SHOULDER
Scapular dysfunction in high school baseball players
sustaining throwing-related upper extremity injury:
a prospective study
Joseph B. Myers, PhD, ATC
a,*
, Sakiko Oyama, PhD, ATC
b,
Elizabeth E. Hibberd, MA, ATC
aa
Sports Medicine Research Laboratories, Department of Exercise and Sport Science, University of North Carolina, Chapel Hill, NC, USA
b
Department of Health and Kinesiology, University of Texas, San Antonio, TX, USA
Hypothesis and background: Though commonly suggested as an injury risk factor, scapular dysfunction has not been established as a prospective cause of throwing-related upper extremity injury in baseball players. The purpose is to determine whether scapular dysfunction identified during preseason screening is predictive of increased risk of throwing-related shoulder and elbow injuries in high school baseball players.
Materials and methods: The presence or absence of scapular dysfunction was obtained prospectively during preseason screenings in 246 high school baseball players over the 2010 and 2011 seasons. Exposure and injury surveillance data were then obtained weekly over the course of each season to determine whether scapular dysfunction was predictive of subsequent throwing-related upper extremities sustained. Results: There were 12 throwing-related upper extremity injuries sustained in the 246 participants, yielding an injury rate of 1.0 per 1,000 athlete exposures. There were no significant differences in injury rates between the participants with normal scapular function versus subtle scapular dysfunction (P¼ .62), normal scapular function versus obvious scapular dysfunction (P¼ .26), or subtle versus obvious scapular dysfunction (P¼ .45).
Conclusion: This study showed that scapular dysfunction identified during preseason screenings is not associated with subsequent throwing-related upper extremity injury.
Level of evidence: Level I, Prospective Design, Prognosis Study. Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.
Keywords: Baseball; shoulder; elbow; scapula; throwing; clinical assessment
Throwing-related upper extremity injuries, which comprise more than half of all injuries occurring in base-ball, affect a large number of competitive baseball players,5,10,16,17,20,21,23,26,28,31 resulting in significant time loss from participation.23 Epidemiologic studies show that approximately 20% and 70% of baseball players have shoulder and elbow pain, respectively,6,9-11,16,17,20,21,26,30
The Institutional Review Board at the University of North Carolina at Chapel Hill approved this study (No. 09-1268). The protocol is entitled ‘‘Identification of Physical Characteristics Associated With Upper Ext-remity Injuries in High School Baseball Players: A Prospective Study.’’
*Reprint requests: Joseph B. Myers, PhD, ATC, Department of Exer-cise and Sport Science, University of North Carolina at Chapel Hill, CB 8700 Fetzer Hall, Chapel Hill, NC 27599-8700, USA.
E-mail address:[email protected](J.B. Myers).
www.elsevier.com/locate/ymse
1058-2746/$ - see front matterÓ 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.
with many of these injuries occurring in high school–aged players.4,7,20,26,28,30-32The shoulder and elbow not only are the most frequently injured body parts but also result in the longest time loss when injured.23
Although there are various proposed intrinsic and ext-rinsic factors suggested as risk factors for throwing-related upper extremity injury, scapular dysfunction has been re-peatedly suggested to be a contributing factor.3,13,14Scapular dysfunction, often termed scapular dyskinesis,13,14,36 is an alteration in position and motion of the scapula during shoulder girdle motion. Scapular dysfunction is believed to be problematic in baseball players because of the integral role that the scapula plays in shoulder movement. The resulting dysfunctional scapular movement affects the upper extremity kinetic chain and can lead to overuse injuries by altering the stress pattern on the structures surrounding the shoulder and elbow joint. However, whether the scapular dysfunction is linked to increased risk of throwing-related upper extremity injuries has not been shown.
In addition, a majority of the work on scapular dysfun-ction to date has used sophisticated laboratory equipment (electromagnetic tracking devices primarily) to measure scapular motion. Yet, visual assessment of scapular motion is performed as part of clinical practice during preseason screening and injury evaluations, to identify the presence of scapular dysfunction. Currently, there are several pub-lished descriptions of clinically based visual scapular assessment methodology,15,22,25,29 with the scapular dys-kinesis test (SDT) described by McClure and associates22,33 appearing to have the highest level of both reliability22and validity.22,33If the scapular dysfunction identified with the SDT is associated with increased risk of injuries, this test could be used as a screening tool to identify baseball players who are at risk of throwing-related shoulder or elbow injuries. Therefore, the purpose of this study is to determine whether scapular dysfunction detected during preseason screening is associated with increased risk of throwing-related shoulder and elbow injuries in high school baseball players.
