This study showed that the COR for flexion / extension is located below disc level, but not at a stationary position and changes its position over the motion period. Flexion / extension is not a circular motion but is facilitated through a completely variable COR. For lateral bending a separate COR was found above disc level. This COR remains constant throughout motion and is located at an oblique sagittal axis, therefore allowing coupled lateral bending and rotation.
Cervical disc prostheses are used to preserve motion after discectomy. It is generally accepted that they should preferably resemble physiological motion as close as possible. Nevertheless, they come in a great variety of completely different biomechanical concepts. At least at the beginning, the basic idea was to use ball-socket designs understanding that flexion / extension is a circular motion guided by the curvature of the facet joints. But even if the surface of the facet joints roughly appears spherical and therefore might determine a circular motion [28], these joints can only guide motion to a certain extent, but they cannot function as a rigid rail and force the vertebral bodies in a strictly circular track. Moreover, it is not a simple torque which is applied to the cervical vertebral bodies that causes flexion / extension, but rather a complex symphony of forces including sagittal translation, axial compression and/or tension together with rotation. And there is a considerable inter-individual variety both for the anatomy but especially for the strength of the muscles and the ligaments which finally initiate and guide cervical spine motion. Therefore, the concept of simple circular motion is not suitable anymore in cervical arthroplasty, and research on qualitative motion must be translated into better biomechanical design of these devices.
A considerable number of studies investigated the ROM of the cervical spine (Tables 1 and 2) [2, 4, 6, 15, 17, 25, 27]. The ROM for flexion / extension found in our study is within the lower range of previously published data [2, 4, 17, 25, 27], probably due to the MRI investigation technique, where the volunteers must remain in maximum flexion and extension for a longer period of time than for functional X-rays. The ROM for lateral bending found in our study is slightly below the values reported in the literature for studies referring to functional X-rays [6, 25, 29], but it is similar to the values that were reported from a functional-MRI-study [15] .
Less information is found in the literature regarding the COR. Studies published by Penning, Amevo and van Mameren [9, 16, 18, 19] contribute to the understanding of the motion pattern of the cervical spine. Especially the scientific work from Bogduk [17] reveals that the COR for flexion / extension is found at different locations from C3/4 down to C6/7, and that there is a separate COR for lateral-bending located more superiorly than the COR for flexion / extension. The data found in our study is congruent to these findings, but more than that it describes that – and how - the COR for flexion / extension changes its position during motion.
Frobin described flexion / extension as a combined rotation and translation [30], but still regarded it as a circular motion following an orbit with a given COR.
Van Mameren presented a biomechanical analysis based on data derived from a cineradiographic study: 25 radiographic frames were taken during flexion / extension and separately analyzed [9]. However, also this study does not reveal whether the COR remains constant during motion.
Anderst and Baillargeon contributed in their studies to a better understanding on 3D-motion of the cervical spine during flexion / extension, lateral bending and rotation [20,21,22]; Flexion / extension is described as motion following the sagittal plane with the COR (centrode, as it is called in his studies) being both level-dependent and showing translation during motion, but the path of the COR is finally not illustrated in a manner that could easily be translated into an improved disc-prosthesis design. In our work we describe the respective positions of the CORs together with their motion paths during flexion / extension, therefore we believe that our work is more illustrative for a discussion how a disc prosthesis could be designed in order to replicate this COR motion path during flexion extension.
For lateral bending and rotation Anderst precisely described coupled motion in his 3D-analysis. However, this study shows that lateral bending and rotation are not two separate complex 3D-motions, but they rather appear as one relatively constant rotation around a sagittal oblique axis. The steeper the angle of this axis, the higher is the rotational component, a flat angle - as it is found in the lower cervical motion segments – causes a higher bending component than rotational component [22]. Therefore, when investigating whether the COR changes its position during lateral bending, also 2D-analysis can reliably answer this question: the intersection-point between this oblique sagittal axis with a frontal plane (the plane where the respective frontal MRI pictures were taken) remains independent from coupled rotation. Therefore it can be detected in 2D-analysis whether the COR for lateral bending remains constant or changes its position during motion. However, it should be mentioned that an oblique rotational axis does not allow anymore to give y-coordinates for a COR for lateral bending at all, because – other than in flexion / extension where the rotational axis is strictly perpendicular to the sagittal plane – the y-coordinates are found along the oblique rotational axis and therefore are dependent from the respective x-coordinates where the frontal plane cuts this oblique axis. Therefore, the respective y-coordinates given in our data are reliable for detecting changes of the COR for lateral bending, but they do not define its actual position.
