Joint Reaction Forces

For Medical Students

The Principle

The joint reaction force is the force the two bones press against each other with. It is what the cartilage between them has to carry.

It is the sum of the external load and the muscle forces holding the joint together. The muscle part is usually the larger.

A pair of scissors shows why. A small force at the handles produces a much larger force at the pivot, because of the distances involved.

At the Knee

Two joints have to be counted separately. The tibiofemoral joint and the patellofemoral joint carry different loads in the same activity.

Conventional teaching gives tibiofemoral force as roughly two to three times body weight in walking, and three to four in stair climbing.

Conventional teaching gives patellofemoral force as roughly half body weight in level walking, three to four times in stairs, and up to seven or eight times in a deep squat.

The patellofemoral figure rises steeply with flexion, because both the quadriceps force and the angle between the tendons increase together. 3.12 covers the joint.

What Changes It

Flexion angle, particularly for the patellofemoral joint.

Quadriceps force, which is the largest single contributor in most activities.

Body weight, which scales the external part of the sum.

Co-contraction, which adds compression without adding movement.

Clinical Relevance

Deep flexion under load is the highest patellofemoral demand in daily life, which is why squatting and kneeling are the activities patients with anterior knee pain avoid.

Stairs load both joints more than level walking, which is why stairs appear in almost every outcome score. 3.3.12 covers the kinematics.

Reducing quadriceps demand reduces joint force, which is the mechanical basis of modifying an activity rather than stopping it. Section 6 covers management.

A joint force figure is only meaningful with its activity and its joint named.

MRCS Level

The Principle

Separate force from stress. Joint reaction force is a load; stress is that load divided by the area carrying it, and cartilage responds to the second. 3.5 covers contact mechanics.

A joint can therefore raise its force and hold its stress steady by enlarging its contact area, which is what the patellofemoral joint does through flexion.

The force is computed rather than measured in an intact knee, from inverse dynamics plus an assumption about how muscle force is distributed.

At the Knee

Tibiofemoral force is divided between the compartments, and conventional teaching gives the medial compartment the larger share in normal alignment, commonly quoted as around 60 per cent.

That division is why medial compartment osteoarthritis is commoner than lateral, and why varus alignment accelerates it. 3.13 covers alignment.

Patellofemoral force is the resultant of the quadriceps and patellar tendon tensions, so it depends on the angle between them as well as on their magnitude.

The patella increases the quadriceps moment arm, which lowers the muscle force needed for a given extension moment and therefore lowers the tibiofemoral force. 3.4.4 covers it.

What Changes It

Alignment, which shifts the compartment division directly.

Patellar height and trochlear geometry, which change the patellofemoral contact and the lever arm.

Meniscal integrity, which changes the area the force is carried over rather than the force.

Previous patellectomy or extensor mechanism surgery, which changes the lever arm and therefore the muscle force required.

How It Is Measured

Instrumented tibial implants measure tibiofemoral force directly in vivo and are the only such measurement, in a small, elderly, replaced population.

Musculoskeletal modelling estimates the force in intact knees, and its figures depend on the muscle force distribution it assumes.

Cadaveric rigs with pressure sensors measure force and area together, without muscle activity and with an applied load the investigator chose.

Published ranges are therefore wide, and the wide range is the honest state of the evidence rather than a failure to agree.

Clinical Relevance

Name the joint and the activity with any joint force figure, and treat the range as wide.

Expect the medial compartment to carry more load in a normally aligned knee, and much more in a varus one.

Preserve meniscal tissue to preserve contact area, since the force will not fall.

Account for a changed extensor lever arm after patellar surgery when explaining persistent load related symptoms.

FRCS Level

At the Knee

Treat compartment load division as the quantity unicompartmental and osteotomy surgery manipulate, and treat the published division as a population statement rather than a patient's.

Overload of a lateral compartment after medial opening wedge osteotomy is the predicted consequence of overcorrection, and it is the kinetic account of a technical error.

Instrumented implant work reset several expectations, including that the lateral compartment can unload almost completely in parts of the gait cycle, which means the 60 to 40 division describes an average rather than an instant.

What Changes It

Correction magnitude in osteotomy, which moves load between compartments and can overshoot.

Component alignment and constraint in arthroplasty, which set how load is distributed and how much is transmitted to fixation.

Soft tissue balance, which determines whether one side carries the joint through part of the arc.

Patient mass and activity, which scale and multiply the cycles the construct sees.

How It Is Measured

Instrumented implant telemetry gives force and its distribution in vivo, in a handful of series worldwide.

Modelling scaled to an individual's anatomy estimates the same quantities with assumptions that the telemetry work is used to validate.

Retrieval and wear analysis shows where load actually fell, after the event.

Intraoperative pressure sensing measures balance in an anaesthetised knee, which is a different muscular state from the working one.

Clinical Relevance

Plan osteotomy correction to a target that avoids lateral overload rather than to the largest achievable correction.

Read the compartment load division as an average over a cycle, not a constant.

Treat intraoperative balance measurement as informative about an anaesthetised knee.

Expect activity and mass to multiply the loading cycles a construct must survive.