During this course we listed the Differential diagnosis of Patellofemoral Pain. You can see that this is a complex subject. In the previous section of the course we discussed those conditions that caused pain by interfering with the mechanics of the knee. In this section will look at those conditions which do not necessarily affect the mechanics of the knee.
Neuroma under the skin - A neuroma is a localized swelling and irritation of a nerve. Blunt trauma (a blow) to the front of the knee can bruise the fine sensory nerves that lie just under the skin and can lead to neuroma-like pain.
Gentle scratching of the skin with a thumbnail or squeezing of the skin is painful. The diagnosis is confirmed by relief being obtained with an injection of an anesthetic under the skin.
This is a common condition among people who bang their knee on a hard object such as the dashboard of a car or simply the ground.
Note that neuroma-like conditions will not appear on any imaging study.
Osteochondritis dissecans (OCD) - This is a somewhat mysterious condition occurring in teenagers and young adults. It affects the bone that lies at the very end of a bone (such as the femur), directly under the articular cartilage ('gristle').
The area of bone that is affected is usually small. It becomes more or less separated from the rest of the bone and is painful. It rarely occurs in the knee cap, but when it does it is not likely to heal on its own.
Surgery is likely to be recommended to keep it in position and prevent it from breaking off into the joint.
Tumors - The patella is not a common site for tumors, but a number of benign and malignant neoplasms (cancers) have been reported.
Infection - Osteomyelitis (infection of the bone) in adults is relatively uncommon, and this is particularly true of the patella.
If other signs and symptoms of infection are present, however, or if the host is particularly immuno-compromised (unable to fight infection), the diagnosis should be considered. A patellectomy (removal of knee cap) is an option in severe or intractable cases.
Osteochondritis dissecans is not common, but it is very important to exclude it. Failure to do so in an early stage may make the difference between a cure and a disaster.
The same applies to infection and tumors.
Careful examination of the x-ray is mandatory. In particular, one needs to be aware of discrete signs which may indicate osteochondritis dissecans, tumor or infection.
The key feature of this condition is pain out of proportion to any obvious physical problem. It can occur in any part of the body. It would be the equivalent of sneezing and sneezing and sneezing even after the pepper's been removed from under your nose.
Let's break this down a little, to make it easier to understand. The condition is caused by abnormal activity of the 'sympathetic' fibers (special nerve fibres in the spine). The disorder is beyond the control of the patient (hence 'reflex') and in its worst form leads to wasting ('atrophy') and dysfunction (put these together and you get 'dystrophy') about the knee . In its archetypal ('classical') form, the skin is exquisitely sensitive ('allodynia'), it is unusually warm or cool, and exhibits either red or blue discoloration in a marble-like pattern. These features are present in varying degrees.
Sometimes x-rays show thinning of the bone (osteoporosis), in the knee cap itself. A so-called bone scan is occasionally positive. A bone scan is a test whereby a slightly radioactive material is injected into the subject's vein. It is then picked up by all the bones in the body. If bone is particularly active in one part of the body, the nuclear material will preferentially be picked up by this hyper-active bone. The scan is said to be 'hot' in that part of the body.
Another test is called a 'sympathetic block'. It is a special type of injection that is given just off the side of the spine. When the injection provides pain relief, the patient can often be said to suffer from CRPS. The good news is that the injection can in and of itself provide lasting relief. Thus sympathetic blocks can provide both the diagnosis and the treatment!
It is important to recognize CRPS early on in its course, for as time goes on, it becomes more resistant to treatment.
It surprises many people when they are told that the pain they experience around the patella is coming from somewhere else, often the hip.
Too often a practitioner homes in on the patella as the cause of peri-patellar pain (pain around the knee cap) without giving due consideration to the other structures which can give similar discomfort.
This leads to the patella and the rest of the extensor mechanism receiving inappropriate attention while the primary problem gets neglected. The situation then becomes extremely complicated.
The practitioner must develop a routine of questioning and examination which covers all these conditions.It requires discipline and vigilance.
Problems about the hip and spine can lead to knee pain even in the absence of pain about the hip or spine! For example, a pinched nerve in the lower back may not cause back pain, but it may cause pain about the knee!
This is called 'referred pain', because the pain is referred away from where the true problem lies. A full examination of the knee should always include an examination of the hip and spine.
Now we have reached the end of this important lesson. To conclude -