The freedom to heal: nonrigid immobilization by a halo orthosis

Technical note

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Halo orthoses present a paradox. On the one hand, the nominally rigid immobilization they provide to the head aims to remove loads on the cervical spine following injury or surgery, and the devices are retightened routinely to maintain this. On the other hand, bone growth and remodeling are well known to require mechanical stressing. How are these competing needs balanced? To understand this trade-off in an effective, commercial halo orthosis, the authors quantified the response of a commercial halo orthosis to physiological loading levels, applied symmetrically about the sagittal plane. They showed for the first time that after a few cycles of loading analogous to a few steps taken by a patient, the support presented by a standard commercial halo orthosis becomes nonlinear. When analyzed through straightforward structural modeling, these data revealed that the nonlinearity permits mild head motion while severely restricting larger motion. These observations are useful because they open the possibility that halo orthosis installation could be optimized to transfer mild spinal loads that support healing while blocking pathological loads.

Abbreviation used in this paper:ASTM = American Society for the Testing of Materials Standard.

Abstract

Halo orthoses present a paradox. On the one hand, the nominally rigid immobilization they provide to the head aims to remove loads on the cervical spine following injury or surgery, and the devices are retightened routinely to maintain this. On the other hand, bone growth and remodeling are well known to require mechanical stressing. How are these competing needs balanced? To understand this trade-off in an effective, commercial halo orthosis, the authors quantified the response of a commercial halo orthosis to physiological loading levels, applied symmetrically about the sagittal plane. They showed for the first time that after a few cycles of loading analogous to a few steps taken by a patient, the support presented by a standard commercial halo orthosis becomes nonlinear. When analyzed through straightforward structural modeling, these data revealed that the nonlinearity permits mild head motion while severely restricting larger motion. These observations are useful because they open the possibility that halo orthosis installation could be optimized to transfer mild spinal loads that support healing while blocking pathological loads.

Halo orthoses support the cervical spine to facilitate healing after a spinal fracture or dislocation, or after surgery on the upper spinal column.21 These devices and their improvements over the past decades are clearly effective.12 However, the mechanism by which these devices affect healing favorably is not certain. Halo orthoses can reduce net rotation of the upper cervical spine by somewhere between 30% and 95% in normal physiological motion,14,15,18 far more than alternatives such as the Miami J collar or the Minerva brace.22 Although halo design does affect the degree of stabilization in some motions,5 conventional halo designs are largely similar in their degree of restraint.20,25 Intervertebral rotation (“angulation”) at the injury site due to shifting from a supine to an upright position is observed to be more than 3° in 78% of patients wearing halo orthoses, but over the range of intervertebral rotations measured in one study, no correlation existed between healing outcome and the magnitude of angulation a halo orthosis permitted at the patient's injury site.25

If not fully rigid immobilization of the spine, then what is the appropriate mechanical function of a halo orthosis? Although the level of support that best promotes healing has not yet been identified, evidence from the literature suggests that a degree of mechanical load sharing between a fracture site and a fixation device can improve healing of both bone13 and attachments of bone to soft tissue.10 A large body of literature exists that quantifies relationships between mechanical loading and healing.13 Connecting to this body of literature requires an estimate of this load sharing. Because load sharing is determined by the relative stiffnesses of the halo orthosis and the neck,9 our objective was to quantify the stiffness of a halo orthosis and its variation over the course of repeated loading.

