The flexor tendons are the tendons on the palmar (lower) side of the hand that bend or flex the fingers and thumb. There are two flexor tendons for each finger and one flexor tendon for the thumb. 

The Anatomy of the Tendons & Hand

When discussing anatomy we must remember that tendons attach muscles to bone. With that in mind, the muscles for the flexor tendons of the hand are located in the forearm and become tendons just prior to the wrist joint.

  • The long flexor tendon (flexor digitorum longus or FDL) attaches to the end of the finger at the last bone, or distal phalanx. It bends the last joint, or DIP joint.
  • The short flexor tendon for each finger (flexor digitorum superficialis or FDS) attaches to the middle bone, or middle phalanx, of each finger. It bends the middle joint or PIP joint.
  • Each flexor tendon is a long but tough band-like structure made of collagen. It is covered by tenosynovium, which makes the surface smooth for gliding.
  • The flexor tendons are contained in a tunnel called the flexor tendon sheath. At the wrist (carpus), this tunnel and sheath are called the carpal tunnel. In the hand the primary sheath separates into 5 separate flexor tendon sheaths for each individual finger. 
  • In each finger flexor tendon sheath, there are bands of thickened tissue called pulleys that help hold the tendon in place (to avoid bowstringing of the tendon out of place with grip).
  • The anatomy and mechanics in the finger flexor tendon sheath are very complex. Both the FDS and FDP tendons slide back and forth with grip and extension. The two tendons also slide past each other during different activities. 
  • The FDS tendon is more superficial in the wrist and palm. It splits near the middle PIP joint. 
  • The longer FDP tendon travels underneath the FDS and goes through that split in the FDS at the level of the PIP joint. It then travels out to the end of the finger where it attaches. (This segment of the flexor tendon sheath containing the pulleys and where the flexor tendons move past each other is called “no man's land” for the difficulty of operating in this area. )
  • Both flexor tendons and the pulleys must operate correctly for you to completely flex your finger.

The Causes & Types of Injury

Lacerations (i.e. deep cuts) on the palm side of the hand (where the flexor tendons are located) can be devastating to the function of the hand. Repair and restoration of function after a flexor tendon injury are most often performed by hand surgery specialists for this reason.

It’s important to note that all lacerations to the hand and flexor tendon lacerations are different and unique:

  • Lacerations may be classified by location. There are 5 different zones from one to 5, starting at the fingertip and moving proximally towards the wrist. Each zone has its own problems and is treated differently. 
  • Lacerations may be classified by the orientation of the laceration (longitudinally or transverse).
  • Lacerations may be classified by whether they are clean and sharp (i.e. caused by a knife or glass) or jagged (i.e. caused by a table saw or chainsaw). 
  • The wounds from a laceration may be clean or dirty. 
  • There may be associated injuries (such as a fracture, nerve injury or loss of skin and soft tissue coverage). 
  • There may be a patient history of arthritis or stiffness of the surrounding joints.

In addition to these various types of laceration, the age and general health of the patient (i.e. circulation healthy, smoking history, malnutrition, etc.) can also affect a patient as they address a serious injury. We must consider these factors and their impact on a patient’s ability to recover long-term as we develop treatment plans. The ability of a patient to cooperate with the critical but complex postoperative therapy is also crucial for the success of flexor tendon surgery.

The Treatment

In general, treatment of flexor tendon lacerations is surgical. Exceptions may be made, however:

  • When only the FDS is cut, leaving an intact and functional FDP, which is capable of flexing the finger. 
  • When there is only a small partial laceration of the flexor tendon, which is still functional and may require only therapy. 
  • Additionally, a patient who has very stiff arthritic finger joints that are incapable of motion may not benefit from flexor tendon repair.

Flexor tendon surgery is usually performed on an outpatient basis with general anesthesia or axillary block anesthesia. During surgery:

  • The laceration wound is explored to evaluate and repair any possible damage to other structures such as nerves or arteries. 
  • If a fracture is present, that is usually stabilized first by inserting small pins or screws. 
  • The flexor tendon sheath is opened in the area of the laceration and the tendons inspected. 
  • The exact repair is different in each zone. For example, in zone one, near the end of the finger, the FDP tendon may be repaired directly back to the bone with a small anchor. In zone 5, a simple repair of the tendon may be completed. In zone two (or “no man's land”) the anatomy is more complex and demanding. Usually, in this zone, both tendons have been lacerated as well as a pulley. In most cases, both tendons must be repaired as well as the flexor tendon sheath and pulleys. This can be the most challenging to repair. 
  • Often the flexor tendon will retract into the palm or forearm due to tension from the muscle. The tendon must be retrieved and woven down the flexor sheath in order to complete the repair. 
  • The tendon and sheath must be handled delicately under magnification in order to avoid damage that will increase the risk of scar tissue or adhesions of the tendon.

After the surgery is completed, the hand and wrist are placed in a protective cobra splint on the top (dorsum) of the hand, which flexes the wrist. This is to protect the completed repair. The tendon will take 6 weeks to heal and must be completely protected during this time. The patient is instructed not to perform any active gripping during this time. Active gripping of any finger may pull apart and the repaired tendon, since the flexor tendons work together in concert.

Basically, the patient is not to use the hand all for any lifting or gripping during the 6 week period.

Additional Important Treatment Information

A Different Treatment Protocol

A very complicated but elegant program of therapy has been developed for treating flexor tendon repairs. This newer flexor tendon protocol is designed to allow motion of the repaired flexor tendons in the sheath without putting excess tension on the repair and pulling apart the repair. Too much motion or stress will result in tearing apart the repair. No motion will result in scarring of the flexor tendons and no motion. There must be a delicate balance between these two. 

Compliance with the therapy program is absolutely critical to the success of the repair. During this program, the patient is allowed supervised active extension of the finger and flexion of the finger by a therapist. There is no active gripping during this time. After 6 weeks, the tendon repair is sufficiently healed to remove the splint and start active range of motion and gripping exercises.

Additional Care May Be Needed

In approximately 20% of our flexor tendon repair cases, an additional surgery may be required to free up the flexor tendon from scar tissue (tenolysis), release joint contracture (shortening) to relieve a stiff joint, or repair an initial flexor tendon repair that has failed.

In cases of chronic rupture of the flexor tendon (after failed primary flexor tendon repairs), a two-stage flexor tendon reconstruction is required. In that case, the scarred, damaged tendon is removed and replaced with an artificial Kevlar and Silastic tendon, which is attached to the bone and muscle. Approximately 3 to 6 months later, the artificial tendon is removed and replaced with a tendon graft from the leg. The hand must be protected for 6 weeks after each surgery.

Practice Note: Dr. Bowman and Dr. Hunter (the inventor of this active tendon in Philadelphia) was responsible for some of the clinical research regarding its use and were the first surgeon in western Pennsylvania to use the active artificial tendon.