Cath Lab Digest

   Podiatric physicians routinely see disorders of the forefoot, especially the lesser metatarsophalangeal joints (MPJs). In fact, they are often the presenting complaint of the patient or the reason for the medical referral. Lesser MPJ disorders can also occur as a result of preexisting conditions such as trauma, infection, faulty biomechanics and previous podiatric surgery.

   Unfortunately, the term metatarsalgia has been used as a catch-all term to describe a condition without a true etiology. This can be very frustrating to both the patient and the doctor. Indeed, having a firm grasp of the forefoot anatomy is essential when it comes to understanding lesser MPJ pathology.

   The forefoot consists of the bones and all soft tissue structures from the distal aspect of all five digits to the bases of all five metatarsals at the metatarsocuneiform/cuboid (Lisfranc’s) joint. The MPJs are composed of the articulation between the base of the proximal phalanx of each digit and each metatarsal head. These joints are described as ellipsoid joints, which are created by the rounded metatarsal heads and the shallow cavities on the base of the proximal phalanx of each toe.¹

   Articular cartilage covers the distal and plantar portions of the metatarsal heads. There is no articular cartilage on the dorsal surface of the metatarsal. The articular cartilage of the phalangeal bases encompasses the entire cavity of the bone. Two collateral ligaments, running from dorsal proximal to plantar distal, maintain the integrity of the MPJ. These ligaments allow sagittal dorsiflexion and plantarflexion of the proximal phalanx on the metatarsal heads but prevent excessive transverse plane motion of abduction and adduction. Minimal frontal plane motion of inversion and eversion is possible. Disruption of these ligaments causes transverse deviation of the digits as well as dorsal contraction.

   The fibrous capsule surrounds the joints and is relatively thin dorsally but is synonymous with the thick plantar ligaments that comprise the plantar plate. All the MPJs are connected plantarly by the deep transverse intermetatarsal ligament.

   The lesser MPJs are located approximately 2.5 cm proximal to the web of the digits. The amount of joint dorsiflexion of the lesser MPJs can approach 90 degrees and is necessary for the extremes of forefoot function in walking, but most importantly in sports. The lesser MPJs are at a relative 25 degrees of dorsiflexion in stance, owing to the 20- to 25-degree plantarflexion of the metatarsals in stance.

   Muscles associated with this area are divided into extrinsic and intrinsic muscles. The extrinsic muscles at this level are composed entirely of tendinous material. Dorsally, the extensor digitorum longus tendon passes over the lesser MPJ and divides into three slips. The central slip inserts into the base of the intermediate phalanx. The remaining two slips rejoin to insert into the base of the distal phalanx. The extensor hood is comprised of soft tissue at the joint level that connects the extensor tendon to the plantar plate. The extensor tendon does not insert into the proximal phalanx but dorsiflexes the proximal phalanx on the metatarsal head through the extensor hood mechanism. The tendons of the extensor digitorum brevis tendon attach to the long extensor tendons at the level of the second, third and fourth MPJs.

   Plantarly, the tendon of the flexor digitorum longus courses along the long axis of the joint and inserts into the base of the distal phalanx. The tendon of the flexor digitorum brevis inserts into the center area of the middle phalanx. This aids in plantarflexion of the proximal and distal IPJ. There is no insertion of the flexor tendons on the proximal phalanx.