Joints are divided according to the type of tissue that binds the bones: fibrous joint, cartilaginous joint, synovial joint.
< Fibrous joint >
Suture is a representative fibrous joint where fibrous tissue connects the skull bones. Since the fibrous tissue is very tough and narrow, the suture is immobile (Fig. 1-7).
Fig. 2-3. Fontanelle becoming suture.
However, when we were infants, the suture used to be the fontanelle. Since the fibrous tissue in the fontanelle is wide, the fontanelle is mobile. Thanks to the mobility of the fontanelle, a newborn’s head that is relatively big can pass through the narrow birth canal of the mother (Fig. 8-42).
Fig. 2-4. Tooth in dental alveolus.
As another fibrous joint, the teeth are firmly situated in the dental alveoli of mandible and maxilla (Fig. 4-2) by the periodontal ligament, a fibrous tissue.
Fig. 2-5. Interosseous membrane between ulna, radius.
The other kind of the fibrous joint is syndesmosis. It contains broad fibrous tissue between bones and is slightly mobile unlike suture. A good example is the interosseous membrane between the ulna and radius (Fig. 3-53) that allows pronation and supination (Fig. 2-23). However, the interosseous membrane between the tibia and fibula (Fig. 3-89) does not allow such movement.
< Cartilaginous joint >
The cartilaginous joint is more mobile than the fibrous joint.
Fig. 2-7. Two types of cartilaginous joint.
The cartilaginous joint is categorized into synchondrosis (with one hyaline cartilage) and symphysis (with two hyaline cartilages and one fibrous cartilage).
Fig. 2-8. First costal cartilage.
An example of synchondrosis is the 1st costal cartilage between R1 and sternum (Fig. 1-18). The 1st costal cartilage slightly moves during inhalation and exhalation (Fig. 3-24). However, the joints between R2–R7 (exactly, the 2nd–7th costal cartilages) and sternum are synovial joints (plane joint) having the articular cavity (Fig. 2-41).
Fig. 2-9. Symphysis between vertebral bodies.
The representative of symphysis is the intervertebral disc (fibrous cartilage) (Fig. 1-14) intervening between the two thin hyaline cartilages. One can flex, extend, and rotate one’s neck/trunk because each intervertebral disc is slightly movable.
The thickness of intervertebral disc is slightly variable like the 24 hours circadian rhythm.
An intervertebral “disc” (There is no “disk” in anatomy.) is composed of “anulus” fibrosus (There is no “annulus” in anatomy.) and nucleus pulposus. The disc resembles a ring-shaped doughnut that contains fruit jam in its center.
Just as jam leaks when one bites into the doughnut, the nucleus pulposus can herniate through a torn anulus fibrosus.
Herniation usually occurs in the lumbar vertebrae that are suffering from the pressure while keeping mobility (Fig. 1-15).
The pubic symphysis between bilateral pubes is another kind of symphysis (Fig. 1-34) (Fig. 1-37) (Fig. 1-38).
< Movement terms >
Prior to the synovial joint, movement and other related terms are introduced.
Fig. 2-14. Anatomical position.
In the anatomical position, the great toes meet together, and the upper limbs are laterally rotated (Fig. 2-23), so that the thumbs are pointed away from the body. The anatomical position is the basis for description of the locational relationship (superior, inferior, anterior, and posterior) of body. For example, the palm faces anteriorly, unlike daily life posture.
Fig. 2-15. Plane terms.
Sagittal plane is the sectioning plane which divides the body into right and left parts. Midsagittal plane (or median plane) is the sagittal plane dividing in halves. Horizontal and coronal planes can be easily understood with the figure above.
Fig. 2-16. Difference of transverse and horizontal planes.
Transverse (or cross) plane of the leg is a horizontal plane, while transverse plane of the foot is a coronal plane. Thus, we can remind ourselves that transverse plane is not always horizontal plane.
Fig. 2-17. Lateral, medial, proximal, distal.
The terms, lateral and medial are conveniently used in anatomy. The terms, proximal and distal mean parts that are close to and distant from the trunk, respectively. For instance, the arm is proximal to the hand (Fig. 2-18).
Fig. 2-18. Regions, joints of upper and lower limbs.
Unlike ordinary usage, the word “arm” in anatomy is restricted to the area between the shoulder and elbow joints. The region between the elbow and wrist joints is called the forearm. Likewise, “leg” in anatomy means only the region between the knee and ankle joints.
In case of the elbow joint, flexion is to move the forearm forward.
In case of the shoulder joint, flexion refers to the movement of the arm forward from the anatomical position; hyperextension is opposite.
Anterior and posterior movements of the vertebral column are the flexion and hyperextension, respectively. Excessive flexion or hyperextension are restricted by the longitudinal ligaments (Fig. 2-9).
Fig. 2-22. Flexion of the upper limb.
The wrist joint range for moving forward and that for moving backward are similar. However, flexion of the wrist joint is defined as forward movement, because the flexion direction matches that of the elbow joint, metacarpophalangeal joints, and interphalangeal joints (Fig. 1). Simply put, in all upper limb joints, flexion is forward movement.
Fig. 2-23. Movement terms.
