and movements communicated accidentally by the patient or bystanders, or intentionally by the surgeon in the effort to reduce the dislocation. Thus, in illustration of the last, it is not very rare to see an iliac transformed into an ischiatic or a thyroid dislocation of the hip in the effort to reduce by flexion, abduction, and rotation.

In the great majority of cases a dislocation is produced suddenly by external violence or by extreme muscular action, or by the two acting together upon a healthy joint, and when thus produced it is called traumatic. In other cases the joint has been diseased for some time previous to the occurrence of the dislocation, and this latter is effected by the gradual action of the muscles or even by gravity; these are known as spontaneous, and present many varieties. (See Chapter X.) A third class, congenital dislocations, is composed of those in which the dislocation occurs during intrauterine life, presumably as the result of a malformation or defective development. Dislocations produced during delivery are traumatic. The second and third classes will be separately considered. (See Chapters IX. and X.)

Statistics. Compared with other surgical injuries, dislocations are infrequent. The statistics compiled by Gurlt' from the annual reports of the London Hospital for the years 1842 to 1877, give 51,938 fractures, 5212 dislocations, 98,373 wounds, 23,180 contusions, 39,947 sprains, 20,396 scalds and burns, 3715 dog-bites, and 975 suicidal attempts. The proportion of dislocations to fractures in this table is 1 to 10. Most of the statistics that have been published, especially the earlier ones, are defective in that they are based upon hospital, to the exclusion of dispensary, records, and therefore do not contain the due proportion of the less important varieties, or because of errors of diagnosis, notably in regard to dislocation of the wrist and ankle. It is only within the last forty years that the common injury now known as fracture of the lower end of the radius has been recognized as a fracture; it was formerly deemed a dislocation. And in like manner Pott's fracture at the ankle is often found recorded as a dislocation. Krönlein's statistics have the merit of being based upon the records for six and a half years (18741880) of a single hospital and polyclinic, and yet containing a number larger than that of any other except Malgaigne's, and large enough in itself to make it probable that the percentages do not vary widely from those that a larger number collected at the same place would give. They are as follows:

1 Gurlt: Archiv für klin. Chirurgie, 1880, p. 467.

2 Loc. cit., p. 5.

TABLE I-TABLE OF 400 RECENT TRAUMATIC DISLOCATIONS (KRÖNLEIN).

In addition, I quote those made up by Prahl' from the records of the Breslau Hospital and Polyclinic for the years 1830 to 1880, although they are probably affected by the errors of diagnosis above mentioned. Compared with Knönlein's, they show some important differences in the percentages, which may, perhaps, be due to an insufficient number of cases, or to differences in the habits and occupations of the population from which the cases were drawn.

TABLE II.-TABLE OF 420 TRAUMATIC DISLOCATIONS (PRAHL).

1 Inaug. Dis. Breslau, 1880. Abstract in Centralblatt für Chirurgie, 1881, p. 57. 2 As divided according to age, the numbers of the cases of the thumb and fingers amount to 22 and 16, respectively, instead of 21 and 17 as given in the main table.

The total number of cases on the records was 453, of which 420 were traumatic, 100 females, 290 males, and 23 were congenital; of the latter, 18 were of the hip, 15 females, 3 males, 2 each of the elbow and patella, and 1 of the knee. During the same period 2958 fractures were treated, a proportion of 7 to 1, reckoning the traumatic dislocations alone.

The following table has been made by combining Krönlein's statistics with a compilation made by Prahl of his own and others to show the direction in which the inclusion of dispensary or polyclinic statistics affects the percentages.

The following table summarizes the others with Malgaigne's statistics of the Hôtel Dieu.

These tables show the great relative frequency of dislocations of the upper extremity as compared with those of the lower. Each set of statistics shows that dislocation of the shoulder is far more common than that of any other joint, the percentages varying from 45.2 to 60.7; that next in frequency come dislocations of the elbow, with percentages varying from 10 to 27.2. These two dislocations may be estimated as together comprising from two-thirds to three-fourths of all cases; of the remaining one-third or one-fourth, dislocations of the fingers and hip form the majority.

