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The bones of the lower limb are larger, stronger, and heavier than those of the upper limb because they support the body weight and are adapted for locomotion.
They are arranged in four main parts:
Hip bone – forms the pelvic girdle.
Femur – bone of the thigh.
Tibia and Fibula – bones of the leg.
Tarsal, Metatarsal, and Phalangeal bones – form the foot.
The pelvic girdle connects the lower limbs to the trunk and transmits the body weight to the lower extremities.
The hip bone (also called coxal bone or innominate bone) forms the lateral part of the pelvis.
It is a large, irregular, flat bone formed by the fusion of three parts:
Ilium (upper large part)
Ischium (posteroinferior part)
Pubis (anteroinferior part)
The three parts meet and fuse at the acetabulum, a deep cup-shaped cavity for articulation with the head of the femur.
To place the hip bone in anatomical position:
The acetabulum should face laterally.
The obturator foramen should lie inferomedially.
The symphysial surface (pubic symphysis) should face medially.
The bone has two surfaces, four borders, and three main parts (ilium, ischium, pubis).
The pelvic surface is smooth and concave.
The gluteal surface is convex and roughened by lines for muscular attachment.
The acetabulum is directed laterally, downward, and forward.
The obturator foramen is a large opening bounded by pubis and ischium.
The ilium is the upper expanded part of the hip bone.
It forms the superior two-fifths of the acetabulum.
It consists of a body and an ala (wing).
Forms the upper part of the acetabulum.
Contributes to the acetabular fossa and acetabular margin.
Gives attachment to the rectus femoris from the anterior inferior iliac spine (AIIS).
The broad, fan-shaped portion above the body.
Presents three surfaces:
Gluteal surface – convex; has three gluteal lines (posterior, anterior, inferior) that mark muscle attachments:
Gluteus maximus between posterior and anterior lines.
Gluteus medius between anterior and inferior lines.
Gluteus minimus below inferior line.
Iliac fossa – smooth concavity on internal surface for iliacus muscle.
Sacropelvic surface – medial aspect with:
Auricular surface for sacroiliac joint articulation.
Iliac tuberosity above it for ligaments.
Anterior border:
Has ASIS (anterior superior iliac spine) and AIIS (anterior inferior iliac spine).
ASIS gives attachment to inguinal ligament, sartorius, and tensor fasciae latae.
Posterior border:
Has PSIS (posterior superior iliac spine) and PIIS (posterior inferior iliac spine).
Between PIIS and ischial spine lies the greater sciatic notch.
Superior border (Iliac crest):
Curved ridge forming the upper margin of ilium.
Divided into outer lip, intermediate zone, inner lip.
Attachments:
Outer lip – tensor fasciae latae (anteriorly), latissimus dorsi (posteriorly).
Intermediate zone – internal oblique.
Inner lip – transversus abdominis and quadratus lumborum.
Medial border:
Forms part of the linea terminalis (arcuate line).
The iliac crest is used as a landmark for:
Bone marrow biopsy and lumbar puncture (level of L4 spine).
ASIS is palpable and used to locate McBurney’s point and inguinal ligament.
Iliac tuberosity and auricular surface are involved in sacroiliitis (painful inflammation of sacroiliac joint).
The pubis forms the anteroinferior part of the hip bone.
It consists of:
Body – flattened and medially placed.
Superior ramus – extends laterally from body to join ilium and ischium.
Inferior ramus – passes downward and laterally to join the ischial ramus.
Pubic crest: Anterior border of the body of pubis; ends medially as the pubic tubercle.
Attachments: Inguinal ligament and rectus abdominis.
Pubic symphysis: Medial surface articulates with opposite pubic bone by fibrocartilage.
Pectineal line (pecten pubis): Ridge on superior ramus forming part of the pelvic brim; gives attachment to pectineus muscle.
Obturator crest: Located below pectineal line; gives attachment to obturator membrane.
Obturator groove: Becomes the obturator canal for passage of obturator nerve and vessels.
Inferior ramus: Joins ischial ramus to form the ischiopubic ramus, bounding the obturator foramen.
Fracture of pubic rami may injure urinary bladder or urethra.
Pubic symphysis diastasis can occur during childbirth due to hormonal softening of ligaments.
