Strategies for Improving the Prosthetic Trochlea Design Based on Differences in Trochlea Morphology Between Femoral Components Set in Kinematic and Mechanical Alignment and the Native Knee

Overview

This chapter reviews differences between the morphology of the prosthetic trochlea for four different femoral components set in kinematic alignment (KA) and the native knee in the same femur-cartilage model. Trochlear morphology is described along the arc length of the native trochlea by the medial-lateral location of the trochlear groove, the radial distance of the trochlear groove from the flexion-extension (F-E) axis of the tibiofemoral joint, and the sulcus angle of the trochlea. Differences between the prosthetic trochlear morphology aligned in KA and the native trochlea were generally consistent for all four femoral components. Differences were a more proximal extension of the prosthetic trochlea, several centimeters above the native trochlea; a more lateral location of the prosthetic groove proximally and a more medial location distally, understuffing as indicated by a lower radial distance for the prosthetic trochlear groove than the native knee; and a prosthetic sulcus angle that differed particularly at the extremes of the arc of the trochlear groove. Owing to these differences, in conjunction with the wide variability in the medial-lateral location of the native trochlear groove and in the Q-angle, strategies for improving the prosthetic trochlea for use in KA are to widen the proximal trochlea laterally, orient the groove so that it is more lateral proximally, and widen the groove proximally. Additional strategies are to increase the radial distance of the trochlear groove from the F-E axis to correct understuffing and steepen the sulcus angle, particularly inferiorly, to provide more definitive patellar tracking.

Differences in Trochlear Morphology Between Four Femoral Components Set in Kinematic Alignment and the Native Knee

To quantify differences in trochlear morphology between femoral components set in KA and native femurs, 10 three-dimensional (3D) femur-cartilage models were created by combining computed tomography (CT) scans and laser scans of native human cadaveric femurs. Four different femoral components were kinematically aligned on each 3D femur-cartilage model. ^, The four femoral components were Persona (Zimmer-Biomet, Warsaw, IN), NexGen (Zimmer-Biomet, Warsaw, IN), Vanguard (Zimmer-Biomet, Warsaw, IN), and GMK Sphere (Medacta, Castel San Pietro, Switzerland). The latter design differed from the other three designs in that the prosthetic trochlea interfaced with an anatomic patellar implant, whereas the other three trochlear designs interfaced with modified dome (i.e., sombrero hat) patellar implants.

Measurements of the prosthetic and native trochlea were made along the arc length of the native trochlear groove. The best-fit of a cylinder to the cartilage surface of the medial and lateral femoral condyles of the 3D femur-cartilage model established a coordinate system ( Fig. 10.1 ). Eleven cross-sections of the 3D femur-cartilage model were constructed at 10% increments along the arc length of the native trochlear groove by rotating about the axis of the cylinder ( Fig. 10.2 ). These cross-sections were propagated onto the prosthetic trochlea, and tracings of the native and prosthetic trochlea were generated ( Fig. 10.3 ). The deepest point represented the groove, and the two highest points on the medial and lateral facets represented the boundary of the sulcus. These three points were used to characterize the morphology of the trochlea by determining the medial-lateral and radial locations of the groove, and the sulcus angle.

Figure 10.1, Diagrams illustrating the three dependent variables used to characterize the morphology of the trochlea. (A) A three-dimensional distal femur-cartilage model with the cylindrical axis and an arbitrary cross-section passing through the trochlea. The cylindrical axis is the axis of coaxial cylinders best fit to the posterior articular surfaces of the femoral condyles from 10 degrees to 110 degrees of flexion. (B) An outline of the articular surface of the trochlea for an arbitrary cross-section and the three dependent variables used to describe trochlear morphology. The three dependent variables are the medial-lateral location of the trochlear groove measured as the medial-lateral distance from the midpoint of the cylindrical axis (medial positive), the radial location of the trochlear groove measured as the radial distance from the cylindrical axis to the trochlear groove, and the sulcus angle of the trochlea. The dependent variables were determined at 11 cross-sections in 10% increments along the arc length of the native trochlear groove. An arbitrary cross-section is shown for illustrative purposes.

Figure 10.2, Image showing the relationship of 11 cross-sections along the arc length of the native trochlear groove with respect to the cylindrical axis on an oblique view of the three-dimensional femur-cartilage model. The 0% cross-section was set coincident to the proximal edge of the trochlear groove, and the 100% cross-section was set at the most distal edge. Not shown are the projections of the cross-sections for prosthetic trochleae.

Figure 10.3, Diagram of a representative cross-section of the distal femur showing the relationship between tracings of the articular surface of the native trochlea (gray) , kinematically aligned ( KA ) prosthetic trochlea (green) , and mechanically aligned ( MA ) prosthetic trochlea (blue) . The landmarks of the deepest point of the groove and the highest point ( HP ) of the medial and lateral facets (only shown on the native trochlea) were used to determine the medial-lateral and radial distances of the groove and the sulcus angle of the trochlea for the native and prosthetic femurs. The medial-lateral distance was positive in the medial direction.

For all four femoral components, mean medial-lateral locations of the prosthetic groove were within 2.5 mm of native ( Fig. 10.4 ). However, the paths of the trochlear grooves differed; the prosthetic groove was a straight line when unwrapped and projected onto the coronal plane, whereas the native groove curved with a lateral bulge ( Fig. 10.5 ). As a result, the prosthetic groove was more lateral than native proximally between 0% and 30% of the arc length and more medial than native beyond about 30% of the arc length. Mean radial locations of the prosthetic groove were as large as 5-mm less than native. Sulcus angles of the prosthetic trochlea were steeper than native proximally and flatter than native distally for three of the four femoral components. For one femoral component (Vanguard), the sulcus angle was flatter than native over the full arc length. The general consistency in differences from native observed for the four femoral components highlights the need to reassess the design of prosthetic trochleae for use in KA.