CT Evaluation of Patellar Instability

Transcription

1 CT Evaluation of Patellar Instability Poster No.: C-2157 Congress: ECR 2014 Type: Educational Exhibit Authors: R. Ruef, C. Edgar, C. Lebedis, A. Guermazi, A. Kompel, A. Murakami; Boston, MA/US Keywords: DOI: Epidemiology, Dysplasias, Congenital, Surgery, Physiological studies, Education, CT, Conventional radiography, Musculoskeletal joint, Extremities, Anatomy /ecr2014/C-2157 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 17

2 Learning objectives To review the pathophysiology, physical exam, imaging findings, and treatment of patellar instability. In this poster, we will focus on the role of CT in dynamically tracking patellar alignment, more definitively characterizing trochlear dysplasia, and diagnosing subluxation and malalignment. Page 2 of 17

3 Background Maintaining the stability of the patellofemoral joint is biomechanically complex, with contributions from the bony architecture of the patella and trochlea, muscles, and ligaments. The articulation is dynamic, and characterized by varied levels of patellofemoral engagement with varied degrees of flexion. The patella is most vulnerable to lateral dislocation in extension and early flexion. Several anatomic factors contribute to this: The trochlear groove is most robust along its lateral aspect, proximally. This contributes maximally to stability in early flexion. In full extension, the patella is completely disengaged from the trochlea. At extension, the quadriceps exerts its maximal lateral force on the patella (measured by the q angle). This lateral force vector decreases with flexion. The quadriceps also exerts a posterior force vector, which helps pull the patella into the trochlea. This vector is maximal in flexion, and minimal in extension. In patella alta, the patella engages later in flexion. The maximal lateral force vector and minimal posterior force vector seen in extension, combined with delayed engagement with the trochlea predisposes patients with patella alta to lateral dislocations. In some patients, the trochlea has a shallow, dysplastic groove resulting in reduced osseous constraint of the patella, and decreased stability of th articulation. The medial patellofemoral ligament also restrains the patella, primarily in extension and early flexion. This is often injured in lateral dislocations, contributing to chronic instability. This may be a candidate for repair in some cases to prevent recurrent dislocations. A more lateral tibial tubercle, relative to the patella, results in a higher lateral force vector on the patella throughout its course, predisposing patients to dislocation. Page 3 of 17

4 Findings and procedure details In patients with patellar instability, on examination, the patella may be seen to move laterally with full extension (the J sign). Pain with compression of the patella as it moves through the trochlea is suggestive of articular cartilage injury, which often occurs with a subluxation event. Patients with instability, often also show apprehension when lateral pressure is exerted on the patella in early flexion. CT is an important modality in quantifying the severity of trochlear dysplasia and patellar malalignment. Patellar tracking CT is preformed at our institution as follows: The patient is placed supine. 4 contiguous 1.5 mm axial images are obtained through the mid patella, perpendicular to the long axis of the patella at 0, 10, 20, 30, 40, 50, and 60 degrees of flexion. Following this, an acquisition through the entire knee is preformed at 0 degrees of flexion. These images can be used assess the trochlear sulcus angle and patellar tilt angles at varying degrees of flexion, as well as the trochlear depth. A reconstructed image is used to measure the tibial tubercle-trochlear groove distance. This measures the laterality of the tibial tubercle relative to the trochlear groove, on superimposed axial CT images, with >20 mm considered abnormal. A more lateral tibial tuberosity may be surgically medialized to help reduce the lateral force vector on the patella. The patellar height ratio, also known as the Insall-Salvati ratio is used to diagnose patella alta on the lateral radiograph. This is the ratio of the length of the patella, to the length of the patellar tendon, with a ratio >1.3 considered abnormal. Measurement Trochlear depth Patellar tilt angle Patellar height ratio Trochlear sulcus angle Tibial tubercle - trochlear groove Table 1 Vaules <3 mm indicative of a shallow, dysplastic trochlea >5 degrees indicative of lateral patellar tilt >1.3 indicative of patella alta >145 degrees indicative of a shallow, dysplastic trochlea >20 mm associated with patellar instability Page 4 of 17

5 Images for this section: Fig. 1: Patellar height ratio measurement. The ratio is measured as the length of the long axis of the patella, to the length of the patellar tendon. >1.3 is considered abnormal. Page 5 of 17

6 Fig. 2: CT acquisitions acquired from 0 to 60 degrees of flexion allow dynamic assessment of the patellofemoral articulation. Page 6 of 17

7 Fig. 3: The trochlear sulcus angle is measured using lines crossing through both condylar eminences, and the trochlear sulcus. Page 7 of 17

8 Fig. 4: The patellar tilt angle is measured using a line through the long axis of the patella, and a line parallel to a line through the posterior aspect of the femoral condyles. Angles >5 degrees are considered abnormal. Page 8 of 17

9 Fig. 5: Trochlear groove depth is measured as the distance between a line drawn tangential to the anterior margin of the medial and lateral femoral condyles and the deepest part of the femoral notch, with less than 3 mm considered abnormal. Page 9 of 17

10 Fig. 6: The tibial tubercle - trochlear groove distance is measured on superimposed axial images. Page 10 of 17

11 Fig. 7: A shallow, dysplastic trochlea. Page 11 of 17

12 Fig. 8: This patient's trochlear depth is at the lower limit of normal, and the patellar tilt angle of 16 degrees is abnormal. Page 12 of 17

13 Fig. 9: This patient had previously dislocated her patella. The tibial tubercle - trochlear groove ratio of 25 mm is abnormal. This increases the lateral force vector seen by the patella, and predisposes patients to lateral dislocation. Page 13 of 17

14 Fig. 10: The femur on the left is normal. The femur on the right is dysplastic. The facets appear more prominent, because the are flatter and pointed coronally. Page 14 of 17

15 Conclusion Patellar instability is a predisposing factor for patellar dislocation. Patellar stability is maintained by various stabilizing muscles and ligaments, particularly the medial patellofemoral ligament. It is further affected by bony anatomic variations of the trochlear groove, and the relative location of the tibial tuberosity. Evaluation for the etiology of instability predisposing patient's towards dislocation is primarily preformed with CT. The ability to dynamically characterize the tibiofemoral articulation confers an advantage to CT, and consequently, helps guide treatment approach. Page 15 of 17

16 Personal information Page 16 of 17

17 References Alexis Chiang Colvin, Robin V. West. Patellar Instability. The Journal of Bone & Joint Surgery. December 2008, 90:12, Christian W. A. Pfirrmann, Marco Zanetti, José Romero, and Juerg Hodler. Femoral Trochlear Dysplasia: MR Findings. Radiology. September :3, Daniel E. Redziniak, David R. Diduch, William M. Mihalko, John P. Fulkerson, Wendy M. Novicoff, Shahin Sheibani-Rad, Khaled J. Saleh; Patellar Instability. The Journal of Bone & Joint Surgery. September 2009, 91:9, Brian Schulz, Marc Brown, Christopher S. Ahmad. Evaluation and Imaging of Patellofemoral Joint Disorders. Operative Techniques in Sports Medicine, June 2010, 18:2, Samuel R. Ward, Michael R. Terk, Christopher M. Powers. Patella Alta: Association with Patellofemoral Alignment and Changes in Contact Area During Weight-Bearing. The Journal of Bone & Joint Surgery. August 2007, 89:8, Page 17 of 17