Force Transmission

For Medical Students

The Principle

A force entering a joint has to leave through something. The load path is the sequence of structures it passes through.

Each structure in the path carries a share. If one stops carrying its share, the others take it, and none of them volunteered.

A bridge is the useful comparison. Removing one support does not reduce the traffic; it redistributes the weight onto the remaining supports.

At the Knee

Load passes from the femur, through the cartilage and the menisci, into the tibial cartilage, then into the subchondral bone and the shaft.

The menisci carry a substantial share by spreading the load over a larger area. Conventional teaching gives them roughly half the load transmission in the extended knee.

The compartments do not share equally. Conventional teaching gives the medial compartment around 60 per cent in a normally aligned knee.

The patellofemoral joint is a separate path, carrying the extensor mechanism's force across the trochlea. 3.12 covers it.

What Changes It

Meniscal integrity. A meniscectomised compartment carries its load over a smaller area.

Alignment, which shifts the share between compartments.

Cartilage thickness and quality, which change how load is spread at the surface.

Subchondral bone stiffness, which changes how the deeper part of the path behaves.

Clinical Relevance

Meniscectomy raises the load per unit area in that compartment, which is the mechanical account of why it leads to degeneration. 3.6 covers meniscal biomechanics.

Repairing or preserving a meniscus preserves a load path rather than only a piece of tissue.

Osteoarthritis in one compartment of a malaligned knee is a load path story before it is a biological one.

Describing a treatment as protective means saying which part of the path it protects.

MRCS Level

The Principle

Distinguish load from stress at every step. The load through a compartment may be unchanged while the stress on its cartilage doubles, because the area carrying it halved. 3.5 covers contact mechanics.

The path is also a stiffness problem. Load follows the stiffer route, so a stiffened subchondral plate changes how the overlying cartilage is loaded.

Hoop tension is the mechanism by which the menisci participate. Compression of the wedge becomes circumferential tension carried into bone by the roots. 3.6 covers it.

At the Knee

Conventional teaching holds that total or near total meniscectomy reduces contact area substantially and raises peak contact stress by a comparable factor, which is the quantitative form of the clinical observation.

A root tear behaves mechanically like a meniscectomy, because an unanchored ring carries no hoop tension. 2.5.7 and 2.6.4 cover the roots.

Extrusion is a functional finding on the same logic: a meniscus outside the joint is not in the load path.

Subchondral sclerosis and bone marrow lesions sit in the deeper part of the path and are a marker of where load has been concentrated.

What Changes It

The amount and location of meniscus lost, and whether the ring is still anchored.

Alignment, which sets the compartment share before any of this applies.

Cartilage integrity, which changes the surface's ability to spread load.

Previous osteotomy or arthroplasty, which redirect the path deliberately.

How It Is Measured

Cadaveric rigs with pressure sensitive film or sensors measure area and peak stress together, which is where the meniscectomy figures come from.

Instrumented implants measure compartment load division in vivo, in replaced knees.

Finite element modelling estimates stress distribution in intact knees and depends on its material assumptions.

Imaging shows the structures and not the load, so the path is inferred rather than measured in a patient. 5.6 covers meniscal imaging.

Clinical Relevance

Preserve meniscal tissue and repair roots where feasible, on load path grounds.

Read extrusion as a functional loss rather than an incidental measurement.

Expect subchondral change to mark where load concentrated, and look for the cause.

Address alignment before or with any procedure intended to protect a compartment.

FRCS Level

At the Knee

Treat every reconstructive decision as a load path decision. Meniscal transplantation, cartilage repair and osteotomy each restore or redirect part of the path, and combining them is standard because the path has several parts.

The order matters. Conventional teaching holds that correcting alignment before or with a cartilage or meniscal procedure protects it, because the uncorrected limb reloads the same spot. 3.13 covers alignment.

Subchondral bone is the part of the path most often ignored and is increasingly treated as a target, which is a mechanical argument about where load concentrates rather than a new biology.

What Changes It

Alignment correction, which changes the compartment share directly.

Meniscal substitution, which restores area rather than tissue.

Cartilage procedure type, which restores a surface of differing stiffness from the original.

Bone quality and prior marrow lesions, which change the deeper path's behaviour.

How It Is Measured

Cadaveric and modelling work quantifies the effect of each intervention on contact area and stress.

Weight bearing imaging measures joint space and extrusion, which are structural proxies.

Second look arthroscopy and retrieval show the result where load actually fell.

Long term cohort and registry data measure the clinical consequence, over the timescale the mechanics predict.

Clinical Relevance

Correct alignment before or with a cartilage or meniscal procedure.

Plan reconstruction as a combination when the path has more than one failed part.

Consider subchondral bone when a surface procedure has failed without a technical cause.

Describe each procedure by the part of the load path it restores.