Pelvic girdle, gluteal region and thigh

Published on 17/03/2015 by admin

Filed under Basic Science

Last modified 17/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 11043 times

CHAPTER 80 Pelvic girdle, gluteal region and thigh

The pelvic girdle consists of the paired hip bones (each consisting of the ilium, ischium and pubis) and a single sacrum. The two pubic bones articulate anteriorly at the pubic symphysis and the sacrum articulates posteriorly with the two iliac bones: however, the bones are virtually incapable of independent movement except in the female during parturition. The pelvic girdle is massively constructed and serves as a weightbearing and protective structure, as an attachment for trunk and limb muscles, and as the skeletal framework of a birth canal capable of accommodating a large-headed fetus.

The gluteal region or buttock is an area demarcated by the gluteal fold inferiorly, a line joining the greater trochanter and the anterior superior iliac spine laterally, the iliac crest superiorly and the midline medially. It contains a large bulk of skeletal muscle that covers several vulnerable neurovascular structures, and incorporates junctional zones between the lower limb, pelvis and perineum at the sciatic foramina. Direct and indirect musculoskeletal injuries in this region may damage the sciatic nerve and gluteal vessels.

The thigh consists of a cylinder of compact bone, the femoral shaft, surrounded by muscle groups traversed by important neurovascular structures. The muscles are grouped according to function and lie within osteofascial compartments that are defined by fascial septa running between the femur and an enveloping tube of tough fascia, the fascia lata. The femoral artery gives off its major branch, the profunda femoris, in the anterior compartment, and the sciatic nerve usually divides into its main branches, the tibial and common fibular nerves, as it passes through the posterior compartment. The femoral nerve divides soon after entering the anterior compartment beneath the inguinal ligament; the obturator nerve enters the region proximally and medially from the pelvis and divides into its main branches which run anterior and posterior to adductor brevis.

SKIN AND SOFT TISSUES

SOFT TISSUE

Fascia

Deep fascia

The deep fascia covering the gluteal muscles varies in thickness. Over the maximus it is thin, but over the anterior two-thirds of the medius it forms the thick, strong gluteal aponeurosis. This is attached to the lateral border of the iliac crest superiorly, and splits anteriorly to enclose tensor fasciae latae and posteriorly to enclose gluteus maximus.

Fascia lata

The fascia lata, the wide deep fascia of the thigh, is thicker in the proximal and lateral parts of the thigh where tensor fasciae latae and an expansion from gluteus maximus are attached to it. It is thin posteriorly and over the adductor muscles, but thicker around the knee, where it is strengthened by expansions from the tendon of biceps femoris laterally, sartorius medially, and quadriceps femoris anteriorly. The fascia lata is attached superiorly and posteriorly to the back of the sacrum and coccyx, laterally to the outer margin of the iliac crest, anteriorly to the inguinal ligament and superior ramus of the pubis, and medially to the inferior ramus of the pubis, the ramus and tuberosity of the ischium, and the lower border of the sacrotuberous ligament. From the iliac crest it descends as a dense layer over gluteus medius to the upper border of gluteus maximus, where it splits into two layers, one passing superficial and the other deep to the muscle, the layers reuniting at the lower border of the muscle.

Saphenous opening

The saphenous opening is an aperture in the deep fascia, inferolateral to the medial end of the inguinal ligament, which allows passage to the long saphenous vein and other smaller vessels (Fig. 80.1). The cribriform fascia, which is pierced by these structures, fills in the aperture and must be removed to reveal it. Adjacent subsidiary openings may exist to transmit venous tributaries. In the adult the approximate centre of the opening is 3 cm lateral to a point just distal to the pubic tubercle. The length and width of the opening vary considerably. The fascia lata in this part of the thigh displays superficial and deep strata (not to be confused with the superficial and deep layers of the superficial fascia described above). They lie, respectively, anterior and posterior to the femoral sheath; with the saphenous opening situated where the two layers are in continuity. This serves to explain the somewhat oblique and spiral configuration of the saphenous opening.

