Comparison of three-dimensional helical axes of the cervical spine between in vitro and in vivo testing

The range of motion is a well-accepted parameter for the assessment and evaluation of cervical motion. However, more qualitative data of the kinematics of the cervical spine are needed for the development and success of cervical disc arthroplasty.

The aim of this study was to provide basic information about helical axes of human cervical spine under in vitro conditions. Furthermore, it should clarify whether the three-dimensional helical axes of cervical motion gained from in vitro experiments are in agreement with those gained from in vivo experiments, and therefore to prove its reliability.

An in vitro test with pure moments and mono-segmental specimens was designed to investigate and compare the helical axes of the cervical spine.

Six human cadaveric specimens (three male and three female) with an average age of 47.5 years (range: 34-58 years) were carefully selected. Each specimen was divided into three motion segments: C2-C3, C4-C5, and C6-C7. We performed 3.5 full cycles of rotation about all axes, flexion-extension, lateral bending, and axial rotation, by applying pure moments of 1.5 Nm without any preload. Following the in vitro tests, the three-dimensional helical axes were calculated and projected into the x-ray images.

Rotation analysis of all three directions revealed similar results for all six specimens. All calculated helical axes were similar to the published in vivo data. Furthermore, the instantaneous centers of rotation were in agreement with in vivo data.

The data gained from this study verify cervical kinematics during in vitro testing using pure moments. It can be assumed that other soft tissue such as muscles are not necessarily needed to simulate cervical kinematics in vitro.