Patellar instability means an unstable kneecap. It occurs when the patella (kneecap) moves out of the groove at the end of the thighbone (femur) that holds it in place.A patellar subluxation means that the kneecap has briefly slid out of its normal place in the groove at the center of the bottom end of the thigh bone. When you bend and straighten your knee, the kneecap moves up and down in a V-shaped notch called the trochlear groove. Patellar instability is the term given to a range of injuries that occur when the patella, or kneecap, is displaced from its intended resting place. Causes include a traumatic dislocation, such as occurs during a sports activity, or a displacement caused by daily activities.
This test is used to evaluate the instability. A medial/lateral displacement of the patella greater than or equal to 3 quadrants, with this test, is consistent with incompetent lateral/medial restraints. Lateral patellar instability is more frequent than medial instability.
This exercise is meant to be repeated daily to strengthen your patellar tendon and improve your range of motion. Step 1: Lie flat on your back, making sure your impacted knee is flat and straight. Tighten your quadriceps muscles and lift your leg 45 degrees off the ground. Hold for one to five seconds.
With a partial dislocation, a tear may have occurred in one of the ligaments or muscles around the knee, which results in a slight displacement, or a feeling that it’s about to dislocate again if the knee cap if literally pushed. Or it may be that you can’t bend or straighten the knee without feeling knee pain. Arthroscopic surgery can correct this condition. If the kneecap is only partially dislocated, your doctor may recommend nonsurgical treatments, such as exercises and braces. Exercises will help strengthen the muscles in your thigh so that the kneecap stays aligned.
Other Names
- Unstable Patella
Pathophysiology
- General
- Defined as recurrent dislocations or sensation that the patella is going to dislocation
- The majority of first-time dislocations occur with the foot planted, tibia externally rotated
- Most patellar dislocations occur in the teen years with patellar instability in the 20s-30s
Causes
- Acute traumatic
- Episodic/ recurrent instability/ chronic patholaxity
- The most common type, occurs after an initial dislocation
- Seen in adolescent athletes, more commonly in women
- Alternative patterns described by Chatel in pediatrics
- Congenital dislocation
- Permanent dislocation
- Habitual dislocation in knee flexion
- Habitual dislocation in knee extension
- Syndromic instability
- Refers to neuromuscular, connective tissue, or other disorders
- Examples include: Cerebral Palsy, Ehlers Danlos Syndrome, Marfan Syndrome, Down Syndrome
- Patellar Dislocation
- Trochlear Dysplasia
- Patella
- Medial Patellofemoral Ligament
- Primary static restraint to lateral instability during first 30° of flexion
- Prevents excessive lateral movement of the patella
- Medial Retinaculum
- Vastus Medialis Obliquus (VMO)
- Most distal portion of the medial quadriceps muscle
- Exerts a medially directed force that helps keep the patella in position
- Trochlear Groove of the Femur
- Variants in trochlear morphology can predispose the patella to maltracking[4]
- Gross subluxation/dislocation
- Can influence recurrent patellar instability
- Tibial Tubercle
- Arises from the lateral aspect of the proximal tibia
- Excessive lateralization increases tibial external rotation, severe genu valgum, or even increased femoral anteversion[5]
- All affect patellar tracking
- Age 15-19
- Race
- African American
- Caucasian American
- Orthopedic
- History of Patellar Dislocation
- Weak Vastus Medialis Obliquus (VMO)
- Tight lateral structures: Iliotibial Band, Vastus Lateralis
- Anatomic/ Biomechanical
- Trochlear Dysplasia (OR: 18.1)[6]
- Patella alta (OR: 10.4)
- Patellar tilt
- Lateralization of tibial tubercle or increased elevated TT-TG distance (OR: 2.1)
- Lateral femoral condyle hypoplasia
- Miserable Malalignment Syndrome: femoral anteversion, genu valgum, external tibial torsion
- Connective Tissue Disorders
- Cerebral Palsy
- Ehlers Danlos Syndrome
- Marfan Syndome
- Down Syndrome
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)
Diagnosis
- History
- Important to review the history of previous patellar dislocations, episodes of patellar instability
- A patient will complain of anterior knee pain
- Physical Exam: Physical Exam Knee
- In the acute setting, hemarthrosis is often present
- Tenderness over MPFL, tenderness over the medial femoral condyle
- Special Tests
- Special tests may be deferred in the acute setting
- 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 the path of the patella during flexion and extension
- Patellar Grind Test: Apply pressure to the patella with a knee in extension, patient contracts quadriceps
- Patellar Glide Test: Passively glide patella medially and laterally
- 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 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 ((Laurin’s angle))
- 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 (CDI): 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 [8]
- 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
- The 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 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
- Common Findings[9]
- 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
Parikh Classification of Patellar Instability
- Type I: first patellofemoral dislocation with (A) or without (B) osteochondral fracture[10]
- Type II: recurrent subluxation (A) or dislocation (B)
- Type III: dislocate patella by the examiner or patient which is either passive (A) or habitual in flexion/extension (B).
- Type IV: dislocated patella that is either reducible (A) or irreducible (B).
Treatment
Prognosis
Nonoperative
- Indications
- Majority of cases
- Absence of loose bodies or osteochondral fragments, other soft tissue injuries
- NSAIDS
- Physical Therapy
- Emphasis: Quadriceps/ VMO Strengthening, core and hip strengthening
- Kinesiology Taping
- Knee Immobilizer
- May be indicated early in acute setting
- Patellar Brace
- Patellar stabilizing sleeve or “J” brace
Operative
- Indications
- Failure of conservative management
- Associated osteochondral fragment (≥ 5 mm)
- Associated osseous avulsion of the MPFL
- Associated meniscus tear
- Techniques
- Medial patellofemoral ligament reconstruction
- MPFL reconstruction with autograft vs allograft
- Trochleoplasty
- Tibial tubercle osteotomy
- Arthroscopic debridement (removal of loose body) vs Repair
- Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer)
- Tibial tubercle distalization
- Lateral release
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