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A titanium expandable pedicle screw improves initial pullout strength as compared with standard pedicle screws
The Spine Journal 11 (2011) 777–781
Basic Science
A titanium expandable pedicle screw improves initial pullout strength as compared with standard pedicle screws Srilakshmi Vishnubhotla, MSa, William B. McGarry, MSa, Andrew T. Mahar, MSa,b,*, Daniel E. Gelb, MDc a Biomechanical and Clinical Research, Alphatec Spine Inc., 5818 El Camino Real, Carlsbad, CA 92008, USA Department of Orthopaedic Surgery, University of California San Diego, 350 Dickinson St, San Diego 92103, CA, USA c Department of Orthopaedics, University of Maryland Medical Center, 22 S. Greene St, Baltimore, MD 21201-1595, USA b
Received 16 August 2010; revised 5 April 2011; accepted 14 June 2011
Abstract
BACKGROUND CONTEXT: Pedicle screws are now standard for spinal arthrodesis as they provide three-column spinal stabilization. Decreased vertebral body bone density because of aging reduces the stability of the bone-screw interface, potentially increasing screw pullout or pseudarthrosis. Modifications to standard pedicle screw designs to improve screw stabilization may help to compensate for the detrimental effects of decreased vertebral bone density. PURPOSE: To evaluate differences in initial pullout strength of an expandable titanium pedicle screw as compared with a standard titanium pedicle screw. STUDY DESIGN: In vitro human cadaveric biomechanical investigation. METHODS: Fresh thoracolumbar spines from four human cadavers were imaged using quantitative computed tomography to obtain standard lumbar osteoporosis (Dual-energy X-ray absorptiometry [DXA]) T scores. Six bodies were sectioned per spine, and standard titanium 6.5-mm diameter pedicle screws and expandable 6.5-mm diameter titanium screws (maximum expanded diameter510 mm) were randomized to right and left sides. Screw testing, in axial pullout at 25 mm/min, was randomized to reduce the effects of testing order. Data for stiffness (N/mm), yield load (N), ultimate load (N), and energy (N mm) (area under the load-displacement curve) were analyzed using a one-way analysis of variance (p!.05). RESULTS: Lumbar DXA scores averaged 3.6. There were no statistical differences between screw types for stiffness. Yield load was not statistically different between groups, although the expandable screw yield load was nearly 25% greater than that of the standard screw. Ultimate load was found to be statistically greater (~30%) for the expandable screw compared with the standard screw (p!.05). The energy required to cause bone-implant failure was also statistically greater for the expandable screw compared with the standard screw (p!.0001). CONCLUSIONS: Expandable titanium pedicle screws demonstrated improved screw pullout stability compared with standard titanium screws in osteopenic or osteoporotic bone. Further studies are warranted examining other loading methods to evaluate the stability provided by an expandable pedicle screw. Ó 2011 Elsevier Inc. All rights reserved.
Keywords:
Expandable titanium pedicle screw; Osteoporosis; Spinal fusion; Biomechanical stability
FDA device/drug status: Approved (Zodiac pedicle screw system). Not approved for this use (Osseoscrew pedicle screw system). Author disclosures: SV: Stock Ownership: Alphatec Spine, Inc. (5,000 shares); Trips/Travel: Alphatec Spine, Inc. (Nonfinancial); Other Office: Alphatec Spine, Inc. (Salary). WBM: Stock Ownership: Alphatec Spine (2,000 shares, 1%); Other Office: Alphatec Spine (Salary). ATM: Stock Ownership: Alphatec Spine (2,500 shares, 0%); Consulting: Alphatec Spine (Salary); Trips/Travel: Alphatec Spine (Financial). DEG: Royalties: Globus Medical (B), Synthes Spine (B); Private Investments: ASIP (0 shares, 6%); Consulting: Synthes Spine (C), Alphatec Spine (B); Speaking Teaching Arrangements: AO Spine North America (C); Fellowship Support: Synthes Spine (E, Paid directly to institution). 1529-9430/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2011.06.006
The disclosure key can be found on the Table of Contents and at www. TheSpineJournalOnline.com. This study was funded by Alphatec Spine. The authors disclose a financial relationship associated with the product discussed in this manuscript. The information contained in this manuscript was presented as an e-poster at the 2009 North American Spine Society meeting in San Francisco. * Corresponding author. Biomechanical and Clinical Research, Alphatec Spine Inc., 5818 El Camino Real, Carlsbad, CA 92008, USA. Tel.: (760) 494-6651; fax: (760) 493-9132. E-mail address: [email protected] (A.T. Mahar)
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S. Vishnubhotla et al. / The Spine Journal 11 (2011) 777–781
Introduction
Methods