The superficial stratum, lateral and superior to the saphenous opening, is attached, in continuity, to the crest and anterior superior spine of the ilium, to the whole length of the inguinal ligament, and to the pecten pubis and lacunar ligament. It is reflected inferolaterally from the pubic tubercle as the arched falciform margin, which forms the superior, lateral and inferior boundaries of the saphenous opening: this margin adheres to the anterior layer of the femoral sheath, and the cribriform fascia is attached to it. The falciform margin is considered to have superior and inferior cornua. The inferior cornu is well defined, and is continuous behind the long saphenous vein with the deep stratum of the fascia lata.

The deep stratum is medial to the saphenous opening and is continuous with the superficial stratum at its lower margin. Traced upwards, it covers pectineus, adductor longus and gracilis, passes behind the femoral sheath, with which it is blends, and continues to the pecten pubis.

Fascial compartments

There are three functional groups of muscle in the thigh, namely, anterior (extensor), posterior (flexor) and medial (adductor). The anterior and posterior groups occupy separate osteofascial compartments that are limited peripherally by the fascia lata and separated from each other by the femur and the medial and lateral intermuscular septa (Fig. 80.2). The adductor muscles, though distinct in terms of function and innervation, do not possess a separate compartment limited by fascial planes. Nevertheless it is customary to speak of three compartments: anterior, posterior and medial. The muscles of the three compartments are described below. Adductor magnus, adductor longus and pectineus could each be considered to be constituents of two compartments, i.e. adductor magnus in the posterior and the medial compartments, and adductor longus and pectineus in the anterior and the medial compartments.

The nerve supply to the compartments of the thigh mainly follows the ‘one compartment – one nerve’ principle. Thus the femoral nerve supplies the anterior compartment muscles, the obturator nerve supplies the medial compartment muscles, and the sciatic nerve supplies those in the posterior compartment. The dual functional and compartmental attribution of adductor magnus and adductor longus and pectineus are reflected in their dual nerve supplies.

In contrast to the motor innervation, the arterial supply to the compartmental muscle groups does not exhibit such a direct relationship. All groups receive a supply from the femoral system, particularly from the profunda femoris and its branches. The adductors receive a contribution from the obturator artery, and the hamstrings receive a proximal supply from the inferior gluteal artery. Further details are given in the descriptions of the individual muscles.

Femoral sheath

The femoral sheath is a funnel-shaped distal prolongation of extraperitoneal fascia, formed of transversalis fascia anterior to the femoral vessels, and of the iliac fascia posteriorly (Fig. 80.3). It is wider proximally and its tapered distal end fuses with the vascular adventitia 3 or 4 cm distal to the inguinal ligament. At birth the sheath is shorter; it elongates when extension at the hips becomes habitual. The femoral branch of the genitofemoral nerve perforates its lateral wall. The medial wall slopes laterally and is pierced by the long (great) saphenous vein and lymphatic vessels. Like the carotid sheath, the femoral sheath encloses a mass of connective tissue in which the vessels are embedded. Three compartments are described: a lateral one containing the femoral artery, an intermediate one for the femoral vein, and a medial compartment, the femoral canal, which contains lymph vessels and an occasional lymph node embedded in areolar tissue. The presence of this canal allows the femoral vein to distend. The canal is conical and approximately 1.25 cm in length. Its proximal (wider) end, termed the femoral ring, is bounded in front by the inguinal ligament, behind by pectineus and its fascia and the pectineal ligament, medially by the crescentic, lateral edge of the lacunar ligament and laterally by the femoral vein. The spermatic cord, or the round ligament of the uterus, is just above its anterior margin, while the inferior epigastric vessels are near its anterolateral rim. It is larger in women than in men: this is due partly to the relatively greater width of the female pelvis and partly to the smaller size of the femoral vessels in women. The ring is filled by condensed extraperitoneal tissue, the femoral septum, which is covered on its proximal aspect by the parietal peritoneum. The femoral septum is traversed by numerous lymph vessels that connect the deep inguinal to the external iliac lymph nodes.

