Abstract
Purpose of Review
The topic of acute patella dislocations is controversial. Discussions revolve around which individuals need early surgery, identification of risk factors, and rehabilitation protocol. The purpose of this review is to discuss the current recommendations for non-operative and/or operative management of first-time dislocators.
Recent Findings
Recent studies have made it clear that not all patellar dislocations are the same, not all patients do well with conservative treatment, and risk stratification can identify individuals at high risk of recurrence who would benefit from early surgical intervention. Risk factors that have been identified include younger age, skeletally immature, contralateral instability, trochlear dysplasia, patella alta, increased tibial tubercle-trochlear groove distance, and increased patella tilt. The PAPI (Pediatric and Adolescent Patellar Instability) RCT study and JUPITER (Justifying Patellar Instability Treatment by Early Results) prospective cohort study have been carefully developed, are under way, and will provide further guidance.
Summary
In summary, the management of acute patellar dislocations is evolving. Surgery for patients with osteochondral loose bodies should include fixation as well as soft tissue stabilization. The standard of care for patients with an acute patellar dislocation without osteochondral loose bodies or fracture is non-operative treatment. However, imaging for all first-time dislocators is indicated to stratify risks and determine risk profile. If an individual is at high risk, soft tissue stabilization may be considered. Still, most patients will be treated non-operatively.
Keywords: Patellar instability, Acute dislocation, Risk factors, Non-operative, Operative
Introduction
Patellofemoral disorders encompass a large spectrum of disease including pain, instability, focal chondral disease, and arthritis while impairing patient function. Patellar dislocations are second only to anterior cruciate ligament tear as a major cause of traumatic knee hemarthrosis [1]. Importantly, first-time dislocations frequently cause injury to the joint surface. An arthroscopic study documented chondral or osteochondral injuries in 71% of patients in acute dislocation events [2]. In young patients with dislocations, this is concerning due to the potential to accumulate chondral injuries leading to early posttraumatic arthritis if the patellar instability is not corrected.
The treatment of patients who have experienced a first-time dislocation is controversial. Although both non-operative and operative treatment can be chosen, there is a paucity of literature to support these management decisions. However, it is understood that most acute patellar dislocations are the result of aberrant anatomical architecture. With proper understanding of the pathology leading to the dislocation event, patient-specific management options may be selected.
The purpose of this review is to discuss the current recommendations for the non-operative and/or operative management of first-time dislocators. The review will include a discussion on anatomy, biomechanics, and aberrant patellofemoral anatomy as risk factors for instability and an acute dislocation. An understanding of these topics will be the foundational knowledge for the management decisions of which individuals need early surgery, when non-operative treatment is safe, and when soft tissue stabilization should be considered.
Anatomy and Biomechanics of Patellar Instability
The biomechanics of the patellofemoral joint require a complex interplay between the bony and soft tissue structures. The osseous anatomy including the patella and trochlea as well as the soft tissue structures including the quadriceps mechanism, patella tendon, medial soft tissues, and lateral soft tissues should be evaluated for architectural abnormalities. Anatomic malformations of the bones cause malalignment and may predispose to maltracking of the patella. Additionally, abnormalities of the dynamic and static soft tissue structures have significant effects on patellofemoral alignment and motion [3•].
Bony Structures
The patella resides within the distal femoral trochlea and links the extensor mechanism through connections to the quadriceps tendon at its superior pole and the patellar tendon at its inferior pole [4]. A high-riding patella, or patella alta, requires more extensive knee flexion prior to the patella engaging the trochlea which may result in increased risk of dislocation episodes or chondromalacia [5, 6]. A low-riding patella, or patella baja, results in increased joint force on the patella which may lead to a restriction in motion and early arthritis. Many values may be used to measure patellar height. The Caton-Deschamps index (CDI) is widely utilized and has the added benefit of its value not varying with knee flexion. The trochlea is formed by the anterior aspect of the distal femur and is composed of a centralized trochlear groove with associated medial and lateral facets. Trochlear dysplasia is characterized by a loss of normal concave anatomy and therefore trochlear groove depth. This creates a flat trochlea with asymmetrical facets. During knee flexion, trochlear dysplasia predisposes to the loss of patellar restraint and a subsequent patellar dislocation. Dejour and colleagues quantified trochlear dysplasia radiographically. They defined pathologic features as a trochlear bump > 3 mm and trochlear depth at 4 mm or less [7]. The tibial tuberosity-trochlear groove (TT-TG) distance is utilized as a more accurate method of quantifying axial anatomy of the patellofemoral articulation. A TT-TG value of > 15 mm is associated with increased risk of patellar instability due to a lateralized force vector [8].
