Figures for:
Biomechanics of Spinal Deformity

[Neurosurg Focus 14(1), 2003. © 2003 American Association of Neurological Surgeons]

Figure 1. A: A force vector in three-dimensional space. B: If a force (F) is applied at a distance (d) from a fulcrum (IAR), a bending moment (M) is created.

Figure 2. The six fundamental segmental movements, or types of deformation, of the spine along or around the IAR are: 1) rotation or translation around the long axis (A); 2) rotation or translation around the coronal axis (B); 3) rotation or translation around the sagittal axis (C); 4) translation along the long axis (A); 5) translation along the coronal axis (B); and 6) translation along the sagittal axis of the spine (C).

Figure 3. A twisting of the spine around its long axis (A) can result in a rotatory deformation around the axis (B). Curved arrow depicts applied bending moment.

Figure 4. Sagittal- (A), coronal- (B) and axial-plane (C) deformities are the three fundamental deformations that contribute to all spinal deformities, either individually or in combination.

Figure 5. A: A fixed (old) spinal deformity caused by two contiguous VB fractures. The neutral axis is depicted by the black line and the load-bearing axis by the gray line. Note that compensatory spinal curves have developed. B: This deformity may be inappropriately managed by the placement of a ventral short-segment weight-bearing strut near the neutral axis, rather than ventral to the neutral axis near the load-bearing axis. This is problematic because the strut does not span the entire length of the injured and deformed portion of the spine, nor does it bridge the deformity from neutral vertebra to neutral vertebra. C: A longer strut may be required. The location of the neutral axis usually influences this decision-making process. In this case, however, the neutral axis diverges from the load-bearing axis. The ventral weight-bearing strut should not be placed behind the load-bearing axis, as is the case in B and C. Rather, it should be placed well ventral to the neutral axis and in line with the load-bearing axis. D: This may require an even longer construct that extends well beyond the fractured levels. With such a deformity, an interbody graft that is positioned well ventral to the neutral axis and in line with the load-bearing axis and that extends to the neutral vertebra (that between kyphotic and lordotic curves) above and below the deformity neutralizes its negative effect. Deformity progression will thus be unlikely.

Figure 6. The load-bearing axis (neutral axis; shaded region) (A) is generally considered to be located in the region of the middle column of Denis. In extension, however, the load-bearing axis is shifted dorsally in the cervical spine (B). In flexion it is shifted ventrally (C), and in lateral bending it is shifted laterally toward the concavity of the curve.

Figure 7. Sagittal balance. A: A spine in sagittal balance, with a generous but not excessive cervical lordosis, thoracic kyphosis, and lumbar lordosis. B: A plumb line that is dropped from the mid-C-7 VB (SVA) in the standing position falls in the region of the lumbosacral pivot point (dorsal L5-S1 disc interspace). If this normal spinal contour is disturbed by a focal deformity, balance may be achieved by compensatory mechanisms. C: Note that if loss of lumbar lordosis is present, the SVA falls through the region of the sacral promontory. D: Significant imbalance, however, may be develop, resulting in the SVA falling at a significant distance ventral to the sacral promontory. The CSL is used to assess balance in the coronal plane. E: The CSL is a line that is perpendicular to a line passing through both iliac crests, ascending rostrally, in line with sacral spinous processes. The vertebrae bisected by this line are termed stable vertebrae (shaded vertebrae).

Figure 8. Upper: An apical vertebra occurs at the horizon or apex of a curve, either in the sagittal (A) or coronal plane (B). It is associated with adjacent disc interspaces that have the greatest segmental angulation (alpha) of all interspaces in the curve, as depicted. Lower: The neutral vertebrae are located between curves, be it sagittal (A) or coronal (B). There is little or no angulation at its rostral and caudal disc interspaces (beta), as depicted.

Figure 9. A: A long implant should usually not terminate at or near an apical vertebra. B: A longer implant may be required. C: Spine deformation at the termini of the implant is to be expected if the implant terminates at the apex of a curve. D: This is also shown for correction of a scoliotic deformity. E: Note postoperative progression of deformity. The implant was placed up to, but not beyond, the apical vertebra.

Figure 10. The King classification scheme for idiopathic scoliosis. A: Type I is a double concave deformity in which the lumbar curve is larger and more rigid than the thoracic curve. B: Type II is a double concave deformity in which the thoracic curve is more rigid. C: Type III is a thoracic curve. D: Type IV is a long thoracic deformity that tilts into the curve. E: Type V is a double thoracic curve that tilts into the concavity.

Figure 11. The definition of complex deformity may be enhanced by the additional use of the scheme proposed by Lenke. This scheme emphasizes the CSL. A: The CSL between pedicles up to the stable vertebra with minimal or no lumbar or scoliosis (Lumbar modifier A). B: The CSL touches the apical VB or pedicles (Lumbar modifier B). C: The CSL does not touch apical VB or the VBs immediately above and below the apical disc (Lumbar modifier C) (arrows denote apical vertebrae).

