We harvested 14 fresh axes from human cadavers (eight male, six female) with an average age of 77.9 years (range, 60–98 years) at the anatomic department of Lübeck University. We randomly assigned the specimens to two groups (seven specimens each; four males, three females): in Group I, only one fracture compression screw (FCS) was used to fix the Type II dens fracture models, and in Group II, two FCSs were used. After dissection of all soft tissue and cartilage, radiographs were obtained to rule out the possibility of pathologic lesions. BMD was scanned at three levels on each specimen: on the top and base of the dens and on the anteroinferior part of the axis. The BMD of the axis was defined as the mean BMD data of the three levels. There were no differences in the mean donor age and BMD between the two groups. The specimens were sealed in double-layered plastic bags and kept frozen at −20° C. On the testing day, they were fully thawed at room temperature and kept moist by spraying the specimens with 0.9% physiologic saline solution during testing.
The necessary sample size for this study was determined a priori using the software G*Power 3.1.2 (Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany). McBride et al. used six cadavers and found no differences in stiffness with one or two screws [30
]. We found no literature to suggest what differences in stiffness might result in clinically important differences in maintenance of fixation or healing and many factors influence the failure of fixation or nonunion. We therefore arbitrarily selected an effect size of 0.8. With a level of significance set to 0.05 and a power targeted at 0.8, considering mechanical failure during testing, we estimated a requirement of seven specimens per group.
The FCS used in the study (Königsee Implantate GmbH, Königsee, Germany) (proximal end, 4.0 mm; shank, 3.0 mm) has been used to treat Type II dens fractures in patients at the University Hospital of Schleswig-Holstein. It is a double-threaded, headless, cannulated, self-tapping, and self-drilling titanium screw. The double-threaded structure comes with different gradients and pitches, with the finer pitch at the proximal end and the wider pitch at the distal end. It produces compression between fracture fragments by the distal end passing across the fracture line and the proximal thread entering the proximal bone fragment, drawing the two bone fragments together during insertion of the FCS. As the distal thread has a diameter of 3.0 mm (with a core diameter of 2.0 mm) and the proximal thread has a diameter of 4.0 mm (with a core diameter of 2.9 mm), both threads can cut without interfering with each other during insertion.
We embedded the C2 in an interior columniform metal container with resin (Technovit 4006; Heraeus Kulzer GmbH, Wehrheim, Germany) to provide a firm base of support. Before embedding, Plasticine® (Flair Leisure Products PLC, Cheam, UK) was placed around the anterior, lateral, and anteroinferior surfaces of the C2 to prevent resin-bone and resin-screw head interaction from lending extra stability to the specimen. The pivot axis of the dens was kept perpendicular to the base of the container at the central point under two laser line generators, which were used for monitoring during embedding. Once the resin was tightly cured, the Plasticine® was removed and the C2 was reinforced by three screws, two from the superior articular surface and one from the spinous process to the resin. In this way, the C2 achieved enough stabilization for testing without any negative effects from the resin (Fig. ). The embedded C2 specimens then were mounted on the testing device of the Zwick 14 5670 universal mechanical testing machine (UTM; Zwick International, Ulm, Germany).
(A) A specimen with Plasticine® was embedded in resin. (B) After the resin was cured, the specimen was taken out and the Plasticine® was removed. (C) The specimen then was reinforced by three screws.
One hundred Newtons is reportedly the in vivo physiologic load of the cervical spine in a relaxed neutral posture [31
] and 1.5 Nm moment is a good approximation to the maximum physiologic rotational load [25
]. Although direct anterior screw fixation cannot restore the original strength of the intact dens [2
], based on the above factors, we established the following parameters for testing. When testing shear stiffness, the maximum load was 40 N and the load speed was 0.1 mm/second. The shear stiffness and torsional stiffness were calculated from the slope of the most linear portion of the load-linear displacement curve under a nondestructive low-load test.
