Knee replacement surgery
The surgery involves exposure of the front of the knee, with detachment of part of the quadriceps muscle (vastus medialis) from the patella. The patella is displaced to one side of the joint allowing exposure of the distal end of the femur and the proximal end of the tibia. The ends of these bones are then accurately cut to shape using cutting guides oriented to the long axis of the bones. The cartilages and the anterior cruciate ligament are removed; the posterior cruciate ligament may also be removed but the collateral ligaments are preserved. Metal components are then impacted onto the bone or fixed using polymethylmethacrylate (PMMA) cement. A round ended implant is used for the femur, mimicking the natural shape of the bone. On the tibia the component is flat, although it often has a stem which goes down inside the bone for further stability. A flattened or slightly dished high density polyethylene surface is then inserted onto the tibial component so that the weight is transferred metal to plastic not metal to metal. During the operation any deformities must be corrected, and the ligaments balanced so that the knee has a good range of movement and is stable. In some cases the articular surface of the patella is also removed and replaced by a polyethylene button cemented to the posterior surface of the patella. In other cases, the patella is replaced unaltered.
The current classification of AAOS divides prosthetic infections into four types.
• Type 1 (Positive intraoperative culture): 2 positive intraoperative cultures
• Type 2 (early postoperative infection): Infection occurring within first month after surgery
• Type 3 (acute hematogenous infection): Hematogenous seeding of site of previously well-functioning prosthesis
• Type 4 (late chronic infection): Chronic indolent clinical course; infection present for >1 month
Hip replacement prosthesis includes—
• a metal (originally stainless steel) femoral component,
• a Teflon acetabular component which was replaced by Ultra High Molecular Weight Polyethylene or UHMWPE in 1962, both of which were fixed to the bone using
• PMMA (acrylic) bone cement, and/or screws.