 Research article
 Open Access
 Open Peer Review
Comparison and prediction of pullout strength of conical and cylindrical pedicle screws within synthetic bone
 WenChi Tsai^{1, 2},
 PoQuang Chen^{3},
 TungWu Lu^{1},
 ShingSheng Wu^{2},
 KaoShang Shih^{1, 4} and
 ShangChih Lin^{5, 6}Email author
https://doi.org/10.1186/147124741044
© Tsai et al; licensee BioMed Central Ltd. 2009
 Received: 28 August 2008
 Accepted: 30 April 2009
 Published: 30 April 2009
Abstract
Background
This study was designed to derive the theoretical formulae to predict the pullout strength of pedicle screws with an inconstant outer and/or inner diameter distribution (conical screws). For the transpedicular fixation, one of the failure modes is the screw loosening from the vertebral bone. Hence, various kinds of pedicle screws have been evaluated to measure the pullout strength using synthetic and cadaveric bone as specimens. In the literature, the Chapman's formula has been widely proposed to predict the pullout strength of screws with constant outer and inner diameters (cylindrical screws).
Methods
This study formulated the pullout strength of the conical and cylindrical screws as the functions of material, screw, and surgery factors. The predicted pullout strength of each screw was compared to the experimentally measured data. Synthetic bones were used to standardize the material properties of the specimen and provide observation of the loosening mechanism of the bone/screw construct.
Results
The predicted data from the new formulae were better correlated with the mean pullout strength of both the cylindrical and conical screws within an average error of 5.0% and R ^{2} = 0.93. On the other hand, the average error and R ^{2} value of the literature formula were as high as 32.3% and 0.26, respectively.
Conclusion
The pullout strength of the pedicle screws was the functions of bone strength, screw design, and pilot hole. The close correlation between the measured and predicted pullout strength validated the value of the new formulae, so as avoid repeating experimental tests.
Keywords
 Pedicle Screw
 Polyurethane Foam
 Pullout Strength
 Screw Insertion
 Bone Chip
Background
Transpedicular screw fixation has been extensively used for the treatment of instability due to degenerative disorders, trauma, tumor metastasis, and deformity correction. It provides immediate stability, enhances bony fusion, corrects deformity, and preserves the anatomic profile. However, breakage and the loosening of pedicle screws from the vertebral bone are two main clinical concerns [1].
In the literature, synthetic bones were selected for the measurement of the screw resistance to loosening [2–16]. Chapman et al. [4] used an analytical formula to predict the pullout strength of cancellous and/or cortical screws inserted into the synthetic bone. The Chapman's formula was confirmed by some reports to be valuable because of the close correlation between the predicted and the measured pullout strength [3, 4, 7, 11, 17]. However, pedicle screws of the spine usually have different profiles. The Chapman's formula was derived for the cancellous/cortical screw, and the thread design was different from the pedicle screw with conically distributed inner and/or outer diameters. Furthermore, the effects of the bone removal by predrilling a pilot hole and subsequently squeezing the bone chip into the thread surroundings were not considered in the Chapman's formula.
The purposes of the current study were threefold. Firstly, the pullout strength of six cylindrical and conical pedicle screws within the synthetic bone were measured and compared. Then, the measured pullout strength data were tested to prove the accuracy of the Chapman's formula in predicting the pullout strength of various pedicle screws. Finally, special emphasis was put on deriving a new formula that takes into consideration the effects of the pilot hole and the squeezed bone chip at the thread surroundings.
Methods
The design parameters and the equivalent diameter functions of the six pedicle screws.
Pedicle Screw  Threaded Length  Outer Diameter  Inner Diameter  Screw Pitch  Equivalent Diameter Function (0≤x≤30) 

UPS4  37  6.0 (0≤ x≤ 37)  3.8 (x = 0) 5.4 (x = 30)  2.4  D _{ o } (x) = 6.0

UPS3  37  6.0 (0≤ x≤ 37)  4.0 (0≤ x≤ 37)  2.5  D _{ o } (x) = 6.0 D _{ i } (x) = 4.0 
Diapason  40  4.6 (x = 0) 6.0 (x = 30)  2.6 (x = 0) 5.0 (x = 30)  1.75 

HCD  40  6.0 (0≤ x≤ 40)  4.2 (0≤ x≤ 30) 5.6 (x = 40)  2.7  D _{ o } (x) = 6.0 D _{ i } (x) = 4.2 
CCD  30  6.0 (0≤ x≤ 30)  4.5 (0≤ x≤ 30)  2.8  D _{ o } (x) = 6.0 D _{ i } (x) = 2.8 
MossMiami  35.5  6.0 (0≤ x≤ 35.5)  4.0 (0≤ x≤ 35.5)  2.9  D _{ o } (x) = 6.0 D _{ i } (x) = 2.9 
Synthetic bone made from polyurethane foam was used as the testing specimen for its consistent and homogeneous structural properties (Wuzhou Co, Taipei, Taiwan). The screw was inserted perpendicularly into the testing block following predrill pilot holes preparation. Pilot holes were prepared with a 3.2mm drill bit, which was smaller than the inner diameter of the screw at the screw tip, except for the Diapason. All screws were engaged into the synthetic bones with a consistent 30mm thread length. Tapping was not used during screw insertion.
