Correction of Adolescent Idiopathic Scoliosis Using a Convex Pedicle Screw Technique

Abstract

Background:

We describe our convex segmental pedicle screw technique for the treatment of adolescent idiopathic scoliosis. We developed this technique to achieve optimum 3-dimensional deformity correction while reducing the surgical risks of an inherently dangerous procedure.

Description:

The surgery involves a wide posterior subperiosteal exposure across the deformity levels to the tips of the transverse processes. Posterior releases are performed through facetectomies. Pedicle screws are placed using a freehand technique based on anatomical landmarks. Adequate screw positioning is assessed with an image intensifier before rod engagement. Segmental pedicle screws are placed across the convexity of each curve included in the fusion. Proximal and distal fixation of the rods on the contralateral side is performed across 2 pedicle screw anchors. We use titanium rods bilaterally. Curve correction is done using the convex pedicle screws by applying segmental vertebral translation and derotation starting with the main thoracic curve followed by the lumbar curve. Segmental compression or distraction is performed at the proximal and distal ends of the construct to level the end vertebrae included in the fusion. Maximum correction of the main thoracic scoliosis is done, whereas the lumbar scoliosis (which is usually more flexible) is corrected to the point that results in a globally balanced spine in the coronal plane. The rod attached on the convex side of the main thoracic scoliosis is overbent to restore thoracic kyphosis, and the aim is always to achieve regional and global sagittal balance. An interfacetal, intertransverse, and interlaminar fusion is performed with use of locally harvested bone supplemented by allograft bone.

Alternatives:

With previous techniques, the use of bilateral segmental pedicle screw fixation has been advocated as a requirement to achieve adequate deformity correction in patients with adolescent idiopathic scoliosis.

Rationale:

This technique is associated with low risks of neurological and vascular complications because the screws are placed at the convex pedicles, away from the spinal cord/cauda equina and the aorta. The use of far fewer pedicle screws compared with previous techniques reduces surgical time and blood loss, which is related to lower postoperative morbidity. It may also decrease the risk of deep wound infection, which is associated with the number of implants used. Low implant density (1.2, with a density of 2 representing placement of pedicle screws bilaterally at every instrumented segment) with our technique can achieve satisfactory scoliosis correction, improved thoracic kyphosis, and normal global sagittal balance. Our use of this technique has resulted in excellent patient satisfaction and functional outcomes with no neurological complications or intraoperative neuromonitoring events, deep wound infections, detected nonunions, or need for revision surgery.

Introductory Statement

Scoliosis correction over a convex rod has several advantages: (1) it involves placement of convex pedicle screws, which carry less risk for pedicle wall penetration; (2) larger-diameter and longer pedicle screws can be used on the convex side; (3) a medial or lateral pedicle wall breach on the convex side does not bring the screw into direct contact with the neurological structures or anterior vascular structures, respectively; and (4) the concave supportive rod sits higher than the spine and occupies little space, allowing a large area for bone-grafting on the curve concavity and thus reducing the risk of nonunion.

Indications & Contraindications

Indications

• The convex correction technique can be used for patients with adolescent idiopathic scoliosis during the phase of their spinal growth as well as for those who have reached skeletal maturity, including adults (Figs. 1-A through 1-D).

• Patients with single thoracic (Lenke type-I) or single thoracolumbar/lumbar (Lenke type-V) adolescent idiopathic scoliosis can be treated using the convex correction technique¹. The rod placed across the convexity of the scoliosis (corrective rod) requires segmental pedicle screw fixation to allow application of corrective maneuvers, while the rod on the contralateral concave side (supportive rod) requires only 2-level proximal and distal pedicle screw fixation as its role is to augment the construct and increase stability, which will enhance an osseous fusion.

• Patients with double thoracic (Lenke type-II), double thoracic and lumbar (Lenke type-III or VI), or triple thoracic and lumbar (Lenke type-IV) adolescent idiopathic scoliosis can also be treated with the convex correction technique². Correction is achieved through maneuvers applied on the convexity of each structural curve starting with the main thoracic scoliosis, followed by the lumbar curve, and (if present) ending with the upper thoracic scoliosis. Two proximal and distal pedicle screw fixation anchors are placed on the contralateral side to provide stability of the construct.

• The same principles of convex deformity correction have been effectively applied to patients with scoliosis of different etiologies—including early-onset idiopathic, syndromic, or neuromuscular scoliosis—in our practice.

