biomechanic in simple version

1. KNEE BIOMECHANICS
by K. SANJAIDAS, PHYSIOTHERAPIST
BIOMECHANICS OF KNEE, SANJAIDAS .K

2. Joint Positions
Resting position knee
25 degrees flexion
Close packed position knee
Full extension
Capsular pattern of the knee
Gross limitation of flexion (e.g. 90 degrees), mild limitation of
extension (e.g. 5-10 degrees)
ROM values
Flexion 150 degrees
Extension 0-5 degrees
BIOMECHANICS OF KNEE, SANJAIDAS .K

3. Arthrokinematics
• Flexion and extension are a combination of
rolling and gliding in the opposite direction
• Convex femur articulating with a concave tibia
BIOMECHANICS OF KNEE, SANJAIDAS .K

4. Flexion Arthrokinematics
• From full extension, the initial phase of flexion is
pure rolling.
• The degree of pure rolling varies per condyle
• The medial condyle rolls for 10-15 degrees, the
lateral condyle rolls for 20 degrees as it has a
larger joint surface
• The 15-20 degrees of rolling corresponds with the
normal range of flexion/extension in walking.
• The tibia glides posterior after the initial phase of
flexion
BIOMECHANICS OF KNEE, SANJAIDAS .K

5. Extension Arthrokinematics
• Tibia rolls and glides in anterior direction
BIOMECHANICS OF KNEE, SANJAIDAS .K

6. Rotation Arthrokinematics
• External rotation
Lateral femur condyle glides anterior, medial
condyle glides posterior
• Internal rotation
Lateral femur condyle glides posterior, medial
condyle glides anterior
BIOMECHANICS OF KNEE, SANJAIDAS .K

7. Function Cruciate Ligaments
With the knee flexed at 90 degrees:
• The PCL prevents backward sliding of the tibia on the femur
• The ACL prevents forward sliding of the tibia
In flexion
• The PCL takes on a more vertical orientation
• ACL becomes more horizontal
In extension
• Both cruciates are stretched
• PCL becomes less vertical
Some of the fibers of the cruciates are always in a stage of tension because:
• There is no sliding motion in any position
• The 2 ligaments do not change length when from flexion to extension,
when the condyles stay in contact with the tibialplateau
BIOMECHANICS OF KNEE, SANJAIDAS .K

8. Role of muscles assisting ligaments
• Iliotibial band assists LCL
• Sartorius, gracilis, semitendinosus assist
MCL
• Quadriceps influences stability medially
and laterally
• Straight and oblique fibers form a fibrous
canopy over the anterior aspect of the
joint. The straight fibers prevent opening
on the same side, the oblique fibers
prevent opening on the opposite side.
• Flexor muscles (biceps femoris,
gastrocnemius) play an active role in
limiting extension
BIOMECHANICS OF KNEE, SANJAIDAS .K

9. Rotatory stability of the knee
• Axial rotation can only occur when the knee is flexed.
• In full extension, rotation of the knee is prevented by
tension in the collaterals and the cruciates
• The cruciates are wound counter clockwise. Therefore if the
tibia is rotated medially, they are wound up more, and will
tighten. During lateral rotation of the tibia they unwind
• The collaterals are wound clockwise. If the tibia is rotated
laterally, the ligaments are wound up more and will resist
further rotation
• So, the collateral ligaments prevent lateral rotation, the
cruciate ligaments prevent medial rotation.
BIOMECHANICS OF KNEE, SANJAIDAS .K

10. The “screw home” mechanism
• Terminal extension is associated with a small
measure of external rotation.
• Occurs automatically in the absence of any
voluntary movement.
• Flexion is associated with an automatic
internal rotation of 20 degrees
• This motion is conjunct, it cannot be
performed independently
BIOMECHANICS OF KNEE, SANJAIDAS .K

