While the goals remain the same, the variety of generic braces available for treating scoliosis, each with distinct mechanical functions, complicates the situation. In truth, the scoliotic curve can be corrected through various methods, guided by numerous criteria. Consequently, the prescribing doctor and the Certified Prosthetist Orthotist (CPO) team must select from more than 20 distinct mechanisms of action, strategically combining these mechanisms to achieve the best compliance. We have compiled and categorized these mechanisms of action as described in both English and French academic literature. Physiotherapists must possess a keen understanding of these mechanisms of action, which they have the ability to either enhance or counterbalance effectively.

The ways in which action mechanisms operate can be categorized into three distinct types: passive, active, and secondary. Passive mechanisms are those that directly address and rectify the scoliotic curvature. Active mechanisms, on the other hand, engage with principles of growth, the physiological functions of ligaments, and require either active participation from the patient or involve breathing exercises. Secondary mechanisms have dual aspects; they can be advantageous, serving to benefit the child, or detrimental, such as causing a decrease in Forced Vital Capacity.

To begin with, we will explore mechanisms of passive action. These systems operate independently, requiring no external energy supply. The correction of the curve is achieved directly through the tightening of the brace. Unlike surgical procedures that rectify scoliosis at the vertebral body level, the brace exerts its corrective influence through the ribcage above the T10 vertebra and engages soft tissue below this point. Generic designs of braces are adjustable to accommodate a diverse range of curves and angles. The Milwaukee brace is particularly versatile, capable of adapting to nearly any curvature, but it is specifically indicated for angulations exceeding 40 degrees. In contrast, shorter braces are more suited to lower lumbar curves, which have an angulation of less than 40 degrees.

The initial six mechanisms that operate passively are associated with criteria that are anatomical, geometric, and biomechanical in nature. A key anatomical factor is whether the curvature is situated above or below the T10 vertebra. Scoliosis presents itself as a deformity occurring in three dimensions. Typically, X-rays are captured in the frontal and sagittal planes, which led to the natural progression of braces utilizing plane geometry. Most of these braces employed a three-point system targeted at the plane where the curvature was most pronounced. It was not until later advancements that detorsion braces were introduced, prominently featuring Dubousset’s 3D brace, which applied solid geometry principles for correction along the X, Y, and Z axes.

Regarding the segment of the curve positioned beneath T10, the transfer of soft tissue fundamentally encompasses three distinct mechanisms. The initial mechanism involves directing soft tissue towards the concave side within the frontal plane. This principle finds its application in Chêneau braces. The subsequent mechanism is characterized as a push-up approach. Instead of expanding into the concave side, a sort of wall is formed, facilitating the transfer from the base upwards, which is illustrated by the Boston and Sforzesco braces. The dynamics occur within the frontal plane and are executed through translation along the Z axis. This outcome can be further enhanced by incorporating a convex iliac plateau, a concept initially introduced in the Michel 3-point brace and subsequently adopted in both the GTB short brace and the ARTbrace.

For the section of the curve situated above T10, the mechanical influence on the spine is primarily facilitated through the ribs. The Chêneau brace operates on the principle of translation, featuring marked expansion within the concavity. Asymmetry might be restricted to bending, with contact occurring in the concavity during deep inhalation, similar to the ARTbrace. Alternatively, the brace can be designed symmetrically without any concave expansion, as exemplified by the Sforzesco brace. In this case, a convex pad enables the transfer, allowing the concave ribs to revert to their natural position upon making contact with the brace. Symmetrical braces prove to be more effective when the angulation is pronounced.

