Genetics (Family History)
Bunions are considered to be hereditary but since the condition is very common, it would not be surprising to have other members of a family with the deformity. It is likely that we inherit certain foot bone shapes, joint flexibility, even body morphology, that lead to the development of a bunion, rather than inheriting the deformity itself.
Faulty foot mechanics such as flat feet (also called over-pronation) are thought to causes bunions. When weight is transferred correctly through the ankle to the foot, the foot joints remain stable and can absorb ground contact forces, adapt to the shape of the ground and power the body forwards without over-stressing the bones or soft tissues. However, if a person over-pronates, the body weight overloads the foot bones along the inside of the foot and prevents normal transmission of weight through the 1st metatarsal and into the great toe. The abnormal direction of the forces start to change the alignment of the 1st metatarsal and the soft tissues around the joint fail to stabilise it and eventually exacerbate the developing deformity.
The abnormal biomechanics are worsened by ligament laxity. Bunions are more likely to occur in people with unusually flexible joints since this makes the joints less stable when loaded abnormally.
Childbirth and Pregnancy
Childbirth and pregnancy has also been reported to lead to bunions. The hormone relaxin, which loosens up ligaments to allow the pelvis to widen for childbirth, can also affect the feet. Many women find that their feet widen during pregnancy. Changes in the feet during pregnancy may also be related to the increase in weight and changes in walking style.
Tight calf muscles
Can cause the foot to pronate abnormally and also lead to greater forces on the forefoot, can contribute to the deformity by leading to an abnormally functioning toe joint.
Causes chronic overloading of the foot joints, often resulting in an abnormally pronated foot posture and a change in gait patterns that exaggerate the forces acting on the great toe joint.
Conditions / Syndromes
Conditions associated with loose ligaments, flexible joints and low muscle tone could increase the likelihood of bunions. An example would be Marfan syndrome which is a connective tissue disorder. Other conditions associated with increased muscle tone and a change in gait with forefoot overloading will cause bunions, such as cerebral palsy (a neuromuscular condition). Inflammatory arthropathies are known to result in bunions: Rheumatoid arthritis is a chronic inflammatory disease where the immune system attacks the joint lining causing swelling and pain in the joints. The inflammation results in a change in foot structure and this can lead to deformity at the great toe. Psoriatic arthritis has a similar effect and can cause marked joint destruction. Gout is a crystal arthropathy and occurs when excess uric acid is deposited as monosodium urate crystals in the joints, typically the great toe joint. The joint swelling can cause the joint to deform.
Poorly fitting footwear
A bunion is something a person will have a predisposition to, but poor footwear in terms of toe box shape and heel height will exacerbate the problem if worn for extended periods of time. Bunions are rare, but do occur in populations that do not wear shoes.
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