Muscle:Interossei

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Interosseous Muscles of the Hand, Lumbricals, and Abductor Digiti Minimi are a group of intrinsic hand muscles whose trigger points (TrPs) produce finger pain readily mistaken for arthritis, radiculopathy, or digital nerve entrapment. The hallmark clinical clue is the Heberden's node — a dorsal enlargement at the distal interphalangeal (IP) joint that is closely associated with TrPs in the interossei and frequently resolves in tenderness immediately after TrP injection. TrP injection is the most reliably effective treatment for this muscle group; spray and stretch are generally ineffective except for the first dorsal interosseous.

Anatomy

Interossei

The interossei lie between adjacent metacarpal bones. There are four dorsal interossei (D1–D4) and three palmar interossei (P1–P3).

Each dorsal interosseous arises by two heads with structurally distinct fibre arrangements:

  • The head nearest the middle finger is pennate — covers nearly three-quarters of its metacarpal bone; has a long endplate zone running nearly the length of the muscle belly (designed for strength)
  • The other head has a more parallel fibre arrangement with a transverse endplate zone near mid-muscle (designed for speed and range of motion)

Both heads of each dorsal interosseous attach distally to the base of the proximal phalanx of the related finger on the side away from the midline of the hand, and to that finger's extensor aponeurosis.

The first dorsal interosseous is larger than the others. One head arises from the ulnar border of the thumb metacarpal; the other from nearly the entire radial border of the second metacarpal. Both attach to the proximal phalanx of the index finger on the radial side. This muscle fills the dorsal web space of the thumb.

Each palmar interosseous arises from the palmar surface of one metacarpal bone and attaches distally to that finger's extensor aponeurosis and the base of its proximal phalanx on the side closest to the midline (the middle finger).

Lumbricals

The four lumbricals attach proximally to the four tendons of the flexor digitorum profundus in mid-palm, and distally to the radial side of the extensor aponeurosis of each finger. They are not interosseous muscles but function similarly. Spatially: the first and second lumbricals lie palmar to the first and second dorsal interossei (with the transverse head of the adductor pollicis interposed); the third and fourth lie palmar and adjacent to the second and third palmar interossei.

Abductor Digiti Minimi

Arises from the pisiform bone; attaches distally to the ulnar side of the base of the first phalanx of the little finger and its extensor aponeurosis. Presents a parallel fibre arrangement with a transverse endplate zone at mid-muscle.

Innervation

  • All interossei and abductor digiti minimi: ulnar nerve (medial cord, lower trunk; C8, T1)
  • First and second lumbricals: median nerve
  • Third and fourth lumbricals: ulnar nerve

Referred Pain Patterns

First Dorsal Interosseous

TrPs refer pain strongly down the radial side of the index finger and deeply through the dorsum and palm of the hand. Pain may also extend along the dorsal and ulnar sides of the little finger, and as spillover into the dorsal wrist and hand. Pain is most intense at the distal IP joint, where a Heberden's node may be present. The first dorsal interosseous is the second most frequent source of referred palm pain (exceeded only by the palmaris longus). Patients may have difficulty deciding whether the pain is more severe on the palmar or dorsal aspect.

Remaining Dorsal and Palmar Interossei

TrPs refer pain along the side of the finger to which that interosseous muscle attaches, extending as far as the distal IP joint. No distinction is made between dorsal interossei, palmar interossei, and lumbrical pain patterns — they are clinically equivalent for a given finger. The exact pattern varies with TrP location. An active TrP may be associated with a Heberden's node within its zone of referred pain and tenderness.

Abductor Digiti Minimi

TrPs refer pain along the outer (ulnar) aspect of the little finger to which it attaches.

Heberden's Nodes — Clinical Significance

Heberden's nodes are dorsolateral or dorsomedial soft-tissue enlargements (sometimes partly bony) at the distal IP joint, appearing on the side of the finger to which the involved interosseous muscle attaches. They are a common finding in patients with interosseous TrPs and serve as a guide to TrP location.

Key clinical facts:

  • Nodes may be acutely tender soon after they appear; they tend to become pain-free over time
  • Tenderness of a Heberden's node typically disappears immediately after TrP injection of the corresponding interosseous; the node diminishes in size more slowly
  • Idiopathic nodes are most common on the index and middle fingers; they are 10 times more prevalent in women, closely related to menopause, and may have an autosomal sex-influenced inheritance pattern
  • Similar nodes at the proximal IP joints are called Bouchard's nodes (present in only 25% of individuals with Heberden's nodes)
  • A Heberden's node on the ulnar side of the thumb is associated with TrPs in the adductor pollicis (see Chapter 39)
  • The proposed mechanism: sustained taut band tension on tendons → trophic changes at the distal IP joint, possibly via an autonomic component within the TrP reference zone; early radiographic sign is small calcium deposits in extensor tendons near the distal phalanx before clinical appearance

