Approximately, one-third of skeletally mature patients experiencing one episode of patella dislocation will experience recurrent patellar instability over time.1–4 The biomechanics of the patellofemoral joint are complex with a combination of factors contributing to overall stability. The multifactorial nature of patellar instability requires the clinician to understand the individual risk factors for recurrence, and to recognize that certain combinations of these factors may considerably increase that risk. A detailed patient history and examination is essential in identifying these risk factors and individualizing a treatment plan for patients. Diagnostic imaging can further identify anatomic abnormalities predisposing to recurrent instability and help guide treatment decisions. In this article, the approach to the first-time patella dislocator will be presented and will include an assessment of risk factors for recurrence and the author’s preferred treatment algorithm.
HISTORY AND DEMOGRAPHICS
Patient age is an important consideration when evaluating primary patellar instability. Although the mechanism of association has not been clearly defined, younger patient age and skeletal immaturity at initial presentation have both been well-established as increasing the risk for a recurrent event.1–3,5–7 Christensen et al1 reported that patients below 18 years of age with primary dislocation have more than twice the risk of recurrence than similar adult patients. The risk was found to be >11 times higher for patients younger than 16 years of age compared with those older than 16.6 Likewise, prior studies have reported that patients with open physes are at over twice the risk for recurrent patellar dislocation when compared with skeletally mature knees.2,4 Skeletal maturity may be determined by assessing the patency of the distal femoral and proximal tibial physes on plain film radiographs. Patients with open or partially open physes are classified as skeletally immature and those with closed physes are considered skeletally mature. Because of inconsistency in reported results, there is currently a lack of consensus with regard to the influence of sex on risk of recurrence.1–5,7,8
It is important to elicit a detailed history including prior instability of the ipsilateral or contralateral patella. A history of contralateral patellar dislocation has been shown to increase the risk for recurrent ipsilateral dislocation by 3 times compared with those without prior events.4,6 This association may simply be attributed to underlying anatomic risk factors for instability, which are expected to be symmetric in the majority of patients. This concept is further supported by the findings of Christensen et al1 who found both trochlear dysplasia and patella alta to be independently associated with nearly 9-times the risk of subsequent instability of the contralateral patella. In addition, both developmental dysplasia of the hip and a family history of patellar instability have been described as risk factors for contralateral instability during follow-up.8
The mechanism of injury causing primary patellar instability must be considered and grouped into 1 of 2 categories: sport versus nonsport. Approximately 60% of first-time patella dislocations occur during sporting activity.2,8 In high-school level athletes, injury rates have been reported to be highest in female gymnastics, followed by male football and wrestling.9 These sports-related dislocations have been associated with a nearly 2-fold increase in the risk of recurrent instability compared with nonathletic mechanisms.2 Furthermore, the 5-year recurrence-free estimate for a sports-related mechanism of injury was 53% versus 76% for nonsport injuries. Relative to the nonsporting injuries, sports-related injuries may impart more damage to the stabilizing structures owing to the high-energy mechanism, contributing to the observed increased rate of recurrence. All of these risk factors require careful consideration and form an important part of the treatment decision process.2
Examination of a patient with a first-time patella dislocation can be limited in the acute setting due to patient discomfort and apprehension. Initial evaluation should focus on ruling out more severe pathology such as ligament disruption, meniscus injury, osteochondral loose bodies, or neurovascular injury. In the absence of clinical or radiographic indications for more urgent intervention, it can often be advantageous to immobilize the knee and re-examine the patient ~1 week after injury.
Assessment should begin with a determination of standing alignment and an evaluation for the presence of any coronal plane (varus/valgus) or rotational abnormalities such as increased femoral anteversion or tibial torsion. Feet should be examined for pes planus and a Beighton score can be calculated for generalized ligamentous laxity. Patella mal-tracking is assessed as the knee is taken through flexion and extension both passively and actively. Mal-tracking is often associated with a “J-sign” whereby the patella is subluxated laterally in full extension and then reduces into the trochlea during flexion. Flexion instability that reduces in extension is more commonly seen in congenital patellar dislocation and may suggest extensor mechanism tightness.
