Patellar Instability | Diagnosis & Treatment

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Patellar Instability

Patellar Instability | Diagnosis & Treatment


Introduction & Pathomechanism

Patellar instability is mainly affecting active adolescent patients and peaks at the age between 10 and 20 years. It accounts for up to 3% of acute knee injuries and recurrence rates are variable according to the source but frequently seen.



The quadriceps tendon exerts a force on the patella that is slightly lateral to the midline and this is opposed by the vastus medialis and medial patellofemoral ligament. When anatomical characteristics like trochlear dysplasia, an increased Q-angle, patella alta, increased tibial tuberosity-trochlear groove distance and torsional abnormalities are present, they may further increase the risk of patellar instability or dislocation. Because of patellar subluxations or dislocation(s), the retropatellar cartilage collides with the lateral femoral condyle. As such cartilage defects are common and in about 90% of cases the medial patellofemoral ligament is ruptured or elongated. Traumatic dislocations are most frequently associated with more cartilage damage due to the high-energy mechanism.

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Clinical Picture

Patellar dislocations occur most frequently during sports but in some cases can occur atraumatically. In a traumatic event, the knee is mostly flexed and subject to a valgus force or receives an anterior or medial blow to the knee. The patient will most likely tell you about the knee giving way and a “popping” sensation or sound that may be followed by swelling and possibly hemarthrosis. The majority of these injuries spontaneously reduce and sometimes the presence of hemarthrosis may be the only sign indicating that a luxation has occurred.

Two patient “types” experiencing patellar instability are proposed by Hiemstra et al. (2014)

  1. WARPS: Stands for “Weak, Atraumatic, Risky anatomy, Pain and Subluxation”. This group includes younger patients with diminished quadriceps strength and neuromuscular control (especially of the vastus medialis) and reduced core stability. This group is further associated with anatomical issues that contribute to their patellar instability and recurrency such as trochlear dysplasia, shallow and short trochlear groove, pes planus, a large tibial tuberosity-trochlear groove distance, increased ligamentous laxity, amplified patellar tilt, valgus limb alignment and rotational abnormalities of the tibia and femur. They present more frequently with symptoms of patellofemoral pain and recurrent subluxations rather than frank dislocation episodes. Movements requiring minimal force usually lie at the base of their instability symptoms and subluxations or dislocations occur often without trauma.
  1. STAID: “STrong, Anatomy normal, Instability and Dislocation”. This group includes more patients sustaining a patellar dislocation at a later age, and more unilaterally. They tend to have stronger quadriceps strength and core stability and no clear predisposing anatomical factors. The event of the dislocation is traumatic in nature and frequently they do not have any complaints of patellofemoral pain prior to sustaining the dislocation.




In case the patella is still dislocated, it will be most likely displaced laterally. In more subtle cases of patellar instability with recurring subluxations, signs of quadriceps weakness and wasting are often visible. Assess limb alignment and the presence of an increased Q-angle. Many patients will have their limb in a valgus position, which can result from femoral anteversion, hyperpronation of the foot, or external tibial torsion, but equally from weak hip musculature

Functional assessment

Range of motion and lower extremity strength should be assessed bilaterally but comparison to normative values may be interesting in case bilateral strength deficits are present.


Active examination

The J-sign can be indicative of patellar maltracking. Further, the (in)ability to move and load the knee joint as well as signs of apprehension during movement can be noted.

  • Passive examination

Pain and swelling often impede a passive assessment. When possible, the examination often reveals tenderness at the medial epicondyle of the femur and patella, and apprehension during the lateral displacement of the patella. The lateral femoral epicondyle may be tender from the collision with the patella during dislocation and/or reduction. Tenderness over the origin of the medial patellofemoral ligament (Bassett sign) may be indicative of a ligamentous disruption. An increased lateral glide of the patella (2 or 3 quadrants of the patellar width) accompanied by apprehension can give an idea about ligamentous laxity or rupture.