Iliac fascia

The iliac fascia covers psoas and iliacus. It is thin above, but thickens progressively towards the inguinal ligament. The part covering psoas is thickened above as the medial arcuate ligament. Medially, the fascia over psoas is attached by a series of fibrous arches to the intervertebral discs, the margins of vertebral bodies, and the upper part of the sacrum. Laterally, it blends with the fascia anterior to quadratus lumborum above the iliac crest, and with the fascia covering iliacus below the crest.

The iliac part is connected laterally to the whole of the inner lip of the iliac crest and medially to the pelvic brim, where it blends with the periosteum. It is attached to the iliopubic ramus, where it receives a slip from the tendon of psoas minor, when that muscle is present. The external iliac vessels are anterior to the fascia but the branches of the lumbar plexus are posterior. The fascia is separated from the peritoneum by loose extraperitoneal tissue. Lateral to the femoral vessels, the iliac fascia is continuous with the posterior margin of the inguinal ligament and the transversalis fascia. Medially it passes behind the femoral vessels to become the pectineal fascia, attached to the pecten pubis. At the junction of its lateral and medial parts it is attached to the iliopubic ramus and the capsule of the hip joint. It thus forms a septum between the inguinal ligament and the hip bone, dividing the space here into a lateral part, the muscular space, containing psoas major, iliacus and the femoral nerve, and a medial part, the vascular space, transmitting the femoral vessels (Fig. 80.3). The iliac fascia continues downward to form the posterior wall of the femoral sheath.

Obturator membrane

The obturator membrane (Fig. 80.4) is a thin aponeurosis that closes (obturates) most of the obturator foramen, leaving a superolateral aperture, the obturator canal, through which the obturator vessels and nerve leave the pelvis and enter the thigh. The membrane is attached to the sharp margin of the obturator foramen except at its inferolateral angle, where it is fixed to the pelvic surface of the ischial ramus, i.e. internal to the foramen. Its fibres are arranged mainly transversely in interlacing bundles; the uppermost bundle, which is attached to the obturator tubercles, completes the obturator canal. The outer and inner surfaces of the obturator membrane provide attachment for the obturator externus and internus respectively. Some fibres of the pubofemoral ligament of the hip joint are attached to the outer surface.

BONE

The pelvic girdle is an entity consisting of the two hip bones and the sacrum (strictly speaking, the sacrum is part of the vertebral column). These bones are virtually incapable of independent movement except in the female during parturition. The pelvic girdle is massively constructed and serves as a weightbearing and protective structure, an attachment for trunk and limb muscles, and as the skeletal framework of the birth canal.

HIP BONE

The hip bone is large, irregular, constricted centrally and expanded above and below (Fig. 80.5A,B). Its lateral surface has a deep, cup-shaped acetabulum, articulating with the femoral head, anteroinferior to which is the large, oval or triangular obturator foramen. Above the acetabulum the bone widens into an undulant plate surmounted by a sinuously curved iliac crest.

The bone articulates in front with its fellow and posteriorly with the side of the sacrum to form the pelvic girdle. Each hip bone has three parts, ilium, ischium and pubis, connected to each other by cartilage in youth but united as one bone in adults. The principal union is in the acetabulum. The ilium includes the upper acetabulum and expanded area above it; the ischium includes the lower acetabulum and bone posteroinferior to it; the pubis forms the anterior acetabulum, separating the ilium from ischium, and the anterior median region where the pubes meet.

Acetabulum

The acetabulum (Fig. 80.5A,C) is an approximately hemispherical cavity situated about the centre of the lateral aspect of the hip bone. It faces anteroinferiorly and is circumscribed by an irregular margin deficient inferiorly at the acetabular notch. The acetabular fossa forms the central floor and is rough and non-articular. The articular lunate surface is widest above (the ‘dome’), where weight is transmitted to the femur. Consequently, fractures through this region tend to be associated with unsatisfactory outcomes. All three components of the hip bone contribute to the acetabulum, although unequally. The pubis forms the anterosuperior fifth of the articular surface, the ischium forms the floor of the fossa and rather more than the posteroinferior two-fifths of the articular surface, and the ilium forms the remainder. Occasionally a linear defect may be seen to cross the acetabular surface from the superior border to the acetabular fossa This does not correspond to any junction between the main morphological parts of the hip bone.

