DEFINITION

A number of osteotomy techniques have been described to treat severe or rigid sagittal plane spinal deformity.

These include multilevel anterior interbody radical discectomy and release, posterior pedicle subtraction osteotomy (PSO), vertebral column resection (VCR), and Smith-Petersen osteotomy (SPO).

This chapter reviews the SPO (also known as chevron or Ponté osteotomy), a mainstay in the treatment of sagittal deformity since it was first described in 1945 by Smith-Petersen and associates.16

 

 

ANATOMY

 

The SPO is indicated for correction of a fixed or partially fixed sagittal plane spinal deformity, including hyperkyphosis typified by Scheuermann kyphosis (FIG 1).

 

Although commonly used in the thoracic spine, it has also been used in the lumbar region to correct flat back syndrome or loss of normal lordosis.

 

 

 

FIG 1 • Preoperative clinical photograph (A) and lateral radiograph (B) of 100-degree Scheuermann kyphosis.

 

PATHOGENESIS

 

The various causes of flat back syndrome include Harrington distraction instrumentation,5,11,12 anterior column degeneration, chronic vertebral compression fractures, adjacent segment degeneration, and iatrogenic causes with pseudarthrosis resulting in loss of sagittal plane correction.1,4

 

Additionally, the concepts behind SPO have been applied at the cervicothoracic junction for kyphosis such as in ankylosing spondylitis.

 

Regardless of the etiology, the clinical presentation of patients with sagittal plane spinal deformity is quite similar.

 

PATIENT HISTORY AND PHYSICAL FINDINGS

 

Patients usually complain of back pain due to muscle fatigue but can also present with the inability to stand erect without compensating by bending their knees, stumbling while walking, and a feeling of leaning forward (FIG 2).10

 

 

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FIG 2 • Clinical photograph of a patient with sagittal plane deformity.

 

IMAGING AND OTHER DIAGNOSTIC STUDIES

 

The flexibility of the deformity should be evaluated by both physical examination and preoperative planning radiographic evaluation.

 

Radiographically, sagittal spinal deformity is evaluated with anterior posterior (AP), posterior anterior (PA), and lateral full-length radiographs with the knees extended and the hands resting on the clavicles (FIG 3A,B).9

 

 

 

FIG 3 • A. Thirty-six-inch film with arms straight out obscuring view of C7-T4 area. B. Demonstration of correct position for 36-inch film to allow view of upper thoracic spine and not affect balance. C. Supine bolster lateral x-ray.

 

 

The bolster supine hyperextension lateral radiograph or the push prone radiograph is also helpful to assess the rigidity of the deformity (FIG 3C). Further detailed analysis of coronal plane and segmental anatomy can be determined by computed tomography (CT) scan.

 

Sagittal imbalance is usually determined by the vertical plumb line technique9,10,13 as assessed on 36-inch plain film.

 

Neutral sagittal balance: Vertical plumb line falls at the center of dens or middle of C7 vertebral body aligned with the posterior-superior aspect of the S1 endplate on standing upright films.

 

Positive sagittal balance: Vertical plumb line falls anterior to posterior-superior aspect of S1 by a minimum of 2 to 3 cm.

 

Types of sagittal imbalance include the following:

 

 

Compensated abnormalities with neutral sagittal balance

 

Uncompensated abnormalities with positive sagittal balance that can be rigid or fixed

 

Attention must also be placed on the femurs and on pelvic parameters in evaluating global balance and in preoperative planning.14

SURGICAL MANAGEMENT

 

Standard SPO essentially involves complete resection of the facet complex bilaterally as well as any overlapping lamina and spinous process.

 

The posterior column bone resection must extend from pedicle to pedicle in a cephalocaudal direction. Facetectomies allow for shortening of the posterior column and a component of subsequent lengthening of the anterior column with middle column as fulcrum (FIG 4).

 

If done in the thoracic spine, the rib head and costovertebral articulation will also act with the middle column as the fulcrum for extension.

 

 

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FIG 4 • Schematic illustration of the bony resection required for and the angular correction that is obtainable with SPO.

