The impact of upper instrumented vertebra device type on shoulder balance in Lenke type 1 scoliosis: A comparison of hooks vs. screws

Abstract

Study design

Retrospective Cohort Study.

Objectives

This study aims to evaluate how using hooks versus screws at the upper instrumented vertebra (UIV) influences postoperative shoulder alignment in individuals with Lenke Type 1 adolescent idiopathic scoliosis (AIS).

Methods

A total of 61 patients with Lenke Type 1 adolescent idiopathic scoliosis who underwent posterior spinal fusion (PSF) between 2017 and 2022 were analyzed in this retrospective cohort study. Subjects were grouped based on UIV device type: hook or screw. Shoulder imbalance was measured prior to surgery, three days after the procedure, and at the one-year postoperative follow-up using radiologic shoulder height (RSH) as the primary measure. Statistical analysis compared outcomes between the groups.

Results

Both hook and screw instrumentation effectively corrected shoulder imbalance. Nonetheless, there were no statistically significant differences between groups in correction outcomes either immediately post-surgery (P = 0.301) or at the one-year follow-up (P = 0.431). Likewise, changes in RSH from preoperative to postoperative and from postoperative to follow-up were not significantly different (P = 0.784 and P = 0.201, respectively). These findings suggest the choice of UIV device does not significantly impact shoulder balance correction in the short to mid-term.

Conclusions

Hook and screw constructs appear equally effective in correcting shoulder imbalance in Lenke Type 1 AIS. The choice of instrumentation at the UIV may have minimal influence on shoulder alignment after surgery. Further prospective research involving larger cohorts and extended follow-up durations is needed to validate these results and explore the long-term effects of UIV constructs on shoulder balance.

1. Introduction

Adolescent Idiopathic Scoliosis (AIS), affecting 2–4 % of individuals aged 10–18 years, often leads to shoulder imbalance, a condition that affects both aesthetics and function.^{1, 2, 3, 4} Correcting this imbalance is critical for achieving optimal surgical outcomes and enhancing quality of life.^{5, 6, 7} In the surgical correction of AIS, particularly Lenke type 1 curves, the choice of upper instrumented vertebra (UIV) instrumentation—hooks versus screws—has been thought to influence postoperative shoulder alignment. While hooks and screws each offer distinct biomechanical advantages, their specific effects on shoulder balance remain controversial. For example, Akazawa et al. evaluated 25 Lenke type 1 patients and reported that hooks at the UIV were associated with improved long-term radiographic shoulder height compared with pedicle screws. In contrast, a larger multicenter study of 354 AIS patients found no significant differences between hooks and screws in terms of shoulder asymmetry correction or proximal junctional kyphosis at 2-year follow-up.^{8, 9, 10} These conflicting results, compounded by limitations such as small sample size and heterogeneity of curve types, underscore the ongoing uncertainty regarding the optimal UIV anchor choice. This study aims to assess how hook-based versus screw-based instrumentation at the UIV affects shoulder alignment following surgery in patients with Lenke Type 1 AIS. By focusing on this key component of surgical planning, the goal is to inform clinical decisions and optimize patient outcomes.

2. Materials and methods

This retrospective cohort study, approved by our institution's Ethical Committee, analyzed AIS Lenke Type 1 patients who underwent posterior spinal fusion (PSF) between 2017 and 2022. Participants who had AIS Lenke Type 1 with Cobb angle between 50° and 75°, a minimum follow-up of one year, and provided informed consent were included. This range of Cobb angle was selected because more severe scoliosis curves are typically rigid and may require different surgical approaches. Additionally, the correlation between shoulder balance and scoliotic curve is often different in these severe, rigid cases. Patients were excluded if they had incomplete or missing clinical or radiological data. Whole spine standing x-ray radiograph in anterior-posterior (AP) and lateral view were taken from all cases before surgery, 3 days after surgery, and the last medical visit. Demographic and radiological information were extracted from medical records. The variables included the main thoracic (MT) Cobb angle, thoracic kyphosis (TK), cervical lordosis, lumbar lordosis, radiologic shoulder height (RSH), the level of the UIV, the level of the lower instrumented vertebra (LIV), UIV inclination, sagittal and coronal alignment. Each variable was measured in preoperative, postoperative, and one year follow-up on x-rays of patients.

