Musculoskeletal pains are common in pediatric patients and range from benign conditions to cases requiring immediate attention [1]. Among them, growing pains, first recognized in 1823, are unexplainable, recursive, and self-limiting musculoskeletal pains [2]. The pains usually occur at night and are usually accompanied by no clinical signs of inflammation. Growing pains also improve at the end of childhood [3,4].

The prevalence rate of growing pains is reportedly 2.6%-49.4% [5,6,7,8,9,10,11], and known causes include growth [9], hypermobility and fatigue [12,13,14,15], anatomical/biomechanical factors [16,17], and psychological problems [9,18]. While clinicians face these symptoms relatively more often, the cause of the disease is not clearly understood and appropriate treatments are not always provided. There is no special test for diagnosing growing pains. Instead, in most cases, clinical decisions are made according to inclusive or exclusive criteria [15,19,20,21]. Criteria for the diagnosis of growing pains were suggested in a cohort study of 720 students divided into three groups [9]. The criteria included pain lasting longer than 3 months, symptom-free periods of several days to several months with only intermittent pain episodes, pain occurring in the late afternoon or during sleep, lack of arthralgia, pain so severe that it interferes with sleep, and normal X-ray, blood test, physical examination findings. Other suggested criteria to diagnose growing pains included pains that are intermittent and bilateral, localized at the front thigh or in the shin mostly in the afternoon, and no abnormal findings on radiological or hematological tests [16].

Massage, nonsteroidal anti-inflammatory drugs, and orthoses are helpful for patients who complain of benign limb pain, for whom initial reassurance with a comment that symptoms should improve with age helps [1]. However, appropriate treatments to remove causes have not yet been established, and studies of treatment are lacking, even though various methods like those described above have been proposed to control growing pains.

Anatomical factors, fatigue, and psychological factors are thought to be causes of growing pains. Hypermobility and flat feet have been proposed anatomical factors of growing pains [16]. Presently, we studied children who complained of recursive growing pains in the lower extremities, investigated malalignment conditions of the lower extremities, and comparatively analyzed the frequency of pain occurrence, pain degree, and changes in their ability to perform static and dynamic balancing and functional activities after wearing customized foot orthoses that controlled biomechanical malalignment of the feet and lower limbs.

Pediatric patients who complained of growing pains wore foot orthoses for 3 months. When the frequency of growing pains and degree of the pains were evaluated after 3 months, both exhibited statistically significant improvements. Growing pains are frequently found in the age groups of childhood and adolescence, and are often faced clinically.

Massage, stretching, or drug administration are often used to lessen symptoms [13]. A small case study attempting to control growing pains through a biomechanical approach to correct overpronated feet with foot orthoses and reported that foot orthoses effectively controlled growing pains [16]. A later study argued that anatomical factors like foot posture are irrelevant to growing pains [23].

This present hypothesis was that if customized foot orthoses using the inverted orthotic technique were used in pediatric patients complaining of growing pains to correct foot misalignment and instability, growing pains in the feet and lower extremities would be reduced. Seventeen of 20 (75%) subjects had overpronated feet with an RCSP ≤-5°, and exhibited statistically significant improvements in pain frequency and degree 1 and 3 months after wearing foot orthoses (Figs. 3, 4). In addition, since all pediatric patients complained of chronic and recursive pains for more than 6 months, it was difficult to expect natural improvements of growing pains within 1 month. So, reduction of growing pains is considered to be an effect of foot orthoses.

Growing pains caused by fatigue or myalgia in the muscles of the lower limbs, rather than by damage to the bone itself [12,13,14,15]. Myalgia is caused by overuse of the muscle in the lower limbs by excessive training, severe fatigue in proximal parts of tibialis posterior muscle or tibialis anterior muscle by excessive pronation, and hyperextension caused by a lack of flexibility in the peroneal muscle [24].

Since there was no athlete among the participating children, the growing pains were not likely correlated with excessive training. Non-athlete children are rarely exposed to the situation of excessive training. Therefore, it is assumed that growing pains are likely correlated with fatigue or pain in the peroneal muscle that occurred in tibialis posterior or tibialis anterior muscles by excessive pronation. A number of children were excluded due to age. Most of the pediatric patients who were selected for this study exhibited overpronation (RCSP ≤-5°) on biomechanical testing. Since the peroneal muscle is the most usual site of pains, it seems that imposing extension stress on the tibialis posterior muscle was effective. Since the tibialis posterior muscle, which is located at the center of the peroneal portion of the leg, functions as the primary muscle that maintains the arch of the foot and controls overpronation, muscle fatigue, and myalgia are highly prone to occur within this muscle. In addition, since overpronated feet of children are mostly flexible flat feet, mechanical instability of the feet occurs frequently. In this foot condition, intrinsic and extrinsic muscles of the foot need to be more functional, so that the risk of myalgia would be increased due to overuse of the corresponding muscles.

