Arthritis Care & Research Vol. 67, No. 1, January 2015, pp 80 – 88 DOI 10.1002/acr.22380 © 2015, American College of Rheumatology

SPECIAL THEME ARTICLE: MOBILITY AND THE RHEUMATIC DISEASES

Functional Impairments Characterizing Mild, Moderate, and Severe Hallux Valgus SHEREE E. HURN,1 BILL VICENZINO,2

AND

MICHELLE D. SMITH2

Objective. Hallux valgus (HV) has been linked to functional disability and increased risk of falls, but mechanisms underpinning functional disability are unclear. This study investigated functional performance, muscle strength, and plantar pressures in adults with mild, moderate, and severe HV compared to controls, while considering the influence of foot pain. Methods. Sixty adults with HV (classified as mild, moderate, and severe on dorsalplantar radiographs) and 30 controls participated. Measures included hallux plantar flexion and abduction strength, walking performance, postural sway, and forefoot plantar pressures. Multivariate analysis of covariance and pairwise comparisons (P < 0.05 after Bonferroni adjustment) were used to investigate differences between groups, adjusting for age, sex, body mass index, and foot pain. Results. Hallux plantar flexion and abduction strength were significantly reduced in those with moderate (mean differences: plantar flexion ⴚ45.8 N, abduction ⴚ12.3 N; P < 0.001) and severe HV (plantar flexion ⴚ50.1 N; P < 0.001, abduction ⴚ11.2 N; P ⴝ 0.01) compared to controls. A significant reduction in hallux peak pressure and pressure-time integral was evident in moderate (peak pressure ⴚ90.8 kPa; P < 0.001) and severe HV (peak pressure ⴚ106.2 kPa; P < 0.001) compared to controls. Those with severe HV also demonstrated increased mediolateral postural sway in single leg stance compared to controls (3.5 cm; P ⴝ 0.01). Conclusion. Moderate to severe HV is associated with reduced hallux plantar pressures and strength measures, while relatively normal function compared to controls was found in those with mild deformity. Greater understanding of specific functional deficits associated with different stages of HV will help inform clinical management and future research.

INTRODUCTION Hallux valgus (HV) is a common foot deformity (1) characterized by progressive lateral deviation of the hallux, first metatarsophalangeal (MTP) joint subluxation, and development of osteoarthritis (OA) (2). Increasing HV severity has been shown to negatively impact on health-related quality of life and self-reported function (3–5), and HV has been linked to increased falls risk in older adults (6,7). However, the underlying mechanisms linking altered forefoot structure in HV to functional disability are unclear.

Supported in part by an Australian Podiatry Education and Research Foundation (APERF) grant. Dr. Hurn’s work was supported by a Sir Robert Menzies Memorial Scholarship in the Allied Health Sciences. 1 Sheree E. Hurn, PhD: Queensland University of Technology, Brisbane, Queensland, Australia; 2Bill Vicenzino, PhD, Michelle D. Smith, PhD: The University of Queensland, Brisbane, Queensland, Australia. Address correspondence to Sheree E. Hurn, PhD, Queensland University of Technology, School of Clinical Sciences, Kelvin Grove, QLD 4059, Australia. E-mail: sheree.hurn@ qut.edu.au. Submitted for publication January 1, 2014; accepted in revised form June 3, 2014.

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In order to investigate potential mechanisms, it is important to increase our understanding of a range of physical parameters, including gait and muscle strength, that have been linked to HV in previous literature. It is plausible that altered forefoot structure in feet with HV may affect muscle function, which in turn may impact on gait parameters, specifically the smooth transfer of body weight required for efficient toe-off (8). Unfortunately, the findings of gait studies using plantar pressure analysis have been inconsistent, with some reporting increased (9 –11), decreased (12), or unchanged (13) pressures under the medial forefoot in HV compared to controls. None of these studies have accounted for different levels of HV severity, and only 2 have considered the presence of foot pain (11,14). Regarding muscle function, 2 previous studies have found reduced hallux plantar flexion strength in people with HV (7,15), but no prior work has quantitatively measured hallux abduction strength in adults with HV, although electromyography studies have observed imbalances of intrinsic foot muscles (16,17). If gait function and muscle strength are impaired in HV, then other functional performance measures, such as postural sway and walking speed, may be useful indicators of functional impairment. Functional performance measures

Impact of HV on Functional Parameters in Adults

Significance & Innovations ●

Adults with moderate to severe hallux valgus (HV) present with hallux plantar flexion and abduction weakness, which is significant after adjusting for foot pain.

●

Adults with severe HV display increased mediolateral postural sway in single leg stance compared to controls.

●

There appear to be no significant differences between adults with mild HV and controls in terms of muscle strength, plantar pressures, or postural sway, indicating that early intervention may help prevent these specific functional impairments.

have been used in prior work surrounding HV with inconsistent findings. Some studies have found walking performance and postural stability to be impaired (18 –21), while other authors have reported no difference in functional performance between those with HV and controls (10, 22–24). Most of these studies have not considered the impact of different levels of HV severity, which is an important consideration as structural deformity may not impact on function until it has progressed to a severe state (22). Foot pain has been shown to negatively affect functional performance (22,25), and the majority of studies to date have not considered the influence of foot pain on functional performance in HV. Therefore, it is unknown whether pain may have been a contributing factor in studies reporting poor functional performance in individuals with HV. Considering the importance of maintaining an active lifestyle in otherwise healthy individuals with HV, greater understanding of potential functional impairments associated with HV is needed. Evidence of the degree of hallux angulation associated with a decrease in function would assist clinicians in making decisions regarding when to recommend intervention for mild or moderate HV. Therefore, this study aimed to investigate balance, walking performance, muscle strength, and plantar pressures in adults with mild, moderate, and severe HV compared to controls, while considering foot pain, age, sex, and body mass index (BMI) as covariates.