Based on the classification of the UIV device employed, patients were stratified into two distinct groups: the Hook and pedicle screw (PS) (Fig. 1.). The RSH was quantified as the differential measurement of the soft tissue shadow directly superior to the acromioclavicular joint. The RSH was deemed positive when there was an elevation of the right shoulder. In addition to quantitative evaluation, RSH was also categorized qualitatively. An absolute RSH greater than 10 mm was defined as clinically noticeable shoulder imbalance, and patients were classified as having left shoulder up, right shoulder up, or balanced shoulders. The UIV inclination was defined as the angular measurement of the upper endplate of the UIV relative to the horizontal plane. In alignment with the RSH, the UIV inclination was considered positive when the right side demonstrated an elevation. Coronal balance was defined as the spatial distance between the central sacral vertical line (CSVL) and the C7 plumb line. All surgeries were conducted by spine surgeons who each have a minimum of 10 years of specialized experience in scoliosis surgery in one institution.

Fig. 1.

Preoperative (a, c) and final follow-up (b, d) standing anteroposterior radiographs of two patients with Lenke type 1 AIS, treated with hook (a, b) or screw (c, d) instrumentation at the UIV. The yellow lines indicate the radiographic shoulder height (RSH) measurement. As illustrated, most patients presented preoperatively with either right shoulder elevation or balanced shoulders, while at final follow-up the majority were either balanced or had mild left shoulder elevation.

2.1. Statistical analysis

All data were methodically recorded and analyzed using SPSS version 19 (IBM SPSS Statistics 19). Continuous data were presented as mean ± standard deviation (SD), whereas categorical data were summarized using frequencies and percentages. The Spearman correlation test was applied to assess relationships between qualitative variables. For comparing the two groups, the Student's t-test was used for parametric data, and the Mann–Whitney U test for nonparametric data. A P-value below 0.05 was considered statistically significant.

3. Results

Our cohort consisted of 61 patients, predominantly female (77 %), with a mean age of 15.16 ± 2.32 years. Hooks constituted the most frequently used Upper Instrumented Vertebra (UIV) devices, utilized in 57.4 % of surgeries, while screws were employed in 42.6 %. UIV placement was primarily between T2 and T4, with T2 being the most common site (42.6 %). Lower Instrumented Vertebra (LIV) levels were situated between T11 and L4, as summarized in Table 1.

Table 1.

Patient's demographic and surgical details.

Variable	Hook (n = 35)	Screw (n = 26)	Total (n = 61)	P.Value
Age (years)	15.46 ± 2.33	14.62 ± 2.29	15.10 ± 2.33	0.20
Sex (M/F)	11/24	3/23	14/47	0.06
UIV Level				0.17
T1	1	0	1 (1.6 %)
T2	18	8	26 (42.6 %)
T3	12	9	21 (34.4 %)
T4	4	8	12 (19.7 %)
T5	0	1	1 (1.6 %)
LIV Level				0.23
T11	1	1	2 (3.3 %)
T12	2	4	6 (9.8 %)
L1	5	9	14 (23.0 %)
L2	10	5	15 (24.6 %)
L3	14	5	19 (31.1 %)
L4	3	2	5 (8.2 %)

A significant shift in shoulder balance was noted post-operatively. Initially, a substantial proportion of participants (57.4 %) presented with a ‘Right Up’ shoulder imbalance, which markedly transitioned to a ‘Left Up’ imbalance in 78.7 % of the cases post-surgery (P = 0.003). This alteration in shoulder balance was sustained at the follow-up, with ‘Left Up’ imbalance prevalent in 54.1 % of participants. The progression from the immediate post-surgery phase to the follow-up also demonstrated a statistically significant trend towards a more balanced shoulder alignment.

