The ankle is a common site for musculoskeletal injury for both the general public and specific sporting populations. Indeed, foot and ankle injuries are reported to represent over a quarter of musculoskeletal injuries in elite athletics [
]. Foot and ankle pathology is not isolated to young athletes, with a recent systematic review revealing pooled prevalence estimates for frequent foot pain of 24% and for frequent ankle pain of 15% [
]. Ankle sprains represent a common cause of ankle pain, accounting for between 3 and 5% of all Emergency Department visits in the UK, equating to approximately 5600 incidences per day [
]. Despite the high prevalence and potential severity of painful symptoms that follow the acute episode [
], ankle sprains are commonly regarded as benign injuries that will resolve quickly with limited treatment [
]. However, outcomes of conservative treatment range widely, with as many as 74% of patients with ankle sprains experiencing symptoms up to 4 years post injury [
]. Foot and ankle pathology typically results in pain, reduced range of motion and reduced quality of life [
]. Restricted range of motion can affect mobility and participation in sports and social activity. Thus, improving range of motion in the joint is one of the primary goals of therapy.
Accurate measurement of joint range of motion (ROM) is necessary in both research and clinical settings [
]. Indeed, it is commonly used as an outcome measure to assess the efficacy of therapies [
]. The assessment of ROM during weight-bearing is considered to be most related to activities of daily living [
], replicating tasks such as stepping, walking and moving from sitting to standing. The evidence based standard for accurate measurement of ankle joint ROM involves radiographic measurement [
]. However, this technique is not suitable for regular use in clinical practice for a number of reasons including exposure to radiation. This has resulted in the development of a number of non-invasive measurement approaches, although no one method can be regarded as the preferred methodology [
Goniometry is commonly used in the clinical setting to measure non-weight bearing ankle dorsiflexion ROM [
]. It involves the measurement of joint angles by an examiner, who places the arms of the device along the bones immediately proximal and distal to the joint, providing an estimated angle in degrees. However, evidence suggests that goniometry may not be a reliable method in either a research or clinical context [
]. In addition, the technique requires a high degree of handling proficiency, with the starting position, centre of rotation and axis positions being prone to investigator error [
]. The inclinometers are also frequently used to assess weight-bearing ankle ROM, providing a digital display of inclination of the tibia bone relative to the ground [
]. Recently, inclinometer applications have been developed for a range of smartphones, which make them a popular choice to examiners. However, there is limited evidence regarding the validity and reliability of these devices [
]. Indeed, the Tiltmeter and iHandy represent the only validated smartphone applications to measure ankle dorsiflexion to date [
There is a need to provide a standardised measurement technique for weight bearing ankle ROM measurement, with ease of use and portability are also critical factors for clinical translation. A new prototype device, known as the D-Flex, has been designed for application in any location, without the need for walls or flooring markings. The D-Flex provides a standardised method of ankle fixation with an automatic measurement system removing the potential for human error. However, there is no evidence regarding its performance when compared with goniometry or inclinometer devices. Accordingly, the study was designed to investigate the validity and reliability of D-Flex for the assessment of ankle ROM.
The study aimed to assess the reliability and validity of a novel ankle dorsiflexion measurement device, the D-Flex, in comparison to conventional goniometer or inclinometer systems. Findings revealed that the D-Flex demonstratied both excellent inter and intra-rater reliability. However, the measures of ankle ROM from the D-Flex were much lower than those observed from the Goniometer and inclinometer. This could have been due to the design of the device, with a foot and ankle support isolating tibiotalar movements. Indeed, the order of magnitude in movement is similar to that observed in radiographic assessments of tibiotalar ROM [
] and other devices such as the Achillometer® device which also constrain the mid and forefoot during measurement [
The results of this study indicated that the D-Flex was a more reliable device to measure dorsiflexion ROM than the goniometer and inclinometer. This could have been a consequence of the relatively novice investigators using the devices. While it is difficult to compare between reliability coefficients from different studies, the weight-bearing tests provide the same or higher ICC values when they are compared with other tests using the inclinometer or the goniometer [
]. Goniometric intra-rater reliability is more widely accepted among clinicians, although results vary considerably between studies [
]. For example, Konor et al. (2012) reported very good intra-reliability (ICC = 0.85) correlating with the present results (ICC = 0.85, 0.75) [
], while by contrast, Popoff et al. (2012) reported poor to good intra-rater reliability (ICC = 0.50–0.75) [
]. This discrepancy may be explained by the fact that goniometry is dependent on the experience of the examiner due to the degree of technical proficiency involved [
]. The D-Flex revealed good to excellent intra-rater reliability (ICC = 0.76–0.90). Although this was the first study to examine the performance of the D-Flex, it is anticipated that future studies would maintain the high standard of reliability, due to the device’s standardised procedure and decreased risk of human error. The standard test method, ergonomic design and biofeedback (heel pressure) provide an ideal means for reliable testing across a range of experienced raters.
This standardisation of measurement protocol was a significant factor in the high inter-rater reliability of the D-Flex devices (ICC 0.88–0.95). These reliability values were higher than that of a digital inclinometer (ICC ranging from 0.77 to 0.88), when authors compared novice and experienced raters [
]. The low measurement error for the D-Flex (SEM = 0.9–1.1°) is comparable to that of other newly designed dorsiflexion devices which have been examined [
]. Although limited comparison can be made due to the varying metrics of measurement, these low measurement error values are in accordance with the values provided by Konor and colleagues (intra-rater SEM ranging from 40 to 60 mm) and Bennell and colleagues (intra-rater SEM ranging from 50 to 60 mm) for measurements taken using the tape measure.
The results of this study are limited to the healthy participants, so the results may not be extrapolated directly to injured populations. Indeed, further research using participants with ankle pathologies will establish whether the D-Flex could prove a beneficial tool for those patients with ROM deficits. The use of a rater with basic experience could be a limitation, particularly when evaluating reliability. However, the present data does infer highly reliable results that are similar to previous literature demonstrating that an individual with basic experience may perform the test with high reliability. It should be noted that the D-Flex relies on battery life. In addition, measures from the D-flex typically took three seconds, which could problematic for individuals with pain or pathology. The translation of the device will also be cost dependent, to date manufacture has limited to a few prototypes.
The differences observed between measurement devices were greater than values reported to be clinically relevant i.e. greater than minimal detectable change (MDC) values of ~ 5° [
]. The low measures observed in the D-Flex will inevitably create a ceiling effect regarding the detection of clinically relevant changes in ankle dorsiflexion. Indeed, our estimated MDC values for D-Flex ranged between 2.4–3.1°. The corresponding MDC values for goniometry (ranging from 7.1–7.8°) are similar to those reported in the literature [
]. However, the MDC values estimated for inclinometer assessment (11.5–15.3°) were far greater than values previously reported by Konner et al. (3.7–3.8°). This could be due to the difference in the experience of the raters and the technology used during the assessment.
The present study has revealed that the D-Flex is a highly reliable method of measurement for ankle ROM, with very good intra-rater reliability, and excellent inter-rater reliability. In addition, it was identified as a more reliable device than both the universal goniometer and inclinometer, which are currently used in practice. It is anticipated that the D-Flex will prove a superior measurement tool for ankle dorsiflexion ROM, due to its ergonomic design and biofeedback mechanisms.