Here I will explain the basic knee joint anatomy for runners. I think there is enough information here for most amateur runners to understand a little more about the structures of the knee joint and how injuries can occur.
Knee Joint Anatomy: The Bones
The knee joint itself is a hinge joint formed between the Femur (thigh bone) and the Tibia (shin bone). Hinge joints typically only allow flexion and extension (bending and straightening) movements, although there is an element of rotation available too.
There is a secondary joint at the knee as well, which is the Patellofemoral joint – formed between the Patella (kneecap) and the Femur. The Patella slides up and down on the front of the joint (within the trochlea groove) when the knee is bent and straightened.
Injuries to the bones within the knee joint are pretty rare in runners unless due to a fall or other impact. Stress fractures occasionally occur but these tend to be either in the shaft or neck of the Femur (higher than the knee) or in the mid-Tibia.
Patella dislocations do sometimes occur, but again are not that frequent in runners unless you twist or hyperextend the knee by accident.
Knee Joint Anatomy: The Joints
As with all Synovial joints the knee joint is surrounded by a capsule and the ends of the bones within the joint are covered in hyaline cartilage which helps with smooth movement of the joint.
Within the knee joint there are two additional rings of cartilage known as the Meniscus (medial and lateral). The Meniscus sit on top of the flat upper surface of the Tibia and their role is shock absorption and cushioning of the knee joint. The meniscus can be injured through rotation and hyperextension forces at the knee and so again this is not that common in runners as a ‘running specific’ injury. The Meniscus can also be the victim of degenerative changes which can result in spall tears through repeat wear and this can be more common in older runners.
The knee joint has four main ligaments which are responsible for its stability. There are the:
- Anterior Cruciate Ligament – probably the most famous from an injury point of view, but not for runners as it tends to be torn through twisting/hyperextension or forceful impacts. The ACL is found deep inside the knee joint.
- Posterior Cruciate Ligament – less well known but can still be injured with forceful movements. Is less important for knee stability than the ACL. The PCL is also found deep inside the joint.
- Medial Collateral Ligament – found on the inner side of the knee joint, connecting the Femur and Tibia. The MCL is injured through forces on the outside of the knee or pushing the lower leg outwards.
- Lateral Collateral Ligament – found on the outer side of the knee joint this is the least commonly injured ligament. Forces on the inside of the knee or pushing the lower leg inwards can damage the LCL.
Knee Joint Anatomy: The Muscles
There are a number of muscles which affect the knee joints. Here are the basics:
The Quadriceps (quads) – these four muscles are found at the front of the thigh and their role is to straighten the knee. All four cross the knee joint as one (they converge together just above the kneecap) and insert via the Patella tendon to the Tibia.
The Hamstrings – There are three hamstring muscles which are located at the back of the thigh. They have two roles – one is to extend the hip joint and the other is to flex the knee. All three cross the knee joint, two medially (Semimembranosus & Semitendinosus) and one (Biceps Femoris) laterally.
The Adductors – There are two adductor (or groin) muscles which cross the knee joint – the Adductor Magnus and Gracilis. These two muscles act on move the leg inwards when the knee is straight.
Gastrocnemius – Gastrocnemius is the biggest muscle found in the calf and functions to plantarflex the ankle (point the foot down or lift the heel off the ground). It does actually cross the knee joint on both sides of the popliteal fossa and so also has a weak role in assisting knee flexion.
Popliteus – A relatively small muscle found at the back of the knee joint. It attaches to the lateral femoral condyle and femoral head and crosses diagonally across the back of the knee and down to the medial Tibia. It’s role is quite complex but basically it helps to rotate the Tibia and unlock the knee joint when fully extended. This muscle can become troublesome in runners – see Popliteus injury.
Knee Joint Anatomy: Other Structures
IT Band – The IT band is like a big tendon which courses all the way down the lateral thigh (from the lateral pelvis, TFL and Glute Max muscles) and inserts into the lateral Tibia, just below and to the outside of the patella. It does also pass fibres medially towards the kneecap which become part of the lateral retinaculum (important in IT band syndrome and Patellofemoral Pain Syndrome). It plays such an important role in knee joint stability and injury, especially in runners.
Bursae – Bursae (Bursa – singular) are small sacks filled with fluid which lie between bone and underlying tendons or other soft tissue. They help to reduce friction and so injury, but sometimes can become inflamed themselves.
There are several Bursae around the knee joint and some which are involved in injuries. The most prominent in runners is the Pes Anserine Bursa on the outer knee, below the level of the knee cap. See more on Pes Anserine Bursitis here. There are also bursae above (suprapatella), below (infrapatella) and on top of (prepatella) the patella.
Fat Pad – The Infrapatella fat pad – also sometimes known as Hoffa’s fat pad sits between the Patella tendon and the anterior aspects of the Tibia and Femur. It’s role is similar to cartilage – padding the joint and allowing smooth motion. It is sometimes injured after a direct impact such as landing on the knees, or from repeated forceful extension of the knee joint. This second scenario is caused fat pad impingement (or Hoffa’s syndrome) and may occasionally develop in runners.
Plicas – These are folds in the synovial lining of the joint. You may occasionally hear these come up when talking about knee injuries in runners. The repetitive motion of running can sometimes cause irritation of the medial plica to the inside of the Patella, but this condition is hard to distinguish from other injuries. Generally treatment involves resting and reducing the inflammation, followed by a rehabilitation course to strengthen the hip and knee musculature.