Pubic tubercle serves as a palpable landmark for the superficial inguinal ring.
The ischium forms the posteroinferior part of the hip bone.
It contributes the posterior two-fifths of the acetabulum.
It consists of:
Body – upper thick part joining the ilium and pubis.
Ramus – thinner part projecting forward to unite with the inferior ramus of pubis (forming the ischiopubic ramus).
Forms part of the acetabulum and gives attachment to several muscles and ligaments.
On its posterior surface, a strong ischial spine projects medially and posteriorly.
Above the spine → Greater sciatic notch.
Below the spine → Lesser sciatic notch.
The ischial spine gives attachment to:
Sacrotuberous ligament (partly).
Coccygeus muscle.
Levator ani (iliococcygeus part).
Superior gemellus muscle.
Large, roughened projection below the lesser sciatic notch.
It bears the weight of the body when sitting.
It gives origin to:
Semimembranosus (upper lateral facet).
Biceps femoris (long head) and semitendinosus (upper medial facet).
Adductor magnus (inferior surface).
Provides attachment for the sacrotuberous ligament at its posterior margin.
Extends anteriorly and medially to unite with the inferior ramus of pubis, forming the ischiopubic ramus.
The outer surface gives origin to the adductor magnus, gracilis, and obturator externus muscles.
The inner surface is part of the wall of the ischiorectal fossa.
Greater Sciatic Notch
Lies above the ischial spine.
Converts into greater sciatic foramen by the sacrospinous ligament.
Structures passing through:
Piriformis muscle.
Sciatic nerve.
Superior and inferior gluteal vessels and nerves.
Pudendal nerve and internal pudendal vessels (they exit here and re-enter through the lesser foramen).
Lesser Sciatic Notch
Lies below the ischial spine.
Converted into lesser sciatic foramen by the sacrospinous and sacrotuberous ligaments.
Transmits:
Tendon of obturator internus.
Nerve to obturator internus.
Internal pudendal vessels and pudendal nerve (entering the perineum).
Avulsion fractures of the ischial tuberosity occur in athletes due to strong hamstring pull.
The ischial spine is used as a landmark in obstetrics to measure the station of the fetal head.
Tenderness over the ischial tuberosity is seen in weaver’s bottom (ischial bursitis).
The acetabulum is a deep cup-shaped cavity on the lateral surface of the hip bone.
It articulates with the head of the femur to form the hip joint.
Formed by the fusion of:
Ilium – upper two-fifths.
Ischium – posterior two-fifths.
Pubis – anterior one-fifth.
Acetabular margin (rim):
Incomplete inferiorly due to acetabular notch.
Gives attachment to the acetabular labrum, a fibrocartilaginous rim that deepens the cavity.
Acetabular fossa:
Central non-articular depression; lodges a fat pad and attachment of ligamentum teres (ligament of head of femur).
Lunate surface:
Crescentic articular part covered by hyaline cartilage.
Articulates with the head of femur.
Thicker superiorly where body weight is transmitted.
Acetabular notch:
Located inferiorly between the two ends of the lunate surface.
Converted into a foramen by the transverse acetabular ligament.
Congenital dislocation of the hip results from shallow or defective acetabulum.
Fracture of acetabular wall may accompany posterior dislocation of the femoral head in accidents.
The acetabular fossa serves as a useful landmark during hip replacement surgery.
A large, oval opening in the anteroinferior part of the hip bone.
Bounded by:
Pubis (superiorly and medially).
Ischium (inferiorly and laterally).
Covered by the obturator membrane, a thin fibrous sheet closing the foramen except for a small obturator canal at its upper part.
Formed between the obturator membrane and the groove on the inferior surface of the superior ramus of pubis.
Transmits:
Obturator nerve.
Obturator artery.
Obturator vein.
The obturator membrane gives origin to:
Obturator externus (external surface).
Obturator internus (internal surface).
It strengthens the anterior pelvic wall and reduces unnecessary weight of the bone by maintaining a light but strong structure.
Obturator hernia: Protrusion of abdominal contents through the obturator canal, common in elderly thin females.
The obturator nerve may be compressed, causing pain in the medial thigh.
