The influence of foot orthoses on foot mobility magnitude and arch height index in adults with flexible flat feet

Abstract

Background:

Flexible flat foot is described as a reduction in the height of the medial longitudinal arch and may occur from abnormal foot pronation. A foot orthosis is thought to modify and control excessive pronation and improve arch height.

Objective:

To compare the immediate effect of three types of orthoses on foot mobility and the arch height index in subjects with flexible flat feet.

Study design:

A quasi-experimental study.

Method:

The dorsal arch height, midfoot width, foot mobility and arch height index were assessed in 20 participants with flexible flat feet (mean age = 23.2 ± 3 years) for three different foot orthosis conditions: soft, semi-rigid and rigid University of California Biomechanics Laboratory (UCBL).

Results:

Maximum midfoot width at 90% with arch mobility in the coronal plane was shown in the semi-rigid orthosis condition. The semi-rigid orthosis resulted in the highest mean foot mobility in 90% of weight bearing, and the rigid orthosis (UCBL) had the lowest mean foot mobility. The soft orthosis resulted in foot mobility between that of the rigid and the semi-rigid orthosis. UCBL orthosis showed the highest arch height index, and the semi-rigid orthosis showed the lowest mean arch height index.

Conclusion:

Due to its rigid structure and long medial–lateral walls, the UCBL orthosis appears to limit foot mobility. Therefore, it is necessary to make an orthosis that facilitates foot mobility in the normal range of the foot arch. Future studies should address the dynamic mobility of the foot with using various types of foot orthoses.

Clinical relevance

Although there are many studies focussed on flat foot and the use of foot orthoses, the mechanism of action is still unclear. This study explored foot mobility and the influence of foot orthoses and showed that a more rigid foot orthosis should be selected based on foot mobility.

Keywords

Foot orthosis foot mobility magnitude arch height index

Background

The foot complex includes not only the medial and lateral longitudinal arches but also the transverse arch.^1,2 The medial longitudinal arch plays a significant role in shock absorption and energy transfer during walking.^1,2 Increasing or decreasing the height of the medial longitudinal arch may compromise foot function and stability as well as alter the mechanical alignment of the lower limb.¹ During weight-bearing conditions, a flexible flat foot presents with a reduction in the height of the medial longitudinal arch. This in turn is linked with abnormal foot pronation, whereby the calcaneus is in a valgus position relative to the talus, and the navicular is dorsally subluxed to the talus.^3,4

Despite individual differences, biomechanics of the foot are considerably influenced by the form and shape medial longitudinal arch.⁵ Prescribing foot orthoses for flat foot aims to optimise the natural orientation of the structure of the foot and its related function. This in turn protects the medial longitudinal arch from abnormal stresses, preventing further deformity while attempting to promote optimal foot function and stability.⁶ Foot orthoses can aim to promote and/or control foot pronation and lower limb alignment due to the coupling mechanism of the subtalar joint and tibia. In addition, they can also aid in improving the direction and function of the arch.^7,8

The foot is regarded as the terminal segment of the body that adapts to uneven terrain and supports the body during single limb and double stance. In essence, changes in the length and width of the foot as well as the height of arch are dependent on the rate of weight acceptance of the limb.⁹ This view is supported by McPoil et al.⁸ and Nilsson et al.,¹⁰ who showed that the length and width of the foot changed by 10 mm and 8 mm, respectively, and increased the height of the arch by 13 mm and longitudinal arch angle (LAA) by 14° during walking.

Foot posture and its mobility are one of the necessary elements of evaluating foot performance and play important role in prescribing an orthosis.^8,11 In recent years, evaluating foot mobility and foot type has been considered as a critical factor when prescribing a foot orthosis. For example, the foot mobility magnitude (FMM) suggests changes in the arch height in the sagittal plane as well as changes in the midfoot width in the coronal (medial–lateral) plane in both static weight-bearing and non-weight bearing.⁸ Studies have shown that the mobility of the arches during weight bearing and walking^11,12 is considered less during the prescribing and manufacturing process.

Overtime, individuals with a flexible flat foot can lose the elastic properties of some ligaments (e.g. deltoid ligament) within the foot. As a consequence, the longitudinal arch of the foot can lose its ability to return to its primary height during weight-bearing activities. It therefore seems that a foot orthosis should be able to regulate arch height in different states of weight bearing and prevent over pronation (vertical, medial and lateral mobility). In contrast, however, a foot orthosis used for individuals with flat foot must be flexible to facilitate adequate motion of the joints of the foot.¹³ Recent studies have discussed flexible orthoses that improved foot direction during a weight-bearing state and increased the arch height during standing.¹⁴ The arch height index (AHI) is a measure that has gained attention in recent years. The AHI includes a normal arch height which is truncated by foot length of approximately 0.34 ± 0.03 mm.¹⁵ The truncated foot length (TFL) is the distance from the distal end of the heel to the first metatarsophalangeal joint.^16,17 Using this index measure with foot orthoses and barefoot would enable the clinician to determine the amount of height that has been reduced during weight bearing. Additionally, it may also establish the influence of the foot orthosis in creating an arch to facilitate normal arch orientation.

