biomechanics of knee and implant design
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BIOMECHANICS OF KNEE AND IMPLANT DESIGN
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KINEMATICS OF KNEE
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VARYING TRANSVERSE AXIS OF KNEE
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EACH GAIT CYCLE
• Flexion and extension -70 * swing 20 * stance • Abduction and adduction 10 * • internal and external rotation - 10 to 15*• 67* flexion for swing phase• 83* flexion for climbing stairs• 90* flexion for descending stairs• 93* flexion for rise from chair
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NORMAL POST TRANSLATION OF KNEE IN FLEXION
• Medial condyle = 2mm• Lateral condyle = 21 mm• Medial based pivoting of the knee
• In flexion - tibia undergoes internal rotation• In extension - tibia undergoes ext rotation
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• MA is 3 degrees of valgus from vertical axis of body.• AA is in 6 * of valgus from MA
•9 degrees of valgus vertical axis of body.
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• The tibial articular surface is in 3* of varus.
• The distal femur is in 9* of valgus
• So in order to get the neutral mechanical axis in TKR we insert the femoral component in
5-7* of valgus.
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• Proximal tibial cut is perpendicular to the mechanical axis
• Posterior condylar axis is 3*• In order to create a rectangular flexor space
the femoral component should be in 3* ER
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PATELLO FEMORAL JOINT
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Q ANGLE
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JOINT REACTION FORCE
• It’s the force experienced by the trochlea due to posterior displacement of the patella during flexion
• JRF increases with flexion• Normal ADL it is 2-5 times• Squatting and 120* flexion 8 times
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EVOLUTION OF KNEE PROSTHESIS
• Interpositional • Mold arthroplasty• Hinged knee implants- they did not account
for the complex knee motion.• Bi-compartmental prosthesis-1. Gunston- polycentric knee
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ARGUMENTS IN FAVOUR PCL RETAINING
• > range of motion with effective femoral roll-back
• restraint to translational displacement • more symmetrical gait.• less bone resection• improved function of the patellofemoral joint• proprioceptive role of the PCL
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PCL substituting design the displacement must be resisted by the prosthetic articular
geometry↓
Inc stress on the prosthesis↓
Inc stress transfer to the bone cement interphase
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GAIT
• SYMMETRICAL GAIT more so in stair climbing• PCL substituting 1. decreased knee flexion 2. tendency to lean forward in a quadriceps-
sparing posture• Contradicting studies both in favor of and
against PCL retaining.
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• The patella to the joint line is less altered with PCL-R
• Improved patello-femoral joint function• Patellar clunk syndrome with PCL-S.
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ARGUMENTS IN FAVOUR PCL SUBSTITUTION
• PCL is diseased with arthritis and contracture• Technically surgery is less demanding• No problems related to a too loose or a too
tight PCL• Better deformity correction• No problems with excess femoral rollback• Less polyethylene wear.