Evaluation of spinal instrumentation rod bending characteristics for in‐situ contouring

Abstract

Bending characteristics were studied in rods used for spinal instrumentation at in-situ contouring conditions. Five groups of five 6 mm diameter rods made from: cobalt alloy (VITALLIUM), titanium-aluminum-vanadium alloy (SDI™), β-titanium alloy (TNTZ), cold worked stainless steel (STIFF), and annealed stainless steel (MALLEABLE) were studied. The bending procedure was similar to that typically applied for in-situ contouring in the operating room and included two bending cycles: first--bending to 21-24° under load with further release of loading for 10 min, and second--bending to 34-37° at the previously bent site and release of load for 10 min. Applied load, bending stiffness, and springback effect were studied. Statistical evaluation included ANOVA, correlation and regression analysis. TNTZ and SDI™ rods showed the highest (p < 0.05) springback at both bending cycles. VITALLIUM and STIFF rods showed mild springback (p < 0.05). The least (p < 0.05) springback was observed in the MALLEABLE rods. Springback significantly correlated with the bend angle under load (p < 0.001). To reach the necessary bend angle after unloading, over bending should be 37-40% of the required angle in TNTZ and SDI™ rods, 27-30% in VITALLIUM and STIFF rods, and around 20% in MALLEABLE rods.

Choice of Rods in Surgical Treatment of Adolescent Idiopathic Scoliosis: What Are the Clinical Implications of Biomechanical Properties? – A Review of the Literature

“…This intraoperative bending of the rod introduces cracks or dents in the material, which can reduce the endurance limit of the rod, also termed the “notch effect”. Slivka et al [ 32 ] found that the endurance limit of CoCr was at least 25% higher than UHSS, SS, or TI in response to repetitive bending whereas Noshchenko et al [ 33 ] showed the highest “springback” (or yield strength) in Ti rods compared to SS. Wedemeyer et al [ 34 ] in a bovine model, confirmed that Tis can withstand higher strains and yield than the relatively brittle SS before failure occurs after bending.…”

Risk factors for rod fracture after posterior correction of adult spinal deformity with osteotomy: a retrospective case-series

“…The first notion states that primary contouring or metal bending (including the use of French benders) has shown to weaken metal and decrease fatigue strength [ 20 , 21 ]. Further, repeated bending or rod contouring to an extreme angle has been hypothesized to decrease rod fatigue strength [ 20 – 25 ]. While biomechanical studies have shown a link between “excessive contouring” and decreased fatigue strength, no clinical study to date has confirmed if this “excessive contouring” poses a significant risk in real-world surgical applications.…”

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

“…Studies were conducted in different countries across continents that increase the external validity of the study. Four studies were done in USA [ 21 , 23 – 25 ], two studies in France [ 12 , 16 ], two studies in South Korea [ 17 , 18 ], two studies in Japan [ 19 , 22 ], and one study in Malaysia [ 20 ].…”

“…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.…”