An appreciation of the complexities of human joint design may be gained by considering the nature of the bony compo- nents and the functions that the joints must serve. The hu- man skeleton has about 200 bones that are connected by joints. Bones vary in size from the pea-sized distal phalanx of the little toe to the long femur of the thigh. The shape of the bones varies from round to flat, and the contours of the ends of the bones vary from convex to concave. The task of design- ing a series of joints to connect these varied bony components to form stable structures would be difficult. The task of de- signing joints that are capable of working together to provide both mobility and stability for the total structure represents an engineering problem of considerable magnitude.
Joint designs in the human body vary from simple to complex. Simple human joints usually have stability as a primary function; the more complex joints usually have mobility as a primary function. However, most joints in the human body serve a dual mobility-stability function and must also provide dynamic stability. The human stability joints are similar in design to table joints in that the ends of the bones may be contoured either to fit into each other or to lie flat against each other. The bracing of human joints is accomplished through the use of joint capsules, ligaments, and tendons. These are components of synovial joints, which are designed primarily for mobil- ity. Synovial joints are constructed so that the ends of the bony components are covered by hyaline cartilage and enclosed in a synovial sheath and fibrous layer that to- gether constitute the joint capsule. The capsules, liga- ments, and tendons located around mobility (synovial) joints not only help to provide stability for the joint but also guide, limit, and permit motion. Wedges of cartilage, called menisci, discs, plates, and labra, in synovial joints help increase stability, provide shock absorption, and facilitate motion. A lubricant, synovial fluid, is secreted in all synovial joints to help reduce friction between the articulating surfaces.102
In the traditional method of joint classification, the joints (arthroses or articulations) of the human body are divided into two broad categories on the basis of the type of materials and the methods of uniting the bony compo- nents. Subdivisions of joint categories are based on mate- rials used, the shape and contours of the articulating surfaces, and the type of motion allowed. The two broad categories of arthroses are synarthroses (nonsynovial joints) and diarthroses (synovial joints).6,29
Synarthroses
The material connecting the bony components in synarthrodial joints is interosseous connective tissue (fi- brous and/or cartilaginous). Synarthroses are grouped into two divisions according to the type of connective tissue in the union of bone to bone: fibrous joints and cartilaginous joints. The connective tissue directly unites one bone to an- other, creating a solid connective tissue-bone interface.
Fibrous Joints
In fibrous joints, the fibrous tissue directly connects bone to bone. Three different types of fibrous joints are found in the human body: sutures, gomphoses, and syndesmoses. A suture joint is one in which two bony components are united by a collagenous sutural ligament or membrane. The ends of the bony components are shaped so that the edges interlock or overlap one another. This type of joint is found only in the skull and, early in life, allows a small amount of movement. Fusion of the two opposing bones in suture joints occurs later in life and leads to the formation of a bony union called a synostosis.
Cartilaginous Joints
The materials connecting the bony components in cartilagi- nous joints are fibrocartilage and/or hyaline cartilage. These materials directly unite one bony surface to another, creat- ing a bone-cartilage-bone interface. The two types of carti- laginous joints are symphyses and synchondroses.
In a symphysis joint (secondary cartilaginous joint), the two bony components are covered with a thin lamina of hyaline cartilage and directly joined by fibrocartilage in the form of discs or pads. Examples of symphysis joints include the intervertebral joints between the bodies of the verte- brae, the joint between the manubrium and the sternal body, and the symphysis pubis in the pelvis.
Figure 2–17 The coronal suture. The frontal and parietal bones of the skull are joined directly by fibrous tissue to form a synarthrodial suture joint.
Coronal Suture
The serrated edges of the parietal and frontal bones of the skull are connected by a thin fibrous membrane (the sutural ligament) to form the coronal suture (Fig. 2–17).
At birth, the fibrous membrane allows some motion for ease of passage through the birth canal. Also, during infancy, slight motion allows for growth of the brain and skull. In adulthood, the bones grow together to form a synostosis and little or no motion is possible.
Example 2-5
The conical process of a tooth is inserted into the bony socket of the mandible or maxilla. In the adult, the loss of teeth is, for the most part, caused by disease processes affecting the connective tissue that cements or holds the teeth in approximation to the bone. Under normal condi- tions in the adult, these joints do not permit motion between the components.