The symbols in the formula were respectively denoted as: F _{ pullout }= predicted pullout strength (N), S _{ shear }= ultimate shear strength of synthetic bone (MPa), L = length of thread engagement in synthetic bone (mm), D _{ o }= thread outer diameter (mm), d = thread depth (mm), and p = thread pitch (mm). The equivalent crosssectional area of the Chapman's formula equals and, that is, the thread shape factor is .
The definition of the symbol is respectively as: D _{ o } (x) = outerdiameter function along the screw shaft (mm), D _{ i } (x) = innerdiameter function along the screw shaft (mm), and d _{ p }= diameter of the pilot hole (mm). For the six screws used in this study, the functions of D _{ o } (x) and D _{ i } (x) within 0 ≤ x ≤ 30 mm are given in Table 1, and schematically shown in Figure 3.
The terms D _{ oequ }and D _{ iequ }were respectively the equivalent outer and inner diameters. The symbol d _{ equ }was the equivalent thread depth (mm). The term of the modified Chapman's formula was the factor that incorporated the effects of the ratio of the inner and outer diameters, squeezed bone mass, and pilot hole into calculating pullout strength. In this study, within the 30mm engaged depth, the inner core and outer thread peaks of UPS3, CCD, HCD, and MossMiami have nearly constant diameters (Figure 1 and Table 1). Hence, the equivalent outer and inner diameters are constant along the screw shaft. However, the equivalent outer and inner diameters of the conical screws, UPS4 and Diapason, are calculated as: D _{ oequ }= D _{ o }(x) and D _{ iequ }= D _{ i }(x) for three cases: x = 7.5 mm, 15.0 mm, and 22.5 mm. According to the ASTM testing standard [19], the shear strength and the material constant of the polyurethane foam used in this study were experimentally evaluated and have the value of 290 MPa and 0.84, respectively. All the predicted pullout strengths using the Chapman's, the Integral, and the modified Chapman's formulae were calculated by the software Mathematica, Ed. 5 (Wolfram Research, Champaign, IL).
For pullout tests, each pedicle screw was tested six times, and the mean and one standard deviation were calculated. The statistical analyses were performed using analysis of variance with multiple comparisons between groups (StudentNewmanKeuls test) to test the significance of the variation between six pedicle screws. A p value less than 0.05 was considered to be statistically significant. For the analytical formulae, the coefficient of determination, R ^{2}, between the measured and predicted pullout strengths was calculated to check their correlation.
Results
Measured Pullout Strength
The measured and predicted pullout strengths of the six pedicle screws.
Group  Pullout Strength (mean value ± one standard deviation)  R ^{2} Value  

UPS4  UPS3  Diapason  HCD  CCD  MossMiami  
Experiment  1904 ± 72  1600 ± 53  1568 ± 46  1583 ± 59  1610 ± 33  1570 ± 100    
Chapman's Formula  1/4L  1175  1198  1067  1035  1073  1146  0.40 
1/2L  1096  1198  1082  1035  1073  1146  0.26  
3/4L  1017  1198  1090  1035  1073  1146  0.71  
Integral Formula  1861  1483  1411  1509  1636  1481  0.93  
Modified Chapman's Formula  1/4L  1561  1482  1067  1509  1622  1418  0.38 
1/2L  1742  1482  1356  1509  1622  1418  0.83  
3/4L  2079  1482  1725  1509  1622  1418  0.88 
Table 2 lists the measured and predicted pullout strengths of six pedicle screws using three formulae in three equivalent diameter cases. The prediction error for each screw between the experimental and the Chapman's data was 38.3% (UPS4), 25.1% (UPS3), 40.0% (Diapason), 34.6% (HCD), 33.4% (CCD), 27.0% (Moss Miami) for the x = 7.5 mm case, 42.4% (UPS4), 25.1% (UPS3), 31.0% (Diapason), 34.6% (HCD), 33.4% (CCD), 27.0% (Moss Miami) for the x = 15.0 mm case, and 46.6% (UPS4), 25.1% (UPS3), 30.5% (Diapason), 34.6% (HCD), 33.4% (CCD), 27.0% (Moss Miami) for the x = 22.5 mm case. The minus values of the prediction error revealed that the Chapman's formula underestimates the pullout strength of even the cylindrical pedicle screw.
