Dislocated Kneecap

Pathophysiology

• General
  • It is important to take a thorough history to determine if the occurrence was a first-time dislocation
  • Patients may describe a popping sensation without obvious dislocation suggesting subluxation or dislocation with spontaneous reduction
• Lateral Dislocation
  • Most common by far
  • Occurs with contact during contact or collision sport
  • Without contact during aggressive cutting or pivoting maneuver
• Other Directions
  • Medial dislocations, intra-articular and superior dislocations are rare
  • Associated with congenital conditions, quadriceps atrophy or iatrogenic

Causes

Sudden knee pain can result from a traumatic injury, stress injury, or flare-ups from another underlying condition. Remember that it doesn’t take a severe injury to cause a partial tear of your ligament or to wear down your cartilage.

• Typically traumatic from:
  • Non-contact twisting injury to the knee
  • Direct blow to the medial aspect of the knee
  • Tibia is often externally rotated with foot planted, knee slightly flexed with a valgus position
• Generalized ligamentous laxity
  • These patients may have more benign etiology
  • Often subluxation events rather than frank dislocation

Pathoanatomy

• Vastus Medialis Obliquus (VMO)
  • The most distal portion of the medial quadriceps muscle
  • Exerts a medially directed force that helps keep the patella in position
• Medial retinaculum
• Medial Patellofemoral Ligament (MPFL)
  • Primary static restraint to lateral instability during the first 30° of flexion
  • Prevents excessive lateral movement of the patella
  • Almost universally torn during dislocations

Associated Conditions

• Miserable Malalignment Syndrome
• Osteochondral Defect Knee
• Patellar Instability
• Patellofemoral Pain Syndrome
• General
  • Female > male
  • Younger age
• Muscle related factors
  • Weakness or atrophy of Vastus Medialis Obliquus
  • The excessive tone of the Vastus Lateralis or Iliotibial band
• Anatomic factors
  • Increased Q Angle
  • Patella Alta or high riding patella
  • Trochlear dysplasia
  • Excessive lateral patellar tilt
  • Lateral femoral condyle hypoplasia
  • Laterally located tibial tubercle
• Generalized ligamentous laxity
• Connective Tissue Disorder
  • Marfan Syndrome
  • Ehlers Danlos Syndrome
• Down Syndrome
  • Due to congenitally small patella, hypoplastic condyle

Differential Diagnosis

• Fractures
  • Distal Femur Fracture
  • Patellar Fracture
  • Tibial Plateau Fracture
• Dislocations & Subluxations
  • Patellar Dislocation (and subluxation)
  • Knee Dislocation
  • Proximal Tibiofibular Joint Dislocation
• Muscle and Tendon Injuries
  • Quadriceps Contusion
  • Iliotibial Band Syndrome
  • Quadriceps Tendonitis
  • Patellar Tendonitis
  • Popliteus Tendinopathy
  • Extensor Mechanism Injury
    • Patellar Tendon Rupture
    • Quadriceps Tendon Rupture
    • Patellar Fracture
• Ligament Pathology
  • ACL Injury
  • PCL Injury
  • MCL Injury
  • LCL Injury
  • Meniscal Pathology
  • Posterolateral Corner Injury
  • Multiligament Injury
• Arthropathies
  • Knee Osteoarthritis
  • Septic Arthritis
  • Gout
• Bursopathies
  • Prepatellar Bursitis
  • Pes Anserine Bursitis
  • MCL Bursitis
  • Infrapatellar Bursitis
• Patellofemoral Pain Syndrome (PFPS)/ Anterior Knee Pain)
  • Chondromalacia Patellae
  • Patellofemoral Osteoarthritis
  • Osteochondral Defect Knee
  • Plica Syndrome
  • Infrapatellar Fat Pad Impingement
  • Patellar Instability
• Neuropathies
  • Saphenous Nerve Entrapment
• Other
  • Bakers Cyst
  • Patellar Contusion
• Pediatric Considerations
  • Patellar Apophysitis (Sinding-Larsen-Johansson Disease)
  • Patellar Pole Avulsion Fracture
  • Tibial Tubercle Avulsion Fracture
  • Tibial Tuberosity Apophysitis (Osgood Schalatters Disease)

Symptoms of a dislocated patella

Diagnosis

• History
  • Most describe the pain and potentially a noticeable deformity of the knee
  • Usually preceded by direct trauma to the knee or a sudden change in direction
  • Many describe a sense of giving way or instability when the dislocation occurs
  • Feel a pop or multiple pops and there may be generalized pain in the anteromedial knee
• Exam: Physical Exam Knee
  • The acutely dislocated patella is typically easy to see
  • May have joint effusion or hemarthrosis (#2 cause in pediatrics behind ACL^[6])
  • Carefully identify the patellar poles, medial and lateral joint lines, and retinaculum
  • Palpable tenderness along the medial retinaculum or MPFL may be appreciable
  • Patellar laxity, looseness, or increased motion may be noted
  • Important to stress other structural ligaments if possible
  • Range of motion is very limited in dislocation, may be intact in subluxation
  • Strength testing can be limited in knee extension, hip abduction
  • Normal patellar glide medially and laterally is between 25-50% of the width of the patella
  • Other findings include femoral anteversion, patella Alta, tibial torsion, genu recurvatum, genu valgum or varum, pes planus, and general ligamentous laxity
• Special tests
  • Special tests may need to be deferred in the acute setting depending on how symptomatic the patient is
  • Consider evaluating for ligamentous laxity using the Beighton Score^[7]
  • Patellar Apprehension Test: Apply medial and lateral pressure to the patella testing for apprehension
  • Patellar J Sign: Evaluate path of the patella during flexion and extension

