Foot pain and functional limitation in healthy adults with hallux valgus: a cross-sectional study

Thirty volunteers with HV (aged 20 years and older) and 30 controls matched for age (± 5 years), gender, and body mass index (BMI) (± 5 kg/m²) were recruited for participation in this study through community advertisements. Exclusion criteria were: history of foot or ankle surgery or fractures, hallux limitus (self-reported or first MTPJ passive dorsiflexion range of motion < 50 degrees)[26], inflammatory disease, neurological conditions, and a history of falls. Because radiographs were required to measure HV angle and determine inclusion into HV or control groups, pregnant or breastfeeding women were also excluded from this study. Weight-bearing dorsoplantar radiographs of both feet were obtained for all eligible participants by the same radiographer using a GE Definium 6000 Digital X-ray system. The HV angle (measured as the angle between the first metatarsal and proximal phalanx)[27] was determined from digital radiographs using computer software developed for telemedical applications[28], and HV cases were defined as an angle greater than 15 degrees. To be eligible for the control group, participants were required to have a radiographic HV angle less than 15 degrees on both feet. Radiographs were further examined by another examiner for signs of osteophytes and joint space narrowing with reference to a radiographic atlas developed by Menz et al.[29]. The case definition defined by these authors was used to classify cases of first MTPJ OA. Ethical approval was gained from the institutional Medical Research Ethics Committee, and all subjects gave written informed consent prior to participation.

Height, weight and body mass index (BMI) were recorded in order to match HV participants with controls, and demographic data were obtained via questionnaire. All examinations and questionnaires were administered by a single examiner (SN). Intra-rater reliability for physical measures was determined from pilot work. Refer to Additional file1: Table S1 for intraclass correlation coefficients (ICC_3,1), standard error of measurement (SEM), and minimum detectable change at the 90% confidence limit. Reliability was considered good for ICCs greater than 0.75, and very good for ICCs greater than 0.9[30].

General health and well-being were assessed using the Short Form 36 Health Survey (SF-36v2®), which includes eight subscales[31]. Habitual physical activity levels were assessed using the Baecke Questionnaire[32] to calculate a work index, sport index, and leisure index.

To investigate functional disability related to foot pain, participants completed the Foot Health Status Questionnaire (FHSQ)[33] and Manchester Foot Pain and Disability Index (FPDI)[34]. The FHSQ contains four domains: foot pain (4 items), foot function (4 items), footwear (3 items), and general foot health (2 items). This questionnaire has been validated and shown to have good test-retest reliability[33]. FHSQ subscale scores are converted to a scale ranging from 0 to 100, with higher scores indicating better foot health. The original FPDI contained 19 items[34]; however, recent psychometric evaluation[35, 36] has shown that 17 items cluster around three main constructs: functional limitation (10 items), pain intensity (5 items), and appearance (2 items). While different methods have been used for FPDI scoring[36], we used the approach described by Menz et al.[37]. Responses to questionnaire items were coded as follows: “none of the time” = 0, “some days” = 1, “on most/every day” = 2. A total score was calculated as the sum of item responses, resulting in an ordinal scale ranging from 0 to 34, and subscales were calculated for pain (score range 0 to 10) and function (score range 0 to 20).

Visual analogue scales (VAS) were used to investigate foot pain intensity and concerns about appearance. Worst and average foot pain intensity over the past four weeks were evaluated using a 100 mm VAS, with 0 mm described as “no pain” and 100 mm described as “worst pain ever,” and the pain location was identified on a foot diagram. The pain VAS has well-established reliability and validity in lower limb musculoskeletal research[38]. Participants were also asked to indicate how satisfied they were with the appearance of their feet on a 100 mm VAS, with 0 mm representing “completely satisfied with appearance” and 100 mm representing “completely dissatisfied with appearance.” This method was adapted from a technique described by Saro et al.[39].

Participants were asked to wear their regular footwear to the examination session. Shoes were assessed using a steel ruler to measure relative heel height (the difference between heel height and forefoot sole thickness)[25] and using digital callipers for relative ball width (the difference between forefoot width across the widest point of the MTPJs and the width of the shoe upper at the same point). A positive value for relative ball width indicated that the shoe upper was wider than the forefoot. Participants who wore sandals that were unable to be measured in this manner (n = 12) were excluded from this analysis. Finally, participants were asked whether they had ever regularly worn shoes with a heel height greater than two inches (yes/no), and how often they currently wore this style of shoe (never, seldom, sometimes, often, always)[25].