Example 2-6
The shaft of the tibia is joined directly to the shaft of the fibula by an interosseous membrane (Fig. 2–18). A slight amount of motion at this joint accompanies movement at the ankle joint.
Example 2-7
CASE APPLICATION
Disruption of the Interosseous
Membrane case 2–7
In George’s ankle fracture, the interosseous membrane was disrupted, allowing some separation of the tibia and fibula.
Restoration of normal talocrural anatomy is crucial after such injuries, which is one of the major reasons open reduc- tion and internal fixation is used to treat these fractures.
The surgeon must be careful that the screw connecting the tibia and fibula is not too tight, in order to leave enough space to accommodate the talus in full dorsiflexion. The
“give” of the joint may take some time to recover, making it difficult for patients like George to jog or run comfortably for some months.
A gomphosis joint is a joint in which the surfaces of bony components are adapted to each other like a peg in a hole. In this type of joint, the component parts are con- nected by fibrous tissue. The only gomphosis joint that exists in the human body is the joint between a tooth and either the mandible or maxilla.
A syndesmosis is a type of fibrous joint in which two bony components are joined directly by an interosseous lig- ament, a fibrous cord, or an aponeurotic membrane. These joints usually allow a small amount of motion.
Figure 2–18 The shafts of the fibula and tibia are joined di- rectly by a membrane to form a synarthrodial syndesmosis.
Tibia
Interosseous membrane
Fibula
The Symphysis Pubis
The two pubic bones of the pelvis are joined by fibrocar- tilage. This joint serves as a weight-bearing joint and is responsible for withstanding and transmitting forces;
therefore, under normal conditions, very little motion is permissible or desirable. During pregnancy, when the connective tissues are softened, some slight separation of the joint surfaces occurs to ease the passage of the baby through the birth canal. However, the symphysis pubis is considered to be primarily a stability joint with the thick fibrocartilage disc forming a stable union between the two bony components (Fig. 2–19A).
Example 2-8
Diarthroses
The joint construction in diarthrodial (synovial) joints differs from that found in synarthrodial joints. In synovial joints, the ends of the bony components are free to move in relation to one another because no connective tissue directly connects adjacent bony surfaces. The bony com- ponents are indirectly connected to one another by means of a joint capsule that encloses the joint. All synovial joints are constructed in a similar manner and have the following features: (1) a joint capsule that is composed of two lay- ers8; (2) a joint cavity that is enclosed by the joint capsule;
(3) synovial tissue that lines the inner surface of the capsule; (4) synovial fluid that forms a film over the joint surfaces; and (5) hyaline cartilage that covers the surfaces of the enclosed contiguous bones13 (Fig. 2–20). Synovial joints may also include accessory structures, such as fibro- cartilaginous discs, plates, or menisci; labra; fat pads; and ligaments. Articular discs, menisci, and the synovial fluid help to prevent excessive compression of opposing joint surfaces by spreading applied forces over larger areas. Ar- ticular discs and menisci often occur between articular surfaces where congruity is low, thus increasing surface A synchondrosis (primary cartilaginous joint) is a type
of joint in which the material used to connect the two com- ponents is hyaline cartilage. The cartilage forms a bond be- tween two ossifying centers of bone. The function of this type of joint is to permit bone growth while also providing stability and allowing a small amount of mobility. Some of these joints are found in the skull and in other areas of the body at sites of bone growth. When bone growth is com- plete, some of these joints ossify and convert to bony unions (synostoses).
A
B
Figure 2–19 Cartilaginous joints. A. The two pubic bones of the pelvis are joined directly by fibrocartilage to form a symph- ysis joint called the symphysis pubis. B. The first rib and the sternum are connected directly by hyaline cartilage to form a synchondrosis joint called the first chondrosternal joint.
The First Chondrosternal Joint
The adjacent surfaces of the first rib and sternum are connected directly by articular cartilage (Fig. 2–19B).
Example 2-9
contact area. In some cases, articular discs extend all the way across a joint and actually divide it into two separate cavities, such as the articular disc at the distal radioulnar joint. Menisci usually do not divide a joint but provide lubrication and increase congruity. Ligaments (and the tendons associated with muscles crossing the joint) play an important role in keeping joint surfaces aligned and in guiding motion. Excessive separation or translation of joint surfaces is limited by passive tension in ligaments, the fibrous joint capsule, and tendons (passive stability).9 Active tension in muscles (dynamic stability) also limits the separation or translation of joint surfaces.