It is well known that the demographics of aging are shifting with an expected 54% of people older than 65 years by 2050 [1,2]. This trend has been associated with an increase in orthopedic-related diseases, such as osteoporotic vertebral compression fractures and degenerative scoliosis [3–5]. It has also been shown that patients treated for degenerative scoliosis may have further deformity progression because of the aging process [3]. Despite surgical stabilization of degenerative scoliosis, patients may remain symptomatic because of pseudarthrosis and possibly curve progression. Curve progression, instrumentation failure, and pseudarthrosis may all be related to poor fixation of the pedicle screw bone-implant interface as fixation strength has been shown to be affected by bone density [6–10]. Attempts at improving the bone-implant interface strength of pedicle screws have focused on screw expansion or screw augmentation [11–16]. These modifications have generally shown encouraging in vitro results. Screw augmentation has demonstrated improved fixation strength when compared with expansion [11]. However, in vitro cement leakage was noted using perforated screws [13]. Cement technique and handling were both cited as critical to achieve improvements in strength [11,13]. Previous designs of expanding screws have demonstrated only moderate improvements in fixation strength, but the screw itself only expanded by 1.5 mm at the very distal portion of the screw [11]. The bone quality in the anterior vertebral body may not be sufficient to provide additional strength beyond the purchase achieved within the pedicle. The smallest diameter screw used was 7.0 mm, obviating placement in the upper thoracic spine or within smaller vertebral bodies. An expandable titanium screw was recently developed that increases in diameter by approximately 50% to address issues of bone fixation strength. The expansion region on the expandable titanium screw is located at the vertebral body–pedicle junction. The purpose of this study was to evaluate the differences in initial biomechanical stability of this expandable titanium pedicle screw as compared with a standard titanium pedicle screw when placed in osteopenic/osteoporotic vertebral bodies.
Brief description of surgical technique Anatomic preparation for insertion of pedicle screws is completed via typical surgical techniques. Once prepared, a placement probe may be used to visualize the location of the center (maximum diameter) of the expansion region of the screw (Fig. 1, Left). The probe also contains cutting flutes so that further anatomic preparation can be completed at the surgeon discretion to ensure the ball of the probe (and thus the center of the expansion region) is anterior to the pedicle and not within the pedicle itself. After probe location verification, screws may then be placed. An actuation handle and actuation rod are then fixed to the driver handle. The actuation rod fixes to a thread within the cannulated tip of the screw. The actuation handle is used to pull the front of the screw, via the actuation rod, posteriorly to expand the commercially pure titanium ‘‘skin’’ over the titanium alloy inner component. Fluoroscopic imaging can be used to visualize each phase, and the expanded screw is visible via fluoroscopy (Fig. 1, Middle and Right). If removal or repositioning is desired, a separate removal instrument with the actuation rod may be used to fix to the distal tip, and the actuation rod is reversed to collapse or undeploy and remove the screw. With regard to bone ingrowth/ongrowth limiting the ability to remove a screw, a previously presented animal survival study indicated similar bone formation in or on the screw as compared with standard pedicle screws [17]. In addition, the same study confirmed that the screw could be collapsed and retrieved without issue after a 6-week survival period in an ovine model.
Biomechanical testing Four fresh thoracolumbar (T12–L5) human cadaveric spines were used for this study. Six vertebral bodies per spine were disarticulated from each spine and reviewed for previous fractures, deformity, or instrumentation. One vertebral body was noted to have a Type B vertebral compression fracture and was eliminated from the study. The spines had previously been measured for bone density using quantitative computed tomography (General Electric HiSpeed CT/i with QCT-5000 Bone Densitometry Software; General Electric
Fig. 1. (Left) Placement of the locating probe demonstrates location of maximum expansion, and cutting flutes may be used to advance the probe to the desired location anterior to the pedicle, (Middle) lateral fluoroscopy may be used to visualize screw placement and expansion region, and (Right) anteroposterior fluoroscopy also demonstrates screw expansion.
