The Fifth Lumbar. - The fifth lumbar vertebra presents some points of importance.  Its massiveness is an evidence of its weight-carrying capacity.  The depth of its anterior margin is markedly greater than that of the posterior portion of its body.  The intervertebral disc between the fifth and the sacrum still further accentuates, by the relatively great thickness of its anterior margin, the angle formed by the articulation of the fifth with the sacrum.  The inferior articular processes are wider apart than those of other lumbars.  The transverse processes are usually heavily developed, but the spinous process is apt to be smaller than those of the other lumbars.  This vertebra joins the sacrum at a rather abrupt angle forming a decided projection, the sacro-vertebral angle.  A line drawn through the intervertebral disc between the fifth lumbar and the sacrum would form an angle with the horizontal of about 30 degrees.  It is evident that the inferior articular processes of this vertebra have a considerable function of weight carrying.  If it were not for the bracing action of these processes, the superincumbent weight would tend to slide the body of the fifth forward on the base of the sacrum.

    Loss vs. Exaggeration of Normal Curves. - As a general proposition, it may be stated that, the loss of a normal curve in the spinal column is apt to cause more discomfort than would the exaggeration of a normal curve.  There is probably no better example of this, than the effects noted in changes of the lumbo-sacral articulation.  It is manifest that extension in the arthrodial articulations, between the articular processes of these two bones, serves to hold them more firmly together and make the sacro-vertebral angle more prominent.  This serves to make the lower abdomen more prominent and makes the line of division between abdomen and pelvis more marked.

    Motion in Lumbo-Sacral Articulation. - Flexion, of the fifth on the sacrum, compresses the thick anterior margin of the intervertebral disc and slides its articular processes upward on those of the sacrum, thus tending to greatly decrease the sacro-vertebral angle and make the spinous process of the fifth become more prominent.  It is conceivable that forced flexion in this articulation could cause a complete dislocation of the articular surfaces.  Flexion and extension are so free in this articulation that much of the movement, ascribed to the lumbar region as a whole, is contributed by it.  Loss of motion here, as in lumbago, is characterized by a rigidity which causes the stride in walking to be greatly shortened.

    Adaptation in Lumbo-Sacral Articulation. - In cases of unequal length of legs as a result of injury, flat-foot, slight bend of an inflamed knee or hip, there is a tilting of the fifth, on the base of, the sacrum, in order to balance the weight of the body.  There is unequal movement in the
arthrodials formed by the articular processes, i. e., the joint on the side of the shorter leg extends, while the opposite one flexes, thus producing a tendency to rotate.  This rocking action permits a great range of adaptation in this joint, an action which is absolutely essential to the maintenance of balance in the upright position.

    Stability of the Lumbo-Sacral Articulation. - The anterior common ligament is so placed as to lend support to this articulation in the extended position.  Ligaments ordinarily limit motion but are extensible tissues when under continuous strain, hence the weight of the body tends always to be transmitted from bone to bone.  To change this arrangement and thus put the strain continuously on ligamentous tissue, leads to relaxation in the joint.  There are many joints in the body which, so far as the adaptation of the articulating surfaces of the bones which form them are concerned, furnish no stability.  The knee joint is a good example of this.  It has sixteen ligaments which serve to furnish it a stability not warranted by the form of the articulating surfaces of tibia and femur.  The lumbo-sacral articulation has a stability in its normal angle due to the locking of its articular processes.  The more these processes are locked, as in hyperextension, the greater t e tendency to transmit weight through them.  This is unnatural and hence produces fatigue, both by continuous pressure on the articular surfaces and by stretching of the anterior common ligament.  This is the condition caused by a pendulous abdomen.

    Decompensation of the Lumbo-Sacral Articulation. - Flexion of the lumbo-sacral articulation causes a straightening of the lumbar thus bringing the weight of the body more completely on the column of bodies and changing the lumbo-sacral angle, so that the axis of the pelvic cavity is brought more nearly in line with that of the abdomen.  The obliteration of the normal lumbar curve produces a general curve, i. e., coincides with the dorsal and thus becomes part of a general posterior curve.  This puts a great strain on the posterior spinal ligaments.  This is a state of decompensation of the normal spinal curves, which necessitates a decided effort to balance the body.

