The Femur and Patella: The Longest Bone and the Body's Biggest Sesamoid
One bone carries you. Every step you have ever taken, every stair, every sprint and every stumble has been transmitted through a single shaft of bone in each thigh — the longest, heaviest and strongest bone you own. It is engineered so precisely that its internal struts follow the exact lines of force that pass through it, and it is angled so cleverly that it brings your knees under your centre of gravity so you can walk on two legs instead of waddling. And yet a few centimetres of its upper end are so precariously supplied with blood that a fall in a bathroom at the age of eighty can end a life. Riding on its front is a small triangular bone that most people never think about, which quietly makes your knee thirty per cent stronger. This is the story of both.
An eighty-two-year-old woman gets up at night, catches her slipper on a rug, and falls onto her side. She does not hit her head; she is talking, alert, apologising for the fuss. But when her daughter pulls back the blanket, one leg is unmistakably wrong: it is shorter than the other, and the foot has rolled outward so the toes point at the wall. No swelling, no wound, nothing dramatic — just a leg that has quietly changed its address. In the same hospital that night, a twenty-year-old footballer arrives after a collision, and a young man is told his kneecap has slipped sideways out of its groove for the second time this year. Three injuries, one bone and its small companion. And each of them is explained not by force alone, but by shape: by where a neck meets a shaft, by which artery climbs which surface, and by the direction in which a thigh muscle happens to pull.
A bone built for a lifetime of load
The femur is the archetype of a long bone — and the clearest proof that bone is a living engineering material. The femur (Latin: os femoris) is the longest, heaviest and strongest bone in the human body, and roughly a quarter of your standing height. It has the classic long-bone plan described in bones: classification and structure: a tubular diaphysis of dense cortical bone surrounding a marrow cavity, flaring metaphyses at each end, and epiphyses capped with articular cartilage. Cut its upper end lengthwise and you see something remarkable — the trabeculae of the spongy bone inside are not random. They run in two arcing systems, one sweeping from the medial calcar up into the head to resist compression, the other arching from the lateral shaft into the greater trochanter to resist tension, with a relatively weak triangle between them where fractures like to start. The bone has, over a lifetime, been rebuilt along the very lines of force it carries. Nothing in the skeleton illustrates Wolff's law more beautifully.
The upper end: head, neck and two trochanters
The head of the femur is about two-thirds of a sphere, covered in hyaline cartilage except for a small pit near its centre — the fovea capitis — which anchors the ligament of the head (ligamentum teres). It sits deep inside the acetabulum of the hip bone to form the hip joint. From the head runs the neck, angled upward and medially, and the angle it makes with the shaft — the neck–shaft (collodiaphyseal) angle — is about 125 degrees in the adult. Too small an angle is coxa vara, which shortens the limb and reduces the abductors' leverage; too large is coxa valga. Where the neck meets the shaft stand the two trochanters: the greater trochanter laterally, a broad palpable block that receives gluteus medius and minimus, piriformis, obturator internus and the gemelli; and the lesser trochanter posteromedially, the cone-shaped insertion of iliopsoas, the great hip flexor. Connecting them in front is the roughened intertrochanteric line, the attachment of the iliofemoral ligament and the capsule; behind is the sharper intertrochanteric crest, carrying the quadrate tubercle for quadratus femoris. That difference in front and behind is not trivia — it is exactly where the joint capsule stops, and therefore where a fracture becomes intracapsular or extracapsular.
