PATELLAR DISLOCATION
Background
The knee cap (patella) sits within the quadriceps tendon at the front of the knee and is firmly attached to the shin bone (tibia) by the patellar tendon. The patella protects the knee joint from direct trauma (e.g. landing on the knees) and acts as a biomechanical pulley to improve the efficiency of the quadriceps muscle as the main extensor and decelerator of the knee. The cartilage on the under surface of the patella is the thickest in the body, which reflects the large loads that are placed on the patella during activities of daily living (e.g. stairs, squatting, running, and jumping).
The patellofemoral compartment is part of the knee joint and is formed by the patella and the groove (femoral trochlea) at the lower (distal) end of the thigh bone. As the knee bends (flexion) from a fully straightened position, the patella descends into the trochlea, but does not fully enter this groove until approximately 20-30° of knee flexion. Within the first 20-30° of knee flexion, a significant amount of patellar stability is provided by the surrounding soft tissues (quadriceps muscle and tendon, retinaculum and surrounding ligaments) but once the patella engages with the trochlea the stability is increased by the bony ‘walls’ of the groove.
Mechanism of injury
Patellar dislocation occurs when the bone is forced out of its normal position, so that the patella and trochlear surfaces are no longer in contact with one another (image 4); the patella almost always dislocates outwards (laterally). Patellar dislocations are usually caused by a non-contact twisting injury to the knee, or direct contact to the inner (medial) aspect of the patella. In some individuals, the injury can be trivial and in recurrent episodes, less force may be required to re-dislocate the bone. Individuals with naturally lax joints (e.g. hypermobility syndrome, Ehlers-Danlos syndrome) are more susceptible to patellar instability due to the reduced support provided by surrounding soft tissues. Individuals that are under the age of 18 at the time of first dislocation are also more likely to experience further dislocations.
Risk factors
Specific anatomical risk factors have been associated with patellar dislocation:
- Patella alta (alta = high): a naturally ‘high-riding’ patella means the patella has further to descend before engaging with the trochlea and less bony support is provided whilst the knee is relatively straight.
- Trochlear dysplasia (dysplasia = abnormal development): a shallow or misshapen trochlea decreases the bony support for the patella, or a bony bump at the top of the trochlea (supratrochlear spur or boss) can prevent the patella from tracking correctly as it enters the groove.
- Increased tibial tuberosity-trochlear groove (TT-TG) distance exerts a greater lateral ‘pull’ on the patella when the quadriceps muscle contracts.
A recent study found that 87% of individuals that suffered a first-time patellar dislocation had at least one of these anatomical risk factors: research is currently being conducted to determine whether individual anatomy can ‘predict’ the risk of re-dislocation, but no clinically useful prediction tool is available at this present time.
History
Patellar dislocation occurs most often in younger patients, especially those in the second decade of life, with the incidence decreasing with increasing age. The individual typically recalls a memorable incident that caused the patella to ‘pop out’ laterally. This usually occurs with the knee relatively straight, where the patellar has not yet engaged the trochlea and there is less traction provided by the quadriceps. The patella may have ‘popped back in’ by itself or been relocated by someone else.
An individual may report some or all of the following history elements; however, patients that have injured their knee without tearing their ACL may also report a similar history.
- deformity
- isolated anterior knee pain
- feeling/hearing a sensation/noise (e.g., pop, crack, snap)
- knee giving way/moving out and back in at the time of injury
- inability to continue the activity
- difficulty weight bearing
- rapid onset of knee joint swelling
- difficulty fully straightening the knee
- lack of trust in the knee and/or knee instability.
Patients with naturally unstable joints (e.g. Ehlers-Danlos syndrome) may not recall a specific, traumatic event and may not experience knee joint swelling. one-third of conservatively managed patients will re-dislocate after a first patellar dislocation; after a second dislocation, more than 50% will have further episodes of instability.
Clinical tests
Clinical tests that quantify lateral translation of the patella or assess for apprehension whilst attempting to displace the patella laterally are used to assess for patellar instability. These include the moving patellar apprehension test, quadrant test, and the inverted J-sign. Palpation of the medial patellofemoral ligament (MPFL) insertions may also produce tenderness, but the diagnostic accuracy of these tests is poor, or have not been adequately investigated. Therefore, it is generally recommended that several different positive tests, in combination with the history and exclusion of other conditions, are needed to diagnose patellar instability clinically.
