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12.5: Introduction to Joints - Biology

12.5: Introduction to Joints - Biology


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Learning Objectives

  • Discuss both functional and structural classifications for body joints
  • Describe the characteristic features for fibrous, cartilaginous, and synovial joints and give examples of each
  • Define and identify the different body movements
  • Discuss the structure of specific body joints and the movements allowed by each
  • Explain the development of body joints

The adult human body has 206 bones, and with the exception of the hyoid bone in the neck, each bone is connected to at least one other bone. These joints are designed for stability and provide for little or no movement.

Joint stability and movement are related to each other. This means that stable joints allow for little or no mobility between the adjacent bones. Conversely, joints that provide the most movement between bones are the least stable. Understanding the relationship between joint structure and function will help to explain why particular types of joints are found in certain areas of the body.

The articulating surfaces of bones at stable types of joints, with little or no mobility, are strongly united to each other. For example, most of the joints of the skull are held together by fibrous connective tissue and do not allow for movement between the adjacent bones. This lack of mobility is important, because the skull bones serve to protect the brain.

Similarly, other joints united by fibrous connective tissue allow for very little movement, which provides stability and weight-bearing support for the body. For example, the tibia and fibula of the leg are tightly united to give stability to the body when standing. At other joints, the bones are held together by cartilage, which permits limited movements between the bones. Thus, the joints of the vertebral column only allow for small movements between adjacent vertebrae, but when added together, these movements provide the flexibility that allows your body to twist, or bend to the front, back, or side.

In contrast, at joints that allow for wide ranges of motion, the articulating surfaces of the bones are not directly united to each other. Instead, these surfaces are enclosed within a space filled with lubricating fluid, which allows the bones to move smoothly against each other. These joints provide greater mobility, but since the bones are free to move in relation to each other, the joint is less stable. Most of the joints between the bones of the appendicular skeleton are this freely moveable type of joint. These joints allow the muscles of the body to pull on a bone and thereby produce movement of that body region. Your ability to kick a soccer ball, pick up a fork, and dance the tango depend on mobility at these types of joints.


9.2 Fibrous Joints

At a fibrous joint, the adjacent bones are directly connected to each other by fibrous connective tissue, and thus the bones do not have a joint cavity between them (Figure 9.2.1). The fibers joining the bones may be short or long, thus the gap between bones at fibrous joints vary from narrow to wide. There are three types of fibrous joints. A suture is the narrow fibrous joint found between most bones of the skull. At a syndesmosis, the bones are more widely separated but are held together by a strap of fibrous connective tissue called a ligament or a wide sheet of connective tissue called an interosseous membrane. This type of fibrous joint is found between the shaft regions of the long bones in the forearm and in the leg. Lastly, a gomphosis is the narrow fibrous joint between the roots of a tooth and the bony socket in the jaw into which the tooth fits.

Figure 9.2.1 – Fibrous Joints: Fibrous joints form strong connections between bones. (a) Sutures join most bones of the skull. (b) An interosseous membrane forms a syndesmosis between the radius and ulna bones of the forearm. (c) A gomphosis is a specialized fibrous joint that anchors a tooth to its socket in the jaw.


Background

Osteoarthritis is the most common type of joint disease, affecting more than 30 million individuals in the United States alone (see Epidemiology). It represents a heterogeneous group of conditions resulting in common histopathologic and radiologic changes. It has been thought of as a degenerative disorder arising from biochemical breakdown of articular (hyaline) cartilage in the synovial joints. However, the current view holds that osteoarthritis involves not only the articular cartilage but also the entire joint organ, including the subchondral bone and synovium.

Osteoarthritis predominantly involves the weight-bearing joints, including the knees, hips, cervical and lumbosacral spine, and feet. Other commonly affected joints include the distal interphalangeal (DIP), proximal interphalangeal (PIP), and carpometacarpal (CMC) joints. This article primarily focuses on osteoarthritis of the hand, knee, and hip joints (see Pathophysiology). For more information on arthritis in other joints, see Glenohumeral Arthritis and Wrist Arthritis.

Although osteoarthritis was previously thought to be caused largely by excessive wear and tear, increasing evidence points to the contributions of abnormal mechanics and inflammation. In addition, some invasive procedures (eg, arthroscopic meniscectomy) can result in rapid progression to osteoarthritis in the knee joint. [11] Therefore, the term degenerative joint disease is no longer appropriate in referring to osteoarthritis. (See Pathophysiology.)

