The Role of Motion Capture Analysis in Evaluating Postoperative Functional Outcomes in Adolescent Idiopathic Scoliosis: A Systematic Review

Abstract

Introduction: This systematic review evaluates the application of motion capture analysis (MCA) in assessing postoperative functional outcomes in adolescent idiopathic scoliosis (AIS) patients treated with spinal fusion. Material and Methods: A comprehensive search of PubMed, Scopus, Embase, and Cochrane Library was conducted for studies published between January 2013 and September 2024. Eligible studies included original research examining AIS patients’ post-spinal fusion, specifically assessing kinematic outcomes via MCA. Key outcomes included gait parameters, range of motion (ROM), and trunk–pelvic kinematics. Results: Nine studies comprising 216 participants (81.5% female), predominantly with Lenke 1 and 3 curve types. MCA revealed significant improvements in gait symmetry, stride length, and trunk–pelvic kinematics within one year of surgery. Enhanced mediolateral stability and normalized transverse plane motion were commonly observed. However, persistent reductions in thoracic–pelvic ROM and flexibility highlight postoperative limitations. Redistributing mechanical loads to adjacent unfused segments raises concerns about long-term compensatory mechanisms and risks for adjacent segment degeneration. Conclusions: While spinal fusion effectively restores coronal and sagittal alignment and improves functional mobility, limitations in ROM and dynamic adaptability necessitate targeted rehabilitation. Future research should standardize MCA methodologies and explore motion-preserving surgical techniques to address residual functional deficits.

1. Introduction

Adolescent idiopathic scoliosis (AIS) is a complex three-dimensional spinal deformity characterized by lateral curvature and axial rotation of the spine, with no identifiable etiology [1]. It affects approximately 1–3% of adolescents worldwide, with a higher prevalence in females, particularly during periods of rapid growth [2]. AIS can lead to cosmetic deformity, pain, psychological distress, and, in severe cases, respiratory and cardiac complications due to thoracic insufficiency [3,4]. Early detection and management are crucial to prevent curve progression and mitigate long-term adverse outcomes. Although the precise etiology of AIS remains unknown, current evidence suggests a multifactorial origin involving genetic predisposition, abnormal melatonin signaling, asymmetric growth modulation, and neuromuscular control dysfunction [5]. Genetic studies indicate that AIS may have a heritable component, with loci associated with scoliosis susceptibility identified in genome-wide association studies. Additionally, alterations in proprioception, vestibular function, and central nervous system processing have been hypothesized to contribute to the three-dimensional spinal deformity [6]. Furthermore, biomechanical theories suggest that asymmetric spinal loading and differential vertebral growth rates may influence the progression of scoliotic curves. These factors collectively contribute to the characteristic spinal rotation and lateral curvature observed in AIS patients. AIS predominantly affects females, with a female-to-male ratio of approximately 4:1 for curves requiring intervention. The severity of the condition is commonly assessed, with surgical intervention considered for major curves angles exceeding 45–50 degrees [2]. Conservative management, including bracing and physiotherapy, is generally recommended for mild-to-moderate cases, while spinal fusion remains the current standard for progressive and severe curves. Modern diagnostic modalities, such as EOS low-dose stereoradiographic imaging, allow for three-dimensional assessment of spinal deformities while minimizing radiation exposure. Functional evaluation, including MCA and gait analysis, is increasingly used to complement radiographic findings and provide dynamic insights into the biomechanical impact of AIS and its surgical correction [7].

Surgical intervention is often indicated for patients with severe curvature, typically those with major curve angles exceeding 45–50 degrees, to halt progression, correct deformity, and improve quality of life [8,9]. Advances in surgical techniques, such as segmental spinal instrumentation (SSI), primarily using pedicle screws, have significantly enhanced the ability to achieve optimal coronal and sagittal plane correction [10]. However, achieving satisfactory radiographic outcomes does not necessarily correlate with the restoration of normal functional abilities or improvements in daily activities [11]. Therefore, assessing postoperative functional outcomes has become increasingly important to fully understand the impact of surgical interventions on patients’ lives.

