Background
Hip joint range of motion (ROM) is a basic clinical parameter for diagnosing hip diseases, such as osteoarthritis [
1,
2] or femoroacetabular impingement (FAI) [
3,
4], and for monitoring the efficacy of a treatment [
5]. Hip joint ROM is widely assessed using low-technology tools such as manual goniometers or inclinometers. The advantages of goniometry are the simplicity in assessing ROM, the direct measurement of joint angles without any data reduction process and the low cost of the instrument. The two-arm goniometer is still the most commonly used, economical and portable device for the evaluation of ROM [
6], despite acknowledged limitations. Major drawbacks of goniometry are that the starting position, the center of rotation, the long axis of the limb and the true vertical and horizontal positions can only be visually estimated; moreover, conventional goniometers must be held with two hands, leaving neither hand free for stabilization of the body or the proximal part of the joint [
6]. There are also difficulties in monitoring joints that are surrounded by large amounts of soft tissue, such as the hip [
7]. In addition, manual goniometers assess joint flexibility only in two dimensions; however, as most of the hip ROM measures in clinical practice are practically in-plane movements, this limitation is minor. The validity (i.e., the degree to which a measurement actually measures what it claims to measure) and reliability (i.e., the degree to which a measurement is consistent and stable) of manual goniometers have therefore been questioned, especially for measuring hip flexion. Bohannon et al. [
8] showed that in the hip flexion movement, as measured in a clinical setting, more than a quarter of the ROM can be attributed to pelvic tilt, leading to an immense misinterpretation of this movement due to the insensitivity of manual goniometers for secondary pelvic movement. Elson and Aspinall [
9] proposed a new method for measuring range of hip flexion by palpating the lumbosacral junction to allow early identification of lumbar spine flexion which accompanies hip flexion.
Three-dimensional measurement tools based on electromagnetic tracking have recently been used to precisely measure shoulder [
10‐
12] and spine [
13,
14] ROM, as well as patellofemoral [
15] and hip joint [
16] kinematics. Electromagnetic tracking systems (ETS) enable the direct measurement of a three-dimensional position and the orientation of multiple sensors referred to a stationary source (transmitter). As ETS may well provide the reference standard to assess the ROM of the musculoskeletal system [
17] in a clinical setting, concurrent use of ETS and simple two-dimensional measurement devices is a possible method to determine the validity of goniometers to yield plausible and useful objective ROM data [
18‐
20].
Thus, the aims of this study were (i) to verify the validity of a conventional manual goniometer (i.e., the standard instrument for clinical assessments) to measure passive hip ROM against a criterion standard instrument (ETS) (concurrent validity) and to discriminate between individuals with and without FAI (known group construct validity), and (ii) to examine the test-retest (intra-rater) reliability of hip ROM goniometric and ETS assessments.
Discussion
This study examined whether manual goniometers (i) are sensitive enough to discriminate between individuals with and without FAI (construct validity), (ii) measure the anatomical correct hip joint ROM (concurrent validity) and (iii) produce consistent results (test-retest reliability). The major findings of this study were that goniometric measurements of passive hip motion provided greater ROM data than the criterion instrument ETS. Interestingly, the agreement between the two devices was high for hip abduction and internal rotation, but low for flexion, adduction, and external rotation. The finding that goniometers are particularly valid for measuring hip abduction was also confirmed by the comparison between FAI patients and healthy controls. Moreover, it was demonstrated that goniometric evaluation of passive hip joint angles was reliable between days, with similar reliability scores compared to the ETS. Manual goniometers can therefore be used with confidence during longitudinal assessments, which rely on repeated measurements over time.
