Statics Of The Pelvis PDF

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Statics Of The Pelvis Structure The structure of the lower limb is specialized for support of the body’s weight, locomotion, and maintenance of body stability (balance). The pelvic girdle is an entity consisting of the paired hip bones (each composed of the ilium, ischium and pubis) and the sacrum (...

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25. Statics Of The Pelvis

Structure The structure of the lower limb is specialized for support of the body’s weight, locomotion, and maintenance of body stability (balance). The pelvic girdle is an entity consisting of the paired hip bones (each composed of the ilium, ischium and pubis) and the sacrum (strictly speaking, the sacrum is part of the vertebral column). The two pubic bones articulate anteriorly at the pubic symphysis, a secondary cartilaginous joint that may display a slight degree of mobility during hip and sacroiliac movement, and during childbirth. Posteriorly, the sacrum articulates with the two iliac bones at the sacroiliac joint; the bones are virtually incapable of independent movement, except in the female during parturition or as a result of pathological change. The pelvic girdle connects the lower limb to the axial skeleton via the sacroiliac joint, a plane synovial type of joint in which mobility has been sacrificed for stability and strength, to allow for effective weight transmission from the trunk to the lower limb.

Functions The pelvic girdle is massively constructed and serves as a weight-bearing and protective structure, as an attachment for trunk and lower limb muscles, and as the skeletal framework of a birth canal capable of accommodating passage of the fetus. The pelvic girdle is a basin-shaped ring of bones that connects the vertebral column to the two femurs. The primary functions of the pelvic girdle are to: 1. Bear the weight of the upper body when sitting and standing. 2. Transfer that weight from the axial to the lower appendicular skeleton for standing and walking. 3. Provide attachment for the powerful muscles of locomotion and posture and those of the abdominal wall, withstanding the forces generated by their actions. Consequently, the pelvic girdle is strong and rigid, especially compared to the pectoral (shoulder) girdle. Other functions of the pelvic girdle are to: 4. Contain and protect the pelvic viscera (inferior parts of the urinary tracts and the internal reproductive organs) and the inferior abdominal viscera (e.g., intestines), while permitting passage of their terminal parts (and, in females, a full-term fetus) via the perineum. 5. Provide support for the abdominopelvic viscera and gravid (pregnant) uterus. 6. Provide attachment for the erectile bodies of the external genitalia. 7. Provide attachment for the muscles and membranes that assist the functions

Statics and Mechanics of the Pelvis (Pelvic Mechanism) Functional considerations: the sacroiliac joints perform two functions: (1) a stress relief mechanism within the pelvic ring (important during walking and running and, in women, during childbirth) and (2) a stable means for load transfer between the axial skeleton and lower limbs.

The skeletal pelvis constitutes the major mechanism for transmitting the weight of the head, trunk and upper limbs to the lower limbs. It may be considered as two arches divided by a coronal transacetabular plane (a vertical plane passing through the acetabular cavities). The anterior arch, formed by the pubic bones and their superior rami, connects these lateral pillars as a tie beam to prevent separation; it also acts as a compression strut against medial femoral thrust. The posterior arch is the one chiefly concerned in the function of transmitting the weight of the trunk. It consists the upper three sacral vertebrae and two strong pillars of bone running from the sacroiliac joints to the acetabular fossae. For the reception and diffusion of the weight each acetabular fossa is strengthened by two additional bars running towards the pubis and the ischium. In order to lessen concussion in rapid changes of distribution of the weight, joints (sacroiliac articulations) are interposed between the sacrum and the iliac bones: an accessory joint (symphysis pubis) exists in the middle of the anterior arch.

