Forces in the halo-vest apparatus

The freedom to heal: nonrigid immobilization by a halo orthosis

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.

Experimental validation of 3D printed patient-specific implants using digital image correlation and finite element analysis

With the dawn of 3D printing technology, patient-specific implant designs are set to have a paradigm shift. A topology optimization method in designing patient-specific craniofacial implants has been developed to ensure adequate load transfer mechanism and restore the form and function of the mid-face. Patient-specific finite element models are used to design these implants and to validate whether they are viable for physiological loading such as mastication. Validation of these topology optimized finite element models using mechanical testing is a critical step. Instead of inserting the implants into a cadaver or patient, we embed the implants into the computer-aided skull model of a patient and, fuse them together to 3D print the complete skull model with the implant. Masticatory forces are applied in the molar region to simulate chewing and measure the stress-strain trajectory. Until recently, strain gages have been used to measure strains for validation. Digital Image Correlation (DIC) method is a relatively new technique for full-field strain measurement which provides a continuous deformation field data. The main objective of this study is to validate the finite element model of patient-specific craniofacial implants against the strain data from the DIC obtained during the mastication simulation and show that the optimized shapes provide adequate load-transfer mechanism. Patient-specific models are obtained from CT scans. The principal maximum and minimum strains are compared. The computational and experimental approach to designing patient-specific implants proved to be a viable technique for mid-face craniofacial reconstruction.

Stimulated muscle force assessment of the sternocleidomastoid muscle in humans

The aim of this study was to configure a force assessment device and determine potential testing protocols for quantitative evaluation of human neck muscles. The study design consisted of non-randomized control trials, with repeated measures; data from 12 normal subjects were obtained. Several apparatuses were designed, constructed and tested, i.e. single or short trains of supramaximal stimuli were used to activate sternocleidomastoid muscles in a seated position with strain gauges (6.2% variability with double-pulse stimulations) or in supine positions with load cells (5.2% variability with similar activation). Using a final configuration, maximum elicited peak forces were 1742 +/- 323 g for single-pulse and 3976 +/- 484 g for double-pulse stimulations (n = 12). There were no significant differences in maximum recorded peak torques between sessions per individual. Yet, detectable muscle activities were simultaneously recorded in the contralateral sternocleidomastoid muscles. This non-invasive, quantitative assessment approach has novel value for determining treatment efficacy, disease progression, and/or approach has novel value for determining determining treatment efficacy, disease progression, and/or relative distribution of muscle strength in patients with abnormal neck muscle function.

Determination of minimum required halo pin force

The motivation for this study is to provide design guidance for a new halo system that minimizes pin loosening. If halo pin loosening can be substantially decreased with a new halo system, then the current standard of care of overtightening halo pins will not be necessary. Accordingly, there is a need to determine the halo pin force that should be applied to ensure adequate fixation. A biomechanical test was performed using cadaver head constructs, a custom halo fixture, and a tensile testing machine in an attempt to determine the relationship between the force required to dislodge a halo ring and the initial halo pin force. Three cadaver head constructs were tested at initial pin forces of 120, 240, and 360 N. For each test, the halo was pulled from a cadaver head with a displacement rate of 2.5 mm/min until the halo ring disengaged from the head. The vertical force that caused disengagement of the halo from the head was determined from the resulting load-displacement curves. A linear regression of the data predicts disengagement forces of 80, 320, and 570 N, respectively, for initial pin forces of 120, 240, and 360 N. The 95% prediction interval of disengagement forces for initial pin forces of 120, 240, and 360 N were +/-130, 120, and 130 N, respectively. A previously published study reported the maximum vertical load on a halo orthosis during patient usage to be 186 N. The lower 95% prediction interval from this study indicates that an average initial pin force of 230 N is necessary to prevent halo pin disengagement from a 186-N vertical load.

Surgical management of odontoid fractures

Odontoid fractures account for approximately 20% of all cervical fractures, with the majority being type II fractures according to the Anderson and D'Alonzo classification. The treatment of odontoid fractures is determined by multiple factors, including fracture type, presence of associated injuries, patient age, and patient comorbidities. It is generally well accepted that type I and type III injuries heal well with non-operative treatment. However, some type I injuries can be seen in association with occipito-atlantal dislocation; and some type III fractures can be closer to the neck of the odontoid (high and shallow based), and may act like a type II fracture, that is, with an increased probability of nonunion. The treatment of type II fractures remains controversial. Over the past decade, internal fixation has become an accepted treatment for unstable injuries of the cervical spine. Multiple surgical approaches have been proposed. This article reviews the various alternatives for treating odontoid fractures, attempting to give to the reader a broad perspective on the current techniques, including information taken from evidence-based medicine.

