Patellar Instability

Patellar instability is a broad topic that encompasses a continuum of patellar abnormalities ranging from asymptomatic maltracking to debilitating recurrent dislocations. To address this complex topic, it is important to first define several terms. During the normal knee flexion cycle, the patella tracks in the center of the trochlea of the distal femur. Maltracking occurs when the course of the moving patella deviates from the bony constraints of the trochlear groove. Maltracking may or may not be symptomatic. The term subluxation is used to describe a specific episode in which the patella abruptly leaves the trochlear groove. Subluxation episodes are usually transient and uncomfortable. When the patella displaces completely from the trochlear groove, it is considered a dislocation . First-time dislocation episodes are typically the result of indirect or direct trauma. After dislocating, the patella may spontaneously reduce, or a manual reduction may be necessary. Chronic patellar dislocation describes the rare situation in which the patella remains dislocated for months or years. In the acute setting, a patella dislocation usually causes substantial pain and morbidity. Patients with recurrent patellar instability episodes, whether subluxation or dislocation, usually have demonstrable risk factors. Patients with abnormal skeletal anatomy, defined as malalignment, are predisposed to maltracking and instability. Similar to maltracking, malalignment implies a deviation from normal biomechanics or anatomy, but it does not always result in patellar instability.

Patellar dislocations represent 2% to 3% of all knee injuries and are the second most common cause of traumatic knee hemarthrosis. The annual risk of a first-time patellar dislocation is 5.8 per 100,000. Although most persons who experience a first-time dislocation will have no further instability, the literature reports a recurrence rate of 15% to 60% and an annual risk of recurrence of 3.8 per 100,000.

Despite the prevalence of patellar instability, ideal management recommendations remain controversial, and evidence-based literature is limited. The great variability of patellar pathologies and patient symptoms makes it difficult to form global treatment recommendations. A thorough understanding of the specific patellar abnormality, the level of functional disability, and the patient’s desired activity level must be taken into account when evaluating and assessing a patient with patellar instability. Identification and correction of anatomic and biomechanical risk factors form the basis for developing successful nonoperative and operative treatment strategies.


In the assessment of a patient with patellar instability, it is imperative to determine the mechanism of injury. Sports-related activities account for 61% to 72% of first-time dislocations. Patellar instability may result from direct trauma to the medial knee or, more frequently, an indirect injury, such as when the leg rotates around a planted foot ( Fig. 104-1 ). In most cases of patellar dislocation, the patella will spontaneously reduce. Often, patients will report feeling the knee “give way” and the kneecap “pop” or “clunk” in or out of place. They may describe an abnormal shape to the inside of the knee, sometimes confusing the prominent medial femoral condyle for what they mistakenly think is a medially dislocated patella. Patients may state that the patella reduced when they extended their knee or that they pushed it back into place. Swelling will develop soon after the injury in most patients unless their patella is very unstable; in those cases, subsequent swelling and pain may be minor or absent.

FIGURE 104-1

Mechanisms of acute patellar dislocation. A noncontact dislocation occurs by external rotation of the lower leg relative to the body ( A ), whereas contact injuries are caused by a direct blow to the medial side of the knee ( B ).

(Modified from The Cleveland Center for Medical Art & Photography, Copyright 2008.)

It is important to distinguish pain secondary to patellar instability from that of other patellofemoral disorders. Patellofemoral pain syndrome (PFPS) is the most common cause of anterior knee pain and can be confused with patellar instability. Patients with PFPS typically describe anterior knee pain exacerbated by prolonged sitting or when descending the stairs. The pain may be bilateral, or it may change from one knee to the other over time. In addition, the patients may feel like the knee is “unstable,” but when questioned carefully, they do not describe the same mechanisms of injury that would result in frank instability or they do not experience episodes in which the patella actually leaves the trochlear groove. Distinguishing between the two clinical entities is imperative because isolated PFPS is rarely treated successfully by surgical means. Furthermore, it is not uncommon for patients with instability to also experience PFPS.

Pain may also be the result of complex regional pain syndrome. The classic presentation is an exaggerated pain response, burning in nature, with intolerance to cold stimuli. The pain is typically continuous, deep, diffuse, and nonanatomic in distribution. Complex regional pain syndrome may also result in decreased patellar mobility, tenderness and hypersensitivity of the patella and retinacula, changes in skin temperature and color, and associated hair growth. Finally, localized pain can be the result of a neuroma, in which case it typically causes burning or sharp, stabbing pain in the distribution of the affected cutaneous nerve. Other causes of anterior knee pain include osteochondritis dissecans of the patella or trochlea, patellofemoral osteoarthritis, patellar or quadriceps tendinosis, or pigmented villonodular synovitis.