Motion capture analysis (MCA) is a sophisticated biomechanical tool that quantitatively evaluates human movement by tracking markers placed on anatomical landmarks [12]. These markers are recorded by high-speed infrared cameras, allowing for three-dimensional reconstruction of movement patterns. MCA provides kinematic parameters such as joint angles, range of motion, and spatiotemporal gait characteristics, while kinetic analysis (if integrated with force plates) assesses ground reaction forces and joint moments. In AIS patients, MCA can offer valuable insights into how spinal deformities and their surgical correction affect movement dynamics [13,14].

Previous studies have utilized MCA to assess gait abnormalities in AIS patients, revealing alterations such as reduced pelvic rotation, asymmetrical step lengths, and compensatory mechanisms to maintain balance [15,16]. For instance, AIS patients exhibit significant differences in lower limb kinematics compared to healthy controls, highlighting the impact of spinal deformity on gait [17,18]. Postoperatively, some studies have reported improvements in gait parameters, while others have noted persistent abnormalities despite surgical correction [17,18]. However, the application of MCA in evaluating postoperative outcomes remains underexplored, and existing studies often have small sample sizes and varying methodologies [18,19].

For several reasons, a comprehensive systematic review of the current evidence on the use of MCA in surgically treated AIS patients is warranted. First, it can synthesize available data to provide a clearer understanding of postoperative functional outcomes, which is essential for optimizing rehabilitation protocols and enhancing surgical techniques [20]. Second, identifying gaps in the literature can guide future research efforts to address unresolved questions. Finally, given the global prevalence of AIS and the widespread adoption of surgical interventions, this review can offer valuable insights for an international audience. It can facilitate the exchange of knowledge across different healthcare systems and cultural contexts, ultimately contributing to improved patient care worldwide [21,22].

This systematic review aims to evaluate the current evidence on motion capture analysis in surgically treated adolescent idiopathic scoliosis patients. By synthesizing available data, we seek to determine how MCA has been utilized to assess postoperative functional outcomes, identify patterns and discrepancies in findings, and highlight areas for future research.

2. Material and Methods

2.1. Eligibility Criteria

This systematic review adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Eligible studies included those examining adolescent idiopathic scoliosis (AIS) patients who underwent surgical correction, specifically spinal fusion, and assessed postoperative functional outcomes exclusively through motion capture analysis (MCA). Only original research, such as clinical trials, cohort studies, and case-control studies, published in English between January 2013 and September 2024, were included. Only studies with a minimum follow-up of one year postoperatively were included to ensure adequate assessment of functional recovery. While gender-specific and severity-based subgroup analyses would be valuable, inconsistencies in data reporting across studies limited our ability to conduct such comparisons. Exclusions encompassed non-original research, case reports, and studies lacking MCA-based outcome data.

2.2. Information Sources

A comprehensive search was performed across four databases: PubMed, Scopus, Embase, and the Cochrane Library. The final search was conducted in December 2024. Additional relevant records were identified through reference lists of included studies to ensure a thorough literature capture.

2.3. Search Strategy

Search terms targeted AIS, surgical interventions like spinal fusion, and motion capture analysis. Search strategies for each database incorporated a combination of Medical Subject Headings (MeSH) and keywords to ensure precision and comprehensiveness. Filters limited results to studies on human subjects published in English and within the specified date range. Complete search strings are detailed in Appendix A.

2.4. Study Selection

Two independent reviewers (both orthopedic surgeons specializing in spinal deformities) screened all studies for inclusion. Both reviewers had prior experience with motion analysis methodologies and AIS research. In cases of disagreement, a senior researcher with expertise in scoliosis biomechanics adjudicated the decision.

Two full texts were obtained for studies that met initial criteria or appeared eligible upon abstract review. A PRISMA flow diagram illustrates the study selection process and exclusion reasons at each stage (Figure 1).

Figure 1. PRISMA flow chart.

2.5. Data Collection Process

Data extraction was performed independently by two reviewers using a standardized form. Extracted information included study characteristics (authors, year, design), participant demographics, surgical intervention specifics, MCA protocols, and outcome measures. Each data entry was cross-verified by a second reviewer to ensure accuracy and completeness.

2.6. Data Items

Primary outcomes were MCA-derived kinematic parameters related to postoperative functional performance, focusing on parameters like joint angles, range of motion, and symmetry in dynamic movement. Secondary data items included participant demographics, surgical intervention characteristics, and follow-up duration. Assumptions were minimized and based solely on details explicitly reported in each study.

