New HSS Study Identifies Knee Shape Linked to Higher ACL Injury Risk in Young Athletes

Researchers at Hospital for Special Surgery (HSS) used physics-based computer simulations to demonstrate that an individual's three-dimensional (3D) knee articular shape (encompassing bone, cartilage, and menisci) influences anterior cruciate ligament (ACL) loading, independent of the properties of the surrounding soft tissue, such as ligaments. The results provide new insights into why some people are more prone to ACL injuries and why females experience significantly higher risk than males.

The findings were published in Communications Medicine, a Nature Portfolio journal. At the heart of the study is a personalized, physics-based virtual model of the knee built from magnetic resonance imaging (MRI) of each individual. These computational models are key components of broader digital twin frameworks, which integrate models with patient data to simulate how the body functions and responds to different conditions or treatments. In sports medicine, this approach could eventually help identify athletes at greater risk of ACL injury and guide surgeons in identifying those who may benefit from additional procedures during ACL reconstruction.
"Our findings lay the groundwork for injury prevention and treatment based on how each person's knee shape drives movement and ACL loading," said Carl W. Imhauser, PhD, associate scientist at the Department of Biomechanics at HSS and senior author of the study. "These findings help the field move beyond looking at the biomechanical impact of one or two anatomical shape features toward understanding how the entire knee works together as a whole."

The researchers used MRI data from 168 young, active individuals (120 females, 48 males) to build digital twins of their knees. Half of those individuals had sustained acute ACL injuries and the other half, the control group, did not. Then, using computer models, identical forces were applied to each knee while holding ligament properties constant. By measuring the force carried by the ACL, the results showed that differences in the 3D shape of the knee alone produced:

• 20% greater ACL force in injured females compared to uninjured females;
• 26% greater ACL force in injured males compared to uninjured males;
• and 17% greater ACL force in females overall compared to males.

"Thanks to our findings, now we know that two athletes with the same technique and the same training can experience different loads on their ACL because of the joint they were born with, and that changes the conversation," said Andrew D. Pearle, MD, sports medicine surgeon at HSS and co-author of the study. "We're not yet at the point of handing an individual athlete a risk score, but we may someday be able to identify the at-risk knee. This work is the mechanistic foundation for getting there."
The study also identified different biomechanical pathways driving ACL loading between sexes. While internal tibial rotation (inward twisting of the tibia) contributed to increased ACL force in both males and females, anterior tibial translation (forward displacement of the tibia relative to the femur) additionally affected loading in females. This suggests females may experience greater ACL load through distinct mechanical mechanisms during dynamic movements. These findings obtained from computationally created individual digital twins may help explain the well-documented higher ACL injury rates among female athletes.

"A digital twin lets us test how a person's specific knee responds to load before an injury occurs," said Mark Amirtharaj, MD, sports medicine surgeon at HSS and co-author of the study. "We proved we can build and run these models at scale across a large cohort, and critically, that they align with direct measurements from cadaver knees. That validation gives clinicians confidence when translating simulation to practice."

Future work at HSS will focus on translating these findings into practical applications and move the field from one-size-fits-all ACL care toward truly individualized knee care.

"This research represents an exciting step toward bringing truly personalized care to patients coming to HSS with ACL injuries," said Danyal H. Nawabi, MD, FRCS, sports medicine surgeon at HSS and co-author of the study. "As surgeons, we envision a future where we can use a patient's MRI to create a digital model of their knee, identify their individual risk factors, and even simulate different surgical procedures to determine which approach might offer the best outcome."

"Dr. Imhauser and the entire research team have tackled a complex biomechanical question with remarkable rigor, and the results speak for themselves," said Riley J. Williams III, MD, Chief of the Sports Medicine Institute at HSS. "By isolating anatomy as a driver of ACL loading, they've opened doors to precision prevention strategies we couldn't imagine before. HSS is committed to funding and fostering this caliber of translational research."

This work was part of a longstanding multidisciplinary and multi-institutional team effort bringing together the Sports Medicine Institute and Department of Biomechanics at HSS with close collaborators at the University of Vermont, led by Prof. Bruce Beynnon, director of Orthopaedic Research at Robert Larner M.D. College of Medicine.