Materials and methods
Procedures
All participants in this study were varsity-level, male baseball players who participated at 1 of 17 high schools from across the state of North Carolina during the 2010 or 2011 spring baseball season. Before participation, a parent or guardian consented to his or her son’s study participation by providing a signature on a university institutional review board–approved parental consent form. In addition, university-approved consent/assent was ob-tained on the day of testing from each participant. All consenting players on a team participated in the study regardless of playing position.
All testing was conducted at each team’s high school facility (athletic training room, gymnasium, or classroom), allowing for an
entire team to be screened during 1 testing session. All testing sessions were conducted from mid January to mid February, to capture the desired data before the beginning of the spring base-ball season (spring season starts the second week in February in the state of North Carolina). Each participant started the testing session by completing a participation history survey developed by use of Teleform document scanning and recognition software (Autonomy Cardiff, Vista, CA, USA). The survey captured a history of the amount of baseball participation during the player’s entire career, current and past playing positions, and throwing-related upper extremity injury history. Each player then underwent a clinical-based scapular dysfunction assessment. Then, over the course of the season, each participant’s game and practice exposure and injury were captured weekly with the aid of the certified athletic trainer, over the course of the entire spring season.
Scapular dysfunction assessment
The SDT described in studies by McClure and associates22,33was used to assess scapular dysfunction. The SDT has been shown to be both reliable and valid when compared with electromagnetic tracking methodology.22,33Each participant was asked to remove his shirt and stand 2 to 3 meters away from a tripod-mounted video camera (Sony MiniDV Handycam Camcorder [model DCR-HC62]; Sony Corporation of America, San Diego, CA, USA). The camera was positioned to obtain a posterior view of the participant’s torso and arms (Fig. 1). Each participant was given 2 handheld weights whose mass was dependent on his body mass. For participants weighing less than 68.1 kg, 1.4-kg (3-lb) handheld weights were used, whereas for those with a mass greater than 68.1 kg, 2.3-kg (5-lb) weights were provided. Each participant was instructed on how to perform bilateral shoulder flexion and abduction raises and provided an opportunity to practice. Once the participant successfully demonstrated the task, video recording was initiated and each participant was instructed to perform 5 bilateral shoulder flexion repetitions immediately followed by 5 abduction repetitions. All repetitions were performed at a speed of 3 seconds for elevation and 3 seconds to descend. All videos were saved for later analysis.
After completion of data collection, 2 certified athletic trainers with extensive clinical experience (working with overhead athletes for>5 years) and PhD-level training in upper extremity biome-chanical/neuromuscular research (video analysis, electromagnetic tracking, electromyography, and diagnostic ultrasound) indepen-dently graded each participant’s video using the operational definitions of normal scapular function, subtle dysfunction, and obvious dysfunction.22,33When there was disagreement between the 2 observers, the videos were reviewed together and discussed and a consensus was obtained.
Injury surveillance
Over the course of the 2010 and 2011 seasons, the exposures and injuries sustained by each participant were tracked with the aid of the certified athletic trainer employed at each high school. During each week of the season, the athletic trainer received a Web link by E-mail from the study coordinator for an online injury surveillance survey. Each week, the athletic trainer reported practice and game exposures, where an athlete exposure was
defined as full participation in a practice session or a game. When a participant missed an exposure or had a limited exposure, additional information was obtained, including the reason for the missed exposure (because of either injury or another reason). When an injury was listed as the reason for a missed or limited exposure, the survey would obtain injury-specific data including diagnosis (made by the athletic trainer and/or physician), injury mechanism, and exposure loss/modification. This study focused on throwing-related upper extremity injuries, which was defined as a shoulder or elbow injury that resulted from the act of throwing/pitching and resulted in at least 1 missed or limited exposure. All injury and exposure data were verified at the end of the baseball season with the help of each high school’s athletic trainer (injury data) and coach (attendance, practice, and game participation).
Statistical analyses
Initially, agreement between the 2 independent raters of scapular dysfunction (normal, subtle, and obvious) was determined using calculation of weighted and maximumk scores. The injury rates for the participants with normal scapular function, subtle scapular dysfunction, and obvious scapular dysfunction were calculated as the number of injuries reported divided by the number of expo-sures and were reported per 1,000 athlete expoexpo-sures. The injury rates among groups were compared by use of Poisson regression with a generalized estimating equation and natural log of the total number of exposures (practices and games) as an offset, as well as an estimation of rate ratios.
