On the temporo-maxillary articulation / by J. Smith.

  • Smith, J
Date:
[between 1800 and 1899?]
Catalogue details

Licence: Public Domain Mark

Credit: On the temporo-maxillary articulation / by J. Smith. Source: Wellcome Collection.

    4/6 (page 208)
    The articulating surface of the condyle thus acquires some of the properties of a screw: that screw being a conical screw, or helix, and having its axis lying at right angles to the plane of vertical motion in the jaw, or some way nearly so; and being from the direction of the thread (that is the articular surface), a right-hand screw on the left side, and a left-hand screw on the right side; the base of the cone being placed externally. In using the word screw here, it is not of course meant that either a perfect screw, as defined in mechanics, exists, or that the power of one is imparted to the joint. In fact, only a limited portion of such a conical screw is presented in the condyle,—half, or a third of a turn of the thread being all that is necessary for the purposes of the joint, the remaining portion being unnecessary, and consequently absent. This absence of that complementary portion indeed, leading to some difficulty in recognising that which is present. The Glenoid cavity will, in all such examples, be found to correspond to the characters of the condyle, constituting as it were a longitudinal section of a spiral chamber: the condyle and glenoid cavity together thus constituting a certain portion of a conical tap, moving in a corresponding portion of a conical die. Regarding the action of the joint, viewed as thus con¬ structed, it will be seen that as the respective surfaces of a conical tap and die are never wholly in contact unless the screw is completely home, so in many stages of motion in this joint, the condyloid and glenoid surfaces must be to some extent separate from each other. And on attentively ob¬ serving the action of the condyle within the glenoid ca¬ vity, this will be found to be the case ; as in opening the jaws the point of contact between the surface of the con¬ dyle and glenoid cavity recedes, inwards and backwards, de¬ scribing a spiral track over the condyle, corresponding to the articular surface, and passing from the anterior external until it only exists at the posterior internal aspect of the articulation; and returning again until the whole is in contact, as the mouth is closed. By this means a great amount of friction is avoided ; what would otherwise be a