2.7. Risk of Bias Assessment

The quality of the included studies was evaluated using the Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2) tool, chosen for its suitability in assessing diagnostic accuracy and reliability in studies involving motion capture analysis [23]. Two reviewers independently evaluated each study across four key domains: patient selection, execution of the motion capture test, reference standards, and timing and flow of data collection. Discrepancies between reviewers were resolved through consensus. Studies were categorized by their risk of bias level, as summarized in Figure 2.

Figure 2. QUADAS-2 Risk of Bias.

2.8. Summary Measures and Data Synthesis

The primary summary measure focused on differences in kinematic parameters captured by MCA pre-and post-surgery. Outcomes were summarized as mean differences or percentages where applicable. Due to the included studies’ heterogeneity, it was impossible to perform a meta-analysis.

3. Results

3.1. Study Characteristics and Design

A total of nine studies were considered eligible for this systematic review. The included studies predominantly consisted of prospective cohort studies (n = 6) and cross-sectional comparative studies (n = 3), with an overall mean level of evidence of II. No randomized controlled trials (RCTs) were identified in the current literature. A summary of the levels of evidence for each included study is provided in Table 1. A total of 216 patients were included across nine studies, with a mean sample size of 23 ± 9 participants per study (range: 14–44). They were mostly females (81.5%). Studies primarily assessed Lenke types 1 and 3 curves, though subtypes varied. Three cross-sectional studies and six prospective studies were included. Study characteristics are resumed in 1. The analysis of motion capture data revealed consistent patterns in postoperative functional improvements and residual limitations, which are categorized below.

Table 1. Study characteristics.

Authors	Year	Level of Evidence	Sample Size	F/M	Type of Study	Group Comparison	Type of Scoliosis	Type of Scoliosis
Delpierre et al. [20]	2019	III	44	18/4	CS	Group 1: 22 AIS patients Group 2: 22 healthy controls	Lenke 1:9 Lenke 3:3 Lenke 4:2 Lenke 5:8	AIS
Holewijn et al. [22]	2017	II	18	NA	PSo	Group 1: 14 Single curve Group 2: 4 Double curve	NA	AIS
Holewijn et al. [23]	2019	II	18	NA	PSo	Group 1: 14 Single curve Group 2: Four Double curve		AIS
Nishida et al. [24]	2018	II	18	18/0	PSo	Group 1: 18	Lenke 1A or 1B	AIS
Patel et al. [25]	2018	II	28	Group 1: 10/5; Group 2: 7/6	PCS	Group 1: 15 PSF Group 2: 13 healthy subjects		AIS
Paul et al. [21]	2014	II	16	12/4	PSo	Group 1 PSF: 10	Lenke 1:10 Lenke 2, 2 Lenke 3:1 Lenke 5:3	AIS
Schmid et al. [26]	2015	III	29	Group 1: 12/2; Group 2: 7/8	CS	Group 1: 14 AIS Group 2: 15 healthy subjects		AIS
Wong-chung et al. [27]	2017	III	24	24/0	CS	Group 1: 18 Group 2: 6	Group 1: Lenke 1A, 1B, 2A Group 2: Lenke 1C, 3C, 4C	AIS
Xun et al. [28]	2019	III	21	Group A 3/3 Group B 5/2 Group C 5/3	CS	Group A: Six healthy subjects Group B: Seven Untreated AIS > 50° Group C: Eight PSF		AIS o EOS (>5 years)

AIS: Adolescent Idiopathic Scoliosis; EOS: Early Onset Scolios; CS: Cross-Sectional Study; PCS: Prospective Cohort Study; PSF: Posterior Spinal Fusion; PSo: Prospective Observational Study.

3.2. Motion Capture Timing

Preoperative MCA assessments were conducted in the majority of included studies. MCA characteristics were reported in Table 2. These evaluations revealed characteristic AIS-related gait deviations, including reduced pelvic rotation, asymmetrical step lengths, and altered trunk–pelvic coordination. Postoperatively, MCA demonstrated significant improvements in these parameters, although residual limitations in thoracic–pelvic range of motion persisted. Measurements were taken preoperatively and at follow-ups ranging from 3 months to 2 years, with most studies conducting final evaluations around 12 months. Vicon motion capture systems were the most employed technology, with marker sets ranging from 23 to 56 retro-reflective markers. Study variables and results are summarized in Table 3 and Table 4.

