- 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