Patellar instability by definition is a disease where the patella bone pathologically disarticulates out from the patellofemoral joint. This most often involves multiple factors from acute trauma, chronic ligamentous laxity, bony malalignment, connective tissue disorder, or anatomical pathology. Overtime patients that have patellar instability can have debilitating pain, limitations in basic function, and longterm arthritis [1] [2] [3] [4]. 

Epidemiology:

Patella dislocations account for 3% of all knee injuries [5]. The majority of injuries and pathology occurs in young individuals. In particular, most patients with patellar instability are age 10-16 years old and female [6] [5]. The incidence of patellar instability in the general population is 5.8 per 100,000 and 29 per 100,000 in the 10-17-year-old age group[5] [2]. Many cases of first-time dislocations without loose bodies or articular damage are treated conservatively. However, the recurrence rate after conservative treatment can be up to 15-44% [2]. Patients with a history of two or more dislocations have a 50% chance of recurrent dislocation episodes [5]. Having a previous patellar dislocation is associated with the highest risk of persistent patellar instability later in life. Furthermore, patients with a known medial patellofemoral ligament (MPFL) injury confirmed on MRI, the rates of recurrence are even higher [7]. With these recurrence rates, first-time dislocators can continue to have pain, functional limitations, and instability. 

Patellar instability can be summarized, and each entity will be discussed further below:

• Young patients (10-17 years-old)
• Acute traumatic episode
• Chronic patholaxity - Ehlers-Danlos syndrome
• Bony malalignment - valgus alignment, femoral anteversion, tibial external rotation
• Anatomical pathology - trochlear dysplasia
• Eventual progression to pain, functional decline, and long term arthritis

Classification:

Patellofemoral instability is classified descriptively [8] [9]:

• Acute (first dislocation)
• Subluxation or dislocation
• Traumatic
• Patellar instability
• Recurrent
• Habitual dislocation - involuntary dislocation of the patella
• Passive patellar dislocation - with aid of apprehension maneuver
• Syndromic - patellar dislocation associated with a neuromuscular disorder, connective tissue disorder, or syndrome

Mechanism:

• Traumatic mechanisms can occur with a direct blow with a knee to knee collision or helmet to the side of the knee injury
• Noncontact twisting injury with the knee extended and the foot externally rotated

Evaluation:

