This study establishes an association between tissue adaptations of the anterior band of the UCL identified during MRI evaluation as a response to the medial elbow distraction experienced during pitching. In 2 comparable groups of asymptomatic pitchers in terms of age and years of playing experience, those presenting with unilateral UCL thickening of the throwing elbow exhibited a significantly greater internal adduction moment compared with those with symmetrical UCL appearance. The interpretation of the tissue adaptation in this asymptomatic population remains unclear. It is possible ligament thickening represents a positive adaptation, with unilateral thickening equating with a stronger ligament.24
Conversely, the unilateral thickening apparent in this young group may represent the initiation of a degenerative UCL tear that does not become symptomatic until adulthood. In the absence of prospective, longitudinal studies, it is unclear what these adaptations represent beyond a response to the repetitive stresses associated with pitching.
Tissue adaptations of the throwing elbow have been described in asymptomatic throwers from youth through professional levels of play.14,17,22,23
Kooima et al17
reported that of 16 asymptomatic major league players, 14 demonstrated UCL abnormalities that were visible on MRI. In earlier work, we identified asymmetrical thickening of the UCL in 65% (15/23) of uninjured high school–aged pitchers.13
These changes in ligament appearance have been attributed to chronic exposure to stress and considered a normal finding in the overhead athlete in the absence of symptom complaints.17
Thus, these MRI findings must be interpreted in context with the player’s history and physical examination when the athlete presents with complaints.17
An association between the medial elbow distraction forces experienced during pitching and injury has been described in adult pitchers. Anz et al3
calculated elbow kinetics obtained from a videotaped pitching analysis in 23 professional pitchers and subsequently tracked injuries during the next 3 competitive seasons. In this sample, 9 players went on to sustain an elbow injury. The authors reported differences in peak external elbow valgus torque that approached statistical significance when comparing injured (98.8 N·m) and uninjured pitchers (91.1 N·m), as well as a significant correlation of elbow injury with higher elbow valgus torque during the late cocking phase.3
Anz et al3
concluded that these data indicated higher levels of torque can result in an increased injury risk, and manipulation of pitching mechanics to alter this torque may help decrease injury rates. Additional work will be necessary, however, to determine the magnitude at which elbow valgus torque becomes a risk factor for subsequent injury.
The average peak elbow adduction moment during pitching for participants in this study is comparable with previously reported values. Fleisig et al11
evaluated pitching kinetics and kinematics in uninjured baseball pitchers from youth to professional levels of play. The high school group, consisting of 33 participants ranging from 15 to 20 years, exhibited an average peak internal elbow varus torque of 48 N·m.11
The magnitude of the peak varus torque in the high school–aged pitchers was significantly greater than youth pitchers (28 N·m) and significantly less than collegiate (55 N·m) and professional (64 N·m) pitchers. The authors concluded that because there were no differences in kinematic or timing characteristics across the different age groups, the progressive increase in joint kinetics was a consequence of increases in strength and muscle mass associated with increasing age.11
We partially agree with this conclusion. Increases in muscle mass typically accompany an increase in whole body mass and height, which are components of kinetic calculations. The values reported by Fleisig et al11
did not implement anthropometric normalization. Failure to normalize kinetic values obtained during 3-dimensional motion analysis studies limits the ability to make valid comparisons across participants, as differences in nonnormalized data may be a consequence of body dimensions and not biomechanical characteristics.12,19
The effect of normalized data is illustrated in a study by Sabick et al,25
who evaluated elbow valgus torque in 14 youth pitchers who were on average age 12 years. Sabick et al reported a mean peak elbow valgus torque for the group of 18 N·m with body weight the strongest predictor of valgus torque (61% of variability). After subsequently controlling for body size, the authors identified the most significant biomechanical contributors to elbow valgus torque as maximum shoulder abduction and shoulder internal rotation torque.25
By normalizing kinetic values, we are able to evaluate the association between UCL appearance and the peak elbow adduction moment as a function of pitching mechanics and not body dimensions.
This was not an exhaustive investigation of factors contributing to UCL tissue adaptations. Although the current study identified an association between the magnitude of the internal elbow adduction moment and UCL appearance, symptomatic tears in throwers have been attributed to repetitive trauma.15,22
Thus, the volume of throwing an athlete performs is likely to also contribute to tissue adaptations. We did not identify a difference in age among athletes with and without unilateral UCL thickening. Age may not, however, accurately reflect the volume of throwing an athlete has experienced. Pitch type has also been associated with elbow injury, with breaking pitches hypothesized to introduce greater loading to joints than a fastball or change-up pitch. The potential effect of both pitch volume and pitch type on tissue adaptations is supported by a study by Lyman et al,18
who reported a significant correlation between the number of pitches thrown in a game and over the course of a season and the rate of elbow pain among youth pitchers. The investigators also reported that throwing a slider was associated with an 86% increased risk of elbow pain.18
Finally, pitch velocity has been associated with elbow injury in professional baseball athletes. In a prospective study, Bushnell et al4
reported that uninjured pitchers who subsequently sustained an elbow injury threw on average 4 mph faster than pitchers who did not experience elbow injury. In the current investigation, there was a 5-mph difference between groups with and without UCL adaptations. Although this did not reach statistical significance, the work by Bushnell et al4
suggests the difference may be clinically meaningful. On the basis of the results of these studies, it is reasonable to conclude that adaptations in UCL appearance are likely to be multifactorial.