Lecture 6 - Hip Biomechanics (part 1)

Question 1

What are the bones of the hip?

Answer 1

1. Innominate bone
2. Femur

Question 2

What are the structures of the innominate bone?

Answer 2

1. Acetabulum
2. Labrum

Question 3

What are the structures of the femur bone?

Answer 3

Head of the femur

Question 4

What is the acetabulum formed by?

Answer 4

3 fused bones:
1. Ilium
2. Pubis
3. Ischium

Question 5

What are the different surfaces of the acetabulum?

Answer 5

1. Acetabular fossa (non articular depressed region on floor of acetabulum)
2. Articular/lunate surface (makes contact with head of femur)

Question 6

What is the orientation of the acetabulum?

Answer 6

Faces laterally and inferiorly

Question 7

What is the labrum?

Answer 7

A fibrocartilaginous ring that helps…
1. Stabilize hip joint
2. Increase contact area
3. Decreases joint stress

Question 8

What is the structure of the femur?

Answer 8

Femoral head: 2/3rds of a sphere
Articular cartilage: Makes more spherical

Question 9

What improves joint congruence?

Answer 9

Cartilage
(Increase in congruence = Increase in stability)

Question 10

What is the orientation of the femoral head and neck?

Answer 10

• Large head narrows abruptly into neck
• Neck enclosed by hip joint capsule

Question 11

What does the orientation of the femoral head and neck influence?

Answer 11

• Hip motion and weight bearing
  -> Frontal plane: 125 degree angle between neck and femur
  -> Transverse plane: 15 degree angle between neck and femoral condyles

Question 12

What is Wolff’s law?

Answer 12

• Form follows function
• Femoral neck sustains large bending moments and tensile compressive forces
• Neck reinforced by thickened cortical bone and trabecular bone

Question 13

What is the important landmark of the femur?

Answer 13

Greater trochanter
- Insertion site of muscles
- Lengthens moment arm (improves mechanical advantage)

Question 14

What do the landmarks of the femur help understand?

Answer 14

• Hip joint anatomy
• Muscle function

Question 15

What type of joint is the hip joint?

Answer 15

Ball and socket joint
-> Synovial
-> Triaxial
-> 3 dof

Question 16

What are the structures of the hip joint?

Answer 16

Joint capsule:
- Attached to bony rim of acetabulum and intertrochanteric crest
- Composed of multiple bands of fibers
- Encloses femoral neck and head

Question 17

What reinforces the joint capsule?

Answer 17

Three ligaments…
Ligament of the head of the femur (one of them)

Question 18

What is hip joint mobility?

Answer 18

The range of motion of the hip motion linked to motion of the low back and pelvis

Question 19

What is the typical hip rom in the sagittal plane for flexion?

Answer 19

• 135 degrees
• Largest amplitude

Question 20

What is the typical hip rom in the sagittal plane for extension?

Answer 20

• 45 degrees
• Limited by ligaments

Question 21

What is the typical hip rom in the frontal plane for abduction?

Answer 21

• 45 degrees
• Can be improved by stretching

Question 22

What is the typical hip rom in the frontal plane for adduction?

Answer 22

• 25 degrees
• Limited by contralateral limb

Question 23

What is the typical hip rom in the transverse plane for internal rotation?

Answer 23

• 35 degrees
• Limited by ligaments

Question 24

What is the typical hip rom in the transverse plane for external rotation?

Answer 24

• 45 degrees
• Limited by ligaments

Question 25

What are the possible orientations of the pelvis in the sagittal plane?

Answer 25

a) Retroversion (posterior tilt - extends hip) b) Neutral c) Anteversion (anterior tilt - flexes hip)

Question 26

What is the possible orientation of the pelvis in the frontal plane?

Answer 26

- Pelvic tilt (hike and drop) can occur - Elevated side: hip adduction - Dropped side: hip abduction

Question 27

What is the orientation of the pelvis in the transverse plane?

Answer 27

a) Pelvic internal rotation (contralateral medial) b) Pelvic external rotation (contralateral lateral)

Question 28

What are spine-hip interactions?

Answer 28

- Patients/athletes compensate for lack of mobility with other joints - Compensations occur in frontal/sagittal plane

Question 29

What are the joint reaction forces of the articulating surfaces?

Answer 29

- JRF on femur are transmitted from the acetabulum to the femoral head - JRF creates a torque on the neck of femur - Wider femoral necks withstand torques better - Men have wide necks than women

Question 30

What is the alignment of the articulating surfaces in the frontal plane?

Answer 30

1. Femoral neck and shaft: 125 degrees (optimal) 2. Coxa valga > 125 degrees: - JRF parallel to neck - More compression - Less bending 3. Coxa vara < 125 degrees: - Increased bending moment *Angle decreases with age*

Question 31

What is the alignment of the articulating surfaces in the transverse plane?

Answer 31

Anteversion: 15 degrees - Decreases during maturation - Excessive anteversion places head of femur too anteriorly Retroversion: less common

Question 32

What is the alignment of the articulating surfaces in the transverse plane?

Answer 32

Medial rotation: compensates for excessive femoral anteversion

Question 33

What causes lateral tibial torsion?

Answer 33

Excessive anteversion

Question 34

What is cerebral palsy?

Answer 34

- Affects a persons ability to move and maintain balance/posture - Can result in femoral anteversion - Solution: femoral osteotomy (cut femur, realign head, stabilize)

Question 35

What are the similarities/differences between the hip and shoulder joints?

Answer 35

Difference: - Greater stability/less mobility in hip (greater boney congruence and support from ligament/capsule) Similarities: - Movement limited by ligaments and muscles