This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
On the outer side of the joint there may be present (1) a bursa beneath the external head of the gastrocnemius which may communicate with the bursa between the popliteus tendon and external lateral ligament. (2) One between the biceps tendon and external lateral ligament, (3) another between the popliteus tendon and external lateral ligament, and (4) one beneath the popliteus, usually an extension of the synovial membrane of the joint. On the inner side: (1) one beneath the internal head of the gastrocnemius, which usually communicates with the joint and sends a prolongation between the gastrocnemius and the semimembranosus. This is the most important posterior bursa. (2) There is one beneath the tendons of the sartorius, gracilis, and semitendinosus muscles. (3) One beneath the tendon of the semimembranosus, between it and the tibia; it rarely communicates with the knee-joint. (4) One between the tendons of the semimembranosus and the semitendinosus.
Sometimes a rounded tumor that is called a ganglion appears in the popliteal space. When the knee is flexed it is felt as a round, movable tumor which is hard and cystic. If the knee is extended it slides inward to the edge of the inner condyle and becomes hard and fixed. It usually originates from the bursa beneath the inner head of the gastrocnemius, is prolonged between it and the semimembranosus, and, when the knee is flexed, it either disappears entirely by its contents going into the joint or it can still be felt in the popliteal space. It may be a difficult matter to excise these cysts on account of their ramifications, and when this is impossible it is better to open them up, clean them out, and then sew the wound shut in order to avoid infecting the joint. Care should be taken not to mistake them for solid tumors or enlarged lymph-nodes, both of which are less common than ganglion.
The patella may be fractured in two ways, producing different lesions and requiring different treatment. Fracture is produced either by muscular contraction or by direct violence; the former is the more common.
As pointed out by Humphry, when the knee is fully flexed only the upper third or fourth of the articular surface of the patella is in contact with the condyles of the femur - the remaining two-thirds or three-fourths of the projecting portion of the bone resting on the pad of fat. When semi-flexed the greater part of its surface is in contact with the condyles, or at least the whole of its middle third. In full extension only the lower third or fourth or even less remains in contact.
When semi-flexed the patella is subjected to the greatest leverage strain; hence it is that fractures most often occur in this position and that the fracture occurs so frequently at the junction of the lower and middle portions. When the bone is fractured by indirect force (muscular) the line of fracture traverses its whole thickness and consequently the joint is always involved. Usually there are but two fragments. The extent of separation depends on the amount of laceration of the capsule on each side of the line of fracture (Fig. 551).
On each side of the patella the fibrous expansion of the quadriceps tendon, fascia lata, and joint capsule, if intact, will prevent separation of the fragments. If it is ruptured widely it will permit a separation of about 2.5 cm. (1 in.). It is rare that the primary injury produces a wider separation, and those cases in which the fragments are wider apart are usually those in which the upper fragment has been subsequently-pulled up by the contraction of the quadriceps. A fracture which when recent may have had only 1 cm. separation may subsequently show 7.5 to 10 cm. (3 to 4 in.). When the union is fibrous subsequent stretching may occur, also refracture increases the tendency to wide separation.
Fig. 550. - Housemaid's knee or enlargement of the prepatellar bursa.
Fracture by direct violence is due to the direct impact of a blow or a crushing of the patella between the femur and some foreign body. In this case the capsule on the sides is but little torn and although there may be several fragments they do not become widely separated.
Macewen has pointed out that the torn fibrous portion of the quadriceps over the patella may hang down between the fragments and hinder union.