The pitching of a baseball is a strenuous action that applies exaggerated kinematic and kinetic influences at the elbow of the athlete. The repeated application of these influences can gradually degrade the mechanical integrity of the native biological support structures over time and eventually lead to failure by fatigue. One such structure is the anterior bundle of the ulnar collateral ligament (AMCL).

The fatigue properties of a material are often characterized by its respective fatigue life, where the amount of repetitive loading possible for a given load state before failure is defined. It is also important to predict how the damage is progressing throughout the material for cycles which are less than the predicted failure life. This study seeks to determine these properties of fatigue for the elbow AMCL and implement them into the execution of a Finite Element (FE) simulation for the purpose of predicting the magnitude and location of damage to the AMCL, per magnitude of pitching action performed. For applicability to the general pitching athlete, these results will be formulated using a non-dimensional material model.

Twenty-two cadaveric elbow specimens were used for this study. Eleven were tested in vertical elongation to make measurement of the AMCL elastic mechanical properties and subsequently tested to failure. Eleven contralateral specimens were tested in valgus fatigue to make measurement of the AMCL fatigue properties. These results were incorporated into a transversely isotropic hyperelastic constitutive equation and used to define the material properties of a 3D elbow model within a FE environment. Simulated displacements mimicking pitchin-induced joint opening were applied and both the stress and strain outputs were analyzed.

The average tensile failure load was 595.25 N ± 201.93 N. For specimens tested at 90% and 80% of their estimated one-cycle failure moment, the average cycles to failure were 3211 ± 4721.33 and 25063 ± 30487.58, respectively. The middle band of the AMCL increased in maximum stretch by an average margin of 0.89% ± 1.46%, with an average failure stretch of 1.077 ± 0.019. The FE fatigue simulation utilizing the nonlinear damage accretion function produced fatigue-induced stretch increase results that agreed with experimental observation to within 1%.