9.3.1 Gender-based Differences

The gender-specific characteristics of the pelvis are presented in almost every anatomy book. In summary, these characteristics are based on the difference in form and are most distinctly seen in the ala of the ilium and the ischial tuberosities. In total, the male pelvis is described as being long and slender and the female pelvis as being wider and shorter. The dimensions of the female pelvis are therefore seen as a phylogenetic adaptation to the requirements of the birth canal during childbirth.

The differences in detail:

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  The alae of the ilia are higher and more slender in the male pelvis.

  
  
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  The inner pelvic ring, the level of the pelvic inlet, or the arcuate line tend to be rounder in the male pelvis and more transversely elliptical in the female pelvis.

  
  
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  The two inferior pubic rami form an arch (pubic arch) in the female pelvis. It has been described as more of an angle (pubic angle) in the male pelvis.

Naturally, these different characteristics in the bony anatomy of the pelvis also have a meaning for local surface-anatomy. They determine what is to be expected topographically when searching for a specific structure (▶ Fig. 9.10 ):

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  The iliac crests are readily used for quick orientation in the lumbar area. The most superior aspect of the iliac crest is found higher up in males than in females:

  
  
  
  
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    According to McGaugh et al. (2007), the line connecting the most superior aspect of the iliac crests (Jacoby’s line)

    
    
    
    
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      is located at the level of the L4 spinous process in 59% of men. In 22% it is at the level of the L4/L5 interspinous space and in 14% at the level of the L5 spinous process.

      
      
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      in 46% of women, it is located at the level of the L4 spinous process, in 28% at the level of the L4/L5 interspinous space, and in 26% at the level of the L5 spinous process.

      
      

      However, identifying the L4 level requires very precise palpation of the bony edge. Chakraverty et al. (2007) and Pysyk et al. (2010) have cautioned that manual palpation tends to access a higher level (L3 or L3/L4)

    
    
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    As with the iliac crests, the anterior superior iliac spine (ASIS) is preferably located to determine levels within the pelvis. It can be assumed that the female ASISs are found significantly further apart than their male counterparts. Therefore, it is necessary to search for them more laterally.

  
  
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  The inferior pubic rami meet at a significantly smaller angle in the male pelvis. It is therefore expected that the ischial tuberosities can be palpated significantly more medially in the male pelvis than in the female pelvis.

9.3.2 Coxal Bone

The coxal bone is the largest fused bony entity in the musculoskeletal system once skeletal growth has been completed. Two surfaces extend superiorly and inferiorly from a central collection of bony mass in the acetabular area:

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  Superior surface = ala of the ilium. This surface is entirely osseous. Its borders are strengthened by strong edges and projections (iliac crest and diverse spines). Although the middle of the ala of the ilium is osseous as well, it tends to be thinner and can be perforated in some cases.

  
  
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  Inferior surface = the rami of the ischium and the pubis with a central collagen plate (obturator membrane).

When planes are drawn at a tangent over these superior and inferior surfaces, these planes are seen to be found at a 90° angle to each other (▶ Fig. 9.11).

The protruding edges, spines, and flattened areas of both surfaces of the coxal bone act as possible sites of origin or insertion for muscles and ligaments. Anatomical specimens show that the ilium is almost completely enclosed by the small gluteal muscles and the iliacus. The obturator membrane is likewise located between the obturator externus and the obturator internus. Thus, a series of active dynamic forces act on the coxal bone.

Other sections with significantly spongy thickening (▶ Fig. 9.12) can be identified in addition to the above-mentioned bony edges at the edge of both coxal bones and the central bony mass:

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  The body’s weight in standing is transferred from the SI joint to the acetabulum and vice versa along the arcuate line. The arcuate line divides the greater from the lesser pelvis (1).

  
  
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  Weight is transferred in sitting between the SI joint and the ischial tuberosity (2).

  
  
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  Pressure and tensile stresses are transmitted from the coxal bone onto the symphysis via the superior pubic ramus (3).

The weight of the body is transferred from the vertebral column onto the pelvis at the SI joint. This is approximately 60% of the entire body weight in an upright position.

9.3.3 Sacrum

The sacrum is the third and central part of the bony pelvis. It is well known that the sacrum is a fusion of at least five originally distinct vertebrae. The final ossification into a single bone occurs in the fifth decade of life. Remnants of cartilaginous disks are existent prior to this.

Location and Position

The location and position of the vertebral column’s kyphotic section at the pelvis can be identified in the median cut of the pelvis. The recognizable tilt of the sacrum into the pelvic space can be calculated by using the angle between the transverse plane and a line extending from the end plate of S1 (Kapandji, 2006). This generally amounts to approximately 30° (▶ Fig. 9.13).

