Muscle:Extensor Digitorum

From Painwiki
Jump to navigation Jump to search

Extensor Digitorum is the primary long finger extensor of the dorsal forearm, arising from the lateral epicondyle and distributing via four tendons to the second through fifth digits. The extensor indicis is a deeper, independent extensor of the index finger that shares a tendon compartment with the extensor digitorum and has its own clinically distinct TrP pattern. Both muscles are covered here as they form a functional and anatomical unit in the dorsal extensor compartment.

TrPs in the extensor digitorum are among the most prevalent in the human body — nearly all adults harbour a latent TrP in the middle finger extensor fibres, making this muscle one of the most reliable for teaching TrP palpation and LTR elicitation. Active TrPs are a common cause of pain misidentified as tennis elbow, finger arthritis, or C7 radiculopathy, and a common cause of impaired finger flexion that is not attributed to the extensor group at all.

Anatomy

Extensor Digitorum

  • Proximal attachment: Lateral epicondyle of the humerus; intermuscular septa; antebrachial fascia — shares this common origin with the extensor carpi radialis brevis and extensor carpi ulnaris
  • Distal attachment: Four tendons pass under the extensor retinaculum to the second through fifth digits; each tendon expands into an extensor hood (aponeurotic expansion) over the proximal phalanx, where it is joined by tendons of the lumbrical and interosseous muscles; the hood then divides into a central slip (to the base of the middle phalanx) and two lateral slips that unite to attach to the dorsal surface of the distal phalanx
  • The tendons across the dorsum of the hand are united by highly variable oblique bands that limit independent finger movement — individual finger control depends on lumbricals, interossei, and individual finger flexors, not the extensor digitorum alone
  • Fibre architecture: Fibre-length-to-muscle-length ratio 0.42–0.50 for different digitations — diagonal endplate zone across the middle of the muscle belly. TrPs can be located anywhere in this endplate zone.
  • Innervation: Deep branch of the radial nerve (posterior interosseous nerve); posterior cord; spinal nerves C6, C7, and C8

Extensor Indicis

  • Proximal attachment: Dorsal and lateral surface of the body of the ulna and the interosseous membrane
  • Distal attachment: Passes under the extensor retinaculum in the same compartment as the extensor digitorum tendons; at the level of the head of the second metacarpal, joins the ulnar side of the index finger slip of the extensor digitorum and attaches into the extensor expansion
  • Innervation: Deep radial nerve (dorsal interosseous nerve), C6, C7, C8

Anatomical Variants of Clinical Importance

The extensor digitorum brevis manus is a rare variant (1.1% of hands, 38 of 3,304 examined) that is symptomatic in 50% of cases when present. It originates from the distal radius or dorsal wrist capsule and inserts on the index finger's dorsal aponeurosis. It presents as a painful dorsal wrist mass, may be misdiagnosed as a ganglion cyst or tumour, and is best demonstrated with the wrist flexed 30° and fingers fully extended. When this muscle is present, the extensor indicis proprius is usually absent. If TrPs are present they may contribute significantly to symptoms.

Referred Pain Patterns

Extensor Digitorum

Pain is projected down the dorsum of the forearm to the back of the hand and into the fingers dorsally, consistently stopping short of the ends of the fingers — the last phalanx and nail bed are pain-free. (This distinguishes the extensor digitorum from the long finger flexors, which project pain to and beyond the fingertips.)

By fibre group:

  • Middle finger extensor fibres (most common): Pain forms a line extending from the dorsum of the forearm, wrist, and hand, intensifying at the MCP and proximal IP joints of the middle finger. Occasional volar wrist spillover. Pain felt in the hand and middle finger, with stiffness and soreness of the middle finger joints.
  • Ring finger extensor fibres: Pattern similar to the middle finger, referring to the ring finger. Additionally — unlike the middle finger extensor — TrPs in the ring and little finger extensors project pain and tenderness proximally into the lateral epicondylar region. Pain with firm handshaking at the "elbow" should raise suspicion for ring/little finger extensor involvement.
  • Little finger extensor fibres: As per ring finger pattern; also refers to the lateral epicondyle.