Structure

The thicker parts of the hip bone are trabecular, encased by two layers of compact bone, while the thinner parts, as in the acetabulum and central iliac fossa, are often translucent and consist of a single lamina of compact bone. In the upper acetabulum and along the arcuate line, i.e. the route of weight transmission from the sacrum to the femur, the amount of compact bone is increased and the subjacent trabecular bone displays two sets of pressure lamellae. These start together near the upper auricular surface and diverge to meet two strong buttresses of compact bone, from which two similar sets of lamellar arches start and converge on the acetabulum. The anterior part of the iliac crest has been much studied with regard to distribution of cortical and trabecular bone. For a survey of these studies consult Whitehouse (1977). Additionally, Whitehouse’s observations, based on scanning electron micrography, indicate that the cortical bone is very porous, being only 75% bone, decreasing to 35% near the anterior superior iliac spine. Denser cortical bone starts at the margins of the crest and thickens rapidly below it on both aspects of the iliac blade.

Studies of the internal stresses within the hip bone have revealed a pattern of trabeculae that corresponds well with the theoretically expected patterns of stress trajectories (Holm 1980). These patterns are considerably more complex than in any other major bone. Stresses are higher in the acetabular than in the iliac region. In the ilium, the pelvic surface is subjected to considerably less stress than is the gluteal surface.

Ossification

Ossification (Figs 80.6, 80.7) is by three primary centres, one each for the ilium, ischium and pubis. The iliac centre appears above the greater sciatic notch prenatally at about the ninth week; the ischial centre in its body in the fourth month, and the pubic centre in its superior ramus between the fourth and fifth months. At birth the whole iliac crest, the acetabular floor and inferior margin are cartilaginous. Gradual ossification of the three components of the acetabulum results in a triradiate cartilaginous stem extending medially to the pelvic surface as a Y-shaped epiphysial plate between the ilium, ischium and pubis, and including the anterior inferior iliac spine. Cartilage along the inferior margin also covers the ischial tuberosity, forms conjoined ischial and pubic rami, and continues to the pubic symphysial surface and along the pubic crest to its tubercle.

The ossifying ischium and pubis fuse to form a continuous ischiopubic ramus at the seventh or eighth year. Secondary centres, other than for the acetabulum, appear about puberty and fuse between the 15th and 25th years. There are usually two for the iliac crest (which fuse early), and single centres for the ischial tuberosity (in cartilage close to the inferior acetabular margin and spreading forwards), anterior inferior iliac spine (although it may ossify from the triradiate cartilage), and symphysial surface of the pubis (the pubic tubercle and crest may have separate centres). Progression of ossification of the iliac crest in girls is an index of skeletal maturity and is useful in determining the optimal timing of surgery for spinal deformity.

Between the ages of 8 and 9 years three major centres of ossification appear in the acetabular cartilage. The largest appears in the anterior wall of the acetabulum and fuses with the pubis, the second in the iliac acetabular cartilage superiorly, fusing with the ilium, and the third in the ischial acetabular cartilage posteriorly, fusing with the ischium. At puberty these epiphyses expand towards the periphery of the acetabulum and contribute to its depth (Ponseti 1978). Fusion between the three bones within the acetabulum occurs between the 16th and 18th years. Delaere et al (1992) have suggested that ossification of the ilium is similar to that of a long bone, possessing three cartilaginous epiphyses and one cartilaginous process, although it tends to undergo osteoclastic resorption comparable with that of cranial bones. During development the acetabulum increases in breadth at a faster rate than it does in depth.

Avulsion fractures of pelvic apophyses may occur from excessive pull on tendons, usually in athletic adolescents. The most frequent examples of such injuries are those to the ischial tuberosity (hamstrings) and anterior inferior iliac spine (rectus femoris).

