PMID: 9168713. In order for the patella to engage with the femoral trochlea, a higher degree of flexion than normal is needed. MRI, given its superior soft tissue contrast and multi-planar capability, has emerged as the modality of choice in evaluating articular cartilage abnormalities. Operative lateral retinaculum release is indicated in refractory cases. What is the treatment for a patellar retinaculum tear? The lateral trochlear articular surface is usually more prominent than its medial portion. Sports Med Arthrosc Rev 25:7277, Gillespie H (2012) Update on the management of patellar instability. Nonoperative treatment is generally recommended in first time dislocators unless there are MRI findings of severe predisposing dysplasia and the presence of a chondral or osteochondral body. The injured retinaculum had an indistinct, irregular appearance associated with edema and hemorrhage. 5). However, it lacks sensitivity with 40% of sizable osteochondral lesions being missed on initial presentation after patellar dislocation [16]. The MPFL is best seen on axial MRI on the slice just distal to the VMO. Edema and thickening compatible with tears of the MPFL and medial retinaculum (red arrows) are apparent both anteriorly and posteriorly. Stretching a Lateral Retinaculum of the Knee | livestrong Int Orthop 34:305309, Biedert RM, Albrecht S (2006) The patellotrochlear index: a new index for assessing patellar height. They include: pain with compression of patella and moderate lateral facet tenderness, inability to evert the lateral edge of the patella, mainstay of treatment and should be done for extensive period of time, closed chain short arc quadriceps exercises, pain refractory to extensive rehabilitation, ideal candidate has no symptoms of instability, medial patellar glide of less than one quadrant, lateral patellar glide of less than three quadrants, only elevate 1 cm or else risk of skin necrosis, indicated only for instability with lateral translation (not isolated lateral tilt), viewing through superior portal will show medial facet does not articulate with trochlea at 40 degrees of knee flexion, postoperatively the patella should be able to be, patellar instability with medial translation, Spontaneous Osteonecrosis of the Knee (SONK), Osgood Schlatter's Disease (Tibial Tubercle Apophysitis), Anterior Superior Iliac Spine (ASIS) Avulsion, Anterior Inferior Iliac Spine Avulsion (AIIS), Proximal Tibiofibular Joint Ganglion Cysts, Pre-Participation Physical Exam in Athlete, Concussions (Mild Traumatic Brain Injury). It is measured at the mid-point of the patella on the axial slices [48]. (18a) A 13-year-old female following acute patellar dislocation. Deep lacerations are often associated with this type of injury. Observer Agreement on the Dejour Trochlear Dysplasia Classification: A comparison of true lateral radiographs and axial magnetic resonance images. Early detection particularly in the stage preceding the development of significant cartilaginous loss and osteoarthritis is critical. Fluid extending into the VMO myotendinous junction is generally seen in the setting of coexisting MPFL/retinacular tears. Axial PDFS MR image showing MPFL disruption (open arrow) and trochlear dysplasia (arrowheads). AJR Am J Roentgenol 167:339341, Caton JH, Dejour D (2010) Tibial tubercle osteotomy in patello-femoral instability and in patellar height abnormality. Pediatric Lateral Patellar Retinacular Sleeve Avulsion - JOSPT The medial patellar retinaculum and MPFL are best seen on MRI on the axial fluid-sensitive sequences. The ratio of the medial to the lateral facet length defines trochlear facet asymmetry (MT/LT) *100%. Given the lack of history of direct trauma, a reliable diagnosis can be made. Femoral condylar chondral injuries occur during the dislocation phase due to impaction shearing forces of the patella upon the flexed femur, typically occurring at the weightbearing surface. 28 A buckle transducer was clamped on the lateral patellofemoral ligament and the transducer was calibrated to zero with the knee at full extension. James M. Provenzale, Rendon C. Nelson, Emily N. Vinson. Cochrane Database Syst Rev 2:CD008106, Woo R, Busch MT (1998) Management of patellar instability in children. Knee 13:2631, McNally EG (2001) Imaging assessment of anterior knee pain and patellar maltracking. Unauthorized use of these marks is strictly prohibited. PubMed Some controversy exists regarding whether female gender is a definite risk factor for patellar instability with certain studies identifying a 33% increased likelihood of first-time dislocation as well as three times high re-dislocation rates than males, whereas others have found roughly equal rates [2, 12,13,14]. California Privacy Statement, Bull NYU Hosp Jt Dis 67:2229, Dupuy DE, Hangen DH, Zachazewski JE, Boland AL, Palmer W (1997) Kinematic CT of the patellofemoral joint. In the seven patients with hyperextension injuries, three had associated meniscal and cruciate ligament tears. 