Conditions interfering with patellofemoral mechanics

 

In previous parts of this course - Patellofemoral Anatomy and Important Patellofemoral Concepts - I outlined relevant anatomy and some basic concepts.

In the part on - Differential Diagnosis of Patellofemoral Pain - I listed the conditions which may cause patellofemoral pain. I will refer back to that list several times during the rest of the course, and I will start here to talk about the first group of conditions - those that interfere with the mechanics of the knee. This is going to be a long section. Before I start tackling it, let me just mention 'chondomalacia' again (often mis-spelled as 'chrondomalacia'...

Just a quick word first, though. Adolescents can develop anterior knee pain for no apparent reason. There are no focal lesions, no malalignment, and no distant pathology referring pain to the knee . The pain comes from an imbalance of the various muscle groups growing and developing at different rates. The pain is treated symptomatically, and assurance can be given that the condition is likely to be self-limited.

 

Tight structures around the knee

Let's start with the first topic on our differential diagnosis list - 'tight structures around the knee'. It has long been appreciated that tightness about the knee can cause pain around the patella. Tight structures may include -

Tight structures at the side of the patella - the lateral retinaculum and the ITB - may cause the patella to tip over to one side, like a beret on someone's head. This is called 'patellar tilt'.

The key features of patellar tilt on the physical examination are a tilted patella and tenderness about the lateral facet when the examiner's fingers are curled under the knee cap.

[Reprinted with permission, The Adult Knee, Chapters 59-60, Lippincott Williams & Wilkins, 2003.]

Tilting of the knee cap is often well appreciated on a good set of x-rays (you'll learn more about this in the Imaging Lesson).

The large majority of patients with such a patella malalignment can be effectively treated with a non-operative approach. This includes a traditional program of hamstring and quadriceps stretches, muscle strengthening, anti-inflammatory medications, bracing, and activity modification. Newer concepts include taping, ITB stretching, orthotics for select patients, and muscle coordination.

We've just looked in more detail at the tight structures which can cause pain by interfering with the mechanics of the knee. Run over the list of 'tight structures' again quickly.

Then move down to remind yourself of the list of 'loose structures around the knee'.

We'll go on to discuss the loose structures now.

 

Loose structures around the knee

Loose structures may facilitate patellar subluxation and dislocation, with consequent joint surface damage and pain around the knee cap.

• Constitutional laxity - Genetic disorders, like Ehlers Danlos Syndrome, where the joints are abnormally lax ('double jointed'), may allow frequent dislocation of the patella.

• Patella alta - As you saw in the Anatomy section, the knee cap sits in a groove called the trochlea. When the knee is straight, the knee cap sits at the top edge of this trochlea, perhaps a little above it. As soon as the knee bends, the knee cap slides into the groove.
  
  Some people are born with a knee cap that sits a little high. This is called patella alta. The knee cap is a little farther up the thigh, and, more importantly a little farther from the trochlea. When the knee starts to bend, the knee cap is still not engaged in the groove. This makes it easier for the knee cap to slide off to the side. Thus patients with patella alta are prone to instability, whereby they feel the knee cap slip out.
  
  Recurrent episodes leads to damage of the joint surface, with pain around the kneecap.

 

Factors increasing the Q-angle

 

 

I'm moving on to the third group of conditions which can affect the mechanics of the knee - 'factors affecting the q-angle'. Before we get there, do you want to have another quick look at that q-angle picture we discussed in the Part II? It is an important concept.

The Q-angle is the angle formed between two lines -

• from the prominent bump on the pelvis bone above the hip through the centre of the patella
• from the prominent bump on the tibia (tibial tubercle) through the centre of the patella

[Reprinted with permission, The Adult Knee, Chapters 59-60, Lippincott Williams & Wilkins, 2003.]

It gives an idea of the angulation of femur bone to tibial tubercle, and the consequent stresses which may be contributing to bowstringing - i.e. pulling the patella to the outer (lateral) side of its groove.

I want to stress that it is the tibial tubercle, not the tibia bone itself, which forms the lower reference point. The tibial alignment can be relatively normal but the q-angle increased if the tibial tubercle is displaced too far to the side.

The existence of the Q angle (see Anatomy in Part I) causes the patella to move laterally when a person tightens their thigh muscles. When the Q angle is elevated, the patella has a greater tendency to move laterally. This can contribute to a painful patella and/or lead to an unstable patella, where the patella slips out of its groove. The following conditions affect the Q angle -

• Flat feet - The collapse of the foot's arch leads to twisting of the tibia, femur and extensor mechanism when the person takes a step.
• Lateral positioning of the tibial tuberosity - The main determinant of the Q angle is the position of the tibial tuberosity. If the tibial tuberosity is laterally positioned (too far to the right on a right knee), the Q angle by definition will be increased.
• Valgus - A knock-kneed position moves the tibial tuberosity laterally.
• Tibial rotation ('miserable malalignment') - 'Miserable malalignment' is the colloquial name given to 'complex torsional variations' (abnormal femoral and/or tibial rotation) in limb alignment from the hip down to the ankle.
  
