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Comparison of pullout strength of small-diameter cannulated and solid-core screws
Comparison of Pullout Strength of Small-Diameter Cannulated and Solid-Core Screws Charles G. Kissel, DPM, FACFAS,1 Scott C. Friedersdorf, DPM,2 Douglas S. Foltz, DPM,3 and Todd Snoeyink, DPM4 The purpose of this study was to determine the difference in pullout strength between cannulated and solid-core small-diameter bone screws. Cannulated screws from different manufacturers were compared against solid-core screws with 2.0-mm, 2.4-/2.5-mm, and 3.0-mm diameters. A synthetic material made to simulate bicortical bone was used as the test medium. The screws were extracted under servohydraulic control. There was no statistically significant difference between any of the cannulated and solid-core 2.0-mm screws used in this study ( P ⬍ .05). In the 2.4-/2.5-mm screw tests, both of the cannulated screw designs had a significantly higher pullout strength when compared with the solid-core screw ( P ⬍ .05). In the testing of 3.0-mm screw test, 1 of the cannulated screw designs showed a significantly higher pullout strength than the other cannulated and solid-core screws that were tested ( P ⬍ .05). The results of this study suggest that small-diameter cannulated bone screws are similar in mechanical pullout strength to solid-core screws. ( The Journal of Foot & Ankle Surgery 42(6):334 –338, 2003) Key words: cannulated screws, internal fixation, pullout strength
D uring the past 2 to 3 decades, the bone screw has become the most commonly used orthopedic implant device (1, 2). The compression generated by bone screws allows for primary bone healing and for earlier mobilization of the affected area. The effectiveness of bone screws is determined by the holding (pullout) strength in bone (2–5). The holding strength is dependent on different aspects of screw design, including pitch, outer diameter, thread length, and ratio of core/outer diameter (3–5). In recent years, the cannulated bone screw has been introduced and has become increasingly more popular (6). The cannulated bone screw has several advantages. 1) Guide pins can be used for provisional fixation, allowing more accurate placement of the screws. 2) Cannulated screw placement consists of
From the Section of Podiatric Surgery, Department of Orthopedic Surgery, Detroit Medical Center, Wayne State University, Detroit, MI. Address correspondence to: Charles G. Kissel, DPM, FACFAS, 29433 Ryan Rd, Warren, MI 48092. 1 Diplomate, American Board of Podiatric Surgery; Section Chief and Director, Podiatric Residency Program, Detroit Medical Center, Wayne State University. 2 Submitted while third-year resident. 3 Submitted while second-year resident. 4 Submitted while third-year resident. This study was not funded in any manner. The authors will not receive either directly or indirectly any benefits from a commercial party in connection with this study. Copyright © 2003 by the American College of Foot and Ankle Surgeons 1067-2516/03/4206-0004$30.00/0 doi:10.1053/j.jfas.2003.09.007
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fewer steps and can even be performed percutaneously. 3) The cannulated screw reduces the chance of angulational errors because there is no need to view the instrumentation in 3 dimensions (4,7,8). There are several studies that compare the holding strength of cannulated with solid-core screws (4, 8 –10). Hearn et al (9) compared a 7.0-mm cannulated screw with a 6.5-mm solid-core screw both in an animal and in synthetic material. They found no significant differences in extraction strength in either bovine femora or synthetic cancellous material. They concluded that the changes in thread-to-core ratios that accommodate cannulation have no effect on extraction strength. Brown et al (10) calculated the theoretic pullout strength of several different 4.0-mm cannulated and solid-core screws based strictly on screw dimensions. They also performed experimental tests on polyurethane foam bone to compare the various screw constructs. Both their theoretic and experimental results showed that cannulation of the bone screw did not inherently diminish its mechanical performance. Leggon et al (4) conducted testing that compared cannulated with solid-core screws in both cortical and cancellous bone. Using adult canine femora, the midshaft was used as cortical bone and the condyle used as cancellous bone. In each location, they used 3.5-mm cannulated and solid-core screws. In addition, 7.0-mm cannulated and 6.5-mm solidcore screws were also used for a large fragment perspective. This study concluded that holding power is similar for
