Basic Structure and Function of Human Joints

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Chapter 2

Basic Structure and Function of Human Joints

A joint is the junction or pivot point between two or more bones. Movement of the body as a whole occurs primarily through rotation of bones about individual joints. Joints also transfer and dissipate forces produced by gravity and muscle activation.

Arthrology, the study of the classification, structure, and function of joints, is an important foundation for the overall study of kinesiology. Aging, long-term immobilization, trauma, and disease all affect the structure and ultimate function of joints. These factors also significantly influence the quality and quantity of human movement.

This chapter focuses on the general anatomic structure and function of joints. The chapters contained in Sections II to IV of this text describe the specific anatomy and detailed function of the individual joints throughout the body. This detailed information is a prerequisite for understanding impairments of joints as well as employing the most effective rehabilitation of persons with joint dysfunction.

CLASSIFICATION OF JOINTS BASED ON MOVEMENT POTENTIAL

One method to classify joints focuses primarily on their movement potential. Based on this scheme, two major types of joints exist within the body: synarthroses and diarthroses (Figure 2-1).

Synarthroses

A synarthrosis is a junction between bones that allows slight to essentially no movement. Based on the dominant type of periarticular connective tissue that reinforces the articulation, synarthrodial joints can be further classified as fibrous or cartilaginous.63

Fibrous joints are stabilized by specialized dense connective tissues, usually with a high concentration of collagen. Examples of fibrous joints include the sutures of the skull, the distal tibiofibular joint (often referred to as a syndesmosis), and other joints reinforced by an interosseous membrane. Cartilaginous joints, in contrast, are stabilized by varying forms of flexible fibrocartilage or hyaline cartilage, often mixed with collagen. Cartilaginous joints generally exist in the midline of the body, such as the symphysis pubis, the interbody joints of the spine, and the manubriosternal joint.63

The function of synarthrodial joints is to strongly bind and transfer forces between bones. These joints are typically well supported by periarticular connective tissues and, in general, allow very little movement.

Diarthroses: Synovial Joints

A diarthrosis is an articulation that allows moderate to extensive motion. These joints also possess a synovial fluid–filled cavity. Because of this feature, diarthrodial joints are frequently referred to as synovial joints. Synovial joints comprise the majority of the joints within the musculoskeletal system.

Diarthrodial or synovial joints are specialized for movement and always exhibit seven elements (Figure 2-2). Articular cartilage covers the ends and other articular surfaces of bones. The joint is enclosed by a peripheral curtain of connective tissue that forms the joint (or articular) capsule. The joint capsule is composed of two histologically distinct layers. The external, or fibrous, layer is composed of dense connective tissue. This part of the joint capsule provides support between the bones and containment of the joint contents. The internal layer of the joint capsule consists of a synovial membrane, which averages 3 to 10 cell layers thick. The cells within this specialized connective tissue manufacture a synovial fluid that is usually clear or pale yellow, with a slightly viscous consistency.63 The synovial fluid contains many of the proteins found in blood plasma, including hyaluronan and other lubricating glycoproteins.63,75 The synovial fluid coats the articular surfaces of the joint. This fluid reduces the friction between the joint surfaces as well as providing some nourishment to the articular cartilage.

Ligaments are connective tissues that attach between bones, thereby protecting the joint from excessive movement. The thickness of ligaments differs considerably depending on the functional demands placed on the joint. Most ligaments can be described as either capsular or extracapsular. Capsular ligaments are usually thickenings of the articular capsule, such as the glenohumeral ligaments and deeper parts of the medial (tibial) collateral ligament of the knee. Capsular ligaments typically consist of a broad sheet of fibers that, when pulled taut, resist movements in two or often three planes. Most extracapsular ligaments are more cordlike and may be partially or completely separate from the joint capsule. Consider, for example, the lateral (fibular) collateral ligament of the knee or the alar ligament of the craniocervical region. These more discrete ligaments are usually oriented in a specific manner to optimally resist movement in usually one or two planes.

