bajekal hip bio mechanics
TRANSCRIPT
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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!