The instant center of rotation (also, instantaneous velocity center, instantaneous center, or instant center) is the point fixed to a body undergoing planar movement that has zero velocity at a particular instant of time.

The physiological cervical spine kinematics involves not only the amount of motion but also the nature of motion. The nature of motion, known as quality of motion, is difficult to evaluate clinically, although it can be assessed by measuring the mean instant center of rotation 1) 2) 3) 4) 5).

1)
Galbusera F, Bellini CM, Brayda-Bruno M, Fornari M. Biomechanical studies on cervical total disc arthroplasty: a literature review. Clin Biomech (Bristol, Avon) 2008;23(9):1095–1104.
2)
Heller JG, Sasso RC, Papadopoulos SM, et al. Comparison of BRYAN cervical disc arthroplasty with anterior cervical decompression and fusion: clinical and radiographic results of a randomized, controlled, clinical trial. Spine (Phila Pa 1976) 2009;34(2):101–107.
3)
Powell JW, Sasso RC, Metcalf NH, Anderson PA, Hipp JA. Quality of spinal motion with cervical disk arthroplasty: computer-aided radiographic analysis. J Spinal Disord Tech. 2010;23(2):89–95.
4)
Amevo B, Aprill C, Bogduk N. Abnormal instantaneous axes of rotation in patients with neck pain. Spine (Phila Pa 1976) 1992;17(7):748–756.
5)
Amevo B, Worth D, Bogduk N. Instantaneous axes of rotation of the typical cervical motion segments: a study in normal volunteers. Clin Biomech (Bristol, Avon) 1991;6(2):111–117.