Extensor Indicis

TrPs in the midportion of the belly refer pain toward the radial side of the dorsum of the wrist and hand, but generally not into the fingers themselves. This TrP is seldom found in isolation; when all other extensor group TrPs have been treated and wrist pain persists, an extensor indicis TrP is the likely remaining source.

Activation and Perpetuating Factors

TrPs in the finger extensors are activated by:

  • Forceful repetitive finger movements: professional musicians (pianists in particular), carpenters, mechanics
  • Repeatedly stretching a rubber band with the finger extensors
  • Any activity requiring sustained forceful grip
  • Forearm fracture — activation by fracture has been observed clinically
  • Local infection of a finger (observed with ring finger)
  • Extensor tendon "jumping its trolley" — loss of tendon mooring over the MCP joint causing ulnar deviation of the finger and muscular strain (requires surgical repair)

The ring and little finger extensor fibres form a functional unit with the supinator for twisting motions (opening jar tops, doorknobs). These three muscles — ring/little finger extensors plus supinator — frequently develop TrPs together.

Key satellite TrP relationships: Key TrPs in either the scalene muscles or the serratus posterior superior can induce satellite TrPs in the extensor digitorum. These satellite TrPs resolve with inactivation of the key TrPs.

The common spread pattern for lateral epicondylar involvement is: (1) supinator, (2) brachioradialis, (3) ECRL, (4) extensor digitorum (especially middle and ring fingers) — at this stage, gripping and hand-twisting motions become painful and the ECU may develop secondary TrPs. Epicondylitis develops as an enthesopathy secondary to the taut band tension of the central TrPs at the lateral epicondyle — the primary TrP cause is often not recognised and not treated.

Clinical Examination

History and Functional Assessment

Key features of the history:

  • Pain in the dorsum of the hand and/or fingers (dorsal surface, not fingertips)
  • Stiffness and tenderness of the proximal interphalangeal joints — often attributed to arthritis
  • Weakness of grip, sometimes without pain (middle finger extensor involvement alone can produce grip weakness without pain)
  • Pain or difficulty performing repetitive fine finger movements: typing, playing a musical instrument, milking, handwriting
  • Pain at the elbow with firm handshaking → suspect ring/little finger extensor fibres
  • Impaired finger flexion — finger extensors contribute to powerful flexion synergistically; TrPs in the extensors impair flexion

Finger-Flexion Test (Active ROM)

The primary clinical test for extensor digitorum TrP tension:

  1. Patient flexes the interphalangeal joints to bring fingertips against the palmar pads
  2. The metacarpophalangeal joints are held straight (not flexed) simultaneously

Positive: One or more fingers stand out from the others, unable to fully flex — the affected finger's extensor muscle cannot lengthen sufficiently. Passive flexion of the involved finger beyond this point is painful.

Since these muscles cross the wrist and all finger joints, the examiner must passively flex all joints — fingers first, then wrist, then wrist into ulnar deviation — to fully reveal TrP-related stretch restriction. Curling only the fingers, or bending only the wrist, does not sufficiently stretch the long extensors.

Bilateral Handgrip Test

Weakness due to finger extensor TrPs is detected during a handshake by testing both hands simultaneously. The test is more sensitive with the patient's hands in ulnar deviation and wrist flexion. This may reveal weakness without pain when TrPs are latent.

Extensor Pollicis Test (when thumb extensors are suspected)

Although the thumb extensors rarely develop TrPs, when they do they are easily missed. To test for extensor pollicis TrP involvement:

  1. Flex the hand passively
  2. Pronate the forearm fully
  3. Adduct the thumb passively beneath the index finger
  4. Passively flex the MCP joint
  5. Test by passively flexing the IP joint

Positive: Flexion of the IP joint is limited and causes pain dorsal to the first CMC joint and radial to the second metacarpal. The thumb tends to move into radial abduction as the IP joint is flexed.

