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AN14.1-4 | Features of individual bones (Lower Limb) — Self-Directed Learning

The Hip Bone (Os Coxae)

The hip bone (os coxae) is a large, irregular bone formed by the fusion of three primary bones: the ilium (superior), ischium (posteroinferior), and pubis (anteroinferior). All three meet at the acetabulum — a cup-shaped socket that articulates with the femoral head to form the hip joint.

Key landmarks of the ilium:
- Iliac crest — the curved upper border; palpable as the "hip bone" you rest your hand on; used for bone marrow biopsy
- Anterior superior iliac spine (ASIS) — attachment for sartorius and inguinal ligament; palpable landmark in clinical examination
- Posterior superior iliac spine (PSIS) — marked by a skin dimple; overlies the sacroiliac joint
- Iliac fossa — smooth concavity on medial surface; site of origin of iliacus muscle; in tuberculosis, pus from a psoas abscess can track here

Key landmarks of the ischium:
- Ischial tuberosity — the bony prominence you sit on; origin of hamstrings; pressure sores develop here in bedridden patients
- Ischial spine — divides the greater and lesser sciatic notches; landmark for pudendal nerve block in obstetrics
- Lesser sciatic notch — converted into the lesser sciatic foramen by the sacrospinous and sacrotuberous ligaments; transmits the obturator internus tendon and pudendal nerve

Key landmarks of the pubis:
- Pubic symphysis — secondary cartilaginous joint; can widen in pregnancy under relaxin
- Pubic tubercle — attachment of the inguinal ligament; landmark for identifying femoral hernia (below and lateral) versus inguinal hernia (above and medial)
- Obturator foramen — large oval opening bounded by pubis and ischium; covered by the obturator membrane; transmits obturator nerve and vessels through the obturator canal

Joints formed: The hip bone forms the hip joint (with the femur), sacroiliac joint (with the sacrum), and pubic symphysis (with the opposite hip bone). Together, both hip bones and the sacrum form the bony pelvis.

The Femur

The femur is the longest, strongest bone in the body. It transmits the entire body weight from the hip joint to the knee joint.

Parts and key features:
- Head — spherical; bears a pit (fovea capitis) for the ligament of the head of the femur (carries the obturator artery — the only blood supply to the femoral head in adults; critically compromised in intracapsular neck fractures)
- Neck — connects head to shaft at the neck-shaft angle (~126° in adults; reduced in coxa vara, increased in coxa valga); the neck is intracapsular — fractures here risk avascular necrosis
- Greater trochanter — large lateral projection; attachment for gluteus medius and minimus (hip abductors); palpable and used as a landmark for hip replacement surgery
- Lesser trochanter — posteromedial projection; attachment for iliopsoas (the primary hip flexor)
- Intertrochanteric line (anterior) and intertrochanteric crest (posterior) — mark the junction of neck and shaft; anterior line is attachment for the capsule of the hip joint
- Linea aspera — prominent posterior ridge on shaft; attachment for multiple muscles
- Medial and lateral condyles — articulate with the tibia; bear hyaline cartilage
- Intercondylar fossa — posterior groove between condyles; attachment for cruciate ligaments
- Adductor tubercle — on medial condyle; attachment for adductor magnus

Ossification of the femur:
- Lower end of femur: epiphysis appears at 9th month of intrauterine life → used to confirm term pregnancy (≥36 weeks) radiologically
- Upper end of femur (head): appears at 1 year after birth
- Lower epiphysis appears first → by law of ossification, it should ossify last — and it does (age ~18 years)

Joints formed: Hip joint (head with acetabulum), knee joint (condyles with tibia), and patellofemoral joint (patellar surface with patella).

Tibia and Fibula

Tibia is the medial, weight-bearing bone of the leg — the only bone that directly transmits body weight from the femur to the foot.

Key features of the tibia:
- Upper end: medial and lateral condyles with their tibial plateaus (articular surfaces for femoral condyles); tibial tuberosity (anterior projection; attachment for patellar ligament; site of Osgood-Schlatter disease in adolescents)
- Shaft: triangular in cross-section; sharp anterior border and medial surface are subcutaneous — fractures here cause open wounds
- Lower end: bears the medial malleolus (the inner ankle bump); articulates with the talus at the ankle joint
- Ossification: upper epiphysis appears first (at birth) → fuses last (~18 years); lower epiphysis appears at 1 year → fuses at ~16 years — this follows the law of ossification

Fibula is the lateral, non-weight-bearing bone of the leg.
- Primarily for muscle attachment (origin of many leg muscles)
- Head — proximally, palpable below and behind the knee; common fibular nerve winds around it (susceptible to injury in knee dislocation or tight plaster casts)
- Lateral malleolus — the outer ankle bump; forms part of the ankle mortise; more distal than the medial malleolus
- Ossification: lower epiphysis appears first (at 1–2 years) and should ossify last — but in fibula, the lower epiphysis fuses first (~16 years) and upper epiphysis fuses later (~20 years). This is the violation of the law of ossification by the fibula, explained by the lower end being functionally more important (ankle joint).

Joints formed by tibia: Knee joint (tibial condyles with femoral condyles), superior and inferior tibiofibular joints (with fibula), ankle joint (lower end with talus).
Joints formed by fibula: Superior tibiofibular joint (head with lateral tibial condyle), inferior tibiofibular joint (fibrous), ankle joint (lateral malleolus with talus).

Bones of the Foot — Tarsals, Metatarsals, Phalanges

The foot has 26 bones: 7 tarsals, 5 metatarsals, and 14 phalanges (2 in the great toe, 3 in each of the remaining 4 toes).

Seven tarsal bones (proximal to distal):
1. Calcaneus (heel bone) — largest tarsal; bears the body weight; site of Achilles tendon insertion; calcaneal spur in plantar fasciitis
2. Talus — sits on the calcaneus; articulates superiorly with tibia and fibula at the ankle joint; has no muscle attachments (entirely covered by articular cartilage or capsule); fractured in high-impact ankle injuries
3. Navicular — medial; articulates with the talus; medial projection is the navicular tuberosity (attachment for tibialis posterior); accessory ossicle here (os naviculare) can mimic fracture on X-ray
4. Cuboid — lateral; articulates with calcaneus; groove on its plantar surface for the peroneus longus tendon
5. Three cuneiforms (medial, intermediate, lateral) — articulate with the 1st, 2nd, and 3rd metatarsals respectively

Metatarsals (I–V):
- Base of 5th metatarsal has a prominent tuberosity — attachment for peroneus brevis; avulsion fracture here is common after ankle inversion injury (Jones fracture vs avulsion fracture of the tuberosity — important distinction)
- Each metatarsal has a head, shaft, and base

Key muscle attachments on tarsal bones (for AN14.4):
- Calcaneus: Achilles tendon (posterior surface), plantar fascia (medial tubercle), extensor digitorum brevis (superior surface)
- Navicular tuberosity: tibialis posterior
- Cuboid: peroneus longus (via groove)
- 1st metatarsal base: peroneus longus, tibialis anterior
- 5th metatarsal tuberosity: peroneus brevis