Physical Examination

Pertinent Bony Anatomy

Lower extremity alignment is determined primarily by the relationship between the femur and tibia. Abnormalities in the position and relationship of these bony structures result in malalignment that can predispose patients to patellar instability. Normally, the knee has a tibiofemoral angle of approximately 5 to 7 degrees of valgus. With excess knee valgus, or genu valgum, the mechanical pull of the quadriceps muscle changes, increasing the normal laterally directed force vector on the patella. The rotation of the femur and tibia also influences patellar stability. Normally, the femoral neck has 7 to 20 degrees of anteversion and the tibia has 15 degrees of external rotation ( Fig. 104-2 ). Increases in femoral anteversion and external tibial torsion will further increase the laterally directed forces on the patella.

FIGURE 104-2

Torsion of the femur and/or tibia.

(Modified from The Cleveland Center for Medical Art & Photography, Copyright 2008.)

The patella is a sesamoid bone within the extensor mechanism of the knee that articulates with the distal femur. It is stabilized superiorly and inferiorly by the quadriceps and patellar tendons, respectively, and medially and laterally by the ridges of the femoral trochlea. At the distal end of the femur is the trochlea, which articulates with the patella and provides a bony restraint to excessive medial and lateral translation. The lateral trochlear ridge, which is typically larger, more proximal, and more anterior than the medial trochlear ridge, helps keep the patella centered in the trochlea by resisting pathologic lateral patellar excursion. Anatomic variations (including trochlear dysplasia, resulting in a shallow trochlear floor relative to the medial and lateral condyles) and hypoplasia of the lateral condyle decrease the resistance to lateral patellar translation. Under normal conditions, the patella “engages” in the trochlea at 20 degrees of flexion. Patients with a relatively longer patella tendon, called patella alta, have less osseous stability because more knee flexion is required before the patella is “engaged” and stabilized by the bony constraints of the trochlear groove. In addition, patients with patella alta have a decreased patellofemoral contact area compared with patients who have knees of normal patellar height.

The bony anatomy of the tibia and foot also affects patellar instability. Normally, the tibial tuberosity is approximately 9 to 13 mm lateral to the center of the patella. This relationship changes during the knee flexion arc in most patients. The distance is greatest during terminal knee extension in part because of the “screw home mechanism,” where the tibia externally rotates on the femur. Any further lateralization of the tuberosity relative to the center of the patella will result in an increase in the laterally directed force on the patella. Furthermore, hindfoot valgus and excessive pronation of the foot place a valgus force on the knee, which also increases the laterally directed force on the patella.

Pertinent Soft Tissue Anatomy

The quadriceps complex, consisting of the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis muscles, is the most important dynamic stabilizer of the patella ( Fig. 104-3 ). All four muscles converge in the distal thigh and insert through the quadriceps tendon at the proximal pole of the patella. Each of the individual muscles contributes a different force vector based on its angle of insertion. The vastus medialis and lateralis muscles provide additional connections to the tibia through attachments to the medial and lateral patellar retinacula, respectively. The vastus medialis oblique (VMO) is a distinct part of the vastus medialis muscle that originates off of the lateral intermuscular septum and inserts at a high angle, up to 65 degrees, on the proximal third of the medial border of the patella. The VMO is an important medial patellar stabilizer that counterbalances the pull of the vastus lateralis muscle. In addition, the VMO may exert a force on the medial patellofemoral ligament (MPFL), adding additional medial stability to the patella. Atrophy, hypoplasia, and impaired motor control of the VMO will therefore result in decreased opposed activity of the vastus lateralis muscle, producing increased lateral patellar displacement. The dynamic muscular stabilizers can compensate to some degree for anatomic bony and soft tissue deficiencies that predispose to patellar instability. Conversely, weakness of the dynamic stabilizers may predispose athletes to patellar instability episodes during athletic activities even if they have “normal” anatomy and tracking under less stressful circumstances.

FIGURE 104-3

Quadriceps musculature.

(Modified from The Cleveland Center for Medical Art & Photography, Copyright 2008.)