Small blood vessels with capillaries penetrate the joint capsule, usually as deep as the junction of the fibrous layer of the joint capsule and the adjacent synovial membrane. Sensory nerves also supply the external layer of the capsule and ligaments with receptors for pain and proprioception.

To accommodate the wide spectrum of joint shapes and functional demands, other elements may sometimes appear in synovial joints (see Figure 2-2). Intra-articular discs, or menisci, are pads of fibrocartilage imposed between articular surfaces. These structures increase articular congruency and improve force dispersion. Intra-articular discs and menisci are found in several joints of the body (see box).

A peripheral labrum of fibrocartilage extends from the bony rims of the glenoid fossa of the shoulder and the acetabulum of the hip. These specialized structures deepen the concave member of these joints and support and thicken the attachment of the joint capsule. Fat pads are variable in size and positioned within the substance of the joint capsule, often interposed between the fibrous layer and the synovial membrane. Fat pads are most prominent in the elbow and the knee joints. They thicken the joint capsule, causing the inner surface of the capsule to fill nonarticulating joint spaces (i.e., recesses) formed by incongruent bony contours. In this sense, fat pads reduce the volume of synovial fluid necessary for proper joint function. If these pads become enlarged or inflamed, they may alter the mechanics of the joint.

Bursae often form adjacent to fat pads. A bursa is an extension or outpouching of the synovial membrane of a diarthrodial joint. Bursae are filled with synovial fluid and usually exist in areas of potential stress. Like fat pads, bursae help absorb force and protect periarticular connective tissues, including bone. The subacromial bursa in the shoulder, for example, is located between the undersurface of the acromion of the scapula and the head of the humerus. The bursa may become inflamed because of repetitive compression between the humerus and the acromion. This condition is frequently referred to as subacromial bursitis.

Synovial plicae (i.e., synovial folds, synovial redundancies, or synovial fringes) are slack, overlapped pleats of tissue composed of the innermost layers of the joint capsule. They occur normally in joints with large capsular surface areas such as the knee and elbow. Plicae increase synovial surface area and allow full joint motion without undue tension on the synovial lining. If these folds are too extensive or become thickened or adherent because of inflammation, they can produce pain and altered joint mechanics. The plicae of the knee are further described in Chapter 13.

CLASSIFICATION OF SYNOVIAL JOINTS BASED ON MECHANICAL ANALOGY

Thus far in this chapter, joints have been classified into two broad categories based primarily on movement potential. Because an in-depth understanding of synovial joints is so crucial to an understanding of the mechanics of movement, they are here further classified using an analogy to familiar mechanical objects or shapes (Table 2-1).

A hinge joint is generally analogous to the hinge of a door, formed by a central pin surrounded by a larger hollow cylinder (Figure 2-3, A). Angular motion at hinge joints occurs primarily in a plane located at right angles to the hinge, or axis of rotation. The humero-ulnar joint is a clear example of a hinge joint (see Figure 2-3, B). As in all synovial joints, slight translation (i.e., sliding) is allowed in addition to the rotation. Although the mechanical similarity is less complete, the interphalangeal joints of the digits are also classified as hinge joints.

A pivot joint is formed by a central pin surrounded by a larger cylinder. Unlike a hinge, the mobile member of a pivot joint is oriented parallel to the axis of rotation. This mechanical orientation produces the primary angular motion of spin, similar to a doorknob’s spin around a central axis (Figure 2-4, A). Two examples of pivot joints are the humeroradial joint, shown in Figure 2-4, B, and the atlanto-axial joint in the craniocervical region.

An ellipsoid joint has one partner with a convex elongated surface in one dimension that is mated with a similarly elongated concave surface on the second partner (Figure 2-5, A). The elliptic mating surfaces severely restrict the spin between the two surfaces but allow biplanar motions, usually defined as flexion-extension and abduction-adduction. The radiocarpal joint is an example of an ellipsoid joint (see Figure 2-5, B). The flattened convex member of the joint (i.e., carpal bones) significantly limits the spin within the matching concavity (i.e., distal radius).

