A new reference line in diagnosing a high-riding patella on routine digital lateral radiographs of the knee

Objective

To propose a new reference line in diagnosing a high-riding patella (patella alta) on routine digital lateral radiographs of the knee.

Materials and methods

On 64 routine digital lateral radiographs of adult knees with no bone pathology or surgery and over the range of knee flexion between 1.6° and 79.1° (34.4°± 16.6°), the long axis of the superimposed egg-shaped femoral condyles was drawn and moved upward to pass through the point where the posterior contours of the femoral diaphysis and the femoral condyles meet—the posterior reference point—to become a new reference line. Any part of the patella crossing the new reference line indicated a high-riding patella. The new reference line was compared with the four most common patellar height ratios (Insall–Salvati, Grelsamer–Meadows or the modified Insall–Salvati, Caton Deschamps, and Blackburne–Peel) for the drawing time, accuracy, and reproducibility (intra- and interobserver agreement).

Results

The new reference line required approximately one-third to one-half of the drawing time compared to the most common patellar height ratios, with equal accuracy and complete reproducibility. On the contrary, the reproducibility of the most common patellar height ratios ranged from poor to moderate, with the best results for the Insall–Salvati ratio.

Conclusions

The new reference line proved to be a very simple, accurate, and reproducible tool in diagnosing a high-riding patella. We believe that our preliminary results are an encouraging impetus to more complex research.