Hana mua a paʻa ke kahua ma mua o ke aʻo ana aku iā haʻi.

Build yourself a firm foundation before teaching others.

ʻŌlelo Noʻeau, compiled by Mary Kawena Pukui, #276

Introduction

The adult human body has roughly 206 bones, and except for the hyoid bone in the neck, each bone is connected to at least one other bone. Joints (articulations) are formed where bones or bone and cartilage come together or articulate with each other to form a connection. The root word arth- is used in medical terms to describe joints.

Many joints allow for movement between the bones, such as at the knee joint or the elbow joint. At these joints, the articulating surfaces, the areas of the adjacent bones that touch each other, are not directly united to each other. Instead, these surfaces are enclosed within a space filled with lubricating fluid, allowing for a wide range of motions and smooth movements. These joints provide greater mobility, but since the bones are free to move in relation to each other, the joint is less stable.

Other joints in the body, however, may be joined to each other by connective tissue or cartilage, such as the sutures in the skull. These joints are designed for stability and provide for little or no movement. The articulating surfaces of bones at stable types of joints, with little or no mobility, are strongly united to each other.

Joint stability and movement are related to each other. Stable joints allow for little or no mobility or movement between the adjacent bones. Immovable and slightly moveable joints tend to be located primarily in the axial skeleton. Conversely, joints that provide the most movement between bones are the least stable. Freely moveable joints are most commonly seen in the appendicular skeleton (the limbs). Understanding the relationship between joint structure and function will help to explain why particular types of joints are found in certain areas of the body.

9.1 Joint Classification

Joints are classified based on either structure or function. The structural classification, which describes the joint based on the type of tissue that binds the bones together, has three categories. The functional classification is also divided into three broad categories.

Figure 9.2: Types of Synovial Joint

Functional Classification of Joints

The functional classification of joints is based on the amount of mobility found between the adjacent bones. Joints are functionally classified as a synarthrosis or immobile joint, an amphiarthrosis or slightly moveable joint, or as a diarthrosis, which allows for increased movement and is considered “freely” moveable. (arthroun = “to fasten by a joint”).

Synarthrosis

An immobile or nearly immobile joint is called synarthrotic. The immobile nature of these joints provides for a strong union between the articulating bones. Synarthrotic joints often unite bones that surround internal organs. Thus the strong union between the articulating bones protects internal organs, such as the brain, lungs, and heart.

Amphiarthrosis

An amphiarthrotic joint has limited mobility. The limited movement provided by amphiarthrotic joints aids in the stability of the joint. An example of this type of joint is the cartilaginous joint that unites the bodies of adjacent vertebrae. Filling the gap between the vertebrae is a thick pad of fibrocartilage called an intervertebral disc (Figure 9.3). Each intervertebral disc strongly unites the vertebrae but still allows for a limited amount of movement between them. However, the small movements available between adjacent vertebrae can sum together along the length of the vertebral column to provide for large ranges of body movements.

Figure 9.3 Intervertebral Disc: An intervertebral disc unites the bodies of adjacent vertebrae within the vertebral column. Each disc allows for limited movement between the vertebrae and thus functionally forms an amphiarthrosis type of joint. Intervertebral discs are made of fibrocartilage and thereby structurally form a symphysis type of cartilaginous joint. [OpenStax]

Diarthrosis

A freely mobile joint is classified as a diarthrotic. These types of joints include all synovial joints of the body, which are very numerous and provide the majority of body movements. Diarthrotic joints are the most common joints in the body and are found in the appendicular skeleton, giving the numerous articulations of the limbs a wide range of motion. These joints are divided into three categories, based on the number of axes of motion provided by each. An axis in anatomy is described as the movements in reference to the three anatomical planes: transverse, frontal, and sagittal. Thus, diarthroses are classified as uniaxial (for movement in one plane), biaxial (for movement in two planes), or multiaxial joints (for movement in all three anatomical planes).

Structural Classification of Joints

The structural classification of joints is based on whether the articulating surfaces of the adjacent bones are directly connected by fibrous connective tissue or cartilage, or whether the articulating surfaces contact each other within a fluid-filled joint cavity. These differences serve to divide the joints of the body into three structural classifications. A fibrous joint is where the adjacent bones are united by fibrous connective tissue. At a cartilaginous joint, the bones are joined by hyaline cartilage or fibrocartilage. At a more complex synovial joint, the articulating surfaces of the bones are not directly connected but instead come into contact with each other within a joint cavity that is filled with a lubricating fluid. Synovial joints allow for “free” movement between the bones and are the most common joints of the body.

Fibrous Joints

Fibrous joints are joints in which the bones are anchored by fibrous connective tissue. These joints do not have a synovial cavity between them. In general, most fibrous joints are synarthrotic (immovable), and the amount of movement allowed is based upon the length of the connective tissue fibers that bind the bones together. The three types of fibrous joints are sutures, syndesmoses, and gomphoses.

Figure 9.4: Fibrous Joints Fibrous joints form strong connections between bones. (a) Sutures join most bones of the skull. (b) An interosseous membrane forms a syndesmosis between the radius and ulna bones of the forearm. (c) A gomphosis is a specialized fibrous joint that anchors a tooth to its socket in the jaw.

Oregon State University A&P OER also in OpenStax image

Sutures

The word suture means to bind or to sew, and sutures literally look like a stitch or seam. Sutures occur only between the bones of the skull (except for the mandible) and this oftentimes makes it appear that the bones of the skull are stitched together.

