Deformity correction techniques

General

After Instrumentation and osteotomies various intraoperative techniques are employed to achieve the desired alignment.

Types of correction

Single correction

Concave-sided or convex-sided correction

Dual rod correction

Under/over correction of deformity

Under-correction of lumbar lordosis can lead to

Compensatory pelvic retroversion

causing dorsally directed forces at the uppermost instrumented vertebra (UIV) and ventrally directed forces in the non-instrumented spine, increasing the risk of proximal junctional failure (PJF).

Tension rather than compression through the instrumented construct, particularly in the lower lumbar spine, → pseudarthrosis

Over-correction of lumbar lordosis (the body attempts to achieve a C2 Tilt of 0)

Results in a negative global sagittal alignment, leading to

Pelvic anteversion OR
PJK (more common)

Cantilever Technique

Definition:

In engineering: is a projecting beam or girder fixed at one end.
In ASD surgery: Applying a moment arm to a fixed structure, such as a pedicle screw.

Application:

The most common application involves using the insertional force from the cantilever to reduce the spine to a fixed rod.

Mechanism:

This is typically achieved by fixing the rod to the sacrum and/or ilium, and then sequentially reducing the rod to the fixed pedicle screws.

Effectiveness:

It is highly effective for correction in both the coronal and sagittal planes. It is particularly effective for reducing kyphosis in the thoracolumbar or thoracic spine.

Considerations:

Enormous stress is placed on the interface between the screw and the bone.
To minimise these stresses, load sharing the rod across multiple pedicle screws at a time, while slowly and sequentially tightening them down, should be considered.
The use of reduction or extended tab screws is particularly helpful.

Vertebral Translation

Definition:

This technique involves the reduction of the spinal column to a fixed rod.

Mechanism:

Vertebral translation involves fixation of a rod to both the cephalad and caudal anchors (ends)→ The intervening vertebral bodies are then sequentially translated to the rod using reduction instruments or reduction pedicle screws.

Unlike the cantilever technique where only one end of the rod is rigidly fixed

In cases of severe deformity, the rod can be initially positioned to capture all of the apical screws and then rotated into the desired sagittal plane.

Effectiveness:

It is a particularly effective technique for inducing kyphosis in the thoracic spine.
It can be used in the convex pedicle screw technique, where segmental vertebral translation is a key initial corrective manoeuvre to pull vertebrae towards the convex rod.

Considerations:

The rod is often over-contoured to allow for viscoelastic creep in the system.

Viscoelastic materials:

eg tendons and ligaments
exhibit creep

Molecular rearrangement under stress—allowing them to sustain deformation and maintain the new shape unless subjected to counteracting forces.
This property makes time a crucial factor in corrective maneuvers, as prolonged application of stress enhances deformity correction

Similar to the cantilever technique, vertebral rotation can place significant stress on the screw-bone interface.
Reduction should be performed in a slow, sequential manner with load shared across multiple screws at once.
Surgeons should be vigilant in assessing for screw pullout, especially in patients with poor bone quality or dysplastic pedicle anatomy where fixation is tenuous.

Compression/Distraction

Definition:

These techniques allow the surgeon to increase (distraction) or shorten (compression) the interpedicular distance between two vertebrae along a fixed rod./2

Application:

Can be applied to a single segment or over multiple segments to induce a global corrective manoeuvre.

Mechanism

Biplanar correction: For sagittal correction

Posterior compressive force is lordogenic.
Posterior distraction is kyphogenic.

Uniplanar Correction: For coronal correction

Compression/distraction can also be applied for uniplanar correction, such as achieving even shoulder balance or horizontalising the lower instrumented vertebrae (LIV).

Bone-Screw Interface:

The inferior pedicle pole is stronger than the superior pole.

Lehman 2012: The caudad half of the pedicle is denser and withstands higher forces compared with the cephalad aspect → repetitive forces applied against the inferior pole (e.g., compressing a pedicle screw caudally) provide better correction before bone-screw interface failure than forces applied to the superior pole (e.g., compressing a pedicle screw cephalad).

In-Situ Contouring

Definition:

This is the process of inducing scoliosis, lordosis, or kyphosis on a rod after its fixation to pedicle screws.

Timing:

Typically utilised following one of the previously described manoeuvres when there is residual scoliosis or kyphosis.

