Biomechanics In this study of motions, this essay will

 

Biomechanics is
the study of how the systems and structures of biological organisms respond to different
forces. In humans, it is the study of skeletal and muscular system working when
it is moving and explaining the forces which produce human’s motions. It plays
an important role to prevent injury. In this study of motions, this essay will
be focusing on kinetics and kinematics in joints. Kinematics describes motion
with no regards to what causes it, such as distance, angle, position and etc. Meanwhile
Kinetics analyze and examine forces and torques that cause motion or to ex,
such as sum of forces and moments. In brief, kinetics is used to describe the
relationship between the forces and the motion produced.

There are
hundreds of joints in human, an example of a joint that requires biomechanics
is knee joint. Knee joint is one of the largest and most complex joint in human’s
body The roles of biomechanics keeps the knees stable when forces transmitting
across it, allowing it to gait, flex and rotate. Knee joint comprises of two
distinctly separate joints which are Tibiofemoral (TF) joint and Patellofemoral
(PF) joint. It acts as a pivot between the longest bones in the human body
whilst the quadriceps muscles act across it. The main roles of knee joint are to
allow movement with minimum energy requirements from the muscles and stability,
and also to transmit, absorb and redistribute forces caused during the
activities of daily life.

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Knee
joint kinematics

The TF
joint has a wide range of motion, mainly in the rotation in the sagittal plane.
In short, it acts as a hinge (ginglymus) joint in the sagittal plane. Hinge
joints are called ginglymus,  ginglymus is a synovial joint that allows movement in only one
plane, through strong collateral ligaments which restrict movement to a planar
causing it to produce hingelike motion. The ligaments
provide static stability while muscles like quadriceps and hamstrings
contractions provide dynamic stability. The TF joint can be
described through six degrees of freedom (three rotations and three
translations) in a joint coordinate system. Degrees of freedom is defined as
the number of independent movements it has.

 

At 0? flexion, the long axes of
the tibia and femur are aligned in the sagittal plane, this is defined as full
extension. When at full extension, the knee allows for optimized weight support
and stability. If the body was at rest in some other angle of flexion, then the
vertical line of action of the ground reaction force would pass posterior to
the knee joint, the quadriceps muscles would need to do work to maintain
posture; this would be inefficient and the muscle
force would increase the load on the TF joint
and causing strain in the tissue. Thus, flexion contracture is disabling.
Similarly, hyperextension becomes stable only because
of large tensions in the posterior capsular
structures when the line of action of the body
weight passes anterior to the flexion extension
axis of the knee.

The most frequent knee movement occurs
during gait. Knee motion is dictated by energy considerations that require the
centre of gravity of the body to move forward with minimal other upper body
movement (up-down or medial-lateral), and by optimal capacity to absorb the
impact at heel strike. In order for the toes of the swinging leg not to be
dragged on the ground, the knee flexes up to approximately 70? during the swing phase
of gait. When the leg has swung past the other and just before heel strike the
quadriceps muscles contract to bring the knee to full extension and the foot forwards.
After heel strike the knee does not remain extended as this would mean upward
movement of the body and the leg acting as a rigid strut unable to absorb the
impact load; therefore, the knee joint flexes up to 15? in mid stance phase by stretching the quadriceps muscles, thus
absorbing energy. Flexion is not the only rotation that takes place during
gait. Prior to heel strike, as the knee extends from approximately 30 to 0? flexion, the tibia
externally rotates by up to 30?. This is known
as the ‘screw-home mechanism’ and is thought to occur
in order to tighten the soft tissue structures
and lock the knee geometry prior to
accommodating the impact load of weight bearing. This rotation is linked to corresponding rotations at the hip
and ankle.

