Biomechanical Aspects

and Uwe Spetzger¹

(1)

Department of Neurosurgery, Klinikum Karlsruhe, Karlsruhe, Baden-Württemberg, Germany

The cervical spine is the most flexible part of the spinal column with six degrees of freedom in total. Every motion segment makes up a complex balanced system which enables several functions: posture with simultaneous flexibility, balance by providing compensation movements, bearing of axial loads and protection of the spinal cord. The biomechanical spinal research uses more and more the principle of finite elements. Furthermore, the latest implants for total disc replacement have to be tested in terms of their in vivo kinematics.

3.1 Motions of the Cervical Spine

The cervical spine enables motion in six different directions:

Anterior flexion and posterior extension (inclination and reclination of the head) in the sagittal plane
Lateral bending to the left and right in the coronal plane
Rotation to the left and right in the axial plane

Motions of the cervical spine under physiological conditions are usually combinations of these basic movements. A special feature is the fact that additionally to the actual motion (e.g. flexion/extension in the sagittal plane), the instantaneous centre of rotation (ICR) dynamically changes within the disc. Thus, there is an additional translation movement. This phenomenon hast to be considered in the design of artificial discs for total disc replacement. Furthermore, Lee (1997) could show that there is a displacement of the ICR in cases of spinal instability.

A synopsis of the range of motion of the cervical levels in the different planes is given in Table 3.1.

Table 3.1

Segmental motion of the cervical spine with mean values and minimum/maximum (angle degrees); Panjabi and White (1990)

Direction of motion	C2/3	C3/4	C4/5	C5/6	C6/7	Whole cervical spine
Flexion/extension	10 (5–16)	15 (7–26)	20 (13–29)	20 13–29)	17 (6–26)	75
Unilateral bending	10 (7–20)	11 (9–15)	11 (0–16)	8 (0–16)	7 (0–17)	35
Unilateral axial rotation	3 (0–10)	7 (3–10)	7 (1–12)	7 (2–12)	6 (2–10)	45

The physiological shape of the cervical spine in the neutral position is a lordosis which means an arrangement of the vertebrae with a dorsal concavity in the sagittal plane.

3.2 The Functional Spinal Unit and Its Function

One cervical spinal motion segment is considered as the smallest functional spinal unit (FSU). The FSU consists of two adjacent vertebrae including facet joints, intervertebral disc, anterior and posterior longitudinal ligament, interspinous ligament as well as the autochthonous muscles. These anatomical components make up a complex balanced system that enables the basic functions of the spine: