Regional biomechanics of the knee joint

Practical
Dr. Ahmed Mohamed Mostafa
LECTURER OF PHYSICAL THERAPY FOR PEDIATRICS
FACULTY OF PHYSICAL THERAPY
MERIT UNIVERSITY.
 Muscle and joint interaction
Extensors of the knee:
• Quadriceps femoris muscle
• The large vastus group of muscles produces about 80% of the total extension torque at
the knee, and the rectus femoris produces about 20%
• Together, the quadriceps muscle, patella, and patellar tendon are referred to as the
knee extensor mechanism.
• The knee extensor muscles produce a torque about two thirds greater than that
produced by the knee flexor muscles
• Through isometric activation, the quadriceps stabilizes and protect the knee
• Through eccentric activation, the quadriceps controls the rate of descent of the body’s
center of mass, such as when sitting, squatting, or landing from a jump. Eccentric
activation of these muscles also provides shock absorption to the knee
• Concentric contraction of this muscle, accelerates the tibia or femur toward knee
extension. This action is often used to raise the body’s center of mass, such as during
running uphill, jumping, or standing from a seated position.
External Torque Demands Placed against the Quadriceps:
Contrasting “Tibial-on-Femoral” with “Femoral-on-Tibial” Knee Extension
• External torques applied to the knee by a constant load vary, based on knee angle
• During tibial-on-femoral knee extension, the external moment arm of the weight
of the lower leg increases from 90 to 0 degrees of knee flexion.
• During femoral-on-tibial knee extension (as in rising from a squat position), the
external moment arm of the upper body weight decreases from 90 to 0 degrees of
knee flexion
• External torques are relatively large from 90 to 45 degrees of flexion via femoral-
on-tibial extension, and from 45 to 0 degrees of flexion via tibial-on-femoral
extension. Reducing these external torques can be accomplished through several
strategies. An external load, for example, can be applied at the ankle during tibial-
on-femoral knee extension specifically between 90 and 45 degrees of flexion. This
activity can be followed by an exercise that involves rising from a partial squat
position, a motion that incorporates femoral-on-tibial extension between 45 and 0
degrees of flexion.
Internal Torque–Joint Angle Relationship of the Quadriceps Muscle
• Maximal knee extension (internal) torque occurs between 45 and 70 degrees of
knee flexion, with less torque produced at the near extremes of flexion and
extension
• the maximal-effort knee extensor torques produced between about 90 and 5
degrees of flexion.
• The internal moment arm (leverage) used by the quadriceps is greatest between
about 60 and 20 degrees of knee flexion.
• Knee extensor torques are produced isometrically by maximal effort, with the hip
held in extension.
• Maximal-effort knee extension torque remains at least 90% of the maximum
between 80 and 30 degrees of flexion. This high-torque potential of the
quadriceps within this range of motion is used during many functional activities
that incorporate femoral-on-tibial kinematics, such as ascending a high step,
rising from a chair, or holding a partial squat position while participating in sports
• 50% to 70% reduction in maximal internal torque as the knee approaches full
extension
Functional Role of the Patella.
• The patella functions as a “spacer” between the femur and
quadriceps muscle, which increases the internal moment arm of the
knee extensor mechanism, the presence of the patella augments the
extension torque at the knee.
• the knee extensor moment arm is greatest between about 20 and 60
degrees of knee flexion
• Factors that affect the length of the knee extension moment
arm across the sagittal plane range of motion. These include
1. The shape and position of the patella
2. The shape of the distal femur (including the depth and slope of the
trochlear groove)
3. The migrating medial-lateral axis of rotation at the knee (the
evolute)
 Patellofemoral joint kinetics
• The patellofemoral joint is routinely exposed to high magnitudes of compression
force.
• These forces include:
• 1.3 times body weight during walking on level surfaces.
• 2.6 times body weight during performance of a straight leg raise.
• 3.3 times body weight during climbing of stairs.
• Up to 7.8 times body weight during performance of squats.
Two Interrelated Factors Associated with Compression Force
on the Patellofemoral Joint
1. A force within the quadriceps muscle
2. Knee flexion angle
Compression Force on the Patellofemoral Joint
• These compression forces originate from active forces produced by the
quadriceps, their magnitude is influenced by the amount of knee flexion at the
time of muscle activation
• Forces within the extensor mechanism are transmitted proximally and distally
through the quadriceps tendon (QT) and patellar tendon (PT), like a cable
crossing a pulley. The resultant, or combined, effect of these forces is directed
toward the trochlear groove of the femur as a joint compression force (CF).
• Increasing knee flexion by descending into a deeper squat raises the force
demands throughout the extensor mechanism, and ultimately on the
patellofemoral joint. The increased knee flexion associated with the deeper squat
also reduces the angle formed by the intersection of force vectors QT and PT,
reducing the angle of these forces increases the magnitude of the CF directed
between the patella and the femur.
• In theory, if the QT and PT vectors were oriented in opposite directions, the
muscular-based compression force on the patellofemoral joint would be zero.
• Both the compression force and the area of articular contact on the
patellofemoral joint increase with knee flexion, reaching a maximum between 60
and 90 degrees
Factors affecting the tracking of the patella across the patellofemoral
joint
Role of the Quadriceps Muscle in Patellar Tracking
• As the knee is extending, the contracting quadriceps pulls the patella not only
superiorly within the trochlear groove of the femur, but also slightly laterally and
posteriorly. The slight but omnipresent lateral line of force exerted by the
quadriceps results, in part, from the larger cross-sectional area and force
potential of the vastus lateralis
Quadriceps angle, or more commonly the Q-angle:
• The Q-angle is determined first by constructing a line representing an estimation
of the resultant force vector of the different heads of the quadriceps. This line
connects a point between the anterior-superior iliac spine and the midpoint of
the patella. The second line is drawn representing the long axis of the patellar
tendon, made by connecting a point on the tibial tuberosity with the midpoint of
the patella. The Q-angle is formed at the intersection of these two lines, typically
measuring about 13 to 15 degrees (±4.5 degrees) within a healthy adult
population.
Factors That Naturally Oppose the Lateral Pull of the Quadriceps on the
Patella
• Optimal tracking is defined as movement between the patella and
femur across the greatest possible area of the articular surface with the
least possible stress.
• Local factors that act directly on the patellofemoral joint. the overall
line of force of the quadriceps is often estimated by the Q-angle.
Biomechanically, the overall lateral pull of the quadriceps produces a
lateral “bowstringing” force on the patella. A larger Q-angle creates a
larger lateral bowstringing force. A large lateral bowstringing force has
the tendency to pull the patella laterally over a region of reduced
contact area, increasing the stress on its articular surfaces and
potentially increasing the likelihood of dislocation
• Global factors, are related to the alignment of the bones and joints of
the lower limb.
the interaction of locally produced
forces acting on the patella as it moves
through the trochlear groove of the
femur. Each force has a tendency to pull
(or push in the case of the raised lateral
facet of the trochlear groove of the
femur) the patella generally laterally or
medially. Ideally, the opposing forces
counteract one another so that the
patella tracks optimally during flexion
and extension of the knee.
In theory, if the line of force of the
quadriceps is collinear with the patellar
tendon force, the lateral bowstringing
force would be zero
Global Factors
• The magnitude of the lateral bowstringing force applied to the patella is
strongly influenced by the frontal and horizontal plane alignment of the bones
associated with the knee extensor mechanism. As a general principle, factors
that resist excessive valgus or the extremes of axial rotation of the
tibiofemoral joint favor optimal tracking of the patellofemoral joint. These
factors are referred to as “global”

