an effusion, but this effusion is the consequence of the separation rather than a favoring, precedent, and causative condition, for it is presumably drawn from the surrounding tissues by suction, just as cedema appears under a dry cup.

At the hip they are produced by the unopposed contraction to those muscles which have not been paralyzed. In Roser's three cases of spinal caries, mentioned above, the dislocation was dorsal, and the immediate cause was the contraction of the adductors no longer opposed by the pelvitrochanteric muscles. The opposite form, dislocation upon the pubis, due to paralysis of the adductors and the consequently unopposed contraction of the muscles on the outer side and back of the hip, has been reported by Bradford' and Reclus.2

Another variety may be mentioned, in which by the unequal growth of parallel bones, the tibia and fibula or the radius and ulna, one of them is slowly dislocated.

Voluntary dislocations, those which the individual can produce and reduce at will, may be mentioned in connection with this class. Those in which the peculiarity has originated in a previous traumatic dislocation are due to rupture of some of the ligaments or attached muscles and have been described among the consequences of traumatic dislocations; but a number of cases have been recorded in which this cause could not be invoked in explanation. (See Chapter II.) The only case I have seen was a man about thirty years of age who, a few years ago, frequented the medical schools of New York and added to his income by exhibiting his peculiar power before the classes.

Dislocations by destruction and Dislocations by deformity are of less practical interest to the surgeon because less amenable to treatment, and are to be regarded rather as incidents in, or symptoms of, other diseases than as morbid entities.

In the former, dislocations by destruction, Volkmann included those dislocations which occur in the course of chronic fungous or carious disease of joints or as a consequence of acute traumatic suppurative arthritis. Frequent examples are seen at the hip and knee.

In consequence of the destruction of the articular ligaments or of the bones themselves an abnormal mobility is created which allows the bones readily to be displaced by the action of gravity or by muscular contraction. At the hip this displacement is usually upward and backward; at the knee the well-known subluxation of the tibia backward or upward is produced by the contraction of the hamstring muscles, or, if the patient lies long upon one side and the destruction is well advanced, the displacement may be lateral to the distance of an inch or even more.

In the latter, dislocations by deformity, Volkmann included the dislocations which occur in the course of such affections as the morbus coxœ senilis and in the arthropathies of nervous origin, "Charcot's disease," in which the articular ends of the bones disappear by absorption without suppuration.

The remaining form has been specially studied, so far as I know, only

Bradford: Boston Med. and Surg. Journal, 1883, vol. 108, p. 73.

2 Reclus: Revue de Méd. et de chir., 1878, p. 176.

by Madelung,' and only at the wrist; the dislocation was always of the carpus forward, and was accompanied by marked changes in the shape of the radius of the bones of the first row of the carpus. The cause appeared to be overexertion, or, rather, prolonged and frequently repeated exertion in patients who, presumably, were predisposed to the change by defective vitality of the bones. Volkmann includes such cases under the general head of disturbances of growth of joints, loc. cit., p. 692.

1 Madelung: Deutsche Gesellschaft für Chirurgie, 1878, p. 259, and Arch. für klin. chir., vol. 23.

PART III.

SPECIAL DISLOCATIONS.

CHAPTER XI.

DISLOCATIONS OF THE LOWER JAW.

DISLOCATIONS of the lower jaw are infrequent, constituting about four per cent. of all dislocations, according to Table III., Chapter I. They may be bilateral or unilateral, the former being the more common, in the proportion of about 5 to 2 according to Malgaigne, who found 54 bilateral in a total of 76 cases which he collected. Of these 54, 31 were in women, and this greater frequency in the female sex is universally recognized. The injury is rare in infancy and old age because, it is thought, the rami of the bone are not so nearly at right angles with its body as in adult life. It has been observed in patients eighteen and seventy-two years old, and has been caused in the newborn child by obstetric manipulations.

In the great majority of cases the dislocation is forward, the condyle of the jaw passing in front of the articular eminence at the root of the zygoma. A few instances have been reported of double or single dislocation backward with fracture of the wall separating the articular cavity from the external auditory meatus, of dislocation upward into the cavity of the cranium, and of unilateral dislocation outward with, or perhaps without, fracture of the body of the jaw. These are, however, entirely exceptional and may be briefly described before proceeding to the consideration of the common form.

Dislocation backward with fracture of the posterior wall of the articular cavity is caused by great violence received upon the chin and acting from before backward. One or both condyles may be driven through the wall into the external auditory canal, breaking the bone and lacerating or pushing backward the outer cartilaginous portion. The production of the lesion is probably easier when the molar teeth are lacking from the upper or lower jaw, or if the mouth is partly open when the blow is received. The symptoms are pain in, and bleeding from, the ear, immobility of the jaw, the mouth being held partly open, and displacement backward, as shown by the relations of the front teeth to each other. The absence of the condyle from its normal position can be recognized by the touch, and the auditory canal is seen to be obstructed by the displacement of its anterior wall.

