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Compressive Preload Improves the Stability of Anterior Lumbar Interbody Fusion Cage Constructs

Investigation performed at the Musculoskeletal Biomechanics Laboratory, Department of Veterans Affairs, Edward Hines Jr. Veterans Affairs Hospital, Hines, Illinois

Avinash G. Patwardhan, PhD
Alexander J. Ghanayem, MD
Department of Orthopaedic Surgery and Rehabilitation, Loyola University Medical Center, 2160 South First Avenue, Maywood, IL 60153. E-mail address for A.G. Patwardhan: apatwar{at}lumc.edu

Gerard Carandang, MS
Robert M. Havey, BS
Leonard I. Voronov, MD, PhD
Musculoskeletal Biomechanics Laboratory, Department of Veterans Affairs, Edward Hines Jr. Veterans Affairs Hospital, 5th Avenue and Roosevelt Road, Hines, IL 60141

Ben Cunningham, MD
Section of Orthopaedic Surgery, The University of Chicago, 5841 South Maryland Avenue, Chicago, IL 60637

Frank M. Phillips, MD
Rush Presbyterian St. Luke's Medical Center, 1725 West Harrison Street, Suite 1063, Chicago, IL 60612

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from the Rehabilitation Research and Development Service, Department of Veterans Affairs (grants A2219RA and A2259RA). The interbody fusion cages were provided by Sulzer Spine-Tech (Minneapolis, Minnesota). None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Background: Insertion of an anterior lumbar interbody fusion cage has been shown to reduce motion in a human spine segment in all loading directions except extension. The "stand-alone" cages depend on compressive preload produced by anular pretensioning and muscle forces for initial stabilization. However, the effect that the in vivo compressive preload generated during activities of daily living has on the construct is not fully understood. This study tested the hypothesis that the ability of the cages to reduce the segmental motions in flexion and extension is significantly affected by the magnitude of the externally applied compressive preload.

Methods: Fourteen specimens from human lumbar spines were tested intact and after insertion of two threaded cylindrical cages at level L5-Sl. They were subjected to flexion and extension moments under progressively increasing magnitudes of externally applied compressive follower preload from 0 to 1200 N. The range of motion at level L5-S1 after cage insertion was compared with the value achieved in the intact specimens at each compressive preload magnitude.

Results: The cages significantly reduced the L5-S1 flexion motion at all preloads (p < 0.05). They decreased flexion motion by 29% to 43% of that of the intact specimens for low preloads (0 to 400 N) and by 69% to 79% of that of the intact specimens under preloads of 800 to 1200 N. In extension, in the absence of an externally applied preload, the cages permitted 24% more motion than the intact segment (p < 0.05). In contrast, they reduced the extension motion at preloads from 200 to 1200 N. Under preloads of 800 to 1200 N, the reduction in extension motion after cage placement was 42% to 48% of that of the intact segment (p < 0.05). The reduction of motion in both flexion and extension after cage placement was significantly greater at preloads of 800 to 1200 N compared with the motion reductions at preloads of 400 N (p < 0.05).

Conclusions: In contrast to the observed extension instability under anular tension preload only, the two-cage construct exerted a stabilizing effect on the motion segment (a reduction in segmental motion) in flexion as well as extension under externally applied compressive preloads of physiologic magnitudes. The external compressive preload significantly affected the stabilization provided by the cages. The cages provided substantially more stabilization, both in flexion and in extension, at larger preloads than at smaller preloads.

Clinical Relevance: The study suggests that the segment treated with an anterior lumbar interbody fusion cage is relatively less stable under conditions of low external compressive preload. The magnitude of preload required to achieve stabilization with stand-alone cages may be only partially achieved by anular pretensioning. Since the magnitude of the preload across the disc space due to muscle activity can vary with activities of daily living, supplemental stabilization of the cage construct may provide a more predictably stable environment for lumbar spine fusion.

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