Restore Normal Knee Kinematics

FIGURE 1 |Distal Radius (Rd)=
Posterior Radius (Rp)

FIGURE 2 | Constant radius from
0° to 90° flexion

FIGURE 4 | Patella groove
comparison

FIGURE 5 | Anatomic patellar
track with constant radius

In the normal knee, kinematic analyses show the tibia rotates in flexion about a constant axis.^22,23 Further anatomic evaluation of the femur indicates the distances between this axis and the distal and posterior condylar surfaces are nearly equal (Figure 1).^24,39 Contemporary designs which do not reproduce this constant radius may not functionally restore the natural axis of rotation. The ADVANCE® femoral component was designed to restore the normal sagittal curvature of the femur with constant radii extending from full extension to 90 degrees flexion, thus reproducing the normal anatomic geometry (Figure 2).

In most femoral components the sagittal curvature consists of a series of arcs that decrease in diameter from full extension into flexion (Figure 3). Many of these designs incorporate a large distal radius and/or a long distance between the chamfers creating a more flat or "boxy" curvature.^19,21,40 Components with these varying sagittal curvatures, or "j-curves", shift the location of tibio-femoral contact posteriorly in early flexion and can slide forward in deeper flexion. Furthermore, "boxy" femoral implants can "overstuff" the joint anteriorly causing increased incidence of patella subluxation and the necessity to release the lateral retinacular tissues (Figure 4).⁵

FIGURE 3 | multiple radii of
standard femoral component

A number of features have been incorporated into the ADVANCE® femoral component design to restore normal patello-femoral articulation and improve long term implant function. Design factors that can affect patellar function include the shape and depth of the trochlear groove, the thickness of the lateral anterior condyle and the sagittal curvature of the femoral implant.^5-8 To help restore normal patello-femoral articulation, the curvature of the ADVANCE® Knee patella groove was designed with a constant sagittal radius from full extension to 90 degrees of flexion to more closely match the normal anatomy (Figure 5).³⁹ Furthermore, the anatomic patellar groove depth is restored by deepening the trochlear groove on the femoral implants (Figure 6). Restoration to a more normal anatomic level prevents "overstuffing" of the joint anteriorly and allows proper function of the extensor mechanism.

FIGURE 6 | Trochlear groove depth comparison

FIGURE 7 | Raised lateral
anterior flange

A lateral anterior flange which rises 3mm-4mm above the floor of the trochlear groove provides resistance to lateral subluxation (Figure 7).⁴³ The importance of the raised lateral flange has been previously cited as a necessary design feature to maintain patellar tracking in the early stages of flexion.¹⁷ Additionally, the trochlear groove has been extended posteriorly so the patella has full area contact with the femoral implant into deep flexion (Figure 8).

The trochlear groove is angled laterally 3.6 degrees to minimize strain in the lateral retinacular tissues (Figure 9). Previous reports have shown this to be an advantage in decreasing the need for lateral retinacular releases.¹⁸

FIGURE 9 | 3.6° laterally angled
patellar track

FIGURE 8 | Extended trochleargroove on ADVANCE ® Knee

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