Viscoelastic Effect in Three Cycle Biomechanical Testing: Range of Motion during First Cycle Compared to Third Cycle

Presented at SMISS Annual Forum 2014
By Mark Moldavsky MS
With Brandon Bucklen PhD,

Disclosures: Mark Moldavsky MS E; Globus Medical Inc. Brandon Bucklen PhD E; Globus Medical Inc.,

Cadaveric biomechanical testing is a useful resource for clinicians. Testing can help answer questions regarding injury mechanisms, immediate postoperative stability of multiple constructs, adjacent level effects, kinematics of disc replacement prosthesis and many more. Due to the viscoelastic nature of human tissue, it is common practice to test constructs for three full cycles and acquire data from the third cycle.

It is accepted knowledge that motion increases from the first to third cycle but no study has been performed to quantify the increase in motion and if the increase significant. The objective is to perform a meta-analysis using biomechanical testing data, of lumbar spines from a single institution, and compare the range of motion (ROM) of the first cycle to the third cycle.

A sample size of 74, 79, and 75 was gathered for flexion-extension, lateral bending, and axial rotation respectively. Cadaveric lumbar spines were tested within standard limits of 7-10Nm. Intact ROM between single lumbar motion segments was analyzed. The first and third hysteresis curves were isolated. Abnormal hysteresis curves or motion with missing data was excluded. A two sample t-test assuming equal variances was used to determine statistical significance (p≤0.05).

Flexion-extension ROM was 8.03±2.86° in the first cycle and increased to 8.25±2.93° in the third cycle. Similarly, lateral bending increased from 7.90±3.00° to 8.11±3.12° from the first to third cycle, respectively. Finally, axial rotation motion increased from 5.12±2.37° to 5.29±2.47°, respectively. A one-tail statistical analysis resulted in p-values of 0.32, 0.33, and 0.33 for flexion-extension, lateral bending, and axial rotation, respectively.

The largest increase in motion from the first to third cycle was 0.22 deg in flexion-extension. The effect of testing for three cycles did not result in a significant difference in motion in all loading modes. A large sample size comparing first and third cycle motion of intact lumbar segments did not show a significant difference. Biomechanical testing may be restricted to one cycle because there is no significant viscoelastic motion change. Future studies should validate this trend across multi-institution six degree-of-freedom test set-ups.