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 Table of Contents  
Year : 2020  |  Volume : 3  |  Issue : 2  |  Page : 173-184

Shoulder balance in adolescent idiopathic scoliosis: Current concepts and technical challenges

1 Department of Spine Surgery, Kothari Medical Centre and Park Clinic, Kolkata, West Bengal, India
2 Department of Orthopaedics, Maulana Azad Medical College and Lok Nayak Hospital, New Delhi, India

Date of Submission01-Feb-2020
Date of Decision23-Mar-2020
Date of Acceptance08-Jun-2020
Date of Web Publication13-Jul-2020

Correspondence Address:
Dr. Tarun Suri
Department of Orthopaedics, Maulana Azad Medical College and Lok Nayak Hospital, New Delhi.
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/isj.isj_8_20

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Cosmesis and self-image perception are important aspects of adolescent idiopathic scoliosis (AIS). Shoulder balance plays an important role in this aspect. In this article, we provide a broad narrative review of the clinical and radiological assessment of shoulder balance along with various factors affecting the postoperative shoulder imbalance (PSI). Systematic literature search was done using specific keywords, and studies were screened using PRISMA flowchart. Concept of medial and lateral shoulder balance, which are two distinct and independent entities, has been discussed. The trapezial angle and area are medial shoulder parameters, and are of more concern as they directly affect the cosmesis. Choice of the upper instrumented vertebra (UIV), behavior of the proximal thoracic curve, and preoperative levels of shoulder are important determinants of PSI. The relative correction of middle and distal curves also plays an important role to prevent imbalance and adding-on, especially in type 5 and 6 curves. Hooks at the UIV may provide a soft landing and help in the adjustment of any PSI.

Keywords: Adolescent idiopathic scoliosis, postoperative shoulder imbalance, shoulder balance

How to cite this article:
Basu S, Suri T. Shoulder balance in adolescent idiopathic scoliosis: Current concepts and technical challenges. Indian Spine J 2020;3:173-84

How to cite this URL:
Basu S, Suri T. Shoulder balance in adolescent idiopathic scoliosis: Current concepts and technical challenges. Indian Spine J [serial online] 2020 [cited 2021 Jan 23];3:173-84. Available from: https://www.isjonline.com/text.asp?2020/3/2/173/289661

  Introduction Top

Adolescent idiopathic scoliosis (AIS) is a three-dimensional deformity created by axial rotation and coronal curvature. Apart from arresting curve progression and adequate curve correction, cosmesis and improvement of self-image are important goals of treatment of this deformity. Factors such as shoulder level, rib and lumbar hump, and scar size, have an important role in determining patient as well as surgeon’s satisfaction.[1] Postoperative shoulder imbalance (PSI) not just has an effect on patient’s appearance and satisfaction but can also cause functional problems due to trunk imbalance.[2],[3] The success of the corrective surgery is dependent on choosing correct proximal and distal fusion levels. While choosing upper instrumented vertebra (UIV), consideration has to be given to the shoulder levels and nature of proximal thoracic (PT) and main thoracic (MT) curves. Assessment of shoulder balance is important preoperatively and should rely on both clinical and radiological parameters. This review article aims to address various parameters described to assess shoulder balance, intraoperative strategies used to correct it, and factors affecting the final outcome.

  Materials and Methods Top

A systematic online search of literature was made using PubMed and Cochrane library. Boolean search was done using the following keywords: “shoulder balance” AND “scoliosis” and “shoulder imbalance” AND “scoliosis” and “shoulder level” AND “scoliosis.” Articles written in English language, found using above keywords, were further analysed.

  Results Top

A total of 46 articles were found using the aforementioned keywords [Figure 1]. Four articles were found to be duplicated in the search and were excluded. By the study of topics only, four articles were found not relevant to the topic. Full text of the remaining 38 articles were obtained and included in the study. After this screening, 38 articles were included to write this review. Of those, 18 were retrospective studies, 14 prospective studies, five cross-sectional studies, and one meta-analysis. One reference was taken from a book chapter.
Figure 1: PRISMA diagram outlining the data retrieval and analysis

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  Discussion Top

Shoulder balance is a complex, yet important aspect in the management of AIS. Although preoperative imbalance is a cause of cosmetic and self-image concerns, a postoperative imbalance can result in poor cosmetic result, patient dissatisfaction, and in some cases require reoperation. Knowledge of clinical and radiological parameters is important to evaluate patients objectively for better outcomes. They are often poorly known and not used by many scoliosis surgeons. Further, newer concepts such as distinction between medial and lateral shoulder balance have also been described in literature. Balancing of shoulders is an intricate art, which requires consideration of many preoperative parameters and intraoperative techniques, which have been discussed in detail subsequently.

