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What Causes Bunions?

I am currently writing a book on the pathomechanics of common foot and ankle disorders. In my opinion, one of the most fascinating subjects of this book is the bunion deformity, also known as hallux abductovalgus (HAV). This topic is both controversial and enigmatic simply because we still do not fully understand what causes this condition. I became interested in HAV deformity during my fourth year of podiatric medical school when I participated in a landmark research study of motion of the first ray of the human foot.1

After reviewing hundreds of published articles about HAV deformity, it is clear that the most common risk factors are either heredity, ill-fitting footwear or both.2-9 In terms of pathomechanics, it is interesting to observe that there are two distinct theories regarding HAV deformity.A distal theory focuses primarily on the hallux, which subluxes in a lateral and valgus direction due to some type of malalignment or instability of the first MPJ.10 A more proximal theory focuses on instability of the first tarsometatarsal joint (i.e. first metatarsocuneiform joint), which is proposed to be unstable, particularly in the transverse plane.11-13 The question is: how would heredity or improper footwear contribute to either of these mechanical aberrations?

A recent study conducted at LaTrobe University in Australia sheds light on this question.14 Munteanu and coworkers studied a group of 74 monozygotic and 56 dizygotic female twin pairs and evaluated the incidence of HAV deformity as well as the use of footwear across their lifespans. Surprisingly, there was not a strong correlation with genetic factors but rather with environmental factors such as constrictive footwear. There was a strong familial tendency toward HAV among the twins but the increased risk for HAV appears to be due to the twins sharing similar habits and lifestyles rather than the fact that they also share certain genetic traits. In this study, the chance for HAV to appear in both identical (monozygotic) twins was essentially the same as between two fraternal (dizygotic) twins. 

This study showed thatwhenwomen wear shoes with a narrow or very narrow toe box, particularly during the fourth decade of life, have a significant risk for developing HAV deformity.14 Conversely, there was no correlation between shoe heel height and increased risk of HAV. These findings mirror those from another recent study by Menz and colleagues.15

Wearing shoes, by itself, has been identified as a risk factor for developing HAV deformity. There are published studies demonstrating that risk of HAV increases when women who are barefoot during early life change to a shoe-wearing lifestyle.2-5 

In a classic study, Shine assessed the incidence of HAV deformity in an excellent comparison of a large group of habitually shod versus unshod people.16 The study was conducted on the island of St. Helena, where roughly half the population wears shoes on a daily basis and half remain barefoot all their life. When comparing these two groups, Shine found that risk of developing HAV increased in almost a linear fashion with each year of wearing shoes. Hallux valgus deformity, regardless of severity, occurred in only two percent of the unshod population while it was prevalent in 16 percent of men and 48 percent of women who had worn shoes for over 60 years. 

Why women were at far greater risk is confounding in this study as the style of shoes worn by men and women on the island of St. Helena is identical and would not be described as constrictive or “inappropriate” in terms of design.16 This propensity for women to develop HAV at a significant higher frequency in comparison to men cannot be fully attributed to footwear. 

Footwear is more likely to contribute to the distal pathomechanical mechanism of HAV rather than proximal. Any pressure applied medially on the hallux could cause gradual lateral subluxation at the first MPJ.17 However, medial translation of the first metatarsal, a hallmark finding with HAV deformities, could not be directly explained by constrictive footwear. Indirectly, lateral subluxation of the hallux caused by shoe pressure could weaken the medial capsule of the first MPJ, allowing “escape” of the first metatarsal medially.18

On the other hand, the relationship between pes planus, foot pronation and risk of HAV has been gaining more and more credibility over the past 10 years.19-24 This is the basis for the proximal mechanism in the pathomechanics of HAV.11,12 The first ray of the human foot will move dorsally and in the direction of inversion with overloading from rearfoot pronation.1,25,26 The medial shift is thought to occur primarily at the first tarsometatarsal joint.13 The first ray itself does not “pronate” in HAV deformity while the foot itself appears to be more pronated in patients with this condition.

The strongest validation of the relationship between rearfoot pronation and incidence of HAV was provided by the Framingham Foot Study.27 This study of an extraordinary large group of human patients allowed comparison of planar pressure and force among a group of patients with hallux valgus (1,123 patients), hallux valgus combined with other foot disorders (641 patients) and people without hallux valgus (3,707 patients). This high-powered study evaluated foot posture, center of pressure excursion during gait and distribution of peak pressure and force in eight regions of the foot. Patients with hallux valgus demonstrated higher peak pressure under the medial rearfoot as well as a smaller center of pressure excursion index and a lower arch profile than healthy subjects.  

All of these findingsfrom the Framingham Foot Study link hallux valgus to pes planus and a pronated rearfoot alignment during gait. While it has been commonly accepted that there is a genetic risk factor for both pes planus and HAV, there are no studies that have attempted to measure the familial pattern of HAV deformity in patients with pes planus.

Although HAV is a very common deformity podiatrists see in practice, more research is necessary to ensure a better understanding of the seemingly elusive etiology or etiologies for this condition. Patients commonly ask how they got the condition and how it could have been prevented.  Thus far, wearing of constrictive shoes, rather than high heeled shoes is a proven causative factor.  Being female is a risk factor, but we are not sure why. Heredity seems to be another factor, but recent research has given reason to be cautious about the role of genetics and HAV.

References

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