Location and Topography
The palmar interosseous muscles belong to the middle group of hand muscles. There are three of them in total, and they lie in the 2nd, 3rd, and 4th intermetacarpal spaces.
The middle fascial compartment of the palm (compartimentum palmaris medius) is the central region of the hand, bounded by four walls:
- Anterior wall: palmar aponeurosis (aponeurosis palmaris).
- Lateral wall: vertical part of the lateral intermuscular septum.
- Medial wall: medial intermuscular septum.
- Posterior wall: deep fascia covering the palmar interosseous muscles and the horizontal part of the lateral intermuscular septum.
Structure: Origin and Insertion
The palmar interosseous muscles are unipennate. Their anatomical landmarks:
- Origin: bodies of the II, IV, and V metacarpal bones.
- Insertion: into the dorsal digital expansion of the extensor tendons and the bases of the proximal phalanges of the II, IV, and V digits; insertion is directed toward the axial (III) digit.
Biomechanics and functions
- Adduction: adduct the V, IV, and II digits toward the III digit.
- Flexion and extension: flex the proximal phalanx and extend the middle and distal phalanges.
Innervation
All interosseous muscles of the hand—palmar and dorsal—are innervated by the deep branch of the ulnar nerve (ramus profundus n. ulnaris).
The deep branch of the ulnar nerve passes into the subsyndesmal/subtendinous space of the middle compartment from the medial side.
It also innervates the 3rd and 4th lumbricals, the adductor pollicis muscle, and the deep head of the flexor pollicis brevis.
Do not confuse: Other meanings of the abbreviation PAD
In medical literature, the abbreviation PAD is used as a mnemonic for the palmar interosseous muscles and has other meanings:
- Padua Prediction Score: used to assess the risk of venous thromboembolic events in hospitalized non-surgical patients, including during exacerbation of Crohn's disease and for assessing risk when continuing menopausal hormone therapy. The PADUA system is also used to evaluate the renal margin in renal cell carcinoma.
- PAD regimen: bortezomib + doxorubicin + dexamethasone; recommended for plasma cell leukemia and listed among the treatment regimens for multiple myeloma.
To memorize the function of the interosseous muscles, it is convenient to use the "DAB–PAD" rule. PAD stands for Palmar ADduct: the palmar interosseous muscles adduct the digits toward the III digit. DAB stands for Dorsal ABduct: the dorsal interosseous muscles abduct the II, III, and IV digits away from the axis of the hand. The Plantar ADduct rule applies to the plantar interosseous muscles of the foot, which adduct the III–V digits toward the axis of the II digit.
Class and exam questions
Which muscle group of the hand do the palmar interosseous muscles belong to?
They belong to the middle group of hand muscles, along with the lumbricals (1–4) and dorsal interosseous muscles.
What structures form the posterior wall of the middle compartment of the palm?
The posterior wall is formed by the deep fascia covering the palmar interosseous muscles and the horizontal part of the lateral intermuscular septum.
Which nerve innervates these muscles?
All palmar interosseous muscles are innervated by the deep branch of the ulnar nerve (ramus profundus n. ulnaris), which passes into the subtendinous space from the medial side.
What else to study on this topic
- Dorsal Interosseous Muscles of the Hand (mm. interossei dorsales)
- Lumbrical muscles of the hand (mm. lumbricales)
- Middle Compartment of the Palm (compartimentum palmaris medius)
- Deep Branch of the Ulnar Nerve (r. profundus n. ulnaris)
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