The Knee and Patellofemoral Joints 257Movements of the Patella
During flexion and extension of the knee joint
proper, the patella undergoes a complex gliding
movement that includes movements up and down
with very slight medial, lateral, and rotational com-
ponents. This leads some authors to consider this a
synovial, modified plane joint (Magee, 1997), while
others do not consider it a true joint. The kneecap
remains at a relatively constant distance from the
tibia because of the patellar tendon. Thus, the
change of position of the patella occurs in relation
to the femur; the patella slides down during flexion
and up during extension within the femoral groove.
An excursion of the patella on the femoral condyles
of approximately 2 3/4 inches (7 centimeters) occurs
from extension to full flexion (Frankel and Nordin,
1980). As this excursion proceeds, there is also a
continuous transition in contact surface size and
location that has important implications for potential
cartilage injury.
Connective Tissue Constraints
In addition to being anchored to the tibia by the
patellar tendon below, the patella is further stabilized
by various other connective tissue structures as seen
in figure 5.14A. For example, lateral extensions from
the quadriceps tendons (termed patellar ligaments)
and fibrous expansions of the vasti and iliotibial band
(medial and lateral patellar retinaculum) also pro-
vide lateral and medial stability for the patella and
are key in preventing the patella from coming out of
its groove (patellar subluxations or dislocations).Muscles of the Patellofemoral Joint
Since the quadriceps femoris directly attaches to the
patella, adequate and balanced strength of its com-
ponents is essential for stability and proper move-
ment of the patella. Other muscles, including the
pes anserinus group and hamstrings, can affect the
patella less directly. The interplay of these musclesThe Locking Mechanism of the Knee
Perform the following observations and refer
to figure 5.13 to clarify the locking mecha-
nism of the knee.- Demonstrate the rotation of the tibia
accompanying knee extension in an open
kinematic chain movement. While sitting at
the edge of a table with the knees bent to
90°, use a pen to mark an “x” on the midpoint
of your patella and another “x” on your tibial
tuberosity on your left side. Slowly extend
your left knee. Note the movement of the
tibial tuberosity relative to the mark over your
patella. Redraw the tibial mark with the knee
in full extension, and note that it has moved
laterally, demonstrating the slight external
rotation of the tibia relative to the femur that
occurs in the final phases of knee extension
with the foot free as shown in the figures. - Demonstrate the rotation of the femur
accompanying knee extension in a closed kinematic chain movement. Stand in first-position parallel
and bend the knees about 30°. From this position of slight flexion, slowly straighten your knees, and
again note the relative change of your tibial tubercle and mid-patella. The mid-patella mark should
now be located more medially, demonstrating the slight internal rotation of the femur relative to the
tibia that occurs in the terminal phases of knee extension when the foot is fixed.