38


PART I FOUNDATION CONCEPTS


TABLE 2.2 Comparison of Bone and Cartilage Characteristics Bone


Cartilage Fibers Cells Ground substance Strength Mostly collagen Osteocytes


Chondroitin sulfate in proteoglycan and water


Strong resistance to deformation until reaches breaking point


Repair potential


Signifi cant capability


Collagen, elastin, and reticular fi bers


Chondrocytes


Insoluble calcium salts in a small amount of liquid


High deformation capabilities with limited strength


Minimal capability


in terms of how they move. Table 2.3 summarizes joint classifi cations utilizing both systems.


STRUCTURAL CLASSIFICATION


In the structural classifi cation, joints are described as fi brous, cartilaginous, or synovial. Fibrous Joints


Fibrous joints consist of two bones connected by dense connective tissue. The main role of this type of joint is to provide stability through the strength of the connective tissue. There are three types of fi brous joints—syndes- mosis, suture, and gomphosis. Figure 2.8 illustrates each of the three types of fi brous joints.


Syndesmosis Joints


Despite their close proximity and necessity of both cartilage and bone in the formation and function of joints, there are many differences between the two structures. Table 2.2 compares cartilage and bone in terms of their composition, strength, and repair capabilities after injury.


Clinical Connection 2.5


The German anatomist and surgeon Julius Wolff (1836–1902) theorized that bone responds to exter- nal stresses in a predictable manner. Wolff’s law states that bone responds to stress and overloading by forming more bone, often in the form of bone “spurs,” or osteophytes. Osteophytes often form in the weight-bearing joints of the knees and spine. These spurs can limit range of motion and com- press or impinge surrounding tissues. In the spine, bone spurs can press on nerve roots, causing irrita- tion and possibly pain and weakness.


Syndesmosis joints consist of two bones united by a fi brous ligament or fi brous membrane. The bones are held in a close relationship to each other by this mem- brane, which allows either a small amount of movement or no movement between the bones. An example of a syn- desmosis joint is between the radius and ulna where the interosseous membrane maintains the spatial relationship between the shaft of the radius and ulna. The shafts of the tibia and fi bula in the lower leg also share a similar interosseous membrane.


Suture Joints


Suture joints are another type of fi brous joint and consist of a thin layer of fi brous tissue holding two bones as they fi t together. The suture joints found in the skull are an example of this type of joint. The various portions of the skull fi t tightly together and have a stable, secure relationship to one another.


Gomphosis Joints


In gomphosis joints, fi brous tissue holds bones together like a peg in a hole. An example of a gomphosis joint is a tooth bound to the mandible or maxilla.


Cartilaginous Joints JOINT CLASSIFICATION


Because joints can be classifi ed based on how they func- tion or on their design and structure, it is necessary to be familiar with two common classifi cation systems. The anatomist classifi es joints according to their anatomical structures. However, because the shape of the individual bones of a joint and their articulating surfaces are often linked to their function, the kinesiologist organizes joints


Cartilaginous joints consist of bones held together by fi brocartilage or hyaline cartilage. The joints provide stability to a region and have either limited mobility or a specifi c type of motion. Symphysis and synchondrosis joints are types of cartilaginous joints (Fig. 2.9).


Symphysis Joints


A symphysis joint is a type of cartilaginous joint, such as the intervertebral discs formed from fi brocartilage that unites one vertebral bone to another. The discs act as shock absorbers as loads are transmitted through the spine. They also improve the fi t of each vertebra with another. Another example of a symphysis joint is the


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