    Part of the Pelvis. - Obstetricians count the fifth lumbar as a part of the pelvis, since it is bound to the innominates by ilio-lumbar ligaments, which extend from the tips of its transverse processes to the crests of the ilia.  These ilio-lumbar ligaments tend to compel the fifth lumbar vertebra to act somewhat as though it were a portion of the solid pelvis.

    Characteristics of the Sacro-Iliac Articulations. - The articulations between the sacrum and innominates are normally immovable, They may become physiologically movable, in the pregnant woman, in order to facilitate the birth of the child, i. e., they exhibit functional adaptation.  Following the act of parturition they normally become immobile, i. e., exhibit functional adaptation to weight carrying.  Failure of either of these forms of adaptation is an abnormality.  In case the articulations do not relax in the parturient woman, the whole process of adapting the birth canal and its contents, is exhibited by the head of the child.  Normally the bony birth canal and the child's head mutually undergo adaptive changes.  In case these articulations do not regain comparative immobility, following parturition, a condition of instability will exist, which will express itself in a disturbance of the statics of the body.  Balancing and weight-carrying functions will be injured.

    Physiological Relaxation. - The menstrual periods in many women are characterized by relaxation of the pelvic ligaments, with consequent disturbance of the weight-carrying power of the sacroiliac articulations.

    The Male Pelvis. - The male pelvis never exhibits any form of normal relaxation of ligaments, therefore the existence of any instability in the sacroiliac articulations is pathological, i. e., due to debility or trauma.  The trauma may be direct and forceful enough to strain the ligaments suddenly, or it may consist in a form of fatigue, which eventually allows the ligaments, engaged in the weight-carrying functions of these joints, to become strained.

    Loss of Stability. - It is axiomatic that loss of stability, in the pelvic girdle, will weaken its weight-carrying capacity and hence disturb the normal static condition of the whole body.  In view of this fact, we must make a rather careful study of the structure of these joints and note any evidences of inherent weakness, i. e., observe at what points unusual force might most easily produce a lesion.

    Analysis of Sacro-Iliac Articulations. - Dissection of these joints discloses the existence of the same structures found in other joints, i. e., bone, cartilage, synovial membrane and ligaments.  The fact that these structures do exist in the sacroiliac articulations, naturally classifies these joints as having possible mobility.  These joints serve to absorb shocks transmitted through the legs to the pelvic girdle.  The slight movement, normally possible in them, subjects them to much the same conditions which serve to injure other joints.

    Relation of Sex to Sacro-Iliac Lesions. - Clinically we have found disturbances of these joints in both men and women, hence we are forced to believe that sex does not control the character of the lesions.  They are much more frequent in women than in men.  This is undoubtedly due to the necessarily greater functional adaptability of the female pelvis.

    Inherent Weakness in the Character of the Structure. - The sacroiliac articulations are inherently weak, so far as any bony interlocking is concerned.  Their stability is a matter of ligamentous strength.  The sacrum is wedge-shaped from above downward and from anterior to posterior.  The anterior surface, being broader than its posterior, does not serve well to offer resistance to the superincumbent weight of the spine.  The sacrum articulates by its auricular surfaces with those of the ilia.  The articulating surfaces of both bones are covered with cartilage.  The joints are surrounded by capsular ligaments and contain synovial sacs.  The opposing auricular surfaces are reciprocally, slightly, uneven but not enough so to sustain any weight without ligaments.  The illustration, Fig.  90, shows clearly the relation of the form of the sacrum to the direction of the weight it sustains.  The structure of the sacro-iliac synchondroses indicates that movement is possible and, in fact, probable.  The primary object of the movement is to produce elasticity in the pelvic girdle and interrupt shocks which would be transmitted from the legs to the trunk.  A further object would be, in the female, adaptation of the birth canal to its contents.

    Causes of Subluxations. - Clinically we recognize the existence of disturbances in these joints as due to relaxation of ligaments due to pregnancy, menstruation, general debility, or trauma.  Functional adaptability in the female pelvis makes women easily subject to changes in these joints, and likewise permits easier correction.  The male pelvis is practically never disturbed except as result of debility or trauma, and is therefore more difficult to correct.

