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The Effect of Local Halo Pin Care Reagents on Cranial Radial Compressive Stress (When Tightening Halo Pins)

Stephen H. Liu, MD
Thomas R. Lunsford, MSE, CO
Robert L. Waters, MD

ABSTRACT

The halo fixation device is used widely for treating traumatic injury to the cervical spine. Its major complications, pin loosening and infection, can be minimized by retightening, increasing torque and practicing local pin care. Local pin care includes applying a reagent to the skin at the pin site and to the halo ring hole or pin threads.

The coefficient of friction at the halo ring/pin thread interface is a major determining factor in the relationship between the applied torque and the radial compressive stress applied to the skull. This relationship was determined for halo pins coated with reagents in common clinical use.

The type of reagent used had a profound effect on the coefficient of friction and the radial compressive stress. At 0.9 N-m (8 in.-lbs.) the mean radial compressive stress using pins coated with betadine, hibiclens and hydrogen peroxide averaged 52 MPa (7,550 psi), 49.1 MPa (7,130 psi) and 53.5 MPa (7,770 psi), respectively, compared to uncoated pins and pins coated with antibiotic ointment, which averaged 66. 7MPa (9,680 psi) and 70.3 MPa (10,200 psi), respectively. These latter two radial compressive stresses were precipitously close to the ultimate radial compressive strength of cranial bone.

Introduction

Since its introduction by Perry and Nickel in 1959, the halo device has become widely used in treating traumatic injury to the cervical spine (1,2). Biomechanical studies show the halo-vest is the most effective means of externally immobilizing the cervical spine (3,4).

Complications of halo fixation include pin loosening, infection, osteomyelitis, skull penetration and intra/extradural abscess (5,6,7,8). The most common complications, pin loosening and/or infection, can be decreased by increasing the torque at the pin-skull interface to 0.9 N-m (8 in.-lbs.), re-tightening the pin one or two days after initial placement and practicing aggressive local pin care (6,9). Commonly used reagents such as hydrogen peroxide', hibiclens, betadine or antibiotic ointment4 are used to cleanse the skin prior to halo pin insertion and to provide local pin care.

From a mechanical perspective, the addition of a reagent to the threads of the halo pin or halo ring hole acts as a coating that could change the coefficient of friction at the thread interface. The friction between the pin thread and ring hole thread is a major factor in the relationship between tightening torque and the radial compressive stress on the skull. If the thread friction is reduced (i.e., the threads are lubricated), then a standard applied torque will exert excessive radial compressive stress on the skull. Conversely, if the friction of the threads is increased, then a standard applied torque will produce less radial compressive stress than desired.

The pertinent consideration when "torquing" the skull pins is that the radial compressive stress at the halo pin interface be less than the radial compressive strength of the cranial bone at the point of pin insertion. Bone quality varies among individuals and with age (10). Cranial bone is stronger in healthy adults than it is in elderly individuals with osteoporosis.

No data are available on the lubricating effect of local care reagents on the amount of radial compressive stress produced by the halo pin. However, McElhaney et al. tested 237 cranial bone specimens (26 donors) and found the ultimate radial compressive strength to be 73.7 +/- 35.1 MPa (10,700 +/- 5 100 psi)(10). The large standard deviation indicated wide variations in the porosity and internal arrangement of the trabeculae (10). With a 20 percent decrease in porosity, the ultimate radial compressive strength of cranial bone is reduced by 60 percent (10).

Therefore, two worrisome scenarios exist. First, halo pin or ring hole threads may be lubricated, and the applied stress may exceed the bone strength in patients who have weakened cranial bones in compression due to osteoporosis. Second, if a high-friction reagent is used on the threads, the recommended pin-tightening torque may produce inadequate radial force, causing the pins to loosen.

The purpose of this study was to access the relationship between applied torque and radial compressive stress on the skull following application of commonly used reagents to the pin/halo threads.