 

Articular kinematics

The outline of the femoral
condyles in the sagittal plane is longer than the anterior-posterior dimension
of the tibial plateau. This means that if flexion was occurring purely by a
rolling motion, then the femur would roll off the tibial plateau well before
the knee reached full flexion. At full extension the femur has a large contact
area with the tibial plateaus and presses anteriorly on the meniscal horns. As
the knee flexes, contact moves posteriorly towards the posterior meniscal horns
and the contact area with the tibial plateaus is reduced as lesser radii of curvature
of the femoral condyles are sequentially coming into contact. Tibial rotation
occurs during flexion (the reverse of the screw-home); the medial tibial
plateau is slightly concave whereas the lateral tibial plateau is flat or
slightly convex in the sagittal plane; this means that the centre of contact in
the medial side remains relatively constant in terms of anterior-posterior position,
but the lateral condyle rolls posteriorly towards the posterior horn of the
mobile lateral meniscus until the meniscus starts resisting further translation
by stretching circumferentially. Thus, tibial rotation is essentially occurring
about a medial axis. From then on the femoral condyles have almost circular
sagittal sections and the anterior cruciate ligament (ACL) is in tension, resisting
further posterior translation of the femur relative to the tibia; this results in
the femur sliding anteriorly and rolling posteriorly at the same time. In deep
flexion contact occurs almost solely between the femoral condyles and the
posterior meniscal horns with very little cartilage-to-cartilage contact, especially
in the medial side where the meniscus is constrained from displacing further
posteriorly off the tibial posterior rim; further tibial internal rotation occurs
as well (up to approximately 15?), which causes the
posterior horn of the lateral meniscus to displace off the posterior rim of the
tibial plateau.

PF joint has a complex, three-dimensional range of motion
across TF joint flexion in order to allow for minimal quadriceps contraction to
extend the knee. This complex mechanism of knee joint motion means that the
geometry itself is not adequate to maintain stability, requiring input from
passive soft tissue (e.g. ligaments) and muscle tensions. It also means that large
forces acting on small articulating areas generate high articular stresses,
commonly called joint contact pressure. The PF
joint articulation occurs between the patella and the femoral trochlear groove.
The complex articular surface of the patella can be roughly split into lateral
(larger) and medial slightly concave facets of congruent shape to that of the
femoral trochlea when the knee is flexed; these facets are separated by a
convex ridge.

At full TF joint extension the PF joint
contact occurs at the distal end of the patella. As flexion increases the
patella engages into the femoral trochlear groove and the contact area spreads
across the width of the patella and moves proximally. In deep flexion contact
occurs only laterally and medially on
the patella; the lateral facet articulates on the distal aspect of the lateral
femoral condyle, and the ‘odd’ medial facet contacts against the medial femoral
condyle at the edge of the intercondylar notch. The increase of PF contact area
with knee flexion is a clever mechanism that controls the magnitude of stresses
by spreading the ever increasing PF joint load with knee flexion over a larger
area. Analysis of the patellar kinematics requires all six degrees of freedom
to be accounted for as the three dimensional movement is complex. The reader is
referred to textbooks for more detail, but the most frequent clinical problems relate to abnormal
medialelateral translations and rotations (patellar tilting) during knee
flexioneextension. These abnormalities can result from factors including a
shallow trochlear

groove, inadequate passive
soft tissue restraints, or abnormal balance of tensions between the components
of the quadriceps especially deficient vastus medialis obliquus tension.

 

Kinetics
in loading and articular mechanics

 

With the
force of gravitation, most of the organisms including human, constantly
experiences force to keep us standing on the ground. The loads that
the joint surfaces experience are a result of external forces. such as ground
reaction forces, and the muscle forces that are required to maintain posture
and facilitate body movement. Ligament forces are passive internal forces which
are developed in response to joint motion or external loading and provide
alternative pathways for load transmission through the joint.

 

In the PF joint, when the
knee is near extension, the lines of action of the patellar tendon (PT) and quadriceps muscle
(Q) are almost co-linear in the sagittal
plane, resulting in a small joint

force. However, as the knee
flexes, the angle between the lines of action of the PT and Q reduces,
resulting in an ever increasing PF joint force up to approximately 70? knee flexion,

where the PT tension is
approximately 70% that of Q. As the contact area between the
femur and the patella increases with knee flexion, therefore maintaining
physiological contact stresses (since stress = force/area). Beyond 70? knee flexion, the
quadriceps wraps around the trochlea, so this effect does not increase.

A daily activity that generates
large knee joint forces requires up to 100? of knee flexion to bring the body to the upright position and,
hence, entails maximal quadriceps tension. The line of action of the BW is
approximately 200 mm behind the knee. In order to maintain

equilibrium between the flexing
and extending moments, and assuming that half of BW is transferred through each
leg, the PT tension needs to reach approximately 3 BW, resulting in a TF joint
force of approximately 3.5 BW and a PF joint force of approximately 5.5 BW.
Thus, although the PF joint is not interposed in the weight-bearing structure
of the leg, it cannot be

taken to be less heavily-loaded than the
TF joint.

