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Patellar imaging.

Advanced imaging techniques such as computed tomography, magnetic resonance imaging, ultrasound and arthrography are excellent tools for diagnosing specific soft-tissue conditions, but routine film radiography still is common and very useful in illustrating patellar abnormalities, particularly those stemming from acute trauma or degenerative arthritic changes. The rule of thumb in extremity radiography is always to obtain, at a minimum, 2 good 90[degrees] projections of the structure or structures being examined. Standard projections of the knee generally include anteroposterior (AP), posteroanterior (PA), or both, plus lateral, oblique and tangential (eg, Settegast method and Kuchendorf method). (1,2) The lateral projection is important because when performed properly, patellar thickness, height and general morphology can be determined readily. (See Fig. 1.) Evidence of dislocations, fractures and degenerative pathology also can be well demonstrated on the lateral view. (3-5)

[FIGURE 1 OMITTED]

However, obtaining an AP or PA projection of the patella completely free of bony obscuration from the relatively dense condylar and epicondylar portions of the distal femur sometimes is difficult because of the position of the patella in relation to the femur. (See Fig 2.) In such cases, the ordering physician might request routine oblique projections to better demonstrate the patella in a relatively unobscured manner. Unfortunately, routine oblique projections of the patella might not demonstrate subtle arthritic pathology. (See Figs. 3 and 4.)

[FIGURES 2-4 OMITTED]

The Kuchendorf and Settegast tangential projection methods are beneficial in many instances, but these methods involve physical manipulation of the patella, flexion of the knee joint or both. Such manipulation or movement can cause patient discomfort, and in some cases can inadvertently cause further trauma to the injured patella and knee joint. (4,5) The projections presented here are designed to optimally demonstrate the patella in the frontal axial oblique plane with minimal movement of the patient and body part.

Methods

This method consists of 2 axial oblique projections, each using a double tube angle. Because of this, the procedure must be performed tabletop without a grid so as to avoid grid cutoff. Adequate patellar imaging using this method is highly dependent on proper body part and tube positions. The patient is prone for both projections.

Posterolateral Axial Oblique Projection

The posterolateral axial oblique projection requires the patient's affected leg to be rotated medially 10[degrees] to 15[degrees] so that the medial surface of the patella is closest to the film (anteromedial oblique position). The knee should be flexed as little as possible; 5[degrees] or less is satisfactory. An 8 x 10-inch or 10 x 12-inch nongrid film cassette is placed on the tabletop beneath the affected knee. The tube is angled 40[degrees] to 45[degrees] caudally and 40[degrees] to 45[degrees] medially, with the central ray directed through the posterolateral aspect of the patella toward the center of the cassette. A cylinder cone can be used to improve image quality. The recommended source-to-image distance is 40 to 44 inches. (See Fig. 5.) A posterolateral axial oblique projection of the patella relatively free of superimposition from the femur is shown in Fig. 6.

[FIGURES 5-6 OMITTED]

Posteromedial Axial Oblique Projection

The posteromedial axial oblique projection requires the patient's affected leg to be rotated laterally 10[degrees] to 15[degrees] so that the lateral surface of the patella is closest to the film (anterolateral oblique position). Once again, the knee should be flexed only slightly, about 5[degrees]. An 8 x 10-inch or 10 x 12-inch nongrid film cassette is placed on the tabletop beneath the affected knee and the central ray is directed 40[degrees] to 45[degrees] caudally and 40[degrees] to 45[degrees] laterally from the midsagittal plane. The central ray enters the posteromedial aspect of the patella and is directed toward the center of the film. (See Fig 7.) A cylinder cone can be used to improve image quality, and the recommended source-to-image distance is 40 to 44 inches. The posteromedial axial oblique projection of the patella in Fig. 8 is almost totally free of superimposition from the femur.

[FIGURES 7-8 OMITTED]

Conclusion

The 2 projections described in this article demonstrate the apical, lateral and medial aspects of the patella with minimal movement of the body part. In addition, these projections reduce condylar and epicondylar overlap, which can be beneficial in demonstrating patellar fractures or other subtle types of acute injury or degenerative pathology not adequately demonstrated by other projections.

References

(1.) Daffner RH, Tabas JH. Trauma oblique radiographs of the knee. J Bone Joint Surg. 1987;69A: 568-572.

(2.) Martensen KM. Alternate AP knee method assures open joint space. Radiol Technol. 1992;64(1):19-23.

(3.) Fulkerson JP, Hungerford DS. Disorders of the Patellofemoral Joint. 2nd ed. Baltimore, Md: Williams & Wilkins; 1990:IX.

(4.) Bontrager KL, Lampignano JP. Textbook of Radiographic Positioning and Related Anatomy. 6th ed. St. Louis, Mo: Elsevier Mosby; 2005:246-253.

(5.) Ballinger PW, Frank ED. Merrill's Atlas of Radiographic Positions and Radiologic Procedures. 9th ed. St. Louis, Mo: Mosby; 1999:289-310.

Victor N. White, M.S.R.S., R.T. (R), is an associate professor and director of the radiographic science program at Lewis-Clark State College in Lewiston, Idaho.
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Title Annotation:ON THE JOB
Author:White, Victor N.
Publication:Radiologic Technology
Geographic Code:1USA
Date:May 1, 2007
Words:857
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