Flexor Tendon Lacerations

Fibrous flexor sheath

Roughly half of all flexor tendon injuries occur in zone II. The sheath commences at the palmar plate of the metacarpophalangeal (MCP) joint with the A1 pulley. A condensation of the palmar aponeurosis (PA) results in the so-called 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 (anular pulleys A2 and A4).

Additional anular pulleys overlie the palmar plates of the MCP, proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints (A1, A3, and A5 pulleys, respectively). These are continuous with the transverse retinacular ligaments dorsally. (See the 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 the two heads of the flexor pollicis brevis (FPB) insert, 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 the exception of the A1, A3, and A5 pulleys. Repair is impossible because of 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 a Hunter rod reconstruction is being performed, it is often useful to reconstruct the pulley first before placing the rod so as to achieve sufficient tension on the pulley. After the graft has been sutured, it is rotated around so that a synovial surface overlies the tendon. Such grafts have been demonstrated to continue secreting synovial fluid.

For more information, see Flexor Tendon Anatomy.

Tendon nutrition

The tendon derives its nutrition from the following two sources:

• Diffusion
• Perfusion

Diffusion takes place 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 is accomplished 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 (FDS), and V3 and V4 supply the flexor digitorum profundus (FDP). 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 FDP is more dependent on diffusion than the FDS is.

Tendon healing

A debate persists as to the nature of tendon healing — that is, whether it is extrinsic or intrinsic:

• 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.

Various methods of expressing the results of tendon repair or grafting have been developed, including the following:

• Littler method
• Boyes technique
• Total active motion (TAM)
• Ratio of TAM to total passive motion (TPM)
• Lister technique
• Buck-Gramcko technique

In the Littler method, each joint's individual range of motion (ROM) is measured. The Boyes technique uses the pulp-to-midpalmar crease measurement (nail-to-table measurement can be added).

The TAM method incorporates the summed ROM of the interphalangeal (IP) joints minus the extension deficit, as a fraction of 175. ^{[8, 9]} Some incorporate MCP measurement. Strickland reported 50% of repairs achieved 50% of TAM. He reported 80% improvement following tenolysis, with 3% rate of rupture. Lister reported 71% improvement but with 21% rupture.

The TAM-to-TPM ratio expresses the relation of the postoperative TAM2 to the preoperative TPM1 for tendon grafts. In flexor tendon repair, the postoperative TAM2/175 is used. Lister believed this to be the best method for reporting results and reported 76% TAM/TPM for grafts. Strickland's formulae were as follows:

• Flexor tendon repair - TAM2/175 as a percentage
• Tendon grafting - TAM2/TPM1 as a percentage
• Tenolysis - 100 – (TPM1 – TAM2)/(TPM1 – TAM1) as a percentage

Both the Lister technique and the Buck-Gramcko technique incorporate the TAM (with MCP) and the distance from pulp to midpalmar crease. These are used widely.

A retrospective review presented a functional outcome score in which tendon function, opposition, intrinsics, deformities, and sensation are assessed to evaluate the results of both tendon and nerve repair in patients with a "spaghetti wrist" combined injury of the flexor tendons, nerves, and vessels at the wrist. ^{[10, 11]}

Lister reported 80% good or excellent results in zone II with 85% outside of zone II. The FDS was excised in only 25% of patients with zone II injuries, and only 45% of those 25% achieved good or excellent results. This result is difficult to explain, but it may reflect improved blood supply or greater strength with the intact FDS. Only 12% of patients required tenolysis. Singer and Maloon reported 80% excellent or good results.

Rigo et al, in a retrospective review of flexor tendon repair outcomes in zones I, II and III (N = 291; 356 fingers), reported excellent or good function in 95 (30%) of 322 fingers at 8 weeks and 107 (48%) of 225 fingers at the last follow-up (mean, 7 months; range, 3-98 months). ^[12] Variables determined to be negative outcome predictors included the following:

• Age
• Smoking
• Injury localization between subzones IC and IIC
• Injury to the little finger
• Extent of soft-tissue damage
• Concomitant skeletal injury
• Delay to surgery
• Use of a two-strand Kessler repair technique
• Attempted suture or preservation of the tendon sheath-pulley system
• Resecting or leaving the concomitant superficial flexor tendon cuts untreated

Edsfeldt et al investigated the effect of concomitant nerve transection, combined FDP-FDS tendon transection, and patient age on digital ROM in 273 patients (311 fingers) more than 1 year after FDP tendon transection and repair in zone I and II. ^[13]  They found that age greater than 50 years was significantly associated with impaired digital ROM during the first year but not at 2 years. Concomitant nerve transection and combined transection of FDP and FDS were not found to affect digital mobility.