Pedicle subtraction osteotomies (PSOs) have been used in the treatment of multiple spinal conditions involving a fixed sagittal imbalance, such as degenerative scoliosis, idiopathic scoliosis, posttraumatic deformities, iatrogenic flatback syndrome, and ankylosing spondylitis. The procedure was first described by Thomasen1 for the treatment of ankylosing spondylitis. More recently, multiple centers have reported the expanded use and good success of PSO in the treatment of fixed sagittal imbalance of other etiologies.2,3 According to Bridwell and colleagues,2 lumbar lordosis can be increased 34.1°, and sagittal plumb line can be improved 13.5 cm.
PSO is a complex, extensive surgery most often performed in the revision setting. Multiple authors have described the technique for PSO.4,5 There are significant technical challenges and many complications, including neurologic deficits, pseudarthrosis of adjacent levels, and wound infections.6 Short-term challenges include a large loss of blood, 2.4 L on average, according to Bridwell and colleagues.6 Time of closure of the osteotomy gap is a crucial point in the surgery. Blood loss, often large, slows only after the gap is closed and stabilized.
In this article, we describe a technique in which an additional rod or pedicle screw construct is used at the periosteotomy levels to close the osteotomy gap during PSO and simplify subsequent instrumentation. In addition, we report our experience with the procedure.
Materials and Methods
Seventeen consecutive patients (mean age, 58 years; range, 12-81 years) with fixed sagittal imbalance were treated with lumbar PSO. The indication in all cases was flatback syndrome after previous spinal surgery. Mean follow-up was 13 months. Mean number of prior surgeries was 3. Thirteen PSOs were performed at L3, and 4 were performed at L2.
Radiographic data were collected from before surgery, in the immediate postoperative period, and at final follow-up. All the radiographs were standing films. Established radiographic parameters were measured: thoracic kyphosis from T5 to T12, lumbar lordosis from L1 to S1, PSO angle (1 level above to 1 level below osteotomy level), sagittal plumb line (from center of C7 body to posterosuperior aspect of S1 body), and coronal plumb line (from center of C7 body to center of S1 body).2
Good clinical outcomes in the treatment of spinal disorders require careful attention to the alignment of the spine in the sagittal plane.7,8 When evaluating the preoperative radiographs, we measured and documented pelvic parameters. Figure 1A shows how pelvic incidence was determined. We measured this as the angle between a line drawn from the center of the S1 endplate to the center of the femoral head and the perpendicular off the S1 endplate. Figure 1B shows pelvic tilt as determined by the angle between a line drawn from the center of S1 to the femoral head and a vertical line originating from the center of the femoral head. Figure 1C shows the sacral slope, which we measured as the angle between a line drawn parallel to the endplate of S1 and its intersection with a horizontal line.
Surgical Technique
The overall surgical technique for PSO has been well described.4,5 Here we describe the “outrigger” modification to osteotomy closure (Figures 2, 3).
Most of our 17 cases were revisions. In these cases, new fixation points are first established. All fixation points that will be needed for the final fusion are placed. If a pedicle above or below the osteotomy level is not suitable for a screw, it can be skipped.
Wide decompression of the involved level is performed from pedicle to pedicle, ensuring that the nerve roots are completely decompressed. The dissection is then continued around the lateral wall of the vertebral body. While the neural elements are protected with gentle retraction, the pedicle and a portion of the posterior aspect of the vertebral body are removed with a combination of a rongeur and reverse-angle curettes. Resection of the vertebral body can be facilitated by attaching a short rod to the pedicle screws on either side of the osteotomy level and using it to provide gentle distraction.
Once sufficient bone has been removed to close the osteotomy, short rods are placed in the pedicle screws in the level above and the level below the osteotomy site. These rods are attached with offset connectors that allow the rods to be placed lateral to the screws. Before the surgical procedure is started, the patient is positioned on 2 sets of posts separated by the break in the table. The break in the table allows flexion to accommodate the preoperative kyphosis and allows hyperextension to help close the osteotomy site. Now, with the osteotomy site ready for closure, the table is gradually positioned in extension along with a combination of posterior pressure and compression between the pedicle screws above and below the osteotomy. Once the osteotomy is adequately compressed, the short rods are tightened, holding the osteotomy in good position. With the osteotomy held by the short rods and table positioning, decompression of the neural elements is confirmed and hemostasis obtained.