Functional Anatomy of the Hip-Bones and Ligaments

Original Editor - Ewa Jaraczewska

Top Contributors - Ewa Jaraczewska, Jess Bell, Kim Jackson, Lucinda hampton and Kirenga Bamurange Liliane

Description

The hip joint is considered one of the largest joints of the human body. It connects the pelvis to the lower extremities. The primary roles of the hip joint are to allow lower extremities weight-bearing and stability in standing and during mobility such as walking or running.

"Our goal as therapists turns out to be that we alleviate the pain, enhance the movement mechanics, and also to reduce the work of the body, the energy expenditure, in order for the body to move effortlessly, seamlessly as much as possible". Rina Pandaya

Structure

The hip joint is a ball-and-socket joint that provides stability for the body and sustains forces that exceed the total body weight multiple times. This ability to accept a significant load is due to a combined work of the static and dynamic elements of the joint paired with a proper body alignment. The static stabilisers include bones, ligaments, capsules and bursae. Muscles, fascia, tendons, nerves and vascular supply are all dynamic elements of the hip joint. The movements that occur in this joint are:

• 

  Hip Joint
  Sagittal plane: flexion/extension
• Frontal plane: abduction/adduction
• Transverse plane: medial/lateral rotation

Hip (thigh) circumduction is a movement performed in two planes: sagittal and frontal

Bones

The hip joint has two bony structures: the acetabulum and the femoral head connected to the femoral shaft via the femoral neck. They contain the trabeculae, supportive and connective tissue element that forms in cancellous bone. This tissue growth follows the stress lines along the bone and accepts the high forces that cross the joint. For example, during a one-leg stance, these forces are two and a half times bodyweight, and during running, they increase to 5 times body weight. Wolff's law explains the progress of the trabecular pattern.

"The knowledge of the trabeculae is extremely important because it will teach us and the body how to dissipate weight in the most energy-efficient manner". Rina Pandaya

Garden Classification of Fractures:

• Stage one: the medial trabeculae form an angle of 180 degrees.
• Stage two : the medial trabeculae of the head form an angle of approximately 160 degrees to the femoral neck.
• Stage three: the medial trabeculae are out of alignment with the pelvis
• Stage four: medial trabeculae are in alignment with the pelvis.

Wolff's law: a living bone will react to the mechanical forces exerted on it. These forces can be static or dynamic, internal or external. Gravity is an example of static force. A weight-bearing is considered a dynamic force. The bone will go through the process of remodelling when the load is increasing. As a result, it becomes stronger and can resist the loading.

Ward's Triangle

Ward's triangle is located in the femoral neck at the intersection of three trabecular bundles and it defines the area of the lowest bone mineral density thus susceptible to trauma. Based on the systematic review and meta-analysis of over 20 randomised controlled trials some forms of exercises (eg. Tai Chi) can improve bone mineral density in Ward's triangle.

Acetabulum

Acetabulum

The acetabulum is a lateral part of the pelvis. Its concave shape allows for the fitting of the femoral head. The femoral head is located at the proximal aspect of the femoral bone which distally connects to the tibia and the fibula.

What is referred to as a normal pelvis and acetabulum are in fact difficult to define.A better understanding of the topographic map of the acetabulum is important for creating a good anatomic implant design when replacement or reconstruction of the acetabulum is needed. The superior and posterior walls of the acetabulum are in anteversion and abduction. The face of the acetabulum has an average of 20.7 degrees of anteversion and 39.8 degrees of abduction. The anterior portion of the acetabulum is flat while the posterior portion is angled with its highest point at the sciatic notch.

The margin of the acetabulum is 75% circle, except for its anteroinferior aspect called the acetabular notch. The transverse ligament of the hip closes the circle at this notch. This is where the acetabular labrum attaches allowing for an increase of the articular area of the acetabulum. Because of this anatomical structure, 50% of the femoral head can fit into the acetabulum.

The acetabulum is divided into anterior and posterior columns. The anterior column includes the anterior ilium, anterior wall, the anterior dome of the acetabulum, and superior pubic ramus. The posterior column consists of greater and lesser sciatic notches, posterior wall, the posterior dome of the acetabulum, and ischial tuberosity.

Femoral Head

The femoral head completes the distal aspect of the femur which is the strongest bone in the human body. The head is pointed in a medial, superior, and slightly anterior direction. It connects to the femur via the femoral neck which extends to the femoral shaft on its other end. The shaft has a mild anterior arch. Two femur bony protrusions, the greater and the lesser trochanters serve as attachment points to muscles responsible for hip and knee motion. Medial and lateral condyle join the femur to the tibia. The 128 degrees of inclination angle defines the angle between the neck and shaft. This angle changes with age and tends to decrease as the person gets older.