 

 

The end objective is increased lordosis by shortening the posterior column to restore the sagittal balance such that the head is centered over the sacrum.13

 

Modification of the SPO involves placement of an interbody graft or spacer in the disc space after complete discectomy and interbody arthrodesis. This method permits a greater degree of lordosis without compromising neural foraminal height and can be used to address coronal plane deformity by placing the interbody spacer

asymmetrically in the disc space.13

 

It must be recognized that the degree of correction is governed by the flexibility of the anterior column and the effective preoperative disc height.

 

Ankylosis or bridging anterior osteophytes may significantly block the correction (FIG 5). In cases of rigid deformity, true anterior column osteoclasis helps to achieve a correction of up to 40 to 50 degrees, as may be seen with ankylosing spondylitis.15

 

Although it is commonly estimated that 1 mm of posterior bone resection results in approximately 1 degree of sagittal correction, this may vary depending on the flexibility through the disc. If a radical anterior release is performed before extension osteotomy, the combined anterior distraction and posterior shortening can

 

increase the segmental correction by a factor of 2.5.14 Indications2,3,13

 

 

Type I smooth thoracic and/or lumbar kyphosis Type I sharp, angular kyphosis in the thoracic spine

 

 

 

 

FIG 5 • A. Patient with ankylosing spondylitis with only one disc that might move with SPO. B. Patient with ossification of the ligamentum flavum, which if not resected, might prevent SPO closure and neurologic compromise.

 

 

Type II smooth kyphotic deformity of thoracic and/or lumbar spine when associated with minor (6 to 8 cm) positive sagittal balance

 

 

Type II smooth kyphotic deformity of thoracic spine when associated with major sagittal imbalance (>12 cm) In scoliosis, for three-dimensional deformity correction

 

Contraindications

 

 

Sharp, fixed angular type II deformity that cannot be corrected by SPO2,3,13

 

Anterior fixation, a collapsed or immobile disc space, or an anterior bridging osteophyte at the level of a planned SPO (an open mobile disc space is prerequisite)

 

Posterior wound infection

 

 

Anterior or lateral bridging osteophytes or congenital bars that cannot be released Relative contraindications

 

 

Calcification of the great vessels Ossification of the dura

 

Inability to achieve appropriate segmental control with fixation

 

Preoperative Planning

 

The more caudal the level considered for an SPO, the greater the effect will be on overall alignment.

 

More mobile and taller interbody disc spaces allow for greater correction than severely degenerated, less mobile disc spaces.

 

Due to the potential for natural long-term postoperative degeneration of adjacent disc levels that may have a kyphogenic effect, overestimating the required correction by a few degrees is preferred.

 

Disperse SPO over multiple levels.

 

Avoid SPO adjacent to the lowest instrumented vertebra (LIV) or upper instrumented vertebra (UIV) to minimize

the risk of end vertebrae fixation pullout.

Equipment

Posterior-based segmental instrumentation system

Open frame that will allow the kyphosis to reduce radiolucent operating table Neurologic intraoperative monitoring is highly recommended.

Transcranial motor evoked potential (TcMEP): essential in kyphosis correction Somatosensory evoked potentials (SSEP): essential in kyphosis correction

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Electromyography (EMG): free running of lower extremities and evoked with pedicle screw fixation (optional)

D-wave monitoring

 

 

Patient Positioning

 

The patient is placed in the prone position with the hips flexed initially.

 

For lumbar deformities, the hips can be extended in the case during correction to help close the osteotomy.

 

Approach

 

Posterior extension osteotomies are done at the apex of the deformity. The procedure's objective is to shorten the posterior column by closing down the disc space posteriorly pivoting on the posterior longitudinal ligament

(PLL) and to thereby extend the anterior column (see FIG 5).16

 

The rule of thumb is for every 1 mm of posterior bone resection, 1 degree of correction can be expected.8

TECHNIQUES

• Smith-Petersen Osteotomy

Obtain meticulously clean, wide exposure of spinous process, lamina, pars, facets, out to the tips of transverse process (TP), as well as the lateral border of any old fusion. Strive for meticulous hemostasis; use bipolar cautery respecting bilateral segmental perfusion dynamics. Avoid Gelfoam, or if needed, it should be removed before osteotomy closure (TECH FIG 1).

Remove overlapping spinous processes to cleanly delineate ligamentum flavum and its midline raphe. Remove inferior facets bilaterally at the desired spinal level, exiting inferior to the base of the TP.

Undertake ligamentum flavum resection from pedicle to pedicle to avoid any mass effect from redundant ligament dorsally on the neural structures once the SPO is closed down.

