Longitudinal Changes in Spinopelvic Alignment and Sagittal Balance After Bariatric Surgery in Patients With Morbid Obesity: A Prospective Observational Study

Obesity is a complex chronic disease associated with substantial metabolic, cardiovascular, respiratory, and musculoskeletal consequences. In addition to increasing the absolute mechanical load transmitted through the axial and appendicular skeleton, severe obesity may alter body posture, trunk position, pelvic orientation, spinal curvature, and global balance. These adaptations may develop as compensatory mechanisms that allow an individual to maintain an upright posture and preserve the center of gravity within the base of support despite marked changes in body mass and body habitus.

Bariatric surgery is an established treatment option for appropriately selected patients with severe or morbid obesity and can result in substantial and progressive weight reduction. Although the metabolic and systemic effects of bariatric surgery have been extensively investigated, the longitudinal effects of major postoperative weight loss on spinal and pelvic alignment remain less clearly characterized. In particular, the extent to which changes in body mass are accompanied by changes in pelvic orientation, lumbar and thoracic alignment, spinopelvic relationships, and global sagittal balance has not been fully established.

The present prospective longitudinal observational study is designed to evaluate changes in spinopelvic alignment and sagittal balance in patients with morbid obesity undergoing bariatric surgery. Participants will be evaluated before surgery and subsequently at approximately 3, 6, and 12 months after surgery. This repeated-measures design will allow the same individuals to serve as their own longitudinal reference and will enable assessment of both the magnitude and the temporal pattern of postoperative changes.

Scientific Rationale Maintenance of an energy-efficient upright posture requires a coordinated relationship between the spine, pelvis, lower extremities, and surrounding musculature. The pelvis represents the mechanical link between the spine and lower extremities and plays a central role in sagittal balance. Variations in pelvic orientation may be accompanied by compensatory changes in lumbar lordosis, thoracic alignment, hip position, and overall trunk inclination.

In individuals with severe obesity, increased abdominal and truncal mass may shift the body's center of gravity and increase the moment arms acting on the lumbar spine and pelvis. Compensatory postural mechanisms may therefore develop to maintain balance. These mechanisms can potentially include changes in pelvic tilt, sacral orientation, lumbar curvature, thoracic curvature, and global trunk position.

However, the skeletal and postural response to obesity is unlikely to be uniform among all individuals. Some spinopelvic characteristics are predominantly anatomical and remain relatively constant after skeletal maturity, whereas other parameters reflect positional or compensatory adaptations and may change according to posture, mechanical loading, muscle function, pain, or body composition.

Major weight loss after bariatric surgery provides a unique clinical model in which a substantial change in mechanical loading occurs over a relatively short period of time. Longitudinal evaluation of patients during this period may therefore provide information about which components of spinopelvic alignment are relatively fixed and which are capable of adaptation following major changes in body mass.

Primary Objective The primary objective of this study is to determine whether spinopelvic alignment and global sagittal balance change during the first 12 months after bariatric surgery in patients with morbid obesity.

Preoperative radiographic measurements will be compared with measurements obtained at approximately 3, 6, and 12 months following surgery.

The longitudinal design is intended to identify not only whether measurable changes occur but also when these changes become apparent during postoperative weight reduction.

Secondary Objectives Secondary objectives include evaluating the relationship between the magnitude of postoperative weight loss and changes in spinopelvic radiographic parameters.

The study will investigate whether patients who experience greater reductions in body weight or body mass index demonstrate different patterns or magnitudes of spinopelvic change compared with patients who experience smaller reductions.

The study will also characterize the temporal behavior of different spinopelvic parameters. Some parameters may change predominantly during the early postoperative period, whereas others may demonstrate progressive changes over several months or remain stable throughout follow-up.

Another objective is to evaluate the interaction between regional spinal alignment and global balance. Changes occurring in one anatomical region may be accompanied by compensatory changes elsewhere in the spinopelvic chain. Accordingly, interpretation will focus on the overall relationship between the pelvis and spinal segments rather than on isolated measurements alone.

Study Design This is a prospective, longitudinal, observational study.

The study does not assign participants to bariatric surgery for research purposes and does not determine the type of bariatric procedure performed. Patients will undergo bariatric treatment according to their routine clinical indications and treatment plans.

The study will observe and quantify musculoskeletal and radiographic changes occurring during the normal clinical course following bariatric surgery.

Participants will undergo serial assessment at the following general time points:

Preoperative assessment before bariatric surgery Approximately 3 months after surgery Approximately 6 months after surgery Approximately 12 months after surgery The use of repeated measurements in the same participant will reduce the influence of interindividual anatomical variability and permit evaluation of longitudinal changes.

Study Population The study population will consist of adults with morbid obesity undergoing bariatric surgery and participating in the planned longitudinal follow-up program.

