Spinal alignment in surgical, multisegmental, transpedicular correction of adolescent idiopathic scoliosis

Background

AIM OF THE STUDY:

This paper presents comprehensive knowledge combined with practical experience of the authors in surgical treatment of adolescent idiopathic scoliosis.

This information was gained by the authors by participating in development of the new operative techniques, together with assimilating the knowledge of different scoliosis centers across the globe.

The senior author’s experience began in the early 1970s. Since that time, knowledge about scoliosis biology and biomechanics has evolved. It started with Harrington rod constructs, evolved with Cotrel-Dubousset multilevel hook application and finally transpedicular stabilization is nowadays routinely performed. Material include surgical treatment of over 2200 cases of idiopathic scoliosis, among which adolescent idiopathic scoliosis (scoliosis idiopathica adolescentium) occurs in about 85% of all cases treated surgically.

An evolving array of available instruments has allowed a more physiological approach in surgical treatment, with greater ability to correct particular aspects of deformation.

The problem of post-operative decompensation remains significant in some patients, when proper logical approach had been neglected in pre-operative planning.

Discussion

TERMS USED IN THE ANALYSIS:

The impact of multisegmental correction on the alignment of scoliotic spine justifies explaining basic issues related to spinal alignment.

The Central Sacral Vertical Line (CSVL) plays an important role in radiological assessment of lumbar curves in the coronal plane and in determining the distal stable vertebra of a curve. The spine is fully aligned in this plane if a vertical line beginning at the centre of the C7 body and the CSVL overlap (Figure 1).

If the vertical line is situated on the left side of the CSVL, we deal with spinal decompensation towards the left side (−). If the vertical line beginning at C7 is located on the right side of the CSVL, it points to right spinal decompensation (+). A clinical reflection of spinal decompensation in the coronal plane, in which it is most frequently diagnosed, is mostly a shift of the chest and shoulders against the pelvis. Measurement of thoracic trunk shift assessed on X-ray in antero-posterior projection in patients in a standing position is referred to as vertical trunk reference (VTR) against the Central Sacral Vertical Line (CSVL) (Figure 2) [20].

Global sagittal balance of the spine is assessed on a lateral X-ray performed in a standing position. It is determined by the course of a vertical line beginning at the dens of the axis or the centre of the C7 body. The vertical line that most often crosses the posterior-superior corner of S1 is the sagittal vertebral axis (SVA) (Figure 3).

Positive values point to a shift of the SVA to the front against the promontory (the posterior-superior corner of the S1 vertebral body); whereas negative values point to a shift to the back against it (Figure 4).

From a clinical point of view, the vertical line (centre of gravity) falls slightly to the back of the straight line connecting the centres of the heads of the femoral bones, and determines the central system of coordination of the human body. It is the axis of gravity of the crossing coronal and sagittal planes.

ELEMENTS OF THE DEFORMATION IN SCOLIOTIC SPINE:

Difficulties in the assessment of a scoliotic deformation (deformation of vertebras) result from the fact that it does not have an axis of symmetry. The same curves in the coronal plane may have different 3-dimensional configurations. Stagnara described this situation as an overlapping of a structural deformation and the axis of symmetry of the spine. Simplifying the issue for practical purposes, Stagnara’s plan d’élection is a plane of actual, maximal curve formed between the coronal and the sagittal planes [21].

Anatomically, scoliosis is a lateral curve of the spine; according to the clinical definition, it is idiopathic scoliosis. A mathematical definition is based on geometry, and scoliosis is determined as a lateral shift of the line of the vertebral bodies away from their normal, symmetrical position in the line of the middle of the sagittal plane [19].

Progression of rotation of the vertebras in the transversal plane increases the deformation of scoliosis in the coronal and sagittal planes and has an impact on general decompensation of the spine [16,17].

Lack of linear alignment (imbalance, decompensation, uncompensation) can also occur when the major (original) curve is too long, or when secondary curves are too short and are not “flexible” enough to provide physiological alignment of the position of the spine (stay in shape). Decompensation can also occur in spite of sufficient length and flexibility of compensatory curves if they do not show a “normal” compensatory reaction. Patients with single thoracic curves of type III and IV according to King’s classification (with lack of compensatory curves) are usually “decompensated” before surgery. Decompensation also occurs in the case of double curves of a significant angle (<80°) of type I and II (according to King) towards major curves (in type I to the left, in type II to the right).

