Cortical Margining Capabilities of Fins when the cortical portion of a graft is positioned in line with the ventral vertebral body cortical sur- face, thereby using the cortical vertebral margins to help buttress axial loads via taking advantages of the boundary effect.12,13 The DOC ventral cervical stabilization system (DOC VCSS; Depuy-Acromed, Raynham, Massa- chusetts) is one of several dynamic cervical de- vices that permits and controls subsidence by al- lowing the movement of a rostral platform along the vertical axis, or rather, the axis defined by the curvature of the spine and the implant.3 This implant also has a unique fin-platform design that is based in part on the theory of the boundary effect. The fin components of the DOC system take advantage of the axial load-bearing ability of the dense cortical margin along the periphery of the vertebral body. Although this implant can be used to allow controlled subsidence (also known as controlled dynamism), it should also buttress axial loads when necessary. Nevertheless, the biomechanical efficacy of the DOC VCSS remains unclear. There are no published biomechanical data authenticating the clinical value of fins. Therefore, this study was designed to examine the biomechanical contributions and load-bearing capacity of the fin components of the DOC VCSS. MATERIALS AND METHODS Specimen preparation Eighteen fresh cadaveric upper thoracic verte- brae (T1-T3) were obtained for this study. The vertebral bodies were disarticulated and cleaned of all soft tissue. The vertebral endplates to be studied were meticulously cleaned of all disc material. The spines had a mean age of 60 ± 6 years. Bone mineral density (BMD) data, deter- mined via dual energy x-ray absorptiometry (DEXA; Hologic QDR 4500A, Waltham, MA), was available for 13 of the vertebral bodies. The mean BMD for these specimens was 0.66 ± 0.16g/cm2. Each segment was embedded in a polyester resin for use in customized gripping fixtures designed for the materials testing system. Specimens were kept moist with saline soaked gauze until the time of testing. Specimen instrumentation Each specimen was instrumented with a DOC VCSS that was 51 mm in length. Twenty-degree screw trajectory platforms were utilized. This size proved optimal for the instrumentation of the thoracic vertebrae. The ratio of fin area to verte- bral body cross-sectional area was calculated. This ratio was maintained for use on the upper thoracic vertebrae, so that a truly clinical compari- son of the buttressing effect of the fins on the cortical margin of the vertebral bodies could be made. Three test conditions were devised to quan- tify the strength contributed by the fins at the endplate (Fig. 1A-C). Each specimen was instru- mented using a method randomly selected from three techniques: Condition A: The rostral platform was placed A B C Fig. 1. A: Condition A instrumentation; platform was placed so that the fins were resting against the superior endplate, and no screws were inserted. B: Condition B instrumentation; platforms without fins were placed on the vertebral bodies, and two screws were inserted. C: Condition C instrumentation; platform was placed so that the fins were resting against the superior endplate, and two screws were inserted. Yonsei Med J Vol. 46, No. 3, 2005