MATERIALS AND METHODS Participants. Sixty adults with HV (7 men, 53 women) and 30 controls (5 men, 25 women) were recruited between March 2009 and December 2010 to participate in this study via community advertisements seeking volunteers age ⱖ20 years with and without HV. All potential participants (n ⫽ 192) were screened and excluded if they had any previous foot or ankle fractures or surgery, hallux limitus, any neurologic condition that may impair balance or gait, inflammatory disease, or a history of falls. Due to risks associated with exposure to ionizing radiation, potential participants were also excluded if pregnant or

81 breastfeeding. No study exclusion criteria were applied regarding foot pain or previous treatment. This study was approved by the institutional medical research ethics committee, and all participants gave written informed consent prior to participation. Measurement procedure. Participants attended on a single occasion and all measurements were obtained by the principal examiner. Intrarater reliability for these outcome measures was acceptable and has been reported previously (26,27). Participant characteristics. Demographic information for study participants was obtained via questionnaire. Participants’ general health status was investigated using the Short Form 36 health survey, version 2 (SF-36v2), general health domain transformed score; physical activity levels were assessed using the Baecke physical activity questionnaire (28); and self-reported foot pain and disability scores were obtained using the Manchester Foot Pain and Disability Index (FPDI) (29). The FPDI includes subscales for pain (score range 0 –10) and function (score range 0 –20), with higher scores indicating more pain or self-reported functional disability. HV assessment. 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, defined as the angle between the longitudinal axis of the first metatarsal and the longitudinal axis of the proximal phalanx of the hallux (30), was measured from digital radiographs by the principal examiner (a podiatrist) using computer software developed for telemedical applications (31). HV cases were defined as having an HV angle greater than 15° on one or both feet. To be eligible for the control group, participants were required to have a radiographic HV angle less than 15° on both feet. Dorsoplantar radiographs were also examined for presence of OA at the first MTP joint, with reference to a published radiographic atlas of foot OA, using a standard case definition of OA as “present” or “absent” (32,33). Foot posture and mobility. Foot posture was assessed using the Foot Posture Index (FPI-6) (34). Scores may range from ⫺12 to ⫹12, indicating supinated (negative) or pronated (positive) foot types. Dorsal arch height (DAH) and midfoot width (MFW) were measured in both weight bearing and non–weight bearing, using a protocol described previously (35). The calculated difference between weight bearing and non–weight bearing DAH and MFW provides an indication of foot mobility, and foot mobility magnitude (FMM) is a composite measure representing change in both DAH and MFW. Finally, the weight-bearing lunge test was used to assess dorsiflexion range of motion (ROM) at the ankle joint (36), and passive dorsiflexion ROM at the first MTP joint was assessed in non–weight bearing using a small goniometer (37). Hallux plantar flexion and abduction strength. Hallux plantar flexion and abduction strength was evaluated using 50 kg load cells (GK 2126-50, Gedge Systems) mounted in a custom-built frame. This testing protocol has been previously described (26). While seated with the knee in

82 30° of flexion and the lower leg and foot stabilized using Velcro straps, participants performed 3 isometric maximum voluntary contractions in hallux plantar flexion and abduction, and the maximum force achieved over 3 trials was used for analysis. Plantar pressures. The Pedar-X system (Novelgmbh) was used to capture in-shoe plantar pressure data. Participants were asked to bring a pair of sports shoes or walking shoes suitable for fitting the Pedar insoles. Participants walked at a self-selected comfortable speed along a 10-meter flat walkway, and 5 trials were completed. The first and last steps of each trial were removed, leaving an average of 23 steps across the 5 trials for analysis. Five forefoot regions were identified using a relative mask based on prior work by Putti et al (27): hallux, lesser toes, first metatarsal, second metatarsal, and third to fifth metatarsal heads. Peak pressures (kPa) and pressure-time integrals (kPa ⫻ seconds) were calculated. Timed walking. Participants were asked to walk as fast as they could along a 10-meter walkway, and to ascend and descend a set of 10 stairs (17.5 cm high, 26 cm deep) as quickly as possible. Each test was completed barefoot. Time to complete each task was recorded in seconds using a stopwatch, and the fastest of 3 trials was used for analysis. Postural sway. Postural sway was examined using a force plate (Model 4060-07, Bertec) in 6 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 70second trial was completed for each condition (38). If a participant was unable to complete the trial, this was noted during data collection, and trials less than 30 seconds were excluded from analysis. Force-plate data were processed in Matlab (version 7.9, MathWorks) to calculate mediolateral and anteroposterior center of pressure range (cm). Statistical methods. For variables measured separately on each leg (postural sway in single leg stance, hallux abduction and plantar flexion strength, and plantar pressures), only the right or left foot was used for analysis to maintain independence of observations (39). The foot with the greater HV angle was chosen for HV participants (n ⫽ 60; 28 right feet, 32 left feet), while for control participants (n ⫽ 30) the right or left foot was chosen at random using a random number generator (15 right feet, 15 left feet). HV participants were classified as mild, moderate, or severe in accordance with clinical convention (40). K-means cluster analysis was performed to partition observations of radiographic HV angle into 3 groups, which were defined as mild, moderate, and severe. This categorization of HV severity, compared to controls, was used as the grouping variable in multivariate analysis of covariance (MANCOVA), with functional outcomes entered as dependent variables in the following groups: hallux muscle strength, forefoot peak pressures, forefoot pressure-time integrals, walking performance, anteroposterior postural sway, and mediolateral postural sway. Assumptions of

Hurn et al MANCOVA were checked by examining univariate normality within groups via histograms and Shapiro-Wilk tests. Log transformations and removal of outliers were performed for skewed variables; however, as conclusions of hypothesis testing were not altered when transformed variables were used, interpretation was based on untransformed data. Levene’s test and Box’s test were used to assess univariate homogeneity of variances and multivariate homogeneity of covariance matrices, respectively. The following variables were considered as covariates: sex, age, BMI, and self-reported foot pain (FPDI pain subscale). Pairwise comparisons with a Bonferroni adjustment were used to examine differences between groups (P ⬍ 0.05). Finally, participant characteristics, foot posture, and mobility were compared across groups using the following tests: chi-square for categorical variables, analysis of variance for normally distributed continuous variables, and the nonparametric Kruskal-Wallis test for ordinal variables that could not be transformed into a normal distribution. Statistical analyses were conducted using Stata, version 10, and PASW Statistics 18, release 18.0.2 (IBM). Post hoc power calculations showed that a sample size of 90 gave 89% power to detect an effect size of F ⫽ 0.4 (␣ ⫽ 0.05). For the purpose of determining clinical significance, minimum detectable change at the 95% confidence level (MDC95) was calculated based on the standard error of measurement.

