November 24, 2019
patellar dislocation cover

Patellar Dislocation Introduction and Diagnosis

After recent prominence in the media due to a prominent quarterback in the NFL suffering a patellar dislocation, a background of this injury along with which imaging modalities are used will be reviewed this week. Many sports fans were watching and questioning the recovery time and whether surgery is needed for what some call a “knee dislocation.” A patellar dislocation occurs when the patella disengages completely from the trochlear or femoral groove. It can reduce on its own and sometimes will need manually reduced. The overall incidence is around 8 to 23 per 100,000 and has the highest among adolescents aged 14 to 18 years. Males and females seem to be affected equally [24].
Patellar dislocations most commonly occur in the lateral direction. Medial dislocations, intra-articular and superior dislocations are rare. The vastus medialis obliquus or VMO is the most distal portion of the quadriceps muscle and exerts a medially directed force that helps keep the patella in position. Weakness or atrophy of this muscle makes dislocation more likely. The medial retinaculum is reinforced by the medial patellofemoral ligament, or MPFL, which prevents excessive lateral movement of the patella. Other risk factors include an increased Q angle, patella alta or high riding patella, trochlear dysplasia, excessive lateral patellar tilt and lateral femoral condyle hypoplasia [1, 9].
knee anatomy illustration patella dislocation

Figure 1. The relationship of the MPFL to other structures.VML: vastus medialis longus, AM: adductor magnus, VMO: vastus medialis obliquus, PMC: posteromedial capsule;MPFL: medial patellofemoral ligament; SMCL: superficial band of medial collateral ligament. (Adopted from Amis et al. 2004)

History may differ depending on the mechanism of injury, but most describe pain and potentially a noticeable deformity of the knee. This is usually preceded by direct trauma to the knee or a sudden change in direction. Many also describe a sense of giving way or instability when the dislocation occurs. Patients often feel a pop or multiple pops and there may be generalized pain in the anteromedial knee.

On physical examination, the first step is to inspect the knee. An acutely dislocated patella is typically easy to see but there may be a large effusion or hemarthrosis that make it more difficult. The provider should also inspect for patella alta, tibial torsion, genu valgum or varum or general ligamentous laxity. General ligamentous laxity is assessed with the Beighton hypermobility score [2]. There has been a term dubbed “miserable malalignment syndrome” for patients with excessive femoral anteversion, excessive tibial outward rotation, and genu valgum [3]. The knee should be palpated including each pole of the patella, medial and lateral joint lines and retinaculum. The bulk of the VMO should be evaluated and quantified if possible. A palpable defect along the medial retinaculum or MPFL may be appreciable. Tenderness over the MPFL origin has been called the Bassett sign, which is consistent with ligamentous disruption [4,5]. The collateral ligaments should also be palpated and assessed along with the cruciate ligaments. 

