A review of the trunk surface metrics used as Scoliosis and other deformities evaluation indices
© Patias et al. 2010
Received: 3 May 2010
Accepted: 29 June 2010
Published: 29 June 2010
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© Patias et al. 2010
Received: 3 May 2010
Accepted: 29 June 2010
Published: 29 June 2010
Although scoliosis is characterized by lateral deviation of the spine, a 3D deformation actually is responsible for geometric and morphologic changes in the trunk and rib cage. In a vast related medical literature, one can find quite a few scoliosis evaluation indices, which are based on back surface data and are generally measured along three planes. Regardless the large number of such indices, the literature is lacking a coherent presentation of the underlying metrics, the involved anatomic surface landmarks, the definition of planes and the definition of the related body axes. In addition, the long list of proposed scoliotic indices is rarely presented in cross-reference to each other. This creates a possibility of misunderstandings and sometimes irrational or even wrong use of these indices by the medical society.
It is hoped that the current work contributes in clearing up the issue and gives rise to innovative ideas on how to assess the surface metrics in scoliosis. In particular, this paper presents a thorough study on the scoliosis evaluation indices, proposed by the medical society.
More specifically, the referred indices are classified, according to the type of asymmetry they measure, according to the plane they refer to, according to the importance, and relevance or the level of scientific consensus they enjoy.
Surface metrics have very little correlation to Cobb angle measurements. Indices measured on different planes do not correlate to each other. Different indices exhibit quite diverging characteristics in terms of observer-induced errors, accuracy, sensitivity and specificity. Complicated positioning of the patient and ambiguous anatomical landmarks are the major error sources, which cause observer variations. Principles that should be followed when an index is proposed are presented.
Our interest in the study of the trunk surface (TS) deformity is recently increased due to a variety of reasons.
The cosmetic improvement of the trunk after any treatment is of paramount importance to the child under treatment and his family. The TS symmetry is what it is seen and praised by them and not the radiograph itself which is traditionally used by the physician. TS symmetry is also one of the elements intergrading and improving the quality of life of patients, an issue vital for any human being . This was actually the motivation behind both the development of a variety of devices for documentation and evaluation of TS shape and the creation of a variety of indices that are currently used to access the state of such deformities.
The concept is how to collect data related to TS on physiology, to document the pathology, to assess the effect on the TS deformity of any surgical or conservative treatment comparing the pro- to post-treatment state. The characterization of the threshold of normality to pathology is a complex issue that also needs investigation. Although not yet sensitive enough to detect small changes for monitoring of curve natural progression, TS analysis can help to document the external asymmetry associated with different types of spinal curves in scoliosis as well as the cosmetic improvement obtained after surgical interventions .
The review and the evaluation of the TS metrics used as Scoliosis or any deformity evaluation indices would be very useful and would offer some objective assessing tools for the interested physicians.
Scoliosis is a deformity of the spine in which there are one or more lateral curvatures deviating from the midline in the coronal plane. Although scoliosis is characterized by lateral deviation of the spine, a 3D deformation actually is responsible for geometric and morphologic changes in the trunk and rib cage .
The goal of scoliosis screening is to detect scoliosis at an early stage, when the deformity is likely to go unnoticed and there is an opportunity for a less invasive method of treatment, or less surgery, than would otherwise be the case. What in reality scoliosis school screening program does, using the scoliometer or any other surface measuring device, is reveal children with surface, mainly thoracic, deformity. It does not reveal the scoliosis per se. It is now definitely accepted that the surface deformity does not accurately predict the magnitude of scoliosis, especially in younger children. As Bunnell characteristically states  "it has become apparent from many reports that, although there is a significant correlation between clinical deformity and radiographic measurement, the standard deviation is so high that it is not possible to reliably predict the degree of curvature from surface topography in any given patient by any technique".
Traditionally, scoliosis screening is done either by Adam test or using other optical techniques, while the radiographic measurement of Cobb angle is considered the golden standard.
The first step in the scoliosis examination is simple inspection. This includes inspection of a standing patient from behind and optical evaluation of asymmetries in shoulders, scapulae, waistline and the distance of the arms from the trunk, as well as the "balance" of the head.
▪ Thoracic scoliosis with Cobb angle ≥10° - sensitivity 74% - 84%, specificity 78%-93%
▪ Thoracic scoliosis with Cobb angle ≥20° - sensitivity: 92% - 100%, specificity 60% - 91%
▪ Lumbar scoliosis with Cobb angle ≥20° - sensitivity 73%, specificity: 68%
▪ Scoliosis with Cobb angle ≥40° - sensitivity 83%, specificity 99%
High Sensitivity means low rate of false negatives, ie. the number of scoliotic patients classified as normal is small.
High Specificity means low rate of false positives, ie. the number of normal patients classified as scoliotic is small.
It is very important to note that in younger children the concordance of the surface and spinal deformity is weak and it becomes stronger as the children are growing up. Therefore, in younger children with surface trunk asymmetry, the prediction of the spinal deformity alone from the surface topography is inaccurate, simply because surface topography reveals the thoracic cage and the spinal deformity together.
