Differences in Trochlear Morphology of a New Femoral Component Designed for Kinematic Alignment from a Mechanical Alignment Design

Abstract

Because kinematic alignment (KA) aligns femoral components in greater valgus and with less external rotation than mechanical alignment (MA), the trochlear groove of an MA design used in KA is medialized, which can lead to complications. Hence, a KA design has emerged. In this study, our primary objective was to quantify differences in trochlear morphology between the KA design and the MA design from which the KA design evolved. The KA and MA designs were aligned in KA on ten 3D femur-cartilage models. Dependent variables describing the morphology of the trochlea along the anterior flange, which extends proximal to the native trochlea, and along the arc length of the native trochlea, were determined, as was flange coverage. Along the anterior flange, the KA groove was significantly lateral proximally by 10 mm and was significantly wider proximally by 5 mm compared to the MA design (p < 0.0001). Along the arc length of the native trochlea, the KA groove was significantly lateral to the MA design by 4.3 mm proximally (p ≤ 0.0001) and was significantly wider proximally by 19 mm than the MA design. The KA design reduced lateral under-coverage of the flange from 4 mm to 2 mm (p < 0.0001). The KA design potentially mitigates risk of patellofemoral complications by lateralizing and widening the groove to avoid medializing the patella for wide variations in the lateral distal femoral angle, and by widening the flange laterally to reduce under-coverage. This information enables clinicians to make informed decisions regarding use of the KA design.

Keywords: patellar tracking; patellofemoral complications; patellofemoral instability; patellofemoral joint; total knee arthroplasty; total knee replacement.

Figure 1

Coronal views of the of the MA design and the KA design. Two major design modifications were widening the trochlear groove from 6° to 20.5° and decreasing the angle of the lateral border from 14.7° to 11.0°. Red shows areas of resection uncovered.

Figure 2

Images showing the standard planes and the relationship of the cylindrical axis with respect to the standard planes on a posterior oblique view of the 3D femur-cartilage model (left), and the origin, medial-lateral (M-L) axis, radial axis, and reference plane of the cylindrical coordinate system on an anterior oblique view of the 3D femur-cartilage model (right). The cylindrical axis (black line) passes through the center of a cylinder (green) best-fit to the central third of the cartilage on each femoral condyle (left). The origin of the cylindrical coordinate system was on the M-L axis (i.e., cylindrical axis) midway between the most medial and lateral points on the femoral condyles (right). A radial axis set at the proximal edge of the groove of the native trochlea defined the plane of the 0% cross-section along the arc length of the native trochlear groove. The relationship of the cylindrical axis and coordinate system to the prosthetic trochlea is not shown.

Figure 3

Image showing the relationship of eleven cross-sections along the arc length of the native trochlear groove with respect to the cylindrical axis on an oblique view of the 3D 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 on the KA and MA prosthetic trochleas.

Figure 4

Additional cross-sections in planes parallel to the plane of the cross-section at 0% of arc length were used to describe the geometry of the anterior flange, which extends above the proximal edge of the groove. The −100% cross-section of the anterior flange corresponded to the most proximal point of the MA design. Although cross-sections in −10% increments are shown, analysis was performed only at cross-sections in −20% increments.

Figure 5

Diagrams of a representative cross-section of the distal femur showing the relationship between tracings of the articular surface of the native trochlea (black), the prosthetic trochlea for the MA design (green), and the prosthetic trochlea for the KA design (red). (Top) The landmarks of the deepest point (DP) of the groove and the highest point (HP) of the medial and lateral facets (only shown on the prosthetic trochlea for the MA design) were used to determine height variables, which included the radial distance of the groove and the heights of the medial and lateral peaks. (Middle) Medial and lateral slopes were tangents to the cross-section profile in the regions nearest the DP. (Bottom) Medial-lateral (ML) variables included the ML position of the groove, the ML flat width, and the medial and lateral peak ML distances from the DP.

Figure 6

Graphs showing the mean ± one standard deviation for differences in medial-lateral variables between the KA and MA designs along the anterior flange at intervals from −100% (proximal) to 0% (distal) of the normalized length of the flange of the MA design.

Figure 7

Graphs showing the mean ± one standard deviation for differences in lateral and medial heights and the radial distance between the KA and MA designs along the anterior flange at intervals from −100% (proximal) to 0% (distal) of the normalized length of the flange of the MA design.

Figure 8

Graphs showing the mean ± one standard deviation for differences in lateral and medial slopes between the KA and MA designs along the anterior flange at intervals −100% (proximal) to 0% (distal) of the normalized length of the flange of the MA design.

Figure 9

Graphs showing the mean ± one standard deviation of the coverage of the lateral and medial regions of the anterior resection for the KA and MA designs along the anterior flange at intervals from −100% (proximal) to 0% (distal) of the normalized length of the flange of the MA design.

Figure 10

Graphs showing the mean ± one standard deviation for differences in medial-lateral variables between the native knee and the KA and MA designs along the native trochlear groove at intervals from 0% to 100% of normalized length of the native groove.

Figure 11

Graphs showing the mean ± one standard deviation for differences in medial and lateral heights and the radial distance between the native knee and the KA and MA designs along the native trochlear groove at intervals from 0% to 100% of normalized length of the native groove.

Figure 12

Graphs showing the mean ± one standard deviation for differences in lateral and medial slopes between the native knee and the KA and MA designs along the native trochlear groove at intervals from 0% to 100% of normalized length of the native groove.