The primary soft tissue static stabilizers of the patella are the patellofemoral, patellotibial, and patellomeniscal ligaments ( Fig. 104-4 ). Studies have shown that the MPFL is the primary passive soft tissue restraint on the medial side of the patella, contributing 50% to 60% of the total restraining force against lateral patellar displacement. Importantly, the MPFL is almost always disrupted in first-time patellar dislocations. The MPFL has been shown to be a distinct structure in the knee. The exact origin of the MPFL is controversial, but it appears to be in the saddle between the adductor tubercle and medial epicondyle of the femur. From its attachment near the medial epicondyle, the ligament then courses anteriorly and laterally, inserting on the proximal two thirds of the medial border of the patella. It is 4.5 to 6.4 cm long and 1.9 cm wide, with a tensile strength of 208 N.

FIGURE 104-4

Medial patellofemoral ligaments. MPFL, Medial patellofemoral ligament; MPML, medial patellomeniscal ligament; MPTL, medial patellotibial ligament.

(Modified from The Cleveland Center for Medical Art & Photography, Copyright 2008.)

Examination Overview

Acute Patellar Instability Episode

Clinical assessment of a patient who has recently sustained an episode of patellar instability can be difficult. Knee range of motion is usually limited because of soft tissue swelling and joint effusion, and patients have difficulty relaxing for examination because of pain and fear. Aspiration of a joint effusion is occasionally indicated for comfort. The presence of hemarthrosis suggests one of a limited number of injuries to the knee, including ligamentous or meniscal tears, patellar dislocations, and intraarticular fractures. The ligamentous, bony, and muscular structures of the knee should be systematically examined. Palpation along the course of the quadriceps, VMO, medial retinaculum, and MPFL is necessary to localize any site of failure. The MPFL will be tender at its site of rupture, which can be anywhere along the course from the medial epicondyle to the patella. Tenderness at the medial epicondyle, the most common site of MPFL failure, is called the Bassett sign. Pain and tenderness at the inferior pole of the patella may represent tendinosis or an incomplete or complete patellar tendon tear. Similarly, tenderness at the superior patellar pole may represent quadriceps tendinosis or tendon failure. The competence of the extensor mechanism is confirmed by asking the patient to extend his or her knee against resistance from a flexed position. Rarely, osteochondral or chondral loose bodies may be palpated in the suprapatellar pouch or medial and lateral gutters. Concomitant injury to the collateral or cruciate ligaments or meniscus can be determined with use of the standard ligamentous and meniscal tests. Resolution of acute pain and swelling allows for a more comprehensive examination.

Standing Examination and Gait Testing

With the patient standing, the lower extremity is visualized. Malalignment abnormalities, including genu valgum, pes planus, hindfoot valgus, and pronation of the foot, may be identified. Patients may have “squinting patella” (patellae that point toward each other) as a result of increased femoral anteversion. In addition, core strength testing is performed. A single-leg squat test is used to assess hip strength and trunk control by requiring the patient to slowly lower the body over a single planted foot. Poor control suggests core and hip rotation weakness that can predispose to instability. Gait testing provides insight into the dynamic factors influencing patellar tracking. A “kneeing-in” gait may be the result of increased femoral anteversion, external tibial torsion, or foot pronation. These anatomic abnormalities, combined with core and hip rotation strength deficits, lead to the development of a valgus thrust, which generates an external rotation moment about the knee, resulting in a dramatic lateral force on the patella.

Sitting Examination: J Sign

The J sign refers to the shape of the track that the patella follows as the knee extends from a flexed position. As the patient extends the knee from 90 degrees of flexion, the patella will move laterally as it passes the proximal-most portion of the lateral trochlear ridge, which usually happens at approximately 20 degrees of knee flexion; however, it occurs earlier (at a greater flexion angle) in patients with patella alta. The presence of a J sign may be a normal variant, especially if seen bilaterally in asymptomatic patients. In patients with patellar instability, a positive J sign has been associated with lateral retinacular tightness or medial retinacular insufficiency, VMO deficiency, soft tissue imbalance, and patella alta and hypoplastic lateral condyle. The precise cause and clinical significance of the J sign, particularly in asymptomatic patients, remains unclear.

Supine Examination

Muscle bulk, especially of the VMO, should be evaluated for evidence of atrophy or hypoplasia. Strength deficits are determined by asking the patient to extend the knee against resistance. Positioning of the tibial tuberosity relative to the center of the patella is visualized and compared with the other knee. A hip examination, including muscle strength testing and range of motion, should be performed. Tightness of the iliotibial band is examined with use of Ober’s test. An excessively tight ITB can cause increased tension in the lateral retinaculum, resulting in patellar maltracking and instability.

