c. Luxation of the upper extremity of the radius backwards.—This luxation would appear to be the most frequent the upper extremity of the radius is liable to, although it cannot be considered a common accident. When, how ever, we consider the functions of this joint and its form, we shall not be surprised to find the luxation backwards more usual than that forwards. The articulation is less sustained posteriorly by muscular parts than in front, when the fleshy bellies of the supinators cover and support it. There is also much latitude given to the movement of pronation, and the pronators are very powerful muscles. During a forced pronation, the radius becomes very oblique, and its upper extremity has a strong tendency to pass behind the axis of the hu merus.
The motion of supination, on the contrary, is not so frequent, the muscles to effect it are not so powerful, and the oblique and interos seous ligaments, which afford no restraint in the motion of pronation, are, on the contrary, soon rendered tense, and oppose a forced supination, which is the movement most likely to be followed by the luxation forwards. We think, therefore, we have physiological grounds for our belief that the luxation of the radius backwards ought to be the most frequent lux ation of the radius at the elbow-joint. When the luxation of the upper extremity of the radius backward has occurred, the patient feels at the moment a severe pain in the region of the joint. The fore-arm is flexed, and the hand remains fixed in a state of pronation. Supination cannot be effected either by the voluntary action of muscles or by force ap plied, and each effort, tending to produce this effect, is attended with a considerable augmen tation of pain. The hand and fingers are held in a moderate state of flexion. Finally, the superior extremity of the radius forms a mani fest prominence behind the capitulum or small head of the humerus.
When the bone is left unreduced, many of the motions of the fore-arm are rendered im perfect, particularly supination ; but the shoul der articulation becomes somewhat more free, and in some degree this circumstance makes up for the deficiency.
Sir A. Cooper, who has not seen any example of this luxation of the radius backwards in the living subject, has given us an account of a dis section of this injury. lie informs us that in the winter of 182 t a subject was brought for dis section into the theatre of St. Thomas's Hos pital, in which was found this luxation, which had never been reduced. The head of the radius was thrown behind the external condyle of the humerus, and rather to the lower extre mity of that bone. When the arm was ex tended, the head of the radius could be seen as well as felt behind the external condyle of the humerus. On dissecting the ligaments, the coronary ligament was found to be torn through at its fore part, and the oblique ligament had also given way. The capsular ligament was partially torn, and the head of the radius would have receded much more had it not been supported by the fascia which extends over the muscles of the fore-arm.
d. Sub-luxation of the upper extremity of the radius, with elongation of the coronary Boyer denies the possibility of any partial luxation of the upper extremity of the radius, he describes very clearly an abnormal condition of the radio-humeral joint, of which we have seen many examples, and which perhaps we may call a sub-luxation.
The ligaments which connect the head of the radius to the ulna, in the cases above alluded to, undergo a gradual relaxation and elonga tion, so that whenever an unusual effort is made to produce a strong pronation of' the fore-arm, the head of the radius is permitted to pass backwards, somewhat behind its na tural situation ; but as soon as the effort ceases, the radius resumes its natural position in the lesser sigmoid cavity of the ulna. A true lux ation in these eases cannot be said to happen, unless the effort of pronation is sufficient to bring the superior extremity of the radius behind the small head of the humerus; when ever this has occurred, then the sub-luxation is converted into the complete luxation of the radius backwards, and presents nll the cha racters of this accident, and it cannot be re placed without the assistance of art. It is known to anatomists that the radio-cubital joint is not advanced much in its development in infants ; that the lesser sigmoid cavity is as yet small and shallow ; and that the coronary ligament of the radius is proportionally longer and more yielding than it is destined to be in after life. This articulation, however, is fully equal, even at this earliest period of life, to sustain any efforts that its own pronator muscles can communicate to it ; but it is by no means constructed so as to be able to resist those forced movements of pronation and stretching we see too frequently given to the fore-arms of infants of a tender age, by their attendants, who in lifting them from the ground usually seize them by the fore-arms, these being at the time in a full state of pro nation. Thus we find that in delicate children the foundation is laid for that elongation of the coronary ligament, which ends in the con dition of this joint we have denominated sub luxation. We have usually observed that the subjects of this affection were delicate from their youth, and that sometimes only one, and that frequently both arms were affected ; that in all cases the extremity was more or less deformed, having a bowed appearance, the convexity being external ; that a very evident protuberance could be seen and felt in the situaticn of the head of the radius ; and that the patient had nearly perfect use of the arm, although he could neither fully flex nor extend it. Vi en the surgeon places his thumb on the external condyle of the humerus and head of the radius in one of these cases, and at the same time has the fore-arm supinated, the head of the radius is felt to rotate in its proper place, and on its axis, as in its perfect condition ; but if now a forced movement of pronation be given to the head of the radius, the latter will be observed to slip backwards towards the olecranon process: every time the patient him self fully pronates the fore-arm, the sub-lux ation occurs, and in supination the radius resumes its place again. This relaxation of the ligaments of the radio-cubital joint, no matter how produced, at all events predisposes those affected with it to the more complete luxation of the radius backwards.