Analysis of the gliding pattern of the canine flexor digitorum profundus tendon through the A2 pulley

Abstract

Friction between a tendon and its pulley was first quantified using the concept of the arc of contact. Studies of human tendons conformed closely to a theoretical nylon cable/nylon rod model. However, we observed differences in measured friction that depended on the direction of motion in the canine model. We hypothesized that fibrocartilaginous nodules in the tendon affected the measurements and attempted to develop a theoretical model to explain the observations we made. Two force transducers were connected to each end of the canine flexor digitorum profundus tendon and the forces were recorded when it was moved through the A2 pulley toward a direction of flexion by an actuator and then reversed a direction toward extension. The changes of a force as a function of tendon excursion were evaluated in 20 canine paws. A bead cable/rod model was developed to simulate the canine tendon-pulley complex. To interpret the results, a free-body diagram was developed. The two prominent fibrocartilaginous nodules in the tendon were found to be responsible for deviation from a theoretical nylon cable gliding around the rod model, in a fashion analogous to the effect of the patella on the quadriceps mechanism. A bead cable/rod model qualitatively reproduced the findings observed in the canine tendon-pulley complex. Frictional coefficient of the canine flexor tendon-pulley was 0.016+/-0.005. After accounting for the effect created by the geometry of two fibrocartilaginous nodules within the tendon, calculation of frictional force in the canine tendon was possible.

Figure 1

Mid section of the canine flexor profundus tendon in the sagittal plane (top) and from the volar aspect (bottom). The two distinguished fibrocartilaginous nodules are apparent (asterisk). D: distal, P: proximal.

Figure 2

Testing device consists of one mechanical actuator with a linear potentiometer, two tensile load transducers, a mechanical pulley on the right of the figure and a weight. A: The canine flexor profundus tendon is passed through the A2 pulley. B: The same tendon as A is passed under the rod. C: A bead- cable unit is passed under the rod.

Figure 3

Relationship between the canine flexor profundus tendon and the A2 pulley is shown. A: The A2 pulley partly covers the distal part of the proximal fibrocartilaginous nodule (colored black). The measurement starts from this position. B: At the end of excursion, the A2 pulley partly covers the proximal part of the distal nodule (colored black).

Figure 4

The effect of excursion on F1 and F2 in flexion and extension for one representative case is shown. In flexion, F2 starts at a value less than F1. As excursion progresses, F2 increases almost linearly. At the end of the excursion, F2 is greater than F1 as the distal nodule came into the A2 pulley. The resulting gliding resistance is calculated to be negative. In extension, F2 begins greater than F1 and decreases linearly to a value less than F1.

Figure 5

The effect of excursion on F1 and F2 for the canine tendon-pulley complex when an unphysiologically long excursion is tested. When the tendon is translated toward an actuator (flexion), F2 changes significantly depending on the site of the fibrocartilaginous nodule. As the proximal nodule comes into the A2 pulley, F2 increases linearly (a). F2 decreases down to below F1 level as the proximal nodule comes out of the pulley (b). Then F2 increases up again above F1 level as the distal nodule comes into the pulley (c). As the nodule comes out of the pulley, F2 decreases again (d). Physiological tendon excursion is from 15 mm to 20 or 23 mm at the bottom axis.

Figure 6

Effect of excursion on F1 and F2 when the canine flexor profundus tendon is translated under the rod. F2 starts at a value less than F1 level and increases linearly as excursion progresses. Above excursion 9 mm, F2 was greater than F1 as the distal nodule came in contact with the rod.

Figure 7

The effect of excursion on F1 and F2 when a bead-cable unit is translated under the rod. When the bead comes in contact with the rod, F2 increases (excursion 6–18 mm). As the bead passes beyond the rod, F2 decreases to a value less than F1 (excursion 18–33 mm), followed by an increase as cable-rod contact continues (excursion 33–47 mm).

Figure 8

Free body diagrams of the bead-cable unit at the four excursion points described in Fig. 7. A: Contact only between cable and rod. At excursion 1, the reaction force faces inferiorly. B: Bead and cable are in contact with the rod at right side. At excursion 2, the reaction force is directed toward the bead and F2 is greater than F1. C: Only the bead is in contact with the rod. At excursion 3, the reaction force faces inferiorly toward the bead and F2 was almost equal to F1. D: Bead and cable are in contact with the rod at the left side. At excursion 4, the reaction force faces toward the bead and F2 is smaller than F1, recording a negative force difference between F2 and F1. The direction of the reaction force is approximately toward the bead as long as the bead remains in contact with rod.