31 Lumbricals

Jaehon M. Kim and Chaitanya S. Mudgal

31 Lumbricals

31.1 Introduction

The lumbricals have the unique feature of being the only muscle in the body which originates from its own antagonistic muscle. The lumbricals originate from flexor digitorum profundus (FDP) and insert onto the radial lateral band at the level of proximal phalanx. ¹ The primary function of the lumbricals is to extend the interphalangeal joints, but there exists a complex interplay of tension between flexor and extensors. In reality, both origin and insertion of lumbricals are highly variable, and the detailed report of anatomy and function in literature is sparse. The purpose of this chapter is to summarize what we know about lumbricals in existing literature along with illustrative cases of pathomechanics.

31.2 Detailed Anatomy

31.2.1 Origin

A detailed account of lumbrical anatomy was carefully demonstrated in a cadaveric study by Eladomikadachi et al. ² There are four cylindrical lumbricals in each hand, based on their insertion site (index to small finger). The origins of each muscle are diverse and do not correspond to their respective tendon. The first lumbrical is the most consistent with its origin at the radial aspect of index finger FDP at the level of proximal half of the metacarpal bone. The second lumbrical receives contributions from neighboring tendons over 20% of the time. ² The most variability is seen with third and fourth lumbricals, where over 50% derived origins from other fingers (▶Fig. 31.1a, b). The third and fourth lumbricals commonly share fibers from adjacent FDP and occasionally derive their entire fibers from the neighboring tendons. This, in part, may explain the less independent extension of ring and small fingers compared with index and long fingers, especially with extension.

**Fig. 31.1** There is a high frequency of variability in the origins of fourth lumbrical. Multiple combinations of fiber origins were observed, including additional derivation from the ulnar aspect of ring flexor digitorum profundus (FDP), few fiber contributions from the adjacent lumbrical, and entire origin from the ring FDP. (a) Standard description of lumbrical origin and insertion. (**b–j**) Variations. (From Schmidt H-M, Lanz U. Surgical Anatomy of the Hand. Stuttgart, Germany: Thieme; 2004.)

31.2.2 Insertion

The distal lumbrical insertions always attach to the radial side of the lateral band at the level of proximal phalanx. A shared insertion to the oblique or transverse fibers of the extensor apparatus is common and is seen in up to 60% of lumbrical muscles. ² Minor contributions to the volar plate and proximal phalanx are also present. The highest frequency of variations is observed once again in the ulnar fingers. The fibrous hood of the dorsal apparatus, which is a broad interconnection of various tendons and ligaments, crosses majority of the joints in the fingers (▶Fig. 31.2). The central portion corresponds to the extensor digitorum communis (EDC), tethered centrally by the sagittal bands on either side. The tendons of interossei and lumbrical run volar to the axis of the metacarpophalangeal (MCP) joint, and once they become the lateral band, they swoop dorsally around the proximal phalanx forming the conjoint lateral band (▶Fig. 31.2). The attachments of lumbricals are distal and dorsal to the interossei. The subsequent crossing of PIP and DIP joints occurs dorsal to the flexion–extension axis of these joints, and volar shift of the conjoint lateral band is prevented by the triangular ligament at the level of middle phalanx (▶Fig. 31.3). To prevent dorsal dislocation, transverse retinacular ligaments (▶Fig. 31.4) anchor the lateral bands to the pulley of the flexor tendons on either side of the PIP joint. The terminal tendon, which combines the EDC and two lateral bands, attaches to a bony ridge at the base of the distal phalanx.