BioMechanics
February 1999

Surgery:
Hammer toe may be linked to anatomical anomaly

By David Miller, Stephen Mielech, Keith Myrick, and Kathy Siesel, DPM

The extensor digitorum brevis (EDB) muscle is a small muscle located over the dorsolateral aspect of the foot that shares a close proximity to the extensor hallucis brevis muscle. Its origin is situated at the anterolateral aspect of the floor of the sinus tarsi and the lateral aspect of the distal part of the calcaneus.

The EDB muscle is considered a stance-phase muscle. It contracts from the early midstance period to the late propulsive period. During gait cycle, the muscle stabilizes the midtarsal joint about the oblique axis in a pronatory manner during the early propulsive period and it assists the lumbricale and the extensor digitorum longus muscles in stabilizing the second, third, and fourth digits. While it is considered a small muscle, the EDB muscle does not have to be large to stabilize this position, as ground reactive forces are already creating the pronation of the midtarsal joint. In addition, it stabilizes the second, third, and fourth metatarsals against the lesser tarsus, and the lesser tarsal bones against each other in a posterior direction. This muscle also assists in stabilizing the second, third, and fourth digits against their respective metatarsal heads. Lastly, it stabilizes the second, third, and fourth intermediate phalanges against their proximal phalanges.³

This muscle could act as a prime mover during swing phase. However, during swing phase it assists the extensor digitorum longus muscle in dorsiflexing the digits. During stance phase, it stabilizes the joints.

The insertion of the muscle is normally threefold, with each tendon inserting on the plantarlateral aspects of the second, third, and fourth tendons of the extensor digitorum longus muscle. In a study by Chaney et al¹ the extensor digitorum brevis was found to have a variable number of tendons (generally three or four), some extending to the hallux and others to the fifth digit. Of the extra tendon slips encountered in the gross anatomy laboratory at Ohio College of Podiatric Medicine, the majority have been to the fifth digit. We have yet to see one to the hallux, other than the extensor hallucis brevis. Of the 336 limbs studied over the last six years, we have seen only one with an accessory tendon such as that described here.

Sarrafian² reported that normal division of the extensor digitorum brevis occurs in 26% of limbs and that in 72.5% additional muscular slips were found. It should be noted that the normal number of EDB muscle tendons reported in most anatomy textbooks is three, however textbooks usually also state that additional tendon slips are not unusual. Sarrafian reports 34% of accessory slips go to the second toe. Accessory tendon slips are most commonly found extending to the second digit. Somewhat less common are slips extending to the third and fourth digits.

Practitioners involved with foot care should be aware of the possibility of an accessory tendon slip. Accessory tendon slips from the extensor digitorum brevis muscle are common with approximately one-third being to the second pedal digit.² Practitioners should be aware of this anomaly, as its presence may be responsible for pedal pathology otherwise thought to be idiopathic.

Description

Clinical significance

During the propulsive period of gait, the extra tendon gives the extensors of the foot a mechanical advantage that causes stronger contraction, mainly of the second digit, and increased pronation about the oblique midtarsal joint axis. If the extensors are pulling more strongly than the flexors, the oblique midtarsal joint will remain in a more pronated position throughout the gait cycle.

It is this mechanical advantage of the extensors over the flexors that leads to the hammer digit. This type of extensor substitution is frequently associated with severe digital deformity and lesser metatarsophalangeal joint contraction and subluxation.⁵ Since the oblique midtarsal joint is in a more pronated position, the metatarsus is unstable at propulsion and the increased ground reactive forces traumatize the forefoot, leading to such symptomatology as plantar hyperkeratosis and the subluxation of joints in the forefoot. The oblique midtarsal joint is being supinated by the flexor digitorum brevis muscle during propulsion.⁴ It may not attain its fully supinated position if it began in a more pronated position.

External leg rotation must also occur through propulsion in order to abduct and dorsiflex the rearfoot. This motion can occur only if there is supination around the oblique midtarsal joint axis. Since oblique midtarsal joint supination is slightly limited, the leg is unable to maximally externally rotate to get the needed amount of supination at the subtalar joint, thus further decreasing joint stability.³

Proximal instability causes the digits to buckle with the proximal phalanx displaced dorsally on the metatarsal head; the intermediate phalanx is plantar-flexed in relation to the proximal phalanx. Normally, the interossei muscles exert equal and opposite transverse plane force upon the base of the proximal phalanx. Because of the insertion of the accessory tendon, mainly to the medial aspect of the extensor hood apparatus, the delicate balance of force created by dorsal interossei muscles one and two is disrupted. The proximal phalanx of the second digit is forced into an abnormal adducted position, resulting in metatarsophalangeal joint instability. The proximal phalanx will buckle. The hallux may block adduction of the second digit, but in propulsion, as weight transfers from lateral to medial, the second proximal phalanx will dorsiflex.

An extra tendon creates an increased adductory force at the metatarsophalangeal joint. This is not equally countered by the second dorsal interosseous muscle into dorsiflexion as the flexors contract during propulsion, leading to a more severe hammer-toe deformity in this digit than in the remaining lesser digits.³ Because of the consequences of such an anomaly, the podiatric medical practitioner must be aware of its possible existence as a cause for what is otherwise termed an idiopathic foot deformity.

Clinically, the only possible ways to diagnose this condition would be intraoperatively or with the use of magnetic resonance imaging. Conservative treatment would include debridement or padding for the heloma dura that may result from the digital contracture. The surgical option would be to lengthen or resect the accessory tendon slip when the digital procedure for the hammer toe is performed.

David Miller, BA, Stephen Mielech, BA, and Keith Myrick, BA, are fourth-year podiatric medical students at the Ohio College of Podiatric Medicine in Cleveland. Kathy Siesel, DPM, is an assistant professor in the department of podiatric biomechanics/orthopedics at the same institution.

References

1. Chaney DM, Lee MS, Khan MA, et al. Study of ten anatomical variants of the foot and ankle, JAPMA 1996;86(11):535.
2. Sarrafian, S. Anatomy of the Foot and Ankle, 2nd ed., Angiology. Philadelphia: JB Lippincott, 1993.
3. Root ML, Orien WP, Weed JH. Clinical Biomechanics, Vol. II, Normal and abnormal function of the foot. Los Angeles: Clinical Biomechanics, 1977.
4. Valmassy RL. Clinical biomechanics of the lower extremities. St. Louis: Mosby, 1996.
5. Downey MS. Digital deformity associated with subluxation of the lesser metatarsophalangeal joint. In: McGlamry ED, ed. Doctors hospital podiatric education and research institute: Update ’88. Tucker, GA: Podiatry Institute Publishing, 1988.