Predicted Pullout Strength
For the screws with constant inner and outer diameters within the engaged length, such as the UPS3, HCD, CCD, and MossMiami, the predicted pullout strengths using the modified Chapman's and Integral formulae was the same as that shown in Table 2 and Figure 5. For the conical screw, such as the UPS4 and Diapason, the Integral formula prediction had a more accurate result than that of the modified Chapman's formula.
Discussion
In a report by Esses et al. [20], of the 617 cases using various pedicle screws, the rate of screw breakage was 2.9%, and that of screw loosening was 0.8%. Yuan et al. [21] reported that screw breakage was observed in 2.6% and screw loosening in 2.8% of the 2153 patients treated for degenerative spondylolisthesis. The rate of screw breakage ranged from 6% to 21%, while the loosening rate ranged from 18% to 27% [1, 21–23]. It is mandatory that the implants are rigid enough for fixation of the spinal segments. However, if screw loosening or breakage occurs before rigid fusion is achieved; it will increase the likelihood of nonunion with subsequent morbidity. Therefore, some kinds of predictive guidelines to assure the pullout strength of pedicle screws are necessary before fabrication and clinical usage.
Synthetic bone has been extensively used in the biomechanical evaluation of bone screws [2–16]. In this study, the advantageously consistent property of the solid polyurethane foam was reflected in the relatively small standard deviations in pullout strength, which were less than 7% of mean values (Table 2). The continuous bone chips stripped off the screw thread provide clear observation and insight into the loosening mechanism of the bone/screw construct (Figure 4). In this study, the length of the threaded portion was different for six pedicle screws (Figure 1). During testing, the engaged length of all screws within the synthetic bone was consistently 30 mm. The measured pullout strength in this study cannot be directly used as a basis for comparison of the resistance to loosening of the six pedicle screws fully immersed in the vertebral bone. The experimental comparison in pullout strength of the six pedicle screws was only to validate the reported mathematical formula and provided a basis for deriving a new formula with more accurate prediction in the early stage of screw design.
The prerequisite of the Chapman's formula is that the screw is pulled out from the synthetic bone along an ideally cylindrical surface. This assumption is straightforward, as shown in Figure 4. The outer surface of the bone chip peeled off the UPS4, UPS3, HCD, CCD, and MossMiami screws formed a spiral with a cylindrical outer diameter. For the Diapason screw, the spiral of bone chip has the appearance of linearly variable outer and inner diameters. However, the pullout strength in the Chapman prediction was proportional to the crosssectional area of the assumed shearing surface. Hence, the discrepancy between the experimental observation and the Chapman's assumption about the tearing surface of the bone/screw construct may induce a prediction error for the pullout strength of conical screws, such as the Diapason.
Table 2 lists the measured and predicted pullout strengths of six pedicle screws using three formulae in three equivalent diameter cases. Even for the UPS3, HCD, CCD, and MossMiami screws with a cylindrical chip spiral, the average prediction error of the three cases (x = 7.5 mm, 15.0 mm, and 22.5 mm) was still around 30%. The average prediction error of the UPS4 screw with cylindrical chip spiral is even up to 42.4%. These findings regarding cylindrical pedicle screws differed from those of previous reports, which claimed that a strong correlation did exist between the Chapman prediction and experimental data [3, 4, 7, 11, 17]. Hence, the factor of assuming the tearing surface between the bone and screw, alone, cannot account for the quite large inaccuracy of the Chapman prediction. Consequently, the ratio of inner to outer diameter and the effect of the squeezed bone chip and pilot hole were taken into consideration to improve the prediction accuracy of the analytical formula.
