This study was conducted to find the most suitable tangential axial radiography method for the patellar and patellofemoral joints by comparing PJR with the conventional radiographic methods. Among the radiographic conditions using PJR weres compared with the conventional imaging methods of Settegast and Merchant. In the DMLP evaluation, when the knee of the phantom was bent at 70° to 20° at 10° intervals using PJR, the DMLP value decreased as the angle of flexion was increased. This implied that the inclination or tilt of the patella decreased when the knee was extended than when the knee was flexed and that the half lateral image of the patella was more accurately displayed with the former. Laurin method with a knee flexion angle of 20° had the smallest inclination of the patella and the half lateral image of the patella showed the best results. But, based on the DFGT results, as the value representing the distance between the femoral trochlear groove and the tibial tuberosity increased to a positive value, the tibial tuberosity invaded the patellofemoral joint area and resulted in narrowing of the patellofemoral joint space. On the other hand, as the distance between the femoral trochlear groove and the tibial tuberosity was reduced to a negative value, the inclination of the patella became larger. In addition, among the radiographic conditions using PJR, 20° knee flexion, which is similar to that in the Laurin method, and 160° angle of the long axis of the femur to the long axis of the lower leg resulted in the largest invasion of the tibial tuberosity to the patellofemoral joint and the narrowest patellofemoral joint spacing (Fig. 6a). Although the Settegast method gave the smallest inclination of the patella on imaging, it required the largest knee bending angle and may cause further discomfort to a patient with knee pain. When the distance between the femoral trochlear groove and the tibial tuberosity was the largest and the tibial surface invaded the patellofemoral joint space, the resulting image had the narrowest joint spacing. The reason for the result of the smallest inclination of the patella with the Settegast method than with the Merchant and PJR methods was thought to be close contact of the patella with the detector in the prone position. The object to image receptor distance (OID) between the patella and the detector was the target area for radiographic imaging. If the OID was short, distortion in the radiographic image was reduced. On the other hand, if the OID was increased, the image was enlarged and had a degraded quality, resulting in blurring and low contrast [13–15]. The Merchant method showed the greatest patellar inclination, and the overlap between the patellofemoral joint space and the femoral trochlear groove and the tibial tuberosity was not as good as the PJR method, but better than the Settegast method. With the results of the DMLP, DPFG, and DFGT evaluations taken together, 40° knee flexion with 140° angle between the long axis of the femur and the long axis of the lower leg was the most beneficial radiographic condition using PJR (Fig. 6b).
Tangential axial radiography must be performed in order to detect patellar fracture and injury and patellofemoral joint stenosis and wear and to determine the correlation between the femur and tibia. However, conventional radiography methods have several disadvantages. First, although the Settegast and Hughston methods are relatively frequent in the clinical setting because of the relatively short procedure time, the required prone position and knee flexion may be difficult in patients complaining of pain from patellar fracture or dislocation [3, 16–18]. In the prone position, the patellar area comes into contact with the examination table and a load is applied; this may worsen the pain in the area near the knee and should be avoided [3, 17]. Second, the Laurin and Merchant methods are radiographic methods that can be used to diagnose patellofemoral joint disease in a sitting or supine position without the need to prone. The Laurin method mainly measures the lateral patellofemoral angle and checks whether patellar is normally open to the outside. The Laurin method was said to require the smallest bending angle of the knee and was the closest to the actual structure of the patellar [10, 11]. However, as the knee bending angle decreases, the distance between the femoral trochlear groove and the tibial tuberosity increases, the tibial surface invades the patellofemoral joint space, and the gap narrows. Therefore, this is not suitable for viewing minute damages to the patellofemoral joint. Moreover, the Laurin method necessitates postural instability, because the patient must bend the knee in a sitting position, directly hold the detector, and perform radiography. In addition, in a sitting position, special care is required to avoid radiation exposure of areas other than the patellofemoral joint .
Most of the existing studies on patellofemoral joint observation determined patellar dislocation and subluxation by measuring the congruence and sulcus angles [15, 19, 20]. To measure this in the Merchant method, the long axis of the femur must be parallel to the surface of the examination table using an auxiliary device. In the Merchant method, each knee is radiographed separately. When both legs are shot simultaneously, one leg may not be fixed and the knee may be adducted or abducted; these may increase the matching angle and distort the image . In addition, when the distance from the source is doubled, the radiation intensity is proportional to the inverse square of the distance, where the radiation level per unit area decreases to 1/4 . In the case of similar source to image receptor distance (SID), as the distance from the source of the X-ray tube to the object part (SOD) increases, the entrance surface dose decreases. Conversely, as the OID becomes closer, enlargement of the image is prevented [13, 23]. The set OID was about 30 cm for the Merchant method and about 20 cm for PJR (Fig. 7). In this research, because the tangential axial radiography method using PJR had 10-cm longer SOD and 10-cm shorter OID, compared with those by the Merchant method, the latter will create more exposure to radiation doses. Comparison of radiation doses in tangential axial radiography of the patellofemoral joint requires more in-depth experiments. Nevertheless, radiographic imaging using PJR may be a good means of solving some of the shortcomings of the conventional radiographic methods. Specifically, PJR may allow patients to comfortably undergo radiographic imaging in a supine position and may enable a stable position to prevent shaking or distortion of the image at the target site. Compared with the conventional radiographic methods, PJR showed radiographic images in which the patellofemoral joint space was larger, the gap between the trochlear groove and the tibial tuberosity was consistent, and the half lateral image of the patella was clearly shown (Fig. 8).
One limitation of this study was that comparison of different patellofemoral joints among humans was not possible. However, by performing several patellofemoral joint radiography imaging on the phantom knee joint, we were able to quantify the most optimal patellar and patellofemoral joint shapes. In the future, studies that measure the shape of several patellofemoral joints in humans are needed.