Lenke 5 curves are the third most common adolescent idiopathic scoliosis (AIS) curve type[1]. These curves are characterized by a single structural curve in lumbar/thoracolumbar region with non-structural curves (defined as bending to a Cobb angle of less than 25°) in thoracic and low lumbar (lumbosacral) regions of the spine. Common surgical treatment of Lenke 5c curves involves selective fusion (where the proximal level of fixation is distal to the thoracic apex) of the thoracolumbar curve[1] with the expectation that the non-structural thoracic curve will spontaneously correct[1-4]. There appears to be good literature support for selective fusion of thoracolumbar curves[2,4-6]. Ilgenfritz et al[4] studied 21 patients undergoing selective fusion of Lenke 5 curves and identified a spontaneous correction of the uninstrumented thoracic curves of 42% at 1 year. Thirty percent was the correction maintained at five years follow-up[4]. These authors and others felt that extension of fusion to include thoracic spine provided no significant advantages[7]. However, there appears to be a state of equipoise in the literature as to whether an anterior or posterior surgical approach is best suited for the selective fusion of Lenke 5 curves.

The anterior approach was initially popularized by Allen Dwyer et al[8] and became increasingly popular with advancements in anterior instrumentation[9-11]. The anterior thoracoabdominal approach was reported to be highly efficacious at improving clinical and radiographic measurements of trunk rotation[10,12]. One important potential advantage of anterior approach was the possibility that surgeons could obtain equal or better correction with shorter fusion constructs and consequently preserve more spinal motion[12-17].

The posterior approach, however, is more familiar to spine surgeons and the growing popularity of pedicle screws constructs for posterior spinal segmental instrumentation provided a very viable alternate to anterior approach[18]. Additionally, widespread use of osteotomies[3] has resulted in better coronal and axial correction[19-25]. Geck et al[23] compared Lenke 5 AIS correction in 31 patients with posterior pedicle screw instrumented fusion to an equal number of patients undergoing anterior instrumented fusion. The authors[23] reported significantly better curve correction, less loss of correction over time, and shorter hospital stays with the posterior approach. However, this data represented an AIS cohort that underwent an anterior instrumented fusion from a single institution in comparison to an AIS cohort that underwent a posterior pedicle screw instrumented fusion from a different institution, which makes it difficult to know if differences in blood loss, length of hospitalization, and magnitude of correction are due to differences in surgeon skill or management protocols. Bennett et al[20] reported maintenance of correction with posterior spinal fusion at five years follow-up for a heterogeneous group of Lenke 3c, 5c, and 6c curve types. However, these results cannot be generalized to Lenke 5 curves, as a systematic review by Helenius[26] suggests that the most appropriate use of the anterior approach is for Lenke 5 curves with a distal level of fixation (dLOF) at third lumbar vertebrae (L3). Evidence suggests that dLOF is significantly correlated with 2-year correction and balance after spinal fusion for Lenke 5 curves[27], however, previous studies have note matched anterior vs posterior cases by dLOF.

To effectively compare these two different approaches in regards to the magnitude of correction, the preservation of motion segments, and patient oriented outcomes for Lenke 5 AIS curves, data is required from multiple surgeons (multi-centered study) with careful regard to match cases according to curve magnitude while standardizing the dLOF at L3. Therefore, our purpose was to compare the posterior vs anterior approaches for the instrumentation and fusion of Lenke 5 AIS curve types for cases that were matched by curve magnitude, to compare cases where surgeons used a selective posterior approach (where the proximal level of fixation was distal to the thoracic apex) vs anterior cases and to compare selective posterior cases to anterior cases where the dLOF was standardized to the L3. We hypothesized that the anterior approach would result in fewer vertebrae fused and would provide better or comparable correction of radiographic curve parameters when the dLOF was standardized to L3.

A primary goal of spinal fusion for idiopathic scoliosis is to maximize correction, while preserving as many motion segments as possible[4]. Lenke 5 curves are unique in having a thoracolumbar or lumbar curve as the dominant curve in association with a flexible, non-structural thoracic curve, which is expected to spontaneously correct with a selective fusion. We have now provided evidence that for matched Lenke 5 cases and for cases where a selective fusion is performed with similar baseline curve parameters, surgeons performing a posterior approach will include more motion segments in the fusion construct when compared to those performing an anterior approach. In our analyses, including more motion segments did not improve radiographic or patient oriented outcomes[30]. These findings are particularly important in the context of current evidence highlighting significant reductions in sagittal, coronal, and transverse planes of motion following instrumented spinal fusion[31]. Furthermore, their results suggest that the more distal the fusion construct goes, the greater reductions in forward flexion post-operatively[31,32], which underscores the importance of standardizing to the dLOF. Surgeons should continue to rigorously evaluate surgical approaches in clearly defined cohorts to elucidate potential options for maximizing curve correction while maintaining spinal mobility. To our knowledge, this is the first study to compare selective instrumentation and fusion of matched Lenke 5c curves using either a posterior approach vs an anterior approach with the dLOF standardized (as recommended)[26] to the third lumbar level.

