Biomechanical Principles of the Hip and the Application to THA

1. BiomechanicalPrinciples of the Hip and the Application to THA Ian MacNiven PGY-1 Arthroplasty Rounds

2. Hip Joint Biomechanics • background for the diagnosis and treatment of hip disorders • Knowledge of: • kinematics • loading experienced during static and dynamic activities • transmission of mechanical stresses • various tissues and structures comprising the hip Goal: Aim of joint replacement is to restore the soft tissues to their pre-disease tension, reduce pain, prevent degeneration, restore position and anatomy

3. Anatomical Stability • 27 muscles cross hip joint • Most Important: abductors, specifically gluteus medius • Extends from the lateral aspect of the ilium to the greater trochanter allowing single leg stance to occur • Acetabular labrum • Osseous Anatomy – depth of the acetabulum • Ligaments: iliofemoral, pubofemoral and ischiofemoral ligaments

4. Anatomical Features of the Femur Influencing Hip Biomechanics • Femoral neck anteversion • Anterior rotation of femoral head on shaft 2. Femoral offset • distance between the centre of rotation of the femoral head and the shaft 3. Head-Neck ratio • difference between the circumference of the femoral head and the femoral neck

5. Anatomical Importance of Acetabulum Anteversion • There is a wide anatomical variation in the anteversion angle. The anatomical landmark defining this angle has recently been described by Archbold et al. as being the transverse acetabular ligament

6. Hip Biomechanics Miller

7. Mechanical Forces • The center line of the body is not directly over the hip • In order to prevent one from falling over in single leg stance the hip abductors work against the weight of the body • This leads to more force on the hip joint than just body wt. alone

8. The hip joint acts as a fulcrum balancing the force of body weight with the force generated by the hip abductors

9. Hip Reaction Force • W = weight of body • M = moment force of abductors • R = joint reaction force • Assume b = 12.5 • And a = 5 • JRF: • compressive force experienced at the femoroacetabular articulation • the balance of the moment arms of the body weight with the pull of the hip abductors at the greater trochanter to maintain a level pelvis

10. Hip Reaction Force • May reach 4-6x the body weight • JRF; R decreases with • Medialization of acetabulum • Laterization of GT • Long neck prosthesis • Since A < B; the force of the Abductors > body W

11. When the body is in equilibrium, the sum of the forces acting across the hip will also be zero. Therefore, the joint reaction force can be represented as: JRF= Abd + BW During single leg stance; body in equilibrium, the sum of the rotational torque (force x distance) acting around the hip will be zero. Therefore: BW x b = Abd x a Reducing JRF – Reduces Pain. How can we achieve this? Reducing BW (weight loss) or by reducing the lever arm of BW (b). ‘‘b’’ can be reduced by tilting the body towards the affected hip during single leg stance in the gait cycle = positive Trendelenburg test. Why? The forces acting opposite BW are reduced (abductor muscle weakness) the rotational torque around the hip can only be balanced by tilting towards the affected limb (thus reducing b). How do we reduce the abducting force? A cane. Held in the opposite hand, it acts to reduce the influence of body weight and consequently lower the abductor force.

12. Kinematics after hip replacement Goal: restore function by restoring normal anatomy and thus normal hip biomechanics 5 Factors: • Center of rotation of the hip • Anteversion (femoral and acetabular components) • Femoral Offset • Leg length • Bearings These parameters will be considered individually, but they are all inter-related.

13. Center of Rotation of the Hip • Careful planning is necessary to restore the geometrical center of rotation to its pre-disease position • Prostheses allow selection of the appropriate neck length, neck-shaft angle and neck off-set • Restoration of the hip centre confers both anatomical and biomechanical advantages and usually the best acetabular bone stock is found in the true acetabulum, which permits medialisation of the acetabular cup. • Correct positioning increases the lever arm for the abductor muscles, thus restoring biomechanics • ‘High hip center’ and cup lateralization have both been implicated in early failure

14. Anteversion • The combined anteversion of the acetabular cup and the femoral stem should be aided by the transverse acetabular ligament to reflect patient anatomy. • Therefore, optimum movement with minimal impingement • Relative retroversion of the acetabular cup predisposes to dislocation, exacerbated by the posterior approach due to weakness of posterior muscles and anterior impingement during IR

15. Femoral Offset: Distance between center of the femoral head and a line down the shaft of the femur • Femoral offset is “A” from the center of the femoral head to the long axis of the femur. • The neck-shaft angle is “B,” between the long axis of the femoral neck and the long axis of the femoral shaft. • Davey et al. • A) Avg 43.9 mm (Range 27 to 57 mm) • B) Neck shaft angle Avg 124.7 degrees • (Range 105 to 155)

16. History • Sir John Charnley • First to address offset/tension in arthroplasty • Goal - restore femoral offset • Medialiaze the acetabular component • Avoid excessive anteversion of femoral component • Maintain 135 degrees of neck-shaft angle • Advance greater trochanter

17. Effect of increasing Offset • Increases abductor lever arm (increasing ‘a’) • Decreases hip joint reaction force • Increases soft tissues tension • Less impingement • Decreases wear • Increased torque on stem (as neck-shaft angle decreases)

18. Effect of decreased offset • Abductor function weakened • Trendelenburg Limp, Early fatigue • Increases wear • Decreased soft tissue tension predisposes joint to dislocation due to laxity • Increased impingement

19. 5 ways to increase offset • Increase neck length • Leads to leg length discrepancy • Decrease angle • Abductors more efficient, Decreases leg length • Alter geometry of the bearings • Acetabulum modularized offset • Medialize femoral neck with concomitant increase of neck length

20. Decreasing the neckshaft angle reduces the height of the femoral head and thus the limb length while increasing offset. • Benefits: • Increases abductor tension • More efficient muscle Dual or high-offset femoral components either vary the neck-shaft angle of the implant or medialize the neck to vary offset.

21. Radiograph of a sixty-seven-year-old man with severe osteoarthritis of the right hip associated with superolateral migration of the femoral head and disruption of the Shenton line.

22. Following total hip arthroplasty with a high-offset femoral component, the joint center has been displaced medially coincident with an increase in the abductor lever arm (A*) and a corresponding reduction in the joint reactive force.

23. Final Thoughts • Lower limb biomechanics during gait do not return to normal following total hip arthroplasty • Intraoperative measurement of LLD • Shuck Test, Drop Kick Test, Leg to Leg Comparison, Special Tests • Imaging, Navigation Techniques