Applied Surgical Anatomy of the Finger Flexor Tendons
Applied Surgical Anatomy of the Finger Flexor Tendons
This section describes only the anatomy of the finger flexor tendons. For a general description of the palmar anatomy, see Applied Surgical Anatomy of the Volar Aspect of the Wrist.
Overview
The anatomy of the finger flexor tendons provides the key to the treatment
and prognosis of flexor tendon injuries. Nowhere else in the body are the links between anatomy, pathology, and treatment illustrated so clearly. The structure of the tendons, their blood supply, and their special relationship to other structures all relate to the pathogenesis of injury and repair.
The anatomy of the finger flexor tendons encompasses zones, each of which is separated from the others by anatomic landmarks. The zones all must be treated differently in cases of tendon laceration. We shall consider the anatomy from the proximal to the distal aspect, from zone 5 to zone 1, as devised by Milford (Fig. 5-60).33
Zone 5
Zone 5 is in the anterior compartment of the forearm, proximal to the flexor retinaculum and the carpal tunnel (see Fig. 5-60). At that point, nine distinct tendons run into the hand toward the digits. Each finger has two tendons, one each from the flexor digitorum superficialis muscle and the flexor digitorum profundus muscle. The thumb has one long flexor, the flexor pollicis longus.
The tendons in zone 5 are not enclosed in a tight canal, but are surrounded by a synovial sheath in the distal part of the forearm. Tendon repairs carried out in this area generally are successful, and independent finger flexion usually returns.
Zone 4
Zone 4 encompasses the tendons as they run through the carpal tunnel. Eight tendons remain in a common synovial sheath throughout the carpal tunnel.
Tendon repairs carried out in zone 4 have a good prognosis, but not as good as the prognosis of those carried out in zone 5, because the tendons are enclosed in a fibroosseous tunnel. The tunnel must be opened for repairs, and adhesions may form after surgery.
Zone 3
Zone 3 is the zone of the lumbrical origin. As the flexor digitorum profundus tendons traverse the palm, a lumbrical muscle arises from each tendon. The radial two lumbricals arise from a single head, from the radial side of the profundus tendons to the index and middle fingers. The ulnar two lumbricals arise from two heads, from the adjacent sides of the profundus tendons between which they lie. The tendons of the lumbricals pass along the radial sides of the metacarpophalangeal joints before they insert into the dorsal expansion. They pass volar to the axes of the
metacarpophalangeal joints; thus, they act as flexors of those joints, even as they extend the interphalangeal joints (see Fig. 5-41).
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Figure 5-60 The zones of the wrist and hand (according to Milford).
Lacerations in zone 3 almost invariably involve damage to the lumbrical muscles. Most surgeons do not recommend repairing the lumbricals; the increased tension on the muscles caused by the repair produces fixed flexion at the metacarpophalangeal joints and limited flexion at the interphalangeal joints, resulting in an intrinsic plus hand.
Zone 2
Zone 2 stretches from the distal palmar crease to the middle of the middle phalanx. In this area, the two tendons for each finger run together in a common fibroosseous sheath.
The sheaths run from the level of the metacarpal heads (the distal palmar crease) to the distal phalanges. They are attached to the underlying bone and prevent the tendons from bowstringing.
Thickenings in the fibrous flexor sheath are constant (Fig. 5-61). They act as pulleys, directing the sliding movement of the tendons. There are two types: Annular and cruciate. Annular pulleys are composed of a single
fibrous band (ring); cruciate pulleys have two crossing fibrous strands (cross). Annular pulleys act much like the rings on a fishing rod. Without the ring, the fishing line would pull away from the rod as it bends. This effect is known as bowstringing; in human terms, it results in the loss of range of movement and power in the affected finger. Annular pulleys include the following:
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The A1 pulley, which overlies the metacarpophalangeal joint. It is incised during trigger finger release.
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The A2 pulley, which overlies the proximal end of the proximal phalanx. It must be preserved (if at all possible) to prevent bowstringing.
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The A3 pulley, which lies over the proximal interphalangeal joint.
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The A4 pulley, which is located about the middle of the middle phalanx. It must be preserved to prevent bowstringing.
