Anterior Release

Complete anterior release as performed in our institution is done through an open approach to the thoracic or lumbar spine or both, to allow improved mobilization of a curve and correction of sagittal and coronal deformities. In the thoracic spine, the convex rib heads are resected and an attempt is made to rupture the concave costovertebral joints. In both the thoracic and lumbar spine the disc and posterior annulus are removed, with release of the posterior longitudinal ligament. The convex inferior endplate is then resected with or without resection of the convex superior endplate. This allows mobilization and correction in the coronal plane. In addition, the sagittal profile of the thoracic spine, which is often hypokyphotic in thoracic AIS, can be corrected to its normal degree of kyphosis by essentially shortening the anterior column. Anterior structural support is often recommended in the lumbar spine and at the thoracolumbar junction to prevent the development of kyphosis. After complete anterior release, the patient may be instrumented anteriorly if the curve is not too large or rigid. Generally, thoracic curves of up to 70 degrees that have flexibility of >25% may be corrected well with anterior release and instrumentation. An additional posterior release with instrumentation may be required in more rigid deformities.

Posterior Release

First described by Hibbs in 1924, the posterior facetectomy involves removal of the inferior articular process of the facet joint with curretage of the joint cartilage.1 The technique has become a standard in posterior scoliosis surgery because it allows some increased mobility, facilitating curve correction while improving the fusion bed. Howarth, in 1943, added resection of intraspinous ligaments and spinous processes to this technique, further improving curve mobility and the amount of local bone available for fusion.2

Smith-Petersen Osteotomy

This osteotomy was described by Smith-Petersen and colleagues in 1945 for use in treating the deformity in lumbar flexion that can result from ankylosing spondylitis (“rheumatoid arthritis”).3,4 A modified posterior resection is now used in treating spinal deformity of many etiologies. Smith-Peterson osteotomies (SPOs) allow mobilization and correction primarily of deformities in the sagittal profile, but may be useful in obtaining coronal mobilization as well. The SPO procedure closes the posterior column, hinges on the middle column, and lengthens the anterior column of the spine. This results in a posterior shift of the gravity line, shortening the moment arm for posteriorly applied corrective forces. As originally described, the SPO required a fracture of the ankylosed anterior column. The modified SPO in common use today requires a mobile anterior column; the resultant lengthening of the anterior column may require anterior structural support.

The procedure in SPO produces a “V”-shaped osteotomy with the “V” directed caudally. The spine is exposed through a standard posterior approach. The spinous processes are resected. The ligamentum flavum is detached from the inferior margin of the lamina and the inferior articular process. An oblique osteotomy is made through the superior articular process of the caudal vertebra and inferior articular process of the cephalad vertebra, directed 45 degrees to the frontal plane. The intervening facet joint is excised. An extension or posterior compression force is applied gradually to the posterior elements to obtain correction. One millimeter of bone resection corresponds to roughly one degree of sagittal plane correction. From 5 to 15 degrees of sagittal-plane correction can be expected per osteotomy ( Fig. 16.1 ). Although SPO is primarily used for sagittal-plane correction, coronal correction can be achieved with asymmetric osteotomies, especially when multiple SPOs are used. It has been noted by some that asymmetric PSOs may be more effectively used for this purpose.5

When applying the SPO to fixed coronal deformities, care must be taken to prevent worsening of the deformity. Inter-vertebral-body fusion by means of a transverse intervertebral approach may be useful in correcting the deformity. Asymmetric placement of an interbody spacer may help in correcting a coronal deformity. By using the interbody spacer as a fulcrum, one may obtain the same amount (or more) of correction of a sagittal deformity with less neuroforaminal compromise.6

Reported complications of SPO include pseudarthrosis, which may result from creating a gap in the disc space. This alters the integrity of the anterior column, which bears 80 to 90% of the compressive forces on the spine in the standing position. Degeneration of adjacent segments has also been reported. Neurological complications can be significant, and have been reported in as many as 30% of patients undergoing SPO.3,6 Radiculopathy may result from compression of nerve roots as they exit the foramina narrowed by closure of the SPO. Care must be taken to perform wide foraminotomies at the level of an SPO. Pedicle fractures may occur with overzealous compression during the closing of an SPO.

Ponte Osteotomies

The osteotomy described by Alberto Ponte allows mobilization and correction of the sagittal profile.7 It was initially described for use in the thoracic spine in cases in which no ankylosis exists, such as in Scheuermann’s kyphosis and osteoporosis. In contrast to the originally described SPO, Ponte osteotomies are posterior shortening procedures associated with minimal lengthening of the anterior column of the spine. This is achieved through a generous posterior resection of the superior and inferior laminae as well as the facet joint. It has been suggested that the center of rotation moves anteriorly in Ponte osteotomies, lengthening the moment arm of the posterior corrective forces.8 Because of the compression exerted on the middle column of the spine, it is also imperative to rule out the presence of disc herniation before using this purely posterior technique, to prevent possible spinal-cord compression from a herniated disc when posterior compressive forces are applied to the spine.

The technique of the Ponte osteotomy is similar to that of the modified SPO commonly utilized today, in that it initially involves a standard posterior approach to the spine and resection of the spinous processes. The soft tissues of the interspinous ligaments and ligamentum flavum are removed. The standard excision of the inferior articular process is complemented by removal of a portion of the superior articular process as well. Thus, at each level of a Ponte osteotomy, a 4- to 6-mm interlaminar gap is created ( Fig. 16.2 ). Generous undercutting of the laminae is crucial to prevent spinal-cord compression when the resulting interlaminar gaps are closed by means of compression. Resection is performed laterally into the neural foramen ( Fig. 16.3 ). Compression forces are applied across the instrumentation; segmental multiple compressions may be needed to obtain the desired correction ( Fig. 16.4 ).

Geck and co-workers9 reported on the sagittal-plane correction achieved in Scheuermann’s kyphosis with segmental pedicle-screw instrumentation and Ponte osteotomies in 17 patients. No neurological complications were reported. However, care should be given to avoiding overcorrection in the sagittal plane.

Debate exists about the most appropriate terminology for the procedure involving complete release of the posterior vertebral elements in the surgical correction of a rigid spinal curve. When done in the thoracic spine over multiple levels for the correction of Scheuermann’s kyphosis, the procedure is clearly most accurately called a Ponte osteotomy. When a single level, 30- to 45-degree correction is performed in the lumbar spine with a marked opening of the anterior column, the procedure is most appropriately called an SPO. Unfortunately, the terms are often used interchangeably for the often multilevel thoracic or lumbar excision of the inferior and superior articular processes as well as all intervertebral posterior soft tissues. These releases may also be used for coronal-plane correction, for which they were not initially described.