Patella instability is a generic term that describes abnormal lateral subluxation or dislocation of the patella out of the trochlear groove of the femur. This condition is normally found in young, active individuals. An acute patella dislocation typically results in a painful, swollen knee that is at risk for further patellar instability.
Structure and function
The patella is a sesamoid bone found within the powerful extensor mechanism of the leg. The quadriceps tendon connects the quadriceps muscle complex to the patella, and the extensor mechanism continues from the patella to the tibial tubercle as the patellar tendon. Proper patella tracking and mechanics is essential to correct function of the extensor mechanism.
The patella articulates with the trochlear groove of the femur throughout flexion and extension of the knee. The undersurface of the patella contains a medial and lateral articular facet covered with articular cartilage. In full extension, the patella sits superior to the trochlear groove. As motion is initiated in early knee flexion, the distal aspect of the patellar articular surface begins to enter the superior aspect of the trochlear groove. Variations in the bony anatomy of the trochlea and tibial tubercle may have an effect on how the patella enters and tracks within the trochlea.
The medial patellofemoral (MPFL) ligament is the primary soft-tissue restraint to lateral patellar translation. Additionally, the quadriceps, particularly the vastus medialis oblique (VMO), provides dynamic stabilization. Once engaged within the trochlear groove the patella is stabilized by both soft tissue and the bony confines of the trochlear groove.
The MPFL is the primary static stabilizer to lateral translation of the patella. It serves as a checkrein to guide the patella correctly into the trochlear groove as the knee is flexed. The MPFL originates from the anterior aspect of the medial femoral condyle to the superomedial margin of the patella.
Patella instability affects between 7 and 49 people per 100,000, accounting for 11% of musculoskeletal pathology seen in an office setting. It is also responsible for 16% to 25% of all injuries in runners, with a higher predilection for females.
Patients with an acute patella dislocation typically present to an emergency department with a painful swollen knee. Descriptions of the event include a painful twisting moment to the knee with or without an audible “pop” and a sensation of the knee giving out. The description of the knee giving way should alert the physician to the possibility of a patella dislocation. Patients or witnesses may or may not visualize the kneecap on the lateral aspect of the knee during the injury. As the leg is straightened, the patella relocates and immediate swelling occurs. It is not uncommon for patients to present with the patella found in the dislocated position.
Diagnosis can be difficult following an acute injury. Many patients, especially younger patients, are unable to give a clear history of the injury mechanism. Careful inquiry about the mechanism of injury may reveal an internal rotation of the femur and tibia with the foot fixed on the ground and relatively low forces as compared to other ligamentous and soft tissue injuries. This position of the knee increases the Q angle (discussed below) and the strong contraction of the quadriceps pulls the patella laterally, causing the dislocation.
Physical examination following an acute injury may be best performed following aspiration of the joint to alleviate pain and allow palpation of anatomic structures. If the patella remains in a dislocated position, the leg will be held in a flexed position. Gentle extension of the leg with medial force directed on the patella will relocate the patella within the groove. Tenderness over the medial retinaculum and medial femoral epicondyle is common. Further exam of the knee may be difficult in the acute setting due to excess pain as well as patient anxiety.
When a patient presents in an office setting with patella instability or a remote history of patella dislocation, a more thorough physical exam is possible. Standing alignment of the lower limbs is assessed for genu varum or valgum. The knee is taken through active and passive range of motion to visualize patellar tracking. A thorough ligamentous exam is performed to rule out additional pathology.
The mobility of the patella can be assessed by attempting to displace the patella medially and laterally with the knee in full extension. In a normal knee, the patella should not displace more than half its width in either direction.; Lateral translation may produce apprehension in a patient with instability. The patellar grind test is performed by directly applying downward pressure to the patella and manually displacing it in all four directions. This may produce pain indicative of an articular cartilage injury. Patella tilt, or lateral patellar compression syndrome, is due to a tight lateral retinaculum. This is tested with the knee in 20 degrees of flexion (ie, with a pillow under the knee to relax the extensor mechanism). The examiner holds the patella between the thumb and index finger and pushes the patella down in an attempt to flip the lateral edge upward. Limited upward movement indicates a tight lateral retinaculum which can contribute to lateral patella subluxation.
