Biomechanics Notes
07-09-98
KNEE
the second most commonly injured joint after the ankle
not just a hinge joint, it does some lateral rotation, translation, etc.. it allows a
great deal of flexion and not much extension (0-5 degrees)
Forces between the femur & patella (reference figure 6-21, page 130 in the text)
The largest, thickest cartilage in the body is on the back of the patella.
After Lumbar facets, the most common site of osteoarthritis is in the
patellofemoral joint.
There is a great deal of force put on the knee.
With the knee flexed 5 degrees, the patellofemoral joint force is 574 N.
With the knee flexed 90 degrees, the force increases to 985 N, almost a two fold
increase in force.
The more you bend the knee, the greater the moment arm. As the center of gravity goes
further and further away from the knee, the greater the force is needed from the quads.
In addition to this, at a larger angle of flexion, there is less surface area for the patella to
contact (due to the angle of the femur) and this causes an increase in stress, since stress is force
per area. The stress continues to increase as the area decreases.
In order to avoid this problem, weight lifters will wrap their knees to keep the patella from
flipping and thereby decreasing the stress in this joint. This, however, compromises the tendons
which can more easily tear in this position.
Carpet layers have problems with their knee as well due to the working a great deal with
their knee flexed greater than 90 degrees. Similarly, it is better to kneel than to squat while
gardening to decrease the load on this joint.
Factoid – Flabella: a sesamoid bone in one of the tendons in the back of the knee!
Three things that hurt the knee joint:
1. Decrease in surface area
2. Increased moment arm
3. Increased force on the patella
Patella on X-Ray (sunrise view)
This view shows the joint space
There should be a short facet and a long facet of the patella. If there is a shallow
intercondylar notch in the femur, there is a greater chance for dislocation. In regard to anatomy,
the angle of the patella (the angle at which the two planes along the posterior patella intersect) are
different for different people.
Patella function:
1. Increase moment arm to increase quadriceps efficiency
2. To replace the quad tendon with a material that can better withstand the repeated
rubbing along the femur.
If you remove the patella, this really limits activities.
Menisci of the Knee
Function:
to spread out the surface area so that the load is essentially cut in half at
any given point
to help in nutrition of cartilagenous surfaces
to provide a guide for motion of the femur and the tibia
If you remove the meniscus:
Increased stress at the joint
Decreased nutrition
Decreased alignment
(these all, in turn, lead to osteoarthritis)
Doctors used to take the meniscus out if there was a problem. They thought that the menisci were
unnecessary artifacts. HELLO! We now know its importance.
Surgery factoid: the outside of the meniscus has some blood supply so if the meniscus is cut there,
it bleeds. It has no blood supply in the middle, however, and if it is cut there, there is no bleeding!
There is a new theory that chondroitin sulfate supplements help to decrease hyaline cartilage
degeneration.
Most meniscal injuries are medial. The meniscus doesn’t heal well because it is a lot like a disc – not
many blood vessels or nerves.
ACL/PCL
Tests for ACL/PCL – there are a lot of orthopedic tests for this:
Sag Sign: tests for a PCL tear but since the examiner can bring the tibia forward a
great deal, this sign can be mistaken for a torn ACL.
Ask: what were you doing at the time of your injury? Often, the injury was caused
by twisting (for example planting cleats in the ground and twisting the knee
in soccer). This is mostly meniscal. ACL/PCL injuries are mostly stop and go injuries.
The PCL is a lot bigger and stronger than the ACL.
Anterior Drawer sign: the movement should be less than 7 mm. (Grade I sprain is
supposedly about 2-7 mm of movement. If the movement is greater than 7
mm, there has been a tear.) Make sure that the patient tells you before you
do this that the knee hurts. Don’t increase the injury!! And make sure that
you’re not starting from the sag position.
When testing knees, do the healthy knee first for two reasons:
1. for a good reference for comparison
2. so that the patient will realize that the pain they have in their affected knee is
abnormal
Lachman’s test: 35 degree knee bend, pull anterior and medial. For ACL injury.
Valgus/Varus – for medial and lateral collateral ligaments.
Meniscus tests:
Mc Murray’s – two part test, know which part tests for which part of the meniscus.
(click or pop at 30 degrees).
Apley’s Compression/Distraction
Look for pain but meniscal tears usually are not very painful
This test isn’t used much except for boards.
Table 6-1, Page 117 in the text. Descending the stairs requires more flexion (90 degrees) so this is
therefore harder on the quads than ascending stairs where flexion is only 83 degrees. In walking
down the stairs, we lean back to avoid falling which has our center of gravity further away from the
knees, which also increases stress upon the quads.
Women Vs. Men:
Women:
2.6 times more ACL tears in soccer
6 times more ACL tears in basketball
Stress on joints – due to muscles that cross the joints. Before “toe-off” phase of walking, there is
the most force upon the knee. (reference Figure 6-17, page 127 in the text)