Force Application:

If force is applied in a purely coronal or sagittal plane, the correction will be one-dimensional.
If force is applied in an oblique plane, the bending will induce a degree of both coronal and sagittal correction.

Rod Diameter:

When using large diameter rods, simultaneous bending between the surgeon and assistant is recommended to maximise the force imparted on the stiff rod.
In-situ contouring is less effective with smaller diameter or titanium rods due to their higher elasticity.

Cons

If excessive force is applied, can places significant stress on the bone–implant interface, increasing the risk of implant failure
Additionally, in situ contouring is generally less effective with titanium rods due to their elasticity, which causes them to recoil toward their original shape.

Rotational correction

Global rod derotation

General

A surgical method primarily used in Adolescent Idiopathic Scoliosis (AIS) to correct spinal deformity.

Principle:

The coronal deformity in AIS can be approximated to the desired sagittal alignment when rotated 90 degrees.
Aim here is to rotate the deformity curve and not rotate the vertebral body.

Mechanism and Execution:

After pedicle screws are inserted on both the correction and support sides of the curve:

Where to place screws

At each segment on the correction sides (thoracic concave)
Every second or third segment on the support side (thoracic convex) of the curves.

Be careful as there is significantly narrower pedicles on the concave side at the thoracic curve apex, increasing the risk of cortical wall penetration.

A precontoured rod is inserted as below

For thoracic curves: The contoured rod is positioned on the concave side. When rotated toward the concavity, it generates a strong postero-medial traction force, causing apical and juxta-apical vertebrae to translate toward the midline and posteriorly.
For lumbar curves: The rod is applied to the convex side of the curve and rotated clockwise (as viewed from the caudal end) to restore lumbar lordosis.

This rod is precontoured with a slight exaggeration (about one-third) of the normal sagittal contour for the instrumented segment.

It is advantageous to use large-bore, stiff rods with an exaggeration of the normal sagittal profile, as considerable straightening can occur during derotation.
If a rod straightens, it should be removed, rebent, and reinserted until the desired sagittal curve is achieved.

The rod is then rotated 90 degrees (e.g., counterclockwise in the thoracic spine) using clamps or rod holders.

for this rotational correction to be effective, minimal friction must exist at the screw–rod interface, allowing free movement of the screw along the rod.

This manoeuvre is designed to transform the scoliosis into kyphosis in the thoracic spine and lordosis in the lumbar spine, thereby restoring the sagittal profile.
Correction of the deformity is performed solely by derotation, without additional compression or distraction.
A critical principle is that the rod should always be rotated toward the side where it is applied (concavity in the thoracic spine, convexity in the lumbar spine) to avoid pedicle screw breach through the weaker lateral wall.

Impact on Correction:

Sagittal Plane Correction:

Substantial sagittal plane correction by restoring the sagittal profile, with movements of the upper and lower instrumented vertebrae during the 90-degree rotation contributing to this realignment.

Coronal Plane Correction:

Achieves substantial coronal plane correction.

Axial Plane (Rotational) Correction:

The effect of rod derotation on rotational correction is considered negligible, as it primarily results from the translation of the vertebra due to friction between the rod and screws.
Advanced imaging has shown that while previously thought to induce rotational correction at the apex, it primarily results in translational adjustment.
In smaller, less rigid curves, theoretical apical vertebral derotation is possible if there is minimal friction at the screw-rod interface. However, in most cases, significant friction may paradoxically increase rotational deformity and exacerbate the rib hump.
The technique does not correct axial plane deformities.

Advantages and Limitations:

This approach benefits from the stability offered by multiple fixation points, distributing mechanical stress and reducing localized anchor strain.
It is particularly effective for thoracic hypokyphosis, enabling simultaneous correction in both the coronal and sagittal planes.
However, the ideal sagittal contour may not always align with the coronal deformity, potentially leading to suboptimal sagittal apex placement post-rotation.

Variations and Related Techniques:

Convex Rod Derotation Technique:

An alternative approach that uses convex-side pedicle screw for derotation

For improved coronal and sagittal alignment
Has reduced neurological risks

as the cord is away from the convex side when inserting the screw.

It produces comparable outcomes to traditional concave-side corrections but also does not correct axial plane deformities.

