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 axis of rotation, usually mediolateral. The elbow joint is one example of a hinge joint. Saddle (sellar) joints are characterized by opposing surfaces that are concave and convex, but along opposite planes so that they are contoured to fit together. The osteokinematics of saddle joints are inconsistently described as either biaxial (motion about two primary axes in two planes) or triaxial. This inconsistency can be explained by the fact that the majority of motion typically occurs in two planes (usually flexion‐extension and abduction‐adduction), while there is a small amount of internal‐external rotation. The carpometacarpal joint of the thumb is an example of a saddle joint in which flexion‐adduction‐internal rotation combine to produce the action of opposition. Condyloid joints are composed a one nearly spherical convex surface opposing a shallow, nearly flat concave surface. These joints are considered biaxial owing to the predominance of movement about two axes and in two planes. The arthrokinematics during movement of condyloid joints are described by the “concave–convex rule,” which specifies that to maintain congruence of joint surfaces during bone movement, the convex condyloid component must roll in the direction of bony movement and glide in the opposite direction with respect to the concave component. Examples of condyloid joints include (a) the metacarpophalangeal joints of the fingers; (b) the knee joint, in which the distal femoral condyles articulate with the shallow, concave tibial plateaus; and (c) the atlanto‐occipital joint between the occipital condyles and the atlas. Ball and socket joints are distinguished by one bone with an ovoid or spherical convex surface that moves within a relatively deep concave surface. Ball and socket joints allow movement about all three axes and in all three planes of motion, flexion‐extension, abduction‐adduction, and internal‐external rotation. The coxofemoral (hip) and glenohumeral (shoulder) joints are examples of ball and socket joints. Lastly, pivot joints are characterized by one bone with a rounded process that moves within a sleeve or ring formed by the opposing bone. They permit rotation about a single axis and are, therefore, uniaxial joints. Examples include the proximal radioulnar and atlantoaxial joints.
Summary
This chapter discussed the main non‐neural tissue types that comprise the musculoskeletal system. Tables 3.1–3.5 summarize each tissue type in detail under normal uninjured conditions, Table 3.7 summarizes joints by their main function versus arthrokinematics, while Table 3.8 provides a general summary overview. Differences in these tissues characteristics after injury are discussed in Chapter 11, and their interaction with the nervous system is discussed in Chapter 4.
Table 3.8 Summary of Non‐Neural Tissues of the Musculoskeletal System
| Tissue | Description | Main functions |
|---|---|---|
| Loose connective tissuesAdiposeAreolarReticular | Fibers are loosely woven with many cells, all embedded in a semifluid ground substance. | Provides cushioning, support, elasticity, and immune functions |
| Dense connective tissues ‐IrregularDermis of skinDeep fasciaPeriosteumPerichondrium ‐ RegularTendonCartilageBone | Characterized by regularly or irregularly arranged collagen fibers, and low intercellular substance. Primary cells are fibroblasts or fibroblast like cells in dense irregular connective tissues. Primary cells in tendons are tenocytes, chondrocytes in cartilage, and a number of cells including osteoblast and osteoclasts in bone. |
Tendon: Transfer of tensile forces created by muscles onto bone; absorbs sudden shocks to limit muscle damage.Cartilage: Hyaline: Protection of bony surfaces, especially at points of movement; Fibrocartilage: Strength and rigidity, joint support and fusion; Elastic cartilage: Resilience and pliabilityBone: Strength, stability; lever
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