Ankylosing Spondylitis Treated with Pedicle Subtraction Osteotomy

History:
The patient is a 31 year-old male with a long history of intractable lumbar back pain. He was diagnosed with ankylosing spondylitis and has been unable to work for the last 10 years secondary to pain. All of his pain is in the back and he denies any pain in his legs. He notes that he has become increasingly "pitched forward" over the last several years and that he is unable to see more than 20 feet in front. He has almost no motion throughout his spine and walks with his knees flexed to allow him to see forward.

Radiographs of the patient's entire spine demonstrate almost complete autofusion through the thoracolumbar spine. The lateral radiograph demonstrates a marked loss of lumbar lordosis and global sagittal imbalance with the C7 vertical plumb line roughly 10 cm. in front of the pelvis (yellow line).

Surgical Options:
Traditionally ankylosing spondylitis has been treated with extension osteotomies at the level of the deformity. Originally described by Smith-Petersen in 1945, these osteotomies are done by taking the posterior elements completely off at the level of the disc space; the spine is then extended through the osteotomized segments and disc spaces. The amount of correction attainable through each disc space is roughly 10-15°. After the osteotomy is done, it is usually necessary to perform anterior structural grafting at the levels of osteotomy to prevent anterior collapse and settling.

Another option for improving the sagittal imbalance is with a pedicle subtraction osteotomy. In a pedicle subtraction osteotomy, the posterior elements are carefully resected along with a decancellation of the body via a transpedicular route. The lateral margins of the pedicle and body are carefully removed and the entire spine is extended through the osteotomy. The advantages of a pedicle subtraction osteotomy are that it is entirely accomplished through a posterior approach without the need for an anterior procedure. As well, 35-40° of lordosis can be expected with an adequate osteotomy.

Surgery:
Because of the dramatic global sagittal imbalance and the fact that the patient already had a complete autofusion through his thoracolumbar spine, it was decided that a pedicle subtraction osteotomy was the best option. He underwent an L3 PSO with instrumentation and fusion from T12-S1. The amount of correction that was achieved in the sagittal plane as a result of the PSO was roughly 45 degrees.

Lateral radiographs demonstrate a much more physiologic lordosis. Note the wedged L3 vertebral body and the absence of pedicles at the L3 vertebral body. Also the C7 plumb line is at the back of the sacral endplate, a correction of 10 cm. In the lateral x-ray, the resected pedicles are seen at L3 (yellow arrow). A generous central decompression (red curve) was also performed to avoid kinking of the dura during closure of the osteotomy.

Clinical photos taken 6 weeks postoperatively. The patient is happy with his progress and has much better vision because of improved posture.

Discussion:
Patients with fixed sagittal imbalance (FSI) are complicated cases who often require fairly involved surgeries to achieve an acceptable position. There are many etiologies of FSI including ankylosing spondylitis, postlaminectomy kyphosis, iatrogenic flatback, and posttraumatic kyphosis. Many of these patients have previously had one or more surgeries, further complicating their treatment.

Evaluation of sagittal imbalance is best done with a standing long cassette of the entire spine. A plumb line is dropped from the middle of C7. This plumb line should intersect the back or the middle of the lumbosacral disc. If the plumb line is in front of the lumbosacral disc, the patient is defined as having forward or anterior sagittal imbalance. To evaluate the flexibility of the deformity, a hyperextension lateral x-ray taken over a bolster is invaluable.

In determining the optimal thoracic kyphosis/lumbar lordosis ratio in a patient, a good rule of thumb is that a patient should generally have 30° more lumbar lordosis than thoracic kyphosis. Strict attention should be paid to the relationship of the C7 plumbline to the lumbosacral disc as previously discussed.

Options for increasing lordosis include Smith-Petersen osteotomies or a pedicle subtraction osteotomy. Smith-Petersen osteotomies will achieve 1 degree of correction for each mm of bone resected. The average osteotomy will achieve 10-15° of correction; a patient with a great deal of kyphosis will then need osteotomies over several levels to achieve reasonable balance. As well, an anterior procedure is often necessary to fill in the disc spaces at the osteotomized levels which will be extended open by the posterior column resection and closure. A pedicle subtraction osteotomy can be expected to achieve 35-40° of correction and potentially avoid the need for an anterior procedure, because the anterior and middle columns are closed with closure of the posterior column osteotomy.

When performing osteotomies, they should general be done at the level of maximal deformity. This is usually the L3/4 level, which also corresponds to the normal apex of lordosis. This also is safer in terms of neurologic damage as it is well away from the spinal cord. A pedicle subtraction osteotomy achieves a great deal of correction through one level and there is a risk of "kinking" of the nerve roots and dura. It is imperative to perform a generous central decompression both superiorly and inferiorly to avoid neurologic injury. After closure of the osteotomy we perform a wake-up test to assure that no untoward neurologic events have occurred.