1. Biomechanics
of the
Knee Complex : 4
DR. DIBYENDUNARAYAN BID [PT]
THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY,
RAMPURA, SURAT

2. Ligaments
 The roles of the various ligaments of the knee have
received extensive attention, which reflects their
importance for knee joint stability and the frequency
with which function is disrupted through injury.

3.  Given the lack of bony restraint to virtually any of
the knee motions, the knee joint ligaments are
variously credited with resisting or controlling:
 1. excessive knee extension
 2. varus and valgus stresses at the knee (attempted
adduction or abduction of the tibia, respectively)
 3. anterior or posterior displacement of the tibia beneath
the femur
 4. medial or lateral rotation of the tibia beneath the
femur
 5. combinations of anteroposterior displacements and
rotations of the tibia, together known as rotatory sta-
bilization of the tibia

4.  The large body of literature available on ligamentous
function of the knee joint can be confusing and
appears contradictory.
 This may be due to some confusion in terms as to
whether the tibia or the femur is being referenced,
but it is more likely due to complex and variable
functioning and to dissimilar testing conditions.

5.  It is clear that ligamentous function can change,
depending on the position of the knee joint, on how
the stresses are applied, and on what active or
passive structures are concomitantly intact.

6. Medial Collateral Ligament
 The MCL can be divided into a superficial portion
and a deep portion that are separated by a bursa.
 The superficial portion of the MCL arises proximally
from the medial femoral epicondyle and travels
distally to insert into the medial aspect of the
proximal tibia distal to the pes anserinus (Fig. 11-15).

8.  The deep portion of the MCL is continuous with the
joint capsule, originates from the inferior aspect of
the medial femoral condyle, and inserts on the
proximal aspect of the medial tibial plateau.
 Throughout its course of travel, the deep portion of
the MCL is rigidly affixed to the medial border of the
medial meniscus (see Fig. 11-10).

9.  The MCL, specifically the superficial portion, is the
primary restraint to excessive abduction (valgus) and
lateral rotation stresses at the knee.
 The knee joint is best able to resist a valgus stress at
full extension because the MCL is taut in this
position.
 As joint flexion is increased, the MCL becomes more
lax and greater joint space opening is allowed
(medially gapping).47

10.  With the knee flexed, the MCL plays a more critical
role in resisting valgus stress despite the permitted
joint gapping.
 Grood et al. determined that at close to full
extension, the MCL accounted for 57% of the
restraining force against valgus opening, but at 25°
of knee flexion, the MCL accounted for 78% of the
load.

11.  This difference is likely due to the greater bony
congruence and inclusion of other soft tissue
structures (e.g., posteromedial capsule, ACL) that at
full extension can more effectively assist with
checking a valgus stress.
 The MCL also plays a supportive role in resisting
anterior translation of the tibia on the femur in the
absence of the primary restraints against anterior
tibial translation.

12.  The MCL has the capacity to heal when ruptured or
damaged, because of its rich blood supply.
 An isolated injury, therefore, does not often
necessitate surgical stabilization but is often left to
heal on its own, although this remodeling process
can take up to a year.

13. Lateral Collateral Ligament
 The lateral collateral ligament (LCL) is located on
the lateral side of the tibiofemoral joint,
beginning proximally from the lateral femoral
condyle.
 The LCL then travels distally to the fibular head
(Fig. 11-16), where it joins with the tendon of the
biceps femoris muscle to form the conjoined
tendon.

15.  Unlike the MCL, the LCL is not a thickening of the
capsule but is separate throughout much of its length
and is thereby considered to be an extracapsular
ligament.
 The LCL is primarily responsible for checking varus
stresses, and like the MCL, limits varus motion most
successfully at full extension.

16.  Grood et al. reported that at 5° of knee flexion, the
LCL accounted for 55% of the restraining force
against varus stress.
 This capacity increased to 69% with the knee flexed
to 25°.
 Although the LCL’s primary role is to resist varus
stresses, its orientation enables the LCL to limit
excessive lateral rotation of the tibia as well.

17. Anterior Cruciate Ligament
 The relatively high rate of injury of the ACL by
athletes and other active individuals has resulted in
the ACL’s being one of the most highly researched
ligaments in the human body.
 The ACL is attached to the anterior tibial spine (see
Fig. 11-9), where it extends superiorly and
posteriorly to attach to the posteromedial aspect of
the lateral femoral condyle (Fig. 11-17).

19.  The ACL courses posteriorly, laterally, and superiorly
from tibia to femur.
 In addition, the ACL twists inwardly (medially) as it
travels proximally.
 The ACL may also be considered to consist of two
separate bands that wrap around each other.

