Patellar instability can fall on the spectrum of frank dislocation to subtle subluxation, the dislocation is typically lateral.
- Patellar Instability most commonly occurs in 2nd-3rd decades of life.
- Recurrence rate following first-time dislocation is between 15% and 60%.
Risk factors for Patellar Instability include:
- Incompetence of the medial patellofemoral ligament (MPFL): The main factor that results in recurrent patellar dislocation.
- Ligamentous laxity (Ehlers-Danlos syndrome).
- Increased Q angle.
- Patella alta: causes patella to not articulate with sulcus, losing its constraint effects.
- Trochlear dysplasia.
- Excessive lateral patellar tilt.
- Lateral femoral condyle hypoplasia.
- Dysplastic vastus medialis oblique (VMO) muscle.
- Overpull (contracture) of lateral structures: iliotibial band and vastus lateralis muscle.
Mechanism of injury is usually noncontact twisting injury with the knee extended and foot externally rotated or a direct blow to medial aspect of the knee.
See Also: Anterior Shoulder Instability
Patella Anatomy & Biomechanics
Patella is the largest sesamoid bone, proximally it’s attached the the quadriceps tendons, while distally it’s attached to the patellar tendon.
Patella functions include the following:
- Fulcrum for quadriceps to facilitate knee extension.
- Protects knee joint.
- Enhances lubrication and nutrition of knee.
- Medial patellofemoral ligament (MPFL): it is attached proximally between adductor tubercle and medial femoral epicondyle (Schottle point), and distally to superomedial patella. It resists lateral translation of patella.
- Retinacular Ligament: it is attached proximally to Vastus medialis and vastus lateralis and tibial condyles, and distally to medial and lateral patella.
Brattström described the Q angle as an angle formed by the line of pull of the quadriceps mechanism and that of the patellar tendon as they intersect at the center of the patella.
- Clinically, this angle is represented by the intersection of a line drawn from the anterior superior iliac spine to the center of the patella with a second line drawn from the center of the tibial tuberosity to the center of the patella.
- For this measurement to be accurate, the patella must be centered on the trochlea by flexing the knee 30 degrees.
- This valgus angle gives a lateral force vector to the patellofemoral joint as the knee is extended.
- Normal Q angle:
- In males it should be 8 to 10 degrees.
- In females, the normal angle is 15 degrees ± 5 degrees.
- The factors that can increase this Q angle are:
- Genu valgum.
- Increased femoral anteversion.
- External tibial torsion.
- A laterally positioned tibial tuberosity.
- A tight lateral retinaculum.
- Any of these factors that increase the Q angle can be a contributing factor in Patellar Instability.
- Miserable Malalignment Syndrome: 3 anatomic characteristics that lead to an increased Q angle
- Femoral anteversion.
- Genu valgum.
- Pronated feet.
See Also: Ankle and Foot Anatomy
- The vastus lateralis pulls laterally to the frontal plane of the femur.
- The vastus medialis is divided into two parts:
- The vastus medialis longus.
- The vastus medialis obliquus.
Patella bone has static and dynamic stabilizers:
Static stabilizers of the patella include:
- Shape of the patella
- The femoral sulcus
- A patellar tendon of appropriate length
- Medial patellofemoral ligament (MPFL)
Dynamic stabilizers of the patella include:
- The vastus medialis obliquus muscle.
Static and dynamic forces tend to displace the patella laterally.
- Patients who have Patellar Instability frequently report diffuse pain around the knee (usually is located anterior in the knee) that is aggravated by going up and down stairs or hills.
- Patellar crepitation and swelling of the knee are common.
- A feeling of insecurity in the knee and occasionally of “giving way” or “going out” of the knee may be present.
Physical Examination includes:
- Large hemarthrosis in acute dislocation.
- A positive patellar apprehension test.
- A positive J sign.
- Passive patellar translation:
- Measured in quadrants of translation (midline of patella is considered “0”).
- 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.
See Also: Knee Examination
Patellar Instability Radiology
Radiographs are necessary to identify fracture, loose bodies, arthritis, malalignment and abnormal anatomy.
- The anteroposterior AP radiograph rarely provides any information regarding patellofemoral problems.
- It may reveal a bipartite patella, which is a variant of normal, and evaluate lower extremity alignment and version.
- The lateral view of the knee is more helpful in determining patella alta and Trochlea dysplasia.
- Sunrise / Merchant views: assess for lateral patellar tilt, the shape of the patella, the shape of the femoral trochlea and the relationship of the patella to the femur.
- Most frequently used to obtain patellar tilt and measure the tibial tubercle-trochlear groove distance (TT-TG):
- A measurement of the lateralization of the tibial tubercle.