Range of motion and patellar glide should be tested if pain permits. The patella can normally be moved medially and laterally between 25-50 percent the width of the patella. A “J sign” may also be seen with knee extension in which the patella tends to deviate laterally and may “pop” into the trochlear groove. Lastly, the moving patellar apprehension test is most specific and sensitive for instability. The examination begins with the knee held in full extension and the patella is manually translated laterally with the thumb. The knee is then flexed to 90 degrees and then brought back to full extension while the lateral force on the patella is maintained. For the second half of the test, the knee is started in full extension, brought to 90 degrees of flexion, and then back to full extension while the index finger is used to translate the patella medially. Part one should have apprehension and possibly tightening of quadriceps muscle and the second half should not elicit apprehension. Sensitivity was 100 percent with specificity 88 percent in one study with 51 patients [6].
The first step in imaging patellar dislocations is plan radiography. Standard anteroposterior weight bearing radiographs of both knees and posteroanterior radiographs at 45 degrees flexion may aid in the assessment of the coronal alignment of the tibiofemoral joint or possibly the presence of arthrosis. Lateral views and sunrise, also called Merchant, views should also be performed to help with assessment. The lateral and sunrise radiographs provide information in regards to trochlear morphology, patellar height, and patellar tilt. These radiographs will also help identify fractures of the patella or avulsion fractures, which sometimes occur with a rupture of the MPFL from the middle third of the patella. Loose bodies and sometimes large cartilage defects can also be seen with some dislocations or subluxations.
Other risk factors can also be quantified or identified with radiographs and are important in both acute and recurrent dislocations. Lateral patellar tilt is assessed by the lateral patellofemoral angle on sunrise view. This is an angle is measured between a line along the subchondral bone of the lateral trochlear facet and posterior femoral condyles with normal being an angle greater than 11 that opens laterally [7]. Abnormal angles are parallel or open medially. There are numerous methods to identify or measure patellar height. Patellar height can be assessed by direct or indirect methods. Indices such as Insall-Salvati (IS), Blackburne-Peel(BP) and Caton-Deschamps (CD) are indirect methods which measure patellar height by ratios based on the length of the patellar tendon or some reference points on the proximal part of the tibia (Figure 2). The Insall-Salvati ratio is the ratio measuring the length of the patella ligament and the patellar length. A normal ratio is 1.0; a ratio of 1.2 suggests patella alta and 0.8 patella baja. According to the description by Caton [8], the Caton-Deschamps index is the distance between the distal point of the patellar articular surface and the anterior superior margin of the tibia, divided by the patellar articular surface length. A normal ratio is 1.0; a ratio of less than 0.6 suggests patella baja and a ratio of 1.3 suggests patella alta. The Blackburne-Peel method (BP) measures the ratio of the height of the lower pole of the articular surface above a tibial plateau line to the articular surface length of the patella [9]. Usually range between 0.54- 1.06. A ratio of less than 0.54 is considered to be patella alta. The technique described by Blumensaat in 1938, which uses the roof of the intercondylar notch as a reference line, is one of the most commonly used direct methods for the assessment of patellar height [10, Figure 3]
measure Blumensaat line patella dislocation

Figure 3. Blumensaat line is drawn on the linear radiopacity of the roof of the intercondylar notch on the lateral knee radiograph. In this method, the patellar height is assessed according to the distance between the lower pole of the patella and the Blumensaat line in millimeters. The lower pole of the patella should normally lie on the Blumensaat line; if it is more than 10 mm above the line, it is classified as patella alta (adopted form Seyahi 2006)

True lateral radiographs and sunrise views may also identify trochlear dysplasia by the crossing sign, evidence of a supratrochlear spur, and a double contour, which denotes a hypoplastic medial condyle. The trochlear findings were elucidated by Dejour and Le Coultre and were subsequently revised to create the trochlear dysplasia classification system [11, Figure 4]. The crossing sign occurs when the trochlear groove lies in the same plane as the anterior border of the lateral condyle, which represents a flattened trochlear groove. The double contour sign occurs when the anterior border of the lateral condyle lies anterior to the anterior border of the medial condyle, which represents a convex trochlear groove or hypoplastic medial condyle. A supratrochlear spur can arise from the proximal aspect of the trochlea and can also indicate a risk factor. Cross-sectioning imaging is also used for recurrent patellar dislocations.

Figure 4. Dejour classification of trochlear dysplasia 1. Type A indicates the presence of the crossing sign with a shallow trochlea of >145. Type B indicates the presence of a supratrochlear spur and a flat or convex trochlea. Type C indicates the presence of a double contour sign with a hypoplastic medial femoral condyle. Type D indicates the presence of a supratrochlear spur and a double contour sign with a cliff pattern between condyles. Adopted from Dejour et al. 2007.

