Medial Patellofemoral Ligament (MPFL) Injury

The medial patellofemoral ligament (MPFL) is the primary soft-tissue restraint to lateral displacement of the patella in the first 30° of knee flexion. It is a thin, fan-shaped ligament that runs from the superomedial border of the patella to a point on the femur just distal and slightly posterior to the adductor tubercle. In the vast majority of acute lateral patellar dislocations the MPFL is torn, usually at its femoral or patellar attachment rather than in midsubstance. This tear is the essential lesion that permits the patella to escape laterally.

For the exam the story is straightforward: understand the anatomy and isometry, recognise the injury on MRI, know which patients can be treated non-operatively after a first dislocation, and be able to describe an anatomic reconstruction when surgery is indicated. The common viva traps are assuming every dislocator needs a reconstruction, forgetting to assess bony factors, and placing the femoral tunnel in a non-isometric position.

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The MPFL is a capsular ligament that lies in the second layer of the medial knee, deep to the sartorius fascia and superficial to the capsule. It is approximately 50-60 mm long and 10-20 mm wide at its midpoint, fanning out toward the patella. Its fibres blend with the distal vastus medialis obliquus (VMO) aponeurosis, which is why VMO strengthening is part of every rehabilitation protocol.

Femoral origin: located in a sulcus between the adductor tubercle (proximal) and the medial epicondyle (distal). On a true lateral radiograph it lies roughly 1-2 mm anterior to the posterior cortical extension line of the femur and 2-3 mm proximal to the posterior condylar flare. This radiographic landmark is the single most useful intraoperative check for tunnel position.

Patellar insertion: the proximal third to half of the medial patellar border, with superficial fibres continuing into the VMO fascia. The insertion is broader than the femoral origin.

Biomechanical role: sectioning studies (Amis 2003) show that the MPFL provides 50-60% of the total resistance to lateral patellar translation at 0-30° of flexion. Beyond 30° the trochlear groove and lateral femoral condyle become the primary bony restraints. The ligament is tightest in extension and early flexion and slackens as the patella engages the trochlea.

Injury mechanism: the typical injury occurs during a twisting valgus movement or a direct blow to the medial patella with the knee in slight flexion. The patella translates laterally, the medial retinaculum and MPFL are loaded to failure, and the patella either spontaneously reduces or remains locked laterally. The classic bone-bruise pattern on MRI (medial patella and anterolateral femoral condyle) is the footprint of the dislocation event.

Why recurrence happens: once the MPFL is torn or stretched, the medial checkrein is lost. If the trochlea is dysplastic or the TT-TG distance is increased, the patella sits more laterally even in early flexion and the risk of repeat dislocation rises sharply. Over time, repeated subluxations can cause chondral wear on the medial patella and lateral trochlea and stretch the medial tissues further.

Healing and chronic changes: acute tears may scar in a lengthened position. Femoral avulsions in good tissue can heal with repair if addressed early; patellar avulsions with small bony fragments may require reattachment. Midsubstance tears have limited healing capacity. Chronic insufficiency leads to secondary changes in the medial retinaculum, VMO atrophy or inhibition, and progressive lateral facet overload.

There is no single universally used classification for MPFL injury, but three practical axes matter for decision-making.

The topic refers repeatedly to "Dejour B-D" and the crossing sign and spur - the examiner will expect you to actually grade the trochlea. Trochlear dysplasia is the single most important bony risk factor for patellofemoral instability, and the Dejour classification (true lateral radiograph plus axial CT or MRI) is the standard.

The three radiographic signs on a true lateral:

• Crossing sign - the line of the trochlear floor crosses the anterior border of the lateral femoral condyle, meaning the groove becomes flat (flush with the condyle) at that point. This is the hallmark of dysplasia.
• Supratrochlear spur (boss) - a bony prominence projecting anteriorly above the trochlea.
• Double-contour sign - a second line behind the crossing line, representing a hypoplastic, medialised medial facet.

Quantitatively, a trochlear depth under 3 mm, a sulcus angle over 145 degrees, or a lateral trochlear inclination under 11 degrees support the diagnosis. Grades B and D (those with a spur) are the high-grade patterns most associated with failure of isolated soft-tissue surgery and are the ones in which a trochleoplasty may be considered in expert hands.

First-time dislocation: the patient usually describes a twisting injury or fall with the knee giving way laterally. There is often an audible or palpable "pop", immediate swelling, and inability to bear weight comfortably. The patella may have reduced spontaneously or required manual reduction in the emergency department. Haemarthrosis is common.

Recurrent instability: patients report repeated "giving way" or "slipping" episodes, often with minimal trauma, a sense that the knee is "unstable" or "wobbly", and anterior or medial knee pain after episodes. They may describe the patella "jumping out and back in".

