Anatomy, Shoulder and Upper Limb, Hand Interossei Muscles

Structure and Function

Palmar Interossei

The palmar (volar) interossei are unipennate muscles originating from the metacarpals of the respective digit to which they are associated (see Image. Palmar Interossei of the Hand). These muscles adduct the 1st, 2nd, 4th, and 5th digits about an axis through the midline of 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.

When present as a distinct muscle, the 1st palmar interosseous originates from the medial, palmar surface of the 1st metacarpal and inserts into the base of the proximal 1st phalanx and the extensor hood. This muscle is frequently considered rudimentary, with many anatomical sources classifying it as part of either the adductor pollicis or the flexor pollicis brevis. However, recent studies identify the 1st palmar interosseous—termed the “pollical palmar interosseous”—in more than 85% of specimens.[2] This muscle is believed to originate from the oblique head of the adductor pollicis, with research suggesting an evolutionary role.[3]

The 2nd palmar interosseous originates from the medial surface of the base of the 2nd metacarpal and inserts into the medial portion of the extensor hood and base of the proximal 1st phalanx. The 3rd and 4th palmar interossei originate from the lateral aspects of the 4th and 5th metacarpals and insert into the lateral portions of their respective extensor hoods and the bases of their corresponding proximal phalanges.

Dorsal Interossei

Among all intrinsic hand muscles, the dorsal interossei are the most dorsally located (see Image. Dorsal Interossei of the Hand). These bipennate muscles serve as the primary abductors of the 2nd, 3rd, and 4th digits at the MCP joints (see Image. Dorsal Interossei on Magnetic Resonance Imaging). The dorsal interossei also contribute to MCP joint flexion and extension at the PIP and DIP joints.[4]

The 1st dorsal interosseous originates from the adjacent surfaces of the 1st and 2nd metacarpals and inserts into the lateral base of the 2nd proximal phalanx and the extensor hood. The 2nd dorsal interosseous originates from the medial aspect of the 2nd metacarpal and the lateral aspect of the 3rd metacarpal. This muscle inserts into the lateral base of the 3rd proximal phalanx and its extensor hood.

The 3rd dorsal interosseous originates from the medial surface of the 3rd metacarpal and the lateral surface of the 4th metacarpal, inserting into the medial base of the 3rd proximal phalanx and the extensor hood of the 3rd digit. The 4th dorsal interosseous originates from the lateral aspect of the 4th metacarpal and the medial side of the 5th metacarpal. This muscle inserts into the lateral base of the 4th proximal phalanx and the extensor hood of the 4th digit.[5]

Embryology

Upon completion of the 4th week of embryonic development, 4 limb buds arise from somites and mesenchyme of the lateral plate mesoderm that is covered by a layer of ectoderm. Development of the upper extremities is propagated by a plethora of protein factors, with fibroblast growth factors (FGF) and Sonic Hedgehog (Shh) playing vital roles. The gene expression and subsequent interactions of these various proteins contribute to the development of 3 spatial limb axes: proximodistal, anteroposterior, and dorsoventral.[6]

Development of the hand begins with flattening of the distal upper extremity buds around days 34 to 38 of embryonic development.[7] Somites form the limb musculature while the mesenchyme of the lateral plate mesoderm forms bone and cartilage. Somitic mesoderm of the hand divides into superficial and deep layers. Both palmar and dorsal interossei muscles develop from the deep layer of this mesoderm. By the 12th week of development, tendons are fully developed and functional.[8]

Blood Supply and Lymphatics

Blood supply to the palmar interossei comes from the palmar metacarpal arteries and drains into the palmar metacarpal veins. The palmar metacarpal arteries arise from the deep palmar arch, which consists of the terminal portion of the radial artery and the deep branch of the ulnar artery. Additionally, anastomoses are present between the palmar metacarpal arteries and the common palmar digital arteries, which originate from the superficial palmar arch.

