Biomechanics of the Patellofemoral Joint
ABSTRACT
At last! ….. An easy to understand insight into the applied biomechanics of the
patella. Along with discussions of various exercise and treatment modalities
make this online course immediately applicable in your clinical practice.
FUNCTIONS OF THE PATELLA
cartilage of the trochlea as well as the condyles
Some authors believe that the patella is not very
in acting as a bony shield.
important in extensor mechanism mechanics (1)
recommended
Most authors seem to have taken for granted an
patellectomy. Others, on the contrary, attribute
essential function of the patella, the loss of which
to
role2,3
results in patients seeking treatment. One of the
whenever
capital characteristics of hyaline cartilage is its
and
therefore
the
patella
recommending
readily
a
more
its
prominent
preservation
absence of a nerve supply. Healthy cartilage
possible.
allows the transmission of forces to subchondral
Perhaps the patella’s most important function is
and cancellous bone in such a way that the pain
facilitating extension of the knee by increasing
threshold of the richly innervated bone is not
the distance of the extensor apparatus from the
surpassed. It is also well known that tendons are
axis of flexion and extension of the knee.
capable of withstanding great tensile loads but
Throughout the entire range of motion, the
not high friction or compression. The presence of
patella increases the force of extension by as
the patella in the extensor apparatus protects the
much as 50% . Hyaline cartilage with its very low
tendon from friction and permits the extensor
compressive stiffness and coefficient of friction 5
apparatus to tolerate high compressive loads.
4
is indispensable for transmitting the quadriceps
Finally, the patella plays a role in the aesthetic
force around the distal femur to the tibia.
appearance of the knee. This can be appreciated
The patella acts as a guide for the quadriceps
in
tendon in centralizing the divergent input from
flattened ends of the condyles are easily visible
the four muscles of the quadriceps, transmitting
with the knee flexed. Of all these functions, the
these
This
most important role of the patella is in extension
decreases the possibility of dislocation of the
of the knee. Patellectomy results in weakened
extensor apparatus and controls the capsular
extension of the knee or even incomplete knee
tension of the knee. The patella also protects the
extension.
forces
to
the
patellar
tendon.
1
the
patellectomized
Some
knee
muscle
in
atrophy
which
the
inevitably
Biomechanics of the Patellofemoral Joint
follows patellectomy in spite of sustained and
the joint move in relation to one another during
intensive physical therapy.
function.
APPLIED BIOMECHANICS
PATELLOFEMORAL
JOINT
BIOMECHANICS
DURING OPEN AND CLOSED CHAIN EXERCISES
Despite the high incidence of patellofemoral pain
in the general population, the pathophysiology of
Since Patellofemoral pain is typically reproduced
this disorder is not clearly understood. The most
with activities that are associated with high
commonly accepted hypothesis is related to
patellofemoral joint reaction forces (i.e. stair
abnormal
increases
climbing and squatting)9,10 it would appear that
subsequent
an exercise program should be designed to
patellar
Patellofemoral
tracking
joint
stress
which
and
enhance quadriceps strengthening while keeping
articular cartilage wear6,7
joint stress to a minimum. This is particularly
Although articular cartilage is aneurzl and has
important
been
patellofemoral
dismissed
as
a
possible
source
of
during
the
acute
phase
pain when symptoms can
of
be
symptoms, it has been proposed that the sub-
readily exacerbated. To accomplish this task,
adjacent
knowledge
endplate
is
exposed
to
pressure
of
the
biomechanics
of
the
patellofemoral joint is necessary.
variations that would normally be absorbed by
healthy cartilage8. This mechanical stress is
believed to stimulate pain receptors in the
PATELLOFEMORAL JOINT REACTION FORCE
subchondral bone8.
The patellofemoral joint reaction force is the
measurement
Ultimately
health
patellofemoral
biomechanical
and
must
terms.
be
In
disease
of
understood
carrying
out
of
compression
of the patella
against the femur and is dependent upon the
the
in
angle
its
tension
of
knee
flexion
as
well
as
muscle
. The resultant of the quadriceps
11,12,13
functions, the patella must accommodate the
force vector and patellar tendon force vector is
forces that normal activity brings to bear on this
equal and opposite to the patellofemoral joint
joint. The capacity to accommodate these forces
reaction
may be reduced by abnormalities of anatomy or
stresses on the patellofemoral articular cartilage
disease, or overcome by injury or overloading.
(Figure 1).
force7,
which
evokes
compressive
Alterations of the joint due to aging may also
have an adverse effect. Understanding the failure
Hungerford and Barry20 and Reilly15 calculated
to meet the mechanical demands of daily activity
the patellofemoral reaction force to be 0.5 times
requires
is
body weight during level walking; 3–4 times
transmitted across the patellofemoral joint, how
body weight during stairs climbing; and 7–8 times
that joint is stabilized, and how the two parts of
body weight in the squats position.
an
understanding
of
how
load
2
Biomechanics of the Patellofemoral Joint
Resistance against a 9 kg boot equates to 6–7
times body weight.
