For our experiments, we had 32 lumbar spine specimens (L2-S2) from human donors (3 females and 29 males), two of which (the ones from the oldest donors) were used in the preliminary tests. The ethical standards in the Helsinki Declaration of 1975, as revised in 2000 (5), as well as the national law were respected. The average age was 38.3 years (28–44 years) in the female and 36.8 years (18–54 years) in male cohort. In this way, we largely excluded 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. It has a semiconstrained design, which allows a limited translation of an ultra high molecular weight polyethylene (UHMWPE) inlay in the sagittal plane. The endplates are made of Cobalt Chrome (CoCr) alloy. In the spiked version (Fig. 1) there is a row of three spikes along the front edge while the keel on the other version (Fig. 2) is attached along the midline in an antero-posterior direction. Controlled translational motions of the core in the antero-posterior direction lead to the displacement of the center of rotation, physiological approximation and normal mobility.
(Fig. 1) (Fig. 2)
The experiments were carried out in the laboratory for Biomechanics and Experimental Orthopaedics of Ludwig-Maximilians-University in Munich. An existing simulator (Fig. 3) 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 [2].
(Fig. 3)
From our 32 cadaver specimens we chose 10 male specimens with similar L4 and L5 vertebral body dimensions fitting to the Acitve L prosthesis size M to allow comparability of results.
The segment L4/5 was deducted from the remaining spine specimens in order to be our test segment. It was prepared with removal of the tissue around the vertebral body anteriorly and laterally including the anterior longitudinal ligament and periosteum. The disc tissue was completely removed and the top and bottom endplates were cleared of cartilage remnants. Care was taken to preserve the subchondral bone. The prosthesis was implanted with a proper surgical technique using the original instruments provided. For the experiments, we used prosthesis of only size M with a superior plate angulation of 6° and polyethylene (PE) inlay of 8.5 mm or 10 mm. In combination with the described selection of the specimens this allowed a nearly anatomical reconstruction of the motion segments. Therefore, it can be assumed that the obtained measurement results correspond to the values in vivo.
For the measurement of the micro-motions specially attached measuring sensors were used, which were connected via a measuring module to a computer. This allows very precise recording of the motion amplitudes accurate to 1/1000 µm. The construct with the implanted prosthesis and the sensors were fixed with cement to specially designed adaptors and these in turn to the motion simulator (Fig. 4).
(Fig. 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 [3].
Goal was a simulation of the natural movement sequence, under physiological conditions, in the lower lumbar spine. Each axis was moved with the frequency of one Hz. The axis were not coupled.
Table 1
Range of motion and values of the axial load according to ISO/CD 18192-1.3 for a motion segment in the lumbar spine.
| Flexion/Extension | Lateral rotation | Axial rotation | Axial load |
Maximum | + 60 | + 20 | + 20 | 2000 N |
Minimum | -30 | -20 | -20 | 600 N |
The recording of the data was done via the measuring sensors connected to the receiver module. The processing and presentation of the results was done with the software Catman (HBM Germany).
For both implants 5 cycles of measurements were performed. The measurement of the micro-motion started parallel to the movement simulation. The measurement data were recorded with a frequency of 50 Hz in all three axis. In each experiment, the simulation ran for more than 1000 cycles (on average about 1050). The graphical representation of the measured values showed the stabilization of the measured amplitudes after passage of about 400 cycles. In the phase between the 540th and the 600th cycle, 60 representative cycles with 3000 values were selected for the evaluation of the results. Thus, we had 60 micro-motion's amplitude values per experiment per plane. A calculation of a representative mean value for the amount of movement of the prosthesis in one particular axis was done from these determined values. All mean values of the tested prosthesis were grouped according to the axis in an Excel spreadsheet and were fed for statistical analysis in the GraphPad Prism 6 program.
For the statistical analysis (IBM SPSS 25.0®) methods of descriptive statistics were used. At first, we tested our values with the Kolmogorov-Smirnov normality test and we found out that our results follow a normal distribution. Then the Student’s T-test was performed to investigate the significant differences in the micro-motion of the intervertebral disc prosthesis in each axis for the two anchoring types. The significance level was set at 0.05. The graphic presentation of the results of the tested prosthesis was made in the form of box plots representing the three axis of motion. In particular, for the presentation of localization and dispersion, we used the median and interquartile range (Q3 minus Q1) respectively. The median represents the movement level. The interquartile range defines the motion profile.