Quadriceps Angle

The quadriceps angle, or Q angle, is measured as the angle between the vector of action of the quadriceps tendon and the patellar tendon. Clinically, the measurement is taken as the angle between lines from the anterior superior iliac spine of the pelvis to the middle of the patella and from the middle of the patella to the tibial tuberosity ( Fig. 104-5 ). A larger Q angle theoretically results in a greater laterally directed force on the patella on contraction of the quadriceps muscle. Normal Q angles in males and females range from 8 to 16 degrees and 15 to 19 degrees, respectively, in the supine position and 11 to 20 degrees and 15 to 23 degrees, respectively, in the standing position. Q-angle measurements vary widely as a result of differences in measurement techniques (supine, standing, sitting, and knee flexion angle) and poor inter- and intraobserver reliability. Failure to precisely identify the center of the patella or to keep the leg in neutral rotation may also lead to inaccurate measurements of the Q angle. In addition, patients with patella alta, a positive J sign, or persistent patellar subluxation or dislocation may have falsely low Q angles. Although it has been typically accepted that higher Q angles are associated with a higher risk of patellar instability or pain, this phenomenon has not been consistently found in the literature. In fact, Sanfridsson et al. found lower Q-angle values in patients with a history of patellar instability, and Cooney et al. showed a negative relationship between the quadriceps angle and the tibial tuberosity–trochlear groove (TT-TG) distance. Therefore the usefulness of the Q angle remains controversial, and clinicians must be mindful that the Q angle is only one piece of information; the Q angle alone should not dictate treatment for a patient.

FIGURE 104-5

The quadriceps angle (Q angle) formed by the intersection of a line from the anterior superior iliac spine ( ASIS ) to the midpoint of the patella, with a line from the midpoint of the patella to the midpoint of the tibial tuberosity.

(Modified from Livingston LA, Mandigo JL: Bilateral Q angle asymmetry and anterior knee pain syndrome. Clin Biomech [Bristol, Avon] 14[1]:7–13, 1999.)

Patellar Tilt Test

The patellar tilt test is used to evaluate the lateral soft tissue restraints, including the lateral retinaculum and the iliotibial band. With the knee fully extended and the quadriceps relaxed, the examiner lifts the lateral border of the patella ( Fig. 104-6 ). If the patella does not correct to at least neutral with the anterior surface of the patella parallel to the floor, it suggests tight lateral structures.

FIGURE 104-6

Patellar tilt test.

(Modified from The Cleveland Center for Medical Art & Photography, Copyright 2008.)

Patellar Glide Test

The integrity of the medial and lateral structures can be assessed with the patellar glide test. With the knee in full extension and the quadriceps relaxed, the patella is translated medially and laterally ( Fig. 104-7 ). Using the width of the patella divided into quadrants, the amount of patellar glide is reported as the number of quadrants worth of translation that the patella exhibits on examination (i.e., if the patella translates a distance equivalent to 75% of the patellar width, the patellar glide would be graded as 3). With a laterally directed force, a patellar glide grade of 3 or 4 may suggest a deficiency in the medial soft tissue patellar restraints, mainly the MPFL, and a medial translation of one quadrant or less suggests tightness of lateral structures. However, the clinician should compare the patellar mobility to the asymptomatic knees because increased patellar glide bilaterally is more indicative of generalized hyperlaxity and does not necessarily mean that the patient has patellar instability. The patellar glide test is also subject to a high degree of interobserver variability. Although measurement instruments have been developed to allow for improved inter- and intraobserver reliability, they are not routinely used in clinical practice.

FIGURE 104-7

Patellar glide test. Note that the patella is displaced laterally.

Patellar Apprehension Test

The most commonly used provocative test for determining patellar instability is the apprehension test. The test is positive if the patient has a feeling similar to a subluxation or dislocation episode as the patella is manually translated laterally while the quadriceps is relaxed. Importantly, it is the apprehension, not the presence of pain alone, that makes the test positive. Although lateral patellar instability is more common, medial patellar instability may occur, especially in the setting of previous patellar realignment surgery. Medial subluxation of the patella is evaluated using the Fulkerson relocation test. While the patient is supine with the knee extended, the patella is manually displaced medially. With knee flexion, the patella falls into the trochlea. The test is positive if relocation of the patella reproduces pain or a sense of instability.

Prone Examination

Femoral anteversion is measured using Craig’s test. The patient is placed prone, and the knee is flexed to 90 degrees. While the greater trochanter is palpated, the leg is rotated until the greater trochanter is positioned as far laterally as possible, placing the head into the center of the acetabulum. The angle between an imaginary vertical line and the axis of the tibia is the amount of femoral anteversion present.