2. Depends on how bad: Small tears are observed and heal. On the other hand, frank patellar dislocation is a significant risk factor in the development of patellofemoral osteoarthritis with an incidence of 49% at 25years after the patellar dislocation incident in comparison with 8% in a control group without a dislocation history [60]. The MPFL is attached to the region of the adductor tubercle or medial femoral epicondyle extending deep to the vastus medialis obliquus (VMO) and attaching to the superior two thirds of the patella [52]. (21a) A corresponding anterior coronal slice reveals the large displaced osteochondral fragment (arrow) that occurred as a result of this injury. A lateral patellar sleeve fracture can be misdiagnosed ity. Distances between 15 and 20 mm are borderline, and distances of more than 20 mm indicate significant lateralization of the tuberosity.5. The .gov means its official. Less commonly, a direct laterally or medially orientated blow to the patella can precipitate dislocation. (16a) An axial fat-suppressed T2-weighted image reveals numerous typical findings of recent lateral patellar dislocation. Lateral Patellar Dislocation. 8 Lippacher S, Dejour S, Elsharkawi M, et al. government site. Other structures that blend in this region include the medial patellotibial ligament, the medial patellomeniscal ligament, the investing fascia, and the medial joint capsule. Int Orthop 34:311316, Donell ST, Joseph G, Hing CB, Marshall TJ (2006) Modified Dejour trochleoplasty for severe dysplasia: operative technique and early clinical results. Stretching exercises for Patellofemoral pain. Rethy Chhem, Etienne Cardinal. Acta Orthop 76:699704, Nikku R, Nietosvaara Y, Kallio PE, Aalto K, Michelsson JE (1997) Operative versus closed treatment of primary dislocation of the patella. The natural history. First, a line is drawn paralleling the posterior femoral condyles surfaces. Value of the tibial tuberosity-trochlear groove distance in patellar instability in the young athlete. 7). 35 Dislocation typically occurs in the setting of internal rotation of the femur on a fixed, externally rotated tibia. Clin Sports Med 21:521546 x, Article The ensuing loss of medial restraint favors future patellar dislocations, especially if additional risk factors are present. AJR Am J Roentgenol 194:721727, Kalichman L, Zhang Y, Niu J et al (2007) The association between patellar alignment and patellofemoral joint osteoarthritis features--an MRI study. The close association of the MR with the MCL is also apparent. Another study noted an association between abnormal trochlear morphology and high-grade patellofemoral cartilage damage [58]. Am J Sports Med 16:244249, Hawkins RJ, Bell RH, Anisette G (1986) Acute patellar dislocations. What is the diagnosis? The Medial Patellofemoral Ligament (MPFL) is an hour-glass shaped ligament made of bands of retinacular tissue. Open Orthop J 9:463474, Article AJR Am J Roentgenol 195:13671373, Jarraya M, Diaz LE, Roemer FW, Arndt WF, Goud AR, Guermazi A (2018) MRI findings consistent with peripatellar fat pad impingement: how much related to patellofemoral maltracking? 2012 Feb;41(2):137-48. doi: 10.1007/s00256-011-1291-3. Radiology 216:582585, Ward SR, Terk MR, Powers CM (2007) Patella alta: association with patellofemoral alignment and changes in contact area during weight-bearing. Early diagnosis is essential, as chronic maltracking will lead to patellofemoral cartilage damage and osteoarthritis [3]. Am J Knee Surg 13:8388, Izadpanah K, Weitzel E, Vicari M et al (2014) Influence of knee flexion angle and weight bearing on the Tibial Tuberosity-Trochlear Groove (TTTG) distance for evaluation of patellofemoral alignment. 