In the present study, we evaluated the mid-long term (average 4.5 years follow-up) clinical outcome of posterior only instrumented fusion surgery for NF-1 patients with dystrophic EOS. The major curve was corrected from 66.1º±16.2º (43º to 90.3 º ), preoperatively, to 31.1 º ±14.6 º (13.4 º to 51.2 º) post-operatively (P=0.00). However, the major curve at the last follow-up fell back to 41.0 º ±16.0 º (17 º to 70.3 º ). The T1-S1 length increased by 2.8cm ±1.0 cm after surgery and increased at a speed of 0.6cm ±0.3 cm per year. However, the incidence of the alignment complications was relatively high during the follow-up.
About 10% of children with NF-1 develop scoliosis that predominantly involves the cervical and thoracic spine.(13)Dystrophic scoliosis with NF-1 has a high risk of rapid progression.(5) The progression of the dystrophic curve can be neither stopped nor relieved by corset therapy.(14) Therefore, aggressive surgical treatments of dystrophic scoliosis in NF-1 patients was widely recommended.(3, 15)
Traditionally, the combined anterior and posterior fusion was supported by majority of people and was recognized as the most reliable method.(16-18) The clinical outcome of dystrophic scoliosis patients treated with posterior-only fusion by using hooks and rods demonstrated that the pseudarthrosis rate was high and curve progression was common.(17) Recently, some good results of posterior only fusion surgeries in NF-1 patients with dystrophic scoliosis have been reported.(9, 10, 19) In this study, we reported our results about the posterior only fusion procedure in NF-1 patients with EOS.
In addition to our study, there are currently only two articles specifically described the outcomes of fusion procedures to treat EOS in NF-1 patients. Greggi et al. (11) reported that NF-1 EOS patients either underwent posterior fusion if the thoracic kyphosis was less than 50°, or underwent anterior-posterior spinal fusion surgery if the thoracic kyphosis was 50° or more. In their study, the average correction rate was 60%, slightly higher than our results. And the follow-up results were good as well, showing no significant progress. Ryoji Tauchi and his colleagues applied the anterior-posterior fusion techniques to all EOS patients before they aged at 10 and the orthopedic effect was more noticeable. The main curve was corrected from 71.2° to 24.1° (66.2%). After a follow-up averaged on 14 years, the result showed that there was no significant progress in scoliosis. Their correction rate was superior to our study in both groups, which might be related to the better anterior release due to the anterior spinal surgery in addition to the posterior fusion procedure.
In contrast to Greggi's criteria for grouping patients with kyphotic angles, we used posterior orthopedic fusion procedures for all patients and did not separate the patients by kyphosis degrees or deformity disposal. In our patients with a kyphotic angle greater than 50°, the initial postoperative correction rate was 44.5%, the last follow-up correction rate was 31.1%, and the loss rate was 13.4%. In patients with a kyphotic angle less than 50°, the initial postoperative correction rate was 67.5%, the last follow-up correction rate was 43.8%, and the loss rate was 23.7%. Patients with a kyphotic angle more than 50° had both a lower initial correction rate and a lower last follow-up correction rate than those patients who had a kyphotic angle of 50° or less. But the two groups were similar in terms of subsequent orthopedic maintenance. Compared with the anterior and posterior fusion, simple posterior only orthopedic surgery may not be able to satisfy orthopedic maintenance requirement in NF-1 patients with EOS, while simple posterior orthopedic fusion in patients with larger kyphosis may not even meet the requirements of initial orthopedic surgery.
Five years of clinical follow-up showed that crankshaft occurred in 6 out of 10 patients and the Cobb’s angle of fusion segments increased by 10°. The incidence rate of the crankshaft was also significantly higher in children aged at 7 or younger than that of patients who were 7 to 10 years old. Using a single posterior approach orthopedic fusion surgery was at very high risk of the crankshaft, which was consistent with the previous non-surgical observation of such patients(2).
