Human skeleton
Front view of a skeleton of an adult human
Back view of a skeleton of an adult human
The human skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs and heart.
The longest and heaviest bone in the body is the femur and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 13% of the total body weight [1], and half of this weight is water.
Fused bones include those of the pelvis and the cranium. Not all bones are interconnected directly: There are six bones in the middle ear called the ossicles (three on each side) that articulate only with each other. The hyoid bone, which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments.
Development
Early in gestation, a fetus has a cartilaginous skeleton from which the long bones and most other bones gradually form throughout the remaining gestation period and for years after birth in a process called endochondral ossification. The flat bones of the skull and the clavicles are formed from connective tissue in a process known as intramembranous ossification, and ossification of the mandible occurs in the fibrous membrane covering the outer surfaces of Meckel's cartilages. At birth a newborn baby has approximately 270 bones, whereas on average an adult human has 206 bones (these numbers can vary slightly from individual to individual). The difference comes from a number of small bones that fuse together during growth, such as the sacrum and coccyx of the vertebral column. The sacrum (the bone at the base of the spine) consists of five bones which are separate at birth but fuse together into a solid structure in later years. An infant is born with zones of cartilage, called epiphyseal plates, between segments of bone to allow further growth. Growing is usually completed between ages 13 and 19, at which time the epiphyseal plates of long bones close allowing no further growth.
Organization
- See also: List of bones of the human skeleton
Much of the human skeleton maintains the ancient segmental pattern present in all vertebrates (mammals, birds, fish, reptiles and amphibians) with basic units being repeated. This segmental pattern is particularly evident in the vertebral column and in the ribcage.
There are 206 bones in the adult human skeleton, a number which varies between individuals and with age - newborn babies have 270 bones[2] some of which fuse together. These bones are organized into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached.[3]
Axial skeleton
The axial skeleton (80 bones) is formed by the vertebral column (26), the thoracic cage (12 pairs of ribs and the sternum), and the skull (22 bones and 7 associated bones). The axial skeleton transmits the weight from the head, the trunk, and the upper extremities down to the lower extremities at the hip joints, and is therefore responsible for the upright position of the human body. Most of the body weight is located in front of the spinal column which therefore have the erector spinae muscles and a large amount of ligaments attached to it resulting in the curved shape of the spine. The 240 skeletal muscles acting on the axial skeleton position the spine, allowing for small movements in the thoracic cage for breathing, and the head, where they control the minute and complex facial movements.[3]
Appendicular skeleton
The appendicular skeleton (126 bones) is subdivided into the upper and lower extremities: The axial skeleton is connected to the upper extremity (60) through the pectoral girdle (4) and to the lower extremity (60) through the pelvic girdle (2). Some 300 muscles attach to the appendicular skeleton.[3]
The only joint between the pectoral girdle and the thorax is between the clavicle and the sternum (i.e. the sternoclavicular joint), the scapula instead being controlled by muscles. The humerus articulates to the scapula at the shoulder joint and to the two parallel bones of forearm, the radius and ulna, in the elbow joints (humeroulnar, humeroradial, and radioulnar). The distal ends of the forearm bones form the wrist joints with the hand. In the hand, eight carpal bones arranged in two rows articulate with the metacarpal bones of the palm which articulate with the 14 finger bones (the phalanges).[3]
The pelvic girdle is a composite structure which includes bones from both the axial skeleton, the sacrum and the coccyx, and the lower extremities, the two hip bones. Because the lower limbs have to bear the weight of the human body — reaction forces at the feet can be 5-10 times the body weight during sprinting and jumping — the bones of the pelvic girdle, the thigh, and the lower leg are massive and have a network of fibers and many strong muscles to maximize strength and stability. Similarly, and in contrast to the elbow, in the knee the kneecap, a sesamoid bone formed in the tendon of the large quadriceps muscles of the thigh, protects the knee from damage while helping the muscles in extending the knee. In each ankle there are 7 tarsal bones, including the heel bone, and in each foot 5 metatarsal bones and 14 phalanges.[3]
Function
The skeleton has six main functions:
Support
The skeleton provides the framework which supports the body, and maintains its shape. The joints between bones permit movement, some allowing a wider range of movement than others, e.g. the ball and socket joint allows a greater range of movement than the pivot joint at the neck.
Movement
Movement in vertebrates is powered by skeletal muscles, which are attached to the skeleton by tendons. Without the skeleton to give leverage, movement would be greatly restricted. However, biologically speaking, the skeleton does not enable movement.
