Biomechanics of THA AIIMS JODHPUR SEMINAR

The PowerPoint presentation titled "Biomechanics of Total Hip Arthroplasty (THA)" from the All India Institute of Medical Sciences Jodhpur provides an in-depth exploration of the forces acting on the hip joint and their implications for THA.

### Key Sections:

1. **Introduction to Biomechanics**
- Definition and study of forces acting on the body.
- Explanation of torque and its role in biomechanics.

2. **Forces Acting Across the Hip Joint**
- Detailed analysis of gravitational force, abductor muscle force, and joint reaction force (JRF).
- Discussion on the stability of the hip and the relationship between body weight and abductor tension.

3. **Static Biomechanical Concepts**
- Examination of bilateral and single-leg stances.
- Impact of body weight distribution and center of gravity on hip stability.

4. **Biomechanics of Total Hip Arthroplasty**
- Comparison between the lever arm ratios in normal and arthritic hips.
- Surgical modifications to reduce the load on the hip joint.

5. **Implant Design and Components**
- Detailed descriptions of cemented and cementless femoral stems.
- Analysis of femoral neck designs, head sizes, and acetabular liners.
- Comparison of cemented vs. cementless acetabular components.

6. **Impact of Biomechanical Variations**
- Effects of varus and valgus configurations on the proximal femur.
- Influence of femoral version and offset on hip function and stability.

7. **Strategies to Reduce Joint Reaction Force**
- Techniques to optimize femoral and acetabular component placement.
- Importance of restoring combined offset for maintaining abductor muscle tension.

8. **References**
- Cited works providing additional context and supporting information for the topics discussed.

The presentation utilizes diagrams and images to illustrate biomechanical concepts, lever systems, and implant designs, providing a comprehensive overview of the biomechanical principles relevant to total hip arthroplasty.

1. Biomechanics of THA ALL INDIA INSTITUTE OF MEDICAL SCIENCES, JODHPUR

2. Introduction Biomechanics- study of forces acting on and generated within the body and of the effects of these forces on the body. Torque- a force that tends to cause rotation

3. BIOMECHANICS First class lever Hip joint- Fulcrum Forces act on both sides of fulcrum i.e. body weight & abductor tension

4. Forces acting across hip joint Gravitational Force Abductor muscles force Joint reaction force(JRF) In a stable hip, τ BW= τ AB

5. Forces acting across hip joint Joint reaction force(JRF)- Force generated within a joint in response to external forces acting on it JRF = FBW+ FAB Management of painful hip disorders aim to reduce the JRF Determinants of JRF- Body weight Body weight moment arm Abductor force Abductor force moment arm 2W during SLR 3W in single leg stance/ walking 4W while sitting on chair (unaided) 5W in walking 7W in fast walking 10W while running

6. Static biomechanical concept Bilateral Stance Superincumbent body weight is transmitted through the SI joints and pelvis to both the femoral heads. Resultant vectors are vertical The gravitational moment arms for the right hip and the left hip are equal Weight is evenly distributed bilaterally

7. Bilateral Stance Centre of gravity Midline Slightly posterior to the hip joint Anterior to S2 Hip joints in extension

8. Single leg stance ( or right side in stance phase of gait) Effective body weight= HAT + Lt LL weight = 2/3 rd the BW + ½ of 1/3 rd BW = 5/6th TBW Centre of gravity shifts toward swing leg (Left side). Body weight acts eccentrically. Tilt the hip in adduction Rotational motion in hip, Balanced by abductors

9. Biomechanics of total hip arthroplasty In an arthritic hip, the ratio of the lever arm of the body weight to that of the abductors may be 4 : 1(NORMAL 2.5: 1) The lengths of the two lever arms can be surgically changed to make their 1 : 1 Theoretically, this reduces the total load on the hip by 30%

10. Biomechanics of total hip arthroplasty- permanent changes in the joint alteration to soft tissue and muscles Alteration to joint mechanism Stability and range of motion depends on : 1. Head size 2. Head-neck ratio and 3. Implant design- femoral stem, acetabular liner and cup

11. Cemented Femoral Stem Rod inside two tubes (cement and bone) Composite beam/ shape closed- stick up and stay Stem is rigidly bound to cement Initial primary stability, not intended to any micromotion any micromotion results in cement damage Anatomical, collared, rough

12. Cemented Femoral Stem Loaded taper/ force closed – taper slip Secondary stability by one year Controlled subsidence 1mm in one year, Smooth, straight, tapered, void + (centraliser), no collar

13. Cemented Femoral Stem

14. CPT stem Summit stem Spectron EF stem

15. Cementless stem Pre-requisites Immediate stability Intimate contact with bone

16. Cementless femoral components Khanuja and Mont

17. Short femoral stem Presere bone stock Less thigh pain Less stress shielding No proximal distal mismatch DAA Less than 120 mm length

