The thigh bone is connected to the…

This particular article will focus on the osteology, vascular and nerve supply of the hip joint. My intention is to provide an anatomical framework and highlight some of the important anatomical points regarding this joint.

Osteology

The skeletal framework of the lower limb begins with the hip joint and pelvic girdle. The hip joint is a typical synovial ball and socket joint, consisting of the acetabulum articulating with the head of the femur. The acetabulum is the socket, its latin name means vinegar cup, as demonstrated by it’s shape. The acetsbulum, is formed by the confluence of the three bones, the ischium, ilium, pubis and the triradiate cartilage, that form the innominate bone. The acetabulum’s circumference is not complete, instead there is an inferior notch (see Figure 1), in life this is covered by the transverse ligament. The bony socket is further deepened by the presence of an additional connective tissue lip around the bony edge (also deficient at its inferior aspect), this is known as the acetabular labrum and functionally deepens the socket, providing a deeper articulation for the head of the femur – see Figure 3.

Figure 1: Lateral view of the innominate bone, adapted from Gray’s Anatomy

Figure 2: Anterior view of the hip joint, adapted from Atlas of Anatomy

Figure 3: Superior view of the hip joint, adapted from Atlas of Anatomy

The ball part of this joint is formed by the head of the femur. The non articulating part of the head tapers to form the neck of the femur, which widens distally to join the shaft. Interestingly the head has a central depression facing the acetabulum, from which a ligament arises, the ligamentum teres. This attaches to either side of the acetabular notch and blends with the transverse ligament (Figure 4). Ligaments usually have poor blood supply, however, this ligament carries the acetabular branch of the obturator artery to nourish the head of the femur. It is important to appreciate that the acetabular branch is a significant source of blood supply to the femoral head in children and less so in adults, more about this later. Other crucial ligaments that help stabilise the hip are the ischiofemoral, iliofemoral and pubofemoral ligaments. These ligaments are key contributors, along with the acetabular labrum and synovial capsule in helping to provide stability to this joint. The synovial capsule, acetabular labrum and ligaments form such a comprehensive seal that there exists a vacuum effect within the joint. Hence during arthroscopy a needle is inserted into the capsule to abolish this vacuum seal. The ischiofemoral, iliofemoral and pubofemoral ligaments limit movements of the hip, The strongest is the iliofemoral, this anterior ligament blends with the pubofemoral and resists anterior dislocation of the hip. The ligament extends all the way down the neck, unlike the weaker posterior ligament, the ischiofemoral which extends only half way; this is one of the contributing factors as to why posterior dislocations of the hip joint are more common.

Figure 4: Lateral view of the hip joint opened up, adapted from Atlas of Anatomy

Figure 5: Lateral view of the ligaments of the hip joint, adapted from Atlas of Anatomy

Figure 6: Anterior ligaments of the hip joint, adapted from Atlas of Anatomy

Figure 7: Posterior ligaments of the hip joint, adapted from Atlas of Anatomy

In fact, this is a good point to compare the hip joint to another ball and socket joint, the shoulder joint. The latter has evolved to sacrifice stability for movement, while the opposite is true of the hip joint. Another interesting comparison is between the shoulder and pelvic girdles. The shoulder girdles are attached anteriorly to the torso via the sternoclavicular joints, however there is no connection posteriorly, the scapulae thus can move independently, greatly aiding shoulder movement, in particular think about abduction beyond 90 degrees and the action of serratus anterior in throwing a punch. The hip on the other hand is attached anteriorly (symphysis pubis) and posterior by the two sacroiliac joints.

The next thing to notice is the angle of the femurs to the midline. Comparing our femurs to some of our evolutionary cousins, we notice that the quadrupeds have almost vertically orientated femurs. While, humans have femurs with their distal ends angled medially (we are knock kneed), this is thought to aid our ability to walk and run as bipeds, try walking normally with your legs shoulder width apart.

Vascular supply

The hip joint is supplied by two anastomoses, the trochanteric and cruciate, both of which are formed by tributaries of the internal iliac and femoral arteries – see Figure 8. The femoral artery commences as the external iliac crosses deep to the inguinal ligament. The femoral artery gives off the profunda femoris, medial and lateral circumflex arteries. Both of these circumflex arteries form a ring around the femur and their branches contribute to both anastomoses.

The trochanteric anastomosis is also supplied additionally by the superior gluteal artery (arises from the posterior division of the internal iliac artery) and the inferior gluteal artery (arises from the anterior division of the internal iliac artery). The cruciate anastomosis is also supplied by ascending branches of the perforator arteries (the perforator arteries are branches of the profunda femoris) and descending branch of the inferior gluteal artery.

