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Probing and Tapping: Are We Inserting Pedicle Screws Correctly?
Spine Deformity 4 (2016) 395e399 www.spine-deformity.org
Probing and Tapping: Are We Inserting Pedicle Screws Correctly? Vishal Prasad, FRCS (Tr & Ortho)a, Addisu Mesfin, MDb, Robert Lee, FRCS (Tr & Ortho)c, Julie Reigrut, MSd, John Schmidt, PhDd,* a Medway Maritime Hospital, NHS Foundation Trust, Gillingham, Kent, United Kingdom Department of Orthopaedics and Rehabilitation, University of Rochester, Rochester, NY, USA c Royal National Orthopaedic Hospital NHS Trust, Brockley Hill, Stanmore, Middlesex HA7 4LP, United Kingdom d K2M Inc., 751 Miller Drive, Leesburg, VA 20175, USA Received 10 November 2014; revised 25 May 2016; accepted 11 June 2016 b
Abstract Purpose: Although there are a significant number of research publications on the topic of bone morphology and the strength of bone, the clinical significance of a failed pedicle screw is often revision surgery and the potential for further postoperative complications; especially in elderly patients with osteoporotic bone. The purpose of this report is to quantify the mechanical strength of the foam-screw interface by assessing probe/pilot hole diameter and tap sizes using statistically relevant sample sizes under highly controlled test conditions. Methods: The study consisted of two experiments and used up to three different densities of reference-grade polyurethane foam (ASTM 1839), including 0.16, 0.24, and 0.32 g/cm3. All screws and rods were provided by K2M Inc. and screws were inserted to a depth of 25 mm. A series of pilot holes, 1.5, 2.2, 2.7, 3.2, 3.7, 4.2, 5.0, and 6.0 mm in diameter were drilled through the entire depth of the material. A 6.5 45-mm pedicle screw was inserted and axially pulled from the material (n 5 720). A 3.0-mm pilot hole was drilled and tapped with: no tap, 3.5-, 4.5-, 5.5-, and 6.5-mm taps. A 6.5 45-mm pedicle screw was inserted and axially pulled from the material (n 5 300). Results: The size of the probe/pilot hole had a nonlinear, parabolic effect on pullout strength. This shape suggests an optimum-sized probe hole for a given size pedicle screw. Too large or too small of a probe hole causes a rapid falloff in pullout strength. The tap data demonstrated that not tapping and undertapping by two or three sizes did not significantly alter the pullout strength of the screws. The data showed an exponential falloff of pullout strength when as tap size increased to the diameter of the screw. Conclusion: In the current study, the data show that an ideal pilot hole size half the diameter of the screw is a starting point. Also, that if tapping was necessary, to use a tap two sizes smaller than the screw being implanted. A similar optimum pilot hole or tap size may be expected in the clinical scenario, however, it may not be the same as seen with the uniform density polyurethane foam tested in the current study. Ó 2016 Scoliosis Research Society. All rights reserved. Keywords: Probe hole; Pilot hole; Tapping; Screw pullout; Polyurethane foam
Introduction A PubMed Central search of bone mechanical properties revealed well over 10,000 articles including the effects of genetics, biochemical signals, man-made chemical effects, and a variety of disease states. In spite of all this research,
Author disclosures: VP (none); AM (none); RL (none); JR (other from K2M, Inc., during the conduct of the study; other from K2M, Inc., outside the submitted work.); JS (other from K2M Inc., during the conduct of the study). *Corresponding author. K2M, Inc., 751 Miller Drive, Leesburg, VA 20175, USA. Tel.: (703) 554-1242; fax: (703) 779-7537. E-mail address: [email protected] (J. Schmidt).
spine surgeons still do not know the quality of the patients’ bone until they open the surgical site and ‘‘poke around.’’ In many cases, the patient undergoing spine surgery has osteoporosis. As has been stated previously, ‘‘Osteoporosis is a major generalized bone disease characterized by a low bone mass and the development of non-traumatic fractures, especially of the vertebral bodies as a direct result of osteopenia’’ [1]. Probing of the vertebral bodies is the best indicator of whether the bone is normal, osteopenic, or osteoporotic. Osteoporotic (weak) bone must be managed more carefully than normal bone. The clinical significance of a failed bone screw is often revision surgery and the potential for further postoperative complications. The holding power of a screw is also critical in
2212-134X/$ - see front matter Ó 2016 Scoliosis Research Society. All rights reserved. http://dx.doi.org/10.1016/j.jspd.2016.06.001
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V. Prasad et al. / Spine Deformity 4 (2016) 395e399
deformity corrections, and although this is less of a problem in young healthy bone, it is an issue in elderly patients with osteoporotic bone. The importance of correct and positive fixation of bone screws is evident based on the number of publications on the subject. Many of these publications emphasize one particular aspect of fixation, such as probe/ pilot hole [2-8] or tapping [9-12]. Typical shortcomings of these studies include the small sample size, use of cadaveric or animal bone (with very large scatter in the data), and an insufficient number of independent variables tested. To combat the shortcomings of cadaveric studies, polyurethane foam has often been used in biomechanical studies to mitigate the variability of cadavers (cite). The intent of this study is to quantify the mechanical strength of the foam-screw interface by testing under highly controlled conditions. Testing included: Varying the probe/pilot hole diameter and then inserting a standard-size screw. Using one size pilot hole/screw and varying the tap size.
