Spondylometry

Spondylometry is the measurement of metric and angular parameters characterizing the condition of the spine. The use of objective quantitative values in vertebrology is necessary to predict the course of deformations, identify local pathological processes, as well as to enable independent reproduction of the same parameters by different researchers and the exclusion of the subjectivity factor when examining a patient and assessing treatment results.

Absolute metric and angular parameters, as well as some relative indicators expressed in decimal fractions and percentages, are calculated clinically, based on data from X-rays, computed tomography and magnetic resonance imaging.

The importance of quantitative indicators should not be overemphasized. There is a known fact when three independent radiologists analyzed the same radiographs of a deformed spine to determine the magnitude of scoliosis. The fluctuations in the measured angular values averaged 3.5°, and in some cases they reached 9°. Then, one radiologist, who did not participate in the first study, determined the magnitude of scoliosis on the same radiograph at fairly long intervals (several months). The differences in the results were similar to those in the first study. This allows us to consider a value close to 4° as an acceptable measurement error associated with subjective reasons. However, if, during multiple dynamic studies, a unidirectional repeatability of the error is noted (for example, in the direction of increase), then this value reflects the true dynamics of the process.

Considering it unnecessary to describe all known methods of quantitative evaluation of radiographs, we have limited ourselves to those that are currently most widely used in vertebrology and traditional orthopedics, and, in addition, are of fundamental importance for the characterization of spinal pathology. Special methods of spondylometry used in the evaluation of specific nosologies - congenital deformities, spondylolisthesis, etc. are given in the corresponding sections of the book.

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Clinical methods of spondylometry

The mobility of the spine in the frontal plane is measured with the trunk tilted to the right and left. The normal range of lateral mobility of the thoracic spine, confirmed by X-ray data, is 20°-25° (10°-12° in each direction), and of the lumbar spine - 40°-50° (20°-25°).

The mobility of the thoracic and lumbar spine in the sagittal plane is measured in a standing position by the change in the distance between the spinous processes of the T1-T12 and T12-L5 vertebrae. When bending forward, these distances in an adult normally increase by 4-6 cm (Ott's test) and 6-8 cm (Schober's test), respectively. According to X-ray data, the sagittal mobility of the thoracic spine is 20°-25°, and of the lumbar spine, 40°.

Spinal torsion is clinically assessed at the apex of the deformation with the patient standing on straight legs with the torso tilted forward (Adams test). At the level of the greatest asymmetry of the paravertebral muscles or ribs, the height of the sections symmetrically removed from the spinous process is measured relative to the horizontal line (the so-called determination of the height of the hump) or the angle of deviation of the tangent to the posterior sections of the chest (the Schultes method for determining the torsion angle).

For clinical qualitative and quantitative assessment of the spine, the concepts of compensation and stability of deformation in the frontal plane are also used. Deformation is considered compensated if the plumb line, dropped from the spinous process of the C7 vertebra, passes along the intergluteal fold of a standing patient. The magnitude of decompensation (in mm) is determined by the magnitude of the plumb line deviation from this position to the right or left. Deformation is considered clinically stable if the plumb line is projected at the middle of the distance between the feet.

Radiation methods of spondylometry

Standard X-ray examination of the spine should be performed in two projections with the patient lying on his back and on his side. It is important to emphasize that when measuring the magnitude of deformation, it is necessary to refer to the method by which it was performed, since the difference in results obtained using different methods can be 10° or more.

Determination of the magnitude of spinal deformation in the frontal plane. Methods for calculating the magnitude of spinal deformation in the frontal plane are based on determining either the magnitude of the deformation arc between neutral vertebrae (Cobb and Fergusson methods) or the sum of the deformation components - the wedge shape of the vertebral bodies and intervertebral discs (E.A. Abalmasova method). Due to its complexity, E.A. Abalmasova's method has not found wide practical application and is used mainly to assess the functional mobility of individual vertebral-motor segments.

The most widely used method in orthopedics is the Cobb method, based on measuring the angle formed either by the intersection of straight lines drawn tangent to the roots of the arches or along the cranial or caudal endplates of the upper and lower neutral vertebrae, or by perpendiculars restored to them. It should be noted that the term "Cobb method" was formed historically, thanks to the active practical work of J. Cobb (American orthopedist), who popularized the Lippmann method (1935) for assessing the magnitude of scoliosis.