In order to obtain a quantifiable measurement of tenderness around the first MTPJ, as a surrogate for clinical palpation, pressure-pain threshold (PPT) was measured at the medial and plantar aspects of the joint. A digital pressure algometer (Somedic AB, Farsta, Sweden) was used to measure the pressure applied at a rate of 40kPa/s by a rubber-tipped probe (area 1cm²)[40]. An average of three measurements from each site was used for analysis.

Participants were asked to walk along a 10 metre walkway, and ascend and descend a set of 10 stairs (17.5cm high and 26cm deep) as quickly as possible. Each test was completed barefoot (without shoes or socks). The tests were recorded in seconds and the fastest of three trials was used for analysis. Similar functional performance tests have been used with good reliability in previous research[8]. Postural sway was examined using a force plate (Model 4060–07, Bertec, USA) and six different standing conditions: both feet on a firm surface with eyes open and closed, both feet on high-density foam (0.10 kg/cm³; 15 cm thickness) with eyes open and closed, and single leg stance on a firm surface with eyes open (left and right). A 70 second trial was completed for each condition[41]. If the subject was unable to complete the trial, a minimum of 30 seconds was required for the trial to be included in analysis. Data was analysed using Matlab (version 7.9; MathWorks, Natick, USA), and variables analysed were range of centre of pressure (COP) in both mediolateral and anteroposterior directions. Hallux plantarflexion and abduction strength were evaluated using 50 kg load cells (GK 2126–50, Gedge Systems, Melbourne, Australia) mounted in a custom-built frame (Figure1). The load cells were calibrated prior to each measurement session, and the signal converted to force (N). Participants were seated with the knee in 30 degrees of flexion and the lower leg and foot stabilised using Velcro straps. After familiarisation with the required movements, participants were asked to perform three maximum isometric voluntary contractions in hallux plantarflexion and abduction. The examiner ensured that the subject’s heel remained in contact with the base of the frame and that there was minimal activity of lower leg muscles. The maximum force achieved over three trials was used for analysis.

Sample size was based on a priori power calculations. Using standard deviations obtained from preliminary data analysis (n = 26), we determined that 29 subjects in each group would provide 80% power to detect a difference of 11 mm between groups on the 100 mm pain VAS (alpha 0.05)[42, 43].

Age and BMI were first compared using independent t-tests to ensure that there were no significant differences between HV and control groups. For variables measured bilaterally (appearance VAS, hallux strength, PPT, and postural sway in single leg stance), data from only one limb was analysed[44]. For HV subjects the foot with a greater HV angle was chosen (16 right feet and 14 left feet used for analysis), while for control subjects the right or left foot was chosen at random using a random number generator (15 right feet and 15 left feet used for analysis). All variables were examined for normality of distribution using boxplots, histograms, and quantile-normal plots. Continuous variables showing a skewed distribution were transformed wherever possible using log, square root, or inverse square root transformations, as appropriate. Differences between groups were then examined using independent t-tests for continuous variables, Wilcoxon rank-sum tests for ordinal variables and continuous variables for which no adequate transformation was available, and Chi-squared statistics for categorical data. In the HV group (n = 30), the relationship between HV angle and other variables was investigated using Spearman’s correlation coefficient. Correlations are reported for variables showing statistically significant correlations with HV angle. Spearman’s rho was interpreted as follows: low correlation (rho = 0.26 to 0.49), moderate correlation (rho = 0.5 to 0.69) or high correlation (rho > 0.7)[45]. Results for continuous variables are presented as means, standard deviations (SD), mean differences (MD), and 95% confidence intervals (CI), while ordinal data are presented as medians (min – max). All analyses were conducted using STATA version 10[46], and the level of significance was set at p < 0.05.

Mean age of the total sample was 44.4 years (SD 15.1, range 20 to 76 years) and mean BMI was 24.5 kg/m² (SD 3.8, range 18.0 to 35.4 kg/m²), with HV and control groups matched on these characteristics. The mean absolute difference between matched subjects for age was 1.6 years and for BMI was 1.7kg/m². There were 25 women and five men in each group. Subject characteristics, as well as radiographic HV angle and presence of first MTPJ OA in each group are presented in Table1.

