Design and Testing of a Novel Fusion Construct for Atlantoaxial Instability

Abstract

The atlantoaxial joint of the upper cervical spine is unique in its anatomy due to the absence of an intervertebral disc. Instead, the atlantoaxial joint is comprised of the odontoid process and associated ligaments that together create a pin joint that allows the large range of motion present at the C1-C2 segment. Atlantoaxial instability occurs when there is excessive motion at the C1-C2 segment and is most commonly caused by traumatic fracture of the odontoid process. The conventional surgical treatment for atlantoaxial instability in the geriatric patient population is posterior fusion, which consists of implanting a screw/rod construct to stabilize the C1-C2 segment of the upper cervical spine and promote long term fusion. However, controversy exists regarding the existing fusion procedures due to high intraoperative risks, extended operating times and invasiveness of the surgery that results in a large portion of the patient population being unfit for surgical treatment and instead are treated conservatively with poor outcomes. The purpose of the present thesis is to design a less invasive implant for the surgical treatment of atlantoaxial instability that uses the posterior arch of C1 as a fixation site. This novel implant could then be used with existing C2 fixation methods such as translaminar screws to create a C1-C2 fusion construct that presents less intraoperative risks, shorter operating times and a less invasive surgery with the result of being able to treat more geriatric patients surgically. An optimization study based on minimizing the mean squared difference between experimental load rotation curves and computed load-rotation curves was done to determine ligament laxity values which improved the physiological motion response of an upper cervical spine finite element model. Atlantoaxial instability was simulated and simplified fusion constructs were then implemented in the finite element model for the purpose of predicting the stability of novel constructs that used the C1 posterior arch as a fixation site and comparing these values to the predicted stability of clinically available fusion constructs. The stability predictions of the finite element model suggested that a fusion construct that used the C1 posterior arch as a fixation site was feasible if fixation in C2 was achieved at either the pedicle or lamina. The finite element model was also used to calculate the loads in each construct. Through a series of iterations in the design phase, the novel implant was embodied as a clamp that grasped the posterior arch of C1 and could be connected to C2 translaminar or C2 pedicle screws through the use of rods, thus creating the full fusion construct. Several prototypes were manufactured using a 3D printing technique (selective laser melting) and benchtop test methods were performed to ensure that the implant could withstand the loads predicted by the FE model. The stability of the novel constructs relative to existing, clinically successful constructs was examined through biomechanical cadaveric testing. The results of this study showed that constructs that used C2 translaminar screws were significantly more stable than existing constructs that used C2 translaminar screws. Additionally, novel constructs that used C2 pedicles screws were significantly more stable than existing constructs that used C2 pedicle screws. These results present a strong clinical promise for the novel C1 posterior arch clamp in terms of high fusion rates, reduced operating times, less intraoperative risks and less invasive surgeries.