The standard series of conventional radiographs includes standing anteroposterior, lateral at 30 degrees of flexion, tunnel, and sunrise views. These views may reveal persistent subluxation, osteochondral fractures, and abnormal anatomy of the knee, including patellar positioning and trochlear morphology. The anteroposterior radiograph is useful for examining the tibiofemoral alignment in addition to identifying bipartite patella and patellar fractures. Assessment of tibiofemoral arthritis is also best assessed using this view. The tunnel view radiograph is helpful in identifying the presence of loose bodies or osteochondritis dissecans lesions in the knee.

The sunrise or axial radiograph is usually obtained in 45 degrees or more of knee flexion. This view is most useful for identifying displacement of the patella from the normal location in the trochlear groove. It may also show the bony irregularities of the lateral trochlear ridge or medial facet of the patella that can occur with patellar dislocations. Occasionally, a loose body in the lateral gutter will be apparent only on axial views. The Merchant view is obtained with the knee flexed 45 degrees over the end of the table and the x-ray beam angled 30 degrees downward. Measurement of the sulcus angle on this view is one way to characterize trochlear dysplasia. The sulcus angle is measured as the angle between two lines originating from the deepest part of the femoral trochlear to the highest points of the medial and lateral condyles. Values greater than 145 degrees suggest trochlear dysplasia.

Relative patellar height is best assessed on a lateral radiograph obtained with the knee flexed to 30 degrees. Patella alta is associated with patellar instability. A more proximally situated patella requires a greater degree of knee flexion before it engages the bony constraints of the trochlea of the femur. Various methods of assessing relative patellar height have been described ( Table 104-1 ), including the Insall-Salvati, modified Insall-Salvati, Blackburne-Peel, Caton-Deschamps, and Labelle-Laurin indices. The Blackburne-Peel ratio was found to be the most reproducible with the least interobserver error. It is defined as the ratio of the perpendicular distance from the lower articular margin of the patella to the tibial plateau divided by the length of the articular surface of the patella. A ratio of 0.8 is considered normal, whereas values above 1.0 indicate patella alta. Compared with the other indices of patellar height, the Blackburne-Peel ratio relies on consistent osseous landmarks rather than the variable anatomy of the patella and location of the tibial tuberosity.

TABLE 104-1


Measurement Normal Alta Baja
Insall-Salvati 1.0 >1.2 <0.8
Modified Insall-Salvati >2.0
Blackburne-Peel 0.8 >1.0 <0.5
Caton-Deschamps <1.2 >1.2 <0.6
Labelle-Laurin Visual Visual

Trochlear dysplasia, another known risk factor of patella instability, is best evaluated on a true lateral radiograph. Three anterior lines are important in determining the trochlear morphology: the most anterior line is the medial femoral condyle, the middle line is the lateral femoral condyle, and the remaining line is the floor of the trochlea ( Fig. 104-8 ). Normally, the line of the lateral ridge terminates proximal to the line of the medial ridge. However, a shallow, flattened trochlea will cause the trochlear line to prematurely cross the anterior aspect of the femoral condyles, known as the crossing sign. Dejour et al. reported the presence of the crossing sign in 96% of patients with history of a true patellar dislocation compared with 3% in the control group. The presence of a supratrochlear spur (trochlear prominence) and double contour (medial condyle hypoplasia) are additional findings of trochlear dysplasia.

FIGURE 104-8

Lateral view of the knee for trochlear dysplasia.

(Modified from The Cleveland Center for Medical Art & Photography, Copyright 2008.)

Additional quantitative measurements of trochlear dysplasia can be made, including trochlear bump and depth. Using a true lateral radiograph, a straight line is drawn tangential to the anterior femoral cortex. The trochlear floor may be anterior, posterior, or flush with the anterior femoral cortex, forming the trochlear bump. The size of the bump can be measured; positive values indicate anterior positioning of the trochlear floor relative to the anterior femoral cortex, which suggests a shallow trochlea. In the normal knee, the value of the trochlear bump is near zero (i.e., the anterior femoral cortex is flush with the trochlear floor). However, in one study values of more than +3 mm were present in 66% of patients with objective patellar instability. Trochlear depth is the distance between the trochlear floor and most anterior condylar contour line measured along a line 15 degrees from the perpendicular to the tangent of the posterior femoral cortex ( Fig. 104-9 ). Trochlear depth measurements less than 4 mm were found in 85% of knees with patellar instability compared with only 3% of control subjects.