In newborns and infants, the areas of connective tissue between the bones are much wider, especially in those areas on the top and sides of the skull that will become the sagittal, coronal, squamous, and lambdoid sutures. These broad areas of connective tissue are called fontanelles (Figure 9.6). During childbirth, the structure of the fontanelles allows for flexibility of the skull, so that as the fetus travels through the birth canal, the skull bones can push closer together or overlap slightly, which aids the movement of the infant’s head through the birth canal. After birth, these regions of connective tissue permit rapid growth of the skull to accommodate the significant enlargement of the brain that occurs in childhood. The fontanelles greatly decrease in width during the first year after birth as the skull bones enlarge. When the connective tissue between the adjacent bones is reduced to a narrow layer, these fibrous joints are called sutures.

Figure 9.6 The Newborn Skull: The fontanelles of a newborn’s skull are broad areas of fibrous connective tissue that form fibrous joints between the bones of the skull. [OpenStax]

The skull bones have wave-shaped edges that interlock and the articulating surfaces are completely filled with very short connective tissue fibers. The rigid interlocking joints provide protection but the connective tissue allows for the skull to expand as the brain grows during youth. During development, certain sutures ossify, causing them to fuse into a single unit. The fusion between bones is called synostosis (bony junction) and serves to protect the brain from damage. For example, the metopic suture in the frontal bone of the cranium ossifies shortly after birth and is no longer visible when examining the skull of a child.

Syndesmosis

Syndesmoses are a type of fibrous joint that are connected by ligaments, cords, or bands of fibrous connective tissue. Though syndesmoses do not have a joint cavity, a gap exists between the bones at this joint. The gap between the bones is either wide or narrow and is filled by a broadsheet of connective tissue called an interosseous membrane.

In the forearm, the wide gap between the shaft portions of the radius and ulna bones is strongly united by an interosseous membrane. Similarly, in the lower leg, the shafts of the tibia and fibula are united by an interosseous membrane. In addition, at the distal tibiofibular joint, where the articulating surfaces of the bones lack cartilage, the narrow gap between the bones is anchored by fibrous connective tissue and reinforced by ligaments on both the anterior and posterior aspects of the joint. Together, the interosseous membrane and these ligaments form the tibiofibular syndesmosis.

The syndesmoses found in the forearm and lower leg unite the parallel bones and prevent the bones from separating. However, a syndesmosis does not prevent all movement between the bones (some movement is necessary), and thus this type of fibrous joint is functionally classified as an amphiarthrosis (slightly moveable). In the forearm, the fibers of the interosseous membrane are long and thus flexible enough to allow for rotation of the radius bone during forearm movements. In the leg, the syndesmosis between the tibia and fibula consists of shorter fibers than that in the forearm. The short fibers strongly unite the bones of the lower leg, allowing for very little movement. This provides strength and stability to the leg and ankle and these are important during weight-bearing. Thus in contrast to the stability provided by the tibiofibular syndesmosis, the flexibility of the antebrachial interosseous membrane allows for the much greater mobility of the forearm. In addition to reinforcing the joint structure, the interosseous membrane between the bones of the forearm (radius and ulna) and the interosseous membrane between the bones of the lower leg (tibia and fibula) provides a surface area for muscle attachment. It is important to note that powerful muscle contractions require ample space for muscle attachment.

Gomphosis

A gomphosis (fastened with bolts) is a type of peg and socket joint that only exists between a tooth and its boney alveolar socket in the maxillary bone (upper jaw) or mandible bone (lower jaw) of the skull. It is a specialized fibrous joint that anchors the root of a tooth into its bony socket. Between the bony walls of the socket and the root of the tooth are many short bands of dense connective tissue, each of which is called a periodontal ligament. The short fibers of the periodontal ligament render the gomphosis joint immobile, and thus this type of joint is functionally classified as a synarthrosis.

Cartilaginous Joints

In cartilaginous joints, the articulating surfaces of the bones are united by cartilage [See chapter 4]. Cartilaginous joints do not have a joint cavity and have little movement. There are two structural classifications of cartilaginous joints: synchondrosis and symphysis.

Figure 9.7: Cartilaginous Joints At cartilaginous joints, bones are united by hyaline cartilage to form a synchondrosis or by fibrocartilage to form a symphysis. (a) The hyaline cartilage of the epiphyseal plate (growth plate) forms a synchondrosis that unites the shaft (diaphysis) and end (epiphysis) of a long bone and allows the bone to grow in length. (b) The pubic portions of the right and left hip bones of the pelvis are joined together by fibrocartilage, forming the pubic symphysis.

Synchondrosis

A synchondrosis is a cartilaginous joint where the bones are joined by hyaline cartilage [See chapter 4 image of hyaline cartilage]. Almost all synchondrosis joints are immovable and are functionally classified as synarthrosis. An example of synchondroses is the first sternocostal joint, where the first rib is anchored to the manubrium by its costal cartilage.

Symphysis

A cartilaginous joint where the bones are joined by fibrocartilage is called a symphysis (growing together). Fibrocartilage [See chapter 4 image of fibrocartilage] is very strong because it contains bundles of thick collagen fibers. The presence of the collagen provides tensile strength, giving a much greater ability to resist pulling and bending forces. Fibrocartilage is also compressible and thus very resilient to such forces. This gives symphyses the ability to strongly unite the adjacent bones, but can still allow for limited movement to occur. Thus, a symphysis is functionally classified as amphiarthrosis. Examples of a symphysis include the pubic symphysis, which is designed to bear the weight of the body during movement but also to be flexible enough for ambulation.

Epiphyseal Cartilages

The epiphyseal growth plate present in the proximal and distal ends of long bones in children is often classified as a temporary synchondrosis joint. As described in the chapter on osseous tissue, the epiphyseal plate is the region of growing hyaline cartilage that unites the diaphysis (shaft) of the bone to the epiphysis (end of the bone). Growth in bone length occurs in childhood and eventually becomes synostosis as the epiphyseal plate (hyaline cartilage) fuses into the epiphyseal line (bone).

