bajekal hip bio mechanics

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Biomechanics of the hip

Rajiv BajekalConsultant Orthopaedic SurgeonBarnet General Hospital

Barts and the London Basic Science

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Terms

• Statics

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

resulting action – Kinesiology- study of human motion

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Kinematics

• Flexion- 120 degrees• Extension 30

• Abduction 50• Adduction 30• External rotation 45

• Internal rotation 45

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Kinetics

• Joint reaction force- 3 times in single legstance

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

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Free body analysis and diagrams

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• Lever systems

• Class I• Class II

• Class III

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Normal hip

• Joint reaction force (JRF) calculation

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

JRF is always higher thanthe body weight

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Normal hip

• Joint reaction force (JRF) calculation

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

This equation plays aparamount role indetermining the modality of

management in hipdisorders.

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Normal hip

• Determinants of JRF (Key elements inhip biomechanics):

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

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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.

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Hip disorders

• Pathomechanical factors:1. Increase body weight

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

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Quick reminder

• Determinants of JRF (Key elements inhip biomechanics):

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

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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

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Hip disorders

• Reducing Body Weight orits moment arm

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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 .

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Hip disorders

• Help Abductor Force or its moment arm.

1. Provide additional

moment

• Walking stick inopposite hand(practical)

Mr. Denham’s original drawing

from his 1959 paper

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Hip disorders

• Help Abductor Force or its moment arm.

1. Provide additional

moment

• Walking stick inopposite hand(practical)

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Hip disorders

• Help Abductor Force or its moment arm.

1. Provide additional

moment

• Walking stick inopposite hand(practical)

• Suitcase in ipsilateralhand (theoretical )

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Hip disorders

• Help Abductor Force or its moment arm.

2. Increase abductor lever

arm

• Increase offset• Osteotomy• Greater trochanter lateral

transfer• Varus placement of THR

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Hip disorders

• Help Abductor Force or its moment arm.

3. Improve abductor line of

action

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Hip disorders

• Help Abductor Force or its moment arm.

Osteotomies aims

1. Increase weight bearingarea

2. Improve congruency

3. Improve lever arm

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Hip disorders

• Help Abductor Force or its moment arm.

Aims of osteotomy

1. Increase weight bearingarea

2. Improve abductor lever

arm3. Improve congruency

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Hip disorders

• Help Abductor Force or its moment arm.

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)

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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)

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Hip Replacement

• Design philosophies• How do they work ?

• Why do they fail ?

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Hip Replacement

• Charnley & HarrisPhilosophy.

– Proximal collar toprevent sinking

– Rough surface topromote bonding

– ‘composite beam’

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Hip Replacement

• Ling & Lee philosophy

– No proximal collar,polished and tapered

– Sinkage convertsshear stress intoradial compression

– ‘polished taper’design

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Stem design and terms

Offset

Neck length

Head diameter

Head neck ratio

Stem length

Morse taper

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Hip Replacement

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

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Hip Replacement

• Aseptic losening

1. Femoral stem (Modes of failure)

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Hip Replacement

• Aseptic losening

1. Femoral stem:

Mode IA: Pistoning of stemin cement mantle

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Hip Replacement

• Aseptic losening

1. Femoral stem:

Mode IB: Pistoning of stemand cement mantle incanal.

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Hip Replacement

• Aseptic losening

1. Femoral stem:

Mode II: Medial stem pivot.

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Hip Replacement

• Aseptic losening

1. Femoral stem:

Mode III: Calcar pivot(Windscreenwipe effect).

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Hip Replacement

• Aseptic losening

1. Femoral stem:

Mode IV: Cantilevarbending

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Hip Replacement

• Instability: Four major factors

1. Patient2. Surgeon3. Component design4. Component position

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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.

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Head and neck size

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

dislocation

• Excursion distance-distance head travelsbefore dislocation

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Soft tissue tensioning

• Abductor complex• Offset

• Neck length• Short neck length=trochantericimpingement=dislocation

• Reduced offset and neck length is theworst case scenario

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Conclusions

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

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