TABLE 1
Screw dimensions
Screw Size (mm)
Screw Type
Diameter (mm)
Thread Length (mm)
Core Diameter (mm)
Head Diameter (mm)
Screw Length (mm)
Pitch (mm)
2.0 2.0 2.0 2.4 2.4 2.5 3.0 3.0 3.0
Osteomed lag Osteomed cannulated Vilex cannulated Osteomed lag Osteomed cannulated Vilex cannulated Osteomed lag Osteomed cannulated Vilex cannulated
1.96 2.13 2.16 2.35 2.46 2.49 3.00 3.04 3.02
7.10 8.34 6.29 9.00 8.05 6.52 8.82 8.08 6.46
1.39 1.67 1.68 2.00 1.93 2.01 2.25 2.20 2.13
3.00 2.69 3.18 3.66 3.23 3.74 4.42 4.01 4.32
18.00 17.96 17.98 18.05 17.93 18.00 18.00 18.00 18.00
1.00 1.00 0.90 1.20 1.00 1.00 1.50 1.14 1.11
cannulated and solid-core screws of similar size and in similar types of bone. Thompson et al (8) compared large fragment cannulated and solid-core screws of various lengths and various thread patterns. The investigators used a synthetic polyurethane foam that was designed to have the same mechanical properties as human cancellous bone. They concluded that cannulated and solid-core screws of similar dimension and thread length have similar holding strengths. The authors also stated that total thread surface area is a better predictor of pullout strength than functional thread-surface area, because all long-threaded screws showed a greater holding power than short-threaded screws. They also concluded that there was no demonstrable effect on holding power when screws were inserted with or without tapping. To our knowledge, there are no studies on pullout strength of screws with diameters ⬍3.5 mm. The aim of this study is to determine if these cannulated screws will have similar holding strengths to the solid-core screws of the same diameter. The authors questioned whether cannulation of these small diameter screws would compromise the pullout strength. Materials and Methods Three sizes of cannulated screws from 2 different manufacturers were selected for use in the study. The 2.0-mm, 2.4-mm, and 3.0-mm diameter cannulated and solid-core screws (Osteomed, Addison, TX) and the 2.0-mm, 2.5-mm, and 3.0-mm cannulated screws (Vilex, Pittsburgh, PA) were tested because these sizes are commonly used in foot and ankle surgery. All screws used were 18 mm in length and partially threaded. In addition, they were all self-drilling and self-tapping screws with fluted tips. All screws in the study were made of a titanium alloy with the exception of the 3.0-mm Vilex screw, which is made of stainless steel. The screws used in this study were donated by their respective manufacturers. One sample screw from each of the tested screw sets was manually measured by using an electronic
FIGURE 1
Insertion of cannulated screw into testing material.
micrometer accurate to 0.01 mm. Exact screw dimensions are recorded in Table 1. The testing model chosen was a synthetic bone material consisting of an inner cancellous layer and 2 outer layers to simulate cortices (Pacific Research Laboratories, Vashon, WA). The inner layer is constructed of polyurethane foam (LAST-A-FOAM; General Plastics Manufacturing, Tacoma, WA) with a density of 20 lb/sq ft (.32 g/cm3). This inner layer measures 14 mm in thickness. The outer cortical layers consist of E-Glass epoxy sheets (Pacific Research Laboratories) with a density of 106 lb/sq ft (1.7 g/cm3). Each outer layer of the bicortical construct measures 2 mm in thickness. The screws were inserted in a standard lag technique, following manufacturers’ instructions (Fig. 1). The guide wires for the cannulated screws were placed with the aid of a wire-guide block to assure that the screws were placed perpendicular to the testing material. Each screw was inserted to the level of the run-out, such that all threads were purchased, as described by the American Society for Testing and Materials (ASTM) standards (11). This was deter-
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Results
FIGURE 2
Assembly of testing apparatus on the Instron.
