Biomechanics of the Hip.pptx0000000000000

Biomechanics of the Hip.pptx0000000000000

1. Biomechanics of The Hip Yaser Emam Khalifa, MD Mohamed Mahran, MD Assiut Basic Orthopaedics Course 28-29 March, 2018

2. Biomechanics The science concerned with the internal and external forces acting on the human body and the effects produced by these forces.

3. Aim of the Talk Forces acting on the hip joint Abnormal biomechanics and OA Restoration of normal biomechanics in osteoarthritis by Osteotomy or THR

4. Forces Acting on the Hip Joint The body weight acts as a load applied to a lever arm extending from the body's center of gravity to the center of the femoral head. The abductor musculature , acting on a lever arm extending from the lateral aspect of the greater trochanter to the centre of the femoral head.

5. Distance (B) is about three times distance (A); Thus, to maintain static equilibrium, the abductor muscle force must be three times the partial body weight.

6. The abductor musculature must exert an equal moment to hold the pelvis level when in a one-legged stance, and a greater moment to tilt the pelvis to the same side when walking or running.

7. Because in a static equilibrium the sum of all forces is zero, the abductor muscle force plus the partial body weight must equal the joint reaction force. Thus the total compressive force, or the joint reaction force , is about three times the partial body weight.

8. Joint contact force is estimated to be of 2.5 to 2.8 times the body weight during single-limb stance phase of gait. The force changes as the muscle positions alter during the walking cycle, the angle of abductors and the femoral axis changes. Joint Reaction Force

9. Straight leg raising in supine position 2 times the body weight During gait : 3.5 - 5 times the body weight. Ascending and descending stairs 5-6 Acceleration and deceleration 6-7 Joint force equals body weight only when the patient used a walker for support

10. Arthritis of the Hip Biologic : abnormal tissue (cartilage) resistance to even normal load (inflammatory and metabolic arthritis) Mechanical : abnormal load leading to failure of normal tissues. (Secondary OA, DDH, Perthe ’ s disease, slipped epiphysis)

11. Stress is defined as internal force per unit surface area as the result of an external load. Biomechanics in OA

12. Therefore, the congruency of the total surface area of the hip joint and the magnitude of Stress in that area become important parameters that affect cartilage function. Biomechanics in OA

13. Cartilage is able to attenuate internal stress as a result of applied loads because of its viscoelastic properties.

14. When a load is applied to cartilage, there is an initial deformation as a result of that load ( strain ). With time, additional deformation results because of creep (deformation or change in strain of material over time as a result of the constant load).

15. OA begins when the magnitude of unit load experienced by the joint exceeds the tolerance level of the articular cartilage and the subchondral bone.

16. The Normal Hip Geometrically normal – able to withstand the physiologic range of joint loading (20-25 kg/cm 2 ). The acetabular weight bearing surface (WBS) or the sourcil is horizontal in the coronal plane. There is symmetrical subchondral density in the AP radiograph.

17. The Normal Hip Femoral head is spherical and there is dynamic congruency. Neck shaft angle is 130 ° and anteversion is 12 ° . This means normal shape containment (coverage) and congruency

18. Reduction of Abnormal hip joint forces Malfunctioning hip joint Abnormal hip joint forces OA If JRF are reduced prevention or treatment of OA

19. Horizontal WBS (sourcil) = medial shear = equilibrium

20. Oblique WBS (sourcil) = shear superolateral

21.   Sourcil angle

22. 1. Decrease body weight 2. Decrease body weight moment arm 3. Decrease abductors force. 4. Increase abductors moment arm Conservative Strategies to reduce JRF

23. Weight reduction Weight reduction is the simplest principle for reducing the force on the femoral head and thus on the hip joint. For each pound lost, the force is reduced by approximately 2.5 to 3 pounds; therefore a reduction of even a few pounds in weight is significant.

24. A simple way to reduce forces in the hip is to have the patient lean over the affected hip . The center of gravity then moves laterally toward the center of the femoral head, thus shortening its lever arm. Trendelenburg Gait.

25. Using a Cane Another way to reduce the weight on the hip joint is to have the patient use a cane in the opposite hand.

26. Surgical Osteotomy to increase the WBS in the joint. This procedure places a major portion of the head underneath the weight-bearing zone of the acetabulum . Joint reconstruction : in late cases by THR

27. Rationale of Osteotomy Restoration of the normal biomechanics by correction (re-distribution) of the load over the articular cartilage to reduce stress and prevent or delay OA.

28. Types of pelvic osteotomies Reconstructive Salvage Double innominate Triple innominate Single innominate Salter Pemberton Sutherland Periacetabular Steel T ö nnis Jackob Dial Wagner Mow Ninimiya and Tagawa Azuma and Taneda Ganz and B ü chler (Bernese) Chiari osteotomy Shelf Procedure 1 2 3 4 1 2

29. Restoration of Normal Biomechanics in THR Joint reconstruction. Design of the prosthesis .

30. The biomechanical situation in THA is more complicated. The surgeon directly controls the surgical technique, component placement and alignment . Biomechanics in Hip Replacement

31. Applied Biomechnaics in THA All joint parameters are influenced by the operation Joint center Component orientation (stem, cup) Neck-shaft angle Offset and lever arms Range of motion and impingement Bearing material and wear.

32. Lateral Offset Distance between the center of the femoral head and the center of the medullary canal (anatomic axis of the femur).

33. Offset by Design

34. Long Lateral Offset Advantage : improves the abductor function, which may reduce joint reaction force and increase stability. Disadvantage : increased bending moment increased stress on the medial neck, medial femoral cement, and stem, with potential for loosening and stem failure.

35. Factors affecting global offset Cup position, Cup size Femoral head size, Stem size, Neck length, Stem version, and Stem position within the medullary canal.

36. Neck-shaft Angle

37. Restoration of normal hip biomechanics Understanding the biomechanics Defining the problem Finding the best solution – whether conservative or surgical.

38. Thank You