Concurrent validity of photogrammetric and inclinometric techniques based on assessment of anteroposterior spinal curvatures

The shape of spinal curvatures is of interest for various fields of science [21, 22], and is an element of posture assessment. In the literature focusing on spinal curvatures assessment, we can encounter various methods, which suggests that a perfect technique has not yet been found. The results reported by various researchers are difficult to compare due to the fact that different methods and measuring tools are applied; therefore, it is necessary to continue studies designed to compare results obtained with specific methods. Among the specialist publications we can encounter articles discussing concurrent validity of various techniques designed for assessing body posture [12, 23, 24, 25], yet no research has previously been reported to compare the methods based on photogrammetry and gravitational inclinometer, which has been done in the present study. The two methods were used in assessing the same subjects, which made it possible to verify the assumption regarding the concurrent validity of these measurements. Additionally, this is the first study focusing on senior citizens with increased BMI, which made it possible to verify the accuracy of these methods in this group of patients. Previously conducted comparisons related to younger individuals with no excess weight [12, 16], while measurements of spinal curvatures are necessary in older people or overweight individuals, and in fact assessments are more error prone in such cases.

The inclinometer applied in this study is a small and affordable measuring tool. It may be reused many times, which makes the diagnostic process easier. Numerous authors have shown high reliability and usefulness of this method for clinical examinations, e.g. Czaprowski [16], Saur et al. [25], Kużdżał et al. [5] and Walicka-Cupryś et al. [23]. The other device used in the present study for assessing body posture is a system whose operation is based on photogrammetric method taking advantage of Moiré phenomenon. An advanced optical system makes it possible for the computer to map out a three-dimensional image of the subject’s back and to analyse over 50 parameters in the frontal and sagittal plane. The digital data may be analysed, compared, stored and subjected to statistical processing [7].

Results of the present study did not show significant differences in angles of lumbar lordosis and thoracic kyphosis in measurements based on photogrammetric technique and performed with gravitational inclinometer. Yet, the findings showed that measurements of some fractional parameters were not compatible. No statistically significant differences were found in the measurements of ALFA and GAMMA angles, performed with the use of the two methods. A significant difference related to the assessment of BETA angle. The above finding may have resulted from certain factors connected with conducting assessment with the use of photogrammetric and inclinometric techniques. In assessments based on photogrammetry, carried out with CQ device, the location of BETA angle depends on the ranges of curvatures. This angle is marked by the measuring system automatically, at a location where two curvatures overlap. In inclinometer the examiner marks BETA angle at a fixed point of thoracolumbar transition. It is also important to remember about other measurement errors discussed by Borek et al. [4], i.e. inaccurately determined anthropometric points necessary for performing the measurement, because in practice the error of determination amounts to 15 mm (± 5 mm) [26]; inadequate application of the inclinometer and error in its calibration due to incorrectly defined vertical alignment, resulting from miscalculation; all these add up in the measurement results (positively or negatively).

In photogrammetry the final result is affected by vertical alignment of the device, performed by means of the line levels on the camera, and then by errors in marking anthropometric points on the subject’s skin and in defining measuring points on the screen.

Of importance here is the examiner’s experience, as well as screen resolution, size of luminous spot and contrast of the defined points [4]. Moreover, the patients in this study had excessive body weight which may have distorted measurements of anteroposterior curvatures based on photogrammetry, due to excessive fatty tissue. The above factors may explain the differences in BETA angle measurements acquired in photogrammetric assessment and with gravitational inclinometer.

The presented findings are not consistent with the research results reported by Walicka-Cupryś et al. who assessed spinal curvatures with liquid-based inclinometer and photogrammetric technique [12, 23]. These authors did not find the two methods to be compatible. The results of examination may differ in this case due to the type of the applied device, because in assessments performed with liquid-based inclinometers there are two additional measurement errors, i.e. concave or convex meniscus, depending on inclinometer positioning, as well as error resulting from the viewing angle, because of which the readout may differ from the actual condition. In view of the above, the authors of the present study applied gravitational inclinometer which does not present these errors. The present findings are consistent with those reported by other authors who assessed the angle of kyphosis and lordosis using photogrammetry and imaging technique applying X-rays. When they compared photogrammetry results with X-ray images these authors found positive correspondence in the case of thoracic kyphosis angle, yet measurement of angle of lordosis with photogrammetric method carried significant error [27, 28]. Different findings related to measurement of lumbar lordosis were shown by Drzał-Grabiec et al., who reported that these two methods produced consistent results, yet the angular values identified with the non-invasive method were lower [10]. Leroux and Zabijek compared measurements of thoracic kyphosis; analysis of the results showed high correlation between the above methods. In the case of thoracic kyphosis the correlation coefficient was 0.89, while for lumbar lordosis it was slightly lower, at the level of 0.84 [29]. Saad et al. found high compatibility and repeatability of results in studies comparing values of Cobb angle, measured with photogrammetric technique and in X-ray examination [19, 30]. Comparative assessments of lumbar lordosis with methods based on radiography and photogrammetry were performed by Van Maanen et al. [31] and Iunes et al. [32] who confirmed concurrent validity of the two methods, and reliability of photogrammetry in assessing body posture.

As a result of technological progress a number of varied devices are available for assessing spinal curvatures. Notably, a number of non-invasive methods make it possible to perform posture assessment quickly, accurately, and safely, without putting a patient at risk of side effects of exposition to X-rays.

Analysis of the present findings and those reported by other authors confirms that both photogrammetry and inclinometer enable reliable posture assessment. An important argument is the compatibility of results obtained during photogrammetric assessment and X-ray examination, which currently is recognized as the most accurate method of posture evaluation. Inclinometric and photogrammetric assessments of anteroposterior spinal curvatures are based on the same assumptions as the commonly used Cobb method, which essentially involves identification of angle of inclination in the examined areas with respect to vertical axis [5]. The present study did not find significant differences in the inclination of the specific spinal segments, i.e. lumbosacral, and upper thoracic areas, or in the angle of lumbar lordosis and thoracic kyphosis identified during photogrammetric and inclinometric assessment. Therefore, it can be concluded that gravitational inclinometer may effectively be used for assessing spinal curvatures in sagittal plane, without the inevitable high expenditures linked with photogrammetric methods. Yet, to explicitly confirm whether or not the two methods produce consistent and reliable results it is necessary to conduct further comprehensive research with the use of more than two measuring methods, including inclinometric method, photogrammetry and X-ray.