There is an adduction moment
in the TF joint in the frontal plane when walking. The normal leg has
approximately 6? valgus
angle between the anatomical axes of the femur and tibia in the frontal plane
in the extended knee, whereas the mechanical axis – which joins the centres of
the hip, knee and ankle e is approximately straight. Hip adduction is needed to
position the foot of the load-bearing leg under the BW in the stance phase of
gait. Thus, the line of action of the BW passes medial to the centre of the
medial femoral condyle, which means that all compressive joint force should be
transferred through the medial condyle. However, the quadriceps muscles act
also to extend the knee during stance, and their line of action passes through
the middle of the joint, therefore balancing the load between the two
compartments to

some extent. The line of
action of the TF joint force is located at the medial compartment and moves by
up to 40 mm in the medial-lateral sense during gait, with knee stability
maintained by tension in the ilio-tibial tract. Thus, the medial condyle force
is larger than the lateral, and this is reflected by the relative areas and
congruence of the two articulations.

 

Looking at the extended PF
joint in the frontal plane, there is a resultant lateral force acting on the
patella stemming from the lines of action of the PT and Q; this is often
referred to as the

Q-angle effect. The Q-angle
is defined as the angle between the line of action of the PT and the resultant
line of action of the quadriceps muscles; it is approximately 12-15? in males and 15-18? in females in the extended knee and reduces with flexion due to
reversal of the screw-home mechanism, which causes medialization of the tibial
PT attachment (the tibial tubercle). The Q-angle effect tends to sublux the
patella laterally.

 

This is resisted
geometrically by the depth of the trochlear groove, with the sulcus angle being
the single best predictor of symptoms of instability.7 Thus, trochleoplasty to
deepen the groove in a dysplastic knee is mechanically logical and increases objective
patellar stability significantly. The patella is most unstable at approximately
10-30? of knee
flexion, where it has not yet engaged into the trochlear groove and the Q-angle
effect albeit smaller than in full extension, is resisted solely by the soft tissues
(the retinacular structures).

 

 

 

 

 

Articular
mechanics

Looking closer into the mechanics of the
articulation, the forces discussed above are transferred to the articulating
surfaces and can be split into compressive and frictional/shear components.

The compressive force is distributed
over the contact area of the articulation to result in contact stresses within
the structures involved. The magnitude of these stresses depends on both the

magnitude of the force and the contact
area. The articulating geometry does not allow for full conformity between the compartments
in contact in TF or PF joints, and therefore is not ideally designed to
minimize contact stresses. In the PF joint this is accommodated for by the
thick articular cartilage (the thickest in the body) and an ever increasing
contact (conforming) area

with knee flexion up to approximately 90? flexion (as
aforementioned, the joint force increases with knee flexion). In the TF joint
this is accommodated for by the presence of the medial and

lateral menisci. These are crescent-shaped
fibrocartilages, wedge-shaped in cross section. Their tissue includes strong collagen
fibres running primarily in a circular manner around their periphery. They sit
on the rim of the tibial plateau and attached to tibia in order to increase
mobility in the articulation. The insertional ligaments act as anchors to the
tibial plateau, any loss of an insertional ligament would mean complete loss of
the load-bearing function of the meniscus. The mobility of the menisci allows
them to maximize the degree of conformity of the articulation across knee
flexion, as the menisci increasing the contact area, hence reducing the contact
stresses on the articular cartilage.

When knee motion entails high velocities
and light loads, a film of fluid is present between the surfaces, and that ensures
very low friction. When short-term impact loads are involved (as seen in heel
strike), the fluid is trapped between the surfaces as there is not enough time or
space for it to escape, thus generating a ”squeeze film” effect which
protects the surfaces from direct contact and assists in distributing and
absorbing the shock load. In the case whereby

the squeeze film effect is insufficient
the surfaces are protected at a molecular level by the large protein molecules
contained in the synovial fluid; this molecular layer acts as a boundary

lubricant.

 

 

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