Tibial Motion on Femur

For Medical Students

The Principle

Open chain motion is motion with the distal segment free, which at the knee means the femur held and the tibia moving.

The relative motion is the same as in closed chain. What differs is which bone travels through space and therefore what has to be controlled.

A gate on a hinge and the wall swinging past a fixed gate would look identical from the hinge. Only one of them is how a gate works.

At the Knee

Sitting and straightening the knee, the swing phase of walking, and kicking are open chain, so the arrangement is real rather than artificial.

As the knee flexes in open chain the tibia swings posteriorly and the femur stays put, with the contact point moving posteriorly on the tibia.

Axial rotation appears as the tibia rotating under the femur, which is the direction almost every clinical test and every published figure is written in.

The patella travels with the tibia through the extensor mechanism rather than with the femur, so its path through the trochlea is set by the tibia's position. 3.12 covers it.

What Changes It

Gravity, which assists flexion when sitting and resists extension, so the same movement demands different muscle work depending on body position.

Hip position, because the hamstrings and the rectus femoris cross the hip as well as the knee and their length changes with it.

Speed, since a fast open chain movement generates momentum the joint has to absorb at the end of range.

Whether a load is attached at the ankle, which multiplies the demand through a long lever arm.

Clinical Relevance

Examination of the knee is almost all open chain, which is why examination findings and function can differ. Section 4 covers the tests.

A patient who can straighten the knee against gravity when seated has shown something about the extensor mechanism and little about standing function.

Open chain measurement isolates the knee, which is useful when the knee is the question and misleading when the task is the question.

Published ranges of motion are usually open chain, so comparing them with a standing measurement is comparing two different things.

MRCS Level

The Principle

Open chain isolates one joint, which is its value and its limit. A single joint measurement cannot describe a task that distributes work across three.

The convention for reporting knee kinematics describes tibial motion relative to the femur, so open chain is the native language of the literature even for closed chain tasks.

Reading a closed chain finding into open chain terms is legitimate and has to be done deliberately, because the sign of the rotation flips.

At the Knee

Conventional teaching gives roughly 25 to 30 degrees of total axial rotation at 90 degrees of flexion in open chain, falling to near zero in full extension.

The screw home relationship appears in open chain as external rotation of the tibia through terminal extension. 3.3.5 covers it.

Anterior and posterior translation is measured in open chain by every clinical and instrumented test, which is why those figures are not weight bearing figures.

The extensor mechanism works at its worst lever arm in terminal extension in open chain, which is a kinetic fact with a kinematic cause.

What Changes It

Hip position, through the two joint muscles, which changes the available range and the force the knee can produce at a given angle.

Guarding, which reduces measured laxity and is the reason examination under anaesthetic finds more.

Effusion, which alters resting position and the comfortable range without any structural change.

Graft or implant constraint, which shows up more clearly in open chain because nothing else is sharing the work.

How It Is Measured

Instrumented arthrometry measures open chain anteroposterior translation and is the only clinical instrument in common use.

Goniometry and inertial sensors measure open chain flexion range adequately and rotation poorly.

Isokinetic dynamometry measures open chain strength through range, which is a kinetic measurement in a kinematic arrangement.

Cadaveric and robotic testing is open chain by construction, since no specimen bears weight, and that is a standing limitation on the apportionment literature.

Clinical Relevance

Record the hip position when measuring open chain range or strength, since the two joint muscles change the answer.

Expect guarding to reduce the measured laxity, and plan around the larger figure from examination under anaesthetic.

Treat an effusion as a cause of an altered range before attributing the range to a structure.

Keep open chain and closed chain figures in separate columns rather than combining them into one range.

FRCS Level

At the Knee

Treat the open chain measurement as a structural question and the closed chain one as a functional question, then expect them to disagree in the patients who are difficult.

A graft can be tight in open chain and inadequate in closed chain, because compression and muscle change the demand. 3.11 covers the constructs.

Open chain rehabilitation after cruciate reconstruction loads the graft at a different part of the range from closed chain work, which is a kinematic argument and not a preference.

What Changes It

Graft position and tension, which determine the angle at which the construct is tightest in open chain.

Implant constraint, which is most apparent in open chain and most consequential in closed.

Extensor mechanism integrity and patellar height, which change the open chain lever arm through the arc.

Time after surgery, since the measured open chain limit changes as the graft remodels.

How It Is Measured

Arthrometry gives a reproducible open chain number and has a reported limit of agreement that matters when a side to side difference is small.

Examination under anaesthetic removes the muscular contribution and is the cleanest open chain structural measurement available clinically.

Robotic testing answers which structure carries what share, in open chain, in a specimen, and with a sectioning order that changes the answer. 3.2.9 covers that limitation.

No open chain measurement predicts a closed chain symptom reliably, which is the honest position to hold when counselling a patient.

Clinical Relevance

Measure under anaesthetic when the structural question decides the operation.

Expect a tight open chain measurement to coexist with closed chain symptoms, and counsel accordingly.

Stage rehabilitation by the range at which each chain loads the graft rather than by time alone.

Avoid treating a small arthrometer difference as a finding without the instrument's limit of agreement in mind.