2.1. Patients
This single-center study was approved by the institutional review board (approval no. 17-R069) and conducted in accordance with the tenets of the Declaration of Helsinki. Between April 2012 and December 2015, all patients with metatarsalgia longer than 2 months without remission, regardless of treatment, were included in this study. Metatarsalgia was defined as weightbearing pain and/or tenderness on the plantar side of the lesser metatarsal head. Sixty patients were screened and examined according to the subject selection criteria by a single foot and ankle surgeon (KA) study investigator. As this was a retrospective case series, a statistically powered sample size was not calculated. Simple radiography of the patients’ feet was carried out, during weightbearing in the dorsoplantar view. The rate of patients' complications of the hallux valgus, defined as a 20-degree or greater angle of the hallux valgus, was examined. Moreover, the forefoot length was assessed in the first metatarsal relative to the second metatarsal. Patients were divided into 3 groups according to this length: in index minus, the first metatarsal was shorter than the second and the following metatarsals became progressively shorter; in index plus, the first metatarsal was larger than the second; and in index plus-minus, the first and second metatarsals were approximately the same length. Furthermore, all patients underwent MRI and were confirmed not to have rupture of plantar plates or Freiberg's disease. The exclusion criteria were as follows: patients with conditions such as rigid forefoot deformities (e.g., hammer toes, claw toes), patients with infection, crystal arthritis, previous foot surgery, major trauma, rheumatic disease, and neurological disease (e.g., cerebral infarction, parkinsonism, radiculopathy, and Morton's neuroma). After implementing the exclusion criteria, we included 56 feet (41 patients) older than age 20 who were not allowed to take medications or receive any insole treatment during the study period. All patients provided written informed consent.
2.2. Toe exercise
Patients received an 8-week toe functional exercise program conducted under a physiotherapist's guidance. We administered simple exercises such that elderly individuals could understand them. The patients sat in an upright position, placed a towel on the floor, and placed one foot on it, one shoulder-width apart. Then, they used their toes to scrunch up the towel, making sure to keep the rest of their foot in contact with the ground. Three sets of 15 scrunches were performed on each foot. In addition to the towel exercise, curling and spreading out of all toes was done. Patients were instructed to perform the exercises for 10 minutes 2 times a day for 2 months.
2.3. Toe-grip strength meter
TGS was measured using the push-type toe-grip strength meter (Figure 1) [13]. Using the strain gauge installed in the cantilever, the device measures the strength of the toes pressing the floor. For the measurement, a foot was placed on the positioning bar of the device and strapped tightly to restrict upward motion during measurement. The measurement was started after the confirmation of zero on the device's screen. The second foot could be measured by turning the measurement board upside down. We measured all the TGS values before and after the exercise program in both upright and sitting positions. In the upright position, patients stood upright on the measurement board facing forward. Special care was taken not to have the patient's head down, causing extra weight to be placed on the toes. During measurement in the seated position, patients sat on a chair adjusted to their height with their knee and hip joints precisely at 90 degrees. Then, the patients gripped their toes using maximal force in both positions. The highest value (Newton: N) of force with which toes were gripped was automatically recorded as the maximal peak force on the device's screen (Figure 2). We undertook the measurement twice on the same day. The average of the two measured values was recorded.
2.4. Assessment
Patients were also assessed by using the visual analog scale (VAS) for pain, the American Orthopedic Foot and Ankle Society (AOFAS) hallux metatarsophalangeal–interphalangeal scale (scale of 100 points), the number of marbles that the patient could pick up using their toes in 10 s while seated (Picking Up the Marbles [PUM]), and observing the number of seconds the patient was able to stand on a single leg for up to 60 s (Single Leg Standing Time [SLST]). The VAS, consisting of horizontal lines of a 100-mm length, was self-recorded. For pain intensity, the scale is anchored by “no pain” (scale of 0) and “worst imaginable pain” (scale of 100). To investigate whether the duration of disease affects the outcome of treatment, patients were divided into two groups based on disease durations of more than 1 year or less, and the average difference in the degree of improvement between the pre- and post-treatment measures was examined. Multivariate analysis was conducted to investigate which background factor correlated with the degree of improvement in the TGS in the upright and sitting positions, degree of improvement in AOFAS score, and improvements in VAS scores. The reliability of the push-type toe-grip strength meter was assessed with the Bland-Altman plot using the intra-rater correlation coefficient (ICC). During the measurement of TGS, the inter-rater reliability was not examined because the value is only read off of the screen of the device.
2.5. Statistical analysis
To compare the pre-exercise (pre-ex) and post-exercise (post-ex) values in relation to the parameters evaluated, the paired t-test for independent samples was used. The average difference in the degree of improvement between the pre- and post-treatment measures was also evaluated by the paired t-test. A multivariate generalized linear model, using normal distribution, was used to investigate the relationship between the background factors, including age, sex, body mass index (BMI), duration of disease, affected side, forefoot morphology and outcomes, including improvement of TGS in upright and seated positions, VAS scores, and AOFAS scores. In the affected-side analysis, the right and left feet were evaluated as a comparison based on bilateral feet measurements. In the forefoot morphology, plus and plus-minus feet were evaluated as a comparison based on minus feet. To determine the inter-rater reliability of the push-type toe-grip strength meter, the Bland–Altman plot was constructed to assess the agreement. Intra-class correlation coefficients for agreement were calculated. The data were analyzed by SPSS statistical software (version 7, Chicago, Illinois). The level of significance was set at p < 0.05.