Hip Joint The Biggest Joint In The Body
Hip Joint : The Biggest Joint In The Body
Authored by Dr.Surya Rao Poodipeddi on 6 May 2014 - 15:42.
The Hip joint is the biggest amongst all the joints in the human body. The hip joint is unique, not only in size but also its functions. It has wide range of functions with ability to perform six different types of movements namely:
Flexion (Movement of thigh forwards
Extension (Movement of the thigh backwards)
Adduction (Moving the thigh inside)
Abduction (Moving the thigh outside)
Medial Rotation (Rotating the thigh inwards)
External Rotation (Rotating the thigh outwards)
The Hip Bone Structure:
The hip joint is the joint between the femur (thigh bone) and the acetabulum of the pelvis (hip bone). The pelvis is the largest bone formed by three bones namely Ilium, Ischium and Pubis.  The Ishium and ileum join behind with a single piece of a big bone called the sacrum. The pubis on either side, join in the front forming the pubic arch.
The whole cavity formed by these bones one for each side is called the pelvic cavity. The outer part of the pelvis has a circular cavity called the acetabulum into which the head of the femur (thighbone fits to serve the purpose of ball and socket joint with an ability to perform several movements as mentioned above.
The head of the femur is the continuation of the thigh bone into a small portion called the neck of the femur. Fractures of the neck of the femur are most common in the elderly group of persons.
The primary function of the hip joint is to support the weight of the body in both static (eg standing) and dynamic (eg walking or running postures).
The hip joint is also termed as a synovial joint. A synovial joint that can produce movement in more than one axis is called a multi-axial joint. The hip joint therefore is also termed as a multi-axial joint.
Anatomy of Hip Joint
The hip joint is a multi-axial ball and socket synovial joint formed by the articulation of the rounded head of the femur (Thighbone) and the cuplike acetabulum of the pelvis. It acts as a pivot forming the primary connection between the bones of the lower limb and the axial skeleton of the trunk and the pelvis.
The joint surfaces of both the acetabulum and the head of the femur are covered with a strong but lubricated layer called the articular hyaline cartilage. As explained the cuplike acetabulum is formed at the union of the three pelvic bones and the joint may not be fully ossified (ossification means the process of forming bone) before the age of 25 years.
The depth of the acetabulum needs to be increased to firmly hold and secure the femoral head (head of the femur) and this is accomplished by a fibro-cartilaginous rim called the labrum that grips the head of the femur and secures it in the joint.
The large head of the femur is completely covered in a hyaline cartilage leaving a small area called the fovea meant for the attachment for an intra-capsular ligament . The head of the femur is attached to the shaft of the bone through a thin neck which is prone for fractures especially the elderly.
Though the hip joint is considered as the largest joint in the human body it is only second to the shoulder joint in respect of the range of the movements and yet support the entire weight of the body, arms and the head.
This is where the nature's engineering skill comes into the picture, with the line of gravity falling behind the axis of the hip joint, giving it the capacity to extend the joint over the hip (the act of bending backwards and yet remain in a standing position).
As we perform this unique action the anterior joint capsule may get overstretched. However, the two strong anterior ligaments prevent this from happening unless they are torn during sudden movements over the joints or due to any injury. 
A joint like this with capability to perform multiaxial movements needs strong ligaments all round the joint to firmly secure it and at the same time prevent its exit to the outside during  its wide range of movements. To accomplish this objective the hip joint is reinforced by three main ligaments.
Iliofemoral ligament: This ligament runs anteriorly (front side) from the pelvis to the femur. It is a Y-shaped ligament and is also known as ligament of Bigelow and is often considered as the strongest ligament in the human body.
 
Pubofemoral ligament, also running frontward, as the name implies runs from the pubis to the femur and is oriented more inferiorly(downwards) and reinforces the inferior part of the hip joint capsule. It also blends with the inner parts of the iliofemoral ligament.
 
Ischiofemoral ligament: The posterior (backside) aspect of the capsule of the hip joint is reinforced by this ligament that attaches from the ischial part of the acetabulum (remember that the acetabular rim is formed by the union of three bones including the ischium) to the femur.
 
