Interpedicular kinematics in an in vitro biomechanical assessment of a bilateral lumbar spondylolytic defect
Introduction
Spondylolysis, an osseous defect of the pars interarticularis of the vertebral arch, occurs in nearly 6% of the population (with some ethnic variations) particularly during childhood and adolescence (Fredrickson et al., 1984, Morita et al., 1995, Simper, 1986). While the majority of cases are developmental in nature without any known cause, the defect is prominent amongst athletes involved in sports requiring repetitive flexion–extension of the spine, such as gymnasts, football linemen and weightlifters (Ferguson et al., 1974, Jackson et al., 1976, Rossi and Dragoni, 1990, Standaert and Herring, 2000).
Most commonly affecting the L5 and L4 vertebrae, spondylolysis is postulated to arise from an accumulation of repetitive microtrauma (Farfan et al., 1976, Standaert and Herring, 2000). The inferior facets of the cephalad vertebra and the superior facets of the caudal vertebra impose contact stresses on the lamina of the index vertebra during lumbar extension, which may induce a stress fracture. This more commonly occurs in people with congenitally weak or dysplastic pars interarticularis during early childhood when the posterior arch is not completely ossified and the intervertebral disc is still very elastic, making the pars susceptible to fatigue failure (Roche and Rowe, 1951, Simper, 1986, Wiltse and Jackson, 1976).
Spondylolysis can progress to spondylolisthesis, but progression is uncommon in the presence of < 30% slippage at prognosis, and rarely occurs after adolescence (Fredrickson et al., 1984, Seitsalo et al., 1991). In a 45 year follow-up evaluation study of 500 first-grade school children, Beutler et al. (2003) found that bilateral spondylolysis was more prevalent (73%) than unilateral spondylolysis (27%). The authors further found that unilateral spondylolysis never progressed to spondylolisthesis or disability, and in some cases self healing of the defect was also observed (Beutler et al., 2003).
From a biomechanical standpoint, it remains unclear whether a bilateral spondylolytic defect in lumbar spine intrinsically results in increased intervertebral translations during physiologic bending motions, or does the defect induces secondary changes in the load distribution and motion patterns amongst spinal elements, or a combination of both which may eventually lead to spondylolisthesis. A few biomechanical in vitro studies have quantified kinematic changes following a bilateral spondylolytic defect and immediate stability offered by various surgical treatment procedures, mostly using conventional metrics viz. range of motion (ROM), linear and angular extensions (Deguchi et al., 1999, Fan et al., 2010, Mihara et al., 2003). Additional metrics may however be necessary to capture full extent of the three dimensional trajectories traced by vertebral bodies, and any changes in intervertebral translatory motions following the defect. In this biomechanical in vitro assessment of a bilateral spondylolytic defect in lumbar spine, interpedicular metrics proposed by Cook et al. (2012) were further developed to capture bending-plane and out-of-plane projections of intervertebral translations.
The tests were performed on cadaveric kangaroo lumbar spine segments, which like other mammalian spines have morphological adaptations to accommodate specific biomechanical demands (Boszczyk et al., 2001). Despite perceived morphological differences, Alini et al. (2008) reported similarities in median axial torsion range of motion (ROM) between kangaroo and human lumbar spine segments. Kangaroo lumbar spines have been used previously in an in vitro biomechanical study to test the efficacy of a nucleus replacement implant (Sabet et al., 2010). Bipedal locomotion, comparable adult stature and mass, upright posture, and similarities in vertebral anatomy are some of the factors that make kangaroo an ideal animal model for the study of human lumbar spine (Brown et al., 1992, Hopwood, 1976, Kostuik, 1992).
The main objectives of this study were:
- a)
Using conventional metrics (range of motion (ROM) and neutral zone (NZ)), to evaluate global and segmental (defect and cephalad level) kinematic changes following a bilateral spondylolytic defect in lumbar spine during bending motions of flexion-extension (FxEx), bilateral bending (LB) and axial torsion (AXT).
- b)
Using modified interpedicular kinematics, to further assess whether a bilateral spondylolytic defect in lumbar spine results in increased bending-plane and/or out-of-plane intervertebral translations during the three different bending motions.
Section snippets
Specimen preparation and in vitro testing
Fourteen fresh frozen lumbar spines (L1–L6) were obtained from kangaroo cadavers of undetermined age and sex (Maverick Biosciences, Dubbo, Australia). Specimens were first externally examined for any damage, and then screened for bony abnormalities or fractures under a C-arm X-ray scanner (Zeihm Solo Mobile C Arm, Nuremberg, Germany). Seven specimens were selected after screening, and stored in double plastic bags until preparation and testing. The specimens were thawed overnight at 4 °C
Angular measurements recorded by the motion tracking device and the spine simulator
A strong positive correlation (R2 = 0.999) was found between angular measurements recorded by the opto-electronic motion tracking device and the kinematic spine simulator. The slope of the regression line (0.984, 95% confidence interval: 0.983–0.985) and a relatively small root mean square error (0.005) suggested that the opto-electronic motion tracking device recorded angular measurements with a high degree of accuracy.
Global
Following the defect, median global ROM increased significantly from 27.4° to
Discussion
To the best of our knowledge, interpedicular metrics have not been used previously in the biomechanical assessment of a bilateral spondylolytic defect in the lumbar spine. In this study, we further developed interpedicular metrics to investigate whether a bilateral spondylolytic defect in the lumbar spine causes any significant changes in bending-plane and/or out-of-plane intervertebral translations during different bending motions.
Our results suggest that following the defect, changes in
Conclusions
This biomechanical study exhibited that a bilateral spondylolytic defect at L4 results in a significant increase in global lumbar ROM during FxEx and AXT motions. At the defect level (L4–L5), ROM increased significantly in all three bending motions; but at the cranial adjacent level (L3–L4) ROM increased significantly during AXT motion and decreased during FxEx motion. The defect did not cause any significant change in bending-plane and out-of-plane translations at L4–L5. However, a significant
Conflict of interest
None declared.
Acknowledgments
The authors thank Patrick Lam for help with statistical analysis, Nerida Grewal for assistance with C arm X-ray scanning, and Naomi Tsafnat for providing constructive critical feedback which greatly improved this manuscript. This work was supported by an International Postgraduate Research Scholarship from the Australian Government—DIISRTE and UNSW to the first author (UC), and an internal research grant from Spine Service.
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