Palpation often reveals tenderness along the course of the proximal medial patellar restraints [medial quadriceps tendon-femoral ligament and medial patellofemoral ligament (MPFL)] between the medial border of the patella and medial femoral condyle. The lateral femoral condyle may also be painful from collision with the patella during dislocation and/or reduction. A chondral injury may manifest as pain with a patella grind test. Fixed lateral tilt of the patella may suggest a tight lateral retinaculum. Finally, the patella can be translated passively medially and laterally in full extension and at a 30-degree flexion. The patella is divided into quadrants and translation >2 quadrants [approximately equal to the medial patella border translating to the lateral edge of the trochlear groove (TG)] is considered an increased amount of translation. Patient apprehension during this maneuver is another important finding during examination.
ANATOMIC RISK FACTORS AND RADIOGRAPHIC EVALUATION
In the initial workup of primary patellar instability, thorough evaluation with appropriate imaging is crucial for informed decision making. It allows direct assessment of the anatomic risk factors, both osseous and soft tissue, for re-dislocation. Plain radiography, magnetic resonance imaging (MRI) and computed tomography (CT) are the most common imaging modalities used, often in combination. Along with plain radiographs, advanced imaging with MRI is generally recommended in the initial workup. The role of CT in the absence of significant bony pathology is controversial, as much of the same information can be obtained with MRI, which has the added benefit of detailed soft tissue assessment without radiation exposure. Anatomic risk factors for recurrent patellar instability that are evaluated with imaging include patella alta, trochlear dysplasia, patellar tilt and subluxation, lateralization of the tibial tubercle (TT), and integrity of the MPFL.
Patella alta has long been associated with patellar instability, with recent literature suggesting the risk is increased 1.6 to 10.6 times.1,2,5,7,10,11 Patellar height is regularly assessed on lateral radiograph with the knee in 30 degrees of flexion using the Insall-Salvati (IS) and Caton-Deschamps (CD) indices (Fig. 1).13,14 Cutoff criteria for patella alta range from >1.2 to >1.5 for IS and >1.2 to >1.3 for CD.15 Although there is no current agreement with regard to which index is most reliable, it is recognized that pediatric and adolescent populations require higher cutoff values in order to be classified as abnormal due to the chronology of patellar ossification.16 In addition, one must consider imaging with sagittal plane MRI for evaluation of “functional engagement” between the articular surfaces of the patella and trochlea.17,18 This measurement allows identification of patients in whom the patella is not properly engaged in the trochlea during early (0 to 30 degrees) knee flexion.
Although many authors believe that patella alta has a causal relationship with patellar instability, others suggest that the instability itself may lead to the finding of increased patellar height, with normalization observed following MPFL reconstruction.19–21 Specifically, Woodmass et al22 demonstrated that isolated MPFL reconstruction can decrease the CD index by a mean of 0.1 postoperatively (mean preoperative 1.13±0.2 vs. postoperative 1.04±0.1). Allen et al23 reported that the combination of MPFL reconstruction and anteromedialization TT osteotomy will decrease the CD index by a mean of 0.2. Lastly, one must recognize that after distalization osteotomy of the TT, the CD should normalize while the IS index may remain unchanged.
Trochlear dysplasia contributes to instability by interfering with patellar tracking and alignment throughout knee range-of-motion. It has been identified as one of the strongest predictors of recurrent instability in recent literature, with risk increased 2.6 to 23.7 times in comparison with knees without dysplasia.1–7 It is defined on imaging by a sulcus angle >145 degrees and is commonly described using the Dejour classification method (Fig. 2).24,25
The Dejour classification of trochlear dysplasia can be utilized on standard lateral radiographs or on axial plane MRI or CT slices. Dejour type A is characterized on lateral radiographs by an isolated “crossing sign,” which represents the point at which the TG flattens and crosses the anterior border of the femoral condyles. Dejour types B to D signify increasing degrees of trochlear dysplasia and are characterized by the “crossing sign” plus a supratrochlear spur (type B), double contour line (type C), or both (type D). The supratrochlear spur represents radiographic projection of the superolateral trochlea and the double contour heralds asymmetry of the trochlear facets due to a hypoplastic medial facet.11,12 On axial MRI images trochlear dysplasia is classified based on a shallow groove (type A), flat or convex trochlea (type B), convex lateral facet and hypoplastic medial facet (type C), and height asymmetry of the medial and lateral facets (“Cliff” pattern, type D).
