1. Biomechanics
of the
Knee Complex : 6
DR. DIBYENDUNARAYAN BID [PT]
THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY,
RAMPURA, SURAT

2. Tibiofemoral Joint Function

3. Joint Kinematics
 The primary angular (or rotatory) motion of the
tibiofemoral joint is flexion/extension, although both
medial/lateral (internal/external) rotation and
varus/ valgus (adduction/adduction) motions can
also occur to a lesser extent. These motions occur
about changing but definable axes.

4.  In addition to the angular motions, translation in an
anteroposterior direction is common on both the
medial and lateral tibial plateaus; to a lesser extent,
medial and lateral translations can occur in response
to varus and valgus forces.
 The small amounts of anteroposterior and
medial/lateral displacements that occur in the
normal knee are the result of joint incongruence and
variations in ligamentous elasticity.

5.  Although these translations may be seen as
undesirable, they are necessary for normal joint
motions to occur.
 Excessive translational motions, however, should be
considered abnormal and generally indicate some
degree of ligamentous incompetence. We will focus
on here on the normal knee joint motions, including
both osteokinematics and arthrokinematics.

6. Flexion/Extension
 The axis for tibiofemoral flexion and extension can
be simplified as a horizontal line passing through the
femoral epicondyles.
 Although this transepicondylar axis represents an
accurate estimate of the axis for flexion and
extension, it should be appreciated that this axis is
not truly fixed but rather shifts throughout the ROM.
 Much of the shift in the axis can be attributed to the
incongruence of the joint surfaces.

7.  The large articular surface of the femur and the
relatively small tibial condyle create a potential
problem as the femur begins to flex on the fixed
tibia.
 If the femoral condyles were permitted to roll
posteriorly on the tibial plateau, the femur would run
out of tibia and limit the flexion excursion (Fig. 11-
25).

9.  For the femoral condyles to continue to roll as
flexion increases without leaving the tibial plateau,
the femoral condyles must simultaneously glide
anteriorly (Fig. 11-26A).

11.  The initiation of knee flexion (0 to 25), therefore,
occurs primarily as rolling of the femoral condyles on
the tibia that brings the contact of the femoral
condyles posteriorly on the tibial condyle.
 As flexion continues, the rolling of the femoral
condyles is accompanied by a simultaneous anterior
glide that is just sufficient to create a nearly pure
spin of the femur on the posterior tibia with little
linear displacement of the femoral condyles after 25
of flexion.

12.  Extension of the knee from flexion is essentially a
reversal of this motion.
 Tibiofemoral extension occurs initially as an anterior
rolling of the femoral condyles on the tibial plateau,
displacing the femoral condyles back to a neutral
position on the tibial plateau.

13.  After the initial forward rolling, the femoral condyles
glide posteriorly just enough to continue extension of
the femur as an almost pure spin of the femoral
condyles on the tibial plateau (see Fig. 11-26B).

14.  This description of the interdependent
osteokinematics and arthrokinematics indicates that
the femur was moving on a fixed tibia (e.g., during a
squat).
 The tibia, of course, is also capable of moving on a
fixed femur (e.g., during a seated knee extension or
the swing phase of gait).

15.  In this case, the movements would be somewhat
different. When the tibia is flexing on a fixed femur,
the tibia both rolls and glides posteriorly on the
relatively fixed femoral condyles.
 Extension of the tibia on a fixed femur incorporates
an anterior roll and glide of the tibial plateau on the
fixed femur.

17. Role of the Cruciate Ligaments and Menisci
in Flexion/Extension
 The arthrokinematics associated with tibiofemoral flex-
ion and extension are somewhat dictated by the presence
of the cruciate ligaments.
 If the cruciate ligaments are assumed to be rigid
segments with a constant length, posterior rolling of the
femur during knee flexion would cause the “rigid” ACL to
tighten (or serve as a check rein).
 Continued rolling of the femur would result in the taut
ACL’s simultaneously creating an anterior translational
force on the femoral condyles (Fig. 11-27A).

19.  During knee extension, the femoral condyles roll
anteriorly on the tibial plateau until the “rigid” PCL
checks further anterior progression of the femur,
creating a posterior translational force on the
femoral condyles (see Fig. 11-27B).

20.  The anterior glide of the femur during flexion may be
further facilitated by the shape of the menisci.
 The wedge shape of the menisci posteriorly forces
the femoral condyle to roll “uphill” as the knee flexes.
 The oblique contact force of the menisci on the
femur helps guide the femur anteriorly during
flexion while the reaction force of the femur on the
menisci deforms the menisci posteriorly on the tibial
plateau (Fig. 11-28).

21.  Posterior deformation occurs because the rigid
attachments at the meniscal horns limit the ability of
the menisci to move in its entirety.
 Posterior deformation also allows the menisci to
remain beneath the rounded femoral condyles as the
condyles move on the relatively flat tibial plateau.

22.  As the knee joint begins to return to extension from
full flexion, the posterior margins of the menisci
return to their neutral position.
 As extension continues, the anterior margins of the
menisci deform anteriorly with the femoral condyles.

