The typically recommended treatment for Lenke 5 curves is fusion only of the primary TL/L curve.3,13 However, nearly 27% of fusions for these curves were treated nonselectively by experienced adolescent idiopathic scoliosis surgeons. This is in contrast with 17% of patients with primary thoracic scoliosis undergoing nonselective fusion by surgeons with similar experience as reported by Newton et al.11 This suggests that protecting thoracic motion segments may not be approached with the same vigilance as preserving lumbar motion, or that the Lenke classification criteria for a structural minor curve are interpreted differently for the thoracic and lumbar regions.
One of the primary purposes in performing a selective thoracic fusion is to minimize the loss of lumbar motion and theoretically the risk of lumbar degeneration or pain. Multiple studies have demonstrated the negative long-term risks of extending a scoliosis spinal fusion into the lower lumbar spine.14–19 Reducing the number of fused levels maximizes spinal flexibility and distributes stress across more distal lumbar motion segments.20 Theoretically, this may diminish the long-term risk of disc degeneration at adjacent distal levels. Therefore, it is clear why many studies have focused on the topic of selective thoracic fusion.4–12
Some attention has been given to the concept of selective TL/L fusion. In their multicenter study, Sanders et al21 attempted to determine when the associated thoracic curve could be left unfused in a major TL/L curve. They determined that patients who had a TL/L to thoracic Cobb angle magnitude ratio of 1.25 or greater, a flexible thoracic curve (bending to 20° or less), and closed triradiate cartilage had good results and did not require further surgery. Another study retrospectively evaluated whether the Lenke classification could predict a successful selective TL/L fusion based on the criteria for a Lenke 5 curve.13 In their patients, the radiographical and clinical outcomes supported the suggestions for a selective fusion of a Lenke 5 curve. The authors, however, did report on 1 patient (of 21) who required a revision operation for a progressive thoracic curve.
The concept of minimizing fusion levels in TL/L curves is not novel in the literature. Many of the proponents for anterior fusion have suggested that an anterior approach can save fusion levels compared with a posterior approach.22–24 On average, the number of saved levels range from 1 to 1.6. The majority of these studies have focused on the caudal extent of the fusion. Other authors have suggested that the combination of wide posterior releases combined with segmental instrumentation can result in similar or improved results, without some of the problems associated with an anterior approach.25
The present study is the first to evaluate the effects of extending the fusion into the thoracic spine in a matched set of patients. Those fused into the thoracic spine had significantly better postoperative thoracic Cobb angle measurements. This allowed for greater lumbar curve correction in the long fusion group. In both groups, the resulting thoracic and lumbar curve corrections had a similar effect on the coronal alignment.
In the sagittal plane, fusion of the thoracic spine caused a decrease in thoracic kyphosis compared with a gain in kyphosis in the short fusion group. Overall, there was a significant difference in the thoracic kyphosis between the 2 groups. A great deal of attention has been given to sagittal profile after fusion of a main thoracic curve, especially with the use of modern segmental instrumentation. Multiple studies have demonstrated the influence of surgical approach and instrumentation type.26–31 The specific concerns discussed are the risk of junctional kyphosis and reduced lumbar lordosis associated with postoperative thoracic hypokyphosis.32,33 It is unclear what the relative effects of decreasing thoracic kyphosis are, when fusing into the lumbar spine in patients with long fusion.
Clinically, the most significant finding was the relative decrease in flexibility associated with the long fusion. Patients who underwent a short fusion were relatively more flexible than those who had their fusion extended into the thoracic spine. As already discussed, decreased lumbar mobility associated with a lumbar fusion may increase the risk of degenerative changes and low back pain.14–19 It is unclear if the increased immobility of a longer thoracic and lumbar fusion will also increase the risk of degeneration or low back pain compared with a patient with a short lumbar fusion. Long-term studies will be needed to address this question. In the more immediate future, the importance of flexibility in the patient's postoperative activity (sport, dance, etc.) should be considered when selecting fusion levels for a Lenke 5 curve.
One of the limitations of this study is the number of anterior and posterior approaches used to treat the patients. Our intent was to obtain a very close match of patients with similar preoperative values. As such, we did mix anterior and posterior approaches to obtain the closest match possible. Some may consider that an anterior-only approach may result in better flexibility, because there is no dissection through the posterior paraspinous musculature. We did analyze the anterior versus posterior selective fusions separately, but were underpowered to report the results (n = 6, posterior selective fusions). The data did suggest, however, that the flexibility measures were influenced most by the long fusion as opposed to the surgical approach.
It is unclear why 27% of the patients in our cohort had their thoracic spine included in the fusion. Although the patients matched similarly, specific clinical factors or patient concerns (skeletal maturity, shoulder balance, rib prominence, etc.) may have influenced the specific treatment plan. Interestingly, at short-term follow-up, whether the patient had a short or long fusion did not influence patient outcome as measured by the Scoliosis Research Society questionnaire. Overall, the relative importance of minor differences in coronal and sagittal correction as well as truncal flexibility need to be balanced with individual surgeon and patient criteria that determine fusion levels for Lenke 5 curves. Only long-term follow-up will help us to understand the risk and benefit of preserving thoracic motion segments in primary TL/L curves.
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