Historically, osteoarthritis has been divided into primary and secondary forms, though this division is somewhat artificial. Secondary osteoarthritis is conceptually easier to understand: It refers to disease of the synovial joints that results from some predisposing condition that has adversely altered the joint tissues (eg, trauma to articular cartilage or subchondral bone). Secondary osteoarthritis can occur in relatively younger individuals (see Etiology). [12, 13, 14, 15, 16, 17, 18, 19]

The definition of primary osteoarthritis is more nebulous. Although this form of osteoarthritis is related to the aging process and typically occurs in older individuals, it is, in the broadest sense of the term, an idiopathic phenomenon, occurring in previously intact joints and having no apparent initiating factor.

Some clinicians limit the term primary osteoarthritis to the joints of the hands (specifically, the DIP and PIP joints and the joints at the base of the thumb). Others include the knees, hips, and spine (apophyseal articulations) as well.

As underlying causes of osteoarthritis are discovered, the term primary, or idiopathic, osteoarthritis may become obsolete. For instance, many investigators believe that most cases of primary osteoarthritis of the hip may, in fact, be due to subtle or even unrecognizable congenital or developmental defects.

No specific laboratory abnormalities are associated with osteoarthritis. Rather, it is typically diagnosed on the basis of clinical findings, with or without radiographic studies (see Workup).

The goals of osteoarthritis treatment include pain alleviation and improvement of functional status. Nonpharmacologic interventions are the cornerstones of osteoarthritis therapy and include the following:

Intra-articular pharmacologic therapy includes corticosteroid injection and viscosupplementation, which may provide pain relief and have an anti-inflammatory effect on the affected joint. (See Treatment.) Oral pharmacologic therapy begins with acetaminophen for mild or moderate pain without apparent inflammation.

If the clinical response to acetaminophen is not satisfactory or the clinical presentation is inflammatory, consider nonsteroidal anti-inflammatory drugs (NSAIDs). (See Medication.) If all other modalities are ineffective and osteotomy is not viable, or if a patient cannot perform his or her daily activities despite maximal therapy, arthroplasty is indicated.

The high prevalence of osteoarthritis entails significant costs to society. Direct costs include clinician visits, medications, therapeutic modalities, and surgical intervention. Indirect costs include time lost from work.

Costs associated with osteoarthritis can be particularly significant for elderly persons, who face potential loss of social interactions and independence, leading to a need for help with activities of daily living. As populations of developed nations age over the coming decades, the need for better understanding of osteoarthritis and for improved therapeutic alternatives will continue to grow. (See Epidemiology.)


Bones

Bone, although strong, is a constantly changing tissue that has several functions. Bones serve as rigid structures to the body and as shields to protect delicate internal organs. They provide housing for the bone marrow, where the blood cells are formed. Bones also maintain the body's reservoir of calcium. In children, some bones have areas called growth plates. Bones lengthen in these areas until the child reaches full height, at which time the growth plates close. Thereafter, bones grow in thickness rather than in length, based on the body's need for additional bone strength in certain areas.

Flat (such as the plates of the skull and the vertebrae)

Tubular (such as the thighbones and arm bones, which are called long bones)

Some bones have combinations of these. All bones have essentially the same structure. The hard outer part (cortical bone) consists largely of proteins, such as collagen, and a substance called hydroxyapatite, which is composed mainly of calcium and other minerals. Hydroxyapatite is largely responsible for the strength and density of bones. The inner part of bones (trabecular bone) is softer and less dense than the hard outer part but still contributes significantly to bone strength. A reduction in the amount or quality of trabecular bone increases the risk of fractures (breaks). Bone marrow is the tissue that fills the spaces in the trabecular bone. Bone marrow contains specialized cells (including stem cells) that produce blood cells. Blood vessels supply blood to the bone, and nerves surround the bone.

Did You Know.

Bone structure adjusts throughout life in response to activity and mechanical stress (for example, weight-bearing exercise).

Bones undergo a continuous process known as remodeling (see Osteoporosis). In this process, old bone tissue is gradually replaced by new bone tissue. Every bone in the body is completely reformed about every 10 years. To maintain bone density and strength, the body requires an adequate supply of calcium, other minerals, and vitamin D and must produce the proper amounts of several hormones, such as parathyroid hormone, growth hormone, calcitonin , estrogen, and testosterone . Activity (for example, weight-bearing exercises for the legs) helps bones strengthen by remodeling. With activity and optimal amounts of hormones, vitamins, and minerals, trabecular bone develops into a complex lattice structure that is lightweight but strong.

Bones are covered by a thin membrane called the periosteum. Bone injuries are painful because of pain-sensing nerves located mostly in the periosteum. Blood enters bones through blood vessels that enter through the periosteum.


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