Abduction refers to a lateral motion that pulls a part away from the midsagittal plane (Fig. 2-15). Reversely, adduction refers to a medial motion that pulls a part toward the midsagittal plane.
Fig. 2-24. Midsagittal planes of hand and foot.
In terms of the fingers and toes, the midsagittal planes pass the middle finger and the 2nd toe, respectively. Abduction is to move outward from the middle finger (Fig. 2-25) and the 2nd toe.
There is a universal rule for movements of fingers including the thumb.
Movement to make the thumb and little finger encounter each other is called opposition. This unique movement, which is important in grasping stuff (Fig. 1-28), is possible thanks to the 1st carpometacarpal joint (saddle joint) (Fig. 2-50) and the developed hand muscles of human.
Lateral rotation, medial rotation, supination, and pronation are illustrated in Fig. 2-23. The former two movements of the shoulder joint (Fig. 3-43) are independent from the latter two movements of the ulna and radius (Fig. 2-46).
Fig. 2-27. Extension of lower limb.
When sitting down, the hip and knee joints are flexed. To stand up, they are extended.
Fig. 2-28. Posture of fetus.
If one flexes all the joints of upper limbs (Fig. 2-22), lower limbs (Fig. 2-27) and vertebrae (Fig. 2-21), he/she will look like a fetus (Fig. 8-42).
Dorsiflexion decreases the angle between the leg and dorsum of foot. Plantar flexion increases the angle.
Eversion means movement of the sole of foot away from the midsagittal plane. Inversion is the opposite (Fig. 1-48) (Fig. 2-23).
Fig. 2-31. Protraction of mandible.
Protraction and retraction are the anterior and posterior shifts, respectively.
< Synovial joint >
In the synovial joint, articular capsule consists of an outer fibrous membrane and an inner synovial membrane. The “articular” surface of bone is covered by the “articular” cartilage, whereas the rest surface of bone is covered by the periosteum (Fig. 1-4).
The “synovial” membrane secretes “synovial” fluid that reduces friction and absorbs shocks (Fig. 1-51).
Harsh movement of a synovial joint which suddenly expands the articular cavity causes a cracking sound.
Some articular cavities are divided by the articular disc, which relieves impacts on the joint. Articular disc contains supplementary synovial membrane (Fig. 2-32), which facilitates joint movement (Fig. 2-33).
Unlike the articular disc, the articular meniscus imperfectly divides the articular cavity.
Fig. 2-37. Knee joint.
The menisci also serve to stabilize the knee joint between the femur and tibia (Fig. 1-43).
A thickened portion of the fibrous membrane is the ligament connecting two bones.
The ligament, a dense regular connective tissue (Fig. 16-26), limits mobility of the joint, and prevents inappropriate movement (Fig. 2-21).
Synovial joints are classified into six types, depending on the characteristics of ligament and articular surface. Some are relatively immobile; others have multiple degrees of freedom.
The plane joint allows only a little sliding movement. An example of the plane joint is the joint between the scapula (acromion) (Fig. 1-21) and clavicle (Fig. 3-40).
The hinge joint has one axis for movement. The best example is the interphalangeal joints having collateral ligaments.
Fig. 2-43. Temporomandibular joint.
In the temporomandibular joint, the articular cavity inferior to articular disc (Fig. 2-35) is a hinge joint used for dicing up food (elevation and depression of the mandible) (Fig. 1-12). On the other hand, superior articular cavity is a plane joint (Fig. 2-41) used for grinding food (protraction (Fig. 2-31), retraction, and lateral movement of the mandible).
The pivot joint also has one axis. Unlike the hinge joint, it permits rotating in situ of bone.
The atlantoaxial joint between CV1 (atlas) and CV2 (axis) is a pivot joint. There is no intervertebral disc (Fig. 2-9) between CV1 and CV2.
Fig. 2-46. Pronation.
The proximal ulna and radius form another pivot joint to induce pronation and supination (Fig. 2-23).
Fig. 2-47. Ellipsoid joint (wrist joint).
The ellipsoid joint has a concave ellipsoid articular surface on one side and a convex ellipsoid articular surface on the other side.
Fig. 2-48. Circumduction of shoulder joint.
Circumduction is the combined movement of flexion, extension, abduction and adduction. Try to circumduct the wrist joint excessively. And it will not move as smoothly as the shoulder joint (ball and socket joint) (Fig. 2-51).
The ellipsoid joint such as the wrist joint moves in two directions: flexion/extension and abduction/adduction (Fig. 3-58). Movements of the saddle joint are as same as those of the ellipsoid joint (Fig. 2-47). A difference is shape of the articular surfaces. The only remarkable saddle joint in our body is the 1st carpometacarpal joint, which facilitates flexion/extension and abduction/adduction of the thumb (Fig. 2-25).
The ball and socket joint, the most mobile type of the synovial joint, can move along three axes.
Fig. 2-52. Hip joint.
Typical ball and socket joint is the hip and shoulder joints (Fig. 2-18). Regarding the hip joint, the acetabular labrum make the acetabulum of hip bone deeper (Fig. 1-31), in order to stabilize the ball and socket joint. Regarding the shoulder joint, the glenoid labrum deepens the glenoid cavity of scapula (Fig. 1-21).