Byung-Ho Jin, et al. with the fins resting against the superior endplate. Bone screws were not inserted. Condition B: The rostral platform without fins was placed on the vertebral bodies, and bone screws (14 mm in length) were inserted. (The fins of each platform were completely removed using an electric saw). Condition C: The rostral platform was placed with the fins resting against the superior endplate. Bone screws (14 mm in length) were then inserted. To facilitate the testing of these three condi- tions, the DOC assembly was modified by in- verting the superior platform on the rods. This permitted the fins to rest securely against the end- plate and permitted appropriate screw trajectory (20 caudal angulation). The platform was al- lowed to slide freely along the bilateral rods con- stituting the DOC assembly during loading appli- cations. The insertional torques were measured for the last three to four screw turns using a torque wrench calibrated to ± 3% (Sturtevant Richmont, Franklin, IL). The caudal platform and cross connector were fixed with locking screws to a distal portion of the implant to prevent the rods from rotating. In conditions B and C, each plating system was fixed with two anchor screws (14 mm in length). A construct securing pin was used to properly position the platform on the vertebral body while the screws were inserted. For condi- tion A specimens, construct securing pins were used to ensure that the platform fins were pro- perly positioned against the vertebral endplate until the specimen was embedded into the grip- ping fixture. The caudal region of each specimen was secured in a polyester resin (Bondo/Mar- Hyde Corp., Atlanta, GA) in such a manner so that the rostral disc surface was horizontal. Specimens were embedded to their inferior end- plates, and the embedding material was permitted to infiltrate the canal and to engulf the dorsal elements and the lower portion of the DOC as- sembly. This maintained vertebral body align- ment, and secured the implant against the specimen in order for a load to be applied to the movable platform only. The ventral surface of the vertebral body was kept free of the embedding material to allow the upper platform to travel during testing. Yonsei Med J Vol. 46, No. 3, 2005 Biomechanical testing The instrumented vertebral bodies were tested on an MTS materials testing machine (MTS Alliance RT/10, MTS Corp., Eden Prairie, MN). A probe descending from the load cell applied a pure axial compressive load to the superior plat- form of the DOC construct (Fig. 2). Testing was conducted at a rate of 20 mm/min using displace- ment control, and data were sampled at a rate of 20 Hz using Testworks 4 software (MTS Corp., Eden Prairie, MN). Compression continued until the DOC platform traveled towards the inferior endplate of the vertebral body, up to a maximum of 10 mm travel. The stiffness of the construct was determined based on the initial linear data from the load-displacement curves. Due to the discon- tinuous yielding seen during compressive tests to assess the failure at bone-implant interfaces such as this, the 0.2% offset yield was used to deter- mine the yield load (yield strength) of the con- struct. Statistical analysis A one-way analysis of variance (ANOVA) with a Newman-Keuls multiple comparison test was Fig. 2. Specimen setup on materials testing system; a probe was used to axially compress the superior DOC platform.