In the surgical treatment of Lenke type 5 AIS, SF of the structural TL/L curve has been considered to be the leading treatment method [5, 8, 14]. Many studies have reported satisfactory radiological and clinical outcomes, and also spontaneous T curve correction with SF. However, Lenke type 5 AIS is unique, and it differentiates from other types as the T kyphosis cannot be controlled by only fusing the structural TL curve. Although Lenke classification identifies the T kyphosis as (+), N, and (-), it made no recommendation regarding kyphosis. Contrary to the recommendations of the Lenke Classification, some spine surgeons have been reported to perform NSF in 27% of patients with Lenke type 5 AIS [11]. The main reason for performing NSF has been to control the T coronal plane deformity [11]. However, it has been stated that maintaining sagittal balance is crucial for favorable radiological and clinical outcomes, and it should not be neglected in AIS [12, 15]. T5-T12 T kyphosis and T1-T4 sagittal alignment were determined as the criterion to be considered in achieving sagittal balance [12, 16]. In a study by Connolly et al., sagittal plane parameters have been stated to be more substantial in the long-term health of the spine [17]. Consistent with this statement, in another study by Takayama et al. [18], it was reported that patients with low functional scores were the ones whose sagittal balances could not have been restored. Considering that the importance of the sagittal plane in the treatment of AIS has been supported by current publications [6], another reason to extending the fusion to T spine may be to control and restore the T kyphosis [6]. Accordingly, it was planned to investigate effects of T kyphosis on the treatment choice and radiological outcomes in Lenke type 5C patients in the current study. Moreover, the importance of the correction of TL kyphosis and T hypokyphosis was also emphasized in a study by Suk et al. [16] In our study, hypokyphotic patients were not included in the study due to underpowering, thus only normokyphotic and hyperkyphotic patients were compared.
In a study by Lark et al., 58 Lenke Type 5 patients underwent SF or NSF, and a significant difference was reported in both postoperative TL/L Cobb and T Cobb angles in the matched groups. (The mean TL/L Cobb angle was 19° ± 6° in SF group and it was 13° ± 6° in NSF group, p < 0.001; mean T Cobb was 22° ± 9° in SF and it was 12° ± 6° in NSF, p < 0.001). In our study, there was no significant difference between the groups in terms of postoperative TL/L Cobb and T Cobb angles. (The mean TL/L Cobb angle was 10.24° ± 9.97° in SF and 10,93° ± 8.12° in NSF group [p = 0.718]; the mean T Cobb angle was 5.35°± 4.52° in SF group and 6,80° ± 5.34° in NSF group [p = 0.198]). In that study, TK increased in the group SF and it decreased in the group NSF postoperatively. In parallel, TK increased in the group SF (5,2%) and decreased in the group NSF (56,4%) in our study (p < 0.001). Contrary to reported postoperative hypokphosis (mean: 18° ± 6°) by Lark et al. in the group NSF, the patients in both groups in our study had normokyphosis postoperatively. The reason for this difference may be that patients who underwent NSF were hyperkyphotic ones in our study, whereas they were normokyphotic ones in that study. Another reason for that may be the longer time required for the normalization of the sagittal profile, as stated in a meta-analysis by Pasha et al. [19]. Unlike that study, the other spinopelvic parameters were also evaluated in our study. Coherent with our findings, in the preoperative evaluation of Lenke type 5 patients, the mean values of spinopelvic parameters reported by Farshad et al. [20] were as 48° ± 13°, 36° ± 9°, 12° ± 7°, 50° ± 12° for the PI, SS, PT, LL; respectively. As reported in the literature, increased sacral slop was present in our patient series as well to probably compensate for increased LL (Table 4) [20–22]. In our study, PI did not change similar to previously reported [20–22], yet SS decreased while PT increased significantly postoperatively (p = 0.434, p = 0.037, p = 0.001) in both groups. However, the observed difference between the groups was not statistically significant.
Lonner et al. reported that the frequency of complications has been increasing related to AIS surgery [19]. In another study, it was reported that post junctional kyphosis (PJK) developed in 28% of patients with AIS. In a study in which PJK was reported as 8.5% in Lenke type 5 AIS, hyperkyphosis was defined as the main risk factor. Also, Wang et al. reported that PJK was frequently seen in short-segment instrumentation [6]. In our study, it has been demonstrated that SF can be performed in patients with Lenke type 5 and T sagittal profile can be restored better with NSF in patients with TK. Contrary to those reported in the literature, the reason for developing PJK in none of the 32 patients in our series with a mean of 42,3 months follow-up might be due to the consideration of the sagittal plane analysis in the preference of surgical treatment method.
In our retrospective case series, the patients with Lenke type 5 AIS who underwent SF or NSF were compared radiologically. When the Cobb angles are evaluated in both groups, the mean T curves angles was not high, so they can be considered as non-structurally. Whereas SF can be performed for both groups according to the Lenke Classification, the surgeon included the T region into the fusion area in those who have high TK based on his own experience. While the sagittal modifier of Lenke remains N in patients undergoing SF, it changed from (+) to N in most of the patients who underwent NSF. These findings show us that the sagittal plane evaluation of the Lenke Classification System may be insufficient to guide the treatment. In this study, in the mid-to-long term follow-up, it has been shown that SF can be performed for the patients with Lenke Type 5 AIS, additionally the sagittal plane is restored better with NSF in patients with TK.
To the best of the authors' knowledge, this study is the first study to demonstrate that TK, which the Lenke Classification does not consider in the treatment recommendation, can be an important determinant in the choice of SF versus NSF in patients with Lenke 5C AIS by evaluating TL/L, T Cobb angles, TK, LL as well as PI, SS, PT. It must be noted that the findings of this study should be supported by prospective randomized controlled trials involving a larger number of patients.
This study was retrospective in nature and it contains similar deficiencies with other retrospective studies. First, the outcomes of SF and NSF treatments were evaluated only radiologically. The sample size was relatively small. The decision to perform a SF versus NSF was based on TK and there was no control group. However, this potential study bias was minimized by the preoperative similarity in terms of age, gender, follow-up duration, and preop coronal and sagittal plane parameters except for T5-T12 TK of both groups being compared. The patients in the study were homogeneous and they underwent posterior instrumentation and fusion by the same surgeon with the same pedicle screw instrumentation system. Also, a considerable length of follow-up duration a mean of 41,7 months is another strength of the study.