Many people experience metatarsalgia, pain of the plantar forefoot region of the foot with weightbearing. Researchers have suggested that metatarsalgia occurs when repetitive high-pressure loading under vulnerable metatarsal heads exceeds physiologic limits.1 Biomechanical factors that may increase forefoot loading include tight ankle plantar flexors, wearing high-heeled shoes, claw- or hammertoes, and dysfunction of the first metatarsal phalangeal joint. The last factor contributes to increased pressure during forefoot rocker, because the great toe is a major transducer of propulsive force to the ground during late stance phase of gait. When it does not share in its normal contribution to the propulsive vector due to hallux valgus or hallux rigidus, the result is a greater load on the metatarsal heads during walking.2,3
Other factors that decrease the load that the plantar forefoot is able to bear include atrophy or anterior migration of the plantar fat pad under the metatarsal heads,4 and pathologies such as rheumatoid arthritis, ankylosing spondylitis, gout,1 nerve entrapment,5 and diabetes mellitus. When the symptoms are attributable to a pathology that is not exclusively musculoskeletal, the problem is described as “secondary metatarsalgia.”4
Nonoperative interventions consist of modification of weightbearing within the patient's shoes to decrease the pressure on the painful structures, including a variety of custom and prefabricated foot orthoses.6 Kang et al.1 reported that the use of a metatarsal pad (a tear drop-shaped pad, rounded end facing distally, placed just proximal to the second metatarsal head) was able to decrease peak and total pressure on the second metatarsal during walking. Hodge et al.7 compared a variety of functional foot orthotic variations in trials with patients with metatarsalgia secondary to rheumatoid arthritis. The orthotic conditions included shoe only (control), prefabricated orthosis, custom foot orthosis (constructed using a positive cast of the patient's foot), custom orthosis with metatarsal pad (such as used by Kang et al.1), and custom orthosis with metatarsal bar. They found that all orthotic conditions decreased the total pressure measured at the patients' second and lateral metatarsal heads compared with the “shoe only” condition, although there were not significant differences in pressure measured between orthotic conditions. The authors' opinion based on the patients' subjective reports was that the custom orthosis with metatarsal pad was most effective in relieving symptoms.
One limitation of these established conservative methods of treating metatarsalgia is they require a device inside the shoe of the patient. As such, they require patient compliance in their use and adequate space available for the forefoot in toe box. This is a problem in footwear which tends to have inadequate volume by style, such as high-heeled footwear;8 and for patients with deformities of the lesser toes, such as claw toe or hammertoe deformities, which are frequently seen in patients with rheumatoid arthritis and diabetes mellitus.9 To address these two problems, one of the investigators in this study (GCH) conceived of a modification to be applied to the outsoles of the shoes of patients with metatarsalgia. In our review of literature, we found no examples of a similar intervention applied to the symptom of metatarsalgia. Our study investigated two questions regarding this treatment approach; first, does the use of the Hunt Metatarsal External Shoe Cut-out (HMESC) decrease pressure measured on the symptomatic metatarsal heads during walking?, and second, is the use of shoes modified with the HMESC associated with an improvement in the functional level of patients with metatarsalgia?
MATERIALS AND METHODS
Five patients (one of whom had bilateral symptoms) were referred to physical therapy and pedorthic management for treatment of primary or secondary metatarsalgia by one of the investigators (JMS), an orthopedic surgeon specializing in treatment of patients with foot and ankle problems. The patient sample included four women and one man (average age of 50.6 ± 14.5 years, range, 28–62). All patients referred by the investigating orthopedic surgeon were offered participation in the study sequentially to avoid selection bias.
The PEDAR insole pressure measurement system (Novel Electronics, Munich, Germany) was used to measure pressure. The data gathering components of this instrument are insoles that are shaped like the insole of a shoe and are constructed of a matrix of 99 sensors, each with an effective sensor area of ∼1.5 cm2. Each sensor consists of two electromagnetic surfaces separated by a 2 mm foam core between the superior and inferior surfaces. When the insole is compressed, the current between the sensors decreases in proportion to the decrease in distance between them, thereby converting capacitance into pressure data.
The sampling rate of the PEDAR used in this study was 50 Hz. Researchers have previously demonstrated that the shapes of vertical force profiles during walking generated by the PEDAR at 50 Hz correlated closely to those generated by force plates sampling at 99 Hz, r = 0.99,9 and at 1,000 Hz, r = 0.95.10
The Lower Extremity Functional Scale (LEFS) is a 20-item questionnaire designed to measure musculoskeletal lower limb-related disability as influenced by the patients' main complaint.11 In this patient series, metatarsal head pain was the main complaint and reason for referral. With this tool, respondents rated their ability to do each item on a scale from “0” (unable to do that activity) to “4,” (able to do that activity without any difficulty) (Appendix).
The patients' physical therapy intervention included ankle plantar flexor stretching exercise with pedorthic treatment consisting of the application of the HMESC to the outsoles of the patient's most frequently worn shoes (see Figure 1 for HMESC and Figure 2 for HMESC applied to a shoe). The HMESC was constructed of 5-mm thick compressible crepe soling (durometer rating = 60 Shore) with 4–6 cm oval shaped cut out to reduce metatarsal head pressure. The size and exact shape of the cut out were determined by the clinician based on a Harris mat impression, which was used to determine the area of highest forefoot pressure (Figure 3), by the patient's description of his or her pain symptoms, and the clinician's palpation of the most tender metatarsal head or heads.
