An intact rib cage bolsters clinical relevance
By Andrew T. Healy, MD, and Thomas E. Mroz, MD
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Procedures that decompress the thoracic spinal cord are common and inevitably convey some degree of change to spinal kinematics at the surgical level. Whether or not to proceed with instrumented fusion following these procedures — to prevent spinal instability or long-term degeneration and pain — is a critically important yet largely unexplored question.
Recent research1-3 from Cleveland Clinic’s Spine Research Lab has begun to help quantify the biomechanical effects of these types of procedures in the thoracic region. That research and its potential implications are summarized here.
Despite its specialized and resilient design, the spinal column is a frequent source of pain and disability from degeneration, disk herniation, infection, tumor and traumatic pathologies. The thoracic spine, as the longest of the spinal segments, frequently incurs these pathologies.
Part of what makes the thoracic spine unique are the stenocostovertebral articulations and continuity of the rib cage, which afford increased stiffness and stability relative to the cervical and lumbar spine. As a testament to the contribution of the rib cage, in vitro testing shows that the thoracic spine will achieve over 700 percent greater motion in extension simply if the sternum is removed.4
Because previous platforms for cadaveric spinal testing were not equipped to test the full thoracic spine with associated rib cage, the bulk of the historical data on thoracic biomechanics has been obtained by testing specimens disarticulated from the rib cage. Therefore, biomechanical data quantifying the consequences of decompressive procedures on the thoracic spine in the clinically relevant scenario — with an intact rib cage — have been limited, in contrast to biomechanical data regarding the cervical and lumbar spine.
Figure 1. The robotic spine testing system with a cadaveric specimen.
In 2013 we set out to fill some of this knowledge gap surrounding the stability of the thoracic spine following decompressive procedures. Specifically, we used an industrial robot manufactured by KUKA Systems GmbH (Augsburg, Germany) to perform multidirectional flexibility tests on 19 fresh frozen human cadaveric thoracic spine specimens with the rib cage intact (Figure 1).
The specimens were tested first in their intact state, then after each of three sequential surgical decompressive procedures at T4-5 or T8-9 — (1) laminectomy, (2) unilateral facetectomy and (3) unilateral costotransversectomy — and then after instrumented fusion from T3 to T7 (Figure 2).
CT and dual-energy X-ray absorptiometry (DXA) scans of each specimen were carried out to determine pre-existing spinal pathology or fusion and the bone mineral density of each specimen. Custom-designed spinal fixtures were used to secure the spine cranially and caudally onto the robotic spine testing system. The cranial (C7-T1) and caudal (T12-L1) levels were mounted onto the custom test fixtures using pedicle screws and rods.
Figure 2. The sequential decompressive procedures used in the study: laminectomy (top left), unilateral facetectomy (top right), unilateral costotransversectomy (bottom left) and instrumented fusion (T3 to T7) (bottom right). Reprinted from Healy et al.1
A six-axis, force-moment sensor (Gamma, ATI Industrial Automation, Apex, North Carolina) was used to measure the applied load and provide feedback for the robot. Three-dimensional motion was monitored continuously using an optoelectronic camera system (Optotrak Certus®, Northern Digital Inc., Waterloo, Ontario, Canada) at a rate of 20 Hz. The camera system measured the vertebral motion by tracking the relative motion between infrared markers placed on rigid body vertebral segments. This system has a measurement accuracy of ±0.1 mm in translation and ±0.1 degrees in rotation.
Collectively this testing enabled us to measure the change in range of motion across the surgical levels.
We found that in all three planes of motion, the sequential decompressive procedures caused no statistically significant change in motion across the surgical level when compared with the intact state, likely due to the tremendous stability afforded by the thoracic rib cage.
We also found that despite the presence of the semirigid rib cage, the addition of pedicle screw fixation dramatically and effectively decreased the range of motion across surgical levels if necessary (see Figure 3). Complete results are available in our full-length publications.1-3
Figure 3. Mean range-of-motion values for all specimens in the intact state and after each of the surgical decompressive procedures. Reprinted from Healy et al,1 ©2014, with permission from Elsevier.
Our studies showed that laminectomy, unilateral facetectomy and unilateral costotransversectomy at the level of both the true and false ribs did not significantly alter the range of motion in our cadaveric model, suggesting that such procedures may not require instrumentation. The potential to avoid instrumentation, if confirmed, would be significant, since proceeding with instrumentation carries additive operative risk for the patient and added cost to the healthcare system.
At this stage of investigation, long-term consequences such as gradual deformity or patient discomfort cannot be ruled out and require future investigation. Since the completion of these studies, we continue to use the robotic testing system to answer fundamental biomechanical questions directly translatable to spinal surgery and patient care.
Dr. Healy is chief resident in neurosurgery in Cleveland Clinic’s Center for Spine Health.
Dr. Mroz is a spine surgeon and Co-Director of the Center for Spine Health.
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