Definition

By: Gregory R. Waryasz, MD

The lumbricals of the musculoskeletal system is characterized by being 4 muscles. The name originates from their appearance as earthworms.

It is part of the intrinsic hand muscles.  It consists of skeletal muscle fibers.

The unique structural features include being four separate muscles that look like worms.

The origin of the 1st and 2nd lumbricals are the lateral two tendons of the flexor digitorum profundus.  The origin of the 3rd and 4th lumbricals are the medial three tendons of the flexor digitorum profundus.

The insertions are the lateral sides of the extensor expansions of the 2nd through 5th fingers.

The blood supply is from the arterial branches of the palmar arches  and venous drainage is from the accompanying veins.

The innervation is from the median nerve for the 1st and 2nd lumbricals and from the deep branch of the ulnar nerve for the 3rd and 4th lumbricals.

The lumbricals as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in the embryo.

The function of the lumbricals is to flex the metacarpophalangeal joints, extend the interphalangeal joints of the 2nd through 5th digits. The synergists are the dorsal interossei, palmar interossei, flexor digitorum profundus, and the flexor digitorum superficialis.

Common diseases include ulnar nerve palsy and carpal tunnel syndrome.

Ulnar nerve palsy can occur at any point of the course of the ulnar nerve leading to weakness and sensory changes.  Numbness and tingling can occur in the distribution of the ulnar nerve.

Carpal tunnel syndrome is a compressive neuropathy of the median nerve at the wrist.  It can lead to weakness in the hand and decreased sensation in the palm.  Patients often report numbness and tingling in the palm and distribution of the median nerve.  Long-standing carpal tunnel can lead to atrophy of the muscles innervated by the median nerve distal to the compressive neuropathy.

Commonly used diagnostic procedures include clinical history, physical exam, EMG, and MRI.

It is usually treated with bracing, NSAIDs, physical therapy, and corticosteroid injections for ulnar nerve palsy syndrome and carpal tunnel syndrome.  A carpal tunnel surgical release procedure can often reduce the compressive neuropathy.

References

Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.

Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.

Wheeless’ Textbook of Orthopaedics: Lumbricals (http://www.wheelessonline.com/ortho/lumbricals)