Primary lesions are due to a sudden straining of an articulation beyond its normal range of motion so that it is unable to return spontaneously.  There is then found to be a double deviation, a deviation in two directions, from mid position, as though having reached the limit of normal motion, and being strained farther, it turned in some abnormal way.  In such position the articular surfaces are no longer parallel but assume an angle to each other.  Some part of one side then engages against the opposite surface, and makes a dent, so that when released it does not slide back normally, but under the tension of the stretched ligaments assumes even a sharper angle,  restrained by the dent it has made.  The ligaments, radially disposed, permit this abnormal motion and even provide for the secondary deviation.  In the resulting position the fibres of this ligament are not necessarily all stretched, but possibly only a few of the fibres are stretched, the rest relaxed.

    The factors in lesion then are:  motion beyond normal under high tension; assuming of an angle; indentation of a surface by a projecting portion; high tension of part or all of restraining ligament; in partially returning toward normal, the assuming of position still farther from normal.

    The principles of technic here described apply to all such lesions.  They are necessarily mechanical principles, and are exceedingly obvious as principles, though not so easy of application as they are of understanding.

    First, it is necessary to have tension or pressure or fixation on both of the bones involved in lesion; as on both sacrum and ilium.

    Since, however, it is rarely possible to take hold of the two bones, and in most cases is impossible to get effective hold of either of them, as in the dorsal and lumbar spine, it is therefore necessary to use other factors—positions in which there is ligamentous drag on the one, through the weight of the body; and motions in which all of the joints are at the limit of their motion, between the lesion and the point that can be taken hold of.  This principle—that joints must be at the limit of their possible normal motion before corrective force can be transmitted through them or made effective on them is the point that seems to me most neglected, and to be responsible for most of the failures in technic.  As for instance it is necessary to have all of the joints of the spine and of the shoulder at the limit of their motion before tension can be transmitted through them to the sacrum.

    Third, it is necessary to so direct this tension or pressure that it comes to a focus at the point desired.  As in breaking a stick held in the two hands it is possible to break it in the middle or near either end by differently adjusting the force of the two hands, so is it in directing the force of correction.

    For instance in the first ethnic described for correction of anterior sacral lesion, if the pressure of the left arm is over the middle of the crest instead of over the anterior superior spine, the effect is not to cause gapping of the lesion, but to cause increased pressure over the point in lesion.  The technic then takes effect at the last lumbar joint.  So also if the spine be flexed, the effect of rotation of the shoulder backward is to cause strain at the apex of the posterior curve rather than at the point desired.

    Fourth, it is necessary so to direct the force that it serves to release the engaged part from its indentation against the opposite surface; as in pressing down on the anterior superior spine the articulation is gapped open in the back, the front edge being the fulcrum therefor.  To do this it is usually necessary to use one part of the articulation as the fulcrum for the other part.  In a rotated dorsal vertebra, for instance, the engaged side is the side toward which rotation has occurred; as in a spine rotated to the right, the articular surface on the right is caught in flexion, the left side extends alone, producing secondary rotation to the right.  To release this it is necessary to carry the left side to the limit of its motion and with that limitation as a fulcrum, to gap open and release the engaged part.

    Fifth, with all articulations at their limit and the leverage adjusted, a quick spring is much more effective than steady force; it needs only a fraction of the force, with less danger of injury to soft parts, and gives a cleaner and more perfect correction.  This because the parts engaged are elastic.  The quickness of the spring should be in proportion to the elasticity.  It is also easier to apply just the right degree of force, calculated beforehand, in the quick spring than in the steady pressure.  A quick and shallow springing motion does not go so far beyond the instant of actual correction as a steady force, because in the latter it is impossible to calculate the moment of release, and so to check the corrective force.
 
 
MEANING OF THE “POP”

    The instant of correction of a lesion is usually indicated by a “pop.”  This is not always the case.  Also the pop may signify the making as well as the correction of a lesion.  Let us examine the mechanical factors in the “pop.”

    In the lesion we have articular surfaces at an angle with each other, and some edge of one surface engaged against the other surface, making an indentation or a wrinkle of the tissue, by which it is held as a lesion.  But this means that whereas at one point there is compression, at another there is separation, with a vacuum, or at best negative pressure, suction as we would call it, a potential vacuum.  What fills this potential vacuum?  The tissue of the joint we will remember is elastic; it is compressible, but it is also expansible.  Gas or fluid may collect there under the differential pressure, but also the tissue may expand to fill the negative space.  All of the bony tissue involved is under high pressure from the stretched ligament, whose elastic tension holds them in their false position.  As Dr. McConnell discovered, if all of the ligaments of a joint be cut, the lesion will correct itself.  (See also A. T. Still Research Institute Bulletin No. ___.)

    When, therefore, under the sudden spring of the corrective force the engaged point is released, the readjustment takes place under all of these elastic forces.  The “pop” is in proportion to the suddenness of the release and t he degree of elastic tension of the ligaments and of the elasticity of the tissues compressed.

    These principles of correction may be thus summarized:

    To transmit force, articulations must be at the limit of their motion.

    Effort must be properly focused on lesion.

    It must be so directed as to release the engaged points.

    One side of a joint used as fulcrum for the other.

    Elastic spring to overcome elasticity of parts.

    Many operators carefully avoid the “spring” technic, finding it painful and giving a shock to the patient.  They use a technic that involves so stretching the ligaments that release comes spontaneously.  If cutting of all of the ligaments allows a lesion to correct itself, so will sufficient stretching of them bring the same result.  Meanwhile the patient profits by the stimulation that comes from the stretching.  The choice here is between the radical technic with positive results and the gentle technic with much greater expenditure of labor but less pain to the patient, and less immediate results, with greater danger of recurrence.  Probably a combination of the two is best.  Our purpose at present is to build a mental picture of the mechanics of lesions and their correction, for which purpose the radical technic is described.  Different description of the gentle technic is not necessary; it consists in a gentler modification of the radical technic.  The principle of this gentler technic is like that of moving a heavy barrel, by very slightly tilting one side and then rolling it.  With a clear picture of all the leverages, with a practiced control of all tension, and with persevering practice, this gentler and more ideal technic may be mastered.