The Knee

This article is not an exhaustive anatomical analysis of the knee, rather it focuses on some key anatomical concepts of the knee, which will be relevant to healthcare professionals

Osteology

The first thing to clarify about the knee joint, is that it is actually two joints, an articulation between the condyles of the femur and the tibial plateau (Tibiofemoral joint) and the anterior aspect of the condyles of the femur and the patella (Patellofemoral joint), see Figure 1. Posteriorly, between the condyles of the femur (figure 2) is the intercondylar notch and on the tibial plateau in the middle is the intercondylar eminence and the anterior and posterior intercondylar areas respectively (Figure 3).

ant knee

Figure 1: Anterior view of the knee, taken from Atlas of Anatomy

distal axial view of knee

Figure 2: Axial view of the distal end of the knee, taken from Atlas of Anatomy

tibial plateau anatomy 2

Figure 3: Axial view of the tibial plateau and the head of the fibula, taken from Atlas of Anatomy

An important thing to appreciate are the contours of the surfaces of the condyles of the femur and the tibia. Next time you are in the anatomy lab take a look. You will find the femoral condyles are convex and smooth, essential for rolling forwards on the tibial condyles during extension. Now look at the tibial plateau, in particular the anterior surfaces of each tibial condyle. The anterior aspect of the medial tibial condyle slopes up, while the anterior aspect of the lateral tibial condyle is either flat or slopes down. The importance of this will become clear later.

The fibula is not part of the joint, rather the fibular head articulates with the lateral aspect of the tibia (Figure 1 & 3). Remember the fibula is a non weight bearing bone, which is the point of origin for various muscles of the lateral compartment of the leg. Also, remember that the common fibular nerve wraps around the neck of fibula; where it is susceptible to compression and damage in proximal fractures of the fibula.

The patella develops with in the quadriceps tendon. The patella is the largest sesamoid bone in the human body, it articulates with the anterior surface of the femoral condyles. Occasionally the patella may be appear as two fragments, sometimes confused for a fractured patella, this is known as a bipartite patella, this is usually due to failure of fusion during development.

The knee joint is a typical synovial joint, of the hinge type. The joint allows extension and flexion of the knee, but also a small degree of external and internal rotation, which we will cover later on. Typical synovial joints are those in which the articular surfaces are covered with hyaline cartilage (as opposed to fibrocartilage, found in atypical synovial joints) and the joint is encapsulated in a fibrous capsule, with an inner synovial layer (the synovium), which secretes synovial fluid. The synovial capsule of the knee is interesting, in that it is actually two compartments, united anteriorly. Therefore the intercondylar eminence is not within the capsule, this is best understood by looking at Figure 4.

tibial plateau anatomy

Figure 4: Tibial plateau anatomy, the dotted line represents the synovial capsule. Image adapted from Clinical Anatomy, Snell.

Movements

Flexion and extension are the obvious movements of the knee; however, is it the rotational movements of the knee that are crucial for unlocking and locking the knee joint. When standing, the knee in full extension is locked with respect to the articulating surfaces; this permits keeping the knee joint stable without the inefficient use of constant muscle tension to achieve the same effect. The locking of the knee requires no additional muscle action, unlike unlocking, where the popletius muscle pulls the lateral meniscus out of the way to allow the joint to unlock. Let’s look at both of these processes in more detail.

During locking, with the foot on the floor, the femur rotates medially on the tibia. With the foot off the floor, the tibia rotates laterally on the femur (producing the same relative movement as the foot on the floor). This medial movement of the femur occurs because the anterior part of the medial tibial condyle is sloped up, limiting movement of the medial femoral condyle sooner than the lateral condyle of the femur on the lateral condyle of the tibia, where the anterior part of the lateral tibial condyle is sloped downwards or flat. Thus as the knee is extended, both femoral condyles begin rolling forward on their respective tibial condyles. As we approach full extension, the medial condyle of the femur reaches the end of it’s movement first, the lateral femoral condyle continues to move, but now in a medial direction, locking the knee once the end of the range of movement is reached between the lateral condyle of the femur, the lateral meniscus and the lateral condyle of the tibial plateau. In the locked state, all the ligaments are pulled taut, giving the knee its stability in extension.