When customized foot orthoses produced using the inverted orthotic technique are applied to overpronated feet of pediatric patients, they control overpronation and limit the overpronation response and secondary response in the lower extremities, leading to reduced fatigue or pain in the peroneal muscle including the tibialis posterior and anterior muscles. Customized foot orthoses are also thought to decrease the secondary genu valgum of the knee joint and maltracking of the knee joint caused by overpronation, resulting in eliminated femoral patellar pain (the most common cause of front gonalgia). In other words, customized foot orthoses produced using the inverted orthotic technique are thought to confer biomechanical stability to the feet and the lower extremities, resulting in a significant reduction of pain frequency and degree in the lower limbs.

In the evaluation of static balancing ability, the COG sway velocity on a unilateral stance test was significantly reduced. In the evaluation of dynamic balancing ability, movement velocity area and endpoint and max excursion area showed statistically significant improvements. In the evaluation of functional balancing ability, step speed on the tandem walk test exhibited statistically significant improvements (Table 4).

This study was performed to determine whether posture maintenance and balancing are improved by the foot orthoses produced using the inverted orthotic technique to provide the feet and lower extremities biomechanical stability. In the evaluation of static balancing ability (posture maintaining ability), sway was consistently reduced in multiple directions on the COG sway velocity test that was performed with each participant standing on one leg. In particular, sway was significantly reduced when the compensation for visual sense was removed by closing the eyes. Maintaining the posture of standing with one leg with eyes closed is a situation that requires an individual to maintain their posture only with proprioception of the foot and the ankle and tactile sense of the sole. Improvement of balancing ability after wearing foot orthoses in that situation indicates that wearing foot orthoses further stimulated the proprioceptive and tactile senses of the sole, or at least provided better information to them. In fact, foot orthoses produced using the inverted orthotic technique directed the subtalar joint to a near-neutral position and the midtarsal joint close to the normal position. By restoring joint position this way, high-quality afferent position sense information should be transduced from the ankle joints and tarsal bone to the central nervous system.

Another reason the posture maintaining ability was improved was because the foot orthoses in this study conferred biomechanical stability of the feet. Children with flexible flat feet frequently experience biomechanical instability within the bones of the feet. Thus, a customized foot orthoses made of rigid plastic should lift up the entire arch of the ankle and foot including subtalar and midtarsal joints, resulting in great reduction of mechanodynamic instability of the bones of the feet. A reduction of mechanodynamic instability will be able to decrease fatigue in the intrinsic and extrinsic muscles that control foot position.

Use of foot orthoses can improve joint position sense by increasing the afferent feedback from the skin receptors within the foot and ankle [25]. Since the inverted orthotic technique was designed to place major joints in the foot into normal positions and confer mechanical stability to the bones of foot and the joints, the use of the inverted orthotic technique using a solid material helped improve balancing ability through transduction of normal afferent signals from proprioception receptors of the ankle or skin receptors of the sole.

Among the analyses used to test dynamic balancing ability, movement velocity, endpoint excursion distance, and maximum excursion distance also displayed significant improvements. In these evaluations, subjects actively moved their COG following the order provided on a computer screen. The improvements in these sections meant that the body moved as the subject intended, which was likely enabled by mechanical stability of the feet, and appropriate transmission of the afferent joint position sense signal of the foot and ankle.

In the tandem walk test for evaluating functional balancing ability, step speed improved significantly. A significant improvement in test results meant that the foot with the balancing function of touching the ground was better able to support the body when the orthoses was applied. This was believed to be enabled by mechanical stability and proper afferent signal transduction from the foot and the ankle. The customized foot orthoses produced by the inverted orthotic technique markedly reduced pain frequency and degree in pediatric patients with growing pains and improved their static, dynamic, and functional balancing abilities.

This study had a few limitations. First, it had a small cohort size (20 subjects). Second, it was necessary to limit participant age to evaluate balance ability. If only pain change data had been used without an age limitation, a larger group (50 children) would have contributed to the results. In addition, the period of follow-up was 3 months, which is relatively short. As a follow-up study, results obtained 1 year later will be published. In this study, evaluation of balancing ability 3 months after treatment was performed with subjects wearing foot orthoses. As a compensatory study, balancing ability will be evaluated with and without wearing it in the follow-up study to find out the effect of wearing the foot orthosis for 1 year. In addition, this study did not consider patient body type or body mass index. However, since none of our patients appeared obese, we believe that this limitation did not affect the results. Future follow-up studies should be performed to compensate for these limitations.

In this study, 75% of pediatric patients complaining of growing pains had overpronated feet. When they wore customized foot orthoses produced using the inverted orthotic technique, pain frequency and degree were significantly improved. Static, dynamic, and functional balancing abilities also improved. Therefore, custom molded foot orthoses using the inverted orthotic technique will be an excellent treatment method for pediatric patients complaining of growing pains.