1- Excessive genu valgum

2- excessive external rotation of the knee

(A) Neutral alignment of the
knee, showing the characteristic
lateral bowstringing force acting
on the patella.

(B) Excessive knee valgus and

knee external rotation can
increase the Q-angle and
thereby increase the lateral
bowstringing force on the
patella.
Knee flexor-rotator muscles
• All muscles that cross posterior to the knee have the ability to flex and to
internally or externally rotate the knee. except gastrocnemius
• Many of the functions of the flexor-rotator muscles of the knee are expressed
during walking and running activities
• Functions are considered separately for tibial-on-femoral and femoral-on-tibial
movements of the knee.
• Control of Tibial-on-Femoral Osteokinematics An important action of the flexor-
rotator muscles is to accelerate or decelerate the lower leg during the swing
phase of walking or running.
• Control of Femoral-on-Tibial Osteokinematics. The muscular demand needed to
control femoral-on-tibial motions is generally larger and more complex than that
needed to control most ordinary tibial-on-femoral knee motions. A muscle such
as the sartorius, for example, may have to simultaneously control up to 5 degrees
of freedom (i.e., 2 at the knee and 3 at the hip).
Maximal torque production of the knee flexor-rotator muscles
The maximal-effort knee flexor
torques produced between 5 degrees
and about 90 degrees of flexion. The
internal moment arm (leverage) used
by the knee flexors (hamstrings) is
greatest between about 50 and 90
degrees of knee flexion. Knee flexor
torques are produced isometrically by
maximal effort, with the hip held in
extension