A study on the diagnostic value of knee MRI parameters for lateral patellar compression syndrome: clinical application of P-PTA, LP and ISR

Abstract

Background: Lateral patellar compression syndrome (LPCS) is characterized by increased lateral patellofemoral joint pressure due to chronic lateral patellar tilt, tightened lateral retinaculum, and imbalanced stress between the lateral and medial femoral condyles. However, there is currently no well-established or widely accepted diagnostic standard for LPCS. This study aimed to explore the feasibility of various structural measurement parameters of magnetic resonance imaging (MRI) of the knee to diagnose LPCS and to identify new MRI diagnostic indicators as references and guidance for LPCS clinical diagnosis.

Methods: This study enrolled 168 patients, who were divided into three groups: the LPCS group, the knee osteoarthritis (KOA) group, and the structurally normal group (n=56 participants per group). A standardized magnetic resonance scanning protocol was used, including sagittal and coronal fat-suppressed proton density-weighted imaging and sagittal T1-weighted imaging. Two radiologists analyzed the MRI and measured the patellar-patellar tibial angle (P-PTA), the quadriceps-patellar angle (Q-PA), the length of patellar (LP), the length of patellar tendon (LT), the LP/LT ratio, the Insall-Salvati ratio (ISR).

Results: The LPCS group had significantly lower P-PTA and LP values, but higher LT and ISR values, compared with those in the normal and KOA groups (all P<0.05). Compared with those in the structurally normal group, the LPCS groups' Q-PA value was higher (P=0.034). According to receiver operating characteristic analysis, the optimal cut-off values for P-PTA, LP, LP/LT, and ISR were 146.45°, 41.10 mm, 0.85, and 1.19, with sensitivities and specificities of 67.86%/59.82%, 78.57%/55.36%, 67.86%/58.93%, and 66.07%/60.71%, respectively.

Conclusions: Measurement parameters of MRI, particularly P-PTA, LP and ISR, can serve as important tools to assist in the diagnosis of LPCS. Assessment of these parameters should be included in the clinical diagnostic process for LPCS to improve diagnostic accuracy.

Keywords: Lateral patellar compression syndrome (LPCS); diagnosis; knee joint; magnetic resonance imaging (MRI); patella tilt.

Figure 1

X-ray (A) and MR (B) images of a patient with LPCS. X-ray and MR images showed that the patella tilted laterally. MR images showed thickening and tightening of the lateral patellar retinaculum, and cartilage injury in the lateral space of the patellofemoral joint. LPCS, lateral patellar compression syndrome; MR, magnetic resonance.

Figure 2

Measurement methods of LP, LT and P-PTA of knee joint. L1 is the LP, that is the distance between the upper pole and the lower pole of the patella. L2 is the LT, that is the connection through the lower pole of the patella and the tibial tubercle. The angle between L1 and L2 is P-PTA. LP, patellar length; LT, patellar tendon length; P-PTA, patellar-patellar tibial angle.

Figure 3

Q-PA measurement method of knee joint. L3 is a tangent along the inner edge of the quadriceps tendon, and L4 is a tangent along the upper edge of the patella. The angle between L3 and L4 is Q-PA. Q-PA, quadriceps-patellar angle.

Figure 4

ROC curves of P-PTA, LP, LP/LT and ISR for differentiating LPCS from KOA and structurally normal groups. (A) ROC curves of P-PTA for differentiating LPCS from KOA and structurally normal groups. (B) ROC curves of LP for differentiating LPCS from KOA and structurally normal groups. (C) ROC curves of LP/LT for differentiating LPCS from KOA and structurally normal groups. (D) ROC curves of ISR for differentiating LPCS from KOA and structurally normal groups. ISR, Insall-Salvati ratio; KOA, knee osteoarthritis; LP, patellar length; LPCS, lateral patellar compression syndrome; LT, patellar tendon length; P-PTA, patellar-patellar tibial angle; ROC, receiver operating characteristic.

Figure 5

ROC curves of P-PTA, LP, LP/LT, ISR and Q-PA for differentiating LPCS from structurally normal group. (A) ROC curves of P-PTA for differentiating LPCS from structurally normal group. (B) ROC curves of LP for differentiating LPCS from structurally normal group. (C) ROC curves of LP/LT for differentiating LPCS from structurally normal group. (D) ROC curves of ISR for differentiating LPCS from structurally normal group. (E) ROC curves of Q-PA for differentiating LPCS from structurally normal group. ISR, Insall-Salvati ratio; LP, patellar length; LPCS, lateral patellar compression syndrome; LT, patellar tendon length; P-PTA, patellar-patellar tibial angle; Q-PA, quadriceps-patellar angle; ROC, receiver operating characteristic.