The shaft and the great ridge on its back
The shaft is smooth and almost featureless in front — and crowded with attachments behind. Running down the posterior surface is the linea aspera, the "rough line", a raised double ridge with a medial and a lateral lip separated by a narrow interval. This single ridge is the anchor of the thigh: the adductor group (adductor longus, brevis and magnus, and pectineus higher up) grips its medial lip, the vastus lateralis and the short head of biceps femoris take the lateral lip, and vastus medialis sweeps off the medial lip and the spiral line. Trace it upward and the lateral lip becomes the gluteal tuberosity for gluteus maximus, while the medial lip continues as the pectineal line for pectineus. Trace it downward and the two lips diverge as the medial and lateral supracondylar lines, enclosing the flat triangular popliteal surface that forms the floor of the popliteal fossa. The muscles that use all of this are set out in the muscles of the thigh. Note also that the shaft is not vertical: because the hip joints are set far apart and the knees are brought close together, the femur slants downward and medially, and this obliquity is greater in the wider female pelvis — the anatomical basis of the Q-angle at the knee.
Think of the femoral shaft as the mast of a sailing ship. A bare mast would snap under the first strong wind; what keeps it upright is the rigging, the ropes tensioned along its length. The linea aspera is the cleat rail where every one of those ropes is tied — adductors pulling from the inside, vasti from the outside, gluteus maximus from above. That is why the ridge exists only on the back of the bone: it is where the tension lines converge, and bone thickens wherever it is repeatedly pulled. Strip the muscles away and the femur is just a tube; leave them attached and it becomes a rigged mast that can carry eight times body weight through a running stride.
The lower end: condyles, epicondyles and a groove
Distally the femur expands into two great knuckles, the medial and lateral condyles, which together with the tibia and the patella form the knee joint. Behind and below they are separated by the deep intercondylar fossa, roofed by the intercondylar line, and it is on the walls of that fossa that the cruciate ligaments take their femoral attachments — the anterior cruciate from the medial surface of the lateral condyle, the posterior cruciate from the lateral surface of the medial condyle. In front the two condyles fuse into a single smooth, saddle-shaped patellar surface (the trochlea), whose lateral lip is deliberately taller and more prominent than the medial. On the sides sit the medial and lateral epicondyles, the origins of the collateral ligaments; just above the medial epicondyle rises the adductor tubercle, the small bump where the tendinous ischiocondylar part of adductor magnus inserts and a reliable landmark for finding the medial epicondyle and the adductor hiatus. Each condyle is also broader behind than in front, so as the knee extends the joint surfaces "screw home" into their most stable, close-packed position.
The blood supply — and why a broken hip is a different injury
Everything clinically important about the femoral neck follows from one fact: its blood runs the wrong way. The head of the femur is fed from three sources, and they are wildly unequal. The dominant supply comes from the retinacular arteries — branches of the medial circumflex femoral artery (with a smaller contribution from the lateral circumflex femoral), described among the arteries of the lower limb. These vessels form a ring around the base of the neck, then send fine branches that climb UP along the neck, underneath the reflected capsule, to reach the head. A second, minor source is the artery to the head of the femur running inside the ligamentum teres — significant in childhood, usually trivial in adults. A third, the nutrient artery of the shaft, contributes little to the head. So the head's arterial blood must travel retrograde, distal to proximal, hugging the very bone that is at risk. Break the neck inside the capsule and you tear those retinacular vessels off the bone. The head is left supplied by little more than the ligamentum teres — and avascular necrosis of the femoral head follows in a substantial proportion of displaced fractures, sometimes months later, as the dead bone slowly collapses.
This is why surgeons care so much about a line most students skim past. An intracapsular (subcapital or transcervical) fracture of the neck endangers the head's blood supply, and in an older patient with a displaced break the head is usually replaced altogether — a hemiarthroplasty or total hip replacement. An extracapsular fracture — intertrochanteric, through the well-vascularised cancellous bone between the trochanters, or subtrochanteric just below them — leaves the retinacular vessels intact, heals reliably, and is fixed with a dynamic hip screw or an intramedullary nail. Same fall, same bone, two entirely different operations, and the whole decision turns on where the capsule ends: attached in front along the intertrochanteric line, but only to the middle of the neck behind.
