Femoral Motion on Tibia

For Medical Students

The Principle

Relative motion is the same whichever bone is called fixed, but the description changes completely. Naming the fixed segment is therefore the first step.

Closed chain motion is motion with the distal segment fixed, which at the knee means the foot planted and the femur moving.

Think of a ladder against a wall. Moving the top and moving the base produce the same change in angle and entirely different tasks.

At the Knee

Standing, squatting, landing, stair climbing and the stance phase of walking are all closed chain, so this is the knee's usual working arrangement.

As the knee flexes in closed chain, the femoral condyles roll posteriorly on a fixed tibia and the femur descends.

The pelvis and trunk move with the femur, which is why hip and trunk position change the knee's path in a way they cannot in open chain.

Axial rotation in closed chain appears as the femur rotating on the tibia, and relative internal rotation of the tibia is the same motion as relative external rotation of the femur.

What Changes It

Foot position, since a planted foot transmits rotation from the ground up the limb.

Hip and trunk position, which move the line of the body weight relative to the knee.

Ankle range, particularly dorsiflexion, which limits how far the tibia can incline and therefore how a squat is achieved.

Muscle activity across two joints, because the hamstrings and the gastrocnemius cross the knee and one other joint each.

Clinical Relevance

Watch the whole limb when a knee moves in closed chain, since the knee's path is partly set at the hip and the ankle.

Expect a patient to compensate at the hip or the ankle when the knee cannot do its share.

Rehabilitation that tests a knee in closed chain tests it in the arrangement it normally works in. Section 6 covers management.

A description of motion has to say which segment was fixed, or the numbers cannot be compared.

MRCS Level

The Principle

Treat the limb as a linked chain rather than as a joint. In closed chain the segments cannot move independently, so a change at one joint obliges a change at another.

That coupling is the practical difference between the two chains. Open chain isolates, closed chain distributes.

Weight bearing is not the same thing as closed chain, although the two usually coincide. The defining feature is the fixed distal segment.

At the Knee

Posterior femoral rollback in closed chain is the mechanism that lets the knee reach deep flexion without the femur leaving the plateau. 3.3.2 covers the roll and glide relationship.

The quadriceps works against body weight through a lever arm that changes with flexion, which is a kinetic consequence of a kinematic fact. 3.4 covers the moments.

Rotation in closed chain is driven partly from the ground. A planted foot and a turning body rotate the femur on the tibia, which is the arrangement most pivoting injuries occur in.

Patellofemoral contact migrates proximally on the patella as the knee flexes in closed chain, which 3.12 covers.

What Changes It

Hip and ankle range, which set how the chain can be arranged.

Trunk inclination, which moves the ground reaction force line relative to the knee.

Foot and tibial rotation, which in closed chain reach the knee rather than dissipating.

Previous surgery at an adjacent joint, since a stiff hip or ankle redistributes work to the knee.

How It Is Measured

Optical capture with a force plate measures closed chain tasks and is the standard laboratory arrangement for them.

Fluoroscopy during a weight bearing squat or step gives bone level kinematics, over the range the equipment can follow.

Weight bearing magnetic resonance and computed tomography give loaded static positions at chosen angles, which is a series of snapshots rather than a path.

Non weight bearing laboratory data describes open chain and should not be read across, which is a frequent error in applying kinematic figures.

Clinical Relevance

Assess the hip and the ankle when a closed chain knee task is abnormal, because the chain distributes the problem.

Expect ground derived rotation to reach the knee in a planted foot, which is the mechanical setting for pivoting injury.

Use a closed chain measurement when the question is about function and an open chain one when the question is about the knee alone.

State the chain whenever a range or a translation is recorded.

FRCS Level

At the Knee

Treat closed chain kinematics as the condition most surgical decisions are really about, since patients present with symptoms in standing tasks rather than in seated ones.

The ground reaction force line relative to the knee is the quantity alignment surgery changes, and it only exists in closed chain. 3.13 covers alignment.

A chain has more than one solution. A patient with a deficient knee can achieve the same task with a different segment arrangement, which is why function and laxity dissociate.

What Changes It

Coronal and sagittal alignment, which set where the force line falls through the arc.

Adjacent joint pathology, which constrains the available solutions and loads the knee differently.

Neuromuscular control, which selects among the available solutions and is modifiable.

Previous meniscal or cartilage surgery, which changes what the chosen solution costs the joint surface.

How It Is Measured

Instrumented gait and task analysis with a force plate is the reference for closed chain work, and it measures a laboratory version of the activity.

Biplanar fluoroscopy during a step or a lunge gives bone level detail for a short range of a single task.

Long leg standing radiography measures the static alignment the dynamic measurement is usually inferred from, and the two can disagree.

Compensation is visible in a kinematic record and is not separable from capacity by measurement alone, which is where examination and history still decide.

Clinical Relevance

Measure alignment in a standing long leg film before attributing a closed chain symptom to the soft tissues alone.

Expect compensation to mask a deficit in a kinematic record, and read the record alongside the examination.

Consider adjacent joint pathology as a cause of knee overload rather than only as a comorbidity.

Set rehabilitation goals in the chain the patient's symptoms occur in.