Muscle atrophy after treatment with Halovest

STUDY DESIGN

A prospective study was conducted on the sequential changes in the muscles around the cervical spine following external fixation for an extended period.

OBJECTIVE

Muscular atrophy around the cervical spine following prolonged external fixation with a Halovest and subsequent recovery from the atrophy were examined.

SUMMARY OF BACKGROUND DATA

There are a number of reports on the usefulness of the Halovest. However, there have been no descriptions about muscular atrophy following fixation by the Halovest.

METHODS

The study participants were 10 patients who were conservatively treated by the Halovest. Imaging by CT was performed periodically following fixation by the Halovest. The cross sections of the sternocleidomastoid muscle and the nuchal muscle group at C5-C6 were computed by using NIH Image Software. The results were used to evaluate muscular atrophy following fixation by the Halovest and the subsequent recovery after removal of the fixation.

RESULTS

Following fixation by the Halovest, atrophy of the muscles around cervical spine sequentially progressed. Three months after fixation, muscular atrophy was about 15% at the sternocleidomastoid muscle and 22% for the nuchal muscle group, but the patients recovered from the atrophic state following the removal of the device.

CONCLUSION

Following fixation by the Halovest, muscular atrophy around the cervical spine sequentially progressed. After the device was removed, however, the muscles recovered from the atrophic state. Muscular atrophy caused by prolonged fixation by the Halovest was a reversible change.

Atlantoaxial stabilization: clinical comparison of posterior cervical wiring technique with transarticular screw fixation

Symptomatic atlantoaxial instability requires atlantoaxial stabilization. In this study the authors compared clinical, radiographic, and cervical outcome questionnaire results in 67 such patients who underwent 71 separate procedures. Thirty-eight patients had traditional posterior C1-C2 cervical wiring and halo-vest immobilization (group 1), whereas 33 were alternatively managed with transarticular screw fixation without rigid external immobilization (group 2). Mean follow-up in group 1 was 53.2 months and mean follow-up in group 2 was 41.0 months. Radiographic evaluation demonstrated seven pseudoarthroses and four fibrous unions in group 1, with six patients subsequently undergoing reoperation. There were no pseudoarthroses and two fibrous unions in the transarticular screw group (p = 0.015). In those that fused, >2-mm displacement occurred in six of the group 1 patients (p = 0.027). There was a trend toward fewer complications in group 2 patients (p = 0.085) with four complications, as compared with 12 complications in group 1, including a 21% incidence of halo-vest-related complications. These results demonstrate the significant benefits of transarticular screw fixation over posterior cervical wiring techniques in the management of atlantoaxial instability.

Halovest dynamic loads: full crossover comparison of three vest types

STUDY DESIGN

Ten cervical spine trauma subjects were studied during halovest treatment. Each subject wore each of three different vests.

OBJECTIVES

To compare the effect of vest design on loads between halo and vest during various activities.

SUMMARY OF BACKGROUND DATA

Complications during halovest use may be related to vest design, a variety of which are available. Loads between halo and vest have been shown to vary with activity type, but no comparison between vest types has been reported previously.

METHODS

Loads between the halo and the vest were measured during performance of activities of daily living and during load application to the halo, using custom-built four-channel transducers and a PC-based data acquisition system.

RESULTS

Substantial variations between subjects exist in loads between the halo and the vest. At rest, neck distraction loads were significantly greater for the supine posture than for either the sitting or standing postures. Loads applied to the halo by the investigators are carried by the neck and not by the structure connecting the halo to the vest. During activities of daily living all four measured load components are generally nonzero. During activities of daily living substantial differences in loads occurred between vest types. For all activities of daily living combined, the relative neck distraction load values were as follows: 4PAD 100%, Bremer 159%, and PMT 180%.

CONCLUSIONS

Previous evidence supports a connection between certain complications of halovest wear (such as pin loosening) and loads on the pins. This study shows that different halovests are associated with quite different loads between the vest and the halo (and thus loads on the pins and the neck). This is encouraging for prospective, clinical comparison of different halovests and for improvements in clinical performance through halovest design improvements.

