knee biomechanics
TRANSCRIPT
KNEE BIOMECHANICS
Patellofemoral joint
Articulation: Posterior surface of the patella Femoral sulcus or patellar surface of femur
Patellar surface
Triangular with apex downwards
divided by a vertical ridge into medial and lateral facets.
a second vertical ridge toward the medial border separates the medial facet from an extreme medial edge, known as the odd facet of the patella
Femoral surface
The femoral sulcus has a groove that corresponds to the ridge on the posterior patella
it divides the sulcus into medial and lateral facets.
The lateral facet of the femoral sulcus is slightly moreconvex than the medial facet and has a more highly
developed lip than does the medial surface
Patellofemoral contact area
Changes with knee position Extension:only the inferior Pole is in
contact with the femur. along the inferior margin of both the medial and lateral facets of the
patella at 10 to 20 of knee flexion. At 45 degree covers middle of
patella and spreads outward to cover the medial and lateral facet.
At 90 superior pole beyond 90 the area of contact
begins to migrate inferiorly. smaller odd facet makes contact with the medial femoral condyle for the first time.
At full flexion, the patella is lodged in the intercondylar groove, and contact is on the lateral and odd facets, with the medial facet completely out of contact.
MUSCLES
Muscles crossing knee are basically flexors or extensors, and have additional function of rotation/ abdn addn
Knee flexor
7 muscles flex the knee:3Hamstrings:the semimembranosus, semitendinosus, biceps
femoris(long and short heads) sartoriusgracilis popliteusgastrocnemius musclesPlantaris also flexes the knee when
present
Other than short head of biceps femoris and popliteus all other are two joint muscles, thus their flexing ability depends on angle of the other joint where it is attached.
Other functions of knee flexors
Rotation:the popliteus, gracilis,
sartorius, semimembranosus, and semitendinosus are also medial rotators
Biceps femoris is a lateral rotator
Varus & valgus moment: The lateral muscles (biceps femoris, lateral head of the
gastrocnemius, and the popliteus) are capable of producing
valgus moments Muscles on the medial side of the joint (semimembranosus, semitendinosus,
medial head of the gastrocnemius, sartorius, and
gracilis) can generate varus moments.
Action on menisci and locking
Popliteus is known as the unlocking muscle although without it also locking will take place.
active assistance of the semimembranosus and popliteus muscles ensures that tibio femoral congruence is maximized throughout the range of knee flexion by acting on menisci
Knee extensor
RF, VI pull upwards. pull of vastus
lateralis muscle is 35 degree laterally, whereas the pull of the vastus medialis muscle is 40degree medially.
The combined action produces a force directly upwards.
Role of patella
the patella lengthens the MA of the
quadriceps by increasing the
distance of the patellar tendon from the axis of the knee joint. The patella, as an
anatomic pulley, deflects the action line of the quadriceps
femoris muscle away from the joint center, increasing
the angle of pull
at semi flexion, the patella is primarily responsible for increasing the quadriceps angle of pull.
In full knee flexion, the patella is fixed firmly inside the intercondylar notch of the femur, which reduces the pulley action of patella. Still the quadriceps maintains a fairly large MA because the rounded contour of the femoral condyles deflects the muscle’s action line and because the axis of rotation has shifted posteriorly into the femoral condyle.
in the final stages of knee extension, patella’s effect on the quadriceps’ MA is diminished but the small improvement in joint torque provided by the patella is important.
because near end range extension, the quadriceps is in a shortened position, which reduces its ability to generate active tension.
Quadriceps lag
If there is substantial quadriceps weakness or if the
patella has been removed because of trauma (a procedure
known as a patellectomy), the quadriceps may not be able to produce adequate torque to
complete the last 15degree of non–weight-bearing knee
extension. This is called as “quadriceps lag” or “extensor
lag.”
Q angle The Q-angle (or "quadriceps
angle) is formed in the frontal plane by two line segments: from tibial tubercle to the middle of the patella
from the middle of the patella to the ASIS
The q-angle in adults is typically 15 degrees. Increases or decreases in the q-angles are associated in cadaver models with increased peak patellofemoral contact pressures (Huberti & Hayes, 1984). Insall, Falvo, & Wise (1976) implicated increased q-angle, along with patella alta, in a prospective study of patellofemoral pain.
VMO
Oblique fibers of vastus medialis.
Credited to locking of knee joint and terminal knee extension.
Soleus and gmax can produce knee extension although they do not have any attachment in knee.