20.  Each of these bands is thought to have a different
role in controlling tibiofemoral motion.
 The anteromedial band (AMB) and the
posterolateral band (PLB) are each named for their
origins on the tibia.
 The major blood supply to the ACL arises primarily
from the middle genicular artery.

21.  The ACL functions as the primary restraint against
anterior translation (anterior shear) of the tibia on
the femur.
 This role, however, belongs to either the AMB or the
PLB, depending on the knee flexion angle.
 With the knee in full extension, the PLB is taut; as
knee flexion increases, the PLB loosens and the AMB
becomes tight, as demonstrated by the data plotted
in Figure 11-18.

23.  This shift in tension between the bands allows some
portion of the ACL to remain tight at all times.
 In the intact joint, forces producing an anterior
translation of the tibia will result in maximal
excursion of the tibia at about 30° of flexion when
neither of the ACL bands are particularly tensed.
 The ACL is also responsible for resisting
hyperextension of the knee.

24.  There appears to be essentially no anterior
translation of the tibia possible in full extension
when many of the supporting passive structures of
the knee are taut (including the PLB of the ACL).

25.  In addition to its primary restraint against anterior
shear, the ACL can act as a secondary restraint
against either varus or valgus motions (adduction
and abduction rotations respectively) at the knee.
 With valgus loading, the lengths of both bands of the
ACL increase as knee flexion increases.
 After injury to the MCL, a valgus moment will
increase the strain on the ACL throughout the flexion
range.

26.  Although the ACL may not make an important
contribution to limiting medial rotation of the tibia,
medial rotation of the tibia on the femur increases
the strain on the AMB of the ACL, with the peak
strain occurring between 10° and 15°.
 This is most likely due to the orientation of the ACL,
inasmuch as it winds its way medially around the
PCL, becoming tighter with medial rotation.

27.  Regardless of the rotational effect on the ACL’s
loading pattern, injury to the ACL appears to occur
most commonly when the knee is slightly flexed and
the tibia is rotated in either direction in weight-
bearing.
 In flexion and medial rotation, the ACL is tensed as it
winds around the PCL. In flexion and lateral
rotation, the ACL is tensed as it is stretched over the
lateral femoral condyle.

28.  The muscles surrounding the knee joint are capable
of either inducing or minimizing strain in the ACL.
 With the tibiofemoral joint in nearly full extension, a
quadriceps muscle contraction is capable of genera-
ting an anterior shear force on the tibia, thereby
increasing stress on the ACL.

29.  Fleming et al. reported that the gastrocnemius
muscle similarly has the potential to translate the
tibia anteriorly and strain the ACL
because the proximal tendon of the gastrocnemius
wraps around the posterior tibia, effectively pushing
the tibia forward
when the muscle becomes tense through active
contraction or passive stretch.

30.  The hamstring muscles are capable of inducing a
posterior shear force on the tibia throughout the
range of knee flexion, becoming more effective in
this role at greater knee flexion angles.

31.  The hamstrings, therefore, have the potential to
relieve the ACL of some of the stress of checking
anterior shear of the tibia on the femur.
 With the foot on the ground, the soleus muscle may
also have the ability to posteriorly translate the tibia
and assist the ACL in restraining anterior tibial
translation (Fig. 11-19).

33.  Given the potential of individual muscles to either
increase or decrease loads on the ACL, it is not
surprising that co-contraction of multiple muscles
across the knee can influence the strain on the ACL.

34.  For example, co-contraction of the hamstrings and
quadriceps muscles will allow the hamstrings to
counter the anterior translatory effect of the
quadriceps and reduce the strain on the ACL.

35.  In contrast, activation of both the gastrocnemius and
the quadriceps muscles results in greater strain on
the ACL than either muscle alone would produce,
unless the hamstrings also co-contract to mitigate
the anterior translation imposed by the
gastrocnemius.

36.  Although muscular co-contraction will limit the
strain imposed on the ligaments of the knee, it comes
at a price.
 Co-contraction will reduce the anterior shear force
on the tibia, but it increases joint compressive loads.

37. Posterior Cruciate Ligament
 The PCL attaches distally to the posterior tibial spine
(see Fig. 11-9) and travels superiorly and somewhat
anteriorly to attach to the lateral aspect of the medial
femoral condyle (see Fig. 11-17).
 Like the ACL, the PCL is intracapsular but
extrasynovial.
 The PCL is a shorter and less oblique structure than
the ACL, with a cross-sectional area 120% to 150%
greater than that of the ACL.