Six-pin halo fixation and the resulting prevalence of pin-site complications

BACKGROUND

In spite of the many advances in halo application technique, the prevalence of complications associated with the use of halo fixation remains high, particularly at the pin sites. Many practitioners do not use more than four pins for halo application in adults because they believe that it increases the risk of complications. The purpose of this study was to investigate the use of six pins in halo application, in order to determine if the extra pins increased fixation strength without increasing the overall pin-site complication rate.

METHODS

The first part of our study consisted of force-deflection tests conducted on models of the skull fitted with either a four or a six-pin halo to determine if the six-pin halo provided greater fixation strength. Each skull model was placed in a servocontrolled hydraulic test machine; an axial distraction force was then applied until failure occurred. The second part of the study was a retrospective analysis of sixty-three patient records to document the prevalence of pin-site complications in patients treated with a six-pin halo system; these findings were then compared with established complication rates associated with four-pin halos.

RESULTS

In the force-deflection tests, the mean load to failure of the six-pin halo construct (2879 N [647 lb]) showed the system to be significantly stronger (p = 0.0033) than the four-pin halo construct (1681 N [378 lb]). Of the sixty-three patient records reviewed, five (8% [95% confidence interval, 1% to 15%]) revealed pin-loosening; no infection was recorded for these five patients. One of the sixty-three patients had redness and erythema at "multiple sites," but these areas healed well. Another presented with infection at all six sites; this was recorded as an allergic reaction.

CONCLUSIONS

Six-pin halo fixation results in greater halo strength and cervical spine stabilization without increasing the risk of pin-site complications.

CLINICAL RELEVANCE

Our findings are relevant for current clinical practice as the high complication rates associated with halo application have deterred some practitioners from using this type of fixation. The use of six pins, along with an improved protocol for halo application and care, may contribute to a more successful treatment outcome with fewer complications.

Biomechanical evaluation of two halo pin designs, with, and without, intact periosteum

The presence of periosteum has been hypothesized to adversely affect halo pin penetration and performance (Voor, 1992. Ph.D. Dissertation, Tulane University, New Orleans, LA). However, biomechanical testing of halo pins has historically been conducted on bone specimens with periosteum removed. This may have lead to an unrealistic measure of biomechanical pin performance. Our study compares the biomechanical performance of two halo pin designs on bovine bone specimens with, and without, intact periosteum. The two pin designs included in this study were the conventional pin (Bremer Medical) with conical tip, and a newly released trochar-style pin (DePuy AcroMed). Results showed the mean peak load before failure of the trochar-style pin (mean +/- 95% confidence interval: 656+/-29 N) to be significantly higher than the conventional pin (517+/-53 N) on bone with intact periosteum (p = 0.001). With the periosteum removed, the mean peak load of the trochar-style pin (655+/-99 N) remained statistically the same (p = 0.987), while the mean peak load of the conventional pin (634+/-65 N) increased significantly (p = 0.026). Variation of the data of the conventional pin significantly decreased from 32 to 19% on removal of periosteum (sigma = 165-103 N, respectively, p = 0.0967), while variation of the trochar-style remained statistically the same at 30-29% (sigma = 193-188 N, respectively, p = 0.954). These results show that the trochar-style pin may be biomechanically superior to the conventional pin for vertical forces experienced during immobilization. The performance of this new pin style may also not be significantly affected by overlying soft tissue. Use of this new pin style may, therefore, improve overall stability and fixation of the halo apparatus.

Pin loosening in a halo-vest orthosis: a biomechanical study

STUDY DESIGN

The cranial pin force history of a halo-vest orthosis was measured using an instrumented halo in a clinical study with three patients. Pin force values at the time of halo-vest application and at subsequent clinical visits during the halo-vest wear period were compared.

OBJECTIVES

To document the pin force reduction in the cranial pins of a halo-vest orthosis in vivo.

SUMMARY OF BACKGROUND DATA

The halo-vest is an orthosis commonly used to immobilize and protect the cervical spine. An important problem with halo-vest use is pin loosening. There have been no previous reports of pin force history in vivo.

METHODS

A custom-built strain-gauged, open-ring halo was used to measure the compressive force and superiorly-inferiorly directed shear forces produced at the tips of the two posterior pins. The instrumented halo was applied to three patients with cervical spine fractures. Pin force measurements were recorded at the time of halo application and at subsequent follow-up visits during the entire treatment period.

