Patellar Instability

Excerpt

Patellar instability, by definition, is a condition where the patella bone pathologically disarticulates out from the patellofemoral joint, either subluxation or complete dislocation. This most often involves multiple factors, from acute trauma, chronic ligamentous laxity, bony malalignment, connective tissue disorder, or anatomical pathology. Over time, patients with patellar instability can have debilitating pain, limitations in basic function, and long-term arthritis.

Epidemiology

Patella dislocations account for 3% of all knee injuries. The majority of injuries and pathology occurs in young individuals. In particular, most patients with patellar instability are aged 10 to 16 years old and female. The incidence of patellar instability in the general population is 5.8 per 100,000 and 29 per 100,000 in the 10 to 17-year-old age group. 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 to 44%. Patients with a history of two or more dislocations have a 50% chance of recurrent dislocation episodes. A previous patellar dislocation is associated with the highest risk of persistent patellar instability later in life. Furthermore, in patients with a known medial patellofemoral ligament (MPFL) injury confirmed on MRI, the recurrence rates are even higher. 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:

1. Young patients (10 to 17 years old)

2. Acute traumatic episode

3. Chronic patholaxity - Ehlers-Danlos syndrome

4. Bony malalignment - femoral anteversion, genu valgum, and external tibial torsion / pronated feet. The three bony malalignments combined are termed "Miserable Malalignment Syndrome" and lead to an increased Q angle.

5. Anatomical pathology - trochlear dysplasia

6. Eventual progression to pain, functional decline, and long-term arthritis

Classification

Patellofemoral instability is classified descriptively. These classifications are listed below:

1. Acute (first dislocation)

2. Subluxation or dislocation

3. Traumatic

4. Patellar instability

5. Recurrent

6. Habitual dislocation - involuntary dislocation of the patella

7. Passive patellar dislocation - with the aid of apprehension maneuver

8. Syndromic - patellar dislocation associated with a neuromuscular disorder, connective tissue disorder, or syndrome

Mechanism

1. Traumatic mechanisms can occur with a direct blow with a knee-to-knee collision or a helmet to the side of the knee injury

2. Noncontact twisting injury with the knee extended and the foot externally rotated

Evaluation

1. Presentation:

   1. Patient age and gender

      1. More likely in females

      2. More likely in younger age groups (10-17 years old)

   2. Record the number of previous dislocation or subluxation events

   3. Complaints of instability

   4. History of general ligamentous laxity

   5. Any previous surgery

   6. Pain location

      1. Anterior knee pain

2. Physical Examination: Examination will evaluate a number of areas.

   1. Evaluate overall limb alignment

   2. Hip and knee rotation should be noted

      1. Excessive femoral anteversion will show the patient's toes pointed in or "pigeon toed"

   3. Presence of large hemarthrosis

      1. Evidence of an acute injury

      2. The absence of signs of trauma supports a chronic ligamentous laxity mechanism or a habitual mechanism

   4. Medial-sided tenderness over the medial patellofemoral ligament (MPFL)

   5. Increase in passive patellar translation compared to the contralateral side

      1. Midline is considered '0' quadrants of movement

      2. Normal is < 2 quadrants of patellar translation

      3. Lateral translation of the medial border of the patella to the lateral edge of the trochlea is '2' quadrants of motion and considered abnormal

   6. Apprehension sign - patella apprehension with passive lateral translation results in guarding and lack of a firm endpoint

   7. J sign - excessive lateral translation in extension, which then causes the patella to "pop" into the trochlear groove as the patella engages the trochlea early in flexion

   8. Assess the Q-angle

      1. 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

      2. 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, which is more reliable and accurate.

3. Imaging: Radiographic examination will divulge several factors.

   1. Radiographs will rule out loose bodies

      1. Most common is the medial patellar facet

      2. Lateral femoral condyle

   2. AP radiographs

      1. Best for evaluating overall lower extremity alignment

   3. Lateral radiographs

      1. Patellar height (Patella Alta versus Baja)

         1. Blumensaats line should extend to the inferior pole of the patella at 30 degrees of knee flexion

         2. Multiple ratios can be calculated and give an idea about the level of the patella. Ideally, the following ratio should be calculated with the knee in 30 degrees of flexion. Either on a lateral radiograph, Sagittal CT, or MRI images.

