The precise etiology of bunions is unknown, but it is well documented that a bunion is often the result of medial deviation of the first metatarsal and lateral deviation and pronation of the hallux. There may be a genetic component to bunions. In some instances, bunions may be associated with inflammatory joint disease or cerebral palsy.
Wearing narrow or ill-fitting shoes may exacerbate damage to the MTP joint.
Classic history and presentation: Typically, patients present with a noticeable bump and pain on the medial aspect of the forefoot. An overlying, erythematous callus or bursae may also be present from friction and rubbing against footwear. Patients may also present with transfer metatarsalgia secondary to altered gait pattern with overloading the central metatarsal heads, or pain in the lessor toes due to associated hammertoe deformity.
Patients often complain of difficulty or increased pain with certain types of footwear.
Prevalence: There is increased prevalence of bunions in females compared to males, with reported ratios ranging as low as 2:1 and as high as 15:1.
The prevalence of bunions increases with age, with an estimated prevalence of 23% in adults aged 18-65 years, and 35.7% in adults older than age 65 years.
Risk factors: Predisposing factors include female sex, age, use of constricting footwear, and a family history of bunions. Additional potential predisposing factors include metatarsus adductus, pes planus, equinus contracture, first ray hypermobility, ligamentous laxity, and length of the first metatarsal.
Pathophysiology: There are several static and dynamic structures at the first MTP and first tarsometatarsal (TMT) joint that impact the stability of the hallux. The initial stage in hallux valgus deformity is thought to be weakening of the medial supporting structures, leading to medial deviation of the first metatarsal and lateral deviation and pronation of the hallux. Over time, this results in a progressive varus deformity of the first TMT joint.
As the head of the metatarsal drifts medially and rotates, it rests on the medial sesamoid. The lateral sesamoid then rests in the first intermetatarsal (IM) space. This deformity at the MTP joint then allows the hallux flexor and extensor tendons to bowstring laterally, exerting forces that further deform the hallux.
Over time, bunions can lead to additional problems, including hammertoe of the second toe, synovitis, degeneration of cartilage and metatarsal heads, entrapped nerve, or unstable gait from shifting weight-bearing regions of the foot.
Grade / classification system: The traditional classification system utilizes weight-bearing anteroposterior (AP) radiographs to determine the severity of the hallux valgus deformity using the hallux valgus angle (HVA), 1-2 intermetatarsal angle (IMA), and hallux valgus interphalangeus (HVI) angles (specific numbers vary in the literature).
- Normal: HVA < 15 degrees, IMA < 9 degrees.
- Mild hallux valgus: HVA < 20 degrees, IMA < 11 degrees.
- Moderate hallux valgus: HVA 20-40 degrees, IMA 11-16 degrees.
- Severe hallux valgus: HVA > 40 degrees, IMA > 16 degrees.
- Type 1: Increased HVA and IMA; no first metatarsal pronation; no evidence of first MTP joint degeneration.
- Type 2a: Increased HVA and IMA; first metatarsal pronation without sesamoid subluxation on axial radiographs.
- Type 2b: Increased HVA and IMA; first metatarsal pronation with sesamoid subluxation on axial radiographs.
- Type 3: Increased HVA and IMA; first metatarsal pronation; metatarsus adductus > 20 degrees.
- Type 4: Increased HVA and IMA; with or without first metatarsal pronation; degenerative joint disease of the first MTP joint.
