Section Two
Joints

Ⅰ．Classification and Structure of Joints

Joints （ articulations ，骨连接） are the regions which a bone joints another bone．According to how much they move，joints are classified into two types：direct and indirect joints．

In the direct joints （直接连接） ，bones are held together by fibrous connective tissues or cartilages（Figure 2.20）．They have no a joint cavity，and are immovable or slightly movable．A suture is that form of joint where bones in the skull are connected by a thin layer of fibrous tissue．A cartilaginous joint is also that form ofjoint，for an example，two adjacent verterbral bodies are jointed by a piece of cartilage（intervertebral disc）．

Indirect joints （间接连接） are also termed synovial joints （滑膜关节） or diarthroses （可动关节） ，which are connected by ligaments and have a joint cavity（Figure 2.21）．They granted free movement．This class includes the greater number of the joints in the body．A synovial joint has three basic structures：the articular surface，capsule and cavity．The articular surface （关节面） is the portion of the bone that comes into contact with one another within a joint．It is covered with articular cartilage．The cartilage is a strong and smooth hyaline cartilage．It prevents the bones touching one another and granting them space to move freely and smoothly．The articular capsule （关节囊） is composed of connective tissue，which surrounds the articular cavity．It has two layers，the outer layer is a layer of dense connective tissue attached to the periphery of articular surfaces，the inner layer is also termed synovial membrane．Synovial fluid is secreted by the synovial membrane．It can lubricate the articular cartilage and reduce friction between them．The articular cavity （关 节腔） is a closed cavity filled with synovial fluid．

The accessory structures of synovial joints include ligaments，articular discs，articular labra，synovial folds and bursae． Ligaments （韧带） ，which consist of bundles of dense connective tissue，connect bone to bone and stabilize the joints．There are intracapsular and extracapsular ligaments．The articular disc （关节 盘） is a fibrocartilage and located between the articular surfaces of joints．It facilitates the articular surfaces fitting to each other．The articular labrum （关节唇） is a fibrocartilaginous ring and deepens the articular fossa．The synovial folds （滑膜襞） are the extension of synovial membrane and increase the area of it．The synovial bursae （滑膜囊） are saclike structures formed by synovial membrane．They，found between the skin and a bone or a tendon and a bone，reduce friction and allow easy movement of one part on another．

Ⅱ．Movements of Synovial Joints

Flexion and extension： Flexion （屈） means the decrease of the angle between two bones． Extension （伸） means the increase of the angle between two bones．

Adduction and abduction： Adduction （收） means the movement toward the median plane of the body in the coronal plane． Abduction （展） means the movement away from the median plane of the body in the coronal plane．

Rotation： Rotation （旋转） means moving a part of the body around its long axis．Medial rotation turns the anterior surface medially，and lateral rotation turns this surface laterally． Pronation （旋前） is a medial rotation of the forearm and hands so that the palm faces posteriorly． Supination （旋后） is a lateral rotation of the forearm and hands so that the palm faces anteriorly．

Inversion and eversion： Inversion （内翻） is rotating the sole of the foot inwards，and eversion（外翻） is rotating it outwards．

Circumduction： Circumduction （环转） is the circular movement of the body，combining the movements of flexion，extension，abduction and adduction in sequence．

Ⅲ．Joints

They are divided into joints of the trunk，the upper limb，the lower limb and the skull based on their locations．

The joints of the trunk They primarily include the vertebral column and bony thorax．24 vertebrae，one sacrum and one coccyx are jointed one by one to form the vertebral column （脊柱） （Figure 2．22）．The vertebral arches are mainly connected by ligaments．The vertebral bodies are connected by the intervertebral discs and ligaments．The intervertebral discs （椎间盘） are interposed between the bodies of adjacent vertebrae except that between the first and second cervical vertebrae．They are thickest in the lumbar region and thinnest in the superior thoracic region．Each intervertebral disc consists of an inner nucleus pulposus （髓 核） and a peripheral annulus fibrosus （纤维盘 ）（Figure 2.23）．It provides strong attachments between the vertebral bodies and acts like a shock absorber for axial forces．Sometimes，the annulus fibrosus ruptures，and the nucleus pulposus projects backwards and outwards into the vertebral canal，where it may press on the spinal nerve，or then the spinal cord．This condition refers to intervertebral disc herniation．The vertebral column is about 70 cm in length in the adult．From anterior view，bodies of the vertebrae become larger and thicker from the cervical to the second sacral vertebra because the lower vertebrae support more weight．From lateral view，the vertebral column presents natural curvatures．Cervical and lumbar curvatures are convex；thoracic and sacral curvatures are concave．Infants lack convex curvatures．The convex curvatures develop enabling infants to hold their heads erect，sit and stand．These curvatures increase the capacities of vertebral column to weight-bearing and shock-absorbing．The vertebral column can perform flexion，extension，lateral bending and rotation．The cervical and lumbar regions allow further extension of movements when compare to that of the thoracic and sacral regions．