Further advanced imaging is also used in some cases. A computed tomographic (CT) scan more accurately characterize the morphology of the trochlea and can assess femoral and tibial torsion. Relative rotation is assessed with measurement of the tibial tubercle to trochlear groove or TT-TG distance. This is the distance between two perpendicular lines; one from the posterior cortex to the tibial tubercle and one from the posterior cortex to the trochlear groove [9, Figure 5]. This average 8-10 mm in pediatric and adult patients. A TT-TG distance of greater than 20 is highly associated with patellar instability.
trochlear groovetibial tubercle TT-TG distance patella dislocation CT

Figure 5. Superimposed CT axial scans of the trochlear groove and the tibial tubercle demonstrating measurement of the TT-TG distance. Line A denotes the perpendicular to the deepest portion of the trochlear groove, and line B denotes the prominence of the tibial tubercle. The TT-TG measurement is the distance between lines A and B. Adopted from Weber et. al 2016.

Magnetic Resonance Imaging (MRI) is used to assess the soft tissues and chondral surfaces. Prior studies have suggested that the MPFL is more often disrupted at the femoral origin and the medial femoral condyle should be evaluated closely for disruption [12-13]. Following acute injuries, cartilage damage can be assessed and bone contusions on the medial patellar facet and lateral femoral condyle can also be seen. Adjacent soft tissue damage, such as edema in the VMO and bony changes such as osteochondral lesions can be seen. MRI-based estimates of TT-TG distance have been shown to underestimate the distance by an average of 3.8 mm compared with CT-based estimates [14]. Recent findings have suggested that trochlear dysplasia may be more accurately and reproducibly described as high grade versus low grade on MRI rather than by the Dejour classification on lateral radiographs [15-16].
Ultrasound has also been used to identify soft tissue injuries following and acute patellar dislocation. In one study comparing 97 patients following a lateral patellar dislocation, ultrasound had 90 percent sensitivity, 96 percent specificity and 95 percent accuracy in regard to partial tearing of the MPFL. This was compared to MRI that showed 82 percent sensitivity, 96 percent specificity and 95 percent accuracy for partial tears. Similar numbers were also shown for complete tears [17]. In a case series of 50 patients with acute lateral patellar dislocation who underwent ultrasound before surgery, Felus et al. reported sensitivity, specificity, and accuracy of greater than 90 % in the diagnosis of partial and complete MPFL tear [18]. Other smaller case series also exist with similar findings [19-23]. During a dynamic examination, a force applied to the medial patellar border by the examiner’s hand, sonographic visualization of increasing distance between the torn ends of the MPFL can be observed, which aids the examiner in correctly justifying the degree of damage and provides more advantages compared to MRI. The oblique course of the MPFL not parallel to the standard planes used for knee MRI may make unambiguous diagnosis difficult in some cases. Many providers use ultrasound as a diagnostic tool and confirm findings with more advanced imaging if needed.