Figure
1.
Compressive
forces
on
the
patellofemoral joint are determined by the
resultant of M1, (quadriceps tendon force vector)
and M2 (patellar ligament force vector), (from
Fulkerson JP. Hungerford DS: Disorders of the
patellofemoral Joint (2nd Ed), Balitmore: Williams &
Wilkins, 1990, O John, P. Fulkerson, MD, reprinted
with permission).
Figure 2. Contract areas on the patella as a
function of knee flexion at 20, 45, 90 and 135°.
(A-B=Path of medial margin of contact zone, I =
Inferior, S = Superior, L = Lateral, M = Medial).
(From Goodfellow J, Hugnerford DS, Zindel M:
Patellofemoral joint mechanics and pathology. J Bone
Joint Surg 58B(3):287-290, 1976, reprinted with
permission)
PATELLOFEMORAL CONTACT AREAS
The fact that the quadriceps force and the
Another important variable to consider regarding
contact area changes with varying knee flexion
the biomechanics of the patellofemoral joint is
angles has significant implications for prescribing
the area of contact between the patella and
different types of therapeutic exercise.
femur. As reported by Goodfellow et al18 the
patellofemoral contact area increases with knee
During
flexion,
being
extension with resistance applied at the ankle),
approximately twice that at 90° (Figure 2). When
the amount of quadriceps force required to
considering the forces transmitted through the
extend the knee steadily increases as the knee
patellofemoral joint, an assumption is made that
moves from 90° to full knee extension. A study
the joint reaction force is equally distributed
performed by Lieb and Perry19 demonstrated that
across the entire contact area.
a
with
the
total
area
at
90°
60%
an
open
increase
chain
in
exercise
quadriceps
(i.e.
force
knee
was
necessary to complete the last 15° of knee
3
Biomechanics of the Patellofemoral Joint
extension. This increase in force was attributed
also
estimated
patellofemoral
contact
stress
to a decrease in mechanical advantage of the
during both open and closed chain exercise.
extensor mechanism. In addition to the increase
in quadriceps force as the knee extends, the
patellofemoral contact area steadily decreases.
This combination of increased quadriceps force
and decreased contact area during terminal knee
extension, results in greater knee flexion angles
where the quadriceps force is not as great and
the contact area is larger20.
Conversely, during closed chain exercise (i.e.
squatting), the quadriceps force is relatively
minimal as the knee is extended and steadily
increases as the knee flexes20. This increase in
force is distributed over a greater surface are as
the contact area also is increasing as the knee
flexes.
The
greater
contact
area
Figure 3. Comparison of patellofemoral joint
stress in millipascals (MPa) during open and
closed chain exercise at four knee flexion angles.
Error bars represent one standard deviation.
(From Steinkamp LA, Dilingham MF, Markel MD, Hill
JA, Kaufman KR: Biomechanical considerations in
Patellofemoral joint rehabilitation Am) Sports Med
21(3):438-444, 1993, reprinted with permission).
prevents
excessive patellofemoral joint pressure during
flexed knee activities20.
An analysis of patellofemoral joint stress during
open and closed chain exercises was undertaken
These
by Steinkamp et al21. The results of this study
studies
strengthening
found that compared with knee extension against
suggest
can
be
that
quadriceps
safely
performed
throughout 0-90° knee flexion range by varying
resistance, the patellofemoral joint stress while
the mode of exercise. Since both forms of
performing a leg press maneuver was less at
exercise can be used to promote quadriceps
knee flexion angles from 0 to 48°. Beyond 48° of
hypertrophy, it appears that a comprehensive
knee flexion, however, the patellofemoral joint
strengthening program would incorporate both
stress for the closed chain exercise was greater
open
than the open chain exercise (Figure 3).
and
closed
chain
exercise
so
that
strengthening can be performed throughout a
large arc of motion.
The results of Steinkamp et al 21 are similar to
that reported by Hungerford and Barry20, who
4
Biomechanics of the Patellofemoral Joint
DYNAMICS
In full extension, the patella articulates with the
supratrochlear
fat
pad.
Normal
knee valgus
creates an angle between the line of pull of the
quadriceps and the patellar tendon, the so-called
“Q angle” (Figure 4).
Figure 5. The Q angle imposes a valgus vector on
the terminal degrees of extensor movement.
It is dangerous, however, to focus too much on
the Q angle. The clinician must recognize that
this on only one of many factors affecting
patellar balance. An increased Q angle provides
no direct correlation with patellofemoral pain.