was one deviation from the standard, which was that the screws were inserted to a depth of 25 mm, not 20 mm. A standard metal bushing housed the screws prior to insertion. The bushing provided an interface with the testing machine and allowed for full contact with the head of the screw without slippage or tilting during pullout. All screws were inserted to a depth of 25 mm, using a bushing and spacer (see Fig. 1). Screws were never backed out. Axial pullout strength tests were performed on an Electropuls E3000 using Bluehill2 Software (Instron Corporation, Norwood, MA). The bushing around each inserted screw was placed in a custom axial pullout fixture whereas the polyurethane foam block was positioned in a base fixture clamped to the test frame base platen. Both fixtures were designed to ensure that each pedicle screw was centered directly under the load cell, Figure 1. An axial preload of 20 5 N was established followed by a tensile load at a rate of 5 mm/min until the screw released from the test block. The ultimate axial pullout strength was collected from the recorded load (N) versus displacement (mm) curve.
Materials and Methods Two ASTM standards were used to guide the testing: ASTM F1839 and ASTM F543 [13,14]. ASTM F1839-08e1 e Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopedic Devices and Instruments. Based on a literature review and previous testing, a general consensus ranking of the foam can be made. The 0.16-g/cm3 foam is similar to osteoporotic whereas the 0.32-g/cm3 foam is near normal bone mineral density and the 0.24-g/cm3 foam is in between [15-21]. All foam was purchased as sheet stock, 62 cm 244 cm 5 cm in thickness (Last-A-Foam, General Plastics, Tacoma, WA) and cut to final dimensions per ASTM F543. Pullout testing was performed parallel to the foam rise in 0.16-, 0.24-, and/or 0.32-g/cm3 foam per section A3 of the standard. There
Probe/Pilot hole size To assess the effect of the initial pilot/probe hole diameter, a series of pilot holes were drilled through the entire depth of the foam blocks. Pilot hole diameters were 1.5, 2.2, 2.7, 3.2, 3.7, 4.2, 5.0, and 6.0 mm in diameter. A 6.5 45-mm pedicle screw (K2M MESA) was inserted. Axial pull testing was performed with n 5 30 for each diameter pilot hole in each density of foam: 0.16, 0.24, and 0.32 g/cm3 (n 5 720). Tap size Based on the results of the Probe/Pilot hole study a 3.0-mm pilot hole was drilled and then tapped with the following size taps: no tap, 3.5, 4.5, 5.5, and 6.5 mm.
Fig. 1. (A) Insertion of the screw to the standardized depth. Screws were fitted as shown and inserted by hand to a depth of 25 mm. The spacers held the bushing level during screw insertion, ensuring screws were perpendicular to the foam surface. (B) Illustrates how the bushing/test block were inserted into the axial pullout fixture (right). The test block free floats in the base fixture, which is mounted to the load frame.
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A 6.5 45-mm pedicle screw was inserted. Axial pull testing was performed for each tap size in 0.16 and 0.32 g/cm3 foam (n 5 300). All test data were analyzed using JMP 11.0 and SAS 9.4 (SAS Institute, Cary NC). Results For all plots the Maximum Load is defined as the peak pullout force in Newtons. Probe/Pilot hole size Table 1 summarizes the mean pullout strength for each probe/pilot hole tested in each of the three foams. Note that eight different pilot holes (independent variables) were assessed. Figure 2 shows the results of the testing in 0.16 g/cm3 foam (osteoporotic). The results for both the 0.24- and 0.32-g/cm3 foam show a similar shape, and all follow the general form of Y5b0 þ b1 X þ b2 ðX lÞ
ð1Þ
2
A look at the raw data shows that the probe/pilot hole diameter has nonlinear effect and a parabolic shape. The parabola indicates that there is an optimum probe/pilot hole for any given screw size. For the 6.5-mm-outer-diameter
Fig. 2. Effects of pilot hole size on pullout strength in 0.16 g/cm3 foam. Note the nonlinear, parabolic shape indicating an optimal pilot hole size. The pedicle screw was a 6.5-mm-diameter screw. The solid line represents the predicted values from the regression equation.
solid line represents the regression equation and the data fits the form of: Max loadðNÞ5a b expðlTap
ð3Þ
sizeÞ
Max load ðNÞ5551:1 48:90 Hole Diam 25:3 ðHole Diam 3:58Þ ; r2 50:954; n5240
ð2Þ
2
screw tested, that optimum appears at 2.7 mm. The regression equation and correlation coefficient (r2) for 0.16 g/cm3 foam were as follows: A nonlinear regression shows a plateau at 345.9 N for the 0.16-g/cm3 foam and 1248.93 N for the 0.32-g/cm3 foam.
