Imaging of the patellofemoral joint is controversial. Anterior knee pain is a common source of referral of young patients to the orthopaedic clinic. The first imaging modality available to most surgeons will be plain radiography. Numerous techniques for taking the skyline radiograph have been suggested by various authors. Settegast10 proposed positioning the patient prone with the knee flexed beyond 90° (Fig 1A) and this technique was advocated in a review article on patellofemoral imaging.9 Merchant et al8 originally described a technique with the patient supine and the knee flexed to 45° (Fig 1B). In later work however, Merchant7 reported cases in which skyline radiographs obtained at 45° knee flexion seemed normal whereas radiographs taken at 30° showed patellar subluxation. Laurin et al6 proposed that the patient be positioned supine with the knee flexed to 20° (Fig 1C). Ficat et al4 required a series of three radiographs be taken with the knee in 30°, 60° and 90° flexion (Fig 1D). Ficat et al4 also reported cases in which skyline radiographs obtained at 60° knee flexion seemed normal whereas radiographs taken at 30° showed patellar subluxation. One study of clinical practice in the United Kingdom11 showed that more than 19% of orthopaedic surgeons would not request a skyline image whereas 21% would request more than one image at various knee flexion angles. Eight knee flexion angles for skyline radiography are in current clinical use.
Various radiologic parameters have been defined to allow measurement of numerous patellofemoral disorders. Brattstrom2 defined the sulcus angle as the angle formed by the trochlear groove (Fig 2A). The sulcus angle has a normal value of 138° with a standard deviation of 6°. The congruence angle was defined by Merchant et al8 to measure the degree of patellar subluxation (Fig 2B). It measures the angular distance between the articular ridge of the patella and a reference line that bisects the sulcus angle. If the apex of the patellar articular ridge is lateral to the zero line, the congruence angle is designated positive. If it is medial, the congruence angle is negative. The normal congruence angle was defined at 45° knee flexion as -6° with a standard deviation of 11° (Fig 2B). The lateral patellofemoral angle was described by Laurin et al6 at 30° knee flexion as being the angle between the intercondylar line and the lateral patellar facet (Fig 2C). This angle is normal if it opens laterally. It is a measure of patellar tilt. Pure lateral patellar displacement was measured at 30° knee flexion by Laurin et al6 by calculating the distance between a perpendicular to the intercondylar line at the level of the medial femoral condyle and the medial edge of the patella (Fig 2D). It is abnormal if the patella lies lateral to this line. The facet angle was defined by Wiberg12 as the angle formed between the medial and lateral facets of the patella (Fig 2E). It is a measure of the depth of the patella and therefore its stability in the trochlear groove. The purpose of the current prospective study was to determine the intraobserver and interobserver reliability of the five parameters at each of the three knee flexion angles studied. We studied the changes in these parameters as knee flexion increased and determined at which knee flexion angle the greatest number of abnormal parameters could be detected. Using this method we determined the optimum knee flexion angle for skyline patellofemoral radiographs.
MATERIALS AND METHODS
Ethical committee approval was granted before the start of the study. Written informed consent was obtained from all subjects before participation in the study. Between September 1999 and June 2000, 72 new patients between 12 and 30 years presented to our institution with symptoms of anterior knee pain. After informed consent was obtained, these patients had a series of three skyline (tangential patellar) radiographs of their patellofemoral joints taken at 30°, 50°, and 90° knee flexion. The method of Laurin et al6 was used to obtain the skyline patellofemoral images. A set of knee rests was made at the three angles of knee flexion, to ensure consistency of knee flexion angle and to support the knee, which allowed the quadriceps to relax at the time that the images were taken. As is standard practice in our department, AP radiographs of the knee with the patient standing and lateral radiographs with the patient supine also were taken.
The skyline radiographs were evaluated by two observers who were blinded to the study. The sulcus angle, lateral patellofemoral angle, facet angle, lateral displacement, and congruence angle were calculated on each image obtained. One observer (APD) evaluated every radiograph on two occasions separated by several days to calculate intraobserver correlation coefficients.
All data were entered into a MicrosoftTM Excel spreadsheet file (Microsoft Corporation, Seattle, WA) and submitted for independent statistical analysis. The intraclass correlation coefficient was calculated to assess the reliability of measurements scored on a continuous scale (sulcus angle, congruence angle, lateral displacement, facet angle). The kappa statistic was used to assess agreement with respect to categorical diagnoses (abnormal patellofemoral angle). Statistical significance was set at the 5% level.