The shaft has a mild anterior arch. At the distal femur, the shaft flares out in a cone-shaped manner onto a cuboidal base made up of the medial and lateral condyle. Medial and lateral condyle join the femur to the tibia, forming the knee joint.

Both hips and knees are synovial joints that are covered by cartilage to reduce friction and optimize the range of motion. Bony features are landmarks for measuring the axis along the femur.

Ligaments of the Hip Joint

Ligaments

The ligaments reinforcing the hip are divided into intra- articular and extra-articular. There are two intra-articular ligaments:

1. Ligament of the femoral head is also called a round ligament of the femur, ligamentum teres femoris, or the foveal ligament. This ligament attaches between the peripheral inferior acetabular notch and the fovea of the femoral head. The main functions include carrying small vessels and nerves to the femoral head and providing proprioception and structural stability to the hip joint by limiting abduction and external rotation when the hip is flexed.
2. Transverse ligament of the acetabulum is in fact a fibrous structure that transforms the acetabular notch into a foramen. It channels nerves and vessels into the joint.

The extra-articular ligaments are primary hip joint stabilisers and they include:

1. Iliofemoral ligament (also known as the Y ligament of Bigelow) contains lateral and medial fibrous branches. It stretches between the anterior inferior iliac spine/ the acetabular rim and the intertrochanteric line/ the greater trochanter. The primary function of this ligament is to prevent hip hyperextension and external rotation in the upright position. In addition, it serves as the anterior capsule reinforcer.
2. Ischiofemoral ligament is the weakest of the four ligaments and consists of a triangular band of fibres that form the posterior hip joint capsule. It inserts in the ischium, behind the acetabulum and attaches to the femur, medial to the greater trochanter. The ligament has two roles: it limits internal rotation and extension and reinforces the posterior portion of the capsule during internal rotation.
3. Pubofemoral ligament originates at the pubic part of the acetabular rim and the superior pubic ramus and attaches to the lower part of the femoral neck. It limits abduction and external rotation during hip extension and reinforces the inferior capsule.
4. Zona orbicularis primary role is to stabilise the hip joint. This ligament is formed by the circular fibres from the capsule which contains fibres from all extra-articular ligaments.

Joint Capsule

The hip joint capsule has two layers:

1. External layer: The fibrous joint capsule is a supporting structure of the hip joint contributing to its stability.It contains mechanoreceptors monitoring vibration and tensile loading in the hip joint. Pacinian and Ruffini corpuscles have the largest representation in the hip joint capsule, particularly in its superior-lateral aspect. In addition to Pacinian and Ruffini corpuscles, there is a smaller number of the Golgi Tendon Organs mechanoreceptors. These findings support the capsule's role in fast muscular response to the joint movement required to maintain its stability and with pain perception of the hip joint. Some research data suggests that there is a reduction in mechanoreceptors density in hip osteoarthritis with no relationship found with demographic information.
2. Internal layer: Synovial membrane producing synovial fluid that lubricates the joint and carries the nutrients to the hip joint. It stretches between the margin of the articular surface of the femoral head and the ligament of the head of the femur covering the internal surface of the capsule.

Bursae

There are 5 bursae in the hip joint whose primary role is friction reduction between the bony components of the joint and the surrounding muscles.

The following is a list of hip bursae and their locations:

• Trochanteric bursa: lateral insertion of the medial gluteus muscle and posterior trochanteric facet
• Ischial bursa: between the ischial tuberosity and the gluteus maximus
• Subgluteus medius bursa:between the tendon of gluteus medius and the lateral facet
• Subgluteus minimus bursa: between the gluteus minimus tendon and the anterior facet of the femur
• Iliopsoas bursa: between the musculotendinous part of the iliopsoas muscle and the anterior capsule of the hip

Clinical relevance

1. Coxa profunda is a pathology related to an increased depth of the acetabular socket. This is a problem more commonly present in females.
2. Acetabular retroversion refers to an abnormal posterior angulation of the superolateral acetabular rim. This pathology is causing the femoral head to be excessively covered and predisposes to femoroacetabular impingement. It is also considered a risk factor for the osteoarthritis of the hip.
3. Deficits in the passive structures of the hip joint may lead to increased femoral head translation, or shearing forces.
4. Development of larger cross-sectional areas and tighter capsular ligaments can be the result of progressive arthritis.
5. Surgical procedure involving capsular closure restores the biomechanics of the hip, a study has found.
6. The pathology of the iliopsoas bursa can be a consequence of hip osteoarthritis, hip inflammatory disease, or primary bursitis.

Resources

1. Hip Joint. The Lecturio Medical Concept Library: https://www.lecturio.com/concepts/hip-joint/
2. Hip Joint. Radiology Reference Article: https://radiopaedia.org/articles/hip-joint-1
3. Anatomy, Bony Pelvis and Lower Limb, Hip Joint: https://www.ncbi.nlm.nih.gov/books/NBK470555/