  When the patient is examined standing the knee cap can point inwards ('squinting') or outwards ('grasshopper patella'). The hip joint is usually excessively turned out, the end of the thigh bone can twist inwards or outwards, the upper tibia is commonly twisted outwards, the tibia displays a bow-legged appearance, and finally even the ankle can be twisted outwards. And when all is said and done, one twist compensates for another from the hip all the way down to the foot until the foot points forward!

[Reprinted with permission, The Adult Knee, Chapters 59-60, Lippincott Williams & Wilkins, 2003.]

Such anatomy can be associated with pain at the front of the knee . Clearly, the knee cap is an innocent bystander in this complex malalignment. Imaging will often reveal a normally-positioned knee cap overlying a rotated femur. Fortunately, pain can often be controlled with activity modification, stretching and strengthening exercises. For the patient requiring surgery, so-called small procedures such as a lateral retinacular release are not usually successful. On the other hand, the untwisting bone cutting procedures (osteotomy) are substantial, and there are no long-term reports of their results.

Now, I am aware that you have had to take in a lot so far with this lesson on 'differential diagnosis'. The end of this page is quite a good place to take a break if you would like that.

I'll just round up about 'malalignment' which we have already mentioned several times.

Outside of overuse and simple tendinitis, malalignment is arguably the greatest source of persistent knee pain in patients between the ages of 20 and 50.

Patella malalignment is a translational or rotational deviation of the patella relative to any axis. This includes a knee cap that sits too high or too low in the trochlea, as well as an elevated Q angle, patellar tilt and 'miserable malalignment'.

You should know that the very existence of patella malalignment as a clinical entity remains a subject of dispute within the orthopedic world. Otherwise knowledgeable health professionals state that tilted knee caps are normal, and they base that argument on the fact that they often see tilted knee caps that are painless.

It is my feeling that they are confusing the term normal in the everyday sense with the term normal as it might be used in the world of medicine. 'Normal' in everyday day language means common or acceptable. In that sense, it is indeed 'normal' for a segment of the population to develop cancer or heart disease. But a medically normal condition is one that does not lead to death, disease or pain, and in that sense of the word, cancer and heart disease are clearly not normal. Kneecap malalignment is normal in the 'common' sense of the word, but not in the medical sense. An analogy here can be made with flat feet: they are common, they are not necessarily painful, but they are not normal.

Patellar malalignment appears to be related to a number of factors that appear in varying degrees. These include constitutional laxity (being loose-jointed), paradoxical tightness of the ilio-tibial band and of the lateral retinaculum (these are tight while every other joint is loose!), abnormal positioning and contractions of the VMO muscle, anatomic variations of the trochlea, a high riding knee cap, and a tibial tuberosity that is too far off to the outside (elevated Q angle).

The pain is presumed to result from excessive pressure on part of the knee cap (right side of a right knee cap, the left side of a left knee cap). However tilt itself does not completely account for the onset of pain as evidenced by the observation that not every patient with malalignment is symptomatic. Pain appears to be related to multiple other factors that have yet to be identified.

Blunt trauma to the front of the knee and overuse are two of the many potential triggering mechanisms. Nerve abnormalities have been noted in the lateral retinaculum, but it is not clear whether these are always present. As with tightness of the lateral retinaculum, it is also unclear whether the nerve abnormalities are the cause or the result of the condition.

 

Irritation of soft tissues inside the knee.

Pain in the front of the knee can arise from the soft tissues -

• Plica syndrome - A plica is a fold of joint lining inside the knee. It can become thick or inflamed and begin to rub against the femoral condyle as the knee bends and straightens.
• Fat pad syndrome - The fat pad is the lump of fatty tissue under and on the sides of the patella. It can become inflamed and painful. If the fat pad is larger than usual it can become caught ('impinged') in the machinery of the knee.
• Synovial impingement - A certain amount of synovium (joint lining) in the knee is normal. If the synovium becomes exuberant it can be pinched as the knee bends and straightens.
• Neuroma in the lateral retinaculum - Examination of the lateral retinaculum, the dense tissue connecting the knee cap to the femur, has revealed nerve changes in patients whose knee cap is 'malaligned' (not sitting right). Whether this is the cause or the result of the malalignment has not been determined.
  
