Bone Cartilage and Ligaments

Bones are the parts of the human body that are most resistant to deformation. Unless they are broken or fractured, bones do not undergo significant shape changes during short periods. As such, they can be considered as rigid bodies in the analysis of movement and motion. In a rigid body neither the distance between any two points nor the angle between any three points changes during motion.

The bone matrix is composed of collagen fibers and inorganic calcium salts decorating these fibers (Fig. 1.6a). Collagen is the most abundant structural protein in the body. Collagen fibers bend easily when compressed but resist stretching; they have enormous tensile strength. The salts are primarily calcium phosphate and, in lesser amounts, calcium carbonate. The salt crystals can withstand large compressive forces but they are brittle and inflexible. However, when deposited on flexible collagen fibers, the resultant composite behaves differently. The bone composite possesses the best structural features of the collagen and the salt: it can withstand large compressive forces and has considerable strength against tension and torsion.

Bone is not a homogeneous material; that is, its physical properties vary with location. In a long bone, compact bone tissue (relatively dense and solid) forms the walls of the cylindrical shaft (Fig. 1.6b). The compact bone constitutes the surface layer of other bones. An internal layer of spongy bone, an open network of struts and plates, surrounds the marrow cavity. Spongy bone is also present at the expanded areas (heads) of long bones. Both compact and cancellous (spongy) bone have the same matrix composition but they differ in weight density and three-dimensional microstructure. In general, spongy bone is found where bones are not heavily stressed or where stresses arrive in many different directions. On the other hand, compact bone is thickest where the bone is stressed extensively in a certain direction.

Using shape as a criteria, bones of the human body have been classified into six categories. Long bones are found in the upper arm and forearm, thigh and lower leg, palms, soles, fingers, and toes. They play a crucial role in movement, functioning as lever systems. Short bones such as

Compact And Spongy Bone

compact bone spongy bone

Figure 1.6 a,b. The microstructure of a thin section of a long bone (a). The thin, branching lines in the figure represent the collagen fibers decorated with calcium salts. Roughly one-third of the matrix of bone consists of collagen fibers. The balance is primarily a mixture of calcium salts. The bone cells called osteocytes are usually organized in groups around a central space that contains blood vessels. The lamellar organization in a long bone (b) shows that the walls of the shaft of the femur are of compact bone whereas the heads are composed of spongy bone.

compact bone spongy bone

Figure 1.6 a,b. The microstructure of a thin section of a long bone (a). The thin, branching lines in the figure represent the collagen fibers decorated with calcium salts. Roughly one-third of the matrix of bone consists of collagen fibers. The balance is primarily a mixture of calcium salts. The bone cells called osteocytes are usually organized in groups around a central space that contains blood vessels. The lamellar organization in a long bone (b) shows that the walls of the shaft of the femur are of compact bone whereas the heads are composed of spongy bone.

those found in wrists and ankles are boxlike in appearance. Flat bones form the roof of the skull, sternum, the ribs, and the scapula. They protect the underlying soft tissues from the forces of impact. They also offer an extensive surface area for the attachment of skeletal muscles. Irregular bones such as the vertebrae of the spinal column have complex shapes with short, flat, and irregular surfaces. Sutural bones are small, flat, and oddly shaped bones of the skull in the suture line. Finally, sesamoid bones such as the patellae are usually small, round, and flat. They develop inside tendons.

Bone is a living tissue. Cells constitute approximately 2% of the mass of a typical bone. Among the bone cells, osteoblasts excrete collagen and control the deposition of inorganic material on them. They are responsible for the production of new bone. Osteoclasts, on the other hand, secrete acids that dissolve the bony matrix and release the stored minerals of calcium and phosphate. During this activity, osteoclasts are tightly sealed to the bone surface. They dissolve bone mineral by active secretion of hydrogen ions. Bone degradation products are then transported within vesicles across the cell and emptied out to the extracellular space. This process, called resorption, is fundamental to the regulation of calcium and phosphate concentration in body fluids. In the human body, regardless of age, osteoblasts are adding to the bone matrix at the same time os-

teoclasts are removing from it. The balance between the activities of these two cell types is important: if too much salt is removed, bones become weaker. When osteoblast activity predominates, bones become stronger and more massive.

On the average, the turnover rate for bone is quite high. Approximately one-fifth of the adult skeleton is demolished and then rebuilt or replaced in a year. The turnover rates vary from bone to bone, possibly depending on the function of the bone. The rate of remodeling also varies with the spatial location on a bone. For example, the spongy ends of long bones of human limbs remodel at a much higher rate than the shaft of a long bone.

The bone growth and remodeling appear to be tightly regulated in the human body by hormones and steroids. Electrical fields are known to stimulate bone repair and stimulate the self-repair of bone fractures. Heavily stressed bones become thicker and stronger, whereas bones not subjected to ordinary stresses become thin and brittle. Regular exercise serves as a stimulus that maintains normal bone structure.

