Lumbar Spine Intervertebral Centers of Rotation During Lifting Motion
George Kontogiannis1, Ameet K. Aiyangar, PhD2, William J. Anderst, MS1, Xudong Zhang1.
1
University of Pittsburgh, Pittsburgh, PA, USA, 2Univbersity of Pittsburgh, Pittsburgh, PA, USA.
Disclosures:
G. Kontogiannis: None. A.K. Aiyangar: None. W.J. Anderst: None. X. Zhang: None.
Introduction: Abnormal spine motion may result in excessive loading to the intervertebral discs, leading to disc degeneration.
Intervertebral range of motion is a standard metric used to evaluate spine motion. However, range of motion only describes the
amount of motion, not the quality of motion. The instantaneous center of rotation (ICR) has been proposed as a reliable, stable
measurement of the quality of vertebral motion through which abnormalities could be explored [1]. Characterizing the quality of
lumbar spine motion may be important for identifying differences in function due to gender, external load, way to handle the
load, pathology or surgical intervention. The ICRs for spine motion segments have been derived from end-range-of-motion static
single [2] or biplane [3]radiographs, and surface-based measurements [4], which may be inadequate for accurate
characterization of in vivo functional motion. The purpose of this study was to determine the ICR at each lumbar motion
segment during continuous lifting movements in young, healthy subjects as they lifted different weights. It was hypothesized
that the average location and range of the ICR would vary significantly across motion segments and weight levels.
Methods: Ten asymptomatic subjects (24±3 years; 6M, 4F) provided informed consent to participate in this IRB-approved study.
Participants performed a continuous straight-legged lifting (i.e., back-lift) movement where their lumbar region was imaged by a
bi-plane X-ray system. The biplane radiographs were collected at 30 frames per second for 2 seconds as subjects lifted a weight
(10 lb, 20 lb, and 30 lb) from ankle-level to the upright position. Two trials were performed for each weight. Subject-specific
bone models (L1-S1) derived from CT were used with a computerized tracking program to reproduce bone orientation and
location in 3D space for each x-ray frame with sub-millimeter accuracy [5]. The continuous motion path of the ICR at each
lumbar motion segment was calculated using the perpendicular bisector method with a step size of 2.0º and expressed as a
percentage of inferior bone size. Thus, the ICR was calculated approximately 25-35 times during each continuous lifting
movement, and interpolated at one-degree increments of intervertebral rotation. Repeated measures analysis of variance was
used to assess the effect of weight and lumber motion segment on the average anterior-posterior (AP) and superior-inferior (SI)
location and motion range of the L2/L3 to L4/L5 ICR.
Results: The average locations of the ICRs of L1 through L4 were slightly superior and posterior to the geometric center of the
respective inferior vertebra (Table 1). The L5 ICR was near the superior endplate of S1 (Figure 1). No significant differences in
average SI or AP location of the ICR were identified with respect to weight (p = 0.306 and p = 0.630, respectively) or vertebral
levels (p = 0.170 and p = 0.553, respectively). Likewise, no significant differences in average SI or AP range of the ICR (Table 2)
were identified with respect to weight (p = 0.768 and p = 0.452, respectively) or
with respect to vertebral level (p = 0.468 and p = 0.644, respectively). Intervertebral range of motion ranged from 9° at L1/L2 to
14° at L4/L5 (Table 3).
Table1: Average (±SD) SI and AP locations of the ICRs in % of lower vertebra size
SI
L1/L2
L2/L3
L3/L4
L4/L5
L5/S1
21.2 ± 20.8
28.3 ± 29.6
32.2 ± 13.0
38.4 ± 21.3
-0.23 ± 8.73
-0.500 ± 27.2
-2.58 ± 12.4
-4.04 ± 17.2
-15.1 ± 35.2
AP -5.03 ± 14.5
Table 2: Average (±SD) SI and AP ICR ranges of the path in % of inferior vertebral body size.
L2/L3
L3/L4
L4/L5
L5/S1
SI
57.5 ± 55.2
52.0 ± 43.0
66.4 ± 57.2
26.7 ± 24.1
AP
69.5 ± 37.0
60.1 ± 39.5
61.1 ± 49.8
60.5 ± 47.8
Table 3: Average (±SD) intervertebral ROM (in degrees).
L1/L2
L2/L3
L3/L4
L4/L5
L5/S1
9.0 ± 2.5 12.0 ± 7.6 11.3 ± 2.3 14.2 ± 6.2 10.3 ± 4.1
Discussion: The average location of the ICR and the range of the ICR path remained consistent across lumbar motion segments
L1-L2 to L4-L5. ICRs presented by Pearcy [2] are currently the most widely used dataset for representing spinal joints as spherical
joints in rigid body dynamics simulations[6,7]. Mean ICRs from the current study were, comparatively, more inferior and anterior
compared to Pearcy [2], but were closer to values presented by Xia et al. [3]. Additionally, the magnitude of weight lifted did not
seem to affect ICR location or path length. These finding suggest consistent motion quality across lumbar motion segments,
regardless of vertebral level or weight magnitude. However, consistent migration patterns of ICR during the motion could not be
identified, even though it has been previously shown that accompanying translation in motion segments is substantial [8].
Significance: This is the first study that characterized the lumbar intervertebral centers of rotation based on high-accuracy
skeletal kinematic data in vivo. The new data challenge the existing knowledge about normal spine motion and its clinical
ramifications.
Acknowledgments:
References: 1. Bogduk, Clin Biomech., 2000. 2. Pearcy, Spine 1988., 3.Xia, J. Biomech., 2010. 4 2012. 4. Zhang, J. Biomech., 2003.
5. Anderst, Spine, 2011. 6. DeZee, J. Biomech., 2007. 7. Christophy, Biomech. Model. Mechanobiol., 2012., 8. Aiyangar, ORS,
2012.
ORS 2014 Annual Meeting
Poster No: 0092