Protection
The skeleton protects many vital organs:
Blood cell production
The skeleton is the site of haematopoiesis, which takes place in red bone marrow.
Storage
Bone matrix can store calcium and is involved in calcium metabolism, and bone marrow can store iron in ferritin and is involved in iron metabolism.
Endocrine regulation
Bone cells release a hormone called osteocalcin, which controls the regulation of blood sugar (glucose) and fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat.[4]
Sex-based differences
An articulated human skeleton, as used in biology education
There are many differences between the male and female human skeletons. Most prominent is the difference in the pelvis, owing to characteristics required for the processes of childbirth. The shape of a female pelvis is flatter, more rounded and proportionally larger to allow the head of a fetus to pass. Men tend to have slightly thicker and longer limbs and digit bones (phalanges), while women tend to have narrower rib cages, smaller teeth, less angular mandibles, less pronounced cranial features such as the brow ridges and external occipital protuberance (the small bump at the back of the skull), and the carrying angle of the forearm is more pronounced in females. Females also tend to have more rounded shoulder blades.
Disorders
- See also: List of skeletal disorders
There are many disorders of the skeleton. One of the more prominent is osteoporosis.
Osteoporosis
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Main article: Osteoporosis
Osteoporosis is a disease of bone, which leads to an increased risk of fracture. In osteoporosis, the bone mineral density (BMD) is reduced, bone microarchitecture is disrupted, and the amount and variety of non-collagenous proteins in bone is altered. Osteoporosis is defined by the World Health Organization (WHO) in women as a bone mineral density 2.5 standard deviations below peak bone mass (20-year-old sex-matched healthy person average) as measured by DXA; the term "established osteoporosis" includes the presence of a fragility fracture.[5] Osteoporosis is most common in women after the menopause, when it is called postmenopausal osteoporosis, but may develop in men and premenopausal women in the presence of particular hormonal disorders and other chronic diseases or as a result of smoking and medications, specifically glucocorticoids, when the disease is craned steroid- or glucocorticoid-induced osteoporosis (SIOP or GIOP).
Osteoporosis can be prevented with lifestyle advice and medication, and preventing falls in people with known or suspected osteoporosis is an established way to prevent fractures. Osteoporosis can also be prevented with having a good source of calcium and vitamin D. Osteoporosis can be treated with bisphosphonates and various other medical treatments.
Gallery
References
See also
Bones (Axial skeleton, Appendicular skeleton) |
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VERTEBRAL COLUMN |
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THORAX |
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cranial bones of SKULL |
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facial bones of SKULL |
nasal - maxilla - lacrimal - zygomatic - palatine - inferior nasal conchae - vomer - mandible - THROAT: hyoid (greater cornu, lesser cornu, body)
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UPPER EXTREMITY |
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LOWER EXTREMITY |
PELVIS: pelvis (ilium, ischium, pubis) - LEG: femur - patella - fibula - tibia - FOOT: tarsals (calcaneus - talus - navicular - cuneiform - cuboid ) - metatarsals - phalanges (prox - int - dist)
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MIDDLE EAR OSSICLES |
malleus - incus - stapes
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Bones of head and neck: the facial bones of the skull |
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Maxilla |
Surfaces
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Anterior: fossae (Incisive fossa, Canine fossa) - Infraorbital foramen - Anterior nasal spine
Infratemporal: Alveolar canals - Maxillary tuberosity
Orbital: Infraorbital groove - Infraorbital canal
Nasal: Pterygopalatine canal
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Processes
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Zygomatic process
Frontal process (Agger nasi, Anterior lacrimal crest)
Alveolar process
Palatine process (Incisive foramen, Incisive canals, Foramina of Scarpa, Incisive bone, Anterior nasal spine)
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Other
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Body of maxilla - Maxillary sinus
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Zygomatic |
Orbital process (Zygomatico-orbital) - Temporal process (Zygomaticotemporal) - Lateral process (Zygomaticofacial)
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Palatine |
Fossae
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Pterygopalatine fossa - Pterygoid fossa
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Plates
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Horizontal plate (Posterior nasal spine) - Perpendicular plate (Pterygopalatine canal, Sphenopalatine foramen, Pyramidal process)
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Processes
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Orbital - Sphenoidal
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Mandible |
Body
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external surface (Symphysis menti, Mental protuberance, Mental foramen) - internal surface (Mental spine, Mylohyoid line, Sublingual fovea, Submandibular fovea) - Alveolar part of mandible
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Ramus
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Mylohyoid groove (Mandibular canal, Lingula) - Mandibular foramen - Angle
Coronoid process - Mandibular notch - Condyloid process - Pterygoid fovea
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Minor/
nose |
Nasal bone: Internasal suture - Nasal foramina
Inferior nasal concha: Ethmoidal process - Maxillary process
Vomer: Wing
Lacrimal: Posterior lacrimal crest - Lacrimal groove - Lacrimal hamulus
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Bones of head and neck: the cranium of the skull |
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Occipital |