There are various studies that have explored arch height indices.^{15,16,18–20} However, while previous studies of foot mobility have used healthy (asymptomatic/no deformity) individuals, no data exist on foot mobility in patients with flexible flat feet. In addition, the influence of orthoses intervention on FMM and the AHI in individuals with a flexible flat foot is uncertain. Payne et al.²¹ explored the response of the foot to various foot orthoses conditions (three rigid orthoses with different height, two soft orthoses and a shoe) with different arch heights. Using a digital calliper, the height of the navicular was measured (sagittal plane). The findings showed that the position of the calcaneus which altered when placed in each foot orthosis, and a statistically significant change in the position of the calcaneus in the direction of inversion, was noted, as well as a significant increase in the arch height.²¹

The aim of this study was to determine the influence of foot orthoses with various forms of rigidity on the mobility of the foot. Awareness of the type of foot orthoses that create reduced or greater mobility could assist the clinician during the prescription stage, which in turn can facilitate optimal foot function and patient satisfaction.

Methods

Participants

A total of 20 students with flexible flat feet (12 females, 8 males; mean age = 23.2 ±3 years) from the rehabilitation faculty of Tehran University of Medical Science took part in this study. All participants met the following inclusion criteria: bilateral flexible flat foot as judged from the 6 item version of Foot Posture Index²² (score 6+ to 10+ = moderate; 10+ to 12+ = severe); no history of fracture to the lower limbs; present with a body mass index between 18.5 and 25; report pain and symptoms in the foot or lower limb after 1 h of walking; and no systematic disease that may influence the lower limb (e.g. cerebral palsy, poliomyelitis). Prior to participation and the start of the study, ethical approval was obtained from the University of Tehran and informed consent was obtained from all participants.

Orthotic device

Design characteristics and structure of the orthoses were required to be standardised for all participants. All foot orthoses were made by the same orthotist. The functional orthosis was manufactured as semi-rigid orthosis that was made from polypropylene with a heel post. The rigid orthosis was a University of California Biomechanics Laboratory (UCBL) orthosis and therefore custom moulded. A polyurethane material was used for the soft orthosis and the arch was contoured with small edges (Figure 1 (a, b, c)).

Figure 1.

Orthotic devices used in this study: (a) UCBL orthosis, (b) functional foot orthosis and (c) soft foot orthoses.

Apparatus and test procedure

The dorsal arch height measurement system was used, which incorporates the heel cup and uses a mobile digital calliper and ruler (Figure 1). The digital calliper is connected to a wooden part with dimensions of 30 × 30 × 2 cm moving vertically. It measures foot arch height up to 50% of foot length.^15,23 An additional digital calliper was used for measuring foot width in both states (with and without the orthosis) during static stance. The distribution of weight (kg) was conducted by a scale (Model EB 9005, China) for distributing the weight when standing.

For measuring the medial longitudinal arch height from dorsal area of the foot in the weight-bearing state, the participant was asked to initially put his or her foot on the device with the heel positioned completely on the heel cup. The left foot was positioned on the scale. The participant was then instructed to stand in a relaxed stance position, looking straight ahead with his or her arms by their side. In this state, the scale must indicate half of his weight. The digital calliper was moved downwards and on 50% of foot length the calliper touched the skin of the foot with no pressure placed upon it. In thisstate, the figure indicated on the calliper was recorded (Figure 2(a)).

Figure 2.

Dorsal arch height measurement in (a) 50% WB and (b) 90% WB.

For measuring in 90% of weight bearing, the participant was asked to put his or her entire weight on their right foot. In order to maintain balance, the left foot was placed on the scale to the value of 10% of its whole weight. The figure indicated by calliper was recorded¹⁶ (Figure 2(b)). These measurements were repeated by each participant standing on the rigid, semi-rigid and soft orthoses. The TFL^16,17 was also recorded by a ruler on a wooden plate. For measuring in the non-weight-bearing state, the participant was asked to sit on the edge of a chair to allow the feet to hang and maintain them in a slightly plantarflexed position. In this state, the participant was requested to lift the wooden plate in such a way that the plate only touched the plantar level of foot. In this position, the digital calliper was placed midway of the height of the foot to allow the arch to be documented (Figure 3(a)).

