Dynamic Characteristics of the Intact, Fused, and Prosthetic-Replaced Cervical Disk

Abstract

Cervical degenerative disease is an almost assured pathology when aging. It is the most prevalent cause of cervical cord and nerve root disruption, and found in 75%–90% of patients older than 60 years of age (1). A common method to alleviate this pathology consists of the vertebral interbody fusion. While the United States leads the world in vertebral interbody fusion operations, performing approximately 125,000 each year, this current surgical solution is not ideal since it involves the loss of movement and shock absorption at the affected spinal level (2-3). Studies indicate that 25% of the patients develop adjacent segment disease within ten years of the procedure, two thirds of which require additional surgery (4-10). An alternative solution to disk fusion is the prosthetic replacement of the affected disk. Since the function of the natural intervertebral disk is to accommodate motion while attenuating shock which accompanies dynamic loads, it follows that a prosthetic disk which emulates these dynamic mechanical properties may mitigate adjacent disk disease. A few studies, most notably Kasra et al. (11), describe a series of dynamic lumbar tests determining the dynamic stiffness, viscous damping coefficient, energy absorption, and natural frequency of the intervertebral disk (12-15). LeHeuc et al. (16) evaluated the dynamic characteristics of prosthetic implants for the lumbar region but did not incorporate the study in vitro or take physiological masses into account. Indubitably, there is little data on the dynamic characteristics of the cervical spine, especially studies determining these properties in a fused or prosthetic implanted spine model.