Method

The radial compressive stress applied to the cranial bone can be calculated by the formula

Where

S = radial compressive stress in Pascal, Pa, (psi)
F_c = radial compressive force in Newtons, N, (lbs.)
A_p = area of halo pin tip making contact in m² (in.²)

Some of the factors in the above equation are shown in Figure 1 and defined below.

T = applied torque N-m (in.-lbs.)
D = pin thread pitch distance = .576 cm (.227 in.)
u (mu) = thread friction (no units)
P = thread pitch distance = .091cm (1/28 in.)
a (alpha) = thread angle = 30 degrees

Substituting the constants into equation B provides a relationship between applied torque (T) and radial pressive force (F_c) that depends solely on thread coefficient of friction, u (mu).

The coefficient of friction u (mu) must be determined experimentally.

The last variable to be determined in the calculations of radial compressive stress is the pin's area of contact as a function of applied torque and thread friction (reagent) As the halo pin is tightened, the skull is penetrated proportionately deeper (see Figure 2 ).

Area of Contact

The relationship of halo pin tip contact to the diameter determined from the cranial bone hole diameter measurements using the following relationship (11).

Where

A = diameter of halo pin tip contact m² (in.)
D = diameter of hole cm (in.)

This relationship has been plotted so that the bone sample hole diameter measurement could be translated into areas of contact of the halo pin tip (see Figure 3 ). Pin contact area was determined from the cranial bone sample hole diameter, given the curve shown in Figure 3 . Diameter was measured three times to ensure reliability. The mean (+/- standard deviation) areas of the halo pin tip are shown in Table A .

Apparatus and Materials

An apparatus employing a tensiometer⁵ was developed to allow the radial compression force of a halo pin to be measured as a function of applied torque (see Figure 4 ). Torque was applied to the halo pins with an analog type of torque wrench⁶. Because the apparatus used a 4:1 lever system between the tensiometer and the halo pin, the halo pin's radial compression force was four times the force indicated on the tensiometer. A standard 300 series stainless steel halo ring⁷ was sectioned, autoclaved and then mounted on the apparatus (see Figure 5a ).

Autoclaved 300 series stainless steel halo pins⁸ containing slotted bases and concave radial tips were used. Testing was performed using uncoated threads and threads coated with the following reagents: 3 percent hydrogen peroxide solution, topical betadine solution, hibiclens cleansing solution and bacitracin-neomycin sulfate-polymycin B sulfate ointment.

The human skull bone used for testing was obtained from autopsy. Six male frontal skulls (right and left) specimens were obtained from three different calvaria of 24, 50 and 74 years. These specimens were sectioned into samples of approximately 1.5 cm x 1.5 cm x .8 cm thick (see Figure 5b ).

Procedures

The effects of variations in thread friction were evaluated using uncoated halo pins and ring holes. Four different halo ring holes were tested with four different halo pins (16 combinations) at 0.23 (2), 0.45 (4), 0.68 (6), 0.9 (8), 1.0 (9) and 1.11 N-m (10 in.-lbs.) of torque.

All halo pins and rings were autoclaved prior to and after application of each of the five conditions: four reagents and no reagent. Skull specimens were mounted between the metal bar and the halo pin tip (see Figure 5b ). Torque values of 0.23 (2), 0.45 (4), 0.68 (6), 0.9 (8), 1.0 (9) and 1.11 N-m (10 in.-lbs.) were applied, and radial compression forces were recorded for each reagent and uncoated thread.

Bone samples were removed from the apparatus after each torque tightening, and the diameters of the resulting indentations were measured with the aid of a micro scale8 with .0254-cm (.01-in.) divisions and a lOX magnifying eye piece⁹. The area of contact of the tip of the halo pin (A_p) was related to this diameter. Radial compressive stress (S_c) was calculated from equation A since both radial compression force (F_c) and area of contact (A_p) were known.