Perform bilateral superior articular facet resection, which ends flush to the top of the TP.

 

 

 

 

TECH FIG 1 • Intraoperative photographs before (A) and after (B) SPO.

 

 

 

TECH FIG 2 • A,B. Sequential temporary apical rod technique for segmental reduction.

 

 

 

Smooth out cut edges to avoid any obstruction to closure or possible iatrogenic injury to dura or nerves. If there is no coronal deformity to be corrected, the osteotomy should be symmetric.

 

To address a coronal deformity, an asymmetric SPO can be used with the wider osteotomy on the side of convexity.13

 

Removal of laminar bone should be adequate and account for possible desired contact between the superior and inferior lamina after closure that may aid fusion. Typically, the osteotomy width is between 6 and 10 mm cephalocaudally.

 

The gap created by the osteotomy is compressed, hinging on the posterior aspect of the disc space, causing posterior column shortening and anterior column extension.

 

The compression is held in place with posterior spinal instrumentation (TECH FIG 2).

 

 

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If multiple SPOs are planned, dorsal compression should be performed in a way that redistributes forces over the largest area possible, using either the cantilever reduction technique (TECH FIG 3A) or the apical compression technique (TECH FIG 3B). If using the cantilever method, manipulate two rods simultaneously to distribute corrective loads, and carefully monitor stress on the end vertebrae fixation.

 

TECH FIG 3 • A. Cantilever reduction technique. B. Apical-based sequential temporary apical rod technique.

The lamina and the spinous process of the upper and lower vertebra are decorticated in preparation for arthrodesis.

 

 

 

PEARLS AND PITFALLS
	Attention to cardiovascular parameters can assist with limiting blood loss.   In adults, mild hypotension during exposure can help reduce blood loss; however, during osteotomy and manipulation, the pressure should be elevated; mean arterial pressures over 80 mm Hg are ideal.   In the pediatric population, mean arterial pressures are maintained between 50 and 60 mm Hg during the approach with an increase from 70 to 80 mm Hg before deformity correction.6   Avoid bone wax where it can inhibit fusion.     Carefully plan for soft tissue coverage. Initial exposure should meticulously preserve full-thickness flaps especially at the deformity apex. Flaps may be necessary to get tensionless closure in revision cases. Low-profile instrumentation or monoaxial screws may need to be considered in kyphosis with severe gibbus deformity and thin tissue cover.     Beware of ossification of the ligamentum flavum, which can best be seen on preoperative scans (see FIG 5B). These areas are frequently stenotic and adhered to dura dorsally. Ligamentum flavum must be fully resected before any closure is attempted.     During the SPO, the pedicles should remain undisturbed. One must be especially careful with osteotomes, which may propagate a crack into the pedicle if dull or not well directed. Invasion of pedicles may weaken critical fixation points requiring an alternate approach to adequately allow for closure of the osteotomy.

 

 

• Avoid the use of large Kerrisons; size 2 or 3 mm is recommended, particularly when working on the concavity of deformity with associated scoliosis.

   

• Look out in rare cases with dorsal displacement of spinal cord or roots, which may have frequent adhesions that need to be resected.

   

• End segments are most likely to pullout, so optimization of screw length and diameter is critical.

   

• In higher degree thoracic kyphotic patients, cranial traction may be helpful. In severe lumbar deformity, extending the table during correction may facilitate osteotomy closure; but, beware of pressure on the thighs and tibiae or the patient shifting under the drapes.

   

• Before closing any osteotomy, the neural foramina should be probed gently to ensure they are patent. It is common to discover a small superior tip of a resected superior facet retained in the foramen (especially in thoracic spine) or a remaining spike of either facet base.

   

• In cases where osteotomy fails to close down, check for residual lateral facet bone or bridging osteophytes or anterior column obstruction. You may need to do more SPOs or convert to PSO if the anterior column will not release.

   

• During closure, anteriorly directed manual pressure near the apex can help provide additional corrective force. Any time instruments are used for pushing down on the apex to assist reduction, be certain that the canal is safe and there is no risk if an instrument inadvertently slips.

   

• In the lumbar spine where lordosis increases during closure, the dura may buckle slightly. If this is seen, create additional central opening by resecting and beveling the midline lamina to provide extra space.

