Osteology

Orientation: Large, irregular bone formed by fusion of three bones at the acetabulum; forms the pelvic girdle with its contralateral partner and the sacrum.

Side determination:

• Acetabulum faces laterally and slightly inferiorly/anteriorly.
• Obturator foramen is anteroinferior to the acetabulum.
• Ischial spine and greater/lesser sciatic notches are posterior.

Three constituent bones (fuse at the acetabulum by adulthood):

• Ilium: superior, fan-shaped; forms the upper two-fifths of the acetabulum.
• Ischium: posteroinferior; forms the lower two-fifths of the acetabulum.
• Pubis: anteroinferior; forms the anterior one-fifth of the acetabulum; meets its contralateral partner at the pubic symphysis.

Major landmarks:

• Iliac crest: superior border of ilium, extends from ASIS to PSIS; palpable along its entire length.
• Anterior superior iliac spine (ASIS): origin of sartorius and inguinal ligament attachment.
• Anterior inferior iliac spine (AIIS): origin of the straight head of rectus femoris.
• Posterior superior iliac spine (PSIS) and posterior inferior iliac spine (PIIS): posterior landmarks, PSIS often marked by a skin dimple.
• Greater sciatic notch: converted to the greater sciatic foramen by the sacrospinous ligament; transmits piriformis and the major gluteal neurovascular structures.
• Lesser sciatic notch: converted to the lesser sciatic foramen by the sacrospinous and sacrotuberous ligaments; transmits obturator internus tendon and the pudendal neurovascular bundle.
• Ischial spine: separates greater and lesser sciatic notches; landmark for pudendal nerve block.
• Ischial tuberosity: weight-bearing point in sitting; origin of the hamstrings.
• Acetabulum: deep, cup-shaped socket for the femoral head; horseshoe-shaped articular surface (lunate surface) surrounding a non-articular acetabular fossa (contains fat pad and the ligament of the head of femur).
• Obturator foramen: largest foramen in the body, mostly closed by the obturator membrane, small canal superiorly transmits obturator nerve/vessels.
• Pubic tubercle: attachment of the inguinal ligament.
• Pubic crest: medial to the tubercle, attachment for rectus abdominis.

Muscle attachments:

• Sartorius: ASIS.
• Rectus femoris: AIIS (straight head) and superior acetabular rim (reflected head).
• Gluteus maximus, medius, minimus: external surface of ilium (posterior to anterior gluteal lines respectively).
• Piriformis: internal surface of ilium/sacrum, exits via greater sciatic foramen.
• Hamstrings (biceps femoris long head, semitendinosus, semimembranosus): ischial tuberosity.
• Adductor magnus (hamstring part): ischial tuberosity.
• Obturator internus: internal surface around obturator foramen.
• Obturator externus: external surface around obturator foramen.
• Pectineus, adductor longus/brevis, gracilis: pubis (body and rami).
• Iliacus: iliac fossa (internal surface of ilium).

Ligament attachments:

• Inguinal ligament: ASIS to pubic tubercle (thickened lower border of external oblique aponeurosis, not a true bone-to-bone pelvic ligament but attaches to the hip bone at both ends).
• Sacrospinous ligament: ischial spine to sacrum/coccyx.
• Sacrotuberous ligament: ischial tuberosity to sacrum/coccyx/PSIS.
• Iliofemoral, pubofemoral, ischiofemoral ligaments: hip joint capsule (covered under Hip Joint).

Articulations: Sacroiliac joint (auricular surface of ilium), pubic symphysis (pubic bodies), hip joint (acetabulum).

Applied anatomy:

• Pelvic fractures: often high-energy, risk major vessel and pelvic organ injury; classified by stability (e.g., open-book, lateral compression, vertical shear patterns).
• Avulsion fractures: common at apophyses in adolescent athletes (ASIS/sartorius, AIIS/rectus femoris, ischial tuberosity/hamstrings).
• Ischial spine and pudendal nerve relationship is used clinically for pudendal nerve block during labour.

Orientation: Longest and strongest bone in the body; head faces medially, superiorly, and slightly anteriorly.

Side determination:

• Head is proximal and medial; condyles are distal.
• Greater trochanter is lateral, lesser trochanter is posteromedial.
• Linea aspera (rough posterior ridge) helps orient the posterior surface.