To unlock the knee, the femur must rotate laterally first, however to achieve this movement requires the lateral meniscus to be pulled out of the way, so that the lateral condyle of the femur can start to move. Popliteus, pulls the lateral meniscus out of the way, it does this by virtue of its tendon piercing the popliteal fascia and being attached to the lateral meniscus. Once in the flexed position the ligaments are no longer held taut. It is the locking mechanism that makes engineering a knee prosthesis difficult.

Bursae

The word bursa comes from the Latin for bag or purse; bursae (plural) consist of small fibrous sacs, with an internal lining that secretes and fills the bursae with a viscous fluid. Their main function appears to aid friction less movement, between bone and either tendon, ligament or skin.

The knee has multiple bursae (about 11), I have focused mainly on the anterior bursae and one posterior bursa, which most commonly present in conditions affecting the knee and hence are clinically relevant. The suprapatellar bursa, which actually communicates with the synovial capsule and sits between the femur and quadriceps femoris, allowing the quadriceps tendon to slide smoothly over the femur during movement. The deep infrapatellar bursa sits between the tibia and the quadriceps ligament. The superficial infrapatellar bursa sits between the quadriceps ligament and the skin. The prepatellar bursae, as the name suggests is anterior to the patella and is sandwiched between the patella and skin, it permits smooth movement of the skin over the patella.

The semimembranosus bursa is located between the semimembranosus muscle and the medial head of gastrocnemius. This bursa may communicate with a deeper bursa, which does communicate with the joint capsule. In conditions where excessive synovial fluid is produced the bursa may enlarge and herniate between the medial head of gastrocnemius and the tendon of semimembranosus, this is known as a Baker’s cyst. If it bursts and leaks fluid, this can spread into the calf causing symptoms and signs similar to a deep vein thrombosis. Obviously, a deep vein thrombosis should be excluded first!

Repetitive and excessive trauma can cause a bursa to become inflamed, presenting with swelling, erythema and pain around the knee; known as bursitis. Inflammation of the prepatellar bursa, usually due to a person spending excessive amount of time on their knees causes housemaid bursitis (prepatellar bursitis). The superficial infrapatellar bursa may also become inflamed in those who spend a lot of time genuflecting, a position associated with clergyman and plumbers, hence the name Clergyman’s or Plumbers’ knee.

Menisci and ligaments

The menisci (singular: meniscus) are two C shaped fibrocartilaginous discs which act as shock absorbers between the condyles of the femur and the tibia. Both menisci are with within the synovial cavity. The medial meniscus is significantly larger and less mobile (as it is attached to the medial collateral ligament) than the lateral meniscus, see Figure 3. Hence due to its lack of mobility, the medial meniscus is more commonly injured (though when the anterior cruciate ligament is injured then the lateral meniscus is more commonly injured – explained later), resulting in joint line pain, tenderness and if a bucket handle shaped tear occurs, then this may trap itself in the joint causing the knee joint to lock.

The ligaments (see Figure 5) can be thought of straps that connect different parts of the joint and prevent excessive movement which may cause disruption of the joint and normal function. Both cruciate ligaments are intra-capsular (but extra-synovial) as opposed to the collateral ligaments, which are extra-capsular.

There are two cruciate ligaments, which as their name suggests, cross one another to form a cross in the sagittal plane. The cruciate ligaments are named with respect to where they insert on the tibial plateau. The anterior cruciate ligament (ACL) is attached to the anterior intercondylar space, the posterior cruciate ligament (PCL) to the posterior intercondylar space.

ligaments of the knee

Figure 5: Anterior view of the knee ligaments, with the quadriceps tendon divided and reflected, adapted from Atlas of Anatomy.

ligaments of the knee posterior

Figure 6: Posterior view of the knee ligaments, adapted from Atlas of Anatomy.

The ACL originates on the medial aspect of the lateral condyle of the femur as two bands and insert on to the anterior intercondylar area of the tibia and blends with the anterior horn of the lateral meniscus. The ACL is the strongest ligament when the knee is at 90 degrees and also the most commonly injured. Damage to the ACL is a significant injury, causing the patient’s knee to give out.

The posterior cruciate ligament (PCL) originates on the lateral aspect of the medial condyle and extends posteriorly inserting at the middle of the posterior edge of the tibial plateau (posterior intercondylar area). The PCL prevent the femur from displacing anteriorly off the tibia and in the flexed position prevents the tibia from displacing posteriorly. The PCL ruptures less commonly and when it does, tends to cause less issues than rupture of the ACL.

