Biomedical Engineering Reference
In-Depth Information
that the displacement under applied tension across the insertion is region-
dependent (Figure 17.3A), with the highest displacement found at the ACL, then
decreasing in magnitude from the interface to bone.
These regional differences suggest an increase in tissue stiffness from liga-
ment to bone. In addition, both tensile and compressive strain components were
detected at the insertion while the knee was loaded in tension. These fi ndings are
in agreement with fi nite-element model analyses of the medial collateral ligament
(MCL) performed by Matyas et al., which predicted that the maximum principal
tensile stresses are located in the MCL mid-substance, while the maximum prin-
cipal compressive stresses occurred near the distal edge of the MCL- to - bone
insertion (Matyas, Anton, Shrive, and Frank 1995).
Fibrocartilage is often localized in the anatomical regions subjected to com-
pressive loading (Vogel 1995; Vogel and Koob 1989). It is thus likely that the
insertion fi brocartilage will also bear the compressive strain detected in the elas-
tography analysis (Spalazzi, Gallina, Fung-Kee-Fung, Konofagou, and Lu 2006).
Recently, Moffat et al. performed the fi rst experimental determination of the
compressive mechanical properties of the ACL-bone interface (Moffat et al.
2005). Combining microscopic mechanical testing with optimized digital image
correlation methods (Wang et al. 2003), the region-dependent changes in inter-
face mechanical properties were quantifi ed. In this method, the cells residing in
the interface were fi rst stained with Hoechst nuclear dye in order to enhance
texture correlation following the methods of Wang et al. (Wang et al. 2002).
The samples were subsequently loaded in a custom unconfi ned compression
microscopy device. Displacement under the applied load was then imaged using
epifl uorescence microscopy and digital image analysis was performed to deter-
mine the mechanical properties. As shown in Figure 17.3B, the incremental
displacement decreased gradually from the non-mineralized to mineralized fi bro-
cartilage to bone, indicating an increase in tissue stiffness across these regions.
The interface also exhibited a region-dependent decrease in strain and a signifi -
cantly higher elastic modulus for the mineralized fi brocartilage when compared
to the non-mineralized fi brocartilage zone (Moffat, Chahine, Hung, Ateshian, and
Lu 2005). These regional mechanical responses enable a gradual transition rather
than an abrupt increase in tissue strain across the insertion, and are likely intrin-
sic to the multi-tissue ACL-to-bone interface, as they were evident under both
the applied tension and compression. These observations collectively demon-
strate the functional importance of the fi brocartilage interface in mediating load
transfer between soft and hard tissue.
Given the structure-function dependence inherent in the biological system,
the region-dependent changes in mechanical properties reported by Moffat et al.
(Moffat, Chahine, Hung, Ateshian, and Lu 2005) are likely correlated to changes
in matrix organization and composition across the interface. Characterization of
the insertion site using Fourier Transform Infrared Imaging (FTIR-I) (Spalazzi,
Boskey, and Lu 2007) and X-ray analysis (Moffat, Chahine, Hung, Ateshian, and
Lu 2005) revealed an abrupt change in calcium and phosphorous content pro-
gressing from ligament, to interface, then to bone (Moffat et al. 2006; Spalazzi
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