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TKR: Bone Defects in Primary Knees



- Discussion:
    - defects are peripheral or central
    - peripheral defects offer no peripheral support for tibial component;
    - central defects with a cortical rim that does not support the tibial componennt act like a peripheral defect;
    - in OA varus knees defect is peripheral and posteromedial;
    - in valgus knees a central defect is most common;
    - in RA knees central defect may be present medially as well as laterally;
    - defects are characterized by the presence of eburnated bone;
    - area and depth are related to the duration of arthritis, instability of knee, and, especially in central defects, the strength of bone;
    - if defect occupies 50% or more of the component support surface (particularly peripheral) or requires cement column > 5 mm,bone 
           graft should be done;
    - on tibia, bone should not be cut to the level of the defect;
    - tibial surface is weakened by removal of subchondral bone;
    - tibial surface rapidly becomes weakened as one cuts distally under the subchondral bone;
    - if one cuts the tibia deeper than 5 mm below medial subchondral bone or 8 mm below lateral subchondral bone, fixation is compromised;
    - exposed cancellous bone of the proximal tibia has no protection during load bearing except the metal or plastic of tibial tray;
    - tibial bone is more at risk than femoral bone, because femoral bone is twice as strong as tibial bone;
    - for above reasons, the bone must not be cut to the level of defect rather the defect must be built to the level of the cut;
    - significant bone loss may be present in either the medial or lateral tibial plateau;
    - may need more extensive than normal bone resection to achieve a level proximal tibial surface, cement buildup of the deficient condyle 
           with or without reinforcing screws, and the use of bone grafts (procedure of choice);
           - first 2 options have not proved successful because of implant loosening;
           - bone grafts are easily fashioned from bone normally removed while fashioning the distal or posterior femoral condyles;
           - any cartilage or fiberous tissue is removed from both the graft and the host bed;
           - graft is then tailored to fit precisely in tibial defect;
           - it is securely fixed by 2 screws, thereby providing a secure flat base for the tibial component of good subchondral bone plus the graft