Tap size Table 2 and Figure 3 show the results of tapping a 3.0mm pilot hole. Figure 3 shows an exponential curve with an everincreasing rate of decline from an asymptotic value. The Table 1 Mean pullout strengths. Probe hole (mm) n
1.5 2.25 2.7 3.2 3.7 4.25 5 6
30 30 30 30 30 30 30 30
0.16 g/cm3
Max loadðNÞ5345:9 0:203 expð1:011Tap
0.24 g/cm3
0.32 g/cm3
Table 2 Maximum Load pullout strengths after tapping. Tap size (mm)
Mean
SD
Mean
SD
Mean
SD
377.34 394.51 373.31 405.87 367.74 341.15 255.45 104.80
15.86 23.21 14.67 17.78 21.83 16.71 14.31 9.41
767.54 771.84 767.76 737.33 682.04 607.16 463.50 256.99
45.47 26.90 51.94 46.87 65.30 28.90 22.27 21.37
1,314.66 1,252.65 1,426.15 1,219.27 1,154.41 1,117.47 875.60 507.65
90.31 95.82 25.80 76.26 65.60 48.89 46.33 66.39
SD, standard deviation. For each of the designated probe/pilot holes and foam densities. All values are in Newtons (N), ntotal 5 720.
0.00 3.50 4.50 5.50 6.50
N 30 30 30 30 30
0.16 g/cm3
n
Mean
SD
343.86 334.39 339.07 285.94 201.90
9.43 18.15 11.74 12.39 11.08
30 30 30 30 30
SizeÞ
ð4Þ
0.32 g/cm3 Mean
SD
1,220.36 1,235.42 1,219.63 1,032.25 687.01
31.29 25.82 27.33 24.17 25.85
SD, standard deviation. Student t-testing was performed with alpha 5 0.05 and a critical T value of 1.97646. Note that the greatest decrease in strength for a 6.5-mm screw occurs at tap sizes greater than 4.5 mm. All values are in newtons (N); ntotal 5 300.
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Fig. 3. The effect of tap size on probe/pilot hole tapping in 0.16-g/cm3 foam. There were small and statistically significant differences noted in the pullout strengths for each tap size. But the primary feature of the curve is the ‘‘cliff’’ that occurs just after the 4.5-mm tap size. The solid line represents the predicted values from the regression equation. The pedicle screw was a 6.5-mm-diameter screw.
Discussion of Results Probe/Pilot hole The concept of an optimum probe/pilot hole has been previously proposed [2-5]. However, in each of these studies, only three or four pilot hole sizes were tested. In the testing described by George [4], the method of hole preparation involved either a drilled hole or probe hole. Eight cadaveric spine segments from three separate cadavers provided vertebral bodies. They found no difference in pullout strength based on how the probe/pilot hole was made: drilled (mean 907.38 208.95N) or probed (mean 919.75 191.31N). This was confirmed by the work of Zdeblick [5], who also reported no difference in pullout strength based on how the probe/pilot hole was made. These studies showed that it does not matter how the hole is created; it is the size of the probe/pilot hole that determines the final holding power of the screw. Two nonlinear models were analyzed for the pilot hole data: a parabolic model, Eq. (1), and an exponential one very similar to Eq. (3). The exponential model took the form Max load5b0 b1 expðb2 Hole
DiameterÞ
ð5Þ
Both models were fit to the data, and the important metric is the residual error. Residual error is the error that cannot be accounted for by the fit of the model to the data and just like in golf, the low score wins. In all cases, the parabolic data had the lowest residual error. For the data of Figure 2, that error
was as follows: parabolic 5 101,809 and exponential 5 125,994. Therefore, the parabola is the best fit. By testing eight pilot hole sizes, the true, parabolic shape of this relationship becomes apparent. The parabolic shape indicates there is an optimal size hole that should be made prior to screw insertion. For the 6.5-mm screws tested in the PU foam, the peak of the parabola, which is the optimum-sized hole, occurs at 2.6 mm. But why does the pullout strength drop off at pilot hole sizes smaller than this? The answer has to do with the materials involved. The screw is made of metal, typically stainless steel, Ti-6Al-4V, or cobalt/chrome. The modulus of the metal (Young’s modulus, E) is a measure of the stiffness, and the metals used in bone screws are roughly 10 higher than bone. Because of this mismatch, the screw will force its way, fracturing the underlying material. With the substrate fractured, the pullout strength will decrease. The idea of making a pilot hole exactly at the optimum hole size is tempting. However, because it is unusual to know the true quality of the material, a probe/pilot hole slightly larger than optimum should be considered. For the case of the 6.5-mm screw tested in the current study tested in PU foam, a pilot hole size of 3.2 mm would provide approximately a 25% margin of safety (from fracturing the underlying material) while sacrificing only 10 N in pullout strength. (These values come from the predicted values of the regression equation of Figure 2: peak 399.6 N at the 2.6-mm pilot hole, 390.9 N at the 3.2-mm pilot hole.) Tap size This aspect of the study