RESULTS Participant characteristics. The mean ⫾ SD age of participants in the total sample (n ⫽ 90) was 49.1 ⫾ 15.4 years, range 20 –76 years, and the mean BMI was 25.0 ⫾ 4.2 kg/m2, range 17.5–36.8 kg/m2. Three HV subgroups were identified by cluster analysis, with 21 participants categorized as having mild HV (mean ⫾ SD HV angle 21.1 ⫾ 3.0°), 25 participants categorized as moderate HV (mean ⫾ SD HV angle 30.8 ⫾ 2.3°), and 14 HV participants categorized as severe HV (mean ⫾ SD HV angle 39.9 ⫾ 5.4°). Participant characteristics, including number of males and females in each group, general health status, physical activity levels, presence of first MTP joint OA, and foot pain and disability scores are presented in Table 1. Those with mild HV had a significantly higher total activity index compared to controls (P ⫽ 0.028). There was a significantly higher frequency of first MTP joint OA in those with severe HV compared to controls (P ⫽ 0.004), and FPDI scores indicated that all HV groups (mild, moderate, and severe) reported significantly more foot pain and disability than controls (P ⬍ 0.001). Characteristics of foot posture and mobility are shown in Table 2. Participants with moderate and severe HV had significantly higher FPI-6 scores compared to controls, indicating a more pronated foot type (moderate HV versus control: mean difference [MD] 2.6 [P ⫽ 0.014]; severe HV versus control: MD 3.4 [P ⫽ 0.004]). Greater foot mobility, as determined using the FMM, was found in participants with moderate HV compared to controls (MD 3.0 mm; P ⫽ 0.035), but no difference was found in individuals with mild or severe HV compared to controls. Neither first MTP

Impact of HV on Functional Parameters in Adults

Table 1.

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Participant characteristics for each identified HV cluster compared to controls*

HV angle, ° Men:women, no. Age, years BMI, kg/m2 Physical activity (score range 3–15) SF-36 general health score First MTP joint OA, no. present FPDI scores, median (min.–max.) FPDI function (range 0–20) FPDI pain (range 0–10) FPDI total (range 0–34)

Control (n ⴝ 30)

Mild HV (n ⴝ 21)

Moderate HV (n ⴝ 25)

Severe HV (n ⴝ 14)

Total (n ⴝ 90)

9.8 ⫾ 3.5 5:25 44.2 ⫾ 15.3 24.7 ⫾ 4.3 7.6 ⫾ 1.1 78.8 ⫾ 13.1 2

21.1 ⫾ 3.0† 2:19 50.3 ⫾ 14.1 23.6 ⫾ 4.2 8.5 ⫾ 1.5† 79.2 ⫾ 17.0 1

30.8 ⫾ 2.3† 5:20 50.3 ⫾ 16.6 26.3 ⫾ 3.9 8.2 ⫾ 1.0 78.1 ⫾ 14.0 4

39.9 ⫾ 5.4† 0:14 55.4 ⫾ 13.8 25.2 ⫾ 4.6 7.8 ⫾ 1.0 78.9 ⫾ 14.4 6†

22.9 ⫾ 11.6 12:78 49.1 ⫾ 15.4 25.0 ⫾ 4.2 8.0 ⫾ 1.2 78.7 ⫾ 14.2 13

0 (0–5) 0 (0–2) 0 (0–9)

2 (0–10)‡ 3 (0–6)‡ 6 (0–18)‡

2 (0–11)‡ 3 (0–8)‡ 6 (1–23)‡

3 (0–14)‡ 3 (0–7)‡ 7 (0–24)‡

1 (0–14) 1 (0–8) 4 (0–24)

* Values are the mean ⫾ SD unless indicated otherwise. Interpretation of questionnaire scores is as follows: physical activity: higher scores indicate greater levels of habitual physical activity; general health: higher scores (closer to 100) indicate better general health; Manchester Foot Pain and Disability Index (FPDI): higher scores indicate more foot pain and greater self-reported functional disability. HV ⫽ hallux valgus; BMI ⫽ body mass index; SF-36 ⫽ Short Form 36 health survey, version 2; MTP ⫽ metatarsophalangeal; OA ⫽ osteoarthritis. † Significant difference compared to control group (P ⬍ 0.05, based on analysis of variance or Pearson’s chi-square test). ‡ Significant difference compared to control group (P ⬍ 0.001, based on Kruskal-Wallis rank test).

joint passive dorsiflexion nor ankle dorsiflexion showed any significant differences between groups (P ⬎ 0.05). Hallux plantar flexion and abduction strength. Multivariate analysis showed significant differences in hallux muscle strength between groups after adjusting for sex, age, BMI, and foot pain (MANCOVA F6, 164 ⫽ 4.55, P ⬍ 0.001) (Table 3). Results for pairwise comparisons of hallux flexion and abduction strength are shown in Table 4. Interestingly, there were no significant differences in muscle strength between those with mild HV and control participants (P ⬎ 0.05). However, compared to control participants, hallux muscle strength was significantly reduced in those with moderate HV (P ⬍ 0.001 for plantar flexion and abduction) and severe HV (P ⬍ 0.001 for plantar flexion, P ⫽ 0.012 for abduction). Furthermore, compared to the mild HV group, those with moderate HV demonstrated significant hallux abduction weakness (P ⫽ 0.043), and those with severe HV had significant hallux plantar flexion weakness (P ⫽ 0.044). Plantar pressures. Multivariate analysis showed significant differences between groups for both forefoot peak pressures (MANCOVA F15, 240 ⫽ 2.6, P ⫽ 0.002) and pressure-time integrals (MANCOVA F15, 240 ⫽ 2.8, P ⬍ 0.001) Table 2.

(Table 3). Pairwise comparisons revealed no significant differences in forefoot loading between participants with mild HV and controls (P ⬎ 0.05). However, those with moderate and severe HV showed significantly reduced hallux peak pressures (P ⬍ 0.001), and reduced hallux pressure-time integrals compared to controls (moderate HV versus controls: P ⫽ 0.003; severe HV versus controls: P ⫽ 0.001) (Table 4). Individuals with severe HV also had lower pressure-time integrals under the hallux (P ⫽ 0.033) and lateral forefoot (metatarsal heads 3–5) (P ⫽ 0.016) compared to those with mild HV. Timed walking. Multivariate analysis showed no significant differences between groups with regard to the 10meter walk or stair ascent and descent times (MANCOVA F6, 164 ⫽ 0.82, P ⫽ 0.554) (Table 5). Similarly, pairwise comparisons did not show any significant differences between groups (P ⬎ 0.05) (Table 6). Postural sway. Multivariate analysis demonstrated a significant difference in mediolateral postural sway across groups (MANCOVA F15, 240 ⫽ 2.0, P ⫽ 0.018). However, differences in anteroposterior sway did not reach statistical significance in the multivariate model (MANCOVA F15, 240 ⫽ 1.7, P ⫽ 0.050) (Table 5). As seen from the

Foot posture and mobility for each identified HV cluster compared to controls*

Foot posture and mobility Foot Posture Index (score range ⫺12 to 12) Foot mobility magnitude, mm Ankle dorsiflexion, cm First MTP joint dorsiflexion, °