Data Analysis

The data were screened prior to testing using univariate summaries. Means and standard deviations were calculated for radial compressive force and stress. The mean radial compression forces and stresses were compared by analyzing variances. The Scheffe' test was used for the multiple-paired comparisons. Statistical significance was determined at the .05 level.

Simple regression was used to determine the linear relationship between radial compressive force and torque for the uncoated pins and pins coated with different reagents. Polynomial regression was used to determine the relationship between radial compressive stress and torque for each of the coating reagents and the uncoated condition.

Results

Friction Variation in Pins and Holes

The means (+/- standard deviation) radial compressive forces resulting from the applied torques for 16 combinations of four uncoated halo pins and four uncoated halo ring holes are shown in Figure 6 . No significant differences among the combinations were noted at any one of the applied torques. Therefore, the differences observed for the reagents tested were independent of slight variations in friction in any one of the four holes and/or four pins.

Radial Compressive Force

Figure 7 shows the mean (+/- standard deviation) radial compressive forces resulting from the applied torques for different reagents tested. The ointment-coated and uncoated threads produced significantly higher radial compression forces at torque values above 0.45 N-m (4 in.lbs.). Moreover, ointment-coated threads produced significantly greater radial compressive forces than uncoated pins. The regression in Figure 7 shows two groups: low thread friction (uncoated threads and ointment-coated threads) and high-thread friction (hibiclens-, betadine- and hydrogen peroxide-coated pins).

Radial Compressive Stress

The mean radial compressive stress versus appplied torque for the u ncoated threads and reagent=coated threads are shown in Figure 8 . The radial compressive stresses were obtained by dividing the radial compressive force by the corresponding halo pin tip contact area, equation A. As was the case with radial copressive force, the calculated radial compressive stress restults could be sorted into two groups: higher friction (hydrogen peroxide, betadine, and hibiclens) and lower freiction (ointment and uncoated) (see Figure 8 ).

Analysis of variances of the means of the radial copressive stresses indicates ointment-coated threads had substantially lower thread friction, resulting in significantly greater radial compressive stress than the other reagent-coated threads at all torque values tested except 0.45 N-m (4 in.-lbs.). Uncoated threads generated the next-highest radial compressive stresses. At 0.9 N-m (8 in.-lbs.) of applied torque, the mean stress due to ointment-coated and uncoated threads were 66.7 MPa (9,680 psi) and 63.9 MPa (9,270 psi), respectively, compared to approximately 51.7 MPa (7,500 psi) for the threads coated with the other three reagents.

At intermediate torque values 0.45, 0.68 and 0.9 N-m (4, 6 and 8 in-lbs.), hydrogen peroxide-coated threads had significantly lower thread friction than betadine- or hibidens-coated threads but significantly higher thread than the lower-friction group (i.e., ointment-coated or uncoated threads). For the three coatings in the high-thread friction group (hydrogen peroxide, betadine and hibiclens), the thread friction and the resulting radial compressive stresses converged at the higher torque values 0.9, 1.0 and 1.11 N-m (8,9 and 10 in.-lbs). Coefficient of Friction The linear regression for the radial compressive force versus applied torque (see Figure 7 ) are of the form

Where

y = radial compressive force, F_c
m = slope
x = applied torque, T
b = y-axis intercept or radial compressive force axis intercept

Comparing these expressions with the general relationship, equation C, it is possible to calculate the corresponding coefficients of friction (see Table B ). The range of the low-friction group was .263 to .288, and the range for the high-friction group was .475 to .545. A twofold difference exists between the high- and low-friction groups. Ointment produced the lowest coefficient of friction and the hibiclens the highest.

