All across the nation’s soccer fields, basketball courts, and myriad athletic arenas, it starts with a pop. Next comes a limp and a trip to an orthopedic surgeon. Injuries of the anterior cruciate ligament (ACL) comprise more than half of serious knee injuries in the United States. The ACL, which connects the femur with the tibia, helps us launch, land, and change direction. Surgery and physical therapy can have people back in play within a year, but the likelihood of retaining mobility in the long run is a coin toss. The injury puts more than half of patients on a slow, stealth course toward a debilitating degenerative joint condition called post-traumatic osteoarthritis (PTOA) and there is currently no way of telling who will have it.
Innovative interdisciplinary research now underway at NYU Langone Health aims at identifying biomarkers of PTOA at the time of ACL surgery—usually within three months of injury—by combining novel magnetic resonance imaging (MRI) methods with genomic approaches inspired by cancer research. The ability to predict who is at high risk has the potential to delay disease progression and accelerate the search for effective therapies. The project is supported by a five-year, $3.4 million award from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) and led by two investigators: Thorsten Kirsch, PhD, professor of orthopedic surgery and cell biology at NYU Grossman School of Medicine and professor of biomedical engineering at NYU Tandon School of Engineering; and Amparo Ruiz, PhD, assistant professor of radiology at NYU Grossman School of Medicine and scientist at the Center for Advanced Imaging Innovation and Research and the Tech4Health Institute.
“The big problem now is that more young people get injured,” said Dr. Ruiz. Numerous peer-reviewed studies have shown a steady increase in the incidence of ACL injuries among adolescents and young adults, with women found to be more vulnerable than men. The trend holds across the athletic spectrum, from recreational pursuits to high school sports to professional leagues. Researchers don’t know the reason but suspect that a growing popularity of year-round sports and insufficient or incorrect attention to injury prevention are contributing factors. In recognition of this trend, last year the Hospital for Special Surgery and the Aspen Institute launched a National ACL Injury Coalition, aiming to halve the number of injuries among high school athletes. Meanwhile, for those who rupture their ACL, “you do the surgery, you do post-surgery treatment, however long that takes, and that is that,” said Dr. Ruiz. But the story doesn’t end there.
A 2023 systematic review and meta-analysis of scientific studies spanning almost six decades’ worth of data found that within 10 years of an ACL tear, a fifth of the patients developed PTOA; within 15 years, more than a third did; within 20 years, more than half. The outcome did not vary by type of treatment and wasn’t better for those injured more recently, indicating that the progress made in surgical techniques since the 1960’s has had little bearing on long-term prognosis.
“If you rupture your ACL at 18 and within twenty years you get post-traumatic osteoarthritis, at 38 years old you have so much pain you want a knee replacement,” said Dr. Kirsch, referring to the only available treatment for advanced PTOA. “That’s a problem because the prosthesis lasts maybe twenty years,” after which the patient needs a replacement through what is known as revision surgery, a more complex operation to put in a more extensive implant that doesn’t last as long as the original. “Even if they do [a revision] at 58, in another twenty years, at 78, you probably need a third one, which would be even more complicated.”
“We cannot predict the patients who are at risk of developing this early osteoarthritis,” said Dr. Kirsch. “In breast cancer, you can go [get screened] every year and detect early. We don’t have this.”
Although there is no cure for PTOA, advance knowledge could empower doctors and patients to delay the condition’s onset and slow down its progression through a variety of lifestyle changes, including nutrition, exercise, and activity choices. “All these things have a really big impact,” said Dr. Ruiz. “They’re not going to stop you from developing OA, but you can get it in your sixties instead of your forties. You just have to be around somebody who has osteoarthritis—it’s daunting.”
Novel Imaging Brings Early Changes into Focus
To find early markers of PTOA, Dr. Kirsch and Dr. Ruiz are using a novel type of diffusion tensor imaging developed at NYU Langone.
Diffusion tensor imaging (DTI), an MRI method that probes the random motion of water molecules, is best known for its use in the study of the brain’s white matter, where water tracks tightly packed nerve fibers. Articular tissue has been known as a candidate for DTI applications since the mid 2000s. Water makes up about 70 percent of cartilage, where its motion is heavily restricted by a rigid matrix composed of collagen and proteoglycan. By the early 2010s, ex vivo studies established DTI’s sensitivity to articular tissue’s collagen architecture and proteoglycan content, suggesting promise for early detection of osteoarthritis. But bringing lab findings to clinical research still required two major pieces: the development of high-resolution DTI suitable for in vivo scans; and validation persuasively linking in vivo imaging to disease.
Both have emerged from investigations at NYU Langone and its Center for Advanced Imaging Innovation and Research, where a team led by José Raya, PhD, embarked in the early 2010s on a decade-long program of systematic study, development, and validation of DTI for osteoarthritis. In the early 2020s, work by Dr. Raya, Dr. Ruiz, and colleagues, found that a custom DTI sequence designed for in vivo knee imaging predicted disease progression confirmed three years later on X-rays—the clinical standard for evaluating OA—with 79 percent accuracy. (The research, supported by a separate NIAMS award, was presented at the 2022 meeting of the International Society for Magnetic Resonance in Medicine; the authors are working on a manuscript to report full findings in a peer-reviewed journal.)