Major landmarks:

• Head: two-thirds of a sphere, articulates with the acetabulum; fovea: small pit for the ligament of the head of femur.
• Neck: connects head to shaft, at an angle of inclination of approximately 125° to the shaft (varies with age/sex); common fracture site.
• Greater trochanter: lateral, palpable; site of insertion for gluteus medius, gluteus minimus, and piriformis.
• Lesser trochanter: posteromedial, at the base of the neck; insertion of iliopsoas.
• Intertrochanteric line: anterior, connects the trochanters; attachment of the iliofemoral ligament.
• Intertrochanteric crest: posterior, connects the trochanters; contains the quadrate tubercle for quadratus femoris.
• Linea aspera: posterior ridge along the shaft, diverging proximally into the gluteal tuberosity (laterally, gluteus maximus insertion) and pectineal line (medially); diverges distally into the medial and lateral supracondylar lines, bounding the popliteal surface.
• Adductor tubercle: most distal point of the medial supracondylar line; insertion of adductor magnus (hamstring part).
• Medial and lateral condyles: distal, articulate with the tibia; separated posteriorly by the intercondylar fossa/notch.
• Medial and lateral epicondyles: proximal to the condyles; medial epicondyle carries the adductor tubercle just above it, lateral epicondyle has a groove for the popliteus tendon.
• Patellar surface: anterior, between the condyles, articulates with the patella.

Muscle attachments:

• Iliopsoas: lesser trochanter.
• Gluteus medius, minimus: greater trochanter.
• Piriformis, obturator internus/externus, gemelli (superior/inferior): trochanteric fossa and surrounding area of greater trochanter.
• Quadratus femoris: quadrate tubercle, intertrochanteric crest.
• Vastus lateralis: greater trochanter and linea aspera (lateral lip).
• Vastus medialis: intertrochanteric line and linea aspera (medial lip).
• Vastus intermedius: anterior/lateral shaft.
• Adductor longus, brevis, magnus (adductor part), pectineus: linea aspera region and pectineal line.
• Adductor magnus (hamstring part): adductor tubercle.
• Gluteus maximus: gluteal tuberosity.
• Short head of biceps femoris: lateral supracondylar line.
• Popliteus: lateral condyle (groove below the epicondyle), extends onto the posterior tibia.
• Gastrocnemius (medial and lateral heads): posterior surface, just above the condyles.

Ligament attachments: Ligament of the head of femur (fovea), iliofemoral ligament (intertrochanteric line), collateral ligaments of the knee (epicondyles, covered under Knee Joint).

Articulations: Hip joint (head), knee joint (condyles), patella (patellar surface, patellofemoral joint).

Applied anatomy:

• Femoral neck fracture: common in osteoporotic elderly patients after a fall; risks disruption of the retinacular vessels (branches of the medial circumflex femoral artery) running along the neck, causing avascular necrosis of the femoral head, especially with displaced/intracapsular fractures.
• Intertrochanteric fracture: extracapsular, better blood supply preserved, generally lower avascular necrosis risk than neck fractures.
• Shaft fracture: high-energy trauma, significant blood loss risk (femoral vessels/profunda femoris branches), can injure the sciatic nerve posteriorly if severely displaced.
• Supracondylar femoral fracture: risks the popliteal artery, which is tethered at the adductor hiatus and popliteal fossa, and can be pulled/compressed by the distal fragment (typically displaced posteriorly by gastrocnemius pull).

Orientation: Largest sesamoid bone in the body, triangular, apex pointing distally, embedded within the quadriceps femoris tendon.

Side determination: Base (broad, superior) vs apex (narrow, inferior); posterior articular surface has a larger lateral facet than medial facet (matching the deeper/larger lateral femoral condyle).

Major landmarks:

• Base: superior border, receives quadriceps tendon.
• Apex: inferior, continuous with the patellar ligament.
• Anterior surface: subcutaneous, rough.
• Posterior (articular) surface: smooth, divided by a vertical ridge into a larger lateral and smaller medial facet, articulating with the femoral patellar surface/condyles.

Muscle/ligament attachments:

• Quadriceps femoris tendon: base and sides (rectus femoris, vastus lateralis/medialis/intermedius converge here).
• Patellar ligament: apex to tibial tuberosity (functionally the continuation of the quadriceps tendon distal to the patella).
• Medial and lateral patellar retinacula: sides, formed by expansions of vastus medialis/lateralis, contribute to knee extensor mechanism and joint stability.

Articulations: Patellofemoral joint (posterior surface with femoral patellar surface/condyles), part of the knee joint synovial cavity.