mined by visual inspection. The solid-core screws were inserted by first drilling a pilot hole of the appropriate size. A drill-guide block was used to keep the drill bit perpendicular to the testing surface. The screw was then manually inserted into the testing surface with the proper screwdriver until the thread to shaft junction of the screw was reached. The testing fixture was designed according to ASTM standards and used a grip span of 20 times the major diameter for each screw size (11). Pullout testing was performed on an Instron model 8500 testing machine (Instron, Canton, MA) with the screw axially aligned with the load actuator. The attached chuck was then positioned to grasp the screw underneath the screw head (Fig. 2). Tensile linear load was applied to the screw at a rate of 5 mm/min. Force applied during the distraction process was then recorded in units of kilopounds (Kp) and plotted in a force-time curve with time measured in units of seconds. An example of this force-time curve is shown in Fig. 3. The force-time curve was then used to determine the failure point of each sample. Ten trials of each screw type were performed and the mean value for each of the 10 trials was determined. Statistical Analysis. Statistical comparisons were performed by using the SPSS/PC (SPSS, Chicago, IL) software package to assess differences between pullout forces among the various screw sets. Group comparisons were performed by using an analysis of variance test for independent samples, with post-hoc testing using the least significant difference algorithm. Statistical significance between groups was defined if P ⬍ .05. Power analysis was used to estimate the number of samples required in our study. The software package PS Power and Sample Size Calculations (courtesy of W.D. Dupont and W.D. Plummer, available at: www.mc.vanderbilt.edu/prevmed/ps.htm) was used to calculate the required sample size for testing. 336
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The results of the testing are displayed in Tables 2– 4. The failure point of each test sample was defined as the point at which the force-time curve first becomes negative. In the 2.0-mm screw group trials, there was no statistically significant (P ⬎ .05) difference in pullout strength among the pairings (2.0-mm solid-core Osteomed: n ⫽ 10, 109.31 ⫾ 26.55 Kp; 2.0-mm cannulated Osteomed: n ⫽ 10, 115.33 ⫾ 24.15 Kp; 2.0-mm cannulated Vilex: n ⫽ 10, 114.73 ⫾ 30.25 Kp) (Table 2). The 2.4-/2.5-mm cannulated screw groups both displayed a higher pullout strength (P ⬍ .05) than their solid-core equivalent (2.4-mm solid-core Osteomed: n ⫽ 10, 122.43 ⫾ 24.80 Kp; 2.4-mm cannulated Osteomed: n ⫽ 10, 133.94 ⫾ 21.90 Kp; 2.5-mm cannulated Vilex: n ⫽ 10, 128.64 ⫾ 22.05 Kp) (Table 3). Among the 3.0-mm screws tested, the 3.0-mm cannulated Osteomed screw displayed a higher pullout strength (P ⬍ .05) than the Vilex cannulated and Osteomed solid-core screws tested (3.0-mm solid-core Osteomed: n ⫽ 10, 144.69 ⫾ 20.15 Kp; 3.0-mm cannulated Osteomed: n ⫽ 10, 193.33 ⫾ 33.80 Kp; 3.0-mm cannulated Vilex: n ⫽ 10, 149.74 ⫾ 15.50 Kp) (Table 4). Screw head failures were directly observed by the authors in 8 trials (two 2.4-mm cannulated Osteomed and six 2.0-mm cannulated Osteomed screws).
Discussion There are some factors that may have influenced the findings in this study. First, drilling the required pilot hole for the solid-core screw sets compromised the amount of screw thread to bone interface, which may have decreased the overall pullout strength of these screws. This phenomenon may occur because drill-bit excursion and vibration create a pilot hole that is slightly larger than the actual core diameter of the screw. The amount of drill-bit excursion occurring in vivo may be even greater because of the fact that, in this study, a drill-block guide was used to minimize excursion and vibration and to keep the drill bit perpendicular to the bicortical construct. In contrast, no drilling was required with the cannulated sets, thereby maximizing the amount of thread to bone contact. Another point is that the most distal screw thread in the cannulated screw design engages bone, whereas the most distal thread on a solid-core screw has no significant bone purchase because of pilot-hole drilling. Furthermore, the authors feel that the correct combination of thread pitch, incidental angle of pitch, thread length, and thread-to-core diameter has been achieved through careful engineering of the cannulated screw systems so that they may perform on a par with or better than the solid-core sets. As an example, an ideal pitch would maximize the amount of bone to screw
