Primary stability of the Activ L® intervertebral disc prosthesis in cadaver bone and comparison of the keel and spike anchoring concept

Background: High primary stability is the key prerequisite for safe osseointegration of cementless intervertebral disc prosthesis. The aim of our study was to determine the primary stability of intervertebral disc prosthesis with two different anchoring concepts – keel and spike anchoring. Methods: 10 ActivL intervertebral disc prosthesis (5 x keel anchoring, 5 x spike anchoring) implanted in human cadaver lumbar spine specimens were tested in a spine movement simulator. Under axial load flexion, extension, left and right bending and axial rotation were applied on the lumbar spine specimens through a defined three-dimensional movement program as per ISO 2631 and ISO/CD 18192-1.3 standards. Micromotion of the implants covering every single movement axis were measured for both anchor types and compared using Student’s T-test for significance after calculating 95% confidence intervals. Results: In the transverse axis, the keel anchoring concept showed lower statistically significant (p<0.05) mean values of micromotion compared to spike anchoring concept. The highest micromotion values for both types were observed in the longitudinal axis. The data achieved the threshold of primary stability (150-200 μm).Conclusions: Both fixation systems fulfil the required criteria of primary stability. Independent of the selected anchorage type an immediate postoperative active mobilization doesn’t compromise the stability of the prostheses.

Testing of primary stability of the Activ® L intervertebral disc prosthesis in cadaver bone and comparison of the two different anchoring concepts – keel and spike anchoring

Background: High primary stability is the fundamental prerequisite for safe osseointegration of cementless intervertebral disc prosthesis. The aim of our study was to determine the primary stability of intervertebral disc prosthesis with two different anchoring concepts – keel and spike anchoring. Methods: 10 human cadaveric lumbar spine specimens with an ActivL intervertebral disc prosthesis (5 x keel anchoring, 5 x spike anchoring) were tested on a spine simulator. Under axial load, moments of flexion, extension, left and right bending and axial rotation were applied on the lumbar spine specimens through a defined three-dimensional movement program as per ISO 2631 and ISO/CD 18192-1.3 standards. Micro-motion of the implant was measured in every axis for both anchor types and compared using statistical test for significance after calculating 95% confidence intervals. Results: In the transverse axis, the keel anchoring concept showed lower mean values of micro-motion , which was statistically significant (p<0.05) compared to spike anchoring concept. In the sagittal axis, the results were again in favour of the keel anchoring, but did not reach statistical significance (p>0.05). The highest micro-motion values were observed in the longitudinal axis. Both concepts showed values around the threshold of primary stability (150 μm) with the spike concept showing lower mean values, but without a statistically significant difference.Conclusions: Both types of anchors met the criteria of primary stability. The keel anchoring shows a slight advantage compared to anchoring with spikes. Direct postoperative active mobilization doesn’t seem to compromise the primary stability of the prosthesis.

Wear of a total intervertebral disc prosthesis

In this study we characterized the wear behavior of a tribological system with material combinations of the total intervertebral disc prosthesis InDisc (Institute of Medical and Polymer Engineering) – consisting of two laser sintered titanium alloy plates with an elastomer inbetween. For the tests a translatory oscillation test bench was used which simulates the kinematic and the physiological environment of the lumbar spine. In the interface between the elastomer (Silopren LSR 2670) and the laser sintered titanium alloy (Ti6Al4V) abrasion cords out of silicone rubber with a maximal area of 500 × 700 μm2 were generated as well as individual particles with a size of 10 × 20 to 40 × 50 μm2. In the second prosthesis interface of Ti6Al4V and bone, bone particles with an area of 0.5 × 0.5 to 1 × 1 μm2 were detected. The wear particles and the amount of wear volume influence the biological response of a total intervertebral disc prosthesis in the body.

Current Situation of Design Criteria for Intervertebral Disc Prosthesis

This paper presents the literature review on the design criteria for intervertebral disc prosthesis. The design criteria relate to the design features that intervertebral prosthesis must accomplish (i.e. fixation to bones, spine mobility, energy absorption and etcetera). The need to improve the performance has led to changes in the features which reflect in the current design criteria. Currently, the disc prosthesis technology is experiencing a generational change. The first generation was thoroughly studied while the second generation is in the clinical tests stage. During the time the first generation prostheses were applied in patients there was not a clear trend in the clinical results which produced a lack of trust and reliability in the performance of the disc prosthesis. The changes in the design features of one generation compared to the next generation are based in the deepening in the knowledge of the problem and the results obtained with the first generation prostheses. Some design criteria were identified for the first generation. These criteria were not completely characterized since there was not enough information to be used by the designer. This lack of characterization of most design criteria produced many different versions without a clear focus which help to define the basic design features of disc prostheses. This document presents the necessary information to thoroughly characterize the design criteria outlining the missing information for the design criteria found. An analysis is done of the design criteria in the second generation of prosthesis. Finally if the information contained in the design criteria is enough, the clinical results would be better focused to achieve a more repeatable, reproducible and reliable process for a total disc intervertebral prosthesis replacement as now is considered the vertebral fusion, this is a gold standard.