Characterization of L4-L5 disc strains during in-vitro loading using 3D optical digital imaging correlation techniques: what is normal?

Introduction: Very little data exists describing the sub-regional behavior of the intact intervertebral disc (IVD) under load. Illumination of factors that lead to excessive or deviated IVD stresses and strains, and potential biomechanical rationales for IVD failure (e.g. adjacent segment degeneration), first requires a methodology to quantify and establish normal disc strains.

Aims/

Objectives: To characterize for the first time, IVD surface strains using a noncontact optical measurement method under standardized in-vitro loading.

Methods: 14 human L3-S1 spine segments were tested in multiple directions (7.5 Nm flexion-FL, extension-EX, lateral bending-RLB/LBL, axial rotation-RAR/LAR, and 400N compression-C). Principal strains and strain directions across the disc surface were captured by application of a fine speckled pattern on the IVD surface which was optically tracked and analyzed using a 3D digital imaging correlation (DIC) system (Vic-3D). Cameras were positioned laterally (Fig1). The left side (only) L4-5 IVD was divided into four similar sized quarters (Q1-Q4 anterior to posterior), and mean principal strains were determined in each between rest and peak loads. Differences were analyzed using 1-way ANOVA (p < 0.05). By definition maximum principal strain (Pmax) is strain along the direction of maximal stretch or elongation, while minimum principal strain (Pmin) is strain at 90° to Pmax. Pmin is usually negative and denotes the maximum amount of contraction (e.g. a stretched rubber band elongates while narrowing in width).

Results: The greatest strains occurred posteriorly (Q4) during right lateral bending (RLB) (Pmax 108277±65579uE), right axial rotation (RAR) (Pmax of 44407±30102uE) and flexion (FL) (Pmax of 43532±27803 uE). Pmax decreased in magnitude from posterior (Q4) to anterior (Q1) during RLB, with significant differences between all Q’s (p≤0.046). Pmax during RAR was greater posteriorly (Q4) compared to Q1 and Q2 (p≤0.004). Pmax during compression (C) was also greater posteriorly (Q4) compared to the remaining Q’s (p≤0.034). The largest Pmin strains were dependent on load direction, with the greatest magnitudes occurring midposteriorly in Q3 (-76440±40039uE) and posteriorly in Q4 (-72514±38527uE) during left lateral bending (LLB), and anteriorly in Q1 (-69847±26694uE) and mid-anteriorly in Q2 (-55047±20334uE) during FL. The largest magnitude Pmin during C and extension (EX) was seen posteriorly in Q4 (-31921±17578uE and -22852±13652uE, respectively). There were significant differences in the mean Pmax strain direction (gamma) among Qs during all load directions (p≤0.049). RAR resulted in the most vertically oriented Pmax direction (Q3 and Q4), while C (all Q’s) and FL (Q1 and Q2) resulted in the most horizontally oriented Pmax directions likely due to disc bulging.

Conclusions: Sub-regional analysis of principal strains across the intact L4-5 disc showed that different directions of load result in unique and significantly different strain distribution patterns across the anterior-posterior disc gradient. The highest strain magnitudes were observed during lateral bending. Strain tended to be highest in posterior regions in general. Healthy IVD bulging produced distinct anterior-posterior oriented circumferential strain patterns.