Figure 3.

(a) Dorsal arch height measurement in non-WB and (b) midfoot width in non-WB.

For measuring foot width, under non-weight-bearing state (Figure 3 (b)), in 50% and 90 % of weight bearing (Figure 4(a)(b))., two metal bars of the calliper touched the medial and lateral of the foot in midfoot length. The figure on the calliper was then recorded. All of the mentioned measurements were repeated for each of the three foot orthoses and recorded only for the right foot. In addition to these measurements, the comfort of each of condition was measured using a simple visual analysis scale system (10 cm = 0 not comfortable, 10 very comfortable).

Figure 4.

Midfoot width measurement in (a) 50% WB and (b) 90% WB.

To obtain the arch height changes, the measurement of the weight-bearing state was subtracted from non-weight-bearing state. For obtaining changes in foot width, the foot width size in the weight-bearing state was reduced from non-weight-bearing state, and then for obtaining the FMM. A Pythagoras equation and replaced in following formula⁸

FMM = \sqrt{{(difference arch height from non-weight bearing to weight bearing)}^{2} + {(difference midfoot width from non-weight bearing to weight bearing)}^{2}}

Statistical analysis

The mean and standard deviation of variables were calculated for each subject. Repeated measure analysis of variance was used to compare the three conditions. If there was a significant difference between the three conditions, the Bonferroni test was used further as post hoc. Regression coefficient tests were conducted to examine the correlation of the underlying index with other variables. Data analysis was performed, using SPSS.17, and the level of significance was considered to be less than 0.05.

Results

A total of 20 participants presented with bilateral flexible flat foot. The mean age was 23.2 ± 3 years (range = 18–35 years; mean mass = 60.95 kg, mean height = 167.7 cm)

A significant difference was noted in FMM between the barefoot and UCBL orthosis condition (P value = 0.026). There was also a significant difference in foot mobility between the soft orthosis and UCBL orthosis (P value = 0.031) and between semi-rigid orthosis with an UCBL orthosis (P value = 0.037) in 50% weight bearing (Table 1).

Table 1.

P values between barefoot and insoles conditions in 50% weight-bearing.

Test states		Variables
Test states		FMM	AHI
Barefoot	Soft insole	0.05	0.046
	Semi-rigid insole	0.29	0.072
	Rigid insole	0.026	0.011
Soft insole	Semi-rigid insole	0.042	0.05
	Rigid insole	0.031	0.036
Semi-rigid insole	Rigid insole	0.037	0.012

FMM: foot mobility magnitude; AHI: arch height index.

Based on the visual analogue scale – that grade 0 is not comfortable and 10 grade is very comfortable – the comfort degree of UCBL orthosis had a mean value of 3, while the semi-rigid orthosis and polyurethane (soft) orthosis had a mean score of 8 and 6, respectively.

Discussion

This study set out to evaluate the immediate effect offoot orthoses on the mobility of the foot in participants presenting with flexible flat foot. The parameters measured were focussed on arch height, midfoot width, TFL and AHI. It was hypothesised that there would be significant differences between the orthoses in foot mobility measured by arch height, midfoot width, TFL and AHI.

The findings suggested that mean foot mobility in people with flexible flat foot is approximately 1.6 cm. The findings suggest that all three types of orthoses would generally increase the arch height when compared to a barefoot condition alone. The UCBL orthosis according to Pythagoras equation⁸ has minimal foot mobility in the state of non-weight bearing to 50% weight bearing (7.90 ± 1.24 mm), and the functional orthosis has maximum foot mobility in 90% of weight bearing (17.25 ± 2.26 mm) (Table 2). This is somewhat higher compared to the mean AHI in previous studies (0.34 ± 0.03 mm). On the other hand, TFL in people was three times more than the arch height of that person.¹⁶ In this study, the mean AHI in the barefoot state and during weight bearing of 50% was obtained at 0.34 ± 0.35 mm, which is similar to previous studies.

Table 2.

Mean and standard deviation of DAH, MFW, FMM, TFL and AHI.