• Presentation: [10] [1]
  • Patient age and gender
    • More likely in females
    • More likely in younger age groups (10-17 years old)
  • Record the number of previous dislocation or subluxation events
  • Complaints of instability
  • History of general ligamentous laxity
  • Any previous surgery
  • Pain location
    • Anterior knee pain
• Physical Examination : [10] [1]
  • Evaluate overall limb alignment
  • Hip and knee rotation should be noted
    • Excessive femoral anteversion will show the patients toes pointed in or "Pigeon Toed"
  • Presence of large hemarthrosis
    • Evidence of an acute injury
    • Absence of signs of trauma supports a chronic ligamentous laxity mechanism or a habitual mechanism
  • Medial sided tenderness over the medial patellofemoral ligament (MPFL)
  • Increase in passive patellar translation compared to the contralateral side
    • Midline is considered '0' quadrants of movement
    • Normal is < 2 quadrants of patellar translation
    • Lateral translation of medial border of the patella to the lateral edge of the trochlea is '2' quadrants of motion and considered abnormal
  • Apprehension sign - patella apprehension with passive lateral translation results in guarding and lack of a firm endpoint
  • J sign - excessive lateral translation in extension which then causes the patella to "pop" into the trochlear groove as patella engages the trochlea early in flexion
  • Assess the Q-angle
    • The angle formed by a line from the ASIS to the center of the patella and from the center of the patella to the tibial tubercle
    • The Q-angle in full extension can be falsely normal because the patella is not engaged in the trochlea and not on tension. Therefore it is recommended to assess the Q-angle in slight flexion and this is more reliable and more accurate. 
• Imaging: [1]
  • Radiographs will rule out loose bodies
    • Most common is the medial patellar facet
    • Lateral femoral condyle
  • AP radiographs
    • Best for evaluating overall lower extremity alignment
  • Lateral radiographs
    • Patellar height (Patella alta versus baja)
      • Blumensaats line should extend to the inferior pole of the patella at 30 degrees of knee flexion
      • Insall-Salvati measurement - (0.8 - 1.2)
        • Patellar length compared to the patellar tendon length. If the ratio is >1.2, this indicates patella alta
      • Blackburn-Peel measurement - (0.5-1)
        • Compares the articular cartilage surface of the patella and its distance to the line projected toward the tibial plateau. A ratio >1 indicates patella alta
      • Caton-Deschamps - (0.6-1.3)
        • Compares the articular surface length of the patella to the anterior superior surface of the tibial plateau. A ratio > 1.3 indicates patella alta
      • The Caton-Deschamps and Blackburn-Peel measurements have higher reliability and can show change after a tibial tubercle osteotomy is performed
    • Patellar tilt
    • Trochlear dysplasia
      • Crossing sign - seen on lateral radiograph the trochlear groove lies in the same plane as anterior border of the lateral femoral condyle
        • Represents a flat trochlear groove
      • Double contour sign - the anterior border of the lateral femoral condyle lies anterior to the anterior border of the medial femoral condyle
        • Represents a convex trochlear groove/hypoplastic medial femoral condyle
      • Supratrochlear spur
  • Sunrise/merchant views
    • Best assess for patellar tilt
    • Lateral patellofemoral angle
      • A line parallel to the lateral patellar facet and a line drawn across the posterior femoral condyles
      • The normal angle is >11 degrees opening laterally
    • Congruence angle is an index of subluxation
      • Measured from a line through the apex of the patella to a line bisecting the trochlea
      • If the congruence angle is lateral to the congruence line it is considered positive
      • If the congruence angle is medial to the congruence line it is considered negative
      • The normal angle is < (-)6 meaning the more positive the angle, the more subluxed the patella is laterally
  • CT scan
    • Evaluates femoral anteversion
    • Evaluation of tibial rotation
    • TT-TG distance (tibial tubercle to trochlear groove)
      • Must be measured on axial images - it is measured by taking a line on axial CT perpendicular to the posterior femoral condyles through the trochlear notch and a line through the middle of the tibial tubercle
      • TT-TG distance is normally around 9mm [11]
      • TT-TG distance > 20mm is abnormal and has > 90% association with patellar instability [12]
  • MRI
    • Evaluation of loose bodies
      • Osteochondral lesions
      • Medial patellar facet  is the most common
      • Lateral femoral condyle bone bruising
      • Most of the traumatic lesions occur during re-location impact
    • Best for assessing MPFL [13]
      • Location of injury 
        • The most common injury occurs at the femoral origin (Schottles point)
        • Patellar attachment 
        • Midsubstance
        • Combination

Treatment:

