Hip Joint

Bony Structure of the Hip Joint

The hip joint is a multi-axial joint that connects the pelvis to the lower extremities. As compared with the glenohumeral joint (shoulder), the hip has less range of motion and is designed primarily for weight bearing and stability.

• Type: ball-and-socket diarthrodial joint
• Articular surfaces: 
  • Head of the femur
  • Acetabulum of the hip bone (acetabular labrum: a ring of cartilage that increases the depth and stability of the articular surface)
• Supporting structures:
  • Fibrous capsule
  • Intra-articular and extra-articular ligaments
  • Iliopectineal, trochanteric, and ischial bursae
• Functions:
  • Connects the axial skeleton to the lower limbs
  • Bears weight during static (i.e. standing) and dynamic (i.e. walking and running) conditions
  • Allows for flexion-extension, lateral-medial rotation, abduction-adduction, and circumduction of the thigh

Supporting Structures of the Hip Joint

Articular capsule

Consists of 2 layers:

1. Fibrous capsule: 
   • The external layer of the capsule
   • Attaches proximally to the acetabulum, close to the rim
   • Attaches distally around the proximal end of the femur:
     • Anteriorly, to the intertrochanteric line (thickest area)
     • Superiorly, to the base of the femoral neck (thickest area)
     • Posteriorly, about 1 cm from the intertrochanteric crest
     • Inferiorly, on the femoral neck close to the lesser trochanter 
   • Has deep circular fibers that form a collar around the femoral neck, called the zona orbicularis (or annular ligament) 
2. Synovial membrane:
   • The internal layer of the capsule
   • Produces synovial fluid, a viscous substance that lubricates and circulates nutrients to the joint
   • Originates at the margin of the articular surface of the femoral head, covers a portion of the femoral neck, is reflected on the internal surface of the capsule, reaches the fat tissue contained in the acetabular fossa, and encloses the ligament of the head of the femur

Ligaments

Can be divided into 2 groups:

1. Intra-articular: 
   • Ligament of the femoral head or round ligament of the femur: carries the artery to the femoral head
   • Transverse ligament of the acetabulum
2. Extra-articular ligaments, which support the capsule: 
   • Iliofemoral or “Y ligament of Bigelow”: strongest ligament of the human body
   • Pubofemoral: prevents over-abduction
   • Ischiofemoral ligaments: prevent over-extension

Bursae

Bursae are small, synovial fluid-filled sacs that reduce friction between the bony components of the joint and the surrounding muscles. 

• Trochanteric bursa: between the greater trochanter and the iliotibial tract
• Ischial bursa: between the ischial tuberosity and the gluteus maximus 
• Subgluteus medius bursa: between the superior surface of the greater trochanter and the gluteus medius
• Subgluteus minimus bursa: between the superior surface of the greater trochanter and the gluteus minimus
• Iliopectineus or iliopsoas bursa:
  • The largest bursa of the human body
  • Located between the fibrous capsule of the hip and the iliopsoas

Flexor Muscles of the Hip Joint

The primary flexor muscles of the hip are the iliopsoas and rectus femoris.

Extensor Muscles of the Hip Joint

The primary extensor muscle of the hip is the gluteus maximus, assisted by the biceps femoris, semitendinosus, and semimembranosus muscles.

Abductor Muscles of the Hip Joint

Adductor Muscles of the Hip Joint

External Rotator Muscles of the Hip Joint

Neurovasculature of the Hip

Blood supply

The arteries that supply the hip joint originate from the common iliac artery, which bifurcates into the internal and external iliac arteries.

• Internal iliac artery—divides into a posterior trunk and the anterior trunk, which has multiple branches:
  • Gluteal arteries: superior and inferior branches
  • Obturator artery: gives rise to the artery of the head of the femur within the ligament of the head of the femur
• External iliac artery—becomes the femoral artery after passing under the inguinal ligament:
  • Profunda femoris, a deep branch of the femoral artery, provides blood supply to the hip joint along with its branches: 
    • Medial circumflex femoral artery (MCFA)
    • Lateral circumflex femoral artery (LCFA)
• The branches of the internal iliac (inferior gluteal) and the external iliac (profunda femoris) arteries form an important connection at the hip: the cruciate anastomosis.

