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Introduction
  • Can be classified into the following
    • acute traumatic
      • occurs equally by gender
      • may occur from a direct blow (ex. helmet to knee collision in football)
    • chronic patholaxity
      • recurrent subluxation episodes
      • occurs more in women
      • associated with malalignment
    • habitual
      • usually painless
      • occurs during each flexion movement
      • pathology is usually proximal (e.g. tight lateral structures - ITB and vastus lateralis)
  • Epidemiology
    • demographics
      • most commonly occurs in 2nd-3rd decades of life
    • risk factors  
      • general factors
        • ligamentous laxity (Ehlers-Danlos syndrome)
        • previous patellar instability event 
        • "miserable malalignment syndrome" 
          • a term named for the 3 anatomic characteristics that lead to an increased Q angle 
            • femoral anteversion
            • genu valgum
            • external tibial torsion / pronated feet
      • anatomical factors
        • osseous 
          • patella alta 
            • causes patella to not articulate with sulcus, losing its constraint effects
          • trochlear dysplasia
          • excessive lateral patellar tilt (measured in extension)
          • lateral femoral condyle hypoplasia
        • muscle
          • dysplastic vastus medialis oblique (VMO) muscle
          • overpull of lateral structures
            • iliotibial band
            • vastus lateralis
  • Pathophysiology
    • mechanism
      • usually on noncontact twisting injury with the knee extended and foot externally rotated
        • patient will usually reflexively contract quadriceps thereby reducing the patella
        • osteochondral fractures occur most often as the patella relocates
      • direct blow 
        • less common
        • ex. knee to knee collision in basketball, or football helmet to side of knee
Anatomy
  • Passive stability
    • medial patellofemoral ligament (MPFL)
      • femoral insertion origin is between medial epicondyle and adductor tubercle   
        • is usual site of avulsion of MPFL 
      •  is primary restraint in first 20 degrees of knee flexion
    • patellar-femoral bony structures account for stability in deeper knee flexion
      • trochlear groove morphology, patella height, patellar tracking
  • Dynamic stability
    • provided by vastus medialis  (attaches to MPFL)
Presentation
  • Symptoms
    • complaints of instability
    • anterior knee pain
  • Physical exam 
    • acute dislocation usually associated with a large hemarthrosis  
      • absence of swelling supports ligamentous laxity and habitual dislocation mechanism
    • medial sided tenderness (over MPFL)
    • increase in passive patellar translation
      • measured in quadrants of translation (midline of patella is considered "0"), and also should be compared to contralateral side
      • normal motion is <2 quadrants of patellar translation
        • lateral translation of medial border of patella to lateral edge of trochlear groove is considered "2" quadrants and is considered abnormal amount of translation
    • patellar apprehension 
      • passive lateral translation results in guarding and a sense of apprehension
    • increased Q angle
    • J sign post
      • excessive lateral translation in extension which "pops" into groove as the patella engages the trochlea early in flexion
      • associated with patella alta
Imaging
  • Radiographs
    • rule out fracture or loose body
      • medial patellar facet (most common)  
      • lateral femoral condyle 
    • AP views 
      • best to evaluate overall lower extremity alignment and version
    • lateral views
      • best to assess for trochlear dysplasia
      • evaluate for patellar height (patella alta vs baja)
        • Blumensaat's line should extend to inferior pole of the patella at 30 degrees of knee flexion 
        • Insall-Salvati method   
          • normal between 0.8 and 1.2
        • Blackburne-Peel method 
          • normal between 0.5 and 1.0
        • Caton Deschamps method 
          • normal between 0.6 and 1.3
        • Plateau-patella angle 
          • normal between 20 and 30 degrees
    • Sunrise / Merchant views
      • best to assess for lateral patellar tilt
      • lateral patellofemoral angle (normal is an angle that opens laterally )  
      • congruence angle (normal is -6 degrees)
  • CT scan
    • TT-TG Distance 
      • measures the distance between 2 perpendicular lines from the posterior cortex to the tibial tubercle  and the trochlear groove
      • >20mm usually considered abnormal 
  • MRI 
    • help further rule out suspected loose bodies 
      • osteochondral lesion and/or bone bruising 
      • medial patellar facet (most common)
      • lateral femoral condyle
    • tear of MPFL
      • tear usually at medial femoral epicondyle
Adult Treatment
  • Nonoperative
    • NSAIDS, activity modification, and physical therapy
      • indications
        • mainstay of treatment for first time patellar dislocator
          • without any loose bodies or intraarticular damage
        • habitual dislocator
      • techniques
        • short-term immobilization for comfort followed by 6 weeks of controlled motion
        • emphasis on strengthening 
          • closed chain short arc quadriceps exercises
          • Quad strengthening
          • core and hip strengthening to improve limb positioning and balance (hip abductors, gluteals, and abdominals) 
        • patellar stabilizing sleeve or "J" brace
        • consider knee aspiration for tense effusion
          • positive fat globules indicates fracture
  • Operative
    • Arthroscopic debridement (removal of loose body) vs Repair with or without stabilization
      • indications
        • displaced osteochondral fractures or loose bodies
        • may be an indication for operative treatment in a first-time dislocator
      • techniques
        • arthroscopic vs open removal versus repair of the osteochondral fragment 
        • primary repair with screws or pins if sufficient bone available for fixation 
    • MPFL repair 
      • indications
        • acute first time dislocation with bony fragment
      • techniques
        • direct repair when surgery can be done within first few days
          • no clinical studies support this over nonoperative treatment
    • MPFL reconstruction with autograft vs allograft     
      • indications
        • recurrent instability 
        • no significant underlying malalignment
      • techniques
        • gracilis or semitendinosus commonly used (stronger than native MPFL)
        • femoral origin can be reliably found radiographically (Schottle point)  
    • Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer)   
      • indications
        • may be used in addition to MPFL or in isolation for significant malalignment
        • TT-TG >20mm on CT  
      • techniques
        • anteromedialized displacement of osteotomy and fixation
        • correct TT-TG to 10-15mm (never less than 10mm)
    • tibial tubercle distalization
      • indications
        • patella alta
      • techniques
        • distal displacement of osteotomy and fixation
    • lateral release
      • indications
        • isolated release no longer indicated for instability
        • only indicated if there is excessive lateral tilt or tightness after medialization  
      • technique
        • arthroscopic 
    • trochleoplasty
      • indications
        • rarely addressed (in the USA) even if trochlear dysplasia present
        • may consider in severe or revision cases 
      • techniques
        • arthroscopic or open trochlear deepening procedure
Pediatric Treatment
  • Same principles as adults in general but 
    • must preserve the physis
      • do not do tibial tubercle osteotomy (will harm growth plate of proximal tibia) 
Complications
  • Recurrent dislocation
    • redislocation rates with nonoperative treatment may be high (15-50%) at 2-5 years
  • Medial patellar dislocation and medial patellofemoral arthritis
    • almost exclusively iatrogenic as a result of prior patellar stabilization surgery 
 

