BIOMECHANICS OF KNEE AND IMPLANT DESIGN

KINEMATICS OF KNEE

VARYING TRANSVERSE AXIS OF KNEE

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

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

• 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.

• 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.

• 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

PATELLO FEMORAL JOINT

Q ANGLE

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

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

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

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

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.

• The patella to the joint line is less altered with PCL-R

• Improved patello-femoral joint function• Patellar clunk syndrome with PCL-S.

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.