• Nonoperative [10] [14][15]
  • Closed reduction (majority spontaneously reduce on their own), NSAIDs, activity modification, and physical therapy
    • Indications:
      • First-time dislocation
      • No loose bodies or articular damage
      • No osteochondral fragments
      • Habitual dislocators
      • Patients with connective tissue disease - Ehlers Danlos
    • Physical therapy should focus on closed chain exercises and quadriceps strengthening. Core hip strengthening and gluteal muscle strengthening will improve external rotators of the hip, thus externally rotating the femur and decreasing the Q-angle. 
    • Patella sleeve - 'J' sleeve
    • Patellar taping
• Operative [10][1][16][17][18][19][20][21][1]
  • General indications for surgery:
    • Osteochondral injury with loose body
    • Chronic instability 
    • Failure of nonsurgical treatment 
  • Arthroscopic debridement with removal of loose bodies
    • Indications:
      • Loose bodies or osteochondral damage on imaging
    • Open reduction internal fixation if there is sufficient bone available for fixation
      • Screws and pins
  • Medial patellofemoral ligament (MPFL) repair
    • Indications:
      • Acute first-time dislocation with a bony fragment
    • Direct repair with surgery can be performed within the first days after injury
    • No study supports this method over nonoperative treatment 
  • MPFL reconstruction with autograft versus allograft
    • Indications:
      • Recurrent instability and no malalignment or trochlear dysplasia 
    • Gracillus and semitendinosus commonly used
    • Femoral origin can be reliably found (Schottles point)
      • Schottles point is described as 1mm anterior to the posterior cortex line, 2.5mm distal to the posterior origin of the medial femoral condyle, and proximal to the posterior point of Blumensaats line 
    • Tensioning the graft should be done between 60-90 degrees of knee flexion is recommended 
  • Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer)
    • Indications:
      • Malalignment - Patellofemoral maltracking with degenerative changes on the distal and lateral aspects of the patella 
      • TT-TG > 20mm
    • Decreases pressure on the lateral patellar facet and overall trochlea 
    • Fulkerson showed poorer results with Outerbridge grade 3 or 4 lesions and lesions in the center of trochlea or medial aspect of the trochlea
    • Likely will fail when there are large central grade 3 or 4 lesions on the trochlea or medial, proximal, or diffuse patella arthritis
  • Lateral soft tissue release
    • Lateral release has been shown to be ineffective for the treatment of patellar instability
    • Used for lateral compression syndrome where there is combined or isolated patellar tilt or excessive tightness after medialization procedure
    • Usually, this is combined with a medialization procedure and not done in isolation 
  • Trochleoplasty - sulcus deepening of the distal femoral trochlea
    • Limited use in the U.S. due to serious irreversible articular and subchondral injury to the trochlea
    • Indicated for abnormal patellar tracking with J sign caused by femoral trochlear dysplasia
    • Radiographic evidence of trochlear dysplasia
    • The cancellous bone is exposed in the trochlea and elevating a strip of cortical bone on the edge of the trochlea. The new trochlea sulcus is created and the trochlear bone shell is impacted and secured to the new sulcus fixed with staples or suture

The makeup of the patellofemoral joint is the undersurface of the patella and the cartilaginous anterior surface of the distal femur, comprising the trochlear groove. The proper biomechanics of the patellofemoral joint requires an intact and anatomic trochlear groove and inline congruent forces acting on the patella so that it can glide across the trochlear groove smoothly. Any disruption in this mechanism will produce dislocation of the patella out of the trochlear groove.

The bony structures of the patellofemoral joint do provide inherent stability to the patella, and any defect in the bony surface will result in instability. The majority of the patella articulation occurs between the lateral facet and the lateral trochlear groove.  The normal trochlear groove has a large depth and steepness that provides inherent stability to the patellofemoral joint [22]. Trochlear dysplasia and flattening of this groove will cause patellar instability. The cartilage on the undersurface of the patella is 6-7mm thick, the thickest in the body. The patella itself is a sesamoid bone enveloped in the quadriceps tendon. The major function of the patella is to increase the mechanical advantage of the muscle for knee extension [11]. The patella has 7 facets. The major facets being medial and lateral, which are further divided into thirds. A seventh facet is on the most medial edge of the patella and called the odd facet. Any osteochondral fracture or defect will lead to pain and instability and requires fixation.

Besides bony support, even more, important to the stability of the patella is the soft tissue envelope. The quadriceps inserts at the superior patella and encases the patella until it merges with the patellar tendon. The patellar tendon originates at the inferior aspect of the patella and inserts onto the tibial tubercle. The quadriceps tendon, patella, and patellar tendon combined make up the extensor mechanism of the knee. Disruption of the extensor mechanism along its length will result in significant patellar instability and maltracking. 

The medial ligamentous constraints are just as important. The medial patellofemoral ligament (MPFL), patellomeniscal ligament, patellotibial ligament, and retinaculum of the knee capsule all play roles in preventing lateral patella motion and keeping the patella congruent in the trochlear groove. The strongest of these is the MPFL, which originates from the adductor tubercle to insert onto the superior medial border of the patella. The MPFL is the primary restraint to lateral patellar instability [23]. The major focus in treatment when performing an MPFL reconstruction is the location and ideal femoral position to restore normal anatomic knee biomechanics. The width of the femoral attachment of the MPFL is 11-20mm between the medial epicondyle and adductor magnus insertion [24] [25] [26]. Radiographically, Schottles point can be determined on fluoroscopy and is an accurate reproducible location for the femoral attachment. Schottles point is described as 1mm anterior to the posterior cortex line, 2.5mm distal to the posterior origin of the medial femoral condyle, and proximal to the posterior point of Blumensaats line [27]. The MPFL contributes 50-60% of resistance to lateral translation of the patella [28]. Any laceration, avulsion, or traumatic disruption of MPFL will lead to lateral patellar instability. Once the patella begins to enter the trochlea at 20-30 degrees of flexion, the bony anatomy begins to be the major stabilizer as the patella sits in the trochlear groove. With patella alta, the patella does not engage in the groove until higher than normal knee flexion, and this predisposes the patella to dislocation [29][1]. 

