9.3 Cartilaginous Joints
Learning Objectives
By the end of this section, you will be able to:
Describe the characteristic features for fibrous joints and give examples
- Describe the structural features and functional properties of fibrous joints
- Compare sutures, syndesmoses, and gomphoses
- Name an example of each type of fibrous joint and describe its functional properties
As the name indicates, at a cartilaginous joint, the adjacent bones are united by cartilage, a tough but somewhat flexible type of connective tissue. These types of joints lack a joint cavity and involve bones that are joined together by either hyaline cartilage or fibrocartilage (Figure 9.3.1). There are two types of cartilaginous joints. A synchondrosis is a cartilaginous joint where the bones are joined by hyaline cartilage, or where a bone is united to hyaline cartilage. The second type of cartilaginous joint is a symphysis, where the bones are joined by fibrocartilage.
Synchondrosis
A synchondrosis (“joined by cartilage”) is a cartilaginous joint where bones are joined together by hyaline cartilage, or where bone is united to hyaline cartilage. A synchondrosis may be temporary or permanent. A temporary synchondrosis is the epiphyseal plate (growth plate) of a growing long bone. The epiphyseal plate is the region of growing hyaline cartilage that unites the diaphysis (shaft) of a long bone to the epiphysis (end of the bone). Bone lengthening involves growth of the epiphyseal plate cartilage and its replacement by bone, which adds to the diaphysis (see section 6.4). For many years during childhood growth, the rates of cartilage growth and bone formation are equal and thus the epiphyseal plate does not change in overall thickness as the bone lengthens. During the late teens and early 20s, growth of the cartilage slows and eventually stops. The epiphyseal plate is then completely replaced by bone, and the diaphyseal and epiphyseal portions of the bone fuse together to form a single adult bone. This fusion of the diaphysis and epiphysis forms a synostosis and once this occurs, bone lengthening ceases. For this reason, the epiphyseal plate is considered to be a temporary synchondrosis. Because cartilage is softer than bone tissue, injury to a growing long bone can damage the epiphyseal plate cartilage, thus stopping bone growth and preventing additional bone lengthening.
Growing layers of cartilage also form synchondroses that join together the ilium, ischium, and pubic portions of the hip bone during childhood and adolescence. When body growth stops, the cartilage disappears and is replaced by bone, forming synostoses and fusing the bony components together into the single hip bone of the adult. Similarly, synostoses unite the sacral vertebrae that fuse together to form the adult sacrum.
External Website
Visit this website to view a radiograph (X-ray image) of a child’s hand and wrist. The growing bones of child have an epiphyseal plate that forms a synchondrosis between the shaft and end of a long bone. Being less dense than bone, the area of epiphyseal cartilage is seen on this radiograph as the dark epiphyseal gaps located near the ends of the long bones, including the radius, ulna, metacarpals, and phalanges. Which of the bones in this image do not show an epiphyseal plate (epiphyseal gap)?
Examples of permanent synchondroses are found in the thoracic cage. One example is the first sternocostal joint, where the first rib is anchored to the manubrium by its costal cartilage. (The articulations of the remaining costal cartilages to the sternum are all synovial joints.) Additional synchondroses are formed where the anterior ends of the other 11 ribs are joined to their costal cartilage. Unlike the temporary synchondroses of the epiphyseal plate, these permanent synchondroses retain their hyaline cartilage and do not ossify with age. Due to the lack of movement between the bone and cartilage, both temporary and permanent synchondroses are functionally classified as synarthroses.
Symphysis
A cartilaginous joint where the bones are joined by fibrocartilage is called a symphysis (“growing together”). Fibrocartilage contains numerous bundles of thick collagen fibers, thus giving it a much greater ability to resist pulling and bending forces when compared with hyaline cartilage. This gives symphyses the ability to strongly unite the adjacent bones, but can still allow for limited movement to occur. Thus, symphyses are functionally classified as amphiarthroses.
A thick pad of fibrocartilage called an intervertebral disc strongly unites adjacent vertebral bodies at the intervertebral symphysis. The intervertebral symphysis is important because it allows for small movements between adjacent vertebrae. Small movements at many intervertebral joints combine to allow greater mobility of the vertebral column as a whole. In addition, the thick intervertebral disc provides cushioning between the vertebrae, which is important when carrying heavy objects or during high-impact activities such as running or jumping.
At the pubic symphysis, the pubic portions of the right and left hip bones of the pelvis are joined together by fibrocartilage pad. This fibrocartilage provides cushioning similar to the intervertebral disc, thus providing both shock absorption and stability to the pelvis. During pregnancy, increased levels of the hormone relaxin lead to increased mobility at the pubic symphysis which allows for expansion of the pelvic cavity during childbirth.
Chapter Review
There are two types of cartilaginous joints. A synchondrosis is formed when the adjacent bones are united by hyaline cartilage. A temporary synchondrosis is formed by the epiphyseal plate of a growing long bone, which is lost when the epiphyseal plate ossifies as the bone reaches maturity. The synchondrosis is thus replaced by a synostosis. Permanent synchondroses that do not ossify are found at the first sternocostal joint and between the anterior ends of the bony ribs and the junction with their costal cartilage. A symphysis is where the bones are joined by fibrocartilage. The pubic symphysis and the intervertebral symphyses contain fibrocartilaginous pads which are cushioning and allow slight movement making them amphiarthrotic.
Interactive Link Questions
Go to this website to view a radiograph (X-ray image) of a child’s hand and wrist. The growing bones of child have an epiphyseal plate that forms a synchondrosis between the shaft and end of a long bone. Being less dense than bone, the area of epiphyseal cartilage is seen on this radiograph as the dark epiphyseal gaps located near the ends of the long bones, including the radius, ulna, metacarpal, and phalanx bones. Which of the bones in this image do not show an epiphyseal plate (epiphyseal gap)?
Answer: Although they are still growing, the carpal bones of the wrist area do not show an epiphyseal plate. Instead of elongating, these bones grow in diameter by adding new bone to their surfaces.
Review Questions
Exercises
1. Describe the two types of cartilaginous joints and give examples of each.
2. Both functional and structural classifications can be used to describe an individual joint. Define the first sternocostal joint and the pubic symphysis using both functional and structural characteristics.
Glossary
- symphysis
- type of cartilaginous joint where the bones are joined by fibrocartilage
- synchondrosis
- type of cartilaginous joint where the bones are joined by hyaline cartilage
Solutions
Answers for Critical Thinking Questions
- Cartilaginous joints are where the adjacent bones are joined by cartilage. At a synchondrosis, the bones are united by hyaline cartilage. The epiphyseal plate of growing long bones and the first sternocostal joint that unites the first rib to the sternum are examples of synchondroses. At a symphysis, the bones are joined by fibrocartilage, which is strong and flexible. Symphysis joints include the intervertebral symphysis between adjacent vertebrae and the pubic symphysis that joins the pubic portions of the right and left hip bones.
- The first sternocostal joint is a synchondrosis type of cartilaginous joint in which hyaline cartilage unites the first rib to the manubrium of the sternum. This forms an immobile (synarthrosis) type of joint. The pubic symphysis is a slightly mobile (amphiarthrosis) cartilaginous joint, where the pubic portions of the right and left hip bones are united by fibrocartilage, thus forming a symphysis.
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