- Normal values between 9 and 13 mm.
- TT-TG 15 to 20 mm questionably abnormal.
- TT-TG over 20 mm highly associated with patellar instability.
MRI is indicated for acute dislocations in high-level athletes and when an osteochondral loose body or tear of MPFL is suspected.
Radiographic Measurements of the Patella
lateral radiograph to determine patella alta
|With knee flexed 30 degrees, line is drawn through intercondylar notch||Should approximate the lower pole of the patella|
|LT:LP = 1.0||Patella alta if ratio ≥ 1.2|
|Trochlear depth (Dejour) lateral radiograph||Trochlear depth measured 1 cm from top of groove||Should be ≥ 5 mm|
|Patellar height (Caton Deschamps) lateral radiograph||Ratio between articular facet length of patella|
(AP) and distance between articular facet of
patella and anterior corner of superior tibial
epiphysis (AT). Knee flexed 30 degrees.
|AP/AT ratio—normal 0.6-1.3|
Patella infera—ratio < 0.6 Patella alta—ratio >1.3
|Blackburne-Peel ratio||Length of articular surface of patella to length measured from articular surface of tibia to inferior pole of patella||Normal ratio 0.54-1.06|
|Patellar tilt (CT scan)||Angle formed by intersection of the tangent of the posterior condyles and the major axis of the patella on 20-degree flexion scan||Normal angle: < 20 degrees Angle > 20 degrees: dysplasia|
|TT-TG (axial radiograph, CT scan)||Two lines drawn perpendicular to posterior bicondylar line, one line through middle of trochlear groove (TG) and second through tibial tuberosity (TT). Distance between the lines is measured||20 mm = malalignment|
|Crossing sign||Anterior cortical outline of condyle intersects trochlear outline||Dysplastic sulcus|
|Trochlear bump||Trochlear line extends anterior to femoral cortex||Dysplastic sulcus|
Patellar Instability Treatment
Nonoperative treatment in patellar instability is indicated in:
- Acute first-time patella dislocation (if there is no loose body).
- Habitual dislocation.
- Patellofemoral malalignment or recurrent subluxation of the patella.
- Acute first-time patella dislocation treatment include:
- Knee immobilization for 3 to 5 days.
- Hemarthrosis aspiration.
- Ice is applied for 20 minutes every 2 to 3 hours to reduce swelling.
- Rehabilitation after immobilization removal with closed-chain exercises for quadriceps and hamstring muscles.
- Generally, a patellar stabilizing brace is prescribed for the first 6 to 8 weeks during rehabilitation and long-term for sports activity
Some surgeons preferred early surgical treatment with arthroscopic evaluation or débridement and acute repair of the medial patellofemoral ligament (controversial).
Surgical Patellar Instability treatment indications include:
- Acute patellar dislocation with osteochondral fracture and a loose body formation.
- Recurrent Patellar Instability.
- TT-TG >20mm on CT.
- Patella alta.
- Excessive lateral tilt or tightness after medialization
Surgical Procedures for Treatment of Patellar Instability
|Procedures||Indications and Characteristics|
|Medial repair/imbrication||30% failure rate, approximately the same as conservative treatment.|
Indication: first dislocation + repairable chondral defect.
Instability in skeletally mature.
In combination with distal realignment.
|Lateral release||Excessive lateral pressure syndrome.|
In combination with realignment procedure when excessive tightness prevents patellar centering.
May increase risk for both medial and lateral patellar subluxation.
|MPFL reconstruction||Indicated for recurrent MPFL deficiency ± trochlear dysplasia.|
Proximal or anterior femoral placement or overtightening results in medial facet overload.
May combine with distal realignment.
|Elmslie-Trillat procedure||Indicated for instability TT-TG > 20 mm + strong repairable medial structures.|
Healing time and risk for stress or contact fracture of proximal tibia much less than Fulkerson procedure.
|Fulkerson distal realignment||Indicated for symptomatic lateral facet or distal pole arthritis + TT-TG > 20 mm.|
Contraindicated with proximal/medial facet arthritis.
Long healing time, increased risk of proximal tibial fracture with sports.
|Rotational high tibial osteotomy||Indicated for Patellar Instability + severe rotational deformity.|
More normalized gait compared with distal realignment.
|Trochleoplasty||Indicated for dysplastic trochlea.|
Low recurrence rate.
Increased risk for osteonecrosis, DJD, arthrofibrosis.
Lateral condyle: increased pressure; increased DJD of lateral facet.
Good results with less risk reported with MPFL reconstruction.
|3-in-1 procedure: extensor mechanism realignment + VMO advancement + transfer of the medial third of the patellar tendon to the MCL.||Patellar Instability, TT-TG > 20 mm.|