Review Question - QID 8834

The meniscus, anterior cruciate ligament (ACL) and annulus fibrosus of the intervertebral disk are all predominantly composed of type I collagen. Articular cartilage primarily consists of type II collagen.

The knee menisci are crescent-shaped wedges of fibrocartilage composed mainly of type I collagen. Once thought to be a functionless embryonic remnant, the menisci are now known to be a crucial to knee function and long-term joint health. The menisci increase knee stability, distribute axial load, absorb shock and provide lubrication and nutrition to the knee joint. The peripheral third of the menisci is well vascularized by geniculate branches, while the inner two-thirds receive nourishment from synovial fluid, which has important implications for tear healing.

Fox et al. reviewed the structure, composition, and function of the human knee menisci. The meniscus is a dense matrix composed primarily of water (72%) and collagen (22%), interposed with cells. Collagens are responsible for the tensile strength of the meniscus and are predominantly type I collagen (90%) with variable amounts of types II, III, V and VI. The collagen fiber arrangement, running parallel to the peripheral border of the meniscus, enables transfer of a vertical compressive load to circumferential “hoop” stresses, distributing the load and decreasing contact stresses in the femorotibial articulation.

Ahn et al. retrospectively reviewed 32 patients with horizontal cleavage meniscus tears (HCMT) that underwent arthroscopic repair with a marrow-stimulation technique to potentially augment healing. HCMT often undergo meniscectomy rather than attempted repair, due to the involvement of the inner avascular meniscus and tissue degeneration that is often seen with this tear pattern. In this study, arthroscopic repair using all-inside vertical mattress sutures with drilling of the intercondylar notch to express marrow elements was associated with good functional outcomes and 91% healing rate on second-look arthroscopy.

Figure A is an arthroscopic picture of the right knee showing a horizontal cleavage tear of the lateral meniscus [Ahn et al.]. Illustration A is an arthroscopic picture of the same knee showing the completed repair using vertical mattress sutures [Ahn et al.]. Illustration B is a schematic demonstrating the collagen fiber ultrastructure and orientation within the meniscus [Fox et al.]. The superficial network (1) and lamellar layer (2) are composed of radially oriented collagen fibers that provide structural integrity to the meniscus. The central main layer is composed of circumferentially-oriented collagen fibers running parallel to the peripheral border, stabilized by interwoven radial fibers (arrowheads). The central layer is responsible for converting vertical compressive loads into circumferential “hoop” stresses.

Incorrect Answers:
Answers 1-4: The meniscus, ACL and annulus fibrosus are all predominantly composed of type I collagen. Articular cartilage is primarily composed of type II collagen.