*3.3.4 Assessment of patellar height*

The Insall-Salvati [32], modified Insall-Salvati [33], Blackburne-Peel [34], and Caton-Deschamps [35] ratios are commonly used to measure the patellar height. A detailed description of these ratios has been published in standard orthopedic textbooks. The Insall-Salvati [32] index is based on the ratio of the length of the patellar tendon divided by the greatest length of the patella. The normal ratio defined by the authors is 1.0. A ratio of >1.2 indicates patella alta whereas, a ratio of <0.8 denotes patella baja. However, difficulty in determining the exact insertion site of the patellar tendon and abnormal morphology (such as elongated inferior pole of the patella) of the non-articular portion of the patella may falsely alter this ratio. Furthermore, the patellar tendon length varies between sexes in the normal population [59]. In order to eliminate these variables, Grelsamer and Meadows [33] proposed a modified Insall-Salvati ratio. This modified ratio is defined as the distance from the inferior point of the articular surface of the patella to the patellar tendon insertion into the tibial tuberosity, divided by the length of the articular surface of the patella. Using this method, patella alta is defined as a ratio greater

#### *Patellofemoral Instability DOI: http://dx.doi.org/10.5772/intechopen.99562*

than 2.0, a point at which only 3% of controls would be falsely identified as patella alta [33]. Blackburne and Peel [34] reported a ratio that is independent of the length of the patellar tendon. Their index is defined as the ratio of the length of the perpendicular line from the lower end of the articular surface of the patella to the tibial plateau line, divided by the length of the articular surface of the patella. Based on their study, a ratio of 0.8 is considered normal, a ratio of >1.0 indicates patella alta, whereas a ratio of <0.5 denotes patella baja [34]. Caton and Deschamps [35] have also described a ratio to address the difficulty in measuring the length of the patellar tendon. Their ratio is defined as the ratio of the distance from the inferior articular surface of the patella to the anterosuperior border of the tibia, divided by the length of the articular surface of the patella. Based on the Caton-Deschamp index, a ratio of >1.2 indicates patella alta and < 0.6 indicates patella baja. Seil et al. [60] recommended the Blackburne and Peel ratio to measure the patellar height because it showed the most intermediate classification results and the lowest interobserver variability.
