Spine as a Column

The manner by which orthoses exert restrictive effects is perhaps best understood in terms of column mechanics. Several authors have described the spine as a complex variant of an ideal column with a fixed base and free upper end. As a theoretic structure, an “ideal” column is considered a homogeneous rod of constant composition, length, and cross-sectional area. Its behavior when loaded by a balanced axial force has been described mathematically by Euler’s relationship for long-segment column dynamics:

A, Elastic buckling of an ideal column of length L under an axial load P . This column has failed by buckling because P exceeds the critical load for this column as given by Euler’s relationship, where E is the modulus of elasticity for the column material, A is the cross-sectional area of the column, and L is the length. B, Prototype spinal orthosis consisting of at least two end-stabilizing elements, such as circumferential bands, and an interconnecting longitudinal upright. C, Three-point bending strategies of orthoses utilize two horizontal end forces, F _e , which are balanced by a third oppositely directed force and F _a , at or near the axis of rotation for the column. This effectively segments the column into two shorter columns, increasing their respective axial load-bearing capacities.

Prototypical spinal orthoses consist of two end-fixation elements and a connecting longitudinal member ( Fig. 201-1B ). An example is the chairback, thoracolumbosacral orthosis (TLSO), which purchases the rib cage with a thoracic band and the hips with a pelvic band. Uprights interconnect the thoracic and pelvic bands. Sleeve-type orthoses such as the clamshell TLSO also incorporate these elements into their circumferential design. Biomechanically, spinal braces increase the relative cross-sectional area and change the total modulus of elasticity of the spine, thus creating a heterogeneous composite structure that shares axial load.

Most spinal orthoses apply force indirectly, at some distance from the spine. There is an inverse relationship between the thickness of the soft tissue separating the spine from the inner surface of the orthosis and the resulting effectiveness of immobilization. Conformation of the brace to the body helps to maintain the cylindrical body shell, thereby increasing the stability of the spine. Longer braces provide more stability than shorter braces; therefore, the length-to-width ratio of the orthotic significantly affects efficacy (see Fig. 201-1A ).

Orthoses may apply balanced transverse forces by three-point bending that resists rotation and contributes to axial load bearing. The Jewett brace utilizes a three-point bending strategy produced through applied transverse dorsal forces at the sternum and pubis in combination with a ventral force at the apex of the affected thoracic or lumbar vertebra. The long lever arms proportionally decrease the force required to produce a sufficient bending moment. Three-point bending also effectively divides the column functionally into two portions of smaller length, increasing the critical failure load of the whole column to that of each segment ( Fig. 201-1C ).

An underlying principle in long bone splinting is the immobilization of the fractured bone from one joint above to one joint below the site of injury. By extrapolating this concept to the axial skeleton, the spine is composed of five segments, each of which may be considered a long bone (cranial, cervical, thoracic, lumbar, and sacropelvic) (see Figs. 201-1A and B ). Therefore, in practice, one segment above and one segment below the unstable motion segment would be included in the brace.

The in vivo spine differs considerably from an ideal column in its specific composition and mechanical behavior, although this model can serve as a useful paradigm for thinking about the mechanisms underlying the beneficial effects of orthoses.

Dynamic and Passive Control

All orthoses control spinal motion by a combination of dynamic and passive mechanisms. Dynamic control describes the significant role of intrinsic musculature in actively stabilizing the spine and is a major component in the effect of most orthoses. It has been demonstrated experimentally that opposing muscular forces significantly stiffen the spinal column, increasing its load-bearing capacity. If isolated from its muscular support, the osseous and ligamentous spinal column holds only 2 kg of axial load before failure by buckling. In terms of a column model, muscular action directly affects the modulus of elasticity and relative cross-sectional area of the composite spinal column. Orthoses promote muscular stabilization through tactile feedback, guiding the patient to maintain proper positioning of the body. Pressure at the orthosis-skin contact site produces a reminder to maintain a specific position and limit unwanted gross body motion. The patient therefore is able to prevent undesirable motion of the spine using only intrinsic muscular support guided by the orthosis. Stiffer, more securely worn appliances are more effective at limiting motion because of the heightened sensation of resistance that the stiffer appliance produces. Sypert and others noted that the effectiveness of an appliance is directly related to the level of its discomfort. However, brace discomfort may also contribute to higher levels of noncompliance.