Applied anatomy:

• Patellar fracture: direct trauma (fall onto a flexed knee, dashboard injury) or avulsion from forceful quadriceps contraction; can disrupt the extensor mechanism, impairing active knee extension.
• Patellar dislocation: typically lateral, due to the natural lateral pull of the quadriceps (Q-angle) and relative weakness of vastus medialis oblique or a shallow lateral femoral condyle/trochlear groove.
• Chondromalacia patellae: degeneration of the posterior articular cartilage, causing anterior knee pain, especially with a maltracking patella.

Orientation: Medial, larger, weight-bearing bone of the leg; proximal end broad, distal end narrower but with a prominent medial malleolus.

Side determination:

• Medial malleolus is distal and medial; tibial tuberosity is proximal and anterior.
• Anterior border (shin) is subcutaneous and sharp.

Major landmarks:

• Medial and lateral tibial condyles: form the tibial plateau, articulate with the femoral condyles; separated by the intercondylar eminence (two tubercles, sites of cruciate ligament and meniscal horn attachments).
• Tibial tuberosity: anterior, just below the condyles; insertion of the patellar ligament.
• Anterior border (crest): sharp, subcutaneous, palpable along the entire shaft ("shin").
• Medial surface: subcutaneous, palpable.
• Soleal line: posterior surface, proximal third, oblique ridge for soleus origin.
• Medial malleolus: distal, medial projection; articulates with the talus; palpable, forms the medial "ankle bone."
• Fibular notch: distal lateral surface, articulates with the fibula (inferior tibiofibular syndesmosis).
• Inferior articular surface: distal end, articulates with the talar trochlea (superior surface).

Muscle attachments:

• Quadriceps femoris (via patellar ligament): tibial tuberosity.
• Sartorius, gracilis, semitendinosus (pes anserinus), proximal medial surface, below the medial condyle.
• Semimembranosus: posteromedial condyle (small groove).
• Popliteus: posterior surface, above the soleal line.
• Soleus: soleal line.
• Tibialis posterior: posterior surface, lateral to flexor digitorum longus origin.
• Flexor digitorum longus: posterior surface, medial to tibialis posterior origin.
• Tibialis anterior: lateral condyle and proximal lateral shaft.
• Interosseous membrane: attaches along the interosseous border (lateral shaft), connecting to the fibula.

Ligament attachments: Cruciate ligaments and meniscal attachments (intercondylar eminence and adjacent areas), medial collateral ligament of knee (medial condyle/proximal shaft), interosseous membrane (lateral border).

Articulations: Knee joint (proximal, tibial plateau with femoral condyles), superior tibiofibular joint (lateral condyle with fibular head), inferior tibiofibular joint (fibular notch with fibula), ankle joint (distal, with talus).

Applied anatomy:

• Tibial plateau fracture: often from axial loading with a valgus/varus component (e.g., fall from height, vehicle-pedestrian trauma against the bumper, "bumper fracture," typically lateral plateau); can involve the knee joint surface, ligaments, and menisci.
• Tibial shaft fracture: common long bone fracture, high risk of compartment syndrome given the tight fascial compartments of the leg and the tibia's subcutaneous position (open fractures are relatively common).
• Medial malleolus fracture: part of ankle fracture patterns (e.g., bimalleolar, trimalleolar fractures), often from rotational ankle injury.

Orientation: Slender, lateral bone of the leg; non-weight-bearing at the knee, but essential for ankle stability and muscle attachment.

Side determination:

• Head is proximal, lateral malleolus is distal.
• Lateral malleolus extends more distally than the medial malleolus.

Major landmarks:

• Head: proximal, articulates with the lateral tibial condyle (superior tibiofibular joint); has an apical styloid process.
• Neck: just below the head; common fibular nerve winds around it, superficial and vulnerable here.
• Shaft: thin, triangular in cross-section, gives origin to the lateral and posterior leg compartment muscles.
• Lateral malleolus: distal, forms the lateral wall of the ankle mortise; extends more distally and posteriorly than the medial malleolus, an important stabiliser of the ankle against eversion/lateral talar shift.
• Interosseous border: medial shaft, attachment of the interosseous membrane.

Muscle attachments:

• Biceps femoris (short head): proximal shaft.
• Fibularis (peroneus) longus and brevis: lateral surface of shaft.
• Extensor digitorum longus, extensor hallucis longus, fibularis tertius: anterior surface/border.
• Soleus: posterior surface, proximal (soleal line continuation).
• Flexor hallucis longus: posterior surface, distal two-thirds.
• Tibialis posterior: posterior surface (with interosseous membrane).

Ligament attachments: Interosseous membrane (medial border), lateral collateral ligament complex of the ankle (lateral malleolus, anterior talofibular, calcaneofibular, posterior talofibular ligaments), fibular collateral ligament of the knee (head of fibula), anterior/posterior inferior tibiofibular ligaments (syndesmosis, distal shaft/malleolus).