23.  The motion (or distortion) of the menisci is an
important component of tibiofemoral flexion and
extension.
 Given the need of the menisci to reduce friction and
absorb the forces of the femoral condyles that are
imposed on the relatively small tibial plateau, the
menisci must remain beneath the femoral condyles
to continue their function.

24.  The posterior deformation of the menisci is assisted
by muscular mechanisms to ensure that appropriate
meniscal motion occurs.
 During knee flexion, for example, the
semimembranosus exerts a posterior pull on the
medial meniscus (Fig. 11-29), whereas the popliteus
assists with deformation of the lateral meniscus.

27. Flexion/Extension Range of Motion
 Passive range of knee flexion is generally considered to
be 130° to 140°.
 During an activity such as squatting, knee flexion may
reach as much as 160° as the hip and knee are both flexed
and the body weight is super-imposed on the joint.
 Normal gait on level ground requires approximately 60°
to 70° of knee flexion, whereas ascending stairs requires
about 80°, and sitting down into and arising from a chair
requires 90° of flexion or more.

28.  Knee joint extension (or hyperextension) up to 5° is
considered within normal limits.
 Excessive knee hyperextension (i.e., beyond 5° of
hyperextension) is termed genu recurvatum.

29.  Many of the muscles acting at the knee are two-joint
muscles crossing not only the knee but also the hip or
ankle.
 Therefore, the hip joint’s position can influence the knee
joint’s ROM. Passive insufficiency of the rectus femoris
could limit knee flexion to 120° or less if the hip joint is
simultaneously hyperextended.
 When the lower extremity is in weight-bearing, ROM
limitations at other joints such as the ankle may cause
restrictions in knee flexion or extension.

30. Case Application 11-3: Meniscal Entrapment
 Failure of the menisci to distort in the proper direction
can result in limitations of joint motion and/or damage
to the menisci.
 If the femur literally rolls up the wedge-shaped menisci
in flexion (without either the anterior glide of the
femur or the posterior distortion of the menisci), the
increasing thickness of the menisci and the threat of
rolling off the posterior margin will cause flexion to be
limited.

31.  Alternatively, the stress on the meniscus (especially the less
mobile medial meniscus) may cause the meniscus to tear.
 Similarly, failures of the menisci to distort anteriorly during
extension causes the thick anterior margins to become
wedged between the femur and tibia as the segments are
drawn together in the final stages of extension, thus limiting
extension.
 The failure of the meniscus or femoral condyles to move
appropriately on each other may be part of the explanation for
Tina’s original injury to her medial meniscus, although it is
likely that additional stresses to the meniscus contributed.

32. Example 11-1
 Ski boots generally hold the ankle in dorsiflexion, preventing
full knee extension when the foot is on the ground (see Fig. 11-
30A).
 The choice is either to walk with flexed knees or to walk on the
heels. The same problem may be created by a fixed dorsiflexion
deformity in the ankle/foot complex.
 The opposite situation happens with a limitation in dorsiflexion.
 A limitation to ankle dorsiflexion (e.g., caused by tight
plantarflexors) may limit the amount of knee flexion that can be
performed without lifting the heel off the ground.

33.  If there is a fixed plantarflexion deformity at the
ankle, the inability to bring the tibia forward in
weight-bearing may result in a hyperextension
deformity (genu recurvatum) at the knee (Fig. 11-
30B).
 The relationship between ankle and knee motions
when the foot is on the ground can be exploited by
intentionally altering ankle joint motion (e.g.,
through a heel lift or an ankle-foot orthosis) to
prevent or control undesired knee motions.

36. Medial/Lateral Rotation
 Medial and lateral rotation of the knee joint are angular
motions that are named for the motion (or relative
motion) of the tibia on the femur.
 These axial rotations of the knee joint occur about a
longitudinal axis that runs through or close to the medial
tibial intercondylar tubercle.
 Consequently, the medial condyle acts as the pivot point
while the lateral condyles move through a greater arc of
motion, regardless of the direction of rotation (Fig. 11-
31).

37.  As the tibia laterally rotates on the femur, the medial
tibial condyle moves only slightly anteriorly on the
relatively fixed medial femoral condyle, whereas the
lateral tibial condyle moves a larger distance
posteriorly on the relatively fixed lateral femoral
condyle.
 During tibial medial rotation, the medial tibial
condyle moves only slightly posteriorly, whereas the
lateral condyle moves anteriorly through a larger arc
of motion.

38.  During both medial and lateral rotation, the knee
joint’s menisci will distort in the direction of
movement of the corresponding femoral condyle
and, therefore, maintain their relationship to the
femoral condyles just as they did in flexion and
extension.

39.  For example, as the tibia medially rotates (femur
laterally rotates on the tibia), the medial meniscus
will distort anteriorly on the tibial condyle to remain
beneath the anteriorly moving medial femoral
condyle, and the lateral meniscus will distort
posteriorly to remain beneath the posteriorly moving
lateral femoral condyle.

40.  In this way, the menisci continue to reduce friction
and distribute forces without restricting motion of
the femur, as more solid or rigidly attached
structures would do.