18. Femoral Neck Standard offset stem (NSA-135 ) High offset stem (NSA- 127 ) Neck length is adjusted by using modular heads (variable internal bores) Neck length ranges from 25-50mm, adjustment of 8-12 mm for a given stem size is available Morse taper- 12/14 mm When a long neck is required for a head diameter up to 32mm, a skirted head may be used to fully engage the Morse taper

19. Femoral head

20. Head sizes- 22.225 , 28, 32, 36 Increasing the head diameter increases the jumping distance(i.e., the radius of the femoral head) Reduction of rates of revision for dislocation with increasing head size Primary arc of the joint Large diameter head – Less dislocation – more ROM Less impingement Femoral head

21. Acetabular liner Neutral liner- standard hemispherical Elevated liner -10, 15, or 20° lip Face changing liner Lateralised liner Dual mobility liner Constraint liner Size of the liner = inner diameter=size of femoral head Femoral Head size +liner thickness = acetabular sell size Thickness = 16, 18, 20

22. Cemented cup Flanges – compress cement, cup is pressed into position Grooves- increase surface area and bonding Pods- prevent bottoming (thin or discontinuous cement mantle) and ensures a uniform cement mantle

23. Cementless acetabular component Line to line fitting- elliptical cup Press fit- hemispherical cup 1 st generation- Harris- Galante I cup ,Zimmer 2 nd generation- Trilogy cup, zimmer 3 rd generation- pinnacle, continum cup

24. CHARNLEY concept To decrease joint reaction force 1) Shorten lever arm of the body weight by deepening the Acetabulum - preserving subchondral bone in the pelvis 2) Lengthen the lever arm of the abductor mechanism by reattaching the osteotomized greater trochanter laterally- most total hip procedures are now done without osteotomy of the greater trochanter The abductor lever arm is altered by the offset of the stem LFTA- using small size femoral head

25. FO (HO)= perpendicular distance from COR to long axis of femur

26. Biomechanical impact of a Varus and Valgus configuration of the proximal femur (Pauwels) Theoretical reaction force is up to 25% lower in coxa vara compared with the average hip, whereas in coxa valga , it is 25% higher. As the neck-shaft angle increases, the abductor lever arm decreases, thereby requiring a higher abductor force to balance the BW. Standard offset – 0.21mm/year ( poly wear -5 yearrs f/u) 7mm Lateral offset – 0.10mm/year ( due to decreased JRF) Short femoral necks- higher hip forces, people with a wide pelvis- higher hip forces. Women have larger hip forces than men More hip fractures & hip arthritis in female

27. Impact of femoral version on functional FO Inverse correlation (Terrier et al.)

28. Immediate rotational stability of the femoral component to withstand substantial torsional forces

29. Reducing the joint reaction force in total hip arthroplasty Lateralizing the femoral component by increasing the horizontal femoral offset Inadequate femoral offset- hip abductor insufficiency and soft tissue laxity, hip instability Excessive femoral offset- overtightening of the hip(Iliotibial band friction and trochanteric pain), increasing stress on femoral fixation interface (increased micromotion at implant bone interface), resulting in aseptic loosening.

30. Combined offset= femoral offset+ acetabular offset AO= Shortest distance from COR to medial wall of quadrilateral plate/ tear drop Restoration of CO to maintain tension of the abductor muscle complex.

31. Aim- anatomic COR (cup placement ) Moving the cup superiorly has been shown to increase joint loading by 0.1% for every millimeter of superior displacement of the hip centre of rotation, which is however seven times lower than the 0.7% increase/mm when lateralizing the center of rotation of the hip joint.

32. Pauwels also studied the importance of sufficient acetabular coverage and described the inversely proportional relationship between a decrease in weight bearing area and increase in joint pressure in dysplastic hips. cup anteversion = 40 − 0.7 × stem anteversion . anteversion = asin [short axis/long axis]× 180/π

33. neck shaft angle 127-135º Restoration of the neck ante version 10-15º Cup placed in 15-20º of ante version Combined anteversion 25-40º Cup 40- 45º of inclination

34. References Callaghan J, Rosenberg AG, Rubas HE, Eds. The Adult Hip. Philadelphia: Lippincott Williams & Wilkins Atwater AE. Gender differences in distance running In: Cavanaugh PR, Ed Biomechanics of Distance Running Illinois:Human Kinetics Books Champaign 1990 Campbell operative orthopaedics 14 th edition Kim JT, Yoo JJ. Implant Design in Cementless Hip Arthroplasty. Hip Pelvis. 2016 Jun;28(2):65-75. doi : 10.5371/hp.2016.28.2.65. Epub 2016 Jun 30. PMID: 27536647; PMCID: PMC4972888. Houcke JV, Khanduja V, Pattyn C, Audenaert E. The History of Biomechanics in Total Hip Arthroplasty.  Indian J Orthop . 2017;51(4):359-367. Bicanic G, Delimar D, Delimar M, Pecina M. Influence of the acetabular cup position on hip load during arthroplasty in hip dysplasia. Int Orthop 2009;33:397-402

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