is required to preoperatively plan for revision with fixed augments that offer limited intraoperative options. When using a revision component with fixed femoral augments, the surgeon must resect the bone to the level of damage so that the implant makes contact with the component in both condyles as required by the position of the intramedullary stem of the component. Joint line restoration, which is limited by the thickness of the revision component, is usually only 5 mm thicker than the primary implant. Modular revision components make it possible to customize the femoral component with augments of different thicknesses. Joint line restorization and knee stability in flexion are achieved with a proper selection of augments. Distal augments come in thicknesses of up to 16 mm (PFC Sigma Knee, Johnson &Johnson, Raynham, MA) and permit the repair of more severe bone loss than isavailable with fixed (nonmodular) augments. Because bone loss is often asymmetric in the distal femur, modular augments can be used to replace lost bone from only one condyle, whereas a fixed augment requires resecting undamaged bone to the level of the damaged condyle to position the component on the viable bone of both condyles. Tibia1 defects can be managed with either fixed or modular augments or wedges. Fixed augments or wedges require a considerably larger implant inventory as every size of wedge or augment must come with each size of tibial component, as wellas separate right and left components. For instance, to have five sizes of tibial components and augments in three thickness for both right and left knees, a total of 30 augmented components is needed. A modular tibial component requires onlyfive implants but 15 augments to achieve a similar inventory. Although revision surgery with fixed augments requires a greater inventory of componentsand these components are more expensive than modular components, the strength of the component is optimized and the potential for modular augment loosening and secondary wear debris is eliminate

MODULAR VERSUS FIXED STEMS

• If a large bone defect is present, the method of• implant fixation depends on the length and diameter• of stems available for component stability.• Whenever reasonably good bone is present, it is• appropriate to use a short or intermediate length• stem that may or may not be cemented. The increased• load that is transferred to the fixation interface• with a varus-valgus constrained implant• requires stems of at least intermediate length to• enhance fixation stability.• Fixed-stemmed components do not work well• in situations in which large structural or morselized• bone grafts are required. Fixed-stemmed• implants generally come in one or two stem diameters• and in intermediate stem lengths of 50 to 120• mm. This limited number of stem options usually• does not permit good canal fit and fill with the• stem of the component. Therefore, fixed-stemmed• components must be cemented for immediate• component stability when satisfactory bone is not• present at the implant-bone interface. However,• cement may interfere with the allograft union if it• gains entry to the host-graft interface. In addition,• a cemented stem eliminates the axial loads that• encourage bone healing from passing across the• graft-host junction.• A modular implant system with many stem• length and diameter combinations is optimal for• revisions that require major structural allografts.• In this situation, preoperative templating is essential• to determine appropriate stem size and length• to reduce the load placed on the allograft-implant• composite. Canal-filling stems should not be cemented• because bone union is stimulated by physiologic• loads at the allograft-host bone junction