The study is specifically focused on the musculoskeletal consequences of substantial weight reduction rather than the metabolic efficacy of bariatric surgery itself.

Participants will therefore be evaluated from an orthopedic and biomechanical perspective, with emphasis on posture, spinal alignment, pelvic orientation, and sagittal balance.

Anthropometric Evaluation Body weight and related anthropometric variables obtained during the study period will be recorded at the relevant study visits.

These data will allow the investigators to characterize the magnitude and progression of postoperative weight reduction and to investigate potential relationships between changes in body mass and changes in radiographic alignment.

Where appropriate, changes in body weight and body mass index may be evaluated both as absolute changes and as longitudinal changes from the preoperative baseline.

The principal purpose of these analyses will be to determine whether the magnitude of weight reduction is associated with the magnitude of change in spinal or pelvic alignment.

Radiographic Evaluation Radiographic examinations obtained at the defined assessment periods will be systematically evaluated using standardized orthopedic and spinal alignment principles.

The analysis will focus on parameters describing:

Pelvic morphology and orientation Sacral orientation Lumbar alignment Thoracic alignment Overall spinal curvature The relationship between the lumbar spine and pelvis Global sagittal alignment Compensatory postural adaptations Overall trunk balance in relation to the pelvis The investigators will evaluate individual measurements as well as relationships between measurements.

This distinction is important because normal standing balance depends on a coordinated chain of anatomical and compensatory mechanisms. A change in a single parameter may therefore have a different clinical meaning depending on simultaneous changes in other components of the spinopelvic system.

Spinopelvic Parameters The study will assess radiographic parameters that characterize the geometric and functional relationship between the pelvis and spine.

Pelvic incidence represents an important morphological characteristic of the pelvis and provides a framework for interpretation of other sagittal parameters. In contrast, parameters such as pelvic tilt and sacral slope reflect the spatial orientation of the pelvis and may be influenced by postural compensation.

Lumbar lordosis describes the sagittal curvature of the lumbar spine. Its relationship with pelvic morphology is clinically important because lumbar alignment cannot be interpreted independently of pelvic anatomy.

The relationship between pelvic incidence and lumbar lordosis may therefore provide information about whether lumbar alignment is proportionate to the anatomical characteristics of the pelvis.

Global sagittal alignment measurements will be used to characterize the position of the trunk relative to the pelvis. These measurements provide complementary information to regional spinal parameters because an individual may maintain overall balance despite regional abnormalities through compensatory mechanisms.

Where technically measurable on available radiographs, thoracic and other regional spinal parameters will also be evaluated to characterize the complete sagittal profile.

The study is therefore designed to examine the spinopelvic system as an integrated biomechanical unit rather than treating each measurement as an independent radiographic variable.

Longitudinal Assessment A major strength of the study design is the evaluation of participants at multiple postoperative time points.

Weight loss after bariatric surgery is dynamic. The biological and mechanical environment at 3 months after surgery may differ substantially from that at 12 months. A single preoperative-versus-postoperative comparison could therefore fail to identify the timing or trajectory of musculoskeletal adaptation.

The 3-month evaluation will provide information about relatively early changes occurring during the period of rapid postoperative weight reduction.

The 6-month evaluation will allow assessment of intermediate changes as substantial weight reduction continues.

The 12-month evaluation will provide information regarding the cumulative effect of weight loss during the first postoperative year and whether earlier changes persist, progress, regress, or stabilize.

Individual trajectories will therefore be evaluated in addition to group-level changes.

Relationship Between Weight Loss and Alignment The study will specifically explore whether postoperative changes in spinopelvic alignment parallel changes in body weight.

Several possible patterns may occur.

First, some parameters may demonstrate progressive changes proportional to weight reduction, suggesting that they reflect adaptive responses to mechanical unloading.

Second, changes may occur predominantly during the early postoperative period and subsequently reach a plateau.

Third, certain anatomical parameters may remain essentially unchanged despite substantial weight loss.

Finally, considerable interindividual variability may be observed, indicating that factors other than body weight contribute to spinopelvic adaptation.

Identification of these patterns may improve understanding of the biomechanical consequences of severe obesity and its treatment.

Anatomical Versus Functional Parameters An important conceptual aspect of this investigation is the distinction between anatomical and positional parameters.

Certain pelvic parameters primarily reflect skeletal morphology and would not be expected to undergo substantial true anatomical change in skeletally mature adults.

Other measurements reflect orientation, posture, or compensation and may therefore change as loading conditions, body composition, muscular demand, or standing strategy change.

Longitudinal evaluation provides an opportunity to differentiate parameters that remain stable despite major weight loss from those that demonstrate dynamic adaptation.

This distinction may also have methodological importance for future radiographic studies involving obese populations.

Potential Mechanisms of Change The study is observational and is not designed to prove a single biological or biomechanical mechanism. Nevertheless, several mechanisms may contribute to changes observed after bariatric surgery.

