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" Human joints must serve many functions; they are more complex than most man-made designs."

Joint Design

Form Follows Function

The joints of the human body, like those used in the construction of buildings, furniture, and machines, connect different segments together and often allow movement between those segments. The design of the joint reflects these demands. The dictum form follows function, coined by the American architect Louis Sullivan and promoted by the Bauhaus school of design of post–World War I Germany,1 suggests that the appearance (form) of an object or building should allow an observer to determine its function. The form of a chair, for example, with a seat, arms, and a back at appropriate heights and angles, tells us that its function is to support a sitting person. The function of the joint between most tabletops and table legs is support; therefore, the joints form a stable union. The joint between a leg and the top of a folding table, however, has a different function (mobility and stability), and therefore requires a different design, or form. We might design a folding table joint, for example, to have a metal brace fitted with a locking device. The table leg would be free to move when the brace is unlocked; when the brace is locked, the joint would be stable (Fig. 2–1).

Figure 2–1

Folding table joint. A. The table leg is free to move, and the joint provides mobility when the brace is unlocked. B. The table leg is prevented from moving, and the joint provides stability when the brace is locked.

The relationship between form and function means we can often determine the function of joints in the human body by examining their structure; we do this by studying the anatomy (structure) of joints.

Unlike man-made structures, in which form is static, the relationship between form and function in living structures is a two-way street—functional demands often help to determine the form of the structure. For example, the structural elements of the hip joint develop before birth, but the mature shape of the head of the femur and the acetabulum is determined by the functional interaction between these two structures. Decreased contact between the acetabulum and femoral head leads to developmental dysplasia of the hip (Fig. 2–2).2

Figure 2–2

In the normal hip (A) the interaction between the head of the femur and the acetabulum causes the formation of the femoral head's round shape and the acetabulum's cuplike structure. The muscle lever arm enables the abductors to successfully create adequate torque during stance and gait. In the dysplastic hip (B), the femur is not seated in the acetabulum in infancy, and the lack of interaction prevents normal development of the ...

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