The Hip Joint: Stability Bought With Mobility
Stand up straight and stay there. Do nothing — let your muscles go quiet. You will not fall. Something in your hips is holding you up without asking anything of you, and it is not muscle: it is a set of ligaments that wind tight the moment you straighten, like a rope twisted until it can twist no more. That is the hip's whole character. Where the shoulder gambled everything on freedom and accepted a socket so shallow it dislocates from a bad fall, the hip made the opposite bet: a deep cup, a thick capsule, and three of the strongest ligaments in the human body. It gave up reach and bought a lifetime of standing.
Two people arrive at the same emergency department within an hour of each other. The first is eighty-four; she tripped on a rug at home, barely fell at all, and now lies with one leg visibly shorter than the other and rolled outward, the foot lolling to the side. The second is twenty-six and was a front-seat passenger when the car in front stopped; his knee struck the dashboard with his hip flexed, and he now lies with that leg shortened too — but drawn inward and rotated in, the opposite posture, and he cannot lift his foot. Two hips, two mechanisms, two opposite deformities, and the anatomy explains both without a single scan. A joint this strong does not break quietly. When it fails, it fails in patterns — and the pattern tells you exactly which structure gave way, where the blood supply was cut, and which nerve is now lying against bone.
The same bargain, struck the other way
Shoulder and hip are the same joint type solving opposite problems. Both are synovial ball-and-socket joints — the freest design the body owns, capable of movement in all three planes. But evolution asked them for opposite things. The upper limb must reach: place the hand anywhere in a sphere around the body, behind the back, above the head, out to the side. So the shoulder shrank its socket to a shallow saucer that holds barely a third of the humeral head, hung it from a capsule so loose you can pull the surfaces apart in a cadaver, and handed the entire job of stability to muscle — the rotator cuff. It is magnificently mobile and it dislocates in a rugby tackle. The lower limb must carry: hold half your body weight on one leg, thousands of times a day, for eighty years. So the hip sank its socket deep into the pelvis until it swallows more than half the femoral head, thickened the capsule into a sleeve, and wove three ligaments across it that are stronger than anything in the upper limb. The result is a joint you can stand on all day and one that a healthy adult almost never dislocates without the force of a car crash. Range for security: the same trade, made in opposite directions.
A socket that actually holds
The acetabulum is the cup formed where the ilium, ischium and pubis fuse in the hip bone, and it is deep by design. Only its horseshoe-shaped lunate surface is lined with articular cartilage; the central acetabular fossa is non-articular, floored by a fat pad and the ligamentum teres. Inferiorly the rim is interrupted by the acetabular notch. Three refinements turn a deep cup into a grip. First, the acetabular labrum — a fibrocartilaginous collar attached all round the bony rim that deepens the socket further and, being slightly elastic, hugs the femoral head past its equator, creating a genuine suction seal. Second, the transverse acetabular ligament, which bridges the acetabular notch and completes the labral ring into an unbroken circle; vessels and the ligamentum teres slip under it into the joint. Third, the head of the femur itself is more than a hemisphere, so once it is past the labral rim it is captured, not merely rested. The head, neck, angle of inclination and the trochanters are described in the femur and patella; here what matters is that the socket owns more of the ball than any other joint in the body.
The capsule, and the secret it keeps
Where the capsule stops is the single most clinically important line in the lower limb. The fibrous capsule arises from the rim of the acetabulum, just outside the labrum, and runs distally to grip the femoral neck. But it does not grip it symmetrically. Anteriorly it reaches all the way to the intertrochanteric LINE — the whole anterior neck is enclosed. Posteriorly it stops about halfway along the neck, well short of the intertrochanteric CREST, leaving the lateral third of the posterior neck bare. Learn that asymmetry and half of hip pathology becomes predictable, because it means a large part of the femoral neck lies inside the joint cavity, bathed in synovial fluid and covered by no periosteum. A fracture there is an intracapsular fracture: there is no periosteal sleeve to hold the fragments, synovial fluid washes over the fracture haematoma and impairs healing, and — the decisive point — the vessels running up the neck are torn. A fracture below the capsule, an extracapsular or intertrochanteric fracture, is a different animal altogether: it is through well-vascularised cancellous bone in a muscular envelope, it unites reliably, and it does not threaten the head. Along the outside of the neck, the capsule's deep fibres reflect back upwards as the retinacula of Weitbrecht, and the retinacular vessels ride on them like cables on a bridge.
