Painful Arc

For Medical Students

What It Tests

A painful arc is pain occurring through part of the range of movement, with the movement before and after it comparatively comfortable.

It localises differently from tenderness. Tenderness says where a structure is; a painful arc says at which joint position a surface is loaded. 4.6.1 covers palpation.

The position of the arc is the finding. Anterior pain in mid flexion and pain at the extremes of the range point at different parts of the joint. 2.4.3 covers patellar biomechanics.

How It Is Performed

Ask the patient to move the knee actively through its range and to say when it hurts rather than whether it hurts. 4.7.1 covers active motion.

Note the angles at which the pain starts and stops, and record them. 4.1.8 covers the record.

Repeat passively, since pain in the same arc passively suggests a joint surface and pain only actively suggests the extensor mechanism or a muscle. 4.7.2 covers passive motion.

Add compression if the arc is anterior, by pressing the patella against the femur while the knee moves, which increases patellofemoral load specifically. 4.6.5 covers patellar tenderness.

Ask the patient which activity reproduces the same pain, because the activity usually names the arc better than the examination does. 4.2.11 covers aggravating factors.

What a Positive Means

Anterior pain in mid flexion, worse with compression, indicates a patellofemoral problem, since that is the arc in which the patella is most loaded. 2.4.4 covers patellar tracking.

Pain at the extreme of flexion indicates the posterior horn of a meniscus or deep flexion patellofemoral loading, and the location of the pain separates them. 4.6.4 covers joint line tenderness.

Pain in terminal extension indicates an anterior structure or an impinging lesion, which in a reconstructed knee is a specific diagnosis. 4.7.10 covers post-operative range.

Pain throughout the range rather than in an arc suggests inflammation or an effusion rather than a mechanical problem. 4.8.2 covers it.

An arc that is painful actively and not passively indicates the muscles or the extensor mechanism rather than the joint surface. 4.6.6 covers tendon tenderness.

MRCS Level

How It Is Performed

In a viva the useful answer asks the patient when it hurts rather than whether, and records the angles.

The expected points are the active sweep with the angles noted, the passive repeat, the compression for an anterior arc, and the activity that reproduces it.

Say that you would ask about stairs specifically, since descending stairs loads the patellofemoral joint in the mid flexion arc and is the activity most patients name. 4.2.11 covers aggravating factors.

Say that you would distinguish a painful arc from a painful limit, because the two mean different things and are recorded differently. 4.7.5 covers the limit.

Say that you would assess the tracking in sitting, since a maltracking patella produces an arc and the tracking is visible rather than inferred. 4.3.4 covers it.

What a Positive Means

The reasoning expected is that different parts of the joint surface contact each other at different flexion angles, so a lesion is loaded in a particular arc and painless outside it. 2.4.3 covers patellar biomechanics.

The patella engages the trochlea in early flexion and the contact area moves proximally on the patella as flexion increases, which is why the arc's position suggests which part of the patella is involved. 2.4.4 covers tracking.

Deep flexion loads the posterior horns of the menisci and the superior part of the patella, which is why pain at the extreme of flexion has two common sources. 2.5.7 covers root attachments.

Terminal extension loads the anterior structures and brings the intercondylar notch's contents against the roof, which is the mechanism behind an impinging lesion after reconstruction. 4.7.10 covers it.

How Good It Is

A painful arc is more specific than diffuse pain and less reproducible than tenderness, which places it between the two in usefulness.

Its reproducibility depends on the patient reporting consistently, so it is better in an articulate patient and poor in an acutely painful knee.

It is the most useful examination finding for patellofemoral pain, which is otherwise a diagnosis made largely by exclusion. 4.6.5 covers patellar tenderness.

It cannot grade the underlying lesion, which imaging does, and the arc's position is more useful than its severity. 4.1.6 covers that relationship.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the arc's position contributes to localising a surface lesion and to separating patellofemoral from tibiofemoral pain, which changes the operation.

A mid flexion anterior arc with compression pain indicates a patellofemoral problem, and a tibiofemoral operation does not address it. 4.6.5 covers the palpation.

Pain in terminal extension in a reconstructed knee indicates an impinging lesion, which has a reliable arthroscopic answer. 4.7.10 covers it.

Pain at the extreme of flexion in a knee with a meniscal tear contributes to whether the tear is symptomatic, which is the central question in deciding about arthroscopy. 4.9.11 covers meniscal examination in osteoarthritis.

Pain throughout the range moves the assessment away from a mechanical diagnosis and towards inflammation, which changes the specialty rather than the operation. 4.8.9 covers synovitis.

Where It Misleads

The commonest error is recording pain on movement without the arc, which loses the localisation.

The second is not repeating the movement passively, which leaves the extensor mechanism and the surface unseparated.

The third is attributing an anterior arc to the patella without assessing the tracking and the rotational alignment. 4.5.6 covers rotational alignment.

The fourth is reading pain throughout the range as a severe mechanical problem.

The fifth is relying on the finding in an acutely painful knee, where everything hurts.

Fellowship Level

Where It Misleads

In referred practice the arc is often the finding that explains why an arthroscopy did not help.

A patient with an anterior mid flexion arc who had a meniscal resection has had a tibiofemoral operation for a patellofemoral problem, and the arc was the clue. 4.6.5 covers the palpation.

A patient with a terminal extension arc after a reconstruction has an impinging lesion, and the referral is frequently late because the arc was recorded as pain on movement. 4.7.10 covers the range.

A patient with a deep flexion arc and a previous meniscal repair may have a healed repair that is loaded in deep flexion, which is expected for a period and has a timeline rather than a treatment. 4.9.2 covers joint line tenderness.

A patient whose arc has moved between visits has a changing lesion, and in a degenerative knee the arc widening is part of the progression. 4.9.11 covers it.

In medicolegal work a recorded arc is more informative than a recorded pain score, because it is a localising observation rather than a magnitude, and that is worth saying. 4.23.7 covers that setting.