Strength Testing

Test extensor digitorum strength by resisting MCP joint extension of the second through fifth digits with the arm resting on a table. Weakness of this muscle also weakens wrist extension.

Trigger Point Examination

The extensor digitorum scores very high interrater reliability for all TrP criteria including the LTR (Gerwin et al.) — it is one of the easier muscles to examine reliably for TrPs. The middle finger extensor TrP produces one of the most common, most easily elicited, and most easily detected LTRs in the body.

Middle finger extensor TrP: Located by flat palpation 3–4 cm distal to the head of the radius (the head of the radius lies approximately 2 cm distal to the lateral epicondyle). LTR causes extension/deviation movement of the middle finger. Nearly all adults have a latent TrP here. Only when this TrP is active does the patient complain of middle finger pain.

Ring and little finger extensor TrPs: Located deep in the muscle mass beneath the aponeurosis of origin, between the middle finger extensor fibres and the extensor carpi ulnaris (immediately lateral to the palpable border of the ulna). These are difficult to locate by palpation due to depth. LTRs, when obtainable, produce extension of the little and ring fingers. When these central TrPs are present, the lateral epicondyle attachment area is frequently tender — this represents an attachment TrP from sustained taut band tension.

Extensor indicis TrP: Found by flat palpation between or through the extensor tendons, approximately halfway between the radius and ulna as the muscle crosses the forearm.

For the full set of TrP diagnostic criteria, see Concept:Trigger_Point.

Joint Play Assessment

Assess joint play at the elbow, wrist, and hand (MCP and IP joints). Restricted joint play must be identified and restored — TrP treatment alone will not produce full relief if articular dysfunction persists.

Differential Diagnosis

Condition Must be present Must be absent Merely possible
Extensor digitorum TrPs Taut band with spot tenderness in midforearm extensor mass; Finger-flexion Test positive for the affected finger(s); referred pain stopping short of the fingertips; LTR extends the affected finger Dermatomal sensory loss; reflex change; joint swelling Lateral epicondylar tenderness (ring/little finger fibres); dorsal hand/finger pain; grip weakness; PIP joint tenderness
Lateral epicondylitis Localised tenderness at the lateral epicondyle and common extensor origin; pain with resisted wrist or finger extension at the enthesis Extensor digitorum TrP (3–4 cm distal to epicondyle) whose compression reproduces and eliminates the epicondylar pain Elbow pain; grip weakness; dorsal forearm aching
C7 radiculopathy Dermatomal sensory change in C7 distribution (middle finger, dorsal forearm); diminished triceps reflex; positive Spurling's test Taut band in extensor mass reproducing referred finger pain by compression Dorsal hand/finger pain; grip weakness
C6 radiculopathy Dermatomal sensory change in C6 distribution (thumb, index finger); diminished brachioradialis reflex TrP in extensor digitorum reproducing symptoms by compression Lateral epicondylar pain; dorsal forearm pain
De Quervain's tenosynovitis Positive Finkelstein test; tenderness localised to the first extensor compartment (radial styloid area) Extensor digitorum TrP reproducing the pain Base of thumb and radial wrist pain
Finger arthritis / PIP synovitis Joint swelling; radiographic joint changes; morning stiffness > 1 hour Taut band in extensor digitorum reproducing PIP tenderness and relieving it on TrP inactivation PIP joint tenderness and stiffness — this can be caused by extensor digitorum TrPs alone without joint pathology
Extensor digitorum brevis manus Dorsal wrist mass at bases of first/second metacarpals; mass tenses with wrist flexed 30° and fingers extended; confirmed by palpation for increased tension in response to finger extension Exercise-induced dorsal wrist pain; may present as apparent ganglion

Treatment

Spray and Stretch

The finger extensors must be treated as a group. Critically, both the wrist and the fingers must be fully flexed simultaneously — stretching only the fingers or only the wrist does not sufficiently lengthen the long extensors to eliminate TrPs.

