Longitudinal Changes in Lower Extremity Alignment and Mechanical Axis Parameters After Bariatric Surgery in Patients With Morbid Obesity: A Prospective Observational Study

Obesity is associated with increased mechanical loading of the lower extremities and is an important contributor to musculoskeletal morbidity. During standing and walking, forces generated by body weight are transmitted through the hip, knee, ankle, and foot. The magnitude and distribution of these forces are influenced not only by body weight but also by the geometric alignment of the lower extremity. Severe obesity may therefore influence lower-extremity biomechanics through several interacting mechanisms. Increased body mass increases joint loading, while changes in body habitus, soft-tissue distribution, stance, gait, and compensatory posture may alter the functional relationship between the hip, knee, and ankle. Bariatric surgery can produce substantial weight loss within the first postoperative year. This marked reduction in body mass represents a major change in the mechanical environment of the lower extremities. However, it remains uncertain whether weight reduction is accompanied by measurable changes in radiographic lower-extremity alignment and mechanical-axis parameters. The present prospective longitudinal observational study is designed to evaluate lower-extremity alignment before bariatric surgery and at approximately 3, 6, and 12 months following surgery in patients with morbid obesity. The primary scientific objective is to determine whether substantial postoperative weight loss is associated with changes in mechanical alignment of the lower extremities and to characterize the temporal pattern of any observed changes. Scientific Rationale The mechanical axis of the lower extremity represents the weight-bearing relationship between the hip, knee, and ankle. In the coronal plane, the position of the lower-extremity mechanical axis relative to the knee influences the distribution of loads between the medial and lateral compartments. Varus and valgus alignment are therefore important determinants of knee biomechanics. However, overall limb alignment is not determined by a single structure. It reflects the combined geometry and orientation of the femur and tibia, the relationship of the knee joint line to these bones, the position of the hip and ankle centers, and potentially posture-dependent or soft-tissue-related factors. In severe obesity, interpretation of lower-extremity alignment may be particularly important. Excess body mass markedly increases the absolute forces passing through the lower limb. At the same time, large thigh circumference and other changes in body habitus may affect stance and functional limb positioning. Consequently, a radiographic alignment measurement obtained in a patient with morbid obesity may represent a combination of fixed osseous morphology and modifiable functional or postural influences. Major weight loss creates an opportunity to investigate this distinction. Primary Objective The primary objective is to determine whether lower-extremity coronal alignment and mechanical-axis measurements change during the first year after bariatric surgery. Radiographic measurements obtained before bariatric surgery will be compared with those obtained at approximately 3, 6, and 12 months following surgery. The study will evaluate both overall limb alignment and the individual femoral and tibial components contributing to that alignment. Secondary Objectives Secondary objectives include evaluating whether the magnitude of postoperative weight reduction is associated with the magnitude of change in lower-extremity alignment. The study will also examine whether different alignment parameters demonstrate different patterns of change. For example, parameters reflecting true osseous geometry would be expected to demonstrate relatively little change in skeletally mature individuals, whereas measurements influenced by standing position, joint orientation, soft-tissue effects, or functional compensation may show greater longitudinal variation. Another objective is to determine whether changes in overall hip-knee-ankle alignment can be explained by changes in the femoral component, tibial component, joint-line relationship, or other components of the coronal mechanical axis. This approach may help distinguish apparent global alignment changes from true changes in the individual components of the lower limb. Study Design This investigation is a prospective longitudinal observational study of patients with morbid obesity undergoing bariatric surgery. Bariatric surgery is performed according to clinical indications independently of the musculoskeletal research protocol. The study does not allocate participants to a particular bariatric procedure or alter the clinical bariatric treatment strategy. The research focuses on serial orthopedic and radiographic evaluation during the postoperative weight-loss period. Participants will be assessed at the following general time points: Before bariatric surgery Approximately 3 months after surgery Approximately 6 months after surgery Approximately 12 months after surgery The repeated-measures structure allows each participant to be compared with his or her own baseline alignment. Study Population The study population consists of adults with morbid obesity undergoing bariatric surgery and longitudinal