Shear wave elastography reveals elevated infrapatellar fat pad stiffness in patients with early osteoarthritis symptoms after ACL reconstruction

Abstract

Objective: The infrapatellar fat pad (IPFP) plays an important role in knee biomechanics and inflammation, particularly following anterior cruciate ligament reconstruction (ACLR). This study investigated whether IPFP stiffness, measured with shear wave elastography, is associated with early symptoms of osteoarthritis (OA) in individuals within one year after ACLR.

Design: In this cross-sectional study, 24 participants underwent bilateral IPFP stiffness assessments using shear wave elastography. Participants were positioned supine with 20° knee flexion. The stiffness limb symmetry index (LSI) was calculated to normalize stiffness between the ACLR and contralateral limbs. Early OA symptoms were defined as scores ≤85 % on at least two of four subscales of the Knee Injury and Osteoarthritis Outcome Score (KOOS). Independent t-tests were used to evaluate group differences in IPFP stiffness LSI, and receiver operating characteristic curve analysis determined the optimal LSI threshold for discriminating between groups.

Results: Eleven participants (46 %) showed early OA symptoms. Participants with early OA symptoms exhibited a significantly higher IPFP stiffness LSI compared to those without symptoms (49.2 ± 48.7 % vs. -17.3 ± 34.4 %, p < 0.001). An optimal stiffness LSI threshold of 7.1 % was identified, achieving 90.9 % sensitivity, 92.3 % specificity, and an area under the curve of 0.94.

Conclusions: Shear wave elastography shows potential as a non-invasive tool for detecting early IPFP stiffness changes associated with OA symptoms post-ACLR. These findings suggest that IPFP stiffness may be an early marker for OA risk, warranting further longitudinal studies to evaluate its progression and to further examine the clinical utility of shear wave elastography.

Keywords: ACLR; Elastography; IPFP; Rehabilitation; Ultrasonography.

Fig. 1.

Shear Wave Elastography Assessment of Infrapatellar Fat Pad (IPFP) Stiffness. A. Ultrasound probe placement: The examiner conducts a longitudinal scan of the anterior knee with the participant positioned supine and the knee flexed at approximately 20° This position provides a clear view of the middle portion of the patellar tendon and the underlying IPFP. B. Shear wave elastography image of the ACLR limb: A representative image illustrating increased IPFP stiffness in the ACLR limb. The standardized elastography box is positioned beneath the patellar tendon, with warmer colors (red/yellow) indicating areas of higher tissue stiffness. C. Shear wave elastography image of the contralateral limb: A corresponding image from the same participant demonstrating lower IPFP stiffness in the contralateral limb. Cooler colors (blue/green) within the elastography box reflect reduced tissue stiffness. The color scale on the left of images B and C represents tissue stiffness, ranging from 0 to 300+ kPa. These comparative images enable qualitative visualization of stiffness differences between the ACLR and contralateral limbs, highlighting the elevated IPFP stiffness in the ACLR limb of a participant with a high IPFP stiffness LSI.

Fig. 2.

Infrapatellar Fat Pad (IPFP) Stiffness Differences Between Participants With and Without Early OA Symptoms Post-ACLR. Patients with early OA symptoms following ACLR exhibited significantly greater IPFP stiffness relative to their contralateral limb compared to patients without symptoms. Early OA symptoms were defined using the symptom component of the Luyten early OA classification criteria (scoring ≤85 % on at least two of four KOOS subscales). IPFP stiffness limb symmetry index (LSI) was calculated as the percent difference between the stiffness of the ACLR and contralateral IPFP, normalized to the mean of the two limbs. Participants with early OA symptoms had significantly higher IPFP stiffness LSI (t = 3.91, p < 0.001) than asymptomatic participants, with a large effect size (Cohen’s d = 1.60; 95 % CI: 0.66–2.52). The dashed line represents the IPFP stiffness LSI threshold (7.1 %) that best classifies participants with early OA symptoms.

Fig. 3.

Receiver Operating Characteristic (ROC) Curve for IPFP Stiffness LSI Threshold Discriminating Participants With and Without Early OA Symptoms. The ROC curve illustrates the ability of the IPFP stiffness LSI to classify participants with and without symptoms suggestive of early OA. The analysis yielded a high AUC of 0.94, indicating excellent discriminatory performance. An IPFP stiffness LSI threshold of 7.1 % was identified as optimal, maximizing sensitivity (90.9 %) and specificity (92.3 %) for classifying participants based on the presence of early OA symptoms.