suffers from a shortfall of the number of dependent variables, tap sizes. The number of tap sizes used in this study is a reflection of availability; all taps that could be used were utilized in this experiment. Nonetheless, the graph of Figure 3 clearly shows the effect of undertapping. Table 2 shows that the three smallest taps evaluated (0, 3.5, 4.5 mm) result in nearly identical pullout strengths. The shape of the curve in Figure 3 is the important feature. The influence of tap size on pullout strength is obvious for the larger sizes tested; both the 5.5 and 6.5 mm taps. The data illustrate that with a decreasing tap size (undertapping), a plateau is reached where tapping the pilot hole allows for easier insertion of the screw without a decrease in pullout strength. If the tap size is too large, pullout strength literally ‘‘falls off a cliff.’’ Based on the data presented in the current study, a similar optimum pilot hole or tap size may be expected in the clinical scenario; however, it may not be the same as seen with PU foam. Limitations One limitation of this study is that it was not performed in cadavers and therefore is not representative of the
V. Prasad et al. / Spine Deformity 4 (2016) 395e399
clinical situation. This study is not intended to duplicate the clinical situation. Normal bone, found in the spine, consists of a highdensity cortical shell surrounding a cancellous, lower density, interior. The combination of these two types of bone determines the insertion torque and pullout strength of a bone screw. The material used in this study is uniform in properties and as such cannot duplicate the combined cortical/cancellous bone found in the human spine. However, the ASTM standard clearly states that polyurethane foam is ideal for testing bone screws in cancellous bone. Conclusions and Recommendations In the current study, the ideal pilot hole size in PU foam was half the diameter of the screw while data demonstrated that if tapping was necessary, to use a tap two sizes smaller than the screw being implanted. The findings of this study further emphasize the impact of pilot hole/tap sizes on screw pullout strength and suggest there are optimum sizes for both. Although a similar ideal pilot hole and tap size may be expected in a clinical scenario, the optimum sizes will not be the same for bone.
Acknowledgments We thank K2M for providing all of the instrumentation used for this study. We acknowledge the contributions of all of the summer interns who worked on this project over two years. In alphabetical order: Morgan Brown, Karli Johnson, Katherine Kamis, Daniel Schmidt, Kaci Schwarz, Griffin Smith, and Peter Williams. References [1] Ritzel H, Amling M, Posl M, et al. The thickness of human vertebral cortical bone and its changes in aging and osteoporosis: a histomorphometric analysis of the complete spinal column from thirty-seven autopsy specimens. J Bone Miner Res 1997;12:89e95. [2] Battula S, Schoenfeld AJ, Sahai V, et al. The effect of pilot hole size on the insertion torque and pullout strength of self-tapping cortical bone screws in osteoporotic bone. J Trauma 2008;64: 990e5.
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[3] Steeves M, Stone C, Mogaard J, et al. How pilot hole size affects bone-screw pullout strength in human cadaveric cancellous bone. Can J Surg 2005;48:207e12. [4] George DC, Krag MH, Johnson CC, et al. Hole preparation techniques for transpedicular screws. Effect on pull-out strength from human cadaveric vertebrae. Spine 1991;16:181e4. [5] Zdeblick TA, Kunz DN, Cooke ME, McCabe R. Pedicle screw pullout strength, correlation with insertional torque. Spine 1993;18:1673e6. [6] Chatzistergos PE, Sapkas G, Kourkoulis S. The influence of the insertion technique on pullout force of pedicle screws. An experimental study. Spine 2010;35:e332e7. [7] Gantous A, Phillips JH. The effects of varying pilot hole size on the holding power of miniscrews and microscrews. Plast Reconstr Surg 1995;95:1165e9. [8] Halverson TL, Kelley LA, Thomas KA, et al. Effects of bone mineral density on pedicle screw fixation. Spine 1994;19:2415e20. [9] Kuklo TR, Lehman RA. Effect of tapping diameters on insertion of thoracic pedicle screws: a biomechanical analysis. Spine 2003;28:2066e71. [10] Oktenoglu BT, Ferrara LA, Andalkar N, et al. Effects of pilot hole preparation on screw pullout resistance and insertional torque: a biomechanical study. J Neurosurg 2001;1:91e6. [11] Ronderos JF, Jacobwitz R, Sonntag VKH, et al. Comparative pull-out strength of tapped and untapped pilot holes for bicortical anterior cervical screws. Spine 1997;22:167e70. [12] Pfeiffer FM, Abernathie DL. A comparison of pullout strength for pedicle screws of different designs. A study using tapped and untapped pilot holes. Spine 2006;23:e867e70. [13] ASTM F1839dRigid polyurethane foam for use as a standard material