International

International Orthopaedics (SICOT) (1996) 20: 159 – 162

Orthopaedics  Springer-Verlag 1996

Original article CT scanning of the patellofemoral joint The quadriceps relaxed or contracted? A. D. Delgado-Martı´nez1, C. Estrada2, E. C. Rodrı´guez-Mercha´n3, M. Atienza2, and J. M. Ordo´n˜ez1 1

Department of Orthopaedic Surgery, 2 Service of Radiology and 3 Service of Traumatology, La Paz Hospital, Madrid, Spain

Accepted: 13 July 1995

Summary. A study of the patellofemoral joint in patients with patellofemoral pain has been carried out in 14 patients (18 knees) to compare CT scans with the knee extended fully and the quadriceps muscle contracted or relaxed. The purpose was to decide whether both examinations are necessary in young adults with suspected patellar malalignment. The patients were selected randomly. Thirteen knees were also examined in 20° of flexion. The lateral patellar angle and the lateral patellar shift were measured. Linear regression analysis showed a proportional relationship between measurement with the quadriceps contracted and relaxed. Therefore, only one examination is needed and is easier to carry out in the relaxed state. Radiation to the patient and costs are reduced.

tion axiale de la rotule d’apre`s toute les coupes tomographiques. L’e´tude en regression line´aire demontre une proportionalite´ entre les parame`tres obtenus en relaxation et en contraction. La conclusion des auteurs est donc qu’une seule de ces deux explorations suffit pour le diagnostic des de´viations rotuliennes dans l’articulation fe´moropatellaire douloureuse, et ils pre´fe`rent celle en relaxation, plus facile a` re´aliser. Moins de coupes tomographiques sont ne´cessaires ce qui re´duit l’irradiation du patient et le couˆt de l’examen.

Introduction Re´sume´. Les auteurs ont re´alise´ une e´tude comparative entre le scanner de l’articulation fe´moropatellaire douloureuse en e´tat de contraction et en e´tat de relaxation. L’objectif est de savoir s’il faut re´aliser ces deux explorations pour un diagnostic des de´viations axiales rotuliennes chez les jeunes. Un scanner du genou en extension (avec le quadriceps contracte´ puis de´contracte´) a e´te´ re´alise´ chez 14 patients (18 genoux) «choisis par hasard», avec douleur de l’articulation fe´moropatellaire. En outre, cette meˆme exploration fut re´alise´e sur 13 de cas genoux a` 20° de flexion. On a mesure´ l’angle late´ral fe´mororotulien et la de´via-

Reprint requests to: A. D. Delgado-Martı´nez, c/Navas de Tolosa 13-4°, E-23001, Jae´n, Spain

Pain arising from the patellofemoral joint in young adults is commonly associated with patellar malalignment [8]. Diagnosis is based on the clinical history and physical examination [2]. Imaging of the patellofemoral joint with radiographs, CT and MRI is used to confirm the diagnosis and to clarify the nature of abnormality [2]. The tests should be made with the knee near fully extension [3, 8]. As flexion increases, the patellar retinacula become tighter, the patella is displaced towards the intercondylar notch and the patellofemoral relationship appears normal so that the CT scan may lose its diagnostic value and small abnormalities may be overlooked [3, 8]. The CT scan is useful in the diagnosis of patellar malalignment [1] and most authors carry out the examination in different degrees of flexion [1, 6, 8, 9] with the quadriceps relaxed and contracted [1, 6, 7, 8, 9]. A large number of images

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A. D. Delgado-Martı´nez et al.: CT scanning of the patellofemoral joint

Fig. 2. Measurements made from CT images. a The lateral patellofemoral angle is drawn between the lateral patellar facet A and a line passing through the posterior aspect of both femoral condyles B. b The lateral patellar shift is the percentage of the patella remaining outside a line drawn perpendicular to both femoral condyles passing through the apex of the lateral femoral condyle. The lateral patellar shift = AC AB × 100

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Patients and methods

Fig. 1. a Bilateral patellofemoral CT scan with the knees extended and the quadriceps relaxed. b CT scan of the same knees in a similar position, but with the quadriceps contracted. The lateral patellar shift and tilt increase when the quadriceps are contracted

increases both the radiation exposure and cost. Schutzer et al used CT images to classify patellar malalignment as follows: type I – subluxation without tilt; type II – subluxation with tilt, and type III – tilt without subluxation [8, 9]. The purpose of this study is to determine whether it is necessary to obtain CT images of the patellofemoral joint with the quadriceps both relaxed and contracted.

Fourteen patients (18 knees), who all complained of patellofemoral pain, were randomly selected. Their ages ranged from 11 to 31 years. An abnormality of the joint had been suspected for at least 6 months, and 11 knees had had at least one patellar dislocation before the CT scan. A CT scan slice was focused on the central area of the patella, images 5 mm wide were evaluated with a bone window of W = 1500 and C = 150, and both lower limbs were free. The CT scan was carried out with the knee in extension with the quadriceps relaxed and in maximal voluntary contraction (Fig. 1). In 13 knees the same test was repeated with the knee in 20° of flexion, stabilizing the limb in a special apparatus [6]. The lateral patellofemoral angle [8, 9] and the lateral patellar shift [7] were measured in every tomographic image by the same author (ADD) (Fig. 2). The data are shown graphically (Fig. 3). A linear regression analysis was also carried out [5].