8600 Rockville Pike Correspondingly, the patella must shift slightly medially during early flexion to engage the trochlear groove. Rev Chir Orthop Reparatrice Appar Mot 76:4554, CAS Arthroscopy 22:643649, Carrillon Y, Abidi H, Dejour D, Fantino O, Moyen B, Tran-Minh VA (2000) Patellar instability: assessment on MR images by measuring the lateral trochlear inclination-initial experience. Techniques of Medial Retinacular Repair and Reconstruction - LWW The images should be scrutinized for the presence of chondral or osteochondral injury, especially if displaced as an intra-articular body, as this may affect surgical management. Subluxation and dislocation: recurrent. In acute patellar dislocation, CT may demonstrate osseous impaction or fractures of the medial margin of the patella (with or without involvement of the articular surface) and/or the lateral surface of the lateral femoral condyle and intraarticular fragments. We confirm that this article is not under consideration for publication elsewhere and that the authors did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors. The transverse stabilizers include the medial and lateral retinaculum, the vastus medialis and lateralis muscles, the ilio-tibial band, and the medial patellofemoral ligament (MPFL). This injury has been described in conjunction with numerous sports activities, particularly snow skiing. :: KJR :: Korean Journal of Radiology Patellar maltracking occurs as a result of imbalance of this relationship often secondary to anatomic morphologic abnormality. The patellar dislocations were clinically unsuspected in the initial evaluation of eight of these nine patients. Am J Sports Med 45:10591065, Brossmann J, Muhle C, Schrder C et al (1993) Patellar tracking patterns during active and passive knee extension: evaluation with motion-triggered cine MR imaging. (17a) An axial T2 fat-suppressed image in another patient reveals a fluid filled gap (short arrow) at the site of attachment of the medial retinaculum. A tight lateral retinaculum can tilt the patella leading to increased pressure on the lateral facet causing pain (Ficat). Over 100 different procedures have been described for the treatment of patellar instability, and this reflects the various causes for instability and lack of current gold standard [66, 69, 72]. A perpendicular line is measured to the most posterior cortex of the central trochlea. Department of Medical Imaging, The Ottawa Hospital, 501 Smyth Road, Ottawa, ON, K1H 8L6, Canada, Zaid Jibri,Kawan S. Rakhra&Marcos L. Sampaio, Faculty of Medicine, University of Ottawa, 451 Smyth Road, Ottawa, ON, K1H 8M5, Canada, Zaid Jibri,Paul Jamieson,Kawan S. Rakhra,Marcos L. Sampaio&Geoffrey Dervin, Division of Orthopaedic Surgery, The Ottawa Hospital, 501 Smyth Road, Ottawa, ON, K1H 8L6, Canada, You can also search for this author in 1-31. Clin J Sport Med 15:6266, Colvin AC, West RV (2008) Patellar instability. AJR Am J Roentgenol 187:13321337, White BJ, Sherman OH (2009) Patellofemoral instability. PDF ORIGINAL RESEARCH Role of Magnetic Resonance Imaging in Evaluation of National Library of Medicine Alimorad Farshchian M. D.. Farshchian's Orthopedic Regenerative Series. However, patellofemoral tracking is a dynamic process with the spatial relationship between the articular surfaces varying depending on the position of the knee joint [57, 64]. LTI < 11 degrees indicates dysplasia. Magnetic Resonance Cholangiopancreatography MRI PREMIUM Digestive system Illustrations PREMIUM CT axial male abdomen and pelvis CT PREMIUM CT peritoneal cavity CT PREMIUM MRI female pelvis MRI PREMIUM Female pelvis An official website of the United States government. 1. On the other hand, the PTI is significantly altered with knee flexion [37]. b Axial CT image showing tibial tuberosity transfer surgery with screw placement (arrow). Osteochondral fractures are common in acute or recurrent transient lateral patellar dislocation, seen in up to 70% of cases. The medial patellofemoral ligament (MPFL) is a condensation of the medial capsule of the knee joint. ADVERTISEMENT: Supporters see fewer/no ads. All authors (ZJ, PJ, KSR, MLS, GD) contributed to the study design, drafted the work, and revised it critically for final submission. Rheumatology (Oxford) 46:13031308, Sanders TL, Pareek A, Johnson NR, Stuart MJ, Dahm DL, Krych AJ (2017) Patellofemoral arthritis after lateral patellar dislocation: a matched population-based analysis. It is reported in surgical literature that 50-75% of recurrent dislocators, have some form of dysplasia, malalignment or generalized joint laxity. Lateral patellar dislocation results in bone contusion at the medial patella and along the lateral aspect of the lateral femoral condyle. Privacy quadricepsplasty rehab protocol 2002 Dec;225(3):736-43. doi: 10.1148/radiol.2253011578. Other indicated structures: gracilis (G), semitendinosus (ST), and adductor magnus (AM) tendons. During this dislocation phase of injury, shearing forces can damage the articular surfaces of either the patella or the lateral femoral condyle. The Insall-Salvati index is the most widely accepted measurement and is easily performed on radiographs and MRI examinations. Epidemiology Patellar dislocation accounts