RESULTS

A mean compressive force of 343 +/- 64.6 N was produced at the pin tips during halo application with the patient in a supine position. On average, the compressive forces decreased by 83% (P = 0.002) during the typical halo-vest wear period. The compressive forces were substantially greater than the shear forces, which averaged only -11+/-30.2 N at the time of halo application and which did not change significantly with time.

CONCLUSIONS

The study confirmed the hypothesized decrease in the compressive pin forces with time. All patients had developed at least some clinical symptoms of pin loosening at the time of halo-vest removal.

Pin force measurement in a halo-vest orthosis, in vivo

The halo-vest is an orthosis commonly used to immobilize and protect the cervical spine. The primary complications associated with the halo-vest have been attributed to cranial pin loosening. However, the pin force history during day-to-day halo-vest wear has not previously been reported. This paper presents a new technique developed to monitor cranial pin forces in a halo-vest orthosis, in vivo. A strain gaged, open-ring halo was used to measure the compressive and shear forces produced at the posterior pin tips. The strain gages measured the bending moments produced by these forces without compromising the structural integrity of the halo-vest system. The prototype halo measured the compressive and shear force components with a resolution of +/- 15 and +/- 10 N, respectively. To test the feasibility and durability of the device, it was applied to one patient requiring treatment with a halo-vest orthosis. At the time of halo-vest application, the mean compressive force in the two posterior pins was 368 N. Over the 3 month treatment period, the compressive forces decreased by a mean of 88%. The shear forces were relatively insignificant. Using this technology future work will be aimed at determining the causes of pin loosening, optimizing vest and pin designs, and investigating the safety of more rapid rehabilitation.

Halo pin loosening: a biomechanical comparison of experimental and conventional designs

Loosening of the pins is the most common complication associated with use of the halo orthosis. The purpose of this study was to test the hypothesis that a new cylindrical cutting pin tip design which minimizes damage to adjacent bone and does not rely on high axial forces to maintain fixation would perform better mechanically than conventional conical tip pins. Conventional and experimental halo pins were tested for mechanical stability in human cadaveric skull bone using a servohydraulic load frame (Model 858 Bionix, MTS Corp., Minneapolis, MN). A cyclic transverse load of +/-300 N was applied through the pins for 10,000 cycles in a sinusoidal wave form in both fully tightened and reduced axial load situations. Load-to-failure testing was also performed to determine the strength and stiffness of each configuration. Photomicrographs of thin decalcified sections through a hole formed by each pin tip were compared for gross evidence of bony damage. With the pins fully tightened, there was no statistically significant difference in the motion between the experimental design (mean +/- 95% confidence interval: 0.41+/-0.027 mm) and the conventional halo pin (0.38+/-0.075 mm). After the axial pin force was intentionally decreased, there was no significant increase in the motion of the experimental pins (0.43+/-0.032 mm), however, there was a significant increase in the motion of the conventional pins (3.15+/-2.403 mm)(p < 0.05). The failure strength of the experimental pins (2010+/-366.4 N) was significantly greater than the conventional pins (1128+/-94.5 N)(p < 0.005). The pin bone interface stiffness of the experimental pins (1728+/-144.4N/mm) was also significantly greater than that of the conventional pins (1393+/-202.6 N/mm)(p < 0.03) (Fig. 5). Qualitatively, the photomicrographs demonstrated considerably more particulate debris on the boundary of the hole formed by the conventional pin compared to the experimental pin. The data obtained herein support our hypothesis and indicate that the experimental pin design possesses biomechanical characteristics superior to current designs. These characteristics may translate into fewer complications in the clinical setting.

Clinical, radiographic, and kinematic results from an adjustable four-pad halovest

STUDY DESIGN

Charts and radiographs of all patients treated with this halovest at one university hospital were reviewed retrospectively.

OBJECTIVES

To describe the outcomes from an adjustable four-pad halovest and to compare them with those from standard halovests, as previously published.

SUMMARY OF BACKGROUND DATA

With standard halovests, there can be cervical motion up to 70% of normal values, substantial loads between the halo and vest, and complications of pin loosening, pin infections, and scapular pressure sores. The four-pad vest reduces halovest loads and vest-torso motions.

METHODS

The four-pad vest has four independently adjustable pads that completely avoid contact with the scapula, clavicle, and abdomen. Clinical records were analyzed to determine the incidences of halo pin loosening, pressure sores, injury or surgical site nonunion, and loss of cervical alignment. Lateral radiographs were taken with the patient in the upright and supine positions at various times to determine intervertebral rotations (flexion-extension).