Results The data obtained from the measurements is shown in Fig. 3 with the values from the CT scan with the quadriceps relaxed on the X-axis and with the quadriceps contracted on the Y-axis. The results of linear regression analysis are given in Table 1. In every case a statistically significant

Table 1. Results of the linear regression analysis

Knee flexion

Parameters

Statistical significance

0° 0° 20° 20°

LPFA LPS LPFA LPS

p p p p

50.001 50.001 50.001 50.001

LPFA = Lateral Patellofemoral Angle; LPS = Lateral Patellar Shift r2 has been calculated as a square value of r; it indicates the percentage of proportional relationship of the dependent variable

Correlation coefficient (r)

Confidence interval for r = 95%

r2

0.93 0.93 0.96 0.96

0.82 – 0.97 0.82 – 0.97 0.86 – 0.99 0.87 – 0.99

0.87 0.87 0.92 0.92

(in this case the parameter with the quadriceps in contraction) as it is related to the independent variable (parameter with the quadriceps in relaxation)

A. D. Delgado-Martı´nez et al.: CT scanning of the patellofemoral joint

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Fig. 3 a – d. Scatter graphs with linear regression best-fit lines represent the relationship between the CT images with the quadriceps contracted and relaxed. a Lateral patellofemoral

angle with the knee extended. b Lateral patellar shift with the knee extended. c Lateral patellofemoral angle with the knee flexed 20°. d Lateral patellar shift with the knee flexed 20°

Table 2. Formulae correlating the values obtained with the CT scan with the quadriceps in contraction and in relaxation

Discussion

Knee flexion

Parameters

Formula to correlate parameters

0°

LPFA

LPFA (contract) = 0.99 × LPFA (relax) – 6.55 (0.80 – 1.19)a

0°

LPS

LPS (contract) = 1.14 × LPS (relax) + 15.19 (0.92 – 1.37)a

20°

LPFA

LPFA (contract) = 1.38 × LPFA (relax) – 7.28 (1.14 – 1.62)a

20°

LPS

LPS (contract) = 1.39 × LPS (relax) + 3.43 (1.16 – 1.63)a

LPFA = Lateral Patellofemoral Angle; LPS = Lateral Patellar Shift; (contract) = with quadriceps in contraction; (relax) = with quadriceps in relaxation; a indicates the intervals of 95% confidence for the coefficient of multiplying factor

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value was obtained (p 0.001) with a correlation coefficient ranging from 0.93 to 0.96. The formulae derived by correlating the data with the quadriceps contracted and relaxed are summarised in Table 2.

Tomographic images with the quadriceps contracted are said to be a good method of evaluating abnormalities of the patellofemoral joint [1, 6] and this is reported to show the patellar malalignment better because of the greater lateral shift when the muscle is contracted [7]. On the other hand, it has been suggested that there are no significant differences in patellar alignment when the quadriceps are contracted but that CT scanning should be carried out with the muscle both when contracted and when relaxed [8]. Schutzer’s classification was developed without using CT with the quadriceps contracted [8, 9]. Inoue et al did not use the quadriceps contraction in their investigation [4]. We have used the patellofemoral angle to measure lateral tilt with the posterior area of both femoral condyles as the reference point [8], whereas others have used the anterior area of the condyles [7]. We preferred the posterior method since the lateral femoral condyle may be hypoplastic and modify the anterior reference line [2]. Measurement of lateral shift has been according to Sasaki and Yagi (modified) [7], and we have not used Merchant’s congruence angle, although it has been recommended [9]. The sulcus angle is diffi-

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cult to measure with the knee fully extended [4]. Both methods of measurements which we used are independent of image magnification or the size of the bones, and interobserver variability was avoided by them being carried out by the same author. We did not fix the feet together as has been recommended [6] because it seemed better to adopt the natural position [2]; internal rotation is not forced so the Q-angle is not diminished artificially which could lead to negative results. Sasaki and Yagi examined the knee in extension and demonstrated that with the quadriceps contracted there was a greater increase in the patellar shift and tilt in patients compared with a healthy control group [7]. They based this conclusion on differences in absolute values without taking into account the patients’ greater initial values (in relaxation)compared with the control group. The excellent linear correlation in our results (Table 1) shows the presence of a proportional relationship between the measurements with the quadriceps contracted and relaxed which can be expressed by mathematical formulae (Table 2). Our findings indicate that CT scans with quadriceps contracted and relaxed provide the same information for the diagnosis of patellar malalignment. One test, in relaxation or contraction, produces the data needed for mathematical derivation of results. Our data had nearly a homogenous distribution in all ranges of patellofemoral angle and shift (Fig. 3). Since not every patient was diagnosed as having a patellofemoral abnormality, we have a continuous range from those with severe malalignment to those which are normal, so a normal control group was unnecessary. Furthermore, the homogenous distribution allows us to generalise our conclusions to patients with

patellofemoral pain as there is the same proportional relationship in those with and without malalignment. We conclude that it is unnecessary to carry out CT with the quadriceps both relaxed and contracted and only one of these tests is sufficient to make the diagnosis of patellofemoral malalignment. We prefer the test in relaxation as it is easier to perform [8, 9]. Acknowledgments. We are grateful to G. Garrido MD for his advice in the statistical analysis and to Mr Michael M. Walger, medical student, for his help in translating the manuscript.

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