Hip Bone

Hip Bone

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The Hip Bone

The left and right hip bones (innominate bones, pelvic bones) are two irregularly shaped bones which play a role in the overall formation of the pelvic girdle. This bony structure connects the axial skeleton of the body to the lower limbs. The hip bones feature three primary articulations; the sacroiliac joint which creates an articulation with the sacrum, the pubic symphysis which articulates between left and right hip bones, and of course the hip joint which articulates with the head of the femur.

In this article we shall take a look at the anatomy of the hip bones, breaking down the composition of the bone, noting the bony landmarks, and exploring the clinical relevance.

Composition of the Hip Bone

The hip bone is composed of three distinct parts;the ilium, pubis, and ischium. Up until puberty the triradiate cartilage will separate these parts, but at an age of around 15-17 years they will begin to fuse together. When they all form together, the result is a cup-shaped socket known as the acetabulum - a Latin term that literally translates to 'vinegar cup'. Within this cup-shaped socket is where the head of the femur articulates with the acetabulum to create the hip joint.

The Ilium

The ilium is the largest and widest of all three parts of the hip bone, and is found in a more superior position. It forms part of the roof of the acetabulum, and features two distinct surfaces. The inner surface is concave in shape which then produces the iliac fossa - the iliacus muscle will originate from this area. The external surface of the ilium is convex in shape and will feature a number of attachments to the gluteal muscles. Additionally, the superior margin of the wing is thickened, which is known as the iliac crest. This extends from the anterior superior iliac spine to the posterior superior iliac spine, with the latter featuring a wide indentation which is dubbed as the greater sciatic notch.

Clinical Relevance - Anterior Superior Iliac Spine

The anterior superior iliac spine is a key anatomical landmark which has a number of uses in clinical practice. The mid-inguinal point can be found halfway between the ASIS and the centre of the pubic symphysis, and it is at this point where the femoral artery can be palpated. In addition to this, the mid-point of the inguinal ligament is located halfway between the ASIS and the pubic tubercle.

The ASIS is important when measuring the true leg length of a patient. Many people mistakenly measure the apparent leg length, which is measured from the umbilicus to the medial malleolus at the ankle joint, but this can be inaccurate. It is deemed that a true leg length is measured from the ASIS to the medial malleolus at the ankle joint. Leg length discrepancy can be a sign of numerous hip disorders, but it can also be a complication which can follow a hip joint replacement (arthroplasty).

The Pubis

The pubis is the most anterior part of the hip bone and is composed of a body, a superior ramus, and an inferior ramus. The pubic body is located in a more medial position and is connected to the opposite pubic body at the pubic symphysis, with its superior aspect featuring a rounded area known as the pubic crest. This crest will then extend out laterally to become the pubic tubercle.

The superior pubic ramus will move laterally from the body to create part of the acetabulum, and the inferior pubic ramus will project towards the ischium. When these two combine they will form part of the obturator foramen, allowing the obturator nerve, artery and vein to pass through to the lower limb.

Clinical Relevance - Pubic Rami Fractures

Pubic rami fractures can sometimes be observed on x-rays in elderly individuals who have been investigated after a simple low energy fall from standing height. These falls may not seem particularly serious, but they can still cause major damage to the pubic rami.

The Anatomy of the Hip Bone

In this article, we shall consider the individual parts of the hip bone, including the respective bony landmarks. Let us begin at the top with the ilium.

The Ilium

The ilium is the largest and widest part of the hip bone and it is located superiorly. It forms the superior part of the acetabulum, which is a concave socket that provides a stable joint for the femur of the thigh. Immediately above the acetabulum, the ilium widens to form the wing, or ala. The inner surface of the wing is concave, and this forms the iliac fossa. The external surface is convex and provides attachments for the gluteal muscles. The superior margin is thickened and forms the iliac crest, extending from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). On the posterior aspect of the ilium is an indentation known as the greater sciatic notch.

The Pubis

The pubis is the most anterior portion of the hip bone. It consists of three parts: the pubic body, superior pubic ramus, and inferior pubic ramus. The pubic body is located medially and it articulates with the opposite pubic body at the pubic symphysis. The superior aspect of the pubic body is marked by a rounded thickening called the pubic crest, which extends laterally as the pubic tubercle. The superior pubic ramus extends laterally from the body to form part of the acetabulum. The inferior pubic ramus projects towards the ischium. Together, the superior and inferior rami enclose the obturator foramen, which is a bony opening that allows the obturator nerve, artery and vein to pass through to reach the lower limb.

The Ischium

The ischium forms the posteroinferior part of the hip bone. It is composed of a body, an inferior ramus, and superior ramus. The inferior ischial ramus combines with the inferior pubic ramus to form the ischiopubic ramus, which encloses part of the obturator foramen. The posterior-inferior aspect of the ischium forms the ischial tuberosities which support the body when sitting. Near the junction of the superior ramus and body is a posteromedial projection of bone called the ischial spine.

Two important ligaments attach to the ischium. The sacrospinous ligament runs from the ischial spine to the sacrum, thus creating the greater sciatic foramen through which the lower limb neurovasculature, including the sciatic nerve, transcends. The sacrotuberous ligament runs from the sacrum to the ischial tuberosity, forming the lesser sciatic foramen.

In the context of a high-energy major trauma patient, the pelvis can be a major source of bleeding due to fracture. As a result, major trauma patients are assumed to have a pelvic fracture until proven otherwise, and a ‘pelvic binder’ is used to stabilize the pelvis and minimize further bleeding. Circumferential pressure is applied by the binder at the level of the greater trochanters – an important anatomical landmark.

The individual parts of the hip bone are important structural and functional components of the pelvis. The ilium, pubis, and ischium each have unique characteristics that enable them to do their job and contribute to the overall stability of the pelvis. Additionally, any major trauma patient should also be aware of the importance of the greater trochanters as a major site to apply pressure when using a pelvic binder for stabilization.

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