A ball-and-socket joint has a spheric convex surface that is paired with a cuplike socket (Figure 2-6, A). This joint provides motion in three planes. Unlike the ellipsoid joint, the symmetry of the curves of the two mating surfaces of the ball-and-socket joint allows spin without dislocation. Ball-and-socket joints within the body include the glenohumeral joint and the hip joint. As will be described further in Chapter 5, most of the concavity of the glenohumeral joint is formed not only by the glenoid fossa, but also by the surrounding muscle, labrum, joint capsule, and capsular ligaments.

A plane joint is the pairing of two flat or relatively flat surfaces. Movements combine sliding and some rotation of one partner with respect to the other, much as a book can slide or rotate across a tabletop (Figure 2-7, A). As depicted in Figure 2-7, B, the carpometacarpal joints within digits II to V are often considered as plane, or modified plane, joints. Most intercarpal and intertarsal joints are also considered plane joints. The forces that cause or restrict movement between the bones are supplied by tension in muscles or ligaments.

Each partner of a saddle joint has two surfaces: one surface is concave, and the other is convex. These surfaces are oriented at approximate right angles to each other and are reciprocally curved. The shape of a saddle joint is best visualized using the analogy of a horse’s saddle and rider (Figure 2-8, A). From front to back, the saddle presents a concave surface reaching from the saddle pommel in front to the back of the saddle. From side to side, the saddle is convex, stretching from one stirrup across the back of the horse to the other stirrup. The rider has reciprocal convex and concave curves to complement the shape of the saddle. The carpometacarpal joint of the thumb is the clearest example of a saddle joint (see Figure 2-8, B). The reciprocal, interlocking nature of this joint allows ample motion in two planes but limited spin between the trapezium and the first metacarpal.

A condyloid joint is much like a ball-and-socket articulation except that the concave member of the joint is relatively shallow (Figure 2-9, A). Condyloid joints usually allow 2 degrees of freedom. Ligaments or bony incongruities often restrain the third degree. Condyloid joints often occur in pairs, such as the knees (see Figure 2-9, B) and the atlanto-occipital joints (i.e., articulation between the occipital condyles and the first cervical vertebra). The metacarpophalangeal joint of the finger is another example of a condyloid joint. The root of the word condyle actually means “knuckle.”

The kinematics at condyloid joints vary based on joint structure. At the knee, for example, the femoral condyles fit within the slight concavity provided by the tibial plateau and menisci. This articulation allows flexion-extension and axial rotation (i.e., spin). Abduction-adduction, however, is restricted primarily by ligaments.

Simplifying the Classification of Synovial Joints: Ovoid and Saddle Joints

It is often difficult to classify synovial joints based on an analogy to mechanics alone. The metacarpophalangeal joint (condyloid) and the glenohumeral joint (ball-and-socket), for example, have similar shapes but differ considerably in the relative magnitude of movement and overall function. Joints always display subtle variations that make simple mechanical descriptions less applicable. A good example of the difference between mechanical classification and true function is seen in the gentle undulations that characterize the intercarpal and intertarsal joints. Several of these joints produce complex multiplanar movements that are inconsistent with their simple “planar” mechanical classification. To circumvent this difficulty, a simplified classification scheme recognizes only two articular forms: the ovoid joint and the saddle joint (Figure 2-10). Essentially all synovial joints of the body with the notable exception of planar joints can be categorized under this scheme.

An ovoid joint has paired mating surfaces that are imperfectly spheric, or egg-shaped, with adjacent parts possessing a changing surface curvature. In each case the articular surface of one bone is convex and of the other is concave. Most joints in the body fit this scheme. A saddle joint has been previously described. Each member presents paired convex and concave surfaces oriented at approximately 90 degrees to each other. This simplified classification system is functionally associated with the arthrokinematics of roll, slide, or spin (see Chapter 1).