Table 2. Mocap characteristics.

Authors	Year	Timing of Mocap Assessments	Type of Assessment	Marker Placement	Technology
Delpierre et al. [20]	2019	2 months prior surgery 1 year after surgery	10 m walkway self-selected speed	31 retro reflective spherical markers (14 mm) “Plug-in Gait Full Body” model	VICON, Oxford Metrics, Oxford, UK, 100 Hz
Holewijn et al. [22]	2017	Pre-operatively 12 months after surgery	Barefooted on a treadmill at seven different velocities; 0.45, 0.78, 1.06, 1.35, 1.63, 1.94 and 2.22 m/s.	23 markers on the upper body and lower extremities “Plug-in-Gait model”	10 cameras sampling at 100 HzVICON, Oxford Metrics, Oxford, UK
Holewijn et al. [23]	2019	Pre-operatively 12 months after surgery	Barefooted on a treadmill at seven different velocities; 0.45, 0.78, 1.06, 1.35, 1.63, 1.94 and 2.22 m/s.	23 markers on the upper body and lower extremities “Plug-in-Gait model”	10 cameras sampling at 100 Hz VICON, Oxford Metrics, Oxford, UK
Nishida et al. [24]	2018	Pre-operatively 2 years after surgery.	First time: Trunk and pelvic positions were measured for 5 s while patients standing. Second time: Trunk and pelvic kinematics during gait on a 10 m straight path with normal walking speed	31 locations trunk, upper extremity, pelvic, and lower extremity	Oqus, Qualysis, eight cameras, 120 Hz, Gothenburg, Sweden) Two floor reaction force plates (Frequency 600 Hz, Type 4060-10, Bertec, Columbus, OH, USA)
Patel et al. [25]	2018	Pre-operatively 1 years after surgery.	Walking with a 24 ft. walkway	34 markers “Helen Hayes Marker and Plug-In-Gait Model”	Seven-camera system Vicon 512, Vicon Peak, Oxford, UK, at 100 Hz
Paul et al. [21]	2014	Pre-operatively 1 years after surgery	Walking with a 24 ft. walkway Two AMTI OR-6 force-plates (Advanced Mechanical Technology Inc., Watertown, MA, USA)	39 retro-reflective markers “Helen Hayes marker and Plug-In-Gait Model”	Six-camera system Vicon 512, Vicon Peak, Oxford, UK at 60 Hz
Schmid et al. [27]	2015	NA	10 m walkway self-selected speed	56 retro-reflective markers “Plug-in Gait full body and the IfB marker sets”	12-camera MXT20, Vicon, Oxford, UK; frequency: 200–300 Hz
Wong-chung et al. [27]	2017	Pre-operatively 1 years after surgery	Barefooted at their self-selected walking speed in a 10-m walkway	“Vicon’s Plug-in-Gait model”	10 infrared cameras 3D VICON system, Oxford, Metrics
Xun et al. [28]	2019	NA	Movements during quiet and deep breathing in a standing position	Anterior: 37 markers Lateral: 10 markers Posterior: 35 markers	10-camera 3D Motion Analysis Corp., San Diego, CA, USA

Table 3. Study variables and quantitative results.

