DESIGN AND EVALUATION OF A MULTIPLE HEIGHT EXPANDING INTERVERTEBRAL CAGE

Abstract

Introduction The success of spinal fusion is widely accepted. However, room for improvement is possible as complication rates remain between 5%–10% due to tears, device migration, improper sizing and lead to pseudoarthroses.¹ In an attempt to improve outcomes, an expanding intervertebral cage that can be adjusted inter-operatively for proper segmental distraction and support has been designed and undergone preliminary evaluations.

Materials and Methods The main features of the device include a locking spacer that is rotated into position providing both proper distraction and stability based on clinical need. All of the rotating spacers associated with the device possess equivalent minor diameters with distraction height achieved by varying major diameters. Once the appropriate spacer has been identified, a locking mechanism is engaged, locking the spacer in place. In order to ensure parallel distraction while retaining segmental lordosis, the baseplates encompass a variety of angles and are guided bilaterally during distraction.

To evaluate this design, a finite element model (Solidworks, Cosmos, Concord, MA) was employed on a 12° lordosed, 18mm distraction height device under a compressive load of 2745N. This represents the least stable condition as the lordosis angle and height are at the maximum values clinically appropriate. Static and dynamic mechanical testing were performed. Static testing consisted of applying at compressive load at 25mm/min (858 Mini Bionix, MTS, Eden Prairie, MN) until failure of the device or a maximum load of 7000N was sustained. Maximum load, device stiffness and overall deformation were extracted from the load versus deflection data.

Dynamic testing (ELF 3300, Bose, Minnetonka, MN) involved sinusoidal loading from −50N to −300N at a rate of 60Hz. This load represents the approximate mass of the torso. The device was cycled for 5 million cycles with load and displacement data acquired at 250,000 cycle intervals at a rate of 500Hz. Net deflection was computed at 250,000 cycle intervals while compressive stiffness was computed at 500,000 cycle intervals. Non-linear regression analyses were performed for both deflection and stiffness versus cycle number in order to elucidate the behavior of the device.

Results and Discussion: Finite element analysis revealed a maximum stress level of 163MPa equatings to a safely factor of 6.5 in the case of titanium alloy. Static testing revealed that when fully distracted, the device sustained a compressive load in excess of 7160N and displayed a compressive stiffness over 28500N/mm. It has been reported that the vertebral body will fail at levels below 5000N.[2] With respect to fatigue testing, the device achieved the required 5 million cycles. Non-linear analysis of the deformation data (R2> 0.78) displayed a net deflection change of 0.041mm with a subsidence rate of 0.3mm/million cycles. Compressive stiffness (R2> 0.99) was altered at a rate of 0.36N/mm/million cycles.

These results confirm that this novel design can enhance the likelihood for osseointegration by maintaining the micromotion levels below the reported critical value of 75μm.[3]

Conclusions: The design of novel expanding and interoperatively adjustable intervertebral spacer has been realized and appears viable based on preliminary mechanical and finite element analysis.