Pubis

Topography

The pubis (Figs 80.5, 80.6) is the ventral part of the hip bone and forms a median cartilaginous pubic symphysis with its fellow. The body of the pubis occupies the anteromedial part of the bone, and from the body a superior ramus passes up and back to the acetabulum and an inferior ramus passes back, down and laterally to join the ischial ramus inferomedial to the obturator foramen.

Ilium

Topography

The ilium has upper and lower parts and three surfaces (Figs 80.5, 80.6). The smaller, lower part forms a little less than the upper two-fifths of the acetabulum. The upper part is much expanded, and has gluteal, sacropelvic and iliac (internal) surfaces. The posterolateral gluteal surface is an extensive rough area; the anteromedial iliac fossa is smooth and concave; the sacropelvic surface is medial and posteroinferior to the fossa, from which it is separated by the medial border.

Iliac crest

The iliac crest is the superior border of the ilium. It is convex upwards but sinuous from side to side, being internally concave in front, and convex behind. Its ends project as anterior and posterior superior iliac spines. The anterior superior iliac spine is palpable at the lateral end of the inguinal fold; the lateral end of the inguinal ligament is attached to the anterior superior iliac spine. The posterior superior iliac spine is not palpable but is often indicated by a dimple, approximately 4 cm lateral to the second sacral spinous process, above the medial gluteal region (buttock).

The crest has ventral and dorsal segments. The ventral occupies slightly more than the anterior two-thirds of the crest and its prominence is associated with changes in iliac form as a result of the emergence of the upright posture; the dorsal segment, which occupies approximately the posterior third in man, is a feature of all land vertebrates. The ventral segment of the crest has internal and external lips and the rough intermediate zone is narrowest centrally. The tubercle of the crest projects outwards from the outer lip approximately 5 cm posterosuperior to the anterior superior spine. The dorsal segment has two sloping surfaces separated by a longitudinal ridge ending at the posterior superior spine. The summit of the crest, a little behind its midpoint, is level with the fourth lumbar vertebral body. The interosseous and posterior sacroiliac ligaments arise from the medial margin of the dorsal segment.

Sacropelvic surface

The sacropelvic surface, the posteroinferior part of the medial iliac surface, is bounded posteroinferiorly by the posterior border, anterosuperiorly by the medial border, posterosuperiorly by the iliac crest and anteroinferiorly by the line of fusion of the ilium and ischium. It is divided into iliac tuberosity, auricular and pelvic surfaces. The iliac tuberosity, a large, rough area below the dorsal segment of the iliac crest, shows cranial and caudal areas separated by an oblique ridge and connected to the sacrum by the interosseous sacroiliac ligament. The sacropelvic surface gives attachment to the posterior sacroiliac ligaments and, behind the auricular surface, to the interosseous sacroiliac ligament. The iliolumbar ligament is attached to its anterior part. The auricular surface, immediately anteroinferior to the tuberosity, articulates with the lateral sacral mass. Shaped like an ear, its widest part is anterosuperior, its ‘lobule’ posteroinferior and on the medial aspect of the posterior inferior spine. Its edges are well defined, but the surface, though articular, is rough and irregular. It articulates with the sacrum and is reciprocally shaped. The anterior sacroiliac ligament is attached to its sharp anterior and inferior borders. The narrow part of the pelvic surface, between the auricular surface and the upper rim of the greater sciatic notch, often shows a rough preauricular sulcus (that is usually better defined in females) for the lower fibres of the anterior sacroiliac ligament. For the reliability of this feature as a sex discriminant, refer to Finnegan (1978) and Brothwell & Pollard (2001). The pelvic surface is anteroinferior to the acutely recurved part of the auricular surface, and contributes to the lateral wall of the lesser pelvis. Its upper part, facing down, is between the auricular surface and the upper limb of the greater sciatic notch. Its lower part faces medially and is separated from the iliac fossa by the arcuate line. Anteroinferiorly it extends to the line of union between the ilium and ischium. Though usually obliterated, it passes from the depth of the acetabulum to approximately the middle of the inferior limb of the greater sciatic notch.