9.2 Synovial Joints

Synovial joints, freely moving diarthrotic joints, are the most common joints in the body. The key structural difference that sets synovial joints apart from the other types of joints found in the body is the presence of a joint cavity. The joint cavity is a fluid-filled space that exists between the articulating surfaces of the bones. Also unlike fibrous or cartilaginous joints, the articulating bone surfaces at a synovial joint are not directly connected to each other with fibrous connective tissue or cartilage. This gives the bones of a synovial joint the ability to move smoothly against each other, allowing for increased joint mobility. All synovial joints share the same six general features, and many also have bursae and tendon sheaths. The stability of synovial joints is influenced by three main factors and the mobility allowed at each specific joint is directly related to the shape of the joint.

Structure

Common Characteristics of Synovial Joints

All synovial joints share the following six characteristics:

• Articular capsule — a double layer of connective tissue that surrounds and strengthens the joint
• Articular cartilage — a smooth layer of hyaline cartilage that covers the end of the long bones where the bones articulate
• Joint cavity — fluid-filled space that exists between the articulating surfaces
• Synovial fluid — viscous, oily filtrate secreted by the cells of the synovial membrane that provides lubrication, nourishment, and removal of debris
• Reinforcing ligaments — fibrous connective tissue that connects one bone to another providing static stability of the joint
• Nerves and Blood Vessels — provide sensory, as well as extensive capillary beds that supply the synovial membrane

Figure 9.8: Synovial Joints: Synovial joints allow for smooth movements between the adjacent bones. The joint is surrounded by an articular capsule that defines a joint cavity filled with synovial fluid. The articulating surfaces of the bones are covered by a thin layer of articular cartilage. Ligaments support the joint by holding the bones together and resisting excess or abnormal joint motions. [open stax]

Synovial joints are characterized by the presence of a joint cavity. The walls of this space are formed by the articular capsule, a fibrous connective tissue structure that encapsulates each bone just outside the area of the bone’s articulating surface. The bones of the joint articulate with each other within the joint cavity.

Friction between the bones at a synovial joint is prevented by the presence of the articular cartilage, a thin layer of hyaline cartilage that covers the entire articulating surface of each bone. However, unlike at a cartilaginous joint, the articular cartilages of each bone are not continuous with each other. Instead, the articular cartilage acts like a Teflon® coating over the bone surface, allowing the articulating bones to move smoothly against each other without damaging the underlying bone tissue. Lining the inner surface of the articular capsule is a thin synovial membrane. The cells of this membrane secrete synovial fluid (synovia = a thick fluid), a thick, slimy fluid that provides lubrication to further reduce friction between the bones of the joint. This fluid also provides nourishment to the articular cartilage, which is avascular (does not contain blood vessels). In summary, each synovial joint is functionally classified as a diarthroses or “freely moveable” due to the ability of the bones to move smoothly against each other within the joint cavity and allowing the freedom of joint movement.

With synovial joints the bones are connected by ligaments, which are strong bands of fibrous connective tissue that connect bone to bone. These strengthen and support the joint by anchoring the bones together and preventing their separation. Ligaments allow for normal movements at a joint, but limit the range of these motions, thus preventing excessive or abnormal joint movements. Ligaments are classified based on their relationship to the fibrous articular capsule. An extrinsic ligament is located outside of the articular capsule, an intrinsic ligament is fused to or incorporated into the wall of the articular capsule, and an intracapsular ligament is located inside of the articular capsule.

At many synovial joints, additional support is provided by the muscles and their tendons that act across the joint. A tendon is the dense connective tissue structure that attaches a muscle to bone. As forces acting on a joint increase, the body will automatically increase the overall strength of contraction of the muscles crossing that joint, thus allowing the muscle and its tendon to serve as a dynamic stabilizer to resist forces and support the joint. This type of indirect support by muscles is very important at the shoulder joint, for example, where the laxity (movement) in the joint is extensive and the ligaments are relatively weak.

Additional structures located outside of a synovial joint serve to prevent friction between the bones of the joint and the overlying muscle tendons or skin. A bursa (plural = bursae) is a thin connective tissue sac filled with lubricating liquid. These are located in regions where skin, ligaments, muscles, or tendons can rub against each other, usually near a body joint. Bursae reduce friction by separating the adjacent structures, preventing them from rubbing directly against each other. Bursae are classified by their location. A subcutaneous bursa is located between the skin and an underlying bone. It allows the skin to move smoothly over the bone. Examples include the prepatellar bursa located over the kneecap and the olecranon bursa at the tip of the elbow. A submuscular bursa is found between a muscle and an underlying bone, or between adjacent muscles. These prevent the rubbing of the muscle during movements. A subtendinous bursa is found between a tendon and a bone.

Figure 9.9 Bursae

Types of Movements at Synovial Joints

Synovial joints allow the body a tremendous range of movements. Each movement at the synovial joint results from the contraction or relaxation of the muscles that are attached to the bones on either side of the articulation. The type of movement that can be produced at a synovial joint is determined by its structural type.

A uniaxial joint only allows for a motion in a single plane (around a single axis). The elbow joint, which only allows for bending or straightening, is an example of a uniaxial joint. A biaxial joint allows for motions within two planes. An example of a biaxial joint is a metacarpophalangeal joint (knuckle joint) of the hand. The joint allows for movement along one axis to produce bending or straightening of the finger, and movement along a second axis, which allows for spreading of the fingers away from each other and bringing them together. A joint that allows for several directions of movement is called a multiaxial joint (polyaxial or triaxial joint). This type of diarthrotic joint allows for movement along three axes. The shoulder and hip joints are multiaxial joints. They allow the upper or lower limb to move in an anterior-posterior (flexion and extension) direction and a medial-lateral (abduction and adduction) direction. In addition, the limb can also be rotated around its long axis. This third movement results in the rotation of the limb so that its anterior surface is moved either toward or away from the midline of the body.

