Coupled Motions of the Spine
The term "coupling" refers to motion in which rotation or translation of a body about or along one axis is consistently associated with simultaneous rotation or translation about another axis (White & Panjabi 1991). The term "coupled motions" in our view differs from the term "combination of motions." Due to the geometrical properties of the articular facets of vertebrae, in many parts of the spine, the axial rotation is inevitably associated with lateral bending and vice versa (coupled motions). In contrast, the flexion and extension of the bilaterally symmetrical spine by default should occur without coupling with the lateral flexion or torsion. The flexion and extension may, of course, combine with the lateral bending or axial rotation and affect the degree of coupling (Vicenzino 1993, Edmondston 2007). Still, the spine motion in the sagittal plane is relatively independent of motions in other planes. Our approach to building the biomechanical model of the spine relies on this assumption. Therefore, we excluded from the analysis the flexion and extension motions "coupled" with the primary axial rotation and lateral bending.
The concept of coupled motion is implemented in the application "Biomechanics of the Spine" in the following way:
The degree and the pattern of coupling are not universal for all humans and are a matter of variance and discussion. The scientific evidence behind our choice of the coupled motion values applied to our spine model is hereby presented.
To compare different publication data more appropriately, all the graphs on this page rely on the ratio rather than the absolute angular value of primary and coupled motion:
This ratio indicates how many degrees of coupled motion are associated with the 1° of the primary motion. A positive value of ratio means that coupled motion occurs in the same direction as primary. In contrast, the negative value means that the vectors of the coupled and primary motion are opposed. The calculation of the standard deviation for the ratio is quite complicated. So, the whiskers seen on the graphs presented on this webpage indicate the 95% Confidence Interval, calculated according to the methods published elsewhere (Beyene 2005, Motulsky 1995). For those who are not familiar with the statistics, short remark – the 95% Confidence Interval of the Mean is much narrower compared to the corresponding Standard Deviation. If the results are not within the 95% Confidence Interval range but are close, they are highly likely within the Standard Deviation limits.
The Cervical Spine
The axial rotation of C1 in the atlanto-axial joint is strongly coupled with both – axial translation and lateral bending. The main reason is the biconvex shape of articular cartilages on both opposed facets within the lateral atlanto-axial joint (Koebke 1982).
There is a universal consensus about the direction of the coupled motion of the neck vertebrae. The C0 and C1, when axially rotated, demonstrate the lateral flexion to the opposite side (contralateral coupled motion). At the same time, the subaxial cervical spine shows the coupled lateral bending to the side of axial rotation (ipsilateral coupled motion).
The reported variance of this coupled motion is large and is summarized in the following graph:
The graph demonstrates the universal agreement between multiple in-vivo and ex-vivo studies revealing the coupled behavior of the cervical spine. The axial rotation of the occipital bone and atlas rotation (C0 & C1) is coupled with the remarkable lateral bending to the opposite side. In contrast, the coupled lateral bending of the subaxial cervical spine occurs in the same direction as the main axial rotation.
The magnitude of the lateral bending coupled with the axial rotation varies between publications. However, the studies with a coupling ratio close to 1 demonstrate a more compact Confidence Interval range. This is why we implemented the corresponding ratio value ∼1° to our biomechanical model of the subaxial spine.
The following graph demonstrates the opposite coupled motion: the coupled axial rotation induced by the primary lateral neck bending:
Up to date, we were able to localize only one study reporting the primary lateral bending of the craniocervical junction region (C0-C2) (Ishii 2006). The data from this paper suggest that the coupled axial rotation occurs in the opposite direction to the primary lateral bending. Indirectly this finding is supported by multiple studies of the primary axial rotation of C0-C1, univocally indicating on the same qualitative oppositely directed coupled motion of the lateral bending (Ishii 2004, Guo 2021, Salem 2013, Kang 2019).
The lateral bending in the subaxial region, compared to the craniocervical junction, was studied better. Available data suggest strong ipsilateral coupling of axial rotation and primary lateral bending with a ratio close to 0.5.
The Thoracic Spine
The lateral flexion of the thoracic spine variably couples with the axial rotation with no consistency nor in the amount nor the direction of this coupling. The following assumptions could summarize the phenomena related to the biomechanics of the thoracic spine (Edmondston 2007):
Based on these postulations, the coupled motions were not applied to the Anatomy Standard Spine Biomechanical model.
The Lumbar Spine
The available scientific literature meta-analyses declare no evidence about the particular pattern of coupled motion induced by lateral bending (Cook 2013, Legaspi 2007). Regarding the opposite, i.e., the coupling of the lateral bending caused by the axial rotation, the actual evidence is more consistent. The following graph shows the aggregate data from studies of multi-segmental lumbar motion analysis based on accurate 3D registration of vertebra (CT/MRI reconstructions, biplanar radiography, stereophotogrammetry) from patients with no prominent degenerative pathology of the spine:
In-vivo standing double x-ray source studies demonstrate that balanced distribution of the coupled lateral bending following a dynamic axial torsion of the body maintains the global balance of the spine by creating the S-shaped axial curve. The phenomenon is similar to so-called “compensatory scoliosis” – physiologic coordination of local scoliosis to maintain a global balance of the whole body posture (Shin 2013, Panjabi 1994).
Dr. Shin and colleagues reported the dynamic axial rotation test results performed in a standing position and measured using biplanar x-ray. Other studies (Fuji 2007, Ochia 2006) report the measurements obtained from supine individuals in a static and fixed position. For our biomechanical model, we have selected the values for the coupled lateral bending of the lumbar spine close to the values reported in the paper from Dr. Shin, as it lacks the major limitation of many other studies and reproduces more accurate physiological load conditions (Lu 2012).
The screenshots from our application demonstrates the complex and balanced coupled lateral bending of the lumbar spine:
The screenshots of the Application. Click an image toggle between the default and maximally rotated lumbar spine. Please note the bi-directional nature of the coupled lateral bending (contralateral for the upper lumbar spine & ipsilateral – for the lower). This scoliosis-like balanced deformation is the reason why the remarkable angular motion of each lumbar vertebra (absolute sum of angles = 9.2°) results in the negligible lateral bending of the lumbar spine as a whole (-2.2°).