In Search of a New MRI Biomarker of Post-Traumatic Osteoarthritis  


By Pawel Slabiak • June 14, 2023

Center for Advanced Imaging Innovation and Research

PTOA affects millions of people in the U.S. and accounts for billions of dollars in annual healthcare costs. But scientists don't fully understand how PTOA arises.

Post-traumatic osteoarthritis (PTOA) is a degenerative joint disease that can develop after an injury, such as a fracture or an acute sprain.

Amparo Ruiz, PhD, assistant professor of radiology at NYU Langone Health, searches for potential imaging biomarkers of PTOA. 

In recent studies, Dr. Ruiz and colleagues focused on a peptide called P15-1, previously shown to have a protective effect on inflamed cartilage. 

The peptide mimics aspects of a substance called hyaluronan, which is important to the health of articular cartilage but when fragmented tends to promote inflammation.

Histology slide of a rat knee.

Scientists speculate that P15-1 may be blocking hyaluronan fragments from binding to cartilage cells, but the exact mechanism isn't known. 

Cartilage cells, called chondrocytes, seen ex vivo through fluorescence microscopy. Each cell is approximately 8-10 microns in diameter, about the size of a droplet of fog.

In a recent paper, Dr. Ruiz's team reported "the first in vivo evidence that hyaluronan-related inflammatory response in cartilage after injury is a common finding."

An in vivo MRI showing P15-1 accumulation in knee cartilage of a rat with an ACL injury. 



The main question the team had asked was whether imaging the P15-1 peptide could provide a marker of PTOA.

In vitro fluorescence microscopy of chondrocytes viewed on a lab computer. 

To figure this out, the researchers conducted in vitro, in vivo, and ex vivo experiments with seven types of imaging.

The imaging modalities the team used included:  • confocal fluorescence microscopy (in vitro) • fluorescence microscopy (in vitro) • optical imaging (in vivo) • ultrasound (in vivo) • MRI (in vivo) • fluorescence microscopy (ex vivo) • electron microscopy (ex vivo)

First, to determine whether the P15-1 peptide would allow them to distinguish normal and distressed cartilage cells, called chondrocites, the scientists added P15-1 to chondrocite cell cultures. 

Some cultures had a neutral environment; others were placed in proinflammatory conditions that mimic injury.

Fluorescence microscopy showed that P15-1 accumulated in the cultures with the inflammatory media.

Chondrocyte  in normal conditions

Chondrocyte  in inflammatory media

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Next, to find out whether they could image the accumulation of P15-1 peptide in vivo, the researchers studied rat knee joints with and without mechanically induced PTOA. 

All protocols involving the use of animals were approved by the NYU Grossman School of Medicine Institutional Animal Care and Use Committee.

Ultrasound transducer

Syringe with P15-1

Rat under general anesthesia

In order to reliably deliver P15-1 to the intended location in the knee, scientists used ultrasound to guide peptide injections.



Injection site

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Initially, the research team had relied on anatomical landmarks for aim but only half the injections succeeded. With ultrasound guidance, the success rate exceeded 90 percent.

Optical imaging—which is shallow but fast—showed greater accumulation of P15-1 in the injured knees.

Lower intensity in the control knee indicates that healthy cartilage is retaining less of the P15-1 peptide. 

Cartilage in the knee with an ACL rupture shows high retention of P15-1 days after injection. 

MRI shows a post-injection change in a metric known as T1 relaxation in the cartilage of the knee with an ACL rupture. No such change is observed in the control knee. The conclusion is that P15-1 accumulates in the injured joint and that MRI can be used to image this process.





Next, MRI scans—slower but more detailed—also showed differences between injured and control knees.

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To confirm further, the team excised the knees and prepped them for ex vivo imaging by freezing and slicing tissue onto very thin tape.

The method, called cryosectioning, preserves the integrity of the P15-1 contrast agent, which would be lost in traditional histology.

Each cryosection is 10 microns thin. 

To peer into these samples, the scientists used fluorescence microscopy... 

A section of a healthy control knee seen under a fluorescence microscope with tenfold magnification.

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...confirming that the P15-1 peptide accumulated in injured cartilage.

A section of a knee with a ruptured ACL, showing damaged cartilage and white areas with accumulated P15-1.

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At 40-fold magnification, chondrocytes in a knee with an ACL rupture appear bright with P15-1.

An even closer look shows P15-1 retention in chondrocytes.

Another method called electron microscopy (EM) also revealed P15-1 accumulation.

EM and fluorescent microscopy detect different contrast agents, allowing researchers to cross-validate findings.

White arrows point to chondrocytes in the cartilage and the area where cartilage meets subchondral bone, highlighting bright spots with concentrated P15-1.

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Ultimately, "we want to do studies in humans to see that there's really accumulation" of P15-1 during joint injury.

And "we want to use animal studies to understand how to target that process."

Layers of chondrocytes seen through a fluorescence microscope as its focus travels from the surface of cartilage to 605 microns down into the tissue.

"Inflammation is not easy to target with MRI," said Dr. Ruiz.

"Especially at the stage when you can still prevent what unresolved inflammation is going to cause long-term."

Ex vivo fluorescent microscopy (top) and in vivo MRI of P15-1 retention in knee cartilage.

"That's the goal. That's the dream," said Dr. Ruiz.

"It's more complicated than it sounds."