Control (n ⴝ 30)

Mild HV (n ⴝ 21)

Moderate HV (n ⴝ 25)

Severe HV (n ⴝ 14)

ANOVA P

4.4 ⫾ 3.0

5.7 ⫾ 3.6

6.9 ⫾ 2.9†

7.8 ⫾ 1.9†

0.002

14.1 ⫾ 3.0 11.3 ⫾ 2.8 87.8 ⫾ 9.1

14.4 ⫾ 4.0 11.6 ⫾ 3.7 83.6 ⫾ 11.5

17.1 ⫾ 4.4† 11.5 ⫾ 2.2 83.1 ⫾ 10.0

15.7 ⫾ 4.3 11.5 ⫾ 3.6 85.1 ⫾ 12.5

0.030 0.995 0.363

* Values are the mean ⫾ SD unless indicated otherwise. HV ⫽ hallux valgus; ANOVA ⫽ analysis of variance; MTP ⫽ metatarsophalangeal. † Significant difference compared to control group (P ⬍ 0.05), based on ANOVA with Bonferroni correction.

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Table 3.

Estimated marginal means for muscle strength and plantar pressures across mild, moderate, and severe HV groups compared to controls*

Muscle strength, N Hallux plantar flexion Hallux abduction Peak pressure, kPa Hallux Lesser toes MH 1 MH 2 MH 3–5 Pressure-time integral, kPa ⫻ seconds Hallux Lesser toes MH 1 MH 2 MH 3–5

Control (n ⴝ 30)

Mild HV (n ⴝ 21)

Moderate HV Severe (n ⴝ 25) HV (n ⴝ 14)

100.4 ⫾ 6.5 18.7 ⫾ 2.0

80.1 ⫾ 7.0 14.3 ⫾ 2.1

54.6 ⫾ 6.5 6.4 ⫾ 2.0

241.5 ⫾ 14.6 161.5 ⫾ 10.5 231.6 ⫾ 13.1 231.1 ⫾ 11.0 189.9 ⫾ 7.9

192.4 ⫾ 15.7 161.2 ⫾ 11.3 190.5 ⫾ 14.1 215.1 ⫾ 11.8 184.9 ⫾ 8.5

150.6 ⫾ 14.6 162.8 ⫾ 10.5 190.1 ⫾ 13.1 216.0 ⫾ 11.0 189.5 ⫾ 7.9

48.1 ⫾ 3.4 34.4 ⫾ 2.6 56.7 ⫾ 3.6 60.6 ⫾ 3.3 53.8 ⫾ 2.6

39.7 ⫾ 3.6 33.1 ⫾ 2.8 48.1 ⫾ 3.8 59.0 ⫾ 3.6 58.7 ⫾ 2.8

29.8 ⫾ 3.4 33.6 ⫾ 2.6 44.2 ⫾ 3.6 54.8 ⫾ 3.3 50.5 ⫾ 2.6

50.2 ⫾ 8.8 7.5 ⫾ 2.6

P† ⬍ 0.001 ⬍ 0.001

135.3 ⫾ 19.7 ⬍ 0.001 153.3 ⫾ 14.2 0.955 223.1 ⫾ 17.7 0.073 210.3 ⫾ 14.8 0.698 163.6 ⫾ 10.7 0.214 23.7 ⫾ 4.5 28.8 ⫾ 3.5 53.5 ⫾ 4.8 51.2 ⫾ 4.5 45.0 ⫾ 3.5

⬍ 0.001 0.628 0.092 0.355 0.020

* Values are the mean ⫾ SEM unless indicated otherwise. Multiple analysis of covariance, adjusted for age, sex, body mass index, and foot pain (Manchester Foot Pain and Disability Index pain subscale). HV ⫽ hallux valgus; MH ⫽ metatarsal head. † Univariate significance of HV cluster.

pairwise comparisons presented in Table 6, participants with severe HV had significantly increased mediolateral sway in single leg stance compared to controls (P ⫽ 0.013). Surprisingly, individuals with mild HV displayed greater mediolateral sway compared to the moderate HV group when standing on foam with eyes open (P ⫽ 0.025). For all statistically significant results, the mean differences revealed by pairwise comparisons exceeded the MDC95, which indicates that the difference is likely to be Table 4.

clinically significant when measurement error is considered.

DISCUSSION This study shows that moderate to severe HV is associated with significantly reduced hallux plantar flexion and abduction strength and hallux plantar pressures after adjust-

Pairwise comparisons for muscle strength and plantar pressures across mild, moderate, and severe HV groups compared to controls* Mild vs. control

Moderate vs. control

Severe vs. control

Hallux strength measures, N Plantarflexion ⫺20.3 (⫺47.3, 6.7) ⫺45.8 (⫺72.0, ⫺19.6)† ⫺50.1 (⫺81.6, ⫺18.7)† Abduction ⫺4.4 (⫺12.5, 3.8) ⫺12.3 (⫺20.2, ⫺4.4)† ⫺11.2 (⫺20.6, ⫺1.7)† Peak pressure, kPa Hallux ⫺49.1 ⫺90.8 ⫺106.2 (⫺109.7, 11.6) (⫺149.6, ⫺32.1)† (⫺176.9, ⫺35.5)† Lesser toes ⫺0.36 (⫺44.1, 43.4) 1.3 (⫺41.1, 43.6) ⫺8.2 (⫺59.2, 42.7) MH 1 ⫺41.1 (⫺95.7, 13.4) ⫺41.5 (⫺94.3, 11.3) ⫺8.5 (⫺72.0, 55.0) MH 2 ⫺16.0 (⫺61.7, 29.7) ⫺15.1 (⫺59.4, 29.1) ⫺20.9 (⫺74.1, 32.4) MH 3–5 ⫺5.0 (⫺38.0, 28.0) ⫺0.36 (⫺32.3, 31.6) ⫺26.3 (⫺64.7, 12.1) Pressure-time integral, kPa ⫻ seconds Hallux ⫺8.4 (⫺22.4, 5.6) ⫺18.3 (⫺31.9, ⫺4.8)† ⫺24.4 (⫺40.7, ⫺8.2)† Lesser toes ⫺1.2 (⫺11.9, 9.4) ⫺0.75 (⫺11.1, 9.6) ⫺5.6 (⫺18.0, 6.8) MH 1 ⫺8.6 (⫺23.4, 6.2) ⫺12.5 (⫺26.8, 1.8) ⫺3.2 (⫺20.4, 14.0) MH 2 ⫺1.6 (⫺15.4, 12.3) ⫺5.8 (⫺19.2, 7.6) ⫺9.4 (⫺25.5, 6.7) MH 3–5 4.9 (⫺6.1, 15.8) ⫺3.3 (⫺13.9, 7.3) ⫺8.8 (⫺21.5, 4.0)

