Hip joint

General information

The human body has two hip joints, which are symmetrically arranged and are responsible for leg movements and for the dissipation of the forces acting on the body. Furthermore, the hip joints, together with the spine, take over the main tasks of the body’s statics. Numerous ligaments secure the actual hip joint, and further security and stability is provided by the muscles anchored to the thigh.

Like every joint, the hip joint also has a joint head and a socket. Roughly speaking, one can say that the acetabulum in the pelvic bone is a kind of hemispherical recess. The head of the joint is formed by the head of the femur, which dips into the acetabulum.

By definition, the hip joint is formed by the so-called facies lunata acetabuli on the hip bone, as well as by the caput femoris (head of the femur). The facies is the lining of the described hollow ball on the hip bone. In order to ensure stability, the femoral head must find a secure hold in the socket.

In the hip joint, the femoral head is larger than the acetabulum. For this reason, the acetabulum is anatomically enlarged by means of an extension, thus ensuring a safer fit of the thigh in the socket. The enlargement is also known as labrum acetabuli or joint lip.

The joint lip also consists of fibrous cartilage. Together with the facies, they cover 2/3 of the joint head and thus ensure its stability. The acetabular roof is the middle part of the upper edge of the acetabulum.

It is dense and can be easily visualized in an X-ray image. The ligamentum transversum acetabuli, which also contributes to the stability of the hip joint, pulls on the lower part of the acetabulum. The acetabular fossa is covered with a fatty body, which is intended to ensure a smoother movement and to absorb shocks.

The hip joint consists of the femoral head (joint head) and the hip joint bone (acetabulum). The so-called caput femoris is the ball that delimits the femur at the top. It is followed by the neck of the femur (collum femoris), which then forms the transition to the actual femur.

The neck of the femur is often affected by fractures, especially in older patients. The pelvis is the largest bone in the human body. It is very massive and, together with the spinal column, carries the human body.

The pelvis consists of three sections, which are blurred from each other and represent the hip bone (Os coxae) in its entirety. These sections are called pubic bone (Os pubis), ilium (Os ilium) and ischium (Os ischii). In the area where anatomically the three sections join together, we find the acetabular fossa, the socket for the hip joint.

The fossa is delimited by the facies lunata, which gets its name from its crescent-shaped appearance. Furthermore, a small bony cavity is found in this area (Incisura acetabuli). The limbus acetabuli wraps itself in a circle around the socket and limits it to the outside.

The hip joint is secured by numerous ligaments. The strongest ligament in the human body is the iliofemoral ligament. It has a load-bearing capacity of 350 kg and has its starting point at the hip bone and then, turning slightly outwards, pulls down to the thigh bone, where it has its second starting point at the upper part.

There are a total of five ligaments at the hip joint. Four of them lie outside the joint and one inside. The ligaments on the outside form the ring ligament, which is also called the zona orbicularis.

The following ligaments belong to the section located in the joint: the ligamentum ischio-femorale runs from the os ischi to the head of the femur, the ligamentum pubofemorale from the os pubis and the ligamentum iliofemorale from the os ileum to the head of the femur. The ligaments of the hip joint have two main functions. Firstly, they stabilize and strengthen the joint, and secondly, they limit the range of motion and prevent unphysiological movements in the hip joint.

The ring ligament wraps around the narrowest point of the hip joint and acts as a very strong stabilizer. The head of the femur is in the ring band and is held by it. The ligamentum capitis femoris is the only ligament in the joint.

The areas that are not secured by ligaments are considered at risk, since stability is severely limited there and fractures or “dislocation” of the joint can occur mainly there.Capsule: The joint capsule is a rough skin surrounding each joint, which lies close to the joint and protects it or contributes significantly to joint stability. In the hip joint, the joint capsule is outside the labrum acetabuli and attached to the hip bone. The labrum acetabuli projects freely into the capsule.

The capsule and the cartilage edge run at approximately the same height, the area of the neck of the femoral head that is not enclosed by the joint capsule is shorter at the front than at the back. The attachment lines of the joint capsules run close to the anatomical structures of the hip joint. The so-called Linea intertrochanterica should be mentioned in the front area and the Crista intertrochanterica in the back, whereby more precisely the capsule attachment line is about 1 cm away from it.

Like all bones, the bones of the hip joint are supplied with blood via blood vessels leading to the bones. In the area of the head of the femur, vessels called the arteriae capitis femoris enter the thigh bone on each side. Tearing or pinching can cause harmful undersupply of the bone and must be ruled out with every injury and every fracture.

In addition to supplying the thigh, the artery also supplies the ligaments passing by in this area. The pelvis is supplied by the smallest arteries that branch off from the large arteries. The stability of the hip joint is largely determined by the numerous muscles that, in addition to stabilizing the joint, also take over the task of movement.