Synovial joints are subdivided based on the shapes of the articulating surfaces of the bones that form each joint. The six types of synovial joints are pivot, hinge, condyloid, saddle, plane, and ball-and socket-joints (Figure 9.10). See Table 1 with the six types of synovial joints.

Figure 9.10 Types of Synovial Joints

Movements at Synovial Joints

Overall, each type of synovial joint is necessary to provide the body with flexibility and mobility. There are many types of movement that can occur at synovial joints. Movement types are generally paired, with one being the opposite of the other. Body movements are always described in relation to the anatomical position of the body: upright stance, with upper limbs to the side of the body and palms facing forward.

As illustrated in the image, the types of movement include:

OpenStax image

Figure 9.11 Movements of the Body, Part 1 Synovial joints give the body many ways in which to move. (a)-(b) Flexion and extension motions are in the sagittal (anterior-posterior) plane of motion. These movements take place at the shoulder, hip, elbow, knee, wrist, metacarpophalangeal, metatarsophalangeal, and interphalangeal joints. (c)-(d) Anterior bending of the head or vertebral column is flexion, while any posterior-going movement is an extension. (e) Abduction and adduction are motions of the limbs, hand, fingers, or toes in the coronal (medial-lateral) plane of movement. Moving the limb or hand laterally away from the body, or spreading the fingers or toes, is abduction. Adduction brings the limb or hand toward or across the midline of the body or brings the fingers or toes together. Circumduction is the movement of the limb, hand, or fingers in a circular pattern, using the sequential combination of flexion, adduction, extension, and abduction motions. Adduction/abduction and circumduction take place at the shoulder, hip, wrist, metacarpophalangeal, and metatarsophalangeal joints. (f) Turning of the head side to side or twisting of the body is rotation. Medial and lateral rotation of the upper limb at the shoulder or lower limb at the hip involves turning the anterior surface of the limb toward the midline of the body (medial or internal rotation) or away from the midline (lateral or external rotation).

OpenStax image

Figure 9.12: Movements of the Body, Part 2 (g) Supination of the forearm turns the hand to the palm forward position in which the radius and ulna are parallel, while forearm pronation turns the hand to the palm backward position in which the radius crosses over the ulna to form an “X.” (h) Dorsiflexion of the foot at the ankle joint moves the top of the foot toward the leg, while plantar flexion lifts the heel and points the toes. (i) Eversion of the foot moves the bottom (sole) of the foot away from the midline of the body, while foot inversion faces the sole toward the midline. (j) Protraction of the mandible pushes the chin forward, and retraction pulls the chin back. (k) Depression of the mandible opens the mouth, while elevation closes it. (l) Opposition of the thumb brings the tip of the thumb into contact with the tip of the fingers of the same hand and reposition brings the thumb back next to the index finger.

9.3 Selected Joints

Some joints are more complex than others or are injured more frequently due to their structure, location, or range of motion. The following synovial joints are important as general examples of synovial joints and because they are the ones most commonly injured thus requiring medical intervention.

Temporomandibular Joint

The temporomandibular joint (TMJ) is a combined hinge and plane joint that allows for opening (mandibular depression) and closing (mandibular elevation) of the mouth, as well as side-to-side and protraction/retraction motions of the lower jaw. This joint involves the articulation between the mandibular fossa and articular tubercle of the temporal bone, with the condyle (head) of the mandible. Located between these bony structures, filling the gap between the skull and mandible, is a flexible articular disc (Figure 9.13). This disc serves to smooth the movements between the temporal bone and mandibular condyle. The temporomandibular joint is supported by an extrinsic ligament that anchors the mandible to the skull. This ligament spans the distance between the base of the skull and the lingula on the medial side of the mandibular ramus.

Dislocation of the TMJ may occur when opening the mouth too wide (such as when taking a large bite) or following a blow to the jaw, resulting in the mandibular condyle moving beyond (anterior to) the articular tubercle. In this case, the individual would not be able to close their mouth and is typically in a lot of pain. Temporomandibular joint disorder is a painful condition that may arise due to arthritis, wearing of the articular cartilage covering the bony surfaces of the joint, muscle fatigue from overuse or grinding of the teeth, damage to the articular disc within the joint, or jaw injury. Temporomandibular joint disorders can also cause headaches, difficulty chewing, or even the inability to move the jaw (lockjaw). Pharmacologic agents for inflammation or other therapies, including bite guards, are used as treatments.

OpenStax, Figure 9.13 Temporomandibular Joint: The temporomandibular joint is the articulation between the temporal bone of the skull and the condyle of the mandible, with an articular disc located between these bones. During the depression of the mandible (opening of the mouth), the mandibular condyle moves both forward and hinges downward as it travels from the mandibular fossa onto the articular tubercle.

Shoulder Joint

The shoulder joint is called the glenohumeral joint. This is a ball-and-socket joint formed by the articulation between the head of the humerus and the glenoid cavity of the scapula (Figure 9.16). This joint has the largest range of motion of any joint in the body and allows flexion, extension, hyperextension, abduction, adduction, medial rotation, lateral rotation, and circumduction of the arm. However, this freedom of movement is due to the lack of structural support and because the pectoral girdle does not form a complete (stable) ring as does the pelvic girdle. Therefore the enhanced mobility comes at the price of a loss of stability making it one of the joints most often injured in humans.

OpenStax, Figure 9.14 Glenohumeral Joint: The glenohumeral (shoulder) joint is a ball-and-socket joint that provides the widest range of motions. It has a loose articular capsule and is supported by ligaments and the rotator cuff muscles.

