1. McNicholas MJ, Rowley DI, McGurty D, et al: Total meniscectomy in adolescence: A thirty-year follow-up. J Bone Joint Surg Br 2000;82:217-221.
2. Cox JS, Nye CE, Schaefer WW, et al: The degenera- tive effects of partial and total resection of the medial meniscus in dogs’ knees. Clin Orthop 1975;109:178-183.
3. Praemer A, Furner S, Rice DP: (eds): Musculoskel- etal Conditions in the United States, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1999, p 170 (Appendix A: Table B).
4. Day B, Mackenzie WG, Shim SS, et al: The vascu- lar and nerve supply of the human meniscus. Arthroscopy 1985;1:58-62.
5. O’Connor BL, McConnaughey JS: The structure and innervation of cat knee menisci, and their relation to a “sensory hypothesis” of meniscal function. Am J Anat 1978;153:431-442.
6. Aronoczky S, Adams M, DeHaven K, et al: Menis- cus, in Woo SLY, Buckwalter JA (eds): Injury and Repair of the Musculoskeletal Soft Tissues. Park Ridge, IL, American Academy of Orthopaedic Surgeons, 1988, pp 483-537.
7. Arnoczky SP, Warren RF: The microvasculature of the meniscus and its response to injury: An experi- mental study in the dog. Am J Sports Med 1983;11:131-141.
8. Rodeo SA, Seneviratne A, Suzuki K, et al: Histo- logical analysis of human meniscal allografts: A preliminary report. J Bone Joint Surg Am 2000;82:1071-1082.
9. Goto H, Shuler FD, Niyibizi C, et al: Gene therapy for meniscal injury: Enhanced synthesis of pro- teoglycan and collagen by meniscal cells trans- duced with a TGFbeta(1) gene. Osteoarthritis Cartilage 2000;8:266-271.

Fibrochondrocytes – Cells that are able to syn- thesize fibrous extracellular proteins and have the rounded appearance of chondrocytes

Intervertebral disk – The structure located between two moving vertebrae that stabi- lizes the spine, helps maintain its align- ment, allows motion between vertebral levels, absorbs energy, and distributes loads applied to the spine

Meniscus – A crescent-shaped disk of fibrocar- tilage located between the femoral condyle and the tibial plateau

Sharpey’s fibers – The small collagen fibers that attach tendon to bone; in the spine they connect the outer edges of interverte- bral disks to the vertebral bodies

Viscoelastic structure – A structure whose mechanical properties depend on the rate of loading

The spinal disk changes throughout life (be- ginning soon after birth), but these changes accelerate after skeletal maturity, at approximately age 20 years.

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Changes in disk microstructure, composition, size, and vascular supply occur through- out growth and development.

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Like the meniscus, the disk has a relatively poor blood supply and what exists enters from the periphery.

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The intervertebral disk is the primary articulation between the vertebral bodies of two adjacent vertebrae.

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Histology

The disks have an outer ring of fibrocartilage, the anulus fibrosus, and within it, a gelatinous material called the nucleus pulposus (Fig. 8).

Figure 8
Sagittal section view of two vertebral bodies and an intervertebral disk. The three regions of the disk are shown: cartilaginous end plate, outer anulus fibrosus, inner anulus fibrosus, and nucleus pulposus. The pos- terior articular and spinous processes and the articular surface of a facet joint are also shown.
(Reproduced with permission from Ashton-Miller JA, Schultz AB: Biomechanics of the human spine, in Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics. Philadelphia, PA, Lippincott-Raven, 1997, pp 353-393.)

The organic matrix characteristic of each zone is distinct. In the anulus fibrosus, there is an outer layer of approximately 90 concentric lamellae of type I collagen fibers; deep within that is a region of a less dense, type II collagenous matrix. This inner zone is less organized than the outer zone of the anulus fibrosus. The most superficial an- terior fibers of the anulus fibrosus blend with the anterior longitudinal ligament, whereas the most superficial posterior fibers blend with the posterior anterior longitudinal ligament. Protrusion of the nucleus pulposus through tears in the anulus fibrosus is called a herniated disk. Because the anterior anulus fibrosus is thicker than its posterior counter- parts, posterior herniations are more common. When this herniation compresses a spi- nal nerve root, pain that radiates down the leg (known as sciatica) can result.
In contrast to the collagen-rich anulus fi- brosus, the nucleus pulposus is predominantly proteoglycan (Fig. 9).

Micrographs showing the arrangement of the collagen fibrils in the outer anulus fibrosus (A) and the central nucleus pulposus (B). Note the tightly packed, highly oriented lamellae of collagen fibrils in the outer anulus fibrosus and the loose, almost random pattern of collagen fibrils in the nucleus pulposus.
(Reproduced with permission from Buckwalter JA: The fine structure of human intervertebral disc, in White AA III, Gordon SL (eds): American Academy of Orthopaedic Surgeons Symposium on Idiopathic Low Back Pain. St. Louis, MO, CV Mosby, 1982, pp 108-143.)

It is the interaction between large proteoglycan molecules and water that gives the nucleus pulposus its re- sistance to compression. The cells populat- ing the anulus fibrosus are like fibroblasts, while those in the nucleus pulposus are more chondrocytic in appearance and have synthetic characteristics.

Physiologic Function

The intervertebral disk stabilizes the spine, helps maintain its alignment, allows motion between vertebral levels, absorbs energy, and distributes loads applied to the spine.

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The location of the tear (in the vascular or avascular zone) plays a large role in the decision whether to repair or excise a torn piece of meniscus.

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The menisci form during the first 8 weeks of gestation as condensations and differentiation of mesenchymal cells.

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