Three ligaments that let you stand for free
Three thickenings reinforce the capsule, and all three share one beautiful property: they spiral around the neck in the same direction, so that extending the hip winds them tighter and flexing the hip unwinds them. The iliofemoral ligament of Bigelow is the strongest ligament in the human body — it takes over 300 newtons to tear — and it is shaped like an inverted Y, arising from the anterior inferior iliac spine and splaying into two limbs that attach along the intertrochanteric line. It lies directly in front of the joint and it is the principal brake on extension: it is the reason you can lean backwards from a standing position and simply stop, held by ligament rather than by muscle. The pubofemoral ligament runs from the superior pubic ramus and obturator crest to blend into the capsule below, limiting abduction and, with the iliofemoral, extension. The ischiofemoral ligament is the posterior one, arising from the ischium behind the acetabulum and spiralling upwards and laterally to the greater trochanter; it is the weakest of the three and chiefly resists medial (internal) rotation and hyperextension.
Two consequences follow, and both are worth more than the names. First, the close-packed position of the hip — the position of maximum ligamentous tension and maximum bony congruence — is full extension with slight abduction and medial rotation, which is almost exactly the position of standing. That is why standing costs so little: the ligaments are doing the work and the muscles need only make tiny corrections. Second, the position of ease — when every capsular ligament is slack and the joint capacity is greatest — is flexion with abduction and lateral rotation. A hip full of blood or pus will be held exactly there by a patient who has never read an anatomy book, and a hip is at its least stable exactly there, which is why dislocation happens with the hip flexed. Inside the joint, quite separately, runs the ligamentum teres (ligament of the head of the femur), a flat band from the acetabular notch and transverse ligament to the fovea on the femoral head. It contributes almost nothing to stability in an adult; its importance is that it carries the small artery to the head of the femur.
Twist a wet towel between your hands. While it is loose you can bend it any way you like, but as you keep twisting it shortens, hardens and finally refuses to move at all — no grip strength required, the geometry alone locks it. The three capsular ligaments of the hip are that towel, wound around the femoral neck. Straighten your hip and you twist them tight; the joint locks itself and you can stand for hours on almost no muscular effort. Bend your hip and you untwist them; the joint becomes loose and free — which is exactly what you want when climbing stairs, and exactly what makes a flexed hip the one that dislocates. Compare that with the shoulder, whose capsule is so slack it never locks at all, and which therefore must pay a rotator cuff to stay awake every waking second.
Blood that runs the wrong way
The femoral head has a supply problem that no other large joint shares, and it is a problem of direction. Blood does not arrive along the shaft and travel up into the head; it arrives at the base of the neck and climbs retrogradely along the surface of the neck, under the capsule, on the retinacula. Three sources contribute. The dominant one is the medial circumflex femoral artery, a branch of the profunda femoris described in the arteries of the lower limb; it winds round the back of the neck and its posterosuperior retinacular branches supply the great majority of the head. The lateral circumflex femoral artery contributes anteriorly and far less. The third is the artery of the ligamentum teres, a branch of the obturator artery running inside that ligament into the fovea — meaningful in childhood, when it may supply a real share of the head, and trivial in most adults. Now put that together with the capsule's anatomy. An intracapsular fracture of the femoral neck tears the retinacular vessels where they cross the fracture line, and the head is left with only the feeble ligamentum teres supply. The result is avascular necrosis: the bone of the head dies, collapses, and the joint destroys itself over months. That single fact is why a displaced intracapsular fracture in an older adult is usually treated by replacing the head — hemiarthroplasty or total hip replacement — rather than fixing it, while an extracapsular fracture, whose blood supply is intact, is fixed with a dynamic hip screw and left to heal.
Here is the sentence that carries the whole article: the hip is strong enough that healthy bone almost never lets go, so when a hip does fail the question is never "did the ligament tear?" but "where relative to the capsule did the bone break, and did the blood go with it?" Above the line, the head starves. Below the line, it heals. The shoulder, by contrast, almost never starves its humeral head — it simply falls out of its socket, again and again, because it never had bone to lose in the first place. Two ball-and-socket joints, two entirely different failure modes, and both follow directly from the bargain each one struck.
