Summary Statement: Fusion Technologies (2003)

The selection of fusion levels for scoliosis patients remains controversial but extremely critical. Techniques that provide optimal curve correction and a balanced spinal deformity in the coronal and sagittal planes while fusing a minimum number of levels to maximize spinal mobility are the goal. For adolescent idiopathic scoliosis (AIS), classification of curves using the Lenke et al system allows the categorization of curves into similar patterns that can have different treatments more fairly assessed. This system lays the groundwork for performance of a selective fusion of a major curve (largest Cobb measurement) leaving a minor curve that completely deviates from the midline (lumbar apex to center sacral vertical line, thoracic apex to C7 plumbline) unfused. In addition to the classification patterns that predict a selective fusion treatment, an analysis of radiographic and clinical ratios of structural criteria should be carefully performed on patient’s radiographs and clinical examination. When major curve radiographic and clinical parameters are significantly more prominent than the corresponding minor curve (ratios of 1.2 or greater), then selective fusion may be possible. The goal is a balanced spine with matched instrumented correction of the major curve with the unfused minor curve, which will leave a more mobile spine for these patients. Many candidates for selective fusion can be approached anteriorly versus posteriorly. In addition, any major curve, whether there is a corresponding minor curve that is structural or not, can be potentially treated with an anterior instrumentation and fusion. With the anterior approach, thorough anular and disc excision renders the scoliotic apical segments extremely mobile and amenable to correction. Fusion levels for the anterior approach are at most the Cobb measurements, and potentially a bit shorter with hypercorrection of the apical segments. In the thoracic spine, creating kyphosis with a bone-on-bone type of alignment is advantageous for simultaneous coronal and sagittal plane alignment. In the thoracolumbar junction, one must be careful not to kyphose the spine by placing structural interbody spacers (femoral ring or cage) to provide a lordotic contour, which are especially helpful at and below the L1 levels. Good results can be expected using a single or dual rod construct, although those promoting a dual rod construct point to the lack of any implant failure in AIS patients. Correction rates should be similar for both, because the first rod is the correcting rod and the second rod is an additional stabilizing implant meant to increase the biomechanical stability of the construct. Regardless of the type of implant utilized or the specific levels chosen for fusion, an anterior approach almost always requires a shorter fusion than a posterior approach for the same curve pattern and thus patient mobility should be theoretically optimized long-term. For AIS that involves the lumbar region, the ability to “save levels” becomes more problematic, specifically when degenerative changes occur down to the lumbosacral region. It is often difficult, if not impossible, to do short fusions that will provide optimal radiographic and clinical results. A common clinical scenario is that the spine is significantly degenerated to L5, with varying degrees of degeneration of L5–S1. Decisions must then be made regarding the ability to stop a scoliosis fusion at L5, or extend the fusion to include the lumbosacral disc. There are many factors to consider in this decision process. If L5–S1 is degenerated, olisthetic, or L5 foraminal stenosis exists, the decision to extend the fusion to the sacrum is noncontroversial. However, when there is only slight or no degeneration, the decision is more complicated. Intuitively, it would seem logical to preserve the L5–S1 segment in these scenarios. However, the long-term results of a long fusion (upper thoracic spine to L5) appear not to be as favorable as once thought, even if L5 is well seated within the pelvis and with minimal degeneration preoperative. Also, it is unknown exactly how much benefit is obtained by leaving a single lumbosacral disc unfused below a long construct; potentially much of the motion of the spinal-pelvic region will occur at the pelvic–femoral axis. Also, one can expect significant degenerative changes to occur at L5–S1 within 5 to 10 years in many patients following these types of fusions. However, performing long fusions that consistently include the lumbosacral disc will have a higher complication rate because of the additional surgery required, including often a circumferential approach to obtain a solid lumbosacral fusion. Importantly, no long-term adverse ramifications on the sacroiliac joints in patients fused to the sacrum have been clinically seen thus far. Thus, discussion with patients regarding the risk/benefit ratio of including the lumbosacral disc in long adult scoliosis fusions should be conducted to allow informed patient decision-making that is most agreeable to both the patient and treating surgeon, knowing that neither option is ideal. Key Points Selective fusions of the spine for AIS in patients with both thoracic and lumbar regions completely deviated from the midline can provide correction of the major curve with well-balanced minor curves, thereby preserving mobile motion segments to the spine. Anterior approaches to scoliosis can minimize fusion levels (vs. a posterior approach) for many curve patterns, but especially in those patients undergoing a selective fusion of a major thoracic or thoracolumbar/lumbar curve. In addition, most patients treated for an isolated major thoracolumbar/lumbar curve can be approached with a single- or dual-rod anterior instrumentation and fusion. The decision to include the lumbosacral disc in long adult scoliosis deformities involving the lumbar spine is still quite controversial, with each case requiring individual assessment, planning, and precise surgical care. Even with optimal L5–S1 disc health preoperatively, early and significant postoperative L5–S1 degeneration may ensue following a long fusion ending at L5.