The physiological and upper-bound loading ranges for a halo orthosis are well established. Shearing forces supported by the skull pins of existing devices in normal physiological motion are on the order of 100 N.6,24 For a 5-kg head, this corresponds to a downward acceleration of approximately 2g (g = 9.81 m/s2); for comparison, jumping hard on a carpeted floor produces a downward head acceleration of approximately 3g.3 Although compressive pin forces can drop by more than 80% over 3 months, the shearing forces stay relatively stable.7,8 Standard protocols exist for estimating the force needed to dislodge the skull pins from the skull, defined by the American Society for the Testing of Materials (ASTM).1 Experiments on cadavers and physical models following this ASTM protocol indicate that a shear force on the order of 1100 N will dislodge skull pins inserted through an open carbon fiber/epoxy halo ring.16,17

Our strategy was to quantify, within the physiological loading range, the mechanical response of a halo orthosis with the aims of interpreting it using methods of structural analysis and data from the literature for the mechanical response of the head and neck. The experiments involved measurements of responses to defined mechanical loadings applied to a halo orthosis. The halo orthosis was affixed to a fiberglass torso that is rigid relative to a human torso. This experimentation and associated mathematical modeling enabled structural characterization of the halo orthosis itself to physiological loading, and subsequent assessment of its contribution to sharing of loads with the supported neck. We found this response to present nonlinearity of a character that enables small amplitude motions while restricting large amplitude motions. We conclude with discussion of ways that this might be important to healing.

Methods

A commercial halo orthosis (PMT #1233) was fitted using standard procedures to an adult male fiberglass mannequin torso that was secured to a stable base. The halo orthosis model consisted of the following components (Fig. 1 left): 1) a polymer attachment vest with lamb's wool lining, 2) 4 carbon fiber/epoxy composite support rods, 3) 2 polymer ring/rod connectors, 4) a carbon fiber halo ring, and 5) 4 titanium skull pins. We refer to Components 3–5 as the “superstructure.”

Fig. 1.
Fig. 1.

Left: Experimental setup. Right: Mathematical idealization of halo device.

Mechanical loads were applied through wooden loading platforms cut to fit the inside of the halo ring. Loading platforms were held in place by 2-mm penetration with the 4 skull pins. Mechanical dead loads were applied to each loading platform, analogous to the ASTM F1831-971 for testing the strength of halo orthoses. Loads were applied in one of 3 positions on the loading platform: 1) directly above the contact points of the anterior skull pins, 2) directly above the contact points of the posterior skull pins, or 3) centered between these. These positions were chosen to test the range of possible loading conditions.

Rotation and deflection of the components of the halo orthosis were measured from the deflections of anterior and posterior facing lasers mounted at the top of the torso, to measure motion of the torso; across the beam (Fig. 1 right), to measure motion of the attachment vest; and on the halo rings, to measure the overall motion of the entire assembly. Displacements of laser beams were measured at a distance of 3.5 m from the center of the torso.

Two loading regimens were studied. For monotonic loading, loads were increased to a prescribed magnitude in 4 increments for each loading position. For cyclic loading, loads were repeatedly increased incrementally, then decreased by these same increments.

Angular deflections of the halo orthosis were interpreted using a mathematical model (Fig. 1 right). Forces on the anterior and posterior sets of skull pins (FA and FP, respectively) were found for each loading case using static analysis.11 Angular deflection was estimated from these forces by treating the vest as a nonlinear foundation, and the remaining components as linear, elastic, and Euler-Bernoulli beams;2 these assumptions were supported by experimental observations. The angular deflection (θtotal) of the contact points of the skull pins is the sum of the angular deflections of the vest (θvest), beam (θbeam), vertical connector (θconnector), halo (θhalo), and vertical carbon fiber rods (θrods); θrods was negligibly small compared with the other terms.

The angular deflections of the vest, beam, vertical connector, and halo could be written in terms of FA, FP, and their geometry and compliance. These varied linearly with the magnitude of the force on the skull pins:

fd1-spine13747
fd2-spine13747
and
fd3-spine13747
where the geometrical terms defining the configuration of the halo are defined in Fig. 1 right ((xH/L)= 0.56, lA = 7.2 cm, lP = 4.8 cm, h = 4.7 cm),
fd4-spine13747
and
fd5-spine13747
E is the elastic modulus of the component, and I is the area “moment of inertia,” which describes the geometry of the component.11 Applying techniques described in the study by Ginsberg and Genin11 to estimate the area moments of inertia and multiplying by elastic moduli from the study by Ashby and Jones,2 the following 3 bending stiffness estimates were found: (EI)halo= 2400 Nm2, 40 Nm2(EI)connector ≤ 80 Nm2, and 24 Nm2(EI)beam ≤ 48 Nm2.