The shortened, externally rotated leg is the femur telling you exactly what has happened. Once the neck is broken, the head stays locked in the acetabulum while the shaft is free — and the shaft is pulled upward by the powerful hip flexors and adductors and rotated outward by iliopsoas, the short lateral rotators and gluteus maximus, which collectively out-torque the weak internal rotators. Hence a leg that is a few centimetres short with the foot lolling laterally. Learn that posture and you can make the diagnosis from the doorway. It matters more than any other single sign in geriatric orthopaedics, because roughly a third of these patients are dead within a year — not from the bone, but from the immobility, the pneumonia, the delirium and the loss of independence that follow it. This is also why bone-protective treatment and the management of calcium and bone metabolism are not academic footnotes.
The patella: the largest sesamoid in the body
A sesamoid bone is one that develops inside a tendon — and this one develops inside the strongest tendon you have. The patella is a flat, triangular bone embedded in the quadriceps femoris tendon, and it is the largest sesamoid in the human skeleton. Its broad base faces upward, receiving the quadriceps tendon; its pointed apex faces downward and gives origin to the patellar ligament, which runs to the tibial tuberosity — so, strictly, the structure below the kneecap is a ligament (bone to bone), not a tendon, even though everyone calls it the patellar tendon. Its anterior surface is rough, subcutaneous and crossed by vertical striations, separated from the skin by the prepatellar bursa (the one that inflames into "housemaid's knee"). Its posterior surface is almost entirely smooth articular cartilage — the thickest cartilage in the body, up to 5–7 mm — divided by a vertical ridge into a smaller medial facet and a larger lateral facet that match the trochlear groove of the femur. Only the lower part of the posterior surface, just above the apex, is non-articular.
Why have it at all? Two jobs. First and most important, the patella increases the quadriceps' lever arm: by standing the tendon off the front of the femur, it moves the line of pull further from the knee's axis of rotation, boosting the extensor moment by roughly a third — most of that gain in the last 30 degrees of extension, exactly where you need it to lock the knee and to rise from a chair. Second, it protects the front of the joint and lets the tendon glide over the trochlea without abrading. The price of that design is instability. Because the quadriceps pulls slightly laterally (the Q-angle, the angle between the line of pull of the quadriceps and the patellar ligament, is larger in women because of the wider pelvis and more oblique femur), the patella always tends to be dragged sideways. Two things resist it: the horizontal fibres of vastus medialis obliquus, which actively hold it medially, and the taller, more prominent lateral lip of the femoral trochlea, which is a bony wall in its way. Weaken the first or flatten the second and the patella dislocates — and it dislocates laterally, essentially always.
The patella is a pulley wheel on a crane. The cable (quadriceps tendon) does not run straight from the winch to the load; it is lifted away from the tower by a wheel, and that small displacement multiplies the torque the winch can deliver. Remove the wheel and the cable hugs the tower — the crane still pulls, but with much less turning force. That is precisely what happens after a patellectomy: the leg still extends, but extension strength falls by around a third, and the last few degrees are the hardest to regain.
The tapped tendon: strike the patellar ligament with a hammer, the quadriceps is stretched, and the muscle spindles fire a monosynaptic reflex through the L3–L4 roots and back down the femoral nerve — so the knee jerk tests the pathway described in the femoral and obturator nerves. An absent jerk is a very cheap piece of information. Jumper's knee: a volleyball player lands repeatedly and develops tenderness right at the patellar apex — chronic tendinopathy at the origin of the patellar ligament, the point of highest stress. Runner's knee: a young woman gets anterior knee pain going down stairs and after long sitting; the patella is tracking slightly laterally in its groove, and the cartilage of the lateral facet is taking loads it was not designed for (patellofemoral pain, and in time chondromalacia patellae — softening and fibrillation of that cartilage). The motorcyclist: a femoral shaft fracture from a high-speed impact bleeds 1 to 1.5 litres into the thigh from the nutrient and perforating vessels, with no external wound to warn you — two femoral shafts can hold enough of a person's blood volume to cause shock, and the thigh will look merely swollen while the blood pressure quietly falls.