  1. Amis AA, Firer P, Mountney J, Senavongse W, Thomas NP. Anatomy and biomechanics of the medial patellofemoral ligament. Knee. 2003;10:215-20. Erratum in: Knee. 2004;11:73
  2. Beighton P, Solomon L, Soskolne CL. Articular mobility in an African population. Ann Rheum Dis. 1973 Sep;32(5):413-8.
  3. Bruce WD, Stevens PM. Surgical correction of miserable malalignment syndrome. J Pediatr Orthop. 2004;24:392-396
  4. Bassett FH. Acute dislocation of the patella, osteochondral fractures, and injuries to the extensor mechanism of the knee. In: Burke E, editor. Amer Acad Orth Surg Instructional Course Lectures. St. Louis: C.V. Mosby; 1976. p 40-9.
  5. Smith TO, Davies L, O’Driscoll ML, Donell ST. An evaluation of the clinical tests and outcome measures used to assess patellar instability. Knee. 2008 Aug;15 (4):255-62. Epub 2008 Mar 7
  6. Ahmad CS, McCarthy M, Gomez JA, Shubin Stein BE. The moving patellar apprehension test for lateral patellar instability. Am J Sports Med. 2009 Apr;37(4): 791-6. Epub 2009 Feb 3.
  7. LaPrade RF, Engebretsen AH, Ly TV, et al. (2007) The anatomy of the medial part of the knee. J Bone Joint Surg Am 89, 2000–2010.
  8. Caton J. [Method of measuring the height of the patella]. Acta Orthop Belg. 1989;55:385-6
  9. Weber AE, Nathani A, Dines JS, et al. An algorithmic approach to the management of recurrent lateral patellar dislocation. J Bone Joint Surg Am. 2016;98(5):417-427.
  10. Seyahi A, Atalar AC, Koyuncu LO, Cinar BM, Demirhan M. Blumensaat line and patellar height. Acta Orthop Scand Traumatol Turc 2006;40:240-7 (in Turkish).
  11. Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc. 2007 Mar;15(1):39-46
  12. Balcarek P, Ammon J, Frosch S, Walde TA, Schuttrumpf JP, Ferlemann KG, Lill H, Sturmer KM, Frosch KH. Magnetic resonance imaging characteristics of the medial patellofemoral ligament lesion in acute lateral patellar dislocations considering trochlear dysplasia, patella alta, and tibial tuberosity-trochlear groove distance. Arthroscopy. 2010 Jul;26(7):926-35. Epub 2010 Mar 3.
  13. Elias DA, White LM, Fithian DC. Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft-tissue restraints and osteochondral injuries of the inferomedial patella. Radiology. 2002 Dec;225(3):736-43
  14. Camp CL, Stuart MJ, Krych AJ, Levy BA, Bond JR, Collins MS, Dahm DL. CT and
  15. MRI measurements of tibial tubercle-trochlear groove distances are not equivalent in
  16. patients with patellar instability. Am J Sports Med. 2013 Aug;41(8):1835-40. Epub
  17. 2013 Jul 15
  18. Lippacher S, Dejour D, Elsharkawi M, Dornacher D, Ring C, Dreyhaupt J, Reichel H, Nelitz M. Observer agreement on the Dejour trochlear dysplasia classification: a comparison of true lateral radiographs and axial magnetic resonance images. Am J Sports Med. 2012 Apr;40(4):837-43. Epub 2012 Jan 11.
  19. Nelitz M, Lippacher S, Reichel H, Dornacher D. Evaluation of trochlear dysplasia using MRI: correlation between the classification system of Dejour and objective parameters of trochlear dysplasia. Knee Surg Sports Traumatol Arthrosc. 2014 Jan;22(1):120-7. Epub 2012 Nov 30.
  20. Zhang, GY., Zheng, L., Ding, HY. et al. Evaluation of medial patellofemoral ligament tears after acute lateral patellar dislocation: comparison of high-frequency ultrasound and MR
  21. Eur Radiol (2015) 25: 274.
  22. Felus J, Kowalczyk B, Lejman T (2008) Sonographic evaluation of the injuries after traumatic patellar dislocation in adolescents. J Pediatr Orthop 28:397–402
  23. Sallay PI, Poggi J, Speer KP, Garrett WE (1996) Acute dislocation of the patella. A correlative pathoanatomic study. Am J Sports Med 24: 52–60
  24. Sanders TG, Morrison WB, Singleton BA, Miller MD, Cornum KG (2001) Medial patellofemoral ligament injury following acute transient dislocation of the patella: MR findings with surgical correlation in 14 patients. J Comput Assist Tomogr 25:957–962
  25. Nomura E, Horiuchi Y, Inoue M (2002) Correlation of MR imaging findings and open exploration of medial patellofemoral ligament injuries in acute patellar dislocations. Knee 9:139–143
  26. Trikha SP, Acton D, O’Reilly M, Curtis MJ, Bell J (2003) Acute lateral dislocation of the patella: correlation of ultrasound scanning with operative findings. Injury 34:568–571
  27. O’Reilly MA, O’Reilly PM, Bell J (2003) Sonographic appearances of medial retinacular complex injury in transient patellar dislocation. Clin Radiol 58:636–641 17.
  28. Sanders, Thomas L et al. “Incidence of First-Time Lateral Patellar Dislocation: A 21-Year Population-Based Study.” Sports health vol. 10,2 (2018): 146-151.

Leave a Reply

Your email address will not be published. Required fields are marked *