S. Vishnubhotla et al. / The Spine Journal 11 (2011) 777–781
Healthcare, Waukesha, WI, USA). The lumbar vertebral bodies from each spine were scanned to calculate a World Health Organization level of osteoporosis T-score using standard techniques. Expanding 6.5-mm titanium pedicle screws (Osseoscrew; Alphatec Spine, Carlsbad, CA, USA) and standard 6.5-mm titanium pedicle screws (Zodiac; Alphatec Spine) were randomly assigned to right and left sides for each vertebral body. For the expansion screws, the diameter of the shaft at the expansion region increased from 6.5 to 10 mm (Fig. 2). The expandable screws are cannulated to allow for an actuation rod to expand the screw. The anterior portion of the individual vertebral bodies was potted in two-part epoxy resin, making sure that the potting material did not come in contact with either screw. Screws were pulled under displacement control at a rate of 25 mm/minute in line with the screw axis using a servohydraulic load frame (MTS858; MTS Systems Corp., Eden Prairie, MN, USA) (Fig. 3). Tests were stopped when the screw was completely pulled out of the vertebral body or after a failure of bone (pedicle fracture) occurred leading to a rapid drop in the load. Screws were tested in a random order of right and left sides to lower potential confounding effects related to order of testing. Stiffness was calculated from the linear portion of the load-displacement curve. Yield load was identified as the load at which the loaddisplacement curve diverged from its initial linearity. Ultimate load was calculated as the largest load experienced during the test. Failure energy was calculated as the area under the curve identified by the point of permanent loss of fixation. Data for stiffness (N/mm), yield load (N), ultimate load (N), and energy required for failure (N mm) (area under the load-displacement curve) were compared between groups using a one-way analysis of variance (p!.05).
Fig. 2. Insertion of (Top) standard 6.5-mm titanium pedicle screw and (Bottom) expandable titanium pedicle screw.
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Fig. 3. Pullout test setup.
Results The average lumbar bone density T score for the four specimens was 3.6 (samples: 2.9, 61-year-old woman; 2.3, 61-year-old man; 4.3, 74-year-old woman; and 4.7, 80-year-old woman). There were no statistical differences between screw types for stiffness of the bone-implant interface or for yield load (Table). Ultimate load was found to be statistically greater (~30%) for the expandable screw compared with the standard screw (p!.05). The energy required to cause ultimate failure of the bone-implant interface for the expandable pedicle screw was statistically greater than the energy required to fail the bone-implant interface for the standard pedicle screw (p!.0001) (Fig. 4).
Discussion Osteoporosis is a leading causative factor in orthopedicrelated fractures, such as vertebral compression fractures and proximal femur fractures [3–5]. The aging demographic will include more than half the population being older than 65 years by 2050 [1,2]. Some percentage of this population will have the need for spinal reconstruction, and systems that improve fixation in patients with compromised bone quality may lower failures and pseudarthroses. Previous attempts at improving the bone-implant interface have included expanding screws and augmentation with bone cement [11–16]. Augmentation with bone cement has been more efficacious in increasing screw pullout strength but does come with some disadvantages. Cement leakage has been documented during in vitro studies, but it is not clear how dangerous these leaks may be. The in situ use of bone cement also may place the patient at risk with exposure to toxic monomers and possible embolic events. Clearly, improving pullout strength while eliminating the use of bone cement would be preferable. The expandable titanium pedicle screw evaluated in this study demonstrated significantly greater ultimate pullout strength (~30%) compared with a standard titanium pedicle screw. In addition, the energy required to cause ultimate
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Table Biomechanical data for each type of pedicle screw Screw type
Stiffness (N/mm)
Yield load (N)
Ultimate load (N)
Failure energy (N mm)
Expandable pedicle screw Mean SD
548.2 349.7
472.1 195.0
839.0 307.6
8020.7 4339.2
Standard pedicle screw Mean SD % Difference ANOVA result
565.2 320.6 3 N/A
377.6 179.3 25 N/A
651.6 275.2 29 p!.05
3030.1 2431.4 165 p!.0001
SD, standard deviation; ANOVA, analysis of variance; N/A, no applicable difference.
failure of the bone-implant interface was ~160% greater for the expandable pedicle screw compared with the standard pedicle screw. Previously, an expandable screw was evaluated for pullout strength as compared with a standard screw [12]. This study found an approximate 30% increase in pullout strength when using the expandable screw, similar to the data found in the present study. However, the previous study compared a 6.35-mm-fixed shank diameter pedicle screw to the expandable screw, which had an outer shank diameter of 6.5 mm (distal tip expansion to 7.5 mm). It is well known that increasing screw diameter markedly improves pullout strength, so the improvements reported in the previous study are somewhat unclear because of differences in fixed shank diameter between screw types [14]. A subsequent study using a similar screw design (four distal fins expanded by placement of an interior pin) evaluated the use of cement for improving fixation strength and reported a significantly greater pullout strength with cement [11]. This improvement was supported in a more recent study that compared a perforated screw with cement augmentation with standard screws and found an
Fig. 4. Representative load-displacement curve demonstrating the significantly increased energy required to fail the bone-implant interface of the expandable screw compared with the standard pedicle screw with similar shaft diameter.