Movements, and the muscles that make them
Three axes, six movements, and one muscle group that matters more than its size suggests. Flexion is the largest range — about 120° with the knee bent, far less with it straight because the hamstrings tether it — and is produced chiefly by iliopsoas, helped by rectus femoris, sartorius and pectineus. Extension, roughly 20° beyond neutral, is the work of gluteus maximus (a powerful extensor recruited mainly for climbing, running and rising from a chair rather than for level walking) and the hamstrings; in quiet standing it is the iliofemoral ligament, not muscle, that stops you toppling backwards. Abduction, about 45°, belongs to gluteus medius and minimus, assisted by tensor fasciae latae — and these are the muscles that matter most in gait, because their real job is not to lift the leg sideways but to hold the pelvis level when you stand on one leg. Adduction is the adductor group of the medial thigh: adductor longus, brevis and magnus, gracilis and pectineus, supplied by the obturator nerve. Lateral (external) rotation, about 45°, is delivered by a dedicated set of short muscles — piriformis, the obturators, the gemelli and quadratus femoris — described with the rest of the gluteal region. Medial (internal) rotation has no dedicated muscle at all; it is a side-effect of the anterior fibres of gluteus medius and minimus and of tensor fasciae latae, which is why it is the weakest movement of the hip and the first to be lost in osteoarthritis.
When a joint this strong fails
Fractured neck of femur is the classic injury of the osteoporotic elderly, and its posture is unmistakable: the limb is shortened, because the muscles crossing the hip pull the shaft upwards past the broken neck, and externally rotated, because the weight of the free limb and the short lateral rotators roll it outwards. Posterior dislocation is the opposite story and the opposite posture. It takes real violence — classically the dashboard injury, where a flexed, adducted hip is driven backwards along the femoral shaft — and because the ischiofemoral ligament is the weakest of the three, the head is driven out behind the acetabulum onto the ilium. The limb lies shortened, ADDUCTED and INTERNALLY rotated. Lying immediately behind the joint is the sciatic nerve, and in roughly one in ten posterior dislocations it is injured, usually its common fibular division: the patient cannot dorsiflex the foot and has foot drop, exactly as described in the sciatic, tibial and fibular nerves. Anterior dislocation is far rarer and produces the reverse — an abducted, externally rotated limb.
Three more everyday clinical signatures complete the picture. The Trendelenburg sign: ask a patient to stand on one leg and watch the opposite hip. Normally the gluteus medius and minimus of the standing side contract to hold the pelvis level; if they are weak — or if their nerve, the superior gluteal nerve (L4–S1), is injured — the unsupported side of the pelvis DROPS, and over many steps the patient develops the characteristic lurching gait. Referred pain to the knee: the hip and the knee are both supplied in part by the obturator nerve (L2–L4), so a child with hip pathology may complain only of knee pain — a trap that has delayed the diagnosis of slipped upper femoral epiphysis and septic hip more times than any other in paediatric orthopaedics. And osteoarthritis: the great chronic disease of this joint, in which cartilage thins, osteophytes form and the capsule contracts, so the patient loses internal rotation first, then extension, and walks with the hip flexed. Symptoms are managed with analgesia — often the NSAIDs — until pain and function justify total hip replacement, one of the most successful operations in all of surgery.
The eighty-four-year-old who tripped on the rug: her bone was osteoporotic, so trivial force snapped the femoral neck inside the capsule. The shortened, externally rotated leg was visible from the doorway. Her fracture is intracapsular, the retinacular branches of the medial circumflex femoral artery are torn, and fixing the fracture would leave a head with almost no blood supply — so she goes for a hip replacement. The twenty-six-year-old from the car: his bone was normal and it took the entire energy of a collision, transmitted up a flexed femur, to push the head out of the back of the socket. His leg is shortened, adducted and internally rotated, and the fact that he cannot lift his foot tells you the sciatic nerve was stretched over the displaced head. His hip needs urgent reduction — every hour the head sits dislocated raises the later risk of avascular necrosis, because even a dislocation without a fracture can kink the same retinacular vessels. Same joint, same two vessels, two completely different stories.
