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Evaluation of the iPhone with an acrylic sleeve versus the Scoliometer for rib hump measurement in scoliosis
© Izatt et al.; licensee BioMed Central Ltd. 2012
Received: 17 April 2012
Accepted: 12 July 2012
Published: 30 July 2012
Vertebral rotation found in structural scoliosis contributes to trunkal asymmetry which is commonly measured with a simple Scoliometer device on a patient's thorax in the forward flexed position. The new generation of mobile 'smartphones' have an integrated accelerometer, making accurate angle measurement possible, which provides a potentially useful clinical tool for assessing rib hump deformity. This study aimed to compare rib hump angle measurements performed using a Smartphone and traditional Scoliometer on a set of plaster torsos representing the range of torsional deformities seen in clinical practice.
Nine observers measured the rib hump found on eight plaster torsos moulded from scoliosis patients with both a Scoliometer and an Apple iPhone on separate occasions. Each observer repeated the measurements at least a week after the original measurements, and were blinded to previous results. Intra-observer reliability and inter-observer reliability were analysed using the method of Bland and Altman and 95% confidence intervals were calculated. The Intra-Class Correlation Coefficients (ICC) were calculated for repeated measurements of each of the eight plaster torso moulds by the nine observers.
Mean absolute difference between pairs of iPhone/Scoliometer measurements was 2.1 degrees, with a small (1 degrees) bias toward higher rib hump angles with the iPhone. 95% confidence intervals for intra-observer variability were +/- 1.8 degrees (Scoliometer) and +/- 3.2 degrees (iPhone). 95% confidence intervals for inter-observer variability were +/- 4.9 degrees (iPhone) and +/- 3.8 degrees (Scoliometer). The measurement errors and confidence intervals found were similar to or better than the range of previously published thoracic rib hump measurement studies.
The iPhone is a clinically equivalent rib hump measurement tool to the Scoliometer in spinal deformity patients. The novel use of plaster torsos as rib hump models avoids the variables of patient fatigue and discomfort, inconsistent positioning and deformity progression using human subjects in a single or multiple measurement sessions.
Previous studies [3–7] have found the Scoliometer to have adequate or high inter-rater and intra-rater reliability coefficients indicating good measurement reproducibility. The most recent of these by Bonagamba et al. , found the highest intra-rater and inter-rater reliability of the Scoliometer to date, as a result of minimising the major sources of measurement variability in earlier studies. It was suggested the process of patient positioning, vertebral level palpation, patient discomfort and fatigue with being positioned and measured multiple times, and repeat measurements occurring weeks apart, all contributed to Scoliometer measurement variability in prior studies.
The iPhone (Apple Inc, Cupertino, USA) is one of a recent generation of mobile phones which incorporate a MEMS (micro-electro-mechanical-system) accelerometer, which can accurately sense acceleration and inclination. The availability of various software applications for the iPhone which read and display the accelerometer signal allow it to be used potentially in a wide range of clinical applications to replace for example; the goniometer to measure peripheral joint ranges of motion, the protractor to measure Cobb angles , and in this study the Scoliometer for rib hump assessment in spinal deformity patients. The aim of this study was to quantify the measurement performance of the iPhone compared to the standard Scoliometer for the assessment of vertebral rotation in structural scoliosis.
This study also aimed to further minimise the major sources of measurement variability by using a set of plaster rib hump models of actual scoliosis patients retrieved from a specialist spinal orthotist who produces full body plaster torsos in the process of manufacturing custom made scoliosis braces. As a result, the current study eliminated patient inconvenience, the variability of patient positioning and posture, subject discomfort and fatigue, and the possibility of deformity progression between measurement sessions by multiple observers or the same observer. By minimising the known variables that occur when performing multiple measurements, this study aimed to better compare the measurement performance of the devices (iPhone and Scoliometer) rather than the combination of the measurement method and the devices.
Rib hump measurements
The plaster moulds were placed on a standard height bench in random order on four occasions, one week apart. Observers measured the rib humps with either the iPhone or the Scoliometer each week until all observers had measured the humps twice with each device. For all measurements, the observers were unaware of all previous measurements. The observers were free to select the location on the plaster model of the most severe rib hump angle, as would be the case when measuring the rib hump angle on the thorax of a spinal deformity patient in the forward flexed position. The observers were seated during angle measurements to ensure the moulds were consistently positioned at around eye level.
where n and n + 1 are successive measurements. 95% confidence intervals for intra-observer variability were calculated as (1.96×SD intra ) [9, 10] where SD intra is the standard deviation of the intra-observer differences Δα.
The inter-observer variability (standard deviation of the difference between measurements by two different observers) was calculated as √2×SD inter for a single measurement per observer, where SD inter is the standard deviation of the inter-observer differences . The 95% confidence intervals for inter-observer variability were calculated using 2.37×SD (t-distribution with 7 dof) .
In order to allow comparison with previous rib hump measurement variability studies, we also calculated the Intra-Class Correlation Coefficient (ICC) for repeated measurements of each of the eight plaster torso moulds by the nine observers. Both absolute agreement and consistency measures were assessed using a two-way model with SPSS (v 8.0 for Windows, SPSS Inc., Chicago, IL). Note that the absolute agreement measure is a more stringent ICC definition (ICC = 1 requires perfect agreement between all observers).
Assessment of the rib hump measurements using both devices, on eight plaster torsos by nine observers, with two assessments by each observer, gave a total of 280 out of a possible 288 measurements for analysis. One training Registrar failed to perform a second set of Scoliometer measurements. The overall mean rib hump angle for the group of plaster moulds was 16° ± 5.8 (range 6–30) representing the range of torsional deformities seen in clinical practice.
iPhone vs scoliometer comparison
Based on a single reading by each observer, the SD of a rib hump angle measurement was 1.5° for the iPhone and 1.1° for the Scoliometer. The inter-observer error (standard deviation of the difference between measurements by two different observers) is therefore √2×SD = 2.1° for the iPhone and 1.6° for the Scoliometer . The 95% confidence intervals for inter-observer error were ± 4.9° and ±3.8° for the iPhone and Scoliometer respectively, calculated using 2.365×SD (t-distribution with 7 dof).
