Fibril deformation under load of the rabbit Achilles tendon and medial collateral ligament femoral entheses

Abstract

Microscopic visualization under load of the region connecting ligaments/tendons to bone, the enthesis, has been performed previously; however, specific investigation of individual fibril deformation may add insight to such studies. Detailed visualization of fibril deformation would inform on the mechanical strategies employed by this tissue in connecting two mechanically disparate materials. Clinically, an improved understanding of enthesis mechanics may help guide future restorative efforts for torn or injured ligaments/tendons, where the enthesis is often a point of weakness. In this study, a custom ligament/tendon enthesis loading device was designed and built, a unique method of sample preparation was devised, and second harmonic and two-photon fluorescence microscopy were used to capture the fibril-level load response of the rabbit Achilles tendon and medial collateral ligament femoral entheses. A focus was given to investigation of the mechanical problem of fibril embedment. Resultant images indicate a rapid (occurring over approximately 60 μm) change in fibril orientation at the interface of ligament/tendon and calcified fibrocartilage early in the loading regime, before becoming relatively constant. Such a change in fibril angle helps confirm the materially graded region demonstrated by others, while, in this case, providing additional insight into fibril bending. We speculate that the scale of the mechanical problem (i.e., fibril diameters being on the order of 250 nm) allows fibrils to bend over the small (relative to the imaging field of view, but large relative to fibril diameter) distances observed; thus, potentially lessening required embedment lengths. Nevertheless, this behavior merits further investigation to be confirmed. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2506-2515, 2018.

Gaining Insight into the Deformation of Achilles Tendon Entheses in Mice

“…The changes in volume also bring about changes in the angles of the fibers at the insertion to the hard tissues. The volume renderings confirm findings in sliced specimens by Sevick et al, [32] who observed that fibers under load change direction over a short distance. This implicates very small curvature radii – and could lead to high local stress which is in contrast with previous ideas of enthesis function.…”

“…[29]- [31] Slicing significantly alters tissue biomechanics by generating an open surface where pressurized liquid can evade. [32] Therefore, it is still not clear, whether the high strains at the insertion have to be partly attributed to the destruction of tissue structure by slicing.…”

“…[29]–[31] Slicing significantly alters tissue biomechanics by generating an open surface where pressurized liquid can evade. [32] Therefore, it is still not clear, whether the high strains at the insertion have to be partly attributed to the destruction of tissue structure by slicing. A narrow region with high compliance could well be the reason for the broader finding of a compliant insertion. To check this, the local distribution of tissue strain along the transition from tendon to bone has to be investigated in intact specimens at high resolutions. Connizzo and Grodzinsky [30][31] examined tendon surfaces by nanoindentation.…”

“…Volume changes in the tendon and liquid accumulation in its surrounding, e.g. in neighboring bursae, should be measured to provide reference values. Sevick et al [32] reported that fibers change their angle over a very short distance at the insertion into hard tissue under load. They suggest that the fiber curvature radius should not fall below a minimum determined by the bending strength of the fibers.…”

“…Sevick et al [32] reported that fibers change their angle over a very short distance at the insertion into hard tissue under load. They suggest that the fiber curvature radius should not fall below a minimum determined by the bending strength of the fibers.…”