Fig. 1-A Fig. 1-B Posteroanterior (Fig. 1-A) and lateral (Fig. 1-B) radiographs of a patient, 19 years and 9 months old, with severe adolescent idiopathic thoracic and lumbar scoliosis (Lenke type VI) and associated negative global sagittal balance with thoracic hypokyphosis and compensatory cervical kyphosis (in an attempt to balance the head on top of the sacrum/pelvis).

Fig. 1-A.

Fig. 1-B.

Figs. 1-C and 1-D The patient underwent a posterior T4-L4 spinal fusion using the convex segmental pedicle screw correction technique. This achieved excellent scoliosis correction with restoration of normal thoracic kyphosis and global sagittal alignment.

Fig. 1-C.

Fig. 1-D.

Contraindications

• The convex correction technique with low pedicle screw density may not be applicable to older patients with “de novo” degenerative scoliosis. These patients may require a higher implant density and a more rigid construct in the presence of inherent bone weakness and a deformity associated with global sagittal imbalance necessitating performance of spinal osteotomies.

Step 1: Anesthesia

Spinal anesthesia must reduce surgical blood loss and accommodate intraoperative neuromonitoring to reduce the risk of neurological complications during deformity correction.

• Premedicate the patient using midazolam (0.5 mg/kg, up to a maximum of 20 mg) and clonidine (1 µg/kg).

• Use a totally intravenous (IV) anesthetic technique. Induction is with propofol target-controlled infusion and a bolus of atracurium (0.5 mg/kg) to facilitate intubation. Maintain the anesthesia with a propofol and remifentanil infusion and ventilation with an air/oxygen mixture.

• Use a bispectral index (BIS) monitor to optimize the depth of the anesthesia.

• Insert arterial pressure and peripheral venous lines in all patients. Use tranexamic acid to reduce surgical bleeding. Use cell salvage for all patients; this usually prevents the need for allogenic blood transfusion. We do not use autologous blood predonation.

• Use a muscle relaxant (atracurium) to facilitate surgical exposure after baseline intraoperative spinal cord monitoring traces have been established. The muscle relaxant should have worn off by the time the spinal instrumentation is placed in order to allow repetitive motor monitoring.

• During surgical exposure, maintain the mean arterial blood pressure at low levels (mean, 50 to 55 mm Hg) to reduce blood loss. Control the blood pressure through titration of the remifentanil infusion.

• At the time of spinal instrumentation placement, target the mean blood pressure at 60 to 65 mm Hg so that spinal cord perfusion is adequate and intraoperative spinal cord monitoring is stable and reproducible throughout.

Step 2: Patient Positioning

Patient positioning is critical to facilitate surgery; reduce intraoperative bleeding; and prevent pressure areas, brachial plexus injury, and eye injury.

• Position the patient prone on a Jackson table, with an appropriately sized Montreal mattress to accommodate the patient’s individual body type.

• Adequately pad the osseous prominences to prevent pressure areas.

• The abdomen should be free but supported circumferentially to avoid an increase in venous return, which would maximize intraoperative bleeding.

• The face should be well supported on a head rest.

• Place the shoulders and elbows in 90° of flexion in relation to the trunk to prevent brachial plexus injury.

Step 3: Intraoperative Spinal Cord Monitoring

Intraoperative neurophysiological monitoring is mandatory to reduce the risk of spinal cord and nerve root injury during deformity correction.

• Use a multimodal neuromonitoring technique that includes recording of cortical sensory evoked potentials (CSEPs), somatosensory evoked potentials (SSEPs), and upper (abductor digiti minimi) and lower limb (tibialis anterior, abductor hallucis longus, and vastus medialis) motor evoked potentials (MEPs).

• Perform electromyography of the upper and lower limbs. Start monitoring after positioning the patient and continue doing so for a minimum of 20 minutes after all instrumentation is complete. Reference traces are obtained prior to skin incision. We found this approach of multimodal monitoring to be highly sensitive and specific for spinal cord injury.

Step 4: Surgical Exposure

A wide dissection of the soft tissues is needed to allow spinal releases, instrumentation placement, and scoliosis correction.

• Perform a wide exposure from the midline out to the tips of the transverse processes with subperiosteal dissection of the paraspinal muscles (Fig. 2-A).