Figure 3 shows that the Integral formula assumes the tearing surface of the bone/screw construct occurring at the surface formed by the thread peaks along the screw shaft. This is consistent with the bone chip spirals as shown in Figure 4. As aforementioned, the modifications in the geometry of the tearing bone chip alone may not accurately predict the pullout strength of various bone screws. Hence, the term in the Integral formula was added to incorporate the effects of the pilot hole, squeezed bone chip, and ratio of inner to outer diameter into the prediction of pullout strength. The exponent b in the Integral formula was denoted as a material factor in response to the screw insertioninduced change in shear strength of the synthetic bone. The pilothole factor, d _{ p }, was used to consider the removal of the synthetic bone during predrilling of the pilot hole (Figure 3). The simplification of the base expression, , resulted in another thread shape factor , which is the ratio of the thread inner to outer diameter. In the modified Chapman's formula, the term was further simplified to be , which can be interpreted as a thread ratio, pilot hole, and materialinduced factor of the Chapman's formula. For the cylindrical screw, the Integral and modified Chapman's formulae were theoretically identical to each other.
In the literature, the pullout strength of the bone screw has been biomechanically proven to be a function of the screw design (outer/inner diameter, thread pitch, flank angle, threaded length, cutting flute, and cannulated/noncannulated), screw orientation, screwinsertion depth, bonemineral density, pilot hole, bone morphology, surface coating, and loading type [1–7, 13, 15, 16, 23–25]. In general, those studies revealed that 1) outer diameter is an important determinant of pullout strength in a roughly linear manner, 2) pitch is important with a finer thread giving greater purchase, 3) flank angles significantly affect the holding power of the inserted screw, 4) inner diameter and the ratio of inner to outer diameter has a small but significant effect on the pullout strength, and 5) the pilot hole has a significant influence on the pullout strength with nonpilot hole groups, resulting in higher holding power. However, the Chapman's formula was derived for the cylindrically cancellous/cortical screw with the triangular thread shape. This research work was focused on the modification of the Chapman's formula, and, only the effects of tapering profile, predrilling hole, and squeezing chip were formulated to predict the pullout strengths of the six varities of pedicle screws.
Cutting flute at the screw tip facilitates insertion, but this region of the screw has less pullout strength than the fullythreaded region. Theoretically, the shear strength, , of the squeezed bone chip at the thread surroundings is closely related to the tapping function of cutting flute. Yerby et al. [27] have also biomechanically shown that cutting a flute design significantly influences the mean insertion torque and pullout strength. However, the effect of this cuttingflute factor on the shear strength of synthetic bone in the region CDE was complicated and not considered in the new formulae (Figure 3).
In the clinical situation, the inserted regions of the pedicle screw include the vertebral body and the posterior element (pedicle). The vertebral body consists of the cancellous bone with the porous structure in nature. The structure of the posterior element is denser and stiffer than that of the vertebral body. Consequently, the polyurethane foam used in this study was to simulate the hybrid of the vertebral bone and posterior element. In addition, the Asnis's formula cited in this study is more suitable for the polyurethane foam with the lower porosity than the cancellous bone. The applicability of the new formula should be further investigated for predicting the pullout strength of the inserted screw within the cancellous bone. For example, the ultimate shear strength of the squeezing effect bone chips with the higher porosity might be reformulated.
In the literature, a great number of studies have attempted to show experimentally that the conical design of the screw profile increases the pullout strength, and the increasing degree depends on the test medium and design [23, 26–28]. The current testing study also demonstrated that the conicalshaped UPS4 screw had higher pullout strength than its cylindricalshaped counterpart, UPS3, (Table 2 and Figure 3). However, except for the UPS4 screw, there was no significant difference in pullout strength between the Diapason and the other cylindricalshaped screws. The Integral and modified Chapman's formulae also predicted similar results (Table 2 and Figure 5). This meant that the pullout strength of the pedicle screw was the result of a number of varying parameters, not only the conical and cylindricalshaped profile. The isolation of the related parameters was necessary to study the influence of one particular parameter on the pullout strength of the commercially available screws.