FIGURE 3 Example of a force/time failure curve. The screw tested was a 2.0-mm Osteomed cannulated screw. Holding power was defined as the maximum load before the curve slope first becomes negative. TABLE 2
2.0-mm screw pullout strength results
TABLE 3
Trial
2.0-mm Osteomed Solid Core (Kp)
2.0-mm Osteomed Cannulated (Kp)
2.0-mm Vilex Cannulated (Kp)
1 2 3 4 5 6 7 8 9 10 Mean ⫾ SD
110.8 136.2 103.8 83.1 108.3 111.8 111.1 111.3 108.6 108.1 109.31 ⫾ 26.55
138.4 117.6 130 127.9 110.1 115.9 104.8 123.6 90.1 94.9 115.33 ⫾ 24.15
118.4 137.5 134.6 118.8 116.7 102 121.9 123.8 96.6 77 114.73 ⫾ 30.25
interface without being so tight as to prevent proper interposition of bone between adjacent screw threads. The 8 screw-head failures observed during testing occurred at much higher load-failure rates than could be expected for in vivo screws, and the type of direct load stress applied would not even be possible in a clinical situation. The failure values measured were also very close to those of the same screw type measured to full pullout failure. Several reasons may account for these head failures. The cannulation of the head itself may compromise the cross-sectional strength of the head while under pullout load from the testing apparatus. In addition, the diameter of these heads was significantly smaller than the other screws tested (Table 1). Also, the jig apparatus used in our study used a groove into which the screw shaft was seated. This jig design only applied pressure on 2 sides of the screw head, rather than the optimal situation of an equal circumferential
Trial
2.4-2.5-mm screw pullout strength results 2.4-mm Osteomed Solid (Kp)
1 113 2 113.1 3 124.7 4 116.6 5 134.2 6 129.2 7 109.4 8 128.1 9 131.1 10 124.9 Mean ⫾ SD 122.43 ⫾ 24.80
2.4-mm Osteomed 2.5-mm Vilex Cannulated (Kp) Cannulated (Kp) 123.7 133.3 130.6 124.6 121.1 128 126.3 148.3 164.9 139 133.94 ⫾ 21.90
127.4 138.1 148.9 143.8 125.2 113.7 124.1 104.8 131 129.4 128.64 ⫾ 22.05
pull on the screw head. Finally, these screws have a cruciate head design, which may predispose these heads to bending failure paralleling the grooves of the cruciate design. The combination of these factors may explain why head failure occurred only in the 2.0- and 2.4-mm cannulated Osteomed screws. Although the preliminary findings of this study suggest that selected sizes of cannulated screws may have greater pullout strength than their solid-core counterparts, extensive and controlled studies of these screws are still lacking, and this current study has its limitations. In the Hearn et al (9) analysis of larger cannulated versus solid-core screws, screw testing in both a synthetic and an animal model found no significant difference in pullout strength. In contrast, this study used a synthetic bicortical foam block construct only and was not intended to either duplicate in vivo screw conditions or recreate the types of screw failures seen in an
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TABLE 4
3.0-mm screw test results
Trial
3.0-mm Osteomed Solid Core (Kp)
3.0-mm Osteomed Cannulated (Kp)
3.0-mm Vilex Cannulated (Kp)
1 2 3 4 5 6 7 8 9 10 Mean ⫾ SD
131.5 123.8 131.1 155.7 160.5 157.5 158.8 152.9 154.9 120.2 144.69 ⫾ 20.15
181.5 225.2 210.3 191.4 157.6 174.4 204.2 219.2 181 188.5 193.33 ⫾ 33.80
141.3 136.6 164.4 134.5 153.8 165.5 151.2 147.2 149.4 153.5 149.74 ⫾ 15.50
actual surgical setting such as cyclical loading or cantilever loading. In addition, the placement of these screws was through a single cortex rather than the ideal surgical goal of purchasing both cortices. This was necessitated because of a structural limitation of the testing construct. The authors’ intent was to simply test these screws by using a medium with a more consistent structure and density than human bone. The choice of this synthetic material also offers the advantage of low intraspecimen and interspecimen variability that is not obtainable in human bone samples. In this controlled study, the cannulated screws tested displayed a comparable holding strength to their solid-core counterparts; thus, cannulated screws offer the ease of guide-wire use and eliminate a need for drilling while not compromising fixation holding strengths. Conclusion This study has shown that, through proper design and construction, small-diameter cannulated screw sets are equal or better on direct pullout testing when compared with
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conventional solid-core screws. The results of this study suggest that the cannulation of these small-diameter bone screws does not diminish their mechanical performance. Therefore, these cannulated screws make an attractive alternative for surgical fixation applications. Acknowledgment The authors thank Ning Zhiang and Dr Paul H. Wooley for their contributions.