The large range of motion at the shoulder joint is enhanced by the articulation of the large, rounded humeral head with the small and shallow glenoid cavity, which is only about one-third of the size of the humeral head. The socket formed by the glenoid cavity is deepened slightly by a small lip of fibrocartilage called the glenoid labrum, which extends around the outer margin of the cavity. The articular capsule that surrounds the glenohumeral joint is relatively thin and loose to allow for large motions of the upper limb. Some structural support for the joint is provided by thickenings of the articular capsule wall that form weak intrinsic ligaments. These include the coracohumeral ligament, running from the coracoid process of the scapula to the anterior humerus, and three ligaments, each called a glenohumeral ligament, located on the anterior side of the articular capsule. These ligaments help to strengthen the superior and anterior capsule walls, but there is a noticeable difference between the ligamental support in the inferior aspect of the shoulder joint.

The primary dynamic support for the shoulder joint is provided by muscles crossing the joint, particularly the four rotator cuff muscles. These muscles (supraspinatus, infraspinatus, teres minor, and subscapularis (SITS muscles) arise from the scapula and attach to the greater or lesser tubercles of the humerus. The thickening of the capsule formed by the fusion of these four muscle tendons is called the rotator cuff. Two bursae, the subacromial bursa, and the subscapular bursa help to prevent friction between the rotator cuff muscle tendons and the scapula as these tendons cross the glenohumeral joint. In addition to their individual actions of moving the upper limb, the rotator cuff muscles also serve to hold the head of the humerus in position within the glenoid cavity.

Because the shoulder joint has such great mobility and limited stability, injuries to the shoulder joint are common. Most dislocations of the humerus occur in an inferior direction since the coracoid process of the scapula and the rotator cuff tendons protect it superiorly. This can occur when force is applied to the humerus when the upper limb is fully abducted, as when diving to catch a baseball and landing on your outstretched hand or elbow. Repetitive use of the upper limb, particularly in abduction such as during throwing, swimming, or racquet sports, may lead to acute or chronic inflammation of the bursa or muscle tendons, a tear of the glenoid labrum, or degeneration or tears of the rotator cuff. Inflammatory responses to any shoulder injury can lead to the formation of scar tissue between the articular capsule and surrounding structures, thus reducing shoulder mobility, a painful condition called adhesive capsulitis (frozen shoulder).

Elbow Joint

The elbow joint is a hinge joint formed by the trochlea and capitulum of the humerus, the trochlear notch of the ulna, and the head of the radius. Note, however, that it is the deep articulation between the humerus and the ulna that creates the “hinge”. The proximal radioulnar joint is also associated with the elbow. All three of these joints are enclosed within a single articular capsule (Figure 9.15). The elbow joint allows for flexion and extension (classifying it as a monaxial movement) while the proximal radioulnar joint contributes to pronation and supination.

The articular capsule of the elbow is thin on its anterior and posterior aspects and allows for a wide range of motion in regards to flexion and extension. Side-to-side movements are significantly restricted by the strong intrinsic ligaments that thicken the capsule on the lateral and medial margins of the joint. On the medial side is the triangular ulnar collateral ligament. The ulnar collateral ligament may be injured by frequent, forceful extensions of the forearm, especially when the forearm is pronated as is seen in baseball pitchers. Reconstructive surgical repair of this ligament is referred to as Tommy John surgery, named for the former major league pitcher who was the first person to have this treatment.

The lateral side of the elbow is supported by the radial collateral ligament. This arises from the lateral epicondyle of the humerus and then blends into the lateral side of the annular ligament. The annular ligament encircles the head of the radius. This ligament supports the head of the radius as it articulates with the radial notch of the ulna at the proximal radioulnar joint. This is a pivot joint that allows for rotation of the radius during supination and pronation of the forearm.

[OpenStax, Figure 9.15 Elbow Joint: (a) The elbow is a hinge joint that allows only for flexion and extension of the forearm. (b) It is supported by the ulnar and radial collateral ligaments. (c) The annular ligament supports the head of the radius at the proximal radioulnar joint, the pivot joint that allows for rotation of the radius.]

Hip Joint

The hip joint is a multiaxial ball-and-socket joint between the head of the femur and the acetabulum of the hip bone (Figure 9.16) and allows for flexion, extension, abduction, adduction, medial rotation, lateral rotation, and circumduction. The hip carries the weight of the body and thus requires strength and stability during standing and walking. For these reasons, its range of motion is much more limited than that seen at the shoulder joint. The stability of the joint and of the pelvic girdle, which is a complete ring, means that the range of motion of the hip joint is significantly less than that of the shoulder joint. This also means that it is dislocated much less frequently. In fact, in a young healthy individual, it takes an incredible amount of force, such as a car accident, to dislocate the hip.

The acetabulum is the socket portion of the hip joint. This space is deep and has a large articulation area for the femoral head, thus giving stability and weight-bearing ability to the joint. The acetabulum is further deepened by the acetabular labrum, a fibrocartilage lip attached to the outer margin of the acetabulum. The surrounding articular capsule is strong, with several thickened areas forming intrinsic ligaments. These ligaments arise from the hip bone, at the margins of the acetabulum, and attach to the femur at the base of the neck. The ligaments are the iliofemoral ligament, pubofemoral ligament, and ischiofemoral ligament, all of which spiral around the head and neck of the femur. These ligaments thus stabilize the hip joint and allow you to maintain an upright standing position with only minimal muscle contraction. Inside of the articular capsule, the ligament of the head of the femur (ligamentum teres) spans between the acetabulum and femoral head. This intracapsular ligament is normally slack and does not provide any significant joint support, but it does provide a pathway for an important artery that supplies the head of the femur.