  1. Patient seated with elbow supported in extension; hand and fingers hang fully flexed over the edge of the armrest, forearm pronated
  2. Vapocoolant applied in parallel proximal-to-distal sweeps covering the muscle and its referred pain zone
  3. Do not squeeze the fingers tightly — this can hurt the joints; the stretch is achieved by gravity and clinician-assisted passive flexion
  4. When ring and little finger extensor TrPs refer pain to the lateral epicondyle, add an up-sweep (proximal) pattern to cover that region as well
  5. Apply moist heat over the forearm muscles, then 3 slow cycles of full ROM in both flexion and extension

Postisometric relaxation is highly recommended for TrP release in these extensors (Lewit). The patient can perform this at home.

Trigger Point Injection

Patient supine, arm positioned so the hand and fingers hang down limply (moderately stretching the extensors).

Middle finger extensor: Identified by flat palpation; injected at the characteristic site 3–4 cm distal to the radial head. Strong LTRs and clear pain patterns are characteristic.

Ring and little finger extensors: Needle directed toward the point of deep tenderness, located between the middle finger extensor fibres and the ECU. At considerable depth, the TrP may be in the finger extensor or in the underlying supinator — the needle may reach both. Normal grip strength may return immediately after TrP elimination.

Injection warning: Occasional inadvertent deep radial (posterior interosseous) nerve block may occur during injection of the ring/little finger extensors. Warn the patient in advance of possible temporary extensor muscle weakness, which resolves within 15–20 minutes as the dilute procaine solution disperses.

Extensor indicis: TrP lies in the muscle belly approximately halfway between the radius and ulna; injected through the extensor tendons by flat palpation.

After injection: passive stretch to full ROM (usually during vapocooling) → moist heat 5–10 minutes → slow active full ROM × 3 cycles.

For general injection principles see Concept:Trigger_Point_Injection.

Patient Education

Activity Modification

  • When gripping or twisting with the hand (tennis, unscrewing jar tops), maintain the wrist in slight extension with radial deviation (cock-up position) rather than flexed and ulnarly deviated — this is the position of mechanical advantage for the extensors
  • Offer the hand in a receiving line with the palm upward and hand slightly extended so the other person cannot squeeze tightly; alternate hands
  • A leather wrist strap maintaining the wrist in neutral or extension prevents excessive extensor strain during recovery; elastic is not as effective as leather for this purpose
  • Avoid repeatedly testing painful movements — this aggravates TrPs rather than strengthening the muscle

Exercise Programme

Artisan's Finger-stretch Exercise:

  1. Begin with forearms pronated, fingers extended and spread apart
  2. Slowly supinate the forearm while flexing the fingers sequentially, starting with the little finger
  3. Complete the fist with the forearm fully supinated and wrist flexed
  4. Reverse: uncurl fingers and extend the wrist to begin the next cycle

This exercise stretches and activates both finger flexors and extensors, the thumb, and intrinsic hand muscles simultaneously.

Finger-flutter Exercise: Drop the hands to the sides of the body completely relaxed; move the arms and elbows to produce passive relaxed shaking of the hands and fingers. Relaxes and mobilises the entire extensor group.

Passive self-stretch: Fully flex both the wrist and all finger joints simultaneously (Fig. 35.10 in T&S). Addition of postisometric relaxation with a gentle contraction enhances the effect.

Night Positioning

If the patient sleeps with hands fully flexed at the wrist, the finger extensors are held in a sustained stretched position that both maintains TrP activity and can promote carpal tunnel syndrome. A soft splint maintaining neutral elbow, wrist, and finger position at night may be required.

Satellite Trigger Points

Key TrP sources driving extensor digitorum as satellite:

References

  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1: The Upper Half of Body. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 35.
Retrieved from "https://painwiki.com/wiki/index.php?title=Muscle:Extensor_Digitorum&oldid=411"