Initially, the HMESC was fixed to the outsoles of the patients' most frequently worn shoes with double-sided carpet tape (Duck Brand indoor/outdoor carpet tape, Henkel Consumer Adhesives, Avon, OH) for a trial of ∼1 week. If the patient reported that symptoms decreased after that trial period, the HMESC was permanently glued to the shoes and edges of the crepe beveled.
Plantar Pressure Data
Patients with primary or secondary metatarsalgia were recruited to the study by one of the investigators (JWS). Data collection began with the first physical therapy and pedorthic visit. All patients read and signed the consent form, which had been approved by the Institutional Review Boards of CoxHealth Health Care System and Missouri State University, Springfield, MO. After patient evaluation, which included history and subjective information, assessment of foot and ankle structure and range of motion, and Harris mat impression (Figure 3), a HMESC was fabricated for each patient as described previously. If the patient's symptoms were unilateral, crepe soling was applied to the shoe of the nonsymptomatic foot to avoid imposing a leg length discrepancy.
The PEDAR insoles of appropriate size were placed inside the shoes by one of two clinician-investigators before the patients donned them. The data cables from the insoles to the mobile data gathering unit were secured to the patients' ankles and proximal legs with elastic bands. The mobile data gathering unit was connected by a wireless transmitter to a computer. The insole was unloaded on each side by the patient lifting each of his or her feet. This allowed the PEDAR to be calibrated to “zero vertical force” for each pair of insoles before each trial (session of walking with or without the shoe modification). Plantar pressure measurements were done in three 10-m trails per patient for each condition (with and without the HMESC). A crossover design was used with respect to treatment order.
FUNCTIONAL STATUS (LEFS SCORE)
The LEFS was administered to the patients on the initial clinic visit and on the second follow-up (third total) visit. Patients wore the modified shoes for at least 32 h/wk during treatment period, which was 38.2 ± 7.6 days.
The plantar pressure was measured in an area of the forefoot intended to be inclusive of the metatarsal heads and exclusive of most of the rest of the plantar surface. This was done by applying a “mask” to the PEDAR insole data to limit plantar pressure data to the area of interest (Figure 4). The parameters measured included the maximum pressure and nine-step pressure time integral (a mathematical function of pressure over the time needed to complete nine steps on the symptomatic side). Nine steps were described by Kernozek et al.12 to be adequate to establish a reliable maximum pressure mean for walking with the PEDAR system. The effects of the HMESC on each of these parameters were compared using a paired t-test. The change in the patients' LEFS scores between the initial clinic visit and second follow-up were also compared using a paired t-test.
Mean peak pressure was reduced by 120.8 ± 69.0 kPascals (kP) compared with walking in the same shoes without the HMESC, which was a mean reduction of 28.6%. The mean reduction in nine-step pressure time integral was 223.5 ± 186.7 kP, which was a mean reduction of 23.9%. The LEFS score improved during the treatment period by 13.3 ± 3.8 points, which was a mean increase of 29.2%. Paired t-tests yielded probabilities of p = 0.004 for the reduction in peak pressure, p = 0.016 for reduction in pressure time integral, and p < 0.001 for the improvement in LEFS score.
We measured an immediate reduction in both maximum metatarsal head area pressure and in pressure-time integral under the symptomatic metatarsal heads during walking for patients treated for metatarsalgia with the HMESC. The patients also reported a noticeable reduction in pain, which was associated with significant improvement in functional ability during the treatment period.
Figure 5 is one of the participant's “mean value picture,” which is a depiction of the cumulative pressure data generated by the PEDAR system for one trial. It can be seen from the values inside the sensor cells that mean pressure is highest about the area of metatarsal heads two and three for the trials without the HMESC and that pressure has been reduced and distributed posterior with the use of the HMESC. This is likely the mechanism of effectiveness of the intervention in reducing pain.
There are more established interventions for metatarsalgia, such as a metatarsal pad used just proximal to the second metatarsal,7 and a full contact orthoses.5 Conceptually, there are two advantages to the HMESC relative to those more established interventions. First, because the HMESC is applied to the outsole of the shoe, it does not decrease the vertical space of the toe box of the shoe such as may occur with internal shoe modifications. Toe box vertical space is especially important for patients with deformities of the lesser toes, such as claw toes or hammer toes, which require more such space. Furthermore, these individuals may be more likely to have metatarsalgia due to the diminished lever arm of the lesser toes, which have been shortened by the deformities. Lesser toes so shortened are less able to transmit force to the ground during the forefoot rocker stage of gait, and therefore, a greater proportion of force must be borne by the metatarsal heads.3 The preservation of toe box vertical space also is important for types of footwear which tend to be manufactured with inadequate vertical space such as high-heeled women's shoes.6 In this way, the HMESC is a viable intervention for patients who are not compliant with instructions to avoid wearing shoes of this style.
A limitation of this study is the small sample size. Although the large effect size provides adequate statistical power to make a strong conclusion regarding the null hypothesis, with such a small sample of patients we cannot claim that the spectrum of conditions that can contribute to the symptoms of metatarsalgia are represented.
We demonstrated that the HMESC decreased pressure and reduced pain in the metatarsal head region in patients with metatarsalgia and was associated with improved functional abilities during the treatment period. Initial findings are positive, but further study with a much more robust sample is proposed to ensure that this treatment is effective with a group of patients who are more likely to represent the spectrum of biomechanical and contributing pathologies that may lead to these symptoms. Also, further study could include a comparison of the HMESC with more established treatment options such as metatarsal pads, various orthotic designs, and perhaps combinations of the three treatment approaches.
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KEY INDEXING TERMS: metatarsalgia; plantar pressure; shoe modification