A common injury in elderly individuals, particularly those with weakened bones due to osteoporosis, is a “broken hip,” which is a fracture of the femoral neck. This may result from a fall, or it may cause a fall. This can happen as one lower limb is taking a step and all of the bodyweight is placed on the other limb, causing the femoral neck to break and producing a fall. Any accompanying disruption of the blood supply to the femoral neck or head can lead to necrosis of these areas, resulting in bone and cartilage death. Femoral fractures usually require surgical treatment, after which the patient will need mobility assistance for a prolonged period, either from family members or in a long-term care facility. Consequently, the associated health care costs of “broken hips” are substantial. In addition, hip fractures are associated with increased rates of morbidity (incidences of disease) and mortality (death). The complications of a hip fracture including surgery followed by prolonged bed rest may lead to life-threatening complications, including pneumonia, infection of pressure ulcers (bedsores), and thrombophlebitis (deep vein thrombosis; blood clot formation) that can result in a pulmonary embolism (blood clot within the lung).

OpenStax, Figure 9.16 Hip Joint: (a) The ball-and-socket joint of the hip is a multiaxial joint that provides both stability and a wide range of motion. (b–c) When standing, the supporting ligaments are tight, pulling the head of the femur into the acetabulum.

Knee Joint

The knee joint is the largest joint of the body (Figure 9.17). It consists of three articulations. The medial and lateral tibiofemoral joints are the articulations between the rounded condyles of the femur and the relatively flat condyles of the tibia. In addition, the femoropatellar joint is found between the patella and the distal femur.

The tibiofemoral joint is enclosed within a single articular capsule. The knee functions as a hinge joint, allowing flexion and extension of the leg. This action is generated by both rolling and gliding motions of the femur on the tibia. In addition, some rotation of the leg is available when the knee is flexed, but not when extended. Thus, the knee is also sometimes referred to as a modified hinge joint. The knee is well constructed for weight-bearing in its extended position, but is vulnerable to injuries associated with hyperextension, twisting, or blows to the medial or lateral side of the joint, particularly while weight-bearing.

At the femoropatellar joint, the patella slides vertically within a groove on the distal femur. The patella is a sesamoid bone incorporated into the tendon of the quadriceps femoris muscle, the large muscle group of the anterior thigh. The patella serves to protect the quadriceps tendon from friction against the distal femur. Continuing from the patella to the anterior tibia just below the knee is the patellar ligament. Acting via the patella and patellar ligament, the quadriceps femoris is a powerful muscle group that acts to extend the leg at the knee. It also serves as a dynamic stabilizer to provide very important support and stabilization for the knee joint.

Located between the articulating surfaces of the femur and tibia are two articular discs, the medial meniscus, and lateral meniscus (see Figure 9.17b). Each is a C-shaped fibrocartilage structure that is thin along its inside margin and thick along the outer margin. They are attached to their tibial condyles but do not attach to the femur. While both menisci are free to move during knee motions, the medial meniscus shows less movement because it is anchored at its outer margin to the articular capsule and tibial collateral ligament. The menisci provide padding between the bones and help to fill the gap between the round femoral condyles and flattened tibial condyles. Some areas of each meniscus lack an arterial blood supply and thus these areas heal poorly if damaged.

The knee joint has multiple ligaments that provide support, particularly in the extended position (see Figure 9.17c). Outside of the articular capsule, located at the sides of the knee, are two extrinsic ligaments. The fibular collateral ligament (lateral collateral ligament) is on the lateral side and spans from the lateral epicondyle of the femur to the head of the fibula. The tibial collateral ligament (medial collateral ligament) of the medial knee runs from the medial epicondyle of the femur to the medial tibia. As it crosses the knee, the tibial collateral ligament is firmly attached on its deep side to the articular capsule and the medial meniscus, an important factor when considering knee injuries. In the fully extended knee position, both collateral ligaments are taut (tight), thus serving to stabilize and support the extended knee and preventing side-to-side or rotational motions between the femur and tibia.

The articular capsule of the posterior knee is thickened by intrinsic ligaments that help to resist knee hyperextension. Inside the knee are two intracapsular ligaments, the anterior cruciate ligament, and the posterior cruciate ligament. These ligaments are anchored inferiorly to the tibia at the intercondylar eminence, the roughened area between the tibial condyles. The cruciate ligaments are named for whether they are attached anteriorly or posteriorly to this tibial region. Each ligament runs diagonally upward to attach to the inner aspect of a femoral condyle. The cruciate (cross) ligaments are named for the X-shape formed as they pass each other. The posterior cruciate ligament is the stronger ligament. It serves to support the knee when it is flexed and weight-bearing, as when walking downhill. In this position, the posterior cruciate ligament prevents the femur from sliding anteriorly off the top of the tibia. The anterior cruciate ligament becomes tight when the knee is extended and provides resistance to hyperextension.

OpenStax, Figure 9.17 Knee Joint: (a) The knee joint is the largest joint of the body. (b)–(c) It is supported by the tibial and fibular collateral ligaments located on the sides of the knee outside of the articular capsule, and the anterior and posterior cruciate ligaments found inside the capsule. The medial and lateral menisci provide padding and support between the femoral condyles and tibial condyles.

Injuries to the knee are common. Since this joint is primarily supported by muscles and ligaments, injuries to any of these structures will result in pain or knee instability. Injury to the posterior cruciate ligament occurs when the knee is flexed and the tibia is driven posteriorly, such as falling and landing on the tibial tuberosity or hitting the tibia on the dashboard when not wearing a seatbelt during an automobile accident. More commonly, injuries occur when forces are applied to the extended knee, particularly when the foot is planted and unable to move. Anterior cruciate ligament injuries can result in a forceful blow to the anterior knee, producing hyperextension, or when a runner makes a quick change of direction that produces both twisting and hyperextension of the knee. These injuries are also common in hula because of bending, turning, twisting, and hard foot landing during certain movements.