References 1. 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 1:279 –302, 1968 2. Uhthoff HK. Mechanical factors influencing the holding power of screws in compact bone. J Bone Joint Surg 55B:633– 639, 1973 3. Decoster TA, Heetderks DB, Downey DJ, Ferries JS, Jones W. Optimizing bone screw pullout force. J Orthop Trauma 4:169 –174, 1990 4. Leggon R, Lindsey RW, Doherty BJ, Alexander J, Noble P. The holding strength of cannulated screws compared with solid core screws in cortical and cancellous bone. J Orthop Trauma 7:450 – 457, 1993 5. Perren SM, Cordey J, Baumgart F, Rahn BA, Schatzker J. Technical and biomechanical aspects of screws used for bone surgery. Int J Orthop Trauma 2:31– 48, 1992 6. Burns A. Use of small cannulated screws for fixation in foot surgery. J Am Podiatr Med Assoc 90:240 –246, 2000 7. Shaw JA. Biomechanical comparison of cannulated small bone screws: a brief follow-up study. J Hand Surg 16A:998 –1001, 1991 8. Thompson JD, Benjamin JB, Szivek JA. Pullout strengths of cannulated and noncannulated bone screws. Clin Othop Rel Res 341:241– 249, 1997 9. Hearn TC, Schatzker J, Wolfson N. Extraction strength of cannulated cancellous bone screws. J Orthop Trauma 7:138 –141, 1993 10. Brown GA, McCarthy T, Bourgeault CA, Callahan DJ. Mechanical performance of standard and cannulated 4.0mm cancellous bone screws. J Orthop Research 18:307–312, 2000 11. American Society for Testing and Materials. Standard Test Method for Determining Axial Pullout Strength of Medical Bone Screws. West Conshohocken, PA, ASTM, 1996
D uring the past 2 to 3 decades, the bone screw has become the most commonly used orthopedic implant device (1, 2). The compression generated by bone screws allows for primary bone healing and for earlier mobilization of the affected area. The effectiveness of bone screws is determined by the holding (pullout) strength in bone (2–5). The holding strength is dependent on different aspects of screw design, including pitch, outer diameter, thread length, and ratio of core/outer diameter (3–5). In recent years, the cannulated bone screw has been introduced and has become increasingly more popular (6). The cannulated bone screw has several advantages. 1) Guide pins can be used for provisional fixation, allowing more accurate placement of the screws. 2) Cannulated screw placement consists of
From the Section of Podiatric Surgery, Department of Orthopedic Surgery, Detroit Medical Center, Wayne State University, Detroit, MI. Address correspondence to: Charles G. Kissel, DPM, FACFAS, 29433 Ryan Rd, Warren, MI 48092. 1 Diplomate, American Board of Podiatric Surgery; Section Chief and Director, Podiatric Residency Program, Detroit Medical Center, Wayne State University. 2 Submitted while third-year resident. 3 Submitted while second-year resident. 4 Submitted while third-year resident. This study was not funded in any manner. The authors will not receive either directly or indirectly any benefits from a commercial party in connection with this study. Copyright © 2003 by the American College of Foot and Ankle Surgeons 1067-2516/03/4206-0004$30.00/0 doi:10.1053/j.jfas.2003.09.007
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fewer steps and can even be performed percutaneously. 3) The cannulated screw reduces the chance of angulational errors because there is no need to view the instrumentation in 3 dimensions (4,7,8). There are several studies that compare the holding strength of cannulated with solid-core screws (4, 8 –10). Hearn et al (9) compared a 7.0-mm cannulated screw with a 6.5-mm solid-core screw both in an animal and in synthetic material. They found no significant differences in extraction strength in either bovine femora or synthetic cancellous material. They concluded that the changes in thread-to-core ratios that accommodate cannulation have no effect on extraction strength. Brown et al (10) calculated the theoretic pullout strength of several different 4.0-mm cannulated and solid-core screws based strictly on screw dimensions. They also performed experimental tests on polyurethane foam bone to compare the various screw constructs. Both their theoretic and experimental results showed that cannulation of the bone screw did not inherently diminish its mechanical performance. Leggon et al (4) conducted testing that compared cannulated with solid-core screws in both cortical and cancellous bone. Using adult canine femora, the midshaft was used as cortical bone and the condyle used as cancellous bone. In each location, they used 3.5-mm cannulated and solid-core screws. In addition, 7.0-mm cannulated and 6.5-mm solidcore screws were also used for a large fragment perspective. This study concluded that holding power is similar for
