The pennation angle is the angle formed by the individual muscle fibers with the line of action of the muscle, and it is expressed as an average for the entire muscle. When all fibers are essentially parallel to the line of pull as with the rectus femoris or intermedius the pennation angle is zero. At the other extreme would be the pectoralis major or deltoid (at the shoulder) which are more fan shaped. The pennation angle for the vastus lateralis is five degrees.

The physiological cross section is an estimate of a muscle’s maximum force generating capacity and factors in the volume of the muscle, the length of the muscle fibers, and the pennation angle. The physiological cross section is proportional to the overall volume of the muscle and inversely proportional to the length of the individual muscle fibers. It is also proportional to the cosine of the pennation angle. When the pennation angle is low - as with all the muscles of the quadriceps including the VMO- it is a negligible factor as its cosine is close to one. We have recently analyzed the physiological cross section of the VMO and found it to be approximately 30% of the entire vastus medialis complex. The above features contribute to making the VMO a critical dynamic medializing force.]

The patellar cartilage is also unique in that it does not follow the contour of the underlying bone. This can be problematic when one interprets plain x-rays and CT scans. In only 15% of cases does the apex of the bony patella coincide with that of the articular cartilage.

Reflex Sympathetic Dystrophy (RSD)

The nerves involved are the "sympathetic" nerves, nerves that are part of the autonomic nerve system. Along with the parasympathetic nerves they control involuntary body functions such as heart rate, intestinal mobility, blood vessel constriction and dilation (i.e. skin color and temperature) - to name but a few. They respond to, among other things, pain. In addition to responding to pain, it has been suggested that the sympathetic fibers can themselves carry pain impulses via afferent fibers in the sympathetic chain. The autonomic system consists of a central and a peripheral component. The central component consists of the hypothalamus. The peripheral component consists mainly of afferent sensory fibers and efferent motor fibers to all tissues except skeletal muscle. They synapse in ganglia on either side of the spinal cord. Of particular pertinence to the patella and lower extremity, the lumbar sympathetic chain is "formed by the anterior (ventral) divisions of the first, second, third and fourth lumbar nerves, with a branch from the twelfth thoracic nerve and the fifth lumbar nerve. It is located anterior to the transverse processes of the respective lumbar vertebrae".) Interestingly, in RSD the nerves are not doing anything abnormal. The dysfunction consists of a sustained and exaggerated variation on normal physiology - the nervous system equivalent of violent, repeated sneezes.

The specific cause of the persistent sympathetic irritation and exaggerated response is unknown. The ‘gate control theory’ has been invoked: the "gate" which modulates the inhibition and facilitation of pain impulses is dysfunctional in the sense that the pain impulses are not shut out after a normal period of time. This results in a persistent painful stimulus and a reverberating cycle of pain and sympathetic response. Spontaneous depolarization at the exposed, regenerating surfaces of injured nerves has been proposed (when there has been nerve injury) as has a receptive field expansion of the pain-contributing WDR neurons in the dorsal horns of the spinal cord.)

Alternatives include peripheral intravenous injections (Bier blocks) of guanethedine, reserpine or bretylium. An oral, non-steroidal anti-inflammatory medication can be added as can a tricyclic such as amitriptyline and/or a fentanyl patch. More recently the use of an indwelling epidural blockade has been suggested. This provides around-the-clock protection to the knee. It may be more effective than repeated injections especially when no single treatment is long-lasting enough to "hold" the patient until the next injection.)