The side upon which the dislocation took place is so seldom mentioned in the records from which the statistics have been compiled that no posi

tive opinion can be formed as to the relative frequency with which the two sides are affected. Krönlein found in 100 consecutive cases observed by himself the right side was affected 46 times, the left 54.

As between males and females, Malgaigne and Gurlt found the injury three times as frequent in the former as in the latter; Krönlein found it five times as great. Dislocations of the lower jaw are an exception, being four times (Krönlein) as frequent in women as in men.

Age. No age is exempt; dislocations have occurred as early as the moment of birth and as late as the age of ninety years.

The relative liability to the injury at different ages is not shown by simply comparing the number of cases observed at those ages, but by also comparing these numbers with the number of people at those ages living in the community where the observation is made. This comparison has been made by Krönlein for Berlin, with the following results:

TABLE V.-Frequency of DISLOCATIONS at Different Ages.

the

From this it appears that a smaller proportion of individuals between ages of 1 and 10, and 71 and 80 years receive dislocations than in any other decade of life; and the highest proportions are found between the ages 51 and 60 and 61 and 70. It is further to be noticed that dislocation of the shoulder is very rare, and that of the elbow very common, before the age of 21 years. Krönlein's table (Table I.) shows that of 207 cases of the former, in only 2 were the patients less than 21 years old, and that of 109 cases of the latter 80 were no older, the age in 31 being between 1 and 10 years, and in 49 between 11 and 20 years. Prahl's table (Table II.) corroborates this. Compared with fractures, it appears that the liability to dislocation is least during those periods of life in which the liability to fracture is greatest-that is, in infancy and youth and in old age; the latter part of this statement may need some modification, for while dislocations are rare after the age of 70, they are relatively frequent in the preceding decade. The liability to each increases from adolescence through middle life.

CHAPTER II.

ETIOLOGY AND MECHANISM.

THE causes of dislocation may be grouped in two classes: a. Predisposing; b. Immediate or determining.

a. Predisposing Causes.-These are found in certain normal differences of form and function characterizing certain joints, and in accidental or pathological conditions that sometimes arise.

The joint which is most frequently dislocated is the shoulder-joint, and it differs normally from others in the wide range and variety of motion made possible by its form, the laxity of its capsule, and the absence of any firm ligament to hold the bones closely together. A wide range of motion in one direction is not necessarily a circumstance favoring dislocation; on the contrary, it may protect against it by making it difficult to bring into action the fulcrum which is furnished by the edge of the bone when it arrests the motion. In a young healthy person the elbow or knee cannot be dislocated by flexion, because the motion is finally arrested by broad contact of the soft parts, not by the edge of the joint; while, on the other hand, in each case extension is limited by the structures of the joint itself, and hyper-extension at once causes dislocation by rupturing those structures. A long range of motion in one plane does not make the joint insecure, so long as the two bony surfaces rest squarely against each other, as they do in the hinge-joints; but when the change of position makes this contact oblique, as in abduction of the arm, a displacing force exerted in the direction of the long axis of the humerus is resisted only by the capsule. Under certain conditions, therefore, it may be said that freedom of motion in a joint diminishes, and limitation of motion increases, the liability to dislocation. A force which, exerted in the plane of normal motion of a hinge-joint, would be taken up by the muscles without damage to the joint, would, if exerted in another (lateral) plane, rupture the ligaments and dislocate the joint. If the edges of bone or the processes about a joint are exceptionally prominent they may, by virtue of this condition, become predisposing causes of dislocation by arresting motion.

Dropsy of such a joint as the shoulder favors its dislocation by removing the obstacle which the necessity of creating a vacuum between the articular surfaces would otherwise interpose, and by giving to the head of the humerus a different range and character of motion and the possibility of finding new bearings (see Chapter X., Dislocations by Distention).

The destruction of the ligaments by violence or disease, and fracture or disease of the bony constituents of the joint, favor dislocation, and the fracture of an associated or parallel bone may have the same effect, as fracture of the ulna favors dislocation of the head of the radius.