Evidence for foot orthoses for adults with flatfoot: a systematic review

The search strategy was defined by the principles of a systematic search, using the PICO scheme and implied free-text keywords and medical subject headings (Mesh terms) performed by two reviewers. A computerized search was conducted in August 2021, using the databases Medline via PubMed, Scopus, PEDro, Cochrane Library and Cochrane Central Register of Controlled Trials. Major search terms for all databases are represented in Table S1.

Relevant gray literature was derived via Google Scholar. Furthermore, we checked reference lists of included studies, and relevant reviews were identified through the search. The results of the search were imported into the web service Covidence (www.​covidence.​org), which was used for the entire review process.

All intervention studies evaluating any kind of foot orthoses or inserts for flatfoot were eligible for inclusion, with the exception of case studies. Studies evaluating surgical interventions were excluded. We included scientific articles published in peer reviewed journals in English and German. To get the largest possible number of studies, no limitations of the publication year were applied. As an inclusion criterion, studies must have determined outcomes in the form of patient-reported, clinician-reported or laboratory-based measurements. All inclusion and exclusion criteria are shown in Table 1.

Two independent reviewers (MH, DW) assessed titles and abstracts from all search results to identify eligible studies. After potentially relevant articles were selected, full reports were obtained and inclusion and exclusion criteria were assessed. Intervention studies evaluating any kind of foot orthoses for adults with flatfoot were included. Studies evaluating surgical interventions were excluded.

Any disagreement on the eligibility of studies was resolved through discussion to reach consensus or, if required, by involving a third experienced review author (JS).

Data from each study included in this review was independently extracted by two reviewers (MH, DW). In the event of disagreements or discrepancies, a third review author was involved to reach consensus. Data were extracted into standardised tables, including author, publication year, study design, participants, interventions, setting, outcomes, measurements and main findings according to the Cochrane Handbook for Systematic Reviews of Interventions [35].

The methodological quality of all of the included studies was assessed using the Cochrane Risk of Bias tool for randomized controlled trials (RCTs) [35] and the ROBINS-I tool for all other study designs. Two reviewers (MH, DW) independently assessed the methodological quality of the studies included in order to identify any potential sources of bias.

The initial search identified 110 studies. After removing duplicates, 97 studies were investigated. 72 studies were excluded after title and abstract screening. 24 full texts were reviewed and finally 12 studies were included in the study. Two ongoing studies were also included [37, 38]. The process of study selection is presented in Fig. 1.

A total of 170 participants were included. The sample sizes of the included studies ranged from 8 [34] to 80 [39]. Of the 12 included studies, most were repeated-measures intervention studies (n = 9) [16, 34, 40‐46]. Other studies used a randomised-controlled design [39], before-after design [47] or a non- randomised controlled design [48].

Studies were conducted in Asia [16, 34, 39, 43‐46, 48], Australia [42], the United States [41], Iran [40] and Turkey [47]. Most participants were young adults aged between 18 and 45 years (Mean = 31.5) [16, 34, 41, 42, 46‐48]. Two studies recruited only male participants aged 20 and 22 years (Mean = 21) [40, 45] and three studies recruited only college students [43‐45]. Two studies [43, 44] gave no information about age.

Flatfoot was diagnosed differently in the included studies. Chen et al. [16] diagnosed flatfoot by static foot posture, Kido et al. [34] based their diagnosis on radiographs under static loading conditions and Murley et al. [42] investigated their participants radiologically and applied an arch index. Tang et al. [48] diagnosed flatfoot by measuring the arch index described by Cavanagh and Rodgers [49]. Xu et al. [39] diagnosed flatfoot by foot posture index (FPI). Other studies gave no information about flatfoot diagnosis [40, 41, 43‐47].

Only one study [47] described the study setting. None of the studies provided information on wearing time or when the outcomes were measured. Study characteristics are summarized in Table 2.

For the randomised-controlled study of Xu et al. 2019, we judged the risk of bias as low in all items according to the Cochrane risk of bias tool for randomised controlled trials [50]. We were unable to judge the overall risk of bias for the other included studies because of a lack of information in the study reports. There is no clear indication that the studies are at serious or critical risk of bias and there was a lack of information in one or more key domains of bias [51]. None of the included studies [16, 34, 40‐44, 46‐48] gave information about confounding, selection of participants, deviations from intended interventions as well as selection of reported results. In only two studies [47, 48] the risk of bias due to missing data was determined to be low because outcome data was reported for all study participants. In other studies, [16, 34, 40‐44, 46] it was unclear if outcome data included data from all participants. The risk of bias in classification of intervention was judged to be low in all studies [16, 34, 40‐44, 46‐48] because intervention groups were clearly defined. All studies [16, 34, 40‐44, 46‐48] showed a low risk of bias in measurement of outcomes because studies used objective outcome measurements. Results of the risk of bias assessment are summarized in Table 3.