The sacrum, as the summit of the posterior arch, is loaded at the lumbosacral joint. Theoretically, this force has two components: one thrusting the sacrum downwards and backwards between the iliac bones, the other thrusting its upper end downwards and forwards (the sacrum acts as a lever with two arms - just like a seesaw). Sacral movements are regulated by osseous shape and massive ligaments. The frist component therefore acts against the wedge, its tendency to separate iliac bones resisted by the sacroiliac and iliolumbar ligaments and pubic symphysis. The sacrum is not simply fitted like an ordinary keystone in the arch of the bony pelvic ring. Though broader above, it is narrower posteriorly. Any tendency of the sacrum to shift forwards and downwards is checked by the strong interosseous sacroiliac ligaments. Additionally, maximum loading of the sacrum causes it to turn about a transverse axis so that its anterosuperior extremity is depressed and the coccyx is elevated. The sacrospinous and sacrotuberous ligaments are tensed by this movement and they restrain it. At the same time the iliac bones are tightly pulled against the sacrum by the posterior sacroiliac ligaments. This traction is restrained by the symphysis pubis. The symphysis undergoes traction and, to a lesser extent, compression. Thus the weight of the trunk is partly converted into traction, and then resolved into two components, one acting on the symphysis, the other on the thighs. The weight of the body is transmitted to the sacrum anterior to the axis of rotation at the sacroiliac joint. The tendency for increased weight or force to rotate the upper sacrum anteriorly and inferiorly is resisted by the strong sacrotuberous and sacrospinous ligaments anchoring the inferior sacrum and coccyx to the ischium. Weight is transferred from the axial skeleton to the ilia (plural of ilium) via sacro-iliac ligaments then to the femurs during standing, and to the ischial tuberosities during sitting. As long as tight apposition is maintained between the articular surfaces, the sacro-iliac joints remain stable. Unlike a keystone at the top of an arch, the sacrum is actually suspended between the iliac bones and is firmly attached to them by posterior and interosseous sacro-iliac ligaments. When standing, either in a normally relaxed or a fully erect posture, the body as a whole has its center of gravity in the middle of the pelvic cavity. In the normal posture it is below the promontory and at the level of the posterior inferior iliac spines. The vertical plane in this position passes through the centers of the hip joints, the greater trochanters and the promontory of the sacrum. In a relaxed posture the center of gravity moves a little posteriorly. In a fully erect posture it moves a little anteriorly.

In the standing position, the pelvic canal curves obliquely backwards relative to the trunk and abdominal cavity. The whole pelvis is tilted forwards, the plane of the pelvic brim making an angle of 50–60° with the horizontal. The plane of the pelvic outlet is tilted to about 15°. Strictly, the pelvic outlet has two planes: an anterior passing backwards from the pubic symphysis and a posterior passing forwards from the coccyx, both descending to meet at the intertuberous line. In standing, the pelvic aspect of the pubic symphysis faces nearly as much upwards as backwards and the sacral concavity is directed anteroinferiorly. The front of the pubic symphysis and anterior superior iliac spines are in the same vertical plane. While sitting, body weight is transmitted through the inferomedial parts of the ischial tuberosities, with variable soft tissues intervening. The anterior superior iliac spines are in a vertical plane through the acetabular centres, and the whole pelvis is tilted back with the lumbosacral angle somewhat diminished at the sacral promontory. If a series of coronal sections be made through the sacroiliac joints, it will be found possible to divide the articular portion of the sacrum into three segments: anterior, middle, and posterior. In the anterior segment, which involves the first sacral vertebra, the articular surfaces show slight sinuosities and are almost parallel to one another. In the middle segment the width between the dorsal margins of the sacral articular surfaces is greater than that between the ventral margins, and in the center of each surface there is a concavity intro which a corresponding convexity of the iliac articular surface fits, forming an interlocking mechanism. In the posterior segment the ventral width of the sacrum is greater than the dorsal; and the articular surfaces are only slightly concave.

Dislocation downwards and forwards of the sacrum by the second component of the force applied to it is prevented therefore by the middle segment, which interposes the resistance of its wedge-shape and that of the interlocking mechanism on its surfaces; a rotatory movement, however; is produced by which the anterior segment is tilted downwards and the posterior upwards the axis of this rotation passes, through the dorsal part of the middle segment. The movement of the anterior segment is slightly limited by its wedge-form, but chiefly by the posterior and interosseous sacroiliac ligaments that of the posterior segment is checked to a slight extent by its wedge-form, but the chief limiting factors are the sacrotuberous and sacrospinous ligaments. In all these movements the effect of the sacroiliac and iliolumbar ligaments and the ligaments of the symphysis pubis in resisting the separation of the iliac bones must be recognized.

