Bone and cartilage are part of the specialized connective tissues that make up the skeletal system. Bone is a living tissue that replaces itself continuously. The hardness of bone is well suited to the task of providing mechanical support function, and the elasticity of cartilage, to the ability of joints to move. Both cartilage and bone consist of specialized cells that produce and regulate a matrix of material outside the cells. The matrix is abundant in collagens, proteoglycans and non-collagenous proteins. Minerals are present in bone matrix as well.
The external part of bone is called the cortex and is compact bone. The more spongy inner part (trabecular bone) is filled with blood-forming (haematopoietic) bone marrow. The inner and outer parts of the bone have different metabolic turnover rates, with important consequences for late life osteoporosis. Trabecular bone regenerates itself at a greater rate than compact bone, which is why osteoporosis is first seen in the vertebral bodies of the spine, which have large trabecular parts.
Bone in the skull and other selected sites forms directly by bone formation (intramembranous ossification) without passing through a cartilage intermediate phase. The long bones of the limbs develop from cartilage through a process known as endochondral ossification. This process is what leads to the normal growth of long bones, to the repair of fractures and, in late adult life, to the unique formation of new bone in a joint which has become osteoarthritic.
The osteoblast is a type of bone cell that is responsible for synthesis of the matrix components in bone: the distinct collagen (type I) and proteoglycans. Osteoblasts also synthesize other non-collagenous proteins of bone. Some of these proteins can be measured in serum to determine the rate of bone turnover.
The other distinct bone cell is called the osteoclast. The osteoclast is responsible for resorption of bone. Under normal circumstances, old bone tissue is resorbed while new bone tissue is generated. Bone is resorbed by production of enzymes that dissolve proteins. Bone turnover is called remodelling and is normally a balanced and coordinated process of resorption and formation. Remodelling is influenced by body hormones and by local growth factors.
Movable (diarthrodial) joints are formed where two bones fit together. Joint surfaces are designed for weight bearing, and to accommodate a range of motion. The joint is enclosed by a fibrous capsule, whose inner surface is a synovial membrane, which secretes synovial fluid. The joint surface is made of hyaline cartilage, beneath which is a backing of hard (subchondral) bone. Within the joint, ligaments, tendons and fibrocartilaginous structures (menisci in certain joints, such as the knee), provide stability and a close fit between joint surfaces. The specialized cells of these joint components synthesize and maintain the matrix macromolecules whose interactions are responsible for maintaining the tensile strength of ligaments and tendons, the loose connective tissue that supports the blood vessels and cellular elements of the synovial membrane, the viscous synovial fluid, the elasticity of hyaline cartilage, and the rigid strength of subchondral bone. These joint components are interdependent, and their relationships are shown in table 1.
Table 1. Structure-function relationships and inter-dependence of joint components.
|
Components |
Structure |
Functions |
|
Ligaments and tendons |
Dense, fibrous, connective tissue |
Prevents over-extension of joints, provides stability and strength |
|
Synovial membrane |
Areolar, vascular and cellular |
Secretes synovial fluid, dissolves (phagocytoses) particulate material in synovial fluid |
|
Synovial fluid |
Viscous fluid |
Provides nutrients for cartilage injoints, lubricates cartilage during joint motion |
|
Cartilage |
Firm hyaline cartilage |
Constitutes the joint surface, bears weight, responds elastically to compression |
|
Tidemark |
Calcified cartilage |
Separates joint cartilage from underlying bone |
|
Subchondral bone |
Hard bone with marrow spaces |
Provides backing for joint surface; marrow cavity provides nutrients to base of cartilage and is thesource of cells with potential fornew bone formation |
Source: Hamerman and Taylor 1993.
Selected Diseases of Bones and Joints
Osteopenia is the general term used to describe reduced bone substance detected on x rays. Often asymptomatic in early stages, it may eventually manifest itself as weakening of bones. Most of the conditions listed below induce osteopenia, although the mechanisms by which this occurs differ. For example, excessive parathyroid hormone enhances bone resorption, while calcium and phosphate deficiency, which can arise from multiple causes and is often due to inadequate vitamin D, results in deficient mineralization. As people age, there is an imbalance between formation and resorption of bone. In women around the age of menopause, resorption often predominates, a condition called type I osteoporosis. In advanced age, resorption can again dominate and lead to type II osteoporosis. Type I osteoporosis usually affects vertebral bone loss and collapse, while hip fracture predominates in type II.
Osteoarthritis (OA) is the principal chronic disorder of certain movable joints, and its incidence increases with age. By age 80, almost all people have enlarged joints on the fingers (Heberden’s nodes). This is usually of very limited clinical significance. The principal weight-bearing joints which are subject to osteoarthritis are the hip, knee, feet and facets of the spine. The shoulder, while it is not weight bearing, may also suffer from a variety of arthritic changes, including rotator cuff tear, subluxation of the humeral head and an effusion high in proteolytic enzymes—a clinical picture often called “Milwaukee Shoulder” and associated with substantial pain and limitation of motion. The main change in OA is primarily one of degradation of cartilage, but new bone formation called osteophytes is usually seen on x rays.