Authors	Year	Variables Assesed	Quantitative Results
Delpierre et al. [20]	2019	Spatiotemporal gait data (Gait speed, cadence and stride length) ROM: (difference between maximal and minimal angles, noted ROM)	N.R.
Holewijn et al. [22]	2017	ROM of the upper (thorax, thoracic–pelvic, and pelvis) and lower body (hip, knee, and ankle)	Pelvis ROM (transverse) decreased by 13% at 3 months, stable at 12 months post-op. Pelvis ROM (sagittal) reduced by 11% at 3 months, 15% at 12 months. Thorax ROM (sagittal) decreased by 9% at 3 and 12 months post-op. Hip ROM in sagittal plane increased at 3 months, then normalized at 12 months (p > 0.05). Ankle transverse ROM increased at 3 months, returned to baseline at 12 months (p > 0.05).
Holewijn et al. [23]	2019	Phase difference, ROM, and deformity between the shoulders, thorax, and pelvis	Shoulder–thorax ROM decreased at 3 months, returning to baseline at 12 months (p = 0.213). Pre-operative asymmetry in shoulder–thorax motion significantly reduced post-op (p < 0.001). Shoulder–pelvis phase difference decreased at 3 months, also returning to pre-op values at 12 months (p = 0.290).
Nishida et al. [24]	2018	Trunk and pelvic kinematics during gait sagittal trunk inclination angle, coronal trunk inclination angle, trunk rotation angle, pelvic angle) Trunk and pelvic symmetry, sagittal, and transverse planes between concave and convex sides	Pre-op trunk rotation angled left during standing (4.3° ± 2.0°) and gait (8.8° ± 0.6°, p < 0.01), while pelvis angled right (4.0° ± 3.8°) in standing. Post-op, trunk and pelvis achieved symmetrical rotation during gait.
Patel et al. [25]	2018	Axial plane motion of the thorax: Maximum and minimum rotation of the thorax and pelvis Angular difference between rotation of the thorax and pelvis (pelvic–thorax rotational difference) Phase shift between thoracic and pelvic rotational movement	Significant reduction in major curve angles post-surgery (p < 0.05). Thoracic axial rotation range decreased post-op; pelvic rotation remained stable. Pre- and post-op pelvic–thorax ROM showed no significant differences (p > 0.05).
Paul et al. [21]	2014	Clinical outcomes measures, X-rays, and 3D MOCAP Sagittal and coronal center of mass (COM) and center of pressure (COP) offsets and inclination angles were calculated from positional data.	COM peak displacement decreased post-op (42.6 mm to 13.1 mm, p = 0.001); vertical COM displacement unchanged (p = 0.472). Coronal COM-COP inclination angle reduced post-op (p < 0.05); sagittal inclination unchanged
Schmid et al. [27]	2015	ROM of the spinal curvature angles over the full gait cycle; ROM values of the segmental angles over the full gait cycle and selected spatio-temporal gait parameters; Spatio-temporal gait parameters consisted of speed, cadence, left and right step length, left and right step time.	Thoracic curvature: average kyphosis reduced by 10.7°; sagittal ROM increased by 4.9°. Thoracolumbar/lumbar: more lordosis and lateral deviation noted, with minor increase in ROM (p > 0.05).
Wong-chung et al. [27]	2017	Mean kinematic data of the upper body (thorax, pelvis, and trunk) Position and motion of the thorax and pelvis were measured. Mean angle and ROM of lower body kinematic angles (hip, knee, and ankle angles)	Trunk position improved post-op in transverse (7.0° to 2.9°) and frontal planes (5.0° to −0.8°) by 3 months, maintained at 1 year (p < 0.001). Trunk transverse ROM decreased from 9.6° to 7.5° (p < 0.001).
Xun et al. [28]	2019	Thoracic expansion; abdominal wall motion Spinal flexibility in the sagittal plane	Thoracic and abdominal kinematics significantly better in operated patients than untreated severe scoliosis patients (p < 0.05). Breathing kinematics closer to normal in operated subjects (p < 0.05).

N.R.: Not reported.

Table 4. Study qualitative results and conclusions.

