## 4 Discussion  
The patellar height measurement has been shown to be a fundamental clinical tool for surgical planning in patients affected by recurrent patellar instability. Despite several methods being available, a gold standard is still lacking [17; 18; 19]. Specifically, these methods can either relate the position of the patella to the femur (direct assessment) or the position of the patella to the tibia (indirect assessment) [18]. Methods that use an indirect assessment, such as the Insall-Salvati (IS), modified Insall-Salvati (MIS), Blackburne-Peel (BP) ratio and CDI, are the most widely employed [18]. The IS consists of a ratio between the patellar tendon length and the non-articulating patellar surface length [1; 18]. Due to the variability of patellar length, the clinical interpretation of the IS index can be negatively affected [1]. For that reason, [PERSON] described the MIS, which considers the patellar articular surface length rather than the patellar length [18]. However, both for the IS and MIS, the exact location of the distal patellar tendon insertion can be challenging, and it requires perfect lateral radiographs views [1]. [PERSON] and [PERSON] proposed a method consisting of the ratio between the patellar articular length and the articular cartilage lower pole height above the tibial plateau, measured on a 30-degree flexed knee lateral radiograph [9; 20]. This last method has shown to be a valid tool for patellar height evaluation when applied to a total knee arthroplasty, but it does not precisely define the placement of the tibial plateau line in the case of a non-prosthetic knee [1; 20]. The CDI consists of a ratio between the patellar articular surface length and its distance from the tibial plateau anterior border [1]. Generally, it is measured on a lateral radiograph with the knee flexed at 30 degrees, although for the original study by [PERSON], the knee flexion ranged from 20 to 80 degrees, suggesting how the CDI is poorly influenced by the degree of knee flexion in spite of the other indices [1; 11]. Moreover, it can also be performed in children, but age correction should be applied because the patellar ossification begins at its proximal side [21]. Hence, some authors believe that the CDI may represent the best method to measure the patellar height in pre-surgical planning [1; 21]. Although it has been hypothesized that the CDI could be reliable and easily reproducible, this has not been precisely clarified yet [22]. [PERSON] et al. [23] stated that intra-observer reliability of the CDI was higher than the BP and IS. However, they suggested that intra-observer reliability of a measurement method can be likely influenced by the rater's experience, showing how the ICCs of the measurements performed by the medical student were mostly lower when compared to the orthopedic resident, the orthopedic surgeon and the radiologist. In contrast, a study conducted by [PERSON] et al. [19] concluded that the IS ratio shows better intra- and inter-observer reliability than the CDI, BP and MIS ratio, measured on conventional radiographs. [PERSON] et al. [24] evaluated the reliability and inter-observer variability of the IS, MIS, BP, CDI and Labelle-Laurin index, measured by two examiners using standard knee radiographs. A low inter-observer variability with high correlation coefficients and low mean inter-observer errors were found. Furthermore, they recommended the BP as the most reliable method among the ones examined. However, a more recent study conducted by [PERSON] \begin{table}
\begin{tabular}{c c c c} \hline  & **RA** & **IR** & \(p\)**-Value** \\ \hline Seniors & 0.931 & 0.819 & \textless{}0.001 \\ \hline Juniors & 0.888 & 0.725 & \textless{}0.001 \\ \hline \end{tabular}
\end{table} 
Table 4: Seniors and juniors interclass correlation coefficient (ICC) for raters’ average (RA) and individual raters (IR). et al. [25] stated that the BP could be affected by the tibial slope, leading to an improper assessment of the patellar height.