Structure and function: The anatomy of the knee joint
Doctors call the knee joint the articulatio genus. It is one of the largest and strongest joints in the human body. Thanks to its sophisticated design as a rotating hinge joint, the knee is capable of withstanding all loads, large and small, each and every day. We can bend the knee up to 150° (flexion) and hyperextend it up to 10° (extension). It can also be easily swivelled inwards and outwards. Ligaments, tendons and muscles give it stability.
These bones make up the knee joint
The knee joint is essentially made up of three bones: The thigh bone (femur), the shinbone (tibia), and the kneecap (patella). The joint head on the femur has two strong bony projections, the condyles, which are covered with articular cartilage. The articular surface of the tibia is called the tibial plateau.
Two articulations form the knee: The hinge joint between the thigh and shin (tibiofemoral) and the femoropatellar joint between the kneecap and thigh (patellofemoral).
The menisci (outer and inner meniscus) are made of fibrous cartilage and form the connection between the thigh bone and the shin bone. They also provide better force distribution in the knee. The inner meniscus is fused with the inner ligament.
Supporting the knee joint: ligaments, tendons and muscles
The knee joint is held in place by a prominent ligamentous apparatus. The ligaments prevent the bony structures from rubbing against each other too much.
- Collateral ligaments: The collateral ligaments stabilise the knee mainly against bending stresses in the frontal plane. The ligaments therefore prevent the knee from twisting sideways.
- Cruciate ligaments: The anterior and posterior cruciate ligaments are found inside the knee joint. These two ligaments stabilise the knee against various movements of the tibia in the sagittal plane: The anterior cruciate ligament prevents the tibia from sliding forward (anterior drawer). Similarly, the posterior cruciate ligament prevents the tibia from sliding backwards (posterior drawer).
Synovial bursa in the knee: Cushioning for bones, muscles and tendons
The knee also contains bursae that protect the joint from rubbing and from pressure during movements of the knee. They are found above the kneecap, in front of and behind the patellar tendon. Bursae are slidable, elastic, fluid-filled "pads" that can become inflamed. An inflamed bursa – or ‘bursitis’ – can be very painful.
A rather special bone: The kneecap – the largest human sesamoid bone
The kneecap – also known as the patella – is the largest human sesamoid bone. It is fused with the tendon structure that runs from the thigh muscle to the shin bone. The section above the kneecap is called the quadriceps tendon, and the section below the kneecap is called the patellar tendon.
In a sense, the patella acts as a "spacer" between the tendon and the joint, allowing greater leverage and less effort to extend the knee. Moreover, the patella – like all sesamoid bones – prevents the tendons from being damaged by compressive stress when moving back and forth over the joints.
By the way: If we had no kneecaps, incidentally, our thigh muscles would probably be much bigger and we would probably not be able to walk.
Diseases and afflictions of the knee joint
The application of strong force, for example an impact with a hard surface, can cause fractures of the bony components of the knee, such as a patella fracture.
Wear and inflammation in the knee can lead to pain. A few examples:
- Osteoarthritis (knee arthrosis)
- Patellofemoral pain syndrome (pain in the front of the knee)
- Pes anserinus syndrome (overloading of the pes anserinus tendon attachment)
- Patellar tendonitis (jumper’s knee = overloading of the patella attachment)
- Osgood-Schlatter disease (necrosis of the bone where the patellar tendon attaches to the top of the shinbone)