Discussion

The antibiotic ointment- and hibiclens-coated pins produced the highest and lowest compressive stresses. These extremes of radial compressive stress indicate the impact of altered thread friction when coated with these two reagents. At the recommended halo pin-tightening torque of 0.9 N-m (8 in.-lbs.), the mean radial compressive stress was 66.7 MPa (9,680 psi) for ointment-coated threads and 47.5 MPa (6,900 psi) for hibiclens-coated threads, a 40 percent difference (6). The 66.7 MPa (9,680 psi) radial compressive stress at 0.9 N-m (8 in.-lbs.) for the ointment coated threads is precipitously close to the ultimate radial compression strength of cranial bone at 73.7 +/- 35.1 MPa (10,700 +/- 5,000 psi) reported by McElhaney (10) (see Figure 8 ).

The data collected for antibiotic ointment may also be representative of the effect produced by hair grease, mousse and other oil-based hair products, thus cautioning clinicians not to excessively torque pins coated with oil based products. Also, for patients where traction is indicated, halo pins might loosen if coated with reagents that increase thread friction (i.e., hydrogen peroxide, betadine and hibiclens). At the recommended torque of 0.9 N-m (8 in.-lbs.), the mean radial compressive stress for these reagents was approximately 13.8 MPa (2,000 psi) lower than that of uncoated pins.

Radial compressive force variations were relatively small for the low-friction reagents (2.3 to 7.85 N), com pared to that of the high-friction reagents (6.4 to 13 N) (see Table B ). Also interesting to note, the variation increased as torque was increased. This may indicate an inconsistency in "stickiness" of the high-friction-producing reagents, especially hydrogen peroxide.

Conclusion

Any reagent used to prepare the skin prior to pin insertion or to provide local pin-site care should be applied judiciously, avoiding the pin threads near the halo ring hole. This precaution minimizes the alteration of the coefficient of friction at this interface, profoundly effecting radial compressive stress. Finally, all clinical studies on halo pin care must consider the coefficient of friction and the effect of local pin-site care reagents. The radial compressive force exerted by a halo pin on the skull depends not only on the applied torque, but also on the coefficient of friction at the halo ring/halo pin thread interface.

Stephen H. Liu, MD, is with the UCLA department of orthopedic surgery, 10833 Leconte Ave., Room 76-119, Los Angeles, CA 90024.

Thomas R. Lunsford, MSE, CO, is director of the orthotic department at The Institute for Rehabilitation Research, 1333 Moursund, Houston, TX 77030.

Robert L. Waters, MD, is with the Rancho Los Amigos Medical Center, 7601 Imperial Hwy., Downey, CA 90242.

References:

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3. Johnson RM, Hart DL, Simmons EF, Ramsby GR, Southwick WO. Cervical orthoses: a study comparing their effectiveness in restricting cervical motion in normal subjects. JBJS April 1977; 59A:332-9.
4. Koch RA, Nickel VL, The halo vest: an evaluation of motion and forces across the neck. Spine 1978; 3:103-7.
5. Garfin SR, Botte MJ, Triggs KJ, Nickel VL. Subdural abscess associated with halo-pin traction. JBJS 1988; 70A:(9):1338-40.
6. Garfin SR, Botte MJ, Waters RL, Nickel VL. Complications in the use of the halo fixation device. JBJS March 1986; 68A: 3205.
7. Goodman ML, Nelson PB. Brain abscess complicating the use of a halo orthosis. J Neurosurg 1987; 20:27-30.
8. Perry J. The halo in spinal abnormalities. Practical factors and ayoidance of complications. Ortho Clin North Amer 1972; 3:69-80.
9. Botte MJ, Byrne TP, Garfin SR. Application of the halo device for immobilization of the cervical spine utilizing an increasing torque pressure. JBJS June 1987; 69A:5:750-2.
10. McElhaney JH, Fogle JL, Melvin JW, Haynes RR, Roberts VL, Alem NM. Mechanical properties of cranial bone. J Biomech 1970; 3:495-511.
11. Shigley JE. Mechanical Engineering Design, 3rd ed. McGraw-Hill, New York, 1977; 234.

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