Patellar Instability

Article Author:
Steve Wolfe
Article Author:
Matthew Varacallo
Article Author:
Joshua Thomas
Article Author:
Jeffrey Carroll
Article Editor:
Chadi Kahwaji
4/10/2020 12:58:55 AM
PubMed Link:
Patellar Instability


Patellar dislocations are a frequent musculoskeletal injury seen in the emergency department and represent 3% of all knee injuries. Also, patellar dislocations represent a leading cause of knee hemarthrosis. Patellar dislocation is a distinct entity from a patellar subluxation but occurs in a spectrum of disorders termed patellar instability. Patellar dislocations occur more frequently in the second and third decades of life. Patients with obesity, as well as female adolescent athletes, tend to have a higher predisposition to this injury. Patients under the age of 16 tend to have higher reoccurrence rate and associated complications of patellar instability following a patellar dislocation. While the most common occurrences involve sporting activities that involve rapid deceleration and twisting, including internal rotation of leg after foot plantation. However, dislocations can also coincide with traumatic events such as direct impact to a partially flexed knee.[1]

Clinical features in the evaluation of a dislocated patella include obtaining a clear HPI, including a detailed assessment of the mechanism. Also, eliciting a history of patellofemoral instability is also warranted. Once dislocated, the patient will be holding the knee in a guarded position, with the knee in a slightly flexed position.[2][3]

Anatomy and Physiology

The patella is a sesamoid bone with proximal attachments to the quadriceps tendon and distal attachments to the patellar ligament. Soft tissues stabilize the patella in extension and the patellofemoral joint guides the patella in knee flexion. MPFL (medial patellofemoral ligament) restrains the patella medially and joins the patella to the medial femoral condyle. MPFL has the primary positioning restraint in the first 20 degrees of flexion. Dynamic stability of the patella is provided by the vastus medialis (VM) muscle.

Anatomical risk factors Patellofemoral instability include increased Q angle (including an increased distance between the trochlear groove and tibial tubercle as measured on advanced imaging modalities), patella alta, patella tilt, trochlear dysplasia, and vastus medialis weakness. The mechanism for dislocation involves valgus stress with a contracted quadriceps muscle.

An unstable patella can dislocate in any direction (proximal, medial, superior, inferior, and vertically), but may also dislocate intraarticularly or on-axis horizontally and vertically (dorsal fin). In the most common type of dislocation, initial x-ray imaging will show the patella translocated laterally, but could also demonstrate contusions on the lateral femoral condyle anteriorly and contusions or avulsion of the medial patella.


Indications for reduction include a history and physical exam consistent with lateral patellar dislocation. Pre-reduction x-rays are not necessary for lateral patellar dislocation. However, pre-reduction x-rays should be considered in older patients and in patients in whom there may be an uncertainty of a simple dislocation due to mechanism or exam.[4][5]

Patellar dislocations in directions other than laterally frequently require surgery to reduce. However, attempts to reduce dislocations before surgery is warranted and encouraged.

Post reduction x-rays should include AP, lateral and patellar (sunrise/merchant) views.


No known contraindications for reducing patella.


For simple lateral dislocations (most common), many dislocations will spontaneously reduce. A non-analgesic reduction attempt is encouraged. Calm and reassuring education can easily prepare the ideal patient. This education requires only a brief explanation of the presumed diagnosis, a description of the rapid reduction process and the dramatic reduction in pain upon completion. While one may position the patient prone, most references advocate having the patient in a supine seated position. The patient should have their hips in slight flexion helping to avoid contraction of the patellar tendon.[6][7][8]

The clinician will position themselves lateral to the affected knee.


For lateral dislocations, have the patient in a seated position and passively extend knee into slight hyperextension while applying gentle medial pressure to the lateral edge of the patella. For a one-person reduction, maintain one supporting hand near the posterior ankle giving weighted leverage of the patient’s leg, then it is possible to use the other hand with the heel of the hand resting on quadriceps muscle just proximal to the patella and to use the thumb to apply pressure medially. This technique seems to allow both better leverage and monitoring/sensing for resisting quadricep tension development which can make reduction more challenging. There are more challenging patients or patients with increased anxiety/pain that may require a small analgesic to accomplish the same above method. Additionally, successful use of ultrasound-guided femoral nerve block has been reported to achieve adequate pain control before reduction.

For most of the other directional dislocations, a similar approach may be used. For intra-articular dislocations, however, hyperextension is not recommended. Under procedural sedation, a partial movement of the knee to full flexion may relax the surrounding tissues enough to allow traction of the patella out of the joint and into proper alignment.

In a successful reduction, the patient should resume normal flexion/extension with the return of the patella to the tibiofemoral tract.

Post reduction views should include AP, lateral and patellar (sunrise/merchant) views to ensure successful reduction, assess for patellar avulsion or osteochondral injury of the femoral condyle.

