No bone defect classification has been accepted by orthopedic surgeons. Therefore, when the Anderson Orthopaedic Research Institute (AORI) bone defect classification was developed,5 the goal was to make the system easy to understand and apply. The following criteria were the basis of the AORI classification:
1. The same terminology was employed for femoral and tibial defects because of the similarities in the metaphyseal segments of the femur and tibia.
2. The commonly used definitions in most classifications of bone defects, as central or peripheral, cortical or cancellous, contained or uncontained, were eliminated because of the absence of cortical bone in the metaphyseal segments of the distal femur and proximal tibia (Fig. 9.1).
3. Clear and precise definitions were established that minimize ambiguity when bone defects are categorized.
4. Aminimal number of defect types was established to permit clinical investigators to accumulate enough cases to allow meaningful statistical comparisons.
5. This classification was designed to allow retrospective categorization of cases through intraoperative information and postoperative radiographs.
It is important to have quality X-rays when classifying a bone defect. A true lateral view is essential to evaluate the location and extent of osteolysis that may be obscured by the prosthesis on an oblique radiograph. To obtain a "true" lateral view of the knee, the radiograph should be taken in 90 degrees of flexion, placing the entire leg, including the knee and ankle, flat on the radiograph table. If a true lateral view is not obtained, repeat radiographs should be performed after rotating the knee either internally or externally or moving the patient a few inches proximal or distal from the center of the beam.
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