There were no notable distinctions in preoperative radiologic factors between the hook and screw cohorts. When analyzing the influence of the type of UIV device on shoulder imbalance, no substantial differences were detected postoperatively (P = 0.301) or at the follow-up (P = 0.431) between hooks and screws. At final follow-up, mean RSH was −8.39 ± 11.15 mm in the hook group and −5.78 ± 9.04 mm in the screw group (mean difference −2.61 mm, 95 % CI −7.71 to 2.49; p = 0.431). Also, the total amount of RSH change from both preoperative to postoperative, and from the postoperative period to the final follow-up, there were no notable differences observed between the two groups (P = 0.784 and 0.201, respectively). When analyzed qualitatively using a 10 mm threshold, no significant differences in shoulder balance were observed between the hook and screw groups at any timepoint (preoperative p = 0.242; immediate postoperative p = 0.445; final follow-up p = 0.319). At final follow-up, most patients in both groups demonstrated balanced shoulders, while a smaller proportion showed left shoulder elevation, and no cases of right shoulder elevation were identified in the screw group. A detailed distribution is presented in Table 2. In addition to shoulder balance, the impact of the UIV device on other radiological parameters, including the MT curve, TK, cervical lordosis, lumbar lordosis, UIV inclination, LIV inclination, coronal balance, and sagittal balance were evaluated. Similar to the RSH findings, no notable distinctions were identified between the two cohorts for these factors. A more detailed comparison is presented in Table 3.

Table 2.

Distribution of shoulder balance: An absolute RSH greater than 10 mm was defined as clinically noticeable shoulder imbalance.

Timepoint	Group	Left shoulder up	Balanced	Right shoulder up	p-value
Preoperative	Hook (n = 35)	6 (17 %)	17 (49 %)	12 (34 %)	0.242
	Screw (n = 26)	1 (4 %)	13 (50 %)	12 (46 %)
Postoperative	Hook (n = 35)	23 (66 %)	10 (28 %)	2 (6 %)	0.445
	Screw (n = 26)	19 (73 %)	7 (27 %)	0
Final follow-up	Hook (n = 34)	13 (38 %)	20 (59 %)	1 (3 %)	0.319
	Screw (n = 25)	6 (24 %)	19 (76 %)	0

Table 3.

Comparison of radiological parameters between hook and screw groups at the last follow-up.

	UIV Device	N	Mean	Std. Deviation	Std. Error Mean	P-Value
Postoperative UIV Inclination	Hook	35	−1.06	4.958	0.838	0.595
	Screw	26	−0.35	5.359	1.051
Last Follow-up UIV Inclination	Hook	35	−0.42	5.116	1.003	0.986
	Screw	26	−0.40	3.841	0.768
Postoperative MT Cobb angle	Hook	35	9.11	5.373	0.908	0.055
	Screw	26	6.58	4.474	0.877
Last Follow-up MT Cobb angle	Hook	35	9.17	5.607	0.948	0.163
	Screw	26	7.31	4.297	0.843
Postoperative Sagittal Balance	Hook	35	3.69	34.767	5.877	0.184
	Screw	26	15.50	32.729	6.419
Last Follow-up Sagittal Balance	Hook	35	−17.46	35.977	6.081	0.436
	Screw	26	−24.46	29.940	6.112
Postoperative Coronal Balance	Hook	35	13.71	9.012	1.523	0.712
	Screw	26	14.73	12.434	2.439
Last Follow-up Coronal Balance	Hook	35	9.49	8.322	1.407	0.298
	Screw	26	11.88	9.230	1.846
Postoperative Cervical Lordosis	Hook	35	0.91	11.627	1.965	0.928
	Screw	26	0.60	15.384	3.077
Last Follow-up Cervical Lordosis	Hook	35	−0.30	10.876	1.893	0.752
	Screw	26	−1.29	12.499	2.551
Postoperative Kyphosis	Hook	35	29.91	10.135	1.713	0.346
	Screw	26	27.15	12.534	2.458
Last Follow-up Kyphosis	Hook	35	33.17	10.913	1.845	0.860
	Screw	26	32.60	13.982	2.796
Postoperative Lumbar Lordosis	Hook	35	49.06	12.056	2.038	0.099
	Screw	26	43.73	12.594	2.470
Last Follow-up Lumbar Lordosis	Hook	35	50.11	11.116	1.879	0.715
	Screw	26	51.21	11.474	2.342

We also assessed whether there was a correlation between the percentage of curve correction and shoulder imbalance in both the hook and screw groups. However, no meaningful association was identified between the extent of Cobb correction and shoulder balance changes, either postoperatively or at the last follow-up (Table 4.)

Table 4.

Correlation between curve correction and shoulder balance changes in hook and screw groups.