A combination of injuries can occur with a hit to the lateral side of the extended knee (Figure 9.18). A moderate blow to the lateral knee will cause the medial side of the joint to open, resulting in stretching or damage to the tibial collateral ligament. Because the medial meniscus is attached to the tibial collateral ligament, a stronger blow can tear the ligament and also damage the medial meniscus. This is one reason that the medial meniscus is 20 times more likely to be injured than the lateral meniscus. A powerful blow to the lateral knee produces an “unhappy triad” injury, in which there is a sequential injury to the tibial collateral ligament, medial meniscus, and anterior cruciate ligament.

OpenStax, Figure 9.18 Knee Injury: A strong blow to the lateral side of the extended knee will cause three injuries, in sequence: tearing of the tibial collateral ligament, damage to the medial meniscus, and rupture of the anterior cruciate ligament.

Arthroscopy

Arthroscopic surgery involves a small incision and an arthroscope, a pencil-thin instrument that allows for visualization of the joint interior, is inserted into the joint.

Small surgical instruments are also inserted via additional incisions. These tools allow a surgeon to remove debris or repair a torn meniscus or reconstruct a ruptured cruciate ligament. When repairing a cruciate ligament, holes are drilled into the ligament attachment points on the tibia and femur, and the graft, with small areas of attached bone still intact at each end, inserted into these holes. The bone-to-bone sites at each end of the graft heal rapidly and strongly, thus enabling rapid recovery.

9.4 Aging and Joints

With aging, synovial fluid production decreases, the cartilage becomes thinner, and ligaments become less flexible. Genetic and lifestyle factors can affect how much degeneration a person experiences. For example, if someone were to injure their knee playing college sports, then it’s more likely that they will experience degenerative changes in that joint earlier in life than someone without a history of injury.

Arthritis is a common disorder of synovial joints that involves inflammation of the joint. This often results in significant joint pain, along with swelling, stiffness, and reduced joint mobility. There are more than 100 different forms of arthritis. Arthritis may arise from aging, damage to the articular cartilage, autoimmune diseases, bacterial or viral infections, or unknown (probably genetic) causes. In traditional Hawaiian medicine, plants with medicinal properties were used to treat common ailments. Olena (turmeric) has anti-microbial and anti-inflammatory effects, is a powerful antioxidant that has been shown to lower the risk of certain brain diseases and heart disease, and has even been shown to lower the risk of developing cancer.

Osteoarthritis (OA)

The most common type of arthritis is osteoarthritis, which is associated with aging and the “wear and tear” of the articular cartilage (Figure 9.11). Risk factors that may lead to osteoarthritis later in life include injury to a joint, jobs that involve physical labor, sports with running, twisting, or throwing actions, and being overweight. These factors put stress on the articular cartilage that covers the surfaces of bones at synovial joints, causing the cartilage to gradually become thinner. As the articular cartilage layer wears down, more pressure is placed on the bones. The joint responds by increasing the production of the lubricating synovial fluid, but this can lead to swelling of the joint cavity, causing pain and joint stiffness as the articular capsule is stretched. The bone tissue underlying the damaged articular cartilage also responds by thickening, producing irregularities and causing the articulating surface of the bone to become rough or bumpy. The joint movement then results in pain and inflammation. In its early stages, symptoms of osteoarthritis may be reduced by mild activity, but the symptoms may worsen following exercise. In individuals with more advanced osteoarthritis, the affected joints can become more painful and therefore are difficult to use effectively, resulting in increased immobility. There is no cure for osteoarthritis, but several treatments can help alleviate the pain. Treatments may include lifestyle changes, such as weight loss and low-impact exercise, and over-the-counter or prescription medications that help to alleviate the pain and inflammation. For severe cases, joint replacement surgery (arthroplasty) may be required. In recent years stem cell therapy has had promising results. This technique involves harvesting stem cells from a person and injecting them into injured joints.

OpenStax, Figure 9.19 Osteoarthritis: of a synovial joint results from aging or prolonged joint wear and tear. These cause erosion and loss of the articular cartilage covering the surfaces of the bones, resulting in inflammation that causes joint stiffness and pain.

Arthroplasty

Joints that have been damaged by disease or injury can be replaced with artificial components through arthroplasty (arth=joint, plasty=plastic repair). Joint replacement is a very invasive procedure, so other treatments are always tried before surgery. However, arthroplasty can provide relief from chronic pain and can enhance mobility within a few months following the surgery. This type of surgery involves replacing the articular surfaces of the bones with artificial components. For example, in hip arthroplasty, the worn or damaged parts of the hip joint, including the head and neck of the femur and the acetabulum of the pelvis, are removed and replaced with artificial joint components. The replacement head for the femur consists of a rounded ball attached to the end of a shaft that is inserted inside the diaphysis of the femur. The acetabulum of the pelvis is reshaped and a replacement socket is fitted into its place. The parts, which are always built in advance of the surgery, are sometimes custom-made to produce the best possible fit for a patient.

Rheumatoid Arthritis

Other forms of arthritis are associated with various autoimmune diseases, bacterial infections of the joint, or unknown genetic causes. Rheumatoid arthritis is an autoimmune disease that produces arthritis because the immune system of the body attacks the body joints. In rheumatoid arthritis, the joint capsule and synovial membrane become inflamed. As the disease progresses, the articular cartilage is severely damaged or destroyed, resulting in joint deformation, loss of movement, and in late stages, bone fusion. The most commonly involved joints are the hands, feet, and cervical spine, with corresponding joints on both sides of the body usually affected, though not always to the same extent. Rheumatoid arthritis can sometimes also be associated with other conditions such as lung fibrosis, vasculitis (inflammation of blood vessels), and coronary artery disease. With no known cure, treatments are aimed at alleviating symptoms and maintaining range of motion. Exercise, anti-inflammatory and pain medications, various specific disease-modifying anti-rheumatic drugs, immunosuppressants, or surgery are used to treat rheumatoid arthritis.

