Patellar dislocation

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"patellar dislocation" AND management

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Patellar Dislocation

Overview

Patellar dislocation is a common musculoskeletal injury, especially in adolescents and young athletes. The vast majority are lateral dislocations - the patella displaces over the lateral femoral condyle. Other subtypes (superior, horizontal/medial, intercondylar) are rare.

Types of patellar dislocation - lateral, horizontal, superior, and intercondylar

Various types of patellar dislocation. Lateral is the most common. - Roberts & Hedges' Clinical Procedures in Emergency Medicine

Mechanism

Two classic mechanisms:

Quadriceps contraction + valgus/external rotation - e.g., cutting movements in sports or dancing
Direct blow to the anterior or medial patella with the knee flexed

The resulting forces tear the medial retinaculum (and medial patellofemoral ligament - MPFL), allowing the patella to displace laterally. Subluxation (incomplete displacement) typically implies a stretched, rather than torn, medial retinaculum.

Predisposing factors:

Genu valgum
Femoral anteversion
Patella alta
Trochlear dysplasia
Ligamentous laxity / joint hypermobility
Prior dislocation history

Clinical Features

The diagnosis is usually clinically obvious for unreduced dislocations:

Acute pain and inability to bear weight or flex the knee
Knee held in flexion
Patella visible and palpable on the lateral aspect of the knee
Tenting of overlying skin unless significant swelling

Spontaneous reduction in the field is common - the patient may simply describe the knee "going out." Clues to a spontaneously reduced patella:

Knee effusion (may develop hours after injury)
Medial patellar edge tenderness (MPFL injury)
Fairbanks'/Apprehension sign - patient grabs the knee when the patella is pushed laterally, reflecting fear of re-dislocation

Lateral patellar dislocation - clinical photo showing the knee in flexion with the patella visible laterally

Lateral dislocation of the patella presenting to the ED. - Tintinalli's Emergency Medicine

Diagnostic Testing

X-rays (AP + lateral views) are standard. Expected findings:

Empty trochlear groove
Laterally displaced patella
Joint effusion
Possible osteochondral avulsion fracture
Sunrise (Merchant) view: difficult to obtain pre-reduction due to pain; obtain post-reduction
Lipohemarthrosis on x-ray warrants careful search for fracture fragments
Osteochondral fractures occur in ~50% of cases; many are only visible on arthroscopy
CT or MRI may be needed to fully characterize associated injuries

Note: The clinical diagnosis in older patients should be made with caution - patellar dislocations are primarily injuries of the young.

Reduction Technique

Closed reduction is the standard approach. It is usually straightforward and often does not require sedation.

Steps:

Reassure the patient verbally - premedication (analgesia/anxiolysis) as needed
Flex the hip to relax the quadriceps
Gently extend the knee (white arrow in procedure diagrams)
Apply medial pressure on the lateral face of the patella (black arrow), lifting the most lateral edge over the femoral condyle

This produces immediate pain relief. A neurovascular exam should follow every reduction.

If reduction is difficult: gentle downward pressure on the lateral patella may help open the medial patellar facet.

Irreducible dislocation requires orthopaedic consultation and surgical reduction. Clues to irreducibility include:

Older patient age
Pre-existing patellofemoral arthritis
Knee flexion < 45°
Anterolateral (not purely lateral) patellar position
Internal rotation of patellar axis

Non-lateral dislocations (intracondylar, superior, vertical/dorsal fin) are rare and almost always require operative reduction.

Post-Reduction Care

Step	Detail
Immobilisation	Knee immobilizer in full extension
Weight-bearing	Partial weight-bearing; provide crutches
Exercises	Straight leg raises to strengthen quadriceps
Follow-up	Orthopaedics within 1-2 weeks
Post-reduction films	AP + lateral + sunrise view to confirm reduction and check for osteochondral fragments

Complications

Osteochondral fracture - seen in ~50%; may require arthroscopy for fragment removal
Hemarthrosis - more pronounced with associated fractures
Medial retinaculum / MPFL tear - near universal in true dislocations
Recurrent dislocation - 15-71% recurrence rate (widely cited as 15% overall, up to 71% in high-risk populations)

Recurrent Patellar Dislocation

Recurrence is a major concern after first dislocation. The pathogenesis is multifactorial with a genetic component.

Conservative management:

Quadriceps/VMO strengthening physiotherapy
Bracing / taping
Weight loss if applicable
Adequate analgesia

In patients with joint hypermobility, surgery should not be viewed as the first-line approach - joint laxity is a prognostic indicator of poor operative outcomes.

Surgical options (for recurrent instability or associated osteochondral fracture):

MPFL reconstruction - the most common modern approach; evidence supports reconstruction with or without tibial tubercle osteotomy
Tibial tubercle osteotomy (Fulkerson, Maquet, Elmslie-Trillat) - for abnormal TT-TG distance
Trochleoplasty - for severe trochlear dysplasia

Recent Evidence

A 2025 meta-analysis (Recker et al., Arthroscopy 2025 - PMID 39983797) found that operative treatment has greater expected value than nonoperative treatment for first-time patellar dislocations using decision analysis - a shift from the traditional view that both approaches yield similar long-term outcomes. A 2024 systematic review (Su et al., BMC Musculoskelet Disord 2024 - PMID 39143601) found that MPFL reconstruction with and without tibial tubercle osteotomy both achieve good outcomes in patellar instability, with selection based on anatomy.

