The flexor tendon system of the hand and the relationship between the flexor digitorum superficialis and flexor digitorum profundus is complicated and has been the subject of much research1-3. Treatment recommendations for diseases and injuries involving the flexor tendon system have evolved substantially over the past thirty years as a better understanding of its anatomy, function, healing capacity, and rehabilitation protocols has emerged.
Both muscles originate in the forearm, and distally they form the flexor tendons of the digits. The flexor tendons are oval-shaped spiral bands composed of tenocytes and type-I collagen. Distally, beginning at the level of the metacarpal neck, the flexor tendons are enclosed in the flexor sheath, a fibro-osseous sheath with multiple functions. As has been well described, the sheath has thickened regions that form the pulley system. The pulley system is composed of the anular and cruciform pulleys. The A1, A3, and A5 pulleys arise from the volar plate of the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints, respectively. The A2 and A4 pulleys arise from the proximal and middle phalanges. The cruciform pulleys are located between the anular pulleys and are less rigid. The flexor digitorum superficialis tendon of each digit is located volar to the flexor digitorum profundus tendon until the level of the flexor sheath, at which point the flexor digitorum superficialis tendon divides into two slips. One slip passes radial and the other passes ulnar to the flexor digitorum profundus tendon before rejoining dorsal to the flexor digitorum profundus tendon and inserting into the middle phalanx (Fig. 1). At this location, the flexor digitorum superficialis appears, anatomically, to have the capacity to act as an additional pulley for the flexor digitorum profundus. This anatomic layout prompted our investigation.
The flexor tendon pulley system provides a mechanical advantage by maintaining the flexor tendons close to the joint's center of rotation. Absence of, or injury to, the pulleys can result in bowstringing of the tendons. When bowstringing occurs, the tendons fall away from the joint. The mechanical result is that the arc length, or the lever arm, across the joint increases, resulting in an increased torque across the joint.
With an increased arc length, a larger excursion of the tendon is needed to attain the same degree of flexion at the same joint. However, the excursion is limited by muscle physiology and thus rupture of, or damage to, a pulley and the resultant bowstringing leads to a decreased range of motion of the joint. Joint flexion and extension are limited, potentially leading to joint contracture1. Weakness occurs as a result of the tendon being held near the maximal allowable excursion in mid-flexion, where grip is often the strongest4.
In the flexor system, tendon excursion and each joint's moment arm are affected by the distance of the pulleys from the joint axis, the perpendicular distance of the pulley edge from the longitudinal axis of the bone, and the angle of joint motion2. The tighter the radius of the pulley to the bone, the greater the force acting on the tendon sheath. The tendon sheaths, or anular pulleys, travel over each joint close to the center of rotation, maintaining a tight radius. The structure results in a short moment arm for the tensile force of the musculotendon unit to impose on the joint. This minimizes the amount of tendon excursion essential for joint flexion.
Sustaining a short moment arm of the flexor tendons permits efficient use of their available excursion and power, and the pulley system translates a 3-cm flexor tendon excursion into a 260° arc of motion1,3.
Clinically, the study of the pulley system and the relationship between the flexor digitorum superficialis and flexor digitorum profundus has multiple applications. Stenosing synovitis, or trigger finger, is a problem commonly encountered by hand surgeons and is often treated with release of the A1 pulley of the affected finger5. As an adjunct treatment for patients who also have degenerative thickening of the flexor tendons, a release of a slip of the flexor digitorum superficialis tendon is sometimes performed to relieve a persistent fixed flexion deformity of the proximal interphalangeal joint6. Patients who had undergone this procedure were shown to have marked improvement in the range of motion of the proximal interphalangeal joint6, but the effects of this procedure on overall finger kinematics are not well quantified.
A laceration of the flexor tendons—particularly in zone II, which includes the pulley system as well as Camper's chiasma—remains a surgical challenge despite substantial advances in repair and rehabilitation protocols. Treatment has evolved from a recommendation of resection of the lacerated tendons with delayed tendon-grafting to acute repair as the standard of care. However, there must be a delicate balance between providing a strong repair that will withstand early motion and a nonbulky repair that will continue to glide smoothly through the pulley system. To provide a less bulky repair, consideration has been given to resection of the flexor digitorum superficialis, leaving the digit with a single flexor tendon7. In addition to bowstringing, the recognized potential complication of this procedure is devascularization of the flexor digitorum profundus due to injury to the vincula, which carry the blood supply to the flexor digitorum profundus. Investigators have also examined the alternative possibility of leaving one of the two slips of the flexor digitorum superficialis tendon intact to allow improved gliding of the flexor digitorum profundus after repair8. We examined the effect of resecting the flexor digitorum superficialis, such as might be done in a flexor digitorum superficialis slip-resection procedure or to improve gliding in a repair of a flexor tendon laceration, on finger kinematics during flexor digitorum profundus tendon excursion.