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Technique Guides (3)
Questions (7)

(OBQ12.72) Which of the following structures attaches between the medial epicondyle and adductor tubercle of the femur? Review Topic

QID:4432
1

Medial head of gastrocnemius

7%

(289/4318)

2

Medial collateral ligament

18%

(795/4318)

3

Semimembranosus

3%

(113/4318)

4

Adductor magnus

6%

(269/4318)

5

Medial patellofemoral ligament

66%

(2837/4318)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5

The femoral attachment of the medial patellofemoral ligament (MPFL) is located between the femoral medial epicondyle and the adductor tubercle.

During lateral patellar dislocation, the femoral attachment of the MPFL is a common site of injury and avulsion. Traumatic injury or laxity to the MPFL can cause future patellar instability, as the MPFL is the primary restraint to lateral patellar translation in the first 20 degrees of knee flexion. Surgery for reconstruction of the MPFL requires an understanding of the anatomic landmarks for drilling the femoral socket.

Wijdick et al. used radiopaque markers implanted into the femoral and tibial attachments of the superficial medial collateral ligament and the femoral attachments of the posterior oblique and medial patellofemoral ligaments of cadaveric knees. On the AP radiographs, the attachment site of the MPFL was an average of 42.3 mm from the femoral joint line. On the lateral radiograph, the MPFL was an average of 8.9 mm from the adductor tubercle and was located in the anteroproximal quadrant.

