Quantitative Mapping of Matrix Content and Distribution across the Ligament-to-Bone Insertion

Abstract

The interface between bone and connective tissues such as the Anterior Cruciate Ligament (ACL) constitutes a complex transition traversing multiple tissue regions, including non-calcified and calcified fibrocartilage, which integrates and enables load transfer between otherwise structurally and functionally distinct tissue types. The objective of this study was to investigate region-dependent changes in collagen, proteoglycan and mineral distribution, as well as collagen orientation, across the ligament-to-bone insertion site using Fourier transform infrared spectroscopic imaging (FTIR-I). Insertion site-related differences in matrix content were also evaluated by comparing tibial and femoral entheses. Both region- and site-related changes were observed. Collagen content was higher in the ligament and bone regions, while decreasing across the fibrocartilage interface. Moreover, interfacial collagen fibrils were aligned parallel to the ligament-bone interface near the ligament region, assuming a more random orientation through the bulk of the interface. Proteoglycan content was uniform on average across the insertion, while its distribution was relatively less variable at the tibial compared to the femoral insertion. Mineral was only detected in the calcified interface region, and its content increased exponentially across the mineralized fibrocartilage region toward bone. In addition to new insights into matrix composition and organization across the complex multi-tissue junction, findings from this study provide critical benchmarks for the regeneration of soft tissue-to-bone interfaces and integrative soft tissue repair.

Allometry of the Tendon Enthesis: Mechanisms of Load Transfer Between Tendon and Bone

“…Functional grading in mechanical properties across an interface between dissimilar materials decreases the stress concentrations that otherwise would occur at an abrupt change in mechanical properties and allows for more effective stress transfer [6]. In biological systems, graded structures can be found at many interfaces [43], including the beak-tomouth interface in squids [44], the anchor-to-foot interface in mussels [45], and as discussed here, the soft tissue-to-bone interface in mammals [21,22,[46][47][48][49][50]. The width of the supraspinatus tendon-to-bone mineral gradient has been previously examined in mice and found to span a distance of $20 lm [22].…”

Next generation tissue engineering of orthopedic soft tissue-to-bone interfaces

“…[39,40] The mineral phase in calcified fibrocartilage is also mainly carbonate-substituted hydroxyapatite, but the organization of the crystals within the matrix is not as well understood. [11,41] …”

“…These techniques have been more frequently applied to bone, cartilage, tendon, and ligament than the entheses. [11,41,186,187] …”

“…[11–13] Comparative structures of tendon, ligament, and meniscal entheses are highlighted using a tetrachrome stain of sagittal sections of these interfaces (Fig. 3).…”

“…However, recent evidence indicates the presence of a mineral gradient at what has been historically referred to as the tidemark, meaning that this shift in composition may be less distinct. [7,11,18] …”