Authors	Year	Qualitative Results	Conclusions
Delpierre et al. [20]	2019	Asymptomatic subjects showed greater gait speed than IS patients. One year post-fusion, gait speed, cadence, and stride length were similar between groups, although sagittal pelvic mobility was reduced in fused patients compared to controls.	Spinal fusion improves pelvic and thorax–pelvis mobility in the frontal plane. Scoliosis limits thorax–pelvis mobility, leading to upper body stiffness.
Holewijn et al. [22]	2017	Spinal fusion reduced thoracic–pelvic ROM in the transverse plane, especially at higher speeds, while pelvis ROM reduction was consistent across speeds. Lower body ROM, step length, and cadence were unaffected; no compensatory mechanisms were identified for reduced upper body ROM	Future studies should examine potential compensatory mechanisms in spinal segments above and below the fusion. No compensatory mechanisms were identified post-surgery.
Holewijn et al. [23]	2019	Shoulder–thorax phase difference and walking speed were unaffected by surgery. Postoperative improvements in shoulder asymmetry were observed without specific surgical strategies targeting this issue.	No compensatory mechanisms were observed in shoulder–thorax movement post-surgery. Shoulder asymmetry in the transverse plane improved without specific surgical interventions.
Nishida et al. [24]	2018	Trunk and pelvic angles in sagittal and coronal planes remained balanced between concave and convex sides both before and after surgery, indicating maintained alignment.	Posterior correction and fusion successfully corrected asymmetry in transverse trunk and pelvis rotation. Symmetric transverse kinematics were achieved in AIS patients with thoracic curves.
Patel et al. [25]	2018	AIS patients showed abnormal axial gait patterns, which improved significantly after surgical realignment and de-rotation.	AIS patients had abnormal axial gait patterns compared to controls. Significant improvements in axial plane motion were observed post-surgical realignment and de-rotation.
Paul et al. [21]	2014	Thoracic curve angle, kyphosis, and C7 plumb-line were predictors of COM excursion changes. Reduced mediolateral COM excursion post-surgery may reflect decreased kinetic demands from improved alignment; sagittal spinopelvic parameters might impact gait performance more than previously thought.	Spinal fusion affects gait by correcting both coronal and sagittal alignment. Sagittal parameters may play a more critical role in motion performance for AIS patients than previously recognized.
Schmid et al. [27]	2015	Positive correlations between major curve angle and sagittal ROM, as well as between thoracic and lumbar curvatures, were noted, though general trunk kinematics and gait parameters showed no clinically relevant group differences.	Non-invasive optical methods are effective for assessing the dynamic functionality of the scoliotic spine during gait.
Wong-chung et al. [27]	2017	PSF did not alter lower body kinematics. Patients with double-curve patterns exhibited increased axial trunk rotation pre- and post-surgery.	Trunk asymmetry improved to near symmetry by 3 months post-fusion and was maintained at 1 year. Lower body kinematics remained stable despite reduced trunk ROM
Xun et al. [28]	2019	Kinematics were reduced in untreated scoliosis and improved post-fusion, with both closer to normal in operated subjects during quiet and deep breathing (p < 0.05).	Operated AIS patients demonstrated better thoracic and abdominal kinematics than untreated patients. Breathing patterns in operated patients were closer to normal, indicating improved respiratory function.

3.3. Spatiotemporal Gait Parameters and Symmetry

Postoperative MCA demonstrated consistent improvements in gait symmetry among AIS patients following spinal fusion. Several studies reported increased stride length and normalized step cadence within one year post-surgery [27]. Gait speed, a frequently assessed parameter, also showed a tendency to improve, with some studies indicating a return to near-normal levels compared to age-matched healthy controls [24].

In contrast, sagittal pelvic mobility remained reduced postoperatively (mean reduction: 10–15%, p < 0.05), suggesting persistent limitations in dynamic adaptability despite improved coronal and sagittal plane alignment [24]. Additionally, pelvic tilt and obliquity alterations persisted in some patients, particularly in cases of asymmetric spinal fusion constructs.

3.4. Trunk–Pelvic Coordination and Range of Motion (ROM)

AIS patients exhibited postoperative improvements in mediolateral stability, with studies reporting a significant reduction in center of mass (COM) displacement by 30–40% (p < 0.001), indicating improved balance and weight distribution [26]. However, transverse plane motion remained compromised, with a 10–15% persistent reduction in thoracic–pelvic ROM compared to controls [27,28].

Notably, trunk rotation abnormalities seen preoperatively improved post-surgery but did not fully normalize in all cases. One study highlighted that trunk transverse ROM decreased from 9.6° to 7.5° postoperatively (p < 0.001), reinforcing the notion that spinal fusion inherently limits axial rotation [29], potentially affecting sports performance and activities requiring trunk mobility [30].

3.5. Postoperative Compensatory Mechanisms and Adjacent Segment Biomechanics

One of the key concerns in post-fusion AIS patients is the potential for compensatory adaptations in adjacent, unfused spinal segments. While the studies in this review did not directly measure adjacent segment disease (ASD), motion capture data revealed no significant compensatory mechanisms in lower limb kinematics (hip, knee, ankle) in response to reduced thoracic–pelvic mobility [29].