  Concept of Medial and Lateral Shoulder Balance Top

Ono et al.[4] described two types of shoulder balance—medial and lateral. They described the following three clinical parameters measured on photographs of patients taken from the front [Figure 2]:
Figure 2: Parameters for lateral and medial shoulder balance. (A) Clavicle angle represents lateral shoulder balance. (B) Trapezial angle. (C) Trapezial area represents medial shoulder balance

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  1. The clavicle angle (CA): It is measured between the line connecting the top margin of each acromial process and the horizontal line.

  2. The trapezial angle: It is measured between the horizontal line and the line connecting the intersections of sternocleidomastoid muscle and trapezius muscle profiles.

  3. The trapezial area: The area enclosed by the following borders: the line connecting the top margin of acromial processes, the perpendicular line to it through the intersection of sternocleidomastoid muscle and trapezius muscle, and the superior margin of the trapezial muscle.

The trapezial angle and the trapezial area represent the medial shoulder balance, whereas the CA represents the lateral shoulder balance. It was seen in the study that clinical CA, and thus the lateral shoulder imbalance correlated weakly with T1 tilt, first rib angle, and the upper thoracic curve size.

The medial shoulder parameters, trapezial angle, and area represent the trapezial hump, which independently corresponds to cosmetic appearance and may be of greater concern to the patient while looking at oneself in the mirror.

  Radiological Assessment of Shoulder Balance Top

Shoulder imbalance can be evaluated by careful clinical examination, but this method is subject to large variation between observers.[5] Therefore, several radiological parameters have been described for objective assessment of shoulder balance.[5],[6],[7],[8],[9],[10],[11] They are measured on standing full-length anteroposterior (AP) radiograph. For all these parameters, a positive value is assigned when left shoulder is higher, and negative value is assigned when right shoulder is higher.

  1. Radiological shoulder height (RSH)[5],[7],[8] [Figure 3]: It is defined as graded height difference of soft tissue shadows directly superior to the acromioclavicular joints. A grading was suggested by Kuklo et al.[7] based on this difference—significant imbalance (>3cm), moderate imbalance (2–3cm), minimal imbalance (1–2cm), or balanced (<1cm).

  2. Coracoid process height (CPH)[5],[6],[9] [Figure 4]: It is the difference between horizontal lines traced in the superior edge of each coracoids process.

  3. Clavicle–rib cage intersection (CRCI)[5],[6],[9],[11] (also called as clavicle–rib intersection difference, CRID) [Figure 5]:– It is the height difference between the left and right horizontal lines passing through the point of intersection of clavicle and the inner upper part of second rib.

  4. T1 tilt[5],[6],[8],[10],[11] [Figure 6]: It is the angle between the horizontal line and the line through the upper endplate of T1.

  5. First rib angle (FRA)[5],[6],[8],[10],[11] [Figure 7]: It is the tilt of a tangential line connecting both the superior borders of the first ribs.

  6. CA[5],[6],[8],[9],[11] [Figure 8]: It is the angle between the horizontal line and the tangential line connecting the highest two points of each clavicle.

  7. First rib–clavicle height (FRCH)[7],[9] [Figure 9]: It is the difference between the vertical distances of first rib apex to superior clavicle between left and right side.

