The effect of angled insertion on halo pin fixation

Upper cervical spine injuries: indications and limits of the conservative management in Halo vest. A systematic review of efficacy and safety

INTRODUCTION

The integrity of the upper cervical spine is essential for survival and function, because of the neurovascular structures contained within its bony elements. Fractures of the upper cervical spine (C1-C2) are frequent. This systematic review assesses the efficacy and safety of the conservative management in Halo vest for patients with upper cervical spine fractures.

MATERIALS AND METHODS

Two reviewers independently identified studies in English, by a systematic search of CINAHL, Embase, Medline, HealthSTAR, and the Cochrane Central Registry of Controlled Trials, from inception of each database to 28 January 2010, using various combinations of the keywords terms "odontoid fractures", hangman's fractures", "axis fractures", "axis", "atlas", "Jefferson fractures", "C1 arch fractures", "C1 fractures", "C2 fractures", "cervical spine", "injuries", "fracture", "trauma", "neck injury", "surgery".

RESULTS

A total of 43 citations were obtained. An additional 4 papers were obtained from the reference list of the studies included. The 47 studies that were included described a total of 1078 patients with C1-C2 fractures managed by halo fixator.

CONCLUSIONS

The halo fixator has a well defined place in the management of fractures of the cervical spine. Clearly, studies of higher level of evidence, for instance large randomised trials, should be conducted, even though the available evidences suggest that management of upper cervical spine fracture with halo fixator is safe and effective.

Evaluation of skull thickness and insertion torque at the halo pin insertion areas in the elderly: a cadaveric study

BACKGROUND CONTEXT

The halo skeletal fixator provides the most rigid type of immobilization of all the orthoses that stabilize the cervical spine. Sometimes with older patients (>70 years old), the pin penetrates the cortical and cancellous bone of the skull and enters the intracranial space, which can result in serious complications such as brain injury, infection, hematoma, and loss of cerebrospinal fluid from the subarachnoid space. Currently, there is a lack of relevant literature that examines these concerns.

PURPOSE

To evaluate the thickness of the outer table, diploe, and inner table at the anterolateral and posterolateral pin insertion areas of the skull in elderly cadavers by using computed tomography (CT) scans. In addition, insertion torques at the four standard pin insertion areas was determined by applying halo pins at incremental torque in an effort to suggest safe torque levels for the anterolateral and posterolateral pins.

STUDY DESIGN/SETTING

A human cadaveric anatomical and biomechanical study relating to thickness and insertion torques at standard pin areas in the elderly.

PATIENT SAMPLE

Twenty-one elderly cadaveric skull specimens.

OUTCOME MEASURES

Thickness of cortices (tables) and diploe of skull and insertion torque at halo pin insertion areas.

METHODS

Aquarius Image software at the CT scanner's TeraRecon Aquarius Workstation was used to make the necessary skull thickness measurements at the pin insertion areas. Six, 8, 12, 18, and 36 inch lb of torque were used to determine penetration of the pins through the inner table at each of the four locations (two anterolateral and two posterolateral).

RESULTS

The mean anterolateral thickness was 7.36+/-1.57 mm. The average thickness of the outer table, diploe, and inner table were 2.24+/-0.44 mm, 1.52+/-0.41 mm, and 3.59+/-1.70 mm, respectively. The mean posterolateral thickness was 9.47+/-1.12 mm. The average thickness of the outer table, diploe, and inner table were 4.32+/-0.92 mm, 1.88+/-0.35 mm, and 3.27+/-1.21 mm, respectively. No pin penetration was seen at the traditional 8 inch lb of insertion torque in both the anterolateral and posterolateral pin insertion areas. Eighteen inch lb of torque resulted in penetration in 90.48% (19/21) and in 85.71% (18/21) of specimens in the left anterolateral and right anterolateral pin insertion areas, respectively. No penetration was seen even at 36 inch lb of torque in 80.95% (17/21) of the cadavers in both the left and right posterolateral pin insertion areas.

CONCLUSIONS

The current study supported previous research that 8 inch lb of torque is safe for application of halo pins in the elderly. The posterolateral skull is thicker and stronger than the anterolateral skull. The safe maximum torque is 8 inch lb for anterolateral pin insertion area and 18 inch lb for the posterolateral pin insertion area.

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.

Accuracy and reliability of torque wrenches used for halo application in children

BACKGROUND

Halo ring and vest application in children requires torque wrenches capable of delivering a spectrum of torque values ranging from 0.11 to 0.68 N-m (1 to 6 in-lb). Published evaluations of torque wrenches commonly used in adults have shown that the measured torque values were within 10% of the target torque in only 64% of trials. The objective of the present study was to evaluate the accuracy, reliability, and interobserver variability of halo wrenches capable of applying the lower torque levels commonly used in children.

METHODS

Torque wrenches from four distributors (Bremer, Jerome Medical, Mountz, and PMT) were tested with use of a calibrated torque-meter. Five wrenches of each type were tested by a single observer, with fifty trials performed at six different torque settings (0.11, 0.23, 0.34, 0.45, 0.57, and 0.68 N-m). One wrench of each type was then tested by two additional observers at a torque setting of 0.34 N-m, with each observer performing fifty trials per wrench.

