Valgus malformation of the great toe, commonly known as a bunion, is a very common and potentially painful and debilitating condition of unclear etiology. This topic review will provide an overview of the relevant anatomy, pathophysiology, diagnosis, and management of hallux valgus. Toe and foot injuries are discussed elsewhere. (See "Toe fractures in adults" and "Metatarsal shaft fractures".)
RELEVANT ANATOMY AND BIOMECHANICS
Basic forefoot anatomy — By convention, toes and their respective metatarsals are numbered from one (great toe) through five (little toe). The great toe has two phalanges, while the second through fifth toes typically have three (figure 1 and figure 2 and figure 3). Tendons and ligaments insert at the bases of each phalanx. The digital artery and nerve pass together along each side of each toe.
- Hallux valgus deformity – This deformity is defined as a lateral deviation of the hallux (great toe) on the first metatarsal (figure 4). The deviation of the hallux occurs primarily in the transverse plane. The deformity often also involves rotation of the toe in the frontal plane causing the nail to face medially (ie, eversion). These two deviations have led to the use of different terms to describe the deformity. In orthopedic texts, it is often called "hallux valgus" (HV) whereas many podiatry texts prefer the term "hallux abductovalgus (HAV)." The public is more familiar with the expression "bunion."
- Hallux abductus (or hallux valgus) angle – The angle created by the bisection of the longitudinal axis of the hallux and the longitudinal axis of the first metatarsal (figure 4 and image 1). Historically, a hallux abductus (HA) angle of greater than 15 degrees was considered abnormal, but such deformities are not always symptomatic, and some cases of an HA angle greater than 15 degrees occur naturally due to the shape of the articular surfaces involved [1,2]. Contemporary research suggests an HA angle of 20 degrees or greater is abnormal .
- Intermetatarsal (IM) angle – The angle determined by the bisection of the longitudinal axes of the first and second metatarsals (figure 4). An IM angle less than 9 degrees is considered normal.
- Ray – The forefoot consists of five longitudinal projections, called rays, which are comprised of the metatarsal and its respective phalanges, and the bones aligned with and proximal to the metatarsal, such as the cuneiforms or cuboid bones. Hallux valgus involves the first ray.
First ray anatomy — No muscles originate on the first metatarsal and insert into the phalanx to directly stabilize the first metatarsophalangeal (MTP) joint. The abductor and adductor hallucis muscles pass medially and laterally to the MTP joint respectively, but they are located nearer to the plantar surface (figure 5). Thus, any force pushing the proximal phalanx laterally, or the metatarsal head medially, is relatively unrestrained and can create a valgus deformity.
The first metatarsal is held in alignment by a splinting action of the abductor hallucis muscle medially and by the lateral pull of the peroneus longus acting at the base of the metatarsal . Movement at the first MTP joint in the transverse plane is prevented by collateral ligaments running from the metatarsal epicondyles, distally and plantarly, to the proximal phalanx.