  8. Trapezius length (TL)[7],[9] [Figure 10]:– It is defined as the difference of the horizontal distance of the T2 pedicle to second rib–clavicle intersection between left and right side.
Figure 3: Measurement of radiological shoulder height

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Figure 4: Measurement of coracoid process height

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Figure 5: Measurement of clavicle–rib intersection distance

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Figure 6: Measurement of T1 tilt

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Figure 7: Measurement of first rib angle

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Figure 8: Measurement of clavicle angle

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Figure 9: Measurement of first rib clavicle height

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Figure 10: Measurement of trapezius length

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  Clinical Cosmetic Assessment of Shoulder Balance Top

Clinical assessment of shoulder balance is done mostly by visual inspection. Several studies have however described objective parameters for this assessment, to bring more accuracy.[9],[11] Clinical photographs of patients back while standing on a level ground, wearing an underwear, are taken. These images are then transferred to a computer, and an image analysis software, such as Image J (National Institute of Health, Bethesda, Maryland)[11] or image Pro Plus 6.0,[9] is used for further analysis. The following parameters are analyzed[9]:

  1. Shoulder height (SH) [Figure 11]: The horizontal line through the higher axilla intersects the arms at P (right) and Q (left), the plumb line through midpoint of the neck on each PA photo intersects this horizontal line at O, and the trisection lines of OP, OQ intersect the shoulders at A’, B’ (right), and A,B (left). The difference between the heights of A and A’ is defined as the inner shoulder height (SHi), and that between B and B’ is defined as the outer shoulder height (SHo). Positive SH is defined as the left shoulder up and the right shoulder down.

  2. Shoulder area index 1 (SAI1) [Figure 12]: A line “m” is the line connecting the two inflection points (right and left) between the shoulder and neck. Another line “l1” is the horizontal line through the higher axilla. The area surrounded by m, l1, the superior margin of the shoulders, and the outer margin of the upper arms was divided by the plumb line through midpoint of the neck into area “a1” and area “a2,” the ratio a1/a2 was defined as SAI1.

  3. Shoulder area index 2 (SAI2) [Figure 13]: A horizontal line “l2” is drawn through the lower inflection point between the shoulder and the upper arm. The area surrounded by m, l2, the superior margin of the shoulders was divided by the plumb line through midpoint of the neck into area “b1” and area “b2,” the ratio b1/b2 was defined as SAI2.

  4. Shoulder angle (α1) [Figure 14]: The angle between the horizontal line and the line through two inflection points of the shoulders and upper arms was defined as α1. Positive α1 indicates the left shoulder up and the right shoulder down.

  5. Axilla angle (α2) [Figure 15]: The angle between the horizontal line and the line through both axillae was defined as α2. Positive α2 indicates the left axilla up and the right axilla down.
Figure 11: Measurement of inner (SHi) and outer (SHo) height

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Figure 12: Measurement of shoulder area index, SAI1

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Figure 13: Measurement of shoulder area index, SAI2

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Figure 14: Measurement of shoulder angle

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Figure 15: Measurement of axilla angle

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Among the radiographic indices, Bagó et al.[5] found that CRCI most reliably estimated the clinical SH. They also asserted that it is simple to locate the point where clavicle intersects the ribcage and is visible in all radiographs of the spine. Hong et al.[12] did a reliability analysis and comparison of four methods of shoulder balance—Coracoid height difference (CHD), CA, CRID, or CRCI and RSH. They found CA and CHD to be more reliable than other methods. Akel et al.[6] conducted an interesting study on shoulder balance in normal adolescent population and its correlation with radiological parameters. Of 91 adolescents, only 17 (18.7%) had absolutely level shoulders. The average height difference between shoulders was 7.5 ± 5.8mm. None of the subjects however had self-perception of the imbalance. CPH (r = 0.76), CA (r = 0.73), and CRCI (r = 0.74) showed high correlation with clinical pictures, whereas T1 tilt showed only mild correlation. In another study by Qiu et al.,[9] correlating radiographic and cosmetic indices, it was found that none of the correlation coefficient between two groups was greater than 0.8. This indicated that radiographic parameters could only partially reflect the shoulder cosmetic appearances. Thus, the evaluation of SH in the photographs is also very important. FRA and CRCI had highest correlations with SHi and SHo, respectively, in their study. In the meta-analysis published by Zhang et al.,[13] the most commonly used parameter to define shoulder imbalance was RSH.

