30.1 Anatomy and Biomechanics

The interossei are muscles that occupy the space between the metacarpals of the hand. They comprise two groups: one slightly palmar to the mid coronal plane and one slightly dorsal. They are numbered 1 to 4, starting with the most radial and progressing ulnarward (▶Fig. 30.1).

They fill the space between the bones, hence named interossei. There are usually four dorsal and three palmar interossei (▶Fig. 30.2 and ▶Fig. 30.3), but variations do occur, and it is not unusual to find a fourth palmar interosseus. ¹

This also gives rise to a degree of confusion when looking at the muscles in situ, because it is sometimes difficult to tell which muscle is palmar and which is dorsal. The latter are prominent on the dorsum of the hand (▶Fig. 30.4) underneath the extensor tendons, and it is the first dorsal interosseus that one notices immediately, with the others bulging underneath the deep fascia below. It is for this reason that “guttering” is so easily evident in the presence of a chronic ulnar nerve lesion: the “ulnar-minus hand.”

The palmar interossei are unipennate muscles, while the dorsal interossei are bipennate, each one being attached almost throughout its length to the metacarpal and having a very short tendon in relation to its total length. The mean length of the dorsal interossei is 1.3 cm and that of the palmar interossei is 1.7 cm, with the former occupying about 37% of the total volume of the intrinsic muscles of the hand and the latter 16%. ²

The excursion of each muscle/tendon unit is limited, but the proportionate power exerted is very high indeed, because when it contracts, each muscle exerts its effect using a short lever arm; the fulcrum of its action is effectively the middle of the metacarpal head, with the line of pull being about a centimeter away, at the level of the metacarpophalangeal joint. The strength required to hold a piece of paper between two fingers, for example, is proportionately very high.

The dorsal interossei abduct the fingers and the palmar interossei adduct them. This is easily remembered by employing the acronyms DAB, for “dorsal abduct,” and PAD, for “palmar adduct.” Since the abductor digiti minimi abducts the little finger, it functions in the same way and would be referred to as the fifth dorsal interosseus were it not for the fact that it does not lie between two bones.

The interossei combine with the lumbricals at the level of the metacarpophalangeal joint to form a conjoint tendon (▶Fig. 30.5 and ▶Fig. 30.6).

This is joined distally, after it has merged with the extensor expansion just before the proximal interphalangeal joint, by Landsmeer’s oblique retinacular ligament, a structure that begins its course more volarly and takes its origin from the phalanx itself and the fibrous flexor sheath, continuing onward to merge with the lateral slips as they converge and form the terminal extensor tendon (▶Fig. 30.7).

The lumbricals, interossei, and oblique retinacular ligament of Landsmeer run parallel at the level of the proximal phalanx ³ ; the conjoint tendon thereby extends the proximal interphalangeal joint while, at the same time, flexing the metacarpophalangeal joint (▶Fig. 30.8).

It is worth noting that the first palmar interosseus is on the ulnar side of the index, whereas the first lumbrical is on the radial side; therefore, the index does not have an ulnar conjoint tendon. One needs to consider this when contemplating a crossed intrinsic transfer for a patient who has ulnar deviation of the fingers in conditions such as rheumatoid arthritis.

It is easy to mistake the adductor pollicis for the first dorsal interosseus. The former lies deep to the latter and has two “heads,” or sites of origin: one (the deep head) from the thumb metacarpal and the other (the transverse head) from the middle metacarpal. They insert at the base of the proximal phalanx of the thumb. Mardel and Underwood ⁴ considered adductor pollicis to be the real first palmar interosseus muscle, and functionally, it is. The first dorsal interosseus also has two heads: one from the thumb metacarpal and one from the index. It inserts into the base of the index proximal phalanx on its radial side (▶Fig. 30.9 and ▶Fig. 30.10).

The anthropoid apes also have interossei similar to humans, with gorillas, gibbons, and orangutans having the same number. However, chimpanzees appear to have extra three interossei, which may be considered palmar but are in fact intermediate, lying between the dorsal and palmar layers, and can be thought of as short flexors. ⁵ In many ways this resembles the short flexors of the human foot and may reflect the fact that the chimpanzees often use their upper limbs to walk on and to propel themselves.

The interossei are supplied by the deep branch of the ulnar nerve after it has passed through Guyon’s canal into the palm.