One of the goals of spinal deformity treatment is to maximize coronal and axial plane correction while restoring thoracic kyphosis. In recent years, excellent 3-dimensional correction of spinal deformities has been accomplished using a combination of modern strategies, such as direct vertebral rotation, multilevel osteotomies, differential rod contour, and the use of uniplanar screws.1–5 Currently, various rod materials and spinal rod diameters are available; therefore, it is crucial to understand the mechanical properties of the rods. This is especially important because correction loss of the coronal or sagittal plane is occasionally experienced because the contoured rod is bent back during the correction procedure.
Intraoperative rod contouring is usually required to realign the spine. A French bender is a commonly used contouring tool. There have been several reports on the mechanical properties of various rods; however, few reports describe the changes in a rod’s mechanical properties after rod contouring.6–8 The purpose of the current study is to examine the influences of rod contouring on rod strength and stiffness.
Materials and Methods
Spinal rods manufactured with 5.5-, 6.0-, and 6.35-mm titanium (Ti) alloy and 6.0-mm cobalt-chromium (CoCr) alloy were assessed. All rods were cut to a length of 180 mm. Different rod curvatures were evaluated: (1) a no-preparation rod of 0° (control); (2) a 0° rod bent at one point to make tangential angles of 10° and then bent back from the opposite side; (3) a bent rod with tangential angles of 20°; and (4) a 40° bent rod. All rods were contoured using a French bender (Figure 1).
A French bender, which is the most commonly used contouring tool, was used to make the contoured rods.
A 3-point bending test was conducted. The rods were placed on 2 rolls separated by 120 mm. For each rod, the test was accomplished at a constant displacement rate of 10 mm/min with a load applicator (Tensilon RTC-2410; Orientec Co, Saitama, Japan) (Figure 2). A load vs total displacement curve was recorded for each test and used for determining mechanical properties. Bending stiffness (N/mm) was determined by fitting the slope of the initial linear portion. Yield strength (N) was determined by the point of intersection between a load vs total displacement curve and a line parallel to the linear portion. Data are expressed as mean±SD. For comparison between the groups, a one-way analysis of variance was used, followed by Dunnett’s or Scheffe’s test. A P value less than .05 was considered statistically significant.
Three-point bending setup with a bent rod placed on the supporting rolls, separated by 120 mm.
All load vs total displacement curves are shown in Figure 3. The Table shows the yield strength and bending stiffness for all types of rod materials and diameters. The yield strength and bending stiffness of the 6.0-mm Ti rod (0°, control) were 1004.8±16.1 N and 160.3±1.9 N/mm, respectively. After rod contouring with a French bender, the yield strength showed a decrease depending on the degree of bend: 74.5% (0° bend back; P=.28), 54.1% (20°; P=.04), and 50.6% (40°; P=.03). Bending stiffness also decreased after rod contouring: 94.6% (0° bend back; P=.57), 89.2% (20°; P=.12), and 74.9% (40°; P<.01). The bending stiffness of the 6.0-mm CoCr rod was higher than that of the Ti rod with the same diameter on each bend. Meanwhile, the yield strength of the 6.0-mm Ti rod was higher than that of the 6.0-mm CoCr rod. The yield strength and bending stiffness of the 5.5- and 6.35-mm Ti rods also decreased after rod contouring.
Load vs total displacement curve of 6.0-mm titanium alloy rod (A), 6.0-mm cobalt-chromium alloy rod (B), 5.5-mm titanium alloy rod (C), and 6.35-mm titanium alloy rod (D).
Bending Stiffness and Yield Strength of Each Rod
The load vs total displacement curve of 20° and 40° on each rod are shown in Figure 4. When the displacement of the 20° bent rods was 6 mm, simulating an approximate rod reduction from 20° to 0°, the 6.0-mm CoCr rod showed a significantly higher load: 1054.3±68.8 N (6.0-mm CoCr), 915.6±21.2 N (6.35-mm Ti), 748.7±34.2 N (6.0-mm Ti), and 553.0±31.7 N (5.5-mm Ti) (Figure 4A). For the simulation of an approximate rod reduction from 40° to 20° (5-mm displacement), the load of each rod was as follows: 809.5±23.5 N (6.0-mm CoCr), 674.9±32.1 N (6.35-mm Ti), 552.9±18.5 N (6.0-mm Ti), and 400.6±6.3 N (5.5-mm Ti) (Figure 4B).
