bajekal hip bio mechanics

8/8/2019 Bajekal Hip Bio Mechanics

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Biomechanics of the hip

Rajiv BajekalConsultant Orthopaedic SurgeonBarnet General Hospital

Barts and the London Basic Science



Terms

• Statics

• Dynamics – Kinematics- study of motion – Kinetics- action of forces on bodies to their

resulting action – Kinesiology- study of human motion



Kinematics

• Flexion- 120 degrees• Extension 30

• Abduction 50• Adduction 30• External rotation 45

• Internal rotation 45



Kinetics

• Joint reaction force- 3 times in single legstance

• 5 times in walking• Twice during SLR• Upto 10 times while running



Free body analysis and diagrams



• Lever systems

• Class I• Class II

• Class III



Normal hip

• Joint reaction force (JRF) calculation

JRF ( ) = Body Weight ( ) + Abductor Force ( )

JRF is always higher thanthe body weight



Normal hip

• Joint reaction force (JRF) calculation

JRF ( ) = Body Weight ( ) + Abductor Force ( )

This equation plays aparamount role indetermining the modality of

management in hipdisorders.



Normal hip

• Determinants of JRF (Key elements inhip biomechanics):

•Body weight•Body weight moment arm•Abductor force (muscles)•Abductor force moment arm



Normal hip

• Biomechanical study and analysis ofnormal hips are based on these basic

principles.

• They form the basis for the managementof hip disorders and hip arthroplasty.



Hip disorders

• Pathomechanical factors:1. Increase body weight

2. Increase body weight moment arm3. Decrease abductors force.4. Decrease abductors moment arm.



Quick reminder

• Determinants of JRF (Key elements inhip biomechanics):

•Body weight•Body weight moment arm•Abductor force (muscles)•Abductor force moment arm



Hip disorders

• Management of painful hipdisorders- the aim is to reduce

joint reaction forces.JRF = Body Weight + Abductor Force

Strategies to reduce JRF are achieved via:

Reducing Body Weight or its moment arm

Help Abductor Force or its moment arm



Hip disorders

• Reducing Body Weight orits moment arm



Hip disorders

• Help Abductor Force or its moment arm.

1. Provide additional

moment

• Walking stick inopposite hand(practical)

The stick exerts upward forceand thus helping theabductors by reducingbody weight .



Hip disorders



moment


Mr. Denham’s original drawing

from his 1959 paper



Hip disorders



moment




Hip disorders



moment


• Suitcase in ipsilateralhand (theoretical )



Hip disorders


2. Increase abductor lever

arm

• Increase offset• Osteotomy• Greater trochanter lateral

transfer• Varus placement of THR



Hip disorders


3. Improve abductor line of

action



Hip disorders


Osteotomies aims

1. Increase weight bearingarea

2. Improve congruency

3. Improve lever arm



Hip disorders


Aims of osteotomy

1. Increase weight bearingarea

2. Improve abductor lever

arm3. Improve congruency



Hip disorders


Aims of osteotomy

Results best for 5 years but declineafter 10 years (Weisl 1980

JBJS)

Less predictable results than withTHR (Reigstad et al 1984 JBJS)



Hip Replacement

• Aims1. Relieve pain (Via excising the painful joint)2. Improve function (increase offset and the

lever arm will improve line of abductorsaction and tighten them as well ascorrecting limb length discrepancy)



Hip Replacement

• Design philosophies• How do they work ?

• Why do they fail ?



Hip Replacement

• Charnley & HarrisPhilosophy.

– Proximal collar toprevent sinking

– Rough surface topromote bonding

– ‘composite beam’



Hip Replacement

• Ling & Lee philosophy

– No proximal collar,polished and tapered

– Sinkage convertsshear stress intoradial compression

– ‘polished taper’design



Stem design and terms

Offset

Neck length

Head diameter

Head neck ratio

Stem length

Morse taper



Hip Replacement

• How do they fail ? – Infection – Instability – Aseptic losening – Implant failure – Periprosthetic fracture



Hip Replacement

• Aseptic losening

1. Femoral stem (Modes of failure)



Hip Replacement


1. Femoral stem:

Mode IA: Pistoning of stemin cement mantle



Hip Replacement


1. Femoral stem:

Mode IB: Pistoning of stemand cement mantle incanal.



Hip Replacement


1. Femoral stem:

Mode II: Medial stem pivot.



Hip Replacement


1. Femoral stem:

Mode III: Calcar pivot(Windscreenwipe effect).



Hip Replacement


1. Femoral stem:

Mode IV: Cantilevarbending



Hip Replacement

• Instability: Four major factors

1. Patient2. Surgeon3. Component design4. Component position



Hip Replacement

• Component design

– Offset and neck length determine leverarm and soft tissue tension

– Head neck ratio: Larger heads yield agreater arc of movement but largerfrictional forces, disadvantage if usingpolyethylene.



Head and neck size

• Primary arc range- depends on head neckratio – Range of movement before impingement and

dislocation

• Excursion distance-distance head travelsbefore dislocation



Soft tissue tensioning

• Abductor complex• Offset

• Neck length• Short neck length=trochantericimpingement=dislocation

• Reduced offset and neck length is theworst case scenario



Conclusions

• Overview of biomechanics• Focus on basics ‘for life’

• Learn essentials for examination• ‘think about THR’• Pass the examination!

bajekal hip bio mechanics

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