Intra-class correlation coefficients
For iPhone rib hump measurements, the ICC was 0.924 using an absolute agreement definition, and 0.939 using a consistency definition. For Scoliometer rib hump measurements, the ICC was 0.947 using an absolute agreement definition, and 0.950 using a consistency definition.
The increasing popularity of mobile phones and hand held tablets incorporating micro-electromechanical system (MEMS) accelerometers has provided a new technology for accurate angle measurements. The ubiquitous nature of these devices and the ready availability of diverse software applications mean they may have a significant impact on efficiency and convenience in school screening programs and spinal clinics for assessment and diagnosis of spinal deformities. In the medical setting, software applications are available to measure Cobb angles and peripheral joint angles, display computed tomography and magnetic resonance imaging and provide alerts regarding clinical pathology results of individual patients direct to the treating doctor. These mobile technologies offer a convenient tool for the physician; however this necessitates scientific studies to ensure that measurements reported by the smartphone can be relied upon with respect to clinical management decisions for patients. In the current study, we present a novel methodology using static rib hump moulds fabricated from scoliosis patients and use this technique to assess the measurement performance of the iPhone compared to the Scoliometer. The rib humps of the plaster models ranged from 6 to 30° which represented a large range of trunkal asymmetries with the aim of being representative of those which would be encountered in clinical practice. As with the Bunnell Scoliometer, the iPhone together with the Scoliguage software application is a simple, inexpensive and portable method of measuring rib hump progression and a practical way to decrease exposure to radiation from repeated radiographs [3, 4, 12].
When the Spinal Orthopaedic Surgeons at our centre began trialling the iPhone to measure the rib hump of spinal deformity patients, it became clear that the iPhone alone, was for some patients, of inadequate length. For patients with more severe and/or angular rib hump deformities, the length of a mobile phone was unable to cover the full expanse of the ribcage rotational deformity. As a result the rib hump could be underestimated for these cases. The spinal surgeons were of the opinion that additional length was required to ensure measurement accuracy for all rib hump severities which lead to the development of the smartphone acrylic sleeve. The sleeve provides the required additional length and includes the central notch on the inferior edge of the drop-in device to mirror the shape of the traditional Scoliometer which is used to facilitate the placement of the device over the central spinous processes. Due to the larger size of the more recently available hand held tablets (iPad and similar devices), these devices are useful to measure rib hump angles in isolation but were not evaluated as part of the current study.
Although the correlation between trunkal asymmetry and vertebral column deformity is beyond the scope of this paper, it is important to recognise that when considering the correlation between trunk asymmetry measures and spinal deformity, previous work by Grivas et al., found that in children aged 7–13 years the concordance between trunk and spinal deformity was weak but became stronger for children aged 14–18 years. It should also be noted that trunkal asymmetry measurements alone are not sufficient for determining a definitive patient diagnosis and management plan .
In this study, the mean difference between pairs of iPhone and Scoliometer measurements was small, with a mean absolute difference of just over 2°, with a small bias of 1° toward higher rib hump angles with the iPhone and a 95% confidence interval of just over 3°. All of these figures are less than the minimum 5° difference which is widely accepted as signifying a clinically significant change in rib hump deformity. Therefore, we conclude that the iPhone is a clinically equivalent measuring tool to the traditional Scoliometer.
Furthermore, the inter- and intra-observer measurement variability using the iPhone were found to be similar to that of the Scoliometer in the current study. As with nearly all previous studies, the 95% confidence intervals for inter-observer variability were higher than those for intra-observer variability, for both the iPhone and the Scoliometer. Carman et al. note that the intra-observer variability is a more clinically relevant parameter than the inter-observer variability because intra-observer differences can lead to misdiagnosis of rib hump progression, thus influencing clinical treatment decisions. However we note that inter-observer variability may be equally important in large public spinal clinics where different clinicians are likely to assess patients on subsequent clinic review visits. Furthermore, the Intra-class Correlation Coefficients reported in this study (ICC = 0.92-0.95) compare favourably with ICCs reported (0.81-0.95) in previous Scoliometer studies [4–7]. This is to be expected since our use of plaster rib hump moulds eliminates the variability that is due to patient posture, patient fatigue and deformity progression which may all have contributed to the measurement variability results in previous studies. We note that using the iPhone in the clinical setting to measure trunkal asymmetry is subject to patient-positioning variability, and this variability is an unavoidable clinical factor which will occur regardless of the chosen measurement device used.
Clinical judgements as a result of iPhone rib hump measurements can be made with confidence based on readings taken from the iPhone when combined with the acrylic sleeve.
The inter- and intra-observer measurement variability using the iPhone were found to be similar to that of the Scoliometer.
The novel use of plaster torsos as rib hump models avoids the variables of patient fatigue and discomfort, inconsistent positioning and deformity progression using human subjects in a single or multiple measurement sessions.
Sources of support
No financial support was received for this study. None of the authors have any commercial relationship with Apple or with the producers of the Scoliguage software application mentioned in this article. GB Orthopaedics Australia (author GB) designed and provided the acrylic sleeve used in this study.
The authors wish to acknowledge the contribution of Doctors Geoffrey Askin, William Ryan, Robert Labrom, Paul Licina, Alan Carstens, Mostyn Yong, and Physiotherapist, Ms Sally Oliver Lange who with the authors performed the rib hump angle measurements in this study.
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