Apportionment of lumbar L2–S1 rotation across individual motion segments during a dynamic lifting task

Abstract

Segmental apportionment of lumbar (L2–S1) rotation is a critical input parameter for musculoskeletal models and a candidate metric for clinical assessment of spinal health, but such data are sparse. This paper aims to quantify the time-variant and load-dependent characteristics of intervertebral contributions to L2–S1 extension during a dynamic lifting task. Eleven healthy participants lifted multiple weights (4.5, 9.1, and 13.6 kg) from a trunk-flexed to an upright position while being imaged by a dynamic stereo X-ray system at 30 frames/s. Vertebral (L2–S1) motion was tracked using a previously validated volumetric model-based tracking method that employs 3D bone models reconstructed from subject-specific CT images to obtain high-accuracy (≤0.26°, 0.2 mm) 3D vertebral kinematics. Individual intervertebral motions as percentages of the total L2–S1 extension were computed at each % increment of the motion to show the segmental apportionment. Results showed L3–L4 (25.8±2.2%) and L4–L5 (31±3.1%) together contributed a larger share (∼60% combined) compared to L2–L3 (21.7±3.7%) and L5–S1 (22.6±4.7%); L4–L5 consistently provided the largest contribution of the measured segments. Relative changes over time in L3–L4 (6±12.5%) and L4–L5 (0.5±10.2%) contribution were minimal; in contrast, L2–L3 (18±20.1%) contribution increased while L5–S1 (−33±22.9%) contribution decreased in a somewhat complementary fashion as motion progressed. No significant effect of the magnitude of load lifted on individual segmental contribution patterns was detected. The current study updated the knowledge regarding apportionment of lumbar (L2–S1) motion among individual segments, serving both as input into musculoskeletal models and as potential biomechanical markers of low back disorders.

Introduction

A clear understanding of lumbar biomechanics has implications on both the treatment and prevention of low back disorders (LBD). From a clinical perspective, treatment of chronic LBD aims to restore normal functional motion, in addition to the more subjective goal of mitigating pain symptoms. From a prevention perspective, recognition and control of biomechanical risk factors require a thorough understanding of force and stress distributions within the spine during functional tasks. In both cases, an accurate description of dynamic, three-dimensional (3D) vertebral kinematics is necessary (Waters et al., 1993). This is because (a) normal functional motion benchmarks are mainly based on kinematic measures, and (b) given the infeasibility of direct measurement, in vivo forces experienced by lumbar tissue structures are estimated from biomechanical models (Cholewicki et al., 1991, de Zee et al., 2007, Han et al., 2012, Senteler et al., 2015, Stokes and Gardner-Morse, 1995, Waters et al., 1993, Zhu et al., 2013), the process of which is highly sensitive to the quality of kinematic input.

Segmental range of (rotational) motion (ROM) determined from static, lateral radiographs of end-range flexion–extension positions has traditionally been the primary kinematic parameter used in clinical diagnoses—although its efficacy in distinguishing between patients with disorders and asymptomatic ones has been questioned (Ellingson et al., 2013, Lehman, 2004)—and in assessing the success of treatment procedures, particularly those requiring surgical intervention. While ROM is an important, yet simple metric for assessing lumbar joint function, it is, by itself, inadequate for characterizing lumbar motion. For example, the contribution to overall lumbar rotation may differ between individual segments (Ahmadi et al., 2009, Panjabi et al., 1994, Pearcy et al., 1984, Wong et al., 2004) and, more importantly, ROM-based kinematic evaluations may not appropriately represent differences in mid-range rotational characteristics (Anderst et al., 2008, Teyhen et al., 2007) between healthy and pathologically- or surgically-altered spines. Secondly, accurate data regarding the apportionment of lumbar rotation, particularly during functional, daily living activities, are a critical input for musculoskeletal biomechanical models. Musculoskeletal models (Christophy et al., 2012, de Zee et al., 2007, Han et al., 2012, Senteler et al., 2014) often define segmental kinematics based on constant fractions of overall lumbar motion. Given this context, quantifying the apportionment of lumbar rotation across its individual segments and clarifying its time-variant and load-dependent characteristics could have a direct positive impact on the accuracy of model predictions, while also potentially leading to improved biomechanical markers of pathological conditions.

Several studies have attempted to quantify segmental contributions to overall lumbar rotation. These include in vitro cadaveric studies (Goel et al., 1985, Miller et al., 1986, Schultz et al., 1979, Soni et al., 1982, Tencer et al., 1982), in vivo studies based on 2D lateral radiographs or static biplane radiography (Li et al., 2009, Passias et al., 2011, Pearcy et al., 1984, Plamondon et al., 1988), surface marker-based studies (Troke et al., 2001, Zhang and Xiong, 2003), uniplanar continuous radiography (Ahmadi et al., 2009, Harada et al., 2000, Kanayama et al., 1995, Okawa et al., 1998, Wong et al., 2004, Wong et al., 2006), and, more recently, dynamic biplane radiography (Aiyangar et al., 2014, Anderst et al., 2008, Wu et al., 2014). These studies have collectively and progressively improved our understanding of lumbar spinal motion. However, reported results regarding segmental contribution patterns have been inconsistent. Some studies have reported an increased contribution from caudal segments (L4–5, L5–S1) compared to the cephalic segments (Panjabi et al., 1994, Pearcy et al., 1984), others have reported a progressively decreasing contribution from cephalic to caudal segments (Li et al., 2009, Wong et al., 2004, Wong et al., 2006), while still others failed to detect significant differences in contributions (Aiyangar et al., 2014, Goel et al., 1985, Schultz et al., 1979, Tencer et al., 1982, Wu et al., 2014). Differences in experimental protocols do not completely account for these variations as the inconsistency persists even between studies using similar techniques. For example, while Pearcy et al. (1984) and Plamondon et al. (1988) reported an increased contribution from caudal segments compared to cephalic segments from their static biplane X-ray imaging studies, Li et al. (2009) observed the opposite using comparable methods. Harada et al. (2000), using continuous X-ray imaging studies where the pelvis was restrained, reported a phase lag between segments while Wu et al. (2014) and Teyhen et al. (2005) reported simultaneous contributions. Comparing studies that eschewed explicit pelvic restraints, Wong et al. (2004) reported a progressively decreasing contribution from L1–2 to L5–S1, but these results were not replicated in a study by Ahmadi et al. (2009). As a result, how total lumbar rotation is apportioned across its individual segments throughout a movement remains unresolved.

Therefore, the purpose of the current study was to map the continuous segmental percent contributions to lumbar extension motion during load lifting—a common functional activity— and seek answers to the following questions: (1) Do some segments bear a larger share of lumbar rotation compared to others? (2) Does this contribution remain constant over the entire range of motion, or does it vary from beginning to end? (3) Does the amount of external load borne during a particular task affect the distribution of lumbar rotation across its segments?

Note: Due to limitations in the dimensions of the experimental setup, only vertebrae from L2–S1 were observed in this study. Hence readers are informed that the term “lumbar” henceforth refers only to “L2–S1” and does not include T12–L1 and L1–L2 joint information.