In addition, it has been found that higher density of pedicle screw placement can help to improve the postoperative correction rate of EOS in NF-1patients(10, 19). We can only perform regular fluoroscopy due to the lack of O-arm at the early stage of our study, which made it difficult to place the pedicle screws. The average ratio of fixed/surgical segments was 0.69. However, we found that there were three patients whose fixed/surgical segment ratio was above the average, reaching 64% on average. The correction rate at the last follow-up was 49%, and the rate of loss was 15% for these three patients. For the other patients, the average postoperative correction rate was 49%, the last follow-up correction rate was 35%, and the correction loss rate was 15%. It is possible that the insufficiency of placed screws on the spine led to the insufficiency of the correction force, which resulted in the poor initial orthopedic effect in our study. However, lacking of enough screws did not significantly affect the result of the surgery in terms of the prevention of scoliosis progress.
It is generally accepted that scoliosis caused by NF-1 is relatively stiff and the preoperative traction may contribute to the improvement of orthopedic effect. However, no specific study has been published to address this specific issue. For most patients with severe scoliosis, preoperative traction can improve the orthopedic effect, but it also increases the medical costs and the distress of patients. And poor compliance was found in patients with EOS due to their young age. Therefore, further evidence is needed to determine the effect of preoperative traction for EOS in NF-1patients.
The average number of fusion segments we performed was 8.1, while it was 13.1 in Tauchi’s study(20). It suggested that fewer fusion segments in our study led to the poor orthopedic maintenance. In our study, the majority of spine fusion ranges were from upper end vertebrae to lower stable vertebrae, which was adequate for patients with AIS and NF-1 patients with non-dystrophic scoliosis, but insufficient for NF-1 patients with dystrophic EOS. For EOS patients, height retention is one of the factors we must take into account, because maintaining spinal length is a critical factor in allowing adequate lung development, which is a major goal of EOS treatment. The average length of T1-S1 in our patients was 34.5 cm preoperatively, and 39.8 cm at the last follow-up. There was a 5.3 cm growth in length. The length of T1-T12 was 21.4cm preoperatively and 24.5 cm at the last follow-up, resulting in a 3.1 cm growth. In Tauchi's study, the average preoperative and last follow-up of T1-S1 were 30.7 cm and 36.2 cm respectively, resulting in a 5.5 cm growth. The preoperative and last follow-up of T1-T12 length was 18.8 cm and 21.9 cm, respectively, resulting in a growth of 3.1cm. Compared to Tauchi’s study, our surgery was not superior in preserving trunk height. This might be due to the shorter follow-up time, 4.5 years in our study and 14 years in Tauchi’s study, and the fact that patients in our study were young and still have the potential of torso growth.
In this study, there was no significant improvement of lung function brought by the treatment, probably because the patient's preoperative lung function impairment was not significantly associated with the EOS. On the other hand, the early fusion of the spine did not cause damage to the patient's lung function, which may be due to the shorter fusion segment we chose.
There was no neurological complication (transient or permanent neurological deficiency) observed. Given the facts that the complication rate of anterior-posterior approach is as high as 64%, the probability of perioperative pulmonary dysplasia is as high as 45%, and the fact that lung injury and dural tear are also seen during the precedure(19, 20), we suggest that posterior fusion surgery be a good alternative way to get a better postoperative outcome and reduce the postoperative complications.
Until now, none of the current approaches to treat EOS in NF-1 patients has reached a balance between maintaining good orthopedic outcomes and reducing complications and patient distress. Recent studies indicated that non-fusion technology is promising. Jain et al. (21) used the growth-bar technique in 14 NF-1 patients with EOS in 5 centers and performed an average follow-up of 54 months. It was shown that the correction rate of the non-fusion technology at the last follow-up was 51% and the annual spine length increase was 1.1 cm. Considering the correction rate and retaining of the space to grow, it is undoubtedly worth the wait for the longer time.
Of course, there are disadvantages of growth rod technology such as the high incidence of complications associated with internal fixation. Given that further validation of non-fusion technology to treat NF-1 patients with EOS is needed, it is undoubtable that posterior only fusion technology is a promising choice to treat the disease.