FIGURE 104-9

Trochlear depth.

(Modified from The Cleveland Center for Medical Art & Photography, Copyright 2008.)

A classification of trochlear dysplasia has been developed using information from the lateral and axial radiographs ( Fig. 104-10 ). Type A dysplasia is characterized by a shallow trochlear without any changes to trochlear morphology. This type is seen as a crossing sign on the lateral radiograph and sulcus angle greater than 145 degrees on the axial view. Type B dysplasia is characterized by a supratrochlear spur on the lateral view and a flat or convex trochlear on the axial view. Type C dysplasia is present when a crossing sign and double contour are present on lateral radiographs, with medial condyle hypoplasia on axial radiographs. Type D dysplasia represents the most severe form of dysplasia, characterized by a crossing sign, supratrochlear spur, and double contour on the lateral view in addition to asymmetry of the trochlear facets, with a cliff between them on the axial view.

FIGURE 104-10

Dejour’s classification of trochlear dysplasia. Type A: crossing sign (flat or convex trochlea). Type B: crossing sign and supratrochlear spur. Type C: crossing sign and double contour. Type D: crossing sign, supratrochlear spur, double contour, and sharp step-off of the trochlea.

(Modified from Grelsamer RP, Dejour D, Gould J: The pathophysiology of patellofemoral arthritis. Orthop Clin North Am 39[3]:269–274, 2008.)

Computed Tomography

One major limitation of radiographs is the inability to obtain axial images of the patellofemoral joint at angles from 20 degrees of knee flexion to full extension. Computed tomography (CT) imaging allows for great osseous detail and improved accuracy of the measurements compared with those made on conventional radiographs. Patellar tilt is measured as the angle between a line going through the patellar axis and a line tangential to the posterior condyles. Angles greater than 20 degrees are present in more than 80% of patients with patellar instability.

In addition, CT imaging may used to quantify lateralization of the tibial tuberosity, known as the TT-TG distance. Superimposing an axial CT image of the tibial tuberosity and the trochlear groove, the TT-TG distance is that between the center of the tuberosity and groove in a line parallel to the posterior femoral condyles ( Fig. 104-11 ). A TT-TG distance of more than 15 to 20 mm is associated with patellar instability.

FIGURE 104-11

A computed tomography scan showing the technique of measuring the tibial tuberosity–trochlear groove distance (TT-TG). The image of the trochlear groove is superimposed on the image of the tibial tuberosity using the digital radiology software. A line parallel to the posterior femoral condyles is drawn ( line 1 ). A perpendicular to line 1 is then taken through the central portion of the trochlea ( line 2 ) and through the tibial tuberosity ( line 3 ). The difference between line 2 and line 3 (i.e., line 4 ) is the TT-TG. In this case, it is 1.68 cm.

(Modified from Bicos J, Fulkerson JP, Amis A: Current concepts review: the medial patellofemoral ligament. Am J Sports Med 35[3]:484–492, 2007.)

CT imaging also allows for three-dimensional reconstructions to be performed, providing a better spatial understanding of the knee anatomy. Furthermore, dynamic CT has been used to assess patellar tracking, allowing visualization of the patellofemoral joint through clinically relevant ranges of motion. Although studies of patellar instability using dynamic imaging modalities to date have been limited, this imaging protocol has become more widespread in the clinical setting and will likely enhance our understanding of the static and dynamic properties of the patellofemoral joint and guide therapeutic recommendations.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) provides additional imaging details not seen on conventional radiographs and CT imaging. MRI allows assessment of the articular cartilage, trochlear geometry, and soft tissue structures, including the MPFL. The classical MRI findings after an acute episode of patellar instability include impaction injury to the lateral femoral condyle, osteochondral damage to the medial patella facet, and disruption of the medial retinaculum and MPFL ( Fig. 104-12 ). MRI is 85% sensitive and 70% accurate in detecting injury to the MPFL. Characterization using MRI of trochlear dysplasia and localization of an MPFL rupture, if present, may be helpful in cases when MPFL repair is contemplated. Furthermore, MRI can be used effectively to calculate the similar measurements made on radiographs and CT imaging, including TT-TG distance, without radiation exposure. Kinematic MRI has been used to evaluate patellofemoral tracking. The use of kinematic MRI in the clinical setting remains limited as a result of a lack of availability.

FIGURE 104-12

An axial magnetic resonance imaging scan after acute lateral patellar dislocation shows knee effusion, bony bruising of the lateral femoral condyle and medial patellar facet from impaction, and disruption of the patellar attachment of the medial patellofemoral ligament.