Articulations: Superior tibiofibular joint (head, with lateral tibial condyle), inferior tibiofibular joint (distal shaft, with the fibular notch of tibia, syndesmosis), ankle joint (lateral malleolus, with talus).

Common fibular nerve relationship:

• Winds around the neck of the fibula, lying directly subcutaneous and against bone at this point.
• Highly vulnerable to compression (prolonged squatting, leg crossing, tight casts/splints) and to direct trauma or fracture at the fibular neck.
• Injury here produces foot drop and sensory loss over the anterolateral leg and dorsum of the foot (loss of both deep and superficial fibular nerve territories, since the common trunk is affected before it divides).

Applied anatomy:

• Fibular fracture: often part of combined ankle fracture patterns (bimalleolar/trimalleolar, or a Maisonneuve fracture: proximal fibular fracture with syndesmosis disruption from a severe external rotation ankle injury; always examine the proximal fibula in significant ankle injuries).
• Isolated fibular shaft fractures away from the ankle are often managed conservatively, given the fibula's non-weight-bearing role at the knee.
• Fibular neck fracture: common fibular nerve injury risk.

Seven bones, forming the posterior/proximal part of the foot.

Proximal group: Talus, Calcaneus. Intermediate: Navicular. Distal group: Cuboid, Medial cuneiform, Intermediate cuneiform, Lateral cuneiform.

Talus:

• No muscle attachments (unique among tarsals); body, neck, and head.
• Trochlea (body's superior surface) articulates with the tibia and both malleoli (ankle mortise), wider anteriorly, contributing to greater ankle stability in dorsiflexion.
• Head articulates anteriorly with the navicular.
• Blood supply is precarious (similar concept to the scaphoid): largely retrograde from vessels entering the neck/distal talus (branches of posterior tibial, dorsalis pedis, and perforating fibular arteries), so talar neck fractures risk avascular necrosis of the body.

Calcaneus:

• Largest tarsal bone, forms the heel.
• Sustentaculum tali: medial shelf, supports the talar head, grooved on its undersurface for flexor hallucis longus tendon.
• Posterior surface: receives the Achilles (calcaneal) tendon.
• Articulates with the talus (subtalar joint) superiorly and cuboid anteriorly.

Navicular:

• Medial side of the foot, between the talar head and the three cuneiforms.
• Tuberosity: medial, palpable, insertion for most of tibialis posterior tendon (accessory navicular bone is a common normal variant here).

Cuboid:

• Lateral side of the foot, between the calcaneus and the 4th/5th metatarsals.
• Has a groove on its plantar surface for the fibularis longus tendon.

Cuneiforms (medial, intermediate, lateral):

• Wedge-shaped, between the navicular and metatarsals 1–3 respectively.
• Contribute to the transverse arch of the foot (intermediate cuneiform is "recessed," keying the 2nd metatarsal base into a stable mortise, relevant to Lisfranc injury patterns).

Applied anatomy:

• Talar neck fracture: risks avascular necrosis of the talar body, given the retrograde/precarious blood supply.
• Calcaneal fracture: typically from a fall from height (axial loading), often bilateral, associated with spinal compression fractures from the same mechanism (always assess the spine).
• Lisfranc injury: fracture-dislocation at the tarsometatarsal joints, often subtle on imaging, easily missed.

Five bones (I–V, great toe to little toe), each with a base (proximal, tarsal articulation), shaft, and head (distal, MTP joint).

• 1st metatarsal: shortest and stoutest, bears the greatest load; two sesamoid bones (medial, lateral) under its head, within the flexor hallucis brevis tendons.
• 5th metatarsal base: has a prominent tuberosity/styloid process, insertion of fibularis brevis (and fibularis tertius); a common avulsion fracture site ("dancer's fracture" region), distinct from the more distal Jones fracture (metaphyseal-diaphyseal junction, poor healing potential due to a watershed blood supply zone).
• Metatarsal heads form the ball of the foot, weight-bearing during push-off.

Fourteen phalanges per foot: proximal, middle, and distal in digits 2–5; the great toe (digit 1, hallux) has only proximal and distal phalanges (no middle phalanx), mirroring the thumb.

• Structurally similar to the hand phalanges but shorter and more compact, reflecting the foot's weight-bearing/propulsive role rather than fine manipulation.
• Each phalanx has a base, shaft, and head; extensor and flexor tendon attachments are covered under the Foot section.