approximate 44% improvement with cement usage. The ultimate energy required to induce failure at the bone-implant interface was not reported. The curve seen for the regular pedicle screw demonstrates that, once the initial fixation is lost, there is nothing else to prevent full screw pullout. However, the expandable screw showed a very similar curve pattern to the regular screw in the initial loading phase. After initial loss of fixation, the expansion region of the screw compacted the dense cortical region of the pedicle-maintaining fixation as determined by the loaddisplacement curve. This continued until the expansion region exited the pedicle. The load point used for the energy calculation was the load at which point permanent loss of fixation was noted. As would be expected, there are several limitations to the present study. The axial pullout benchtop model is valid for predicting the initial stability at the bone-implant interface. However, the long-term stability provided by any type of pedicle screw cannot be well evaluated using an in vitro benchtop model because of a lack of a remodeling response and difficulties simulating the complex three-dimensional loads that the screw or construct experiences. Also, the retrievability of the screw could not be addressed in an in vitro study, again because of a lack of a remodeling response. However, an animal survival study has shown that the bone formation that occurs in or around the expandable screw is similar to that of standard pedicle screws. In addition, screws were collapsible and retrievable at each instrumented lumbar level after a 6-week survival period in an ovine model. Cyclic loading or windshield wipering was not incorporated into the test method, although this technique may better evaluate the short-term stability of the device. This type of loading may be experienced in the postoperative time period but difficult to simulate in vitro as the loading or remodeling aspects cannot be replicated. Static (ramped) loading methods are commonly used to evaluate the bone-implant interface for anterior and posterior spinal constructs for this reason [11–13,18–20]. The number of samples per group was similar to sample sizes previously published [11], although the variability associated with using human cadaveric tissue can limit the power of in vitro studies. Cadaveric testing, whether single-cycle or even short-term cyclic loading, may not be adequate to evaluate
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potential failure zones within the screw, which should be monitored clinically. Finally, it is unclear what changes in stability or stiffness may be created when using an expanded screw in a multilevel construct.
Conclusion An expanded pedicle screw provided significantly greater initial stability than a standard pedicle screw in axial pullout. Potential applications may be in screw/construct revision surgery, in patients with osteoporosis or in areas of the body with poor screw purchase (ie, the sacrum), and subsequent biomechanical or clinical studies should be considered. It is currently unclear if this type of screw may have application at the ends of longer constructs where large cantilever forces compromise the stability of the bone-implant interface. Future biomechanical testing directions of this device should potentially consider evaluating cyclic loading protocols, different directions of loading, and longer constructs in multidirectional physiologic loading (flexion/extension, lateral bending, and axial torsion). Retrospective and prospective clinical studies could be conducted to capture any screw/construct failures, pseudarthroses, or other unforeseen failures as well as capture information across multiple clinical indications, such as osteoporosis, revision surgeries, and extension of longer constructs.
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[5] Healey JH, Vigorita VJ, Lane JM. The coexistence and characteristics of osteoarthritis and osteoporosis. J Bone Joint Surg Am 1985;67: 586–92. [6] Snider RK, Krumwiede NK, Snider LJ, et al. Factors affecting lumbar spinal fusion. J Spinal Disord Tech 1999;12:107–14. [7] McLain RF, McKinley TO, Yerby SA, et al. The effect of bone quality on pedicle screw loading in axial instability. A synthetic model. Spine 1997;22:1454–60. [8] Halverson TL, Kelley LA, Thomas KA, et al. Effects of bone mineral density on pedicle screw fixation. Spine 1994;19:2415–20. [9] Pfeifer BA, Krag MH, Johnson C. Repair of failed transpedicle screw fixation. A biomechanical study comparing polymethylmethacrylate, milled bone, and matchstick bone reconstruction. Spine 1994;19: 350–3. [10] Wittenberg RH, Shea M, Swartz DE, et al. Importance of bone mineral density in instrumented spine fusions. Spine 1991;16:647–52. [11] Cook