  Incidence of Postoperative Shoulder Imbalance Top

Zhang et al.[2] published the results of his meta-analysis on PSI in AIS in 2017. The incidence of PSI in various Lenke subtypes were as follows: 20% in Lenke 1 AIS, 26% in Lenke 2 AIS, 31% in Lenke 5 AIS, and 27% in mixed AIS. The pooled incidence of PSI combined in all types of scoliosis was 25%. [Table 1] highlights important studies on PSI, with their conclusions/recommendations.
Table 1: Summary of important studies on postoperative shoulder imbalance

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  Factors Affecting Shoulder Balance Top

Choice of upper instrumented vertebra

Careful analysis of PT curve morphology and behavior is essential to achieve satisfactory postoperative shoulder balance.[14] Therefore, selection of UIV becomes one of the most important determinants of shoulder balance. Fusion to T1 or T2 controls the PT curve, but the need for this should be judiciously decided. Concerns of going such high include more blood loss, increased operative time, upper extensor muscle dissection, and denervation and scar visibility in the lower neck.[14],[15] Recommendations for fusion levels have evolved from the time of King’s classification,[16] to subsequently Lenke,[17],[18] and many newer ones after that. The primary disadvantage of Harrington’s technique used by King et al.[16] was that spinal sagittal balance was not taken into account. Cotrel-Dubousset (CD) instrumentation was introduced in 1984, but many authors quickly reported cases of imbalance in the coronal plane using King’s criteria, notably for King type II deformities.[19],[20] Subsequently, new recommendations specific for CD system were advocated by Dubousset and Cotrel[21] and Shufflebarger and Clark.[22] Finally, Lenke et al.,[18] in 1997, came with a comprehensive classification system with revised guidelines for selection of fusion levels. Recently, Bjerke et al.[14] published a literature review of UIV selection with regard to shoulder balance and found three methods for same—Lenke,[17] Ilharreborde,[23] and Trobisch.[24]

Lenke et al.[17] recommends inclusion of the PT curve for Cobb angle greater than 30° that does not correct to below 20°, ≥Grade 1 rotation by Nash and Moe method, apical translation ≥1cm, left shoulder elevation, T1 tilt toward the PT concavity, or when the PT/MT transitional vertebra lies at T6 or below. In borderline cases, push-prone radiographs can be used, and fusion to T2 is recommended if left shoulder elevation occurs. They also recommended that the most consistent criteria are the lack of side-bending flexibility. These criteria are summarized in [Table 2].
Table 2: Lenke criteria for inclusion of the PT curve in fusion

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Ilharreborde et al.[23] subsequently described a system for evaluation of Lenke 1 and 2 curves based on PT Cobb angles for both PA and bending radiographs, T1 tilt, and presurgical shoulder balance. The analysis was done on a prevalidated software. A table was devised for inclusion of the curve to T1, partial inclusion to T2 or T3, or no inclusion (T4 or below). These recommendations are summarized in [Table 3]. Major limitations of this study were lack of reproducibility of the criteria and the software used, no consideration of rib hump, or functional outcome of the patients.
Table 3: Ilharreborde recommendations for choice of UIV based on T1 tilt and shoulder balance

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A recent review from Trobisch et al.[24] described a simplified algorithm for UIV selection using curve type and presurgical shoulder balance [Table 4]. For Lenke curve types 1, 3, or 6, the recommended UIV is to T2 for left shoulder elevation, T3 for level shoulders, and T4 (or below) for left shoulder elevation. [Figure 16] shows one such example of postoperative well-balanced shoulder in a type 6 curve. Fusion to T2 is recommended for all Lenke curve types 2 or 4. Fusion to the upper end vertebra (EV) of the thoracolumbar/lumbar curve is recommended in Lenke curve type 5.[14],[24]
Table 4: Trobisch recommendations for UIV based on Lenke curve type

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Figure 16: A 16-year-old girl with Lenke 6 C N curve—fusion of both curves done leading to good postoperative shoulder balance—note the level shoulders

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Recently, Tang et al.[25] described spontaneous development of cosmetic shoulder balance in Lenke type 1 curves. They observed that despite doing short segment fusion in these curves, and thus not adhering to Lenke’s guidelines, shoulder balance improved spontaneously with time. They theorized that shoulder is adjustable when spine has been instrumented in certain position, and may improve to a more balanced status to keep gravitational stability.