RESULTS

The clinical results with the four-pad vest were at least as good as those for standard vests. Scapular pressure sores were prevented completely by the absence of vest-scapula contact. Kyphosis did not increase significantly with time. The mean segmental rotations were all 3 degrees or less and showed a smoothly decreasing pattern from C1-C2 to C6-C7. The value at Oc-C1 was opposite to that at C1-C2 and is the subject of further analysis.

CONCLUSIONS

The rotations occurring with the four-pad vest are less than or equal to those occurring with standard vests, for overall cervical rotation and for individual motion segment rotations. This is consistent with the smaller halovest forces seen with this vest. Prospective, comparative testing will assess the clinical significance of these findings.

Halo Skeletal Fixation: Techniques of Application and Prevention of Complications

The halo skeletal fixator provides the most rigid cervical immobilization of all orthoses. However, complications such as pin loosening and infection are common. Appreciation of local anatomy and adherence to established application guidelines should minimize pin-related problems. A relatively safe zone for anterior pin placement is located 1 cm above the orbital rim and superior to the lateral two thirds of the orbit. Posterior pin-site locations are less critical; positioning on the posterolateral aspect of the skull, diagonal to the contralateral anterior pins, is generally desirable. Pins should enter the skull perpendicular to the cortex, with the ring or crown sitting below the widest portion of the skull and passing about 1 cm above the helix of the ear. Pins are inserted at a torque of 8 in-lb and retightened once to 8 in-lb at 48 hours. A loose pin can be retightened to 8 in-lb if resistance is met; otherwise, a loose pin should be replaced at a nearby site. Superficially infected pins are managed with local pin care and oral antibiotics. Persistent or severe infections require pin replacement to a nearby site, parenteral antibiotic therapy, and incision and drainage as needed. In-ability to maintain acceptable cervical reduction with a halo fixator is an indication for alternative treatment, such as internal fixation or traction.

The halo skeletal fixator: current concepts of application and maintenance

The halo device provides the most rigid cervical immobilization of all cervical orthoses. Despite its established efficacy, reported complications include pin loosening (36% to 60%), pin-site infection (20% to 22%), severe pin discomfort (18%), ring migration (13%), pressure sores (4% to 11%), unacceptable scars (9% to 30%), nerve injury (2%), dysphagia (2%), prolonged bleeding at pin sites (1%), and dural puncture (1%). Appreciation of skull anatomy and established application guidelines can help minimize these complications. A relative "safe zone" for anterior pin placement is located 1 cm above the orbital rim, superior to the lateral two thirds of the orbit. This position avoids injury to the nearby frontal sinus (medially), temporalis fossa (laterally), and sensory nerves (supraorbital and supratrochlear nerves medially, and zygomaticotemporal nerve laterally). Posterior pin positions are less critical, located roughly diagonal to the contralateral anterior pins. Pins should enter the skull perpendicular to the cortex, with the ring or crown sitting below the equator of the skull, passing about 1 cm above the helix of the ear. Pins are inserted at 8 in-lbs and re-tightened once at 48 hours. A loose pin can be re-tightened to 8 in-lbs if resistance is met; otherwise, a loose pin requires replacement in a nearby site. Superficially infected pins are managed with oral antibiotics and local pin care. Refractory infections require pin removal, parenteral antibiotics, and incision and drainage as indicated. Dysphagia (difficulty in swallowing), produced by exaggerated cervical extension, may necessitate repositioning of the C-spine. Dural pin puncture is managed with hospitalization, antibiotics, and elevation of the head of the bed to decrease cerebrospinal fluid pressure and allow dural healing.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal bone mineral density changes following halo vest immobilization for cervical trauma

In this prospective study we followed the bone mineral density (BMD) changes of the injured cervical spine immobilized with the halo vest. In order to define the natural history of cancellous vertebral bone loss and restoration, dual-energy densitometry was used on each of ten selected cervical spines in the lateral view (1) immediately after the application of the device, (2) at the end of the treatment and (3) 3 months after the removal of the halo vest. The halo vest produces local osteoporosis in the immobilized cervical spine with an overall reduction of BMD averaging 2.83% (P < 0.05). The response of the cervical spine to immobilization was only slightly different from patient to patient and between different vertebral bodies in each particular spine. The type and the level of injury of the cervical spine were not related to the changes of BMD, age or gender of the patient, whereas the local osteoporosis was mostly reversible in the follow-up evaluation of 5-6 months.