Introduction

Hallux Valgus is a deformity of the first metatarsophalangeal joint, often called bunion. Research shows that dancers are at greater risk, with prevalences ranging from 37%–55% whereas in the general population, a prevalence ranging from 8% to 14% is reported. Several risk factors and predisposing factors are known, but no longitudinal data is available on the exact cause of this deformity. As it develops gradually over time, knowing how it develops might aid in prevention. Since dancers are at high risk, the current authors chose this population to unravel certain unanswered questions.

Methods

Uninjured and healthy dancers between 10 and 19 years old were recruited from a dancesport specialty school. They were eligible for inclusion when they had been training for at least one year with the school and in case they were injury-free at the foot or lower limb.

At the Dancesport school, they follow a high-load training program including 4 hours of dance classes, 2 hours of individual practice on weekdays, and 2 hours of individual practice on Saturdays and Sundays, equalling a total of at least 34 hours of dance training per week. Next to this already high-occupied training schedule, they were required to participate in intra-school competitions each semester and to actively engage in national or provincial competitions.

The included participants’ demographics were collected using questionnaires and the school doctor provided information on each participant’s physical data. All measurements were conducted at the school to minimize data loss.

The cohort study was designed to follow the progression of the so-called hallux valgus angle. The angle was measured using the photographic hallux valgus angle described by Omae et al. (2021), which required photographing the foot from a 15° inclination angle of the vertical axis, at 1 meter above the foot. With the participant standing, a line on the photograph was drawn from the first metatarsal head, and another line was drawn from the medial edge of the big toe. The line between point A and B were of equal length as line BC. The angle between AB and BC is the hallux valgus angle, as can be seen in the picture. This measurement was collected at baseline and at the 1-year follow-up.

Next, plantar pressure of the foot was measured with the participants standing upright on a pressure measuring device with their hips and knees in a neutral position. In this position, the system was calibrated for each participant. Next, the participant was required to stand in the demi-pointe position, by turning their lower limbs 60° outward and raising their heels to a stable pose. A division was made between the pressure at the hallux, the toes, the metatarsophalangeal joints, and the medial, middle, and lateral compartments.

Results

A total of 40 dancers with a mean age of 15.5 years were included in this cohort study. They had a mean of 26 weekly training hours and 2.8 years of an athletic career at the time of collecting the measurements

Using a paired t-test, the authors found a significant progression of the hallux valgus angle from 9.4° +/- 3.8° at baseline to 11.5° +/- 5°, representing an increase of 2.1° after one year.

The measurement of foot plantar pressure did not show significant correlations between the baseline plantar pressure measurement and the extent of the hallux valgus in dancers.

The correlation analysis revealed significant negative correlations between the pressure distribution of the foot at the hallux, the toes, and the medial compartment of the foot and the variation of the hallux valgus angle. A significant positive correlation was found between the progression of the hallux valgus angle over 1 year and the pressure distribution in the middle compartment of the foot and the metatarsophalangeal joints.

The multivariate regression analysis showed that the middle compartment and hallux area pressure distribution were significantly associated with the extent of the variation in the hallux valgus in dancers.

Questions and thoughts

In concrete terms, the results indicated that at baseline, no significant relationship was found between the extent of a hallux valgus angle in these healthy dancers and the pressure distribution of the foot.

Over the 1-year period, the dancers had an increase in their hallux valgus angle of 2.1°. The variation in the progression of this angle was negatively correlated with the pressure at the hallux, the toe box, and the medial compartment. This means that those with an increase in the hallux valgus angle over 1 year had a decrease in plantar pressure in the hallux, toes, and medial compartment of the foot. If their hallux valgus increased, the pressure in these areas decreased.

On the other hand, when the hallux valgus in dancers increased, the pressure at the metatarsophalangeal joints and middle compartment of the foot increased. This means that a shift in plantar pressure is observed from the medial, hallux, and toe regions during the demi-pointe position to the metatarsophalangeal joints and middle regions of the foot in those who had an increase in their hallux valgus angle.

It was previously thought that a hallux valgus deformity develops slowly over time but especially in the older population. This study challenges this knowledge and might be important to effectively monitor people at risk for developing hallux valgus deformities already much more early on. Especially in these young dancers who have high-loading training schedules, integrating preventive measures seems of uttermost importance.

On the other hand, much more data is needed to confirm the relevance of the current findings. Sung et al. (2019), for example, found a natural progression of 1.5° per year in children with juvenile hallux valgus who were under 10 years of age but did not find this in children older than 10 years of age. Yet, another study by Liu et al. (2024) showed a significant increase in the hallux valgus in dancers performing at least 20 hours of elite dancesport practice per week over 1 year, although female sex was a much stronger predictor.

Talk nerdy to me

The conclusions should not be generalized to the larger population, since this cohort consisted of elite adolescent dancers who had a very high training load of approximately 26 hours per week. Besides the high training load, their young ages should prevent us from concluding the onset in a broader population. Nevertheless, it provides important information on how young dancers risk being affected by this condition.

These dancers were not asked about pain or problems concerning their first toe or feet during the study period. At baseline, all were pain-free and without injuries to their lower limbs. The study offers insight into how hallux valgus in dancers might develop during youth by prospectively collecting data about this condition. Longer studies seem necessary to confirm the findings of this paper since the reported progression of the hallux valgus angle remained beneath the standard error of the mean and minimal detectable change. However, as this condition develops gradually, an increase of nearly 2° over one year, as found here, should not be overlooked.

You could wonder how reliable this measurement of the hallux valgus in dancers is. The current study calculated the intra-rater reliability of the hallux valgus angle in dancers over a mean of 3 measurement attempts. The ICC values showed good to excellent intra-rater reliability (ICC = 0.910; 95% CI 0.868 – 0.939)

The standard error in the measurement (SEM) and minimal detectable change (MDC), however, indicate that any change below 3.31° of hallux valgus progression might be due to random error and any change below 7.84° could be non-relevant. The authors have correctly interpreted these findings and indicated that changes might be caused by potential measurement errors and a substantial degree of individual variation. More precise measurements may be required and longer follow-up is needed.

The authors propose that their findings “indicate that excessive loading in the midplantar position affects the hallux valgus angle among elite adolescent dancesport dancers.” Yet, this is too simplistic since correlations do not show the direction of the effects, nor of the causational relationship. It might be true that the progression of the hallux valgus angle creates excessive loading in the mid-plantar region and not vice versa.

A relevant side-note to add is that these dancers practiced dancesport in high heels. Therefore, these results can not be transferred to other types of dancers like for example ballet dancers.

Take-home messages

This study evaluated hallux valgus in dancers and measured the progression over a 1-year period. A significant increase in hallux valgus angle was observed of 2°, which was however below the SEM and MDC, indicating that more precise measurements may be required. Yet, a shift in plantar pressure distribution was observed, possibly indicating a true difference in foot biomechanics over the studied period. Since the studied population was at high risk for developing a hallux valgus deformity, the current study provides important insights into the natural progression of deformities of the big toe in otherwise healthy dancers with no pre-existing hallux valgus deformities, indicating the need for effective prevention and follow-up.

Reference

Liu Z, Chen S, Okunuki T, Yabiku H, Hoshiba T, Maemichi T, Li Y, Zhao H, Kumai T. Progression and risk factors of hallux valgus angle in elite adolescent dancers: a cohort study. BMC Musculoskelet Disord. 2024 Nov 30;25(1):983. doi: 10.1186/s12891-024-08005-9. PMID: 39616328; PMCID: PMC11607872.