biomechanics of the knee joint

Tibiofemoral joint

kinematics
Range of motion

·      almost all in sagittal plane

·      10o of extension to 140o of flexion

·      some in transverse plane

·      varies with flexion

·      minimal in extension because femoral and tibial condyles interlock

·      maximal at 90o flexion (10o external rotation and 15o internal rotation possible)

·      minimal in coronal plane

·      few degrees passive abduction and adduction in extension

·      maximal at 30o flexion (10o possible)

Instant centre

·      motion has changing axis

·      instant centres when plotted form J-shaped curve about femoral condyle

·      rotation = spin, rolling = turn, translation = slide, glide

·      fact that instant centres not on joint surface indicates translation (slide) occurs as well as rotation (spin)

·      rotation:translation = 2:1 in early flexion

·      ratio is 4:1 thereafter

Screw-home mechanism

·      femur internally rotates during last 15o extension

·      so that remaining articular surface on medial condyle in contact

·      posterior portion of lateral condyle rotates forward

·      rotation due to bony and ligamentous arrangements

1.   condyles have different configurations

·      lateral condyle is broader than medial

·      medial projects more anteriorly and distally than lateral

2.   ligaments become successively taut during extension

·      as knee moves into full extension, ACL becomes taut

·      this terminates extension of lateral femoral condyle

·      further extension of medial femoral condyle possible by passive rotation forwards of lateral condyle around axis of ACL

·      forces medial condyle to glide back into full extension

·      internal rotation of femur causes tightening of oblique popliteal ligament

·      MCL and LCL tightened simultaneously

·      effect called screw-home mechanism

·      locks knee in fully extended position

·      external rotation required for flexion from locked position

·      provided by contraction of popliteus

Menisci

Functions

1.   load bearing

·      femoral and tibial condyles grossly incongruent

·      menisci act as joint filler

·      contact area decreased by 40% when meniscus removed

2.   shock absorption

·      important in load transmission

·      40% of weight borne by menisci

3.   lubrication

·      distribute synovial fluid through joint

·      aids nutrition of articular cartilage

4.   stabilisation

·      important restraints to rotation and translation

·      assist in smooth transition from glide to roll

5.   proprioception

·      may have feed-back function for joint position sense

Movement

·      displaced slightly forward in full extension

·      move backward with flexion

·      medial meniscus more firmly anchored so moves less

·      menisci actively retracted by

·      popliteus laterally

·      semimembranosus medially

·      menisci move with femoral condyles in flexion-extension

·      move with tibia with rotation

Gait

Sagittal

·      knee never fully extended

·      maximum extension (5o flexion) at

·      beginning of stance phase (heel strike)

·      end of stance phase (just before toe off)

·      maximum flexion (70o) in middle of swing phase

Transverse

·      external rotation

·      during extension in stance phase

·      at end of swing phase just before heel strike

·      internal rotation

·      during flexion in swing phase

Coronal

·      abduction during extension at heel strike

·      adduction during flexion in swing phase

Speed

·      increased cadence produces

·      greater maximal flexion

·      more flexion in stance phase

Daily activities

·      range of 120o for normal function

·      70o for walking

·      80o for climbing stairs

·      90o for descending stairs

·      120o for lifting

Kinetics
Stability

·      ligaments and muscles are primary stabilisers of joint

MCL

·      primary restraint to valgus

·      primary restraint to internal tibial rotation

·      secondary restraint to anterior tibial translation

LCL

·      primary restraint to varus

·      secondary restraint to anterior tibial translation

ACL

·      primary restraint to anterior tibial translation

·      provides 80%

·      greatest effect at 30o flexion

·      secondary restraint to tibial rotation

·      esp. toward full extension

PCL

·      primary restraint to posterior tibial translation

·      greatest effect at 90o flexion

·      secondary restraint to external tibial rotation

·      greatest effect at 90o flexion

Joint forces

Gait

·      peak just after heel strike (3W)

·      associated with hamstring contraction

·      decelerating effect

·      about 2W during knee flexion in stance phase

·      associated with quadriceps contraction

·      prevents knee buckling

·      peak again just before toe-off (4W)

·      associated with gastrocnemius contraction

·      low during early swing phase

·      increases to 1W in late swing phase

·      associated with hamstring contraction

·      shift from medial to lateral plateau

·      medial in stance phase

·      lateral in swing phase

Daily activities

·      4 W in climbing stairs

Menisci

·      function to

·      distribute load over whole joint surface

·      absorb load (take 1/3 to 1/2 of load themselves)

·      when removed, joint reaction force increases 3 x

·      increased force over smaller area

Axes

Mechanical axes

·      mechanical axis of femur passes from centre of femoral head to point midway between femoral condyles

·      mechanical axis of tibia passes from point midway between tibial condyles to centre of talus

·      angle between these axes is 1-2o varus

·      mechanical axis of lower limb is straight line from femoral head to talus

·      passes just medial to centre of knee joint

Anatomical axes

·      anatomical axis of femur passes along shaft of femur

·      anatomical axis of tibia passes along shaft of tibia

·      angle between these axes is 5-7o valgus

Alteration

·      alteration in axes can be caused by

·      alteration in geometric alignment of femur and tibia

·      loss of bone and cartilage in one compartment

·      laxity of ligamentous and other soft tissue structures

·      each mm of joint space change causes 1o angular deformity

Patellofemoral joint

kinetics

·      translates (slide)

·      moves 7 cm with full flexion

·      instant centre is near posterior femoral cortex above condyles

·      both medial and lateral condyles of femur articulate from 0o to 90o

·      from 90o to full flexion, only medial condyle articulates

kinematics
Functions

Patella

·      aids in knee extension

·      increases lever arm of quadriceps tendon

·      maximal effect at 45o flexion (30% increase)

·      30% more quadriceps force required without patella

·      decreases load on femur

·      increases stress distribution

·      protective

·      distal femur from trauma

·      quadriceps from attritional wear

Joint reaction force

·      flexion influences magnitude of force

·      changes angle between quadriceps and patellar tendon

·      reaction force is resultant of both forces added

·      increased angle increases force

·      force is

·      0.5 W during level walking

·      2.5 W with 90o knee flexion

·      7 W during stair climbing with 60o knee flexion

·      7W during running