TABLE 1
Screw dimensions
Screw Size (mm)
Screw Type
Diameter (mm)
Thread Length (mm)
Core Diameter (mm)
Head Diameter (mm)
Screw Length (mm)
Pitch (mm)
2.0 2.0 2.0 2.4 2.4 2.5 3.0 3.0 3.0
Osteomed lag Osteomed cannulated Vilex cannulated Osteomed lag Osteomed cannulated Vilex cannulated Osteomed lag Osteomed cannulated Vilex cannulated
1.96 2.13 2.16 2.35 2.46 2.49 3.00 3.04 3.02
7.10 8.34 6.29 9.00 8.05 6.52 8.82 8.08 6.46
1.39 1.67 1.68 2.00 1.93 2.01 2.25 2.20 2.13
3.00 2.69 3.18 3.66 3.23 3.74 4.42 4.01 4.32
18.00 17.96 17.98 18.05 17.93 18.00 18.00 18.00 18.00
1.00 1.00 0.90 1.20 1.00 1.00 1.50 1.14 1.11
cannulated and solid-core screws of similar size and in similar types of bone. Thompson et al (8) compared large fragment cannulated and solid-core screws of various lengths and various thread patterns. The investigators used a synthetic polyurethane foam that was designed to have the same mechanical properties as human cancellous bone. They concluded that cannulated and solid-core screws of similar dimension and thread length have similar holding strengths. The authors also stated that total thread surface area is a better predictor of pullout strength than functional thread-surface area, because all long-threaded screws showed a greater holding power than short-threaded screws. They also concluded that there was no demonstrable effect on holding power when screws were inserted with or without tapping. To our knowledge, there are no studies on pullout strength of screws with diameters ⬍3.5 mm. The aim of this study is to determine if these cannulated screws will have similar holding strengths to the solid-core screws of the same diameter. The authors questioned whether cannulation of these small diameter screws would compromise the pullout strength. Materials and Methods Three sizes of cannulated screws from 2 different manufacturers were selected for use in the study. The 2.0-mm, 2.4-mm, and 3.0-mm diameter cannulated and solid-core screws (Osteomed, Addison, TX) and the 2.0-mm, 2.5-mm, and 3.0-mm cannulated screws (Vilex, Pittsburgh, PA) were tested because these sizes are commonly used in foot and ankle surgery. All screws used were 18 mm in length and partially threaded. In addition, they were all self-drilling and self-tapping screws with fluted tips. All screws in the study were made of a titanium alloy with the exception of the 3.0-mm Vilex screw, which is made of stainless steel. The screws used in this study were donated by their respective manufacturers. One sample screw from each of the tested screw sets was manually measured by using an electronic
FIGURE 1
Insertion of cannulated screw into testing material.
micrometer accurate to 0.01 mm. Exact screw dimensions are recorded in Table 1. The testing model chosen was a synthetic bone material consisting of an inner cancellous layer and 2 outer layers to simulate cortices (Pacific Research Laboratories, Vashon, WA). The inner layer is constructed of polyurethane foam (LAST-A-FOAM; General Plastics Manufacturing, Tacoma, WA) with a density of 20 lb/sq ft (.32 g/cm3). This inner layer measures 14 mm in thickness. The outer cortical layers consist of E-Glass epoxy sheets (Pacific Research Laboratories) with a density of 106 lb/sq ft (1.7 g/cm3). Each outer layer of the bicortical construct measures 2 mm in thickness. The screws were inserted in a standard lag technique, following manufacturers’ instructions (Fig. 1). The guide wires for the cannulated screws were placed with the aid of a wire-guide block to assure that the screws were placed perpendicular to the testing material. Each screw was inserted to the level of the run-out, such that all threads were purchased, as described by the American Society for Testing and Materials (ASTM) standards (11). This was deter-
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Results
FIGURE 2
Assembly of testing apparatus on the Instron.