Joint Capsule
Joint capsules vary considerably in both thickness and com- position. The capsule enclosing the shoulder joint is thin, loose, and redundant, sacrificing stability for mobility.
Other capsules, such as the hip joint capsule, are thick and dense and thus favor stability over mobility. The thickness, fiber orientation, and composition of the capsule depend on the stresses placed on the joint, illustrating the dynamic na- ture of the joint capsule. For example, in portions of the capsule that are subjected to compression forces, the capsule may become fibrocartilaginous.7,8In patients with shoulder instability in which the joint capsule is subjected to repeated Figure 2–20 A typical diarthrodial joint.
Bone
Stratum fibrosum
Stratum
synovium Articular cartilage
Articular
cartilage Joint cavity Capillary
Articular nerve
CASE APPLICATION
Joints Affected by Our Patient’s
Injury case 2–8
Which joints and joint structures are directly and indirectly affected in the case of George Chen? Consider the talocrural, subtalar, transverse tarsal (talonavicular and cal- caneocuboid), and first metatarsophalangeal (MTP) joints.
tensile deformation, collagen fibrils and elastin fibers are larger and denser compared to normal capsules, adaptations that may increase capsular strength and resistance to stretching deformation.9
The fibrous capsule may be reinforced by and, in some instances, incorporate ligaments or tendons as part of the capsule. For example, the capsule of the proximal interpha- langeal joint of the fingers is reinforced by collateral liga- ments superficially and a central slip of the extensor tendon superficially and posteriorly.10
The joint capsule is composed of two layers: an outer layer called the stratum fibrosum and an inner layer called the stratum synovium (see Fig. 2–20). The stra- tum fibrosum, sometimes referred to as the fibrous cap- sule, is composed of dense fibrous tissue. Water accounts for about 70% of tissue weight; collagen and elastin ac- count for about 90% of the tissue dry weight.8,10The pre- dominant type of collagen is type I, which is arranged in parallel bundles. As the capsule nears its insertion to bone, the tissue changes to fibrocartilage and then miner- alized fibrocartilage and bone, similar to the fibrocarti- laginous (direct) type of ligament or tendon enthesis. The stratum fibrosum is poorly vascularized but richly inner- vated by joint receptors located in and around the capsule (see Table 2–7).12
The inner layer (stratum synovium) is the lining tissue of the capsule. It also consists of two layers: the intima and the subsynovial tissue. The intima is the layer of cells that lines the joint space. It is composed of a layer of specialized fibroblasts known as synoviocytes that are arranged one to three cells deep and set in a fiber-free intercellular matrix.11 Two types of synoviocytes are generally recognized: type A and type B.2 Type A synoviocytes are macrophage-like cells with prominent Golgi apparatus but sparse granular endoplasmic reticulum. Type A cells are primarily responsi- ble for the removal of debris from the joint cavity. During phagocytosis, type A cells synthesize and release lytic en- zymes that have the potential to damage joint tissues.
Type B synoviocytes have abundant granular endo- plasmic reticulum and are twice as numerous as type A cells in normal synovium.13 Type B cells produce sub- stances that inhibit the lytic enzymes, and are responsible for initiating immune responses through the secretion of antigens. Both types of cells synthesize the hyaluronic acid component of the synovial fluid, as well as the matrix in which the cells are embedded. Type A and B cells also secrete a wide range of cytokines, including multiple growth factors. The interplay of the cytokines acting as stimulators or inhibitors of synoviocytes allows structural repair of synovium, response to foreign or autologous antigens, and tissue destruction.14
The subsynovial tissue lying outside the intima is a loose network of highly vascularized fibrous connective tissue. It attaches to the margins of the articular cartilage through a transitional zone of fibrocartilage and blends with the pe- riosteum covering the intracapsular portions of the bones.
Its cells differ from the intima cells in that they are more spindle-shaped, are more widely dispersed among collagen fibrils, and produce matrix collagen.103The subsynovial tis- sue supports the intima and merges on its external surface
thought to provide synovial fluid with the ability to dissi- pate energy.11,102,104,105 Changes in the concentration of hyaluronate or lubricin in the synovial fluid will affect the overall lubrication and the amount of friction that is pres- ent. Many experiments have confirmed that articular coef- ficients of friction in synovial joints are far lower than those created with manufactured lubricants.103The lower the co- efficient of friction, the lower the resistance to movement.