Figure 12. In "bringing the spine to the implant" forces that are oriented along any axis or plane may be used ((for example, the long axis [A], the sagittal plane [B], and the coronal plane [C]). Arrows depict forces applied by the implant.

Figure 13. Bending moments are applied in the sagittal plane by a three-point bending mechanism (upper left) and an applied moment arm cantilever beam mechanism (upper right). Straight arrows depict forces; curved arrows depict bending moments. Lower Left: The three-point bending construct brings the spine to the implant. Lower Right: The terminal three-point bending constructs simply have one long and one short moment arm. Straight arrows depict forces applied.

Figure 14. The crossed-rod technique for correcting thoracic and lumbar kyphotic deformities involving the Harrington distraction rod (A), Luque sublaminar wiring (B), and universal spinal instrumentation (C). The latter technique is facilitated by the use of sequential hook insertion (from E.C.B.). The crossed-rod technique strategy can be used for coronal-plane (scoliotic) deformities as well (D). Two rod translation force application strategies can similarly be used. In this case, a small rod may be applied to the spine and brought to a longer rod that spans the concave side of the deformity, thus partially correcting the deformity (E).

Figure 15. The crossed-rod technique (serially illustrated, A-C) for achieving gradual reduction of a kyphotic deformity.

Figure 16. Short-segment parallelogram reduction of a lateral translational deformity. A: Pedicle screws are placed. B: The pedicle screws are connected by rods. C: The rods are connected (friction-glide tightness) and a torque applied to both rods simultaneously by rod grippers. Reduction is achieved and then maintained using rigid cross-fixation. Distraction, followed by interbody bone graft placement and compression, is used to secure the bone graft in place.

Figure 17. A lateral (upper) and axial view (lower) of the crossed-screw fixation technique. Note that rigid cross-fixation maintains the near 90° screw toe-in angle.

Figure 18. A: Spinal derotation is achieved by careful simultaneous rotation of two rods attached to the spine in its deformed scoliotic state. B: The rotation of the rods by 90° converts a scoliosis (A) to a kyphosis (B). If the resultant kyphotic deformity is unacceptable, it may be corrected by rod contouring. C and D: This strategy can be applied to biconcave curves as well.

Figure 19. Upper: Intrinsic implant bending moment application. Upper Left: In this case, simple distraction of the concave side of the curvature and compression of the convex side achieves the reduction of a scoliotic deformity. Upper Right: Cross-fixation is usually used to assist in the maintenance of the reduction. Lower Left: In this case, laterally placed transverse VB screws are manipulated (distracted and compressed; arrows) to reduce a kyphotic deformity. Lower Right: Compression of the two most dorsal screws and distraction of the two most ventral screws achieves reduction of this deformity. Cross-fixation is usually used to assist in the maintenance of the reduction.

Figure 20. The crossed-rod technique of three-point bending force application in the cervical spine. This can be applied by rods and screws or, as depicted, by rods and wires or cables.

Figure 21. The management of a cervical dislocation with locked facet joint(s) via a ventral approach. After a decompressive discectomy (A) to release/relax the spine, distraction can be performed using a disc interspace spreader (B). This disengages the locked facet joints. Dorsal rotation and relaxation of the applied forces (after the facets have been "unlocked") results in the resumption of the normal spine posture (C and D). Fixation and fusion in normal alignment may then be achieved (E). Caspar pins and distractors can also be used. Pins placed in an angular orientation can be used to exaggerate a kyphosis to disengage the facet joints (F), thus permitting reduction (G). Removal of the distractor and pins then restores normal alignment. Rotational deformity, such as that which occurs with a unilateral locked facet, can be reduced by placing Caspar pins out of the midsagittal plane (H).

Figure 22. A 540° operation is occasionally indicated. Ventral decompression (A), followed by a dorsal reduction (B), and ventral stabilization and fusion (C) may be used to decompress, reduce, and stabilize the spine, respectively.

Figure 23. A: A long implant should perhaps not terminate at the cervicothoracic junction. B: Should this occur, the deformity may become exaggerated at the terminus of the implant.

Figure 24. Coronal-plane deformities may be reduced using compression and distraction (A), the crossed-rod technique (B), the derotation maneuver (C), or a combination of these techniques.

Figure 25. Upper: Long moment arms (d and d´) that pass ventral or caudal to the lumbosacral pivot point (dot) can apply adequate leverage for deformity correction and prevention. An L5-S1 spondylolisthesis (center left) can be managed by performing an L-5 corpectomy (center right) and a reduction and the docking of L-4 on S-1 (lower left). An interbody fusion may be used as a spacer and for fusion acquisition. Dorsal instrumentation maintains fixation (lower right). Care must be taken to ensure that adequate room is provided for both nerve roots at the new L4-S1 juncture.

Figure 26. The axis for sagittal-plane correction is perpendicular to the long axis of the spinal axis (dot in the lateral view). This axis may be located in the region of the spinal canal (A). It may also be located ventrally, in the region of the anterior longitudinal ligament (for example, for dorsal wedge osteotomies [B] or in the middle column region [C]).

Figure 27. Left: Dorsal osteotomy. Center and Right: Egg-shell osteotomy.