Moderate vs. mild

Severe vs. mild

⫺25.5 (⫺51.3, 0.3) ⫺29.8 (⫺59.2, ⫺0.5)† ⫺7.9 (⫺15.7, ⫺0.2)† ⫺6.8 (⫺15.6, 2.1) ⫺41.8 (⫺99.7, 16.2) 1.6 (⫺40.1, 43.4) ⫺0.37 (⫺52.4, 51.7) 0.88 (⫺42.8, 44.5) 4.7 (⫺26.8, 36.1)

⫺57.1 (⫺123.0, 8.8) ⫺7.9 (⫺55.3, 39.6) 32.6 (⫺26.6, 91.8) ⫺4.8 (⫺54.5, 44.8) ⫺21.3 (⫺57.1, 14.5)

⫺9.9 (⫺23.3, 3.4) ⫺16.0 (⫺31.2, ⫺0.9)† 0.48 (⫺9.7, 10.7) ⫺4.4 (⫺15.9, 7.2) ⫺3.9 (⫺18.0, 10.2) 5.4 (⫺10.7, 21.4) ⫺4.2 (⫺17.4, 9.0) ⫺7.8 (⫺22.9, 7.2) ⫺8.2 (⫺18.6, 2.3) ⫺13.7 (⫺25.5, ⫺1.8)†

* Values are the mean difference (95% confidence interval). HV ⫽ hallux valgus; MH ⫽ metatarsal head. † Statistically significant (P ⬍ 0.05).

Impact of HV on Functional Parameters in Adults

Table 5.

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Estimated marginal means for walking performance and postural sway across mild, moderate, and severe HV groups compared to controls*

Walking tests, seconds Stair descent Timed 10-meter walk Anteroposterior sway (range, cm) Both feet, eyes open Both feet, eyes closed Foam, eyes open Foam, eyes closed Single leg stance Mediolateral sway (range, cm) Both feet, eyes open Both feet, eyes closed Foam, eyes open Foam, eyes closed Single leg stance

Control (n ⴝ 30)

Mild HV (n ⴝ 21)

Moderate HV (n ⴝ 25)

Severe HV (n ⴝ 14)

P†

3.6 ⫾ 0.1 5.1 ⫾ 0.1

3.9 ⫾ 0.1 5.2 ⫾ 0.1

3.9 ⫾ 0.1 5.1 ⫾ 0.1

3.8 ⫾ 0.2 5.1 ⫾ 0.2

0.298 0.942

1.9 ⫾ 0.2 2.2 ⫾ 0.2 3.0 ⫾ 0.2 5.3 ⫾ 0.5 2.9 ⫾ 0.2

1.8 ⫾ 0.2 2.3 ⫾ 0.2 3.4 ⫾ 0.2 5.6 ⫾ 0.5 3.7 ⫾ 0.3

1.8 ⫾ 0.2 1.8 ⫾ 0.2 2.5 ⫾ 0.2 4.7 ⫾ 0.5 3.3 ⫾ 0.2

2.2 ⫾ 0.2 2.1 ⫾ 0.3 3.5 ⫾ 0.3 6.5 ⫾ 0.6 3.8 ⫾ 0.3

0.368 0.405 0.018 0.126 0.105

1.4 ⫾ 0.1 1.5 ⫾ 0.2 3.8 ⫾ 0.3 6.9 ⫾ 0.5 4.3 ⫾ 0.6

1.7 ⫾ 0.1 2.1 ⫾ 0.2 4.4 ⫾ 0.3 8.0 ⫾ 0.5 6.3 ⫾ 0.7

1.4 ⫾ 0.1 1.7 ⫾ 0.2 3.2 ⫾ 0.3 6.5 ⫾ 0.5 6.3 ⫾ 0.6

1.7 ⫾ 0.2 2.1 ⫾ 0.2 4.5 ⫾ 0.4 8.8 ⫾ 0.6 7.9 ⫾ 0.8

0.351 0.118 0.011 0.019 0.020

* Values are the mean ⫾ SEM unless indicated otherwise. Multiple analysis of covariance, adjusted for age, sex, body mass index and foot pain (Manchester Foot Pain and Disability Index pain subscale). HV ⫽ hallux valgus. † Univariate significance of HV cluster.

ing for foot pain, age, sex, and BMI. Individuals with severe HV also displayed increased mediolateral postural sway in single leg stance compared to controls. No significant differences were found between those with mild HV and controls in terms of muscle strength, plantar pressures, or postural sway. Furthermore, no significant differences were found across groups regarding the time taken to walk 10 meters and ascend/descend 10 stairs. These findings are novel as few studies to date (11,14,20) have defined different levels of HV severity or considered the influence of foot pain when investigating such outcomes in HV. Table 6.

Our finding of hallux plantar flexion weakness is consistent with previous studies (7,15). Sanders et al (15) reported an inverse relationship between HV angle and hallux plantar flexion strength (Spearman’s rho ⫽ ⫺0.42, P ⫽ 0.03), and lower mean hallux plantar flexion strength in those with painful HV (42 N, range 10 – 80) compared to those with HV without complaints (66 N, range 33–124). While pain may have an effect on motor output, our results demonstrate that increasing hallux deviation is associated with reduced plantar flexion strength after adjusting for pain. Since abductor hallucis is an important dynamic arch stabilizer (41), our study further examined hallux

Pairwise comparisons for walking performance and postural sway across mild, moderate, and severe HV groups compared to controls* Mild vs. control

Walking tests, seconds Stair descent Timed 10-meter walk Anteroposterior sway (range, cm) Both feet, eyes open Both feet, eyes closed Foam, eyes open Foam, eyes closed Single leg stance Mediolateral sway (range, cm) Both feet, eyes open Both feet, eyes closed Foam, eyes open Foam, eyes closed Single leg stance

Moderate vs. control

Severe vs. control

Moderate vs. mild

Severe vs. mild

0.35 (⫺0.2, 0.9) 0.12 (⫺0.4, 0.7)

0.30 (⫺0.2, 0.8) 0.04 (⫺0.5, 0.6)

0.25 (⫺0.4, 0.9) 0.02 (⫺0.6, 0.7)

⫺0.05 (⫺0.6, 0.5) ⫺0.07 (⫺0.6, 0.5)

⫺0.10 (⫺0.7, 0.5) ⫺0.10 (⫺0.7, 0.5)