Injections of hyaluronate and lubricin have been used suc- cessfully to alleviate symptoms of osteoarthritis in both hip and knee joints.
Normal synovial fluid appears as a clear, pale yellow, vis- cous fluid present in small amounts at all synovial joints.105 There is a direct exchange between the vasculature of the stratum synovium and the intracapsular space, where nutri- ents can be supplied and waste products can be taken away from the joint by diffusion.26Usually, less than 0.5 mL of synovial fluid can be removed from large joints such as the knee.6However, when a joint is injured or diseased, the volume of the fluid may increase.26The synovial fluid, like all viscous substances, resists shear loads.102The viscosity of the fluid varies inversely with the joint velocity or rate of shear; that is, it becomes less viscous at high rates. Thus, synovial fluid is referred to as thixotropic. When the bony components of a joint are moving rapidly, the viscosity of the fluid decreases and provides less resistance to motion.102 When the bony components of a joint are moving slowly, the viscosity increases and provides more resistance to motion. Viscosity also is sensitive to changes in tempera- ture. High temperatures decrease the viscosity, whereas low temperatures increase the viscosity.102
Joint Lubrication
The minimal wear shown by normal cartilage, despite varied loads, is the result of the structure of the cartilage matrix and the presence of lubricating fluid.103–105A number of models have been proposed to explain how diarthrodial joints are lubricated under varying loading conditions. The general consensus is that no single model is adequate to explain hu- man joint lubrication and that human joints are lubricated by two or more types of lubrication; the two basic types are boundary lubrication and fluid-film lubrication.28 with the fibrous capsule. The intima is richly endowed with
capillary vessels, lymphatic vessels, and nerve fibers. The blood vessels in the subsynovial tissue transport oxygen, nu- trients, and immunologic cells to the joint.
Branches of adjacent sensory nerves and branches of nerves from muscles near the joint penetrate the fibrous joint capsule.12Large-diameter sensory efferent nerves and thinly myelinated nerves are present in the fibrous capsule;
nonmyelinated C-type fibers are found in the synovium.
The joint receptors found in the fibrous joint capsule are sensitive to stretching or compression of the capsule, as well as to any increase in internal pressure due to increased pro- duction of synovial fluid (joint swelling).
Most of the joint receptors in the knee are located in the subsynovial layer of the capsule close to the insertions of the anterior cruciate ligament (ACL). Mechanoreceptors (predominantly Ruffini receptors) in the subsynovial cap- sule and ACL respond primarily to the stretch involved in terminal knee extension. Pacini receptors are reported less frequently and are thought to be activated by compression.
Free nerve endings are more numerous than mechanore- ceptors and function as nociceptors that react to inflam- mation and pain stimuli. Afferent free nerve endings in joints not only transmit information but also serve a local effector role by releasing neuropeptides.12,15
Synovial Fluid
The thin film of synovial fluid that covers the surfaces of the inner layer of the joint capsule and articular cartilage helps to keep the joint surfaces lubricated and reduces fric- tion.102,103The fluid also provides nourishment for the hya- line cartilage covering the articular surfaces, as fluid moves in and out of the cartilage as compression is applied, then released. The composition of synovial fluid is similar to that of blood plasma, except that synovial fluid also contains hyaluronate (hyaluronic acid) and a glycoprotein called lubricin.26The hyaluronate component of synovial fluid is responsible for the viscosity of the fluid and is essential for joint lubrication. Hyaluronate reduces the friction between the synovial folds of the capsule and the articular surfaces.26 Lubricin is the component of synovial fluid thought to be responsible for cartilage-on-cartilage lubrication; it also is
Table 2–7 Joint Receptors
TYPE NAME SENSITIVITY LOCATION
I
II III IV
Ruffini
Pacini or pacini-form Golgi, Golgi-Mazzoni
Unmyelinated free nerve endings
Stretch—usually at extremes of extension
Compression or changes in hydrostatic pressure and joint movement1 Pressure and forceful joint motion into
extremes of motion
Non-noxious and noxious mechanical stress or biomechanical stress
Fibrous layer of joint capsules on flexion side of joints, periosteum, ligaments, and tendons10
Located throughout joint capsule, partic- ularly in deeper layers and in fat pads Inner layer (synovium) of joint capsules,
ligaments, and tendons10
Located around blood vessels in synovial layer of capsule and in adjacent fat pads and collateral ligaments, tendons, and the periosteum