1. Biomechanics
of the
Knee Complex : 8
DR. DIBYENDUNARAYAN BID [PT]
THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY,
RAMPURA, SURAT

2. Patellofemoral Joint
 Embedded within the quadriceps muscle, the
flat, triangularly shaped patella is the largest
sesamoid bone in the body.
 The patella is an inverted triangle with its apex
directed inferiorly. The posterior surface is divided
by a vertical ridge and covered by articular cartilage
(Fig. 11-39).

3.  This ridge is situated approximately in the center of
the patella, dividing the articular surface into
approximately equally sized medial and lateral
facets.

4.  Both the medial and lateral facets are flat to slightly
convex side to side and top to bottom.
 Most patellae also have a second vertical ridge
toward the medial border that separates the medial
facet from an extreme medial edge, known as the
odd facet of the patella (see Fig. 11-39).

6.  The posterior surface of the patella in the extended
knee sits on the femoral sulcus (or patellar surface)
of the anterior aspect of the distal femur (Fig. 11-40).
 The femoral sulcus has a groove that corresponds to
the ridge on the posterior patella and divides the
sulcus into medial and lateral facets.

8.  The lateral facet of the femoral sulcus is slightly
more convex than the medial facet and has a more
highly developed lip than does the medial surface
(see Fig. 11-2).

10.  The patella is attached to the tibial tuberosity by the
patellar tendon.
 Given the shape of the articular surfaces and the fact
that the patella has a much smaller articular surface
area than its femoral counterpart, the patellofemoral
joint is one of the most incongruent joints in the
body.

12.  The patella functions primarily as an anatomic pulley
for the quadriceps muscle.
 Interposing the patella between the quadriceps
tendon and the femoral condyles also reduces
friction as the femoral condyles contact the hyaline
cartilage-covered posterior surface of the patella
rather than the quadriceps tendon.

13.  The ability of the patella to perform its functions
without restricting knee motion depends on its
mobility.
 Because of the incongruence of the patellofemoral
joint, however, the patella is dependent on static and
dynamic structures for its stability.

14.  We must closely examine the oddly shaped patella
and the uneven surface on which it sits in order to
understand the normal motions of the patella that
accompany knee joint motion and the tremendous
forces to which the patella and patellofemoral
surfaces are susceptible.

15.  The goal of such examination is to understand the
many potential problems encountered by the patella
in performing what appears to be a relatively simple
function.

16.  A comprehension of the structures and forces that
influence patellofemoral function leads readily to an
understanding of the common clinical problems
found at the patellofemoral joint as it attempts to
meet its contradictory demands for both mobility
and stability.

17. Patellofemoral Articular Surfaces
and Joint Congruence
 In the fully extended knee, the patella lies on the
femoral sulcus.
 Because the patella has not yet entered the
intercondylar groove, joint congruency in this
position is minimal, which suggests that there is a
great potential for patellar instability.

18.  Stability of the patella is affected by the vertical
position of the patella in the femoral sulcus, because
the superior aspect of the femoral sulcus is less
developed than the inferior aspect.

19.  The vertical position of the patella, in turn, is related
to the length of the patellar tendon.
 Ordinarily, the ratio of the length of the patellar
tendon to the length of the patella is approximately
1:1 and is referred to as the Insall-Salvati index.

20.  A markedly long tendon produces an abnormally
high position of the patella on the femoral sulcus
known as patella alta, which increases the risk for
patellar instability.
 The interaction of the height of the lateral lip of the
femoral sulcus with patella alta may also be a factor
in patellar instability.

21.  In this condition, the lateral lip is not necessarily
underdeveloped (although it may be), but the high
position of the patella places the patella proximal to
the high lateral wall, rendering the patella less stable
and easier to sublux.

22.  In patients with patella alta, the tibiofemoral joint
must be flexed more before the patella translates
inferiorly enough to engage the intercondylar groove.
 This leaves a larger knee ROM within which the
patella is relatively unstable.

23.  Given the incongruence of the patella, the contact
between the patella and the femur changes
throughout the knee ROM (Fig. 11-41).
 When the patella sits in the femoral sulcus in the
extended knee, only the inferior pole of the patella is
making contact with the femur.114

25.  As the knee begins to flex, the patella slides down the
femur, increasing the surface contact area.
 In this manner, the first consistent contact between
the patella and the femur occurs along the inferior
margin of both the medial and lateral facets of the
patella at 10° to 20° of knee flexion.

26.  As tibiofemoral flexion progresses, the contact area
increases and shifts from the initial inferior location
on the patella to a more superior position.
 As the contact area shifts superiorly along the
posterior aspect of the patella, it also spreads
outward to cover the medial and lateral facet.
 By 90° of knee flexion, all portions of the patella
have experienced some (although inconsistent)
contact, with the exception of the odd facet.