Soft Tissue Structures
The quadriceps mechanism is an important contributor to dynamic patellofemoral joint stability and can also contribute to a lateralized force vector. It is formed by the convergence of the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius. The quadriceps angle, or Q-angle, plays a substantial role in evaluation of patellofemoral forces and tracking. A Q-angle > 20° is considered abnormal and may predispose to both increased lateral displacement forces and increased patellar contact pressures [5]. Vastus medialis obliquus (VMO) dysplasia is a common cause of dynamic patella instability, and strengthening is a foundation of patellofemoral disorder rehabilitation. In addition to the VMO, medial soft tissues include the medial patellofemoral ligament (MPFL), medial patellotibial ligament (MPTL), and medial retinaculum. The MPFL is the primary passive restraint to lateral patellar translation. MPFL laxity may be the result of congenital abnormalities, traumatic lateral subluxations, or patellar dislocation events. The lateral soft tissue restraint to the patella is divided into superficial oblique and deep transverse lateral retinacular layers. The lateral retinaculum is an important secondary static stabilizer of lateral patellar translation. Tightness of the retinaculum may result in patella tilt (> 5°), increased forces between the lateral patellar facet and lateral trochlea, and subsequent arthritis over time [4].
Incidence and Initial Management
In addition to treatment of first-time patella dislocations being a controversial topic, it is also evolving. The incidence of primary patellar dislocations is 5.8 cases per 100,000 population, and in patients 10–17 year old, this increases to 29 cases per 100,000 population [1]. 61–72% of acute patellar dislocations are related to sporting activities. Subsequently, patellofemoral pathology and dislocations have a significant impact on time lost from these sporting activities or work [9]. In any dislocation event, the majority will spontaneously reduce, but it is essential to maintain a high degree of suspicion for underlying pathology. A full imaging work-up should be completed within the initial management. For all first-time dislocators, this includes weight-bearing and mechanical axis radiographs as well as MRI. There is currently no consensus on early mobilization and weight-bearing. Although immobilization in flexion is an option, it is impractical and leads to muscular atrophy, stiffness, and degenerative joint disease. An alternative is early brace protection, range of motion as tolerated, weight-bearing as tolerated with crutches, and aspiration of hemarthroses as needed [3•].
Decision-Making
Who Needs Early Surgery
Traditional indications for early surgical intervention have been limited to osteochondral fractures or loose bodies requiring internal fixation and/or fragment removal [1]. The standard of care for the management of a first-time patellar dislocation has been non-operative treatment. Classic doctrine suggests that patients “do well” with nonsurgical management. However, Magnussen et al. showed that only 26.4% of patients return to unlimited activity at 3 years after conservative treatment of a first-time patellar dislocation [10••]. Additionally, several studies have cited recurrent dislocation rates ranging from 17 to 71% following the index event [1, 11–13, 14•, 15]. Nwachukwu et al. constructed a cost-effectiveness model expressed in quality-adjusted life years and concluded that non-operative treatment was least costly but also least effective. They also concluded that both immediate and delayed surgery were effective [16•]. The data obtained from these studies is provocative and leads us to question the current standard of care. Perhaps surgical stabilization of the first-time patellar dislocation may lead to improved patient outcomes.
Pagliazzi et al. performed a meta-analysis of randomized controlled trials (RCTs) investigating the differences of short- and long-term outcomes between surgical and nonsurgical management of first-time patellar dislocations [17••]. Ten RCTs met the inclusion criteria, and 510 patients were included. Previtali et al. found a reduced risk of recurrent dislocation at short-term and mid-term follow-up favoring the operative cohort. Kujala scores favored the non-operative group at short-term follow-up. However, there were no differences in Kujala scores between the groups at long-term follow-up.