- The hip is a DEEP ball-and-socket: the acetabulum holds more than half the femoral head, deepened by the fibrocartilaginous acetabular labrum and completed inferiorly by the transverse acetabular ligament.
- Only the horseshoe-shaped lunate surface is articular; the central acetabular fossa holds fat and the ligamentum teres.
- The capsule runs from the acetabular rim to the intertrochanteric LINE anteriorly but only to mid-neck posteriorly — so much of the femoral neck is INTRACAPSULAR.
- Three capsular ligaments spiral so they TIGHTEN in extension: iliofemoral (of Bigelow, the Y ligament, strongest in the body, resists extension), pubofemoral (resists abduction and extension), ischiofemoral (posterior, resists medial rotation).
- Close-packed (most stable) position = extension + slight abduction + medial rotation, i.e. standing; position of ease/maximum capacity = flexion + abduction + lateral rotation, the posture of an effusion and the posture that dislocates.
- The ligamentum teres is mechanically trivial in adults but carries the artery to the head of the femur (from the obturator artery) — significant in children.
- Blood supply is RETROGRADE: retinacular branches climb the neck under the capsule, dominated by the MEDIAL circumflex femoral artery, with a smaller lateral circumflex contribution and the artery of ligamentum teres.
- Intracapsular neck fracture tears those vessels → avascular necrosis of the head; extracapsular (intertrochanteric) fracture spares them and heals reliably.
- Movements: flexion (iliopsoas), extension (gluteus maximus + hamstrings), abduction (gluteus medius + minimus), adduction (adductor group, obturator nerve), lateral rotation (piriformis, obturators, gemelli, quadratus femoris); medial rotation has no dedicated muscle and is weakest.
- Fractured neck of femur = SHORTENED and EXTERNALLY rotated limb.
- Posterior (dashboard) dislocation = shortened, ADDUCTED and INTERNALLY rotated limb, with the sciatic nerve at risk (foot drop).
- Trendelenburg sign = failure of gluteus medius/minimus or the superior gluteal nerve (L4–S1); the pelvis drops on the UNSUPPORTED side.
- Assuming the capsule reaches the intertrochanteric crest behind as it reaches the intertrochanteric line in front. It stops at mid-neck posteriorly — the asymmetry is the whole basis of intracapsular versus extracapsular fracture.
- Thinking the ligamentum teres is an important stabiliser. It is mechanically negligible in adults; its significance is vascular, and even that supply is minor after childhood.
- Confusing the two deformities. A fractured neck of femur gives a shortened, EXTERNALLY rotated limb; a posterior dislocation gives a shortened, adducted, INTERNALLY rotated limb.
An 80-year-old woman falls at home and is found with a shortened, externally rotated right leg. Radiographs show a displaced fracture of the femoral neck within the capsule. Which vessel's disruption is most responsible for the risk of avascular necrosis of the femoral head?
- The hip is the mirror image of the shoulder: a DEEP ball-and-socket that trades range for stability — the acetabulum, deepened by the labrum and completed by the transverse acetabular ligament, holds more than half the femoral head.
- The capsule reaches the intertrochanteric line in front but only mid-neck behind, so much of the neck is intracapsular — and the three spiralling ligaments (iliofemoral of Bigelow, pubofemoral, ischiofemoral) tighten in extension, letting you stand with almost no muscular effort.
- Blood reaches the head retrogradely up the neck, dominated by the medial circumflex femoral artery — hence avascular necrosis after an intracapsular neck fracture, while extracapsular fractures heal.
- Signature clinical patterns: shortened + externally rotated leg in a fractured neck of femur; shortened + adducted + internally rotated in posterior dislocation with sciatic nerve risk; Trendelenburg sign from gluteus medius/superior gluteal nerve failure; hip pain referred to the knee via the obturator nerve.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Lower limb: the hip joint.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — The hip joint, fractures of the femoral neck and avascular necrosis.
- Netter FH. Atlas of Human Anatomy — Hip joint: capsule, ligaments and arterial supply.
- Last RJ. Last's Anatomy: Regional and Applied — The gluteal region and hip joint.
- Snell RS. Clinical Anatomy by Regions — Dislocations of the hip and the Trendelenburg sign.
- TeachMeAnatomy — The Hip Joint; Blood Supply to the Femoral Head.