Results

The response of the halo orthosis was dominated by deflection of the vest. For loading applied equally to the anterior and posterior pairs of skull pins, the vest buckled at the attachment points of the carbon fiber rods (the bumps in representative data presented in Fig. 2). Deformation of the superstructure contributed a relatively small amount to overall rotation of the halo ring by a factor of approximately 6 at lower loading levels and slightly more at higher loading levels (Fig. 3). The mechanical response of the superstructure was linear (R2 = 0.97) and well characterized by the mathematical model (Fig. 3 dashed lines). The response of the vest, in contrast, was nonuniform, nonlinear, and relatively compliant. For this loading, the mathematical model predicted that the majority of superstructure rotation came from the vertical connector. For all loading cases, deformation of the halo ring accounted for less than 1% of the overall rotation.

Fig. 2.
Fig. 2.

Response of halo device to loading applied equally to the anterior and posterior skull pins. FA and FP = forces accommodated by the anterior and posterior sets of skull pins.

Fig. 3.
Fig. 3.

Relative contributions of vest and superstructure to the rotation depicted in Fig. 2.

The linearity of the superstructure is seen once more in Fig. 4, which describes the relative contributions of the superstructure and vest in the response of the orthosis to weight applied directly above the anterior skull pins. The dashed line corresponds to the upper bound estimate. The difference between the measurements and predictions in this case was due to a structural nonlinearity: the posterior skull pins contacted the support rods, producing an apparent stiffening of the superstructure.

Fig. 4.
Fig. 4.

Response of halo device to loading applied above the anterior skull pins. The dashed line is the mathematical prediction for the rotation of the superstructure relative to the halo vest. The apparent stiffness of the superstructure was higher than the prediction in this case due to the skull pins contacting the vertical strut upon loading of the halo.

Gradual settlement of the vest was observed when the halo orthosis was subjected to gentle, repeated loading (Fig. 5). Each loading cycle was of 45 N applied equally to all 4 skull pins, as is representative of forces from walking. These caused an incremental increase in the angle of tilt of about 0.01°. The halo orthosis settled into its loaded configuration and stiffened relative to the initial loading for larger rotations (Fig. 5 and Discussion).

Fig. 5.
Fig. 5.

Settling of halo orthosis subjected to mild, repeated loading.

For the experiments conducted here, a linear fit to the first 0.2° of halo deflection over all experiments (n = 4 for loads applied over the anterior pins, posterior pins, and both sets of pins) yielded a rotational stiffness of 11.4 ± 3.57 Nm per degree of rotation of the halo ring. Torques were calculated by considering the positions of loads relative to position of the vertical connector (Fig. 1 right). For mechanical loads beyond this, linear fits to the data for the next 0.2° of rotation indicated that the stiffness decreased subsequently by approximately a factor of 2 (Fig. 2).

Discussion

Nonlinear Response of the Halo Orthosis Was Dominated by Vest Deformation and an Order of Magnitude Stiffer Than the Neck

The mechanical response of the halo orthosis showed significant nonlinearity of 2 types. The first type appeared during initial loading of the halo orthosis. The second appeared in cyclic loading and will be discussed in the next section. A typical loading response of the orthosis involved repeatable “bumps” on the load-deflection curve characteristic of local buckling, and a concave-down shape indicating a decreased resistance to rotation with increasing loading. The halo orthosis rotational stiffness of 11.4 ± 3.57 Nm per degree in the initial linear region was a factor of 45 greater than the stiffness of the neck itself for the motions considered, which is on the order of 0.25 Nm per degree.4

These relative stiffnesses determine the degree to which mechanical loads are shared between the halo orthosis and a patient's neck. For a halo orthosis and the neck resisting mechanical loads in parallel, load sharing can be quantified by the fraction Φ of and applied torque that is transmitted to the neck. This gives the following equation:2

fd6-spine13747
where (khalo/kneck) is the ratio of the measured stiffness of the halo to that of the neck. Applying our data in this equation indicated that the neck should absorb approximately 2.1% of an applied low amplitude torque associated with physiological activity.

