Relationship Between the Medial Elbow Adduction Moment During Pitching and Ulnar Collateral Ligament Appearance During Magnetic Resonance Imaging Evaluation
Hurd WJ, Kaufman KR, Murthy NS. Am J Sports Med. 2011 Feb 18. [Epub ahead of print]
The ulnar collateral ligament (UCL) of the elbow is the most common and serious injury that can occur at the elbow of a baseball pitcher. It has also been seen in other overhead athletes including those performing softball, tennis, volleyball, and javelin throwing. Recently, there has been an increased amount of UCL tears among pediatric athletes, which is a major cause for concern. Throwing frequency has been suggested as a mechanism for UCL tears in both youth and adult athletes, due to the large amount of valgus stress during the late cocking and acceleration phases of throwing. However, no study has examined if a relationship exists between valgus stress during throwing and UCL structural adaptations. Therefore this study examined the relationship between the throwing adduction moment (valgus stress) and the appearance of the UCL on MRI. They included 20 asymptomatic high school baseball pitchers. Bilateral elbow MRIs were recorded and 3D motion analysis was conducted to evaluate the adduction moment during the pitching motion. Based on the MRIs the UCL was graded as either normal or abnormal based on the presence of thickening, signal heterogeneity, and discontinuity. Correlations were then established between the MRI and adduction moment results. They found that the pitchers with elbow MRIs listed as abnormal had a larger adduction moment during pitching. The key characteristic seen in pitchers with abnormal MRIs was UCL thickening.
This is a great study attempting to establish a relationship between structure and function of youth throwers. Their results suggest that young pitchers with a larger adduction moment have a thickened UCL (although the thickness wasn’t directly measured). This is a very simple yet important concept that states that increased stress may result in tissue adaptation; however, studies over time will be required to determine causation. At the present time it is not known if this thickening is a positive or negative adaptation. Longitudinal studies that follow these athletes over time and examine the incidence of elbow injury will help answer this important question. However, from my experience at the shoulder, I feel this is a protective adaptation but only to a critical point. As the elbow is stressed overtime the body will respond by creating more tissue to handle this stress. This can be a good thing that helps to support the increased load and keep the UCL injury free. However, what we don’t know is what are the structural characteristics of the thickened tissue. Normally what we see during microscopic injury and healing is more type III collagen production. This collagen is weaker and more disorganized compared to the type I collagen that was there before the remodeling, so the tissue is getting thicker but it may be that it is also weaker and therefore more prone to injury (This can be seen in research by Mary Barbe https://www.ncbi.nlm.nih.gov/pubmed/19743505 and Louis Soslowsky https://www.ncbi.nlm.nih.gov/pubmed/10810684). I believe a great next step to this study would be to identify if indeed the UCL is being replaced with weak disorganized type III collagen. The results of this study suggest that exposure to abnormal stress can lead to UCL changes. As clinicians we can minimize this stress by limiting the amount of throwing, especially in pediatric athletes. Another important way to prevent UCL tears may be by allow more rest between throwing exposures. What are others thoughts or hypothesis on the mechanism of UCL injury? Do you think changes at the shoulder are affecting the elbow? Or even the hip?
Written by: Stephen Thomas
Reviewed by: Jeffrey Driban