SD, Salkeld SL, Stanley T, et al. Biomechanical study of pedicle screw fixation in severely osteoporotic bone. Spine J 2004;4: 402–8. [12] Cook SD, Salkeld SL, Whitecloud TS 3rd, et al. Biomechanical evaluation and preliminary clinical experience with an expansive pedicle screw design. J Spinal Disord 2000;13:230–6. [13] Becker S, Chavanne A, Spitaler R, et al. Assessment of different screw augmentation techniques and screw designs in osteoporotic spines. Eur Spine J 2008;17:1462–9. [14] Skinner R, Maybee J, Transfeldt E, et al. Experimental pullout testing and comparison of variables in transpedicular screw fixation. A biomechanical study. Spine 1990;15:195–201. [15] Bai B, Jazrawi LM, Kummer FJ, et al. The use of an injectable, biodegradable calcium phosphate bone substitute for the prophylactic augmentation of osteoporotic vertebrae and the management of vertebral compression fractures. Spine 1999;24:1521–6. [16] Vordemvenne T, Schulte T, Petersen W, et al. The PMMA-augmented pedicle screw anchor: a biomechanical comparison of various augmentation techniques. Munich, Germany: German Spine Congress, 2006. [17] Vishnubhotla S, Mahar AT, Arambula J, et al. Histological and retrievability analysis of an expandable titanium pedicle screw compared to a standard titanium pedicle screw. Presented at 17th Annual International Meeting for Advanced Spine Techniques; Toronto, Canada, Poster #404, 2010. [18] White KK, Oka R, Mahar AT, et al. Pullout strength of thoracic pedicle screw instrumentation: comparison of the transpedicular and extrapedicular techniques. Spine 2006;31:E355–8. [19] Mohamad F, Oka R, Mahar A, et al. Biomechanical comparison of the screw-bone interface: optimization of 1 and 2 screw constructs by varying screw diameter. Spine 2006;31:E535–9. [20] Mahar AT, Brown DS, Oka RS, et al. Biomechanics of cantilever ‘‘plow’’ during anterior thoracic scoliosis correction. Spine J 2006;6:572–6.
Basic Science
A titanium expandable pedicle screw improves initial pullout strength as compared with standard pedicle screws Srilakshmi Vishnubhotla, MSa, William B. McGarry, MSa, Andrew T. Mahar, MSa,b,*, Daniel E. Gelb, MDc a Biomechanical and Clinical Research, Alphatec Spine Inc., 5818 El Camino Real, Carlsbad, CA 92008, USA Department of Orthopaedic Surgery, University of California San Diego, 350 Dickinson St, San Diego 92103, CA, USA c Department of Orthopaedics, University of Maryland Medical Center, 22 S. Greene St, Baltimore, MD 21201-1595, USA b
Received 16 August 2010; revised 5 April 2011; accepted 14 June 2011
Abstract
BACKGROUND CONTEXT: Pedicle screws are now standard for spinal arthrodesis as they provide three-column spinal stabilization. Decreased vertebral body bone density because of aging reduces the stability of the bone-screw interface, potentially increasing screw pullout or pseudarthrosis. Modifications to standard pedicle screw designs to improve screw stabilization may help to compensate for the detrimental effects of decreased vertebral bone density. PURPOSE: To evaluate differences in initial pullout strength of an expandable titanium pedicle screw as compared with a standard titanium pedicle screw. STUDY DESIGN: In vitro human cadaveric biomechanical investigation. METHODS: Fresh thoracolumbar spines from four human cadavers were imaged using quantitative computed tomography to obtain standard lumbar osteoporosis (Dual-energy X-ray absorptiometry [DXA]) T scores. Six bodies were sectioned per spine, and standard titanium 6.5-mm diameter pedicle screws and expandable 6.5-mm diameter titanium screws (maximum expanded diameter510 mm) were randomized to right and left sides. Screw testing, in axial pullout at 25 mm/min, was randomized to reduce the effects of testing order. Data for stiffness (N/mm), yield load (N), ultimate load (N), and energy (N mm) (area under the load-displacement curve) were analyzed using a one-way analysis of variance (p!.05). RESULTS: Lumbar DXA scores averaged 3.6. There were no statistical differences between screw types for stiffness. Yield load was not statistically different between groups, although the expandable screw yield load was nearly 25% greater than that of the standard screw. Ultimate load was found to be statistically greater (~30%) for the expandable screw compared with the standard screw (p!.05). The energy required to cause bone-implant failure was also statistically greater for the expandable screw compared with the standard screw (p!.0001). CONCLUSIONS: Expandable titanium pedicle screws demonstrated improved screw pullout stability compared with standard titanium screws in osteopenic or osteoporotic bone. Further studies are warranted examining other loading methods to evaluate the stability provided by an expandable pedicle screw. Ó 2011 Elsevier Inc. All rights reserved.