- The femur is the longest, heaviest and strongest bone in the body; its internal trabeculae follow compression and tension lines, with a weak zone between them.
- Upper end: head with the fovea capitis for the ligamentum teres; neck at a neck–shaft angle of about 125° (coxa vara if less, coxa valga if more).
- Greater trochanter = gluteus medius/minimus, piriformis, obturator internus, gemelli. Lesser trochanter = iliopsoas.
- Intertrochanteric LINE in front (capsule + iliofemoral ligament), intertrochanteric CREST behind — the landmark that defines intracapsular vs extracapsular fractures.
- Shaft: the linea aspera with medial and lateral lips, continuing up as the gluteal tuberosity and pectineal line, and down as the supracondylar lines around the popliteal surface.
- Lower end: medial and lateral condyles and epicondyles, the adductor tubercle, the intercondylar fossa (cruciate attachments) and the anterior patellar surface.
- The femoral head is supplied mainly by RETINACULAR branches of the MEDIAL CIRCUMFLEX FEMORAL artery, which climb the neck retrograde; the ligamentum teres artery adds little in adults.
- Intracapsular neck fracture → torn retinacular vessels → risk of avascular necrosis → usually replace the head; extracapsular (intertrochanteric) → good blood supply → fix it.
- Classic hip-fracture posture: a SHORTENED and EXTERNALLY ROTATED limb, produced by the unopposed pull of the flexors, adductors and lateral rotators on the free shaft.
- The patella is the largest sesamoid, lying in the quadriceps tendon: base above, apex below (giving the patellar LIGAMENT to the tibial tuberosity), with medial and lateral posterior facets.
- Its job is mechanical: it lifts the quadriceps tendon off the femur, increasing the extensor lever arm by about a third — most useful in the last 30° of extension — and shields the joint.
- It dislocates LATERALLY, resisted by vastus medialis obliquus and the prominent lateral lip of the femoral trochlea; the Q-angle (larger in women) is what pulls it that way.
- Calling the structure below the kneecap the "patellar tendon". It runs from the patella (bone) to the tibial tuberosity (bone), so it is properly the patellar LIGAMENT — the quadriceps tendon is the part above the patella.
- Treating every "broken hip" as one injury. Intracapsular and extracapsular fractures differ in blood supply, in healing and in the operation performed; the capsule's attachment is the dividing line.
- Assuming the ligamentum teres keeps the adult femoral head alive. Its artery matters in childhood; in adults it is a minor contributor, which is exactly why a displaced neck fracture is so dangerous.
An 80-year-old woman falls and presents with a shortened, externally rotated right leg. Imaging shows a displaced subcapital fracture of the femoral neck. Which vessels are most likely disrupted, putting the femoral head at risk of avascular necrosis?
- The femur is the longest, heaviest and strongest bone: head (with fovea capitis), neck at ~125°, greater and lesser trochanters joined by the intertrochanteric line in front and crest behind.
- The shaft carries the linea aspera (medial and lateral lips) with the gluteal tuberosity and pectineal line above and the supracondylar lines and popliteal surface below; distally, the condyles, epicondyles, adductor tubercle, intercondylar fossa and patellar surface.
- The head's blood climbs the neck retrograde in the retinacular branches of the medial circumflex femoral artery, so an INTRACAPSULAR neck fracture risks avascular necrosis and is managed differently from an extracapsular one.
- The patella is the body's largest sesamoid: it lengthens the quadriceps lever arm by about a third and shields the joint, and it dislocates laterally unless vastus medialis obliquus and the prominent lateral trochlear lip hold it in its groove.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Lower limb: the femur and patella.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Femoral neck fractures and the blood supply of the femoral head.
- Netter FH. Atlas of Human Anatomy — Bones of the thigh and knee.
- Last RJ. Last's Anatomy: Regional and Applied — The thigh and knee region.
- Snell RS. Clinical Anatomy by Regions — The lower limb: bones and clinical notes.
- TeachMeAnatomy — The Femur; The Patella.