Fig. 9.13 Position of the sacrum.

The sacrum’s position has several consequences:

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  It is the foundation for the lumbar lordosis and therefore the double “S” seen in the vertebral column.

  
  
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  The tip of the sacrum points posteriorly and enlarges the inferior section of the birth canal.

  
  
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  Vertical loading in the upright position is transformed less into translational movement and more into rotational movement (tendency to nutate). This is absorbed by the ligamentous apparatus.

The sacrum’s distinctive form becomes evident in the posterior view (▶ Fig. 9.14). It is characterized by various structures:

Fig. 9.14 General shape of the sacrum.

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  The S1 end plate (base of the sacrum).

  
  
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  The sides of the sacrum:

  
  
  
  
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    S1 to S3 = auricular surface and the sacral tuberosities (neither are palpable).

    
    
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    S3 to S5 = edge of the sacrum (palpable).

  
  
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  The connection between the inferolateral angles.

It is now apparent that the sacrum is not triangular in shape but rather trapezoid.

Detailed Anatomy

The posterior aspect demonstrates additional interesting details (▶ Fig. 9.15 ):

Fig. 9.15 Sacrum, posterior aspect.

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  S1 has not only received the vertebral body end plate, but also the superior articular processes. These form the most inferior vertebral joints with L5.

  
  
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  Generally, it is possible to look through the bony model in four places on each side. The sacral foramina are found posteriorly and anteriorly at the same level and allow the anterior rami and the posterior rami of the spinal nerves to exit from the vertebral column and into the periphery.

  
  
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  Long ridges are found over the entire remaining posterior surface. These ridges are formed by the rudiments of the sacral vertebrae that have grown together. The median sacral crest is the most important of these ridges for palpation. The rudiments of the sacral spinous processes can be seen here as irregular protrusions and can be palpated well. All other crests and the posterior foraminae are hidden under thick fascia and the multifidus muscle.

Apex of the Sacrum and the Coccyx

The apex of the sacrum forms the sacrum’s inferior border. It lies in the middle, slightly inferior to the line connecting the two inferolateral angles. The mobile connection to the coccyx is found here. This is interchangeably labeled a synovial joint or a synchondrosis (with the intervertebral disk) in literature (▶ Fig. 9.16; Rauber and Kopsch, 2003).

Fig. 9.16 Sacrococcygeal transition.

Great variations are seen in the construction of the inferior sacral area. The median sacral crest usually runs down to the level of S4. Normally no rudiments of the spinous processes can be observed at the S5 level. Instead, an osseous cleft can be seen: the sacral hiatus. According to von Lanz and Wachsmuth (2004a), this posterior cleft is only found in approximately 46% of the population at the level of S5 and extends to the level of S4 or S3 in 33.5%. This makes accurate palpatory orientation on the inferior sacrum significantly more difficult.

The S5 arch leading to the hiatus is incomplete and is covered by a membrane (▶ Fig. 9.17). Small osseous horns (sacral horns) form its borders on the side. These horns are easily palpable in most cases but vary greatly in size and are irregularly shaped. They face two small osseous protrusions in the coccygeal bone, the coccygeal cornua, which are also palpable.

Fig. 9.17 Hiatus and membrane.

The covering membrane at the level of S5 is a continuation of the supraspinous ligament and continues onto the coccyx as the superficial posterior sacrococcygeal ligament. The membrane covers the vertebral canal as it peters out inferiorly. It is palpated as a firm and elastic structure, which clearly distinguishes it from the osseous borders.

Additional ligamentous connections between the sacrum and coccyx are (▶ Fig. 9.18 ):

Fig. 9.18 Ligamentous connections between the sacrum and coccyx.

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  The deep posterior sacrococcygeal ligament, the continuation of the posterior longitudinal ligament.

  
  
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  The lateral sacrococcygeal ligament (intercornual and lateral sections), presumably continuations of the former ligamenta flava and the intertransverse ligament.

These ligamentous structures are traumatically overstretched when people fall onto their buttocks and especially onto the coccyx. Their tenderness on pressure can be treated successfully using transverse frictions to relieve pain when they are directly palpated.

9.3.4 The Pelvic Ligaments

The ligaments of the pelvis can be classified according to their position and function. We are therefore familiar with ligaments that:

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  act to maintain contact between the surfaces of the SI joint:

  
  
  
  
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    interosseous sacroiliac ligaments, located directly posterior to the SI joints;

  
  
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  restrict nutation and therefore stabilize the sacrum:

  
  
  
  
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    anterior sacroiliac ligaments (reinforce the capsule);

    
    
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    posterior sacroiliac ligaments;

    
    
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    sacrotuberous ligament;

    
    
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    sacrospinous ligament;

  
  
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  can limit counternutation:

  
  
  
  
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    long posterior sacroiliac ligament.