Our study demonstrates that flexion / extension is not a simple circular motion following an orbit defined by one single constant COR, but the radius for the rotational component in flexion / extension varies within the respective motion segments during motion in addition to the already known segment-dependent decrease from C3/4 down to C6/7.
Figures 4, 5, 6 and 7 illustrate the migration of the COR during flexion / extension in the respective motion segments. From C3 to C6 the respective upper vertebral body mainly translates with little rotation from IE via N to IF, and then it mainly rotates with less translation from ME to IE and from IF to MF. We suppose that during the intermediate part of flexion / extension there is mainly smooth gliding following a greater radius guided by the surface of the facet joints, and that the final part of this motion is facilitated by higher muscle strength and therefore can cause tilting or other non-orbital motion influenced by the increasing tension of the joint capsules and the ligaments together with a compression of the disc. The motion pattern at C6/7 is different; the reason for this is not clear, it might be contributed to the fact that the COR for C6/7 is located closer to the upper endplate of C7 than in the motion segments above.
For the COR in lateral-bending we found no significant changes for the respective CORs during this type of motion. We suppose that the uncovertebral joints together with the facet joints function as a more rigid guidance for lateral bending and therefore keep the COR more constant than in flexion / extension.
Limitations
The major limitation of this study is the small number of volunteers. However, many other studies about biomechanics in cervical arthroplasty published in the literature have similar small cohorts [31,32,33,34,35,36]. It is only the big IDE-studies that investigated more than 100 patients in each cohort, but these studies are sponsored by the respective companies. Our study is completely independent from any company, therefore the small number of 15 volunteers was chosen as we found that other studies also investigated patient cohorts between 15 and 20 subjects [32, 34, 36]. Although strong conclusions may not be drawn because of this limitation, we believe that our study still can contribute towards a better understanding on biomechanics with respect to cervical arthroplasty, and hopefully it will encourage colleagues to further investigate this topic with a larger cohort if possible.
We used MRI for data acquisition and manual digitizing for biomechanical calculation. We are aware that there are more precise techniques, using biplanar radiography plus high resolution CT, for instance, resulting in sub-millimeter precision [21]. However, such techniques lead to a radiation exposure of approx. 4 mSv, which is a considerable burden to healthy volunteers.
For data processing from MRI pictures to AutoCAD software, marking of the 4 corners of the vertebra was performed by a single person. Intra-rater and inter-rater variability of these markings were not determined, which can lead to variability in the COR locations. However, we expect that pooling of the data from the 15 subjects will reduce this variability.
We are also aware that our coordinates were calculated in millimeters and not as a percentage of the respective vertebral body dimension and therefore do not take into account the individual differences of the size of the volunteers‘different vertebral bodies. As shown in Table 3, the x-coordinates for maximum flexion / extension were always found in the posterior third of the respective vertebral bodies, this considerably reduces the possible error; another limitation is that no quantification of eventual degenerative changes in the asymptomatic volunteers cohort was done, which could possibly influence the coordinates of the COR. Therefore, we cannot claim to present a database defining with sub-millimeter precision where the respective CORs are exactly located in the cervical spine, but we believe that the possible resulting error from this limitation has only little influence on our findings that the COR for flexion / extension changes its position throughout motion.
The ROM in our study is in the lower range of previously published data referring to functional x-ray, but even very sophisticated other studies [21] did not include data analysis from the ends of the ROM but used the mid-range of motion. Also, the 4 intervals used for investigating the path of the COR during flexion / extension were not precisely determined but derived from the individual head-position of the respective volunteer. Therefore, these COR coordinates represent an average-interval, but we believe our data still allows a reliable description how the COR moves during flexion / extension as it was never described before in the literature.
Even if we cannot provide coordinates in sub-millimeter precision, we believe the data received from our study is still sufficiently valid to conclude how the biomechanical design of disc prostheses can be further improved.
In summary, our study showed that in flexion / extension the CORs of the investigated intermediate flexion / extension-intervals differ from the COR of the respective maximum flexion / extension for all levels from C3/4 to C6/7. Thus, the study showed that the COR is located below disc level, but not at a stationary position and changes its position over the motion period. Comparing with literature and our findings, flexion / extension is not a simple circular motion. For lateral bending a separate COR was found above disc level. This COR remains constant throughout motion and is located at an oblique sagittal axis, therefore allowing coupled lateral bending and rotation. We believe these findings can influence the design of cervical disc prostheses in future. Simple ball-socket design does not allow physiological motion; however, even simple 2-piece devices – as shown in our work – can replicate physiological motion provided the gliding partners are designed according to the biomechanical findings we presented.