Nonlinear Response Evolved With Loading

A second type of nonlinearity observed was associated with settling of the halo orthosis during repeated loading. Following an initial loading, the halo orthosis settled so that it did not return to its preloading configuration. The consequence of this is that subsequent head motion was little resisted until the halo vest had sufficient time to relax to its preloading state, or until the head reached a limit defined by the previous head motion. This phenomenon is described by the data in Fig. 5 and is illustrated in Fig. 6. For head motion within the “physiologic reloading range” depicted in Fig. 6, nearly all torque associated with head motion would be absorbed by the neck. Beyond this range, the halo orthosis returned to slightly above its initial stiffness, absorbing on the order of 98% of applied torques.

Fig. 6.
Fig. 6.

Resistance rotation provided by the halo orthosis is dramatically reduced over a range of rotation angles that is determined by the peak of the previous loading cycle. For physiological loading, this may enable motion of the neck that is beneficial to healing while restraining the neck from distension beyond a prescribed range.

A limit of settlement was not observed in these tests. However, from experience with adjustment of halo pins, one likely exists. A single readjustment of skull pins after a week of wearing is usually sufficient to keep the orthosis stable for months.16,19,23 With the supposition that pin loosening relates to vest settling, this suggests that a few days of cyclic loading from normal daily activity may drive the vest to a fully settled state.

Load Sharing Can Be Tuned Through Halo Orthosis Design and Installation

The combined experimentation and mathematical modeling suggest that load sharing and both types of nonlinearity can be tuned through design and installation of the halo orthosis. When the head rotates sufficiently to engage the halo orthosis, the halo orthosis shares mechanical torques in parallel with the neck: the inertia and weight of the head are resisted by the sum of resisting torques from the neck and the halo orthosis, and load sharing occurs in proportion to the relative resistances to rotation of the neck and the halo orthosis.

The design of a halo orthosis offers opportunity to tune this resistance and hence tune load sharing. Important factors include the height of the vest and the disposition of the straps. Depending upon the vest lining and the way that the halo vest is strapped onto a patient, the rotational resistance of the vest scales somewhere between the square of its height (appropriate if compression against the torso is concentrated at the top and bottom of the vest) and the cube of its height (appropriate if compression against the torso occurs over the entire length of the torso). The longest commercial vests terminate at the iliac crest (50 cm below the shoulder in the halo orthosis tested in this study), and the shortest at the xiphoid process (approximately 30 cm below the shoulder for a patient of similar size). From Equation 6 and these scalings, a vest of shorter height but otherwise similar to that tested would be expected to share as much as 6%–9% of mechanical torques to the neck (Fig. 7).

Fig. 7.
Fig. 7.

Load sharing can be tuned through the design and installation of a halo orthosis. Mechanical torques can be transferred from the halo orthosis to the neck by using a vest shorter than that tested in the current article. Reducing the degree to which the vest is compressed against the torso can similarly transfer torque from the halo orthosis to the neck, effectively moving the shaded region upwards.

The strapping of a vest onto a patient can affect both the resistance to rotation (and hence load sharing) and the nonlinear response of the halo orthosis. Adjusting the compression of the vest lining against the torso can modulate the vest's resistance to rotation, with reduced compression shifting load to the neck and shifting the shaded range of Fig. 7 upwards. The degree of settling and hence the degree to which the head can rotate with limited resistance (Fig. 6) can be reduced by compressing the vest against the torso.