Cruciate pulleys, none of which are critical for flexor function, include the following:
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The C1 pulley, which is located over the middle of the proximal phalanx
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The C2 pulley, which is located over the proximal end of the middle phalanx
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The C3 pulley, which is located over the distal end of the middle phalanx
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Figure 5-61 The annular and cruciate ligaments of the flexor tendon sheath, lateral view. Note the relationship of the pulleys to the skin creases and joint lines.
The two tendons enter the fibroosseous canal with the superficialis tendon on top of the profundus tendon. Over the proximal phalanx, the superficialis tendon divides into halves, which spiral around the profundus tendon, meeting on its deep surface and forming a partial decussation
(chiasma). The two then run as one tendon underneath the profundus tendon before attaching to the base of the middle phalanx. Thus, the superficialis tendon actually provides part of the bed on which the profundus tendon runs. Distal to the attachment of the superficialis tendon, the profundus tendon inserts into the base of the terminal phalanx (see Fig. 5-77). Within the fibroosseous sheath, the nutrition of the flexor tendons is provided for by blood vessels that enter the tendons from synovial folds called vincula (Fig. 5-62).
Extremely difficult conditions for full recovery exist after lacerations in zone 2, mainly because the flexor tendons are enclosed within a nondistensible fibroosseous canal, and also because, for full function, the tendons must run over each other. It is important to remember that any adhesion between the two can cause malfunction of the involved finger.
Repairs in this zone have the worst prognosis of all the zones.34 It has been nicknamed “no-man’s land” by Bunnell.35
Zone 1
Zone 1 is the area distal to the insertion of the superficialis tendon. Although the profundus tendon still is enclosed tightly within a fibroosseous sheath here, it runs alone. Therefore, the prognosis for the repair of lacerations in this zone is better than that for zone 2, although not as good as that for zones 3, 4, and 5.
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Figure 5-62 The vincula longa and brevia are main blood supplies to the flexor tendons. Note the relationship of the vincula to the flexor tendon synovial sheath (inset).
Vascular Supply of the Tendons
Within the fibrous sheath, the flexor tendons are enveloped in a double layer of synovium (see Fig. 5-62, inset). Each tendon receives its blood supply from arteries that arise from the palmar surface of the phalanges. These vessels are encased in the vinculum (mesotendon). Two vincula supply each tendon, as follows:
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Profundus tendon.
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The short vinculum runs to the tendon close to its insertion onto the distal phalanx.
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The long vinculum passes to the tendon from between the halves of the superficialis tendon at the level of the proximal phalanx.
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Superficialis tendon.
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The short vinculum runs to the tendon near its attachment onto the middle phalanx.
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The long vinculum is a double vinculum, passing to each half of the tendon from the palmar surface of the proximal phalanx.
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Injection studies on fresh cadaveric material have found that this classic arrangement does not always hold true. The long vincula to both tendons may be absent in the long or ring fingers. When they are present, the long vinculum to the superficialis tendon may attach to either or both of its slips, and the long vinculum to the profundus tendon may arise at the level of the insertion of the superficialis tendon.36
These variations should be borne in mind as the flexor tendons are explored within their sheaths. The vincula should be preserved, if possible, to preserve the blood supply to the tendon. They also have some mechanical function,37 which occasionally may mask the diagnosis of acute tendon damage.
Other injection studies have found that the volar aspects of the flexor tendons are largely avascular; their nutrition may be derived from synovial fluid. Therefore, sutures placed in the volar aspects of the tendons do not interfere materially with the blood supply to the tendons themselves.38
Landmarks and Incision
The critical landmarks of hand surgery are the skin creases, all of which are situated where the fascia attaches to the skin. There are four major
creases: The distal palmar crease corresponds roughly to the palmar location of the metacarpophalangeal joints and the location of the proximal (A1) pulley, the palmar digital crease marks the palmar location of the A2 pulley, the proximal interphalangeal crease marks the proximal interphalangeal joint, and the thenar crease outlines the thenar eminence (see Figs. 5-44, 5-60, and 5-61).
The nerve supply to the skin of the fingers comes from two sources: The volar aspect is supplied by the volar digital nerves, and the dorsal aspect is innervated by the dorsal nerves of the radial and ulnar nerves, as well as by the dorsal contribution from the volar digital nerves for the distal 1½ phalanges of the index, long, and ring fingers. The dorsum of the thumb and small finger are served exclusively by the radial and ulnar nerves, respectively. Because of this anatomic arrangement, the midlateral approach to the flexor sheath does not cause skin denervation (see Fig. 5-50).