Passive mechanisms for motion control are important in three-point bending mechanisms and are derived from intrinsic properties of the orthosis itself such as design, size, and material composition. Two common design elements of all orthoses are similar in principle to internal fixation constructs and include end-stabilizing elements (e.g., thoracic bands, pelvic bands) and longitudinal members or uprights that interconnect the end elements. Passive mechanisms apply reactive forces to the body that oppose physiologic and pathologic movement of the head or trunk; viscoelastic forces of ligaments, discs, and muscles; and gravitational force. As summarized by White and Panjabi, passive mechanical strategies form the basis for most orthotic techniques and include (1) spinal distraction, (2) fluid compression, (3) balanced transverse force application, and (4) skeletal fixation. Most appliances use a combination of techniques for motion control.

Distraction

Distracting the ends of a column is an effective method for correcting or preventing deformity. Its effectiveness depends on the efficiency of transmitting the distracting force directly to the column. Spinal distraction is a principal of action of internal fixation devices (e.g., Harrington distraction rods) and is frequently used when correcting and stabilizing deformities. Distraction is also used to reduce acutely unstable spine fractures with tong or halo traction. A braced column in distraction can be considered a composite of an externally applied distracting force plus the axial supporting properties of the original structure.

Distraction orthoses typically act on the head and thorax and do not directly affect individual vertebral segments. Purchase of the head is either indirect, with pads located at the mandible and the occiput (conventional orthoses), or direct by means of skull pins (halo-skeletal fixation). Thoracic purchase is obtained at the sternum and rib cage through a combination of pads, straps, and vest attachment. The effectiveness of a distraction orthosis depends on the efficiency of force transmission to the vertebral segment of interest and the mechanical rigidity of the orthosis material itself. Inefficiency in transmitting external force to the spine has been termed “the transmitter problem” by White and Panjabi and represents the loss of energy that occurs when force is applied to “low stiffness, viscoelastic” structures such as overlying soft tissues, intervening normal joints, and ligaments. In the cervical area, distraction applied to the mandible is compromised by cushioning effects of soft tissue under the chin, the temporomandibular joint, cervical muscle tone, the C0-1 articulation, and each successive segmental articulation above the level at which the force is to have its effect (level of pathology). A more rigid brace with a tighter fit improves the efficiency of transmitting force by compressing intervening soft tissue, but paradoxically it increases the risk of pressure injury to overlying soft tissues. Skeletal fixation improves the effectiveness of force transmission by directly purchasing the skull, minimizing the risk of pressure injury in the head and neck region.

Point-of-contact problems also exist with thoracolumbar braces that involve the shoulder girdle, pectoral muscles, rib cage, and upper abdomen. The shoulders have a significant amount of overlying skin, fat, and muscle and are by definition highly mobile structures involved in arm movement. Because of this mobility, orthoses that rely on shoulder straps or pads to apply a counterforce cannot consistently distract the spine. Changes in body position from sitting to supine also produce shoulder movement contributing to the difficulty of spinal distraction. Koch and Nickel originally studied this effect in six patients wearing the halo apparatus by measuring the forces of distraction and compression exerted through the device with an attached strain gauge. Distraction force varied by more than 20 pounds in a halo vest and 30 pounds in a halo cast when patients changed from supine to sitting positions. Similar variations in distraction with the halo device were noted during shoulder shrugging, coughing, sneezing, and deep breathing. Shoulder purchase is thus a highly variable means of anchoring the caudal end of a distraction orthosis.

Appliances with pads overlying the pectoral areas are compromised by the energy-absorbing effects of fat, muscle, and breast tissue. Movement of the chest occurs with arm motion in a manner similar to that of the shoulders. Although the rib cage is generally a stable structure, deep breathing, coughing, and sneezing produce significant motion that is directly transmitted by all devices purchasing the thorax. Orthoses extending below the thorax to the upper and lower abdomen are at an even greater disadvantage because of the highly elastic nature of this fluid- and air-filled region.

