Chapter 2: Joint Structure and Function

INTRODUCTION

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).

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