Radiographs

• Standard Radiographs Knee
  • Ideally, standard AP and lateral weight-bearing views, as well as sunrise view
  • It May not be possible in the setting of acute dislocation
• Plain radiography
  • Help identify fractures of the patella, avulsion fractures, loose bodies, and sometimes large cartilage defects
  • PA radiographs at 45 degrees flexion may aid in the assessment of the coronal alignment of the tibiofemoral joint
• Lateral views and Sunrise or Merchant views
  • Provide information to trochlear morphology, patellar height, and patellar tilt
• Lateral patellar Tilt
  • Assessed by the lateral patellofemoral angle on sunrise or merchant view
  • Angle is measured between a line along the subchondral bone of the lateral trochlear facet and posterior femoral condyles
  • Normal: angle greater than 11° that opens laterally
  • Abnormal angles: parallel or open medially
• Patellar height
  • Can be measured by both direct and indirect methods
  • The Insall-Salvati Ratio: ratio measuring the length of the patella ligament, patellar length
    • A normal ratio is 1.0; a ratio of 1.2 suggests patella Alta and 0.8 patella Baja
  • Caton-Deschamps index: distance between the distal point of the patellar articular surface and the anterior superior margin of the tibia, divided by the patellar articular surface length
    • A normal ratio is 1.0; a ratio of less than 0.6 suggests patella baja and a ratio of 1.3 suggests patella alta
  • Blackburne-Peel method (BP): ratio of the height of the lower pole of the articular surface above a tibial plateau line to the articular surface length of the patella
    • Normal between 0.54- 1.06; A ratio of less than 0.54 is considered to be patella Alta
  • The technique described by Blumensaat uses the roof of the intercondylar notch as a reference line and is one of the most commonly used direct methods for the assessment of patellar height
• True lateral radiographs and sunrise views can help identify other risk factors
  • The trochlear findings were elucidated by Dejour and Le Coultre and were subsequently revised to create the trochlear dysplasia classification system ^[10]
  • Crossing sign: occurs when the trochlear groove lies in the same plane as the anterior border of the lateral condyle, which represents a flattened trochlear groove
  • Double contour sign: occurs when the anterior border of the lateral condyle lies anterior to the anterior border of the medial condyle, which represents a convex trochlear groove or hypoplastic medial condyle
  • A supratrochlear spur can arise from the proximal aspect of the trochlea and can also indicate a risk factor

CT

• Computed tomographic (CT)
  • Can more accurately characterize the morphology of the trochlea
  • Assess femoral and tibial torsion
• Tibial tubercle to trochlear groove (TT-TG) distance
  • Assesses relative rotation of femur to the tibia
  • The TT-GG distance is between two perpendicular lines; one from the posterior cortex to the tibial tubercle and one from the posterior cortex to the trochlear groove
  • Average 8-10 mm in pediatric and adult patients; a TT-TG distance of greater than 20 is highly associated with patellar instability.

MRI

• Indicated if any evidence of loose body or Osteochondral Lesion on radiographs
• Common Findings^[11]
  • The bruising pattern of the lateral femoral condyle, medial patella
  • Disruption of the MPFL (at the medial femoral epicondyle insertion)
  • Articular cartilage injuries if present

Classification

Dejour Classification of Trochlear Dysplasia

• Type A: flatter than normal with a sulcus angle greater than 145°
• Type B: which is flat
• Type C: which is convex
• Type D: which is convex with a supratrochlear spur

Treatment

Patellar Dislocation Reduction

• Acute dislocations require reduction
• Consideration should be made to transfer the patient to the emergency department depending on the clinical context
• The patient will require some degree of analgesia, possibly procedural sedation
• Procedure
  • A dislocation should be obvious
  • The patient should have his knee in a resting position of slight flexion
  • With a distal hand gently extend the knee
  • Simultaneously, the proximal hand applies gentle medial pressure to the patella
  • As the knee extends, the patella should slide over the lateral femoral condyle and pop into the groove
  • Patients typically feel much better after reduction is complete

Nonoperative

• Indications
  • First-time dislocation
  • No evidence of osteochondral defect, intra-articular damage
• Relative rest from offending activities
• NSAIDS, Ice Therapy
• Patellar J Brace
  • Often helpful initially to provide additional stability
• Kinesiology Tape can be considered
  • Especially McConnell technique
• Physical Therapy
  • Begin around 2-4 weeks

Operative

• Indications
  • Presence of osteochondral defect or loose body
  • Subluxation of the patella on sunrise view
  • Failure to improve with conservative management
  • Anatomic factors predisposing to dislocation
  • Recurrent Dislocation
• Technique
  • Arthroscopy with or without open debridement
  • MPFL repair (re-attachment) or reconstruction (proximal realignment)
  • Lateral release (distal realignment)
  • Tibial Osteotomy (distal realignment)
  • Trochleoplasty