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In order to analyze the congruence of the articular cartilage surfaces and the corresponding subchondral bone, MRI is an ideal tool for imaging all aspects of the patellofemoral joint.

  Thomas Muellner1, Martin Funovics2, Ajsa Nikolic1, Victor Metz2, Rudolf Schabus1, Vilmos Vécsei1

1 University Clinic of Traumatology 2 Clinic of Radiodiagnostic, Surgical Division, University of Vienna Medical School, Waehringerguertel 18-20, A-1090 Vienna, Austria


Patellofemoral disorders encompass a wide range of conditions from anterior knee pain to arthritis. Subtle malalignments are often difficult to diagnose, whereas severe malalignment syndromes and dislocations of the patella are more easily detected. While earlier studies involved the anatomy of the femoral trochlea and the patella (Brattström 1964, Wiberg 1941) more recent studies have focused on the position of the patella using either radiograms (Laurin et al. 1979, Laurin et al. 1978, Merchant et al. 1974) or computerized tomography (CT) (Fulkerson et al. 1987, Inoue et al. 1988). A number of studies focused on the patellar tilt (Grelsamer et al. 1993, Laurin et al. 1979, Laurin et al. 1978), while others involved the congruence angle (Aglietti et al 1983, Fulkerson et al. 1987, Inoue et al. 1988, Merchant et al. 1974). A sensitivity of 30% for the congruence angle (Inoue et al. 1988), and 85% for the modified lateral patellofemoral angle, as described by Grelsamer et al (1993), was found in detecting patellofemoral malalignment. Radiographic studies only allow measurements or descriptions of the bony patellofemoral joint. The articular cartilage surfaces have not been considered in previous reports.

The aim of this study was therefore to analyze the congruence of the articular cartilage surfaces and the corresponding subchondral bone of the patellofemoral joint in the axial plane using magnetic resonance imaging (MRI).

Material and Methods

Twenty healthy volunteers, with no former history or clinical signs of patellofemoral problems or knee operations, were investigated. There were 14 males and 6 females. The mean age was 28 (21-37) years. The length of the patella and the patellar tendon were measured on lateral radiographs, taken in supine position with the knee flexed to 60°, as described by Insall and Salvati (1971). The patellar height was then calculated by dividing the length of the patellar tendon by the length of the patella.

The MRI investigation was performed on a Siemens Magnetom Vision 1.5 Tesla unit with an extremity coil. A cartilage sensitive Flash 3D sequence was employed, with a flip angle of 60°, which allowed a precise differentiation between the subchondral bone and the articular cartilage. Repetition time and echo time were 60ms and 11ms, respectively. The patellofemoral joint was imaged in the axial plane using slice thickness set to 1.5 mm in a field of view of 120x160 mm and a matrix 144x256 pixels. Data acquisition was done with the knee flexed to 20° and 45°. The flexion angle was controlled with a hand-held goniometer. In order to avoid flexion angle variabilities during the MRI investigation, custom-made molds (made of foamed plastics) were used to fix the knee at the prespecified angle. The slice, centered on the midtransverse patella and perpendicular related to the patella, was selected and magnified 3 times so as to define patellar position more accurately. In 20° knee flexion, the cartilage on the highest points of the medial and lateral condyles is so thin that an accurate differentiation between cartilage and bone was not possible. The line drawn across the bony and cartilagineous femoral condyles was therefore defined as the same.

The bony patella was described according to Wiberg´s classification (Wiberg 1941) and the patella index was recorded, as described by Cross et al. (1976). The following angles were calculated for both the subchondral bone and the articular cartilage surfaces. The sulcus and the congruence angles were recorded as described by Merchant et al (1974) (Figure 1). The lateral patellofemoral angle was measured as described by Laurin et al. (1978) (Figure 2). Two radiologists (MF, VM) made the measurements independently and the interobserver variability was determined by using the Pearson correlation coefficient. The limits of agreement were calculated as described by Bland and Altman (1986). The recorded angles were statistically compared using an analysis of variance with repeated measurements and an unstructured variance-covariance matrix (SAS/STAT User´s Guide 1990). A p-value of less than 0.05 was considered statistically significant.


The right knee was investigated in all volunteers. The mean Insall-Savati Ratio 1.06 (SD 0.11). According to the Wiberg´s classification, 3 patellae were classified as Type I, 15 patellae were classified as Type II, and 2 patellae were classified as Type III. A mean patella index of 8.8 (SD 4.8) was recorded.

In 20° of knee flexion only the bony sulcus angle and the bony lateral patellofemoral angles were significantly different when compared to the respective angle derived from the articular cartilage surfaces (Table 1). The mean lateral patellofemoral angle was 8.4° (SD 0.9°) for the subchondral bone, and 9.4° (SD 1.2°) for the articular cartilage surface (p=0.01). The mean sulcus angle was 138.7° (SD 1.7°) for the subchondral bone and 148.2° (SD 2.8°) for the articular cartilage surface (p=0.01). The mean congruence angle was -5.9° (SD 3.5°) for the subchondral bone and -7.4° (SD 4.4°) for the articular cartilage surface (p=0.29).

In 45° of knee flexion both the bony sulcus angle and the bony lateral patellofemoral angle were significantly different when compared to the respective angles derived from the articular cartilage surfaces (Table 1). The mean lateral patellofemoral angle was 10.3° (SD 1.0°) for the subchondral bone, and 13.4° (SD 1.1°) for the articular cartilage surface (p=0.005). The mean sulcus angle was 132.2° (SD 1.5°) for the subchondral bone and 145.4° (SD 2.3°) for the articular cartilage surface (p=0.0001). The mean congruence angle was -13.3° (SD 3.4°) for the subchondral bone and -19.1° (SD 4.1°) for the articular cartilage surface (p=0.7). When comparing the angles derived from the two positions of knee flexion, significant differences were found for all the three angles.

