Specimen Preparation

The experimental setup was similar as previously described (20). However, due to the smaller size of the vertebrae, small adaptions to the testing setup had to be made. The spinal specimens used included C3 to Th1. Vertebrae C3–C4 and C7–Th1 were stabilized using K-wires (1.0–1.4 mm), which were inserted axially in the vertebral bodies leaving segments C4–C5, C5–C6, and C6–C7 mobile. Both ends of the spinal specimens were potted into 3D-printed cylinders (30 mm in diameter) using polymethyl methacrylate (PMMA) (Figure 1). The segments C3–C4 and C7–T1 were completely fixed in neutral position. The neutral position was determined with the specimen lying on its lateral side without any force acting on the spine. Before potting, 2.5-mm K-wires were inserted into the cylinders to allow biomechanical loading. One pin was oriented in the transverse plane perpendicular to the vertebral column and pointing in a laterolateral direction, while the other pin was oriented in the median plane in a dorsoventral direction. Inserting the pins into the PMMA rather than into the vertebrae avoided iatrogenic fractures of the vertebrae. Specimens were loaded in a modified 3D-printed loading jig similar as previously used (20). In comparison with the previously used jig, the openings in the 3D-printed blocks were adapted to the smaller size of the K-wires (2.5 mm). Otherwise, the jig was similar to the one used by Knell et al. (20).

The experimental testing jig for the cervical specimens. Two orthogonal 2.5-mm-diameter Steinman pins were inserted in the transverse and sagittal planes of the potting cylinders. An external skeletal fixator was mounted to maintain different loading positions (neutral, flexion, extension, and lateral bending). Pins in the cranial segment were fixed in a hole in each polyvinylchloride bar, whereas pins in the caudal segment were placed through a slot to direct the applied torque to one direction.