Activation and Perpetuating Factors

Occupational and activity patterns
  • Sustained or repetitive pincer grasp — seamstress, painter, sculptor, mechanic, model-maker (holding small pieces while glue sets)
  • Nervous habits — fiddling with a pen cap while writing with the other hand
  • Weeding, physical therapy manipulation of foot muscles, manicurist retracting nail cuticles
  • Golf hands — constant tight grip on a small-diameter club handle
  • Writing with a ball-point pen (replacing with a felt-tip pen substantially reduces the load)
Not associated
  • Playing piano or batting a baseball do not appear to activate these TrPs
Perpetuation by joint disease
  • Abnormal hand mechanics from arthritis increase strain on the interossei, activating and perpetuating TrPs. Conversely, the TrPs may contribute to osteoarthritis progression. Inactivating TrPs and their perpetuating factors is considered important early management to delay or arrest some forms of osteoarthritis.

Symptoms

Patients characteristically complain of "arthritis pain in my finger" — the presentation is so convincing that the myofascial origin is routinely overlooked. Specific features:

  • Finger pain and stiffness — impairs buttoning, writing, grasping
  • Awkwardness of hand function
  • No numbness or paraesthesia unless the TrP has also entrapped the digital nerve (see Entrapment below)
  • The Heberden's node is reported as a "sore joint that is swollen" — but examination shows no true synovial or bony swelling; tenderness may be referred to the joint

Brief morning stiffness (due to increased viscosity of periarticular structures) is a feature associated with the osteoarthritic context of these TrPs, and subsequent loss of range of motion is often due to muscle shortening from TrP activity rather than joint destruction.

Clinical Examination

Finger Spread and Closure

Finding Muscle shortened Mnemonic
Cannot fully spread the extended fingers apart Palmar interossei (adductors) PAD — Palmar ADduct
Cannot fully close the extended fingers together Dorsal interossei (abductors) DAB — Dorsal ABduct
Small finger persistently abducted / "sticks out" Abductor digiti minimi shortened
Index finger persistently abducted / "sticks out" First dorsal interosseous shortened

Important: Abduction-adduction testing must be performed with the fingers extended at the MCP joints — spreading is severely limited even normally when the fingers are flexed at the MCP joint.

Lumbrical Testing

Lumbrical shortening presents as:

  • Hyperextension of the distal phalanx of the affected finger when fingers are extended
  • Inability to fully close the affected finger into a claw position (fingers flexed at MCP with MCP extended)

Lumbrical overload test: holding a hand of cards, or holding a newspaper, by pressing the middle phalanx of the middle finger against the thumb while avoiding fingertip pressure — this specifically overloads the second lumbrical.

The standard clinical test of intrinsic muscle flexion-extension function is to resist IP joint extension with the MCP joint flexed — this tests both interossei and lumbricals together.

Strength Testing

Muscles shortened by TrPs will show weakness, most evident when tested in a lengthened position.

Trigger Point Examination

Usually only one or two interosseous muscles contain active TrPs at one time; others may harbour latent TrPs.

TrPs in these muscles are difficult to palpate. Technique:

  1. Separate the fingers widely to move the metacarpal bones apart
  2. Pincer palpation between the metacarpal bones
  3. Counter-pressure with a finger against the palm beneath the muscle
  4. Deep tenderness can be localised in the interossei and lumbricals; except for the first dorsal interosseous, referred pain and local twitch responses are rarely induced by palpation alone — needle impalement is usually required to elicit them

When Heberden's nodes are present, they serve as guides — nodes are located over the distal IP joints, on the dorsal side of the finger to which the interosseous attaches.

Entrapment

Cutaneous hypoaesthesia along one side of a finger — reported as numbness — may occur when an active TrP lies in the corresponding interosseous muscle. This apparent neurological deficit disappears following TrP inactivation, consistent with either entrapment of the median or ulnar digital nerve by increased interosseous tension, or sensory inhibition by the TrP itself. Electrodiagnostic testing is required to establish a true entrapment component.

Ulnar nerve motor branch: The deep (motor) branch of the ulnar nerve pierces the opponens digiti minimi before supplying all interossei, the 3rd and 4th lumbricals, the adductor pollicis, and the deep head of the flexor pollicis brevis. Active TrPs in the opponens digiti minimi can therefore produce weakness of this entire muscle group. If unexplained weakness is present in ulnar-innervated intrinsic hand muscles, the opponens digiti minimi must be examined for TrPs.

Joint play dysfunctions at the carpometacarpal or metacarpophalangeal joints associated with interosseous TrPs must be identified and corrected concurrently.