A case described by Croker King' as one of dislocation backward and outward of one condyle was probably such as above described. The patient was a boy eight years old, the lower incisor teeth were one inch behind the upper, the left lower molars just outside the upper ones, and the chin deviated to the left. The reporter accounts for the supposed unilateral dislocation on the left side by the springiness of the jaw in the median line, but as this springiness was not detected until after he had inferred that some such condition ought to exist to explain the production of the dislocation, as no mention is made of recognition of the position of the condyle, and as the boy bled from the ear immediately after the accident, it seems probable that the dislocation was backward through at least the outer part of the auditory canal.

Le Fèvre reported an interesting and very exceptional case in which the injury was caused by a fall from the second story of a building, the blow being received upon the chin. The jaw was displaced slightly backward and to the left, the teeth were close together, and the mouth could not be opened. Slight bleeding from the left ear. The diagnosis of fracture of the condyle was made. The patient was dismissed in the fourth week still experiencing difficulty in mastication and deglutition. Subsequently he suffered from violent headache, had several attacks of convulsions, and died about six months after the receipt of the injury. The autopsy showed that the roof of the glenoid cavity had been fractured, the condyle had passed into the cranium between the fragments, the neck of the condyle was in part destroyed, the dura mater was extensively inflamed and thickened, and there was a large abscess in the middle lobe of the brain.

Robert3 received at the hôpital Beaujon a patient who had been injured by the passage of the wheel of a cart across the right side of his face. The chin was deviated to the right, and the mouth was held open. The left condyle of the lower jaw could be distinctly felt under the skin above the root of the zygoma. Greatly surprised at this displacement Robert sought for and found a vertical fracture of the body of the bone on the right side just in front of the ramus. The left coracoid process remained under the temporal fossa, the sigmoid notch crossing and embracing the zygoma. Reduction was made by pressing the left ramus outward until the condyle was freed from its contact with the upper surface of the zygoma, and then drawing it downward and inward to its place.

Neis had an opportunity to observe a case in which the left condyle was dislocated in the same manner upward and outward, but apparently without fracture either of the jaw or of the temporal bone. The patient was a lad sixteen years old who received the injury by having his head caught between two boats, the pressure being upon the chin and occiput. The lower teeth were displaced backward, the mouth could not be opened, and there was slight oozing of blood from the left ear. Thirteen days

1 King: Monthly Journal of Medicine, 1855, p. 265.

2 Le Fèvre: Journal Hebdomadaire, 1834, vol. 3, p. 333.

3 Robert: Archives générales de Méd. 1845, vol. 7, p. 44.

Neis: Luxation du Maxillaire inf. en haut ou dans la fosse temporale. Thèse

de Paris, 1879, No. 252.

after the accident, when he first came under observation, he was still unable to open the mouth or take solid food. The lower incisor teeth were nearly half an inch behind the upper ones, the jaws could be only slightly separated and could not be brought entirely together. The left condyle could be seen and felt above and in front of the auditory canal in the temporal fossa; it moved with the jaw; there was slight deafness on that side, and a small blood-clot in the canal. No fracture could be found. Reduction was effected with difficulty under chloroform by forcing wooden wedges between the molars. Neis thought the dislocation could be accounted for by the peculiar shape of the face and jaw, the face being short and broad, the chin flat, and the rami of the jaw very divergent upward.

Several other interesting cases are quoted in Neis's thesis.

Dislocation of the jaw forward, the common form, is usually caused by muscular action, as in laughing, scolding, yawning, or vomiting, or exceptionally by violence in widely opening the mouth to introduce some large object, such as an apple or the fist, or in drawing a tooth, or by a blow upon the jaw. Morris' reported a case in which dislocation took place during sleep, the patient being a girl fifteen years old who had long had the habit of sucking her thumb.

In order to understand this mechanism it is necessary to recall the construction and normal action of the joint. The lower jaw is attached to the skull by a synovial capsule which is strong on its outer side (the external lateral ligament), by an internal lateral ligament not in immediate relations with the joint but extending from the spinous process of the sphenoid bone to the margin of the inferior dental foramen, and by the stylo-maxillary ligament, a strong band extending from the styloid process of the temporal bone to the posterior border of the ramus of the jaw. The joint is occupied by an interarticular cartilage or meniscus. which overlies the upper surface of the condyle and accompanies it in its normal movement forward from the glenoid cavity to the eminentia articularis when the mouth is opened. In front of the point to which the condyle thus moves forward the surface of the eminentia articularis is inclined slightly upward to become continuous with the much narrower under surface of the zygoma. The fibres of the muscles attached to the ramus which close the mouth run upward and forward, and only those belonging to the deep posterior portion of the masseter are vertical or inclined backward.

Since the condyle moves forward when the chin descends, the centre of motion of the jaw is not in the condyle, but at a point below it at or near the dental foramen, and as the angle of the jaw is at the same time moved backward the axis of the ramus changes its relations to the direction of the fibres of the masseter much more than it would if the centre of motion was in the joint, and it may become so far inclined forward that the posterior fibres of this muscle lie behind it in such a position. that their contraction would tend still further to raise the angle of the jaw and thrust the condyle forward, thus exaggerating the effect of the action of the external pterygoid, which is to draw the condyle forward

1 Morris British Medical Journal, 1872, II., p. 242.