Decision-Making Principles and Treatment Options

Treatment of patellar instability must be patient-specific. Historically, a plethora of rehabilitation regimens and surgical procedures have been described. Important details from the history, physical examination, and imaging studies will help the clinician determine which treatment options are most suitable for each patient.

Acute Patellar Instability Episode

During the acute phase after a patellar injury, regardless of whether it is a first-time or recurrent injury, the immediate goals of management are to provide relief of symptoms. Relative rest, light compression, and elevation are the initial interventions. Most patients benefit from the use of crutches to limit weight bearing. Pain is generally well controlled with cryotherapy and over-the-counter analgesic medications. A knee immobilizer may be used with patients who are particularly unstable or uncomfortable, followed by a more functional hinged brace within the first 1 to 2 weeks as symptoms allow. Knee aspiration is helpful in relieving pain for some patients with a tense hemarthrosis. Heel slides and quadriceps activation exercises may be initiated within days of the injury, depending on the severity of pain, soft tissue swelling, and effusion. Conventional radiographs should be obtained to look for fractures or large osteochondral loose bodies. MRI may be used selectively to assess the status of the extensor mechanism and to rule out concomitant intraarticular abnormality. Formal referral to a physical therapist allows for a supervised functional progression back to normal sedentary activities in days to weeks and to athletic activities in weeks to months.

Subacute, First-Time Patellar Instability Episode

Nonsurgical treatment remains the mainstay of therapy for first-time episodes of patellar instability. Protocols vary, ranging from a simple period of immobilization to those that involve rapid rehabilitation. Proponents of immobilization suggest that it will help facilitate healing of the torn MPFL. Proponents of early mobilization argue that it helps prevent stiffness and muscle atrophy. In most cases, it is appropriate to initially immobilize the knee for several days and then begin range-of-motion exercises once the quadriceps muscle function returns. Rehabilitation protocols typically include strengthening of the quadriceps, gluteal, and core muscles. Taping and bracing of the patella have been theorized to provide patellar stability by increasing the strength of the quadriceps muscle and by activating the VMO earlier than the vastus lateralis muscle during use of stairs. However, Muhle et al. showed no effect of patellar realignment bracing on patellar positioning. Regardless of the specific nonoperative intervention used, recurrence rates of 15% to 60% have been reported.

Some authors have advocated surgical repair of the MPFL after a first-time dislocation, but the benefit remains unclear. To date, few studies have compared nonoperative versus operative treatment in persons with first-time patellar dislocations. In most studies, no major differences were found in postoperative episodes of instability, activity level or function, and subjective patient measures. Camanho et al. and Bitar et al. have advocated primary surgical intervention, having demonstrated higher Kujala outcome scores and lower rates of recurrence in patients undergoing MPFL repair and reconstruction, respectively.

Well-accepted indications exist for surgical intervention in the acute setting, however, including an irreducible patellar dislocation and a large or displaced osteochondral fracture with an associated loose body. Other relative indications for surgery include disruption of the quadriceps or VMO and the presence of an associated ligamentous, chondral, or meniscal injury. Many clinicians would consider repairing the torn MPFL if surgery was indicated after a first dislocation for other reasons. Additional high-quality studies are needed to delineate the role of primary surgical intervention in the treatment of first-time episodes of patellar instability.

Subacute, Recurrent Patellar Instability Episode

For patients with recurrent patellar instability, surgical stabilization becomes a more compelling option. Rehabilitation programs still may be successful for persons without evidence of mechanical or structural anomalies that would require surgical correction ; however, we are unaware of any studies that have formally assessed the effect of different rehabilitation programs on recurrent patellar instability.

When comprehensive nonoperative treatment fails, surgical intervention is usually recommended. With the use of information from the history, physical examination, and imaging studies, a patient-specific surgical solution can be developed. The goal of surgical intervention is to “individualize, customize, and normalize” a solution tailored to the unique pathologic condition that is leading to the recurrent instability. In most cases, patients undergo a medially based soft tissue repair or reconstruction or a proximal tibial bony realignment procedure. Medial soft tissue procedures, such as MPFL reconstruction, focus on stabilization by reestablishing the deficient medial soft tissue checkrein. Tibial tuberosity osteotomy (TTO) procedures result in realignment of the abnormal bony anatomy. Surgical modification of the position of the tibial tuberosity changes the forces applied to the patella through the patella tendon, allowing the surgeon to customize the procedure so as to address maltracking and unload specific areas of articular cartilage that are experiencing excessive pressure. One or both of these surgical procedures may be used, depending on the needs of the patient.