Figure 4. A, Q angle is demonstrated with the
knee in full extension. B, Knee is flexed 30° in
this photograph. The tibia has rotated medially
nearly completely neutralizing the Q angle. The
patellar tendon is lined up with the anterior
border of the tibia. Note also in B how the fat pad
bulge has disappeared and the patella has
increased in prominence as it is lifted away from
the axis of flexion by the trochlea.
However, clinically it’s the dynamic Q angle that
influence patellar alignment and this may be
responsible for Patellofemoral pain. For detailed
description of the dynamic Q angle refer to
Etiology of Patellofemoral Pain part 1 and part 2.
Observing the course of flexion of the patella,
Hvid and Anderson22 showed that the Q angle
beginning from a position of forced full extension
correlates with internal hip rotation. The “screw
can bring out some of the dynamics associated
home” mechanism of the tibiofemoral joint in
with various pathologic conditions. During the
terminal extension (whereby the tibia rotates
externally in relation to the femur)
first 20° of flexion, the tibia derotates. This
further
significantly
lateralizes the tibial tubercle. The pull of the
decreases
the
Q
angle,
also
decreasing the lateral vector. The patella is
quadriceps then produces a valgus vector (Figure
drawn into the trochlea and the first articular
5) which is resisted by the medial patellar
contact is made by 10° knee flexion. The patella
retinaculum and the vastus medialis.
enters the trochlea from a slight lateral position.
5
Biomechanics of the Patellofemoral Joint
From 20 - 30° of flexion, the patella becomes
the femur26 during knee extension also affects
more prominent as it is lifted away from the axis
patellar tracking. The whole complex system of
of rotation of the knee by the prominence of the
adaptation
trochlea of the femur23. Beyond 30°, the patella
therefore, must be considered to understand
begins to settle into the deepening trochlear
biomechanical
groove.
joint.
Instability
uncommon,
and
problems may
beyond
many
30°
is
relatively
patellofemoral
be associated
with
if
static
alignment
of
the
maintain
function
constant
of
the
unit
load,
Patellofemoral
pain
THE ROLE OF TAPING AND BRACING IN THE
TREATMENT OF PATELLOFEMORAL PAIN
abnormal
patellar tracking in the first 30° of knee flexion.
Also,
to
patella
External
is
patellar
supports
are
commonly
employed in the management of patellofemoral
excessively tilted to the lateral side, flexion of the
pain and are typically used as an adjunct to other
knee will cause posterior movement of the
treatment methods i.e. strengthening50, 57,74,79,83.
iliotibial band, and the lateral retinaculum will
experience abnormally elevated tension as the
The primary goal of Patellofemoral joint taping to
patella is drawn into the trochlea by medial
centralize the patella within the trochlear groove,
retinacular pull. (Figure 6)
thus improving patellar tracking33,57. The patellar
taping technique as described by McConnell 49 has
gained widespread clinical acceptance as an
effective treatment option. In this protocol, rigid
strapping tape is applied to the patella to correct
malalignment,
evaluation)
(as
and
determined
is
followed
by
by
clinical
functional
strengthening of the quadriceps.
Figure 6. With knee flexion, the medial
retinaculum tightens and pulls the patella into
the trochlea. If the lateral retinaculum is
shortened and tightened, there will be lateral
retinacular strain and excessive lateral facet
pressure. (From Fulkerson, J.P.: The Etiology of
Patellofemoral Pain in Young Active Patients. Clin.
Orthop. 179: 132, 1983.)
Utilizing patellar taping, McConnell 50 reported
that 92% of patients were pain free after eight
treatment sessions, which was comparable with
the 96% success rate reported by Gerrard 20 after
only five treatments involving taping.
Many factors, including geometric characteristics
of contact surfaces24, are brought into play to
The fact that the various forms of external
maintain relatively constant unit loads. Cartilage
patellar
thickness and subchondral bone quality
will
effective in reducing symptoms immediately after
affect the patellar response to stress. Rotation of
application32 indicates that such orthoses have a
25
6
supports
have
been
shown
to
be
Biomechanics of the Patellofemoral Joint
mechanical effect on the Patellofemoral joint.
8.
Goodfellow J, Hungerford DS, Woods C: Patellofemoral
joint mechanics and pathology: Chondromalacia
patellae. J Bone Joint Surg 58B: 291–299, 1976.
9.
Douchette SA, Goble EM: The effects of exercise on
patellar tracking in lateral patellar compression
syndrome. Am J Sports Med 20: 434–440,1992.
10.
Levine J: Chondromalacia patellae. Physician Sportsmed
7: 41–49, 1979.
11.
Bentley G, Dowd G: Current concepts of etiology
and treatment of chondromalacia patellae. Clin
Orthop 189:209-228, 1984.
12.
Maquet PG: Biomechanics of the
New York: Springer-Verlag, 1984.