Mean ± SD		DAH (mm)	MFW (mm)	FMM (mm)	TFL (mm)	AHI (mm)
Barefoot	NWB	71.93 ± 6.01	80.95 ± 7.197	–	170.62 ± 1.84	0.33 ± 0.421
	50% WB	58.09 ± 6.68	89.15 ± 8.008	16.08 ± 1.98	170.768 ± 1.84	0.34 ± 0.35
	90% WB	57.89 ± 6.14	90.82 ± 8.57	17.20 ± 1.85	170.84 ± 1.83	0.32 ± 0.338
Soft orthosis	NWB	72.33 ± 4.43	80.25 ± 8.12	–	170.75 ± 1.79	0.315 ± 0.423
	50% WB	65.72 ± 4.59	90.89 ± 8.16	12.52 ± 1.38	170.85 ± 1.85	0.348 ± 0.384
	90% WB	63.05 ± 4.45	92.13 ± 7.88	15.07 ± 1.65	170.93 ± 1.82	0.341 ± 0.368
Semi-rigid orthosis	NWB	73.06 ± 5.97	82.62 ± 7.72	–	175.78 ± 1.17	0.415 ± 0.40
	50% WB	62.96 ± 5.48	95.311 ± 8.21	16.15 ± 2.24	180.02 ± 1.82	0.348 ± 0.044
	90% WB	61.72 ± 5.76	95.64 ± 8.55	17.25 ± 2.26	182.23 ± 1.85	0.36 ± 0.339
Rigid orthosis	NWB	75.32 ± 5.59	85.75 ± 5.47	–	170.09 ± 1.76	0.28 ± 0.443
	50% WB	69.83 ± 5.65	89.229 ± 5.01	7.90 ± 1.24	170.32 ± 1.77	0.034 ± 0.410
	90% WB	67.49 ± 5.67	92.03 ± 7.19	10.09 ± 1.65	170.54 ± 1.79	0.26 ± 0.396

DAH: dorsal arch height; MFW: midfoot width; FMM: foot mobility magnitude; TFL: truncated foot length; AHI: arch height index; SD: standard deviation; NWB: non-weight bearing; WB: weight bearing.

Rigid orthosis: University of California Biomechanics Laboratory (UCBL) orthosis.

In this study, the influence of three types of orthoses on AHI was investigated. The findings indicated that the UCBL orthosis has the highest AHI since it has the maximum arch height, while the semi-rigid orthosis showed the lowest AHI even though it had the maximum mobility in the sagittal plane. AHI in the soft orthosis is closer to a normal AHI. A UCBL orthosis resists against deformation of the medial longitudinal arch and prevents any mobility to foot arches due to its rigid structure in the arch area and long medial and lateral border. In a normal state, the arch during walking and in different postures of the body is dynamic. Therefore, some patients may perceive this as being rigid and painful during running.^24,25 Since the UCBL orthosis facilitates less mobility of the foot, it could be suggested that the intrinsic and extrinsic muscles cannot be active enough in walking, which in turn could compromise function. In addition, excessive external forces applied to the foot may force patients to do compensatory movements until lack of mobility would be recovered. Perhaps for the above mentioned reasons, a UCBL orthosis can be viewed as ‘not as comfortable’.

Since a semi-rigid orthosis has a relatively flexible structure without any wall, it has a very low strength against arch deformation and can achieve maximum arch mobility, which in turn has no influence in modifying the arch and attaining the normal state of the arch. However, having the minimum strength against foot mobility, it is considered as the most comfortable orthosis in patients with higher acceptability. In a study by Mills et al.,²⁶ they investigated the immediate effect of three types of orthoses (hard, medium and soft) using kinematic and electromyography. They noted that an orthosis with the least comfortable was where there was a greater increase in the frontal plane control/support for a mobile midfoot and the opposite for non-mobile foot. The authors also showed that soft orthosis was more comfortable.

A soft orthosis has a relatively semi-rigid structure with a short wall and more strength than semi-rigid orthosis against foot deformation. AHI in soft orthosis and AHI in normal foot are relatively similar. Therefore, in using this type of orthosis, the AHI would become closer to the normal foot. Based on the feel of comfort, it was the semi-rigid and polyurethane orthoses that were most comfortable. Perhaps the reason for this is related to the activity ofthe intrinsic or extrinsic foot muscles during walking. Moreover, since polyurethane orthoses have a low arch height and functional orthoses are very flexible, more mobility is allowed to the foot by these two orthoses rather than UCBL orthoses.

It is acknowledged that the findings of this study are limited to only 20 participants and measurements were taken during a static stance position. However, further clinical studies are required to evaluate the dynamic influence of these forms of foot orthoses in individuals with a flexible flat foot.

Conclusion

This study has shown that UCBL orthosis, due to its rigid structure and long medial–lateral walls, limits foot mobility. Moreover, this type of orthosis also causes some discomfort among some participants with flexible flat feet. Therefore, considering the mobility of foot structure in different movement states, it is necessary to make a foot orthosis with materials and methods by which one can have foot mobility. In addition, this orthosis must control maximum pronation movements in people with a flexible flat foot. Since the therapeutic effect of a foot orthosis still remains unclear, further studies should focus on exploring dynamic foot mobility and the use of various types of foot orthoses in flexible flat foot.

Footnotes

Conflict of interest

None declared.

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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