  Substance P is a chemical associated with irritation and inflammation, and certain investigators believe that there exists an excessive amount of substance P in patients who demonstrate these nerve changes.
• Early rheumatological conditions - Early in their course rheumatological conditions such as rheumatoid arthritis and lupus can cause pain solely about the knee cap joint.

The pain comes from inflammation of the synovium, the inner layer of the capsule that we talked about earlier in the Anatomy lesson. The entire synovium doesn't become inflamed all at once, and in some patients it is the synovium around the knee cap that is first involved.

These patients will present to the doctor with pain at the front of their knee . (This is NOT a common presentation for rheumatoid arthritis, and, it is a rare cause of isolated knee cap pain).

 

Overuse

• Tendinopathy - Tendinopathy (more frequently called 'tendinitis') is a painful irritation of a tendon. The quadriceps and patellar tendons just above and below the knee are prone to tendinitis. The doctor can make the diagnosis by carefully palpating the tissues around your knee cap. Once again, tendinitis will not show up on x-rays or MRIs.
• Simple overuse - Simple overuse is one of the most common sources of anterior knee pain and, except in a military or institutional setting, it rarely brings the patient to the doctor. You've hiked more than usual and the next morning the front of your knee aches. You don't pay it much mind, and in a few days the pain is gone.
  
  The pain was most likely due to irritation of the tissues about your knee cap.

[NB Patellar tendinitis - below the knee cap. Quadriceps tendinitis - above the knee cap]

X-rays in this setting are unremarkable, as is the physical examination. The pain is self-limited. Treatment consists of rest, activity modification, simple pain medication, the application of heat and/or cold, and the gradual resumption of regular activities. Unless the pain persists unabated for more than two weeks or so, the doctor need not order more than a good set of x-rays.

If overuse is pushed to yet a higher level, a stress fracture can ensue. As you will see, however, stress fractures of the knee cap are decidedly uncommon.

 

Deformed bones

It is uncommon for pain around the knee cap ('peri-patellar pain') to be caused by a problem actually originating in the knee cap itself. However, it is important to be aware of this -

• Trochlear dysplasia - The trochlea is normally shaped like a trough. In a person with dysplasia, the trochlea may be flat or even convex.
• Patellar dysplasia - The patella can also feature a number of unusual shapes, including the 'pebble' patella and the 'Hunter's cap'.
• Bi-partite patella - The knee cap is one solid bone. On occasion, however, the knee cap can consist of two bones welded together ('bi-partite' - two parts). Or at least partly welded. The separation between the bones is visible on an x-ray. The problem is that it can appear to be a fracture (break)! One way to differentiate between a fracture and a bipartite patella is to x-ray the other knee! A bipartite patella is usually present in both knees.
  
  To make things more confusing, on occasion the weld between the two bones of a bipartite patella can weaken and become painful. So even though it's not a fracture, it can still be painful!
• Dorsal defect - A dorsal defect is a rare, benign (not serious) condition that classically looks like a 'hole in the bone' on an x-ray. Certain infections can look the same as this on an x-ray, and it's of course the doctor's job to differentiate between the two.

 

Stressed Bones

The patella and tibial tubercle may also lead to pain in these circumstances -

• Stress fracture of patella - A stress fracture is a fracture that develops slowly over time, as opposed to the usual fracture that results from a specific injury.
  
  Relatively few cases of stress fracture of the patella have been reported. It remains open to discussion whether these heal readily or not.
  
  Plain x-rays can remain normal for an extended period of time, an MRI or bone scan being required to make the diagnosis.
  
  The limited literature suggests that recognition and immobilization of the condition within a few weeks of onset leads to uneventful healing, while delays in recognition increase the need for surgery.
• Traumatic fracture of patella - Sudden trauma such as falling down or being struck with a hard object can break the kneecap.
• Osgood Schlatters' Condition - This is an inflammation of the tibial tuberosity, the bump that we all have at the top of our shin, just below the knee . This is where the extensor mechanism, of which the patella is a part, attaches to the tibia bone.
  
  The condition typically occurs in the early teenage years. Tenderness and swelling about the tibial tuberosity are the hallmarks of the condition.
  
  The condition usually resolves on its own. Teenagers and their parents are rarely content to let the condition run its course, however, and it is reasonable to institute a program of physical therapy along with a short course of anti-inflammatory medication and bracing.

This concludes this section on those conditions interfering with the mechanics of the knee, and which lead to pain around the knee cap. In Part IVb I will discuss those conditions which result in pain but do not actually interfere with the mechanics of the knee.

 

---

PREVIOUS PART: Differential diagnosis of patellofemoral pain

NEXT PART: Conditions causing patellofemoral pain but NOT interfering with the mechanics of the knee