Patellofemoral pain and patellar instability

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Clarify whether the problem is pain, instability or both. Patellofemoral pain is typically diffuse anterior or peripatellar discomfort provoked by stairs, squatting, running, jumping or prolonged sitting. Ask about training change, swelling and crepitus. Instability history includes a visible lateral displacement, need for reduction, apprehension or recurrent subluxation. Document mechanism, previous contralateral events, family history and hypermobility because recurrence risk is not determined by the first radiograph alone.

For an unreduced lateral dislocation, provide analgesia, examine distal neurovascular status and extensor continuity and reduce without delay using gentle extension and medial patellar guidance. Avoid force if fracture or tibiofemoral dislocation is suspected. Re-examine straight-leg raise, tenderness, effusion and stability after reduction. BOASt says not to aspirate the associated haemarthrosis routinely. Obtain AP, lateral and skyline views for every reduced first-time dislocation.

Assess patellar glide and apprehension gently after the acute phase, plus effusion, range, tracking, quadriceps, hip strength, foot and lower-limb alignment and global laxity. A J-sign can reflect late engagement but should not be interpreted alone. Examine joint lines, cruciates and extensor mechanism. In patellofemoral pain, reproduce symptoms with a functional task and identify capacity and load factors rather than relying on patellar-grind testing, which can be uncomfortable and non-specific.

Radiographs identify fracture, patellar height, trochlear shape and osteochondral fragment. BOASt recommends MRI reported by a musculoskeletal radiologist when associated injury is suspected, haemarthrosis is present, alternative injuries cannot be excluded, assessment is difficult, surgery is planned or recovery fails. MRI maps cartilage, loose bodies, medial restraint and other ligaments. CT rotational and alignment measurements belong in recurrent-instability surgical planning, not every first presentation.

After first dislocation, do not use a cast. A short splint that permits full flexion may support pain, and unrestricted weight bearing with crutches if needed is encouraged immediately. Restore swelling control, full extension, flexion, quadriceps activation, hip strength and confidence. Arrange knee-clinic review within two weeks and musculoskeletal physiotherapy within three. Patient-initiated follow-up is suitable only when recovery, recurrence risk and alignment do not require planned surveillance.

Patellofemoral pain treatment adjusts aggravating load temporarily while maintaining activity and progressively raises tissue capacity. Combine quadriceps and hip strengthening, range, gait or movement retraining and a graded return to running, squatting and stairs. Taping or a foot orthosis may offer a short adjunct in a selected person but should support rather than replace exercise. Education addresses pain variability and fear; passive modalities alone do not build load tolerance.

Surgery solely to stabilise the patella is not routine after an isolated first event. Fix or remove a significant osteochondral fragment when indicated. Recurrent objective instability despite rehabilitation requires specialist assessment of medial restraint, trochlea, patellar height, tibial-tubercle vector, rotation and skeletal maturity. Medial patellofemoral ligament reconstruction and bony procedures are selected to match abnormality. Isolated lateral release is never indicated for instability under BOASt.

Review redislocation, locking, effusion, extension, strength, apprehension and meaningful function. Return to sport requires full motion, controlled single-leg tasks, strength and confidence without effusion. After surgery, follow the specific cartilage, osteotomy or reconstruction restrictions. Explain urgent return for recurrent unreduced dislocation, locked knee, fever or loss of straight-leg raise. In children, major malalignment and growth influence both timing and procedure.

Key points

Patellofemoral pain is anterior or peripatellar load pain during squatting, stairs, running or prolonged sitting without a true dislocation history; instability produces apprehension, subluxation or a documented lateral dislocation.
Reduce an acute lateral dislocation without delay. BOASt advises against aspirating the associated haemarthrosis routinely.
After reduction obtain AP, lateral and skyline radiographs, document extensor and neurovascular function and inspect for osteochondral fragment or fracture.
Do not use cast immobilisation. A pain-relief splint may permit full flexion, and unrestricted weight bearing with crutches if needed is encouraged immediately.
Arrange age-appropriate knee-clinic assessment within 2 weeks and musculoskeletal physiotherapy within 3 weeks after first-time dislocation.
Obtain MRI when associated osteochondral or ACL injury, haemarthrosis, locked knee, extensor failure, difficult assessment, planned surgery or failure to recover makes it relevant.
Do not routinely stabilise an isolated first-time dislocation surgically; treat significant associated osteochondral injury and discuss any stabilisation plan in an age-appropriate MDT. Isolated lateral release is never indicated.
For patellofemoral pain, manage training load and progressively strengthen knee and hip with movement retraining; imaging is reserved for trauma, instability, effusion, failed care or another diagnosis.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Load-related pain

Rapid training change, quadriceps or hip capacity deficits and repeated flexion load can sensitise patellofemoral tissues without frank dislocation.