The inter-observer study showed that the Pearson correlation coefficients for the bony angles were: sulcus angle: 0.942, congruence angle: 0.983, lateral patellofemoral angle: 0.929. For the cartilage surface the respective Pearson correlation-coefficients were: 0.833, 0.987, 0.812. The calculated correlation was significant at the 0.01 level. The limits of agreement (mean difference ± 2SD) for the respective angles were: sulcus angle: bone: 0.1±3.8; cartilage: 0.1±4.4, congruence angle: bone: 0.2±3.8; cartilage: 0.2±5, lateral patellofemoral angle: bone: 0.9±2.8; cartilage: 0.3±2.7


The purpose of this study was to investigate whether the measurement of the bony patellar alignment correlates with measurements based on the articular cartilage surfaces. Previous radiological methods, ratios, angles, and indices were all based on the subchondral bone (Brattström 1964, Fulkerson et al. 1987, Grelsamer et al. 1993, Inoue et al. 1988, Laurin et al. 1979, Laurin et al. 1978, Merchant et al. 1974, Wiberg 1941). Further attempts to improve measurement of these angles with greater accuracy seems to be fruitless, considering that clinical evaluation is the basis of a successful diagnosis and treatment of gross patellofemoral disorders (Post 1997). Computerized tomography (CT), when introduced to image patellar tracking, improved the accuracy and sensitivity of standard radiographs (Inoue et al. 1988, Stanciu et al. 1994). However, Stäubli et al. (1997) confirmed, in a combined cryosectional and MRI study, that the subchondral bone, as seen on CT scans, is not in accordance with the articular cartilage surface as proposed by Wiberg (1941).

In the present study the volunteers were all free of patellar problems. The measurements revealed no patellar or trochlear abnormalities with a normal patellar height. The MRIs were performed in 20° and 45° knee flexion in order to provide information of the early patellar route (entrance into the sulcus).

The bony sulcus angle in the present study was in accordance with previously published values (Brattström 1964, Merchant et al. 1974). Related to the articular cartilage surfaces, the bony sulcus angle was significantly smaller. The thickness of the cartilage and therefore the increase of the sulcus angle differed among the volunteers. Using the bony sulcus angle, no prediction could be made with regard to the thickness of the cartilage and the shape of the cartilage surfaces.

In all volunteers the lateral patellofemoral angle was open laterally which has been found to be normal (Laurin et al. 1978). The lateral patellofemoral angle showed a significant increase when related to the cartilage and between 20° and 45° knee flexion.

The bony congruence angle was comparable to the values given by Merchant et al (Merchant et al. 1974). Compared to 20° of knee flexion the bony congruence angle was more negative at 45° knee flexion, which was interpreted as a medial movement of the patella with increasing flexion, as described by Hefzy and Yang (1993), and Stein et al (1993).

We found no systematic difference and no correlation factor between the values of the subchondral bone and the articular cartilage surface for all measured angles. Therefore the thickness and the shape of the articular cartilage is unpredictable by measuring angles derived from the subchondral bone. Though the lowest point of the articular surface of the patella is not a point but a curved line, the interobserver variability was low. Therefore it was shown that the MRI of the patellofemoral joint is an investigation which allows a high reproducibility beween different observers.

From the present results some conclusions can be drawn, even though the study population was small and only volunteers with no patellofemoral problems were investigated. First, the measurement of the bony sulcus and lateral patellofemoral angles does not allow conclusions about the articular cartilage surface and thickness. Secondly, as no statistically significant difference was found between the bony and the the cartilaginous congruence angle, the bony congruence angle allows an approximate judgement of the patellar centralization. Routine radiographic examination of the patellofemoral joint seems not to be enough to assess and to quantify the patellofemoral pathology, as the cartilage surface differs significantly from the subchondral bone. Further studies are recommended to find out whether the differences between the cartilage and subchondral bone angles are more pronounced in patients with patellofemoral disorders.


The authors thank Martina Mittlboeck, PhD, for her assistance in statistical analysis.

Table 1: The respective angles were measured in 20° and 45° knee flexion and recorded for both the subchondral bone and the articular cartilage surfaces, mean (SD).

Table 1

Sulcus Angle


Knee Flexion Angle





139° (1.7°)

148° (2.8°)



132° (1.5°)

145° (2.3°)







Congruence Angle



-5.9° (3.5°)

-7.4° (4.4°)



-13.3° (3.4°)

-19.1° (4.1°)







Lateral Patellofemoral Angle 



8.4° (0.9°)

9.4° (1.2°)



10.3° (1.0°)

13.4° (1.1°)






Figure 1A


Figure 1B

Figure 2A 

Figure 2B


Figure 1a,b:

1A The congruence angle is determined by bisecting the sulcus angle (dotted line), and projecting a second line from the apex of sulcus to the lowest point of the patellar ridge (full line); the angle is formed between this line and the bisecting line. (45° of knee flexion)

1B The congruence angle when using the articular cartilage surfaces. (45° of knee flexion)

Figure 2a,b:

2A The lateral patellofemoral angle is the angle between the anterior tangent of the femoral condyles and the tangent of the lateral patellar facet. (45° of knee flexion)

2B The lateral patellofemoral angle when using the articular cartilage surfaces. (45° of knee flexion)


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