Functional examinations with rasterstereography. A new method for the biomechanical analysis of skeletal geometry

Video rasterstereography has been developed far optical back shape measurem ent and for biomechanical analysis of spinal and pelvic geometry. Analysis of one single measurement permits 3-dimensional reconstruction of the back surface and calculation of shape parameters including pelvis tilt and torsi on. In addition, estimates of the lateral deviation of the spinal midline a nd of vertebral rotation are provided. its extended analytic potential make s rasterstereography a very appropriate tool for functional examinations. T he term "functional examinations" refers in this context to biomechanical a nalysis of functional movements of the spine and pelvis caused by quasi-con tinuous changes of posture, if these can be observed as changes in back sha pe. Two examples are given to illustrate the aim and performance of functio nal examinations. Shoe elevation is used to correct leg-length discrepancy and is therefore p rescribed for prevention and correction of scoliosis produced by pelvic obl iquity. In a previous study it was shown that simulating leg-length discrep ancy by raising a foot causes the pelvis to perform a torsional movement ab out the transverse axis. In effect, this movement reduces to some extent th e effect of shoe elevation; thus a larger elevation might give better resul ts. LIZ scoliotic patients underwent functional examination. Leg-length dis crepancies were simulated in 7 steps, and the resulting back shape was anal ysed by rasterstereography. The measurements were corrected for pelvic tors ion. This method provided satisfactory correspondence with radiographically recorded leg-length discrepancies, i.e. 0,7 mm +/- 11,2 mm. One specific a dvantage of this procedure is that it covers aspects relating to spinal lat eral deviation and vertebral rotation. It is concluded, however, that the 7 measurements used are hardly sufficient for this application and that bett er results are therefore to be expected from extended series. Kyphosis a nd lordosis clearly depend on posture. This is confirmed in a fu nctional examination where these angles are measured under voluntary change s of posture, in for-ward bending, the trunk straightens and kyphosis and l ordosis angles decrease. The reverse case applies to backward bending. If t his effect is taken into quantitative consideration,an improved accuracy of measurements is obtained by reference to a standardised, mathematically de fined posture. The resulting rms-error of kyphosis/lordosis measurement is then reduced from 3.10 degrees /2.95 degrees to 1.65 degrees /1.40 degrees. These figures open up new applications in the follow-up of kyphotic and lo rdotic deformities.