In any acutely injured knee, a thorough ligamentous exam should be performed if possible. The differential diagnosis includes:
- Anterior cruciate ligament (ACL) injury
- Ruptured patella or quadriceps tendon
- Acute meniscus tear
Additionally, the medial collateral ligament (MCL) may be injured at the time of a patella dislocation and should be closely evaluated.
Some diagnoses may not be excluded until the knee is less swollen and a more thorough examination is possible. The patient should be able to perform a straight leg raise to rule out rupture of the extensor mechanism. Anterior drawer and Lachman’s exam to evaluate the integrity of the ACL may be difficult in an acutely injured knee but should be attempted.
Radiographs are indicated in any acutely painful, swollen knee to rule out fractures and bony pathology. Anteroposterior (AP), lateral, and axial (Merchant) views are obtained to assess for patella fractures or femur and tibia pathology. Although X-rays are typically negative with patella dislocation, an osteochondral fracture at the superomedial aspect of the patella may be visualized, indicating an avulsion of the MPFL.
Patella alta or baja can be assess on the lateral view of the knee. Patella alta is a risk factor for instability and will be discussed below. The Merchant view may show lateral displacement of the patella within the femoral sulcus. Magnetic resonance imaging (MRI) is not always indicated following an incidence of patella dislocation but can show a torn MPFL as well as characteristic bone bruises on the medial facet of the patella and the lateral femoral condyle. In addition, MRI can be used to rule out other knee pathology as well as show articular cartilage lesions resulting from the dislocation. Computed tomography (CT) has been used to assess tibial tubercle and trochlear alignment.
Risk factors and prevention
Soft tissue factors include ligamentous laxity (such as Ehlers-Danlos syndrome), weak or dysplastic VMO, patella alta, excessive lateral patellar tilt, and a previous history of patellar instability. Bony pathology includes trochlear dysplasia, lateral femoral condyle hypoplasia, and an excessively lateral tibial tubercle.
The Q (quadriceps) angle is defined as the angle between lines joining the anterior superior iliac spine, the center of the patella, and the tibial tubercle. It is normally between 8 and 10 degrees in males and 15-20 degrees in females. An increased Q angle increases the laterally directed force on the extensor mechanism and predisposes to instability. Excessive foot pronation also contributes to patellar instability.
Patellar instability can often be treated conservatively. Rehabilitation and strengthening of the quadriceps play a vital role in conservative management. Stretching of the lateral retinaculum, hamstrings, Achilles tendon, and iliotibial band should also be performed. Weight loss can reduce patellofemoral loading. External methods of stabilization include patellar bracing, patellar taping, and use of orthotic shoes to prevent excessive foot pronation.
Surgical management is indicated following failed conservative treatment. A myriad of surgical treatment options are available based on the individual anatomy, clinical presentation, and surgeon experience. These can be grouped into two main categories: proximal realignment and distal realignment. Proximal realignment addresses the soft tissues, whereas distal realignment modifies the position of the patella through bony transfer of the tibial tuberosity. These procedures can be combined in severe cases of patellar instability. Trochleoplasty, or a surgical deepening of the trochlear groove, is rarely indicated due to high rates of osteochondral disease and early patellofemoral arthritis.
The purpose of proximal realignment is to bring the patella more medially by manipulating the soft tissues, principally the MPFL. This is performed through either primary repair of the MPFL following an acute rupture or by reconstruction with a tendon graft for chronic instability. Release of the lateral retinaculum in patients with patellar tilt has also been described. Shortening or imbrication of the VMO may also be performed.
Distal realignment is the preferred method of treatment if there is an abnormal trochlea, high Q angle, or significant patella alta. The procedure involves a tibial tubercle osteotomy with reattachment distally and medially to correct patella alta and decrease the Q angle. This is contraindicated in children with open physes.
Most patients can be managed conservatively with good outcomes. For the subset of patients who fail conservative management, success rates in the literature range from 85% to 95% for both proximal and distal realignment used separately or together.
The “miserable malalignment syndrome” is named for the three anatomic characteristics that lead to an increased Q angle: femoral anteversion, genu valgum (knock-kneed stance), and external tibial torsion
Patella, dislocation, quadriceps, Q angle
- Recognize clinical history consistent with patella instability
- Be able to perform full knee exam including assessment of ligamentous integrity
- Recognize risk factors for patella instability on exam and radiographs
- Understand basic principles of both conservative and surgical interventions for patella instability