This project aims to develop an [18F] fluorodeoxyglucose (FDG) positron emission tomography (PET) / magnetic resonance imaging (MRI) method to locate the painful inflammation in PFJ OA associated with joint loading. [18F] FDG PET/MRI is an emerging pain imaging approach with enhanced sensitivity to painful hypermetabolic inflammation through evaluation of intracellular glucose utilization rate via ([18F]FDG PET) and fine anatomy details (MRI). The investigative group has shown its promise in revealing previously unidentified or unspecified pain generators in various musculoskeletal pain conditions. The investigators have also demonstrated the feasibility of visualizing the structural changes between unloaded and loaded knee joints with MRI, which can be easily adopted in the current PET/MRI setting. The main challenge in the proposed [18F]FDG PET/MRI approach is to differentiate the normal uptake of FDG for metabolic changes by weight-bearing from abnormal changes indicating eventual pain aggravation by weight-bearing and knee-flexion. The investigators have garnered the following two aims to validate the proposed method by comparing unloaded and loaded knee imaging results between PFJ OA pain patients and asymptomatic, matched controls.

Background:

Imaging of pain from patellofemoral joint osteoarthritis (PFJ OA): PFJ OA is primarily defined as the degeneration of the cartilage of the PFJ; however, the cartilage itself is not innervated and thus cannot be a source of pain. The knee joint is composed of a variety of other innervated tissues, including the synovium, subchondral bone, meniscus, ligaments, tendons, muscles, and infrapatellar fat pad. The degeneration of the patella cartilage may result from or even result in some form of damage to these innervated joint tissues, which eventually leads to the sensation of pain. The main challenge in imaging identification of pain generators in PFJ OA is that structural abnormalities that can cause pain also frequently occur in the asymptomatic population as a natural wear-and-tear process with aging. For example, meniscal tears on MRI are nearly equally prevalent (~60%) in symptomatic and asymptomatic knees. The proposed approach here is to adopt [18F]FDG PET/MRI, a molecular imaging technique highly sensitive to hypermetabolic inflammation. Pain in PFJ OA may stem from inflammatory responses to tissue damage, leading to the increased activities of immune cells causing the local surge of need for energy. [18F]FDG PET is the most widely accepted imaging modality for identifying pathology by visualizing hotspots of energy demand and has recently shown strong promise in detecting painful hypermetabolic inflammation in multiple musculoskeletal pain conditions. Therefore, the proposed implementation of [18F]FDG PET/MRI for PFJ OA will facilitate pinpointing hypermetabolic pain generators that conventional structural imaging techniques have failed to identify.

Loading of PFJ OA: The magnitude and distribution of loads across a joint have the potential to accelerate structural OA decline and result in pain. While biomechanical factors are known to influence the progression of TFJ OA, little is known about the role of biomechanical factors in PFJOA disease progression. These factors are likely to differ from those for TFJ OA progression as a result of the unique functions and design of the different joints. The investigative team has extensive experience in the field of biomechanics and OA. Elevated PFJ stress during squatting was demonstrated in people with patellofemoral pain, a condition affecting younger adults with similar features to PFJ OA. Preliminary work by the investigators confirmed higher PFJ stress during walking in individuals with PFJ OA compared to controls. These data suggest that elevated PFJ stress is likely a contributory factor in PFJ OA. Increased knee extensor moments can result in higher PFJ reaction forces and PFJ stress at a given knee angle and thus may be a potential risk factor for PFJ OA progression. Preliminary work by the investigators showed that higher knee extensor moment during late stance was predictive of structural progression of PFJ OA at one-year follow-up. The current supplement will support further investigation in single-leg squat biomechanics in persons with painful PFJ OA and asymptomatic matched controls. These data are necessary to understand the impact of loading and squat kinematics on pain, leading to novel movement-based interventions for pain management.

AIMS:

Aim 1: Identification of painful inflammation in the unloaded knee with PFJ OA The investigators will 1) compare [18F]FDG uptake measurements of the unloaded knee between symptomatic PFJ OA knees and asymptomatic knees and 2) analyze the correlation between [18F]FDG and biomechanical measurements. The investigators will recruit 25 PFJ OA patients with pain reproducible with a weight-bearing exercise (single-leg squats) and 25 age/sex/BMI-matched asymptomatic controls. The [18F]FDG PET/MRI session of the unloaded knee will yield the max measurements of [18F]FDG standardized uptake value (SUV) in the local hotspots. All subjects will have a post-imaging functional assessment through a battery of physical performance tests for biomechanical measurements. The investigators will compare the [18F]FDG uptake measurements between patients and controls using the t-test and conduct a generalized linear model (GLM) analysis to determine the biomechanical measurements predicting the [18F]FDG uptake measurements.

Aim 2: Identification of painful inflammation in the loaded knee with PFJ OA The investigators will identify abnormal [18F]FDG hotspots in the PFJ of the patients due to knee loading by comparison with asymptomatic controls. After the [18F]FDG PET/MRI session in Aim 1, the participant will be re-scanned using the same PET/MRI protocol while loading the knee continuously with 25% bodyweight using a custom-made device. The investigators will measure the difference of the [18F]FDG SUVmax between the unloaded and loaded sessions in the synovium, femur, tibia, meniscus, fat pad, and muscle around the PFJ. [18F]FDG uptake difference will be compared between PFJ OA patients and asymptomatic controls using the t-test and GLM analysis to determine biomechanical measurements prediction of [18F]FDG uptake difference.

The successful execution of the proposed aims will offer a novel, non-invasive imaging method to identify the painful inflammation in PFJ OA as well as increased inflammatory response due to joint loading, which will eventually aggravate the pain. Together with the ongoing development in the parent R01 project to visualize bone remodeling and cartilage compositional changes, this project will facilitate building a comprehensive imaging method to examine the underlying degenerative process and the symptomatic manifestation of PFJ OA.

Inclusion Criteria:

• Adult with the capacity to give informed consent
• Ability to perform 25 single-leg squats
• No traumatic knee injuries/surgeries since last visit for the parent study (IRB #21-34763)
• No investigational drugs since last visit for the parent study (IRB #21-34763)
• No contraindications to MRI or PET tracer since last visit for the parent study (IRB #21-34763)
• Not taking steroid injections
• Not pregnant
• Additional inclusion criteria for healthy volunteers: Minimal pain (VAS=0-1) during a series of 25 single-leg squats on one or both legs

Exclusion Criteria:

• Inability to consent for themselves
• Inability to perform 25 single-leg squats with the study leg
• New traumatic knee injuries/surgeries since last visit for the parent study (IRB #21-34763)
• Taking investigational drugs since last visit for the parent study (IRB #21-34763)
• New contraindications to MRI or PET tracer since last visit for the parent study (IRB #21-34763)
• Taking steroid injections
• Pregnant
• Diabetic
• Additional exclusion criteria for healthy volunteers: VAS of 2 or greater during a series of 25 single-leg squats on both legs

The study population includes outpatient individuals with patellofemoral joint osteoarthritis (PFJ OA) and healthy asymptomatic volunteers recruited from the University of California, San Francisco (UCSF) and the surrounding community