Extensor Mechanism Ruptures (Quadriceps & Patellar Tendon)

Sudden inability to straight-leg-raise after a knee injury is the single most important finding in urgent care — it means the extensor mechanism is disrupted until proven otherwise, and it changes the pathway from “reassure and review” to “refer now.”

This page is about tendon rupture, not a muscle-belly strain — for quadriceps muscle strain see Acute Muscle Strains & Tears; for the traumatic patellar dislocation that can accompany or mimic this presentation see Patellar Dislocation.

Who gets which

Quadriceps tendon rupture is more common in men over 40, often with an underlying risk factor — chronic kidney disease, diabetes, rheumatoid arthritis, hyperparathyroidism, chronic steroid or fluoroquinolone use, or a connective tissue disorder — that weakens the tendon before a relatively minor mechanism completes the tear.

Patellar tendon rupture needs more force and is more common in athletes under 40; look for pre-existing patellar tendinopathy as a predisposing factor. Overall, quadriceps tendon rupture is about twice as common as patellar tendon rupture. Patellar tendon rupture is itself rare, at fewer than 1 per 100,000 people a year.

Both typically follow eccentric quadriceps contraction against a flexed knee — stumbling, missing a stair, or landing awkwardly from a jump — rather than a direct blow. The mechanism in the older patient with a quadriceps rupture is characteristically a planted foot with the knee slightly bent. In younger patients the mechanism is more often direct trauma. A patellar tendon rupture is usually the end stage of long-standing chronic tendon degeneration, consistent with the often antecedent history of tendon pain.

Recognition

• The inability to actively straight-leg-raise (or hold the leg extended against gravity) is the most specific finding for a complete tear. A partial tear may still extend the knee, but weakly and painfully.
• Patients often report a “pop” at the time of injury, then pain and swelling, sometimes with a haemarthrosis.
• Palpate for a gap in the tendon — above the patella for a quadriceps tear, below it for a patellar tendon tear — though swelling can make this difficult to feel reliably in the first hours. In a quadriceps rupture the defect is usually felt within 2 cm of the superior pole of the patella. In a patellar tendon rupture the gap is felt below the inferior pole of the patella.
• Check patellar height on the lateral X-ray. A quadriceps tear lets the patella sit low (patella baja); a patellar tendon tear lets it sit high (patella alta). Plain films can also show an avulsed bone fragment, an effusion, or be entirely normal — a normal X-ray does not exclude the diagnosis.

Point-of-care ultrasound, when available, is a reasonable adjunct — it can show tendon discontinuity and help distinguish a partial from a complete tear. MRI is a surgical-planning tool, not something to organise from urgent care before referral.

Management

Any loss of active knee extension needs orthopaedic referral — this is a surgical decision, not something to trial conservatively while waiting to see if it improves. Diagnosed and treated early, the prognosis is good; delay allows the retracted tendon to scar down, which makes surgery harder and the outcome worse.

In the urgent-care room:

• Splint the knee in extension (a knee immobiliser) for comfort and to protect against further retraction.
• Non-weight-bearing with crutches until reviewed.
• Analgesia as needed.
• Refer for same-day or next-available orthopaedic assessment — do not defer to routine follow-up.

A partial tear with preserved (even if weak) active extension may be a candidate for non-operative management in an older patient with significant comorbidities, in a knee immobiliser with staged, progressive range of motion — but that decision belongs to the surgical team, not urgent care. Whichever pathway is chosen, expect roughly six months of rehabilitation before a full return to activity.