Squama
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external (Inion/External occipital protuberance, Nuchal lines) - planes (Occipital, Nuchal) - internal (Cruciform eminence, Internal occipital protuberance, Sagittal sulcus, Internal occipital crest)
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Lateral parts
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Condyle (Condyloid fossa, Condylar canal) - Hypoglossal canal - jugular (Jugular process, Jugular tubercle)
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Basilar part
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Pharyngeal tubercle
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Other
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Foramen magnum
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Parietal |
Parietal eminence - Temporal line - Parietal foramen
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Frontal |
Squama
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Frontal suture - Frontal eminence - external (Superciliary arches, Glabella) - foramina (Supraorbital, Cecum) - Zygomatic process - internal (Sagittal sulcus, Frontal crest)
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Orbital part
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Ethmoidal notch - Lacrimal fossa - Trochlear fovea - Frontal sinus - Frontonasal duct
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Temporal |
Squama
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Articular tubercle - Suprameatal triangle - Mandibular fossa - Petrotympanic fissure - Zygomatic process
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Mastoid part
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Mastoid foramen - Mastoid process (Mastoid cells) - Mastoid notch - Occipital groove - Sigmoid sulcus - Mastoid antrum (Entrance)
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Petrous part
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Carotid canal - Facial canal (Hiatus) - Internal auditory meatus - Aqueduct of cochlea - Stylomastoid foramen
fossae (Subarcuate fossa, Jugular fossa) - canaliculi (Inferior tympanic, Mastoid) - Styloid process - Petrosquamous suture
(note: ossicles in petrous part, but not part of temporal bone)
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Tympanic part
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Suprameatal spine
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Sphenoid |
Surfaces
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Superior surface: Sella turcica (Dorsum sellae, Tuberculum sellae, Hypophysial fossa, Posterior clinoid processes) - Ethmoidal spine - Chiasmatic groove - Middle clinoid process - Petrosal process - Clivus
Lateral surface: Carotid groove - Sphenoidal lingula
Anterior surface: Sphenoidal sinuses
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Great wings
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foramina (Rotundum, Ovale, Vesalii, Spinosum) - Spine - Infratemporal crest - Sulcus for auditory tube
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Small wings
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Superior orbital fissure - Anterior clinoid process - Optic canal
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Pterygoid processes
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fossae (Pterygoid, Scaphoid) - pterygoid plates (Lateral, Medial) - Pterygoid canal - Hamulus
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Other
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Body - Sphenoidal conchae
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Ethmoid |
Plates
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Cribriform plate (Crista galli, Olfactory foramina) - Perpendicular plate
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Surfaces
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Lateral surface Orbital lamina - Uncinate process
Medial surface Superior nasal concha - Superior meatus - Middle nasal concha - Middle meatus
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Labyrinth
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Ethmoid sinus - ethmoidal foramina (Posterior, Anterior)
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Bones of head and neck: cranial sutures, fontanelles, and related regions |
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Cranial sutures |
Cranial: frontoethmoidal – frontal or metopic (frontal/frontal) – coronal (frontal/parietal) – occipitomastoid (occipital/temporal) – lambdoid (parietal/temporal) – sagittal (parietal/parietal) – sphenoethmoidal – sphenofrontal – sphenoparietal – sphenosquamosal (sphenoid/temporal) – sphenopetrosal (sphenoid/temporal) – squamosal (temporal/parietal) – petrosquamous (temporal/temporal)
Facial: palatomaxillary suture
Cranial–facial: sphenozygomatic – zygomaticotemporal – zygomaticofrontal
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Fontanelles |
anterior – posterior – sphenoidal – mastoid
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Foramina of multiple bones |
inferior orbital fissure – foramen lacerum – jugular foramen – nasolacrimal canal
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Fossae |
anterior cranial fossa – middle cranial fossa – posterior cranial fossa – cranial cavity
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Other compound structures |
Cranial: asterion – pterion – calvaria – stephanion – bregma – lambda
Facial: nasion
Cranial–facial: dacryon – zygomatic arch – temporal fossa – infratemporal fossa – pterygomaxillary fissure – pterygopalatine fossa
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Bones of upper limbs |
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Pectoral girdle, clavicle |
conoid tubercle – trapezoid line – costal tuberosity – subclavian groove
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Scapula |
fossae (subscapular, supraspinatous, infraspinatous) – suprascapular notch – glenoid cavity
tubercles (infraglenoid, supraglenoid) – spine of scapula – acromion – coracoid process
borders (superior, lateral/axillary, medial/vertebral) – angles (superior, inferior, lateral)
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Humerus |
upper extremity: necks (anatomical, surgical) – tubercles (greater, lesser) – intertubercular sulcus