In the frontal plane, the curvature can be adjusted through bending. This adjustment involves a flexion with a varying volume, characterized by the extension of the concave side and the contraction of the convex side, which is particularly significant at the thoracic level. Translation occurs at a constant volume, facilitating activities such as push-up and lumbar lifting. In the oft-overlooked sagittal plane, corrections made in the frontal plane frequently result in a diminished curvature in the sagittal plane, similar to the effects seen with the Milwaukee brace. For a long time, corrected lordosis was the standard approach, as demonstrated by the PASB brace. The idea of physiologically restoring curves in the sagittal plane emerged with the application of coupled movements in both sagittal and frontal planes to achieve derotation. In the horizontal plane, derotation can be accomplished by applying pressure on the convex side or by exerting anterior pressure with posterior expansion on the concave side. The degree to which the sagittal plane is altered depends on the orientation of the torque applied.

Solid geometry is particularly well-suited for the application of braces, as the structure of the trunk closely resembles a hyperboloid with a central axis of symmetry and horizontal cross-sections that form hyperbolas. In contrast, scoliosis can be visualized as a circled helicoid, characterized by a horizontal generating circle. The pioneering experiment conducted by Lewis Sayre serves as an exemplary demonstration of geometrical detorsion in action. When pressure is applied along the vertical Z-axis, it induces a scoliotic deviation; however, by applying traction in the same Z-axis direction, the scoliosis can be effectively corrected. This fundamental concept underpins Lewis Sayre's method of axial traction plaster correction. Successful detorsion necessitates the use of two rigid bases located at the extremities. Consequently, detorsion braces must be anchored at the scapular and pelvic girdles. This movement along the Z-axis is referred to as geometrical detorsion. Among the exercises employed by physiotherapists, self-extension is the most commonly practiced. Mechanical detorsion is accomplished through the simultaneous movement of the orthogonal X and Y axes. This method offers the advantage of selecting a specific plane, which is typically the frontal and sagittal radiological planes. The process of detorsion arises from the interactions of movements within these sagittal and frontal planes. The greatest effect of mechanical detorsion is observed at the level of the apical vertebra, gradually diminishing as it approaches the limiting vertebrae.

When an individual stands or sits, the presence of sagittal curves is noticeable; however, these curves tend to diminish when the person lies supine, thereby facilitating an emphasis on the brace's action in the frontal plane flexion. These braces, specifically designed for nighttime use, aim for hypercorrection. In cases where there is a singular curve, the bending is considered global, whereas with dual curves, the bending may be localized. The primary function of these nocturnal hypercorrective braces is to realign the nucleus of the intervertebral disc, a task made easier by the absence of gravitational pull. Compliance with brace usage is partly determined by the level of mobility they allow. Symmetrical braces fitted with internal pads significantly limit mobility, while asymmetrical braces without opposing direction pads offer partial movement. In contrast, tissue braces provide nearly unrestricted mobility. The reduction of pressure at the vertebral body level is achieved through the composite beam design of braces, which ensures extensive contact surfaces. The restoration of sagittal plane curves, accompanied by isostatic balance, has been further enhanced through research linking sagittal curves to the lumbopelvic incidence.

The first nocturnal hypercorrective corset to make its mark was the Charleston bending brace, specifically designed for addressing single curvature issues. In contrast, for double curvatures, the Providence brace is employed, as it effectively targets two distinct areas: the thoracic and lumbar regions. These nocturnal hypercorrective braces, however, are not suitable for daytime use. This is due to the fact that the pelvic and scapular girdles are positioned at an angle to facilitate the bending process, which disrupts balance when attempting to stand upright. When there arises a necessity for wearing a brace during daylight hours, the financial burden of purchasing two separate braces—one for nighttime and another for daytime—becomes a significant concern, particularly when considering the complexities of sagittal curves and girdle balance. In response to this challenge, Rigo and Wood have made noteworthy compromises grounded in Chêneau's foundational principles. These adjustments are notably challenging to implement and replicate, as they must harmonize frontal correction with the preservation of sagittal curves while maintaining equilibrium between the scapular and pelvic girdles.