In summary, all orthoses are limited in their ability to distract the spine because of inherent inefficiencies in force transmission at both the rostral and caudal ends of the devices, and because of limitations in exerting pressure through soft tissues. Because distraction is poorly maintained by an orthosis, even when combined with halo fixation, bracing alone is generally not recommended if distraction is required to maintain reduction or to prevent dangerous instability.

Fluid Compression

Fluid compression refers to the ability of a tight circumferential binder, such as a corset, to compress partially fluid-filled soft tissues surrounding the spine, thus creating a fluid cylinder. Because liquids are mechanically incompressible, fluid-filled cylinders have axial load-bearing capacity. For a column model, this technique increases the aggregate cross-sectional area by converting soft tissues into load-bearing structures. Several studies have directly measured the effect of abdominal and thoracic cavity compression, noting little effect of compression on intra-abdominal pressure. In Nachemson and Morris’s classic report, however, a 25% reduction in intradiscal pressure was observed in lumbar segments that were braced with an inflatable abdominal corset. The true unloading effect of fluid compression is thought to be a minor factor for orthotic thoracolumbar stabilization and is beneficial only for restricting sagittal plane motion. Fluid compression is a strategy that is not applicable for the cervical spine, where airway, vascular, and muscular tissues make up a relatively large proportion of the cross-sectional area of the neck and do not tolerate significant compression.

Transverse Loading

Balanced transverse loading describes a common and effective strategy for restricting spinal rotation and translation. Orthoses typically use a three-point bending force application arrangement with two horizontal reactive forces applied at the ends of the column in one direction and a third balancing force in the opposite direction at the fulcrum of the deformity (see Fig. 201-1C ). Because the system is in equilibrium, the sum of all horizontal forces is zero. This prevents translation. Similarly, bending moments generated by the applied forces acting at the axis of rotation for the injured segment also equal zero if rotational motion is adequately controlled. Keys to an effective transverse loading strategy include (1) identifying the axis of rotation at the level of injury or point of instability by using an appliance that is centered at or near this axis of rotation and (2) using an adequately long appliance that maximizes the length of the applied moment arm to control the spinal segment of interest.

Cervical Orthoses

COs are basically soft foam or felt collars with minimal purchase of the mandible or occiput ( Fig. 201-2A ). These collars are light, inexpensive, easy to use, and relatively comfortable to wear. They offer little resistance to cervical motion, however, in any plane of motion, functioning only to remind patients to limit voluntary extremes of neck movement. Because they are only supportive, cervical collars are inappropriate for patients with bony instability. They can provide tactile generated support of cervical musculature and psychological comfort in cases of myofascial strain or sprain or in straightforward postoperative patients without instability.

Cervical and cervicothoracic orthoses.

A, Foam collar. B, Thomas collar. C, Philadelphia collar. D, Miami J collar. E, Two-poster Guilford brace. F , Aspen collar. G, Sternal-occipital-mandibular immobilizer (SOMI). H, Yale orthosis. I, Minerva brace.

Occipital-Mandibular-Cervical Orthoses

OMC orthoses are hard plastic collars that are more rigid than foam collars and offer slightly better purchase of the mandible or occiput ( Fig. 201-2B ). There is no thoracic extension; therefore, while offering an improved cranial point of fixation, they lack true caudal fixation. The addition of an adjustable chin or occipital piece increases resistance to flexion or extension only to a mild degree. Lateral bending and rotation are poorly controlled with these braces. Although rarely used in the modern era they have been applied as a prophylactic measure, in conjunction with a backboard, in acute trauma. Prasarn and Horodyski demonstrated significant segmental motion in cadavers when putting on trauma collars, emphasizing the limitation of these devices for controlling acute traumatic instability and the need for in-line axial traction during placement. Like the soft collars, OMC orthoses do not provide significant immobilization to the cervical spine and are not recommended for patients with instability.