• Excise the inferior facets at every level across the thoracic and lumbar spine with associated decortication of the superior facets before placing bone graft. This increases spinal flexibility and facilitates identification of pedicle entry points for screw placement as well as allows interfacetal bone-grafting.

• Remove the spinous processes down to the deep layer of the ligamentum flavum across the levels of the fusion in order to further mobilize the spine and provide bone graft. Do not violate the spinal canal.

• Very rigid curves may require posterior closing wedge (chevron-type) osteotomies with additional resection of the superior facets to increase curve flexibility and optimize deformity correction.

Figs. 2-A through 2-Y Intraoperative photographs of the patient in Figures 1-A through 1-D, showing the steps of the procedure.

Fig. 2-A.

Wide subperiosteal exposure of the spine across the levels of the 2 curves out to the tips of the transverse processes.

Step 5: Pedicle Screw Placement

Optimum placement of pedicle screws is essential to prevent the risk of injury to neurological, vascular, and visceral structures and to provide stable fixation of the rods, which will in turn allow adequate deformity correction.

• Place pedicle screws using a freehand technique, based on recognition of anatomical landmarks, starting in a distal-to-proximal direction³.

• The pedicle entry point from T1 to T5 is at the junction of the bisected transverse process and the lateral margin of the facet joint. From T6 to T10, it is at the junction of the upper to the middle third of the transverse process and the lateral margin of the facet joint. Pedicle screws at T11 and T12 are inserted with the entry point at the base of the superior facet and in the lumbar spine at the junction of the transverse process, pars interarticularis, and superior facet.

• Prepare the pedicle canal with the use of a pedicle probe, feeling for osseous continuity in all directions before placing the screws.

• Once all pedicle screws are placed, and before rod engagement, use fluoroscopic imaging (coronal and sagittal views) to confirm adequate screw positioning. Minimize radiation exposure to 3 or 4 views in each plane.

Step 6: Construct Design and Correction Technique

Our construct design is based on application of segmental pedicle screws across the convexity of all curves to be included in the instrumented arthrodesis, with the mid-thoracic scoliosis corrected before the lumbar scoliosis.

• Use long-tab reduction pedicle screws, which allow easier capturing of the rod within the screw heads. We used polyaxial reduction screws that can lock into monoaxial screws in our series². Alternatively, uniplanar extended-tab pedicle screws can be used with the same technique (Video 1).

• Place unilateral pedicle screws across the convexity of each individual thoracic or lumbar curve to allow segmental correction (Fig. 2-B). Do not place concave screws at the deformity apex, where there is a maximum risk of neurological, vascular, or visceral injury. Bilateral screws across 2 levels caudally in the lumbar spine and 1 or 2 levels cephalad in the thoracic spine provide proximal and distal stability of the construct (Video 1).

• The rod attached to the convexity of each curve is the corrective rod. Always correct the thoracic scoliosis before the lumbar scoliosis as correction of the lumbar curve can transmit a rotational torque to the thoracic spine and exaggerate the convex rib deformity (Figs. 2-C through 2-K, Videos 2, 3, 4, 5, 6).

• Perform maximum correction of the main thoracic curve, but correct the lumbar scoliosis to the degree required to achieve a balanced effect across the thoracic and lumbar segments with adequate global coronal balance (Figs. 2-L through 2-W, Videos 7, 8, 9 10, 11, 12). Most of the scoliosis correction using our technique is achieved through segmental vertebral translation of the apex of each curve to the midline with partial correction of vertebral rotation accomplished alongside the translatory effect and additionally once the pedicle screws are locked into monoaxial screws.

• The rod attached on the convexity of the main thoracic scoliosis is overbent into thoracic kyphosis, whereas less thoracic kyphosis is bent into the concave rod, which has no apical fixation to the spine, in order to avoid rod prominence under the skin. Overbending the convex rod can restore thoracic kyphosis as we routinely use titanium rods, which tend to partly flatten when cantilevered to the spine across the apex of the scoliosis. Cross-connectors are not used.

Video 1.

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DOI: 10.2106/JBJS.ST.18.00009.vid1

Engagement of the thoracic convex rod.

Fig. 2-B.

Placement of segmental pedicle screws on the convexity across the levels of both curves. Two fixation points were used proximally and distally on the concave side to secure anchor points at the ends of the construct.