Fifty-eight patients (69 knees) fulfilled the entry criteria for the study. Two sets of radiographs (two knees) could not be retrieved for repeat analysis and so the measurements from the remaining 56 patients (67 knees) form the data set for this study. There were 33 male and 23 female patients. Pain was bilateral in eight male and three female patients, which resulted in 41 knees in males and 26 knees from females for the study. The mean age of the patients was 18.3 years (range, 12.5–29.7 years).
The sulcus angle was measured most reproducibly at 30° knee flexion and least reproducibly at 90° knee flexion (Table 1). At all angles of knee flexion studied, the reproducibility of the sulcus angle measurement was high (ICC, 0.865–0.930). This confirms findings from an earlier study.3
The measured sulcus angle showed a statistically significant progression of narrowing of the sulcus as the knee flexion angle increased. The mean sulcus angle at 30° knee flexion was 143.3° which narrowed to 142.5° at 50° knee flexion and additionally to 140.6° at 90° knee flexion. This progression was highly statistically significant p < 0.001 (Table 2). Lateral displacement was measured most reproducibly at 30° knee flexion and least reproducibly at 90° knee flexion (Table 1). The degree of patellar translocation as measured by lateral displacement also was highly reproducible at all knee flexion angles studied (ICC, 0.715–0.796). The measured distance between the medial condyle and the medial border of the patella did not change progressively as the knee flexion angle increased (Table 2). Measurement of the congruence angle was most reproducible at 30° knee flexion and least reproducible at 90° knee flexion (Table 1). The extent of patellar subluxation as measured by the congruence angle showed good reproducibility at 30° knee flexion (ICC, 0.657) but this decreased to only poor reproducibility at 90° knee flexion (ICC, 0.383).
There was a trend toward progressively more negative congruence angles as the knee flexion angle increased. The mean congruence angle was -4.34° at 30° knee flexion, -5.61° at 50° knee flexion, and -6.66° at 90° knee flexion. This did not reach statistical significance (Table 2).
Measurement of the lateral patellofemoral angle was most reproducible at 30° knee flexion and least reproducible at 90° knee flexion. The measurement of patellar tilt by the lateral patellofemoral angle showed fair reproducibility at 30° knee flexion (ICC, 0.589) but decreased to poor reproducibility at 90° knee flexion (ICC, 0.150) (Table 1).
The measured lateral patellofemoral angle became progressively more positive as the knee flexion angle increased. The mean lateral patellofemoral angle was 9.29° at 30° knee flexion and increased to 9.73° at 50° knee flexion and to 11.83° at 90° knee flexion. This trend was statistically significant (p = 0.04) (Table 2).
Measurement of the facet angle of the patella was most reproducible at 30° knee flexion and least reproducible at 90° knee flexion. The facet angle showed fair reproducibility and varied little (ICC, 0.597–0.591)(Table 1). The measured facet angle also varied little as the knee flexion angle increased. The mean facet angle was 131.2° at 30° knee flexion, 131.5° at 50° knee flexion, and 132.2° at 90° knee flexion (Table 2). This was not statistically significant. In every case, the reproducibility of measurement was best for the radiograph taken at 30° flexion, intermediate for the film taken at 50° flexion, and worst for the film taken at 90° flexion. In terms of reproducibility of measurement, the radiograph taken at 30° flexion was superior to the other two angles (Table 1).
In terms of the frequency of detecting abnormal parameters, the radiographs taken at 30° and 50° flexion were similarly able to define abnormal patellar tilt and subluxation. The radiograph taken at 90° flexion was the least sensitive at detecting abnormality. There were 47 knees with at least one radiologic abnormality. The measurements of the remaining 20 knees were within normal limits for all parameters at all angles of knee flexion. The 30°-radiograph detected 11 abnormal sulcus angles, eight abnormal facet angles, four abnormal lateral patellofemoral angles, and 15 abnormal congruence angles (Table 3). The 50° radiograph detected 11 abnormal sulcus angles, 10 abnormal facet angles, five abnormal lateral patellofemoral angles and 15 abnormal congruence angles. The 90°-radiograph detected eight abnormal sulcus angles, 10 abnormal facet angles, one abnormal lateral patellofemoral angle, and 14 abnormal congruence angles.
