Use of a Spine Robot Employing a Real Time Force Control Algorithm to Develop, Simulate, and Compare Spinal Biomechanical Testing Protocols: Eccentric Loading, Pure Moment, and a Novel Head Weight Protocol
Date of Award
Master of Science (MS)
Brian P. Kelly, Ph.D.
Denis J. DiAngelo, PhD Gladius Lewis, PhD
In vitro testing provides a critical tool for understanding the biomechanics of the subaxial cervical spine. Previous common testing protocols used to evaluate the subaxial cervical spine include Pure Moment (PM), follower load, and eccentric lever arm (EL) loading methods. Although these methods are widely accepted, there is always a goal to try to better simulate physiologic loading conditions. While the follower load attempts to simulate compression due to muscle activation, no previous protocol has taken into account the constant vertical force vector applied to C2 produced by the weight of the human head. Furthermore, we are unaware of previous direct protocol to protocol comparisons using the same testing platform and test specimens. A novel protocol, the Head Weight Loading (HWL) protocol, was developed to maintain a constant vertical head weight vector of 65 N on the cranial specimen end throughout an entire range of motion. The objective of this study was to simulate and compare the EL protocol, PM protocol, and the newly developed HWL protocol on a single programmable robotic testing frame with a consistent specimen sample group.
Six fresh subaxial human cadaveric cervical spines (C2-T1) were screened using anteroposterior and lateral radiographs to ensure specimen quality. The EL, PM, and HWL protocols were simulated with global rotation through flexion and extension paths to a nondestructive 3 Nm end limit. Global spinal forces, moment, translational and rotational displacement data were recorded from the robot. Individual vertebral body rotations were measured using an optical non contact motion measurement system. Each motion segment unit’s (MSU) percent contribution to overall motion was used to compare the protocols.
In flexion, the HWL protocol demonstrated significantly less motion at the C7-T1 MSU as compared to the EL and PM simulated protocols. A trend was noted for the HWL protocol to increase motion contributions in the cranial region (C2-C5) and reduce contributions at caudal levels (C5-T1). In extension an opposite trend was noted with motion contribution of the cranial levels, C2-C3 and C3-C4, significantly lower in the HWL protocol, whereas in the caudal, C6-C7 and C7-T1 levels, it was significantly higher.
The Spine Robot’s ability to control end loads in real time enable it to execute a variety of biomechanical tests, making it unique in its ability to directly compare different protocols. The different end load conditions investigated produced significantly different MSU motion responses. The EL protocol has previously been reported to produce a more physiologic moment distribution compared to other standard protocols. However, due to fixturing constraints, it cannot produce loads at C2 that simulate head weight in vivo. The HWL protocol attempted to correct this and simulate an always present vertical force on the spine from the head. Studies incorporating the HWL protocol to study surgical alterations of the cervical spine are in progress, as well as use of the Spine Robot to develop new loading protocols.
Wido, Daniel Mark , "Use of a Spine Robot Employing a Real Time Force Control Algorithm to Develop, Simulate, and Compare Spinal Biomechanical Testing Protocols: Eccentric Loading, Pure Moment, and a Novel Head Weight Protocol" (2011). Theses and Dissertations (ETD). Paper 292. http://dx.doi.org/10.21007/etd.cghs.2011.0348.