Sources: Rosen's Emergency Medicine, 9th Ed. | Tintinalli's Emergency Medicine, 9th Ed. | Roberts & Hedges' Clinical Procedures in Emergency Medicine, 7th Ed. | Rheumatology 2-Vol Set (2022, Elsevier)

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Patellar Height Indices

Patellar height (position) is measured on a lateral radiograph with the knee flexed at 30 degrees. Patella alta (high-riding patella) is a key predisposing factor for patellar dislocation and instability; patella baja (low patella) is a common surgical complication.

Three common methods for calculating patellar height - Caton-Deschamps (AT/AP), Insall-Salvati (LT/LP), and Blackburne-Peel (A/B)

The three main patellar height indices illustrated on a lateral knee view. - Miller's Review of Orthopaedics, 9th Ed.

1. Insall-Salvati Index

View: Lateral radiograph, knee flexed 30°

Measurement:

LT / LP = Patellar tendon length (LT) ÷ Longest sagittal diameter of patella (LP)

Value	Interpretation
~1.0	Normal
> 1.2	Patella alta
< 0.8	Patella baja

Key points:

The most widely known and historically used index
Measures the patellar tendon from the lower pole of the patella to the tibial tuberosity
Limitation: LP (patellar length) can vary with fracture, surgery, or bone resorption, making it less reliable in post-operative knees

Lateral knee x-ray demonstrating the Insall-Salvati ratio: patellar bone length (48 mm) and patellar tendon length (72 mm)

Insall-Salvati ratio on lateral radiograph: LT = 72 mm, LP = 48 mm → ratio = 1.5 (patella alta). - Rheumatology 2-Vol Set, Elsevier 2022

2. Caton-Deschamps Index

View: Lateral radiograph, knee flexed 30°

Measurement:

AT / AP = Distance from lower edge of patellar articular surface to anterosuperior tibial angle (AT) ÷ Length of patellar articular surface (AP)

Value	Interpretation
0.6 - 1.3	Normal
> 1.3	Patella alta
< 0.6	Patella infera (baja)

Key points:

Uses articular surface landmarks rather than total patellar length
Less affected by patellar morphology changes (e.g. after fracture or inferior pole excision)
Widely considered the most reliable index, though this is debated
Preferred for evaluating patellar instability and post-operative height assessment

3. Blackburne-Peel Index

View: Lateral radiograph, knee flexed 30°

Measurement:

A / B = Perpendicular height from tibial plateau tangent to inferior pole of patellar articular surface (A) ÷ Length of patellar articular surface (B)

Value	Interpretation
0.54 - 1.06	Normal
> 1.06	Patella alta
< 0.54	Patella baja

Key points:

Uses the tibial plateau as the reference line (rather than the tibial tuberosity)
Not affected by tibial tuberosity position or tibial slope
Useful when tibial anatomy is abnormal (e.g. post-osteotomy)

Quick Comparison Table

Index	Ratio	Normal	Alta	Baja	Key Reference Point
Insall-Salvati	LT / LP	~1.0	>1.2	<0.8	Tibial tuberosity + total patella length
Caton-Deschamps	AT / AP	0.6-1.3	>1.3	<0.6	Anterosuperior tibial angle + articular surface
Blackburne-Peel	A / B	0.54-1.06	>1.06	<0.54	Tibial plateau tangent + articular surface

Other Related Radiographic Indices (Patellofemoral)

Beyond patellar height, these indices assess alignment and trochlear morphology relevant to instability:

Index	Method	Normal / Threshold
Blumensaat line	Lateral x-ray at 30° - lower patellar pole should approximate this line through intercondylar notch	Patella proximal to line = alta
TT-TG distance	Axial CT/radiograph - distance between tibial tuberosity and trochlear groove (two lines perpendicular to posterior bicondylar axis)	>20 mm = malalignment; surgical relevance for tubercle osteotomy
Patellar tilt	CT at 20° flexion - angle between posterior condyle tangent and major patellar axis	Normal <20°; >20° = dysplasia
Trochlear depth	Lateral x-ray (Dejour) - measured 1 cm from top of groove	Normal ≥5 mm
Crossing sign	Lateral x-ray - anterior condylar cortex intersects trochlear outline	Present = dysplastic sulcus
Trochlear bump	Lateral x-ray - trochlear line extends anterior to femoral cortex	Present = dysplastic sulcus

Clinical Relevance in Patellar Dislocation

Patella alta (high-riding patella) reduces patellofemoral contact area and engagement in the trochlear groove at low flexion angles, predisposing to lateral subluxation and dislocation
TT-TG > 20 mm is the threshold commonly used to select patients for tibial tubercle anteromedialization (e.g. Fulkerson osteotomy)
Caton-Deschamps > 1.2-1.3 may prompt distal tubercle transfer as part of surgical correction of instability
The Caton-Deschamps method is generally preferred in surgical planning for patellar instability because it is not distorted by changes to the inferior patellar pole

Sources: Campbell's Operative Orthopaedics, 15th Ed (2026) | Miller's Review of Orthopaedics, 9th Ed | Rheumatology 2-Vol Set, Elsevier 2022

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