Patellar Dislocation / Instability

The classic acute story is a non-contact pivoting injury (netball, dancing, slipping on ice) with a sensation of the knee giving way or something popping out and back. Patients often report seeing or feeling the kneecap on the side of the knee before it reduced. An immediate haemarthrosis develops within hours, suggesting MPFL rupture and possible osteochondral injury.

If the patella is still dislocated on arrival the diagnosis is obvious: the knee is held in slight flexion, the patella is visibly displaced laterally, and attempts at extension are painful. If reduced before presentation, look for an effusion, medial-sided tenderness over the MPFL (femoral epicondyle and medial patellar border), a positive patellar apprehension test, and quadriceps inhibition. Always assess for generalised joint hypermobility using the Beighton score - this influences both the likelihood of conservative failure and the choice of surgical graft.

Chronic instability presents differently: recurrent giving way on stairs or pivoting, anterior knee pain, and patient-reported episodes of the kneecap slipping. Examination may show a positive J-sign (lateral excursion of the patella as the knee approaches full extension), increased lateral glide, and reduced medial glide.

Differentials: patellofemoral pain syndrome (anterior pain without a dislocation event), quadriceps or patellar tendon rupture (inability to extend, palpable gap, high- or low-riding patella), and ACL injury (haemarthrosis with a pivot mechanism but a positive Lachman rather than apprehension).

Plain radiographs are first-line in acute presentation: AP, true lateral at 20-30 degrees of flexion, and axial skyline (Merchant) view at 20-30 degrees. Look for a loose osteochondral fragment, a marginal fracture of the medial patellar facet (highly suggestive of previous lateral dislocation), patellar height (Insall-Salvati and Caton-Deschamps ratios), and trochlear morphology.

MRI is commonly required after first-time lateral patellar dislocation per BOAST 2024, particularly where there is a haemarthrosis, suspected osteochondral injury, concern for ACL injury, a locked knee, a non-functioning extensor mechanism, difficult clinical assessment, planned surgery, or failure to recover with conservative treatment. MRI assesses cartilage integrity, the kissing bone-bruise pattern (medial patellar facet and lateral femoral condyle), MPFL injury, trochlear dysplasia, patella alta, meniscal and ligamentous pathology, and tibial tuberosity offset (TT-TG and TT-PCL).

CT is reserved for selected surgical planning, particularly femoral or tibial rotational alignment, and is not routine.

For an acute first-time lateral patellar dislocation without a significant osteochondral fragment or other surgical indication, BOAST 2024-aligned UK practice is non-operative. Reduce promptly if still dislocated. Do NOT aspirate the haemarthrosis. Do NOT use cast immobilisation; a removable splint that permits full knee flexion may be used briefly for pain relief immediately after injury. Encourage unrestricted weight-bearing immediately, with crutches if required. Arrange evaluation in an age-appropriate clinic within 2 weeks and physiotherapy assessment within 3 weeks. Document recurrence risk factors including demographics, mechanism, contralateral dislocation, joint hypermobility (Beighton), family history, and lower-limb alignment.

Surgical referral indications: a loose osteochondral fragment requiring fixation or removal (urgent), irreducible dislocation, locked knee, non-functioning extensor mechanism, or suspected associated major injury. Recurrent dislocation warrants specialist referral and surgical discussion but is not always an emergency.

The cornerstone procedure is MPFL reconstruction, usually with a hamstring graft, when bony anatomy is otherwise normal. Adjunctive procedures address underlying bony abnormality:

• •Tibial tuberosity distalisation for significant patella alta with recurrent dislocation, often combined with MPFL reconstruction.
• •Medialisation or anteromedialisation where there is clinically relevant excessive lateralisation; isolated medialisation is not recommended for pure patellar instability.
• •Trochleoplasty for severe trochlear dysplasia in skeletally mature patients (typically Dejour B or D), only by surgeons with appropriate training. NICE HTG328 states that arthroscopic trochleoplasty should only be used with special arrangements for clinical governance, consent, audit, or research.

Isolated lateral retinacular release is never indicated in pure patellar instability. Stabilisation surgery treats instability, not chronic anterior knee pain - be cautious where the dominant symptom is pain without clear instability and in patients with marked generalised joint hypermobility, where outcomes are less predictable.

Non-operative rehabilitation runs over 3-4 months.

Early phase (0-3 weeks): swelling and pain management, immediate weight-bearing as tolerated, full range of motion as comfort allows, and quadriceps reactivation with isometrics and straight-leg raises.

Mid phase (3-8 weeks): closed-chain strengthening, hip abductor and external rotator work (gluteus medius weakness may contribute to dynamic valgus and patellar maltracking), and proprioceptive retraining.

Return to non-contact sport is typically at 3 months and pivoting sport at 4-6 months, gated by symptom-free hop and agility testing.

Post-MPFL reconstruction follows a similar arc with graft protection: full weight-bearing in a brace locked in extension for 2 weeks, progressive flexion to 90 degrees by 6 weeks, full range by 12 weeks. Return to sport is usually at 6-9 months and gated by a limb symmetry index over 90% on strength testing, absence of apprehension, and successful return-to-sport screening.

• •BOAST 2024: Assessment and Management of First-Time Lateral Patellar Dislocation - non-operative pathway with immediate weight-bearing, no aspiration, no cast immobilisation, physiotherapy assessment within 3 weeks.
• •BOA / BASK / British Patellofemoral Society: Surgical Management of Recurrent Patellar Instability - MPFL reconstruction is the cornerstone for normal bony anatomy; adjunctive bony procedures address measurable structural abnormality.
• •NICE HTG328 - Arthroscopic trochleoplasty for patellar instability: evidence is inadequate; use only with special arrangements for clinical governance, consent, audit, or research.
• •Dejour classification (1994) - The standard trochlear-dysplasia framework (A simple, B flat trochlea with supratrochlear spur, C lateral convexity, D combined).
• •Sanders et al. (2018, AJSM) - First-time lateral patellar dislocation recurrence is approximately 40-50% in adolescents and young adults, climbing toward 70% with multiple at-risk anatomical features.

• •The at-risk anatomy quartet: trochlear dysplasia (heaviest weight), patella alta, TT-TG over 20 mm (or TT-PCL over 24 mm), and patellar tilt over 20 degrees.
• •The Dejour classification (A simple, B flat trochlea with supratrochlear spur, C lateral convexity, D combined) is worth knowing.
• •Know the BOAST 2024 don'ts cold: no aspiration of haemarthrosis, no cast immobilisation, no delay to weight-bearing or physiotherapy.
• •First-time dislocator with a loose osteochondral fragment on imaging is urgent surgical referral, not physiotherapy.
• •Recurrent instability with normal bony anatomy is isolated MPFL reconstruction; isolated lateral retinacular release is almost always wrong.
• •TT-PCL is increasingly preferred over TT-TG on MRI because it is not affected by knee flexion position.