Bony thorax （骨性胸廓） is formed by 12 thoracic vertebrae，12 pairs of ribs and one sternum with the joints between them （Figure 2.24）．It is approximately conical in shape and has two apertures．The superior aperture of thorax （胸廓上口） is narrow and inclined anteroinferiorly，and the inferior aperture （胸廓下 口） is wide and irregular．The transverse diameter of thorax is longer than the anteroposterior diameter．The most superior seven pairs of ribs joint directly with the sternum by their own costal cartilages．The most inferior five pairs fail to connect directly with the sternum．Of these，the 8th to 10th ribs are joined to the cartilage of the rib just superior to them and form the costal arch （肋弓） ，the 11th and 12th ribs are termed floating ribs because their cartilages do not connect with either cartilage or the sternum．The intervals between the adjacent ribs are termed the intercostal spaces （肋间隙） ，which are occupied by intercostal muscles，vessels and nerves．The bony thorax is not only to surround and protect the heart，lungs，liver and spleen but also to provide the function of breathing．The elevation of ribs can increase the volume of thorax，which leads to a decrease of the intrathoracic pressure，then pressure changes result in air being drawn into the lungs and produce inspiration．During expiration，the contrary movement decreases the volume of thorax．

The joints of upper limb Most joints of the upper limb are movable．The shoulder joint （肩关节） is formed by the humeral head and the glenoid cavity of the scapula（Figure 2.25）．Because the humeral head is big，the glenoid cavity is shallow，and the articular capsule is thin and loose，movements at the shoulder joint are less limited and most movable．The movements include flexion-extension， abduction-adduction，circumduction，medial and lateral rotation．The humerus joints with the upper ends of the radius and ulna to form the elbow joint （肘关节） ．The movement is limit to flexion and extension（Figure 2.26）．The articulation between the radius and the ulna can perform the supination and pronation of hand（Figure 2.27）．The radius and an articular disc articulate with adjacent carpals to form the wrist （ radiocarpal ，桡腕关节） joint （腕关节） ．The joints of hand provide great dexterity of finger movements（Figure 2.28）．

The joints of lower limb Most joints of the lower limb are stable．The sacroiliac joint is formed between the sacrum and the ilium，strengthened by a number of anterior and posterior ligaments．The pubic symphysis （耻骨联合） is a cartilaginous joint which connects the left and right pubic bones（Figure 2.29）．Two hip bones，the sacrum and coccyx are jointed to form the bony pelvis （骨盆） ．The pelvis has no movement，it thus helps to transfer the weight from the vertebral column to the lower limb and support the body．The male pelvis is different from the female．The male pelvis is heavier and thicker than the female whereas the female pelvis is wider，shallower，has a larger pelvic inlet and outlet（Figure 2.29）．The hip joint （髋关节） is formed by the joints of the head of the femur and the acetabulum of the hip bone．The acetabulum is deep，and encompasses nearly all of the large femoral head．The joint capsule is thick and strengthened by strong ligaments，thus，the joint is stable（Figure 2.30）．The movement of the hip joint includes flexion-extension，abduction-adduction，circumduction，and rotation．However，it is rather than a large range of movement．The knee joint （膝关节） is a complex and unstable joint．It is formed by the joints of the lower end of the femur，upper end of the tibia and the patella（Figure 2.31）．Interposed between the femur and tibia are two pieces offibrous cartilage，the medial meniscus （内侧半月板） and the lateral meniscus （外侧半月板） ．Anterior and posterior cruciate ligaments connect the tibia to the femur，and help to resist twisting and turning movements and prevent displacement of the tibia．Movements of the knee joint are limited to flexion and extension．When the knee is flexed，a slight amount of rotation and circumduction is possible．Quick turns while playing football or basketball may result in knee injuries．These injuries include torn ligaments，dislocated joint，and damage to the menisci．The joints between the tibia and fibula are nearly immovable（Figure 2.32）．Joints of the foot are similar to，but usually less flexible than those in the hand（Figure 2.33）．The ankle joint （踝关节） is formed by the lower ends of the tibia and fibula and the superior part of the talus．This joint is relatively unstable during plantar flexion because the superior part of the rear side of talus is narrower and therefore lies loosely within the mortise．The tarsal and metatarsal bones are arranged in longitudinal and transverse arches，termed the arches of foot （足弓） （Figure 2.34）．They add to the weight-bearing capability and resiliency of the foot．The ligaments and aponeurosis of the sole of the foot are utmost important in maintaining the arches，because they receive the greatest stress of all．

The joints of the skull are sutures，except the temporomandibular joints．In the infant，skull sutures are not completely immovable．At certain places where skull bones have not completely filled in the membrane surrounding them，these are fontanels （囟） （Figure 2.35）．Fontanels are shortly closed after birth．They allow slight compression and the cranium to grow．