knee biomechanics andrew crosby. introduction what kind of joint is it? limits of motion normal...
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KNEE BIOMECHANICS
Andrew Crosby
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Introduction
• What kind of joint is it?• Limits of motion• Normal kinenatics of a step• Plateau & condyles• Patello Femoral articulation• Menisci• Medial, lateral and anterior stability• ACL & PCL
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Knee joint
• Ginglymus (hinge) ?
• Arthodial (gliding) ?
• 6 degrees of freedom– 3 rotations– 3 translations
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• Rotations– flex/ext - -15 to 140 deg– varus valgus - 6-8 deg in extension– int/ext rotation - 25 - 30 deg in flexion
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• Translations– AP 5 - 10mm– comp/dist 2 - 5mm– medio-lateral 1-2mm
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Taking a step
• Just prior to heel strike - max extension & max external rotation
• heel strike - max valgus
• flat foot - flexion & intrenal rotation progress
• swing phase - internal rotation continues, max flexion, max anterior translation.
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Condyles and plateau
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Patellofemoral articulation
• Shape/anatomy of patella
• Anatomy of intercondylar groove
• direction of force
• PFJR vs flexion angle and quads force
• Contact area vs PFJR and stress
• chondromalacia of patella
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• Patella functions – Increases moment arm (increases rotational
torque) 0 - 45 deg– lever at > 45 deg
• Patellectomy?
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Menisci
• Fairbank - 1948
• late 60’s - poor results of miniscectomy
• mid 70’s - load transmission confirmed– 40-60% load is on meniscus– lateral > medial
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• 2 functions– load bearing– stability
• also, joint lubrication
• prevent capsule, synovial impingement
• shock absorbers
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Load bearing -composition;
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Hoop stress
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• Knee joint stability– mainly rotational– miniscectomy +/- ACL and translation– why differences in lat vs med ?
– Structure
– attachments
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Attachment sites
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Medial & lateral stabilizers(mostly ligaments)
• Ligaments– most important static stabilizers– tensile strength - related to composition
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Medial side
• Superficial MCL– Primary valgas restraint -57-78% restraining moment of
knee– femoral attachment fans out around axis of rot.– Lax in flexion
• Semimembranosis (expansion)– internally rot’s tib on femur– tenses post/med capsular structures that are lax in knee
flexion– acts with ACL
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Lateral side
• LCL – Primary varus restraint– lax in flexion– Bicepts passes it and blends with insertion
• maintains tension?
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• Bicepts– flexor(with semimembranosis and pes)– externally rotates tibia– tenses LCL– dynamic assistor of PCL
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Cruciates
• ACL– Primary static restraint to anterior displacement– tense in extension, ‘lax’ in flexion
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• PCL– Primary restraint to post. Displacement - 90%– relaxed in extension, tense in flexion– reinforced by Humphreys or Wrisberg– restraint to varus/valgus force– resists rotation, esp.int rot of tibia on femur
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