In this keynote we will try to clarify the anatomy of this region, and try to explain why diagnosis and surgery may not be easy even in experienced hands.
The posterolateral corner is an important region of the knee because it contributes to the stability of the knee and any damage to this region affects knee stability. Its complex anatomy makes assessment difficult and reparative and reconstructive surgery difficult also. Not only is the anatomy complex but there is considerable variation in the anatomy from person to person.
This article attempts to simplify the subject so that it is more easy to understand the literature, the tests and the issues.
The first thing to appreciate is that the anatomical structures can be considered to exist in three different layers:
The deep layer is the important one for us to understand in depth, but we will touch on the others later.
Several structures are important here -
Let's start with the first two - the popliteus muscle/tendon and the associated popliteo-fibular ligament. The popliteus muscle is always present in every knee while the ligament is present in the majority of knees, and together they play an important role in stability of this part of the knee.
You probably know that a tendon connects muscle to bone and a ligament connects bone to bone? Well the popliteus has both a tendon and a ligament component.The muscle part of the popliteus originates on the shaft of the tibia on the posteromedial side (the inner aspect of the back of the knee). At the opposite end the tendon inserts onto the lower outer side of the femur bone, attaching to the prominence known as the lateral femoral epicondyle and passing under the lateral collateral ligament is it does so.
If you look at the illustration you can see that the muscle/tendon is joined by the popliteofibular ligament, in a kind of 'Y' arrangement. The ligament originates on the head (top) of the fibula bone, and the opposite end merges with the tendon of the popliteus as it inserts on the lateral femoral epicondyle. So you can see how muscle, tendon and ligament are intimately related to one another.
The popliteofibular ligament component play a direct passive role in strengthening this region of the knee, but the muscle also plays an active role in stabilising the side of the knee and preventing it bending sideways ('going into varus') and it also has a specific active role of unlocking the knee when a person as been standing with the knee locked home. When a person is standing relaxed and with the knees straight, the knees don't give way because they 'lock home' [explain this bit further]. The popliteus muscle when it contracts can release the lock and allow normal movement again.
The popliteus has some other more subtle elements too. You can see from the illustrations how closely related the popliteus is to the lateral meniscus. Tiny fibrous slips (or 'fascicles') actually reach from the popliteus tendon to attach to the back of the lateral meniscus, and play a role in stabilising the movement of the lateral meniscus.
This intimate relationship of the popliteus tendon to the lateral meniscus means that the popliteus tendon has to actually breach the capsule at the back of the knee to enter within the capsule, and it is the only tendon in the knee to do this. During arthroscopy (keyhole surgery of the knee) the tendon can be seen inside the joint cavity on its route behind the lateral mensicus as it passes upwards to attach to the bone above.
On the left, this sketch of the menisci and the capsule of the knee (looking down on the menisci from above as if the knee were sliced open) shows the relationship of the popliteus tendon to the lateral meniscus and the capsule. As the tendon is within the joint cavity, the diagnosis of a ruptured popliteus tendon can be made via arthroscopy. The image on the right shows the knee from the front, and you can see the popliteus tendon running adjacent to the lateral meniscus at the back of the joint cavity.
Refer again to the illustration showing the capsule. The point of entrance of the popliteus tendon into the capsule gives one the position of the next of our anatomical structures of the region â€“ the arcuate ligament. The arcuate ligament is a thickening of the capsule where it 'arcs' over the popliteus from its anchoring position on the fibula. The arcuate ligament is not always present - some people do not have an identifiable arcuate ligament. But where it is present it can be torn and contribute to the acquired laxity of a postero-lateral corner injury.
Three further anatomical elements complete the list of deep posterolateral structures contributing to stability in this region -
The lateral collateral ligament (LCL) is also commonly called the 'fibular collateral' ligament. If you look back again to the image showing the capsule, you will see that the lateral collateral ligament is 'extra-capsular', ie it is outside of the capsule and not involved with it - it is a discrete pencil-like chord reaching from the lateral side of the femur to the fibula. The lateral collateral ligament is a consistent structure in the knee, and everyone has one. It acts as a simple passive restraint supporting the outside of the knee.
This is not the case at all with the fabella - which is commonly absent. The fabella is a small bone present at the back of the knee. It mirrors to some extent the patella (kneecap) at the front of the knee - both are 'sesamoid' bones, which means that they form within a muscle or tendon of a muscle. The patella forms within the tendon of the quadriceps muscle, and the fabella, when present, forms within the lateral head of the gastrocnemius muscle - one of the muscles that form the calf.
The fabella, when present, contributes to deep posterolateral corner stability as it anchors the next of our structures - the fabello-fibular ligament - which stretches from the fabella to the fibula, again outside of the capsule. This ligament is not a discrete structure like the lateral collateral ligament but blends into the network of fibrous tissue that forms a supportive network in this plane.
So that completes the list of the deep posterolateral structures - those structures that are often poorly understood by the attending clinicians.
Outside of the deep posterolateral structures are the two more superficial layers supporting the posterolateral corner. These are two sheet-like layers of passive strong supporting fibrous restraints -
The biceps femoris muscle has two parts one arising from the pelvis and the other from the femur, and they join together to form the biceps femoris tendon, more often called the biceps tendon. This tendon loops around the back of the knee and joins the other posterolateral structures where it attaches on the fibula.
Clearly the fibula is important as an 'anchor' to the many supportive structures of the postero-lateral corner - the lateral collateral ligament the popliteo-fibular ligament the arcuate ligament the fabello-fibular ligament the biceps tendon.
There is one other structure that is worth a mention in relation to the fibula and the posterolateral corner, but this is not a fibrous restraint but a nerve. Nerve tissue is delicate and injury to the posterolateral corner frequently involves injury to the nerve that passes just over the head of the fibula - the common peroneal nerve. Damage to the nerve leads to numbness and weakness of the foot - symptoms that often point to the diagnosis of damage to the fibular area and the structures of the posterolateral corner of the knee.
An appreciation of the anatomy of the region allows an appreciation of why surgery to this area may involve both arthroscopy (keyhole surgery) and open surgery - as one explores those structures within the capsule and the others explores those structures outside of the capsule. Arthroscopy is valuable in assessing damage to the popliteus tendon, the capsule, the lateral meniscus, the cruciate ligaments and also the ligament of Wrisberg - a structure that we have not discussed here. Open surgery assesses the lateral collateral ligament, the popliteo-fibular ligament, the arcuate ligament, the fabella and fabello-fibular ligament, the ilio-tibial tract and the biceps tendon.
The diagnostic tests, X-rays and MRI scans are important in alerting the surgeon to the likely damage, but even with both open and arthroscopic surgery, the complicated anatomy and the variability of the anatomy makes it a difficult task for a surgeon to assess and repair (or reconstruct) instability to the posterolateral corner. It is important that the surgeon is experienced in this field.