Load vs total displacement curve of each 20° rod (A) and each 40° rod (B). Abbreviations: CoCr, cobalt-chromium; Ti, titanium.
Currently, various rod materials are available for spine surgery, including Ti, CoCr, commercially pure titanium (CPTi), stainless steel (SS), and ultra-strength stainless steel (UHSS). Studies have described the biomechanical characterization of spinal rods with no preparation from the manufacturer. The mechanical properties of the rods are related to the rod material, diameter, and manufacturing process. In general, Ti and CPTi have higher yield strength and lower Young’s modulus than SS and CoCr.6 Serhan et al7 analyzed the material properties of 4 different 5.5-mm rod materials and reported that 5.5-mm CoCr and UHSS rods had the ability to produce higher correction forces than Ti and SS rods.
Meanwhile, there is minimal literature evaluating the changes in a rod’s mechanical properties after rod contouring with a French bender. Lindsay et al8 evaluated fatigue strength after rod contouring with a French bender and reported that rod contouring procedures significantly reduced the fatigue strength of Ti and CPTi rods. Slivka et al9 evaluated the fatigue strength of a 4.5-mm rod contoured with tube benders and reported that the endurance limits of all types of materials were reduced between 20% and 40% in the bent condition. However, neither study mentioned the yield strength and stiffness of the contoured rods. In the current study, the yield strength and bending stiffness decreased after rod contouring. Dick and Bourgeault10 analyzed a surface notch induced by a French bender under a scanning electron microscope and demonstrated the indentation of the surface and fissures or cracks along the end of the notch in Ti and CPTi materials. In this study, there was also a trend that the extent of decrease in yield strength and bending stiffness was related to the degree of bend. The results indicate that both notches of the rods and geometric effects of the angle influence the mechanical property of the rod. Clinically, rod contouring is inevitable in spinal deformity surgery; however, it may be beneficial to develop various degrees of precontoured rods without surface irregularities.
In the current study’s comparison of load vs total displacement curve, the 6.0-mm CoCr rod showed the highest reduction force, both in 20° and 40°. The bending stiffness of the 6.0-mm CoCr rod was higher than that of the other rods. Even after rod contouring, the CoCr rod had the ability to withstand high acute corrective forces with small amounts of rod deformation. However, the results should be interpreted with some caution. One of the limitations of this study is that the spring-back phenomenon has not been studied. Serhan et al7 reported that CoCr rods had the highest potential for plastic deformation in a highly rigid spine. Noshchenko et al6 demonstrated that ß-titanium alloy and titanium-aluminum-vanadium alloy rods showed the highest springback at rod-bending cycles and that CoCr and SS rods showed mild springback.
The current study evaluated the in-fluence of rod contouring with a French bender on rod strength and stiffness. However, the authors did not analyze the mechanical properties when additional implants (cross-links, multilevel pedicle screws) were attached, which is a limitation of this study. It has been reported that the number of anchor points using multiple hooks11 and rod length between the points affect rod properties. Thus, a biomechanical study with multiple screws is warranted. Furthermore, an investigation using a cadaver or a finite element model may get more detailed information in a clinical setting.
Rod contouring using a French bender reduced yield strength and stiffness in all types of rods. The extent of decrease in yield strength and bending stiffness was related to the degree of bend. In the comparisons of 20° and 40° contoured rods, the 6.0-mm CoCr rod showed the highest reduction force. These results offer a better understanding of mechanical properties after rod contouring and may influence the selection of rod material and diameter.
