Load Distribution

For Medical Students

The Principle

When a joint has two parts side by side, the load divides between them according to where the force line falls.

A force line closer to one side loads that side more. The division is geometric before it is anything else.

Two people carrying a ladder is the comparison. Whoever stands closer to the heavy end takes more of it, however willing the other is.

At the Knee

The ground reaction force line passes medial to the centre of the knee for most of stance, so the medial compartment carries more. 3.4.2 covers the force line.

Conventional teaching gives the medial compartment around 60 per cent of tibiofemoral load in a normally aligned knee, with the lateral taking the remainder.

The medial compartment is built for it. Its tibial surface is concave, its cartilage is thicker centrally, and its meniscus is more firmly attached. 2.5.1 covers the anatomy.

The division is not constant through a step. It changes as the force line moves, and in parts of the cycle one compartment can be almost unloaded.

What Changes It

Alignment. A varus knee moves the force line further medial and raises the medial share. 3.5.6 covers it.

Walking speed and trunk position, both of which move the force line.

Meniscal integrity in either compartment, which changes how that side's share is carried.

Previous surgery that changed alignment, including an osteotomy intended to change this very quantity.

Clinical Relevance

Medial compartment osteoarthritis is commoner than lateral, and the load division is the mechanical reason. 6.73 covers the condition.

Changing the division is what an osteotomy does, which is why it is a mechanical operation with a mechanical target. 3.5.10 covers the effect.

A stick in the opposite hand reduces the medial share, which is a mechanical measure rather than a comfort one.

Lateral compartment disease is less common and often more difficult, partly because the lateral side depends more on its meniscus.

MRCS Level

The Principle

The division is a moment balance, not a force split. The external adduction moment has to be resisted, and it is resisted by a larger medial contact force together with tension in the lateral structures. 3.4.6 covers external moments.

That means the lateral soft tissues participate in the division. A lax lateral side cannot resist the moment, so the medial compartment takes more.

Instrumented implant work showed the division varies through the cycle more than the average suggests, with the lateral compartment close to unloaded at some instants.

At the Knee

Conventional teaching gives a medial share of roughly 60 to 70 per cent in a normally aligned knee under static load, and the figure rises with varus.

Peak medial contact force in walking is higher than the lateral by a comparable factor, and the two peaks occur at slightly different parts of stance.

The lateral compartment carries its share over a smaller area, because the lateral plateau is convex, so its stress is not proportionally lower than its load share suggests. 3.5.1 covers area.

Load division and contact stress division are therefore different quantities, and the second is the one cartilage experiences.

What Changes It

Mechanical axis alignment, which is the dominant term.

Lateral soft tissue integrity, since the moment balance depends on it.

Gait, through foot progression angle and trunk lean, which move the force line.

Meniscal status in either compartment, which changes stress rather than share.

How It Is Measured

Instrumented tibial implants measure compartment force division in vivo, in replaced knees, and are the only direct measurement.

Cadaveric rigs with sensors in both compartments measure it under an applied load.

Inverse dynamics gives the adduction moment, from which the division is inferred rather than measured.

Modelling scaled to an individual estimates both, with assumptions about soft tissue tension.

Clinical Relevance

Separate load share from contact stress, since the lateral compartment carries less load over less area.

Assess lateral soft tissue integrity when a medial overload picture is worse than the alignment explains.

Treat the adduction moment as the measurable proxy for the division.

Expect the division to vary through a step rather than holding at a quoted average.

FRCS Level

At the Knee

Treat the division as the target of unicompartmental and realignment surgery, and treat the published average as a starting point rather than a patient's number.

Overcorrection in osteotomy transfers an excessive share to the other compartment, which is predictable from the same arithmetic that justified the operation. 3.5.10 covers it.

Unicompartmental arthroplasty depends on the other compartment continuing to take its share, which is why its indication includes the state of that compartment and its ligaments.

What Changes It

Correction magnitude, which is the controllable variable and the one that can overshoot.

Ligament integrity, particularly the anterior cruciate, which affects how load is distributed in a knee being considered for unicompartmental replacement.

Component position and thickness in arthroplasty, which alter the division directly.

Progression in the unoperated compartment, which changes the division over time.

How It Is Measured

Long leg standing radiography measures the alignment that the division is inferred from, and is the clinical measurement.

Instrumented implant telemetry measures the division after replacement, in research series.

Stress radiography assesses whether the other compartment opens, which bears on whether it can take its share.

No clinical measurement of the division in an intact knee exists.

Clinical Relevance

Assess the opposite compartment and the cruciate before unicompartmental replacement, since the operation relies on them.

Plan correction to avoid transferring an excessive share, rather than to the largest achievable angle.

Use long leg alignment as the clinical proxy for the division.

Expect the division to change as the unoperated compartment progresses.