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.