Acute lateral dislocation

Valgus rotation or direct medial force displaces the patella laterally and injures medial patellofemoral restraints and cartilage.

Anatomical predisposition

Trochlear dysplasia, patella alta, increased lateral vector, rotational malalignment and generalised hypermobility increase recurrent instability risk.

Previous instability

Prior ipsilateral or contralateral patellar dislocation and a positive family history identify important biological and anatomical recurrence tendency.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Contact-load mismatch
Patellofemoral joint reaction rises with deeper flexion; load exceeding current tissue capacity produces activity pain and avoidance.
2
Medial restraint injury
A first lateral dislocation commonly disrupts the medial patellofemoral ligament, reducing early-flexion restraint and increasing apprehension during biological healing.
3
Trochlear containment failure
A shallow or dysplastic trochlea provides less bony guidance, particularly before the patella engages in flexion.
4
Osteochondral injury
Dislocation and reduction can shear patellar or lateral-femoral-condyle cartilage and release a loose fragment causing haemarthrosis or locking.
5
Fear and motor adaptation
Patellar apprehension changes movement, quadriceps recruitment and limb loading and can perpetuate disability even without another complete dislocation episode.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Patellofemoral pain

Diffuse anterior pain during flexed-knee loading without true instability or large effusion supports a capacity and load syndrome.

Acute lateral dislocation

Visible lateral patellar displacement with severe pain and fixed flexion requires prompt reduction and post-reduction imaging.

Spontaneously reduced event

Medial retinacular tenderness, haemarthrosis and apprehension after a valgus twist can reveal a dislocation that reduced before review.

Osteochondral injury

Large haemarthrosis, locking or a radiographic fragment after dislocation indicates cartilage and bone injury needing MRI and specialist review.

High recurrence risk

Young age, contralateral history, hypermobility, family history, malalignment and dysplasia increase recurrence probability.

Extensor disruption

Absent straight-leg raise, tendon gap or abnormal patellar height redirects care to urgent extensor mechanism assessment.

Red flags requiring action

Failure of reduction, absent straight-leg raise, gross tibiofemoral instability or neurovascular abnormality requires urgent orthopaedic assessment and imaging beyond an isolated patellar pathway.
Large haemarthrosis, locked knee, persistent focal bony pain or visible fragment raises osteochondral fracture and supports MRI and early surgical review.
Fever, erythema and severe passive pain suggests septic arthritis rather than patellofemoral pain and requires urgent aspiration.
Recurrent dislocation in a skeletally immature person with major malalignment, severe trochlear dysplasia or syndromic laxity needs age-appropriate specialist planning.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

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
Post-reduction radiographsFirst step
Why
Confirm reduction and detect fracture or osteochondral fragment.
Interpretation and limitations
BOASt requires AP, lateral and skyline views after first-time lateral dislocation.
02
Functional clinical assessment
Why
Separate pain capacity from instability and extensor failure.
Interpretation and limitations
Assess a provoking task, tracking, apprehension, strength, alignment, laxity and straight-leg raise without relying on one test.
03
MRI when BOASt indications apply
Why
Map osteochondral, ligament and alternative injury.
Interpretation and limitations
Use for haemarthrosis, suspected associated injury, difficult assessment, planned surgery or failed recovery, not automatically for uncomplicated pain.
04
Alignment and rotational imaging
Why
Plan anatomically targeted recurrent-instability correction.
Interpretation and limitations
Long-leg radiographs and selective CT measurements belong in specialist planning and should answer a defined anatomical question.
05
Aspiration for a hot unexplained effusion
Why
Exclude infection or identify crystals.
Interpretation and limitations
Routine aspiration of dislocation haemarthrosis is not recommended, but systemic illness creates a different septic-joint indication.
06
Serial rehabilitation measures
Why
Track recovery and return readiness.
Interpretation and limitations
Use effusion, range, quadriceps and hip strength, single-leg control and apprehension rather than imaging normalisation.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Meniscal tear

Joint-line pain and true locking after rotation differs from diffuse peripatellar load pain, though haemarthrosis can overlap.

Patellar or tendon rupture

Trauma with absent straight-leg raise, palpable defect or altered patellar height requires urgent extensor repair assessment.

Tibiofemoral dislocation

Gross multidirectional instability, major deformity or abnormal vascular findings signals a limb-threatening knee injury rather than isolated patellar displacement.

Osgood-Schlatter or tendon pain

Focal tibial-tubercle, inferior-patellar or tendon tenderness during resisted extension localises an extensor-load disorder outside the articular joint surface.

Septic or inflammatory arthritis

Hot effusion, systemic illness or prolonged stiffness requires aspiration and inflammatory assessment rather than exercise alone.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01DislocatedReduce and image nowFirst stepThe patella remains laterally displaced without evidence of tibiofemoral dislocation or fracture preventing reduction.

