Anatomy, Shoulder and Upper Limb, Hand Palmar Interosseous Muscle

Structure and Function

The palmar interossei are unipennate muscles that originate from the metacarpals of their associated digits (see Image. Palmar Interossei of the Hand). These muscles adduct the 1st, 2nd, 4th, and 5th digits about a longitudinal axis through the 3rd digit. Adduction occurs at the MCP joints. Additionally, the palmar interossei contribute to flexion at the MCP joints and extension at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints of their respective digits.[1]

When present as a distinct structure, the 1st palmar interosseous originates from the medial palmar surface of the 1st metacarpal and inserts into the base of the proximal phalanx and extensor hood of the 1st digit. The 1st palmar interosseous is often considered rudimentary, with many anatomical references classifying it as part of either the adductor pollicis or the flexor pollicis brevis. However, recent studies challenge this classification and report that the “pollical palmar interosseous” appears in over 85% of specimens.[2] This muscle likely derives from the oblique head of the adductor pollicis and may have contributed to thumb function in human evolution.[3]

The 2nd palmar interosseous originates from the medial base of the 2nd metacarpal and inserts into the medial portion of the extensor hood and base of the proximal phalanx of the 2nd digit. The 3rd and 4th palmar interossei originate from the lateral aspects of the 4th and 5th metacarpals, respectively, and insert into the lateral extensor hood and base of the proximal phalanx of their corresponding digits.

Embryology

By the end of the 4th week of embryonic development, 4 limb buds arise from somites and mesenchyme of the lateral plate mesoderm, covered by a layer of ectoderm. Upper extremity development is regulated by multiple protein signals, with fibroblast growth factors (FGF) and Sonic Hedgehog (Shh) playing key roles. Gene expression and interactions among these proteins guide the formation of 3 spatial limb axes: proximodistal, anteroposterior, and dorsoventral.[4][5]

Hand development begins with flattening of the distal upper extremity buds around days 34 to 38 of embryonic development. Somites form the limb musculature, while mesenchyme from the lateral plate mesoderm differentiates into bone and cartilage. The somitic mesoderm of the hand divides into superficial and deep layers. Both palmar and dorsal interossei originate from the deep layer. By the 12th week of development, tendons are fully formed and functional.[6]

Blood Supply and Lymphatics

The palmar interossei receive arterial supply from the palmar metacarpal arteries and venous drainage via the palmar metacarpal veins. The palmar metacarpal arteries arise from the deep palmar arch, which is formed by the terminal segment of the radial artery and the deep branch of the ulnar artery. An anastomosis exists between the palmar metacarpal arteries and the common palmar digital arteries, which originate from the superficial palmar arch (see Image. Arteries of the Hand).[7]

Lymphatic drainage of the upper limb is categorized into superficial and deep systems. Lymphatic plexuses in the skin of the palm and dorsum of the hand ascend alongside the cephalic and basilic veins, ultimately draining into the axillary and cubital lymph nodes, respectively. Deep lymphatic vessels accompany the major deep veins and drain into the humeral lymph nodes.[8]

Nerves

Both the palmar and dorsal interossei receive innervation from the deep branch of the ulnar nerve. This branch arises from the C8 and T1 nerve roots, with T1 serving as the principal contributing segment (see Image. Ulnar Nerve Pathway).[9][10]

Physiologic Variants

Standard anatomy instruction typically teaches that all palmar interossei have a single head, described as unipennate. However, a retrospective study reported that only 62% of palmar interossei were unipennate. The study also noted that most anatomy textbooks oversimplify the attachment sites of both palmar and dorsal interossei, which demonstrate considerable anatomical variability. This variability includes distal insertions into components of the extensor hood, volar plate, bone, or combinations of these structures.[11]

As mentioned, contemporary anatomy texts often describe the 1st palmar interosseous muscle as a rudimentary structure incorporated into either the adductor pollicis or the flexor pollicis brevis, rarely functioning independently. The idea that the 1st palmar interosseous is a distinct and functional intrinsic muscle was first proposed by Henle in 1858, and several recent studies have supported this view. A study involving 72 hand dissections identified the “pollical palmar interosseous” (PPI) muscle in 67 specimens (93%). The same study reviewed 6 additional studies and found the PPI present in over 80% of cadaveric hands. Researchers suggest that the presence of the PPI in humans played a key role in the evolution of dexterous thumb function, distinguishing human motor capability from that of nonhumanoid primates.

Surgical Considerations

Conventional treatment for metacarpal fractures typically involves hand surgery, with metacarpal shortening ranging from 2 to 10 mm. Research indicates that a 2-mm metacarpal shortening results in approximately 8% strength reduction of the respective interossei.[12] Compartment syndromes of the interossei are relatively uncommon, usually occurring with severe ischemia, crush injuries, or burns.[13]

Clinical Significance

The palmar interossei receive innervation from the deep palmar branch of the ulnar nerve. Consequently, injury to the ulnar nerve can lead to weakness or atrophy of the interossei muscles, often resulting from nerve root impingement, brachial plexus compression, or nerve entrapment at the elbow, forearm, or wrist. Ulnar nerve entrapment is the 2nd most common compression neuropathy. Depending on the extent of nerve fiber involvement, patients may experience weakness in finger adduction. While the lumbricals primarily contribute to flexion at the MCP joints and extension at the DIP and PIP joints, the interossei also play a minor role in these movements. A late manifestation of ulnar nerve injury, known as the ulnar claw hand deformity, results from weakness in the 3rd and 4th lumbricals, along with the interossei, causing extension at the 4th and 5th MCP joints and flexion at the 4th and 5th PIP and DIP joints.

Several clinical tests assess ulnar nerve integrity. To elicit the Wartenberg sign, the patient is asked to adduct all fingers. A positive result shows abduction of the 5th digit, indicating weakness in the 3rd palmar interosseous muscle and 4th lumbrical. To test the palmar interossei, a patient may be asked to hold a sheet of paper between the 2nd through 5th digits. If the paper falls, it suggests weakness in the palmar interossei.[14]