Articular surfaces and cartilage cover
The hip joint is formed by the acetabulum of the hip bone (acetabulum) and the head of the femur (caput femoris). The acetabulum is not entirely covered by hyaline cartilage, but only along its lunate surface (facies lunata). The remaining area of the depression is filled with loose adipose tissue and the base of the intracapsular ligament.
A fibrocartilaginous ring—the acetabular labrum (labrum acetabuli)—runs along the margin of the acetabulum. It significantly deepens the joint (making it enclose more than half of a sphere) and spans across the acetabular notch, forming the transverse acetabular ligament (lig. transversum acetabuli). The femoral head is covered with cartilage almost entirely, except for a small pit (fovea capitis femoris) where the ligament attaches.
Features of the fibrous capsule
The articular capsule (capsula articularis) attaches to the hip bone along the entire circumference of the acetabulum, fusing with the transverse ligament. Sealing in the area of the notch is provided by the synovial membrane.
The attachment of the capsule to the femur is of critical clinical significance:
- Anteriorly: along the intertrochanteric line (linea intertrochanterica).
- Inferiorly: at the base of the lesser trochanter.
- Superiorly: at the level of the outer quarter of the neck length.
- Posteriorly: encloses 1/2 to 2/3 of the neck, without reaching the intertrochanteric crest (crista intertrochanterica).
This topography means that most of the femoral neck lies within the joint cavity. In neck fractures, this sharply increases the risk of avascular necrosis of the femoral head due to compromised blood supply.
Ligamentous apparatus
The ligaments of the hip joint are classified into two groups.
The intracapsular ligaments include:
- Transverse acetabular ligament.
- Ligament of the head of femur (lig. capitis femoris). It is covered by the synovial membrane, acts as an elastic cushion that absorbs shocks, and contains an artery supplying the femoral head.
Extracapsular ligaments blend into the fibrous layer of the capsule. They originate from the three pelvic bones:
- Iliofemoral ligament (lig. iliofemorale, ligament of Bertin). Located anteriorly, it runs from the anterior inferior iliac spine to the intertrochanteric line. This is the strongest ligament in the body, restricting extension and preventing the body from falling backward.
- Pubofemoral ligament (lig. pubofemorale). Located on the inferomedial side, it restricts abduction and external rotation.
- Ischiofemoral ligament (lig. ischiofemorale). Reinforces the medial and posterior parts of the capsule.
- Zona orbicularis (zona orbicularis). These are circular fibers in the deep layers of the joint capsule forming a loop around the femoral neck.
«Weak spots» and pathways of pus spread
Areas of the capsule where the margins of the extracapsular ligaments do not meet are called «weak spots». In coxitis (purulent inflammation), pus can break through them into the periarticular tissues.
The anterior weak spot is located between the lig. iliofemorale and lig. pubofemorale, covered by the iliopsoas muscle (m. iliopsoas). Beneath the muscle lies a bursa (bursa iliopectinea), which communicates with the joint in 10% of cases. Pathways of purulent streaks from here:
- Along the muscle proximally (into the lumbar region) or distally (toward the lesser trochanter). The swelling takes on an hourglass shape due to the pressure of the inguinal ligament.
- Into the medial compartment of the thigh.
- Vascular pathway (the most dangerous) — along the femoral vessels into the adductor canal.
The posterior weak spot is located below the inferior margin of the lig. ischiofemorale. The obturator externus muscle overlies it. Pus can spread into the lesser pelvis via the obturator canal or posteriorly into the gluteal region beneath the gluteus maximus muscle.
Clinical landmarks and collaterals
To assess bone alignment in traumatology, the Roser-Nélaton line is used. It is drawn from the anterior superior iliac spine (spina iliaca anterior superior) across the gluteal region to the ischial tuberosity (tuber ischiadicum). Normally, the tip of the greater trochanter lies precisely on this line. Upward or downward displacement of the trochanter indicates a femoral neck fracture or dislocation.
The blood supply to the joint features a rich anastomotic network. In case of major artery occlusion, blood flow is compensated by the gluteal arteries (aa. gluteae), obturator artery (a. obturatoria), and branches of the deep artery of the thigh (a. profunda femoris), specifically the medial and lateral femoral circumflex arteries.
Briefly: answers to topical questions
Which muscles directly adjoin the hip joint capsule?
According to sources, the muscular relationships to the hip joint capsule include the following:
- Iliopsoas muscle (m. iliopsoas) — covers the anterior «weak spot» of the capsule; the bursa iliopectinea is located between the capsule and the muscle.
- Obturator externus muscle (m. obturatorius externus) — lies directly on the posteroinferior «weak spot» of the joint capsule.
The pathways of purulent spread also mention the pectineus muscle (m. pectineus), gluteus maximus muscle (m. gluteus maximus), inferior gemellus muscle (m. gemellus inferior), and quadratus femoris muscle (m. quadratus femoris).
Which arteries form the anastomotic network around the hip joint?
According to sources, the anastomotic network of the hip region includes the following vascular connections:
- Lumbar artery (a. lumbalis) anastomoses with the deep circumflex iliac artery (a. circumflexa ilium profunda).
- Gluteal arteries (aa. gluteae) anastomose with the ascending branches of the lateral and medial femoral circumflex arteries (aa. circumflexae femoris lateralis et medialis).
- Obturator artery (a. obturatoria) anastomoses with the medial femoral circumflex artery (a. circumflexa femoris medialis).
- Medial and lateral femoral circumflex arteries (a. circumflexa femoris medialis et lateralis) provide the primary blood supply to the proximal femur and form an extracapsular vascular ring at the base of the femoral neck.
To easily remember the extracapsular ligaments, recall the three pelvic bones forming the acetabulum: the ilium, pubis, and ischium. Each of them gives rise to a similarly named ligament running to the femur.
Class and exam questions
Why does femoral head necrosis frequently occur in femoral neck fractures?
Since the fibrous capsule attaches far from the articular surfaces, most of the neck lies within the joint. A fracture in this zone often ruptures intracapsular vessels (including the artery in the ligament of the head), depriving the bone of its blood supply.
What forms the posteroinferior weak spot of the capsule?
It is located below the inferior margin of the ischiofemoral ligament (lig. ischiofemorale). The obturator externus muscle directly adjoins this area.
Which pathway of pus breakthrough from the anterior weak spot is the most dangerous?
The vascular pathway is the most dangerous. Pus spreads along the fascial sheath of the femoral vessels into the anterior femoral groove and then descends into the adductor canal.
What else to study on this topic
- Features of collateral circulation of the pelvis and thigh
- Topography of the iliopectineal bursa (bursa iliopectinea)
- Biomechanics of the hip joint in bipedalism
- Muscular apparatus of the anterior and posterior regions of the joint
- Classification of hip dislocations relative to the Roser-Nélaton line
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