41.  Axial rotation is permitted by articular incongruence
and ligamentous laxity. Therefore, the range of knee
joint rotation depends on the flexion/extension
position of the knee.
 When the knee is in full extension, the ligaments are
taut, the tibial tubercles are lodged in the
intercondylar notch, and the menisci are tightly
interposed between the articulating surfaces;
consequently, very little axial rotation is possible.

42.  As the knee flexes toward 90, capsular and
ligamentous laxity increase, the tibial tubercles are
no longer in the intercondylar notch, and the
condyles of the tibia and femur are free to move on
each other.

43.  The maximum range of axial rotation is available at
90 of knee flexion. The magnitude of axial rotation
diminishes as the knee approaches both full
extension and full flexion.
 At 90°, the total medial/lateral rotation available is
approximately 35°, with the range for lateral rotation
being slightly greater (0° to 20°) than the range for
medial rotation (0° to 15°).

44. Valgus (Abduction)/Varus (Adduction)
 Frontal plane motion at the knee, although minimal,
does exist and can contribute to normal functioning
of the tibiofemoral joint.
 Frontal plane ROM is typically only 8° at full
extension, and 13° with 20° of knee flexion.
 Excessive frontal plane motion could indicate
ligamentous insufficiency.

45.  There is evidence that the muscles that cross the
knee joint have the ability both to generate and
control substantial valgus and varus torques.
 When there is ligamentous laxity, the excessive
varus/valgus motion or increased dynamic activity of
muscles attempting to control this excessive motion
could precipitate greater peak stresses across the
joint.

46. Coupled Motions
 Typical tibiofemoral motions are, unfortunately, not
as straightforward as we have described.
 In fact, biplanar intra-articular motions can occur
because of the oblique orientation of the axes of
motion with respect to the bony levers.
 The true flexion/extension axis is not perpendicular
to the shafts of the femur and tibia.

47.  Therefore, flexion and extension do not occur as pure
sagittal plane motions but include frontal plane
components termed “coupled motions” (similar to
coupling that occurs with lateral flexion and rotation
in the vertebral column).
 As already noted, the medial femoral condyle lies
slightly distal to the lateral femoral condyle, which
results in a physiologic valgus angle in the extended
knee that is similar to the physiologic valgus angle
that exists at the elbow.

48.  With knee flexion around the obliquely oriented axis,
the tibia moves from a position oriented slightly
lateral to the femur to a position slightly medial to
the femur in full flexion; that is, the foot approaches
the midline of the body with knee flexion just as the
hand approaches the mid-line of the body with elbow
flexion.
 Flexion is, therefore, considered to be coupled to a
varus motion, while extension is coupled with valgus
motion.

49. Automatic or Locking Mechanism of the Knee
 There is an obligatory lateral rotation of the tibia
that accompanies the final stages of knee extension
that is not voluntary or produced by muscular forces.
This coupled motion (lateral rotation with extension)
is referred to as automatic or terminal rotation.
 We have already noted that the medial articular
surface of the knee is longer (has more articular
surface) than does the lateral articular surface (see
Fig. 11-3).

50.  Consequently, during the last 30° of knee extension
(30° to 0°), the shorter lateral tibial plateau/femoral
condyle pair completes its rolling-gliding motion
before the longer medial articular surfaces do.
 As extension continues (referencing non–weight-
bearing motion of the tibia), the longer medial
plateau continues to roll and to glide anteriorly after
the lateral side of the plateau has halted.

51.  This continued anterior motion of the medial tibial
condyle results in lateral rotation of the tibia on the
femur, with the motion most evident in the final 5 of
extension.
 Increasing tension in the knee joint ligaments as the
knee approaches full extension may also contribute
to the obligatory rotational motion, bringing the
knee joint into its close-packed or locked position.

52.  The tibial tubercles become lodged in the intercondylar
notch, the menisci are tightly interposed between the
tibial and femoral condyles, and the ligaments are taut.
 Consequently, automatic rotation is also known as the
locking or screw home mechanism of the knee.
 To initiate knee flexion from full extension, the knee
must first be unlocked; that is, the laterally rotated tibia
cannot simply flex but must medially rotate
concomitantly as flexion is initiated.

53.  A flexion force will automatically result in medial
rotation of the tibia because the longer medial side
will move before the shorter lateral compartment.
 If there is a lateral restraint to unlocking or
derotation of the femur, the joint surfaces, ligaments,
and menisci can become damaged as the tibia or
femur is forced into flexion.

54.  This automatic rotation or locking of the knee occurs
in both weight-bearing and non–weight-bearing
knee joint function.
 In weight-bearing, the freely moving femur medially
rotates on the relatively fixed tibia during the last 30
of extension.
 Unlocking, consequently, is brought about by lateral
rotation of the femur on the tibia before flexion can
proceed.

55.  The motions of the knee joint, exclusive of automatic
rotation, are produced to a great extent by the
muscles that cross the joint.

56.  We will complete our examination of the
tibiofemoral joint by first examining the individual
contribution of the muscles, emphasizing their role
in producing and controlling knee joint motion.
 We will then reexamine both the passive knee joint
structures and the muscles in their combined role as
stabilizers of this very complicated joint.

57. End of Part - 6