28 Mini-Open Pedicle Subtraction Osteotomy for Deformity Correction

Cory Hartman, Jakub Godzik, Esteban Quiceno, and Juan S. Uribe

Summary

Adult spinal deformity is becoming a commonly diagnosed pathology because of the increasing elderly population. There are many different surgical approaches to correct these dimensional deformities; most of these approaches are associated with high morbidity and high rates of complications. This chapter reviews the indications and surgical technique for a mini-open pedicle subtraction osteotomy (PSO), a component of a hybrid minimally invasive approach to spinal deformity management. PSO is a valid option that can reduce blood loss, prolonged hospitalizations, and prolonged surgical times associated with traditional open surgical approaches for correcting fixed kyphotic deformity.

Adult spinal deformity (ASD) is a common pathology; it has been reported in up to 60% of the older population.1 The surgical treatment associated with correction of sagittal imbalance in ASD is associated with improved quality of life and patient outcome scores. A wide range of osteotomies are employed to achieve an adequate improvement in sagittal balance and spinopelvic parameters; however, these surgical techniques are associated with a complication rate of 39%.^2,3

There are six different types of osteotomies classified by Schwab (Fig. 28.1). (1) Grade 1 is a partial facet resection, such as the Smith-Peterson osteotomy; with this procedure, about 5 to 10 degrees of correction can be gained at each level.² (2) Grade 2 osteotomy is a complete facet resection; the typical example is the Ponte osteotomy, which was described in 1987 and consists of wide resection of the thoracic facet joints and laminae and the complete removal of the ligamentum flavum.⁴ (3) Grade 3 osteotomy is the pedicle subtraction osteotomy (PSO), which extends the resection of the posterior vertebral elements into the vertebral body. (4) Grade 4 osteotomy includes a disc resection. (5) Grade 5 osteotomy includes the complete body and disc resection, and (6) grade 6 includes multiple vertebral body and disc resections.²

The PSO was first described in a series of 11 patients with ankylosing spondylitis. The procedure involves a three-column osteotomy with a transpedicular wedge resection extending from the posterior elements through the vertebral body, but the anterior cortex is left intact.⁶ Currently, the PSO is widely used to correct kyphotic deformities and in, some cases, flat back syndrome. Recent evidence suggests that PSO is more effective in achieving good clinical outcomes in patients with kyphotic deformities than in patients with flat back syndrome who have had previous lumbar surgeries. Patients with flat back syndrome may also develop more surgical complications.

With advances in technology, new minimally invasive surgery (MIS) techniques have been developed to improve clinical outcomes and reduce surgical time, blood loss, and morbidity. MIS techniques, such as interbody fusion associated with posterolateral stabilization, have been used as a treatment for ASD, but these techniques are of limited use when the surgeon faces fixed kyphotic deformities.⁷ The mini-open PSO technique is an element of the hybrid MIS-open approach to management of ASD. The mini-open PSO allows for a PSO to be performed through a smaller opening, reducing blood loss. The remaining construct is created via MIS techniques, such as percutaneous pedicle screws and anterolateral arthrodesis.

We describe a minimally invasive form of PSO that allows for a smaller incision, less blood loss, and less surgical time (Table 28.1).

Table 28.1 Advantages and disadvantages of MIS PSO

e)>10 cm of sagittal imbalance.6

The PSO is commonly performed below the conus in the lumbar spine. In the past, it was performed only between L3 and L5 to recreate physiological lordosis and to avoid spinal cord manipulation during the closure of the wedge osteotomy. However, with the improvement of surgical technique and new technological aids, a PSO can be performed in the cervical and thoracic spine. The PSO should be performed at the level of the kyphotic deformity.⁶ The osteotomy should be performed at the level of the apex of the kyphosis or fixed deformity to maximize lordosis. In cases of lumbar kyphosis or hypolordosis, the PSO should be performed in the lower lumbar spine. This location provides more physiologic lumbar lordosis because 60% of the lordosis is between L4 and S1.

28.3 Contraindications

a)Medical contraindications.

b)Poor bone quality (all patients must have a recent bone density scan).

In cases that require more than 40 degrees of correction, additional methods including interbody cages or additional osteotomies, such as Smith-Peterson, may be used with a PSO to achieve spinopelvic balance.

