Rasterstereographic back shape analysis in idiopathic scoliosis after anterior correction and fusion

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Abstract

Objective. To determine the accuracy of rasterstereographic three-dimensional back surface analysis and reconstruction of the spine in cases of severe idiopathic scoliosis treated by anterior correction and fusion.

Design. Comparison of digitized radiographic curves and rasterstereographic curves by best fit superimposition and calculation of root mean square differences as parameters of similarity.

Background. Rasterstereography has been proven to be accurate in scoliosis up to 50° Cobb angle. Since 1989 the device is in clinical routine use for non-operatively treated patients and reduces the need for otherwise indispensable radiographs significantly.

Methods. Fifty two patients with severe idiopathic scoliosis with Cobb angles up to 88° were examined rasterstereographically and radiographically. Forty eight pre-operative anterior–posterior radiographs and 101 post-operative anterior–posterior radiographs were digitized. Radiographic and rasterstereographic curves were compared and the root mean square differences were calculated as parameters of precision of rasterstereographic reconstruction.

Results. Accuracy of rasterstereography in idiopathic scoliosis with Cobb angles between 50° and 88° is satisfactory. The root mean square difference of the radiographic and rasterstereographic curves was 6.4 mm for lateral deviation and 4.5° for vertebral rotation. After anterior scoliosis surgery the precision of the device is good. The root mean square difference for lateral deviation was 3.4 mm and 3.2° for rotation. Considering both groups an average root mean square of 4.7 mm and 3.7° was calculated.

Conclusions. Accuracy in severe scoliosis up to 88° Cobb angle was satisfactory. The results of this first evaluation of surgically treated severe scoliosis showed a good accuracy after anterior surgery. The system can be used for post-operative follow up examinations and may reduce the number of X-rays considerably. In contrast to radiography, CT or MRI rasterstereography provides an objective quantification and documentation of the post-operative cosmetic improvement of the back shape in standing posture.
Relevance statement

Based on the findings of this study rasterstereography in future enables both objective quantification of cosmetic improvement and significant reduction of X-rays in idiopathic scoliosis with Cobb angles higher than 50° before and after anterior surgical correction and fusion.

Introduction

Rastersterography represents a reliable method for three-dimensional back shape analysis and reconstruction of spinal deformities without ionising radiation exposure (Drerup and Hierholzer, 1994; Drerup and Hierholzer, 1996). The patient can be examined within seconds in standing posture without any markers on the surface of the back. The method uses a system of parallel light lines projected onto the back surface. From the distortion of the raster lines the 3-d shape of the surface can be reconstructed with an accuracy of a fraction of a millimetre, provided that a properly calibrated system is used. By a sophisticated mathematical shape analysis various features such as anatomical landmarks, transverse and sagittal profiles, the symmetry line (as a model for the line of spinous processes), scapula contours, asymmetry etc. can be determined. The device provides print-outs of an overview of the back surface (distorted transversal profiles) (Fig. 1a and b) and of three curves to visualize lateral deviation of the spine (frontal view), sagittal profile (lateral view) and surface (vertebral) rotation on the symmetry line (Fig. 2a and b). Furthermore a three-dimensional model of the whole spine is presented on the computer display. The evaluation procedure needs less than 1 min of computing time. As compared to other examination techniques, rasterstereography is relatively inexpensive, fast and largely objective. Furthermore, it delivers additional information, for example on cosmesis. However, interpretation of the results requires some experience.

It has been demonstrated previously that by utilizing of rasterstereography the number of X-rays needed in the conservative treatment of idiopathic scoliosis with Cobb angles up to 50° could be reduced (Liljenqvist et al., 1998). Based on comparisons of digitised radiographic and rasterstereographic data of 114 patients the reliability of the system could be proven for scoliosis with Cobb angles up to 50° (Drerup and Hierholzer, 1994). The root mean square (RMS) of lateral deviation amounted to 4 mm and of vertebral rotation to 3°. The comparison with frontal radiographs was chosen because this is still the most widely used method of diagnosis and because the rasterstereographic results can well be presented in a form which is easily and directly comparable with X-rays.

