The hip joint is formed like a ball and socket joint, which rotates on more than one axis and is classed as a synovial joint. The ball of the hip joint is comprised of the head of the femur or thigh bone as it is more commonly known, whilst the concavity of the socket is created by the acetabulum, which is a cup-like depression within the pelvic bone.
The femur is both the longest and the heaviest bone in the entire human skeleton. It is comprised proximally of a head that contains a fovea or pit shaped depression and a neck which joins the head to the shaft of the bone. The neck is angled to the long axis of the femur at an average of 126 degrees. Also, the greater and lesser trochanters project laterally and medially respectively, at the junction of the neck and the body of the femur. They are separated by the intertrochanteric line, indicating the exact border of the femoral neck and body. The acetabulum is the female portion of the hip joint; a concavity which encompasses the head of the femur. It is the part of the pelvis in which the ilium, ischium and pubis merge. While it is a concave structure, its bony rim is partially surrounded by a fibrocartilaginous ring known as the labrum. This structure is completed by the transverse acetabular ligament which stretches across the acetabular notch.
The capsule of the hip joint has been described as strong and fibrous, but loose enough to accommodate a range of movements. It attaches to the anatomical structures known as the acetabular labrum, the transverse acetabular ligament and the intertrochanteric line of the femur. The proximal two thirds of the neck of the femur are encapsulated by the hip joint, whereas the distal third remains extracapsular. The iliofemoral ligament, the pubofemoral ligament and the ischiofemoral ligament all act to strengthen the mobile capsule.
- The Iliofemoral Ligament attaches itself to both the anterior inferior iliac spine and the acetabulum superiorly and the intertrochanteric line inferiorly, creating a ‘Y’ shape. It prevents hyperextension of the hip.
- The Ischiofemoral Ligament originates in the rim of the acetabulum and turns laterally as it ascends to insert upon the inferomedial aspect of the greater trochanter. It functions in coherence with the iliofemoral ligament and also fixates the femoral head in the acetabulum. Prevents excessive Medial rotation of thigh
- The Pubofemoral Ligament arises from the pubic ramus and merges inferolaterally into the joint capsule, but also continues into the lower femoral neck. This ligament tightens during extension and abduction in order to limit the latter movement.
- The Transverse Acetabular Ligament runs over the acetabular notch and joins up with the inferior ends of the labrum. This ligament completes the acetabular ring.
- The Ligament of the head of the femur is situated within the capsule but is extrasynovial. It arises from the acetabular notch and inserts upon the fovea of the femur. Within the ligament runs the artery that supplies the head of the femur.
Now a brief overview of the muscle groups, which are organised according to their primary function in relation to the hip joint. The flexor muscles include:
- The iliopsoas muscle which is otherwise known as the psoas major muscle, the iliopsoas originates from the lateral walls of the last thoracic vertebra and each of the lumbar vertebrae. These individual muscle fascicles merge together and insert as a whole upon the lesser trochanter of the femur. The psoas major muscle acts by flexing the thigh at the hip and by reinforcing the hip.
- The sartorius muscle; originating from the anterior superior iliac spine and the notch which is situated just below it, the sartorius muscle inserts itself at the pes anserinus medially from the tibial tuberosity.
- The tensor fascia latae, like the sartorius muscle also originates from the ASIS, however to a lesser extent it also arises from the anterior part of the external lip of the iliac crest. It acts to stabilize the trunk on the thigh and also abducts, medially rotates and flexes the thigh.
- The rectus femoris starts from the anterior inferior iliac spine and the groove that can be found just above the acetabulum. This muscle finishes at the base of the patella and upon the patellar ligament which attaches itself to the tibial tuberosity. The rectus femoris functions by extending the leg at the knee joint and flexing the thigh at the hip joint.
- The gluteus medius muscle originates between the anterior and posterior gluteal lines on the lateral surface of the ileum and inserts upon the lateral surface of the greater trochanter of the femur.
The extensor muscles include:
- The gluteus maximus muscle originates from the sacrum (dorsal part), ilium (behind the posterior gluteal line), the thoracolumbar fascia and the sacrotuberous ligament. It inserts into the iliotibial tract which fills the lateral condyle of the tibia, although a small margin of the fibers also insert into the gluteal tuberosity of the femur.