Keywords:
Expandable titanium pedicle screw; Osteoporosis; Spinal fusion; Biomechanical stability
FDA device/drug status: Approved (Zodiac pedicle screw system). Not approved for this use (Osseoscrew pedicle screw system). Author disclosures: SV: Stock Ownership: Alphatec Spine, Inc. (5,000 shares); Trips/Travel: Alphatec Spine, Inc. (Nonfinancial); Other Office: Alphatec Spine, Inc. (Salary). WBM: Stock Ownership: Alphatec Spine (2,000 shares, 1%); Other Office: Alphatec Spine (Salary). ATM: Stock Ownership: Alphatec Spine (2,500 shares, 0%); Consulting: Alphatec Spine (Salary); Trips/Travel: Alphatec Spine (Financial). DEG: Royalties: Globus Medical (B), Synthes Spine (B); Private Investments: ASIP (0 shares, 6%); Consulting: Synthes Spine (C), Alphatec Spine (B); Speaking Teaching Arrangements: AO Spine North America (C); Fellowship Support: Synthes Spine (E, Paid directly to institution). 1529-9430/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2011.06.006
The disclosure key can be found on the Table of Contents and at www. TheSpineJournalOnline.com. This study was funded by Alphatec Spine. The authors disclose a financial relationship associated with the product discussed in this manuscript. The information contained in this manuscript was presented as an e-poster at the 2009 North American Spine Society meeting in San Francisco. * Corresponding author. Biomechanical and Clinical Research, Alphatec Spine Inc., 5818 El Camino Real, Carlsbad, CA 92008, USA. Tel.: (760) 494-6651; fax: (760) 493-9132. E-mail address: [email protected] (A.T. Mahar)
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S. Vishnubhotla et al. / The Spine Journal 11 (2011) 777–781
Introduction
Methods
It is well known that the demographics of aging are shifting with an expected 54% of people older than 65 years by 2050 [1,2]. This trend has been associated with an increase in orthopedic-related diseases, such as osteoporotic vertebral compression fractures and degenerative scoliosis [3–5]. It has also been shown that patients treated for degenerative scoliosis may have further deformity progression because of the aging process [3]. Despite surgical stabilization of degenerative scoliosis, patients may remain symptomatic because of pseudarthrosis and possibly curve progression. Curve progression, instrumentation failure, and pseudarthrosis may all be related to poor fixation of the pedicle screw bone-implant interface as fixation strength has been shown to be affected by bone density [6–10]. Attempts at improving the bone-implant interface strength of pedicle screws have focused on screw expansion or screw augmentation [11–16]. These modifications have generally shown encouraging in vitro results. Screw augmentation has demonstrated improved fixation strength when compared with expansion [11]. However, in vitro cement leakage was noted using perforated screws [13]. Cement technique and handling were both cited as critical to achieve improvements in strength [11,13]. Previous designs of expanding screws have demonstrated only moderate improvements in fixation strength, but the screw itself only expanded by 1.5 mm at the very distal portion of the screw [11]. The bone quality in the anterior vertebral body may not be sufficient to provide additional strength beyond the purchase achieved within the pedicle. The smallest diameter screw used was 7.0 mm, obviating placement in the upper thoracic spine or within smaller vertebral bodies. An expandable titanium screw was recently developed that increases in diameter by approximately 50% to address issues of bone fixation strength. The expansion region on the expandable titanium screw is located at the vertebral body–pedicle junction. The purpose of this study was to evaluate the differences in initial biomechanical stability of this expandable titanium pedicle screw as compared with a standard titanium pedicle screw when placed in osteopenic/osteoporotic vertebral bodies.
Brief description of surgical technique Anatomic preparation for insertion of pedicle screws is completed via typical surgical techniques. Once prepared, a placement probe may be used to visualize the location of the center (maximum diameter) of the expansion region of the screw (Fig. 1, Left). The probe also contains cutting flutes so that further anatomic preparation can be completed at the surgeon discretion to ensure the ball of the probe (and thus the center of the expansion region) is anterior to the pedicle and not within the pedicle itself. After probe location verification, screws may then be placed. An actuation handle and actuation rod are then fixed to the driver handle. The actuation rod fixes to a thread within the cannulated tip of the screw. The actuation handle is used to pull the front of the screw, via the actuation rod, posteriorly to expand the commercially pure titanium ‘‘skin’’ over the titanium alloy inner component. Fluoroscopic imaging can be used to visualize each phase, and the expanded screw is visible via fluoroscopy (Fig. 1, Middle and Right). If removal or repositioning is desired, a separate removal instrument with the actuation rod may be used to fix to the distal tip, and the actuation rod is reversed to collapse or undeploy and remove the screw. With regard to bone ingrowth/ongrowth limiting the ability to remove a screw, a previously presented animal survival study indicated similar bone formation in or on the screw as compared with standard pedicle screws [17]. In addition, the same study confirmed that the screw could be collapsed and retrieved without issue after a 6-week survival period in an ovine model.