mined by visual inspection. The solid-core screws were inserted by first drilling a pilot hole of the appropriate size. A drill-guide block was used to keep the drill bit perpendicular to the testing surface. The screw was then manually inserted into the testing surface with the proper screwdriver until the thread to shaft junction of the screw was reached. The testing fixture was designed according to ASTM standards and used a grip span of 20 times the major diameter for each screw size (11). Pullout testing was performed on an Instron model 8500 testing machine (Instron, Canton, MA) with the screw axially aligned with the load actuator. The attached chuck was then positioned to grasp the screw underneath the screw head (Fig. 2). Tensile linear load was applied to the screw at a rate of 5 mm/min. Force applied during the distraction process was then recorded in units of kilopounds (Kp) and plotted in a force-time curve with time measured in units of seconds. An example of this force-time curve is shown in Fig. 3. The force-time curve was then used to determine the failure point of each sample. Ten trials of each screw type were performed and the mean value for each of the 10 trials was determined. Statistical Analysis. Statistical comparisons were performed by using the SPSS/PC (SPSS, Chicago, IL) software package to assess differences between pullout forces among the various screw sets. Group comparisons were performed by using an analysis of variance test for independent samples, with post-hoc testing using the least significant difference algorithm. Statistical significance between groups was defined if P ⬍ .05. Power analysis was used to estimate the number of samples required in our study. The software package PS Power and Sample Size Calculations (courtesy of W.D. Dupont and W.D. Plummer, available at: www.mc.vanderbilt.edu/prevmed/ps.htm) was used to calculate the required sample size for testing. 336
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The results of the testing are displayed in Tables 2– 4. The failure point of each test sample was defined as the point at which the force-time curve first becomes negative. In the 2.0-mm screw group trials, there was no statistically significant (P ⬎ .05) difference in pullout strength among the pairings (2.0-mm solid-core Osteomed: n ⫽ 10, 109.31 ⫾ 26.55 Kp; 2.0-mm cannulated Osteomed: n ⫽ 10, 115.33 ⫾ 24.15 Kp; 2.0-mm cannulated Vilex: n ⫽ 10, 114.73 ⫾ 30.25 Kp) (Table 2). The 2.4-/2.5-mm cannulated screw groups both displayed a higher pullout strength (P ⬍ .05) than their solid-core equivalent (2.4-mm solid-core Osteomed: n ⫽ 10, 122.43 ⫾ 24.80 Kp; 2.4-mm cannulated Osteomed: n ⫽ 10, 133.94 ⫾ 21.90 Kp; 2.5-mm cannulated Vilex: n ⫽ 10, 128.64 ⫾ 22.05 Kp) (Table 3). Among the 3.0-mm screws tested, the 3.0-mm cannulated Osteomed screw displayed a higher pullout strength (P ⬍ .05) than the Vilex cannulated and Osteomed solid-core screws tested (3.0-mm solid-core Osteomed: n ⫽ 10, 144.69 ⫾ 20.15 Kp; 3.0-mm cannulated Osteomed: n ⫽ 10, 193.33 ⫾ 33.80 Kp; 3.0-mm cannulated Vilex: n ⫽ 10, 149.74 ⫾ 15.50 Kp) (Table 4). Screw head failures were directly observed by the authors in 8 trials (two 2.4-mm cannulated Osteomed and six 2.0-mm cannulated Osteomed screws).
Discussion There are some factors that may have influenced the findings in this study. First, drilling the required pilot hole for the solid-core screw sets compromised the amount of screw thread to bone interface, which may have decreased the overall pullout strength of these screws. This phenomenon may occur because drill-bit excursion and vibration create a pilot hole that is slightly larger than the actual core diameter of the screw. The amount of drill-bit excursion occurring in vivo may be even greater because of the fact that, in this study, a drill-block guide was used to minimize excursion and vibration and to keep the drill bit perpendicular to the bicortical construct. In contrast, no drilling was required with the cannulated sets, thereby maximizing the amount of thread to bone contact. Another point is that the most distal screw thread in the cannulated screw design engages bone, whereas the most distal thread on a solid-core screw has no significant bone purchase because of pilot-hole drilling. Furthermore, the authors feel that the correct combination of thread pitch, incidental angle of pitch, thread length, and thread-to-core diameter has been achieved through careful engineering of the cannulated screw systems so that they may perform on a par with or better than the solid-core sets. As an example, an ideal pitch would maximize the amount of bone to screw