The veins of the hip joint accompany the arteries in trajectory and name.

Innervation

The femoral and obturator nerves, arising from the lumbar plexus (T12–L4), and multiple smaller nerves arising from the sacral plexus (L4–S4), innervate the hip joint. 

• Anterior/medial thigh:
  • Femoral nerve (L2–L4): innervates the anterior thigh muscles and anterior aspect of the hip joint 
  • Obturator nerve (L2–L4): primarily innervates the adductor muscles and inferior aspect of the joint
  • Saphenous nerve: the terminal cutaneous branch of the femoral nerve
• Posterior thigh/gluteal region:
  • Sciatic nerve (L4–S3): passes through the greater sciatic foramen to the gluteal region; the sciatic nerve is the longest and widest nerve in the body
  • Superior gluteal nerve (L4–S1): innervates the gluteus medius, gluteus minimus, and tensor fascia latae muscles, and the superior aspect of the joint
  • Inferior gluteal nerve (L5–S2): innervates the gluteus maximus muscle
  • Quadratus femoris nerve: innervates the posterior aspect of the joint, inferior gemellus muscle, and quadratus femoris muscle

Clinical tip: The knee joint is also innervated by the femoral, obturator, and sciatic nerves, explaining the pain referral patterns from the knee to the hip.

Clinical Relevance

The following are clinically relevant to the hip joint and region:

• Legg-Calve-Perthes disease: also known as idiopathic avascular necrosis of the proximal femoral head, characterized by idiopathic avascular necrosis of the femoral head. Legg-Calve-Perthes disease presents as a limp with an insidious onset and associated hip pain.
• Slipped capital femoral epiphysis: an orthopedic disorder of early adolescence characterized by the pathologic “slipping” or displacement of the femoral head, or epiphysis, on the femoral neck. 
• Hip fracture: classified according to the fracture’s anatomic location as intracapsular or extracapsular. A low-impact fall is the typical mechanism of injury in the elderly, often associated with osteoporosis. Motor vehicle accidents and other high-velocity trauma are common in younger individuals.
• Fracture and necrosis of the femoral neck: If the fracture is secondary to trauma or osteoporosis, the blood supply of the femoral head can be compromised, which may lead to post-traumatic avascular necrosis.
• Hip dislocation: mostly commonly occurs in cases of high-energy trauma, such as motor vehicle accidents. Posterior dislocation (90%) is the most common, and the extremity generally presents as adducted and internally rotated. Hip dislocations may be associated with avascular necrosis and sciatic nerve injury.
• Hip dysplasia: an innate or acquired malformation of the hip generally seen in newborns. Characterized by hip instability, resulting in subluxation or dislocation. Early diagnosis is essential because late diagnosis may result in irreversible damage to the joint, which may lead to a painful hip and abnormal gait. 
• Osteoarthritis: characterized by hyaline articular cartilage loss but also involves changes to the subchondral bone, synovium, and surrounding joint structures. Osteoarthritis is the most common form of arthritis, and is principally a disease of aging. Patients experience a loss of range of motion and a painful joint.
• Piriformis syndrome: also called deep gluteal syndrome or wallet neuritis, is characterized by a combination of symptoms involving the hip, buttock, and upper thigh. Described as peripheral neuritis of the sciatic nerve and may be related to irritation of the sciatic nerve at the level of the piriformis muscle. May be caused by trauma, hematoma, excessive sitting, and anatomic variations of the muscle and nerve. 
• Trendelenburg gait: abnormal gait secondary to weakness of the hip abductors, primarily the gluteus medius and gluteus minimus muscles, which are essential to maintaining the balance of the pelvis during the gait cycle. Weakness of the hip abductors causes a drop of the contralateral pelvis while walking, or Trendelenburg gait.