Flexor Tendon Lacerations 

Tendon nutrition

The tendon derives its nutrition from the following 2 sources:

• Diffusion, via the synovial lining sheath: Canaliculi pass through the tendon to the surface of the tendon. Movement of fluid into these canaliculi has been demonstrated. This effect is enhanced with digital motion.
• Perfusion, via the segmental arterial supply: The blood supply to the tendon enters distally via the bony insertion and proximally via the vincula. Four vincula, designated V1 to V4, are present. V1 and V2 supply the flexor digitorum superficialis, and V3 and V4 supply the flexor digitorum profundus. They arise at the necks of the proximal and distal phalanges, respectively. In the thumb, the vincula likewise are termed V1 and V2. No flow occurs between adjacent territories of vincula. Presumably, this area is sustained by diffusion through the synovial fluid. The vascular plexus within the tendon occupies the dorsal half. This is important at the time of placement of core sutures during flexor tendon repair.

Logically, diffusion occurs in areas of the tendon that are compressed in flexion, while the other areas are perfused. The flexor digitorum profundus is more dependent on diffusion than the flexor digitorum superficialis.

Tendon healing

A debate persists as to the nature of tendon healing.

• Extrinsic: The original theory was that sheath fibroblasts were responsible for peritendinous adhesions, and the tendons were healed by this route. This was the theory behind total flexor sheath excision and prolonged immobilization for tendon repairs.
• Intrinsic: Tendons bathed in synovial fluid were found to heal satisfactorily. The necessary collagen was produced by the tenocytes.

Modern thinking is that tendon healing is initiated by the proliferation of epitendinous cells, which migrate into the defect, forming a "callus" equivalent. Somewhat later, the tenocytes or fibroblasts from within the tendon invade the callus, producing further collagen that realigns to produce the strong tendon. Peritendinous adhesions are not necessary for either healing or nutrition.

Clinical examination is an essential part of the assessment of any patient who presents with a hand injury. Identification of damaged structures is best performed by a hand surgeon, as several reports have noted underdiagnosis of hand injuries when examined by emergency department staff alone.^[1] Moreover, diagnoses were missed despite examination by a hand surgeon, making the involvement of a surgeon critical, as those injuries can be identified and repaired on formal exploration.^[2] However, use of bedside ultrasonography in the emergency room is more sensitive and specific than physical examination for detecting tendon lacerations. In one study, sensitivity, specificity, and accuracy of US were 100%, 95%, and 97%, respectively. Bedside ultrasonography in the emergency department takes less time to perform than traditional wound exploration techniques or MRI.^[3]

Fibrous flexor sheath

Roughly half of all flexor tendon injuries occur in zone II. The sheath commences at the palmar plate of the metatarsophalangeal (MP) joint with the A1 pulley. A condensation of the palmar aponeurosis results in the so-called palmar aponeurosis (PA) pulley. Where the tendon overlies a joint, the sheath should be sufficiently thin and resilient, resulting in the cruciate (or retinacular) intervals. Where the flexor sheath overlies the phalanges, it is tough and unyielding (annular pulleys A2 and A4). Additional annular pulleys overlie the palmar plates of the MP, proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints, respectively (A1, A3, A5 pulleys). These are continuous with the transverse retinacular ligaments dorsally. See images below.

In the thumb, A1 and A2 pulleys are over the palmar plates, and an oblique pulley is over the proximal phalanx. This passes from proximal ulnar to distal radial; in so doing, it is virtually an extension of the adductor, which inserts into the sesamoids. The sesamoids, into which insert the 2 heads of flexor pollicis brevis (FPB), lie within the substance of the palmar plate.

In the thumb, similar to the A2 and A4 pulleys in the fingers, the oblique pulley is sacrosanct. Because of the obliquity of the oblique ligament and ulnar takeoff, the A1 pulley in the thumb is best divided radially. This is important when surgical release of a trigger thumb is performed. No pulley should be incised during the course of tendon repair, with exception of the A1, A3, and A5 pulleys. Repair is impossible due to the snug fit and the transverse orientation of the fibers.

Tendon sheath and pulley reconstruction

The issue of sheath reconstruction is controversial, and the decision to undertake this is best individualized after thorough assessment of the patient. The sources of fascia include the adjacent fingers, the dorsal wrist retinaculum, and the foot. For pulley reconstruction, place the tendon graft around the phalanx (sutured to itself), either beneath the extensor tendon for the A2 pulley or superficial to it for A4 pulley reconstruction. A transverse strip of dorsal wrist retinaculum is harvested via a longitudinal incision. If performing a Hunter rod reconstruction, reconstructing the pulley first is often useful before placing the rod to achieve sufficient tension on the pulley. Following suture of the graft, it is rotated around so that a synovial surface overlies the tendon. Such grafts have been demonstrated to continue secreting synovial fluid.

See Flexor Tendon Anatomy for more information.

Procedures that involve extension of the tendon are contraindicated for 2 reasons: firstly, the quadriga effect on the other digits is invoked, and secondly, an extension deficit of the involved digit is always present.

Contraindications to 1-stage tendon grafting include less than full ROM, inadequate skin cover, and a hostile bed.