Direct Blow

For Medical Students

What It Tests

A direct blow to the front of the knee, usually a fall onto it or an impact against an object, loads the structures at the front rather than the ligaments at the sides.

Asking about it tests whether the patella, the extensor mechanism, the articular surfaces or the bone were injured, which is a different differential from the ligamentous one.

The commonest serious consequence is an injury the examination can miss, which is why the mechanism is asked about specifically.

How It Is Performed

Ask what the knee hit and from what height, since energy matters more than the fact of the blow.

Ask whether the knee was bent or straight at the time, because a flexed knee presents the patella and an extended one presents the condyles.

Ask whether they can straighten the knee against gravity and whether they could stand up afterwards, which screens the extensor mechanism.

Ask about immediate swelling, bruising and whether the skin was broken.

Ask whether the knee has been injected or operated on recently, because an open or penetrating injury over a prosthetic or recently operated knee is urgent.

What a Positive Means

Inability to straighten the knee against gravity after a direct blow is an extensor mechanism injury until proven otherwise. 4.16.1 and 4.16.2 cover the tendons.

Immediate haemarthrosis after a direct blow raises an osteochondral fracture. 4.17.3 covers osteochondral lesions.

Localised tenderness over the patella with difficulty weight bearing raises a patellar fracture.

A blow to the front of a flexed knee drives the tibia backwards and loads the posterior cruciate, which is the dashboard mechanism. 4.2.21 covers it.

Prepatellar swelling without an effusion is bursal rather than intra-articular, and the treatment differs completely. 4.3.8 covers swelling.

MRCS Level

What a Positive Means

The reasoning expected is that the knee's flexion angle at impact determines what was exposed.

A flexed knee presents the patella and the trochlea, so a fall onto a flexed knee injures those surfaces.

An extended knee presents the condyles and the proximal tibia.

The extensor mechanism is loaded eccentrically during a fall, which is why tendon and patellar injuries occur without a direct blow to the tendon itself.

A direct blow over a recently operated or prosthetic knee is a different problem, and a small wound over a prosthesis is a surgical emergency rather than a dressing.

How Good It Is

A direct blow is reliably recalled and reliably reported, which makes it one of the more trustworthy mechanisms in this group.

It performs well for directing attention to the anterior structures and the bone, and poorly for distinguishing between them.

Its main value is as a prompt for the extensor mechanism assessment, which is the time-critical finding and is the one most often omitted in a swollen painful knee.

Radiographs are indicated more readily after a direct blow than after a twisting injury, and the history is what establishes that.

FRCS Level

How Good It Is

At decision level the direct blow history matters for two time-critical findings and one chronic one.

The extensor mechanism rupture, which needs repair early and whose results deteriorate with delay.

The osteochondral fragment, which may be fixable acutely and not later. 4.17.3 covers the lesion.

And the chronic consequence: a patellofemoral chondral injury from a direct blow is a recognised cause of persistent anterior knee pain in a young patient with an otherwise normal knee, and the history of a fall onto the knee is often the only clue. 4.15.10 covers patellofemoral change.

Where It Misleads

The commonest error is a swollen painful anterior knee in which the extensor mechanism is not formally tested because the knee is too sore to move.

The remedy is to test extension against gravity specifically rather than assessing range, and to repeat it if the first attempt is uninterpretable. 4.7.8 covers extension lag.

The second error is attributing anterior pain months later to the patellofemoral joint generally rather than to a chondral injury from the blow, which changes the prognosis.

And a direct blow in a patient with osteoporosis or on steroids raises a fracture at lower energy than the history suggests.

Fellowship Level

Where It Misleads

In referred practice the direct blow appears as persistent anterior knee pain years later with normal imaging, or as a missed extensor mechanism injury.

A chronic extensor mechanism injury is a far harder reconstruction than an acute repair, and the history that would have caught it is a single question about straightening the knee.

Persistent anterior pain after a direct blow with normal imaging is a presentation where arthroscopy sometimes finds a chondral lesion that imaging did not, and is also a presentation where it sometimes finds nothing.

Deciding between those is the judgement, and the history features that favour looking are a reproducible mechanical symptom at a defined angle and a localised tenderness that has not moved. 4.2.7 covers mechanical symptoms.

No figure here supports that judgement, under decision 023.