for ~3% of all knee injuries and is commonly seen in those individuals who participate in sports activities. MRI Web Clinic, August 2010. https://radsource.us/patella-alta-and-baja/. Kirsch M, Fitzgerald S, Friedman H, Rogers L. Transient Lateral Patellar Dislocation: Diagnosis with MR Imaging. It can not only detect any underlying morphological risk factors but also look for structural damage associated with maltracking including patellofemoral articular cartilage loss, osteochondral defects, or damage to the medial patellar stabilizers [4, 5]. TTTG is the distance between the solid and the dashed lines in (b). Knee Surg Sports Traumatol Arthrosc 22:23882395, Escala JS, Mellado JM, Olona M, Gin J, Sauri A, Neyret P (2006) Objective patellar instability: MR-based quantitative assessment of potentially associated anatomical features. The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). Edema is also present medially in the region of the MPFL and medial retinaculum (arrowheads). Surgical management procedures can broadly be categorized as soft tissue procedures (lateral release, medial imbrication, and MPFL repair or reconstruction) and bony procedures (tibial tubercle transfer procedures and trochleoplasty). More recently, the TT-TG index was developed, which takes knee size into account by assessing the proximaldistal distance between the entrance of the chondral trochlear groove (TE) and the tibial tuberosity (TT). Patellar maltracking: an update on the diagnosis and treatment Radiology 2000; 216: 582-585. Cookies policy. This short surgical video demonstrates an arthroscopic lateral release as seen from inside the knee. Lateral dislocation may also occur, less commonly, as the result of a direct blow to the medial aspect of the knee. A 2015 Cochrane Review concluded that there is no significant increase in functional scores between nonoperative and operative management; however, surgical management does result in a significantly lower risk of recurrent dislocation at the cost of surgical complications [19]. PubMed Central Despite this, there remains considerable variation in surgical technique including graft choice, position, and tension making the literature difficult to compare [8, 15, 79,80,81,82,83,84,85,86]. The typical injury pattern is a tear of the medial patellofemoral ligament (MPFL) and bone bruises of the patella and the lateral femoral condyle. Surgical indications following patellar dislocation include the presence of a chondral or osteochondral body, significant rupture of the medial stabilizers most importantly the MPFL, a persistent laterally subluxed patella, or a second dislocation injury in a patient with malalignment or dysplasia. Physical Therapy. sharing sensitive information, make sure youre on a federal The radiograph can also be useful in detecting osseous morphologic features associated with patellar maltracking such as patella alta and trochlear dysplasia [24, 25]. Trochlear depth assessment. The patellar dislocations were clinically unsuspected in the initial evaluation of eight of these nine patients. From the inside of the knee the lateral retinaculum is incised, from the inside, allowing the kneecap to untilit itself. Nine of 17 patients showed MR evidence of a patellar dislocation. The distance (double-headed arrow) from the deepest point of the trochlea (line B) to the middle of the tibial tubercle (line A) is measured, again by using the posterior plane of the condyles as the reference line (line C). The knee is a complex synovial joint that can be affected by a range of pathologies: Please Note: You can also scroll through stacks with your mouse wheel or the keyboard arrow keys. In this section, we will emphasize the role of MRI and discuss how CT can also have value when assessing patellar maltracking. The patella remains laterally shifted (blue arrow) and an osteochondral injury of the medial patella (arrowheads) is apparent. Concave impaction deformity of the inferomedial patella is a specific sign of prior LPD. It takes a very strong force to tear the patellar tendon. It runs obliquely and transversely and inserts on the patella and the patellar tendon, and is composed of two layers. Subjective: Pain and tenderness on structure (lateral side of patella) (Juhn). Patellar tracking refers to the dynamic relationship between the patella and trochlea during knee motion [1]. Distally, it attaches to the tibial tubercle via the patellar tendon. The patellar apprehension test is used to assess for lateral instability and is positive when pain or discomfort occurs with lateral translation of the patella. Pathology Imaging, particularly