The assessment of construct validity was performed by comparing hip ROM between FAI and healthy hips (known group validity). Considering the advanced number of FAI patients in our clinic and the quickly increasing interest for this pathology worldwide [
3], information regarding limitation of hip ROM is needed. Due to significantly lower hip abduction ROM in the FAI group as measured with both devices, the current results demonstrated strong construct validity of manual goniometers for hip abduction assessment. The finding of lower abduction ROM in FAI patients is also supported by the literature [
4,
33,
34]. For the other motion patterns, tendencies of lower ROMs in the FAI group were observed. However, the small sample size and heterogeneity in patient characteristics limit the interpretation of the present construct validity results. Comparisons to previous studies dealing with hip ROM differences between FAI and healthy hips are difficult because either no information about the measurement technique was provided [
35‐
37] or a CT-based computer-assisted technique, ignoring cartilaginous structures, soft tissue contractures or masses for the calculation of ROM was applied [
4]. Clohisy et al. [
38] found no ROM differences between FAI and non-symptomatic hips, whereas Philippon et al. [
34] reported significantly reduced ROM in injured hips for all directions. Although limited internal rotation in 90° of flexion seems to be the key symptom during clinical examination [
4,
35,
39,
40], the extent of restricted internal rotation still has to be ascertained as the results of this study did not reveal such a significant reduction of internal rotation in the FAI group compared to healthy controls. One possible explanation for this finding could be that in medical routine assessments, hip ROM examination is stopped before the passive limit is reached because of the groin pain that accompanies internal rotation due to sharing forces at the labrum. It is also possible that some subjects in the control group had abnormal bony anatomy, as the estimated prevalence of FAI ranges between 10 and 15% [
40]. Adequately powered studies are needed to verify which ROM movements (together with hip abduction) should be included in physical examination as an indicator of FAI.
Although all hip ROM values measured by the goniometer were significantly greater compared to the ETS, concurrent validity of manual goniometers was particularly good for hip abduction, with high ICC, and low systematic bias and random error. Subjects were positioned supine with the contralateral leg hanging down on the edge of the massage table during hip abduction assessments. Thus, the pelvis was not only stabilized by the belt, but also by the abducted contralateral limb, regardless of any other movements. In this configuration, excessive rotation of the pelvis around a vertical axis was prevented, which was not the case for hip adduction. Considering that the sacrum sensor was assumed to be rigidly attached to the pelvis and therefore representative of the pelvic coordinate system, most of the differences between the goniometer and the ETS can be attributed to this phenomenon. For motion patterns others than hip abduction, it is plausible that the difference between the goniometer and the ETS was largely due to pelvic rotation when the passive limit of motion was reached. Although the co-investigators performing the goniometer assessments tried to minimize this source of error by stabilizing the subject manually, they could not adequately correct for this misalignment. The obtained results for hip flexion are in agreement with Elson and Aspinall [
9], who found a mean value of 85° for true hip flexion. In the same way, Bohannon et al. [
8] stated that a large portion of the hip flexion movement is assumed to be the consequence of pelvic rotation, resulting in pure hip flexion of only 90°. Independent of the arc of motion, pelvic tilt always occurred within the first 10° of hip flexion, indicating that the thigh and pelvis move in synergy with one another. Hence, for what we usually call hip flexion, the ROM generally evaluated is thigh flexion on the trunk, which is a combination of "true" hip flexion and pelvis tilt.
Apart from the uncontrolled pelvic tilt or rotation and neutralization of lumbar lordosis (in case of hip flexion) for goniometric hip ROM assessments, there are other possible factors leading to the disagreement between the goniometer and ETS data. It is unlikely that between-device bias was attributable to differences in the physiological mechanisms underlying hip ROM testing, because measurements were performed simultaneously with the two devices. Rather, the observed discrepancies were certainly due to visual estimation of the true anatomical reference lines (e.g., the long axis of the limb) and potential alignment of the goniometer to the position of the massage table or to the laboratory arrangements, rather than to true bony orientation. Another reason for the disagreement between the goniometer and ETS is the two-dimensional characteristics of goniometric hip ROM measurements. Cheng and Pearcy [
41] showed that an abduction angle measured in the frontal plane and a flexion angle measured in the sagittal plane can be significantly overestimated by the presence of out-of-plane movements. In the setting of this study, both hip abduction and adduction were associated to some degree of flexion, whereas hip flexion was associated to some degree of abduction. Therefore, this kind of error may have occurred, at least to a small extent.
