We recently described a novel surgical treatment option in which posterior dynamic stabilization (PDS) is used to correct coronal plane tilt of a lumbar total disc replacement (TDR).1 Clinical and radiographic outcomes have been satisfactory. Araghi and Ferrara2 have pointed out the potential for system mismatch in hybrid constructs. This is important, though speculative.
As shown by McAfee et al,3 the underlying cause of radiographically depicted disc tilting may be rotational instability. Single-level lumbar TDR was shown to increase axial range of motion (ROM) to 160% of the axial ROM in the intact spine.3 In addition, the neutral zone in axial rotation was increased to approximately 130% that of the intact spine.3, 4 On the basis of an article by Niosi et al,5 the Dynesys system (Zimmer Spine, Warsaw, Indiana) potentially restores axial ROM and the neutral zone. A combination of these systems may therefore be complementary rather than conflicting.
It is also possible that in a hybrid construct, there will be dominance of the more rigid system on overall motion rather than continued interaction between the systems.3, 4, 6 The Dynesys system, while dynamic, has been shown to be motion limiting.5, 7 On the other hand, biomechanical studies comparing TDR with the intact spine do not show such motion limitations.6 Therefore control of motion in a TDRDynesys hybrid construct would likely be dominated by the Dynesys system, with minimal influence from the more motion-tolerant TDR, thus negating concerns regarding potential complications of biomechanically competitive hybrid pairings.
Patient preferences are crucial components of decision making when treating failed TDRs. In circumstances where the patient is strongly against the use of a fusion system, posterior dynamic stabilization may be the only alternative. Accordingly, a more comprehensive biomechanical study should be performed to better understand the influences of such hybrid constructs on instantaneous centers of rotation as well as on the motion-dampening properties of each system.
Wayne K. Cheng, MD
Daniel K. Palmer, BS
Department of Orthopaedic Surgery
Loma Linda University
Loma Linda, California
E-mail address: md4spine@yahoo.com
References
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