In a majority of the RCTs included in the meta-analysis by Pagliazzi et al., patients within the surgical management groups had operations to address MPFL incompetence. An imaging study by Askenberger et al. demonstrated that the medial patellofemoral complex is torn in essentially all cases of first-time dislocations [18]. Biomechanical studies support this, as the force required for a patellar dislocation is greater than that required to tear the MPFL [19]. When operatively addressing acute patellofemoral instability, it is clear that MPFL repair does not reduce recurrence rates of instability [20, 21]. MPFL repair should be rarely considered in cases of a fixable loose body, normal underlying anatomy, good tissue quality, and a distinct zone of injury. In comparison, Bitar et al. showed that MPFL reconstructions reduce recurrence (35% vs. 0%) [22]. Surgery to address MPFL incompetence has become the gold standard. In the setting of an acute dislocation and patellofemoral instability, Liu et al concluded that cartilage injuries should be addressed concurrently [23•]. It is important to note that there is no current role for bony surgery in first-time patella dislocations. However, MPFL isometry is worrisome in severe cases (CD 1.5, TT-TG > 20 mm). Overall, clear surgical indications are irreducible dislocations, loose bodies, and large chondral or osteochondral fractures of the medial patella, lateral trochlea, or lateral femoral condyle weight-bearing zone.
Risk Stratification
There has been increased interest in risk stratification following a first-time patellar dislocation. The objective is to identify individuals at high risk of recurrent patellar instability who may benefit from early surgical intervention. Similar to the evolution of treatment of a first-time shoulder dislocation, risk stratification models for acute patella dislocations may eventually lead to evidence-based treatment recommendations [3•].
Balcarek et al. evaluated risk factors including age younger than 16 years, contralateral instability, trochlear dysplasia, patella alta (CDI > 1.2), TT-TG distance > 16 mm, and patella tilt > 20°. Patients with 4 or more points as positive risk factors showed an odds ratio of recurrence of 4.88 when compared to patients with 3 points or less [13]. Similarly, Jaquith and Parikh performed a multivariate risk factor analysis investigating the role of patella alta (CDI > 1.45), trochlear dysplasia, skeletal immaturity, and contralateral patellar dislocation on the risk of recurrent patellar instability. The presence of all 4 risk factors had a predicted risk of recurrence of 88%, while dropping to 75% for any 3 risk factors, and 55% for any 2 risk factors [14]. Lewallen et al. reported that the 5-year risk of recurrent dislocation with patella alta, trochlear dysplasia, and age younger than 25 years was 70% and the risk for combined solely trochlear dysplasia and age younger than 25 years was 60% [15]. Based on 81 patients, Hevesi et al. developed a multivariate, statistically derived scoring system, the Recurrent Instability of the Patella Score (RIP Score). This scoring system utilized employed age, skeletal maturity, trochlear dysplasia, and TT-TG/PL ratio to predict recurrent instability and can be found on MD Calc. The resulting RIP score provided percent risk of recurrent instability at 5 years and stratified patients into low, intermediate, and high-risk categories [24•].
Other patellar instability factors important to evaluate include age, activity level, personal history of a contralateral dislocation, family history of patella instability, and mechanism of injury. The injury mechanism can be classified as low or high energy as well as contact or non-contact. A patella dislocation in a patient with “normal” patellofemoral anatomy requires a significantly higher level of energy. Consequently, a dislocation with “normal anatomy” is more likely to lead to hemarthrosis, fracture, or severe chondral damage than a patella dislocation in a patient with patellofemoral dysplasia and/or patella alta. However, a patellar dislocation in the setting of abnormal anatomy is at a significantly higher risk for recurrence [3].
It is therefore essential to analyze each parameter of the known risk factors for patellofemoral stability while acknowledging the presence of a broad continuum from normal anatomy to extreme of pathology. The anatomic range of patient populations is variable regarding patellar alta, trochlear dysplasia, position of the tibial tuberosity, and MPFL status. Furthermore, a traumatic patella dislocation does not preclude a full evaluation of underlying patellar instability risk factors which may lead to chronic redislocations and unfavorable non-operative treatment. In addition to the risk for redislocation, these patients with underlying patellar instability risk factors are often at high risk for a patellar dislocation of the contralateral knee [25].
Although risk factors have been identified and patients can be placed into risk categories, this is only one piece of the puzzle. These studies do not provide clear answers regarding the benefits of early surgical stabilization for the first-time dislocator. When deciding between non-operative or operative treatment, it is essential to consider the advantages and disadvantages of both. With non-operative treatment, there is a faster return timetable with no surgical risk. However, non-operative treatment may increase the risk of damage to cartilage and ligaments. Additionally, functional outcomes, quality of life, and return-to-play rates are not clear. With operative treatment, there is a longer time to return and associated surgical morbidities including stiffness and infection. It is not known whether operative treatment is chondroprotective, and functional outcomes, quality of life, and return-to-play rates are also not clear.