follow-up. The population is of particular interest because the magnitude of weight reduction following bariatric surgery may be considerably greater than that generally achieved through short-term lifestyle modification alone. This provides an opportunity to investigate the biomechanical response of the lower extremities to substantial mechanical unloading. Lower-Extremity Mechanical Alignment Mechanical alignment describes the relationship among the centers of the hip, knee, and ankle during weight-bearing. In conventional orthopedic analysis, the mechanical axis of the femur extends from the center of the femoral head toward the knee, whereas the mechanical axis of the tibia extends from the knee toward the center of the ankle. The relationship between these axes provides an overall description of coronal lower-extremity alignment. One of the principal global measurements used in this context is the hip-knee-ankle relationship or hip-knee-ankle angle. This measurement provides information regarding overall varus or valgus alignment. However, a global alignment measurement alone does not identify the anatomical source of the alignment pattern. The study will therefore also evaluate component measurements describing femoral and tibial orientation and joint-line relationships where these measurements can be reliably determined. Mechanical-Axis Deviation Mechanical-axis deviation describes the position of the weight-bearing line relative to the center of the knee. A medially displaced mechanical axis is generally associated with greater medial compartment loading, whereas a laterally displaced axis may increase relative loading of the lateral compartment. In individuals with severe obesity, the biomechanical consequences of even modest malalignment may be amplified because the absolute forces transmitted through the knee are increased. The study will investigate whether the position of the mechanical axis changes as body weight decreases after bariatric surgery. Importantly, any observed change will be interpreted in the context of the entire lower-extremity alignment profile rather than in isolation. Femoral and Tibial Contributions Overall coronal alignment can arise from the femur, tibia, knee joint, or a combination of these structures. Accordingly, measurements reflecting distal femoral and proximal tibial orientation will be considered in the analysis. Parameters such as the mechanical lateral distal femoral angle and medial proximal tibial angle, when available and technically appropriate, can help identify the relative contribution of femoral and tibial morphology to global limb alignment. Joint-line-related parameters may provide additional information about the relationship between the distal femur and proximal tibia. A key scientific question is whether any postoperative change in global alignment is accompanied by corresponding changes in these component measurements. If global alignment changes while measurements representing bony morphology remain stable, this could suggest an important contribution from posture, joint position, loading conditions, or other functional factors. Distinguishing Structural From Functional Alignment This distinction is central to the study. The geometry of the mature femur and tibia cannot be expected to change substantially simply because body weight decreases during a 12-month period. Therefore, the study does not assume that bariatric surgery will anatomically correct established femoral or tibial deformity. Instead, the investigators hypothesize that some aspects of measured weight-bearing alignment may be influenced by modifiable factors. These may include stance width, soft-tissue interference between the thighs, compensatory hip or knee positioning, loading-related joint orientation, and overall postural strategy. If these influences decrease as body weight and body circumference decrease, measured lower-extremity alignment could potentially change despite unchanged underlying osseous morphology. Longitudinal assessment of both global and component parameters is intended to help investigate this phenomenon. Influence of Obesity on Weight-Bearing Posture Patients with severe obesity may adopt a wider stance or altered lower-extremity posture to accommodate body habitus and maintain stability. Increased soft-tissue volume around the thighs may influence the relationship between the femora during standing. Similarly, changes in hip rotation, knee position, or foot placement could influence radiographic measurements obtained during weight-bearing. Following substantial weight reduction, these functional constraints may diminish. Consequently, repeated radiographic evaluation may reveal whether measured lower-extremity alignment remains constant or changes as the patient's body habitus evolves. This issue has practical importance because orthopedic alignment measurements are commonly interpreted as though they represent fixed skeletal characteristics. In reality, some measurements obtained during standing may incorporate both anatomical and functional components. Longitudinal Time Points The inclusion of 3-, 6-, and 12-month postoperative evaluations is intended to characterize the timing of any observed alignment changes. The 3-month assessment represents an early phase of postoperative weight reduction. The 6-month assessment reflects an intermediate period during which substantial additional