Brady Aiken's UCL: A question of stability

Rich Schultz

Brady Aiken's failure to sign with the Houston Astros apparently hinged upon the revelation that the ulnar collateral ligament in his pitching elbow is smaller than average. What are the anatomical and biomechanical ramifications of this finding?

Despite being one of the most congruent joints in the body, it was the elbow joint of San Diego-area high school left-hander and first pick of the 2014 MLB Draft Brady Aiken that brought incongruence between him and the Houston Astros and their contract negotiations. Ultimately, the inability for Aiken and the Astros to strike a deal appears to have revolved around a smaller than average ulnar collateral ligament (UCL), a finding uncovered during Aiken's pre-draft medical examination. While displaying no signs of discomfort and still capably throwing his fastball well into the high-90's up until the end of his high school season, the incidental finding was significant enough to cast doubt in the minds of the Astros brass as to how it might affect the long term health of Aiken's arm, sending contract negotiations towards an impasse that ultimately led to him unsigned.

The presentation of an abnormal finding during a physical exam or an imaging study on an athlete isn't terribly surprising, even at Aiken's young age; however, given that his anatomical quirk is congenital and not one derived from chronic overuse provides more questions than answers when discussing Aiken's future as a pitcher. While there are a plethora of academic studies discussing the consequences of overuse injuries in the throwing athlete—especially those affecting the elbow and UCL—there is little in the way of concrete results outlining the biomechanical implications of a congenital abnormality that allows Aiken to remain asymptomatic and, for all intents and purposes, healthy for the time being, but could possibly lend itself to putting Aiken at increased risk of injury. Adding to this conjecture is even further conjecture of what Aiken's exact condition is and its severity, as the terminology used to describe the issue is vague, with the need for privacy regarding any medical conditions and treatment trumping further inquiries as to the ultimate diagnosis of this abnormality.

Despite the fuzziness of facts, some light can be shed on what an abnormally small UCL could possibly entail in a pitcher. Through some educated assumptions and making use of the current literature on UCL biomechanics and functional anatomy, we can paint a reasonable picture of the effects of suboptimal UCL size.

Anatomically, the UCL originates from the anterior inferior surface of the medial epicondyle of the humerus bone and joins the ulna to the humerus, providing support and resistance in valgus overloads. It is the primary ligamentous stabilizer to valgus stresses in the elbow from 30 to 120 degrees of flexion, In full extension, the UCL provides about 30 percent of the elbow's stability, increasing to about 55-70 percent of stability in 90 degrees of flexion. It is one of the passive stabilizers of the elbow joint and is actually a complex composed of three ligaments—the anterior bundle, the posterior bundle, and the transverse bundle, which is often not present. Visual representation of the bands can be found here, courtesy of instreetclothes.com.

The anterior bundle is the is strongest of the complex and can be further divided into an anterior and posterior band; these bands tighten reciprocally through the entire range of elbow motion, with the anterior band tightening from zero to 60 degrees of flexion and the posterior tightening from 60 to 120 degrees of flexion, allowing the anterior bundle to be taut throughout extension and flexion. The posterior bundle is fan-shaped and is not as well defined or as strong as the anterior bundle. With its origin on the medial epicondyle and inserting on the olecranon, it remains taut in flexion and relaxed in extension and works as a secondary stabilizer of the elbow in lower degrees of flexion (up to roughly 30 degrees). The transverse bundle spans the insertion of the anterior and posterior bundles, but appears to have no role in elbow stability. Additional anatomical details of the anterior and posterior bundles can be found in the table below.

Anatomy Length Width Thickness
UCL complex ~54mm ~10mm (at insertion) ~5-6mm
Anterior bundle ~27mm ~4-5mm ~5mm

With respect to Aiken's finding, if there were to be concerns over a smaller than usual aspect of the UCL, it would be with the anterior bundle, given its location and importance in the overall stability of the elbow joint. While the size concerns could either be with respect to ligament length, width, thickness, or all three in concert, overall, the primary issue is that with a smaller ligament, there is less area for the valgus loads to be dispersed, introducing the potential for increased strain rates and an accelerated reduction in the elastic properties the ligament as it performs and recovers from each throw, thereby putting Aiken's elbow at risk for injury more quickly than someone with a UCL more consistent in size with the rest of their elbow anatomy. Looking at the issue from the perspective of Young's Modulus and the ligament's ability to maintain strength and retain elasticity under deformative forces:

 E \equiv \frac{\mbox {tensile stress}}{\mbox {extensional strain}} = \frac{\sigma}{\varepsilon}= \frac{F/A_0}{\Delta L/L_0} = \frac{F L_0} {A_0 \Delta L}

...we find that the geometry of the ligament to be important, with length, both in terms of original length of the ligament (L0) and how much length changes under stress (ΔL), playing a crucial role in the ultimate health of the ligament under stress. With a smaller ligament lengthwise, there is also the potential for elbow range of motion to be minimized, with high degrees of extension and flexion being comparatively reduced due to the shorter UCL length. Beyond actual ligament length, the width of the ligament and the amount of ligament surface area in contact with the bony origin and insertion points also is potentially affected with a smaller UCL. As an example, the width of the UCL origin involves about 67% of the medial epicondyle, providing a wide and sturdy anchor point for valgus forces to cross via the ligament; this percentage could be compromised and reduced in a smaller ligament, thereby innately weakening the UCL and reducing its ability to withstand valgus forces long term.

With the greater forces placed upon a comparatively smaller anterior UCL bundle comes a greater requirement for the adjacent anatomy to maintain elbow stability. With every pitch, a valgus stress of 64 Newton meters (Nm) is placed on the medial aspect of the elbow, which exceeds the tensile strength of the UCL (34 Nm) significantly. The potential for a reduction in UCL function creates a situation where the muscles of the flexor-pronator mass, the only active stabilizer of the elbow, must play an increased role in force transmission, especially in the late-cocking and acceleration phases of throwing. Research has shown the flexor digitorum superficialis being the greatest contributor to elbow stabilization; however, overall flexor-pronator mass muscle contributions in situations where the UCL is compromised remains unclear. For bony structures, the head of the radius acts as a secondary stabilizer to valgus stress, but becomes important to valgus stability when the UCL is compromised by injury. With a smaller UCL, the need for these structures to play a larger, more primary role in stabilizing the elbow is a possibility, mimicking some of the function seen from these structures when the UCL is injured.

While Aiken's UCL is not injured, the smaller size does provoke questions as to how long the elbow joint can remain so in the light of one of its most important components providing stability potentially compromised, due to its limitations in providing valgus force transfer and dissipation. However, the fact that Aiken's case arises from a congenital abnormality raises the potential for the ligament to function fully and without insult moving forward beyond what is normally seen and expected from a pitcher, due to his anatomy and physiology developing compensatory mechanisms (such as the ones described) as he has matured. In some respects, Aiken's body doesn't necessarily 'know' any better, which lends hope to his elbow functioning normally and without any additional constraints or concerns arising from the smaller UCL. It very well could have the strength of a normal-sized UCL. However, there is little in the way of gold standards with respect to this sort of aberration and to testing the ligament in vivo to provide results that would give rest to the questions raised as to the ultimate strength and health of Aiken's UCL, hence the concern exhibited by the Astros. Despite the initial apprehensions raised due to the finding, it is hoped that between what Aiken' has shown in respect to the possible strength of his elbow along with the advances in the treatment and prevention of elbow and UCL injuries and the understanding of and application of advances in the realm of biomechanics and kinematics, all will be sufficient enough to allay fears that Aiken's predicament is foreboding of a career stinted by his UCL shortcomings.

***

Stuart Wallace is an associate managing editor and writer at Beyond The Box Score. You can follow him on Twitter at @TClippardsSpecs.

Other References:

Dines, J. S. (2012). Sports medicine of baseball. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.

Wheeless, Clifford R. "Medical Collateral Ligament of the Elbow." Wheeless' Textbook of Orthopaedics. Duke Orthopaedics, n.d. Web. 22 July 2014.

Jess G. Alcid, Christopher S. Ahmad, Thay Q. Lee, Elbow anatomy and structural biomechanics, Clinics in Sports Medicine, Volume 23, Issue 4, October 2004, Pages 503-517, ISSN 0278-5919, http://dx.doi.org/10.1016/j.csm.2004.06.008.

Callaway GH, Field LD, Deng XH, Torzilli PA, O'Brien SJ, Altchek DW, Warren RF. Biomechanical evaluation of the medial collateral ligament of the elbow. J Bone Joint Surg Am. 1997 Aug;79(8):1223-31.

Tribst, Marcelo Fernandes, Zoppi Filho, Américo, Camargo Filho, José Carlos Silva, Sassi, Darlene, & Carvalho Junior, Antonio Egydio de. (2012). Anatomical and functional study of the medial collateral ligament complex of the elbow. Acta Ortopédica Brasileira, 20(6), 334-338.

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