27.  As flexion continues beyond 90°, the area of contact
begins to migrate inferiorly once again as the smaller
odd facet makes contact with the medial femoral
condyle for the first time.

28.  At full flexion, the patella is lodged in the
intercondylar groove, and contact is on the lateral
and odd facets, with the medial facet completely out
of contact.

29. Motions of the Patella
 As the contact between the patella and the femur
changes with knee joint motion, the patella
simultaneously translates and rotates on the femoral
condyles.
 These movements are influenced by and reflect the
patella’s relationship to both the femur and the tibia.
 Consequently, the description of motions can appear
quite complicated.

30.  When the femur is fixed and the tibia is flexing, the
patella (fixed to the tibial tuberosity via the patellar
tendon) is pulled down and under the femoral
condyles, ending with the apex of the patella
pointing posteriorly in full knee flexion.

31.  This sagittal plane rotation of the patella as the patella
travels (or “tracks”) down the intercondylar groove of the
femur is termed patellar flexion.
 Knee extension brings the patella back to its original
position in the femoral sulcus, with the apex of the
patella pointing inferiorly at the end of the normal ROM.
 This patellar motion is referred to as
patellar extension.

32.  In addition to patellar flexion and extension, the
patella rotates around a longitudinal (or nearly
vertical) axis and tilts around an anteroposterior
axis.
 Rotation about the longitudinal axis is termed
medial or lateral patellar tilt and is named for the
direction in which the anterior surface of the patella
is moving (Fig. 11-42).

34.  When the tibia medially rotates beneath the femur
during axial rotation, the patella must remain in the
intercondylar groove during the relative lateral
rotation of the femur.
 This relative motion of the femur forces the patella to
face more laterally; this is termed lateral rotation.

35.  Patellar tilt is also dictated somewhat by the
asymmetrical nature of the femoral condyles.
 For instance, the more anteriorly protruding lateral
femoral condyle forces the anterior surface of the
patella to tilt medially during much of knee flexion.

36.  Rotation of the patella about an anteroposterior axis
(termed medial or lateral rotation of the patella) is,
like patellar tilt,
necessary in order for the patella to remain seated
between the femoral condyles as the femur
undergoes axial rotation on the tibia.
 Because the inferior aspect of the patella is “tied” to
the tibia via the patellar tendon, the inferior patella
continually points toward the tibial tuberosity while
moving with the femur (Fig. 11-43).

37. Figure 11-43
■ A. Medial rotation
of the patella.
The inferior pole of the
patella follows the tibial
tuberosity during
medial rotation of the
tibia.
B. Lateral rotation
of the patella.
The inferior pole of the
patella follows the tibial
tuberosity during
lateral rotation of the
tibia.

38.  Therefore, when the knee is in some flexion and there is
medial rotation of the tibia on the fixed femur, the
inferior pole of the patella will point medially;
this is termed medial rotation of the patella.
 In lateral rotation of the patella, the inferior patellar pole
follows the laterally rotated tibia.
 The patella laterally rotates approximately 5° as the knee
flexes from 20° to 90°, given the asymmetrical
configuration of the femoral condyles.

39.  The patella, although firmly attached to soft tissue
stabilizers (for example, the extensor retinaculum),
undergoes translational motions that are dependant
on the point in the tibiofemoral ROM.
 The patella translates superiorly and inferiorly with
knee extension and flexion, respectively.

40.  During active extension, the patella glides superiorly.
 If this glide is restricted, quadriceps function is
compromised, and passive knee extension may be
lost.

41.  During active tibiofemoral flexion, the patella glides
inferiorly.
 A restricted inferior glide could therefore limit knee
flexion.
 There is a simultaneous medial-lateral translation of
the patella that accompanies the superior-inferior
glide that is referred to as patellar shift (see Fig. 11-
42).

43.  The patella is typically situated slightly laterally in
the femoral sulcus with the knee in full extension.
 As knee flexion is initiated, the patella shifts medially
as it is pushed by the larger lateral femoral condyle
and as the tibial medially rotates with unlocking of
the knee.

44.  As knee flexion proceeds past 30°, the patella may
shift slightly laterally or remain fairly stable,
inasmuch as the patella is now firmly engaged within
the femoral condyles (Fig. 11-44).
 Consequently, the patella shifts as the knee moves
from full extension into flexion.

46.  Failure of the patella to slide, tilt, rotate, or shift
appropriately can lead to restrictions in knee joint
ROM, to instability of the patellofemoral joint, or to
pain caused by erosion of the patellofemoral
articular surfaces.
 Therefore, passive mobility of the patella is often
assessed clinically to determine the presence of
hypermobility or hypomobility of the patella with
respect to the femur.