Simultaneous Double-Rod Rotation:

Technique

Involves inserting rods on both sides of the curve and rotating them together.
This synchronised movement corrects both coronal and sagittal profiles

Pros

Prevents rod flattening (especially with flexible titanium rods),
Enhances rotational deformity correction by generating a rotational moment at the apical vertebra.

Outcome

Anekstein et al: 71% major curve correction.

Simple Rod Derotation (SRD)

is a correction technique that utilizes three-column vertebral control.

Direct Vertebral Rotation (DVR)

DVR is performed after Global rod derotation has corrected the coronal and sagittal curves.

Its purpose is to correct the rotational deformity

So to reduce rib hump.

Concept of DVR

Applying a force directed posteriorly in the direction opposite to that of the deformity.
Pedicle screws, by entering the pedicle posteriorly and penetrating the anterior vertebral body, allow for the transmission of rotational force to the entire vertebral body, enabling rotational deformity correction.

Other posterior instrumentation systems like hooks or wires cannot deliver sufficient torque for vertebral rotation because their fixation axis is posterior to the vertebral rotation axis.

Torque is applied to the pedicle screw using long screw derotators on both the concave and convex sides of the curve.

DVR performs a segmental rotation using 2-3 anchors simultaneously to distribute rotational torque, prevent screw pull-out, and achieve more effective derotation.

A. After rod placement, tubes are placed on the apical vertebras both in the convexity and the concavity

B: bilateral derotation of the apical vertebrae toward the convexity is performed

C: final position of the spine where the inners are tightened;

D: Oblique view of the spine with apical tubes;

E: derotation maneuver in oblique view.

F: final position of the spine in oblique view

The direction of DVR is crucial and must be opposite to the direction of the vertebral rotation.

For instance, in a right thoracic curve where apical vertebrae rotate clockwise (seen from below, caudally), the DVR on juxta-apical screws should be rotated clockwise.

For the uppermost 1 or 2 vertebrae, the DVR direction must be opposite to the thoracic DVR to preserve shoulder balance.

For the lowermost 1 or 2 screws in the distal curve, the DVR direction depends on the rotation of the uninstrumented lumbar curve: Suk’s classification

Type A (thoracic concave, compensatory lumbar curve crosses the central sacral vertical line with rotation): The lowermost screws should be rotated in the opposite direction (OD) to the thoracic DVR, aiming to lessen lumbar rotation. (in a thoracic selected fusion)
Type B (distal vertebra rotated in the same direction as thoracic rotation): The DVR should be in the same direction (SD) as the thoracic DVR, with more forceful DVR if lumbar rotation is severe.

Counter-Rotate Technique

Seki 2024.pfd.pdf

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Similarity to DVR: The actual CRT technique is similar to direct vertebral rotation (DVR).

Aim:

The goal of using CRT is to correct the thoracolumbar/lumbar curve as much as possible without the necessity of over-extending the fixation down to the lower lumbar spine, thereby allowing for selective thoracic fusion.

Principles

Correcting the thoracolumbar/lumbar curve using the neutral vertebra (NV) of the caudal side of the thoracic curvature.

Primarily used for Lenke 1-2 B and C curves, it may also be applicable to Lenke 3, 4, and 6.

Focus of Correction:

Correcting the vertebral rotation at the transition from the thoracic to the lumbar spine, and simultaneously corrects the tilting angle of the LIV.

Lower NV +1 or +2 if it is a stiff curve

Tools require

Novel Ratchet Device:

This device is attached to the lumbar region targeted for correction.
Rotational Force Application:

The device applies a rotational corrective force indirectly to the lumbar spine curve located below the neutral vertebra curvature. This correction is achieved by using the opposite rotation of the thoracic spine curve.

Force Maintenance:

The ratchet device is designed to apply a stronger and more direct rotational corrective force to the lower vertebrae. Due to the ratchet structure, once the corrective force is applied (by reducing the distance between points A and B, using B as a fulcrum), the force is maintained and will not return to the original position, even if the surgeon's hand is released

Surgical technique

Preparation and Initial Instrumentation

A skin incision is made in the posterior midline, exposing the posterior spine elements.
Uni-planar pedicle screws are inserted using a free-hand technique.
The facet joint and its articular cartilage within the range of fixation are removed.
A Ponte osteotomy is performed around the periapical region.

