Musculoskeletal Disorders. Sean Gallagher

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 at points of attachment; storage of minerals, lipids and nutrients; blood cell formation
Skeletal muscle	Contains contractile cells (myocytes) arranged in multinucleated striated long fibers (myofibers).	Contraction and then movement of the endoskeleton to which the muscle is attached
Ligaments	Dense regularly connective tissue with many collagenous fibers arranged into bundles; small number of fibroblastic‐like cells	Support and strength to joints

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