Differential Diagnosis

Condition Must-have features Features absent with pure interosseous TrP Key distinguishing test
C6 radiculopathy Dermatomal numbness / paraesthesia in C6 distribution (thumb, index, radial forearm); neck pain; possible biceps reflex change Dermatomal sensory loss; reflex change Cervical screening; MRI spine
C8 or T1 radiculopathy Numbness in C8 (ring, little finger, ulnar forearm) or T1 distribution; intrinsic hand weakness Multiple muscles weak; sensory loss in dermatomal pattern EMG/NCS; cervical MRI
Ulnar neuropathy Numbness in ulnar 1.5 fingers; hypothenar and interosseous wasting; clawing (ring and little finger) Global ulnar intrinsic weakness and wasting; Froment's sign NCS; Froment's sign; Tinel's at cubital tunnel or Guyon's canal
Thoracic outlet syndrome (abductor digiti minimi TrPs) Brachial plexus compression features — see Thoracic Outlet Syndrome Adson's, ROOS test; scalene examination
Digital nerve entrapment (isolated) Hypoaesthesia strictly along one side of one finger Resolves on TrP inactivation alone (if TrP-caused) TrP injection → symptom resolution confirms TrP origin; persistent deficit after injection → NCS
Distal IP joint osteoarthritis / Heberden's nodes Bony enlargement; crepitus; radiographic joint space narrowing Heberden's node tenderness resolves immediately on TrP injection; no true synovial swelling TrP injection; X-ray

When interosseous TrPs are active, look for associated TrPs in:

Finger pain and numbness also may be caused by brachial plexus entrapment by taut scalene muscles or compression of the plexus beneath the pectoralis minor.

Treatment

Spray and Stretch

See Apropos Treatment for general principles.

Spray and stretch is not generally effective for interosseous TrPs because adequate stretch of these muscles is difficult to achieve. TrP injection is preferred.

Exception — First dorsal interosseous: This muscle can be stretched and sprayed. The operator abducts the thumb and adducts the index finger to the point of resistance, while applying down-sweeps of vapocoolant over the involved muscle and pain pattern (including the palmar aspect). Follow with three slow cycles of full active range of motion.

Spray and stretch is more likely to be effective for the interossei when: the TrP is superficial (dorsal interossei); the fingers and metacarpal bones can be separated widely; the down-sweep pattern covers both the involved musculature and the pain pattern. Spray and stretch is applied to these muscles immediately following injection.

Trigger Point Injection

See Trigger Point Injection for general principles. TrP injection is the most rapidly and sustainedly effective treatment for interosseous TrPs.

Dorsal interossei

Each dorsal interosseous has two heads, both of which may need to be explored. For the first dorsal interosseous: hold the patient's index finger between the operator's index and middle fingers, with the middle finger pressed firmly into the web space beneath the muscle in a pincer grasp to fix the TrP. For the second dorsal interosseous: align the needle along the third metacarpal in the second interosseous space and insert into the centre of the tender area; if tenderness remains, re-align with the second metacarpal and probe the other head. A 2.5 cm (1 inch) 25-gauge needle is used.

Palmar interossei

TrPs are difficult to palpate precisely — thorough area exploration with the needle is important. To inject the first palmar interosseous, direct the needle away from the third metacarpal to reach the muscle, which lies beneath the ulnar side of the second metacarpal.

Lumbricals

Injected from the palmar side (no major structure lies between the lumbricals and the palmar skin). Each lumbrical is found at the radial side of its corresponding metacarpal bone, in close association with a flexor digitorum profundus tendon.

Abductor digiti minimi

Locate TrP by flat or pincer palpation. Turn the hand ulnar side up, resting on a pillow. Locate and precisely inject the palpable taut band.

Post-injection outcome: Following TrP inactivation, soreness in the related distal IP joint and joint stiffness disappear. Tenderness of the Heberden's node usually disappears immediately; the node diminishes in size over time.

Corrective Actions

Activity modification
  • Reduce the force and duration of pincer grip activities
  • Replace ball-point pens with freely flowing felt-tip pens (requiring much lighter touch)
  • Interrupt prolonged fine manual activity regularly with break exercises
Home exercises
  • Interosseous-stretch Exercise: Hold forearms in a straight line (arms abducted). Either (A) firmly oppose the palmar aspects of the metacarpal heads and fingers while spreading all fingers and thumbs apart; or (B) contact only the finger pads while spreading fingers and thumbs apart, using uninvolved fingers to assist the stretch. Forearms must remain in a straight line.
  • Finger-flutter Exercise (Fig. 35.9 in T&S): Drop hands at sides with elbows straight; shake fingers in a limp fluttery motion to relax muscles and increase circulation
  • Finger-extension Exercise (Fig. 38.7 in T&S): Interrupt sustained fine activity
  • Artisan's Finger-stretch Exercise (Fig. 35.8 in T&S): Pause during fine manual work
  • Adductor Pollicis-stretch Exercise (Fig. 39.6 in T&S): Also necessary when first dorsal interosseous TrPs are active, given their consistent co-involvement with the adductor pollicis

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 40 (pp. 786–799).
  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1. 2nd ed. Chapter 33 (pp. 685–689).
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