It has also been reported that, in typical screening settings where the prevalence and positive predictive value are relatively low, for every curve >10° detected, there are 1-5 false-positives; similarly, for every curve > 20° detected, there are 3-24 false-positives .
The angle measured by a scoliometer does not correspond to the Cobb angle measured on a radiograph . Furthermore the Cobb angle alone cannot explain the whole of the surface deformity . As a consequence, not all patients with radiographic scoliosis have rotation of the trunk, and not all patients with trunk rotation have radiographic scoliosis . Goldberg  and Kotwicki  agree that "surface parameters corresponding with radiological ones are neither possible nor expedient as both methods focus on different aspects of the deformity. The 3D presentation accompanied by numerical data that is produced in surface topography offers a more complete perspective of the deformity of the back surface and enables a more thorough analysis of the patient's deformity pattern".
▪ The Cobb angle describes only one plane of the 3D deformity.
▪ The Cobb angle is not linearly proportional to the severity of scoliosis in a linear fashion (ie, a curve with a Cobb angle of 40° is more than twice as severe as a curve with a Cobb angle of 20°).
▪ Cobb angle measurement has a reported intra-observer variability of 2.8°-4.9° and an inter-observer variability of 6.3°-7.2° [19, 20] when traditional techniques are used. Recent advances in measurements on digitally acquired radiographs provide far more accurate results, with a reported intra-observer and inter-observer variability of 1.3° .
Back surface mapping for scoliosis screening has been used for many years as a valid alternative to either use of x-rays or scoliometer measurements. From the beginning it became clear that "Because surgeons are so familiar with Cobb angle measurements on radiograph, the introduction of new surface shape measures whose meaning may not be readily apparent to clinicians has been difficult" . This explains the effort over the years to relate surface shape parameters with Cobb angle [eg. [23–25, 71]].
However, over the years it became apparent that the Cobb angle measures only one aspect of the 3D deformity and that the correlation between the Cobb angle and the surface parameters is negligible [26, 6, 27]. However, it is noted that the more severe the Cobb angle the more the surface deformity is pronounced.
Lately, many researchers are seriously questioning the effectiveness of such efforts and strong statements, like this appear "Searching for relationship between radiological Cobb angle and surface parameters with making presumption that the higher correlation with Cobb angle, the better the surface technique may be one of the reasons that introduced the surface topography in a blind alley. In fact, Cobb angle is nothing more than a shadow of two limit vertebrae. It is not clear what would be the rationale to expect that so constructed angle should highly correlate with any of the surface describing parameters." . And as Kotwicki states it "When debating on the role of the surface topography in the evaluation of the body morphology in children with idiopathic scoliosis, one should begin with rejecting the dogma of the radiological Cobb angle, as the only gold standard for scoliosis evaluation. " .
Optical systems have been developed as non-invasive imaging techniques. Examples of such systems are the Moiré-fringe mapping , the structured light techniques like the Integrated Shape Imaging System (ISIS) [30–34], or the Quantec system [35, 14, 36, 37] or the Ortelius  scanners, and devices that scan 360° torso profiles [38–41], ultrasound systems , 3D body scanners (eg. Inspeck, Cyberware, TC2, Minolta Vivid, Vitus 3D, etc) , the Formetric video-raster-stereography system http://www.diers.de[72–76] and last but not least stereo-photogrammetric systems [43–47].
▪ There is no correlation between Moiré asymmetry and the Cobb angle
▪ The risk of obtaining false negatives is low (ie. high sensitivity)
▪ The risk of obtaining false positives is high (ie. low specificity)
In a vast related medical literature, one can find quite a few scoliosis evaluation indices, which are based on back surface data and are generally measured along the three planes (coronal, transverse and sagittal). However, there exist no coherent presentation of the underlying metrics, the involved anatomic surface landmarks and the definition of the planes and the related body axes they refer to.
Generally speaking, the scoliosis parameters which have been used up to now belong to one of the following groups: (a) the first group includes indices which are specific to the measurement technique. These indices depend on the measurement technique, which means that cannot be measured and by other means. Such examples are eg. the angles q1 and q2 in QSIS which are angles formed by the tangents to the corresponding fringes in the Moiré system. Obviously, theses cannot me measured with other means than moiré. (b) The second group are indices independent of the measuring technique. This makes them more useful, since they can be used to evaluate scoliosis given that the back surface topography is known in 3D, regardless of the measuring techniques used. Such example is eg. the Angle of Trunk Rotation index, which can be evaluated by scoliometer measurement, by moiré techniques or by any other 3D surface measurement.
Scoliosis surface parameters after 6th SOSORT consensus paper
Position/view of the patient for surface topography measurement [table eighteen]
Position: standing upright
Anatomic surface landmarks to be taken into consideration systematically [table nineteen]
Posterior iliac spines
Surface parameters recommended for systematic use [table twenty]
Body axis definition
Analogous to radiological VCSL
Frontal plane analysis
Sagittal plane analysis
Relation of C7 to S1
Transverse plane analysis
Trunk rotation main curve
Trunk rotation Compensatory curves
Scoliometer ATR measure for transverse plane deformity
Cobb angle measurement as radiological parameter
In order for any measurement taken at different times to be mutually comparable, either the involved metrics should be coordinate-free or they should refer to the same coordinate system.