AXIS OF ROTATION

In the analogy of a door hinge (see Figure 2-3, A), the axis of rotation (i.e., the pin through the hinge) is fixed because it remains stationary as the hinge opens and closes. With the axis of rotation fixed, all points on the door experience equal arcs of rotation. In anatomic joints, however, the axis of rotation is rarely, if ever, fixed during bony rotation. Determining the exact position of the axis of rotation in anatomic joints is therefore not a simple task. A method of estimating the position of the axis of rotation in anatomic joints is shown in Figure 2-11, A. The intersection of the two perpendicular lines bisecting a to a′ and b to b′ defines the instantaneous axis of rotation for the 90-degree arc of knee flexion.70 The word instantaneous indicates that the location of the axis holds true only for the specified arc of motion. The smaller the angular range used to calculate the instantaneous axis, the more accurate the estimate. If a series of line drawings is made for a sequence of small angular arcs of motion, the location of the instantaneous axes can be plotted for each portion within the arc of motion (see Figure 2-11, B). The path of the serial locations of the instantaneous axes of rotation is called the evolute. The path of the evolute is longer and more complex when the mating joint surfaces are less congruent or have greater differences in their radii of curvature, such as in the knee.

In many practical clinical situations it is necessary to make simple estimates of the location of the axis of rotation of a joint. These estimates are necessary when one performs goniometry, measures torque around a joint, or one constructs a prosthesis or an orthosis. A series of radiographs is required to precisely identify the instantaneous axis of rotation at a joint. This method is not practical in ordinary clinical situations. Instead, an average axis of rotation is assumed to occur throughout the entire arc of motion. This axis is located by an anatomic landmark that pierces the convex member of the joint.

HISTOLOGIC ORGANIZATION OF PERIARTICULAR CONNECTIVE TISSUES

There are only four primary types of tissue found in the body: connective tissue, muscle, nerve, and epithelium. Connective tissue, a derivative of the mesoderm, forms the basic structure of joints. The following section provides an overview of the histologic organization of the different kinds of connective tissues that form capsule, ligament, tendon, articular cartilage, and fibrocartilage. Throughout this textbook, these tissues are referred to as periarticular connective tissues. Bone is a very specialized form of connective tissue closely related to joints and is briefly reviewed later in this chapter.

Very generally, the fundamental materials that comprise all connective tissues in the body are fibrous proteins, ground substance, and cells. Even structures that are apparently as different as the capsule of the spleen, a fat pad, bone, and articular cartilage are made of these same fundamental materials. Each of these structures, however, consists of a unique composition, proportion, and arrangement of fibrous proteins, ground substance, and cells. The specific combination of these materials reflects the structures’ unique mechanical or physiologic functions. The following section describes the basic biologic materials that form periarticular connective tissues.

Fibrous Proteins

Collagen and elastin fibrous proteins are present in varying proportions in all periarticular connective tissues. Collagen is the most ubiquitous protein in the body, accounting for 30% of all proteins.30 At the most basic level, collagen consists of amino acids wound in a triple helical fashion. These spiraled molecular threads, called tropocollagen, are placed together in a strand, several of which are cross-linked into ropelike fibrils. A collagen fibril may be 20 to 200 nm in diameter.75 Many fibrils interconnect to form bundles or fibers. Although up to 28 specific types of collagen have been described based primarily on their amino acid sequences,67 two types make up the majority of collagen found in periarticular connective tissues: type I and type II.75 Type I collagen consists of thick fibers that elongate little (i.e., stretch) when placed under tension. Being relatively stiff and strong, type I collagen is ideal for binding and supporting the articulations between bones. Type I collagen is therefore the primary protein found in ligaments and fibrous joint capsules. This type of collagen also makes up the parallel fibrous bundles that comprise tendons—the structures that transmit forces between muscle and bone. Figure 2-12 shows a high resolution and magnified image of type I collagen fibrils.

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