Most studies [16, 34, 47, 48] measured the effect of foot orthoses via 3-D motion capture. Chen et al. [16] used eight cameras under three conditions: walking barefoot, walking with shoes only and walking with shoes and foot orthoses. A motion analysis system was used by Tang et al. [48] to measure the effect of foot orthoses under the same conditions as Chen et al. Kido et al. [16, 34] used computed tomography. Acak [47] used a podoscope device to make images of the soles of feet. Han et al. [45] used 10 cameras under three conditions: walking with normal foot orthoses, foot orthoses with only arch support function and foot orthoses with both arch support and cushion pads for shock absorbing functions.

Pressure measurement plates were used in four studies [39‐41, 46]. Miller et al. [41] conducted ten trials with and without foot orthoses. Aminian et al. [40] measured three conditions: wearing the shoe only, wearing the shoe with prefabricated foot orthoses and wearing the shoe with proprioceptive foot orthoses. Xu et al. [39] measured three conditions: barefoot, with 3-D and standardized foot orthoses. Jiang et al. [46] measured three different conditions: using an orthotic insole, a flat foot insole and 3-D printed insole. Two studies [43, 44], used imaging as well as pressure plates. In Murley et al. [42], imaging and electromyogram were conducted at baseline. After 12 days, four conditions were investigated: barefoot, walking with shoe only, walking with prefabricated foot orthoses and with 20 degree inverted customized foot orthoses.

All included studies used different outcome measures. None of the studies reported an adverse event.

Aminian et al. [40] measured plantar pressure in the shoe at the medial and lateral heel, medial and lateral midfoot and first, second, third, fourth and fifth rays. For the proprioceptive orthosis, the maximum force was significantly reduced in the medial midfoot, and plantar pressure was significantly increased in the second and third rays compared to the shoe only condition. Similarly, Jiang et al. [46] measured plantar pressure during walking at slow, normal and fast gait speeds on the treadmill at the hindfoot and midfoot while wearing 3-D foot orthoses (PPRI), foot orthoses and flat foot orthoses. The force on the rear and midfoot was significantly increased when wearing flat foot orthoses compared to PPRI and foot orthoses. In addition, the contact area at slow and normal speed in the midfoot area was smaller with PPRI compared to flat foot orthoses. In contrast Park et al. [43] investigated the change in pelvic angle using foot orthoses and reported a significant decrease after wearing the orthotic. Peng et al. [44] investigated the patellofemoral joint and the medial and lateral tibiofemoral joint. After wearing the foot orthoses, the second peak patellofemoral contact force and the peak contact force of the ankle were significantly lower. Foot orthoses also significantly reduced the peak eversion angle and the eversion moment of the ankle. The peak adduction moment of the knee was increased. Rearfoot motion and plantar pressure redistribution were measured in Tang et al. [48]. They reported that foot orthoses may reduce valgus angle and becomes statically similar to normal participants. Furthermore, a reduced foot pressure in the hallux and heel area when wearing foot orthoses was found compared to those wearing only sports shoes. Han et al. [40] reported that hindfoot ankle motion was less with both custom-made foot orthoses than with the normal foot orthoses. Xu et al. [39] measured three conditions: barefoot, with 3-D and standardized foot orthoses.

Acak [47] measured BMI and weight and conducted a 30 m jump test, a vertical jump test, the 12 min Cooper-test and Visual Analog Scale VAS. For the individually designed foot orthoses, they found statistically significant differences in all mentioned categories between pre- and post-test.

Chen et al. [16] investigates spatio-temporal parameters as well as kinematic and kinetic data. For the custom-made foot orthoses, the following was found: an increase in the peak ankle dorsiflexion angle and moment, a reduction in the peak ankle plantarflexion angle and moment as well as an increase in the peak knee varus moment. Kido et al. [34] examines tibia, tarsal and metatarsal bones and medial longitudinal arch. The therapeutic foot orthoses significantly suppressed the eversion of the talocalcaneal joint.

Miller [41] measured dynamic ground reaction forces (GRFs) in three directions: vertical, mediolateral and anteroposterior. The orthotic device reduces vertical and anteroposterior GRFs in the early stages of the stance phase during gait cycle.

In the study conducted by Murley et al. [42] electromyographic activity and foot posture was measured and the prefabricated foot orthosis showed a significant effect.