Fig. 2-C.

The thoracic convex rod is bent in the sagittal plane for thoracic kyphosis and lumbar lordosis and then attached to the pedicle screws. This rod should be placed first because the thoracic scoliosis is corrected before the lumbar scoliosis in order to reduce the rib prominence adjacent to the convexity of the curve.

Fig. 2-D.

With the use of 2 rod-holders, the thoracic convex rod is derotated to restore thoracic kyphosis and lumbar lordosis.

Fig. 2-E.

The thoracic convex rod is secured to the spine distally by tightening the more proximal of the 2 anchor points in the lower lumbar region in order to provide a stable point before starting thoracic scoliosis correction.

Figs. 2-F through 2-I Sequential correction of the thoracic scoliosis is performed across the 5 convex pedicle screws through segmental translation and derotation. Two rod-stabilizing tubes are used: the lower as a stabilizing point and the upper to perform the segmental correction maneuver.

Fig. 2-F.

Fig. 2-G.

Fig. 2-H.

Fig. 2-I.

Fig. 2-J.

The thoracic convex rod is released from the spine distally by loosening the more proximal of the 2 anchor points in the lumbar region.

Fig. 2-K.

At this stage, only the thoracic convex pedicle screws are tightened in order to maintain thoracic scoliosis correction (between the 2 pairs of forceps). The thoracic convex rod is attached loosely at the most proximal thoracic and the 2 lumbar pedicle screws.

Video 2.

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DOI: 10.2106/JBJS.ST.18.00009.vid2

Derotation of the thoracic convex rod.

Video 3.

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DOI: 10.2106/JBJS.ST.18.00009.vid3

The thoracic convex rod is secured to the spine distally.

Video 4.

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DOI: 10.2106/JBJS.ST.18.00009.vid4

Sequential correction.

Video 5.

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The thoracic convex rod is released from the spine distally.

Video 6.

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The thoracic convex rod is attached loosely.

Fig. 2-L.

The lumbar convex rod is bent in the sagittal plane and then is attached to the pedicle screws.

Fig. 2-M.

With the use of 2 rod-holders, the lumbar convex rod is derotated to accommodate the thoracic kyphosis and lumbar lordosis. The lumbar convex rod is bent into less thoracic kyphosis to avoid rod prominence under the skin in the absence of pedicle screw anchors across the concavity of the thoracic scoliosis.

Fig. 2-N.

The lumbar convex rod is secured to the spine proximally by tightening the distal of the 2 anchor points in the upper thoracic region in order to provide a stable point before starting lumbar scoliosis correction.

Figs. 2-O through 2-R Sequential correction of the lumbar scoliosis is performed across the 5 convex pedicle screws through segmental translation and derotation. Two rod-stabilizing tubes are used: the upper as a stabilizing point and the lower to perform the segmental correction maneuver. The lumbar convex rod is loosely attached to the most distal lumbar pedicle screw.

Fig. 2-O.

Fig. 2-P.

Fig. 2-Q.

Fig. 2-R.

Fig. 2-S.

The thoracic convex rod is tightened at the more proximal of the 2 anchor points in the lower lumbar region. This leaves both rods loosely attached distally only across the most caudal pedicle screw anchors.

Fig. 2-T.

Leveling of the most distal vertebra to be included in the instrumented fusion is now performed by compressing the oblique convex side followed by tightening of the distal lumbar convex pedicle screw. Alternatively, distraction could be applied to achieve horizontalization of the distal lumbar vertebra on the elevated lumbar concave side between the most distal 2 pedicle screw levels. However, vertebral compression is always a safer maneuver than distraction in order to reduce the risk of neurological injury.

Fig. 2-U.

The thoracic convex rod is now secured to the lumbar spine by tightening the most distal lumbar concave pedicle screw.

Figs. 2-V and 2-W Both rods are secured proximally by tightening the top-level pedicle screws in the upper thoracic region. If leveling of the most cephalad vertebra is required, compression of the elevated convex side or distraction of the oblique concave side can be performed at this stage. This is the last step of the correction maneuvers.

Fig. 2-V.

Fig. 2-W.

Video 7.

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Engagement of the lumbar convex rod.

Video 8.

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The lumbar convex rod is derotated.

Video 9.

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DOI: 10.2106/JBJS.ST.18.00009.vid9

The lumbar convex rod is secured to the spine proximally.