Stability during Load Transfer: Mechanics of Generating a Nutation Torque at the Sacroiliac Joints. The plane of the articular surfaces of the sacroiliac joint is largely vertical. This orientation renders the joint vulnerable to vertical slipping, especially when subjected to large forces. Nutation at the sacroiliac joints increases the compression and shear forces between joint surfaces, thereby increasing articular stability. For this reason, the close-packed position of the sacroiliac joint is considered to be in full nutation. Forces that create a nutation torque therefore help stabilize the sacroiliac joints. Torques are created by gravity, stretched ligaments, and muscle activation Stabilizing Effect of Gravity. The downward force of gravity resulting from body weight passes through the lumbar vertebrae, usually just anterior to an imaginary line connecting the midpoints of the two sacroiliac joints. At the same time, the femoral heads produce an upward directed compression force through the acetabula. Each of these two forces acts with a separate moment arm to create a nutation torque about the sacroiliac joints (Figure 9-74, A). The torque resulting from body weight rotates the sacrum anteriorly relative to the ilium, whereas the torque resulting from hip compression force rotates the ilium posteriorly relative to the sacrum. This nutation torque “locks” the joints by increasing the friction between the rough and reciprocally contoured articular surfaces. This locking mechanism relies primarily on gravity and congruity of the joint surfaces rather than extra-articular structures such as ligaments and muscles.

Additional (?) -Pregnancy During pregnancy the pelvic joints and ligaments are relaxed, and capable therefore of more extensive movements. When the fetus is being expelled the force is applied to the front of the sacrum. Upward dislocation is prevented by the interlocking mechanism of the middle segment. As the fetal head passes the anterior segment the latter is carried upwards, enlarging the anteroposterior diameter of the pelvic inlet; when the head reaches the posterior segment this also is pressed upwards against the resistance of its wedge, the movement being rendered possible only by the laxity of the joints and the stretching of the sacrotuberous and sacrospinous ligaments. - Pelvic axes and inclination The axis of the superior pelvic aperture traverses its centre at right angles to its plane, directed down and backwards. When prolonged (projected), it passes through the umbilicus and mid-coccyx. An axis is similarly established for the inferior aperture: projected upwards, it impinges on the sacral promontory. Axes can likewise be constructed for any plane, and one for the whole cavity is a concatenation of an infnite series of such lines. The fetal head, however, descends in the axis of the inlet as far as the level of the ischial spines; it is then directed forwards into the axis of the vagina at right angles to that axis. The form of this pelvic axis and the disparity in depth between the anterior and posterior contours of the cavity are prime factors in the mechanism of fetal transit in the pelvic canal.

- Mikulicz angle The femur is essentially a tubular structure with distortions that consist of bows and twists. The most notable is the anterior bow in its mid portion, where the radius of curvature is relatively constant along the length of the femoral shaft. In the coronal (frontal) plane, the femoral neck is inclined obliquely to the shaft at an angle of about 135° (range 120–140°). Although the neck–shaft angle (collo–diaphyseal angle; Mikulicz angle) and neck length are variable, the centre of the neck in the coronal plane is at the level of the apex of the greater trochanter. In the axial (transverse) plane, the femoral neck is anteverted, i.e. rotated anteriorly relative to the posterior surfaces of the femoral condyles; in the adult, this angle is 10–15°. Excessive anteversion may exist when this angle is signifcantly greater than 10–15°. At birth, the angle of anteversion is typically about 35– 40°. As the child develops, forces from muscles and gravity cause the angle of anteversion to

decrease gradually, approaching 15° by young adulthood.