47. Patellofemoral Joint Stress
 The patellofemoral joint can undergo very high
stresses during typical activities of daily living.
 Joint stress (force per unit area) can be influenced by
any combination of large joint forces or small contact
areas, both of which are present during routine
flexion and extension of the tibiofemoral joint.

48.  The patellofemoral joint reaction (contact) force is
influenced by both the quadriceps force and the knee
angle.
 As the knee flexes and extends, the patella is pulled
by the quadriceps tendon superiorly and
simultaneously by the patella tendon inferiorly.

49.  The combination of these pulls produces a posterior
compressive force of the patella on the femur that
varies with knee flexion.
 At full extension, the quadriceps posterior
compressive force on the patella is minimized and
due exclusively to the origin of the vastus medialis
and vastus lateralis muscles on the posterior femur.

50.  Despite the small contact area that the patella has
with the femur in full extension, the minimal
posterior compressive vector of the vastus lateralis
and vastus medialis muscles maintains low joint
stress at full extension.
 This is the rationale for the use of straight-leg raising
exercises as a way of improving quadriceps muscle
strength without creating or exacerbating
patellofemoral pain.

51.  As knee flexion progresses from full extension, the
angle of pull between the quadriceps tendon and the
patellar tendon decreases, creating greater joint
compression (Fig. 11-45).
 This increased compression occurs whether the
muscle is active or passive.
 If the quadriceps muscle is inactive, then elastic
tension alone increases with increased knee joint
flexion.

53.  If the quadriceps muscle is active, then both the
active tension and passive elastic tension will
contribute to increasing the joint compression.
 This compression, of course, creates a joint reaction
force across the patellofemoral joint.
 The total joint reaction force is therefore influenced
by the magnitude of active and passive pull of the
quadriceps, as well as by the angle of knee flexion.

54.  Although the compressive force arising from the
quadriceps increases as the knee flexes from 0° to
90°, the patellar contact area also increases.
 The increase in contact area with increased
compressive force functions to minimize
patellofemoral joint stress until approximately 90° of
flexion.

55.  As knee flexion continues beyond 90°, the contact
area once again diminishes and patellofemoral stress
increases as only the lateral and odd facets make
contact with the femoral condyles.

56.  Patellofemoral joint reaction forces can become very
high during routine daily activities.
 During the stance phase of walking, when peak knee
flexion is only approximately 20° , the patellofemoral
compressive force is approximately 25% to 50% of
body weight.

57.  With greater knee flexion and greater quadriceps
activity, as during running, patellofemoral
compressive forces have been estimated to reach
between five and six times body weight.

58.  Deep knee flexion exercises that require large
magnitudes of quadriceps activity can increase this
compressive force further.
 Although reaction forces at other lower extremity
joints may reach these same magnitudes, they do so
over much more congruent joints;
that is, the compressive forces are distributed over
larger areas.

59.  At the normal patellofemoral joint, the medial facet
bears the brunt of the compressive force.
 Several mechanisms help minimize or dissipate the
patellofemoral joint compression on the patella in
general and on the medial facet specifically.

60.  In full extension, there is minimal compressive force
on the patella; therefore, no compensatory
mechanisms are necessary.
 As knee joint flexion proceeds, the area of patella
contact gradually increases, spreading out the
increased compressive force.

61.  From 30° to 70° of flexion, the magnitude of contact
force is higher at the thick cartilage of the medial
facet near the central ridge.
 This articular cartilage is among the thickest hyaline
cartilage in the human body.
 The presence of this thick cartilage is better able to
withstand the substantial compressive forces
transmitted across the medial facet of the patella.

62.  Within this same ROM, the patella has its greatest
effect as a pulley, maximizing the MA of the
quadriceps.
 With a larger MA, less quadriceps muscle force is
needed to produce the same torque, minimizing
patellofemoral joint compression.

63.  As flexion proceeds, the MA diminishes, which
necessitates an increase in force production by the
quadriceps.
 Beyond 90°, however, the patella is no longer the
only structure contacting the femoral condyles.
 At this point in the flexion range, the quadriceps
tendon contacts the femoral condyles, helping to
dissipate more of the compressive force on the
patella.

64.  The vertical position of the patella can also
significantly influence patellofemoral stress.
 Singerman and colleagues demonstrated that in the
presence of patella alta, the onset of contact between
the quadriceps tendon and femoral condyles is
delayed.

65.  As flexion increases, patellofemoral compressive
forces will therefore continue to rise.
 In contrast to patella alta, the patella can also sit
lower than normal.

66.  If the patella is positioned more inferiorly, it is
termed patella baja and may be due to a shortened
patellar tendon.
 With patella baja, the contact between the
quadriceps tendon and the femoral condyles occurs
earlier in the range,
resulting in a concomitant reduction in the
magnitude of the patellofemoral contact force.

67. End of Part - 8