The Knee and Patellofemoral Joints 259important principles. These principles are the law of
valgus and patellofemoral compression forces.
Law of Valgus
Because the femur normally runs slightly obliquely
inward, the previously described Q angle is formed
with its concomitant tendency to pull the patella lat-
erally as seen in figure 5.17. This predisposition for
lateral motion of the patella is referred to as the law
of valgus. This underlying tendency in the normal
knee can be further exaggerated in the abnormal
knee by many structural and functional factors,
including femoral anteversion, genu valgum, tibial
torsion, a laterally placed tibial tuberosity, a patella
that sits high (patella alta), an excessively mobile
patella (hypermobile), tightness of the lateral sta-
bilizers of the patella, and generalized quadriceps
weakness. Lateral tracking causes the patella to abut
against the lateral portion of the femoral groove.
The resultant excessive shear forces can damage
the cartilage lining the underside of the patella and
cause patellofemoral dysfunction and pain.
It is important to realize that although many of the
factors that tend to increase lateral tracking are struc-
tural and not readily changed, quadriceps strength
can be easily improved and the vastus medialis is
in a perfect position to counter this tendency for
lateral tracking of the patella. Hence, adequate and
balanced strength of the quadriceps is an essential
FIGURE 5.15 Function of patella to increase moment arm and torque of the quadriceps femoris (right knee). (A) Lat-
eral view of the patellofemoral joint, (B) increase in moment arm due to the presence of the patella, (C) decrease in
moment arm if the patella is absent.
FIGURE 5.16 The Q angle (right knee, anterior view).