Occipital-Mandibular-High Thoracic Orthoses

OMHT braces extend caudally to purchase the upper shoulders and utilize more rigid material that enhances motion restriction. Flexion and extension are most effectively limited, and there is decreased motion at all levels as the caudal fixation point is lengthened. However, limitation of motion can produce a parallelogram effect of the midcervical segments. The ends remain fixed along the spinal axis, but motion occurs by the rostral segment translating ventrally and the caudal segment translating dorsally, or vice versa.

Examples of OMHT orthoses are the widely used Philadelphia collar, the Miami J collar, and the Aspen collar. The “poster” braces (e.g., Guilford, Duke) are stiff, cumbersome, and expensive, and they are rarely used in contemporary practice ( Figs. 201-2C–F ). All OMHT appliances control head movement with occipital and mandibular supports and have upper thoracic purchase. Like the simple collars, they are relatively easy to apply, are typically lightweight, and are only moderately expensive.

The popular Philadelphia collar is a Plastazote foam device reinforced with hard plastic. It is available in different sizes and consists of front and back halves connected by Velcro straps. Although generally well tolerated, it can be hot to wear, causing significant sweating and secondary skin maceration. The Plastazote material is less rigid than other OMHT appliances (e.g., post orthoses). Despite its limitations, this device is as effective in controlling upper (C0-3) flexion-extension as the rigid OMHT orthoses. It is less effective in restricting flexion-extension in the midcervical and lower cervical segments and poor in controlling rotation and lateral bending at all levels. The device is frequently used for patients with mild cervical injuries and in postoperative patients with minor instability. Koller and colleagues analyzed atlantoaxial motion in 20 healthy volunteers, comparing the halo thoracic vest and the Philadelphia collar. They found the subaxial spine to be more restricted by the halo vest than that of the Philadelphia collar. However, they found that the atlantoaxial complex was more restricted in sagittal motion between extreme flexion and extension in the Philadelphia collar (mean, 1.3 degrees) than in the halo vest (mean, 3.3 degrees). The author suggested that a Philadelphia collar might be sufficient in the conservative treatment of stable odontoid fractures.

Polin and associates compared the Philadelphia collar and the halo device in odontoid fractures. Their findings indicated no significant difference in the rate of fracture healing between the two orthoses for both type II and type III fractures and suggested that the less invasive collar may be adequate in this setting. Several reports confirm these findings for odontoid fractures in the elderly.

Modifications of the Philadelphia collar have been developed that maintain cervical immobilization yet improve the comfort and convenience of a plastic removable collar. The Miami J and Aspen collars are more rigid than the Philadelphia collar at all cervical levels with the added benefit of removable chin and occipital pads for improved comfort and hygiene. Modern versions of these appliances are now modular with various options of adding thoracic extensions as is warranted by the level of instability.

Schneider and coworkers evaluated reduction of head and intervertebral motion provided by seven contemporary cervical orthoses in 45 individuals. Overall range of motion of the head in three planes and intervertebral motion in the sagittal plane were tested as well as the reported comfort for each brace.

The seven cervical orthoses included the Philadelphia collar (Philadelphia Cervical Collar Co., Thorofare, NJ), Aspen cervical collar (Aspen Medical Products, Irvine, CA), PMT cervical collar (PMT Corp., Chanhassen, MN), Miami J cervical collar (Jerome Medical, Moorestown, NJ), Minerva cervicothoracic orthosis (Seattle Systems, Poulsbo, WA), Lerman noninvasive halo (Seattle Systems), and Sternal-Occipital-Mandibular-Immobilizer (SOMI) (U.S. Manufacturing Corp., Pasadena, CA). The first four were considered to be cervical braces; the last three were cervicothoracic orthoses. All collars were tested in their original configuration without modifications. There was a significant association between the type of brace and patient-reported brace comfort score with the Miami J and the Aspen rated most comfortable. In general, the cervical collars were more comfortable than the cervicothoracic orthoses ( Fig. 201-3A ).

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