The Knee and Patellofemoral Joints 255influenced by ligaments. As previously described,
when the knee is extended, key ligaments are relatively
taut, and little or no rotation of the tibia relative to the
femur is allowed. However, when the knee is bent, the
collateral ligaments are more slack, and 20° to 30°
of internal and 30° to 45° of external rotation of the
tibia are possible (Magee, 1997; Rasch, 1989).
The increased rotation permitted as the knee
bends enhances movement possibilities. For exam-
ple, this rotation is critical for permitting the body to
turn when the foot is in contact with the ground from
a position of kneeling, squatting, sitting, or stand-
ing. It would be very difficult and cumbersome to
change direction if this rotation at the knee were not
available, and the rotation is vital for quick changes
in direction such as a pivot when running or danc-
ing. In non-weight-bearing conditions, this rotation
allows the foot to turn, as when one climbs a pole,
performs inside ankle kicks in soccer, or “presents
the heel” in ballet.
The Locking Mechanism of the Knee
The linking of rotation with the final 20° of knee
extension has particular importance for posture and
knee stability and is termed the “locking mechanism”
of the knee (Hamill and Knutzen, 1995). This auto-
matic mechanism is thought to relate to the restraints
offered by the cruciate ligament and the shape of the
surfaces articulating at the knee joint. In terms of the
latter, the medial femoral condyle projects extensively
both longitudinally and medially. This downward pro-
jection of the medial condyle is necessary to compen-
sate for the lateral-to-medial obliquity of the femoral
shaft as it progresses distally, allowing the knee joint
to be more parallel to the floor than if the condyles
were the same size and shape. Due to this difference
in size, when the knee extends in a standing position
(closed kinematic chain), the excursion of the lateral
condyle is completed (close packed) while that of
the medial remains uncompleted. In essence, all of
the articulating surface has been used on the lateral
side, while leaving about a half inch (1.3 centimeters)
on the medial side. To use the remaining articular
surface on the medial side and reach full extension
(close-packed position), the medial femoral condyle
continues to roll and slide, producing internal rota-
tion of the femur relative to the tibia as shown in figure
5.13B. The extent of this rotation is small, approxi-
mately 5° to 7° (Soderberg, 1986), though significant.
This final rotation that creates a congruous position
of both condyles of the femur relative to the menisci
and underlying tibia is termed the locking mechanism
or “screw-home” movement of the knee.
This locking mechanism of the knee is an
energy-efficient mechanism that allows individuals
to maintain the knee in extension over prolonged
periods of standing without requiring muscular
contraction, as the knee cannot flex without the
knee’s first being “unlocked.” To bend the knee,
the reverse process occurs in which the popliteus
muscle works to externally rotate the femur relative
to the tibia, thereby unlocking the knee and allowing
flexion to occur.FIGURE 5.12 Motions of the femoral condyles on a fixed tibia (right knee, medial view). (A) Posterior rolling accompany-
ing knee flexion, (B) anterior sliding to offset backward motion of femur during flexion, (C) anterior rolling with posterior
sliding to offset forward motion of femur during knee extension.