The coefficient of determination between the measured and predicted pullout strength has been used as an indicator to confirm the accuracy of the Chapman's formula [3, 6, 7, 11]. However, as shown in Table 2, the R ^{2} values between the predicted and measured pullout for three cases (x = 7.5 mm, 15.0 mm, and 22.5 mm) were 0.40, 0.26, and 0.71, respectively. By contrast, the 0.83 and 0.88 of the R ^{2} value in the x = 15.0 mm and 22.5 mm cases proved that the modified Chapman's formula was quite well correlated with the experimental data. In particular, the predicted result using the Integral formula had the highest R ^{2} value (= 0.93). For the cylindrical screws, such as the UPS3, HCD, CCD, and MossMiami, the predicted pullout strengths by the modified Chapman's and Integral formulae were the same as shown in Table 2 and Figure 5. For the conical screws, such as the UPS4 and Diapason, the Integral formula had the best predicted value.
Conclusion
This study was designed to derive the analytical formula for predicting the pullout strength of both conical and cylindrical pedicle screws. The new formula is a function of material (shear strength of synthetic bone), screw (diameter and pitch), and surgery (pilot hole) factors. The strong correlation between the measured and the predicted pullout strength validated the value of the new formula. The usage of the new formulae can eliminate the need for costly and timeconsuming repeated mechanical testing. However, the newly derived formulae were only validated by the synthetic bones. In the future, the detailed investigation and validation about the screwbone interfaces should be studied by the finiteelement method and biomechanical evaluation using cadaver specimens.
Declarations
Authors’ Affiliations
References
 Zdero R, Rose S, Schemitsch EH, Papini M: Cortical screw pullout strength and effective shear stress in synthetic third generation composite femurs. J Biomech Eng. 2007, 129: 289293. 10.1115/1.2540926.PubMedGoogle Scholar
 Ansell RH, Scales JT: A study of some factors which affect the strength of screws and their insertion and holding power in bone. J Biomech. 1986, 1: 279302. 10.1016/00219290(68)900237.View ArticleGoogle Scholar
 Asnis SE, Ernberg JJ, Bostrom MP, Wright TM, Harrington RM, Tencer AF, Peterson M: Cancellous bone screw thread design and holding power. J Orthop Trauma. 1996, 10: 462469. 10.1097/0000513119961000000003.View ArticlePubMedGoogle Scholar
 Chapman JR, Harrington RM, Lee KM, Anderson PA, Tencer AF, Kowalski D: Factors affecting the pullout strength of cancellous bone screws. J Biomech Eng. 1996, 118: 391398. 10.1115/1.2796022.View ArticlePubMedGoogle Scholar
 Steeves M, Stone C, Mogaard J, Byrne S: How pilothole size affects bonescrew pullout strength in human cadaveric cancellous bone. Canad J Surg. 2005, 48 (3): 207212.PubMedPubMed CentralGoogle Scholar
 Cleek TM, Reynolds KJ, Hearn TC: Effect of screw torque level on cortical bone pullout strength. J Orthop Trauma. 2007, 21 (2): 117123. 10.1097/BOT.0b013e318030261e.View ArticlePubMedGoogle Scholar
 Firoozbakhsh KK, DeCoster TA, Moneim MS: Effect of cyclical loading on the holding power of surgical screws. Orthopedics. 1994, 17: 607611.PubMedGoogle Scholar
 Gausepohl T, Mohring R, Pennig D, Koebke J: Fine thread versus coarse thread. A comparison of the maximum holding power. Injury. 2001, 32 (Suppl 4): 17. 10.1016/S00201383(01)001681.View ArticleGoogle Scholar
 Heidemann W, Gerlach KL, Gröbel KH, Köllner HG: Influence of different pilot hole sizes on torque measurements and pullout analysis of osteosynthesis screws. J CranioMaxilloFac Surg. 1998, 26: 5055.View ArticlePubMedGoogle Scholar
 Zhang QH, Tan SH, Chou SM: Effects of bone materials on the screw pullout strength in human spine. Med Eng Phy. 2006, 28 (8): 795801. 10.1016/j.medengphy.2005.11.009.View ArticleGoogle Scholar
 Lin J, Hou SM: Bending strength and holding power of a prototype tibial locking screw. Clin Orthop. 2002, 403: 232239. 10.1097/0000308620021000000034.View ArticlePubMedGoogle Scholar
