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

BACKGROUND: Spinal 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. METHODS: An 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. RESULTS: In 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. CONCLUSIONS: The 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.

Changes in bone mineral density in the intertransverse fusion mass after instrumented single-level lumbar fusion: a prospective 1-year follow-up.

STUDY DESIGN: Prospective cohort. OBJECTIVE: The purpose of this study was to evaluate changes in bone mineral density (BMD) in the intertransverse fusion mass as representative for the process of bone remodeling after spinal fusion. SUMMARY OF BACKGROUND DATA: Intertransverse bone graft is frequently applied to facilitate bony fusion between 2 spinal levels. The biological process of bone graft remodeling leading to eventual fusion is, however, poorly understood. METHODS: In 20 patients with a single-level instrumented posterolateral lumbar fusion for low-grade spondylolisthesis, radiographs, and clinical outcome scores (visual analogue scale for back and leg pain, Oswestry Disability Index, Short Form-36) were obtained. Locally harvested laminectomy bone was used as intertransverse bone graft. The BMD in selected "regions of interest" at both intertransverse fusion areas was assessed on days 4 and 3, after a period of 6 and 12 months after surgery using dual-energy x-ray absorptiometry scans. Eventual fusion status was assessed on computed tomographic scan at 12 months. RESULTS: All clinical outcome scores significantly improved at the final follow-up (P < 0.05). Baseline BMD in both paraspinal fusion areas was expressed as 100%, which significantly decreased from 81% to 75% and 77% to 70% at 3 and 6 months, for regions of interest 1 and 2, respectively (P < 0.001). From 6 to 12 months, there was an increase in BMD from 90% to 80%, for regions of interest 1 and 2 (P = 0.296). On computed tomography scan a complete fusion was noticed in 70% of the patients. CONCLUSION: Repeated dual-energy x-ray absorptiometry was able to elucidate the biological process of bone graft remodeling in the intertransverse fusion mass. An active bone remodeling process was quantified with profound resorption or demineralization of the graft during the first 6 months, followed by subsequent bone apposition and restoration of BMD at the final follow-up. No difference in trend in BMD change between patients with and without fusion could be established; however, no firm conclusions can be drawn from small patient numbers.