The 1st dorsal interosseous muscle is supplied by the 1st dorsal metacarpal artery, which arises directly from the radial artery. The 2nd, 3rd, and 4th dorsal interossei receive blood supply from the 2nd, 3rd, and 4th dorsal metacarpal arteries, which arise from the dorsal carpal arch. All dorsal metacarpal arteries bifurcate into their respective dorsal digital arteries and anastomose with the common palmar digital arteries (see Image. Arteries of the Hand).[9]

Lymphatic drainage of the upper limbs is categorized into superficial and deep drainage. Lymphatic plexuses of the skin on the palm and dorsum of the hand ascend with the cephalic and basilic veins toward the axillary and cubital lymph nodes, respectively. Deep lymphatic vessels follow the primary deep veins and eventually drain into the humeral lymph nodes.[10]

Nerves

The deep branch of the ulnar nerve supplies both palmar and dorsal interossei. The deep branch of the ulnar nerve originates from C8 and T1, with T1 being the primary innervating segment (see Image. Ulnar Nerve Pathway).[11][12]

Physiologic Variants

Classically, students are taught that all dorsal interossei, except for the 3rd, have 2 heads (bipennate), while all palmar interossei possess 1 head (unipennate). However, a retrospective study found that approximately 25% of dorsal interossei and only 62% of palmar interossei were unipennate.[13] This study also concluded that textbooks had oversimplified the attachment sites of both palmar and dorsal interossei, which exhibit a high degree of variability.

As previously mentioned, most contemporary anatomy texts teach that the 1st palmar interosseous muscle is commonly a component of either the adductor pollicis or the flexor pollicis brevis and rarely functional on its own. The notion that the 1st palmar interosseous muscle is an independent and functional intrinsic hand muscle was first suggested by Henle in 1858, with several recent studies supporting this claim.

One study dissected 72 hands and found that the 1st palmar interosseous, referred to as the “pollical palmar interosseous” (PPI) muscle of Henle, was present in 67 hands (93%). The authors also analyzed 6 other similar studies and found that the PPI was present in over 80% of cadaveric hands. Researchers believe the presence of PPI in humans has significant implications for the development of the human species from nonhumanoid primates due to its crucial contribution to the dexterous movements of the thumb.[14]

Surgical Considerations

Metacarpal fractures are commonly treated by hand surgery, with metacarpal shortening ranging from 2 mm to 10 mm.[15] Studies show that with metacarpal shortening of 2 mm, the strength of the respective interossei is reduced by approximately 8%.[16]

Compartment syndromes of the hand interossei are uncommon and difficult to diagnose due to their atypical presentations. A case study found a 1st dorsal interosseous compartment syndrome in a teacher, likely due to overuse injury from writing. This injury went undiagnosed for 8 years but was ameliorated through a simple fasciotomy. Another case report found a 1st dorsal interosseous compartment syndrome in a young, healthy man, also due to overuse injury.[17] The authors measured the compartment pressure at rest and after activity and found a rise in pressure from 12 mm Hg to 60 mm Hg after 10 minutes of hand exercise. This rise in pressure coincided with the patient's hand pain and swelling. After unsuccessful oral pain management, fasciotomy was performed, with subsequent resolution of the patient's symptoms.

Clinical Significance

Both palmar and dorsal interossei receive innervation from the deep palmar branch of the ulnar nerve. Consequently, injury to the ulnar nerve may lead to weakness or even atrophy of the interossei, typically caused by nerve root impingement, brachial plexus compression, or nerve entrapment at the elbow, forearm, or wrist.

Ulnar nerve entrapment ranks as the 2nd most common compression neuropathy in patients. Depending on the compromised nerve fibers, patients may experience weakness in the abduction or adduction of the fingers. 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, the ulnar claw hand deformity, results from weakness of the 3rd and 4th lumbricals, in addition to the interossei. This deformity manifests as extension of the 4th and 5th MCP joints and flexion of the 4th and 5th PIP and DIP joints at rest (see Image. Claw Hand).[18]

Several clinical examinations assess the integrity of ulnar nerve function. When testing for the Wartenberg sign, patients are instructed to adduct all fingers. A positive test result is demonstrated by the abduction of the 5th finger relative to all other digits, indicating weakness of the 3rd palmar interosseous muscle and 4th lumbrical. To examine the palmar interossei, the patient is instructed to hold a sheet of paper between any of the 2nd through 5th digits. Dropping the paper indicates palmar interossei weakness. Additionally, clinicians can evaluate the dorsal interossei by instructing the patient to abduct their 2nd through 5th digits against the clinician’s resistance.[19]