Video 10.

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DOI: 10.2106/JBJS.ST.18.00009.vid10

Sequential correction through segmental translation and derotation.

Video 11.

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Securing rods to the lumbar spine.

Video 12.

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Both rods are secured proximally.

Step 7: Bone-Grafting

Extensive decortication of the posterior elements along with facetectomies, as well as use of abundant morselized autologous and allograft bone, will enhance an osseous fusion and stabilize the instrumented spinal levels.

• Harvest autologous bone from the spinous/transverse processes, facets, and laminae.

• Morselize the harvested bone and place it across the length of the instrumentation to achieve an interfacetal, intertransverse, and interlaminar fusion (Fig. 2-X).

• Perform extensive decortication of the posterior vertebral elements to allow consolidation of the bone graft over a vascular host area (Video 13).

• Supplement the autologous local bone with allograft bone (frozen femoral heads), which is also morselized before application (Fig. 2-Y, Video 14). We do not use bone substitutes or bone morphogenetic proteins in this group of patients.

• We do not use deep or superficial wound drains in primary deformity surgery.

Fig. 2-X.

The construct has been finalized with locally harvested autologous bone placed across the instrumented levels.

Video 13.

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DOI: 10.2106/JBJS.ST.18.00009.vid13

Decortication of the posterior elements.

Fig. 2-Y.

Allograft bone is used to augment the bone graft area and enhance osseous fusion before wound closure.

Video 14.

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Placement of autograft and allograft bone.

Step 8: Postoperative Management

Our patients are cared for in the spinal ward after surgery with initial close neurovascular and systemic monitoring, a change to oral analgesics on postoperative day 1, and early mobilization.

• At completion of the surgery, our patients remain in recovery for 1.5 to 2 hours in order to optimize pain management, fluid balance, and level of consciousness. They are then moved to the orthopaedic/spinal ward with 1-to-1 nursing care overnight. Close monitoring of blood pressure, heart rate, and urine output is performed. Fluid balance and pain management are the priorities along with regular neurovascular observations in the immediate postoperative period.

• Pain management consists of a morphine infusion (20 to 40 µg/kg/hr) with nurse-delivered boluses if required along with IV paracetamol (15 mg/kg 4 times daily). On postoperative day 1, the morphine infusion is stopped and oral morphine as required, as well as 10 mg/kg of oral ibuprofen (up to 400 mg) 4 times daily, is added. The IV paracetamol is also switched to oral administration. The patients are discharged with paracetamol and codeine phosphate.

• Patients are mobilized out of bed on postoperative day 1. Over the following 3 to 4 days, they gradually increase the level of their mobility, practicing walking, sitting, and going up and down stairs. They are usually ready for discharge about 5 to 6 days after the surgery. We do not use a postoperative brace or plaster jacket support in this group of patients.

Results

Surgical correction of adolescent idiopathic scoliosis is indicated to address a severe, progressive curve that produces patient dissatisfaction with cosmetic appearance and occasionally mechanical back pain. The surgical treatment of adolescent idiopathic scoliosis has been revolutionized over the last few decades, from initial non-instrumented in situ fusions to the use of modern pedicle screw constructs that can achieve and maintain dramatic deformity corrections. A new era in the surgical treatment of adolescent idiopathic scoliosis has begun with the introduction of pedicle screw instrumentation, which provides 3-column vertebral fixation and allows major deformity correction in the coronal, sagittal, and axial planes. The application of current advanced correction techniques requires extensive surgical training and a wide multidisciplinary setup of resources in order to secure clinical safety and produce optimum results. A steep learning curve can be expected for safe and consistent pedicle screw placement, especially in the thoracic region. Potential complications, including injury to adjacent neurological, vascular, and visceral structures, can occur as a result of screw misplacement or pull-out during correction maneuvers. In addition, scoliosis correction techniques are not uniform and remain surgeon-specific, which makes it harder to provide any template. These techniques evolve even within individual practices depending on the experience acquired by every surgeon in his/her clinical setting.