Decompensation of the shoulder girdle (shoulders at different heights) occurs most often in type V (elevation of the left shoulder is caused by upper left thoracic curve); it is less frequent in type III of thoracic curve (elevation of the right shoulder). It can also occur as a result of surgical treatment of scoliosis (overcorrection, or inadequate choice of the proximal and distal extent of spondylodesis). Relations between the position of the pelvis and the position of the spine in the sagittal plane (sagittal alignment) are of essential importance for global sagittal balance [22,23].

A shift of the line parallel to the S1 upper end plate (horizontally situated sacrum) causes anteversion of the pelvis and an increase in lumbar lordosis. In such a situation, maintaining alignment is possible by flexion of legs in the hips (Figure 5).

There is a significant shift of the vertical line (centre of gravity) to the front of the promontory of the S1 (vertical sacrum). Retroversion of the pelvis decreases lumbar lordosis (hypolordosis) and limits extension in the hips (Figure 6).

This anterior decompensation in the sagittal plane is the result of, among other factors, surgical distraction of scoliosis within the lumbar spine using Harrington instrumentation when the peripheral hook of the instrumentation most often rested on arches of the L3, L4, and L5 vertebras (flat back syndrome).

EXTENSION OF THE FUSION:

Decrease of lumbar lordosis and leaning of the trunk to the front rarely occurs with current surgical technique using pedicle screws. To restore spinal alignment, it is much more important to surgically restore lumbar lordosis than to improve thoracic hypokyphosis. It should be taken into account that the angles of kyphosis and lordosis differ individually (kyphosis from 40° to 60°, lordosis from 30° to 80°), and the position of pelvis is of essential importance for maintaining global alignment in this plane. One benefit of clinical and radiological identification of major and minor structural curves that should undergo fusion is enabling spontaneous correction of non-structural curves (classification of types of scoliosis according to King and Moe, and Lenke et al) [24,25]. The classification by King and Moe was published in 1983, when Harrington instrumentation was being used. The authors distinguished 5 types of thoracic curves, although they were assessed only in the coronal plane. This classification was the gold standard in the treatment of scoliosis for over 2 decades and it is still very useful clinically.

In 2001 Lenke et al proposed a more extensive and precise classification of adolescent curves comprising “all” curves (not only thoracic), assessing their correction potential and deformation in both the coronal and sagittal planes. The assessment of curves, as in King’s classification, requires performing X-rays of the whole spine in a standing position on a long film (in both planes), completed with corrective X-rays in lateral tilts (right and left) in supine position. Final classification of idiopathic scoliosis according to Lenke includes 42 patterns of curves. The most significant advantage of this classification is the identification of major and minor structural curves that should undergo fusion in order to enable spontaneous correction of non-structural curves.

Posterior spondylodesis of the fused part of the spine should be as vertical as possible and should be extended in the so-called “stable zone”, resting peripherally on a stable vertebra “crossed” by the central sacral vertical line (CSVL). The stable vertebra should, if possible, be neutral in terms of rotation and situated horizontally or almost horizontally against the sacrum (Figure 7).

The choice of the upper level of fusion in type V thoracic curve according to King depends on the position of the shoulders.

The multisegmental instrumentation used nowadays, involving fixation with pedicle screws (mono- and polyaxial), provides control of all 3 columns of the spine, increasing its resistance to compressive and distraction forces. The instrumentation based on screws eliminates the involvement of implants in the vertebral canal; therefore it provides more space for formation of a strong posterior spondylodesis. The derotation manoeuvre is an essential element correcting the curve (restoration of spinal curves in the sagittal plane – thoracic kyphosis and lumbar lordosis), and it also reduces the rib hump (relocation of the whole apical zone posteriorly and medially) (Figure 8).

Surgical derotation involves all the screws simultaneously acting on individual segments within the curve (pedicle screws control all 3 spinal columns). This kind of multisegmental fixation rarely leads to post-surgical decompensation of the spine and loss of correction. This instrumentation eliminates distraction and compressive forces, thus enabling passive elongation of the spine and the spinal cord. This significantly reduces the risk of neurological complications (the spinal cord finds its natural length in the vertebral canal). Our experience also points to frugal use of pedicle screws (not necessarily at each level – segment), while bearing in mind general rules determining the extent (scope) of fusion.