There were no significant differences between HV and control groups for SF-36v2® subscales, although the difference between groups on the vitality subscale (MD = −7.7, CI: -15.6 to 0.11) approached statistical significance (p = 0.05). HV participants reported significantly higher sporting activity (MD = 0.69, CI: 0.28 to 1.1) and physical activity levels at work (MD = 0.17, CI: 0.0 to 0.33), while the leisure index was not significantly different between groups (Table1).

Significant between-group differences were found for all measures of self-reported foot pain and disability, including FHSQ and FPDI subscales (Table2). Participants with HV reported more foot pain over the past four weeks on a 100mm VAS for worst pain (MD = 25.5 mm, CI: 14.3 to 36.6) and average pain (MD = 12.3 mm, CI: 6.2 to 18.3). PPT was lower at the medial first MTPJ in participants with HV (MD = −133.3 kPa, CI: -251.5 to −15.1), although there was no significant difference in PPT at the plantar first MTPJ.

Analysis of foot pain locations showed that 25 HV subjects (83%) reported pain in the first MTPJ or hallux, eight subjects (27%) reported pain in the lesser toes or MTPJs, and six subjects (20%) reported pain in the heel or midfoot area. There were 12 HV subjects who reported foot pain in more than one location, and only two who reported no foot pain. Control subjects reported the following distribution of foot pain: pain under the first MTPJ (one subject, 3%), pain in the lesser toes or MTPJs (nine subjects, 30%), and pain in the heel or midfoot (13 subjects, 43%). Three control subjects reported pain in more than one location, and nine reported no foot pain.

Participants with HV had significant concerns about foot appearance (VAS: MD = 38.1 mm, CI: 23.8 to 52.3) and more difficulty fitting footwear (FHSQ footwear score: MD = −47.5, CI: -60.0 to −34.9) than control subjects. On the FPDI item which states “I feel self-conscious about my feet”, 19 participants with HV (63%) responded “on some days” or “on most/every day”, compared to five participants (17%) in the control group (Chi-squared p = 0.001). Similarly, 13 HV participants (43%) responded positively to the statement “I get self conscious about the shoes I have to wear”, compared to one participant (3%) in the control group (Chi-squared p = 0.001). Fifteen participants in the HV group and 16 control participants reported a history of regularly wearing high heeled shoes (> 2 inches). Eight participants in each group reported wearing high heels “sometimes,” while high heels were worn “often” by four control participants and two HV participants, and “always” by one person with HV. Examination of footwear worn to the examination session showed no significant differences between groups in relative heel height or relative ball width measures (p > 0.05) (Table2).

Functional performance measures are presented in Table3. There were no significant differences in walking performance between groups (p > 0.05). Mediolateral sway while in double-leg stance on a firm surface with eyes closed was the only postural sway parameter that was different between groups, with a significant increase in mediolateral COP range in the HV group compared to controls (p = 0.03). In single leg stance, seven subjects (4 HV, 3 controls) were unable to complete the entire 70-second trial, and two of these trials (< 30 sec) were excluded from analysis. As shown in Figure2, the HV group had significantly weaker hallux plantarflexion strength (MD = −37.1 N, CI: -55.4 to −18.8) and hallux abduction strength (MD = −9.8 N, CI: -15.6 to −4.0) compared to controls.

A significant inverse correlation was found between greater HV angles and lower FHSQ scores indicating poorer general foot health (rho = −0.41, p = 0.03). Greater HV angles were correlated with higher appearance VAS scores indicating less satisfaction with appearance (rho = 0.43, p = 0.02). There was also a significant inverse correlation between greater HV severity and poorer hallux abduction strength (rho = −0.41, p = 0.03). Across the SF-36v2® subscales, low correlations were found between HV angle and bodily pain (rho = 0.39, p = 0.04) and social functioning (rho = 0.49, p = 0.01), indicating that in our sample, HV subjects with more severe deformity reported less bodily pain and better overall social functioning. No significant correlations were found for other SF-36v2® subscales, and HV angle was not correlated with footwear difficulty or any other measure of foot pain and function (i.e. FHSQ, FPDI, VAS, PPT).