Curve Correction (%).		Pre-operative to Post-operative Shoulder Balance Changes	Pre-operative to Last Follow-up Shoulder Balance Changes
Hook	Correlation	0.172	0.175
	P.Value	0.324	0.323
	N	35	34
Screw	Correlation	−0.103	0.161
	P.Value	0.616	0.443
	N	26	25

4. Discussion

This investigation sought to evaluate the differential impacts of hook versus screw instrumentation at the UIV concerning shoulder asymmetry following surgery in individuals with Lenke Type 1 AIS. Our findings indicate that both hook and screw UIV instrumentation effectively correct shoulder imbalance, without any notable distinctions observed between the groups (P = 0.301 post-surgery, P = 0.431 at follow-up). Additionally, the total change in RSH from preoperative to postoperative and from postoperative to the last follow-up showed no significant difference (P = 0.784 and P = 0.201, respectively). These results suggest that the type of UIV device selected might have little impact on the correction of shoulder balance.

Previous studies have shown notable changes in shoulder balance following scoliosis surgery, even among patients with Lenke Type 1 AIS. While some research indicates shoulder imbalance can improve over time, our findings align with these observations, as shoulder balance improved significantly at the last follow-up in comparison to the immediate postoperative period.¹¹

Various risk factors can influence postoperative shoulder imbalance, with UIV level being one of the most frequently cited. However, its impact remains debated. Several studies, including our own, have reported no significant relationship between the UIV level and shoulder imbalance.^{12, 13, 14, 15} Notably, a study by Akazawa et al. observed no difference in 1-week postoperative RSH between hook and pedicle screw (PS) groups, but at 2 years, the Hook group showed better shoulder balance, though the difference was not significant long-term. In contrast, our study found no significant differences between the two groups in short- or mid-term follow-up evaluations.⁸ In a study by Moorthy et al., lateral shoulder asymmetry improved significantly within 6 months post-surgery in AIS patients. Up to 12 months after surgery, the hook group demonstrated superior shoulder balance, despite the screw group achieving more pronounced curve correction. This contrasts with our findings, where no significant variances in shoulder balance were identified between the two groups.¹⁶ Matsumoto et al. reported that, in a cohort of 106 Lenke Type 1A patients, the clavicle angle was notably larger in the pedicle screw group compared to the hybrid construct group.¹⁷ Similarly, Kuroya et al. demonstrated that hooks at the UIV helped maintain shoulder balance in AIS patients over extended durations.⁹

There are several limitations of this study that warrant consideration. First, the relatively modest sample size may restrict the generalizability of our findings. Although our cohort included more patients than several previous studies on this topic, the study remains underpowered to detect small differences in RSH. A larger cohort with a more diverse range of preoperative shoulder imbalance patterns would provide a clearer understanding of the effects of UIV instrumentation on shoulder balance. Furthermore, the retrospective nature of our study introduces potential biases, and therefore, a prospective study would be valuable to confirm our results and minimize such biases. Also, a longer follow-up period could provide more insight into the long-term effects of hook versus screw instrumentation on shoulder balance. Moreover, we did not evaluate how different UIV instrumentation types (hook vs. screw) affect quality of life. Future research should incorporate patient-reported outcomes to evaluate how these two UIV instrumentation techniques affect quality of life concerning shoulder balance.

5. Conclusion

In summary, our research indicates that both hook and screw devices used at the UIV effectively correct shoulder imbalance in patients with Lenke Type 1 AIS, with no notable differences observed between the two. These results suggest that the selection of UIV instrumentation may not significantly affect shoulder balance outcomes in the short to mid-term period.

Author contribution

1. Mohammadreza Chehrassanmade substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data; or the creation of new software used in the work.

2. Farshad Nikoueidrafted the work or revised it critically for important intellectual content;

3. Mohammadreza Shakeridrafted the work or revised it critically for important intellectual content;

4. Ali Valiollahpour Amirimade substantial contributions to the conception or design of the work; Data gathering.

5. Seyed Matin Sadat KiaeiData analyzing and drafted the work.

6. Hasan GhandhariAgree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Statements and declarations

Not applicable.

Compliance with ethical standards

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Ethics Code: IR.IUMS.REC.1402.921.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author.

Funding statement

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

List of abbreviations:

(AIS)

Adolescent Idiopathic Scoliosis,

(UIV)

Upper Instrumented Vertebra,

(PSF)

posterior spinal fusion,

(AP)

anterior-posterior,

(MT)

main thoracic,

(TK)

thoracic kyphosis,

(RSH)

radiologic shoulder height,

(LIV)

lower instrumented vertebra,

(PS)

pedicle screw,

(CSVL)

central sacral vertical line.