Biomechanical comparison between titanium and cobalt chromium rods used in a pedicle subtraction osteotomy model

Abstract

Instrumentation failure is a common complication following complex spinal reconstruction and deformity correction. Rod fracture is the most frequent mode of hardware failure and often occurs at or near a 3-column osteotomy site. Titanium (Ti) rods are commonly utilized for spinal fixations, however, theoretically stiffer materials, such as cobalt-chrome (CoCr) rods are also available. Despite ongoing use in clinical practice, there is little biomechanical evidence that compares the construct ability to withstand fatigue stress for Ti and Co-Cr rods. Six models using 2 polyethylene blocks each were used to simulate a pedicle subtraction osteotomy. Within each block 6.0×45 mm polyaxial screws were placed and connected to another block using either two 6.0×100 mm Ti (3 models) or CoCr rods (3 models). The rods were bent to 40° using a French bender and were secured to the screws to give a vertical height of 1.5 cm between the blocks. The blocks were fatigue tested with 700N at 4 Hz until failure. The average number of cycles to failure for the Ti rod models was 12840 while the CoCr rod models failed at a significantly higher, 58351 cycles (P=0.003). All Ti models experienced rod fracture as the mode of failure. Two out of the three CoCr models had rod fractures while the last sample failed via screw fracture at the screw-tulip junction. The risk of rod failure is substantial in the setting of long segment spinal arthrodesis and corrective osteotomy. Efforts to increase the mechanical strength of posterior constructs may reduce the occurrence of this complication. Utilizing CoCr rods in patients with pedicle subtraction osteotomy may reduce the rate of device failure during maturation of the posterior fusion mass and limit the need for supplemental anterior column support

Comparison of Effectiveness between Cobalt Chromium Rods versus Titanium Rods for Treatment of Patients with Spinal Deformity: A Systematic Review and Meta-Analysis

“…These studies reported the different outcomes of interest concerning our study questions. Four studies compared correction rate [ 12 , 16 , 19 , 20 ]; five studies compared rod fracture [ 17 – 19 , 24 , 25 ]; three studies compared thoracic kyphosis [ 12 , 16 , 20 ], proximal junctional kyphosis [ 12 , 17 , 18 ], and fatigue life [ 21 – 23 ] each; two studies compared lumbar lordosis [ 12 , 16 ] and bending stiffness [ 22 , 23 ] each. Studies comparing similar outcomes were analyzed together in the same forest plot.…”

“…While at the review level, the risk of biases was identified as some studies had our data of interest to be extracted; other studies need calculation to obtain the required data for analysis. The overall mean age of participants in two groups could not be calculated as median was used rather than mean in some studies [ 20 ] and not reported in some studies [ 21 – 24 ]. Also, the overall follow-up time could not be calculated as data were not reported in some studies [ 20 – 24 ].…”

“…The overall mean age of participants in two groups could not be calculated as median was used rather than mean in some studies [ 20 ] and not reported in some studies [ 21 – 24 ]. Also, the overall follow-up time could not be calculated as data were not reported in some studies [ 20 – 24 ]. To reduce biases, the authors firstly evaluated all eligible studies by conducting database search and data extraction from different sources.…”

“…Figure 4(a) illustrates five [ 17 – 19 , 24 , 25 ] of eleven eligible studies that compared the occurrence of rod fracture between cobalt chromium rods and titanium rods. The overall odds ratio between the two implants was 0.15, 95% CI (0.06, 0.40), signifying a higher occurrence rate of rod fracture in titanium rods than that of cobalt chromium rods.…”

Evaluation of Rod Breakage After Three-Column Osteotomy

“…Studies evaluating rod breaking strength 7,13 have shown that cobalt chromium rods withstand around three to five more stress cycles than titanium alloy rods and support a higher load up to breakage. In addition, they showed that larger diameter rods are more resistant.…”