Six human cervical spine specimens (C2–C7) were used in the study. Each specimen was <65 years (mean age 53.3 ± 8.4 years) and was screened visually and with anterior–posterior and lateral radiographs to exclude signs of neoplasm, trauma, severe degeneration, or other factors that could effect their mechanical properties. In addition, each specimen was scanned using dual X-ray absorptiometry (DEXA) (General Electric Medical Systems, Madison, Wisconsin), and those with bone densities of >1.0 standard deviation below the pool of specimens were eliminated.
The specimens were kept frozen at −20°C in sealed plastic bags and were thawed at room temperature for 12 h before testing. On the day of testing, the specimens were prepared by removing all remaining skin and most of the paraspinal cervical musculature; care was taken to keep all ligaments, joint capsules, osseous components, and intervertebral discs intact. To supplement the potting fixation, three drywall screws were inserted radially into the vertebral bodies and lateral masses of C2 and C7. These end vertebrae were then placed into 4-cm deep polyvinyl chloride potting fixtures and were embedded to the middle of the vertebra in a two-part filler compound (Bondo body filler; Bondo, Atlanta, GA, USA).
Prior to testing, lateral mass screws were placed in each specimen at C4 and C5 bilaterally in preparation for the fusion part of the procedure. The screws were placed under fluoroscopic guidance in the standard manner [36
] and did not interfere with normal range of motion (ROM).
Specimens were first tested in load control in our custom multiaxis spine simulator with infrared-emitting diodes screwed into the vertebral bodies of C3, C4, C5, and C6 to monitor angular motion between these vertebrae. Infrared-emitting diodes were additionally secured to the pots to record motion of levels C2 and C7. The spine simulator is composed of six feedback-controlled pneumatic actuators mounted on to two opposing 3-degree of freedom gimbals that control flexion/extension, lateral bending, and axial rotation moments at the upper and lower ends of the specimen. In addition, the upper gimbal is mounted on to a linear actuator to allow for change in height of the specimen as it moves under bending moments applied through either load or position control. A follower load apparatus is attached to the lower gimbal and applies a preload in line with the specimen through its full ROM. The custom spine simulator is controlled by two National Instruments 7344 series motion controllers and a PC computer running a custom-written National Instruments Labview VI program. There are two 6-Axis AMTI load cells, each positioned between the specimen and gimbal on either end, which provide feedback for the pneumatic actuators on that gimbal, and one position encoder per axis of motion for feedback when in position control. Data are collected via a National Instruments AT-MIO-64E-3 data acquisition board. All specimens were initially tested in the intact condition in load control with applied moments of 1.5 Nm in each respective plane of motion and 100 N of axial follower load applied. A 1.5 Nm moment was chosen because it falls within the range of moments used in other biomechanical tests (1.0–4.0 Nm) of the cervical spine and allows for safe, non-destructive testing of the destabilized specimen. Furthermore, this parameter allows for safe testing of the spine in position control after instrumentation has been added, a situation in which forces typically increase to approximately 3.0 Nm. Follower loads were applied through guides attached at each level through which a flexible cable was passed [25
]. Guides are placed in the lateral masses at each segment through the center of rotation. Before any data were recorded, each spine was preconditioned with 30 cycles in each plane of motion to reduce the effects of the specimen freeze/thaw cycle. A second trial of five cycles in each axis was then used for data recording to determine the ROM of segments C2–C7 (ROMC2–C7
). The spine simulator was then placed in position control with the ROMC2–C7
parameters being used for the treated spines under two conditions. A hybrid testing protocol, in which the initial ROM is determined under load control and treated conditions are tested under position control, is based on the assumption that an individual will attempt to regain preoperative function (i.e., ROM) after a procedure. First, a C4–C5 diskectomy was performed to implant the cervical arthroplasty device as described below. Second, a fusion was simulated by securing rods to the previously placed lateral mass screws and placing a custom anterior plate. The spines were subjected to five cycles of ROMC2–C7
in each axis of motion with 100 N of follower load (intact, fusion, implant). Data were collected on the fourth and fifth cycles of each axis (Fig. ).
The cadaveric spine with the Altia TDI implant at the C4/5 level in the multiaxis spine simulator
To place the Altia TDI, a standard diskectomy at C4–C5 was undertaken. Distraction posts were used at the adjacent segments to provide better visualization for the diskectomy and to allow easier insertion of the device. The entire disk was evacuated in each case and the posterior longitudinal ligament was cut. A custom tool was then used to prepare the endplates, remove cartilage, and create parallel surfaces. A sizing probe in the shape of the implant was then inserted to measure the disk space. The implant consists of separate superior and inferior components that combine to form a disk replacement of varying height, width, and depth. Once size was determined, an appropriately sized cutting device was used to create grooves for the keels in the respective vertebral bodies. This device was tamped into the bone with a mallet; the device includes a stop feature to prevent over insertion. Using care to align the keels of the devices with the slots just created, the implant itself was then inserted into the disk space until flush with the anterior surface of the vertebral body.
Torque data from the multiaxis spine simulator and rotation data from the OptoTrak were recorded, processed, and analyzed on a personal computer with a National Instruments AT-MIO-64E-3 board (National Instruments, Austin, TX, USA) and LabView Software (National Instruments) that generated torque–rotation plots. ROM and stiffness were determined from the LabView graphs. Stiffness was determined by measuring the linear slope of the elastic zone from the torque rotation plots. All data were analyzed using a Wilcoxan-signed rank test with P values <0.05 considered statistically significant.