38.  The PCL blends with the posterior capsule and
periosteum as it crosses to its tibial attachment.
 The PCL, again like the ACL, is typically divided into
an AMB and a PLB that are each named for their
tibial origins.
 When the knee is close to full extension, the larger
and stronger AMB is lax, whereas the PLB becomes
taut. At 80° to 90° of flexion, the AMB is maximally
taut and the PLB is relaxed.

39.  The PCL serves as the primary restraint to posterior
displacement, or posterior shear, of the tibia beneath
the femur.
 In the fully extended knee, the PCL will absorb 93%
of a posteriorly directed load applied to the tibia.
 This ability of the PCL to assume such a large load in
full extension restricts posterior displacement to very
minimal amounts.

40.  Unlike the ACL, which resists force better at full
extension, the PCL is more adept at restraining
motion with the knee flexed.
 Maximal posterior displacement of the tibia occurs at
75° to 90° of flexion, however, because with greater
knee flexion, the secondary restraints against
posterior translation become ineffective.
 Sectioning of the PCL, therefore, increases posterior
translation at all angles of knee flexion.

41.  Like the ACL, the PCL has a role in restraining varus and
valgus stresses at the knee and appears to play a role in
both restraining and producing rotation of the tibia.
 The orientation of the PCL may result in a concomitant
lateral rotation of the tibia when posterior translational
forces are applied to the tibia.
 The PCL resists tibial medial rotation at 90° but less so
in full extension.
 The PCL does not resist lateral rotation very well.

42.  In the absence of the PCL, muscles must be recruited to
actively stabilize against excessive posterior tibial
translation.
 The popliteus muscle shares the role of the PCL in
resisting posteriorly directed forces on the tibia and can
contribute to knee stability when the PCL is absent.
 In contrast, an isolated hamstring con-traction might
destabilize the knee joint in the absence of the PCL
because of its posterior shear on the tibia in the flexed
knee.

43.  Contraction of the gastrocnemius muscle also
significantly strains the PCL at flexion angles greater
than 40° ,
whereas quadriceps contraction reduces the strain in
the PCL at knee flexion angles between 20° and 60°.

44. Ligaments of the Posterior Capsule
 Several structures reinforce the “corners” of the posterior
knee joint capsule (Fig. 11-21).
 The posteromedial corner of the capsule is reinforced by
the semimembranosus muscle, by its tendinous expansion
called the oblique popliteal ligament, and by the stronger
and more superficial POL.
 The posterolateral corner of the capsule is reinforced by
the arcuate ligament, the LCL, and the popliteus muscle
and tendon.
 The arcuate ligament is a Y-shaped capsular thickening
found in nearly 70% of knees.
 (Attachments of these ligaments are given in Table 11-1.)

46.  Both the POL and the arcuate ligaments are taut in
full extension and assist in checking hyperextension
of the knee; the POL and arcuate ligaments also
check valgus and varus forces, respectively.
 The orientation of the lateral branch of the arcuate
ligament allows it to become tight in tibial lateral
rotation.

49. Iliotibial Band
 The IT band (or ITB) or IT tract is formed proximally
from the fascia investing the tensor fascia lata, the
glu-teus maximus, and the gluteus medius muscles.
 The IT band continues distally to attach to the lateral
inter-muscular septum and inserts into the
anterolateral tibia (Gerdy’s tubercle),
reinforcing the anterolateral aspect of the knee joint
(see Fig. 11-16).

50.  Despite the muscular attachments to the IT band, it
remains an essentially passive structure at the knee
joint; a contraction of the tensor fascia lata (TFL) or
the gluteus maximus muscles that attach to the IT
band proximally produce only minimal longitudinal
excursion of the band distally.
 The IT band moves anterior to the knee joint axis as
the knee is extended, and posteriorly over the lateral
femoral condyle as the knee is flexed (Fig. 11-22).

51.  The IT band, therefore, remains consistently taut,
regardless of the hip or knee’s position.
 The fibrous connections of the IT band to the biceps
femoris and vastus lateralis muscles form a sling
behind the lateral femoral condyle,
assisting the ACL in checking posterior femoral (or
anterior tibial) translation when the knee joint is
nearly full extension.

52.  With the knee in flexion, the combination of the IT
band, the LCL, and the popliteal tendon crossing
over each other increases the stability of the lateral
side of the joint and
even more effectively assists the ACL in resisting
anterior displacement of the tibia on the femur (see
Fig. 11-22).

54.  Despite its lateral location, the IT band alone
provides only minimal resistance to lateral joint
space opening.
 The IT band also attaches to the patella via the
lateral patellofemoral ligament of the lateral
retinaculum.
 As we shall see, this attachment of the IT band to the
lateral border of the patella may affect
patellofemoral function.

55. End of Part - 4