Chapter Summary

Sources

• OpenStax Anatomy and Physiology (http://openstax.org/details/books/anatomy-and-physiology)
• Principles of Anatomy and Physiology, 15th edition, Gerard J. Tortora; Brayn H. Derrickson
• Human Anatomy and Physiology, 11th edition, Elaine N Marieb; Katja Hoehn

Key Terms

amphiarthrosis

slightly mobile joint

anterior cruciate ligament

intracapsular ligament of the knee; extends from anterior, superior surface of the tibia to the inner aspect of the lateral condyle of the femur; resists hyperextension of knee

articular capsule

connective tissue structure that encloses the joint cavity of a synovial joint

articular cartilage

thin layer of hyaline cartilage that covers the articulating surfaces of bones at a synovial joint

articulation

joint of the body

biaxial joint

type of diarthrosis; a joint that allows for movements within two planes (two axes)

bursa

connective tissue sac containing lubricating fluid that prevents friction between adjacent structures, such as skin and bone, tendons and bone, or between muscles

cartilaginous joint

joint at which the bones are united by hyaline cartilage (synchondrosis) or fibrocartilage (symphysis)

diarthrosis

freely mobile joint

elbow joint

humeroulnar joint

extrinsic ligament

ligament located outside of the articular capsule of a synovial joint

fibrous joint

joint where the articulating areas of the adjacent bones are connected by fibrous connective tissue

fibular collateral ligament

extrinsic ligament of the knee joint that spans from the lateral epicondyle of the femur to the head of the fibula; resists hyperextension and rotation of the extended knee

fontanelles

expanded areas of fibrous connective tissue that separate the braincase bones of the skull prior to birth and during the first year after birth

glenohumeral joint

shoulder joint; articulation between the glenoid cavity of the scapula and head of the humerus; multiaxial ball-and-socket joint that allows for flexion/extension, abduction/adduction, circumduction, and medial/lateral rotation of the humerus

glenoid labrum

lip of fibrocartilage located around the outside margin of the glenoid cavity of the scapula

gomphosis

type of fibrous joint in which the root of a tooth is anchored into its bony jaw socket by strong periodontal ligaments

hip joint

multiaxial ball-and-socket joint between the head of the femur and the acetabulum of the hip bone

interosseous membrane

wide sheet of fibrous connective tissue that fills the gap between two parallel bones, forming a syndesmosis; found between the radius and ulna of the forearm and between the tibia and fibula of the leg

intracapsular ligament

ligament that is located within the articular capsule of a synovial joint

intrinsic ligament

ligament that is fused to or incorporated into the wall of the articular capsule of a synovial joint

joint

site at which two or more bones or bone and cartilage come together (articulate)

joint cavity

space enclosed by the articular capsule of a synovial joint that is filled with synovial fluid and contains the articulating surfaces of the adjacent bones

knee joint

consists of three articulations. The medial and lateral tibiofemoral joints are the articulations between the rounded condyles of the femur and the relatively flat condyles of the tibia. In addition, the femoropatellar joint is found between the patella and the distal femur. 

lateral meniscus

C-shaped fibrocartilage articular disc located at the knee, between the lateral condyle of the femur and the lateral condyle of the tibia

ligament

strong band of dense connective tissue spanning between bones

medial meniscus

C-shaped fibrocartilage articular disc located at the knee, between the medial condyle of the femur and medial condyle of the tibia

meniscus

articular disc

multiaxial joint

type of diarthrosis; a joint that allows for movements within three planes (three axes)

posterior cruciate ligament

intracapsular ligament of the knee; extends from the posterior, superior surface of the tibia to the inner aspect of the medial condyle of the femur; prevents anterior displacement of the femur when the knee is flexed and weight bearing

rotator cuff

strong connective tissue structure formed by the fusion of four rotator cuff muscle tendons to the articular capsule of the shoulder joint; surrounds and supports superior, anterior, lateral, and posterior sides of the humeral head

suture

fibrous joint that connects the bones of the skull (except the mandible); an immobile joint (synarthrosis)

symphysis

type of cartilaginous joint where the bones are joined by fibrocartilage

synarthrosis

immobile or nearly immobile joint

synchondrosis

type of cartilaginous joint where the bones are joined by hyaline cartilage

syndesmosis

type of fibrous joint in which two separated, parallel bones are connected by an interosseous membrane

synostosis

site at which adjacent bones or bony components have fused together

synovial fluid

thick, lubricating fluid that fills the interior of a synovial joint

synovial joint

joint at which the articulating surfaces of the bones are located within a joint cavity formed by an articular capsule

synovial membrane

thin layer that lines the inner surface of the joint cavity at a synovial joint; produces the synovial fluid

temporomandibular joint (TMJ)

articulation between the condyle of the mandible and the mandibular fossa and articular tubercle of the temporal bone of the skull; allows for depression/elevation (opening/closing of mouth), protraction/retraction, and side-to-side motions of the mandible

tendon

dense connective tissue structure that anchors a muscle to bone

tibial collateral ligament

extrinsic ligament of knee joint that spans from the medial epicondyle of the femur to the medial tibia; resists hyperextension and rotation of extended knee

uniaxial joint

type of diarthrosis; joint that allows for motion within only one plane (one axis)