28.4 Preoperative Planning

Proper physical evaluation and clinical history are crucial in the management of patients with ASD. Image evaluation is essential for preoperative planning. Standing spinal radiographs showing posteroanterior and lateral views are mandatory for achieving the proper correction.⁸ Sagittal balance should be measured using these radiographs because it is closely related to quality-of-life scores, and one of the goals of the surgery is to correct the malalignment.⁹

The goals of ASD correction include a sagittal vertical axis of 40 to 50 mm, pelvic incidence minus lumbar lordosis within 10 degrees, and a pelvic tilt of less than 20 degrees; however, older patients require less rigorous alignment objectives.⁹ These preoperative measurements are critical because the surgeon uses them to determine what surgical strategies are needed based on the required restoration of lumbar lordosis. A PSO restores 30 to 40 degrees of lumbar lordosis.⁶

Long flexion and hyperextension radiographs allow one to assess the flexibility of the kyphotic segment of the spine. Supine radiographs can be helpful in determining the flexibility of the deformity in the sagittal and coronal planes. The amount of correction necessary can be reevaluated if the spine corrects significantly in the supine position.⁸ This step can be performed in the operating room during patient positioning.

Magnetic resonance imaging (MRI) provides information on soft-tissue anatomy; it indicates the presence of arachnoiditis, periradicular fibrosis, or pseudomeningocele in patients with a previous history of lumbar surgery—conditions that are associated with a less favorable outcome. The presence of lumbar spinal stenosis, either central or foraminal, can be identified and adequately decompressed during surgery.

Computed tomography (CT) provides a complementary view of the bony anatomy; it is useful for planning the placement of spinal instrumentation and shows disc osteophytes, fixed deformities, and calcifications. We do not advocate the routine use of CT myelography but reserve it for situations in which MRI does not provide adequate visualization or cannot be performed.

28.5 Patient Positioning

4.The remaining pedicle screws may be placed via MIS techniques (Fig. 28.2a). Additionally, deeper screw placement is required for satellite rod placement (discussed below).

5.After removing the spinous process of the index vertebra and the level above, the surgeon proceeds with laminectomy and complete facetectomy of all joints of the index level using a rongeur and Kerrison punch.

6.The decompression laminectomy is performed to expose the pedicles above and below the proposed pedicle resection. At the level of the PSO, the transverse processes are detached from the vertebral body using a Kerrison rongeur, and the ligamentum flavum is also removed. At this point, the vertebral body is isolated, and the nerve roots are completely visualized (Fig. 28.2b).

7.Next, the lateral surface of the vertebral body is exposed with Cobb or Penfield dissectors. If lateral segmental vessels are seen, bipolar cautery is used to control the bleeding. The plane between the lateral aspect of the vertebral body and the adjacent soft tissue is maintained with retractors. The nerve roots are visualized and are protected during the resection of the pedicles, which can be done with a rongeur.

8.A temporary deep-seated, satellite rod is placed across the PSO from the superior to the inferior pedicle screws to prevent translation and neural injury. Eventually, the satellite rods serve as the final rods in the construct, and the primary rods lie above them. It is essential for the surgeon to deeply seat the pedicle screws at the levels above and below the PSO to avoid interference with the primary rods. The wedge osteotomy can be performed using a variety of tools. At our institution, we favor using an ultrasonic oscillating bone scalpel to reduce blood loss and surgical time as well as to maximize the amount of bone available for autografting. Fluoroscopic guidance is used to maintain orientation when creating the wedge. Wedge angulation is dependent on the surgical goals, ranging in the literature from 25 to 40 degrees.⁶ When resecting the vertebral body, one must take care to begin by creating a thin posterior cortical wall anteriorly to facilitate closure of the osteotomy. Using an oscillating bone scalpel instead of a high-speed drill allows one to obtain a large amount of autograft from the vertebral body resection. Once the majority of the osteotomy is performed with the ultrasonic oscillating bone scalpel, curettes may be used to complete the resection (Fig. 28.2c, d).

9.A curved Freer elevator can be used to remove any dural adhesions to the posterior wall to prevent injury to the anterior portion of the dura. Once the resection of the lateral wall is complete, an angled curet or downward pusher can be used to fracture the thinned posterior wall into the osteotomy site. The anterior vertebral cortex must be preserved to avoid vascular and visceral injuries (Fig. 28.2e). Before closing the osteotomy, the surgeon must remove bone fragments that could compress the nerve roots.

10.Closure of the wedge osteotomy can be accomplished by sequential compression maneuvers across the satellite rods bilaterally, bending of the table, and lifting of the patient’s feet. With advances in technology and pedicle screw reduction systems, one may use towers on the tulip heads to reduce both sides simultaneously. This technique allows for compression without the use of a compressor tool, which can slip off the rod if compression is not done appropriately. We recommend this maneuver be done under strict visual inspection and neuromonitoring to prevent injury to neural structures.

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