However, an increased reliance on lumbar and pelvic movements to compensate for restricted thoracic mobility was noted in certain studies, with some patients demonstrating higher-than-expected sagittal pelvic rotation, particularly at faster walking speeds. These findings suggest that while spinal fusion improves global postural alignment, adjacent segment function and long-term adaptations warrant further investigation [30].

3.6. Respiratory Function and Thoracic Expansion

Only Xun et al. assessed thoracic kinematics during respiration, but available data indicated that thoracic expansion improved postoperatively by 25–30% compared to untreated AIS patients [31]. This suggests that spinal fusion may have a positive impact on respiratory mechanics, particularly in patients with preoperative restrictive lung patterns.

3.7. Summary of Findings and Clinical Implications

The findings of this systematic review reinforce the efficacy of spinal fusion in restoring coronal and sagittal alignment and improving gait mechanics in AIS patients. However, limitations in transverse plane mobility and thoracic–pelvic coordination persist, raising important considerations for long-term functional adaptations and rehabilitation strategies.

3.8. Risk of Bias

The risk of bias across studies was assessed using the QUADAS-2 tool, covering four key domains: Patient Selection, Index Test, Reference Standard, and Flow and Timing. Overall, studies demonstrated low-to-moderate risk, with some notable areas of concern:

• Patient Selection: Most studies had low risk and high applicability regarding patient selection, reflecting the target population of adolescents with idiopathic scoliosis undergoing spinal fusion. However, two studies presented a high risk of bias due to selection methods that may limit generalizability.
• Index Test: Motion capture protocols were generally consistent, showing low risk and good applicability. Nonetheless, a few studies showed moderate risk and uncertain applicability due to variations in data acquisition protocols, impacting comparability.
• Reference Standard: The majority adhered to valid reference standards for assessing postoperative functional outcomes, yielding low bias and high applicability. However, some studies showed unclear applicability due to variations in the criteria used for outcome classification, which could affect uniformity in results interpretation.
• Flow and Timing: This domain exhibited the most significant variability, with specific studies rated as high risk due to incomplete follow-up or inconsistent assessment timing, potentially impacting longitudinal robustness.

The majority of studies demonstrated moderate methodological quality based on the QUADAS-2 assessment, with limitations primarily related to sample size heterogeneity, standardization of MCA protocols, and variations in follow-up duration.

4. Discussion

This systematic review synthesizes current evidence on the use of MCA in evaluating functional outcomes following spinal fusion for AIS. While significant progress has been made in correcting spinal deformity and restoring biomechanical symmetry, our findings illuminate critical areas where surgical outcomes could be optimized further. MCA provides an objective, high-resolution assessment of postoperative functional recovery in AIS patients, allowing for quantification of gait symmetry, trunk–pelvic kinematics, and joint range of motion. Compared to radiographic methods such as EOS imaging [7], MCA offers a functional, real-time evaluation without ionizing radiation. However, it does not provide direct structural measurements of spinal alignment and relies on external markers that may introduce variability due to soft tissue artifacts. Other functional assessment tools, such as three-dimensional gait analysis with force platforms, provide ground reaction force data but lack the detailed joint kinematics offered by MCA [26]. Similarly, sEMG allows for the assessment of muscle activation patterns, which can complement MCA findings but does not provide a full biomechanical profile of post-surgical adaptations [32]. The integration of MCA with these complementary modalities may enhance the precision of postoperative functional assessment in AIS patients.

4.1. Functional Gait Restoration: Achievements and Limitations

MCA studies have consistently highlighted significant gait symmetry and stability improvements following spinal fusion. Delpierre et al. [27] and Nishida et al. [33] demonstrated enhancements in stride length, mediolateral stability, and pelvic and trunk kinematics within one year of surgery. These findings suggest that surgical realignment effectively addresses preoperative biomechanical asymmetries, enabling structural corrections and adaptive neuromuscular adjustments in the lower limbs.