⫺0.06 (⫺0.7, 0.6) 0.15 (⫺0.7, 1.0) 0.41 (⫺0.5, 1.3) 0.25 (⫺1.7, 2.2) 0.79 (⫺0.2, 1.8)

⫺0.10 (⫺0.7, 0.5) ⫺0.36 (⫺1.2, 0.5) ⫺0.43 (⫺1.3, 0.4) ⫺0.66 (⫺2.6, 1.2) 0.40 (⫺0.6, 1.4)

0.32 (⫺0.4, 1.1) ⫺0.06 (⫺1.1, 0.9) 0.56 (⫺0.5, 1.6) 1.2 (⫺1.1, 3.5) 0.93 (⫺0.2, 2.1)

⫺0.04 (⫺0.6, 0.6) ⫺0.51 (⫺1.3, 0.3) ⫺0.84 (⫺1.7, 0.0) ⫺0.92 (⫺2.8, 0.9) ⫺0.39 (⫺1.3, 0.6)

0.38 (⫺0.3, 1.1) ⫺0.21 (⫺1.1, 0.7) 0.15 (⫺0.8, 1.1) 0.95 (⫺1.2, 3.1) 0.14 (⫺0.9, 1.2)

0.28 (⫺0.2, 0.8) 0.62 (⫺0.1, 1.4) 0.63 (⫺0.6, 1.8) 1.1 (⫺0.9, 3.0) 2.0 (⫺0.6, 4.6)

0.05 (⫺0.4, 0.5) 0.18 (⫺0.6, 0.9) ⫺0.64 (⫺1.8, 0.5) ⫺0.46 (⫺2.4, 1.5) 1.9 (⫺0.6, 4.5)

0.27 (⫺0.3, 0.9) 0.57 (⫺0.3, 1.5) 0.74 (⫺0.7, 2.2) 1.9 (⫺0.4, 4.2) 3.5 (0.5, 6.6)†

⫺0.23 (⫺0.7, 0.3) ⫺0.44 (⫺1.2, 0.3) ⫺1.3 (⫺2.4, ⫺0.1)† ⫺1.5 (⫺3.4, 0.4) ⫺0.02 (⫺2.5, 2.5)

⫺0.01 (⫺0.5, 0.5) ⫺0.06 (⫺0.9, 0.8) 0.11 (⫺1.2, 1.4) 0.84 (⫺1.3, 3.0) 1.6 (⫺1.2, 4.4)

* Values are the mean difference (95% confidence interval). HV ⫽ hallux valgus. † Statistically significant (P ⬍ 0.05).

86 abduction strength and found that hallux abduction weakness closely mirrored hallux plantar flexion weakness in HV. Interestingly, plantar flexion and abduction strength were both significantly reduced in those with moderate to severe HV, while these measures were similar between those with mild HV and controls. This may suggest that hallux muscle weakness develops secondarily to an abducted hallux position, with lateral displacement of flexor tendons, and loss of mechanical advantage of abductor hallucis. Plantar pressure studies have been an area of inconsistency in HV research. From our results it is evident that plantar loading patterns differ between mild, moderate, and severe HV, and therefore a simple case– control comparison may not be adequate to investigate plantar pressures in this population. Two previous studies have shown an inverse relationship between HV angle and hallux loading (42,43), with greater HV severity associated with reduced hallux loading, and another study found reduced loading under the hallux in HV patients (n ⫽ 229) compared to controls (n ⫽ 35) (11). These reports are consistent with the findings from our study. A number of studies have found higher loading under the first and second metatarsal heads associated with HV (9 –11,13); however, differences in medial forefoot pressures between groups did not reach statistical significance in our study after adjusting for foot pain. It is plausible that the presence of first MTP joint pain may have a greater impact on medial forefoot pressures during gait than lateral deviation of the hallux alone. Our study found that mediolateral sway in single leg stance was increased in individuals with severe HV compared to controls. While Tanaka et al (44) demonstrated that the toe plantarflexors are important contributors to postural control, a more recent study has shown that intrinsic foot muscles such as abductor hallucis are particularly important in controlling mediolateral sway (45). This may explain why mediolateral sway is particularly affected in individuals with HV. Previous studies have reported inconsistent findings regarding postural control in older adults with HV (10,18,21). Menz et al (20) found a significant association between overall HV severity score and balance performance. Our study is the first to distinguish between moderate and severe HV, and no previous studies have considered foot pain as a covariate when investigating postural sway. The potential clinical significance of these findings relates to falls risk in older adults. Hallux plantar flexion weakness has been found to be a risk factor for falls (6,7), and has also been linked to reduced hallux peak pressures during gait (43) and increased postural sway (20). The combination of these 3 parameters worsening with increasing HV deformity is an interesting finding. However, clinical significance should be interpreted in light of the mean differences and confidence intervals presented in Tables 4 and 6, as well as previous literature. For example, Mickle et al (7) reported that hallux plantar flexion strength was reduced by approximately 4% body weight in older adults who fell compared to nonfallers. In our sample that difference would equate to approximately 27 N. The mean differences in hallux plantar flexion strength

Hurn et al (moderate versus control, and severe versus control) (Table 4) do exceed that value, although the upper limit of both confidence intervals is ⬍20 N. Furthermore, while the mean difference for mediolateral postural sway in single leg stance (severe HV versus control, MD ⫽ 3.5 cm) exceeded our calculated minimal detectable change (1.7 cm), the lower limit of the confidence interval is approaching zero, therefore a larger sample of adults with severe HV could be investigated to verify this association. We did not find statistically significant differences in timed short distance walking tests and stair ascent/descent across groups. This is consistent with previous work in older adults (18,22–24). However, Menz et al (20) reported an association between slower walking speed and increasing HV severity in older adults, and in another study (46) these authors found reduced gait velocity and stride length in older adults with moderate to severe HV when walking on irregular surfaces. While simple walking performance tasks may not be affected by HV in healthy adults of all ages, future studies could investigate more challenging walking performance tasks or indicators of efficiency, such as oxygen consumption. We considered foot posture and mobility as participant characteristics in this study. In our sample, those with moderate to severe HV had significantly more pronated feet (FPI-6), and those with moderate HV had increased foot mobility compared to controls (FMM) (Table 2). These measures did not differ between the mild HV group and controls. The foot posture characteristics of our sample should be considered when interpreting our postural sway data. Tsai et al (47) found that postural sway was increased in participants with supinated and pronated foot types compared to neutral foot types. Since FPI scores (mean ⫾ SD 7.8 ⫾ 1.9) in those with severe HV indicated a pronated foot type, this may have contributed to the increased mediolateral postural sway in this group. However, participants with moderate HV also exhibited a pronated foot type (mean ⫾ SD FPI 6.9 ⫾ 2.9), but their postural sway values were similar to controls. One strength of our study was that we accounted for foot pain in our analysis. Previous studies have shown that foot pain has an adverse impact on function (22,25) and may be associated with higher peak plantar pressures (48) and reduced hallux plantar flexion strength (25). Differences across groups remained significant after adjusting for foot pain and other covariates (age, sex, and BMI). Therefore, these factors did not account for differences in muscle strength, hallux plantar pressure, and postural sway observed in individuals with moderate to severe HV. It is interesting to note that 2 individuals in the control group showed signs of OA on dorsoplantar radiographs. While the presence of OA could potentially have affected functional measures such as muscle strength or plantar pressures, these 2 individuals were asymptomatic and therefore the impact on study findings is not likely to be substantial. There are some methodologic considerations in our study that warrant discussion. First, this study was crosssectional in design and therefore causal relationships cannot be inferred from our results. Second, plantar pressure analysis has limitations depending on the equipment used