Apart from these three main ligaments there is also a small ligament called ligamentum teres or the ligament meant for the head of the femur. Though the arterial branch is not present in every one, it can become the only source of blood supply to the bone in the femoral head and therefore disrupt blood supply when the neck of the femur is fractured.
Blood and Nerve Supply to the Hip Joint:
The hip joint has rich blood and nerve supply. The medial and lateral circumflex femoral arteries are the main source of blood to the hip joint. These two are the branches from the deep artery of the thigh called the profunda femoris.
They may also arise directly from the main femoral artery which branches out to the profunda femoris and passes through downwards to pit on the back side of the knee called the poppliteal fossa through which the artery courses as the tibial arteries for the leg. Fracture neck of the femur causes disruption of blood supply leading to necrosis(death of tissue) of the head of the femur.
A number of nerves supply the hip joint which includes the main femoral nerve, obturator nerve, the gluteal nerves and the nerve to the big muscle of the thigh called the quadratus femoris.
Muscles Responsible for Hip Joint Movements:
Muscles are responsible for movements of any joint. The muscle passes over the joint with one end attached to the bone above the joint and the other end forms into a tendon which is attached to the bone below the joint. It is the contraction and relaxation of these muscles which help in the movements of the joint.
Several muscles are responsible for the movements at the hip joint.  These can be divided into four groups according to their orientation around the hip joint namely: Gluteal group, the Lateral rotator group (muscles which help move the thigh outwards), The Adductor group (muscles which help in moving the thigh inward) and the Iliopsoas group.
These groups of muscles produce flexion (bending the thigh frontward), extension (bending the hip backwards), lateral rotation (rotating the hip outwards), medial rotation (rotating the hip inwards) Adduction (moving the thigh inside and abduction (moving the thigh outwards). Most of these muscles are involved in more than one type of movement in the hip joint. This is because different areas of the muscle act on tendons in different ways.
Managing Hip Joint Pain is one of the worst pains which is debilitating and needs instant attention to get relief. The hip hurts for various reasons such as:
a) An injury to the hip joint
b) Diseases of the hip joint
The pain caused due to injury by a fall or an accident could be a hip fracture, hip dislocation and some other common hip injuries:
Broken Hip or Hip Fracture: Normally occurs in women aged over 65 years from a fall causes by weak bones or osteoporosis. In other cases it could be an accident while driving that may lead to a hip fracture of younger individuals causing serious injuries.
Hip fractures are generally treated by individual cases, depending on the nature of the fracture. It could be surgery of the hip or a hip replacement or just a plaster depending on the severity.
Bursitis: When the fluid filled sacs called bursae which cushion the hip joint are inflamed, it hurts. The bursa between the bony bump on the side of the hip and the tendon passing over it causes pain with the slightest activity such as walking. The hip aches the pain radiates into the thigh which is known as trochanteric bursitis .
 Dislocation:  The dislocation of the ball at the top of the femur from the socket causes severe pain restricting movement.  Often the ligaments on the hip joint get damaged due to dislocation. This causes severe pain of the hip. Sometimes due to congenital defects, the hip joint socket may be shallow and lead such a dislocation at the slightest injury.
Labral tear: The labral tear of the hip is damage to the cartilage that surrounds the bony edge of the socket in the pelvis. Twisting of the joint or an injury or repetitive usage of the hip in early stages of osteoarthritis normally causes labral tear which is painful.
Snapping Hip Syndrome: is characterized by a snapping sensation in the hip, when the hip is extended. When bands of connective tissue that support the hip thicken and catch at the top end of the femur, as they slide on the top end of the femur. Often, the bursae underneath also become inflamed and painful. Snapping hip syndrome may occur in athletes, weight lifters, soccer players and other sports persons.
What to do for Pain Relief of the Hip Joint?
Other conditions of the hip joint like osteoarthritis , tendinitis and bursitis also cause severe hip joint pain.
Osteoarthritis:
The spongy tissue that cushions joints called cartilage deteriorates, causing bones to rub against each other. This causes pain in the groin and also in the buttocks. Osteoarthritis can be treated with simple NSAIDs (Non-Steroidal Anti-inflammatory Drugs) like ibuprofen or naproxen as per the instructions on the medicine. If the pain is very severe, seek a doctor’s advice.
Tendinitis:
Tendinitis is caused by inflammation of the tendons (band of tissue that connects muscles to bones) due to overuse. If it is the Hip Tendinitis, a dull pain is felt deep in the groin especially while climbing the stairs or getting up from a chair. In case of the hamstring tendinitis, the severe pain is felt in the buttocks. 
To treat the Tendinitis, NSAIDs and applying ice to that area which hurts for 20 minutes several times a day may help. However, a doctor’s advice may be sought.
Bursitis:
The inflammation of the bursae sacs that cushions the tendons with the lubricating fluid causes the hip joint pain. This can be treated by standing up, crossing your legs and touching your toes, holding for 20 to 30 seconds and switch legs.
Some Interesting Facts about the Hip:
1.       Hip Bone Determines Body Shape: The female hip bones differ from the male hip bone as part of sexual differentiation. After puberty, hips are generally wider than shoulders.Females generally have wider hips, to enable childbirth. At puberty the hip bones are widened by sex hormones called estrogen.  The female pelvis is flatter, more rounded and proportionally larger, so that the head of the fetus may pass through the birth canal during childbirth.
2.       Waist-to-Hip Ratio (WHR), a determinant of health: is the ratio of the circumference of the waist to that of the hips. The WHR has been used as an indicator of health, and the risk of developing serious health conditions. Research shows that people with "apple-shaped" bodies (with more weight around the waist) face more health risks than those with "pear-shaped" bodies who carry more weight around the hips.
3.      The femur is the strongest and the longest bone in the body. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with the tibia and patella forming the knee joint.
4.       The hip bone is  prone to osteoarthritis: It is a large, weight-bearing joint and because of the continued stress placed on it throughout the life, it can develop osteoarthritis in later years, though the cause for osteoarthritis is not known.