Nevertheless, not all aspects of gait return to normal levels. Persistent deficits in sagittal plane motion and restricted thoracic–pelvic mobility have been documented by Holewijn et al. [28] and Wong-Chung et al. [27]. Such limitations may arise from the inherent rigidity of fused spinal segments, which constrain global spine mobility. These findings underline the need for further research into compensatory adaptations in unfused spinal segments and adjacent musculoskeletal regions to better understand their role in mitigating functional deficits.

4.2. Respiratory Function and Thoracic Mechanics

Beyond kinematic outcomes, thoracic cage function is crucial in overall quality of life post-surgery. As shown by Xun et al., improvements in thoracic expansion during deep and quiet breathing represent a significant advancement for AIS patients undergoing fusion [31]. These findings are clinically relevant, given the association between severe scoliosis and restrictive lung disease. However, reduced flexibility in fused segments may pose limitations in adapting to physical activities requiring dynamic thoracic mobility, such as aerobic sports or high-intensity exercise.

4.3. Postoperative ROM and Adjacent Segment Biomechanics

An essential concern following spinal fusion is the redistribution of mechanical loads to adjacent unfused segments, as reduced transverse thoracic–pelvic ROM may lead to compensatory hypermobility. This adaptation, noted in studies like Holewijn et al., raises the risk of adjacent segment disease (ASD), which can manifest as pain, instability, or deformity over time [28]. Moreover, these limitations directly impact dynamic movements such as bending, twisting, or rapid directional changes, which are crucial for functional independence and physical activities, particularly in adolescents. The altered biomechanics of the spine can also affect the pelvis and lower limb joints, potentially disrupting gait patterns and increasing the risk of overuse injuries in the hips and knees. Rehabilitation strategies should prioritize flexibility and core stabilization while addressing potential imbalances across the kinetic chain. Longitudinal assessments, combined with advancements in surgical techniques such as hybrid constructs or motion-preserving devices, may offer solutions to reduce the burden on adjacent segments while maintaining effective deformity correction [33].

These insights highlight the need for a multidisciplinary approach to mitigate long-term biomechanical challenges and optimize overall functional outcomes.

4.4. Rehabilitation as a Cornerstone of Functional Recovery

Rehabilitation strategies are pivotal in addressing these postoperative limitations. The early implementation of targeted physical therapy programs focusing on core stabilization, flexibility, and proprioceptive training can significantly enhance recovery trajectories. Notably, Wong-Chung et al. reported that early rehabilitation efforts correlated with rapid improvements in trunk and pelvic symmetry [30]. Targeted rehabilitation should focus on improving core stability, proprioception, and flexibility to compensate for the restricted range of motion following spinal fusion. Strengthening exercises targeting deep spinal stabilizers (e.g., multifidus, transverse abdominis) may enhance postural control, while proprioceptive training can improve neuromuscular coordination [34]. Additionally, controlled mobility exercises, such as dynamic stretching and aquatic therapy, may help preserve thoracic and pelvic mobility without overloading adjacent spinal segments. Future research should explore the efficacy of rehabilitation programs tailored to specific MCA findings

4.5. Quality of Life and Psychosocial Considerations

While biomechanical outcomes are critical, the broader impact of spinal fusion on psychosocial well-being warrants further exploration. Studies have shown that functional improvements do not always align with patient-reported outcomes, such as satisfaction, body image, or emotional well-being. Integrating validated patient-reported outcome measures (PROMs) into future MCA studies could provide a holistic view of surgical benefits, bridging the gap between objective biomechanical metrics and subjective patient experiences.

5. Study Limitations

The present study has several limitations. Most studies focused on short- to medium-term outcomes, with few evaluating long-term functional changes or complications, such as adjacent segment degeneration. Secondly, the heterogeneity in methodologies, including varying marker placements and motion capture protocols, limits the comparability of results across studies, making it not possible to perform a meta-analysis. Standardizing assessment protocols would enhance the reliability and generalizability of findings.

6. Conclusions

This review highlights the effectiveness of spinal fusion in improving gait symmetry and trunk kinematics in AIS patients. Despite these benefits, limitations in postoperative ROM and compensatory adaptations in adjacent segments remain as concerns. Advancements in surgical techniques are essential to optimize outcomes. Future research should focus on standardizing methodologies and integrating patient-reported outcomes to enhance care and quality of life.