In vivo patellar tracking and patellofemoral cartilage contacts during dynamic stair ascending

Abstract

The knowledge of normal patellar tracking is essential for understanding the knee joint function and for diagnosis of patellar instabilities. This paper investigated the patellar tracking and patellofemoral joint contact locations during a stair ascending activity using a validated dual-fluoroscopic imaging system. The results showed that the patellar flexion angle decreased from 41.9° to 7.5° with knee extension during stair ascending. During first 80% of the activity, the patella shifted medially about 3.9 mm and then slightly shifted laterally during the last 20% of the ascending activity. Anterior translation of 13 mm of the patella was measured at the early 80% of the activity and the patella slightly moved posteriorly by about 2mm at the last 20% of the activity. The path of cartilage contact points was slightly lateral on the cartilage surfaces of patella and femur. On the patellar cartilage surface, the cartilage contact locations were about 2mm laterally from heel strike to 60% of the stair ascending activity and moved laterally and reached 5.3mm at full extension. However, the cartilage contact locations were relatively constant on the femoral cartilage surface (∼5mm lateral). The patellar tracking pattern was consistent with the patellofemoral cartilage contact location pattern. These data could provide baseline knowledge for understanding of normal physiology of the patellofemoral joint and can be used as a reference for clinical evaluation of patellofemoral disorders.

Figure 1

(A) A virtual dual fluoroscopic imaging system was used to reproduce in vivo knee kinematics. The 3D MR-based bony models were matched to their outlines on the fluoroscopic images, (B) Definition of the coordinate systems used to quantify motions of the patella and femur. Femoral coordinate system consists of the transepicondylar axis (TEA) and long axis intersecting at the origin of the femur (midpoint of TEA). Patellar coordinate system consists of superior-inferior (SI), anterior-posterior (AP), and medial-lateral (ML) axes. Lateral shift, anterior shift, superior shift, flexion, lateral tilt, and lateral rotation of patella were considered positive, as shown in the figure.

Figure 2

Definition of the coordinate systems used to quantify the translation of patellofemoral cartilage contact points on the patellar and femoral cartilages (used for normalization of data). (A) An ellipse fitted to the patellar cartilage mesh in posterior view: the ellipse’s axes were used to define the axes of the patellar coordinate system, (B) Rectangular cube of femur: it’s axes were parallel to the TEA and anterior-posterior axis of femoral coordinate system and it enclosed the distal part of femoral condyles.

Figure 3

Tibiofemoral flexion angle measured during stair ascending. The kinematics reported here represents the motion of the tibia relative to the femur. One ascending motion is defined from heel strike on the stair to full knee extension.

Figure 4

6 degrees-of-freedom kinematics of the patellofemoral joint measured during stair ascending. The kinematics represents the motion of the patella relative to the femur. (A) flexion angle, (B) lateral tilt angle (C) lateral rotation angle, (D) medial-lateral shift, (E) anterior-posterior shift, (F) superior-inferior shift.

Figure 5

The average values of translation of the patellofemoral cartilage contact points during the stair ascending activity. (A) The path of the patellofemoral contact points on the patellar cartilage, (B) medial-lateral shift on the patellar cartilage, (C) superior-inferior shift on the patellar cartilage, (D) the path of the patellofemoral contact points on the femoral cartilage, (E) medial-lateral shift on the femoral cartilage, (F) superior-inferior shift on the femoral cartilage. The values on the left side of the figures are absolute kinematic values. The values on the right side of the figures are the normalized kinematic values reported as a percentage (absolute values normalized to their corresponding characteristic lengths). The characteristic lengths were chosen as the length of the main axes of the ellipse fitted to the patellar cartilage and the main axes of the rectangular cube fitted to the femoral condyles for the contact points on the patellar cartilage and the femoral cartilage, correspondingly.

Figure 6

Distance between trochlear groove and the path of the patellofemoral contact points on the femoral cartilage.