The anterior sections of the capsule (anterior sacroiliac ligaments) are very thin (< 1 mm) and have little mechanical relevance (personal correspondence from the IAOM study group). They perforate easily when joint pressure is increased (arthritis). They are not stretched during the iliac posterior test (SI joint test) as the entire function of the ligament is found posterior to the joint.

The interosseus ligaments are very short, nociceptively supplied ligaments that act as pain generators in the presence of sacroiliac pathologies (e.g., instability or blockages). Their function is to maintain the traction in the respective SI joint.

It is easiest to understand the function of the nutation restrictors by looking at the stress on the sacrospinous and sacrotuberous ligaments when the body is in a vertical position (▶ Fig. 9.19). Approximately 60% of the body’s weight bears down on the S1 end plate. This is positioned quite anterior to the nutation/counternutation axis so that the base of the sacrum tends to fall farther into the pelvic space. This tendency is counteracted by the posterior and anterior ligaments positioned very close to the joint. The tip of the sacrum tends to lever itself anteriorly and superiorly. This movement is counteracted by the sacrospinous and sacrotuberous ligaments.

The long posterior sacroiliac ligament (▶ Fig. 9.20) connects both posterior superior iliac spines (PSISs) with the respective edge of the sacrum. It is approximately 3 to 4 cm long, 1 to 2 cm wide, and extends inferiorly into the sacrotuberous ligament. It is the only ligament that counteracts counternutation. It has been described by Vleeming et al. (1996) and already published several times. It has also been mentioned by Dvořák et al. (2008).

The fibers of the multifidus muscle are noticeable as they extend medially into the ligament. A section of the ligament arises from the gluteus maximus on the lateral side.

9.3.5 The Sacroiliac Joint

The significance of the pelvis as central element in the musculoskeletal system has already been described. To understand the exceptional significance of the SI joint, the functional relationship between the various kinematic chains should first be clarified.

Reasons for the Differences in Opinion about the SI Joint

Special Anatomical Factors

The construction of this joint cannot be compared with any “traditional” joint (▶ Fig. 9.21):

Fig. 9.21 Sacroiliac (SI) joint surfaces (according to Kapandji).

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  It is a firm joint (amphiarthrosis) anteriorly and connects the bones posteriorly via a ligamentous structure (syndesmosis).

  
  
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  The joint surfaces are curved at all levels and have ridges and grooves.

  
  
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  The sacral joint surface is very thick, the iliac surface extremely rough.

SI Behavior during Movement

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  The sacrum and the ilium always move against each other in a three-dimensional manner.

  
  
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  Describing the position of the axes during these movements is extremely complicated.

  
  
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  Movement primarily occurs around a frontotransversal (transversal) axis and is very slight (according to Goode et al. [2008], approximately a maximum of 2°). These movements are labeled nutation and counternutation (▶ Fig. 9.22). The extent of joint mobility is influenced by hormones, particularly in women (Brooke, 1924 and Sashin, 1930). Mobility also increases when SI joint disorders are present, for example, with arthritis.

  
  
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  The male SI joint starts to become immobile from around age 50 due to the formation of osseous bridges (Brooke, 1924; Stewart, 1984).

Fig. 9.22 Sacroiliac (SI) joint movements (according to Kapandji).

The complexity of this joint also makes it easy to understand why comparatively few good studies exist that examine standardized assessment methods and treatment techniques. More than 50 tests have been described for assessment alone.

Memorize

For these reasons, the SI joint remains obscure and difficult to comprehend—a mystical structure, a platform for experience and speculation.

9.3.6 Sacroiliac Joint Biomechanics

With an average surface area of 17.5 cm², the SI joint is the largest joint in the human body (Rana et al., 2015).

The SI joint is held together by its structure and the strength of tissues. This can be seen in the frontal plane by looking at the general alignment of the joint surfaces. According to Winkel (1992) the joint surfaces are tilted at approximately 25° from the vertical (▶ Fig. 9.23).

The sacrum’s wedge shape permits the auricular surface to support itself on the similarly shaped iliac joint surface (force closure). Nevertheless, the joint’s construction and the friction coefficient of the uneven and roughened surface are not sufficient to stabilize the sacrum’s position.

It therefore becomes clear that additional strength is needed to keep the joint surfaces together (holding the joint together with the strength of tissues). In particular, this is the function of the interosseous sacroiliac ligaments. These ligaments lie immediately posterior to the joint surfaces and are made of short, very strong, and nociceptively innervated collagen fibers. The SI joint is held together more by the joint structure in males and the strength of tissues in females.