To further analyze non-operative and operative management for acute patellar locations, the PAPI (Pediatric and Adolescent Patellar Instability) RCT study and JUPITER (Justifying Patellar Instability Treatment by Early Results) prospective cohort study have been carefully developed and are under way. The PAPI study includes a total of 240 skeletally immature patients with a first-time dislocation. Half of the group was randomized to conservative, non-operative treatment, while the other half underwent arthroscopy and MPFL reconstruction. Both groups participated in a standardized, intensive rehabilitation protocol [26••]. The JUPITER study is a multi-center, multi-armed study enrolling patients from the age group of 10–25 who sustained a patellar dislocation or subluxation event [27•]. Within the early stages of the study, Bishop et al. reported 137 patients enrolled in the non-operative group and 269 patients enrolled in the operative group. Of note, 71.7% of the patients in the operative group reported more than 1 dislocation event compared to only 20.4% of the patients in the non-operative group. The mean number of recurrent dislocations in the operative group was 8.3 compared to 3.5 in the non-operative group. KOOS QOL scores were significantly lower in those who experienced more than one redislocation (p = 0.01) [28••]. At most recent update, the JUPITER study now has over 1200 patients enrolled. The PAPI and JUPITER studies will provide guidance in the future for patellar instability management. Although treatment remains controversial, a few things have become clear. Patellar dislocations are not all the same, not all patients do well with conservative treatment, and risk factor stratification can identify individuals at high risk of recurrent patellofemoral instability.
Rehabilitation and Return-to-Play
Recommended non-operative treatment is centered on a “core to floor” rehabilitation protocol. The objectives of a rehabilitation program can be divided into different stages: (1) pain, swelling, and inflammation resolution; (2) joint motion and flexibility recovery; (3) muscle strength recovery; (4) motor patterns and coordination recovery; and (5) sport-specific athletic action and return to sporting activity [29]. These stages should be followed for general direction, not as a rigid scheme. Initially, the medial patellofemoral structures need to be kept in a shortened condition to encourage healing. This requires a degree of immobilization, which can be achieved utilizing a sleeve and/or PS brace. To recover knee joint motion, it is necessary to activate and strengthen the involved muscle groups. These are comprised of the quadriceps, core muscles, and posterior chain including the gluteal, hamstrings, and hip rotator muscles. A progression of strengthening should occur while evaluating for neuromuscular imbalance seen in hamstring to quadriceps ratios, dynamic valgus, and sport specific imbalances. Generalized ligament laxity is frequently seen in patellar instability, and therefore it is essential to achieve effective control to prevent recurrent dislocations. Additionally, it is important to begin specific gait training early, since these subjects often develop gait abnormalities as a defense mechanism against patellar instability. To recover functionally, muscle strength, endurance, and proprioception should be translated into athletics movements. A series of performance tests should be utilized in examining the ability to safely return to athletic or sporting action.
The management goal of a first patellar dislocation is to achieve functional recovery. Since patellar instability is commonly a condition affecting young athletes, this requires returning to pre-injury level of fitness and sporting activity. Through progression of the functional stages, the risk of recurrence of dislocation can be reduced. The ability to resume sporting activity should be restricted to the presence of normal results on sport-specific functional tests [29].
Conclusion
The management of acute patellar dislocations is evolving. Surgery for patients with osteochondral loose bodies should include fixation as well as soft tissue stabilization. The standard of care for patients with an acute patellar dislocation without osteochondral loose bodies or fracture is non-operative treatment. However, imaging for all first-time dislocators is indicated to stratify risks and determine risk profile. If an individual is at high risk, soft tissue stabilization may be considered. Still, most patients will be treated non-operatively.
Compliance with Ethical Standards
Conflict of Interest
Joseph M. Rund declares that he has no conflict of interest.
Seth L. Sherman grants from Arthrex, Inc., personal fees from Ceterix Orthopaedics, personal fees from CONMED Linvatec, personal fees from Moximed, personal fees from Neotis, personal fees from Regeneration Technologies Inc., personal fees from Vericel, grants from Zimmer, other from ACL Study Group, other from American Journal of Orthopedics, other from American Orthopaedic Society for Sports Medicine, other from Arthroscopy, and other from Arthroscopy Association of North America, outside the submitted work.
Human and Animal Rights and Informed Consent
This article does not contain any studies with human or animal subjects performed by any of the authors.
Footnotes
This article is part of the Topical Collection on Non-Operative Management of Anterior Knee Pain
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Contributor Information
Joseph M. Rund, Email: jmrqxc@health.missouri.edu
Betina B. Hinckel, Email: Betina.Hinckel@beaumont.org
Seth L. Sherman, Email: shermans@stanford.edu
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