Also of note are factors expected to have little effect on load sharing. Results suggest little opportunity to tune load sharing based upon the position of injury. Vertebrae of the cervical spine resist torque largely in series; with the torques varying little from between levels, the total angulation arises from the summed effects of this torque on each cervical vertebra. The neck's overall resistance to rotation is therefore relatively insensitive to the position of injury, and hence load sharing is affected only weakly. Also, although the disposition of the pins and the superstructure can greatly affect the degree to which inertial forces apply torque to the neck and vest, these factors do not affect the stiffness of the neck or vest directly. The pins and superstructure therefore should have smaller influence on load sharing.

Enablement of Motion Under Physiological Loading May Be Important for Healing

How much rotation is helpful during healing? This question is much asked in orthopedic procedures, often with no clear verdict on whether exercise or isolation is superior.10 The tests conducted show that the halo orthosis in common use at our hospitals allows for rotation of a few degrees under physiological loading. Although we prescribe this to patients for “rigid immobilization of the head,” the device is neither rigid nor immobile and has a changing response over an initial settlement period.

Consistent with a broad literature on the role of mild mechanical stimulus on effecting bone healing,13 the second type of nonlinearity observed, namely the enablement of head motion over a prescribed range, may be of benefit to the healing process. The picture of healing augmentation by a halo orthosis emerging from our observations is one of very strong constraint for motions beyond a physiological limit that is established by day-to-day activity, combined with stimulus of the injury state site through much looser constraints within this physiological range. Data motivate further study to determine what level of head immobilization is best for spinal healing and how varying this over time might improve patient outcomes.

Acknowledgment

This article is dedicated to the memory of Edward R. Fickenscher.

Disclosure

This work was funded in part by the Johanna D. Bemis Trust and by Washington University in St. Louis. The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author contributions to the study and manuscript preparation include the following. Conception and design: all authors. Acquisition of data: Genin, Rosenberg, Seger, Tran. Analysis and interpretation of data: all authors. Drafting the article: Genin, Rosenberg, Seger, Tran, Leuthardt. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Genin. Statistical analysis: Genin, Rosenberg, Seger, Tran. Study supervision: Genin.

This article contains some figures that are displayed in color online but in black-and-white in the print edition.

References

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Article Information

Address correspondence to: Guy M. Genin, Ph.D., Department of Neurological Surgery, and Department of Mechanical Engineering & Materials Science, Washington University in St. Louis, Campus Box 1185, St. Louis, MO 63130. email: genin@wustl.edu.

Please include this information when citing this paper: published online August 22, 2014; DOI: 10.3171/2014.7.SPINE13747.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Left: Experimental setup. Right: Mathematical idealization of halo device.

  • View in gallery

    Response of halo device to loading applied equally to the anterior and posterior skull pins. FA and FP = forces accommodated by the anterior and posterior sets of skull pins.

  • View in gallery

    Relative contributions of vest and superstructure to the rotation depicted in Fig. 2.

  • View in gallery

    Response of halo device to loading applied above the anterior skull pins. The dashed line is the mathematical prediction for the rotation of the superstructure relative to the halo vest. The apparent stiffness of the superstructure was higher than the prediction in this case due to the skull pins contacting the vertical strut upon loading of the halo.

  • View in gallery

    Settling of halo orthosis subjected to mild, repeated loading.

  • View in gallery

    Resistance rotation provided by the halo orthosis is dramatically reduced over a range of rotation angles that is determined by the peak of the previous loading cycle. For physiological loading, this may enable motion of the neck that is beneficial to healing while restraining the neck from distension beyond a prescribed range.

  • View in gallery

    Load sharing can be tuned through the design and installation of a halo orthosis. Mechanical torques can be transferred from the halo orthosis to the neck by using a vest shorter than that tested in the current article. Reducing the degree to which the vest is compressed against the torso can similarly transfer torque from the halo orthosis to the neck, effectively moving the shaded region upwards.

References

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