In earlier times, braces primarily consisted of leather or fabric mounted onto a metal framework. However, with the advent of petrochemical advancements, materials such as celluloid and ultimately plastic were introduced to the design. This evolution significantly enhanced the rigidity and corrective efficiency of the braces but at the expense of diminishing the wearer's mobility within the device. The development of solid geometry has mitigated this trade-off, as evidenced by innovations like the Milwaukee brace, which focuses on geometric detorsion, and the ARTbrace, which adeptly integrates both geometric and mechanical detorsion. In essence, a brace devoid of internal pads can be likened to a sled operating in reverse to counteract the scoliotic deviation. The points of contact within the brace vary depending on whether the individual is standing, sitting, or lying down. This mechanical intervention necessitates the direct correction of spinal curves while on the patient, effectively distinguishing between geometric and mechanical detorsion at both the lumbar and thoracic regions.

In the era when lumbar braces were crafted to address disc ailments, Nachemson documented a notable 30% decrease in intradiscal pressure. This phenomenon is associated with the "composite beam" theory, which emerged from France thanks to Rabishong. The brace's outer casing absorbs a portion of the axial forces, channeling them directly from the shoulder girdle down to the pelvis. The soft tissues help distribute the pressure between the spine and the shell of the brace. This mechanism enhances the function of the soft tissues and diminishes the spine's tendency to buckle, a concept akin to the support provided by a wrestler's belt. The relief in pressure, particularly at the apex of the vertebra, is most pronounced in braces that maintain symmetry. In cases of scoliosis, the nucleus of the apical vertebra often shifts towards the convex side, which hinders the scoliosis correction process. Night-time hypercorrective braces are designed to realign the nucleus with the midline, especially since pressure is minimized when lying down. Furthermore, during the night, the disc, which often compresses throughout the day, rehydrates and aids in its repositioning. Piet van Loon has observed that in some thoraco-lumbar scoliosis conditions, merely inducing thoraco-lumbar lordosis can rectify the scoliosis. The TLI brace demonstrates the critical role of the sagittal plane in correcting deformities on the frontal plane.

Active mechanisms encompass all the brace's interactions with the body's natural processes, such as growth, respiration, postural stability, and viscoplasticity. The engagement of the patient plays a crucial role in enhancing these passive mechanisms. In the Lyon Method, physiotherapy is consistently paired with the use of the brace to activate these mechanisms effectively. Additionally, the brace influences the child's behavior, affecting their interactions within the family, at school, and in social settings. The involvement of a specialized psychologist can further facilitate a positive adjustment to this new dynamic with their surroundings.

The mechanism most frequently highlighted in academic discussions involves the modulation of spinal growth in accordance with Wolff's Law and the Hueter-Volkmann Principle. Restoring balance primarily concerns the paravertebral Tensegrity, which involves realigning the receptors of the postural system to their newly adjusted positions. This, however, requires a brace that is meticulously well-balanced. An immediate correction achieved through the use of a brace can serve as a catalyst for supplemental active correction, which is enhanced by strengthening the muscles. Patients tend to engage more actively in their treatment when this active correction is coupled with reduced pressure at the area's contact zone. Breathing techniques play a pivotal role in the correction of scoliosis, particularly through the Schroth Method's focus on concavity-oriented breathing and the Lyon Method's emphasis on utilizing the expiratory reserve volume. In terms of the sagittal plane, visco-plasticity is instrumental in achieving normalization, while in the frontal plane, such visco-plasticity is initiated during the initial full-time application.

Wolff's law posits that bone development and resorption occur in response to the specific stresses it endures. This principle is further refined at the growth plate level by the Hueter-Volkmann law. Such asymmetry is also evident in the intervertebral discs and surrounding soft tissues. The entirety of this process is encapsulated in Ian Stokes' concept of a vicious cycle. However, when a brace achieves a correction exceeding 50%, it effectively interrupts and reverses this detrimental cycle. During the night, the body experiences growth spurts due to the secretion of growth hormone and melatonin, resulting in a nightly increase of approximately 8 millimeters. This physiological phenomenon underscores the efficacy of nocturnal hypercorrective braces. Ultimately, the success of treatment is determined not by the Cobb angle but by factors such as the initial deformity of the apical vertebral body, the outward protrusion of the intervertebral nucleus, and the stiffness of the paravertebral soft tissues.