The interosseous ligaments are supported by muscular structures and other ligamentous structures that generally act as nutation restrictors. These structures therefore qualify as further SI joint stabilizers:

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  The anterior abdominal muscles (especially the oblique and transverse sections) pull on the ilia anteriorly and place the interosseous ligaments under tension (▶ Fig. 9.24).

  
  
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  The complex thoracolumbar fascia is considered an important stabilizer of the lumbosacral region (Vleeming and Dorman, 1995).

  
  
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  The multifidus acts as a hydrodynamic strengthener. Its swelling during contraction tightens the thoracolumbar fascia.

  
  
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  The gluteus maximus originates on the posterior surface of the sacrum. The superficial fibers cross over the SI joint and likewise radiate into the thoracolumbar fascia.

  
  
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  The piriformis originates on the anterior surface of the sacrum. It crosses over the SI joint.

  
  
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  The pelvic floor muscles, for example, coccygeus and levator ani exert their force onto the posterior pelvis.

  
  
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  The posterior and anterior sacroiliac ligaments, together with the sacrospinous and sacrotuberous ligaments, primarily restrict the nutation of the sacrum. Loading tightens these ligaments and likewise increases the compression of the SI joint.

  
  
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  Several sections of the iliolumbar ligaments cross over the SI joint in the middle. Lumbar lordosis increases the SI joint surface compression (▶ Fig. 9.25).

Pool-Goudzwaard et al. (2001) described in a study the stabilizing role of the iliolumbar ligaments on the SI joint. Gradual transection of the ligaments resulted in a significant increase in SI joint mobility in the sagittal plane.

The ligaments also contribute to sacroiliac movements being transmitted onto the lower lumbar segments and vice versa. Movement within the pelvic ring and movement in L4-S1 must always be regarded as a kinematic chain.

9.3.7 Ligament Dynamization in the Sacroiliac Joint

The interplay between muscles and ligaments near joints has been known for a long time now. The knee joint is a perfect example of this. The extension of muscles into capsular-ligamentous structures is called ligament dynamization. Two examples of pelvic ligaments are presented here to demonstrate how intensive the contact is between muscles and the functional collagen in this region.

Sacrotuberous Ligament

The sacrotuberous ligament is connected to the following:

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  Gluteus maximus from a posterior direction.

  
  
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  Biceps femoris from an inferior direction.

  
  
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  Piriformis from an anterior direction.

  
  
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  Coccygeus from a medial direction.

Vleeming and Dorman (1995) explain the functional significance of the sacrotuberous ligament, dynamized by the biceps femoris, on the SI joint as follows:

We know that the hamstring muscles are most active at the end of the swing phase during gait. The hamstrings slow down the anterior tibial swing a few milliseconds before heel contact, decelerating knee extension.

The long head of the tensed biceps femoris often merges with the sacrotuberous ligament via large bundles of collagen (also without contact with the ischial tuberosity) and dynamizes the ligament (▶ Fig. 9.26). The biceps femoris activity prevents the sacrum from fully nutating and stabilizes the SI joint directly before the landing phase.

Fig. 9.26 Dynamization of the sacrotuberous ligament.

Thoracolumbar Fascia

The thoracolumbar fascia consists of three layers:

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  Superficial layer—posterior layer.

  
  
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  Middle layer—inserted on the lumbar transverse processes.

  
  
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  Deep layer—anterior layer found anterior to quadratus lumborum and iliopsoas.

The posterior, superficial layer contains collagen fibers arising from several muscles that can tighten up this aponeurosis:

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  Latissimus dorsi.

  
  
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  Erector spinae.

  
  
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  Gluteus maximus.

Each of the muscles is able to dynamize the fascia. The fascia forms a diagonal sling between the latissimus dorsi and the contralateral gluteus maximus (▶ Fig. 9.27). The force of the sling acts perpendicular to the joint surfaces, stabilizing the SI joint and the inferior lumbar spine during strong rotation. Consequently, the participating muscles and the fascia belong to the primary SI joint stabilizers. This sling can be especially trained using trunk rotation against resistance.

Fig. 9.27 Alignment of the collagenous fibers in the thoracolumbar fascia.

This fascial layer is also connected to the supraspinous ligament and the interspinous ligament up to the ligamenta flava. Vleeming (personal communication) comments on this: “The entire system is dynamically stabilized.”

Muscles also dynamize the middle and deep layers. It is well known that the transversus abdominis tightens the middle layer (see also the section “Detailed Anatomy of the Ligaments,” in Chapter 10).

The required background information on the pelvic muscles is given in the section “Palpatory Procedures for Quick Orientation on the Muscles” below.

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