for orthopedic devices and instruments. [14] ASTM F543dStandard specification and test methods for metallic medical bone screws (specifically Section A3. Test method for determining the axial pullout strength of medical bone screws). [15] Goel V, Dick D, Rengachary S, et al. Tapered pedicle screw pull out strengths; Effect of increasing screw height outside the pedicle. Summer Bioengineering Conference, 2003. [16] Krenn MH, Piotrowski WP, Penzkofer R, et al. Influence of thread design on pedicle screw fixation. J Neurosurg Spine 2008;9:90e5. [17] Pfeiffer FM, Abernathie DL. A comparison of pullout strength for pedicle screws of different designs. Spine 2006;31:e867e70. [18] Chatzistergos PE, Sapkas G, Kourkoulis K. The influence of the insertion technique on the pullout force of pedicle screws: an experimental study. Spine 2010;35:e332e7. [19] Chao CK, Hsu CC, Wang JL, et al. Increasing bending strength and pullout strength in conical pedicle screws: biomechanical tests and finite element analyses. J Spinal Disord Tech 2008;21:130e8. [20] Patel P. Screw fixation of implants to the spine. PhD Thesis, University of Birmingham, UK. [21] Calvert KL, Trumble KP, Webster TJ, et al. Characterization of commercial rigid polyurethane foams used as bone analogs for implant testing. J Mater Sci Mater Med 2010;21:1453e61.
Probing and Tapping: Are We Inserting Pedicle Screws Correctly? Vishal Prasad, FRCS (Tr & Ortho)a, Addisu Mesfin, MDb, Robert Lee, FRCS (Tr & Ortho)c, Julie Reigrut, MSd, John Schmidt, PhDd,* a Medway Maritime Hospital, NHS Foundation Trust, Gillingham, Kent, United Kingdom Department of Orthopaedics and Rehabilitation, University of Rochester, Rochester, NY, USA c Royal National Orthopaedic Hospital NHS Trust, Brockley Hill, Stanmore, Middlesex HA7 4LP, United Kingdom d K2M Inc., 751 Miller Drive, Leesburg, VA 20175, USA Received 10 November 2014; revised 25 May 2016; accepted 11 June 2016 b
Abstract Purpose: Although there are a significant number of research publications on the topic of bone morphology and the strength of bone, the clinical significance of a failed pedicle screw is often revision surgery and the potential for further postoperative complications; especially in elderly patients with osteoporotic bone. The purpose of this report is to quantify the mechanical strength of the foam-screw interface by assessing probe/pilot hole diameter and tap sizes using statistically relevant sample sizes under highly controlled test conditions. Methods: The study consisted of two experiments and used up to three different densities of reference-grade polyurethane foam (ASTM 1839), including 0.16, 0.24, and 0.32 g/cm3. All screws and rods were provided by K2M Inc. and screws were inserted to a depth of 25 mm. A series of pilot holes, 1.5, 2.2, 2.7, 3.2, 3.7, 4.2, 5.0, and 6.0 mm in diameter were drilled through the entire depth of the material. A 6.5 45-mm pedicle screw was inserted and axially pulled from the material (n 5 720). A 3.0-mm pilot hole was drilled and tapped with: no tap, 3.5-, 4.5-, 5.5-, and 6.5-mm taps. A 6.5 45-mm pedicle screw was inserted and axially pulled from the material (n 5 300). Results: The size of the probe/pilot hole had a nonlinear, parabolic effect on pullout strength. This shape suggests an optimum-sized probe hole for a given size pedicle screw. Too large or too small of a probe hole causes a rapid falloff in pullout strength. The tap data demonstrated that not tapping and undertapping by two or three sizes did not significantly alter the pullout strength of the screws. The data showed an exponential falloff of pullout strength when as tap size increased to the diameter of the screw. Conclusion: In the current study, the data show that an ideal pilot hole size half the diameter of the screw is a starting point. Also, that if tapping was necessary, to use a tap two sizes smaller than the screw being implanted. A similar optimum pilot hole or tap size may be expected in the clinical scenario, however, it may not be the same as seen with the uniform density polyurethane foam tested in the current study. Ó 2016 Scoliosis Research Society. All rights reserved. Keywords: Probe hole; Pilot hole; Tapping; Screw pullout; Polyurethane foam
Introduction A PubMed Central search of bone mechanical properties revealed well over 10,000 articles including the effects of genetics, biochemical signals, man-made chemical effects, and a variety of disease states. In spite of all this research,
Author disclosures: VP (none); AM (none); RL (none); JR (other from K2M, Inc., during the conduct of the study; other from K2M, Inc., outside the submitted work.); JS (other from K2M Inc., during the conduct of the study). *Corresponding author. K2M, Inc., 751 Miller Drive, Leesburg, VA 20175, USA. Tel.: (703) 554-1242; fax: (703) 779-7537. E-mail address: [email protected] (J. Schmidt).
