Introduction

The meniscus, a semilunar fibrocartilaginous tissue located between the femur and tibia, is essential to the mechanical functionality of the knee (Figure 2-1). Besides contributing to joint congruency, stability, shock absorption, and lubrication (Ghosh et al. 1987), it plays a significant role in tibiofemoral load bearing (Shrive et al. 1978). The meniscal extracellular matrix (ECM) contains 85–95% dry weight collagen, of which over 90% is fibrillar type I collagen (Eyre et al. 1983). The remaining dry weight components include types II, III, V, and VI collagens (1–2%) (McDevitt et al. 1992) and proteoglycans (<2–3%) (Adams et al. 1992), with water making up 72–77% of the tissue wet weight (Adams et al. 1992). A sparse population of resident cells, collectively known as meniscal fibrochondrocytes (MFCs), maintains and remodels the ECM via a balance of matrix

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synthesis and catabolism by matrix metalloproteinases (Upton et al. 2003, Verdonk et al. 2005).

Figure 2-1: Anatomy of the knee meniscus. Anterior (left) and top-down (right) views of the medial and lateral menisci within the knee joint. Adapted from www.aaos.org (left) and (Pagnani et al. 1995) (right).

The ultrastructure and biochemical content of the meniscus vary with anatomical location and illustrate the tissue’s adaptations to regional mechanical demands. A meshwork of thin fibrils cover the superficial surface, underneath which lies a lamellar layer of radially distributed fibers (Petersen et al. 1998). Within the tissue center, the circumferentially aligned type I collagen fibers, the primary component of the ECM, give rise to anisotropic (direction dependent) mechanical properties that allow the meniscus to bear greater tensile forces along the fiber direction (Fithian et al. 1990) (Figure 2-2B). During axial loading, the meniscus is displaced radially and converts compressive stresses from the femur to tensile hoop stresses that are transmitted to the circumferential collagen fibers and the horn attachments to the tibial plateau (Shrive et al. 1978) (Figure2-2A). A small fraction of interdigitating radial ‘tie’ fibers constrains the main fiber bundles (Skaggs et al. 1994) and may aid in local strain transmission (Lai et al. 2010) (Figure 2-2B). During normal physiological loading conditions, bulk tensile strains are in the range of 2–6%

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(Jones et al. 1996, Richards et al. 2003), with the tissue transmitting 50% of the load in the knee at full extension and 85% of the load at 90º flexion (Ahmed et al. 1983). The circumferential tensile modulus ranges from 48–259 MPa, whereas the radial modulus is much lower (3–70 MPa) (Bullough et al. 1970, LeRoux et al. 2002, Proctor et al. 1989) (Figure2-2C). Consequently, the meniscus offers less resistance to radial shear forces and is predisposed to longitudinal tears (Kawamura et al. 2003).

Figure 2-2: Anisotropic collagen organization of the extracellular matrix enhances mechanical functionality of the knee meniscus. (A) During axial loading, the meniscus is displaced radially and converts compressive stresses from the femur to tensile hoop stresses (Fc) born by the circumferential collagen fibers and the horn attachments to the tibial plateau. However, lower tensile modulus in the radial direction results in poor resistance to radial shear forces (Fr). Adapted from (Kawamura et al. 2003). (B) Schematic showing the dominant circumferential collagen fiber organization of the meniscus, with an interspersion of radial ‘tie’ fibers. (C) Schematic of circumferential (Circ) and radial (Rad) meniscal samples for tensile testing. Circ samples have >3x higher tensile modulus than Rad samples, demonstrating mechanical anisotropy. Adapted from (LeRoux et al. 2002).

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Load-bearing capability decreases when the fiber-reinforced microstructure is compromised, consequently increasing tibiofemoral contact pressures and predisposing the joint to degenerative osteoarthritis (OA). The incidence of meniscal tears is reported to be 60–70 per 100,000 persons (Hede et al. 1992, Nielsen et al. 1991) and accounts for over 1 million arthroscopic procedures each year (Hasan et al. 2014). While repair success ranges from 63% to 91% for simple peripheral tears (Boyd et al. 2003, DeHaven 1999, Eggli et al. 1995, Hanks et al. 1991, Miller 1988, Morgan et al. 1991), tears in the inner avascular zone of the meniscus have a much poorer prognosis, with up to 75% failing to heal completely and 20% requiring repeat surgery (Rubman et al. 1998).

Figure 2-3: Surgical approaches to meniscal injuries. (A) Arthroscopic image of a suture repair. Repairs stabilize simple tears but rarely result in tissue integration. (B) Illustration of partial resection of damaged tissue. Tissue removal alters joint loading and causes cartilage degradation (C) Replacement options such as allografts or engineered implants restore function in the short-term, but their long-term benefits are unclear. Images courtesy of (A) www.tarlowknee.com, (B) www.cartilagerestoration.org, and (C) www.medimagery.net and www.ivysportsmed.com.

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Partial meniscectomy is the standard treatment after repair failure (Figure 2-3), yet tissue resection also leads to osteoarthritic changes in the affected knee compartment (Aagaard et al. 1999, Englund 2004, Fairbank 1948). Furthermore it appears that degenerative changes positively correlate with amount of meniscal resection. When the tissue is damaged beyond repair, it is completely removed (total meniscectomy). Current replacement strategies include meniscal allograft transplants or engineered substitutes (e.g., collagen or polyurethane scaffolds) (Figure 2-3), although the long-term outcome of these relatively new therapies is unclear (Rongen et al. 2015, Smith et al. 2015, Vrancken et al. 2012). As such, methods that improve meniscal repair or salvage would be a marked advance to the treatment of this common injury.

This chapter will begin by reviewing the basic science understanding of the limited endogenous healing capacity of the meniscus, with an emphasis on the implications of hypovascularity, hypocellularity, and inflammation at the tear site. Following this discussion, we will describe extant and emerging regenerative medicine approaches that are designed to directly address these issues. This section will include a review of repair strategies and experimental methods that are aimed at 1) enhancing vascularity, 2) increasing cellularity, and 3) promoting matrix deposition and new tissue formation via biochemical and/or mechanical cues at the wound site.