Muscle attachments

The attachment of sartorius extends down the anterior border below the anterior superior iliac spine. The iliac crest gives attachment to the anterolateral and dorsal abdominal muscles, and to the fasciae and muscles of the lower limb. The fascia lata and iliotibial tract are attached to the outer lip and tubercle of its ventral segment. Tensor fasciae latae is attached anterior to the tubercle. The lower fibres of external oblique and, just behind the summit of the crest, the lowest fibres of latissimus dorsi are attached to its anterior two-thirds. A variable interval exists between the most posterior attachment of external oblique and the most anterior attachment of latissimus dorsi, and here the crest forms the base of the lumbar triangle. Internal oblique is attached to the intermediate area of the crest. Transversus abdominis is attached to the anterior two-thirds of the inner lip of the crest, and behind this to the thoracolumbar fascia and quadratus lumborum. The highest fibres of gluteus maximus are attached to the dorsal segment of the crest on its lateral slope. Erector spinae arises from the medial slope of the dorsal segment. The straight part of rectus femoris is attached to the upper area of the anterior inferior spine. Some fibres of piriformis are attached in front of the posterior inferior spine on the upper border of the greater sciatic notch.

The gluteal surface is divided by three gluteal lines into four areas. Behind the posterior line, the upper rough part gives attachment to the upper fibres of gluteus maximus and the lower, smooth region to part of the sacrotuberous ligament and iliac head of piriformis. Gluteus medius is attached between the posterior and anterior lines, below the iliac crest, and gluteus minimus is attached between the anterior and inferior lines. The fourth area, below the inferior line, contains vascular foramina.

The reflected head of rectus femoris attaches to a curved groove above the acetabulum. Iliacus is attached to the upper two-thirds of the iliac fossa and is related to its lower third. The medial part of quadratus lumborum is attached to the anterior part of the sacropelvic surface, above the iliolumbar ligament. Piriformis is sometimes partly attached lateral to the preauricular sulcus, and part of obturator internus is attached to the more extensive remainder of the pelvic surface.

Ischium

Topography

The ischium, the inferoposterior part of the hip bone, has a body and ramus. The body has upper and lower ends and femoral, posterior and pelvic surfaces (Figs 80.5, 80.6). Above, it forms the posteroinferior part of the acetabulum; below, its ramus ascends anteromedially at an acute angle to meet the inferior pubic ramus, thereby completing the boundary of the obturator foramen. The ischiofemoral ligament is attached to the lateral border below the acetabulum.

The femoral surface faces downwards, forwards and laterally towards the thigh. It is bounded in front by the margin of the obturator foramen. The lateral border, indistinct above but well defined below, forms the lateral limit of the ischial tuberosity. At a higher level the femoral surface is covered by piriformis, from which it is partially separated by the sciatic nerve and the nerve to quadratus femoris. The posterior surface, facing superolaterally, is continuous above with the iliac gluteal surface, and here a low convexity follows the acetabular curvature. Inferiorly, this surface forms the upper part of the ischial tuberosity, above which is a wide, shallow groove on its lateral and medial aspects. Above the ischial tuberosity the posterior surface is crossed by the tendon of obturator internus and the gemelli. The nerve to quadratus femoris lies between these structures and the bone. The ischial tuberosity is a large, rough area on the lower posterior surface and inferior extremity of the ischium. Though obscured by gluteus maximus in hip extension, it is palpable in flexion. It is 5 cm from the midline and about the same distance above the gluteal fold. It is elongated, widest above, and tapers inferiorly. The ischial posterior aspect lies between the lateral and posterior borders. The posterior border blends above with that of the ilium, helping to complete the inferior rim of the greater sciatic notch, the posterior end of which has a conspicuous ischial spine. Below this, the rounded border forms the floor of the lesser sciatic notch, between the ischial spine and tuberosity. The pelvic surface is smooth and faces the pelvic cavity; inferiorly it forms part of the lateral wall of the ischio-anal fossa.

