A new lumbar posterior fixation system, the memory metal spinal system: an in-vitro mechanical evaluation

Abstract

BackgroundSpinal systems that are currently available for correction of spinal deformities or degeneration such as lumbar spondylolisthesis or degenerative disc disease use components manufactured from stainless steel or titanium and typically comprise two spinal rods with associated connection devices (for example: DePuy Spines Titanium Moss Miami Spinal System). The Memory Metal Spinal System of this study consists of a single square spinal rod made of a nickel titanium alloy (Nitinol) used in conjunction with connecting transverse bridges and pedicle screws made of Ti-alloy. Nitinol is best known for its shape memory effect, but is also characterized by its higher flexibility when compared to either stainless steel or titanium. A higher fusion rate with less degeneration of adjacent segments may result because of the elastic properties of the memory metal. In addition, the use of a single, unilateral rod may be of great value for a TLIF procedure. Our objective is to evaluate the mechanical properties of the new Memory Metal Spinal System compared to the Titanium Moss Miami Spinal System.MethodsAn in-vitro mechanical evaluation of the lumbar Memory Metal Spinal System was conducted. The test protocol followed ASTM Standard F1717-96, “Standard Test Methods for Static and Fatigue for Spinal Implant Constructs in a Corpectomy Model.”1. Static axial testing in a load to failure mode in compression bending,2. Static testing in a load to failure mode in torsion,3. Cyclical testing to estimate the maximum run out load value at 5.0 x 10^6 cycles.ResultsIn the biomechanical testing for static axial compression bending there was no statistical difference between the 2% yield strength and the stiffness of the two types of spinal constructs.In axial compression bending fatigue testing, the Memory Metal Spinal System construct showed a 50% increase in fatigue life compared to the Titanium Moss Miami Spinal System.In static torsional testing the Memory Metal Spinal System constructs showed an average 220% increase in torsional yield strength, and an average 30% increase in torsional stiffness.ConclusionsThe in-vitro mechanical evaluation of the lumbar Memory Metal Spinal System showed good results when compared to a currently available spinal implant system. Throughout testing, the Memory Metal Spinal System showed no failures in static and dynamic fatigue.

Nitinol Memory Rods Versus Titanium Rods: A Biomechanical Comparison of Posterior Spinal Instrumentation in a Synthetic Corpectomy Model

“…Using a previous study comparing a single nitinol square rod construct versus a titanium construct, the effect size was determined. 1 2 , it was determined that the number needed in each group to compare static stiffness and torsional stiffness was 2 and 1, respectively. 2 The nitinol alloy composition selected was SE 508 manufactured by Nitinol Devices and Components (NDC; Freemont, CA).…”

Nitinol Spinal Vertebrae: A Favorable New Substitute

“…Nitinol did not cause spinal canal inflammation and any lymph (and other organs) particle detachment [11]. A single spinal rod of austenitic phase with pedicle screws and a connective bridge has also been presented for scoliosis treatment by Kok et al [14]. They indicated that at the body temperature, the superelastic rod can easily fit with the anatomical shape of the spine.…”

“…As the bridges are made of Nitinol, the height of the rod could be adjusted to the surgeon's desired length with a cutter. According to the authors, the system shows higher fatigue resistance in comparison to other Ti alloys, and better torsional yield strength with 30% increase in the torsional stiffness [14].…”

“…No inflammation observed in the spinal canal [11] No particle detected in lymph and other organs [11] Suitable for scoliosis treatment [14] Nails…”

“…During the past five decades, metallic orthopaedic devices have been used for spinal shape correction in scoliosis disease [7,[11][12][13][14]. Despite good strength, Young's moduli of metals are higher than bone.…”