 Murphy TP, Hill CM, Kapatkin AS, Radin A, Shofer FS, Smith GK: Pullout properties of 3.5mm AO/ASIF selftapping and cortex screws in a uniform synthetic material and in canine bone. Vet Surg. 2001, 30: 253260. 10.1053/jvet.2001.23344.View ArticlePubMedGoogle Scholar
 Carmouche JJ, Molinari RW, Gerlinger T, Devine J, Patience T: Effects of pilot hole preparation technique on pedicle screw fixation in different regions of the osteoporotic thoracic and lumbar spine. J Neurosurg Spine. 2005, 3 (5): 364370. 10.3171/spi.2005.3.5.0364.View ArticlePubMedGoogle Scholar
 Okuyama K, Abe E, Suzuki T, Tamura Y, Chiba M, Sato K: Can insertional torque predict screw loosening and related failures? An in vivo study of pedicle screw fixation augmenting posterior lumbar interbody fusion. Spine. 2000, 25: 858864. 10.1097/0000763220000401000015.View ArticlePubMedGoogle Scholar
 Thompson JD, Benjamin JB, Szivek JA: Pullout strengths of cannulated and noncannulated cancellous bone screws. Clin Orthop. 1997, 341: 241249.View ArticlePubMedGoogle Scholar
 Van LP, Peter B, Beluze L, Lemaître J: Reinforcement of osteosynthesis screws with brushite cement. Bone. 1999, 25: 95S98S. 10.1016/S87563282(99)001428.View ArticleGoogle Scholar
 Brown GA, McCarthy T, Bourgeault CA, Callahan DJ: Mechanical performance of standard and cannulated 4.0mm cancellous bone screws. J Orthop Res. 2000, 18: 307312. 10.1002/jor.1100180220.View ArticlePubMedGoogle Scholar
 American Society for Testing Materials, Standard F1691–96:Standard Test Method for Determining Axial PullOut Strength of Metallic Bone Screws. Philadelphia, PA, USA.Google Scholar
 American Society for Testing Materials, Standard F1839–08:Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments. Philadelphia, PA, USA.Google Scholar
 Hsu CC, Chao CK, Wang JL, Hou SM, Tsai YT, Lin J: Increase of pullout strength of spinal pedicle screws with conical core: biomechanical tests and finite element analyses. J Orthop Res. 2005, 23 (4): 788794. 10.1016/j.orthres.2004.11.002.View ArticlePubMedGoogle Scholar
 Yuan HA, Oarfin SR, Dickman CA, Merdjetko SM: A historical cohort study of pedicle screw fixation in thoracic, lumbar, and sacral spinal fusions. Spine. 1994, 19: 2279S2296S. 10.1097/0000763219941015100005.View ArticlePubMedGoogle Scholar
 Ohlin A, Karlsson M, Duppe H, Hasserius R, RedlundJohnell I: Complications after transpedicular stabilization of the spine. A survivorship analysis of 163 cases. Spine. 1994, 19: 27742779. 10.1097/0000763219941215000007.View ArticlePubMedGoogle Scholar
 Ono A, Brown MD, Latta LL, Milne EL, Holmes DC: Triangulated pedicle screw construct technique and pullout strength of conical and cylindrical screws. J Spinal Disord. 2001, 14: 323329. 10.1097/0000251720010800000007.View ArticlePubMedGoogle Scholar
 Reitman CA, Nguyen L, Fogel GR: Biomechanical evaluation of relationship of screw pullout strength, insertional torque, and bone mineral density in the cervical spine. J Spinal Disord Tech. 2004, 17 (4): 306311. 10.1097/01.bsd.0000090575.08296.9d.View ArticlePubMedGoogle Scholar
 Chao CK, Hsu CC, Wang JL, Lin J: Increasing bending strength of tibial locking screws: mechanical tests and finite element analyses. Clin Biomech. 2007, 22 (1): 5966. 10.1016/j.clinbiomech.2006.07.007.View ArticleGoogle Scholar
 Chao CK, Hsu CC, Wang JL, Lin J: Increasing bending strength and pullout strength in conical pedicle screws: biomechanical tests and finite element analyses. J Spinal Disord Tech. 2008, 21 (2): 130138. 10.1097/BSD.0b013e318073cc4b.View ArticlePubMedGoogle Scholar
 Yerby S, Scott CC, Evans NJ, Messing KL, Carter DR: Effect of cutting flute design on cortical bone screw insertion torque and pullout strength. J Orthop Trauma. 2001, 15: 216221. 10.1097/0000513120010300000012.View ArticlePubMedGoogle Scholar
 Abshire BB, McLain RF, Valdevit A, Kambic HE: Characteristics of pullout strength failure in conical and cylindrical pedicle screws after full insertion and backout. The Spine J. 2001, 1: 408414. 10.1016/S15299430(01)00119X.View ArticlePubMedGoogle Scholar
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