Ischial spine

The ischial spine projects downwards and a little medially (Fig. 80.8). The sacrospinous ligament is attached to its margins, separating the greater from the lesser sciatic foramen. The ligament is crossed posteriorly by the internal pudendal vessels, pudendal nerve and the nerve to obturator internus.

Muscle attachments

Part of obturator externus is attached to the lower femoral surface of the ischial body. Part of obturator externus, the anterior fibres of adductor magnus and, near the lower border, gracilis, are all attached to the anterior surface of the ischial ramus. Between adductor magnus and gracilis the attachment of adductor brevis may descend from the inferior pubic ramus. The posterior surface is divided into pelvic and perineal areas. The pelvic area, facing back, has part of obturator internus attached to it. The perineal area faces medially; its upper part is related to the crus of the penis or clitoris, and its lower part gives attachment to sphincter urethrae, ischiocavernosus and the transverse superficial perineal muscle.

The ischial tuberosity gives attachment to the posterior femoral muscles. Quadratus femoris is attached along the upper part of its lateral border. The upper area of the tuberosity is subdivided by an oblique line into a superolateral part for semimembranosus and an inferomedial part for the long head of biceps femoris and semitendinosus. The lower area is subdivided by an irregular vertical ridge into lateral and medial areas. The larger lateral area is for part of adductor magnus. Superomedial to the tuberosity the posterior surface has a wide, shallow groove, usually covered by hyaline cartilage, with a bursa between it and the tendon of obturator internus. Gemellus inferior is attached to the lower margin of the groove, near the tuberosity. Gemellus superior is attached to the upper margin, near the ischial spine.

The pelvic surface of the ischial spine gives attachment to coccygeus (coextensive with the sacrospinous ligament) and to the most posterior fibres of levator ani. Obturator internus is attached to the upper part of the smooth pelvic ischial surface and converges on the lesser sciatic notch, covering the rest of this surface other than the pelvic aspect of the ischial spine: the muscle and its fascia separate the bone from the ischiorectal fossa.

SKELETAL PELVIS AS A WHOLE

The term pelvis (‘basin’) is applied variously to the skeletal ring formed by the hip bones and the sacrum, the cavity therein, and even the entire region where the trunk and lower limbs meet. It is used here in the skeletal sense, to describe the irregular osseous girdle between the femoral heads and fifth lumbar vertebra. It is large because its primary function is to withstand the forces of body weight and musculature. In this section, its obstetric, forensic and anthropological significance will be considered.

The pelvis can be regarded as having greater and lesser segments, the true and false pelves. The segments are arbitrarily divided by an oblique plane passing through the sacral promontory posteriorly and the lineae terminales elsewhere. Each linea terminalis includes the iliac arcuate line, pectineal line (pecten), and pubic crest. The segments are continuous, and the parts of the body cavity that they enclose are also continuous through the pelvic inlet (pelvic inlet).

Pelvic outlet (inferior pelvic aperture)

Less regular in outline than the pelvic inlet, the pelvic outlet is indented behind by the coccyx and sacrum and bilaterally by the ischial tuberosities. Its perimeter thus consists of three wide arcs. Anteriorly is the pubic arch, between the converging ischiopubic rami. Posteriorly and laterally on both sides are the sciatic notches between the sacrum and ischial tuberosities. These are divided by the sacrotuberous and sacrospinous ligaments into greater and lesser sciatic foramina.

With ligaments included, the pelvic outlet is rhomboidal. Its anterior limbs are the ischiopubic rami (joined by the inferior pubic ligament) and its posterior margins are the sacrotuberous ligaments, with the coccyx in the midline. The outlet is thus not rigid in its posterior half, being limited by ligaments and the coccyx, all slightly yielding. Even with the sacrum taken as the posterior midline limit (more reliable for measurement), there may be slight mobility at the sacroiliac joints. Note also that a plane of the pelvic outlet is merely conceptual. The anterior, ischiopubic part has a plane which is inclined down and back to a transverse line between the lower limits of the ischial tuberosities, and the posterior half has a plane approximating to the sacrotuberous ligaments, sloping down and forwards to the same line.

