HIP JOINT -
SOME DETAILS OF THE ANATOMY
CONTENTS
| 1 | The hip joint skeleton and its ligaments |
| 2 | Some important muscles around the total hip |
| 3 | The anterior access to the hip – the nerves in the way |
| HIP JOINT SKELETON AND ITS LIGAMENTS |
First a note about the pictures. We see the hip joint most often as it is seen from the front, the pictures present the hip as if projected on the so called frontal plane.
Frontal plane is a plane that goes parallel with your forehead and with your kneecaps. Schematic pictures of the hip joint and the illustrations of a total hip device show them almost always as if observed from the front. You stand in front of the hip joint / total hip device and observe them from this point.
Similarly, the absolute majority of the X-ray pictures show the hip joint as if it were projected on the frontal plane. This is called the frontal view of the hip joint.
The surgeons are used to this view; using other views, such as profile view, especially for x-ray pictures, would actually superimpose / project the one hip joint over the other one. The resulting picture would be difficult to decipher and special techniques are then used.
In accordance with this practice, the following pictures of the hip joint show the skeleton parts as viewed from the front. I use also the anatomical names of the skeleton parts of the hip joint – you will encounter them for example in descriptions of x-ray pictures:
Show Picture: Pictures of the femoral head, socket, and labrum (socket’s rim)
A – The socket of the hip joint is a deep hole placed on the outside of the pelvis; the anatomical name is acetabulum. The large part of the socket’s concave surface is covered by smooth joint cartilage (blue in this picture). The hip joint is the best example in the body of the ball-and-socket joint.
Note that whereas the smooth joint cartilage of the head of the femur covers considerably more than a hemisphere the equally smooth cartilage of the socket covers only a U - shaped surface of the acetabulum.
B - The femoral head is the rounded upper ending of the femoral (= thigh) bone covered with smooth joint cartilage. The head continues with the femoral neck (=collum).
At the outside of the femoral neck is a bony protuberance called greater trochanter, an important area for attachment of muscles that move the leg outside (= abductor muscles).
On the inside of the femoral neck is a smaller protuberance called lesser trochanter. It is an important landmark on x-ray pictures for assessing the leg length difference and also an attachment of the muscle that flexes the leg (the Iliopsoas muscle).
C - The edge of the acetabulum is enhanced by a cartilaginous rim called labrum. On the cross-section of the socket as seen on this separate and schematic picture the labrum has a triangular form and violet color. The labrum then continues as a joint capsule that encircles the whole hip joint.
The cartilaginous mass of labrum makes the socket deeper and thus increases the stability of the hip joint. On the other hand, the labrum “stands in the way” when the individual makes extreme hip movement. Recently surgeons discovered that damage of the labrum causes pain in the hip joint and successively leads to damage of the joint cartilage and possibly to the development of osteoarthritis of the whole hip joint.
For more details see the chapter: Hip joint diseases – Hip impingement
Both joint surfaces of the hip joint (the femoral head and the acetabulum) are held together by strong, yet elastic joint capsule enforced by massive joint ligaments that keep the hip joint very stable. It needs a very big violence to dislocate a healthy hip joint.
Show Picture: Hip Joint ligaments
The joint capsule and the joint ligaments on the front side of the hip joint – shown on this picture – are stronger than the ligaments and joint capsule on the backside of the hip joint.
That is why the total hip replacement operations through the weak posterior joint ligaments and capsule (posterior approach) have had higher risk of hip dislocation, especially in cases where the surgeon did not repair the divided soft tissues carefully.
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THE IMPORTANT MUSCLES AROUND THE HIP JOINT |
(The abductor muscles, the tensor fasciae lateae muscle, the iliopsoas muscle, and the adductor muscles)
There are totally nineteen muscles or their parts that cross the hip joint; the hip joint is embedded deep under all these muscles. It is important to realize that these muscles never work in isolation; all movements of the leg in the hip joint (and other joints) are always done by a coordinated effort of whole muscle groups.
Four muscles and muscle groups need to be named especially because their good function is necessary for the good function of the total hip; the contracture (shortening) of these muscles, on the other hand, causes bad function of the total hip joint.
The abductor muscles – the muscles that push the leg to the side.
Show Picture: Muscles that abduct the leg - The middle and the lesser gluteus muscles.
A - The abductor muscles push the leg toward the side. In this picture, you see the training of the weak abductor muscles after total hip surgery. As a rule these muscles are weak after the total hip replacement operation. Their training starts after the operation with the patients lying supine and the operated on leg lies on a powdered, low friction board. The patient pushes the leg into abduction = toward outside.
B - The most important among all abductor muscles are two muscles, the middle and the lesser gluteus muscles, depicted on the lower picture. In this picture you see schematically the middle and small gluteus muscles. They arise from the pelvis and insert at the greater trochanter. When they contract they move the leg toward outside = they abduct the leg.