In our first analysis, we found that surgeons using a posterior approach to the Lenke 5c deformity included more levels in the instrumentation for comparable curves. This is not a surprising finding given that extension of the posterior exposure and instrumentation is technically easier since the anterior extension requires retraction of the lung, incision of the parietal pleura and control of the segmental vessels proximally or control and mobilization of iliac vessels distally. It should be acknowledged, however, that the relative benefit of complete correction of deformity vs the functional loss from fusing more segments has not been fully elucidated. The tendency to fuse more vertebrae with the posterior approach was also demonstrated in our second analysis of selective posterior vs selective anterior cases, yet there was no evidence of superior correction with the posterior approach. Interestingly, half (20 of 40 cases) of our original matched posterior cases were not selective spinal fusions, whereas only 1 of the matched anterior cases had a fusion construct that encompassed the thoracic apex. This finding further illustrates the likelihood of surgeons utilizing a posterior approach to include proximal segments that may or may not be required to improve spinal alignment of the thoracic spine. However, we did identify that surgeons that elected to use a selective posterior fusion fused 5-6 fewer levels than those that utilized a non-selective posterior approach. Finally, when the dLOF was standardized in both groups to L3, the anterior approach provides about a 15% greater correction of the lumbar curve. We also identified moderate to large effect sizes in favor of the anterior approach for outcome measures including number of fused vertebrae, lumbar Cobb percent correction, LSTOA percent correction, absolute change in Th-L apical translation, and change in disc angulation below the dLOF (L3). While the clinical importance of differences of this magnitude is not clearly documented, our results illustrate the potential to maximize post-operative spinal motion with equal or greater radiographic correction with an anterior spinal fusion.

Historically, the anterior approach was considered the preferred approach because of its ability to provide excellent coronal curve correction with significant spine derotation and shorter fusion constructs[5-9]. Tao et al[14] reported superiority of anterior solid rod-screw instrumentation with shorter fusion segments, better sagittal alignment and quality of life measures (SRS scores) than posterior pedicle screw instrumentation. However, trunk scarring, spine pseudoarthrosis, negative impacts on pulmonary function and reduction of lumbar lordosis were reported to be major disadvantages of the anterior approach[24]. More recent studies with newer techniques and implant designs have reported no significant post-operative kyphosis[12] or pulmonary function changes[33] with the anterior thoracoabdominal approach. Our results are consistent with the recent studies in that there were no differences in patient oriented outcomes as reported on the SRS questionnaires or in EBL or length of hospitalization between anterior or posterior approaches for any of our three analyses.

This study has several limitations. Matching was based on radiographic measures but surgeons may have chosen the surgical approach based on the clinical appearance, extending the fusion to include the thoracic vertebra in cases with a more pronounce right scapular prominence. The sagittal plane alignment of either the lumbar and thoracic curves can also influence the decision on the surgical approach. For instance, increased lumbar kyphosis, excessive thoracic kyphosis or thoracic hyper-lordosis, may prompt the surgeon to use a posterior approach for instrumentation to affect sagittal plane correction and this was not analyzed. An argument could also be made that the magnitude of difference between these two approaches is not meaningful to the patient, as we did not identify significant differences in our SRS outcomes or length of hospitalization between approaches. The relative benefits of complete correction of deformity vs the functional lose from fusing more segments has not been objectively studied. However, until we have more objective data on the functional implications of longer fusions or the rate of adjacent level degeneration, we cannot strongly advise an anterior TL approach for the Lenke 5C curve. Given the above considerations, our results do suggest that an anterior approach may be advantageous for severe or rigid deformity where the desired dLOF is the third lumbar level (Figure 1), as it can provide better correction for same levels of fusion with no deleterious effects on patient reported outcomes.

In conclusion, surgeons treating Lenke 5c curves will include more motion segments when employing a posterior approach; when controlled for the dLOF, the anterior approach provides greater correction of the TL curve.