body: radial sulcus – deltoid tuberosity
lower extremity: capitulum – trochlea – epicondyles (lateral, medial) – supracondylar ridges (lateral, medial) – fossae (radial, coronoid, olecranon)
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Forearm |
radius: upper extremity (head, tuberosity) – body – lower extremity (ulnar notch, styloid process)
ulna: upper extremity (tuberosity, olecranon, coronoid process, radial notch, trochlear notch) – body – lower extremity (head, styloid process)
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Hand |
carpus: scaphoid – lunate – triquetral – pisiform – trapezium – trapezoid – capitate – hamate (hamulus)
metacarpus: 1st metacarpal – 2nd – 3rd – 4th – 5th
phalanges of the hand: proximal – intermediate – distal
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Bones of torso |
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Sternum |
Suprasternal notch, Manubrium, Sternal angle, Body of sternum, Xiphisternal joint, Xiphoid process
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Rib |
specific ribs (1, 2, 9, 10, 11, 12, false – 8–12, floating – 11–12) – parts (Angle, Tubercle, Costal groove, Neck, Head)
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Vertebra |
General structures
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body of vertebra, vertebral arch (pedicle, lamina, vertebral notch), foramina (vertebral, intervertebral), processes (transverse, articular/zygapophysis, spinous), spinal canal
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Cervical vertebrae
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C1 (anterior arch, posterior arch, lateral mass), C2 (dens), C3, C4, C5, C6, C7
anterior tubercle, posterior tubercle, foramen transversarium
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Thoracic vertebrae
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T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12
costal facets (superior, inferior, transverse)
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Lumbar vertebrae
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accessory process, mammillary process, L5
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pelvic surface (anterior sacral foramina), dorsal surface (posterior sacral foramina, median sacral crest, medial sacral crest, lateral sacral crest), lateral surface (sacral tuberosity), base, sacral hiatus – presacral space – sacral promontory – sacral canal – ala of sacrum – sacrovertebral angle
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Bones of lower limbs |
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Femur |
upper extremity
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head (fovea) · neck · greater trochanter (trochanteric fossa) · lesser trochanter · intertrochanteric line · intertrochanteric crest
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body
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linea aspera · gluteal tuberosity / third trochanter · pectineal line
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lower extremity
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adductor tubercle · patellar surface · epicondyles (lateral, medial) · condyles (lateral, medial) · intercondylar fossa
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Crus |
Tibia
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upper extremity
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condyles (lateral, medial) · intercondylar eminence (lateral/medial intercondylar tubercle) · posterior/anterior intercondylar area
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body
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tuberosity · soleal line
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lower extremity
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medial malleolus · fibular notch
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head · body · lateral malleolus
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Other
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patella
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Foot |
Tarsus
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calcaneus (sustentaculum tali, trochlear process) · talus (body, neck, head) · navicular · cuboid · cuneiform (medial, intermediate, lateral)
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1st metatarsal · 2nd · 3rd · 4th · 5th
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Other
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phalanges of the foot
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Bones of pelvis / pelvic cavity |
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General |
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Ilium |
body
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arcuate line
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wing
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gluteal lines (posterior, anterior, inferior)
iliac spines (anterior superior, anterior inferior, posterior superior, posterior inferior)
other: crest · tuberosity · fossa
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Ischium |
body
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ischial spine · lesser sciatic notch
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superior ramus
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tuberosity of the ischium
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inferior ramus
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no substructures
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Pubis |
body
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pubic crest
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superior ramus
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pubic tubercle · obturator crest
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inferior ramus
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pectineal line
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Compound |
acetabulum (acetabular notch) · iliopubic eminence / iliopectineal line · linea terminalis · ischiopubic ramus / pubic arch
obturator foramen · greater sciatic foramen / greater sciatic notch · lesser sciatic foramen
lesser pelvis (pelvic inlet, pelvic brim, pelvic outlet) · greater pelvis
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