Patellar tilt and subluxation were historically believed to be major factors leading to patellar instability, and weakness of the vastus medialis obliquus was opined to be the underlying etiology. More recently it has been determined that trochlear dysplasia, patellar morphology, iliotibial band tension, tight lateral retinaculum, laxity or rupture of the MPFL, vastus medialis obliquus dysfunction, and abnormal tibial or femoral rotation can all contribute to this finding.12,26,27 The lateral patellofemoral angle and congruence angle are the commonly utilized methods to assess tilt and subluxation radiographically. Although commonly identified in patients with primary instability, recent literature does not support the notion that patellar tilt is a useful measure for the prediction of recurrent instability.28
Medial Patellofemoral Ligament
The MPFL is the major soft tissue stabilizer of the patella during early knee flexion, contributing ~60% of the force resisting lateralization from 0 to 30 degrees of flexion.29 Secondary rupture of the MPFL has been found in nearly 90% of patients with primary lateral patellar instability, and MRI is the most appropriate imaging modality for assessment of its integrity.30,31 Tears of the MPFL are characterized as either complete or partial, and may occur near the femoral origin, midsubstance, or at the patellar insertion.26,32 Recent studies have sought to relate the location of MPFL injury to the risk for recurrent instability. Sillanpaa et al33 reported an increased risk of recurrent instability following avulsion of the MPFL at its femoral attachment. Zhang et al7 also found complete tears near the femoral attachment and complete combined tears involving >1 location increase the risk for recurrent patellar instability [odds ratio (OR) of 6.0 and 5.9].
TT to TG
A lateralized patellar tendon insertion relative to the TG places the patella at an increased risk for dislocation as the patella is subjected to an inferolateral force vector during knee flexion. This finding may be present due to a lateralized TT, a dysplastic trochlea, or from increased femoral anteversion, which medializes the TG. The most common measure of TT position is the TT to TG (TT-TG) distance. Calculation of the TT-TG is simple, and can be performed on axial imaging using basic viewing programs.34 Although CT scan was historically utilized to measure the TT-TG distance, the use of MRI is increasingly common as it allows simultaneous evaluation of surrounding soft tissues and articular cartilage. Although both imaging modalities have demonstrated excellent interrater reliability, there is tendency to underestimate the TT-TG distance on MRI versus CT scan.35 In addition, severe trochlear dysplasia may make accurate calculation of the TT-TG challenging. A TT-TG distance of ≥20 mm on CT is considered abnormal, with an associated increased risk for recurrent instability (OR of 2.1 to 18.7).1,5,7,10,34,36,37
Although the TT-TG distance has helped guide clinical decision making for many years, recent studies have sought to improve its clinical utility through patient-specific adjustment based on anatomy, rather than using a generic threshold of 20 mm. For example, one study found TT-TG to patellar length (TT-TG/PL) ratio ≥0.5 to be most predictive of recurrent instability (OR of 6.1) while TT-TG to trochlear length (TT-TG/TL) ratio was the most sensitive measure (OR of 4.9).37 A TT to posterior cruciate ligament distance has also been described and may be useful for focused assessment of the tibial contribution to tubercle lateralization.36–40 In select cases, CT scan can be utilized to quantify the degree of rotational deformity originating from both the femur and tibia that may be contributing to the finding of an increased TT-TG.
OVERALL RISK CALCULATION AND TREATMENT ALGORITHM
Each individual anatomic factor described above may increase the likelihood for recurrent instability; however, in combination they can pose even greater risk. Lewallen et al2 reported a 60.2% 5-year risk of recurrence in patients below 25 years of age with trochlear dysplasia. In addition, patients below 25 years of age with both trochlear dysplasia and patella alta were found to have a 70.4% 5-year risk of recurrence. Similarly, Jaquith and Parikh4 reported the combination of patella alta, trochlear dysplasia, skeletal immaturity, and a history of contralateral dislocation resulted in a predicted risk of recurrence of 88%. The presence of any 3 of those risk factors was found to have a predicted risk of 75% and any 2 factors had a predicted risk of 55%. The prognostic value of these findings is important to consider when establishing a plan for treatment.
