Functional Anatomy

The hip joint is a multiaxial ball-and-socket joint that provides support and balance to the upper body during stance and gait. Soft tissues and osseous structures of the hip contribute to the equilibrium of forces that provide controlled hip motion.

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  I. Stability of hip joint is provided by the articular surfaces of the acetabulum and the head of the femur alongside the ligaments, capsule, and labrum.

  
  
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  II. Hip joint orientation: the neck of the femur is angulated in the sagittal and coronal planes in relation to the shaft.

  
  
  
  
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     The femoral neck shaft angle is formed by the femoral shaft axis and a line drawn along the axis of the femoral neck through the center of the head of the femur. Normal angle is 120 to 135 degrees in adults, and can be 20 to 25 degrees greater at birth. ¹

     
     
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     The femoral anteversion is the orientation of the neck in relation to femoral condyles. Normal angle is 30 to 40 degrees at birth and 8 to 14 degrees in adults. ²

  
  
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  III. Long axis of the acetabulum is directed forward, and has 15 to 20 degrees of anteversion with 45-degree inferior inclination. ³

  
  
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  IV. Upper body weight is transmitted to the lower limb through the sacroiliac (SI) joint.

  
  
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  V. The narrower femoral neck in relation to the head aids mobility of the lower limb.

Hip Joint Motions

There is high degree of congruency of the articulating surfaces. Motion between the femoral head and the acetabulum is mostly rotational with minimal or no translation. The pelvis contributes to the hip joint motion to ( Table 5.1 ). ⁴

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  I. Normally, the hip joint moves an average of 120-degree flexion and 15-degree extension. In a position of 90 degrees and neutral adduction, internal rotation (FADIR) ranges from 30 to 40 degrees. External rotation ranges from 40 to 60 degrees. Normal hip abduction and adduction are 30 to 50 degrees and 20 to 30 degrees, respectively. ⁵

  
  
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  II. Hip flexion is limited by the iliofemoral ligament, anterior capsule, and hip flexors. Hip extension puts soft-tissue structures of the hip under tension and limits internal and external rotation.

  
  
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  III. Pelvic Motions

  
  
  
  
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     Anterior pelvic rotation: it includes anterior movement of superior pelvis with the iliac crest tilting forward in a sagittal plane.

     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     

     Table 5.1 Pelvic motion.

     Pelvic motion	Left hip motion	Right hip motion
     Anterior rotation	Flexion	Flexion
     Posterior rotation	Extension	Extension
     Right lateral rotation	Abduction	Adduction
     Left lateral rotation	Adduction	Abduction
     Right transverse rotation	External rotation	Internal rotation
     Left transverse rotation	Internal rotation	External rotation

     
     
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     Posterior pelvic rotation: the superior pelvis moves backward and the iliac crest tilt backward in a sagittal plane.

     
     
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     Left lateral pelvic rotation: the left pelvis moves distally in relation to the right pelvis in the frontal plane and rotates downward.

     
     
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     Right lateral pelvic rotation: the right pelvis moves inferiorly in relation to the left pelvis in the frontal plane and rotates downward.

     
     
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     Left transverse pelvic rotation: in the horizontal plane, the pelvis rotates to the left; the right iliac crest moves anteriorly in relation to the left iliac crest; the iliac crest moves posteriorly.

     
     
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     Right transverse pelvic rotation: in the horizontal plane, the pelvis rotates to the body’s right; the left iliac crest moves anteriorly in relation to the right iliac crest, which moves posteriorly.

  
  
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  IV. Intrapelvic Motions

  
  

  There are three classes of intrapelvic motions:

  
  
  
  
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     Posteroanterior and anteroposterior rotations of the ilia in relation to the sacrum and the pubis.

     
     
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     Sacral movement in relation to the ilia. The SI motion occurs superiorly, inferiorly, anteriorly, and posteriorly, and axial rotation occurs about a transverse axis.

     
     
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     The sitting–standing changes that affect the relationship of the movement of the ilia in relation to each other and the sacrum.

Hip–Spine Kinematics

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  I. Flexion of the lumbar spine to 45 degrees relies on lumbar muscle activity, while flexion greater than 45 degrees requires pelvic rotation. ⁶ The spine contributes greatly to early stage of hip flexion and extension. Lateral flexion of the lumbar spine elicits ipsilateral hip abduction; contralateral hip adduction while twisting movement of the trunk is contributed mostly by the hip.

Lower Limb Axis

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  I. Mechanical axis of the femur: this is assessed by a line passing from the center of the femoral head to the center of the tibial plafond of the ankle joint. Normal is 3 degrees to the vertical axis. ⁷

  
  
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  II. Anatomic axis of femur: this is assessed by a line drawn from the piriformis fossa to the center of the knee joint. It depends on the length of the femur; it increased in shorter femurs and decreased in longer femurs. Normal is 6° from mechanical axis and 9° from the vertical axis.