Results
In this study, 248 varsity-level players from 17 high schools initially were evaluated. Two athletes were subsequently excluded from this study because they were dismissed from the team early in the season, leaving 246 athletes included in the analysis. Complete demographics appear inTable I. To establish the agreement between the independent raters of scapular dysfunction in this study, weighted k scores were calculated. Agreement between independent raters was considered good (0.60-0.80),1yielding a weighted k score of 0.730 (maximum k, 0.885).
Over the course of the 2 seasons, 12 throwing-related upper extremity injuries were sustained and reported. A description of these injuries appears in Table II. The injury rates (with accompanying exposures) in the partici-pants with normal scapular function, subtle scapular dysfunction, and obvious scapular dysfunction are presen-ted inTable III. There were no significant differences in the injury rates between the participants with normal scapular function versus subtle scapular dysfunction (0.71; 95% confidence interval [CI], 0.19-2.70; c2 ¼ 0.25; P ¼ .62), normal scapular function versus obvious scapular dysfunction (0.30; 95% CI, 0.04-2.40;c2¼ 1.29; P ¼ .26), or subtle versus obvious scapular dysfunction (0.42; 95% CI, 0.04-4.04;c2¼ 0.56; P ¼ .45).
Discussion
Given the importance that clinicians and researchers place on scapular function (and dysfunction) as it relates to injury in baseball players (and all overhead athletes), the purpose of this study was to prospectively examine whether scap-ular dysfunction identified during preseason screenings is a risk factor for throwing-related shoulder and elbow injuries in baseball players. It was hypothesized that those individuals who exhibited scapular dysfunction during preseason screening would be more at risk of sustaining a throwing-related upper extremity injury during the season. That hypothesis was rejected in this study. Partic-ipants in this study with subtle or obvious scapular dysfunction were not at a significantly greater risk of sustaining a throwing-related shoulder or elbow injury.
Figure 1 Assessment of scapular dysfunction using SDT.
Table I Participant characteristics (N¼ 246 participants) Data
Age (mean SD) (y) 16.4 1.1
Height (mean SD) (cm) 179.7 7.4
Mass (mean SD) (kg) 76.3 12.5
Length of baseball participation (mean SD) (y)
10.5 2.7 Primary or secondary position
of pitcher
Scapular dysfunction has been linked to various throwing injuries in a cross-sectional study.18 However, a limitation of a cross-sectional study is that it is unable to determine whether the scapular dysfunction predisposed the athlete to injury or whether the injury resulted in alteration of scapular movement. Therefore, this study aimed to identify scapular dysfunction before the injury occurred. Of the 246 participants, 122 were identified as either having subtle or obvious scapular dysfunction, yet only 4 of those individuals went on to sustain an injury. Individuals who were identified as having normal scapular motion patterns sustained the remaining 8 injuries. This observation indi-cates that use of the SDT as a preseason season risk assessment tool might be limited.
There are several possible interpretations for our observa-tion that showed an absence of a link between preseason scapular dysfunction and development of injury over the course of the season. First, alteration in scapular movement may not necessarily be related to injuries. Although many studies have identified scapular dysfunction after the injury or complaints of pain, no study has shown that the presentation of scapular dysfunction precedes pain or injury. An adaptation in scapular position and orientation at rest27and during shoulder elevation24 has been shown in healthy uninjured overhead athletes. In our study, all 122 participants who showed either subtle or obvious dysfunction were asymptomatic at the time of testing. These studies support the interpretation that altered scapular movement may not predispose throwers to injuries but that scapular dysfunction reported in the literature to be present after injury results from the injury.
The second interpretation of this study observation is that 1-time preseason screening of scapular dysfunction cannot detect potential changes in scapular movement occurring during a season that may have led to injuries. As previously stated, all of these measurements were taken at the start
of the season, before injuries occurred. No additional mea-surements were taken after the season or after an injury was sustained. Previous work has shown that scapular movement patterns can change over the course of the season.34,35 Potentially, players who sustained an injury may have ad-apted from their preseason ‘‘normal’’ scapular movement pattern over the course of the season, putting additional stress on the soft tissues about the shoulder and elbow, contributing to injury. However, this could not be determined in our study. Future work should include not only a pro-spective preseason screening data collection session but also subsequent follow-up later in the season or after injury.