The quest to discover active mechanisms that could enhance passive ones first began with geometrical detorsion in the Milwaukee brace. By incorporating a sub-axillary trim line, the brace effectively lowers the shoulders, thereby actively aiding in the process of detorsion. Another active mechanism involves providing costal support through the use of a metal or carbon blade, which works in harmony with respiratory movements. Additionally, posterior closure braces that focus on stabilizing the abdomen make use of abdominal breathing to facilitate the translation of soft tissues.

Most braces exert their primary influence during inhalation, a time when they make the most contact with the body. Those braces that maintain a constant convex contact apply a steady pressure throughout the entire breathing cycle, much like fabric braces do. Despite this, the effectiveness of such a mechanism is not always guaranteed, as evidenced by the ineffectiveness of braces like the Olympus. Traditionally, the reduction of deformities using plaster cast braces was the standard approach until technological advancements allowed for corrections to be made directly on the patient. An exemplary technique is Min Mehta's serial casting, similar in approach to Ponsetti's method for correcting clubfoot. This method achieves a plastic deformity by lengthening the concavity's ligaments. Subsequently, the correction process can be expedited by metaphorically releasing the "handbrake." Once the brace correction surpasses the 50% mark, the initial phase of "full-time" wear becomes beneficial, aiding in brace adjustment. This is akin to the principle of a wristwatch, the presence of which becomes imperceptible due to the continuous pressure it exerts on the skin.

The secondary mechanisms of action are essentially outcomes that arise from the initial two mechanisms. These can manifest in adverse forms, like the tubular thorax seen in some cases of juvenile scoliosis. However, they can also be beneficial. The brace acts as a protective shield, serving as a catalyst for epigenetic changes in the environment. It provides crucial support, helping to shape and stabilize the child during the vulnerable phase of adolescence.

We have identified and organized the six main secondary mechanisms through which braces operate: The impact braces have on respiratory function. The various mechanisms behind the action of diverse plastic materials employed in brace construction. The method by which the brace is secured and closed. The alterations in walking patterns that occur while wearing a brace. The aftereffects and implications of the prescribed brace-wearing protocol. The level of tolerance one has for wearing the brace, which is a critical element for ensuring adherence to use. A brace that lacks wearability will inevitably remain unused.

The reduction in forced vital capacity was found to be 35% for plaster cast braces. In contrast, most braces that exert a comprehensive influence on the thoracic cage result in a 20% reduction. Boston-type braces, which allow full upper thoracic respiration and enhance abdominal breathing, show a 12% limitation. With the Chêneau brace, where translation serves as the main passive mechanism, thoracic volume remains unchanged. Conversely, when bending is the principal passive mechanism, as occurs with certain other braces, the volumes are altered due to the elongation of the concavity, which aids in restoring normal breathing patterns. Anterior-opening braces use anterior valve overlap along with lumbar lordosis to better regulate abdominal respiration. Additionally, employing high-strength yet less rigid polyamide aids in preserving thoracic breathing capabilities.

Our exploration will center around the rigid plastics utilized in trunk braces. Among the most commonly employed materials are polythene and vacuum polypropylene. These materials are favored for several benefits, notably their ability to fit the brace through expansion. However, they are not without drawbacks, such as the propensity to shrink following the thermoforming process. Polymetacrylates offer the unique advantage of allowing screws to be directly fixed by tapping, and their transparency is an added benefit. They necessitate a design akin to a book, featuring hinged openings. In recent developments, polyamides like nylon have emerged, praised for their robust resistance. More importantly, they act as shock absorbers, seamlessly blending transparency and durability, with the added capability of utilizing 3mm thickness and providing greater tolerance, particularly advantageous for adult use.