spine surgeons still do not know the quality of the patients’ bone until they open the surgical site and ‘‘poke around.’’ In many cases, the patient undergoing spine surgery has osteoporosis. As has been stated previously, ‘‘Osteoporosis is a major generalized bone disease characterized by a low bone mass and the development of non-traumatic fractures, especially of the vertebral bodies as a direct result of osteopenia’’ [1]. Probing of the vertebral bodies is the best indicator of whether the bone is normal, osteopenic, or osteoporotic. Osteoporotic (weak) bone must be managed more carefully than normal bone. The clinical significance of a failed bone screw is often revision surgery and the potential for further postoperative complications. The holding power of a screw is also critical in
2212-134X/$ - see front matter Ó 2016 Scoliosis Research Society. All rights reserved. http://dx.doi.org/10.1016/j.jspd.2016.06.001
396
V. Prasad et al. / Spine Deformity 4 (2016) 395e399
deformity corrections, and although this is less of a problem in young healthy bone, it is an issue in elderly patients with osteoporotic bone. The importance of correct and positive fixation of bone screws is evident based on the number of publications on the subject. Many of these publications emphasize one particular aspect of fixation, such as probe/ pilot hole [2-8] or tapping [9-12]. Typical shortcomings of these studies include the small sample size, use of cadaveric or animal bone (with very large scatter in the data), and an insufficient number of independent variables tested. To combat the shortcomings of cadaveric studies, polyurethane foam has often been used in biomechanical studies to mitigate the variability of cadavers (cite). The intent of this study is to quantify the mechanical strength of the foam-screw interface by testing under highly controlled conditions. Testing included: Varying the probe/pilot hole diameter and then inserting a standard-size screw. Using one size pilot hole/screw and varying the tap size.
was one deviation from the standard, which was that the screws were inserted to a depth of 25 mm, not 20 mm. A standard metal bushing housed the screws prior to insertion. The bushing provided an interface with the testing machine and allowed for full contact with the head of the screw without slippage or tilting during pullout. All screws were inserted to a depth of 25 mm, using a bushing and spacer (see Fig. 1). Screws were never backed out. Axial pullout strength tests were performed on an Electropuls E3000 using Bluehill2 Software (Instron Corporation, Norwood, MA). The bushing around each inserted screw was placed in a custom axial pullout fixture whereas the polyurethane foam block was positioned in a base fixture clamped to the test frame base platen. Both fixtures were designed to ensure that each pedicle screw was centered directly under the load cell, Figure 1. An axial preload of 20 5 N was established followed by a tensile load at a rate of 5 mm/min until the screw released from the test block. The ultimate axial pullout strength was collected from the recorded load (N) versus displacement (mm) curve.
Materials and Methods Two ASTM standards were used to guide the testing: ASTM F1839 and ASTM F543 [13,14]. ASTM F1839-08e1 e Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopedic Devices and Instruments. Based on a literature review and previous testing, a general consensus ranking of the foam can be made. The 0.16-g/cm3 foam is similar to osteoporotic whereas the 0.32-g/cm3 foam is near normal bone mineral density and the 0.24-g/cm3 foam is in between [15-21]. All foam was purchased as sheet stock, 62 cm 244 cm 5 cm in thickness (Last-A-Foam, General Plastics, Tacoma, WA) and cut to final dimensions per ASTM F543. Pullout testing was performed parallel to the foam rise in 0.16-, 0.24-, and/or 0.32-g/cm3 foam per section A3 of the standard. There
Probe/Pilot hole size To assess the effect of the initial pilot/probe hole diameter, a series of pilot holes were drilled through the entire depth of the foam blocks. Pilot hole diameters were 1.5, 2.2, 2.7, 3.2, 3.7, 4.2, 5.0, and 6.0 mm in diameter. A 6.5 45-mm pedicle screw (K2M MESA) was inserted. Axial pull testing was performed with n 5 30 for each diameter pilot hole in each density of foam: 0.16, 0.24, and 0.32 g/cm3 (n 5 720). Tap size Based on the results of the Probe/Pilot hole study a 3.0-mm pilot hole was drilled and then tapped with the following size taps: no tap, 3.5, 4.5, 5.5, and 6.5 mm.
Fig. 1. (A) Insertion of the screw to the standardized depth. Screws were fitted as shown and inserted by hand to a depth of 25 mm. The spacers held the bushing level during screw insertion, ensuring screws were perpendicular to the foam surface. (B) Illustrates how the bushing/test block were inserted into the axial pullout fixture (right). The test block free floats in the base fixture, which is mounted to the load frame.