Predictive Scoring Models
In an effort to provide physicians with a reliable and clinically useful method for risk factor evaluation, predictive scoring models have recently been proposed by several authors. These scoring tools consider objective criteria that identify patients who are most at risk for recurrent patellar instability and guide the formation of a treatment plan.6,41,42 A scoring model by Balcarek et al,6 known as the “Patellar Instability Severity score,” incorporates 6 risk factors (age, bilateral instability, patella alta, TT-TG distance, patellar tilt, and trochlear dysplasia) with a maximum score of 7 points. In their retrospective case control study of patients who were initially treated nonoperatively following first-time patella dislocation, patients who scored 4 to 7 points were nearly 5 times more likely to experience an early episode of re-dislocation (within 2 years) than those with a score of 0 to 3 points.6 The authors suggest this scoring tool may help identify which patients are likely to respond well to initial nonoperative treatment.
A novel scoring model by Hevesi et al,43 the “Recurrent Instability of the Patella score,” evaluates long-term risk of recurrent dislocation in patients with first-time instability utilizing 4 risk factors. This model is advantageous as it was derived from data including both operative and nonoperative management of patients followed for an average of over 10 years. The Recurrent Instability of the Patella score incorporates age, skeletal maturity, trochlear dysplasia, and TT-TG/PL ratio into a 5-point scoring system (Table 1). Scores were categorized into low (0 to 1 points), intermediate (2 to 3 points), and high-risk (4 to 5 points) categories with a 10-year recurrence rate of 0.0%, 30.6%, and 79.2%, respectively in the patient population. Their study found stratification as low risk for recurrent instability to be highly sensitive (100% sensitivity, 12.3% specificity) for predicting the absence of recurrent dislocation whereas high-risk stratification was highly specific (89.7% specificity, 68.4% sensitivity) for predicting risk of recurrence. This score may be highly useful in identifying patients at high risk of recurrent instability over the long term and in guiding the treatment decisions.
Because of the multifactorial nature of patellar instability risk assessment, an ideal treatment algorithm has not yet been developed and universally accepted. Certain pathologies necessitate early surgical intervention, such as loose bodies associated with osteochondral injuries of the lateral femoral condyle or patella. Such loose bodies may necessitate early surgical intervention when symptomatic (ie, causing mechanical symptoms) and/or amenable to operative fixation. The decision to pursue operative stabilization in these patients, or in those first-time dislocators without loose fragments, should consider the presence of individual risk factors and involve active discussion with the patient with regard to their goals, concerns, and expectations. Similar to an adolescent male contact athlete who has a >75% chance of recurrent shoulder instability following 1 glenohumeral dislocation,44,45 a skeletally immature 14 year-old with trochlear dysplasia and an elevated TT-TG/PL has an 80% chance of recurrent patellar instability after 1 lateral patella dislocation.43 Recognition of this increased risk of subsequent dislocation that includes the possibility of additional chondral damage is therefore imperative and should frame the discussion of surgical stabilization in these patients.
The authors’ preferred treatment algorithm is presented in Figure 3. Nonoperative management consists of full weight bearing as tolerated and gradual progression of range-of-motion, proprioception, and strength rehabilitation. If a large knee effusion is present on initial presentation, a therapeutic knee arthrocentesis may be considered for pain relief and to reduce tension on the MPFL and medial retinaculum. Fat globules present in the aspirated fluid it may also suggest an osteochondral injury. Full return-to-sport can be achieved between 6 to 12 weeks after injury based on full active range-of-motion, minimal functional strength deficits, and a satisfactory clinical examination. Stabilizing procedures in those identified at high risk for re-dislocation can be utilized based on the presence or absence of the underlying abnormalities described above. MPFL reconstruction is usually performed in all cases, and possible additional procedures include TT osteotomy, trochleoplasty, and lateral lengthening. In the absence of indications for early intervention, surgical timing is individualized, but is often delayed several weeks until the patient has regained full range-of-motion and the acute effusion has subsided.
First-time patella dislocations present a challenge to the treating physician when deciding between operative and nonoperative management. Although many of these injuries can be managed successfully without surgical intervention, certain findings on history, physical examination, and/or imaging should alert the physician to an increased risk of recurrence. A discussion with the patient with regard to their specific risk factors for re-dislocation should follow, and the goals and expectations of treatment should be elicited. Recently published predictive scoring models may be useful to help frame the discussion and guide decision making with regard to the treatment.
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