Fergusson's method is based on measuring the angle formed by the intersection of lines connecting points conventionally taken as the "centers" of the apical, as well as the upper and lower neutral vertebrae. The centers of the vertebrae are determined by the intersection of diagonals drawn on the anteroposterior radiograph through the vertebral bodies.

For the qualitative and quantitative characteristics of the mobility of spinal deformity, A.I. Kazmin proposed a stability index, which is determined by the formula:

Ind _st = (180-a)/(180-a1),

Where a is the magnitude of the scoliotic arc measured in the lying position, a1 is the magnitude of the arc measured in the standing position. In this formula, the magnitude of the angles a and a1 is calculated according to the rules of classical orthopedics, i.e. from 180°, and the measured angle is adjacent to the Cobb angle. For absolutely rigid deformations, the index value is 1.0, for mobile deformations it decreases and tends to 0.

Determining the magnitude of spinal deformation in the sagittal plane. To assess the magnitude of kyphotic deformation, three indicators are most often used - the kyphotic Cobb angle, ventral and dorsal angles. The principle of calculating the kyphotic Cobb angle is similar to determining the scoliotic Cobb angle. On the lateral radiograph, the lines forming the angle are drawn in children - along the discs adjacent to the neutral vertebrae, and in adults (after closure of the apophyseal growth zones) along the endplates of the neutral vertebrae closest to the apex of the kyphosis. The Cobb angle is formed by the intersection of either these lines or perpendiculars restored to them. With regard to kyphosis, a technique similar to the Cobb method was described by Constam and Blesovsky with the only difference that they calculated the deformation value not from 0, but from 180 ° (which corresponds to classical orthopedic canons).

The ventral angle of the kyphosis is formed by the intersection of lines tangent to the anterior surface of the vertebral bodies drawn along the cranial and caudal knees of the kyphosis. The intersection of tangents drawn along the apices of the spinous processes of the upper and lower knees of the kyphosis forms the dorsal angle.

In practical work, the determination of the ventral and dorsal angles of kyphosis is of lesser importance than the determination of the Cobb angle. This is explained by the presence of not always "even" anterior and posterior surfaces of the upper and lower knees of the deformation, and the tangents to them are often not so much straight as very intricately curved curves.

Determining the size of the spinal canal. The shape and size of the spinal canal in the horizontal plane are not constant along the spinal column, differing significantly in the cervical, thoracic and lumbar regions. It is believed that at the level of the C1-C3 segments, the spinal canal is a funnel tapering downwards, in the lower cervical, thoracic and upper lumbar regions it has a cylindrical shape with a uniform increase in sagittal and frontal sizes. At the level of physiological thickenings of the spinal cord (C5-T1 and T10-T12), the spinal canal expands in the frontal plane by 1-2 mm compared to adjacent sections. In the caudal regions (lower lumbar and sacral), the frontal size of the spinal canal prevails over the sagittal one, while the canal cross-section changes from round to irregular ellipsoid.

Changes in the shape and size of the spinal canal or its segments are most often a sign of serious diseases of the spine and spinal cord. Modern technical capabilities of CT and MRI machines allow for direct, accurate calculation of any parameters of the spinal canal, including its area or the area of its segments.

In real practice, however, the doctor most often deals with conventional survey radiographs and uses them to make an approximate assessment of the size of the spinal canal. The main values measured from survey radiographs are the interpedicular distance and the sagittal dimensions of the spinal canal.

The interpedicular distance corresponds to the largest frontal dimension of the spinal canal and is measured on the anteroposterior radiograph between the internal contours of the roots of the arches. Its increase is characteristic of intracanal space-occupying processes, explosive fractures of the vertebral bodies, and spinal dysplasia. The combination of a local increase in the interpedicular distance with a concavity of the internal contour of the root of the arch (normally the latter is visualized as a biconvex ellipse) is described as the Elsberg-Dyke symptom (see terms). A decrease in the interpedicular distance (the so-called frontal stenosis of the spinal canal) is characteristic of some hereditary systemic skeletal diseases (for example, achondroplasia), congenital malformations of the vertebrae, and the consequences of spondylitis suffered at an early age.

