tendon organs (tension receptors).
From Cabuk, H., & Kuşku Çabuk, F. (2016). Mechanoreceptors of the ligaments and tendons around the knee. Clinical Anatomy, 29,(6),789–795. doi: 10.1002/ca.22743.
(b) An image of a ligament joining carpal bones.
Table 3.6 Summary of Cells, Extracellular Matrix (ECM), Subtypes, and Function of Ligaments Under Normal Conditions
| Characteristic | Description |
|---|---|
| Tissue type | Dense regular connective tissues |
| Cells | Fibroblastic synoviocyte‐like type cells |
| ECM | Collagen, proteoglycans, elastin (varies from 4 to 70% of dry weight dependent on ligament) |
| Subtypes | Typical (4–9% elastin) to highly extensible (up to 70% extensible) |
| Function | Support and strength to joints |
Ligament Function
The function of ligaments is to provide support and structure to the joints (while providing some flexibility in joint motion). They also prevent excessive motion and provide feedback on positional information of the joints through sensory endings in the ligaments. Ligaments serve to join two or more bones together; both at rest and during the normal range of motion of the joint in question. Ligaments limit the range of motion through passive stabilization and guiding joints through their normal range of motion under tensile load. Thus, ligaments function to transfer forces from bone to bone in a particular joint, both at rest and during movement, and help to guide motion while providing structural stability to the joint. The range of motion of different joints can vary widely; thus, ligaments can assume a wide variety of shapes and sizes. Ligaments are pliant and flexible, but they also provide strength and support to the joint resulting from forces that may be applied.
Joint ligaments are also highly innervated near the site of attaching tendons and in the outer layer of the joint capsule. This innervation will be discussed further in Chapter 4.
Joints
Bones are too rigid to bend without damage. Fortunately, there are multiple bones throughout the body joined together by joints (the articulation or union between two or more bones or parts of bones). This segmentation of the skeleton allows for movement and segmental growth. As is the case with all aspects of the musculoskeletal system, the structure of a joint, specifically the manner in which the joints are held together, determines its function. Although there are many ways to classify joints, the simplest is a functional classification of little to no movement (synarthroses, syndesmoses, and synchondroses) versus freely movable (diarthroses) (Table 3.7).
Structure of Synarthroses
Synarthroses are bound together by either fibrous (symphyses and syndesmoses) or cartilaginous tissue (synchondroses). Symphyses are joints joined by fibrocartilage, in which the two opposing surfaces are covered by hyaline cartilage (thus, symphyses are also considered cartilaginous joints). The strength of the fibrocartilage allows for only a little movement but much stability, while the hyaline cartilage on the articulating surfaces allows for shock absorption. The pubic symphyses and intervertebral joints are examples of symphyses (Figure 3.15). The adjoining bones of fibrous syndesmoses are bound together by a thin sheet of fibrous tissue, either a ligament or a fibrous membrane. Since the fibrous tissue is flexible, these joints allow partial movement. The amount of movement allowed depends on the length of the fibers uniting the bones. Examples of syndesmoses are the suture joints of the skull and the union of the radius and ulna in the forearm by an interosseous membrane. Synchrondroses are joints joined together by cartilage and permit slight bending in early life. A key example is the joining of the epiphysis of a long bone with the metaphysis by a cartilaginous growth plate (the physis) (Figure 3.14). This is a temporary synchondrosis since the growth plate eventually ossifies in the mature adult.
Table 3.7 Classifications of Joints
| Classification type | Subtype | Description |
|---|---|---|
| Functional | Little to no movement (synarthroses) | Fibrous (symphyses and syndesmoses)Cartilaginous (synchondroses) |
| Freely movable | Diarthroses (all synovial joints) | |
| Arthrokinematics | Plane | Function: Gliding, spinning, or a combination |
| HingeSaddleCondyloidBall‐and‐socketPivot joints | Function: Movement in one plane, usually sagittal, about one axis of rotationFunction: Biaxial (motion about two primary axes in two planes) or triaxial movementFunction: Biaxial movementFunction: Movement in all three axesFunction: Movement in one plane (uniaxial) |
Structure of Diathroses (Synovial Joints)
Diarthroses are designed for movement and include all synovial joints. These are the most common types of joints and are defined as two or more bones whose ends are covered by hyaline cartilage, united by a fibrous tissue capsule that encloses the joint, and separated by a joint cavity. The cavity is filled with synovial fluid produced by a synovial membrane (a vascular connective tissue) lining the interior of the fibrous capsule. The synovial membrane cells produce and secrete synovial fluid, a lubricant that provides a smooth, nearly frictionless, gliding motion of opposing joint surfaces. The synovial fluid also nourishes the articular (hyaline) cartilage covering the bones. This type of joint allows the most movement, although lower stability. As a consequence, extrinsic and intrinsic ligaments usually reinforce synovial joints. Some synovial joints also have other distinguishing features such as menisci, labrums, or fibrocartilage articular discs that allow for shock absorption and/or additional stability. Nearly all of the joints of the upper and lower limbs are synovial.
Function of Joints
Arthrokinematics describes the movements occurring between the joint surfaces, such as rolling, spinning, and gliding of joint surfaces. As such, joints can be divided into plane, hinge, saddle, condyloid, ball‐and‐socket, and pivot joints. Plane joints are characterized by opposing bony surfaces that are flat or nearly so. The arthrokinematics of a plane joint includes gliding and spinning or a combination thereof. Gliding refers to a translation of one surface with respect to another, whereas spinning refers to a clockwise or counterclockwise rotation of one surface with respect to another. Depending on the precise curvature of the surfaces of a plane joint, their orientation, and their constraints by soft‐tissue structures, the osteokinematics of plane joint movement ranges from uniaxial to triaxial. Examples of plane joints include the acromioclavicular joint, which is triaxial, and the vertebral zygopophyseal (facet) joints, which are uniaxial. Hinge joints are constrained to movement in one plane, usually sagittal, about one