01OverviewDefinition, clinical context and the essential points that orientate the chapter.
The extensor mechanism is a functional unit: quadriceps tendon, patella, medial and lateral retinacula, patellar tendon and tibial tubercle. Examination therefore asks both where the injury lies and whether force crosses the knee. Observe resting patellar height, palpate above and below, inspect skin and ask the patient to extend actively. A palpable gap is helpful but can be hidden by swelling and obesity.
Straight-leg raise is central but must be interpreted. Support the heel with the knee initially extended and ask the patient to lift the whole limb. Pain and haemarthrosis can inhibit quadriceps; provide analgesia and repeat without forcing. Intact retinacula can preserve a weak raise despite displaced patellar fracture, so radiographic alignment remains relevant. Inability after analgesia strongly suggests disruption and needs urgent orthopaedic assessment.
Direct blows often create comminuted patellar and cartilage injury; eccentric contraction more often creates a transverse gap or tendon rupture. Examine for dashboard-associated hip, femoral and acetabular injury. In a child, a sleeve avulsion may show only a tiny fragment and abnormal patellar height. In an older adult, systemic disease and steroid exposure increase quadriceps rupture; younger adults more often rupture patellar tendon.
Radiographs include AP and true lateral, with a skyline or sunrise view only when flexion is safe and tolerable. Assess fracture orientation, separation, articular surface and patellar height. A bipartite patella is usually superolateral, well corticated and often bilateral, unlike an acute irregular fragment. Ultrasound dynamically shows tendon continuity, while MRI defines partial tears, retinacula and occult osteochondral injury. CT aids complex fracture planning.
Non-operative care is suitable when the extensor mechanism is functionally intact, displacement and articular incongruity are acceptable and follow-up is reliable. Protect in an extension brace or appropriate splint, allow weight bearing according to the fracture plan and introduce flexion gradually under fracture-clinic and physiotherapy guidance. Re-image after mobilisation or motion if stability is uncertain. Avoid immobilising longer than necessary because stiffness develops quickly.
Surgery restores the extensor chain and articular congruity. Open fracture needs antibiotics and formal debridement. Transverse fractures may use tension-band or screw constructs; comminution may need plates, sutures or partial patellectomy only when reconstruction is impossible. Tendon rupture receives timely repair with protected rehabilitation. The operative goal is stable repair that permits controlled early movement, not radiographic perfection at the cost of skin and blood supply.
Follow-up measures active extension, extensor lag, flexion, patellar tracking, quadriceps strength and functional tasks. A patient may regain a straight-leg raise yet retain major stair or kneeling disability. New wound drainage, increasing erythema, sudden loss of extension, calf swelling or neurovascular change requires urgent review. Address falls, osteoporosis and tendon risk factors where the mechanism was low energy.
Key points
- Inspect anterior skin and wounds, palpate patella, quadriceps and patellar tendon and compare patellar height with the opposite knee.
- Test active straight-leg raise with the hip flexed slightly and knee supported; repeat after adequate analgesia when pain or haemarthrosis limits effort.
- Document distal pulses, peroneal and tibial nerve function and knee-dislocation signs because vascular injury is more urgent than extensor classification.
- Obtain AP and lateral knee radiographs and a skyline view only when safe; look for displacement, articular step, patellar height and osteochondral fragments.
- Use ultrasound or MRI when tendon continuity remains uncertain; CT helps define complex comminuted or articular fracture for surgery.
- An undisplaced or minimally displaced fracture with intact active extension can often use a brace locked in extension, weight bearing as instructed and early specialist-guided motion.
- Disrupted extensor mechanism, open fracture, significant articular incongruity or unstable displacement usually requires operative fixation or tendon repair.
- Monitor skin, wound, extension lag, flexion and quadriceps function and prevent VTE according to immobilisation and patient risk.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Direct patellar impact
A fall onto the knee or dashboard blow produces stellate, transverse or osteochondral fracture and can damage overlying skin and articular cartilage.
Forceful quadriceps contraction
Sudden eccentric contraction against a flexed knee can create transverse fracture or tear quadriceps or patellar tendon.