The main sagittal dimensions of the spinal canal - the midsagittal diameter, the size of the pockets (canals) of the nerve roots and the root openings - can be determined from a lateral radiograph of the spine.

Spinal canal stenosis in the sagittal plane is characteristic of some variants of congenital vertebral defects, degenerative disc diseases, neurologically unstable spinal injuries (burst fractures and fracture-dislocations). Local sagittal expansions of the spinal canal are typical for intracanal space-occupying processes.

Epstein's method - determination of the largest anteroposterior size of the intervertebral foramen - the so-called foraminal size.

The Eisenstein method - determining the smallest distance between the middle of the posterior surface of the vertebral body and a line drawn through the middle of the upper and lower intervertebral joints - corresponds to the size of the nerve root canals.

Hinck's method - the smallest distance between the posterior surface of the vertebral body and the inner surface of the arch at the base of the spinous process - corresponds to the midsagittal diameter of the spinal canal.

It should be remembered that radiographic methods do not allow us to estimate the true dimensions of the canal, but only the distances between their bone walls. Hypertrophied capsules of intervertebral joints and herniated discs are not visualized by radiographic methods, therefore routine radiometry, performed on survey radiographs, tomograms and CT scans of the spine without contrasting the subarachnoid space, has only an approximate value for diagnosing spinal canal stenosis. More accurate data is provided by MRI of the spine.

Determining the amount of vertebral torsion. The amount of torsion, as well as pathological rotation of the vertebrae, i.e. the amount of deformation in the horizontal plane, can be determined most accurately using computed tomography and magnetic resonance imaging. During the development of transpedicular fixation methods for severe scoliotic deformities, surgeons who developed these methods used computed tomography to determine the exact shape of the vertebrae in the horizontal plane and, accordingly, the amount of torsion of each vertebra subject to fixation. However, at the current stage of vertebrology in practical work, determining the absolute amount of torsion of an individual vertebra rarely has independent significance. That is why methods of approximate torsion assessment using an anteroposterior X-ray of the spine have found wide practical application. When determining the amount of torsion, it is important to remember that the anatomical center of the vertebra and, accordingly, the axis around which it is “twisted” is conventionally considered to be the posterior longitudinal ligament.

The Pedicle method (from pedicle - leg, Nash C, Moe JH, 1969) is based on determining the projection position of the vertebral arch root relative to the lateral surface of its body on the convex side of the deformity. Normally, in the absence of torsion, the vertebral arch roots are located symmetrically both relative to the spinous process (its projection shadow) and relative to the lateral sides of the vertebral body. A vertical line is drawn through the middle of the vertebral body, after which half of the vertebra on the convex side of the arch is conditionally divided into 3 equal parts. At the first degree of torsion, only asymmetry of the contours of the arch roots is noted with their usual location within the outer third. At the second and third degrees of torsion, the arch root is projected onto the middle and medial third, respectively, and at the fourth - onto the contralateral half of the vertebral body.

J. R. Cobb (1948) proposed to characterize torsional changes by assessing the position of the spinous process of the vertebra relative to the lateral edge-forming surfaces of its body. However, the visually assessed parameter (the apex of the spinous process) is differently "distant" from the anatomical center of the vertebra (the posterior longitudinal ligament) in different parts of the spine. Moreover, the farther the spinous process is removed from the center of torsion (for example, in the lumbar vertebrae), the greater will be its projection deviation on the anteroposterior radiograph from the midline with the same angular value of torsion, which determines the disadvantage of this method. At the same time, with the same projection displacement of the spinous processes of the vertebrae in the cervical, thoracic, lumbar regions, the true value of torsion will be different. In addition, the method cannot be used in the absence of arches and spinous process - in case of congenital disorders of formation and fusion of arches, as well as in case of postlaminectomy deformities.

The disadvantages of both the Cobb method and the pedicle method are the impossibility of determining the true (angular) value of torsion without special conversion tables. The absolute value of torsion can be determined by the R. Pedriolle method (1979), which is quite accurate, but requires special technical equipment, namely, a torsiometric grid developed by the author. The latter is applied to the vertebra being evaluated on the radiograph in such a way that the edge-forming rays of the grid intersect the centers of the lateral surfaces of the vertebra. The beam of the grid that most centrally intersects the root of the arc on the convex side of the deformation determines the torsion angle.