SELECTIVE FUSION:

In some idiopathic curves it is possible to shorten the spondylodesis, which is called selective spondylodesis. However, in order to provide post-surgical spinal alignment, eg, in the case of a skeletally “mature” double curve (right thoracic and left lumbar), it is possible to immobilize only the major right thoracic curve, leaving the compensatory left lumbar curve free [24–29].

According to our assessment, selective fusion can be performed in about 30% of patients with double curves. In the case of thoraco-lumbar curves (type II according to King), the major right thoracic curve, left lumbar (correction potential ≥50% or <40° in traction or in lateral tilt) in this case only the thoracic curve requires spondylodesis. In the case of lumbar-thoracic curves (type I according to King), the major left lumbar curve and right thoracic compensatory curve with correction potential of ≥50% or <40° in traction and in lateral tilt, only the lumbar curve requires spondylodesis.

In the case of double thoracic curves (type V according to King), when the major curve is a right thoracic curve and the compensatory curve is an upper left thoracic curve (undergoing correction of 50% or <40° in traction or in lateral tilt), only the major right thoracic curve requires fusion. Finally, the decision to perform selective fusion in type V depends on the position of the shoulders. If the left shoulder is situated higher (in this structural left thoracic curve), both thoracic curves should be fused in order to restore the horizontal position of the shoulder girdle (“shoulder girdle alignment”). If the right shoulder is situated higher, spondylodesis should be performed only in the lower right thoracic curve in order to provide alignment of the shoulder girdle [4,20,24,25,30]. Therefore, the following factors should be taken into account while planning selective spondylodesis: bone maturity, correction potential, and rotational changes of compensatory curves.

Another important factor to be considered while making a decision on performing selective spondylodesis is the presence of proximal (in the case of double thoracic curves) and distal (in thoraco-lumbar curves) junctional kyphosis. If it is present, both curves (the major and the compensatory) require fusion. Performing selective fusion within the major curves only (in skeletally immature scoliosis with Risser test = 0–3) may pose a risk of post-surgical decompensation, which may also occur when dealing with fixed structural changes of secondary curves (inadequate assessment of their pre-surgical correction potential) or excessive surgical correction of the major curve (taking into account its pre-surgical correction potential) that exceeds the accommodation potential of the compensatory secondary curve.

The essence of skilful use of multisegmental instrumentation and pedicle screws lies not in aiming to achieve maximal correction of the curve, but in using possibilities offered by segmental derotation manoeuvres that lead to satisfactory correction and restoration of body alignment until a strong spondylodesis is formed within 18 to 22 months [4,15,31–34].

It should be emphasized that an adequate choice of the lower level of fusion is the most important factor influencing the result of surgical treatment. Currently, surgical treatment of thoracic and thoraco-lumbar adolescent idiopathic scoliosis <60° (with the use of multisegmental instrumentation and pedicle screws) is mostly based on a posterior approach, which involves less risk.

COMBINED ANTERIOR-POSTERIOR FUSION:

Combined anterior and posterior fusion is justified in the case of large, “rigid” curves exceeding 80°, as well as in “immature” or decompensated curves. In all such cases a crankshaft phenomenon is likely to develop [35–38] in which we may observe further growth of the frontal spinal column after posterior fusion. The curvature is progressive in patients with immature bones (growth of the frontal column of the spine) around the strong posterior fusion. The fused spine becomes “pulled into” the progressing scoliotic rotational deformity. The post-surgical crankshaft phenomenon occurs mostly in children with open Y-shaped cartilage of the acetabulum and Risser test scores of 0 or 1 and Tanner scale <2. An important factor in preventing the crankshaft phenomenon is an anterior “release” (multilevel discectomy) followed by correction of the curve and its fusion from a posterior approach using multisegmental instrumentation and pedicle screws.

Conclusions

We have discussed essential issues concerning spinal alignment, with particular consideration of post-surgical alignment in adolescent idiopathic scoliosis.