Schottle et al. in a cadaveric study looked at radiographic landmarks for femoral tunnel placement in MPFL reconstruction. A reproducible anatomical and radiographic point, 1 mm anterior to the posterior cortex extension line, 2.5 mm distal to the posterior origin of the medial femoral condyle, and proximal to the level of the posterior point of the Blumensaat line on a lateral radiograph with both posterior condyles projected in the same plane, represented the mean femoral MPFL isometric center.

Illustration A shows a cadaveric dissection of the MPFL femoral attachment between the medial epicondyle and adductor tubercle. Illustration B shows a radiographic example of the MPFL femoral attachment between the medial epicondyle and the adductor tubercle. Illustration C shows Schottle's point which can be reliably found radiographically just anterior to the posterior femoral cortex, and proximal to Blumensaat's line on a lateral radiograph.

Incorrect answers:
Answer 1- Medial head of gastrocnemius originates off posterior aspect of medial femoral condyle.
Answer 2- MCL attaches approximately 3.2 mm proximal and 4.8 mm posterior from the medial femoral epicondyle.
Answer 3- Semimembranosus inserts onto posterior surface of the medial tibial condyle.
Answer 4- Adductor magnus inserts onto adductor tubercle.

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(OBQ11.188) Which of the following best describes the radiographic landmarks on a lateral radiograph for locating the femoral attachment of the medial patellofemoral ligament (MPFL) during reconstruction? Review Topic

QID:3611
1

The intersection of a line extended from the middle of the shaft and Blumensaat's line

7%

(182/2509)

2

Anterior to a line extended from the middle of the shaft and Blumensaat's line

6%

(160/2509)

3

Posterior to a line extended from the posterior cortex of the shaft and distal to Blumensaat's line

7%

(174/2509)

4

Anterior to a line extended from the posterior cortex of the shaft and distal to Blumensaat's line

14%

(348/2509)

5

Anterior to a line extended from the posterior cortex of the shaft and proximal to Blumensaat's line

65%

(1620/2509)

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PREFERRED RESPONSE 5

Correct positioning of a graft for MPFL reconstruction requires accurate placement of the femoral attachment site which is anterior to a line extended from the posterior cortex and just proximal to the posterior extension of Blumensaat's line. Intra-operative fluoroscopy can be used to accurately identify this position.

Schottle et al have described the radiographic landmark to be 1 mm anterior to the posterior cortex extension line, 2.5 mm distal to the posterior origin of the medial femoral condyle, and proximal to the level of the posterior point of the Blumensaat line.

Redfern et al evaluate this radiographic point and found it to be within 4mm of the true attachment on anatomic dissection.

This is demonstrated in Illustration A: the star shows the correct location anterior to the posterior cortical extension line (solid black line), proximal to Blumensaat's (red) line, and between the posterior origin of the medial femoral condyle and the posterior aspect of Blumensaat's line (perpendicular dashed lines).

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(OBQ10.188) A 27-year-old football player sustains an acute lateral patellar dislocation. Which of the following is the most likely site of injury seen on MRI? Review Topic

QID:3281
1

Midsubstance oblique retinacular ligament rupture

17%

(366/2128)

2

Soft-tissue avulsion of medial patellofemoral ligament

56%

(1185/2128)

3

Midsubstance medial patellofemoral ligament rupture

17%

(371/2128)

4

Partial quadriceps tendon rupture

3%

(73/2128)

5

Bony avulsion of medial patellofemoral ligament

6%

(121/2128)

Select Answer to see Preferred Response

PREFERRED RESPONSE 2

The most common site of medial patellofemoral ligament (MPFL) injury is a soft-tissue avulsion injury of the ligament. Both midsubtance and soft tissue avulsions are more common than bony avulsions.

Bony avulsion off the patella can occur as well, and according to the referenced study by Torisuka et al, the MPFL remains attached to the medial patellar fragment and excellent clinical and radiographic results can occur with open reduction and fixation with suture anchors.

The reported study by Nomura et al reported that MRI is >80% sensitive and >80% specific (regarding location) for detecting MPFL injuries as well as their location. In addition, they noted a 96% MPFL injury rate with patellar dislocation.