The prepared C2 was bolted in the metal container, which was mounted on the testing table of the UTM. The base of the container was set perpendicular to the horizontal plane. The load bar of the UTM acted directly on the upper articular surface of the dens, and the tip of the linear variable incremental-optical displacement transducer’s (LDT) guided plunger touched the opposite articular surface. By rotating the resin in the metal container, the shear load could be applied from the anterior, posterior, left, and right directions to the dens with the load rod. The shear load and linear displacement data were transmitted from the UTM and the LDT to the data-collecting computer where the shear load-linear displacement curve was made (Fig. A). When testing torsional stiffness, the maximum torque was 0.75 Nm and the rotational speed was 0.1°/second. The rotational testing device (RTD) and the spring clamp were self-designed and custom-fit devices. We stably fixed the RTD on the testing table of the UTM and attached it to the UTM by connecting the gear wheel to the load bar. Thus, when the load bar of the UTM was moved up and down, the RTD could change the linear load to the left and right of the torsional load. The spring clamp, connected to a Burster Model 8627-5010 torque sensor (Burster Präzisionsmesstechnik, Gernsbach, Germany), was fixed on the circular plate of the RTD in the same pivot axis as the RTD. The dens and the rotational part of the RTD were in the same pivot axis after the dens was held stable by the spring clamp. The resin of the prepared C2 was fixed in the circular metal container, which was mounted on the framework. One marker on the left and right transverse processes of the C2 and one marker on the pivot axis of the RTD were applied. All of them were connected to Megatron® MOB 2500-5-BZ-N rotary encoders (Megatron Elektronik AG & Co, Munich, Germany) by threads tied with 100-g plumbs. The direction of the threads was retained at the plumb line by pulleys. The torque sensor and the rotary encoders were connected to the data-collecting computer. When the load bar of the UTM was moved up and down, torsional loads were applied to the left and right of the dens. The torque and angular displacement data were transmitted from the sensors to the data-collecting computer and the torque-angular displacement curve in left and right rotation was recorded (Fig. B). The shear load was applied from four directions, namely, from anterior to posterior, from posterior to anterior, from left to right, and from right to left. The torsional load was applied in left rotation and right rotation.
Fig. 2A–B The photographs show the apparatus setting for testing (A) shear stiffness and (B) torsional stiffness. 1 = dens; 2 = axis; 3 = loading bar; 4 = LDT; 5 = spring clamp; 6 = marker. (more ...)
We calculated the stiffness of the intact dens in six directions, namely, shear stiffness loading from anterior (SA), shear stiffness loading from posterior (SP), shear stiffness loading from left (SL), shear stiffness loading from right (SR), torsional stiffness in left rotation (TL), and torsional stiffness in right rotation (TR), from the shear load-linear displacement curves and torque-angular displacement curves, respectively. The mean shear stiffness and mean torsional stiffness of the intact dens were compared between the randomized groups and showed no differences (in all tests, p > 0.05).
We then placed guide wires from the anteroinferior edge of the C2 vertebral body to the apex of the dens. In Group I, one guide wire was inserted through the midline of the coronal plane. In Group II, two guide wires were inserted under the tissue-protecting drill apparatus guide (Königsee). According to the tissue-protecting drill apparatus, the distance from the screw entry points to the midline was 4 mm and the angle between the guide wire trajectories and the midline in the coronal plane was 5° in Group II. The appropriate guide wire trajectory in the sagittal plane of Groups I and II was the same as described above. The proper length of the FCS could be measured directly by the gauge over the guide wire. We then removed the guide wires and created an osteotomy at the junction of the dens and vertebra with a thin saw by hand to simulate a Type II dens fracture. The two fracture fragments were reduced anatomically and the guide wires were inserted again through the original trajectory. A clamp held the two fracture fragments tightly with compression between the two fragments. We used the cannulated pilot drill bit to open the bony entry point for the FCS over the guide wires by hand, and no tapping was required for the threads of the FCS. The FCS was introduced by hand over the guide wire and overpenetrated the apex of the dens by one or two threads to eliminate one variable by ensuring all screws engaged the same amount of bone. The tightening was stopped when the thread of the FCS head totally entered the vertebra. We obtained AP and lateral radiographs to prove satisfactory reduction and fixation.
Again, the specimens were mounted and tested for stiffness in six directions in the same positions and orientations on the testing device of the UTM as in intact specimens.
We performed data collection and processing with DIAdem™ 11.0 software (National Instruments Corp, Austin, TX, USA). The data-collecting computer collected load and displacement data continuously throughout the study at a frequency of 100 Hz. Data were expressed as mean ± SD. ANOVA was used for statistical analysis of the differences in the stiffness loading from the same direction between and within groups. Independent-samples t-test was used to detect differences in the stiffness restored ratio in the same loading direction between Groups I and Group II. Bivariate analysis (Pearson correlation coefficients) was used to correlate BMD and stiffness. We performed the statistical analyses using SPSS® 17.0 (SPSS Inc, Chicago, IL, USA).