11. The “screw home” mechanism
Reasons
• The lateral femur condyle is greater than the medial;
therefore it rolls over a greater distance.
• The lateral femur condyle glides more freely on the convex
tibial surface.
• During extension, the MCL is stretched more rapidly than
the LCL, which allows the lateral femur condyle to recede
farther.
• Tension of the cruciates at end of extension produces
external rotation
• Slight lateral pull of quadriceps
BIOMECHANICS OF KNEE, SANJAIDAS .K

12. Patella
• Sesamoid bone in the quadriceps tendon
• Function: increases efficiency of the
quadriceps by shifting the line of its muscular
pull anteriorly.
• Extensor efficiency of the quadriceps is
increased 1.5 times by the presence of the
patella.
BIOMECHANICS OF KNEE, SANJAIDAS .K

13. Patellar position
• Patella should face anterior and slightly lateral
• With a high patella (alta) you’ll see a double
bump when patient is sitting, one from the
high patella, and one from the infrapatellar fat
pad below that.
BIOMECHANICS OF KNEE, SANJAIDAS .K

14. Patellar mobility
• The superior/lateral force of
the quadriceps is turned into
a vertical force by the central
groove of the femoral
patellar surface
• The patella glides proximally
with knee extension
• It moves over a distance 2
times its length,
approximately 8 cm.
BIOMECHANICS OF KNEE, SANJAIDAS .K

15. Knee joint capsule and patella
The knee joint capsule forms 3 recesses in
relation to the patella:
• Superiorly the suprapatellar fold with
the suprapatellarbursa
• On either side, the
parapatellarrecesses
• These unfold in flexion and permit
excursion of patella under the
condyles. Inflammatory lesions can
cause adhesions in the recesses,
leading to restrictions in knee flexion
BIOMECHANICS OF KNEE, SANJAIDAS .K

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17. • The Q-angle is measured by extending a line
through the center of the patella to the
anterior superior iliac spine and another line
from the tibial tubercle through the center of
the patella. The intersection of these two lines
is the Q-angle; the normal value for this angle
is 13 to 18 degrees.
BIOMECHANICS OF KNEE, SANJAIDAS .K

18. Q Angle
• Angle between rectus femoris
and patellar tendon
• Normal: male 13 degrees;
female 18 degrees
• Angle>18 degrees associated
with PFA, patellar subluxation,
femoral anteversion,
genuvalgum, lateral
displacement tibial tuberosity
• Angle <13 degrees associated
with PFA
BIOMECHANICS OF KNEE, SANJAIDAS .K

19. BIOMECHANICS OF KNEE, SANJAIDAS .K

20. Menisci
• Make up for a lack of
congruence of the articular
surfaces by increasing the area
of contact
• Triangular in cross section
• Crescent shaped
• The lateral meniscus forms
almost a complete circle, the
medial meniscus is semi-lunar.
BIOMECHANICS OF KNEE, SANJAIDAS .K

21. Attachments of the menisci
CAPSULAR attached to the deep fibers of the capsule
BONY each horn is attached to the intercondylar
fossa
PATELLAR meniscopatellarfibers, from the lateral edges
of the patella to the lateral border of each
meniscus
MENISCAL anterior horns of the menisci are linked by
transverse ligament
MUSCULAR the semimembranosus has fibers to the
posterior medial meniscus; and the popliteus
sends fibers to the posterior edge of the
lateral meniscus
LIGAMENTS MCL fibers attach to the medial meniscus;
PCL fibers to posterior horn of lateral
meniscus; ACL fibers to anterior horn of
medial meniscus
BIOMECHANICS OF KNEE, SANJAIDAS .K

22. Meniscal Loading
• By increasing the contact surface, transmit
between 30-70% of the load applied across
the joint.
• Meniscal loads increase with increasing flexion
(85% at 90 degrees of flexion)
BIOMECHANICS OF KNEE, SANJAIDAS .K

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24. Menisci Role in Stability
• Minimal to none in the ACL intact knee
• Plays an important role in ACL deficient knee.
• So, what is the possible clinical implication
when an ACL injury occurs?
BIOMECHANICS OF KNEE, SANJAIDAS .K