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(OBQ08.49) A high school softball player has chronic activity-related anterior knee pain without a history of instability. Which radiographic measurement is used to indicate when a lateral retinacular release may be helpful? Review Topic

QID:435
1

Congruence angle

14%

(183/1326)

2

Q angle

23%

(306/1326)

3

Sulcus angle

9%

(123/1326)

4

Lateral patello-femoral angle

53%

(707/1326)

5

Patellar height index

0%

(5/1326)

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PREFERRED RESPONSE 4

The lateral patello-femoral angle is the angle formed by lateral patellar facet and a line drawn across most prominent aspects of anterior portion of the trochlea on a CT scan or Sunrise view radiograph. If there is a negative patellar tilt on this measurement, the patient may benefit from a lateral release for pain relief. Lateral release is not used for instability. The sulcus angle refers to the depth of the trochlea; the congruence angle measures the relationship of the center of the patella to the center of the trochlea. These are used to assess malalignment and instability.

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(OBQ07.30) An athlete sustains a traumatic patellar dislocation. The MRI shows a hemarthrosis with a floating osteochondral fragment. Which of the following is the most likely site of origin for the loose fragment? Review Topic

QID:691
1

The lateral patellar facet

12%

(146/1222)

2

The medial patellar facet

84%

(1032/1222)

3

The odd patellar facet

1%

(11/1222)

4

The medial trochlea

2%

(26/1222)

5

The central trochlea

0%

(6/1222)

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PREFERRED RESPONSE 2

Lateral patellar dislocations are by far the most common. The medial facet of the patella will impact on the lateral trochlear ridge. Either of these would be acceptable answers, but lateral trochlea is not listed. Even without an osteochondral fracture, an MRI of an acute patellar dislocation will generally show bone bruises in these two locations.

Nomura looked at 39 consecutive arthroscopies after lateral dislocation and 95% had patellar chondral defects. They also reported that the medial facet of the patella is the most commonly injured site of the patella.


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(OBQ07.112) You see a patient in the emergency room with an acute lateral patellar dislocation. Which of the following factors is associated with the highest risk of persistent patellar instability? Review Topic

QID:773
1

Younger age

2%

(9/421)

2

Increased Q-angle

13%

(54/421)

3

Male gender

0%

(1/421)

4

Previous patellar instability event

83%

(348/421)

5

Amount of lateral patellar tilt

1%

(4/421)

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PREFERRED RESPONSE 4

Females (not males) have a higher incidence of patellofemoral instability due to their increased Q-angle. The Q-angle or quadriceps angle is the angle formed by the intersection of a line from the ASIS to the patella and from the patella to the tibial tubercle as demonstrated in Illustration A. Normal Q-angle in males is 14 degrees and in females is 18 degrees. A higher angle means that there is a larger lateral vector force on the patella, which predisposes to lateral patellar instability. While an increased Q-angle increases the chance for dislocation, a previous history of dislocation is the strongest predictor.

Fithian et al prospectively followed 189 patients for 2-5 years and found that the risk was highest among females 10 to 17 years old and those with previous instability episodes. Patients with a prior history had 7 times higher odds of subsequent instability episodes during follow-up than first time dislocators.

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(OBQ04.46) All of the following are predisposing factors for lateral patellar dislocation in a native knee EXCEPT? Review Topic

QID:107
1

Excess femoral internal rotation

3%

(14/448)

2

Excess external tibial rotation

16%

(70/448)

3

Lateral femoral condylar hypoplasia

2%

(11/448)

4

Increased Q-angle

2%

(9/448)

5

Insufficiency of the vastus lateralis

76%

(341/448)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5

Predisposing factors to lateral patellar dislocation include: excess femoral internal rotation, external rotation of the tibia, lateral femoral condyle hypoplasia, insufficiency of the VMO, an increased Q angle, a tight lateral retinaculum, patella alta, patella tilt, generalized ligamentous laxity, and patellofemoral dysplasia. The common theme throughout this list of predisposing features is an inequality in forces acting medially and laterally on the patella, with resultant higher rates of lateral patellar dislocation.

Dejour et al examined CT scans on 134 patients treated for patellar instability. They identified 4 common factors the unstable symptomatic knees: 1. Trochlear dysplasia (85%), 2. Quadriceps dysplasia (83%), 3. Patella alta (24%), 4. Tibial tuberosity-trochlear groove, pathological when greater than or equal to 20 mm (56%).


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