Static flexion, in combination with compression, negatively impacts the intralamellar and interlamellar matrix properties of the annulus fibrosus. This is the key finding of a new study, published in the European Spine Journal by Diane Gregory (Wilfrid Laurier University, Waterloo, Canada) et al.
The findings suggest that the mechanical properties of the interlamellar and intralamellar matrices are sensitive to flexion, creating an environment that promotes an increased potential for damage to occur.
The research team selected C3/C4 cervical functional spinal units of porcine specimens. Following preloading, all specimens were loaded under 1200N axial compression in either a neutral or static end range flexion posture (15º) for two hours.
Following loading, six annulus samples were dissected from each disc: four single-layer and two multi-layer samples. The multi-layer samples underwent peel tests to quantify the mechanical properties of the interlamellar matrix while the single-layer samples underwent tensile tests to quantify the mechanical properties of the intralamellar matrix.
Statistical comparisons between properties were performed to determine differences between postural condition, extraction location, and extraction depth.
The study found that flexion elicited a decrease in lamellar adhesive strength (p=0.045) and in single-layer failure strain (p=0.03) when compared to a neutral posture. Flexion also had extraction depth-specific effects namely increased intralamellar matrix stiffness in the inner annulus when compared to neutral (p=0019).
Flexion also resulted in a significant decrease in toe region strain for the inner region of the annulus (p=0.035). The inner region of the annulus was shown to have a significant increase in stress at 30% strain when compared to the outer region after flexion (p=0.041).
Speaking to Spinal News International, Gregory said: “Clinically, it is well known that flexion is linked to an increased risk of disc herniation. Our work has been able to provide empirical evidence to support a possible mechanism. Specifically, flexion seems to most notably impact the adhesion between the annular lamellae thereby increasing the disc’s susceptible to delamination and ultimately herniation.”