         3. Insall-Salvati ratio (0.8 - 1.2)

            1. It is the ratio of the patellar tendon length to the length of the patella (Figure 2)

            2. If the ratio is >1.2, this indicates Patella Alta

         4. Blackburn-Peel ratio (0.5-1)

            1. It is the ratio of the perpendicular distance between the tibial plateau and patellar articular surface to the length of the patella articular surface. (Figure 3) A ratio >1 indicates Patella Alta

         5. Caton-Deschamps - (0.6-1.3)

            1. It is the ratio of the distance between the most inferior point of the patella articular surface to the anterior angle of the tibial plateau and the length of the patellar articular surface. ( Figure 4). A ratio > 1.3 indicates Patella Alta

         6. The Caton-Deschamps and Blackburn-Peel measurements have higher reliability and can show change after a tibial tubercle osteotomy is performed

      2. Patellar tilt

      3. Trochlear dysplasia

         1. Crossing sign - seen on lateral radiograph, the trochlear groove lies in the same plane as the anterior border of the lateral femoral condyle

            1. Represents a flat trochlear groove

         2. Double contour sign - the anterior border of the lateral femoral condyle lies anterior to the anterior border of the medial femoral condyle

            1. Represents a convex trochlear groove/hypoplastic medial femoral condyle

         3. Supratrochlear spur

   4. Sunrise/merchant views

      1. Best assessment for patellar tilt

      2. Lateral patellofemoral angle

         1. A line parallel to the lateral patellar facet and a line drawn across the posterior femoral condyles

         2. The normal angle is >11 degrees opening laterally

      3. Congruence angle is an index of subluxation

         1. Measured from a line through the apex of the patella to a line bisecting the trochlea

         2. If the congruence angle is lateral to the congruence line, it is considered positive

         3. If the congruence angle is medial to the congruence line, it is considered negative

         4. The normal angle is < (-)6 meaning the more positive the angle, the more subluxed the patella is laterally

   5. CT scan

      1. Evaluates femoral anteversion

      2. Evaluation of tibial rotation

      3. TT-TG distance (tibial tubercle to trochlear groove)

         1. Must be measured on axial images - it is calculated 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

         2. TT-TG distance is normally around 9 mm

         3. TT-TG distance > 20mm is abnormal and has > 90% association with patellar instability

   6. MRI

      1. Evaluation of loose bodies

         1. Osteochondral lesions

         2. The medial patellar facet is the most common

         3. Lateral femoral condyle bone bruising

         4. Most of the traumatic lesions occur during re-location impact

      2. Best for assessing MPFL

         1. Location of injury

            1. The most common injury occurs at the femoral origin (Schottles point)

            2. Patellar attachment

            3. Midsubstance

            4. Combination

Treatment

1. Nonoperative

   1. Closed reduction (majority spontaneously reduce on their own), NSAIDs, activity modification, and physical therapy

      1. Indications:

         1. First-time dislocation

         2. No loose bodies or articular damage

         3. No osteochondral fragments

         4. Habitual dislocators

         5. Patients with connective tissue disease - Ehlers Danlos

      2. 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.

      3. Patella sleeve - 'J' sleeve

      4. Patellar taping

2. Operative

   1. General indications for surgery:

      1. Osteochondral injury with loose body

      2. Chronic instability

      3. Failure of nonsurgical treatment

   2. Arthroscopic debridement with removal of loose bodies

      1. Indications:

         1. Loose bodies or osteochondral damage on imaging

      2. Open reduction internal fixation if there is sufficient bone available for fixation

         1. Screws and pins

   3. Medial patellofemoral ligament (MPFL) repair

      1. Indications:

         1. Acute first-time dislocation with a bony fragment

      2. Direct repair with surgery can be performed within the first days after injury

      3. No study supports this method over nonoperative treatment

   4. MPFL reconstruction with autograft versus allograft

      1. Indications:

         1. Recurrent instability and no malalignment or trochlear dysplasia

      2. Gracillis and semitendinosus commonly used

      3. Femoral origin can be reliably found (Schottles point)

         1. Schottle point is described as 1 mm anterior to the posterior cortex line, 2.5 mm distal to the posterior origin of the medial femoral condyle, and proximal to the posterior point of Blumensaats line

      4. Tensioning the graft should be done between 60 to 90 degrees of knee flexion is recommended

   5. Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer)

      1. Indications:

         1. Malalignment - Patellofemoral maltracking with degenerative changes on the distal and lateral aspects of the patella

         2. TT-TG > 20 mm

      2. Decreases pressure on the lateral patellar facet and overall trochlea

      3. Fulkerson showed poorer results with Outerbridge grade 3 or 4 lesions and lesions in the center of the trochlea or medial aspect of the trochlea

      4. Likely will fail when there are large central grade 3 or 4 lesions on the trochlea or medial, proximal, or diffuse patella arthritis

   6. Lateral soft tissue release

      1. The lateral release has been shown to be ineffective for the treatment of patellar instability

      2. Used for lateral compression syndrome where there is combined or isolated patellar tilt or excessive tightness after medialization procedure

      3. Usually, this is combined with a medialization procedure and not done in isolation

   7. Trochleoplasty - sulcus deepening of the distal femoral trochlea

      1. Limited use in the U.S. due to serious irreversible articular and subchondral injury to the trochlea

      2. Indicated for abnormal patellar tracking with J sign caused by femoral trochlear dysplasia

      3. Radiographic evidence of trochlear dysplasia

      4. The cancellous bone is exposed in the trochlea, and a strip of cortical bone on the edge of the trochlea is elevated. The new trochlea sulcus is created, and the trochlear bone shell is impacted and secured to the new sulcus fixed with staples or sutures.