How an Enlarged Nerve Predicts Career-Threatening Elbow Injuries in Professional Pitchers

The career of a professional baseball player can hinge on the health of their throwing elbow. For decades, the primary focus of injury prevention and diagnosis has been on ligament damage, particularly to the well-known ulnar collateral ligament (UCL), the subject of the famous "Tommy John" surgery.

However, a one-year prospective cohort study of 56 professional Japanese baseball players suggests we may have been overlooking more powerful predictors of injury. For context, these athletes were all asymptomatic at the time of their initial screening. This research looks beyond the UCL to analyze the function of key stabilizing muscles and, most surprisingly, the physical size of a critical nerve.

This article will break down the study's most impactful findings, revealing surprising new ways to identify players at high risk for career-altering elbow injuries, before an athlete starts complaining of elbow pain. the pain even starts.

An Enlarged Ulnar Nerve

In this study, researchers used non-invasive MSK ultrasound to measure the cross-sectional area (CSA) of the ulnar nerve, which is essentially a measurement of the nerve's thickness. They measured it at three specific points along the elbow where the nerve is prone to entrapment: the Struthers arcade, the cubital tunnel, and Osborne's ligament.

While the cubital tunnel finding was the most extreme, increased nerve size at the other two points also signaled a significantly higher risk.

The study suggests that the ulnar nerve is exposed to immense mechanical stress from the repetitive, high-velocity pitching motion. The excessive traction and friction can lead to strangulation neuropathy, causing the nerve to swell. That being said, the fact that such a simple, non-invasive measurement can detect this precursor to injury has massive predictive power and could be a game-changer for player screening.

The "Ring-Down Artifact"

Researchers also used ultrasound to look for something called a "Ring-down artifact" (RDA). In simple terms, an RDA is a bright signal that appears on an ultrasound screen, thought to be caused by microscopic gas bubbles forming in a widened joint space—a key indicator of valgus instability in the elbow.

The study detailed two conditions where failing this test was a significant predictor of future injury:

• First, the mere presence of an RDA when a simple gravity load was applied to the elbow increased a player's relative risk of injury by 5.22 times.

• Second, if the RDA was still present even when a player actively contracted their FDS muscle, they had a 3.26 times higher relative risk of injury.

This second finding is especially revealing. A persistent RDA during muscle contraction is a direct visual indicator of functional failure. It suggests that tension from the FDS muscle is not being transmitted effectively enough to stabilize the joint capsule, leaving the elbow vulnerable to the stresses of throwing. It highlights that the dynamic function of supporting muscles, not just the static integrity of the UCL, is a crucial factor in injury prevention.

A More Holistic View of Elbow Health in Baseball Athletes

Taken together, these findings signal a paradigm shift in how we should approach elbow health in baseball players. While the UCL is undeniably important, this study provides strong prospective evidence that the health of the ulnar nerve and the function of the flexor-pronator muscles are significant, independent risk factors that have been underappreciated.

These structures don't exist in isolation. The UCL, FDS muscle, and ulnar nerve are all anatomically connected and work together in the complex system of the medial elbow. For context, the nine players who developed injuries in this study were diagnosed with a mix of conditions, including UCL injuries combined with ulnar neuropathy, UCL injuries combined with FDS injuries, and isolated ulnar neuropathy. In other words, we need to be evaluating all three components, we can create a much clearer and more comprehensive picture of a player's injury risk.

The Value of MSK Ultrasound

A simple, non-invasive ultrasound assessments of ulnar nerve size and elbow instability (identified via the Ring-down artifact) are powerful new tools for identifying at-risk professional baseball players before they experience symptoms. By focusing on nerve health and dynamic muscle function, teams have a new opportunity to move beyond a reactive approach to injury.

As this technology becomes more common, could proactive screening of these "invisible" risk factors fundamentally change how teams protect their most valuable arms?

Reference:

Ikezu M, Kudo S, Noda I, Kubo T, Hayashi H. Risk factors for elbow injuries in professional baseball players based on ultrasound images: a prospective cohort study of 56 players. JSES Int. 2025;9(5):1820-1824. Published 2025 May 22. doi:10.1016/j.jseint.2025.04.029