In vivo patellar tracking: clinical motions and patellofemoral indices

Abstract

Patellar tracking during in vivo weightbearing knee function is not well understood. This study investigated patellar tracking of eight subjects during a full range of weightbearing flexion using magnetic resonance imaging and dual orthogonal fluoroscopy. The data were reported using a clinical description based on patellar and femoral joint coordinate systems and using patellar indices based on geometrical features of the femur and patella. The mean patellar shift was within 3 mm over the entire range of flexion. The patella tilted laterally from 0 degrees to 75 degrees, and then tilted medially beyond 75 degrees of flexion. The mean tilt was within 6 degrees. Similarly, the mean patellar rotation was small at early flexion, and the mean total excursion of patellar rotation was about 8 degrees. The patellofemoral indices showed that the mean sulcus angle and congruence angle varied within 8 degrees over the entire flexion range. The mean lateral patellar displacement was within 6 mm. A consistent decrease in lateral patellar tilt and an increase in lateral patellofemoral angle were observed with knee flexion. In conclusion, patellar motion is relatively small with respect to the femur during in vivo weightbearing knee flexion. These data may provide baseline knowledge for understanding normal patellar tracking.

Figure 1

(A) Knee joint flexion in a dual-orthogonal fluoroscopic image system. (B) A virtual dual-orthogonal fluoroscopic system used to reproduce in vivo knee kinematics.

Figure 2

(A) Coordinate systems used to quantify the clinical motions of the patella. The femoral system consisted of the transepicondylar axis (TEA) and the long axis intersecting at the center of the knee (mid-point of TEA). A box was fit around the patella to determine the patellar center. The patellar system consisted of the SI, AP, and ML axes. Patellar flexion, lateral shift, lateral tilt, and lateral rotation are considered positive as shown. (B) Articulation of the patella on the femur with flexion. The distal portion of the patella is in contact with the femur at low angles and the proximal portion is in contact with the femoral condyles at high flexion angles.

Figure 3

Patellofemoral indices. (A) Sulcus angle (SA); (B) congruence angle (CA); (C) lateral patellofemoral angle (LPA); (D) lateral patellar tilt (LPT); (E) lateral patellar displacement (LPD).

Figure 4

(A) Patellar flexion as a function of knee flexion. (B) ML shift of the patella: initially, from 0° to 30°, the patella shifted medially and beyond 30° shifted laterally. Overall ML movement was small (~2 mm). (C) Patellar tilt with respect to knee flexion. (D) Patellar rotation with respect to knee flexion. The patellar position at 0° was used as reference. Dotted lines represent 95% confidence intervals.

Figure 5

(A) Femoral sulcus angle, (B) congruence angle, (C) lateral patellofemoral angle, (D) lateral patellar tilt, and (E) lateral patellar displacement, all with respect to knee flexion. Dotted lines represent 95% confidence intervals.

Figure A1

(A) Patellar shift, (B) patellar rotation, and (C) patellar tilt along the flexion–extension path of the knee determined using the model matching method and the beads matching method.