Know your knee

Four bones, four ligaments, two kinds of cartilage.

The knee is a hinge that also rotates. That is why it is easy to injure. Each structure below, what it does, and what goes wrong with it.

Diagram Bones Ligaments Cartilage When to seek help

Front view of the knee. Not to scale.

The key

Start with the map

Select a structure to jump to it. Bones are cream, ligaments gold, cartilage green.

Structure one

Bones

Four bones meet at or near the knee. Femur and tibia form the joint. The patella glides across the front. The fibula anchors soft tissue on the outside.

Bone

Femur

Thigh bone. Its lower end forms the top half of the joint.

What can go wrong

Fracture. High force injury when young. Low force fall when bone is thin.
Osteochondritis dissecans. A fragment of bone and cartilage loses blood supply and loosens. Mostly teenagers.
Osteonecrosis. Sudden inner side pain, often at night.
Bone bruising. Deep swelling after impact. Settles over weeks.

Bone

Tibia

Shin bone. Its flat top, the tibial plateau, carries your body weight.

What can go wrong

Plateau fracture. The top surface splits or crushes. Usually needs fixing.
Stress fracture. Hairline crack from repeated loading. Common when mileage climbs fast.
Osgood-Schlatter disease. Painful bump below the kneecap in growing teenagers.
Bone marrow lesions. Swollen bone under worn cartilage. A common source of arthritis pain.

Bone

Patella

Kneecap. Runs in a groove on the femur and levers the thigh muscle.

What can go wrong

Dislocation. Slips to the outside. Often relocates as the knee straightens.
Maltracking. Runs off centre. Pain on stairs and after sitting.
Fracture. Direct fall onto the front of the knee.
Chondromalacia. Softened cartilage behind the kneecap. Grinding and aching.

Bone

Fibula

Slim outer bone. Carries little weight. Anchors the LCL and a hamstring tendon.

What can go wrong

Head fracture. Usually alongside serious ligament damage on the outer side.
Nerve injury. The peroneal nerve wraps the bone. Numbness or a dropped foot.
Stress fracture. Uncommon. Linked to repetitive impact sport.

Structure two

Ligaments

Ligaments join bone to bone. Four control the knee: two cross inside the joint, two sit on either side. Injuries grade one to three. Stretched, partly torn, fully torn.

Ligament

Anterior cruciate (ACL)

Crosses the centre of the joint. Stops the tibia sliding forwards and controls twisting.

What can go wrong

Cause. A pivot, a sudden stop, an awkward landing.
The pop. Many people hear or feel one at the moment of injury.
Swelling. The joint fills with blood within hours.
Giving way. Instability on turning, weeks later.
Rarely alone. Meniscal tears and bone bruising often happen with it.

Ligament

Posterior cruciate (PCL)

The thicker central ligament. Stops the tibia sliding backwards.

What can go wrong

Cause. A blow to the front of the bent knee. Dashboards, or falling onto the knee.
Easy to miss. Aching and stiffness rather than clear instability.
Late effect. Long standing laxity loads the front of the knee.
Outlook. Isolated tears usually do well without surgery.

Ligament

Medial collateral (MCL)

Broad band on the inner side. Resists forces pushing the knee inwards.

What can go wrong

Cause. A blow to the outside of the knee, or a caught ski edge.
Pain site. Along the inner line, not deep in the joint.
Healing. Good blood supply. A brace and physiotherapy are usually enough.
Risk. Scarring limits bending if movement starts too late.

Ligament

Lateral collateral (LCL)

Cord on the outer side, femur to fibula. Resists forces pushing the knee outwards.

What can go wrong

Cause. A blow to the inner side. Less common than MCL injury.
Rarely alone. Often part of wider posterolateral corner damage.
Nerve symptoms. Numbness or a dropped foot, from the peroneal nerve nearby.
Untreated. The knee opens up on uneven ground.

Structure three

Cartilage

Two types. Articular cartilage coats the bone ends. The menisci are separate wedges between them. Neither has much blood supply, so both heal slowly.

Cartilage

Articular cartilage

A few millimetres thick, covering the femur, tibia and back of the patella. Near frictionless.

What can go wrong

Osteoarthritis. Gradual thinning. Stiffness, aching, less movement.
Focal defect. A single pothole, often after injury. Catching and swelling.
No repair. There is no blood supply, so damage does not heal.
Silent early. No nerve supply either, so first losses go unnoticed.
Crepitus. Grating as roughened surfaces pass over each other.

Cartilage

Meniscus

Two C-shaped wedges of fibrocartilage. They spread load, absorb shock and add stability.

What can go wrong

Acute tear. A twist on a planted foot. Joint line pain, clicking, swelling over a day.
Degenerate tear. Wear related splitting after about forty. Often no injury at all.
Locking. A displaced flap blocks full straightening. Needs prompt assessment.
Root tear. Detaches at its anchor and slips out of the joint. Speeds up arthritis.
Repair odds. Only the outer third has blood supply and can be stitched.

Do not wait

When to seek help the same day

Most knee pain can wait. These signs cannot.

Hot, red and swollen, with or without a fever.
You cannot put weight through the leg.
The knee is locked and will not straighten.
The leg looks bent or out of line after a fall.
It swelled within an hour of an injury.
Numbness, pins and needles, or a cold foot.

Put a number on your knee

Anatomy tells you what is there. The test tells you how well it works, and what to do next.

Start the test

Important. General education, not a diagnosis. Knee problems often involve more than one structure, and symptoms overlap. See a doctor or physiotherapist about your own knee.