Alternative procedures have been described with varied results. Isolated lateral retinacular release has been shown to be effective in reducing patellar tilt but not in treating recurrent instability, and excessive lateral release has been associated with increased rates of recurrent instability. Lateral retinacular release is therefore recommended as an adjunct to other patellar stability procedures in patients with a tight lateral retinaculum. A number of proximal soft tissue surgeries have been described, including medial reefing (i.e., tightening of medial structures), VMO advancement (i.e., reattachment of the VMO more distally and laterally on the patella), and the Galleazzi procedure (i.e., passage of the semitendinosus tendon through a tunnel from the medial side of the patella). Many of these procedures have had very good results over the years and continue to be used successfully today. One attribute that all of these procedures seem to have in common is that they recreate the medial soft tissue restraint to excessive lateral translation.

Other surgical procedures have been developed to address bony malalignment. Operations that deepen the trochlea (trochleoplasty) or elevate the anterior portion of the lateral femoral condyle (trochlear osteotomy) were developed to treat trochlear dysplasia. They have been used extensively in Europe, and in some cases have demonstrated excellent early results. These trochlear procedures help improve radiographic indices of trochlear dysplasia and lateral stability, but they may also result in postoperative stiffness and patellofemoral arthrosis.

Another less common approach has been to address malalignment using femoral osteotomies. Excessive femoral anteversion can be corrected with a proximal derotation osteotomy, most frequently in the pediatric population. Excessive valgus alignment can be corrected through distal femoral osteotomies. Although femoral osteotomies have the advantage of directly correcting femoral malalignment, the procedures are associated with greater morbidity compared with knee-based osteotomies and soft tissue reconstructions and should be recommended selectively.

Authors’ Preferred Technique

Medial Patellofemoral Ligament Reconstruction

Injury to the MPFL is considered the essential lesion in recurrent lateral patellar dislocations. MPFL reconstruction provides a reliable method for treating patellar instability in patients without marked bony malalignment. A variety of techniques have been described using different graft sources and a multitude of fixation techniques, but initial enthusiasm has been tempered somewhat by recent descriptions of complications related to the surgical technique.

Surgical Technique

Positioning and Preparation

  • Supine position on the operating room table

  • Induction of general or regional anesthesia

  • Administration of prophylactic intravenous antibiotics before making the incision

  • Examination of both knees after induction of anesthesia

    • Comprehensive ligamentous examination

    • Glide test to determine competence of soft tissue restraints

    • Tilt test to assess for lateral retinacular tightness

  • Placement of a tourniquet on the proximal thigh of the affected leg

  • Placement of a padded valgus bar on the operating room table just distal to the tourniquet

  • Placement of a thigh-high compressive stocking on the contralateral leg

Diagnostic Arthroscopy

  • Observation for loose bodies or meniscal abnormality

  • Patellofemoral joint evaluation

    • Assessment of the articular surface of the patella and trochlea for any chondral lesions

    • Evaluation of patellar tracking in the trochlear groove during flexion and extension

  • Chondral damage addressed with debridement or repair techniques, as indicated.

  • The location and severity of chondral damage is used to determine if a TTO is indicated.

  • Arthroscopic lateral retinacular release if the lateral retinaculum is clinically and radiographically determined to be tight.

  • If a concomitant TTO is needed, it should be performed before the MPFL reconstruction.

Graft Harvest

Hamstring autografts are the preferred graft for most patients. Although smaller than the semitendinosus tendon, even the gracilis tendon is substantially stronger than the native MPFL. Allograft tendons are typically reserved for revision cases or for patients with connective tissues disorders (e.g., Ehlers-Danlos syndrome).

  • The leg is exsanguinated with an Esmarch bandage and the tourniquet is inflated.

  • A short, longitudinal incision is made over the pes anserine insertion.

  • The sartorial fascia is incised and everted to expose the gracilis and semitendinosus tendons.

  • The gracilis or semitendinosus tendon is dissected from the sartorial fascia and tagged.

  • The tendon is harvested with a tendon stripper and placed on a separate table.

  • Muscle and soft tissue debris are removed from the graft.

  • No. 2 FiberLoop (Arthrex Inc., Naples, FL) is woven through the patellar end of the graft in a locking fashion.

Patellar Tunnel

Feb 25, 2019 | Posted by in SPORT MEDICINE | Comments Off on Patellar Instability
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