Impact of HV on Functional Parameters in Adults (49). The Pedar-X insole system comprises 99 capacitive sensors; however, masking to identify forefoot regions is somewhat constrained by the number and positioning of these sensors. A standardized relative mask was used for all participants, consistent with previous studies using Pedar insoles (27). Forefoot morphology is clearly altered in individuals with HV and lateral deviation of the hallux may have resulted in activation of pressure sensors more laterally, therefore explaining reduced pressure under the hallux in individuals with more severe HV. While participants were asked to adopt a comfortable speed to ensure a natural gait pattern, it should be noted that peak pressures may increase with greater walking velocity and this was not accounted for in our study. The influence of footwear on in-shoe pressure analysis and the lack of standardized footwear in our study should also be acknowledged. Third, volunteers with a wide range of ages and degrees of HV severity responded to community advertisements to participate in this study. This sample was considered representative of a clinical population with HV. Some participants had received some previous treatment for HV (n ⫽ 25), and although this was noted by the examiner it was not taken into account in our analysis. Participants who volunteered had relatively high general health (SF-36v2) and habitual levels of physical activity, and all individuals were able to participate in functional tasks. These study results may not be generalizable to older populations with higher levels of physical disability, or populations with concomitant systemic conditions such as rheumatoid arthritis. Finally, while we considered physical activity levels, this variable was not adjusted for as a covariate. The higher habitual physical activity levels observed in the group with mild HV may have contributed to improved performance on some of the functional measures. In conclusion, moderate to severe HV significantly impacts on specific functional parameters. Since there were no differences noted between adults with mild HV and controls, early intervention could target prevention of specific functional deficits, such as poor balance and hallux plantar flexion weakness. Future studies in HV should include foot pain as a concomitant factor likely to influence functional performance. Further research is warranted to investigate whether hallux plantar flexion, and abduction strength and postural stability could be improved by nonsurgical interventions, such as muscle strengthening and balance training in individuals with HV.

AUTHOR CONTRIBUTIONS All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be submitted for publication. Dr. Hurn had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study conception and design. Hurn, Vicenzino, Smith. Acquisition of data. Hurn. Analysis and interpretation of data. Hurn, Vicenzino, Smith.

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REFERENCES 1. Nix S, Smith M, Vicenzino B. Prevalence of hallux valgus in the general population: a systematic review and meta-analysis. J Foot Ankle Res 2010;3:21. 2. D’Arcangelo P, Landorf KB, Munteanu SE, Zammit GV, Menz HB. Radiographic correlates of hallux valgus severity in older people. J Foot Ankle Res 2010;3:20. 3. Menz HB, Roddy E, Thomas E, Croft PR. Impact of hallux valgus severity on general and foot-specific health-related quality of life. Arthritis Care Res (Hoboken) 2011;63:396 – 404. 4. Cho NH, Kim S, Kwon DJ, Kim HA. The prevalence of hallux valgus and its association with foot pain and function in a rural Korean community. J Bone Joint Surg Br 2009;91:494 – 8. 5. Abhishek A, Roddy E, Zhang W, Doherty M. Are hallux valgus and big toe pain associated with impaired quality of life? A cross-sectional study. Osteoarthritis Cartilage 2010;18:923– 6. 6. Menz HB, Morris ME, Lord SR. Foot and ankle risk factors for falls in older people: a prospective study. J Gerontol A Biol Sci Med Sci 2006;61:866 –70. 7. Mickle KJ, Munro BJ, Lord SR, Menz HB, Steele JR. ISB Clinical Biomechanics Award 2009: toe weakness and deformity increase the risk of falls in older people. Clin Biomech (Bristol, Avon) 2009;24:787–91. 8. Nix SE, Vicenzino BT, Collins NJ, Smith MD. Gait parameters associated with hallux valgus: a systematic review. J Foot Ankle Res 2013;6:9. 9. Bryant A, Tinley P, Singer K. Radiographic measurements and plantar pressure distribution in normal, hallux valgus and hallux limitus feet. Foot 2000;10:18 –22. 10. Mickle KJ, Munro BJ, Lord SR, Menz HB, Steele JR. Gait, balance and plantar pressures in older people with toe deformities. Gait Posture 2011;34:347–51. 11. Wen J, Ding Q, Yu Z, Sun W, Wang Q, Wei K. Adaptive changes of foot pressure in hallux valgus patients. Gait Posture 2012;36:344 –9. 12. Kadono K, Tanaka Y, Sakamoto T, Akiyama K, Komeda T, Taniguchi A, et al. Plantar pressure distribution under the forefeet with hallux valgus during walking. J Nara Med Assoc 2003;54:273– 81. 13. Martinez-Nova A, Sanchez-Rodriguez R, Perez-Soriano P, Llana-Belloch S, Leal-Muro A, Pedrera-Zamorano JD. Plantar pressures determinants in mild hallux valgus. Gait Posture 2010;32:425–7. 14. Martinez-Nova A, Sanchez-Rodriguez R, Leal-Muro A, Pedrera-Zamorano JD. Dynamic plantar pressure analysis and midterm outcomes in percutaneous correction for mild hallux valgus. J Orthop Res 2011;29:1700 – 6. 15. Sanders AP, Snijders CJ, van Linge B. Medial deviation of the first metatarsal head as a result of flexion forces in hallux valgus. Foot Ankle 1992;13:515–22. 16. Iida M, Basmajian JV. Electromyography of hallux valgus. Clin Orthop Relat Res 1974;101:220 – 4. 17. Incel AN, Genc H, Erdem HR, Yorgancioglu ZR. Muscle imbalance in hallux valgus: an electromyographic study. Am J Phys Med Rehabil 2003;82:345–9. 18. Spink MJ, Fotoohabadi MR, Wee E, Hill KD, Lord SR, Menz HB. Foot and ankle strength, range of motion, posture, and deformity are associated with balance and functional ability in older adults. Arch Phys Med Rehabil 2011;92:68 –75. 19. Menz HB, Lord SR. The contribution of foot problems to mobility impairment and falls in community-dwelling older people. J Am Geriatr Soc 2001;49:1651– 6. 20. Menz HB, Morris ME, Lord SR. Foot and ankle characteristics associated with impaired balance and functional ability in older people. J Gerontol A Biol Sci Med Sci 2005;60:1546 –52. 21. Menz HB, Lord SR. Foot pain impairs balance and functional ability in community-dwelling older people. J Am Podiatr Med Assoc 2001;91:222–9. 22. Keysor JJ, Dunn JE, Link CL, Badlissi F, Felson DT. Are foot disorders associated with functional limitation and disability among community-dwelling older adults? J Aging Health 2005;17:734 –52. 23. Badlissi F, Dunn JE, Link CL, Keysor JJ, McKinlay JB, Felson

