An Anatomic Study on Whether the Patella is Centered in an Ideal Anteroposterior Radiograph of the Knee

Abstract

Background: Currently, an anteroposterior radiograph of the knee is judged based on a centered position of the patella between the femoral condyles. We are not aware of any anatomic literature supporting this recommendation.

Questions/purposes: Orthogonal images are required for accurate assessment of knee deformity. Although an image with the patella centered at the distal femur is generally accepted as a true anteroposterior (AP) radiograph of the knee, there is minimal anatomic data to support that this view is orthogonal to a true lateral view of the knee where the condyles are overlapped. We designed an anatomical study to test the relationship between these two radiographic views.

Methods: We studied 428 well-preserved cadaveric skeletons ranging from 40 to 79 years of age at death. Centering of the patella was calculated based on distal femoral and patellar widths. Multiple regression analysis was then performed to determine the relationship between patellar centering and age, gender, ethnicity, mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle (MPTA), femoral anteversion, and contralateral centering.

Results: Average patellar centering was 0.13 ± 0.04, indicating that the average patella was laterally positioned in the distal femur. Only mLDFA and contralateral centering showed statistically significant independent correlations with patellar centering with modest standardized beta coefficients of 0.10 and 0.23, respectively.

Conclusions: In the average specimen, the patella is laterally deviated by 13% of the condylar width. Clinicians should be aware that a lateral view with the femoral condyles overlapped is not always orthogonal to a patella-centered AP view when planning and implementing deformity correction.

Keywords: knee alignment; knee anatomy; knee deformity analysis; knee radiographs.

Fig. 1

It is common for radiology technicians to image patients with the patella lateral, despite standard instructions to centralize the patella. This has led us to question whether the anatomic position of the patella is laterally deviated in some patients. a Seventeen-year-old male with bilateral genu valgum, worse on the left, with shortening of the right femur. b Eighteen-year-old male, previously treated for left Blount disease, now with genu valgum and shortening of the femur and tibia.

Fig. 2

Typical anatomic placement of the femur and patella. a The femur rests with the posterior aspect of the femoral condyles and the posterior aspect of the greater trochanter touching the table. The patella fits within the trochlear groove. b In cases with retroversion of the femoral head, the femoral condyles and greater trochanter are elevated on equal platforms to avoid rotational malpositioning secondary to the femoral head.

Fig. 3

Measurements of the distal femur and patella. The width of the femoral condyles, the width of the patella, and the distance from the lateral edge of the patella to the lateral edge of the lateral femoral condyle were measured as illustrated. The specimen was oriented with the distal femoral condyles perpendicular to the vertical lines defining each of these measurements.

Fig. 4

To measure mLDFA and MPTA, the femur was placed resting on the posterior aspect of the greater trochanter and the posterior femoral condyles. The tibial plateaus were then matched rotationally to the femoral condyles to approximate anatomic position, and then the tibia was then shifted distal to allow visualization of the tibial spines. mLDFA was measured as the lateral angle between a line from the center of the femoral head to the intercondylar notch, and a line along the femoral condyles distally. MPTA was measured as the medial angle between a line along the tibial plateau and a line from the center of the tibial spines to the center of the plafond.

Fig. 5

a–c A specimen with average centering ratio of 0.123. a The AP image demonstrates that this specimen is laterally deviated in the standard anatomic position. b The specimens can be rotated to center the patella by elevating the lateral femoral condyle. c This axial view demonstrates that if one attempted to obtain a lateral radiograph orthogonal to a patella-centered AP view, the femoral condyles would not appropriately overlap. d–f A specimen with a high centering ratio of 0.247. d Note the significant lateral deviation of the patella in the AP view. e Significant elevation is necessary to rotate this specimen into a patellar-centered AP view. f An orthogonal lateral in this case would be significantly out of the plane of a true lateral.

Fig. 6

Three specimens with varying patella positions. a Patellar centering ratio of 0.057, one standard deviation below the mean yet still with a laterally positioned patella. b Patellar centering ratio of 0.129, at the mean. c Patellar centering ratio 0.205, one standard deviation above the mean.