New methodology to assess in-vivo quality of motion in cervical spine

Highlights

• A tool that intends to quickly access and compare pre-op and post-op spine balance.

• The Distance to the Ellipse parameter allows to identify unbalanced spine segments.

• A comprehensive biomechanical data is required to address unbalanced spines.

Abstract

Background

Understanding the kinematics of the spine in the interaction with an implanted device is of utmost importance from a clinical point of view. The characterization of the biomechanical movement of the spine occurring at each functional unit is a difficult task as it involves the measurement of complex patterns of motion while identifying more delicate abnormalities that could result in longer-term disease complications.

Center of rotation is a biomechanical parameter that represents the ratio between rotation and translation. It has been recognized as a valid and reliable parameter to identify any delicate abnormal movement of the spine as opposed to the range of motion. However, center of rotation is still not widely used in clinical practice.

Methods

In this study, an algorithm intended to easily identify an imbalanced spine through the center of rotation calculation and a new parameter called distance to the ellipse is presented. In this new approach the distance to the ellipse is a key parameter which represents the distance of the center of rotation lying outside the ellipse that represents the asymptomatic group, from the ellipse itself.

Findings

The presented algorithm allows the comparison of pre-op and post-op outcomes, and the rapid identification of cases needing more attention.

Interpretation

When a comprehensive analysis is required, a dashboard is provided with detailed information for each functional spine unit at each follow-up appointment. It is found that the new approach has the potential to become a new methodology in clinical practice.

Level of Evidence: Biomechanical Study.

Introduction

Each functional spine unit (FSU) is physically represented by the two adjacent vertebrae with the disc and ligaments intact, being the smallest unit representing the general mechanical behaviour in a given region of the spine (Wilke et al., 1998). The spine movement characterization at each FSU is a complex topic as such kinematics may not be effectively described by simple measurements of intervertebral rotation and translation alone. Measurement techniques can be used to characterize complex patterns of motion and identify abnormalities that might result in longer-term complications (Hipp and Wharton, 2008). The center of rotation (CoR) is a pertinent biomechanical parameter able to characterize the quality of motion as opposed to the quantity of motion usually represented by the range of motion (RoM) (Kim, 2015). While the RoM can assume variable values and its calculation depends on whether anteflexion or retroflexion X-Rays are being used, the CoR is independent and can be calculated with a small margin of error while assuming a regular value (Amevo et al., 1992). For each FSU, the CoR can be geometrically obtained by overlapping full flexion and full extension X-Ray pictures (Bogduk et al., 1995). There is a sole axis for each combination of translation and rotation of the vertebra, perpendicular to the plane of rotation, which represents the center around all the points of the vertebra that have no translation and are purely rotated to the new vertebra position (Sears et al., 2006). It is a mathematical concept, as it does not correspond to any biological structure, and its location is a summary of the forces acting on a segment, related to the center of reaction of a vertebra, its rotation and its translation (Bogduk et al., 1995).

However, CoR parameter interpretation has some particularities. Existing literature states that abnormal CoRs significantly correlate with neck pain, acting as an indicator of biomechanical changes that should not occur in asymptomatic persons, but the levels of abnormal CoRs did not match the segment which was the source of pain (Amevo et al., 1992; Dimnet et al., 1982; Mayer et al., 1985). Thus making it difficult to immediately detect the affected FSU, as it might be related to the fact that muscle spasm has the capacity to reduce range of motion and to alter CoRs (Amevo et al., 1992). While the CoR is a recognized and valid parameter, it is not commonly used in daily practice. Hipp and Wharton, 2008, calculated the CoR for each sub-axial cervical spine FSU, illustrating an idealized cervical spine with depicted ellipses for each level, representing the 95% confidence intervals of 129 asymptomatic individuals. The CoR for each FSU could be either inside the correspondent FSU ellipse or outside it, assuming different positions around it, making it hard to interpret. The difficulty of this is only increased when considering multiple FSUs along the spine. From a structural point of view, the cervical spine works as a whole system and, thus, its balance should be considered globally (Le Huec et al., 2015, Le Huec et al., 2018; Smith and Fernie, 1991).

Regarding cervical spine surgery, Anterior Cervical Discectomy and Fusion (ACDF) tend to be related to abnormal changes in the adjacent segments (Sears et al., 2006); At the other end of the scale, the biomechanical outcome (interaction between spine and implant) of Total Disc Replacement (TDR) and the balance of the spine as a whole system is influenced by the mechanical characteristics of the artificial disc (Sears et al., 2006). There is limited knowledge related to how TDRs and ACDFs influence the balance of the spine, either alone or combined (hybrid constructs).

Considering the importance of the CoR when describing the biomechanics of the spine and its complex interpretation, this paper introduces a novel parameter permitting an easier and clearer understanding of how far the CoR position is located from the desirable positioning range and the main reasons for such deviation. Additionally, an algorithm is defined as part of the proposed methodology to clarify not only the biomechanical behaviour pre-op but also the post-op contribution to the restoration of physiological cervical kinematics. The methodology was validated with a total of 20 in-vivo multilevel cases, a collection of TDRs only, or in combination with ACDF - hybrid multilevel constructs. The quality of motion was evaluated using the new parameter designated as Distance to the Ellipse (DE) which establishes a comparison between each functional spine unit CoR and the correspondent control group for each FSU (C2 to C7), together with the RoM at each follow-up visit, allowing us to understand the evolution of quantity and quality of motion.

The result is a tool that enables users to quickly access in-vivo the quality of motion of the cervical spine, so clinicians can easily detect any biomechanical imbalance therein, facilitating a deeper understanding of its possible causes while providing detailed information at each visit.