The DTI technique, called RAISED, is “really good at predicting early progression of osteoarthritis … before it’s really a disease,” said Dr. Ruiz. “Clinical MRI fails to do that.”
But even advanced DTI cannot provide answers at the time of ACL surgery, nor can it explain why some patients deteriorate while others don’t. To figure this out, the investigators are taking a close look at the biology of synovial fluid, a viscuous substance that lubricates the knee joint.
Circulating Biomarkers Come to PTOA Research
“Every cell in the joint releases small vesicles in the synovial fluid,” said Dr. Kirsch. Extracellular vesicles are tiny lipid capsules carrying microRNA and a variety of other biological content capable of prompting the cells that absorb it to alter their behavior. The vesicles “are like a cell communicator,” said Dr. Kirsch. “Like sending out mailmen to tell other cells what to do.”
Genomic sequencing of extracellular vesicles from synovial fluid obtained soon after an ACL rupture and at the time of surgery will give the investigators a glimpse into the turbulent environment of an injured knee. “You have so many cells in an inflamed joint,” said Dr. Kirsch. “That means the vesicles, if we analyze those, give us a picture of what’s really changing.”
In cancer research, sifting through tiny bits of biologic material that float through the body has led to the identification of “circulating biomarkers” that can inform diagnosis and treatment. Drs. Kirsch and Ruiz are looking for similar success in a musculoskeletal disease that has so far defied biomarker research. Because osteoarthritis is accompanied by simmering inflammation, studies have often targeted proinflammatory cytokines, “but none of them have turned out to be good biomarkers,” said Dr. Kirsch. This investigation is different in that no material is preselected based on its known properties. Instead, the research team expects to sequence thousands of substances circulating in an injured knee.
On its own, the genomic data do not indicate whether the joint will heal or begin to deteriorate in the coming decade, but the combination of sequencing and advanced imaging lets the scientists search for a link between vesicle cargo and early cartilage changes picked up by DTI. “That’s what will give us the key,” said Dr. Ruiz, who also leads a separate investigation into an MRI contrast agent for PTOA. “I think that’s the beauty of this project, that it wouldn’t be possible without the collaboration.”
A Predictive Panel for PTOA
The study protocol piggybacks on the current standard of care in which patients with an acute ACL injury have their synovial fluid drawn in order to relieve pressure on the swollen knee and get a clinical MRI exam for radiological evaluation prior to surgery.
Instead of disposing of the fluid, the investigators keep it for analysis—turning what would otherwise be a waste product of medicine into a resource for scientific discovery. The DTI sequence is appended to the standard clinical MRI, extending the scan by about 20 minutes. The only other extras are the drawing of synovial fluid at the time of surgery (i.e. in the post-acute phase), when patients are under anesthesia, and imaging follow-ups with both clinical MRI and DTI in later years.
The investigation also makes use of NYU Langone’s Joint Preservation Registry of the Knee, a bank of synovial fluid samples founded in 2017 by Dr. Kirsch and Eric Strauss, MD, professor of orthopedic surgery, with the goal of enabling researchers to ask new questions about osteoarthritis. By design, Dr. Kirsch and Dr. Ruiz are capitalizing on the 900-patient registry to include scores of samples collected from people aged 18-40 who got an ACL injury up to three years prior to the official start date of the NIAMS-funded PTOA project, which launched in August 2023. The previously banked samples have allowed the scientists to create a long-term cohort slated to receive imaging up to eight years after injury, nearly doubling the investigative span typically feasible on a five-year research grant. Meanwhile, newly enrolled participants form a short-term cohort tracked for up to five years.
“If it’s successful, the outcome is really I think huge, because we would have a real diagnostic,” said Dr. Kirsch. Through correlation with DTI follow-ups, the researchers expect to winnow thousands of microRNA chains down to a few markers for PTOA progression, creating a simple panel that doctors could run for any patient undergoing ACL surgery. If the study succeeds, how soon afterward could the predictive PTOA panel be brought to clinical practice? “Right away,” said Dr. Kirsch, because unlike medical devices, procedures, and drugs, lab tests do not currently require an FDA approval.
Beyond empowering patients to make choices that may delay the onset of PTOA, the ability to identify who is at risk has the potential to quicken and simplify pharmaceutical research on a disease whose slow, uncertain onset has long frustrated clinical trial managers. “Industry cannot wait for 10-15 years to try out a drug—that’s impossible,” said Dr. Kirsch. “If we provide them with something where in two, three years you know the answer, that would be a huge step forward.”
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A team of scientists is using a battery of imaging methods to visualize cells, tissues, and joints in a quest for early noninvasive imaging biomarkers of PTOA.