RESULTS

The measured torque value was within 10% of the target value in 69.2% of the 6400 trials, including 50.7% of the trials performed with the PMT wrench, 51.8% of those performed with the Bremer wrench, 84.5% of those performed with the Mountz wrench, and 90% of those performed with the Jerome wrench. Significant variability (p < 0.05) was found between at least two, and as many as five, wrenches of the same variety at each of three torque settings used for comparison (0.23, 0.45, and 0.68 N-m). Significant interobserver variability (p < 0.05) was found between at least two observers during testing of the Jerome and Mountz wrenches, but no significant differences were shown between observers during testing of the PMT and Bremer wrenches.

CONCLUSIONS

The Jerome and Mountz wrenches are more accurate and reliable at low torque settings than the PMT and Bremer wrenches are. Variability among different wrenches from the same manufacturer may be seen with any of the wrenches studied.

Open versus closed halo rings: comparison of fixation strengths

STUDY DESIGN AND OBJECTIVE

A mechanical skull model was used to compare axial loads to failure for three marketed and one experimental halo ring.

SUMMARY OF BACKGROUND DATA

Open-back halo rings have become increasingly popular; however, to the authors' best knowledge the literature provides no comparison of the fixation strength of an open-back halo versus the traditional closed design.

METHODS

A model biomechanically similar to a human skull was used to compare the axial force necessary to distract each of four halo rings to failure. Three clinically used halo rings were compared: a closed titanium ring, an open titanium ring, and an open graphite ring. One additional open titanium halo ring was constructed with an angular profile; this ring was compared with the clinically used halos.

RESULTS

The mean force to failure for the closed titanium ring significantly exceeded that for the open rings (P < 0.005). No significant differences were noted among the open halo rings.

DISCUSSION

The data obtained by use of this biomechanical model show that the closed halo ring provides distraction strength greater than that of the open rings, suggesting a more rigid system with the closed device. Use of an angular halo did not improve fixation strength in the open ring device. These findings may support use of a closed halo ring in cervical spine traction and immobilization, if this greater strength is shown to be sufficiently clinically important to offset any disadvantages of a closed ring.

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.

A comparison of various angles of halo pin insertion in an immature skull model

STUDY DESIGN

A basic science biomechanical study involving an animal model.

OBJECTIVES

To evaluate the effect of varying angles of halo pin insertion on the force generated at the pin-bone interface, and thereby the stability of the halo pin-bone interaction during insertion.

BACKGROUND DATA

Because of variations in the shape and size of the pediatric skull, halo pins often are inserted at various angles rather than perpendicular to the skull. Concern exists that the high complication rate associated with pediatric halo use may result in part from less than ideal structural properties at the halo pin-bone interface.

METHODS

The authors used a fetal calf skull model to simulate the thickness and structural properties of the pediatric skull. Halo pins were inserted at angles of 0 degree (perpendicular), 10 degrees, 15 degrees, and 30 degrees into skull segments via a halo ring. Load generated at the pin-bone interface was measured using a modified mechanical testing device. Twenty trials were conducted per angle, with the endpoint being specimen failure, pin penetration, or maximum load.

RESULTS

Mean maximum loads per unit thickness were 82.15 +/- 7.54 N/mm at 0 degree, 68.80 +/- 4.79 N/mm at 10 degrees, 51.49 +/- 5.08 N/mm at 15 degrees, and 42.38 +/- 3.51 N/mm at 30 degrees, There was a significant difference between perpendicular insertion (0 degree) and 15 degrees angles of insertion. There was also a significant difference between the 10 degrees and 30 degrees angles of insertion.

CONCLUSIONS

Perpendicular halo pin insertion in an immature skull model was shown to result in increased load at the pin-bone interface. This improved structural behavior may help to reduce the incidence of complications of halo application in children.

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.

Stabilizing properties of the halo apparatus

STUDY DESIGN

A cadaveric cervical spine specimen fixed between a fiberglass torso and a plastic skull was used as a model to determine the effect of halo structural parameters on motion at a lesion simulated at C5-C6. In a second part, nine commercially available halo devices were compared.

OBJECTIVES

To define the contributions of the various components of the halo apparatus to reducing motion in an injured cervical spine and to compare the stability offered by a sample of commercially available halo devices. Controversy exists concerning the ability of the halo apparatus to stabilize the injured cervical spine.

SUMMARY OF BACKGROUND DATA

The halo apparatus has been shown to be the most effective nonsurgical method for stabilizing the fractured spine. Nonetheless, several clinical studies have demonstrated that unacceptably large motions can occur at the injured spinal segment stabilized with a halo apparatus.

METHODS

Each cadaveric cervical spine was mounted onto a fiberglass torso and a rigid plastic skull was attached to the base of the occiput. A posterior ligamentous lesion was created between C5 and C6. The halo ring was fitted to the skull and a vest to the torso. Loads were applied to the skull in flexion, extension, and lateral bending, and relative angulation between C5 and C6 was measured with electroinclinometers. In the first part, the effect of parameters such as vest tightness, vest-thorax friction, vest deformation, and connecting bar rigidity on spinal angulation were measured using one vest. In the second part, the stability offered by each of nine commercially available halo devices was compared.

RESULTS

Increasing chest strap tightness and decreasing vest deformation reduced angulation at the spinal lesion. Once connecting bar joints were tightened to 25% of their recommended torque, increased tightening or adding additional bars had no effect on rigidity. Although specific vests permitted significantly greater motion in specific directions, no vest allowed greater angulation consistently in all loading planes.

CONCLUSIONS

Increasing vest tightness, decreasing the deformability of the vest, and ensuring a good fit can reduce motion in the fractured spine. Most commercially available halo vests provide similar mechanical stability to the injured cervical spine.

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)