Use of bilateral segmental pedicle screw fixation (with an implant density of 2, representing placement of pedicle screws bilaterally at every instrumented segment) was initially recommended to produce maximum scoliosis correction^4-6. Correction in the coronal plane often occurred at the expense of thoracic kyphosis, with bilateral or concave screw correction techniques associated with an inability to restore regional and global sagittal balance. It is now evident that such high implant density is not necessary to achieve and maintain deformity correction^7,8. Strategically placed screw anchors have gained popularity as they reduce the risk of neurological and visceral complications, surgical time and blood loss, implant density directly related to the risk of infection, and implant cost. Over the last 14 years in our practice, we have used segmental screw fixation over 1 side of the construct in order to achieve scoliosis correction, while the second rod has a supportive role and is attached to the spine through 2-level screw stabilization at the proximal and distal ends of the instrumentation (Figs. 3-A through 4-D). In 2 previous series, we documented comparable results, including deformity correction, low complication rates, and high patient satisfaction, between bilateral and unilateral segmental pedicle screw techniques in patients with all types of adolescent idiopathic scoliosis^3,9.

Posteroanterior (Fig. 3-A) and lateral (Fig. 3-B) radiographs of a patient, 14 years and 2 months old, with adolescent idiopathic thoracic scoliosis (Lenke type I) and associated negative global sagittal balance with thoracic hypokyphosis and compensatory cervical kyphosis (in an attempt to balance the head on top of the sacrum/pelvis).

Fig. 3-A.

Fig. 3-B.

Fig. 3-C.

The patient underwent a posterior T2-T11 spinal fusion using the convex segmental pedicle screw correction technique.

Posteroanterior (Fig. 4-A) and lateral (Fig. 4-B) radiographs of a patient, 15 years old, with adolescent idiopathic lumbar scoliosis (Lenke type V) and associated negative global sagittal balance of the spine.

Fig. 4-A.

Fig. 4-B.

Figs. 3-D and 3-E The procedure achieved excellent scoliosis correction with restoration of normal thoracic kyphosis and global sagittal alignment.

Fig. 3-D.

Fig. 3-E.

Figs. 4-C and 4-D The patient underwent a T11-L4 posterior spinal fusion using the convex segmental pedicle screw correction technique, which achieved excellent scoliosis correction with restoration of normal alignment in both planes.

Fig. 4-C.

Fig. 4-D.

It is essential to appreciate that, for any type of scoliosis, the amount of curve correction reflects a measure of technical competence but does not necessarily relate to patient satisfaction, which is the primary goal of surgery. Surgical results have to be assessed through use of outcome measures that represent patients’ perception of the success or failure of treatment. In the present era, recommended treatments have to be based on outcomes as demonstrated through patient quality-of-life assessments. Cost implications are important within modern health-care systems.

In our series, patients with adolescent idiopathic scoliosis who were treated with our convex correction technique obtained and maintained very satisfactory scoliosis correction and a normal coronal and sagittal balance with no major complications². These results were associated with excellent patient satisfaction and functional outcomes. The implant density in our practice (1.2) is much lower than that with bilateral segmental screw techniques. This increases surgical safety as every pedicle screw carries a risk of neurological, vascular, or visceral damage. It also reduces surgical morbidity as demonstrated by decreased operative time and blood loss when compared with our previous series⁹. Lower implant density allows a greater area for bone-grafting, which may decrease nonunion rates. It may also reduce the risk of deep wound infection, which can be related to the amount of instrumentation used. The use of fewer pedicle screws reduces surgical cost, an important consideration at times when health economics influence maximal provision of patient care and surgical results are assessed in conjunction with their anticipated cost-effectiveness.

Pitfalls & Challenges

• Accurate placement of pedicle screws is mandatory to reduce the risk of injury to the spinal cord and nerve roots or the major vessels anterior to the spine. Stable 3-column screw fixation and a clear surgical strategy will allow optimum deformity correction in all planes and achieve a globally balanced spine.

• Rod contouring is essential with overbending of the thoracic convex rod in order to restore thoracic kyphosis (especially when a 5.5-mm titanium rod is used) and underbending of the thoracic concave rod to avoid rod prominence across the posterior chest wall.

• The key to the success of scoliosis surgery is to always achieve a balanced spine across the thoracic and lumbar segments in both the coronal and the sagittal plane. This requires matching thoracic and lumbar scoliosis correction (taking into consideration that the thoracic deformity is usually more rigid and therefore less surgically correctable). Restoring thoracic kyphosis is as essential as preserving lumbar lordosis in order to position the head on top of a level pelvis and correct regional and global spinopelvic parameters.