FIGURE 3 Example of a force/time failure curve. The screw tested was a 2.0-mm Osteomed cannulated screw. Holding power was defined as the maximum load before the curve slope first becomes negative. TABLE 2
2.0-mm screw pullout strength results
TABLE 3
Trial
2.0-mm Osteomed Solid Core (Kp)
2.0-mm Osteomed Cannulated (Kp)
2.0-mm Vilex Cannulated (Kp)
1 2 3 4 5 6 7 8 9 10 Mean ⫾ SD
110.8 136.2 103.8 83.1 108.3 111.8 111.1 111.3 108.6 108.1 109.31 ⫾ 26.55
138.4 117.6 130 127.9 110.1 115.9 104.8 123.6 90.1 94.9 115.33 ⫾ 24.15
118.4 137.5 134.6 118.8 116.7 102 121.9 123.8 96.6 77 114.73 ⫾ 30.25
interface without being so tight as to prevent proper interposition of bone between adjacent screw threads. The 8 screw-head failures observed during testing occurred at much higher load-failure rates than could be expected for in vivo screws, and the type of direct load stress applied would not even be possible in a clinical situation. The failure values measured were also very close to those of the same screw type measured to full pullout failure. Several reasons may account for these head failures. The cannulation of the head itself may compromise the cross-sectional strength of the head while under pullout load from the testing apparatus. In addition, the diameter of these heads was significantly smaller than the other screws tested (Table 1). Also, the jig apparatus used in our study used a groove into which the screw shaft was seated. This jig design only applied pressure on 2 sides of the screw head, rather than the optimal situation of an equal circumferential
Trial
2.4-2.5-mm screw pullout strength results 2.4-mm Osteomed Solid (Kp)
1 113 2 113.1 3 124.7 4 116.6 5 134.2 6 129.2 7 109.4 8 128.1 9 131.1 10 124.9 Mean ⫾ SD 122.43 ⫾ 24.80
2.4-mm Osteomed 2.5-mm Vilex Cannulated (Kp) Cannulated (Kp) 123.7 133.3 130.6 124.6 121.1 128 126.3 148.3 164.9 139 133.94 ⫾ 21.90
127.4 138.1 148.9 143.8 125.2 113.7 124.1 104.8 131 129.4 128.64 ⫾ 22.05
pull on the screw head. Finally, these screws have a cruciate head design, which may predispose these heads to bending failure paralleling the grooves of the cruciate design. The combination of these factors may explain why head failure occurred only in the 2.0- and 2.4-mm cannulated Osteomed screws. Although the preliminary findings of this study suggest that selected sizes of cannulated screws may have greater pullout strength than their solid-core counterparts, extensive and controlled studies of these screws are still lacking, and this current study has its limitations. In the Hearn et al (9) analysis of larger cannulated versus solid-core screws, screw testing in both a synthetic and an animal model found no significant difference in pullout strength. In contrast, this study used a synthetic bicortical foam block construct only and was not intended to either duplicate in vivo screw conditions or recreate the types of screw failures seen in an
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TABLE 4
3.0-mm screw test results
Trial
3.0-mm Osteomed Solid Core (Kp)
3.0-mm Osteomed Cannulated (Kp)
3.0-mm Vilex Cannulated (Kp)
1 2 3 4 5 6 7 8 9 10 Mean ⫾ SD
131.5 123.8 131.1 155.7 160.5 157.5 158.8 152.9 154.9 120.2 144.69 ⫾ 20.15
181.5 225.2 210.3 191.4 157.6 174.4 204.2 219.2 181 188.5 193.33 ⫾ 33.80
141.3 136.6 164.4 134.5 153.8 165.5 151.2 147.2 149.4 153.5 149.74 ⫾ 15.50
actual surgical setting such as cyclical loading or cantilever loading. In addition, the placement of these screws was through a single cortex rather than the ideal surgical goal of purchasing both cortices. This was necessitated because of a structural limitation of the testing construct. The authors’ intent was to simply test these screws by using a medium with a more consistent structure and density than human bone. The choice of this synthetic material also offers the advantage of low intraspecimen and interspecimen variability that is not obtainable in human bone samples. In this controlled study, the cannulated screws tested displayed a comparable holding strength to their solid-core counterparts; thus, cannulated screws offer the ease of guide-wire use and eliminate a need for drilling while not compromising fixation holding strengths. Conclusion This study has shown that, through proper design and construction, small-diameter cannulated screw sets are equal or better on direct pullout testing when compared with
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conventional solid-core screws. The results of this study suggest that the cannulation of these small-diameter bone screws does not diminish their mechanical performance. Therefore, these cannulated screws make an attractive alternative for surgical fixation applications. Acknowledgment The authors thank Ning Zhiang and Dr Paul H. Wooley for their contributions.
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