The design of the Milwaukee and Boston braces, featuring a posterior closure, enhances the movement of soft tissues, allows for precise control over pelvic orientation, and encourages effective abdominal breathing. In numerous cases of scoliosis, mechanical correction is typically executed from the lower sections upward. While an anterior closure offers easier access for the patient, it poses challenges, particularly for young women, due to the chest area complicating the stabilization of the mid-thoracic region. , the presence of which becomes imperceptible due to the continuous pressure it exerts on the skin.

The straps can rapidly diminish in their efficiency. To circumvent these issues, the ARTbrace employs an anterior overlap of the valves combined with rack-and-pinion closure mechanisms. The versatile Lyon brace is designed with screws that are strategically placed by medical professionals, tailored to the patient's growth patterns. This design simplifies the process of maintaining the corrective alignment of the brace. The concept of time, being the fourth dimension of this brace, plays a crucial role. It is imperative that, in addition to the initial X-ray taken while the brace is worn, the brace undergoes continual monitoring and periodic adjustments.

The enduring adaptation of braces to accommodate growth enhances their overall tolerance. Technological breakthroughs in CAD/CAM molding are significantly elevating the accuracy of brace design and their efficacy. However, the most crucial aspect remains the correction performed directly on the patient, reminiscent of the traditional plaster cast braces, which considers the stiffness of the paravertebral structures. This process involves the geometric detorsion of the initial form, followed by the adjustment of this form at both the lumbar and thoracic levels to achieve mechanical detorsion. Consequently, this approach ensures optimal correction within the brace, while the improved precision simultaneously boosts tolerance.

The evolution of braces towards a more compact and lightweight design commenced with the elimination of the Milwaukee superstructure from the Boston brace. By achieving complete stabilization of the girdles, detorsion is facilitated. In contrast, the Chêneau brace offers partial stabilization, permitting translation and, to a certain degree, bending within the confines of plane geometry, making it apt for cases where scoliosis develops along a single plane. However, when scoliosis progresses across multiple planes, the correction process becomes significantly more intricate and unpredictable, as it requires managing support and expansion in numerous directions. The concept of detorsion braces, which incorporates coupled movements in both the frontal and sagittal planes, streamlines the correction process by simplifying geometric adjustments.

The protocol for wearing a brace is tailored specifically to each type of brace and is influenced by various factors. These factors include whether there is an initial recommendation for full-time wearing, the angle of the scoliosis, its ability to be adjusted or its flexibility, and the age of the patient. An essential aspect of this protocol is the hyperbolic dose-response curve, which reveals that wearing the brace for 8 hours a day during the weaning phase can achieve up to 90% of the desired outcome, as would be seen with constant use. Medicine is often described as an art, and defining the precise relationship between the effectiveness and the ultimate results remains a challenge yet to be conquered. The rate of patient drop-out serves as a potential indicator of success, with the aim being to prevent any instances of failure. However, it is important to note that for most braces, wearing them does not significantly impact the quality of life for patients.

When a brace is worn, alterations in one's walking pattern are subtle; however, both speed and rhythm tend to decrease, reminiscent of the cautious pace adopted when wearing a skirt. This adjustment results in a higher energy expenditure while walking. Interestingly, the length of each stride sees an enhancement, which is a beneficial effect, along with the decreased asymmetrical stress often linked to scoliosis.

In summary, this review delves into the mechanical functions of various generic braces, which we have endeavored to categorize systematically. The diverse passive, active, and secondary mechanical actions are designed to address the complex nature of scoliosis, providing a comprehensive solution. It is impractical to converge all potential solutions, necessitating a discerning prioritization that goes beyond merely considering the Cobb angle. Each generic brace is capable of integrating numerous mechanical actions to attain the best possible correction. When physiotherapy is utilized in conjunction with the brace, it can enhance and, to some extent, make up for the mechanical actions that the brace lacks, underscoring the essential need for collaborative teamwork.