The index or ring finger of nine fresh-frozen human cadaver hands was studied. Each hand was disarticulated at the radiocarpal joint and affixed palmar side up to our test apparatus (Fig. 2) with use of two 5/64-in (2.0-mm) Steinmann pins placed through the metacarpal bones. The flexor digitorum superficialis and profundus tendons were identified proximally and sutured to a computer-controlled winch-type servomotor. Small potentiometers were placed at the centers of rotation of the metacarpophalangeal, distal interphalangeal, and proximal interphalangeal joints. These potentiometers acted as rheostats, measuring a change in resistance as the joint was displaced. A computer-controlled motor drew the tendons at a constant velocity (as measured by an encoder on the motor). Previous studies have shown that lower or higher tendon velocities do not have significant effects on excursion or joint displacement9. The data from the potentiometers were recorded, and custom-designed software (LabVIEW Software; National Instruments Corporation, Austin, Texas) then converted this change in resistance to angular displacement.
In each set of experiments, the specimens were taken through a full range of motion starting from an at-rest position and ending when the fingertip touched the palm. In all series of experiments, the angular displacement of the distal interphalangeal, proximal interphalangeal, and metacarpophalangeal joints was measured in response to the tendon excursion of an intact hand. In the control series, this was measured by drawing the flexor digitorum superficialis and profundus simultaneously with the servomotor as well as drawing the flexor digitorum profundus alone. In the experimental series, the ulnar slip of the flexor digitorum superficialis was released and the flexor digitorum profundus was drawn.
The first experimental series involved the flexor digitorum superficialis and profundus tendon excursions of the finger with the flexor digitorum superficialis intact. The excursions of the flexor digitorum superficialis and profundus tendons required to touch the fingertip to the palm were determined as two separate constants. The drive pulleys were set to apply these constants for each trial in this series. The flexor digitorum superficialis and profundus tendons were then pulled simultaneously, with a steady velocity and tension maintained for each tendon. Specimens were put through the same range of motion for each trial, beginning in a neutral, open-palm position and ending when the fingertip touched the palm. The flexion kinematic data were measured and recorded and the finger was then returned to the position at which it started.
In the second series, the finger remained intact but the flexor digitorum superficialis tendon was released from the servomotor. Flexion kinematics were then examined while the servomotor pulled on the flexor digitorum profundus alone. Since the flexor digitorum superficialis tendon was exempt, only the flexor digitorum profundus tendon excursion constant was determined and applied. The previous process was duplicated for all nine trials.
In the third series, we examined the flexion kinematics of the flexor digitorum profundus after release of the ulnar slip of the flexor digitorum superficialis tendon. A Brunner-type skin incision extending from the metacarpophalangeal joint to the proximal interphalangeal digit crease was made, dissection was carried down to the level of the flexor tendons, and care was taken to preserve the anular pulleys. The ulnar slip of the flexor digitorum superficialis tendon was released, again with care taken to preserve the pulley system, and the skin incision was then closed. With use of the previously determined flexor digitorum profundus excursion limit from the second series of tests, the previously described trials were run with the servomotor attached to the flexor digitorum profundus only.
Prior to each trial, a consistent starting point for the range of motion was determined by measuring the distance from the baseplate of the apparatus to the fingertip. The distance of each tendon's start position relative to the motor was also marked. This was to compensate for any slop that might have existed with the motor and thus ensure that the starting position of the digit was consistent and tendon excursion remained constant throughout the trials.
Source of Funding
There was no external funding source for this study.
Each joint was evaluated. The distal interphalangeal joint showed both a more rapid rate of induction of flexion and a greater magnitude of flexion (+5° at 700 ms) when the flexor digitorum profundus was pulled alone as compared with when the servomotor was attached to both the flexor digitorum profundus and the flexor digitorum superficialis. This increased rate and magnitude was unaffected by the release of the flexor digitorum superficialis. The rate of induction of flexion as well as the magnitude of flexion remained constant for the proximal interphalangeal joint in all series, regardless of which of the two tendons was being pulled or whether or not the flexor digitorum superficialis had been released. The metacarpophalangeal joint revealed changes depending on which tendon was being pulled as well as after the release of the flexor digitorum superficialis. The metacarpophalangeal joint flexed less rapidly as well as with less magnitude when the flexor digitorum profundus was pulled alone (-5° at 700 ms). After the flexor digitorum superficialis was released, a further decrease in velocity and magnitude of flexion was noted in the metacarpophalangeal joint (-4° at 700 ms) (Figs. 3, 4, and 5).
Note: The authors thank James C. Chow, MD, Kimberly Balogh, MS, Christopher Graves, and Jonathan Kaplan for their invaluable contributions.