Choice of correction method

Chang et al.[3] compared rod derotation (RD) and RD with direct vertebral rotation (DVR) in an attempt to improve shoulder balance in double thoracic AIS. The RD with DVR was better than the RD-only method in the correction of the PT curves postoperatively, but there was no statistically significant difference at the last follow-up between two groups. SHD was still unsatisfactory with some residual shoulder imbalance after surgery, and shoulder balance had not significantly improved even with additional correction method of DVR. They concluded that the PT curve was rigid and corrected less in both groups. Therefore, less correction of the DT curve effectively achieves better shoulder balance.

Status of preoperative shoulder levels

Scoliotic curvature is composed of two or three structured or compensatory curvatures, which all influence the general body shape. It appears that close relationship exists between curvatures, and this relation may produce the more balanced body shape as well as shoulder level. Apparently, in patients with AIS with minimal shoulder imbalance, these coronal curvatures are in a balanced state with compensation, and correction surgery can collapse this state.[26][Figure 17] shows one such case.
Figure 17: A 15-year-old girl with Lenke 1 A curve—fusion of thoracic curve done leading to postoperative shoulder imbalance—note the elevated left shoulder

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Gotfryd et al.[27] observed that spontaneous correction of the PT curve and hence restoration of shoulder balance did not always occur after fusion of MT curve alone. In a right thoracic Lenke type 1 curve, an elevated right shoulder generally gets balanced by MT curve fusion. In an elevated left shoulder, strong consideration should be given to extend fusion to T2 to control the proximal curve. In case of preoperative level shoulders, careful evaluation of proximal curve should be done. Fusion of MT curve alone may result in iatrogenic PSI and fusion should be done up to T2 or T3.

Hong et al.[26] did a global analysis of all curve types to find out the factors that affect postoperative shoulder balance. They observed that seemingly excessive corrections increase the risk of aggravated shoulder balance in less severely affected patients. The group suggested that surgeons should be careful while performing correction surgery of scoliosis with minimal initial shoulder imbalance, which can lead to unfavorable results regardless of the fusion level or type of scoliosis.

Terheyden et al.[1] found the preoperative shoulder level to be the most relevant factor contributing to the postoperative shoulder level. Earlier reports reported correlations with the postoperative SH for preoperative SH difference,[13],[28] clavicular angle,[7],[23],[26],[29],[30] clavicle height difference,[26] first rib angle,[31] and CRID.[26] The study by Terheyden et al.[1] supported the view that the postoperative SH is influenced by the preoperative shoulder level. The reliable variables CHD, CA, and CRID were investigated. Linear regression analysis showed that CRID was the best of these three predictors of shoulder level at the follow-up examination. As the correlations between preoperative T1 tilt and shoulder level at follow-up were weak, they agreed with previous reports that rejected preoperative T1 tilt as a predictive factor for the postoperative shoulder level.[23],[32]

Middle and distal curvature correction

In the study by Hong et al.,[26] effect of proportion of MT and lumbar curve correction on postoperative shoulder balance was also analyzed. They supposed that in the majority of scoliosis types, major or large structural curvature is located in middle thoracic region rather than in proximal region. Lumbar curve is usually flexible, and might have important role in maintaining a balanced body and shoulder level with compensation. Therefore, excessive correction of distal curve with fusion can lead to collapse of the body balance with different shoulder level. Patients with smaller distal curve had improvement in shoulder level in the study. Therefore, if the distal curvature is not structural, they suggested shorter distal curvature fixation to preserve the compensatory role of the distal curvature. ROC curve analysis of 89 AIS patients (all Lenke types) in this study[26] revealed that in all the patients who had aggravation/new-onset shoulder imbalance after surgery, the curve change ratio between middle and distal (middle curve change/distal curve change) showed the cut-off point at <1.83. Similar observations were also made by Cao et al.,[33] Chang et al.,[3] Koller et al.,[34] Terheyden et al.,[1] and Matsumoto et al.[30] in their studies.