    Rotation. - The motion in these joints is described by various authors. judging from clinical experience the motion seems to be in the nature of rotation.  This rotation takes place on an axis which passes through the articulating surfaces of the sacrum and ilia on a level with the posterior superior spines of the ilia and the second sacral spine.  This makes the second sacral spine and the posterior superior spines of the ilia the bony landmarks indicating the position of the joint surfaces.  Rotation of the ilium forward would make the posterior superior spine less prominent and slightly higher, so that a line drawn across the sacrum through its second spinous process would pass through the lower border of the posterior superior spine, instead of its apex.  Rotation of the crest of the ilium backward makes the posterior superior spine more prominent and slightly lower than normal.  All the positions described by various authors can be reduced by analysis to the two rotations just described.  Since these rotations are unilateral, the pelvic distortion results in a slight apparent difference in the length of the legs so that when the patient lies on the back, on a hard surface, with the legs stretched out as evenly as possible, the heels will be found not to be equal.  In order to compensate for this apparent inequality in length, the pelvis will be found to be tilted, with relation to the spinal column.  This compensatory tilt is the same phenomenon that is present in every case having unequal length of leg support.  In order to make sure which joint is the one at fault, one must use those bony landmarks which are a part of the pelvis, i. e., posterior superior iliac spines and the second sacral spine.  An apparent difference in the length of the legs might be due to a lumbar condition, irrespective of any change in the relation of the bones of the pelvis.

    Compensatory Pelvic Tilt. - It should be remembered that no change in a sacroiliac synchondrosis is ever unaccompanied by a compensatory effort of the body to transmit the body weight through the normal half of the pelvis.  This produces a slight spinal curvature, which is part of the compensatory tilt of the pelvis, to avoid transmitting body weight through the weakened joint of the pelvic girdle.

    Classes of Cases. - Two classes of cases complaing of pain which may be traced to disturbance in these joints.  The first group comprises those of both sexes, who are debilitated, and hence do not have normal tone in muscles and ligaments.  These cases either are bed-fast or inclined to assume the recumbent position.  Cases compelled to lie on the back for a long period following surgical operations are apt to suffer distress in these joints.  The second group comprises those who are over-weighted in the abdomen, and hence tend to lordosis in the lumbar region.  Both of these classes are greatly helped by corrective manipulation and bandages.

    The debilitated individual is toned by corrective manipulation, and the weakened ligaments reinforced by some simple form of girdle which helps to hold the pelvis firm.  The individual with the over-weighted abdomen is physiologically rested by corrective manipulation and the use of a support which will assist the back in carrying the excessive weight which lies anterior to its normal weight-carrying structure.

    The really difficult sacroiliac lesion to correct is the traumatic.  Such a lesion has all the elements which make perfect recuperation problematical in any joint.

    Symptoms. - The symptoms of sacroiliac lesions are usually pains located in the lumbar, gluteal and thigh regions.  The pains are described by patients as being usually a dull heavy ache whenever the weight of the body is transmitted through these joints.  Close analysis will be required to determine whether a given case is in reality a sacroiliac lesion.  The only physical test worth trusting is the alignment of the posterior superior iliac spines and the second sacral spine, when the patient is standing.  The pains may be due to many different strains.  The hyperesthetic points about the sacro-iliac joints may accompany other conditions.  Flat-foot will, in some instances, produce all the sore spots in the lumbar and sacral region which may be present with a sacroiliac lesion.  The backache, due to tilting of the pelvis to compensate for a sacroiliac lesion, is practically similar to that due to the effort to compensate for a change in statics due to flat-foot.

    Plan of Treatment. - A sacroiliac subluxation is due to relaxation of ligaments, or trauma.  To correct such subluxations, the cause is the controlling factor as to the means to be employed, i. e., debility must be controlled by general means, so that local reinforcement of weakened ligaments will not be continuously necessary.  It is usually easy to make a specific correction of the lesion in a debilitated case, but not easy to maintain the correction.  It is difficult to correct a traumatic lesion, but when once corrected, the vitality of the tissues tends to make the correction permanent.  In all debilitated cases voluntary exercise must form an important part of the treatment.  Climbing on rough ground is the best aid in such cases, because no two steps are alike, and hence the tissues are not fatigued by repetitions of similar movements.