V. Prasad et al. / Spine Deformity 4 (2016) 395e399
397
A 6.5 45-mm pedicle screw was inserted. Axial pull testing was performed for each tap size in 0.16 and 0.32 g/cm3 foam (n 5 300). All test data were analyzed using JMP 11.0 and SAS 9.4 (SAS Institute, Cary NC). Results For all plots the Maximum Load is defined as the peak pullout force in Newtons. Probe/Pilot hole size Table 1 summarizes the mean pullout strength for each probe/pilot hole tested in each of the three foams. Note that eight different pilot holes (independent variables) were assessed. Figure 2 shows the results of the testing in 0.16 g/cm3 foam (osteoporotic). The results for both the 0.24- and 0.32-g/cm3 foam show a similar shape, and all follow the general form of Y5b0 þ b1 X þ b2 ðX lÞ
ð1Þ
2
A look at the raw data shows that the probe/pilot hole diameter has nonlinear effect and a parabolic shape. The parabola indicates that there is an optimum probe/pilot hole for any given screw size. For the 6.5-mm-outer-diameter
Fig. 2. Effects of pilot hole size on pullout strength in 0.16 g/cm3 foam. Note the nonlinear, parabolic shape indicating an optimal pilot hole size. The pedicle screw was a 6.5-mm-diameter screw. The solid line represents the predicted values from the regression equation.
solid line represents the regression equation and the data fits the form of: Max loadðNÞ5a b expðlTap
ð3Þ
sizeÞ
Max load ðNÞ5551:1 48:90 Hole Diam 25:3 ðHole Diam 3:58Þ ; r2 50:954; n5240
ð2Þ
2
screw tested, that optimum appears at 2.7 mm. The regression equation and correlation coefficient (r2) for 0.16 g/cm3 foam were as follows: A nonlinear regression shows a plateau at 345.9 N for the 0.16-g/cm3 foam and 1248.93 N for the 0.32-g/cm3 foam.
Tap size Table 2 and Figure 3 show the results of tapping a 3.0mm pilot hole. Figure 3 shows an exponential curve with an everincreasing rate of decline from an asymptotic value. The Table 1 Mean pullout strengths. Probe hole (mm) n
1.5 2.25 2.7 3.2 3.7 4.25 5 6
30 30 30 30 30 30 30 30
0.16 g/cm3
Max loadðNÞ5345:9 0:203 expð1:011Tap
0.24 g/cm3
0.32 g/cm3
Table 2 Maximum Load pullout strengths after tapping. Tap size (mm)
Mean
SD
Mean
SD
Mean
SD
377.34 394.51 373.31 405.87 367.74 341.15 255.45 104.80
15.86 23.21 14.67 17.78 21.83 16.71 14.31 9.41
767.54 771.84 767.76 737.33 682.04 607.16 463.50 256.99
45.47 26.90 51.94 46.87 65.30 28.90 22.27 21.37
1,314.66 1,252.65 1,426.15 1,219.27 1,154.41 1,117.47 875.60 507.65
90.31 95.82 25.80 76.26 65.60 48.89 46.33 66.39
SD, standard deviation. For each of the designated probe/pilot holes and foam densities. All values are in Newtons (N), ntotal 5 720.
0.00 3.50 4.50 5.50 6.50
N 30 30 30 30 30
0.16 g/cm3
n
Mean
SD
343.86 334.39 339.07 285.94 201.90
9.43 18.15 11.74 12.39 11.08
30 30 30 30 30
SizeÞ
ð4Þ
0.32 g/cm3 Mean
SD
1,220.36 1,235.42 1,219.63 1,032.25 687.01
31.29 25.82 27.33 24.17 25.85
SD, standard deviation. Student t-testing was performed with alpha 5 0.05 and a critical T value of 1.97646. Note that the greatest decrease in strength for a 6.5-mm screw occurs at tap sizes greater than 4.5 mm. All values are in newtons (N); ntotal 5 300.
398
V. Prasad et al. / Spine Deformity 4 (2016) 395e399
Fig. 3. The effect of tap size on probe/pilot hole tapping in 0.16-g/cm3 foam. There were small and statistically significant differences noted in the pullout strengths for each tap size. But the primary feature of the curve is the ‘‘cliff’’ that occurs just after the 4.5-mm tap size. The solid line represents the predicted values from the regression equation. The pedicle screw was a 6.5-mm-diameter screw.