The results obtained in the present study suggest that the hip ROMs "read" by the goniometer are in fact intersegmental thigh-trunk angles (e.g., thigh flexion on trunk for hip flexion) rather than true hip joint ROM. This study clearly demonstrates the "harmful" effect of flexing hip while evaluating ROM if pelvic tilt or rotation is not adequately controlled. Future research is needed to find hints allowing the goniometer to measure an angle close to the real ROM and to verify if other ways of goniometric estimation (e.g., hip rotations measured in prone position) would provide similar results compared to the ETS, all the more because one of the goniometer's arms could be placed on the table, potentially adding accuracy due to standardization. Even if manual goniometers are logically preferred in the clinic to more accurate devices, such as ETS, because of the limited time available for routine medical examinations, clinicians should be aware of this misinterpretation and they should try to minimize pelvic rotation. Apart from other advantages (such as simplicity of use, low cost and time saving), the use of conventional manual goniometers for longitudinal evaluations is supported by the present test-retest reliability results, which were good and comparable to those obtained with the ETS.
The excellent absolute and relative reliability estimates for hip flexion are in agreement to those reported on persons with hip osteoarthritis [
1,
2,
42‐
44] and healthy hip subjects [
5,
45]. Results from CV analyses showed that hip flexion measurements had the lowest difference between the two test days. ROM measurements of the hip abductors showed excellent test-retest reliability, with CVs around 5% and ICC estimates exceeding 0.90. A review of the literature did not identify reliability studies with comparable good CV results. Holm et al. [
2] and Pua et al. [
44] both reported CVs of more than 20% for passive hip ROM measurements. Even in a study assessing intra-tester within session reliability [
5], CV was larger compared to the present between session results. The excellent reliability estimates for hip abduction are the result of adequate stabilization of the subject's body and standardized force. Of particular interest in this study was the measurement of hip adduction ROM, as information about the test-retest reliability of this movement pattern is lacking in the literature. Those authors providing information about the repeatability of hip adduction measurements reported CVs of 23% [
2], ICCs around 0.5 [
2,
43,
46], or used the Pearson correlation coefficient, which is a questionable reliability index [
1]. Although superior, the results of this study indicate that hip adduction is the most challenging movement pattern to measure, confirmed by the low ICCs (< 0.9) and the smallest arc of motion (~20°). The ICC estimates for hip internal and external rotation (~0.9) are in agreement with those previously reported on hip osteoarthritis subjects [
2,
44]. In contrast, Croft et al. [
42] reported substantially lower levels of reliability by using six raters and six participants with hip osteoarthritis. They reported inter-tester ICC values of 0.48 and 0.43 for internal and external rotation, respectively. However, because of their small sample size, the results must be interpreted with caution. The present CV values are even slightly superior to those reported in the literature [
2,
5,
44]. Nevertheless, inter-study comparisons for rotational movements are difficult because of differences in the testing positions (prone compared to supine).
The current study has a few limitations. Firstly, human movement analysis based on electromagnetic tracking technology is affected by instrument errors, anatomical landmark uncertainty and skin movement artifacts [
47]. These sources of error were minimized by using a functional approach for calculating the hip joint centers and by adopting a segment coordinate system approach, which reduced anatomical landmark palpation to a minimum. However, it is still unknown if the joint coordinate system solution proposed by Grood and Suntay [
26] represents the clinical reality. Moreover, movements were primarily performed in a single plane, so that error due to inertial effects and skin deformation by direction of movement were both negligible. The authors therefore believe the ETS can be used as the criterion instrument for hip joint ROM assessment in orthopedic research, although it is acknowledged that alternative methods such as fluoroscopy or bone-pins would be more accurate. Secondly, ETS measurements were limited to a single investigator performing the movements. Future research should include more testers in order to determine inter-tester reliability. It can be supposed that this kind of reliability would also yield excellent results for both devices, as standardization of the applied force should eliminate a considerable amount of inter-tester variability [
5,
48,
49]. Finally, the investigator performing the goniometer assessments was not blinded for the goniometric ROM values. This bias was minimized by prohibiting the observer to read the goniometer results until proper alignment of the device was ensured. It is therefore unlikely that this factor influenced the main results.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
SN, NAM, ML and HG were responsible for the conception and design of the study. SN, JFG and SS coordinated the practical work and performed data acquisition. SN prepared the data for analysis and performed together with NAM the statistical analyses, data interpretation and manuscript drafting. ML, HG, JFG and SS assisted with some of the statistical analyses and with the interpretation of the data. All authors read and approved the final manuscript.