The first case is rather rare and refers to metrics like areas, volumes, etc. The second case is the usual case and mainly refers to coordinates, angles, distances and the like. In this latter case there is a need to establish a coordinate system, which is stable between the screening sessions.
The VCSL (Vertical Central Sacral Line) line is also used in QSIS system, in POTSI and DAPI index definition, etc., while the Z axis definition is compatible to that used in SHS and DAPI (see section 6).
One can find quite a number of scoliotic indices in the literature. Here, for methodological reasons, we are going to present them grouped by the plane they refer to. The reason for such a presentation is twofold: first, to present them in a logical way according to the type of deformity they are able to measure; and secondly to lead the discussion to the degree of correlation existing among them.
The spinous process line of Jaremko  and the similar but qualitative indices used in WRVAS (Walter-Reed Visual Assessment Scale) [27, 24] belong to this logic line. Similar to them are also the ASY1 index of  as well as the Integrated Shape Imaging System (ISIS2) LA (Lateral Asymmetry) index . In the latter, a 5th order polynomial is fitted through the spinous process line (as depicted by 19 transversal sections).
Understanding scoliosis or other trunk deformity is a complex issue since it evolves in three dimensional space. Many technologies have been developed and used over the years and each technology offers new approaches in understanding and describing scoliosis through different sets of indices. Out of this massive data the scientific society has to choose measures and define methodologies in order to optimally diagnose, quantify, document and assess the progression of scoliosis for both clinical treatment and cosmetic improvement.
It is clear now that surface metrics have very little correlation to Cobb angle measurements (eg.  regarding POTSI index). In addition, it has also been reported that patients with double curves have significantly less trunk deformity in both the transverse and coronal plane than patients with thoracic and thoraco-lumbar curves of similar Cobb size .
It should also be clear that indices measured on different planes do not correlate to each other. Examples are Cobb angle vs. Scoliometer angle, Cobb vs. Rib and Flank prominence, etc.
Characteristics (observer-induced errors, accuracy, sensitivity and specificity) of different popular indices
Threshold for scoliosis cases
Threshold for change (as suggested by Asher )
It is clear that complicated positioning of the patient and ambiguous anatomical landmarks are the major error sources, which cause observer variations. For instance, moiré techniques generally suffer from errors due to malpositions of the patient and generally require strict and cumbersome protocols for positioning the patient. "A major drawback of moiré topography is that while the shape information is displayed, it is not in a form which can be unambiguously interpreted" . POTSI index is reported  to introduce errors due to the difficulty in situating the points involved for calculating the index, as some of them are located in the shaded areas, while they are not anatomical points easily and uniquely identifiable. "The ISIS system lacked accuracy mainly because of the difficulty of distinguishing adequate landmarks due to shadowing effect" .
Therefore, based on the experience gained from this extended literature review, we think it is useful to lay down the principles that should be followed when an index is proposed.
Indices should be measured with the maximum achievable accuracy and in a direct manner. For instance, Coordinates and Angles are direct measurements whereas areas, volumes etc. are indirectly calculated from other direct measurements. Therefore indices based on direct measurements are more accurate and should be preferable.
Indices should be independent from the method of measuring the back surface deformities. If this is not the case then indices can not be of universal use, and will also highly depend on the current technology.
Indices should be based on robust procedures and automatic measurements and should be evaluated by automatic processing techniques, eliminating as far as possible the human intervention. The reported levels of inter-/intra-observer variability and accuracy of the indices used so far reveals this problem. Only with automation the observer variability, the human induced errors, objectivity, and required experience will be eliminated.
Indices should be based on automatically detectable and uniquely identifiable anatomical landmarks. This is closely connected to point No. 4 above. Both the landmarks used and the measured points on the back surface should be unambiguously positioned, properly signalized and automatically detected and measured on the image.
Indices should require simple measuring protocols. Complicated or demanding protocols are sources of errors. This includes also (and especially) patient position and orientation relative to the sensor, lighting conditions, etc. Indices should be independent from and robust with respect to these parameters as much as possible.
6. Indices should be normalized in order to be comparable among patients. This means that the indices should not depend on the trunk size, on the width of the waist or the length of the arms. In this respect, indices should be unitless, percentages etc.
Indices should provide a stable datum for progress monitoring over time. This means that indices should either be coordinate-system-free of refer to a coordinate system which is stable over time.
Indices should be able to distinguish between different types of surface deformities, ie. Coronal/Transverse/Sagittal, Left/Right semi-trunk, Thoracic/Thoraco-Lumbar/Lumbar, Single/Double curves.
Indices should provide a clear and safe difference in magnitude between normality and pathology, so that pathology can be safely distinguished and diagnosed. This actually means increased sensitivity and specificity. It also means that the indices should have small typical error relative to the smallest change (progression) we would like to detect.
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