The SP-ET index is a new index for assessing the vertical position of patella

Abstract

Background Some parameters in previous studies did not better reflect the vertical position of the patella relative to the femoral trochlear. This study aimed to assess the value of the most superior point of patella-entrance of femoral trochlea distance ratio (SP-ET index) as a newer index in defining the vertical position of patella relative to the trochlea, correlate it with the Insall–Salvati ratio, and investigate the effect of the new index on patellar cartilage lesions. Methods A total of 99 knees of 77 patients with patellar cartilage lesions were retrospectively analyzed using magnetic resonance imaging (MRI) data. The Insall–Salvati ratio and SP-ET index were measured on MR images. Ninety-nine knees just with meniscus rupture were enrolled as the control group. The two parameters of the patellar cartilage lesions were compared with those of the control group. Results The Insall–Salvati ratio and SP-ET index in the patellar cartilage lesions group were significantly higher than those in the control group (p < 0.001). The SP-ET index showed a moderate positive correlation with the Insall–Salvati ratio (r = 0.307, p < 0.001). Receiver operating characteristic (ROC) analysis showed that the diagnostic efficiency of the SP-ET index was better than that of the Insall–Salvati ratio in patients with patellar cartilage lesions. Conclusion The SP-ET index may be a useful complement parameter to define the vertical position of the patella relative to the femoral trochlear. Increased SP-ET index may be an important risk factor for patellar cartilage lesions.

Association between the most superior point of patella-entrance of femoral trochlea distance ratio (SP-ET index) and chondromalacia patella: an investigation via magnetic resonance imaging

“…This method has some similarities with SP-ET index; however, only one plane (the sagittal cross-sectional image with the greatest craniocaudal diameter of the patella) is required to perform the measurement, and this situation minimises the effect of axial patellar displacement. Therefore, this measurement technique was thought to have good applicability with the expectation of higher interobserver agreement in practice [ 12 ].…”

“…A recent study by Li et al . [ 12 ] presented the ratio of the most superior edge of patella-entrance of femoral trochlea interval to the length of the articular surface of the patella (SP-ET) on sagittal MRI as a new index to define the vertical location of the patella relative to the trochlea and studied this new index in relation to CP. This current study aimed to reveal the effectivity of this new index to indicate the CP and present the interobserver difference to depict the usability of this measurement technique.…”

Supratrochlear Rim Is Correlated With Isolated Patellar Chondromalacia on Magnetic Resonance Imaging of the Knee

“…Entrance to Trochlea has been quantified as a line parallel to PH defining the distance from the top of the patella to the first axial plane with the full width of the femoral cartilage, and was used to calculate the recently proposed SP-ET index ( Fig 3 b). 35 Ridge height is a measurement introduced in the 70s 38 and is quantified as the distance from the anterior femoral bone line to the furthest point on the anterior femoral cartilage ( Fig 3 b). In the same plane, the knee flexion angle has been quantified and further used to estimate PTI at a straight knee according to the formula cPTI = PTI – 1.3 a presented Ahmed et al., 39 where cPTI is PTI at 0° knee flexion and a is the measured knee flexion angle.…”

“…Instead, anatomical features other than anterior femoral shape were extensively studied as a cause of isolated patellofemoral pain. These metrics included lateral trochlear inclination (LTI), 24 , 25 , 26 , 27 , 28 , 29 , 30 trochlear angle, 30 patella type, 24 , 31 , 32 , 33 sulcus angle (SA), 26 , 30 , 32 , 34 patellar tilt, 16 , 25 , 34 Insall-Salvati (IS) index, 34 , 35 trochlear depth, 25 , 27 , 36 patellotrochlear index (PTI), 29 , 32 distance ratio between the distance from the most superior point of patella and entrance of femoral trochlea and patellar articular surface length (SP-ET index), 35 and congruence angle. 16 , 32 Despite extensive efforts to identify predictive anatomical features, the only anatomical feature that has shown a strong correlation with isolated patellofemoral pain is a high-lying patella, also known as patella alta .…”

“… 16 , 32 Despite extensive efforts to identify predictive anatomical features, the only anatomical feature that has shown a strong correlation with isolated patellofemoral pain is a high-lying patella, also known as patella alta . 29 , 32 , 34 , 35 There is a paucity of material available to associate abnormal tracking and alignment of the patella to isolated patellofemoral pain.…”