Imaging
X-rays are often used to confirm a dislocated patella when the individual presents with knee deformity. If the individual presents with a history of patellar dislocation that has already been relocated, X-rays should include AP, lateral, and skyline/Merchant views to assess for osteochondral injury, avulsion fractures, or significant lateralisation of the patella.
Magnetic Resonance Imaging (MRI) can be used to identify bone bruising patterns consistent with recent patellar dislocation, articular cartilage injury and medial patellofemoral ligament (MPFL) integrity, while computed tomography (CT) scans have high diagnostic accuracy for bone fracture. However, in the absence of clinical or X-ray findings indicating surgery, MRI and CT do not influence treatment after first-time dislocation and are therefore not routinely performed.
If surgery is indicated, imaging can be used to identify anatomical risk factors and the presence of associated knee injuries, which are used to guide surgical treatment. Since first time patellar dislocations occur frequently in the second decade of the life, X-rays will identify whether the individual’s growth plates (physes) have closed, as open growth plates contraindicate certain surgical procedures.
True lateral view X-rays at 20-30° of knee flexion, MRI and CT can be used to calculate the patella height ratio. The patellar height ratio compares the length of the patella and its distance from a specific point on the tibia to determine whether patella alta is present. This can be measured using various methods including the Insall-Salvati ratio and Caton-Deschamps index. Although there is variability between different imaging modalities, the following thresholds are used to categorise patellar height:
The DeJour classification of trochlear dysplasia is determined by combining slice imaging (CT or MRI) with the true lateral X-ray, and categorised by the presence of a crossing sign (type A), supratrochlear spur (type B), double contour sign (type C), or combinations of type A-C (type D).
Skyline X-ray views at 20-30° of knee flexion, CT and MRI allow measurement of the trochlear depth and the sulcus angle. The trochlea is considered shallow, and therefore dysplastic, if the depth is less than 3mm or the sulcus angle is greater than 145°.
MRI provides the most accurate measurement of the TT-TG distance using tendon and cartilage landmarks, while CT scans provide the most accurate measurements using bony landmarks. TT-TG distances differ between CT and MRI, with MRI values measuring smaller. A recent study found that all patients with PFJ instability had values less than 18mm on MRI, whereas a TT-TG distance above 20mm on CT scan has traditionally been considered the threshold for an abnormal measurement.
Management
Patellar dislocations may be managed with or without surgery. Following first-time patellar dislocation, surgery reduces the rate of re-dislocation when compared with non-surgical management (24% versus 35%) but there is no difference in long-term function, patient satisfaction or recurrent instability (∼33%). With specific reference to children and adolescents, surgery improves sports and quality of life outcomes despite providing no difference in pain, symptoms or function in daily living. Surgery is associated with specific surgical risks, depending on the procedure performed, and non-surgical management is therefore recommended for most first-time patellar dislocations.
Non-surgical management:
Immobilising the knee in a plaster of Paris, or patellar bracing, has been recommended following patellar dislocation but there is no clear evidence to support the use of these modalities. Limited evidence suggests a posterior splint may reduce the re-dislocation rate, while a dynamic brace may provide support to the patella within the first 30° of knee flexion.
Once the initial impairments (e.g., swelling, pain and reduced range of movement) have resolved, exercise therapy is recommended to improve patellar stability and limb alignment whilst moving. Historically, strengthening exercises that attempt to bias the inner quadriceps muscle (vastus medialis oblique) have been advocated but recent evidence suggests these muscles cannot be preferentially targeted, and there is no clinically relevant difference between these exercises and general quadriceps strengthening exercises for first time dislocations. Control of lower limb rotation during weight bearing knee flexion, and joint position (proprioceptive) exercises have also been suggested but this has been more adequately researched in patellofemoral pain rather than patellofemoral instability.
It is important to note that the optimal exercise therapy for patellar dislocation has not been determined. In the early phases following dislocation, open chain knee extension may produce excessive lateral translational forces on the patella and negatively affect healing of the medial soft tissues. Considering patellar dislocations are more likely to occur while the knee is relatively straight, it appears logical to ensure exercises are eventually performed within these vulnerable ranges.
Surgical management:
Surgery is recommended in first-time patellar dislocations if the individual has sustained significant cartilage and/or bone injury, or for recurrent patellar dislocations despite appropriate non-surgical management. The appropriate surgical intervention is dictated by the presentation and may involve fixation of an osteochondral or avulsion (pull off) fracture, ligament reconstruction or additional procedures to address anatomical risk factors identified by imaging.