Biomechanical testing Four fresh thoracolumbar (T12–L5) human cadaveric spines were used for this study. Six vertebral bodies per spine were disarticulated from each spine and reviewed for previous fractures, deformity, or instrumentation. One vertebral body was noted to have a Type B vertebral compression fracture and was eliminated from the study. The spines had previously been measured for bone density using quantitative computed tomography (General Electric HiSpeed CT/i with QCT-5000 Bone Densitometry Software; General Electric
Fig. 1. (Left) Placement of the locating probe demonstrates location of maximum expansion, and cutting flutes may be used to advance the probe to the desired location anterior to the pedicle, (Middle) lateral fluoroscopy may be used to visualize screw placement and expansion region, and (Right) anteroposterior fluoroscopy also demonstrates screw expansion.
S. Vishnubhotla et al. / The Spine Journal 11 (2011) 777–781
Healthcare, Waukesha, WI, USA). The lumbar vertebral bodies from each spine were scanned to calculate a World Health Organization level of osteoporosis T-score using standard techniques. Expanding 6.5-mm titanium pedicle screws (Osseoscrew; Alphatec Spine, Carlsbad, CA, USA) and standard 6.5-mm titanium pedicle screws (Zodiac; Alphatec Spine) were randomly assigned to right and left sides for each vertebral body. For the expansion screws, the diameter of the shaft at the expansion region increased from 6.5 to 10 mm (Fig. 2). The expandable screws are cannulated to allow for an actuation rod to expand the screw. The anterior portion of the individual vertebral bodies was potted in two-part epoxy resin, making sure that the potting material did not come in contact with either screw. Screws were pulled under displacement control at a rate of 25 mm/minute in line with the screw axis using a servohydraulic load frame (MTS858; MTS Systems Corp., Eden Prairie, MN, USA) (Fig. 3). Tests were stopped when the screw was completely pulled out of the vertebral body or after a failure of bone (pedicle fracture) occurred leading to a rapid drop in the load. Screws were tested in a random order of right and left sides to lower potential confounding effects related to order of testing. Stiffness was calculated from the linear portion of the load-displacement curve. Yield load was identified as the load at which the loaddisplacement curve diverged from its initial linearity. Ultimate load was calculated as the largest load experienced during the test. Failure energy was calculated as the area under the curve identified by the point of permanent loss of fixation. Data for stiffness (N/mm), yield load (N), ultimate load (N), and energy required for failure (N mm) (area under the load-displacement curve) were compared between groups using a one-way analysis of variance (p!.05).
Fig. 2. Insertion of (Top) standard 6.5-mm titanium pedicle screw and (Bottom) expandable titanium pedicle screw.
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Fig. 3. Pullout test setup.
Results The average lumbar bone density T score for the four specimens was 3.6 (samples: 2.9, 61-year-old woman; 2.3, 61-year-old man; 4.3, 74-year-old woman; and 4.7, 80-year-old woman). There were no statistical differences between screw types for stiffness of the bone-implant interface or for yield load (Table). Ultimate load was found to be statistically greater (~30%) for the expandable screw compared with the standard screw (p!.05). The energy required to cause ultimate failure of the bone-implant interface for the expandable pedicle screw was statistically greater than the energy required to fail the bone-implant interface for the standard pedicle screw (p!.0001) (Fig. 4).
Discussion Osteoporosis is a leading causative factor in orthopedicrelated fractures, such as vertebral compression fractures and proximal femur fractures [3–5]. The aging demographic will include more than half the population being older than 65 years by 2050 [1,2]. Some percentage of this population will have the need for spinal reconstruction, and systems that improve fixation in patients with compromised bone quality may lower failures and pseudarthroses. Previous attempts at improving the bone-implant interface have included expanding screws and augmentation with bone cement [11–16]. Augmentation with bone cement has been more efficacious in increasing screw pullout strength but does come with some disadvantages. Cement leakage has been documented during in vitro studies, but it is not clear how dangerous these leaks may be. The in situ use of bone cement also may place the patient at risk with exposure to toxic monomers and possible embolic events. Clearly, improving pullout strength while eliminating the use of bone cement would be preferable. The expandable titanium pedicle screw evaluated in this study demonstrated significantly greater ultimate pullout strength (~30%) compared with a standard titanium pedicle screw. In addition, the energy required to cause ultimate
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Table Biomechanical data for each type of pedicle screw Screw type
Stiffness (N/mm)
Yield load (N)
Ultimate load (N)
Failure energy (N mm)
Expandable pedicle screw Mean SD
548.2 349.7
472.1 195.0
839.0 307.6
8020.7 4339.2
Standard pedicle screw Mean SD % Difference ANOVA result
565.2 320.6 3 N/A
377.6 179.3 25 N/A
651.6 275.2 29 p!.05
3030.1 2431.4 165 p!.0001
SD, standard deviation; ANOVA, analysis of variance; N/A, no applicable difference.