Rod Placement and Primary Correction

Rod rotation (RR) is performed on the concave side of the scoliosis.
Differential rod contouring (DRC) is performed on the convex side.
The rod material used is CoCr with a diameter of 5.5 mm.
The setscrews (innies) of the neutral vertebra (NV) are fixed for both the concave and convex rods.

Application of the Counter-Rotate Technique (CRT)

Targeting the Curve: CRT is performed on each vertebra located from the most caudal neutral vertebra (NV) to the lowest instrumented vertebra (LIV), specifically to correct the thoracolumbar/lumbar curve.
The vertebrae below the NV are aggressively applied with corrective force for rotation.
Device Attachment: The novel ratchet device is attached to the lumbar region intended for correction.
Applying Force (Mechanism):

The ratchet device connects to bridges A and B.

The distance between A and B is reduced, using B as the fulcrum.

This action applies a rotational corrective force to the lumbar vertebrae.

The force is applied indirectly to the lumbar spine curve below the NV curvature, utilizing the opposite rotation of the thoracic spine curve.

The device is capable of applying a stronger and more direct rotational corrective force to the lower vertebrae.

Maintaining Correction: Due to the ratchet structure, once the corrective force is applied, it is maintained and will not return to the original position, even if the surgeon's hand is released.
Finalizing Rotation: Correction continues until both device "sticks" are aligned.
The setscrews are then fastened.

Conclusion of Surgery

The coronal balance of the spine is confirmed using fluoroscopy and X-ray after the scoliosis correction is complete.
All patients are evaluated intraoperatively using spinal cord monitoring (motor and somatosensory evoked potentials).

Pros:

When rotational correction is applied to the lumbar spine below the NV, the corrective force is transmitted, albeit indirectly, correcting the rest of the lumbar spinal curve

Vertebral Coplanar Alignment (VCA)

Concept:

In a normal standing spine, the anteroposterior (X-axis) and transverse (Z-axis) axes of the vertebrae are coplanar; scoliosis disrupts this due to rotational and translational deformities.
3D correction in scoliosis surgery.
It operates on the principle of realigning the vertebral axes before rod placement, aiming to reduce mechanical stress on instrumentation and improve precision.
VCA restores coplanarity and simultaneously corrects thoracic hypokyphosis by re-establishing the posterior divergence of the X-axis.

Mechanism and Procedure:

Insertion of monoaxial pedicle screws on the convex side of the curve.
Slotted stainless steel tubes are then attached and aligned with the anteroposterior axis of each vertebra.
A rigid reduction rod is inserted through these slots, which gradually brings the vertebrae into a single rotational axis.
Polyethylene spacers are placed between the tips of the slotted tubes to help restore kyphosis.
Alignment rings are inserted to the stainless steal tubes.
A second alignment rod is then inserted beneath the first, further driving the vertebrae into coplanar alignment.
Once the desired correction is achieved, a definitive rod is secured on the concave side.
Finally, the slotted tubes and the initial convex rod are removed.

Advantages:

VCA offers true three-dimensional correction.
It minimises stress on instrumentation, improves neurological safety by reducing the risk of pedicle screw breach, and provides better load distribution across the instrumented segments.
Achieving correction before rod placement simplifies rod insertion and reduces implant-related complications.

Outcome

Vallespir 2008: N=25 patients with Lenke Type 1 Adolescent Idiopathic Scoliosis (AIS) showed:

Average 73% coronal correction in thoracic curves.
70% correction in thoracolumbar curves.
56% apical vertebral derotation.
65% rib hump reduction without the need for thoracoplasty.
Thoracic kyphosis was preserved, preventing the excessive flattening often seen with other techniques.

Comparison to Other Techniques:

Qiu et al. found that VCA achieved comparable coronal correction to derotation techniques in Lenke 1 scoliosis but provided superior thoracic kyphosis restoration.
Both VCA and derotation techniques had similar safety profiles, though screw pullouts and hemothorax occurred in the derotation group.

Differential Rod Contouring (DRC)

General

DRC is a technique used in scoliosis surgery that supplements traditional rod rotation methods to achieve better three-dimensional correction.
Use in combination with Global rod derotation and direct vertebral derotation techniques

Concept and Purpose:

Using different contouring angles for the concave and convex rods to generate corrective forces.
The goal is to realign the vertebrae in the coronal, sagittal, and transverse planes, with a significant role in vertebral derotation and rib hump reduction.