Tendon vulnerability
Ageing, diabetes, renal failure, inflammatory disease, steroid exposure and previous surgery weaken tendon and permit rupture with modest force.
Sleeve and physeal injury
Children can avulse a cartilage sleeve or patellar pole with little visible ossified fragment, causing extensor failure despite subtle radiographs.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Extensor-chain discontinuity
Quadriceps, patella, retinacula and patellar tendon transmit force to the tibial tubercle; disruption prevents active knee extension and safe gait.
- 2Retinacular compensation
Medial and lateral retinacula may preserve a weak straight-leg raise despite a patellar fracture, so ability alone does not define congruity or stability.
- 3Articular incongruity
Displacement and cartilage injury create a patellofemoral step, altered contact pressure, pain and later post-traumatic arthritis.
- 4Haemarthrosis inhibition
Painful joint distension suppresses quadriceps activation and can mimic tendon disruption, requiring analgesia and careful repeat examination.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Focal bony tenderness, haemarthrosis and an acute fracture line or comminution follows direct impact or forceful quadriceps contraction.
Suprapatellar tenderness or gap, bruising, low patella and extensor failure suggests proximal tendon disruption.
Infrapatellar gap, high patella and inability to extend after a jumping or eccentric event suggests distal disruption.
A weak straight-leg raise despite a displaced fracture can persist through retinacular continuity and does not prove the articular surface is acceptable.
Any anterior wound near patella and tendon can communicate deeply and needs antibiotics, cover and formal operative assessment rather than probing.
A tense haemarthrosis and severe pain can suppress active extension despite intact tissue, making analgesia and repeat testing important.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
First-line knee radiographsFirst stepFirst line - Why
- Define patellar fracture, displacement and patellar height.
- Interpretation and limitations
- Use AP and true lateral views, adding a skyline view only when safe; compare height and look for small sleeve or osteochondral fragments.
- 02
Bedside ultrasound - Why
- Assess quadriceps and patellar tendon continuity dynamically.
- Interpretation and limitations
- A skilled examination can show a gap and retraction quickly, but pain, swelling and operator dependence limit a negative result.
- 03
MRI - Why
- Confirm partial tendon, retinacular and occult osteochondral injury.
- Interpretation and limitations
- Use when examination and simpler imaging remain discordant and the result will change operative or rehabilitation planning.
- 04
CT knee - Why
- Map comminuted and articular fracture for fixation.
- Interpretation and limitations
- CT defines fragments and step more accurately than plain films but does not replace clinical extensor and skin assessment.
- 05
Analgesia-assisted repeat examination - Why
- Distinguish pain inhibition from true extensor discontinuity.
- Interpretation and limitations
- Repeat straight-leg raise after adequate analgesia; persistent inability or a palpable gap supports surgical-pathway escalation.
- 06
Pre-operative assessment - Why
- Identify anaesthetic, bleeding and infection considerations.
- Interpretation and limitations
- Use targeted FBC, renal, coagulation, group and screen and ECG based on age and comorbidity without delaying open-injury care.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Quadriceps tendon rupture
A suprapatellar gap, low-riding patella and inability to extend suggests quadriceps rupture, often after eccentric loading in an older adult.
Patellar tendon rupture
An infrapatellar gap, high-riding patella and acute extensor loss suggests tendon rupture, particularly in younger or systemically predisposed patients.
Tibial-tubercle avulsion
An adolescent with jumping injury, tubercle tenderness and extensor difficulty needs paediatric radiographs and compartment surveillance.
Patellar dislocation
Medial tenderness, lateral displacement or a spontaneously reduced event causes haemarthrosis and osteochondral injury without primary extensor-chain rupture.
Pain-inhibited extension
Large effusion, contusion or intra-articular fracture can prevent a straight-leg raise despite anatomical continuity and improves after appropriate analgesia.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01ImmediateProtect and define the mechanismFirst stepAnterior knee trauma causes haemarthrosis, deformity or extension weakness.+
- 1Perform trauma and distal neurovascular assessment, inspect wounds and support the knee in a comfortable extended position.
- 2Give analgesia, palpate the full extensor chain and test straight-leg raise, repeating after pain treatment when necessary.
