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Research shows how injury to the posterior ligamentous complex reduces stability
Ligamentum flavum found to play primary role in flexion.
PHILADELPHIA — Cadaveric study results demonstrated that distractive-flexion type injuries to the posterior ligamentous complex reduce stability in non-flexion/extension movements.
“Distraction-flexion injuries compose the largest phylogeny of cervical injuries which present clinically, but treatment remains controversial,” Robert A. Hartman, MS, a predoctoral fellow in the Departments of Bioengineering and Physical Medicine & Rehabilitation at the University of Pittsburgh, told Spine Surgery Today in an interview.
Distraction-flexion injuries involve tearing of posterior cervical ligamentous components, which are the supraspinous ligament (SSL), interspinous ligament (ISL) and the ligamentum flavum (LF), facet and/or facet capsule.
Robert A. Hartman
“Characterizing the contribution of these structures to load-sharing and motion restriction may assist in assessment of spinal stability,” Hartman said. In addition, it may “improve understanding of these commonly injured structures, methods of stabilization and inform treatment-rehabilitation planning.”
Robotic-based axial loading
Hartman, Kevin Bell, PhD, and colleagues used a robot-based spine testing system to explain the role of the posterior ligamentous complex components in stability and load resistance in human cervical functional spinal units.
Kevin Bell
Robotic systems offer an alternative method of describing the role of joint structures by determining the 6°-of-freedom loading role of joint structures, according to Hartman, who presented the results at the Philadelphia Spine Research Symposium in November 2013. The system he and his colleagues used consisted of a serial robotic manipulator with an on-board six-axis load cell and a custom-built specimen mounting fixture.
The researchers subjected 12 adult human cervical spine functional units to flexion-extension, lateral bending and axial-rotation to 2.5 Nm with a 10N compressive preload that simulated in vivo stress conditions. They resected six specimens in a posterior-to-anterior order, and the other six specimens in an anterior-to-posterior manner, Hartman said.
Hartman and colleagues then put the spine units through three cycles of flexibility testing for each state and used adaptive displacement control to simulate an in vivo flexibility test. The robotic system replayed the intact motion path for each resection to determine loading changes.
The robotic system used in this investigation
consisted of a serial robotic manipulator
with an on-board six-axis load cell and a
custom-built specimen mounting fixture.
Image: Hartman RA
General increase in motion
Flexibility testing results showed that for flexion-extension and axial rotation, there was a general increase in motion between 20% and 100%, which suggests that a flexion injury may cause instability in axial rotation, according to Hartman.
With regard to the in situ loading results, the researchers found that primary and intact moments decreased with increasing resection.
“The ligamentum flavum was the largest contributor to loading in flexion, suggesting it is the primary posterior tension band toward end range of motion,” Hartman told Spine Surgery Today. “The capsule and facets were primary contributors to load support in axial rotation and lateral bending.”
Although the anterior column, which includes the longitudinal ligaments and disc, played a large role in all motions, it was greatest in extension, he noted.
An assessment of resection order demonstrated some shifting trends. The most notable deviations affected the ligamentum flavum and anterior column in flexion.
“More loading was shifted to the anterior column from the ligamentum flavum in the reverse cutting order,” Hartman said. “This change, along with a similar one in lateral bending and added loading of the capsules in axial rotation points to possible coupling in the posterior ligamentous complex and underscores the effect of different loading histories on viscoelastic structures.” – by Colleen Owens
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Disclosure: Hartman has no relevant financial disclosures.