There are four extra-capsular ligaments, the patellar ligament, oblique popliteal ligament and the two collateral ligaments, see Figures 5 & 6. The medial collateral ligament (MCL) is located on the medial aspect of the knee and prevents lateral displacement of the tibia on the femur. The ligament extends from the medial aspect of the medial condyle and attaches to the medial aspect of the tibial shaft, as it passes the joint downwards, it is also firmly attached to the medial meniscus. Hence damage to the medial collateral ligament, may result in damage to the medial meniscus.

The lateral collateral ligament is more cord like and is distinct from the capsule of the knee and is not attached to its corresponding meniscus. The ligament originates on the the lateral epicondyle of the femur and attaches to the head of the fibula, blending with the biceps femoris tendon.

retinaculum

Figure 7: Anterior view of the knee ligaments and retinacula, adapted from Atlas of Anatomy.

The patellar ligament can be thought of as a continuation of the quadriceps tendon, containing the patella itself, it attaches to the tibia at the tibial tuberosity. The patella functions to increase the leverage of the quadriceps during extension of the knee.  During flexion, the patella is at risk of lateral dislocation, but is normally stabilized by the fibers of vastus medialis, retinacular fibers and the prominence of the lateral femoral condyle (Figure 7).

Vascular Supply

The popliteal artery starts after the femoral artery leaves the adductor hiatus. The artery enters the popliteal fossa – the region behind the knee joint. In this region the popliteal artery is closest to the femur and therefore the deepest structure in the popliteal fossa. As the the artery exits the adductor hiatus, it gives off the medial and lateral superior genicular arteries (see Figure 8). The popliteal artery continues to descend in the popliteal fossa, giving off the medial and lateral inferior genicular arteries. Around on the ventral surface, these genicular branches form an anastomotic ring – see Figure 8. The anastomosis is reinforced by the descending genicular artery, a branch of the femoral artery, the descending branch of the lateral femoral circumflex artery and the anterior tibial recurrent artery from the anterior tibial artery. A fracture of the distal third of the femur can critically impair flow through the popliteal artery, this is due to the pull of the heads of the gastrocnemius causing rotation of the distal fractured fragment backwards, compressing the popliteal artery.

posterior view knee blood supply

Figure 8: Posterior view of knee showing blood supply, adapted from Atlas of Anatomy

knee blood supply anterior.png

Figure 9: Anterior view of knee blood supply, adapted from Atlas of Anatomy

Nerve Supply

Essentially this is the same as the hip joint, hence why pain in either joint, should make the physician assess the other joint too. Remember Hilton’s law, a joint is innervated by the same nerves that innervate the muscle that act on that joint. In this case muscles from all 3 compartments of the thigh act on the knee. Therefore the nerve supply will include the femoral, obturator and sciatic (Tibial and Common fibular nerves).

The Unhappy Triad

unhappy

Figure 10: Image showing exaggerated effect of injury to the knee from a lateral impact to the knee. Taken from Instant Anatomy

Understanding which ligaments are attached to which structures can help us to appreciate what other structures may be damaged in relation to a specific injury. For example, imagine a rugby tackle, where the person being tackled is running, their knee partially flexed (see Figure 10) and the joint being struck from its lateral aspect. This would cause the tibia to abduct from the femur, disrupting the MCL. As discussed the MCL is firmly attached to the medial meniscus, therefore one can expect the medial meniscus to also be damaged. Due to the degree of abduction and or lateral rotational force, the anterior cruciate is also injured, causing a condition known as a blown knee or the Unhappy triad. Research has shown that interestingly the medial meniscus is not commonly injured, instead the lateral meniscus is more commonly injured in this type of trauma. This may well be due to the fact, that the ACL also fuses with the anterior horn of the lateral meniscus, when it attaches to the anterior intercondylar area.

References:

  • Atlas of Anatomy, Anne M. Gilroy
  • Clinical Anatomy By Regions, Richard Snell
  • Clinical Anatomy, Stanley Monkhouse
  • Gray’s Anatomy: The Anatomical Basis of Clinical Practice,  Susan Standring

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