Investigation of how different halos influence pin forces

Indications and Efficacy of Halo-Gravity Traction in Pediatric Spinal Deformity: A Critical Analysis Review

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Halo-gravity traction (HGT) is a well-established technique for correcting severe spinal deformity in pediatric patients.

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HGT induces soft-tissue relaxation and gradually lengthens the spine, and it can be used preoperatively and intraoperatively.

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It is typically indicated for spinal deformity over 90° in any plane and medical optimization.

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There are several complications associated with the use of HGT, and it is critical to follow a protocol and perform serial examinations to minimize this risk.

A novel method for safe and accurate placement of the rocker pins of head immobilization devices utilizing a digital caliber phantom: technical note

BACKGROUND

Immobilization of the head and skull by head immobilization devices (HIDs) is a common practice in neurosurgery. A variety of complications and morbidities are associated with pinning the skull during application of HIDs.

OBJECTIVE

Our aim is to describe a new technique that avoids repeated puncturing of the head and skull during application of HIDs and hence avoiding the potential complications resulting from multiple re-adjustments.

METHODS

We used a pre-adjusted digital millimetric caliber (DMC) as a phantom for the two rocker pins of the HID to mimic and simulate the process of skull pinning. Localization and preparation of the accurate skull pinning sites are safely guided by the pinning phantom.

RESULTS

The technique was applied in different neurosurgical approaches. The pinning phantom was utilized to accurately locate the suitable pinning sites. Contrary to the common practice, there was no need for repeated head and skull puncturing. Minimal manipulations of the head and neck are exerted in this approach as compared with the usual techniques. The head of the patient is allowed to be kept safe on the operating table until the final one-time confident skull pinning by the sterilized skull clamp pins.

CONCLUSION

The process of scalp and skull pinning of HIDs is technically demanding. The DMC utilized as a pinning phantom is a useful technique which provides safe and confident application of the skull rocker pins of HIDs allowing the neurosurgeon to avoid multiple puncturing of scalp and skull and minimizing manipulations of the head and cervical spine.

Complications of Head Immobilization Devices in Children: Contact Mechanics, and Analysis of a Single Institutional Experience

BACKGROUND

Head immobilization devices (HIDs) are a staple of neurosurgical procedures, including in the intraoperative magnetic resonance imaging (iMRI) operating rooms (ORs) where material modifications were necessary for compatibility with the magnets utilized.

OBJECTIVE

To present the experience in this OR environment and discuss the multifactorial nature of the observed adverse events.

METHODS

A retrospective chart review was performed, utilizing the Department of Neurosurgery and iMRI OR databases to identify patients who suffered complications related to HIDs between November 2007 and March 2016. A literature review was also done to identify the magnitude of the problem and the availability of safety guidelines.

RESULTS

Nine hundred and forty patients underwent surgery in the iMRI OR requiring head immobilization. Seven (0.7%) suffered complications related to the HID—depressed skull fractures (n = 7) and epidural hematomas (n = 6). Age at surgery ranged from 1.6 to 10.3 yr. All patients had posterior fossa neoplasms and associated obstructive hydrocephalus. Four patients (57%) suffered permanent neurological deficits. Six patients (86%) underwent a surgical procedure to evacuate the epidural hematomas and repair the depressed skull fracture. In contrast, 1 out of 445 patient (0.2%) suffered HID-related adverse events in the conventional ORs, aged 10.2 yr.

CONCLUSION

HIDs are important to provide stability and support during neurosurgical procedures. Modifications in the material or the shape of the pins can significantly change the pressure exerted. Most of these complications are preventable if certain precautionary measures are taken especially in certain high-risk patients, and the overall benefits of HIDs continue to outweigh the risks. There is a need for consensus on guidelines for the safe use of these devices.

The halo fixator

The halo fixator may be used for the definitive treatment of cervical spine trauma, preoperative reduction in the patient with spinal deformity, and adjunctive postoperative stabilization following cervical spine surgery. Halo fixation decreases cervical motion by 30% to 96%. Absolute contraindications include cranial fracture, infection, and severe soft-tissue injury at the proposed pin sites. Relative contraindications include severe chest trauma, obesity, advanced age, and a barrel-shaped chest. In children, a computed tomography scan of the head should be obtained before pin placement to determine cranial bone thickness. Complications of halo fixation include pin loosening, pin site infection, and skin breakdown. A concerning rate of life-threatening complications, such as respiratory distress, has been reported in elderly patients. Despite a paucity of contemporary data, recent retrospective studies have demonstrated acceptable results for halo fixation in managing some upper and lower cervical spine injuries.

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.

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.