Reduction in anterior truncal mass may alter the position of the body's center of gravity.

Decreased mechanical loading may reduce the compensatory requirement for pelvic or spinal positioning.

Changes in abdominal volume and body habitus may affect standing posture.

Improved mobility or physical activity after substantial weight loss may influence muscular function and postural control.

Changes in pain and functional capacity may also modify the way an individual stands during radiographic assessment.

The study will therefore interpret observed radiographic changes as the integrated consequence of changes occurring after major weight reduction rather than attributing them automatically to a single mechanism.

Clinical Importance Understanding spinopelvic adaptation following bariatric surgery may be relevant to several areas of orthopedic and spinal practice.

Obesity is common among patients presenting with low back pain, degenerative spinal disorders, adult spinal deformity, and other musculoskeletal complaints.

Spinopelvic parameters are increasingly used in the assessment of spinal balance and in planning reconstructive spinal procedures. However, interpretation of these measurements in patients with severe obesity may be complicated by obesity-related postural adaptation.

If substantial weight loss results in measurable changes in positional spinopelvic parameters, the timing of radiographic assessment relative to major weight loss could become relevant when interpreting spinal alignment.

Conversely, if most parameters remain stable despite major changes in body weight, this would suggest that observed spinopelvic characteristics are relatively resistant to mechanical unloading.

Both findings would be clinically informative.

Interpretation of Results The investigators will avoid assuming that weight loss necessarily improves spinopelvic alignment.

A reduction, increase, or absence of change in an individual parameter cannot automatically be classified as beneficial or harmful without considering the overall biomechanical context.

For this reason, the study will focus on objective longitudinal characterization rather than defining every directional change as an improvement.

The overall sagittal profile and relationships among pelvic and spinal measurements will be considered when interpreting the findings.

Analytical Framework The primary analytical approach will compare postoperative measurements with the participant's own preoperative measurements.

Repeated assessments will allow evaluation of changes over time while accounting for the correlated nature of measurements obtained from the same participant.

The magnitude of weight loss will also be examined in relation to changes in relevant spinopelvic measurements.

Depending on the distribution and characteristics of the collected data, appropriate longitudinal statistical methods will be used to evaluate time-dependent changes and associations.

The analysis will distinguish statistical significance from clinical and biomechanical relevance.

Expected Contribution This study is intended to provide a detailed longitudinal description of spinopelvic behavior during substantial weight reduction after bariatric surgery.

Rather than assessing patients only before and after weight loss at a single endpoint, serial measurements at 3, 6, and 12 months will provide information about the trajectory of adaptation.

The findings may clarify:

Which spinopelvic parameters remain stable despite major weight reduction Which parameters demonstrate measurable postoperative change Whether changes occur early or progressively Whether changes are associated with the magnitude of weight loss Whether global sagittal balance changes in parallel with regional spinal or pelvic parameters Whether compensatory relationships between the spine and pelvis are altered after substantial weight loss These findings may generate new hypotheses regarding the relationship between obesity, posture, spinal biomechanics, and musculoskeletal health.

Overall Study Perspective The principal scientific question is not simply whether bariatric surgery causes a numerical change in a particular radiographic measurement. Rather, the study seeks to understand how the integrated spine-pelvis system behaves when one of the major mechanical influences on the body-excess body mass-is substantially reduced.

By prospectively following the same patients from the preoperative period through the first postoperative year, the study aims to characterize the natural evolution of spinal and pelvic alignment during major weight loss.

The results may provide a more precise understanding of the musculoskeletal consequences of obesity and may establish a foundation for future studies examining relationships among weight reduction, spinal symptoms, functional outcomes, degenerative disease, and spinal deformity.

Inclusion criteria

Adults aged 18 years or older. Patients with morbid obesity who are scheduled to undergo bariatric surgery according to routine clinical indications.

Ability to stand independently for standardized weight-bearing radiographic assessment.

Availability of an appropriate preoperative standing whole-spine radiograph for spinopelvic measurements.

Planned postoperative clinical and radiographic follow-up at approximately 3, 6, and 12 months.

Ability and willingness to participate in the prospective follow-up protocol.

Exclusion criteria

Previous spinal fusion, spinal instrumentation, or major spinal deformity surgery that may substantially alter spinopelvic alignment.

Previous major pelvic surgery or structural pelvic disorder preventing reliable spinopelvic measurements.

Neurological or neuromuscular disorders that substantially affect standing posture or sagittal balance.

Inability to obtain technically adequate standardized standing radiographs. Pregnancy at a time when radiographic assessment would otherwise be required. Development of a major spinal, pelvic, or lower-extremity condition or surgery during follow-up that may independently alter spinopelvic alignment.

Incomplete or unavailable radiographic data required for longitudinal analysis.