Examination:

• Look: effusion, bruising over the medial retinaculum, apprehension posture (patient holds the knee extended or in slight flexion and resists movement).
• Feel: tenderness along the medial patellar border or at the femoral origin (adductor tubercle region). Palpable defect in the retinaculum in some acute cases.
• Move: limited range due to pain and effusion; patellar apprehension test positive at 20-30° of flexion (patella is pushed laterally and the patient resists or reports impending dislocation). The test becomes negative or less positive once the patella is engaged in the trochlea in deeper flexion.
• Special tests: J-sign (patella jumps laterally in terminal extension), patellar tilt test (excessive lateral tilt), moving patellar apprehension test, and assessment for generalised ligamentous laxity.

Always examine the contralateral knee and assess for trochlear dysplasia (prominent lateral condyle, flat trochlea on palpation), Q-angle, and foot progression.

Plain radiographs (essential first step):

• AP, true lateral (for femoral tunnel planning and patellar height), and axial (Merchant or sunrise view at 30-45° flexion).
• Look for osteochondral fracture (medial patella or lateral femoral condyle), loose bodies, and the position of the patella on the axial view.
• On the true lateral, assess trochlear morphology (crossing sign, supratrochlear spur) and use the radiographic landmark for the MPFL femoral origin.

MRI (the investigation that changes management):

• Axial T2-weighted or PD fat-saturated sequences are best for the MPFL.
• Direct signs: discontinuity, retraction, wavy fibres, or avulsion at femoral or patellar end; surrounding oedema.
• Indirect signs: bone bruise on the medial patellar facet and anterolateral femoral condyle (the "kissing" contusion of dislocation), cartilage injury, VMO oedema or tear.
• MRI can also grade trochlear dysplasia, measure TT-TG (on superimposed slices), and assess patellar height and chondral surfaces.

CT (selected cases):

• True TT-TG distance when MRI measurement is uncertain or when planning tibial tubercle osteotomy.
• 3D reconstruction for complex trochlear dysplasia or when considering trochleoplasty (rare in most practices).

Ultrasound: can visualise the MPFL dynamically in experienced hands but is not routine for surgical planning.

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The decision to offer surgery after a first dislocation hinges on the risk of redislocation - and that risk is predictable from a handful of factors. This is what the examiner means by "high-risk anatomy".

These factors have been combined into validated tools - the Jaquith and Parikh model (skeletal immaturity, trochlear dysplasia, patella alta and contralateral instability, with recurrence rising from near zero with no factors to the great majority with all four) and the Patellar Instability Severity Score - but the practical message is the same: a young patient with a dysplastic trochlea and patella alta has a high redislocation risk, so early stabilisation is more readily justified, whereas a normal-anatomy adult with a first dislocation usually does well with rehabilitation.

MPFL injury sits at the centre of the patellofemoral instability spectrum. A competent MPFL can compensate for mild trochlear dysplasia or a modestly elevated TT-TG; once it is torn, even normal anatomy may become symptomatic. The modern approach is "anatomic restoration": repair or reconstruct the ligament when tissue is adequate, and address major bony contributors (tubercle osteotomy, trochleoplasty) when they are the dominant drivers of instability. The examiner wants to hear that you can localise the tear, decide between repair and reconstruction, place the femoral tunnel correctly, and recognise when an isolated MPFL procedure is not enough.

• Global epidemiology: Lateral patellar dislocation has an incidence of approximately 5-7 per 100,000 person-years, with the highest rates in adolescents and young adults, particularly females and those participating in pivoting or jumping sports. MPFL disruption is the essential soft-tissue lesion in the great majority of cases worldwide.

• Guidelines and consensus: There is broad international agreement (AAOS, ESSKA, British Patellofemoral Group, and others) that most first-time dislocations without large osteochondral injury are managed non-operatively with structured rehabilitation. MPFL reconstruction is indicated for recurrent instability after adequate rehabilitation or when significant bony malalignment or osteochondral pathology requires addressing. Anatomic femoral tunnel placement using the radiographic landmark on a true lateral image is emphasised in all technical descriptions.

• Registry evidence: Large joint registries (NJR, AJRR, AOANJRR, etc.) do not currently track isolated MPFL procedures in detail. Available case series and systematic reviews (including the 2020 properties review cited above) report good stability and return-to-sport rates after anatomic reconstruction in appropriately selected patients, with lower success when major bony factors are ignored. Revision rates are driven primarily by recurrent instability from technical error or untreated malalignment rather than by implant-related issues.

• Global practice variation: In high-demand athletic or military populations some surgeons offer earlier reconstruction after a first dislocation with a clear, repairable or reconstructible tear. In lower-demand or paediatric populations a longer trial of non-operative care is common. Combined procedures (MPFL reconstruction plus tibial tubercle osteotomy or trochleoplasty) are performed more frequently in centres with expertise in complex patellofemoral dysplasia. No country-specific billing codes are relevant to clinical decision-making or examination content.