Growth plates are the sites of bone growth during childhood and early adulthood. They are positioned at the spongy ends of the long bones. Os-teoblasts proliferate on the surface of the growth plate and make new bone. The long bones of the average infant lengthen by 50% during the first year after birth. The bone growth rate drops to about 7% per year by age 3. The bone growth stops around 30 years of age, and between 35 and 40 the osteoblast activity begins to decline gradually while osteoclast activity continues at previous levels. Nevertheless, among all the mature tissues and organs of adult body, only one has the ability to remake itself and that is bone. When broken, bone reconstructs itself by triggering biological processes reminiscent of those that occur in the embryo. The repair begins when a class of stem cells travel to the damaged site and undertake specific tasks such as producing a calcified scaffolding around the break. Thus, a break or a fracture uncovers the remaking characteristics of bone tissue in adulthood. As discussed in Chapter 6, surgeons have utilized this capacity to lengthen limbs in people with limb abnormalities.

Cartilage is a gelatinous matrix that covers bone surfaces at a large number of articulations. It is glassy smooth, glistening, and bluish-white in appearance. It is found in the connections between the ribs and the sternum, and on the surface of articulating bones of the shoulder and hip joints, elbow, knee, and the wrist. Cartilage pads are positioned between spinal vertebrae. One important function of cartilage is to absorb com-pressive shocks and thereby prevent bone damage. Cartilage drastically reduces friction between opposing bony surfaces and enables rotation of one surface over the other. The only cell type found within the cartilage matrix are chondrocytes. These cells live in small pockets known as lacunae, and all nutrient and waste product exchange occur by diffusion through the matrix. Cartilage is avascular because chondrocytes produce a chemical that discourages the formation of blood vessels. The outer layer of cartilage is composed of a dense irregular connective tissue providing mechanical support and protection. Cartilage does not grow in adults, and in fact decreases in thickness with aging. Unlike other components of the skeletal system, cartilage has a poor self-repair mechanism; most cartilages cannot repair themselves after a severe injury. This is one reason why so many middle-aged runners have "bad knees."

Ligaments connect one bone to another (Fig. 1.7). These are cable-like structures consisting primarily of collagen fibers. Another fibrous protein found in ligaments is elastin. While collagen acts to oppose tensile forces, elastin acts to increase flexibility. A ligament is slightly more compliant

(a) elbow biceps brachii tendon interosseous membrane humerus annular ligament biceps brachii tendon humerus annular ligament interosseous membrane

Ulnar Opposition

articular capsule ulnar collateral ligament articular capsule ulnar collateral ligament

(b) knee, posterior, extended

(c) knee, sagittal medial meniscus tibial collateral ligament

(b) knee, posterior, extended medial meniscus tibial collateral ligament

Lateral Meniscus Popliteus

lateral meniscus fibular collateral ligament posterior cruciate ligament posterior cruciate ligament anterior cruciate ligament posterior meniscofemoral ligament lateral meniscus fibular collateral ligament quadriceps bursa popliteus

(c) knee, sagittal quadriceps anterior cruciate ligament posterior meniscofemoral ligament popliteus

Meniscofemoral Ligament

bursa patella patellar ligament illotibial tract patella patellar ligament illotibial tract

Figure 1.7a-c. The ligaments of the elbow (a) and the knee (b,c). A large number of ligaments are necessary to keep multiple bone segments in place.

than a tendon but is stiffer than a muscle. Ligaments support the joints by holding the ends of bones together. Ligaments also support body organs such as the liver and hold the teeth in the jawbone.

Fibroblasts are the most abundant cells found in ligaments. These cells manufacture and secrete protein subunits to form collagen-rich extracellular fibers. Fibroblasts also secrete hyaluronic acid, a substance that gives tissue matrix its syrupy consistence. Also present are a number of immune system cells and stem cells that respond to local injury by dividing to produce additional cells for self-repair.

The joints of the upper and lower limbs contain an abundance of ligaments positioned in various directions. Seven major ligaments stabilize the knee joint (Fig. 1.7b,c). Tearing of one or two of these ligaments result in increasing mobility and instability of the knee. A news article in the November 15, 1998, issue of the New York Times illustrates this point. The article tells the story of Jason Sehorn, an emerging star of the Giants football team who had landed awkwardly on one knee during an exhibition game with the Jets on August 20. His lower leg lay flat on the ground sideways and his thigh was perpendicular to it. Here is how Sehorn describes the diagnosis: "The doctor came over and grabbed my knee and twisted it in one way. He looked me right in the eye and said, 'They took your a.c.l. (anterior cruciate ligament).' Then he turned it the other way. He said, 'They got your m.c.l. (medial collateral ligament)'." Ligaments are crucial for the strength of the body structure and the control of movement.

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Responses

  • martin
    What is the % of the adult skeleton that was demolished?
    7 years ago
  • jennifer
    What is the percentage of the adult skeleton that is demolished, rebuilt or replaced each year?
    7 years ago
  • alyx
    Where is compact bone found in the body/?
    6 years ago
  • hugh hughes
    What are struts within spongy bone that assist in withstanding stresses in specific direction?
    5 years ago
  • simret
    Does compact or spongy bones have the higher rate of bone turnover?
    5 years ago
  • giacomo
    Does compact or spongy bone have a igher turn over rate?
    4 years ago
  • tewolde
    What lives in small pockets in lacunae and is found in cartilage in matrix?
    4 years ago

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