The obturator vessels are important in pelvic hemorrhage control during surgery.
Three primary centres appear during the 8th–9th week of intrauterine life, one for each component:
Ilium – appears first (around the 8th week).
Ischium – appears about the 3rd month.
Pubis – appears around the 4th–5th month.
These parts remain separate at birth but are joined by cartilage at the acetabulum.
Appear around puberty (14–15 years) and fuse by 20–25 years:
Iliac crest centre – appears near the anterior part of the crest.
Anterior inferior iliac spine centre – gives additional strength to the spine.
Ischial tuberosity centre – appears around 16 years.
Pubic symphysis centre – appears near the crest of pubis.
Acetabular centre – appears at the Y-shaped cartilage within the acetabulum where the three bones meet.
Fusion pattern:
Begins at the acetabulum and spreads outward.
Complete fusion of ilium, ischium, and pubis occurs between 20–25 years of age.
| Centre | Appears | Fuses |
|---|---|---|
| Ilium | 8th week IU | 20–25 yrs |
| Ischium | 3rd month IU | 20–25 yrs |
| Pubis | 4th–5th month IU | 20–25 yrs |
| Iliac crest (secondary) | 14 yrs | 20–25 yrs |
| Ischial tuberosity (secondary) | 16 yrs | 20–25 yrs |
| Pubic symphysis (secondary) | 15 yrs | 20–25 yrs |
| Acetabular Y-cartilage | Puberty | 20–25 yrs |
Pelvic Fractures usually occur at weak points:
Pubic rami, acetabular walls, or sacroiliac joint area.
Because the bones form a ring, a single break is rare — usually two fractures or a fracture with dislocation occur.
Fall injuries can cause:
Acetabular fractures (posterior wall commonly).
Pubic rami fractures — often from anterior-posterior compression.
Ischial tuberosity avulsion — due to hamstring pull.
Iliac crest is used for:
Bone marrow aspiration and grafting.
Landmark for lumbar puncture (L4 vertebral level lies between both crests).
ASIS serves as a surface landmark to locate inguinal ligament, McBurney’s point, and femoral artery pulsation line.
Sacroiliitis (inflammation of sacroiliac joint) causes buttock pain radiating to posterior thigh.
Ischial tuberosity is the weight-bearing point when sitting.
Bursitis (Weaver’s bottom) results from prolonged sitting on hard surfaces.
Avulsion fracture occurs in athletes during violent contraction of hamstrings.
Ischial spine is an important obstetric landmark — indicates the station of the fetal head during vaginal delivery.
Fracture of pubic rami may injure the urinary bladder or urethra (especially in males).
Diastasis of pubic symphysis can occur in pregnancy and childbirth due to relaxin-induced ligament softening.
Pubic tubercle is used as a landmark for superficial inguinal ring and inguinal hernia surgery.
Congenital dislocation of hip results from a shallow or malformed acetabulum, leading to lateral displacement of femoral head.
Posterior dislocation (from dashboard injury) may fracture posterior wall of acetabulum and damage sciatic nerve.
During total hip replacement, correct alignment of the acetabular cup is vital to prevent limb shortening and instability.
Obturator hernia is rare but serious, often seen in elderly, thin females; causes bowel obstruction.
Obturator nerve compression in this hernia produces pain along medial thigh.
Obturator artery is a potential source of pelvic bleeding during hernia repair or hysterectomy.
The pelvic inlet and outlet shape help in determining sex:
Female pelvis: broader, shallow, wide subpubic angle.
Male pelvis: narrow, deep, acute subpubic angle.
Iliac crest ossification centres help in estimating age of adolescents in forensic radiology.
Congenital acetabular dysplasia – shallow socket, leading to early hip dislocation.
Failure of fusion of pubic bones → epispadias or exstrophy of bladder.
Failure of obturator membrane closure → obturator foramen anomalies.
ASIS – most anterior projection on the iliac crest.
Pubic tubercle – 2.5 cm lateral to midline, palpable beneath skin.
Ischial tuberosity – felt in flexed hip, important for gluteal injections landmarking.
Iliac crest level – corresponds to L4 vertebra (spinal tap reference).
Greater trochanter – lateral landmark for hip joint level.
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