   

• Ensure TP decortication laterally, and in the midline, use careful decortication technique mindful of fixation points and any exposed neural elements. Carefully pack generous bone around all osteotomy points avoiding any material into the canal.

   

• Cross-table postreduction lateral imaging should be done to ensure that adequate correction has been obtained.

 

 

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FIG 6 • Clinical photograph (A) and lateral radiograph (B) after SPO correction of 100-degree Scheuermann kyphosis.

 

OUTCOMES

A good result is pictured in FIG 6.

 

 

COMPLICATIONS

Intraoperative

Subluxation

Dorsal root compression Spinal cord traumatic injury Dural buckling Cerebrospinal fluid leak Great vessel injury

Pedicle fracture

Postoperative

 

New neurologic deficits secondary to intraoperative spinal cord or nerve root injury

Radiculopathy from neural foraminal compression (rare unless there is baseline foraminal stenosis)12 Superior mesenteric artery syndrome, intestinal obstruction,7,8 and prolonged ileus

Epidural or intraspinal hematoma

Deep venous thrombosis and pulmonary embolism Pseudarthrosis is another reported complication.

Adjacent segment disease/junctional kyphosis

 

 

REFERENCES

1. Berven SH, Deviren V, Smith JA, et al. Management of fixed sagittal plane deformity: results of the transpedicular wedge resection osteotomy. Spine 2001;26(18):2036-2043.

    

    

2. Booth KC, Bridwell KH, Lenke LG, et al. Complications and predictive factors for the successful treatment of flat back deformity (fixed sagittal imbalance). Spine 1999;24(16):1712-1720.

    

    

3. Bridwell KH. Decision making regarding Smith-Petersen vs. pedicle subtraction osteotomy vs. vertebral column resection for spinal deformity. Spine 2006;31(19 suppl):S171-S178.

    

    

4. Bridwell KH, Lenke LG, Lewis SJ. Treatment of spinal stenosis and fixed sagittal imbalance. Clin Orthop Relat Res 2001;(384): 35-44.

    

    

5. Casey MP, Asher MA, Jacobs RR, et al. The effect of Harrington rod contouring on lumbar lordosis. Spine 1987;12(8):750-753.

    

    

6. Diab MG, Franzone JM, Vitale MG. The role of posterior spinal osteotomies in pediatric spinal deformity surgery: indications and operative technique. J Pediatr Orthop 2011;31(1 suppl): S88-S98.

    

    

7. Dorward IG, Lenke LG. Osteotomies in the posterior-only treatment of complex adult spinal deformity: a comparative review. Neurosurg Focus 2010;28(3):E4.

    

    

8. Gill JB, Levin A, Burd T, et al. Corrective osteotomies in spine surgery. J Bone Joint Surg Am 2008;90(11):2509-2520.

    

    

9. Horton WC, Brown CW, Bridwell KH, et al. Is there an optimal patient stance for obtaining a lateral 36″ radiograph? A critical comparison of three techniques. Spine 2005;30(4): 427-433.

    

    

10. Joseph SA Jr, Moreno AP, Brandoff J, et al. Sagittal plane deformity in the adult patient. J Am Acad Orthop Surg 2009;17(6): 378-388.

     

     

11. LaGrone MO. Loss of lumbar lordosis. A complication of spinal fusion for scoliosis. Orthop Clin North Am 1988;19(2):383-393.

     

     

12. Lagrone MO, Bradford DS, Moe JH, et al. Treatment of symptomatic flatback after spinal fusion. J Bone Joint Surg Am 1988;70(4): 569-580.

     

     

13. La Marca F, Brumblay H. Smith-Petersen osteotomy in thoracolumbar deformity surgery. Neurosurgery 2008;63(3 suppl):163-170.

     

     

14. Lee MJ, Wiater B, Bransford RJ, et al. Lordosis restoration after Smith-Petersen osteotomies and interbody strut placement: a radiographic study in cadavers. Spine 2010;35(25):E1487-E1491.

     

     

15. Simmons EH. Kyphotic deformity of the spine in ankylosing spondylitis. Clin Orthop Relat Res 1977; (128):65-77.

     

     

16. Smith-Petersen MN, Larson CB, Aufranc OE. Osteotomy of the spine for correction of flexion deformity in rheumatoid arthritis. J Bone Joint Surg Am 1945;27:1-11.