Lateral soft tissue restraints also can contribute to patellar instability but to a lesser degree. Studies have shown lateral structures to contribute to 22% fo lateral translation and stability in lateral patellar dislocations [1]. These include the deep lateral transverse retinaculum, epicondylopatellar ligament, and patellotibial band. The epicondylopatellar ligament travels from the lateral femoral epicondyle and attaches directly to the tibia via the iliotibial band (ITB). Therefore, the tightness of the ITB influences lateral stability as well.

Dynamic stability is also a contributor to patellar stability. The vastus medialis muscle confers medial restraint to lateral translation. The vastus lateralis confers a lateral restraint to medial translation.

Knee alignment is a key factor in patellar stability. The normal alignment of the knee joint is in 6 degrees of valgus. The mechanical axis of the knee is a line drawn from the center of the hip to the center of the ankle. This should cross through the center of the knee. The anatomic axis of the knee is a line that goes through the center of the femoral and tibial shaft. In general, the anatomic axis of the femur is 6 degrees from the mechanical axis. Therefore, the knee angle (femoraltibial angle) is approximately 6 degrees of valgus (in relation from the mechanical axis). An increased valgus at the knee off normal will force the patella out of the trochlear groove laterally and contribute to patellar instability [30]. 

Normal femoral anteversion is 15-20 degrees. Femoral anteversion > 20 degrees increases the risk for patellar dislocation laterally because it produces a high laterally directed force across the patella [31] [32]. It also increases the Q-angle leading to increased lateral force across the patella. 

External tibial torsion also will contribute to lateral patellar instability. Normal tibial external rotation during childhood is 15 degrees [33]. The indications for tibial derotational osteotomy are poorly defined. Staheli recommends surgical correction for severe rotational deformities > 30 degrees. Typically external tibial torsion requires surgical correction with either a supramalleolar derotational osteotomy or a proximal tibial derotational osteotomy [33] [34].

The combination of increased femoral anteversion, increased genu valgus, and increased external tibia torsion is named, "miserable malalignment syndrome." All of these entities lead to an increase in the Q-angle and subsequently increased risk for patellar instability [35] [36]. 

Trochlear Dysplasia is a disease characterized by a shallow trochlear groove. It is thought to be caused by either congenital factors or lateral patellar tracking. Lateral patellar tracking will eventually flatten the trochlear groove leading to patellar instability. Regardless of the cause, trochlear dysplasia results in a loss of the inherent bony stability of the patellofemoral joint.

Lastly, disorders that effect collagen include Ehlers-Danlos syndrome or Down Syndrome. These diseases cause a chronic progression of ligamentous laxity. When the soft tissue structures around the knee become lax, the patella loses its restraints and these patients typically will complain of habitual dislocations. These patients will have a classic presentation of Ehlers-Danlos with hyperelastic skin, joint hypermobility, and generalized ligamentous laxity. The classic findings from Beighton are passive hyperextension of the digits >90 degrees, passive abduction of the thumb to the forearm, hyperextension of the knee, hyperextension of the elbows, and forward bending with palms on the floor. 

Other anatomical factors [10]:

• Patella alta - causes the patella to not articulate with the sulcus
• Excessive lateral patellar tilt caused by poor quadriceps flexibility and tight lateral structures
• Lateral femoral condyle hypoplasia
• Muscular
  • Dysplasia of the vastus medialis oblique (VMO) muscle
  • Overpull of ITB or vastus lateralis muscle