Finally, we will consider some of the common causes of lateral knee pain. You may remember from the last blog that lateral means 'away from the mid-line of the body', so this is pain on the outside of the knee.
Many of the structures on the outside of the knee are similar to those on the inside, so they do not need much further consideration here (if you haven't yet, I strongly recommend that you read the first few posts about the knee, as they provide some much-needed background and context) These include:
Lateral meniscal injuries e.g. tears
| As mentioned in the last blog, medial (inside of the knee) tears are significantly more common, but people can (and do) tear their lateral meniscus. Again, the likelihood of it healing on its own depends on where the tear is. Lateral meniscal tears will, like their cousins on the other side of the knee, give pain, swelling and stiffness, The knee may catch, click, or lock, and you will probably have difficulty in straightening your leg. |  The sharper among you may have noticed that this is a diagram of a lesion of the MEDIAL, or inside, meniscus, not the lateral! I made this picture for the last blog and didn't end up using it; and it still shows the lateral meniscus on the right! |
 Image from Summit Medical Group | Lateral Collateral Ligment (LCL) Injuries These may be caused by a blow to the inside of the knee (stretching the ligament on the outside), such as a sliding tackle in football (sorry, soccer), or a strike or sweep in martial arts. It is worth noting, that because the LCL is less vulnerable to injury than the MCL, injuries that do occur to the lateral collateral ligament tend to involve greater force, so are often associated with injuries to other structures whose job it is to stabilise the knee, such as the cruciate ligaments. In the image on the left, please note that the LCL attaches to the fibula, the bone running up the side of the leg. It also attaches into biceps femoris, the outermost hamstring muscle. |
Lateral compartment osteoarthritis
| Unhappily, it is entirely possible to get arthritis in both the outer and inner compartments of the knee, although it is less common in the lateral. In the image on the right, the red marker shows a degenerative lateral compartment. Note the lack of space between the bones, compared to the blue area on the right. As x-rays only show hard things like bone, the space tells us there is cartilage there helping to absorb shock and reduce friction. Please note that having a scan like the one on the right does not necessarily mean you feel pain. |  |
Patellofemoral Pain Syndrome (PFS)
We touched on this in the first common knee conditions blog, talking about anterior or front knee pain where the kneecap does not align properly with the trochlear (meaning 'pulley' in Latin) groove in between the two knobbly bits on the bottom of the thighbone.
Because it is more common that the outer thigh muscles are tighter, as well as the contribution from the iliotibial band (see below), the patella will often track to the lateral side, potentially giving you more pain on the outside of the knee.
Iliotibial Band Syndrome
It extends down the lateral (outside) thigh across the knee, and attaches to the kneecap, tibia (shinbone) and biceps femoris (hamstring, see above) tendon.
 | This is the big one. Particularly in active people, iliotibial band syndrome is one of the most (if not the most), common causes for outside knee pain. The iliotibial band (or 'tract', in older texts) or ITB for short, is a thick band of fascia, or connective tissue, that runs down the outside of the thigh. It is formed at the top by a merging of fascia from a lot of the muscles in the area, but principally gluteus maximus, the large muscle that gives the buttock its shape; and tensor fascia lata. If I told you that 'fascia lata' was another old name for the ITB, you might guess that the job of that muscle is to 'tense' the fascia lata, or ITB. |
| Biceps femoris is the name of the hamstring muscle that goes down the outside of the back of the thigh, The hamstrings (like most muscles) have several jobs. They bend the knee (flexion) and take the hip backwards (extension). They slow the leg down towards the end of the movement while we are kicking or running, to help protect the knee. The lateral collateral ligament also has an attachment into the tendon of biceps femoris, so there is another complicating factor. Repetitive strain of the tendon of muscle can cause pain, swelling and stiffness on the outside of the knee. |  |
Of the muscles whose fascia connects into the ITB, some lift the hip backwards (extension), some out to the side (abduction), and some forwards (flexion). Depending on the portions of the muscle, and the direction of the fibres, they may also rotate the hip inwards or outwards. As you might imagine, this leads to a lot of different forces acting on the ITB.
Repetitive bending and straightening of the knee can cause excessive friction as the ITB rubs over the knobbly bit of bone on the bottom of the thighbone called the lateral femoral condyle. The ITB contacts this bone at around 30 degrees of flexion (bending), so the pain will often be worst in this position. Sufferers of this condition often find themselves walking with a stiff leg to avoid this bending movement.
Hamstring (biceps femoris) injuries
 | Popliteus tendonitis Popliteus is a small muscle that runs across the back of the knee, its tendon attaching just above it on the outside, near where the LCL starts. In downhill running, popliteus and its tendon help to stop the thighbone from sliding forwards on the top of the shinbone, so may be irritated by excessive or unaccustomed downhill running or speed work. There will likely be some swelling, and tenderness along the back border of the femur where it attaches, and the pain will be aggravated by trying to control your speed as you run downhill. Luckily popliteus tendonitis is relatively rare compared to some of the other conditions we are discussing (Petsche & Selesnick 2002) |
References
Petsche TS, Selesnick FH. Popliteus tendinitis: tips for diagnosis and management. Phys Sportsmed. 2002;30(8):27–31
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