4i8 HUMAN ANATOMY. 



To approximate the fragments elevation of the Hmb sometimes suflfices, but 

 occasionally partial section of the lateral expansions of the quadriceps, of the rectus 

 tendon, and of the muscle itself will be required as successive steps. 



In the best of the operations used in recent fractures, and which do not widely 

 open the joint, a silk or silver ligature is carried through an incision at the lower 

 border of the patella behind that bone and between it and the trochlear groove 

 in the femur, is brought out through an incision at the upper border, rethreaded 

 on a needle with an eye near the point, brought down in front of the patella, — 

 beneath the skin, — and tied or twisted so as to hold the fragments together. The 

 blood-clot and synovial exudate are squeezed out 'through the two incisions ; the 

 entangled capsular fibres are removed by attrition of the fractured surfaces against 

 each other. These operations are, of course, not applicable to old fractures in 

 which shortening of the muscle has taken place and approximation and forcible rub- 

 bing together of the fragments are impossible. 



Operations for recent fracture by open arthrotomy permit the direct removal of 

 the fringe of interposed tendinous and capsular fibres and the repair by suture 

 of the rents in the capsule and in the lateral expansions of the quadriceps. The 

 patellar fragments may also be sutured, but this is not always necessary. 



Dislocation of the patella usually occurs from muscular action and as a conse- 

 quence of sudden contraction of the quadriceps. 



The displacement is commonly in the outward direction because the long axis 

 of the quadriceps muscle and tendon is inclined to that of the ligamentum patellae 

 in such a way that the bone is situated at the apex of an obtuse angle which opens 

 outward. When the quadriceps contracts the tendency is to straighten this angle, 

 — i.e., to carry the patella outward, — and this, aided by the greater strength of 

 the vastus externus as compared with that of the inner vastus, is more than suf- 

 ficient to overcome the resistance offered by the greater prominence of the external 

 condyle, as well as the relatively more extensive insertion of the vastus internus into 

 the inner margin of the patella. The bone may even, as in one recorded case, 

 be carried entirely past the condyle, so as to lie behind the centre of motion of the 

 knee when the joint is bent, thus causing the quadriceps extensor to act as a flexor 

 of the leg on the thigh. 



The 'external articular facet on the under surface of the patella is larger than the 

 internal. The patella is in relation, therefore, chiefly with the external condyle, and 

 even if dislocation occurs from direct violence, it is more likely to be driven in that 

 direction (Humphry). If it has once passed beyond the edge of the outer condyle 

 — a "complete" luxation necessarily attended by laceration of the capsule — it is 

 less likely to be replaced than if it had gone in the opposite direction, because of {a) 

 the resistance offered by the prominence of the condyle itself and {b) the greater 

 comparative strength of the vastus externus. 



Outward luxation is not very rare in cases of genu valgum, and, per contra, in 

 congenital cases of patella luxation and in unreduced traumatic luxations genu 

 valgum has followed (Makins). 



The patella may be displaced inward by direct force. It is sometimes turned 

 on edge by a force insufificient to dislocate it completely, and is held in that position 

 by the tension of the soft parts attached to it and by the pressure of the over- 

 lying fascia, "like a stick on end under a tightly stretched sheet" (Stimson). In 

 flexion of the knee the patella lies deeply in the depression between the condyles 

 and the quadriceps tendon is on the stretch. The bone is therefore somewhat 

 removed from danger of direct violence, and is steadied and fixed by the quadri- 

 ceps muscle. In extension the patella rests on the trochlear surface of the femur 

 only by its lower margin ; it is more prominent and thus more exposed to force 

 directly applied ; the quadriceps is relaxed, leaving the bone freely movable. 

 For these reasons extension is the position in which dislocation most commonly 

 occurs. 



