Ethics declarations
While alive all body donors gave their informed and written consent to the donation of their bodies for teaching and research purposes. Being part of the body donor program regulated by the Saxonian Death and Funeral Act of 1994 (third section, paragraph 18 item 8), institutional approval for the use of the post-mortem tissues of human body donors was obtained from the Institute of Anatomy, University of Leipzig. The authors declare that all experiments were conducted according to the principles of the Declaration of Helsinki.
Specimens
To evaluate the method for embedding the vertebral bodies, preliminary tests had to be carried out, as it was not clear whether the planned embedding would be sufficient without additional screws in the respective vertebral bodies. For this purpose, 3D-printed artificial vertebrae made of PLA were produced from CT scans.
For the main experiments, the TH12/L1 segment was taken from six cadavers aged 65-90 years (median: 75) and freshly frozen (-80°C). The mean bone density value (BMD) was 0.862±0.130 g/cm2.
Two artificial bone spinal columns (Sawbones, Pacific Research Laboratories, Vashon, WA, USA) were used for preliminary tests of the test set-up. These were supplied with a screw rod system in Cortical Bone Trajectory (CBT) technology using the freehand technique. The first test unit received a Banana PEEK Cage (DePuy Spine Inc, Raynham, MA, USA) and the second received an Oblique PEEK Cage (TiPEEK, Medacta, Castel San Pietro Switzerland).
Implantation
First, the soft tissue dissection was carefully performed at the human specimens. Afterwards, they were instrumented with a screw-rod system.
To ensure that the treatments can be reproduced for later comparative tests, the preload on the cage was set in a standardised way by a system developed in-house. In this system, the vertebral bodies are clamped between two plates and a defined preload force of 70 N is applied by a thread guide (Figure 1).
The pretension is measured by a force sensor (PCE-FM 1000, PCE Deutschland GmbH, Meschede, Germany). The rods are then locked in the screw heads according to the manufacturer's instructions. This procedure creates a defined initial situation of cage locking.
Embedding
For the preliminary tests, 3D-printed artificial bones were embedded in plaster (Boesner, Witten, Germany) with different configurations with and without additional fixation screws in the upper and lower vertebral plate as well as deep and flat (see Figure 2) according to our planned embedding technique. The different configurations of the embedding were investigated for the upper and lower surface of the vertebra.
Afterwards, the vertebral bodies were axially pulled out of the embedding by a testing machine (ZwickRoell, Ulm, Germany).
Alignment
The embedding in the embedding tubs of the sawbones and human specimens follows a defined protocol. Templates and markers were developed for this purpose, which ensures the exact alignment of the instrumented vertebral bodies (Figure 3).
For dorsal flat embedding of the vertebrae, an inclined plane is placed under the embedding tub. This compensates the cut-off plane of the embedding tub and thus ensures that the embedding is parallel to the table. The vertebrae are fixed to the spinous process by a multi-jointed stand. The vertebra is aligned using an embedding stencil and two thin Kirschner wires. One Kirschner wire is inserted translaterally and one with an angular offset of 90° to dorsal. This creates the plane in which the transverse plane of the cage should be located. The holder is then aligned so that the translateral wire is positioned on the pedicle and the ventral wire is positioned centrally between the two vertebral surfaces. Then, the embedding is carried out in the lower embedding tub with a resin-hardener mixture (Rencast FC52/53, Huntsman, Texas, USA).
For the embedding of the upper segment and to ensure the plane-parallelism of the embedding tubs to each other, the upper embedding tub must be suspended with the same slope angle as the inclined plane. Besides, a marking on the embedding tubs ensures that there is no rotation offset around the cylinder axis. The vortex is now aligned using the positioning template and is also embedded. After curing, it is installed in the test rig.
Test setup
The test set-up was designed in close accordance with the Wilke guidelines [16] for the biomechanical testing of spinal implants. It consists of a frame which is firmly anchored in the machine bed of the servo pneumatic testing machine (DYNA-MESS Prüfsysteme GmbH, Stolberg, Germany). A servo motor with a transmission gear is attached to this frame. This generates the swivel movement of the construction. The motor is torque-controlled and can perform moves cyclically with predefined moments. The angles of the swivel movement are also detected. The construction allows a movement in a flexion-extension direction as well as a lateral bending of the segment by conversion and 90° rotation. To obtain information about the forces acting on the composite, a 6-DOF force sensor (ME-Meßsysteme GmbH, Hennigsdorf, Germany) is installed on the lower embedding (Figure 4).
The control and evaluation are achieved by a self-developed programme.
The embedded vertebral body implant composite was clamped into the test set-up for the examination of cage variants on an artificial bone. A template was used to ensure that the axis of rotation has a reproducible position. During the extension-flexion tests, the rotation axis of the motor should be in the posterior 2/3 area of the vertebral bodies [17]. During rotation for lateral flexion, the axis should be centrally in the vertebral body in the sagittal plane [18]. To ensure the position of the rotation axis, a pointer is used to extend the shaft close to the vertebrae. The pointer is placed on the steel shaft and is guided around the side plate of the swingarm. A telescopic mechanism is installed to guide the pointer close to the vertebrae. For correct positioning of the cage, a rod with an optical measuring cube is screwed firmly into the screw guide of the cage. Further optical markers are attached to the pedicle screw heads and the embedding tubs, as well as to the spinous processes of the vertebrae. After the vertebral bodies have been aligned for the planned direction of movement, the testing starts.
Test execution
For the tests, a compression force of 500 N was applied by the servo pneumatic testing machine. The swivel movement is moment controlled between 7.5 Nm and -7.5 Nm and a frequency of 1Hz. Each vertebral body was loaded with 10,000 cycles each for the two directions of movement. The movements were measured at defined points in time using optical image correlation (Limess Meßtechnik und Software GmbH, Krefeld, Germany; ISTRA 4D Dantec Dynamics A/S, Skovlunde, Denmark). The termination criterion of the fatigue strength test was the exceeding of the ROM of ±10° in one of the three directions of movement.
Evaluation
The data preparation was done by a self-written Matlab routine (MATLAB 2019b, The MathWorks, Natick, MA, USA). The data was evaluated using Excel (Excel 2013, Microsoft Corporation, WA, USA).