The nonincarcerated hemivertebra has been recognized as a risk factor to develop a severe deformity with the growth of the spine. Early surgical treatment is widely suggested [8, 10, 15]. However, earlier hemivertebra resection with short segments fusion had the risk of less certain results in terms of correction and balance during the follow up, as well as complications due to immaturity[7, 15, 16], such as pedicle fracture and displacement of screw, even damage of the spinal cord, nerve, vessel, wound complications because of the weak muscles and fascia of the infant. Furthermore, repeated or prolonged use of general anaesthesia and sedatives in children under 3 years of age may affect the development of the child's brain . But delayed treatment at a later stage may potentially require a long fusion involving more levels which could affect the spine function . So we believe that choosing a proper surgery technique and timing for certain patients individually is very important.
Chang DG et al. Compared the Surgical outcomes by age at the time of surgery in the treatment of congenital scoliosis in children under age 10 years, they concluded that having surgery before 6 years old had significantly better deformity correction and did not cause a negative effect on the growth of vertebral body or spinal canal compared with the group treated after 6 years of age. While in their study, the type and location of hemivertebra were not unified, which will affect the progression of the deformity. In our study, all the patients were diagnosed as scoliosis due to single nonincarcerated thoracolumbar hemivertebra, we found that the preoperative coronal main curve, segmental curve and caudal compensatory curve in the group 1 were slightly larger than that in the group 2, and the cranial compensatory curve and segmental kyphosis were larger in the group 2 than in the group 1, but the difference between the two groups were not statistically. This suggested that not all the deformities caused by nonincarcerated thoracolumbar hemivertebra would progress greatly with the spinal growth, especially when the main curve was the segmental curve, such as cases of group 2 in our series.
Sanchez-Marquez JM et al. reported early thoracolumbar and lumbosacral hemivertebra resection and transpedicular short fusion in patients younger than 5 years allowed for good coronal correction immediately postoperative with a correction rate of 68%, which was difficult to maintain with a correction rate of 56% at midterm follow up. According to the results of our study, There was a better correction of main curve, segmental curve, caudal compensatory curve and segmental kyphosis in the group 2 immediate postoperatively and at last follow-up compared with that of the group 1. This implies that group 2 may have more acceptable radiologic outcomes for deformity correction in our series. This result is different with the previous study reported by Chang DG et al.  above, that was possibly because in our study the fusion segments in the group 2 were more than that in the group 1(3.60 and 2.74, respectively), in this sense, we hold that it is worth to have a little longer fusion in a delayed surgery to get a better correction.
There were no major vascular or neurological complications in our study. The main problem was the loss of correction, emerging adding-on and PJK. Chang, D G et al.  reported that the cause of revision surgery for the curve progression may include inappropriate fusion level, incomplete hemivertebra resection, or failure of fusion. In our series, 6 cases developed distal adding-on in the last follow up, 5 of them in the group 1 had bisegmental fusion, and 1 of them in the group 2 together with a new emerging cure in the cranial segment had a fusion of 5 segments. We found that among the 6 cases with adding-on, 5 of them had the hemivertebra located in thoracolumbar junction (3 with T12 HV and 2 with T12/L1 HV), we speculate that younger age less than 5 years, short segment fusion and hemivertebra located in thoracolumbar junction might be the risk factors of adding-on after hemivertebra resection in younger children of congenital scoliosis.
PJK after posterior hemivertebra resection and short fusion in young children with congenital scoliosis was well studied by Xi Chen et al., they concluded that the incidence of PJK caused by thoracolumbar hemivertebra was 10.3%, and the risk factors including preoperative segmental kyphosis, number of fusion levels, a larger postoperative SVA and ligamentous failure. The incidence of PJK in our study was 8.82%, they all have the risks of 3 to 5 fusion levels and preoperative segmental kyphosis near or more than 30 degree, which were consistent with previous reports. Concluding the risk factors above, we found that the fusion level was a contrary factor for adding-on and PJK, so we recommend that the preoperative segmental kyphosis should be taken into consideration when choosing the fusion levels. We agree with the strategies to avoid PJK given by previous reports, including protection of the soft tissue, proper shaped rods, and brace treatment at least 6 months after surgery [21, 22].
This study has several limitations. First, most of the cases in present study had not yet reached bone maturity at the last follow-up. Second, this is a retrospectively reviewed study with small number of patients included, continuous follow-up and more patients are needed in the future. The third limitation is that the patients were too young to fill out questionnaires of Health-related quality of life by themselves.