Spine rod straightening as a possible cause for revision

Abstract

In a previous study, the authors examined the elastic and short-term anelastic springback of Ti6Al4V, CoCrMoC and A316L stainless steel spine rods to observe how the rods mechanically respond in OR contouring. In that study rods were 200 mm long and only the movement at the tip was recorded. The implication of that work was that rods will straighten in-vivo, however, in order for the mechanism of straightening to be determined, the movement of individual bends over time must first be elucidated. Spine rods used were, commercially pure titanium (CP Ti) a primarily α-phase; Ti-6Al-4V; α/β-phase titanium alloy from two different suppliers (denoted by, Ti-6Al-4V (L) and Ti-6Al-4V); β-phase titanium (TNTZ) and CoCrMoC. Following contouring the rods were aged unconstrained, in normal atmosphere or simulated body fluid (SBF) in a CO incubator for up to 288 h. Elastic springback is significantly different between alloys with different microstructures. Both types of Ti6Al4V rods, while meeting the ASTM F136 industry standard, have significantly different properties, most importantly yield strength, flexural modulus, and springback. Environment showed no significant impact on anelasticity. The anelastic response of Ti6Al4V L sample, which has relatively more beta phase than the Ti6Al4V sample, follows the pure beta phase TNTZ in its extended time response. CoCrMoC and CP Ti have a very reduced anelastic response compared to the other alloys. This potentially can have unanticipated effects on the outcome of spine procedures, as the surgeon is reliant on the rods having similar properties to achieve a desired outcome.

Radius of Curvature in Patient-Specific Short Rod Constructs Versus Standard Pre-Bent Rods

“…More recent studies have shown flattening of rods after implantation in spinal deformity patients. 21 So even if a rod from the standard set is further bent to create greater lordosis, the possibility of an iatrogenic fusion in a flat position may still exist. The smaller radius of curvature in a PSSR maintains and may better match the lumbar curve, potentially improving quality of life outcomes and decreasing the potential for complications.…”

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

“…While these material properties can be easily measured in a laboratory setting the theoretical implications cannot necessarily be transferred to the clinical reality. Furthermore, Ayers et al [ 20 ] found that the biomechanical profile of a given rod was very sensitive to differences in production as individual properties varied greatly between manufacturers for the same material. Pienkowski et al [ 21 ] showed that fatigue life of the implant was explained by implant type rather than material further adding to the complexity of understanding rod biomechanics.…”

Systematic review and meta-analysis for the impact of rod materials and sizes in the surgical treatment of adolescent idiopathic scoliosis

“…However, the potential impact of rod material properties observed in the laboratory setting are not easily extrapolated to the clinical reality [ 94 ]. The clinical performance of spinal rods is susceptible to a complicated interplay of patient, surgeon, and environmental factors [ 95 , 96 ]. It is possible the answer lies in other combinations of stiffness and bending yield strength.…”