13.
Van Eijden TM, Kouwenhoven E, Verberg J, Weijus WAS:
A mathematical model of the patellofemoral joint. J
Biomech 19:219-229, 1986.
14.
Buff HU, Jones LC, Hungerford DS:
Experimental
determination of forces transmitted through the
patellofemoral joint. J Biomech 21:17-23, 1988.
15.
Reilly DT and Martens M: Experimental analysis of the
quadriceps muscle force and patellofemoral joint
reaction force for various activities. Acta Orthop.
Scan.,43:126-137, 1972.
16.
Goodfellow J, Hungerford DS, Zindel M: Patellofemoral
joint mechanics and pathology: Functional anatomy
of the patellofemoral joint. J Bone Joint Surg
58B:287-290, 1976.
17.
Lieb FJ, Perry J: Quadriceps function: An anatomical and
mechanical study using amputated limbs. J Bone
Joint Surg 50A:1535-1548, 1968.
18.
Hungerford DS, Barry M: Biomechanics of the
patellofemoral joint. Clin Orthop 144:9-15, 1979.
19.
Steinkamp LA, Dillingham MF, Markel MD, Hill JA,
Kaufman KR: Biomechaical considerations in
patellofemoral joint rehabilitation. Am J Sports Med
21:438-444,1993.
20.
Hvid I and Andersen LI: The quadriceps angle and its
relation to femoral torsion. Acta Orthop. Scan.
53: 577-579, 1982.
21.
Bandi W: The significance of femoropatellar pressure in
the pathogenesis and treatment of chondromalacia
patellae and femoropatellar arthrosis. In the knee
joint. Edited by Ingwersen. New York: American
Elsevier Publishing Co Inc., 1974.
22.
Ahmed A, Burke D and Hyder A: Force analysis of the
patellar mechanism. J. Orthop. Res., 5(1):69-85,
1987.
For example, Powers et al 32 reported that patellar
taping resulted in small but significant increases
in
loading
response
knee
flexion,
which
suggested more willingness by Patellofemoral
pain subjects to load the knee joint during gait. A
plausible mechanism by which taping affects a
pain patellofemoral joint could be by increasing
its contact areas, thus decreasing the pressure
generating in the joint.
Remember pressure =
force/area. (Powers personal communication).
For further reading on clinical approach for
selection of taping techniques see separate home
study.
ABOUT THE AUTHOR
Professor Christopher Powers
is a world renowned expert
on the subject
of
the
patellofemoral joint.
REFERENCES
1.
Hey Groves, EW: Note on the Extension Apparatus of
the Knee Joint. Br. J. Surg.24:747,1937.
2.
Ficat P.: Patholoogie Femoro-patellaire. Paris: Masson et
Cie, 1970.
3.
Kaufer H.: Mechanical Function of the Patella. J. Bone
Joint Surg.53A:1551, 1971.
4.
Steindler
A.:Kinesiology
of
the
Human
Springfield, IL: Charles C Thomas, 1955.
5.
Mow V,Holmes M, and Lai W.: Fluid transport and
mechanical properties of articular cartilage: a review.
J. Biomech. 17:377-394, 1984.
6.
Grana W, Kiregshauser L: Scientific basis of extensor
mechanism disorders. Clin Sports Med 4:247-257,
1985.
7.
Body.
Heywood WB: Recurrent dislocation of the patella.
Bone Joint Surg 43B:508–517, 1961.
J
7
knee
(2nd
Ed),
Biomechanics of the Patellofemoral Joint
23.
Abernethy PJ, Townsend PR, Rose RM and Radin EL: Is
chondromalacia patellae a separate clinical entity?
S. Bone Joint Surg. (Br.), 60:205-210, 1978.
24.
Sikorski J, Peters J and Watt I: The importance of
femoral rotation in chondromalacia patellae as shown
by serial radiography. J. Bone Joint Surg., 61B:435442, 1979.
25.
McConnell J: The management of chondromalacia
patellae: A long term solution. Aust J Physiother
32:215-223, 1986
26.
Walsh
WM,
Helzer-Julin
M:
Patellar
tracking
problems in athletes. Primary Care 19:303-330,
1992.
8
27.
Hunter LY:
Braces and taping.
4:439-454, 1985.
Clin Sports Med
28.
McConnell J: The advanced McConnell patellofemoral
treatment plan, course notes, The University of
Sydney, Lidcombe, New South Wales, Australia,
1996.
29.
Gerrard B: The patellofemoral pain syndrome in young,
active patients: A prospective study. Clin Orthop
179:129-133, 1989
30.
Powers CM, Landel R, Sosnick T, Mengel K, Perry J: The
effects of patellar taping on stride characteristics in
subjects with patellofemoral pain. J Orthop Sports
Phys Ther 26:286-291, 1997.