- 3Cover any open wound, give immediate network intravenous prophylaxis and obtain urgent orthopaedic review without bedside probing.
- 4Obtain adequate radiographs and use tendon or cross-sectional imaging when the functional and bony findings disagree.
02StableUse protection with early motionActive extension is intact and fracture displacement and articular congruity are acceptable.+
- 1Fit an extension brace or splint and set explicit weight-bearing, skin and range-of-motion instructions.
- 2Arrange fracture-clinic review and repeat radiographs when initial stability or later displacement is a concern.
- 3Begin graded knee flexion and quadriceps activation under specialist guidance while protecting the healing fracture.
- 4Review VTE risk and provide prophylaxis when immobilisation and patient factors outweigh bleeding risk.
03UnstableRepair extensor continuityOpen injury, lost active extension, unstable displacement or important articular incongruity is present.+
- 1Debride open injury formally and choose fixation or tendon repair that restores length, congruity and stable extension.
- 2Protect repair in the prescribed brace and give exact weight-bearing and flexion limits rather than generic immobilisation.
- 3Monitor anterior skin and wound closely and start controlled therapy as soon as fixation and tissue allow.
- 4EscalationEscalate sudden extension loss, drainage, fever or increasing pain for repair failure or infection assessment.
Key medicines and prescribing safety1 treatment · regimens, roles and cautions+
Paracetamol
Give 1 g orally up to four times daily as required, with at least 4 hours between doses and no more than 4 g in 24 hours; reduce the maximum for low weight, frailty, malnutrition or liver risk.Check combination medicines and alcohol or hepatic risk, and do not allow symptom improvement to substitute for examination of an open wound or disrupted extensor mechanism.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Loss of extension
Failed fixation, tendon elongation, retinacular disruption or delayed repair causes extensor lag and difficulty climbing stairs or rising.
Knee stiffness
Pain and prolonged immobilisation cause adhesions and flexion loss, supporting stable fixation and specialist-guided early movement.
Patellofemoral arthritis
Articular cartilage injury, residual step and altered patellar height produce anterior pain, crepitus and reduced function.
Wound and implant problems
Thin anterior skin is vulnerable to necrosis, infection and prominent metal, sometimes requiring revision or later implant removal.
Nonunion
Open injury, displacement, infection and poor biological healing can prevent union, leaving persistent anterior pain, weakness and fixation failure.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Repeat distal perfusion and peroneal and tibial nerve findings after splintage, reduction, surgery and any swelling change.
- Inspect anterior skin and wound under an agreed dressing schedule and escalate blistering, necrosis, drainage or spreading erythema.
- Track straight-leg raise, extensor lag, flexion, quadriceps strength and patellar tracking through rehabilitation.
- Repeat radiographs at the specialist interval to detect displacement, fixation failure and union before advancing motion or load.
- Review calf symptoms, VTE risk and prophylaxis whenever bracing and reduced weight bearing persist.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Straight-leg raise is functional
It tests the entire chain but can be falsely absent through pain and falsely preserved through intact retinacula.
Patellar height localises
A low patella supports quadriceps rupture, whereas a high patella supports patellar-tendon rupture when technique is adequate.
Small paediatric fragment can matter
A sleeve avulsion may remove mostly cartilage, so radiographic size underestimates functional disruption.
Skin influences timing
Thin bruised anterior tissue may make immediate complex fixation unsafe even when the bone pattern is clear.
Motion is part of fixation success
A repair that heals but leaves a stiff painful knee has not restored full extensor function.
11Common pitfallsFrequent interpretation and management errors.
- 01
Interpreting inability to straight-leg raise before analgesia as definitive tendon rupture.
- 02
Assuming a preserved raise excludes a displaced patellar fracture or articular step.
- 03
Missing a small anterior wound that communicates with the fracture or joint.
- 04
Forcing a skyline radiograph when flexion is acutely painful or unsafe.
- 05
Immobilising a stable fracture indefinitely without a graded motion and follow-up plan.
- 06
Documenting union but not extensor lag, stair function, kneeling and quadriceps recovery.