weight loss may have occurred. The 12-month assessment provides information about alignment after a longer period of mechanical unloading and body-habitus change. Several different temporal patterns are possible. Alignment may change early and then stabilize. It may change progressively in parallel with continued weight loss. It may remain stable throughout follow-up. Alternatively, different parameters may exhibit different trajectories. The serial design allows these possibilities to be evaluated more effectively than a single postoperative assessment. Relationship With Weight Loss Body weight and related anthropometric measures obtained during follow-up will be used to quantify postoperative weight reduction. The study will investigate whether patients with greater weight loss exhibit greater changes in lower-extremity alignment. However, a direct linear relationship is not assumed. A patient may experience very substantial weight loss with minimal alignment change if the measured alignment is primarily determined by fixed skeletal morphology. Conversely, relatively modest additional weight loss after an initial threshold may be associated with visible changes in functional stance or mechanical-axis position. The study will therefore explore the relationship between weight reduction and alignment empirically rather than presuming a predetermined pattern. Biomechanical Importance Lower-extremity alignment has important implications for joint loading. At the knee, varus alignment tends to increase the proportion of load transmitted through the medial compartment, whereas valgus alignment may increase relative loading of the lateral compartment. The absolute magnitude of joint loading is also influenced by body weight. Therefore, obesity and malalignment may interact biomechanically rather than functioning as independent factors. Substantial postoperative weight loss will unquestionably reduce the external load associated with body mass. The additional question addressed by this study is whether the geometry of load transmission, as reflected by radiographic alignment measurements, also changes. If both body weight and functional alignment change, the biomechanical effect of weight loss on the lower extremities could be more complex than a simple proportional reduction in force. Relationship to Degenerative Joint Disease Obesity is strongly associated with knee osteoarthritis and other degenerative musculoskeletal conditions. The present study is not designed primarily as an osteoarthritis treatment trial and will not assume that changes in radiographic alignment necessarily translate into short-term structural improvement of osteoarthritis. Nevertheless, understanding alignment changes may be relevant to future investigation of degenerative joint disease. Mechanical alignment is one of several factors influencing compartment-specific loading at the knee. If weight loss affects alignment in addition to reducing overall load, this could have implications for future studies examining symptoms, osteoarthritis progression, gait, or the timing and planning of orthopedic interventions. Conversely, demonstration that skeletal alignment remains essentially unchanged would also be clinically important, because it would indicate that weight reduction decreases loading without necessarily modifying established coronal deformity. Radiographic Assessment Weight-bearing radiographic examinations obtained at the defined study intervals will be evaluated using standardized lower-extremity alignment principles. The analysis will focus on the relationship between the hip, knee, and ankle as well as femoral and tibial components of alignment. Measurements may include, as technically appropriate to the available images: Global hip-knee-ankle alignment Mechanical-axis position or deviation Distal femoral mechanical alignment Proximal tibial mechanical alignment Knee joint-line relationships Additional coronal alignment parameters relevant to interpretation of the mechanical axis The primary emphasis will be on the change in these measurements relative to each participant's preoperative baseline. Importance of Standardized Interpretation Small differences in standing position or limb rotation can influence coronal alignment measurements. For this reason, radiographic findings must be interpreted cautiously and in combination rather than relying on an isolated numerical parameter. When assessing longitudinal change, investigators will consider whether observed differences are coherent across related measurements. For example, an apparent change in the global mechanical axis without corresponding changes in component parameters may have a different interpretation from coordinated changes across multiple alignment measurements. This multi-parameter approach is intended to improve the biomechanical interpretability of the results. Interindividual Variability The study is expected to identify substantial variability among individuals. Patients with similar body mass indices may have very different underlying skeletal alignment. Likewise, patients experiencing similar amounts of weight loss may differ in their postural adaptation, muscle function, activity level, joint degeneration, or baseline lower-extremity geometry. The investigation will therefore assess both average group changes and individual longitudinal patterns. A lack of uniform response would