failure of the bone-implant interface was ~160% greater for the expandable pedicle screw compared with the standard pedicle screw. Previously, an expandable screw was evaluated for pullout strength as compared with a standard screw [12]. This study found an approximate 30% increase in pullout strength when using the expandable screw, similar to the data found in the present study. However, the previous study compared a 6.35-mm-fixed shank diameter pedicle screw to the expandable screw, which had an outer shank diameter of 6.5 mm (distal tip expansion to 7.5 mm). It is well known that increasing screw diameter markedly improves pullout strength, so the improvements reported in the previous study are somewhat unclear because of differences in fixed shank diameter between screw types [14]. A subsequent study using a similar screw design (four distal fins expanded by placement of an interior pin) evaluated the use of cement for improving fixation strength and reported a significantly greater pullout strength with cement [11]. This improvement was supported in a more recent study that compared a perforated screw with cement augmentation with standard screws and found an
Fig. 4. Representative load-displacement curve demonstrating the significantly increased energy required to fail the bone-implant interface of the expandable screw compared with the standard pedicle screw with similar shaft diameter.
approximate 44% improvement with cement usage. The ultimate energy required to induce failure at the bone-implant interface was not reported. The curve seen for the regular pedicle screw demonstrates that, once the initial fixation is lost, there is nothing else to prevent full screw pullout. However, the expandable screw showed a very similar curve pattern to the regular screw in the initial loading phase. After initial loss of fixation, the expansion region of the screw compacted the dense cortical region of the pedicle-maintaining fixation as determined by the loaddisplacement curve. This continued until the expansion region exited the pedicle. The load point used for the energy calculation was the load at which point permanent loss of fixation was noted. As would be expected, there are several limitations to the present study. The axial pullout benchtop model is valid for predicting the initial stability at the bone-implant interface. However, the long-term stability provided by any type of pedicle screw cannot be well evaluated using an in vitro benchtop model because of a lack of a remodeling response and difficulties simulating the complex three-dimensional loads that the screw or construct experiences. Also, the retrievability of the screw could not be addressed in an in vitro study, again because of a lack of a remodeling response. However, an animal survival study has shown that the bone formation that occurs in or around the expandable screw is similar to that of standard pedicle screws. In addition, screws were collapsible and retrievable at each instrumented lumbar level after a 6-week survival period in an ovine model. Cyclic loading or windshield wipering was not incorporated into the test method, although this technique may better evaluate the short-term stability of the device. This type of loading may be experienced in the postoperative time period but difficult to simulate in vitro as the loading or remodeling aspects cannot be replicated. Static (ramped) loading methods are commonly used to evaluate the bone-implant interface for anterior and posterior spinal constructs for this reason [11–13,18–20]. The number of samples per group was similar to sample sizes previously published [11], although the variability associated with using human cadaveric tissue can limit the power of in vitro studies. Cadaveric testing, whether single-cycle or even short-term cyclic loading, may not be adequate to evaluate
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potential failure zones within the screw, which should be monitored clinically. Finally, it is unclear what changes in stability or stiffness may be created when using an expanded screw in a multilevel construct.
Conclusion An expanded pedicle screw provided significantly greater initial stability than a standard pedicle screw in axial pullout. Potential applications may be in screw/construct revision surgery, in patients with osteoporosis or in areas of the body with poor screw purchase (ie, the sacrum), and subsequent biomechanical or clinical studies should be considered. It is currently unclear if this type of screw may have application at the ends of longer constructs where large cantilever forces compromise the stability of the bone-implant interface. Future biomechanical testing directions of this device should potentially consider evaluating cyclic loading protocols, different directions of loading, and longer constructs in multidirectional physiologic loading (flexion/extension, lateral bending, and axial torsion). Retrospective and prospective clinical studies could be conducted to capture any screw/construct failures, pseudarthroses, or other unforeseen failures as well as capture information across multiple clinical indications, such as osteoporosis, revision surgeries, and extension of longer constructs.
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