The blood supply of the femoral head is important to understand, as compromise of its supply risks avascular necrosis of the femoral head. As mentioned earlier, the acetabular branch (AKA Foveal artery) of the obturator artery supplies the head of the femur, in children this is a relatively weak supply and in adults cannot be replied upon at all. In fact the supply to the femoral head arises from retinacular branches of the medial and lateral circumflex femoral arteries. As the name suggests, the retinacular branches travel up towards the head in the retinacular folds of the synovial capsule, reflected on the neck of the femur. The growth plate between the head and neck of the femur is avascular, therefore the head relies on these retinacular branches. Hence any damage to the femur which damages this supply, say a sub capital fracture (where the head is fractured from the neck) risks avascular necrosis of the head of the femur.

Figure 8: Femoral head blood supply, adapted from Atlas of Anatomy

Nerve supply

As discussed in a previous post, don’t forget Hilton’s law; It states that the innervation of the joints will be the same as the innervation of the muscles that cross the joint. In terms of the hip joint, this includes the hip flexors – supplied by the femoral nerve, the hip abductors, supplied by the obturator nerve, the hip extensors, supplied by the sciatic nerve. The lateral rotator, quadratus femoris is supplied by the nerve to quadratus femoris. The gluteal group of muscles, supplied by superior and inferior gluteal nerves.

It is important to appreciate that many of these muscles also insert on to or around the knee and hence the knee joint also shares some of the innervation the hip joint. This means that hip or knee pain may in fact be referred pain. Hence, a patient complaining of pain in the hip may indeed have a problem with his knee or vice versa.

Clinical correlations

As well as fractures of the hip joint, dislocation of the hip joint may occur, this can be either an anterior or posterior dislocation. Due to the strength of the anterior ligaments (iliofemoral and pubofemoral), anterior dislocations are relatively uncommon, while posterior dislocations are seen more  commonly (90% of dislocations); especially in car collisions, where the dashboard is pushed into the flexed (and often adducted hip) knee, transmitting the force along the femoral shaft, disrupting the joint capsule and posterior ischiofemoral ligament. Posterior dislocation risks damage to the sciatic nerve. The resulting leg is rotated medially (gluteus medius) and shortened. In an anterior dislocation the hip and knee are flexed and the leg is externally rotated.

Fractures of the hip can be split into subcapital, trochanteric and shaft fractures. Subcapital fractures tends to occur in patients where there is evidence of osteoporosis and this tends to be post menopausal elderly women.  The lower levels of oestrogen are responsible for thinning of cortical and trabecular bone.  As discussed earlier fractures across the neck, endanger the blood supply to the head and therefore risk avascular necrosis of the femoral head.

Trochanteric fracture occurs between the trochanters and beyond the femoral capsule. These tend to occur in higher energy trauma, usually in younger patients. Combining knowledge of the muscle origins and insertions with their relation to which fragment they act on, can help explain the clinical appearance of hip fractures. In both subcapital and trochanteric fractures, the leg is shortened and externally rotated. This occurs as all of the muscles involved are attached to the distal fragment. The powerful flexors and adductors of the hip joint, pull the distal fragment, shortening the leg and the lateral rotators externally rotate the distal fragment, causing the toes to turn outwards.

Shaft fractures can be divided into proximal, mid and distal shaft fractures. in proximal shaft fractures, the proximal segment is acted on by the flexors (iliopsoas), abductors (gluteus medius and minimus) and lateral rotators, causing the segment to be pulled up, externally rotated and abducted. The distal fragment is adducted and external rotated by the adductors, and both the quads and hamstrings pull on the distal fragment, so the leg again appears shortened and externally rotated. See Figure 9. In mid and distal fragment shaft fractures of the femur, the upward pull of the quads and hamstrings lead to shortening, but now also the pull of gastrocnemius on the distal segment, causes it to rotate backwards; in distal fractures, the rotation can be enough to pull the fragment on to the popliteal artery, potentially compressing  it.

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Figure 9 Forces acting on fracture fragments.  Taken From Clinical Anatomy by Regions, Snell 

Radiology

Plain film radiography is commonly used in emergency medicine to assess the effects of traumatic injury to the hip joint. It is important to understand what the normal appearance is, the head is articulated with the acetabulum and both trochanters are visible. Another important feature to look for is Shenton’s line, an imaginary line traced from the shaft, neck, head of the femur and the inferior aspect of the superior pubic ramus. Disruption of Shenton’s line is suggestive of fracture, dislocation or developmental dysplasia of the hip.

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Figure 10 Shenton’s line. Case courtesy of Dr Jeremy Jones, Radiopaedia.org, rID: 28928

In terms of posterior dislocations (see Figure 11), when looking at the radiograph, the femoral head appears smaller than the contralateral side and the lesser trochanter is no longer obvious, as rotation of the leg medially means the shaft is superimposed over the lesser trochanter.  An anterior dislocation on radiograph, the femoral head appears larger than the contralateral side and head is displaced medially and or inferior to the acetabulum.

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Figure 11: Posterior dislocation of the right hip. Case courtesy of Dr Gagandeep Singh, Radiopaedia.org, rID: 7260

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Figure 12 Anterior dislocation of the left hip. Case courtesy of Dr Sajoscha Sorrentino, Radiopaedia.org, rID: 14836

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