- Kim YJ, Lenke LG, Cho SK, Bridwell KH, Sides B, Blanke K. Comparative analysis of pedicle screw versus hook instrumentation in posterior spinal fusion of adolescent idiopathic scoliosis. Spine (Phila Pa 1976). 2004; 29:2040–2048. doi:10.1097/01.brs.0000138268.12324.1a [CrossRef]
- Suk SI, Lee SM, Chung ER, Kim JH, Kim SS. Selective thoracic fusion with segmental pedicle screw fixation in the treatment of thoracic idiopathic scoliosis: more than 5-year follow-up. Spine (Phila Pa 1976). 2005; 30:1602–1609. doi:10.1097/01.brs.0000169452.50705.61 [CrossRef]
- Lowenstein JE, Matsumoto H, Vitale MG, et al. Coronal and sagittal plane correction in adolescent idiopathic scoliosis: a comparison between all pedicle screw versus hybrid thoracic hook lumbar screw constructs. Spine (Phila Pa 1976). 2007; 32:448–452. doi:10.1097/01.brs.0000255030.78293.fd [CrossRef]
- Lonner BS, Auerbach JD, Boachie-Adjei O, Shah SA, Hosogane N, Newton PO. Treatment of thoracic scoliosis: are monoaxial thoracic pedicle screws the best form of fixation for correction?Spine (Phila Pa 1976). 2009; 34:845–851. doi:10.1097/BRS.0b013e31819e2753 [CrossRef]
- Demura S, Yaszay B, Carreau JH, et al. Maintenance of thoracic kyphosis in the 3D correction of thoracic adolescent idiopathic scoliosis using direct vertebral derotation. Spine Deform. 2013; 1:46–50. doi:10.1016/j.jspd.2012.06.001 [CrossRef]
- Noshchenko A, Xianfeng Y, Armour GA, et al. Evaluation of spinal instrumentation rod bending characteristics for in-situ contouring. J Biomed Mater Res B Appl Biomater. 2011; 98:192–200. doi:10.1002/jbm.b.31837 [CrossRef]
- Serhan H, Mhatre D, Newton P, Giorgio P, Sturm P. Would CoCr rods provide better correctional forces than stainless steel or titanium for rigid scoliosis curves?J Spinal Disord Tech. 2013; 26:E70–E74. doi:10.1097/BSD.0b013e31826a0f19 [CrossRef]
- Lindsey C, Deviren V, Xu Z, Yeh RF, Puttlitz CM. The effects of rod contouring on spinal construct fatigue strength. Spine (Phila Pa 1976). 2006; 31:1680–1687. doi:10.1097/01.brs.0000224177.97846.00 [CrossRef]
- Slivka MA, Fan YK, Eck JC. The effect of contouring on fatigue strength of spinal rods: is it okay to re-bend and which materials are best?Spine Deform. 2013; 1:395–400. doi:10.1016/j.jspd.2013.08.004 [CrossRef]
- Dick JC, Bourgeault CA. Notch sensitivity of titanium alloy, commercially pure titanium, and stainless steel spinal implants. Spine (Phila Pa 1976). 2001; 26:1668–1672. doi:10.1097/00007632-200108010-00008 [CrossRef]
- Orchowski J, Polly DW Jr, Klemme WR, Oda I, Cunningham B. The effect of kyphosis on the mechanical strength of a long-segment posterior construct using a synthetic model. Spine (Phila Pa 1976). 2000; 25:1644–1648. doi:10.1097/00007632-200007010-00007 [CrossRef]
Bending Stiffness and Yield Strength of Each Rod
|0° (Control)||0° (Bend Back)a||20°a||40°a|
| 6.35-mm Ti||192.6±0.96||184.8±12.1 (95.9%)||178.9±14.2 (92.8%)||141.0±25.4 (73.4%)|
| 6.0-mm CoCr||317.0±6.65||278.6±43.6 (87.8%)||261.1±3.69 (82.3%)b||208.7±18.8 (65.8%)b|
| 6.0-mm Ti||160.3±1.86||151.7±4.41 (94.6%)||143.0±1.03 (89.2%)||120.1±15.0 (74.9%)b|
| 5.5-mm Ti||102.2±7.01||100.0±3.22 (97.8%)||109.8±6.28 (106%)||87.0±9.06 (85.1%)|
|Yield strength, N|
| 6.35-mm Ti||1253±2.80||1045±395 (83.3%)||650.4±251 (51.8%)||698.1±235 (55.7%)|
| 6.0-mm CoCr||865±49.4||689.8±210 (79.6%)||488.9±48.1 (56.4%)b||476.0±65.7 (55.0%)b|
| 6.0-mm Ti||1004±16.1||748.7±248 (74.5%)||544.5±128 (54.1%)b||509.7±245 (50.6%)b|
| 5.5-mm Ti||803.9±36.7||757.6±65.6 (94.2%)||324.0±7.24 (40.3%)b||375.1±70.9 (46.6%)|