Three measurements are made for the pelvic outlet. The anteroposterior diameter is usually measured from the coccygeal apex to the midpoint of the lower rim of the symphysis. The lowest sacral point may also be used (on average male 8 cm, female 12.5 cm). The transverse (bituberous) diameter is measured between the ischial tuberosities at the lower borders of their medial surfaces (on average male 8.5 cm, female 11.8 cm). The oblique diameter extends from the midpoint of the sacrotuberous ligament on one side to the contralateral ischiopubic junction (on average male 10 cm, female 11.8 cm). All measurements vary with the individual and racial group.

Pelvic axes and inclination

The axis of the superior pelvic aperture traverses its centre at right angles to its plane, directed down and backwards (Fig. 80.9). When prolonged (projected) it passes through the umbilicus and midcoccyx. 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 infinite series of such lines (Fig. 80.9). 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.

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 symphysis pubis faces nearly as much upwards as backwards and the sacral concavity is directed anteroinferiorly. The front of the symphysis and anterior superior iliac spines are in the same vertical plane. In sitting, body weight is transmitted through 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.

Pelvic mechanism

The skeletal pelvis supports and protects the contained viscera, but is primarily part of the lower limbs, affording wide attachment for leg and trunk muscles. It 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. The posterior arch, chiefly concerned in transmitting weight, consists of the upper three sacral vertebrae and strong pillars of bone from the sacroiliac joints to the acetabular fossae. 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 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. Sacral movements are regulated by osseous shape and massive ligaments. The first component therefore acts against the wedge, its tendency to separate iliac bones resisted by the sacroiliac and iliolumbar ligaments and symphysis pubis.

Vertical coronal sections through the sacroiliac joints suggest division of the (synovial) articular region of the sacrum into three segments. In the anterosuperior segment, involving the first sacral vertebra, the articular surfaces are slightly sinuous and almost parallel. In the middle segment the posterior width between the articular markings is greater than the anterior, and centrally a sacral concavity fits a corresponding iliac convexity, an interlocking mechanism relieving the strain on the ligaments produced by body weight. In the posteroinferior segment the anterior sacral width is greater than the posterior and here its sacral surfaces are slightly concave. Anteroinferior sacral dislocation by the second component (of force) is prevented, therefore, mainly by the middle segment, owing to its cuneiform shape and interlocking mechanism. However, some rotation occurs, in which the anterosuperior segment tilts down and the posteroinferior segment up. ‘Superior’ segmental movement is limited to a small degree by wedging but primarily by tension in the sacrotuberous and sacrospinous ligaments. In all movements the sacroiliac and iliolumbar ligaments and symphysis pubis resist separation of the iliac bones.

SEXUAL DIFFERENCES IN THE PELVIS

The pelvis provides the most marked skeletal differences between male and female. Distinction can be made even during fetal life, particularly in the subpubic arch. In infancy, dimensions of the whole pelvis are greater in males than in females, but the size of the pelvic cavity is usually greater in females. This distinction prevails in childhood, but the difference is maximal at about 22 months. Sexual differences in adults are divisible into metrical and non-metrical features: the range of most features overlaps between the sexes.

Differences are inevitably linked to function. While the primary pelvic function in both sexes is locomotor, the pelvis, particularly the lesser pelvis, is adapted to parturition in females, and these changes variably affect the proportions and dimensions of the greater pelvis. Since males are distinctly more muscular and therefore more heavily built, overall pelvic dimensions, such as the intercristal measurement (distance between the iliac crests), are greater, markings for muscles and ligaments more pronounced, and general architecture heavier. The male iliac crest is more rugged and more medially inclined at its anterior end; in females the crests are less curved in all parts. The iliac alae are more vertical in females, but do not ascend so far; the iliac fossae are therefore shallower and each iliopectineal line more vertical. These iliac peculiarities probably account for the greater prominence of female hips.

Buy Membership for Basic Science Category to continue reading. Learn more here