Instantaneous centers of rotation for lumbar segmental extension in vivo
Introduction
Lumbar spinal motion has predominantly been characterized in terms of relatively simple metrics, such as end-range of rotational motion (ROM) occurring about a fixed, average center of rotation (COR) (Cossette et al., 1971, Freudiger et al., 1999, Fujii et al., 2007, Ochia et al., 2006, Pearcy et al., 1984, White and Panjabi, 1978, Xia et al., 2010). However, lumbar motion comprises rotational and translational components (Aiyangar et al., 2014, Gertzbein et al., 1984, Ogston et al., 1986, Xia et al., 2010), and motion at different intervals may exhibit different characteristics, particularly between healthy and pathologically or surgically altered spines (Ahmadi et al., 2009, Ellingson and Nuckley, 2015, Natarajan et al., 2008, Passias et al., 2011). Hence, comprehensive characterization of vertebral kinematics should ideally incorporate the spatial and temporal variations of both rotational and translational components. Within this context, mapping the locus of instantaneous centers of rotation (ICRs) between two adjacent vertebrae over a given motion – the centrode – has been shown to be a reasonable way to simultaneously quantify the translational component of lumbar motion and its coupling with the rotational component. More importantly, the ICR has been shown to have a biological basis linking aberrations in its location to anatomical and pathological factors, based on its strong association with the center of reaction (Bogduk et al., 1995, Schneider et al., 2005, Zander et al., 2016). In vitro as well as some in vivo studies have shown promising results in applying spatial variations in ICR paths to distinguish between healthy lumbar spines and those with segmental instability (Ahmadi et al., 2009, Gertzbein et al., 1986) as well as different degrees of disc degeneration (Ellingson and Nuckley, 2015, Gertzbein et al., 1985). Modelling studies have demonstrated how even small variations in implant placement and design could elicit notable alterations in load distribution patterns on the facets and adjacent segments (Han et al., 2013, Zander et al., 2009). Furthermore, accurate task-specific ICR information is a crucial input for inverse kinematics-driven biomechanical models, as the estimation of muscle moment arms and, consequently, muscle and joint forces can be highly sensitive to small changes in the presumed location of the COR (Abouhossein et al., 2013, Han et al., 2013, Zhu et al., 2013) and, by association, the center of reaction (Bogduk et al., 1995, Schneider et al., 2005, Zander et al., 2016).
Although ICRs and centrode patterns have been estimated using the method of Reuleaux from a series of static radiographs (Gertzbein et al., 1984, Gertzbein et al., 1985, Gertzbein et al., 1986, Ogston et al., 1986), the lack of sufficient precision has hampered the confidence level in past reported data (Crisco et al., 1994, Pearcy et al., 1984). Developments in dynamic radiographic imaging techniques over the last decade, however, have overcome these limitations to afford more accurate measurement of in vivo vertebral motion during functional activities (Ahmadi et al., 2009, Aiyangar et al., 2015, Aiyangar et al., 2014, Anderst et al., 2008, Teyhen et al., 2007, Wong et al., 2004, Wu et al., 2014).
Our systematic endeavor to map three-dimensional (3D) kinematics of the healthy lumbar spine has been motivated by these developments. The current study investigates lumbar ICR migration patterns during a functional lifting task and poses the following hypotheses:
- (1)
ICR migration patterns vary across the individual lumbar segments, both in the anterior-posterior (AP) and superior-inferior (SI) directions. Specifically,
- a.
The range of ICR migration varies across the lumbar segments.
- b.
The locations of the ICRs, as defined relative to the inferior vertebra of a given intervertebral segment, vary across the segments.
- a.
- (2)
The magnitude of load lifted has a significant effect on the location and the migration range of the ICRs.
Section snippets
Materials and methods
With Institutional Review Board approval, 14 healthy participants (eight male, six female) between the ages of 19 and 30 years (24±2) and a waist size no greater than 89 cm (35 in) [(S)=79±8 cm (31±3 in)] were recruited for the study. Mean height and weight of participants were 175 (±8) cm and 71 (±12) kg respectively. Participants reported no prior history of lower back disorders. All participants provided written, informed consent.
Starting from a trunk-flexed (~75° flexion) position,
Results
Of the 14 participants, data from three were omitted due to poor image quality. Six out of the 122 segmental ICR datasets (4.9%) processed were deemed outliers and excluded, as their AP or SI range of ICR migration exceeded 175% (~3× mean value), or their centrodes exceeded 250% of bone depth (~2× mean value) (Baillargeon and Anderst, 2013), or both.
Substantial migration of ICRs was observed at all segments (Fig. 2, Table 1). Barring a few exceptions, ICRs generally migrated from anterior to
Discussion
In order to better place current findings regarding ICR patterns in context with past studies, results are discussed further within the frame of three aspects of lumbar segmental translation: 1) amplitude of the centrode; (2) average COR location in the sagittal plane; and (3) direction of ICR migration vis-à-vis the corresponding rotational direction.
Conflicts of interest statement
The authors have no conflict of interest related to the manuscript or the work it describes.
Disclaimer
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.
Acknowledgments
The work was funded by a research grant (R21OH00996) from the Centers for Disease Control and Prevention/National Institute for Occupational Safety and Health (CDC/NIOSH). Additional support was received through the Marie Sklowdoska-Curie Cofund postdoctoral fellowship award (EMPAPOSTDOCS 267161) and Ambizione Career Grant Award (PZ00P2_154855/1) from the Swiss National Science Foundation (SNSF). The authors thank Dr. Scott Tashman for technical advice on DSX data acquisition. The authors also
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