Aori Bone Defect Classification

In this system, a defect is only classified when a component has been removed. If both the femoral and tibial components are removed, the femur and the tibia are each assigned a defect classification. Defects are classified from preoperative radiographs for anticipated bone deficiency and then the classification is either confirmed or changed intraoperatively. The femoral epi-condyles, the posterior femoral condyles, and location of the patella relative to the joint line may be used as landmarks to differentiate complex femoral defects. The fibular head and the tibial tubercle should be used as landmarks for tibial defects that are difficult to classify.

Occasionally there is the need to classify a bone defect from postrevision radiographs. The metaphyseal segments of the femur and tibia have a distinct profile (Fig. 9.1). The main criterion to look for is a reduction in this profile and the dimensions of the metaphyseal segments of the femoral condyles and/or the tibial plateaus. The distance from the epicondyle to the end of the femur varies according to an individual's bone structure and size, but this distance is proportional to all other dimensions of the bone. A bone defect, however, alters this relationship. For example, a shortened distance from the epicondyle and metaphyseal flare to the end of the femoral component will be visible if a distal bone defect has not been repaired with a bone graft or an augment to restore a normal joint line. If the bone defects were reconstructed with cement, augments, or grafts and the joint line restored, this will be evident on postrevision radiographs and also in the patient's operative note. On the radiograph, the metaphyseal bone segment should appear as a shortened segment, with an augmented component or bone graft filling the deficient area.

Therefore, the following definitions are the foundation of this classification:

Type 1 Defect (INTACT metaphyseal bone): Minor bone defects that do not compromise the stability of the component. Type 2 Defect (DAMAGED metaphyseal bone): Loss of cancel-lous bone that necessitates an area of cement fill, augments, or bone graft to restore a reasonable joint line level. Type 2 bone defects can 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 compromises a major portion of either condyle or plateau. These defects are occasionally associated with collateral or patellar ligament detachment and usually require bone grafts or custom implants.

In any classification scheme, some cases will fall on the borderline. To classify these cases, it is necessary to evaluate the postoperative radiographs and the surgical treatment mode. For example, if a primary component was used, no bone defect was addressed in the operative note, and the postoperative radiographs demonstrate joint line restoration, an F3/T3 defect would not apply. If a structural bone graft, major cement fill, or a hinged component with condylar resection was used in the revision, we would conclude that the patient had a significant F2/T2 or F3/T3 bone defect.

Bone defects also occur in the patella but are not classified in the AORI bone defect classification. Patellar defects were excluded because they do not affect management decisions with revision surgery. In these cases, bone grafting is not an option and revision components to address such defects are not available, except for the biconvex patellar design of the Genesis Knee (Smith & Nephew, Memphis, TN). In most instances patellar bone defects are managed simply by not resurfacing the damaged patellar bone.

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