Technique:

Bending the concave rod more than the convex rod.

Increasing the difference in contouring angles between the two rods significantly improves apical vertebral rotational correction.

Vertebral derotation improves substantially when the curvature difference between the concave and convex rods exceeds 10°.

DRC has an additive effect, contributing to further reduction in vertebral rotation even after a concave rod rotation has already been performed.

Dynamic schema of apical vertebral body rotation with each surgical method. Estimated additional improvement in apical vertebral rotation was 4° after DRC, and 5° after DVR.

Effectiveness:

Biomechanical studies have shown that increasing the concave rod contouring angle from 35° to 85° can improve vertebral derotation from 35% to 68%.
An analysis of intraoperative CT scans revealed that DRC contributed an average improvement of 6° in apical vertebral rotation after the initial concave rod rotation was completed.

Advantages:

Controlled and predictable vertebral derotation, improving axial plane alignment.
Effectively reduces the rib hump, a key cosmetic concern for patients.
By distributing forces more evenly across the instrumentation, it can potentially enhance the stability of the construct.

Risks

Greater differential contouring results in increased screw pullout forces and heightened mechanical stress on the bone-screw interface.

This increased stress can lead to potential complications like screw loosening or implant failure, especially with highly rigid rods.

Can cause overt thoracic kyphosis.

An increase in thoracic kyphosis from 27% to 144%.

To mitigate risks, DRC should be carefully planned and used in conjunction with other maneuvers, such as osteotomies, to release the spine and balance the mechanical forces.

Convex Pedicle Screw Technique

General

Developed by Tsirikos,
Applies segmental correction and instrumentation primarily on the convex side of the spinal deformity.

This method differs from traditional techniques that focus on the concave side for rod engagement and correction.

Uses a combination of

DVR
Vertebral translation

Pros

Reduced Neurological Risk:

There is a lower likelihood of a medial pedicle breach on the convex side because the spinal cord is positioned at a greater distance from the convex pedicle compared to its concave counterpart.

Superior Anchor Stability:
Convex side pedicle are large in size → able to use larger and longer pedicle screws → provides better purchase and stability for the construct

Efficient Correction:

The method facilitates improved correction in the coronal plane while simultaneously helping to restore thoracic kyphosis, which is often reduced in AIS.

Lower Implant Density:

This technique typically uses fewer implants, which leads to reduced surgical time, blood loss, and potentially lower infection risks without compromising the quality of the deformity correction.

Surgical Procedure and Mechanism

Preparation:

After preoperative planning with radiographs and MRI, the patient is positioned prone.
A midline posterior exposure is performed, followed by routine facetectomies to increase the flexibility of the spine.

Screw Placement:

Pedicle screws are placed segmentally along the convex side of the curve.
To augment the construct and provide additional stability, only two proximal and two distal screws are placed on the concave side.

Correction Manoeuvres:

A convex rod, pre-contoured to restore normal sagittal alignment, is engaged in the segmental screws.
The primary corrective manoeuvre is segmental vertebral translation, where the vertebrae are gradually pulled toward the rod using reduction screws.
This is followed by direct vertebral derotation (DVR), which is applied at every level as the convex pedicle screws are secured, further correcting the axial deformity.

Stabilisation:

Once the correction is achieved and the convex rod is secured, a concave rod is placed for supportive, rather than corrective, purposes.

Finalisation:

The final steps involve tightening the screws, using controlled compression and distraction to optimise spinal balance, and applying bone graft to promote fusion.

Outcome

Tsirikos 2012

found that while both bilateral and unilateral (convex) screw techniques achieved satisfactory scoliosis correction, the unilateral approach offered reduced surgical time and blood loss with no significant difference in clinical outcomes or patient-reported results.

Ferlic 2021

also found comparable correction outcomes between the convex technique (using low implant density) and a traditional bilateral approach, with the convex method involving shorter operating times and fewer implants.

Takahashi 2020

confirmed that convex rod rotation manoeuvres combined with DVR effectively improved vertebral rotation in Lenke types 1 and 2 AIS, supporting the procedure as a viable surgical option.