MRI, can detect subtle features that could lead to the diagnosis, probably even more importantly when there is no clear history of patellar dislocation or before its development. High resolution magnetic resonance imaging of the patellar retinaculum: normal anatomy, common injury patterns . Characterization of the type of medial restraint injury is crucial for surgical planning. (9a) The Insall-Salvati Index is determined by dividing the length of the patellar tendon (PT) by the length of the patella (PL). As a common knee injury, patellar dislocation has a mean annual incidence of 5.8 in 100,000 people and is more prevalent in women. Eur Radiol 10:10511055, Stefanik JJ, Zumwalt AC, Segal NA, Lynch JA, Powers CM (2013) Association between measures of patella height, morphologic features of the trochlea, and patellofemoral joint alignment: the MOST study. Med Sci Sports Exerc 35:20432047, Steiner T, Parker RD (2009) Patella: subluxation and dislocation. Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft-tissue restraints and osteochondral injuries of the inferomedial patella. TT-TG assessment has its own limitations. The most accepted indication for surgical management of patellar instability is the presence of a large displaced osteochondral fracture or loose body. Am J Sports Med 28:472479, Lewallen L, McIntosh A, Dahm D (2015) First-time patellofemoral dislocation: risk factors for recurrent instability. Chondral and Soft Tissue Injuries Associated to Acute Patellar Dislocation: A Systematic Review. (12a) At an axial image 3 cm proximal to the femoral-tibial joint space, the lengths of the medial and lateral trochlear facets are obtained. Patellar sleeve avulsion fracture in a patient with Sinding-Larsen MRI plays a crucial role in quantification and characterization of these predisposing anatomic variations which are key to addressing the patient?s patellar instability operatively. Eur J Trauma Emerg Surg. A tear of the reconstructed MPFL is indicated by fluid interrupting the fibers (27a, long arrow) (27b, arrowheads). 2023 BioMed Central Ltd unless otherwise stated. Identifying edema at the superolateral aspect of Hoffas fat pad on MRI should prompt the reporting radiologist to look for features of patellar maltracking. (3a) Graphical depictions of the mechanism of patellar dislocation: With the knee in flexion, the patella dislocated laterally. Long term follow up studies in patients who have undergone a lateral release have shown an increased incidence of patellofemoral arthritis. MRI and CT are superior modalities in looking for predisposing factors associated with patellar maltracking [26,27,28]. The most important active stabilizer of the patella is the vastus medialis oblique (VMO). There are numerous techniques reported in literature and commonly used to reconstruct the medial sided soft tissue constraints. Diagnosis is made clinically with pain with compression of the patella and moderate lateral facet tenderness and sunrise knee radiographs will often show patellar tilt in the lateral direction. Bone bruise in acute traumatic patellardislocation: volumetric magnetic resonance imaging analysis with follow-up mean of 12 months. Materials and methods: Two radiologists independently reviewed 99 knee MR images for the presence of a focal defect at the lateral patellar retinaculum and . However, the patella starts to engage with the trochlea by 30 and is typically completely engaged by 45. (Figs.1-A 1-A also and1-B). 6. Medial retinacular complex injury in acute patellar dislocation: MR findings and surgical implications. 2023 Lineage Medical, Inc. All rights reserved, Knee & Sports | Lateral Patellar Compression Syndrome. 2. 1 Pain is often described as medial because of soft tissue injuries that occur to the medial retinaculum and/or MPFL. Several methods have been used to assess patella alta. Patella instability in children and adolescents. Romero J, Hodler J. Radiology. Femoral avulsion of the MPFL is a predictor of chronic instability and may be a surgical indication in some patients with acute injury. The axial proton density-weighted image reveals a large osteochondral shearing injury involving the mid to medial patella (arrowheads). (20a) A fat-suppressed proton density-weighted sagittal image in a patient following patellar dislocation reveals an osteochondral injury with a chondral defect (arrows) at the lateral weightbearing surface of the lateral femoral condyle, a finding seen in only 5% of patients. However, the use of this method is not widespread. 