Discussion of Results Probe/Pilot hole The concept of an optimum probe/pilot hole has been previously proposed [2-5]. However, in each of these studies, only three or four pilot hole sizes were tested. In the testing described by George [4], the method of hole preparation involved either a drilled hole or probe hole. Eight cadaveric spine segments from three separate cadavers provided vertebral bodies. They found no difference in pullout strength based on how the probe/pilot hole was made: drilled (mean 907.38 208.95N) or probed (mean 919.75 191.31N). This was confirmed by the work of Zdeblick [5], who also reported no difference in pullout strength based on how the probe/pilot hole was made. These studies showed that it does not matter how the hole is created; it is the size of the probe/pilot hole that determines the final holding power of the screw. Two nonlinear models were analyzed for the pilot hole data: a parabolic model, Eq. (1), and an exponential one very similar to Eq. (3). The exponential model took the form Max load5b0 b1 expðb2 Hole
DiameterÞ
ð5Þ
Both models were fit to the data, and the important metric is the residual error. Residual error is the error that cannot be accounted for by the fit of the model to the data and just like in golf, the low score wins. In all cases, the parabolic data had the lowest residual error. For the data of Figure 2, that error
was as follows: parabolic 5 101,809 and exponential 5 125,994. Therefore, the parabola is the best fit. By testing eight pilot hole sizes, the true, parabolic shape of this relationship becomes apparent. The parabolic shape indicates there is an optimal size hole that should be made prior to screw insertion. For the 6.5-mm screws tested in the PU foam, the peak of the parabola, which is the optimum-sized hole, occurs at 2.6 mm. But why does the pullout strength drop off at pilot hole sizes smaller than this? The answer has to do with the materials involved. The screw is made of metal, typically stainless steel, Ti-6Al-4V, or cobalt/chrome. The modulus of the metal (Young’s modulus, E) is a measure of the stiffness, and the metals used in bone screws are roughly 10 higher than bone. Because of this mismatch, the screw will force its way, fracturing the underlying material. With the substrate fractured, the pullout strength will decrease. The idea of making a pilot hole exactly at the optimum hole size is tempting. However, because it is unusual to know the true quality of the material, a probe/pilot hole slightly larger than optimum should be considered. For the case of the 6.5-mm screw tested in the current study tested in PU foam, a pilot hole size of 3.2 mm would provide approximately a 25% margin of safety (from fracturing the underlying material) while sacrificing only 10 N in pullout strength. (These values come from the predicted values of the regression equation of Figure 2: peak 399.6 N at the 2.6-mm pilot hole, 390.9 N at the 3.2-mm pilot hole.) Tap size This aspect of the study suffers from a shortfall of the number of dependent variables, tap sizes. The number of tap sizes used in this study is a reflection of availability; all taps that could be used were utilized in this experiment. Nonetheless, the graph of Figure 3 clearly shows the effect of undertapping. Table 2 shows that the three smallest taps evaluated (0, 3.5, 4.5 mm) result in nearly identical pullout strengths. The shape of the curve in Figure 3 is the important feature. The influence of tap size on pullout strength is obvious for the larger sizes tested; both the 5.5 and 6.5 mm taps. The data illustrate that with a decreasing tap size (undertapping), a plateau is reached where tapping the pilot hole allows for easier insertion of the screw without a decrease in pullout strength. If the tap size is too large, pullout strength literally ‘‘falls off a cliff.’’ Based on the data presented in the current study, a similar optimum pilot hole or tap size may be expected in the clinical scenario; however, it may not be the same as seen with PU foam. Limitations One limitation of this study is that it was not performed in cadavers and therefore is not representative of the
V. Prasad et al. / Spine Deformity 4 (2016) 395e399
clinical situation. This study is not intended to duplicate the clinical situation. Normal bone, found in the spine, consists of a highdensity cortical shell surrounding a cancellous, lower density, interior. The combination of these two types of bone determines the insertion torque and pullout strength of a bone screw. The material used in this study is uniform in properties and as such cannot duplicate the combined cortical/cancellous bone found in the human spine. However, the ASTM standard clearly states that polyurethane foam is ideal for testing bone screws in cancellous bone. Conclusions and Recommendations In the current study, the ideal pilot hole size in PU foam was half the diameter of the screw while data demonstrated that if tapping was necessary, to use a tap two sizes smaller than the screw being implanted. The findings of this study further emphasize the impact of pilot hole/tap sizes on screw pullout strength and suggest there are optimum sizes for both. Although a similar ideal pilot hole and tap size may be expected in a clinical scenario, the optimum sizes will not be the same for bone.
Acknowledgments We thank K2M for providing all of the instrumentation used for this study. We acknowledge the contributions of all of the summer interns who worked on this project over two years. In alphabetical order: Morgan Brown, Karli Johnson, Katherine Kamis, Daniel Schmidt, Kaci Schwarz, Griffin Smith, and Peter Williams. References [1] Ritzel H, Amling M, Posl M, et al. The thickness of human vertebral cortical bone and its changes in aging and osteoporosis: a histomorphometric analysis of the complete spinal column from thirty-seven autopsy specimens. J Bone Miner Res 1997;12:89e95. [2] Battula S, Schoenfeld AJ, Sahai V, et al. The effect of pilot hole size on the insertion torque and pullout strength of self-tapping cortical bone screws in osteoporotic bone. J Trauma 2008;64: 990e5.
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