Cementless Implant Design Geometry Femoral and Tibial Component Design

An anatomic design with near-normal kinematics is required for successful cementless total knee arthroplasty. Smooth pegs are preferred for all three components. If a tibial stem is required, it should also possess a nonporous surface. The femoral component is a bimetal design as discussed previously. A deep trochlear groove is desirable because it improves range of motion, and minimizes patellar subluxation or dislocation,2 and prevents excess wear and load on the patellar component. It should be angled 6 degrees as in the normal distal femur for proper tracking. Adeep trochlear groove avoids functional shortening of the extensor mechanism seen in knee systems that have shallow grooves.2

The proximal tibia is 5 to 6 mm smaller on the lateral side than on the medial side.26 An asymmetric replacement will provide the

Knee Implant Geometry

FIGURE 7.1. [A] "Blush" of bone chips immediately beneath tibial component 7-days postop. [B] Backscattered electron imaging showing the bone chips interposed between the porous coating (PC) and the host tissue (H). Three-week postmortem retrieval. [C] Tetracycline-labelled bone chips (BC) interposed between the porous coating (PC) and host bone (H) demonstrating viability and incorporation of bone chips at 12 weeks. Human in vivo plug model. [D] Bone ingrowth into retrieved tibial component 6-years postimplantation showing excellent bone ingrowth (B) into porous coating (PC). Osteointegration of bone and porpus coating demonstrated. Substrate (S).

FIGURE 7.1. [A] "Blush" of bone chips immediately beneath tibial component 7-days postop. [B] Backscattered electron imaging showing the bone chips interposed between the porous coating (PC) and the host tissue (H). Three-week postmortem retrieval. [C] Tetracycline-labelled bone chips (BC) interposed between the porous coating (PC) and host bone (H) demonstrating viability and incorporation of bone chips at 12 weeks. Human in vivo plug model. [D] Bone ingrowth into retrieved tibial component 6-years postimplantation showing excellent bone ingrowth (B) into porous coating (PC). Osteointegration of bone and porpus coating demonstrated. Substrate (S).

Knee Implant Geometry
Figure 7.1. Continued.

best coverage of the proximal tibia and avoid soft tissue impingement. With symmetric replacements, the only options are under-sizing the medial side or overhanging the lateral side.27-29 It has been suggested that tibial coverage is inadequate with a symmetric tibial baseplate without overhang.27,28 Tibial fixation and initial stability is also enhanced with two 6.5 mm titanium alloy cancel-lous bone screws that augment the components' four peripherally placed smooth pegs. A smooth central stem is recommended when fixation is required in softer bone (i.e., rheumatoid arthritic patients, osteoporotic females).30

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