7,14 While plain radiography is an important tool to diagnose APD, magnetic resonance imaging (MRI . There are a number of MRI features of trochlear dysplasia including reduction in the trochlear depth, lateral trochlear inclination, and facet asymmetry. Reference article, Radiopaedia.org (Accessed on 02 May 2023) https://doi.org/10.53347/rID-27273, {"containerId":"expandableQuestionsContainer","displayRelatedArticles":true,"displayNextQuestion":true,"displaySkipQuestion":true,"articleId":27273,"questionManager":null,"mcqUrl":"https://radiopaedia.org/articles/lateral-patellar-dislocation/questions/2391?lang=us"}, Case 14: transient lateral patellar dislocation, see full revision history and disclosures, anterior cruciate ligament avulsion fracture, posterior cruciate ligament avulsion fracture, Ahlback classification system in assessing osteoarthritis of the knee joint, Kellgren and Lawrence system for classification of osteoarthritis, anterior cruciate ligament mucoid degeneration, MRI grading system for meniscal signal intensity, medial retinacular abnormalities (ranging from strain to complete disruption) with adjacent periligamentous edema and hemorrhage, lateral displacement of patella (not necessarily seen in transient dislocation), medial patellar contusion +/- corresponding lateral femoral condyle contusion, direct trauma to lateral knee:normally no patellar contusion. Cite this article. Additionally, return to sport can be as low as 45%, leaving many patients searching for further management options [12]. Biomechanical evaluation of lateral patellar dislocations. 2000; 216:858-864. Imaging of Osseous Knee Trauma | Musculoskeletal Key Acta Orthop Belg 72:6571, Panagopoulos A, van Niekerk L, Triantafillopoulos IK (2008) MPFL reconstruction for recurrent patella dislocation: a new surgical technique and review of the literature. A patient with acute first-time transient patellar dislocation without osteochondral lesions and severe risk factors for redislocation is generally treated conservatively. The primary aim of surgery is to repair the knee damage and to correct the anomalies that are predisposing to chronic instability. The MCL is beneath the deep crural fascia (layer 1), from which it is separated by a variable amount of fat (Fig. Medial patellar chondral injuries may occur during either the dislocation or reduction phases of injury. The lateral patellar retinaculum presents a superficial layer attached medially to the patella and patellar ligament, and extending laterally to the iliotibial band and vastus lateralis muscle fascia (Fig. The common peroneal nerve can be localized in the popliteal fossa or identified posterior to the biceps femoris tendon and followed as it courses around the fibular neck. Knee 10:215220, Terry GC, Hughston JC, Norwood LA (1986) The anatomy of the iliopatellar band and iliotibial tract. The patellar tendon may tear when the knee is bent and the foot planted, like when landing from a jump or . Recent literature does not encourage the use of lateral release, since this can increase patellar instability. Lateral patellar dislocation. 1993;161(1):109-13. Between 15 and 45% of patients will develop recurrent patellar instability after acute dislocation, which is both functionally limiting and painful [17,18,19,20]. (26a) An axial proton density-weighted image reveals trochlear dysplasia (line, indicating facet asymmetry and a shallow trochlear groove), ossification in the medial retinaculum and MPFL attachment anteriorly (short arrow) and diffuse scarring of the medial retinaculum (arrowheads)--these findings indicate chronic patellofemoral instability. The patella remains laterally subluxed and tilted, and the patient has an abnormally shallow trochlear groove (line). Patellar maltracking is a disorder that often affects the young active individuals. Trochlear depth of less than 3 mm on MRI has a sensitivity of 100% and a specificity of 96% for trochlear dysplasia.10. From 10 to 20 of flexion, the patella engages the trochlear groove with the contact area being the inferior most portion of the medial and lateral facets. Radiology 225:736743, Kirsch MD, Fitzgerald SW, Friedman H, Rogers LF (1993) Transient lateral patellar dislocation: diagnosis with MR imaging. Am J Sports Med 14:117120, Smith TO, Donell S, Song F, Hing CB (2015) Surgical versus non-surgical interventions for treating patellar dislocation. AJR Am J Roentgenol 161:109113, Lance E, Deutsch AL, Mink JH (1993) Prior lateral patellar dislocation: MR imaging findings. It is the angle between a line tangential to the subchondral bone of the posterior aspect of the femoral condyles and a line along the lateral trochlear facet.
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