Immediately after GR implantation, the mean RSH showed a significant decrease (P = 0.036; Fig. 1a), and the number of patients with shoulder balance increased from 10 to 14. Furthermore, the CA (P = 0.008) and T1T (P = 0.037) improved immediately after GR implantation. These results suggested that GR implantation improved shoulder balance. Studies by Uzümcügil et al.  and Atici et al. [18, 19] reported that GR had a positive effect on shoulder balance. Our findings are consistent with these previous reports. Adjustment of the shoulder balance is part of adjacent compensation, which is often triggered by deformity correction [27, 28]. In view of that, we attributed the adjustment of shoulder balance to the significant correction of the PTC and MC (Table 2) [2, 10, 29].
After multiple lengthening procedures, there was no longer a statistically significant difference in the RSH between the patients with preimplantation shoulder balance and those with shoulder imbalance (Fig. 4a), and there was no statistically significant difference in the radiographic shoulder balance parameters of all patients between before fusion and before implantation (P > 0.05; Fig. 1abc). In general, regular lengthening not only prevents the deterioration of spinal deformities and treats complications but also unleashes spine growth potential and readjusts the balance [20, 30, 31]. However, according to the aforementioned studies and the current study, regular lengthening had no significant effect on improving shoulder balance.
Final fusion was recommended for most GR graduates, but in previous studies, only a small number of patients were GR graduates who underwent final fusion [17,18,19]. In this study, all patients included were GR graduates who completed final fusion and had relatively complete follow-up data. Further analysis showed that final fusion significantly improved shoulder balance in patients with prefusion shoulder imbalance (Fig. 4b). In addition, the number of patients with severe shoulder imbalance decreased after final fusion (Fig. 2). These results might suggest that final fusion enabled rebalancing in patients with shoulder imbalance. In general, final fusion can help patients not only stop the high complication rate during multiple lengthening procedures and further correct deformities but also adjust trunk imbalance and achieve better biomechanical fixation [20, 22, 30, 31]. Furthermore, it is encouraging that final fusion achieved good results for improving prefusion shoulder imbalance according to the results.
Our study revealed a significant difference in the RSH at the last follow-up compared with immediately after fusion (P = 0.021; Fig. 1a), and the number of patients with balanced shoulders increased during the follow-up period after fusion (Fig. 2). A similar trend was found in the CA (P = 0.024; Fig. 1b). These results might suggest that GR graduates might experience ‘spontaneous correction of shoulder balance’ after final fusion. Although there is no uniform language that describes this conjecture in the literature, this phenomenon has been reported in patients with adolescent idiopathic scoliosis undergoing spinal fusion [4, 10, 23, 27]. However, although it has not been previously reported in EOS patients, it might exist in EOS patients who complete final fusion given the similarities between segmental fusion for other types of scoliosis surgery and final fusion. The selective instrumented segments in final fusion often preserve motion segments for the postoperative spine, which enables postoperative balance restoration [6, 32,33,34,35]. In addition, according to previous reports, the early postoperative shoulder balance status might be affected by postoperative pain and malposture, which might improve once postoperative pain is relieved .
Perfect postoperative neutral shoulder balance is not always achieved, but this does not mean that shoulder balance is not favourably improved by surgery. A certain degree of neutral shoulder bias can exist even in the normal population. Previous studies on shoulder balance parameters in normal people without scoliosis have shown that most people’s shoulder balance is not in the absolute neutral position, but the vast majority of people are at approximately the absolute neutral level [28, 36,37,38]. Currently, the maximum sample size that has been used in a study of a normal population (232 adolescents) has shown that the average RSH in the normal population is 0.9 cm . In our study, the patients’ mean RSH improved from 23.8 ± 16.0 mm at preimplantation to 13.2 ± 10.7 mm at the final follow-up, showing a statistically significant improvement and tending toward the neutral shoulder balance level in the general population.
In this study, shoulder imbalance early after fusion was identified as a definite risk factor for final shoulder imbalance in GR graduates. Early postoperative RSH imbalance was previously reported to be a primary predictor of final shoulder imbalance . Notably, early postoperative shoulder imbalance as an independent risk factor seems to contradict the spontaneous correction of shoulder balance after final fusion. However, this study demonstrated limited spontaneous recovery of shoulder balance after final fusion and that not every patient with early postoperative shoulder imbalance eventually showed an improvement, which is consistent with previous literature [4, 10, 27]. Other potential risk factors, including the UIV of final fusion, age at implantation, age at final fusion, and prefusion C7PL-SVA, were identified as not statistically significant. Whether the above factors affect the final shoulder balance of patients with EOS still needs to be further explored.
This study has several limitations. First, the aetiological diversity of EOS made it difficult to assess the efficacy considering the substantial variation in treatment indications and methods among different individuals. The patients included in this study were not only idiopathic but had a variety of curve types. Furthermore, there were some confounding factors leading to bias in this study, including different types of growing rods (single/dual rod(s)) and the replacement of a single rod by dual rods during the GR lengthening process. Moreover, the average follow-up period was relatively short, and more patients in this study were treated with a single rod rather than the most commonly used dual rod, which limited the significance of this study. A prospective multicentre study that focuses on a particular aetiology or type of curvature with more extended follow-up periods and a larger sample size should be developed to overcome the above limitations. Additionally, with the emergence of cosmetic shoulder balance research, it is worthwhile to study patient self-perception, self-evaluations of treatment efficacy, and satisfaction in addition to parameter accuracy and surgical outcomes.
Despite its shortcomings, this study still contributed several novel insights. This study was the first to observe and analyse the shoulder-balancing effect in a relatively large number of GR graduates who had completed final fusion, indicating that final fusion provided the opportunity to readjust the prefusion shoulder imbalance. Furthermore, the current results provide a wake-up call for spine surgeons to focus on the prefusion shoulder balance of GR graduates and to consider the importance of achieving shoulder balance in patients early after fusion. In practice, shoulder balance may not garner as much attention as spinal deformity correction or complication intervention. Extensive literature and a consensus on the surgical strategy for GR implantation and final fusion are still lacking. However, under the current conditions, the treatment results were satisfactory.