Open growth plates indicate skeletal immaturity and contraindicate trochleoplasty or tibial tuberosity osteotomy; widespread osteoarthritic changes also contraindicate trochleoplasty. MPFL reconstruction is permitted with open growth plates. In additional to the rare but potentially serious risks of surgery (e.g. infection, DVT, death), recurrent dislocation and recurrent instability, specific operations are associated with specific risks as described below.
Surgical management:
Post-operative protocols are often dictated by the consultant performing the surgery, therefore any restrictions that are placed on the patient should be clearly communicated between the surgeon and therapist.Our patella instability protocols are available in the protocol section.
Written by: Richard Norris
Peer reviewed by:
Elizabeth A Arendt M.D. | Professor of Orthopaedic Surgery, University of Minnesota Department of Orthopaedic Surgery, South Minneapolis MN 55454
Email: arend001@umn.edu
References
Arendt EA, England K, Agel J, Tompkins MA. An analysis of knee anatomic imaging factors associated with primary lateral patellar dislocations. Knee Surg Sports Traumatol Arthrosc. 2016.
Askenberger M, Janarv PM, Finnbogason T, Arendt EA. Morphology and Anatomic Patellar Instability Risk Factors in First-Time Traumatic Lateral Patellar Dislocations: A Prospective Magnetic Resonance Imaging Study in Skeletally Immature Children. Am J Sports Med. 2017;45(1):50-8.
Becher C, Schumacher T, Fleischer B, Ettinger M, Smith T, Ostermeier S. The effects of a dynamic patellar realignment brace on disease determinants for patellofemoral instability in the upright weight-bearing condition. J Orthop Surg Res. 2015;10:126.
Christensen TC, Sanders TL, Pareek A, Mohan R, Dahm DL, Krych AJ. Risk Factors and Time to Recurrent Ipsilateral and Contralateral Patellar Dislocations. Am J Sports Med. 2017;45(9):2105-10.
Dejour H, Walch G, Nove-Josserand L, Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surg Sports Traumatol Arthrosc. 1994;2(1):19-26.
Erickson BJ, Mascarenhas R, Sayegh ET, Saltzman B, Verma NN, Bush-Joseph CA, et al. Does Operative Treatment of First-Time Patellar Dislocations Lead to Increased Patellofemoral Stability? A Systematic Review of Overlapping Meta-analyses. Arthroscopy. 2015;31(6):1207-15.
Longo UG, Ciuffreda M, Locher J, Berton A, Salvatore G, Denaro V. Treatment of Primary Acute Patellar Dislocation: Systematic Review and Quantitative Synthesis of the Literature. Clin J Sport Med. 2017;27(6):511-23.
Monson J, Arendt EA. Rehabilitative protocols for select patellofemoral procedures and nonoperative management schemes. Sports Med Arthrosc. 2012;20(3):136-44.
Nwachukwu BU, So C, Schairer WW, Green DW, Dodwell ER. Surgical versus conservative management of acute patellar dislocation in children and adolescents: a systematic review. Knee Surg Sports Traumatol Arthrosc. 2016;24(3):760-7.
Saccomanno MF, Sircana G, Fodale M, Donati F, Milano G. Surgical versus conservative treatment of primary patellar dislocation. A systematic review and meta-analysis. Int Orthop. 2016;40(11):2277-87.
Shah JN, Howard JS, Flanigan DC, Brophy RH, Carey JL, Lattermann C. A systematic review of complications and failures associated with medial patellofemoral ligament reconstruction for recurrent patellar dislocation. Am J Sports Med. 2012;40(8):1916-23.
Smith TO, Chester R, Cross J, Hunt N, Clark A, Donell ST. Rehabilitation following first-time patellar dislocation: a randomised controlled trial of purported vastus medialis obliquus muscle versus general quadriceps strengthening exercises. Knee. 2015;22(4):313-20.
Smith TO, Donell S, Song F, Hing CB. Surgical versus non-surgical interventions for treating patellar dislocation. Cochrane Database Syst Rev. 2015(2):CD008106.
Weber AE, Nathani A, Dines JS, Allen AA, Shubin-Stein BE, Arendt EA, et al. An Algorithmic Approach to the Management of Recurrent Lateral Patellar Dislocation. J Bone Joint Surg Am. 2016;98(5):417-27.
Zaman S, White A, Shi WJ, Freedman KB, Dodson CC. Return-to-Play Guidelines After Medial Patellofemoral Ligament Surgery for Recurrent Patellar Instability: A Systematic Review. Am J Sports Med. 2017:363546517713663.