Historical overview and rationale for dynamic fusion

Abstract

The history and rationale of posterior dynamic spinal fusion devices has been derived through the experiences with arthrodesis and devices that serve as adjuncts to such procedures. Dynamic fusion devices provide stabilization through immediate fixation while allowing load sharing. Historically, incorporating these two important concepts in surgical techniques along with the implanted devices facilitates the bone healing process. It stands to reason, spinal fusion techniques that provide immediate stabilization and simultaneously significant graft loading should have the same clinical success seen in other long bone and cervical spine arthrodesis techniques. The load-sharing capability is device-dependent and posterior dynamic constructs theoretically allow dynamic compression of the anterior column higher than that of traditional rigid rod fixation.Interbody graft loading within the functional spinal unit (FSU), affected by dynamic fusion construct, should theoretically be beneficial based on documented experiences with dynamic compression found in orthopedic long bone internal fixators with functional mechanism designs, e.g., Isobar Evolution (Alphatec, Carlsbad, CA). The design of the Isobar dynamic rod is uniquely suited to allow appropriate compressive loads to the interbody graft device based on mechanotransduction healing processes, which can be tied to Wolff’s law. The design attribute requires further clinical validation in order to gauge the physiologic FSU response from which clinical significance can be predicted.Clearly, successful long-term patient outcomes are the most credible validation for the theoretical concepts and design advantages of posterior dynamic fusion constructs. Metrics for dynamic constructs including biomechanical characterization should be used as an indicator for appropriate patient selection and may prove to be important in predicting the success of the posterior dynamic device. However, through appropriate carefully controlled randomized prospective data and the appropriate metrics, clinical validation in terms of Class 1 data will be needed to unequivocally define the benefits of dynamic fusion devices.