Functional attachment of soft tissues to bone: development, healing, and tissue engineering

Abstract

Connective tissues such as tendons or ligaments attach to bone across a multitissue interface with spatial gradients in composition, structure, and mechanical properties. These gradients minimize stress concentrations and mediate load transfer between the soft and hard tissues. Given the high incidence of tendon and ligament injuries and the lack of integrative solutions for their repair, interface regeneration remains a significant clinical challenge. This review begins with a description of the developmental processes and the resultant structure-function relationships that translate into the functional grading necessary for stress transfer between soft tissue and bone. It then discusses the interface healing response, with a focus on the influence of mechanical loading and the role of cell-cell interactions. The review continues with a description of current efforts in interface tissue engineering, highlighting key strategies for the regeneration of the soft tissue-to-bone interface, and concludes with a summary of challenges and future directions.

Figure 1

(a) Tendon and (b) ligament attach to bone across a functionally graded fibrocartilaginous transition site (a toluidine blue–stained section from an adult rat supraspinatus tendon–to-bone insertion is shown in panel a, and fast blue–stained section from a mature bovine anterior cruciate ligament insertion is shown in panel b) (3, 6).

Figure 2

(a) At 18.5 days post conception, scleraxis expression is localized to the supraspinatus tendon and type II collagen expression is localized to the humeral head. A zone of cells at the insertion does not express either marker. (b) Spatially and temporally controlled expression of a number of transcription factors and growth factors drives enthesis development. A question mark (?) indicates that a role for the molecule is expected but has not yet been shown definitively. Abbreviations: BMP, bone morphogenetic protein; Ihh, Indian hedgehog protein; PTHrP, parathyroid hormone–related protein.

Figure 3

(a, left) Spatial gradients in mineral form between supraspinatus tendon and bone at the developing enthesis from the onset of endochondral ossification (age P7 in the mouse, determined via Raman spectroscopy, scale bars = 50 μm for upper left image and 10 μm for lower left image) (38). (right) The mineral gradient migrates from the center of the humeral head at P7 to the tendon attachment by P14. (b) Mineral distribution at the ACL-to-bone insertion increased exponentially across the calcified fibrocartilage region (neonatal bovine, determined via Fourier transform infrared imaging) (72). The significantly higher Ca and P content (*: p < 0.05) in the mineralized fibrocartilage (MFC) versus the nonmineralized fibrocartilage (NFC) region is accompanied by a significant increase in Young’s modulus.

Figure 4

The functionally graded transition between tendon and bone is not regenerated during the healing process (hematoxylin- and eosin-stained images are shown under bright field in the top row and under polarized light in the bottom row) (84).

Figure 5

Coculture of fibroblasts and osteoblasts (upper left) exerts spatial control of cell distribution, resulting in an interface region that contains interacting osteoblasts and fibroblasts. Fibroblast and osteoblast interactions in coculture reduced cell proliferation (*: p < 0.05 versus control) (98).

Figure 6

Biomimetic strategy for engineering a ligament-to-bone interface. Inspired by the native enthesis, a stratified scaffold is designed to mimic the layered tissue regions progressing from ligament to fibrocartilage to bone. Spatial control over cell distribution (fibroblasts, chondrocytes, and osteoblasts) on the triphasic scaffold resulted in the formation of compositionally distinct yet structurally continuous tissue regions mimicking those found at the native ligament-to-bone insertion site (27).