General ligamentous laxity can be assessed with the Beighton score. Some authors describe a palpable defect along the medial retinaculum or medial patellofemoral ligament.

A positive patellar grind test may be suggestive of a chondral injury.

Lateral tilting of the patella may suggest a tight lateral retinaculum.

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Initiate full weight bearing as tolerated and progress range of motion gradually together with proprioception and strength rehabilitation. Hypermobility can increase the risk of sustaining a dislocation of the kneecap, so good strength and neuromuscular control throughout the whole range of movement are important.

  • Stability of the patellofemoral joint is maintained by the static stabilizers (joint anatomy), active stabilizers (M. Quadriceps femoris), and passive stabilizers (retinacular ligaments). Since only the active stabilizers can be trained with conservative management, effective strengthening should initially focus on opposing the patella’s excessive lateral displacement and the knee valgus position. Therefore vastus medialis and gluteal musculature can be targeted already early in rehabilitation. The patient should be strong enough to withstand external rotation moments induced by valgus forces, for example during tackling in rugby. Here also the hamstring musculature plays an important role.
  • Bracing or McConnell taping can help the patient return to participating in sports activities or overcome their fear of loading the knee in the first phase of rehabilitation. Hinged knee braces or lateral stabilization braces may enhance the patient’s sense of stability and increase their trust in the knee.



Petri M, Ettinger M, Stuebig T, Brand S, Krettek C, Jagodzinski M, Omar M. Current Concepts for Patellar Dislocation. Arch Trauma Res. 2015 Sep 1;4(3):e29301. doi: 10.5812/atr.29301. PMID: 26566512; PMCID: PMC4636822.

Hiemstra LA, Kerslake S, Lafave M, Heard SM, Buchko GM. Introduction of a classification system for patients with patellofemoral instability (WARPS and STAID). Knee Surg Sports Traumatol Arthrosc. 2014 Nov;22(11):2776-82. doi: 10.1007/s00167-013-2477-0. Epub 2013 Mar 28. PMID: 23536205.

Johnson DS, Turner PG. Management of the first-time lateral patellar dislocation. Knee. 2019 Dec;26(6):1161-1165. doi: 10.1016/j.knee.2019.10.015. Epub 2019 Nov 11. PMID: 31727430. 

Baryeh K, Getachew F. Patella dislocation: an overview. Br J Hosp Med (Lond). 2021 Aug 2;82(8):1-10. doi: 10.12968/hmed.2020.0429. Epub 2021 Aug 4. PMID: 34431342. 

Ménétrey J, Putman S, Gard S. Return to sport after patellar dislocation or following surgery for patellofemoral instability. Knee Surg Sports Traumatol Arthrosc. 2014 Oct;22(10):2320-6. doi: 10.1007/s00167-014-3172-5. Epub 2014 Jul 22. PMID: 25047793; PMCID: PMC4169614. 

Weber AE, Nathani A, Dines JS, Allen AA, Shubin-Stein BE, Arendt EA, Bedi A. An Algorithmic Approach to the Management of Recurrent Lateral Patellar Dislocation. J Bone Joint Surg Am. 2016 Mar 2;98(5):417-27. doi: 10.2106/JBJS.O.00354. Erratum in: J Bone Joint Surg Am. 2016 Jun 15;98(12):e54. PMID: 26935465.

Martin RK, Leland DP, Krych AJ, Dahm DL. Treatment of First-time Patellar Dislocations and Evaluation of Risk Factors for Recurrent Patellar Instability. Sports Med Arthrosc Rev. 2019 Dec;27(4):130-135. doi: 10.1097/JSA.0000000000000239. PMID: 31688530. 

Ling DI, Brady JM, Arendt E, Tompkins M, Agel J, Askenberger M, Balcarek P, Parikh S, Shubin Stein BE. Development of a Multivariable Model Based on Individual Risk Factors for Recurrent Lateral Patellar Dislocation. J Bone Joint Surg Am. 2021 Apr 7;103(7):586-592. doi: 10.2106/JBJS.20.00020. PMID: 33787553. 

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