1Provide analgesia, document neurovascular and extensor status and reduce gently without delay.
2Obtain AP, lateral and skyline radiographs and reassess straight-leg raise and osteochondral tenderness.
3Avoid routine haemarthrosis aspiration and cast immobilisation; allow weight bearing with crutches as needed.
4Arrange clinic within two weeks and physiotherapy within three, with MRI when an indicated risk or associated injury is present.

02PainBuild patellofemoral capacityAnterior load pain occurs without true dislocation, red flags or a large unexplained effusion.

1Identify provocative loads, hip and knee capacity, alignment and activity change and agree functional goals.
2Temporarily modify dose while progressing quadriceps, hip and movement-control exercise.
3Use taping, orthosis or short analgesia only as adjuncts that enable active rehabilitation.
4Reconsider imaging and alternate diagnoses when swelling, locking or adequate rehabilitation failure persists.

03RecurrentMatch surgery to instability anatomyObjective subluxation or dislocation persists despite appropriate rehabilitation.

1Map trochlea, patellar height, tubercle vector, rotation, alignment, medial restraint and skeletal maturity.
2Discuss the case in an age-appropriate instability MDT and identify the exact corrected abnormality.
3Use soft-tissue and bony procedures selectively and never isolated lateral release for instability.
4Progress return through range, strength, single-leg control, confidence and procedure-specific healing.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

Relieves severe pain and muscle guarding so the patella can be reduced gently and promptly.

Acute reduction analgesia

Give titrated oral, inhaled or intravenous analgesia under the emergency pathway according to pain and reduction conditions; use procedural sedation only with full monitoring and airway-capable staff when simple supported reduction is not tolerated.

Check pregnancy, frailty, cardiorespiratory disease and medicines already received; suspected fracture or tibiofemoral dislocation requires orthopaedic and imaging support rather than force under deeper sedation.

Reduces pain and reactive synovitis during early motion and progressive loading.

Short-course NSAID for rehabilitation

Use a licensed topical NSAID or one oral NSAID at the lowest effective adult dose for the shortest period required to support weight bearing and exercise, with gastroprotection when oral treatment and risk justify it.

Assess gastrointestinal, renal, hepatic and cardiovascular disease, pregnancy, anticoagulants and concurrent NSAIDs; do not inject or mask an unexplained hot effusion or mechanical lock.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Recurrent instability

Repeated patellar subluxation or dislocation damages cartilage, reinforces movement apprehension and progressively limits sport, work and daily activity.

Osteochondral loose body

A sheared fragment can lock or swell the knee and may require early fixation or removal.

Patellofemoral osteoarthritis

Cumulative osteochondral injury and persistent maltracking can produce later anterior pain, crepitus, stiffness and flexed-knee load intolerance.

Stiffness and weakness

Prolonged rigid immobilisation causes marked quadriceps inhibition, flexion loss and delayed functional recovery after a first patellar dislocation.

Failed stabilisation

Uncorrected coronal or rotational alignment, trochlear risk and inappropriate isolated procedures can produce persistent or recurrent instability after surgery.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Confirm maintained reduction, straight-leg raise, neurovascular status and radiographic review after the acute event.
Track effusion, full extension, flexion, quadriceps activation and gait through early rehabilitation.
Document recurrence risk, global laxity, alignment and contralateral or family instability history.
Measure hip and knee strength, single-leg control, apprehension and return-to-running or sport tasks.
Reassess locking, recurrent haemarthrosis or failed recovery for osteochondral and associated ligament injury.
After stabilisation surgery, follow cartilage, osteotomy, growth and reconstruction-specific imaging and restrictions.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Do not aspirate routine haemarthrosis

BOASt specifically advises against aspiration after first lateral dislocation, while septic features create a separate indication.

First events are usually non-operative

Associated osteochondral injury may need surgery, but isolated stabilisation is not routinely offered after one dislocation.

Pain and instability are different

Anterior load pain can occur without displacement and should not be labelled recurrent dislocation from apprehension alone.

Full-flexion splints avoid casting

Short pain support can coexist with early movement and unrestricted weight bearing rather than rigid immobilisation.

Anatomy drives recurrent surgery

Medial restraint, trochlea, height, vector, rotation and growth determine which procedure can address the cause.

Lateral release is not stabilisation

An isolated release weakens lateral restraint and is never indicated as sole treatment for instability.

11Common pitfallsFrequent interpretation and management errors.

01
Delaying reduction while waiting for routine imaging when no fracture or alternate dislocation is suspected.
02
Aspirating every post-dislocation haemarthrosis despite BOASt advice.
03
Using a cast and prolonged non-weight bearing after uncomplicated first dislocation.
04
Missing osteochondral injury in a locked or markedly swollen knee.
05
Operating routinely after one isolated dislocation without an associated lesion or MDT rationale.
06
Treating patellofemoral pain with passive modalities and rest instead of progressive capacity.