The hip muscles are divided into flexors, extensors, abductors and adductors. Together, these muscles press the head of the femur into the acetabulum and thus contribute to the stability and strength of the hip joint.

  • Extensor: The extensor muscles include the gluteal muscles (gluteus maximus, gluteus minimus and gluteus minimus), the adductor magnus and the piriformis muscle.
  • Flexor: the muscles iliopsoas, tensor fasciae latea, pectineus, adductor longus, brevis and muscle gracilis are involved in flexure.
  • Abductors: the muscles responsible for abduction, i.e. abduction of the thigh, are glutaeus medius, tensor fasciae latea, glutaeus maximus, minimus, piriformis and obturatorius.
  • Adductors: the reattachment of the leg (adduction) is performed by the muscles adductor magnus, longus, brevis, M. glutaeus maximus, gracilis, pectineus, M. quadratus femoris, and obturatorius externus.

Numerous nerves also run their course around the hip joint and are mainly used for the sensitive supply of the hip muscles.

Parts of the muscles are supplied by direct nerve endings from the spine (L1-L3 and L2-L4). In addition, the nervus glutaeus superior, nervus glutaeus inferior, plexus sacralis and the nervus obturatorius in the hip region are also involved. As with the vessels, injuries and fractures must always be checked to see whether a nerve has been injured.

Typical signs of paralysis of muscles supplied by corresponding nerves indicate the location of the damage. In the hip joint, external rotation, internal rotation, flexion, extension, abduction and adduction can be performed. Furthermore, there are numerous mixed movements that are possible in the hip joint.

The femoral head stands at a certain angle in the acetabulum. This angle depends on age and changes with increasing age. In a 3-year-old child, the angle is 145 degrees, in adults it decreases to 126 degrees, and in the elderly the angle is only 120 degrees.

The reason for this is the different stability and stages of ossification at the corresponding age. Furthermore, there are numerous diseases and malpositions in which the angle also changes. In the well-known bow legs (coxa vara) the angle can be 90 degrees, while in the bow legs (coxa valga) the angle can be almost 160-170 degrees.

Basically, angles between 120 and 145 degrees are the most stable. However, since the angle changes are slow and not sudden, the body compensates for this instability by active bone remodeling and augmentation. The different angles not only affect the stability of the hip joint, but also have a minor effect on the mobility.For example, people with an angle (also known as the collum corpus angle) of 126 degrees can perform the full range of movement combinations possible in the hip, while very old people with an angle of 120 degrees are restricted in a large number of the movements possible in the hip for mechanical reasons alone.

It is not clear whether a decrease in the collum corpus angle can also lead to a higher susceptibility to fractures. The hip joint is the largest joint in the body, which together with the spine makes a significant contribution to the stability and statics of the body. The hip joint, also known as Articulatio coxae, is composed of the femoral head, which represents the head of the joint, and the hip bone, which represents the acetabulum with a crescent-shaped notch.

To ensure sufficient stability in the joint, it is important that the femoral head fits exactly into the acetabulum. In the case of the hip joint, the femoral head is larger in relation to the socket. In order to guarantee stability in spite of this, there is an anatomical acetabular cup enlargement, also known as a joint lip.

The hip joint is stabilized by numerous ligaments and muscles. The ligaments that stabilize the hip joint extend from the hip bone to the thigh. The most important ligaments in this area are the Ligamentum ileofemorale, the Ligamentum ischiofemorale and the Ligemantum pubofemorale.

Together they form the so-called ring ligament, which holds the femoral head in a buttonhole like a button. One of the 5 hip ligaments runs inside the joint and is also called the Ligamentum capitis femoris. Its joint capsule, which also has a stabilizing effect, surrounds the femoral head and acetabulum.

Numerous muscles in and around the hip joint ensure that all possible movements can be performed and also act as stabilizers in the joint. The most important muscles, among others, are the M. glutaeus maximus, medius and minimus. In addition to the small arteries that supply the hip joint with blood, there is an artery that flows into the head of the femur, also known as the arteria capitis femoris.

In case of injuries or accidents it is always important to check whether the vessels have been injured. If this is the case, a not insignificant bleeding on the one hand and massive undersupply of the hip and thigh bones on the other must be feared. The same applies to injuries to the nerves supplying the hip muscles, which must also be checked for integrity after an accident.

The head of the femur stands at a very specific angle in the hip joint. This angle depends, among other factors, on age. Newborns and young people have an angle of approx.

145 degrees, adults have an angle of approx. 126 degrees, and in old people the angle is approx. 120 degrees.

So the older a person gets, the steeper the head of the femur is in the hip joint. There are still some diseases where the angle is also changed. With bow legs (coxa varum) the angle tends to approach 90 degrees, while with bow legs (coxa valga) the angle becomes steeper and can be around 170 degrees.

Hip joints with angles that are either very steep or very flattened show some instability compared to the normal angles. Due to the slow formation, the body can initially compensate the instability well.