The experiments were performed using 12 human lumbar spine specimens (L2-S2) from body donors. The gender distribution was male in all cases. Two specimens were used for the preliminary tests. All used specimens had a regular L4 and L5 vertebral body size to allow a safe anchor of the AcitvL prosthesis. In all specimens the size M was implanted to allow the comparability of the results and to exclude the potential influence of sizes.
The ethical standards of the Helsinki Declaration of 1975, as revised in 2000 (5), as well as the national law were respected.
10 preparations were available for the actual experiments. The average age in this cohort was 36.4 years with a range from 18 to 48 years. This subpopulation and age group was selected to largely exclude possible orthogeriatric metabolic bone disorders (e.g. osteoporosis).
The ActivL prosthesis (B. Braun/Aesculap, Tuttlingen AG, Germany) consists of three components and is available in two versions. The semiconstrained design allows a limited translation of an ultra high molecular weight polyethylene (UHMWPE) inlay in the sagittal plane. The implant endplates are made of Cobalt Chrome (CoCr) alloy. The spiked version (figure 1) owns three spikes in a row at the front edge. The keel of the second disc version (figure 2) is aligned in the prothesis midline in the antero-posterior direction. Controlled translational motions of the core in the antero-posterior direction let to a displacement of the rotation center, physiological approximation and normal mobility.
The experiments were carried out in the laboratory for Biomechanics and Experimental Orthopaedics of Ludwig-Maximilians-University in Munich. An existing simulator (figure 3, 4) was used, which consists of three major parts: a motion simulator, a control block and a connected computer. The motion simulator allows the simulation in three planes with simultaneous axial load. Thus, there occur six “true moments”: in
the sagittal plane flexion and extension, in the frontal plane lateral-bending and in the transverse plane left and right rotation [3, 4]. The simulator complied with the requirements of DIN ISO 2631 [8] for the testing of spinal implants (figure 5, 6)
The segment L4/5 was dissected from the spine specimen as the test segment. The soft tissue around the vertebral body anteriorly and laterally including the anterior longitudinal ligament and periosteum was removed. The anterior longitudinal ligament was resected in the front plane of the disc L 4/L5. The natural disc itself was completely removed and the top and bottom vertebral endplates were cleared of the intervertebral cartilage. Care was taken to preserve the subchondral bone. All other structures of the segment L 4/5 were preserved. The prosthesis was implanted by only one experienced spine surgeon with a proper surgical technique using the original instruments provided. For the experiments we used only size M protheses with a superior plate angulation of 6° and a polyethylene (PE) inlay of 8.5 mm or 10 mm.
In combination with the described selection of the specimens a nearly anatomical reconstruction of the motion segment was achieved. In particular, the height of the intervertebral disc space was meticulously reconstructed during the implantation. The lordosis angle adjusted itself according to the anatomical conditions and the current position of the mobile segment. Therefore, it can be assumed that the obtained measurement results correspond to the situation in vivo.
For the measurement of the micromotions specially attached measuring sensors were used. The sensors were connected via a measuring module to a computer. The system has a measurement precision for motion of just 1/1000 μm . The sensors were attached to the specimen in three planes. The holders for the specimens were aligned strictly in the sagittal and frontal planes on the L5 vertebral body so that the sensors touched the caudal plate of the prosthesis. For technical reasons it was not possible to install a sensor in the axial line. For this plane the sensors were attached ventrolateral at an a-angle of 45° and the measuring sensor touched the caudal surface of the prosthesis plate near the ventrolateral edge. In order to determine the actual values of the micromotions in the axial axis, a mathematical conversion by the cosine a was necessary.
The construct with the implanted prosthesis and the sensors were fixed with bone cement to the special holding device designed in the motion simulator (figure 4).
The setting of the simulator (table 1) was carried out according to the ISO 2631 standard for defined three- dimensional coordinate systems. The movement areas were set using default values according to ISO/CD 18192-1.3 (figure 5, 6) [10].
The simulation of the natural movement sequence in lower lumbar spine was performed analogue to the physiological conditions (figure 6). Each axis was tested with the frequency of one Hz. The different axes were not coupled. ®
Table 1: Range of motion (ROM) and values of the axial load according to ISO/CD 18192-1.3 for a motion segment in the lumbar spine.
|
Flexion/Extension
|
Lateral bending
|
Axial rotation
|
Axial load
|
Maximum ROM
|
+60
|
+20
|
+20
|
2000 N
|
Minimum ROM
|
-30
|
-20
|
-20
|
600 N
|
The data were recorded via the measuring sensors connected to the receiver module. Processing and presentation of the results was done by the Catman software ® (HBM Germany).
For each of the two implants the experiment was carried out 5 times. The measurement of the micromotion started simultaneously to the movement simulation. The measurement data were recorded with a frequency of 50 Hz in all three axes. Each experimental setting included a simulation run for more than 1000 cycles (on average about 1050). The measurement graph showed a stabilization of the registered amplitudes about 400 cycles. In the phase between the 540th and the 600th cycle, 60 representative cycles each cycle with 3000 measured values were selected for the result evaluation. Thus we received 60 micromotion's amplitude values per experiment per plane (figure 7).
A representative mean value was calculated for the prosthesis movement in one particular axis from the determined values. For each of the two implant types all representative peak-to-valley-values were grouped according to the axis in an Excel spreadsheet and were fed for statistical analysis in the GraphPad Prism 6 program. For both prostheses, the mean peak-to-valley-values for each axis were calculated from these data.
For the further statistical analysis (IBM SPSS 25.0®) methods of descriptive statistics were used. At first, we tested our values with the Kolmogorov-Smirnov normality test to find out whether our results follow a normal distribution. After confirmation of the normal data distribution the Student’s T-test was performed to investigate the significant differences in the micromotion of the intervertebral disc prosthesis in each axis for the two anchoring types. The significance level was set at 0.05. The results of each tested prosthesis are shown as box plots representing the three axes of motion. Median and interquartile range (Q3 minus Q1) were used for the presentation of localization and dispersion. The median represents the movement level. The interquartile range defines the motion profile.