In another study comparing non-digitised radiographs, Liljenqvist et al. (1998) found a RMS difference of 7.9° for the apical vertebral rotation of 95 patients. So far the system was evaluated for non-operatively treated moderate deformities only. This paper reports on the first application of rasterstereography on severe scoliosis with Cobb angles higher than 50° which were operatively treated by Zielke or Halm–Zielke Instrumentation (VDS) (Zielke, 1982; Halm et al., 1998). The VDS is known to enable a 50% vertebral derotation (Halm et al., 1998; Kostuik et al., 1989; Giehl et al., 1989; Lowe and Peters, 1993; Moe et al., 1983; Moskowitz and Trommanhauser, 1993) and sufficient correction of the rib hump. In particular after VDS an objective verification and quantification of the post-operative vertebral derotation and cosmetic improvement appears important.

Other authors (Theologis et al., 1993; Raso et al., 1998) tried to quantify and asses the cosmetic defect of scoliotic back surface previously. They consider the rib hump, scapula asymmetry and the trunk distortion as the major factors affecting the appearance of the back. Iwahara et al. (1998) developed a cosmetic score and considered it for clinical application. Suzuki et al. defined the Posterior Trunk Symmetry Index as an indicator to asses trunk asymmetry in scoliotic deformities (Suzuki et al., 1999; Inami et al., 1999). The evaluation was based on Moiré pictures. Asher et al. examined the trunk asymmetry of patients with scoliosis after posterior spinal instrumentation by rasterstereographic surface analysis to improve the surgical techniques (Asher and Manna, 1999). Jefferson et al. (1988) report on the examination of patients after posterior operation (Harrington instrumentation) by means of the ISIS scanner.

Section snippets

Aims of study

From the biomechanical point of view the aim of the study was the evaluation of the system’s precision in measuring the deformation of the spine in severe scoliosis prior to surgery and following anterior correction and fusion. From the literature no data on three-dimensional back surface analysis after anterior surgical correction of spinal deformities are available. We decided to examine anteriorly instrumented scoliosis first to avoid artefacts by scars and prominent implants after posterior

Methods

Fifty-two patients (mean age 17.7 years, 13–26) with thoracic, thoracolumbar and lumbar idiopathic scoliosis who underwent Zielke or Halm–Zielke Instrumentation were examined. Minimum post-operative follow up was two years (mean 37.6 months, 27–62). Ten patients underwent Zielke Instrumentation and 42 Halm–Zielke Instrumentation. This technique is based on the principles of the original Zielke Instrumentation but uses two instead of one rod.

The patients were examined by rasterstereography and

Results

The Cobb angle of the instrumented primary curves of the deformities could be reduced from 57.2° (52–88°) to 17.2° (6–34°). The non-instrumented thoracic secondary curves spontaneously corrected from 34.5° (16–48° ) to 21.5° (7–43°). At follow-up the Cobb angle of the primary curve showed an increase to 20.8° (9–43°), no loss of correction was registered for the secondary curve.

The maximum difference between the radiographic vertebral rotation of the primary and secondary curve (derotation

Discussion

In contrast to the patient’s view to the cosmetic improvement of the back surface the surgeon assesses the results of surgical correction of spinal deformities predominantly by measuring the Cobb angle and sagittal profile. Clinical assessment of the rib hump and photographs were used for the documentation of derotation effects. Precise and objective data on surface and vertebral derotation after anterior surgery are missing in many publications (Zielke, 1982; Kostuik et al., 1989; Lowe and

Conclusion

The authors consider rasterstereography to be a reliable tool for the objective quantification of cosmesis in severe idiopathic scoliosis treated by anterior surgery. The device provides a reliable tool to calculate vertebral rotation after scoliosis surgery, which is difficult to obtain by means of radiographs. The accuracy of rasterstereography in scoliosis after correction and fusion by anterior surgery is good and comparable with that of non-surgically treated deformities with similar Cobb

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