- The gluteus medius (see flexor muscles)
- The gluteus minimus muscle passes between the anterior and inferior gluteal lines after arising from the lateral surface of the ileum and terminates upon the anterior surface of the greater trochanter of the femur.
- The biceps femoris muscle has two heads. The long head arises from the ischial tuberosity, while the short head arises from the lateral edge of the linea aspera and the lateral supracondylar line of the femur. These heads merge and insert as one tendon into the lateral side of the head of the fibula.
- The semimembranosus muscle comes from both the superior and lateral aspects of the ischial tuberosity and terminates upon the posterior portion of the medial condyle of the tibia.
The abductor muscles include:
- The gluteus medius (see flexor muscles)
- The gluteus minimus (see extensor muscles)
- The piriformis muscle originates upon the anterior surface of the sacrum and also from the sacrotuberous ligament. It inserts upon the superior border of the greater trochanter of the femur.
The adductor muscles include:
- The Adductor Longus muscle originates from the body of the pubis, which is situated just below the pubic crest and inserts into the middle third of the linea aspera of the femur.
- The Adductor Brevis muscle arises from both the body and the inferior aspect of the pubic ramus and terminates on both the pectineal line as well as the proximal part of the linea aspera of the femur.
- The Adductor Magnus muscle comes from the inferior pubic ramus, as well as the ramus of the ischium. It also merges with fibers from the ischial tuberosity, which form part of the hamstring complex. These fibers attach themselves to the gluteal tuberosity, the linea aspera of the femur and the supracondylar line. The portion that adds to the hamstring fibers inserts upon the adductor tubercle.
- The Pectineus muscle starts out at the pecten pubis and finishes at the pectineal line of the femur.
- The Gracilis muscle arises from the body of the pubis and the inferior pubic ramus and terminates upon the superior part of the medial surface of the tibia.
- The Quadratus Femoris muscle comes from the lateral margin of the ischial tuberosity and inserts into the quadriceps tendon that runs above the ventral side and through the periosteum of the patella and finally inserts at the tuberosity of the tibia.
The Internal Rotator muscles include the gluteus medius (see flexor muscles) and the gluteus minimus muscles (see extensor muscles).
The External Rotator muscles include:
- The Obturator Internus muscle originates from the pelvic surface of the obturator membrane and the margins of the obturator foramen. It inserts into the medial surface of the greater trochanter of the femur.
- The Obturator Externus muscle arises from the margins of the obturator foramen and the obturator membrane. It terminates in the trochanteric fossa of the femur.
- The Superior Gemellus muscle comes from the outer surface of the ischial spine and stops at the medial aspect of the greater trochanter of the femur.
- The Inferior Gemellus muscle arises from the upper margin of the ischial tuberosity and terminates upon the medial surface of the greater trochanter of the femur.
- The Quadratus Femoris muscle (see adductor muscles)
- The Piriformis muscle (see abductor muscles)
The range of movements of the hip joint are marked in degrees and are categorised by name:
- Flexion: 140 degrees
- Extension: 15 degrees
- Abduction: 40 degrees
- Adduction: 25 degrees
- Internal rotation: 35 degrees
- External rotation: 45 degrees
These ranges of movement occur when the knee is flexed at a right angle (90 degrees).
There are two sets of arteries that contribute either majorly or minorly to the vascularisation of the joint capsule of the hip. The major contributing set contains the medial and lateral circumflex arteries that arise from the deep branch of the femoral artery. The minor contributing set singularly of the artery of the head of the femur.
The innervation of the hip joint comes anteriorly from the femoral nerve, inferiorly from the anterior division of the obturator nerve, laterally from the articular branch of the sciatic nerve and posteriorly from the nerve that runs to the quadratus femoris as well as the superior gluteal nerve.
Perthes disease, which is correctly termed Legg-Calve-Perthes disease and medically known as avascular necrosis is a disorder that is mostly found in children. The condition affects the head of the femur, when there is inadequate perfusion to the epiphysis, causing the bone to become necrotic. Eventually the blood flow returns to the area through revascularization and bone remodelling takes place. There are no somatic symptoms for this disease and the main complaint from patients with this condition is an increased risk of hip fracture.