not necessarily represent a negative result. Instead, it may indicate that lower-extremity adaptation to weight reduction depends on baseline anatomical and biomechanical characteristics. Analytical Framework The principal analysis will compare serial postoperative measurements with preoperative baseline measurements within the same participants. Appropriate longitudinal statistical methods will be used to account for repeated measurements. Associations between changes in body weight or body mass index and changes in alignment parameters will also be explored. Global alignment variables and component variables will be considered together to determine the probable source of observed changes. The study will distinguish statistical changes from changes that appear biomechanically or clinically meaningful. Possible Study Findings Several scientifically relevant outcomes are possible. First, overall lower-extremity alignment may remain essentially unchanged despite substantial weight loss. Such a finding would indicate that coronal limb alignment is predominantly determined by fixed skeletal anatomy in this population. Second, global mechanical-axis measurements may change while femoral and tibial morphological measurements remain stable. This pattern would support the hypothesis that body habitus and functional standing posture contribute to measured alignment in severe obesity. Third, changes may occur only in a subgroup of patients, potentially related to baseline alignment, degree of obesity, magnitude of weight loss, or other individual factors. Fourth, different parameters may show different time-dependent responses. Each of these findings would contribute useful information regarding the relationship between obesity and lower-extremity biomechanics. Clinical Relevance The study has potential relevance to orthopedic assessment of patients with obesity. Full-length weight-bearing lower-extremity radiographs are commonly used in the evaluation of deformity, osteoarthritis, and surgical planning. If major weight loss changes functional mechanical-axis measurements, clinicians may need to consider the patient's weight trajectory when interpreting alignment studies obtained before or during substantial weight reduction. This could be particularly relevant for patients being considered for osteotomy, arthroplasty, or other procedures in which mechanical alignment is important. The study is not designed to establish new surgical indications or directly modify orthopedic treatment algorithms. Rather, it aims to provide foundational data regarding the stability or variability of alignment measurements during major weight reduction. Broader Musculoskeletal Context The lower extremities do not function independently from the pelvis and spine. Standing balance represents an integrated kinetic chain extending from the trunk through the pelvis, hips, knees, and ankles. The present study focuses specifically on the lower-extremity component of this system. Its findings may subsequently be interpreted alongside complementary investigations of spinopelvic alignment in the same broader clinical setting. This may ultimately contribute to a more integrated understanding of how the entire weight-bearing musculoskeletal system adapts to major postoperative weight loss. Expected Contribution The study is expected to provide prospective longitudinal data regarding lower-extremity mechanical alignment during the first year following bariatric surgery. The serial design may clarify: Whether measured lower-extremity alignment changes following substantial weight loss Whether changes are apparent by 3 months or develop later Whether changes progress between 3, 6, and 12 months Whether global hip-knee-ankle alignment changes Whether mechanical-axis position changes Whether femoral and tibial component measurements remain stable Whether any observed change is associated with the magnitude of weight reduction Whether different baseline alignment patterns demonstrate different postoperative trajectories These results may help distinguish fixed skeletal alignment from potentially modifiable components of weight-bearing posture. Overall Study Perspective The fundamental question addressed by this study is whether the mechanical alignment measured in a patient with morbid obesity represents an entirely fixed skeletal characteristic or a combination of skeletal anatomy and obesity-related functional adaptation. Bariatric surgery provides a natural longitudinal model in which body mass and body habitus change substantially while mature skeletal anatomy remains largely constant. By studying the same participants before surgery and at 3, 6, and 12 months after surgery, the investigators aim to characterize how the lower-extremity weight-bearing axis behaves during this transition. The study therefore extends beyond the simple question of whether patients lose weight after bariatric surgery. It investigates whether major reduction in body mass changes how the hip, knee, and ankle are mechanically aligned during standing and whether any such changes can be distinguished from the fixed osseous geometry of the lower extremity. The findings may improve understanding of obesity-related lower-extremity biomechanics and provide a foundation for future research examining weight loss, gait, joint loading, osteoarthritis, lower-extremity symptoms, and orthopedic surgical planning.