88

24. 25.

26. 27. 28. 29. 30.

31. 32.

33.

34.

35.

Hurn et al DT. Foot musculoskeletal disorders, pain, and foot-related functional limitation in older persons. J Am Geriatr Soc 2005; 53:1029 –33. Chaiwanichsiri D, Janchai S, Tantisiriwat N. Foot disorders and falls in older persons. Gerontology 2009;55:296 –302. Mickle KJ, Munro BJ, Lord SR, Menz HB, Steele JR. Crosssectional analysis of foot function, functional ability, and health-related quality of life in older people with disabling foot pain. Arthritis Care Res (Hoboken) 2011;63:1592– 8. Nix S, Vicenzino BT, Smith MD. Foot pain and functional limitation in healthy adults with hallux valgus: a cross-sectional study. BMC Musculoskelet Disord 2012;13:197. Putti AB, Arnold GP, Cochrane L, Abboud RJ. The Pedar in-shoe system: repeatability and normal pressure values. Gait Posture 2007;25:401–5. Baecke JA, Burema J, Frijters JE. A short questionnaire for the measurement of habitual physical activity in epidemiological studies. Am J Clin Nutr 1982;36:936 – 42. Garrow AP, Papageorgiou AC, Silman AJ, Thomas E, Jayson MI, Macfarlane GJ. Development and validation of a questionnaire to assess disabling foot pain. Pain 2000;85:107–13. Coughlin MJ, Saltzman CL, Nunley JA. Angular measurements in the evaluation of hallux valgus deformities: a report of the ad hoc committee of the American Orthopaedic Foot & Ankle Society on angular measurements. Foot Ankle Int 2002; 23:68 –74. Russell T, Jull G, Wootton R. Can the internet be used as a medium to evaluate knee angle? Man Ther 2003;8:242– 6. Menz HB, Munteanu SE, Landorf KB, Zammit GV, Cicuttini FM. Radiographic classification of osteoarthritis in commonly affected joints of the foot. Osteoarthritis Cartilage 2007;15: 1333– 8. Menz HB, Munteanu SE, Landorf KB, Zammit GV, Cicuttini FM. Radiographic evaluation of foot osteoarthritis: sensitivity of radiographic variables and relationship to symptoms. Osteoarthritis Cartilage 2009;17:298 –303. Redmond AC, Crosbie J, Ouvrier RA. Development and validation of a novel rating system for scoring standing foot posture: the Foot Posture Index. Clin Biomech (Bristol, Avon) 2006;21:89 –98. McPoil TG, Vicenzino B, Cornwall MW, Collins N, Warren M. Reliability and normative values for the foot mobility magnitude: a composite measure of vertical and medial-lateral mobility of the midfoot. J Foot Ankle Res 2009;2:6.

36. Bennell KL, Talbot RC, Wajswelner H, Technovanich W, Kelly DH, Hall AJ. Intra-rater and inter-rater reliability of a weight-bearing lunge measure of ankle dorsiflexion. Aust J Physiother 1998;44:175– 80. 37. Hopson MM, McPoil TG, Cornwall MW. Motion of the first metatarsophalangeal joint: reliability and validity of four measurement techniques. J Am Podiatr Med Assoc 1995;85: 198 –204. 38. Carpenter MG, Frank JS, Winter DA, Peysar GW. Sampling duration effects on centre of pressure summary measures. Gait Posture 2001;13:35– 40. 39. Menz HB. Two feet, or one person? Problems associated with statistical analysis of paired data in foot and ankle medicine. Foot 2004;14:2–5. 40. Garrow AP, Papageorgiou A, Silman AJ, Thomas E, Jayson MI, Macfarlane GJ. The grading of hallux valgus: the Manchester Scale. J Am Podiatr Med Assoc 2001;91:74 – 8. 41. Wong YS. Influence of the abductor hallucis muscle on the medial arch of the foot: a kinematic and anatomical cadaver study. Foot Ankle Int 2007;28:617–20. 42. Mueller MJ, Hastings M, Commean PK, Smith KE, Pilgram TK, Robertson D, et al. Forefoot structural predictors of plantar pressures during walking in people with diabetes and peripheral neuropathy. J Biomech 2003;36:1009 –17. 43. Menz HB, Morris ME. Clinical determinants of plantar forces and pressures during walking in older people. Gait Posture 2006;24:229 –36. 44. Tanaka T, Noriyasu S, Ino S, Ifukube T, Nakata M. Objective method to determine the contribution of the great toe to standing balance and preliminary observations of age-related effects. IEEE Trans Rehabil Eng 1996;4:84 –90. 45. Kelly LA, Kuitunen S, Racinais S, Cresswell AG. Recruitment of the plantar intrinsic foot muscles with increasing postural demand. Clin Biomech (Bristol, Avon) 2012;27:46 –51. 46. Menz HB, Lord SR. Gait instability in older people with hallux valgus. Foot Ankle Int 2005;26:483–9. 47. Tsai LC, Yu B, Mercer VS, Gross MT. Comparison of different structural foot types for measures of standing postural control. J Orthop Sports Phys Ther 2006;36:942–53. 48. Mickle KJ, Munro BJ, Lord SR, Menz HB, Steele JR. Foot pain, plantar pressures, and falls in older people: a prospective study. J Am Geriatr Soc 2010;58:1936 – 40. 49. Urry S. Plantar pressure-measurement sensors. Meas Sci Technol 1999;10:R16 –32.