- Forces acting on the hip In quiet upright standing, the femoral heads support the weight of the trunk, upper limbs and head. About two-thirds of body weight is located above the hips, and so each femoral head normally accepts about one-third of body weight. This force is compressive in nature, as gravity pulls the acetabula against the femoral heads. When viewed in the sagittal plane, minimal muscle forces

suffce to maintain equilibrium as long as the weight of the upper body is directed over the femoral heads. If the upper body leans anteriorly, shifting the upper body weight vector anteriorly beyond the femoral heads and thereby producing a hip flexion moment, posterior thigh muscles can counter such rotation. As the capsular ligaments of the hip slacken in flexion, none of them is able to resist the forward lean. The importance of hip abductor muscle activation during the stance phase of walking can be well appreciated by understanding the simple mechanics of standing on one limb. The lever arm (a) of left hip abductor force (Ab) is about half the length of the lever arm (b) associated with body weight (BW). To balance the competing (coronal plane) gravitational moments about the stance hip, the hip abductors must produce a force about twice superincumbent body weight. Thus, the acetabulum is pulled inferiorly against the femoral head not only by the body weight but also by the force created by the activated hip abductor muscles. The sum of the muscular and gravitational forces equals about 2.5 times the total weight of the person. These downward forces on the head of the femur are counteracted by an upward joint reaction force of equal magnitude. Both magnitude and direction of the joint reaction force are strongly influenced by the pull of the hip abductor muscles.

- Muscles producing hip joint movements Flexion is primarily produced by psoas major, iliacus and rectus femoris, assisted by pectineus, tensor fasciae latae and sartorius. The adductors, particularly adductor longus and brevis, also assist, especially when the hip is near full extension. Effective action of the hip flexor muscles requires strong synergistic activation of the abdominal muscles to stabilize the pelvis. Extension is produced by gluteus maximus, biceps femoris, semitendinosus, semimembranosus and adductor magnus. The posterior fbres of gluteus medius assist with this action (Neumann 2010a). In the fully erect posture, a vertical line through the centre of gravity of the body passes posterior to a line joining the centres of the femoral heads. The body therefore tends to incline posteriorly but this is counterbalanced by ligamentous tension and congruence and compression of the articular surfaces within the hip joints in the close-packed position. Under increased loading of the trunk or leaning posteriorly, these

resistive but passive factors are assisted by active force produced by the hip joint flexors. In swaying anteriorly at the ankles, or when the arms are stretched forwards, and also in forward-bending at the hip, the line of body weight moves anterior to the medial–lateral axis of rotation through the hip joints. The posture adopted, or the rate of change of posture, is largely controlled by the hamstrings, which, besides being powerful flexors of the knee, are strong extensors of the hip. Gluteus maximus becomes particularly active when the thigh is extended against resistance, as in rising from a bending position or during climbing. Abduction is produced by glutei medius and minimus, assisted by tensor fasciae latae, piriformis and sartorius. The motion is limited by adductor muscle tension, the pubofemoral ligament and the extreme medial bands of the descending part of the iliofemoral ligament. The abductor muscles, most notably glutei medius and minimus, are active periodically at precise phases of the walking or running cycle to ensure coronal plane stability of the pelvis. Adduction is produced by adductors longus, brevis and magnus and by gracilis assisted by pectineus, quadratus femoris and the inferior fbres of gluteus maximus. The range of adduction is limited by the increasing tension in the abductor muscles, the transverse part of the iliofemoral ligament and the fascia lata of the thigh. Medial rotation is produced by the anterior fbres of glutei minimus and medius, and assisted by tensor fasciae latae and most adductor muscles. The strength of medial rotation naturally increases as the hip is flexed because this position increases the moment arm of most medial rotator muscles (Delp et al 1999). Medial rotation is limited by tension in lateral rotator muscles such as piriformis, the ischiofemoral ligament and the adjacent posterior joint capsule (Wagner et al 2012). Lateral rotation is produced by gluteus maximus, obturator internus, superior and inferior gemelli, quadratus femoris and piriformis, and it is assisted by obturator externus and sartorius. Lateral rotation, a stronger motion than medial rotation, is limited by tension in the medial rotator muscles and the transverse part of the iliofemoral ligament (Myers et al 2011)....