The chief function of the middle and small gluteus muscles is, however, to keep the pelvis approximately horizontal when you put your whole weight on the leg. When these muscles are weak there will be marked sagging of the pelvis (and buttock) to the opposite site.
Patients with weak gluteus muscles walk with a typical limp, sometimes also called “Trendelenburg” limp
Show Picture: Trendelenburg limp
A – In a patient with strong middle and small gluteus these muscles keep the pelvis stable when the patient tramps on the left leg. The buttock line stays horizontal.
B – In a patient with weak gluteus muscles, the muscles cannot stabilize the pelvis. In this patient the left hip joint has weak gluteus muscles. When this patient tramps on the left hip the pelvis sinks to the right side– the buttock line falls down on the right side. Such walk is unsightly, swinging, duck-like.
The tensor fasciae latae muscle - abducts the leg too
There is another abductor muscle whose contracture may cause problem after total hip surgery. It is called tensor fasciae lateae.
Show picture: The tensor fasciae lateae – apparently longer leg
This muscle, which arises on the outside of the pelvis, inserts in the large band that stretches on the outside of the thigh from the hip to the knee area; this band is called fascia lata. You can feel it yourself on the outside of the thigh. The action of this muscle (together with other muscles) tensions the fascia lata.
Contracture of this muscle and of the fascia lata band keeps the leg abducted in the hip joint after total hip surgery. Such contracture is the most frequent cause of apparently longer leg after the total hip surgery. For more information on leg length difference see the chapter Too long leg.
The iliopsoas muscle – the muscle whose damage causes pain in the total hip joint
Show picture: The iliopsoas muscle
The muscle arises inside the pelvis and attaches its tendon at the lesser trochanter. It passes above the hip joint in the groin area. When you flex your hip joint you may palpate the muscle there.
The iliopsoas muscle is the most powerful flexor of the hip joint; however, its force is not used until a very strong flexion of the hip joint is needed, when flexing the outstretched leg ( for example entering the car). There are many muscles around the hip that flex the leg in the hip joint when less force is needed. This observation explains the fact that even patients whose iliopsoas tendon was divided at operation notice only small reduction of flexion force in the operated on hip joint.
Show Picture: Contracture of the iliopsoas muscle – Pain in the hip joint.
When the patients lies with "flat back" on a flat board, with the legs bent down in the knees over the rim of the board, the contacted iliopsoas muscle flexes the operated on leg. The operated on leg with flexion contracture raises in this position.
In patients with long standing flexion contracture (flexion posture) of the hip joint, this muscle becomes shorter. The surgeon should recognize the contracture of this muscle before the surgery and repair it during the surgery by dividing or lengthening the muscle’s tendon.
If the contracture of the iliopsoas muscle is not corrected at surgery the patient continues with pain in his / her total hip joint even after the surgery. Smaller degree of contracture may be removed by intensive physical therapy.
The iliopsoas muscle may be also squeezed / impinged against the protruding total hip device and cause pain. For more information about the iliopsoas muscle and the pain symptoms it may cause See the chapter: Other THR Complications – Impingement of iliopsoas…
The adductor muscles
Show Picture: The adductor muscles – Apparently shorter leg
Upper Picture: Adductor muscles: –The muscles of the adductor group arise on the pelvis (Pubic bone) and attach on the inside of the thigh bone shaft. When they contract they push the leg toward midline = adduction.
Lower Picture: The adduction contracture of the hip – The adductor muscles are often short in patients whose hip joints were stiff longer time before the total hip replacement operation. These patients kept their leg adducted, i.e. close to the midline, because of pain in the hip joint. These patients cannot abduct the leg = push the leg toward outside. The muscles and other soft structures (joint capsule) become shorter (contracted) in this adducted position.
Because of adduction contracture of the hip joint the whole pelvis tilts to the healthy side, the diseased leg “raises” up. This muscle contracture thus produces the feeling that the leg is shorter than the opposite healthy leg.
This deformity should thus be recognized already before the total hip surgery because it is very difficult to stretch the contracted muscles after the surgery. The surgeon should correct the deformity – divide the short adductor muscles from their bone attachments at pubic bone during the total hip replacement, so that the leg has a free motion toward outside at the end of the surgery.
If this contracture is not recognized and corrected the patient feels that his / her operated on leg is shorter (the apparently shorter operated on leg).
| THE NERVES ON THE ANTERIOR SIDE OF THE HIP JOINT |
Show Picture - Anterior access to the hip and the nerves in the way
One of the risks of the anterior access to the hip are the many small nerves which stretch under the skin past the frontal area of the hip joint. Particularly one of these nerves, a nerve called lateral cutaneus nerve, may be damaged when the surgeon cuts through the soft tissues in the upper part of the operation wound. The damage causes pain and numbness. The pain irradiates from the upper end of the scar down and the skin around the scar is numb and hypersensitive to touch.
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