Zhang et al.[2] also found in their meta-analysis that both undercorrection of PT curve and overcorrection of MT or lumbar curves were significantly associated with PSI. They recommended sufficient correction of PT curve and only moderate correction of MT and lumbar curves to achieve level shoulders.

Preoperative lumbar curve magnitude

Terheyden et al.[1] found a relationship between a larger preoperative LC Cobb angle and less shoulder elevation at the follow-up examination. This correlation was seen in all Lenke types (also in a subgroup analysis for Lenke 1 and 2 curves, which had nonstructural lumbar curves). Similar observations had been made in other studies[26],[34] hypothesizing a compensatory function of unfused lumbar curves reducing shoulder elevation that occurs due to thoracic fusion. In addition to the potentially protective effect of mild lumbar curves in relation to postoperative shoulder elevation, structural lumbar curves of course require instrumentation for purposes of trunk balancing, which can also influence shoulder balance.[35]

Pedicle screws versus hooks at UIV

Although control of PT curve is essential to control the shoulder balance, overcorrection of MT and lumbar curves can be detrimental for shoulder levels. Kuroya et al.[36] suggested that during surgery, it is challenging to adjust the corrections of these three curves (PT, MT, and lumbar curves) while considering the shoulder balance. As the meticulous assessment of shoulder balance requires X-ray in the standing position, it is not as easy as described in the literature to adjust the shoulder balance in harmony with the sufficient scoliosis corrections in the surgical field. They did a retrospective cohort study of 14 patients with AIS who underwent a posterior spinal fusion with hooks at the UIV. They found that hooks at the UIV work as an autoregulator that adjusts the shoulder balance spontaneously postoperatively.

Harm’s study group, however, had different results in their study on assessment of shoulder balance with proximal screws versus hooks in AIS.[37] Pahys et al.[37] reviewed 150 patients with AIS (75 screw group vs. 75 hooks group) with similar shoulder asymmetry preoperatively, and found that shoulder asymmetry to be similar at immediate and 2-year follow-up period.

Preoperative Risser sign

In their meta-analysis, Zhang et al.[2] found Risser sign to be an important risk factor for PSI. This was consistent with the findings of Yagi et al.[28] and Lee et al.[38] The more mature the skeleton, the more rigid the scoliotic curves might be, and the more likely the occurrence of PSI.[2] In addition, if the Risser grade is higher, the ability to compensate for a shoulder imbalance may be reduced.[38]

  PSI and Adding-on Top

“Adding-on” phenomenon caudal to the fusion is defined as an increase in the number of vertebrae within the distal curve from the first erect radiograph postoperatively to the last follow-up.[39],[40] The most common cause of adding-on is inappropriate selection of LIV. It is widely believed that the adding-on phenomenon is a compensation mechanism for PSI.[33] It can lead to an improvement in PSI, but at the cost of correction loss, coronal decompensation, and wedging and degeneration of the adjacent disc. Zhang et al.[2] verified this theory, as patients with PSI in their study were more likely to have the adding-on phenomenon. Cao et al.[33] found that the postoperative shoulder balance and postoperative distal adding-on were weakly but significantly associated with each other, and they recommended that both shoulder imbalance and adding-on needed to be prevented. On the basis of the findings, it was recommended that PSI should be prevented not only for the patients’ postoperative appearance but also for preventing the adding-on phenomenon.

  Conclusion Top

PSI is an important cosmetic issue and should be avoided. The preoperative clinical and radiological assessment is crucial to planning out the UIV and the implant type (hook vs. screw) and implant density. In general, for Lenke types 1, 2, 3, and 4, it is better to err on going up to T2 rather than stopping at T4 to avoid PSI, especially if the preoperative shoulders are in balance. An UIV of T2 is mandatory for preoperative left shoulder at a higher level. For those where UIV of T4 is planned, aggressive horizontalization of T4 should be avoided, and undercorrection of the MT curve often avoids PSI. Finally for Lenke types 5 and 6, the postoperative lumbar curve should be well balanced with the thoracic residual curve (it might be instrumented or uninstrumented)—this will give the best chance of avoiding adding-on and PSI.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

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Conflicts of interest

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  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17]

  [Table 1], [Table 2], [Table 3], [Table 4]


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