The optimum knee flexion angle for skyline radiography is 30°. Measurements of radiographic patellofemoral parameters are most reproducible from radiographs obtained at this angle and the greatest number of abnormalities is detected.
Plain radiography is most likely to remain the primary examination for patients with anterior knee pain attributable to the patellofemoral joint. These images are increasingly being supplemented by CT and MRI scanning, however such examinations are costly and time consuming and are not available to all patients. When ordering any examination involving radiation exposure to the patient it is imperative that the optimum amount of clinically useful information is obtained for the least amount of radiation exposure.
We compared skyline radiographs of knees obtained at various flexion angles using a collection of well-established radiologic parameters to measure different aspects of the patellofemoral joint. These radiologic parameters have defined normal ranges which allowed us to determine the reproducibility of measurements and the frequency of abnormal parameters being detected. The consistency with which these radiologic features can be measured from such radiographs is defined and the efficiency of the radiographs taken at the different knee flexion angles is compared.
The limitations of this study include the number of subjects examined and the choices of the radiologic parameters studied. Numerous radiologic parameters have been described for the patellofemoral joint. The parameters chosen for this study were those used in our clinical practice.
Merchant et al8 defined the congruence angle in 100 normal knees as -6° with a standard deviation of +16° (Table 4). In a population of 25 patients with known recurrent patellar dislocation, Merchant et al8 found the mean congruence angle to be +23°. Aglietti et al1 found the mean congruence angle in a group of 150 healthy subjects to be minus 8° with a standard deviation of +6° (Table 4). In 37 subjects with patellar subluxation, Aglietti et al1 found the mean congruence angle was +16°. In the current study, the mean congruence angle was minus 4.34° with a standard deviation of 16.23° when measured at 30° knee flexion, which is in keeping with the range of values from healthy subjects in the studies of Merchant et al8 and with a standard deviation of 6° (Table 4). Aglietti at al1 found the sulcus angle in their series of normal knees to be 137° with a standard deviation of 6°. In a previous study3 the mean sulcus angle was 140.4° with a standard deviation of 5.2° in knees with no evidence of trochlear dysplasia. In the current study, the mean sulcus angle was 143.3° with a standard deviation of 7.06° when measured at 30° knee flexion (Table 4). At 90° knee flexion, the mean sulcus angle was 140.6° with a standard deviation of 6.48°. These findings indicate that the sulcus angles in the population of symptomatic patients in the current study were widened in keeping with some degree of trochlear dysplasia. Laurin et al5 defined the lateral patellofemoral angle as normal if it opened laterally. In a group of 100 normal knees, the lateral patellofemoral angle was open laterally in 97 knees and the lines were parallel in three knees.5 In 30 knees with patellar subluxation, 24 (80%) showed the lines parallel and in six knees (20%) the angle opened medially.5 In the current study, four of the 67 knees (6%) had a lateral patellofemoral angle that opened medially when measured at 30° knee flexion. This figure decreased to one knee (1.5%) with an abnormal lateral patellofemoral angle when measured at 90° knee flexion, emphasizing the importance of minimal knee flexion angles. Merchant7 and Ficat et al4 published cases in which subluxation was present at 30° and absent at 45° or greater knee flexion. Despite this, the technique of Settegast10 was used to show patellofemoral imaging in one review.9 We included the 90° radiograph in the current study to measure the parameters of the patellofemoral joint across various knee flexion angles and because of the continuing clinical use of this technique.11
The results of our study show that the radiologic parameters studied are most reproducibly measured from skyline tangential patellar images of the patellofemoral joint taken at 30° flexion. With respect to detection of abnormality, the radiographs taken at 30° and 50° flexion were similar. There is no value in obtaining multiple skyline images from a patient because this increases exposure to radiation with no increase in clinical information. Changes were seen in the radiologic parameters measured as knee flexion angle increases. For most parameters, there was a trend toward more normal measurements as knee flexion angle increased (Table 2). This was in accordance with the observations of Merchant7 and Ficat et al4 that knees could appear radiologically normal at knee flexion angles of 45° or greater when there was clear patellar subluxation seen at 30° knee flexion.
One skyline patellofemoral radiograph should be obtained for every patient who presents with anterior knee pain. The minimum achievable angle of knee flexion should be advised when making this image and every effort should be made to take a radiograph with the knee at 30° flexion. This is the optimal flexion angle if reproducible measurements are to be made from the images obtained.
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