Narcotic pain control is not necessary with simple patellar dislocations once the patella is reduced. A short course of NSAIDs, while not necessary, may be prescribed to increase patient comfort.

Before discharge, size the patient for crutches and secure the knee with a knee immobilizer or splint. However, immobilization recommendations are currently under review. Early follow up with an orthopedic or sports medicine specialist within the following week is recommended for early and extensive rehabilitation.[9][10]


No consistent documentation of injury or complications associated with reduction the patella has been identified. However, patellar dislocations can occur with or result in the following complications: patellar fracture, hemarthrosis, anterior knee pain, patellar instability, decreased activity, and patellofemoral arthritis.

Osteochondral injury (of either patella or femoral condyle) rates are high and over 40% may go unnoticed on x-ray but will be visible on later MRI.

Chronic instability and recurrence of the dislocation is approximately 30% and can be as high as 50% in 5 years.

Patellar dislocations with loose bodies and displaced osteochondral fractures are surgical candidates. Surgical consideration should also be given to disruptions of VM insertion and medial patellofemoral ligament. Additionally, complete or extensive rehabilitation with residual functional disability warrants early surgery. Consensus on early surgery as acute management or to prevent recurrent instability is lacking. Limited evidence exists that operative management may reduce recurrent dislocations. There are many case reviews regarding non-operative versus surgical management. However, only a few randomized control trials exist.

There are many rehabilitation options, and a therapeutic exercise is a primary option. However, discerning which rehabilitation approach is the most effective lacks clinical evidence.

Clinical Significance

Simple patellar dislocations are a common musculoskeletal injury with a relatively simple, quick reduction that normally does not require pre-reduction imaging or complicated procedural sedation. Recurrent instability is frequent and prompt follow up should be ensured to start rehabilitation early and to assess for and prevent complications.

Enhancing Healthcare Team Outcomes

Patellar dislocation is quite common and is best managed by an interprofessional team that includes the therapist and orthopedic nurses. Most simple cases of patella dislocation will spontaneously reduce but recurrent cases should be referred to the orthopedic surgeon. Sometimes the dislocation may be associated with a knee pathology.

The outcomes for most patients with simple patella dislocation are good.

  • Contributed by Matthew A Varacallo, MD
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      Contributed by Matthew A Varacallo, MD


[1] Huang JB,Ge H,Zhang YL,Liu CT,Xue C,Chen YC,Wu P,Cheng B, The role of arthroscopic release of gluteal muscle contracture in improving patellofemoral instability. Journal of orthopaedic surgery and research. 2019 May 28;     [PubMed PMID: 31138264]
[2] Ibrahim SA,Shohdy EM,Ramadan SA,Almisfer AK,Abdulsattar WS,Khairat S, Medial Patellofemoral Ligament Reconstruction in Traumatic Patellar Dislocation without Patellar Fixation. The journal of knee surgery. 2019 May 23;     [PubMed PMID: 31121630]
[3] Blanke F,Watermann K,Haenle M,Feitenhansl A,Camathias C,Vogt S, Isolated Medial Patellofemoral Ligament Reconstruction Can Be an Effective Procedure in Patellofemoral Instability with Risk Factors. The journal of knee surgery. 2019 May 23;     [PubMed PMID: 31121628]
[4] Obermeyer C,Hoffmann DB,Wachowski MM, [Patellar dislocation in children and adolescents : Current developments in diagnostics and treatment]. Der Orthopade. 2019 May 17;     [PubMed PMID: 31101964]
[5] Masouros PT,Apergis EP,Babis GC,Pernientakis SS,Igoumenou VG,Mavrogenis AF,Nikolaou VS, Essex-Lopresti injuries: an update. EFORT open reviews. 2019 Apr;     [PubMed PMID: 31057951]
[6] Vermeulen D,van der Valk MR,Kaas L, Plaster, splint, brace, tape or functional mobilization after first-time patellar dislocation: what's the evidence? EFORT open reviews. 2019 Mar;     [PubMed PMID: 30993012]
[7] Purohit N,Hancock N,Saifuddin A, Surgical management of patellofemoral instability. I. Imaging considerations. Skeletal radiology. 2019 Jun;     [PubMed PMID: 30542758]
[8] Arendt EA,Askenberger M,Agel J,Tompkins MA, Risk of Redislocation After Primary Patellar Dislocation: A Clinical Prediction Model Based on Magnetic Resonance Imaging Variables. The American journal of sports medicine. 2018 Dec;     [PubMed PMID: 30398902]
[9] Lee DY,Park YJ,Song SY,Hwang SC,Park JS,Kang DG, Which Technique Is Better for Treating Patellar Dislocation? A Systematic Review and Meta-analysis. Arthroscopy : the journal of arthroscopic     [PubMed PMID: 30301628]
[10] Krebs C,Tranovich M,Andrews K,Ebraheim N, The medial patellofemoral ligament: Review of the literature. Journal of orthopaedics. 2018 Jun;     [PubMed PMID: 29881201]