The evaluation of patellar height: a simple method

Patellar height is evaluated with ratios that have been derived from measurements made on lateral radiographs of the knee. The importance of using a ratio is that magnification, physical size, and flexion angle are eliminated as factors that affect the values. The Insall-Salvati index was the first ratio to be described. It remains the most popular, possibly because normal values are easy to remember. As all of the currently accepted methods are cumbersome to use because they require two measurements as well as a calculation, a single angular measurement was devised (the plateau-patella angle) to offer a simpler alternative. The purpose of this study was to introduce the new method and to assess its validity by comparing it with three classic, commonly used ratios.

In two groups of patients, after exclusions, 269 lateral radiographs of the knee were evaluated. All measurements required for the Insall-Salvati, the Blackburne-Peel, and the Caton-Deschamps indices plus the plateau-patella angle were recorded, along with basic demographics. For validation, the new method was compared with the established methods, and interobserver and intraobserver reliability were computed.

There was excellent correlation between the proposed angle and the three selected indices, especially with the Blackburne-Peel index. Interobserver and intraobserver reliability was high and compared favorably with that reported in the literature for the three classic ratios. The calculated ratios were in line with those described in the literature. The mean plateau-patella angle was 25°. Ninety percent of the measurements fell between 20° and 30°, and one standard deviation above and below was 21° to 29°.

The plateau-patella angle, a new method of evaluating patellar height, can be measured rapidly with use of either a goniometer or digital software. Its range is easy to remember, it is reliably reproduced, and it correlates well with traditional methods. It is thus a valid and easy alternative method for the evaluation of patellar height.