A third interpretation of this study is that other factors besides scapular dysfunction had a larger influence on injury development. Scapular dysfunction is one of many potential risk factors for injuries. Although we hypothe-sized that scapular dysfunction may be a risk factor for injury, we recognize that throwing-related upper extremity injuries result from a multitude of factors, with scapular dysfunction being only one of many potential contributors. Those other factors that have been identified in prospective studies (which were not examined in this study) include patterns of flexibility,32,38 humeral geometry,37 strength patterns,12and actual game/participation factors.6,20,21It is possible that the effects of scapular dysfunction on injury were overshadowed by these other factors.
An additional reason that no association was present might be that there were a low number of throwing-related injuries observed in our study. Compared with the litera-ture, our entire sample group (246 participants) had a lower incidence of injury of 1.0 per 1,000 athlete exposures. The incidence of shoulder and elbow injuries in baseball players reported in the literature is 1.39 to 5.83 per 1,000 expo-sures.2,7-9,19-21,23,26,28,30 The differences in injury rates across the literature may stem from potential disparities in
Table II Injuries reported
No. of participants First/second position Missed exp Limited exp Throwing-related arm injuries reported 12 (4.9%)
Shoulder injury 9 (3.7%)
Elbow injury 3 (1.2%)
Specific injuries reported
1. Elbow: flexor mass strain Catcher/pitcher 0 5
2. Shoulder: rotator cuff strain First base/pitcher 0 2
3. Elbow: flexor mass strain Shortstop/pitcher 5 15
4. Shoulder: rotator cuff strain Shortstop/pitcher 5 1
5. Shoulder: posterior impingement Catcher/shortstop 0 3
6. Shoulder: posterior impingement Outfield/pitcher 15 0
7. Shoulder: biceps tendinosis Pitcher/shortstop 1 9
8. Elbow: medial epicondylitis Pitcher/first base 10 0
9. Shoulder: posterior impingement Pitcher/outfield 6 4
10. Shoulder: posterior impingement Outfield/none 0 1
11. Shoulder: deltoid strain Pitcher/none 0 13
12. Shoulder: deltoid strain Outfield/none 1 0
First/second position, Primary and secondary positions of player injured; Limited exp, number of exposures for which participation had to be modified because of injury; Missed exp, number of missed exposures because of injury.
how injury was defined. In this study, we used a conserva-tive definition of injury (ie, a missed exposure) to eliminate the typical upper extremity aches and pains that are com-mon in high school overhead sports participants. Perhaps this injury definition may have resulted in failure to capture some injuries that throwers were able to play through, which resulted in a lower estimate of injury risks. In add-ition, most of the epidemiology work to date has focused on older players (collegiate and professional level) who play the pitching position. Our study included not only pitchers (who have a higher risk of injury) but also all position players who are of high school age. The result might have been decreased injury incidence.
A limitation that must be acknowledged is that this study only used 1 method of assessing scapular dysfunction (the SDT described by McClure and associates22,33). Yet, there are other methods published that may better identify injury risk.15,22,25,29This is an area for future research. We opted for the methods described by McClure and associates be-cause of the higher level of reliability and validity estab-lished for their scapular assessment.
Clinical significance
The results of this study provide some valuable information that can be applied clinically. First, this study showed that scapular dysfunction identified during screenings is not associated with subsequent injury. This would suggest that its use as a preseason season risk assessment tool might be limited. However, our observation does not necessarily refute the previous studies linking scapular movement to injury, because scapular dysfunction that developed during the season, which was not examined in this study, may have led to injuries. Periodic screening of scapular motion and close monitoring of shoulder and elbow symptoms during a season may be necessary to determine whether the alteration in scapular movement is associated with injury development. However, the assessment of scapular dysf-unction can be administered with reliability. We established good reliability in this study, supporting previous literature showing its reliability and validity. Thus, it appears to be a good tool for identifying atypical scapular movement patterns. So, although the tool might not identify risk before injury, it may reliably identify scapular dysfunction as part of a clinical examination for scapular dysfunction that may be present when injury occurs.
Conclusions
Though commonly suggested as an injury risk factor for throwing-related shoulder and elbow injury in baseball players, scapular dysfunction (as measured with a clin-ical screening tool before the start of the season) could not be established as a prospective risk factor in this study. This study showed that scapular dysfunction is commonly seen in high school baseball players.
Acknowledgments
We acknowledge both Amber Sauls and Casey Shutt for their contribution to data collection and data reduction. We also acknowledge the certified athletic trainers at the participating high schools for the contribution they made to the injury surveillance portion of this study.
Disclaimer
The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
The project described was supported by grant No. R03AR055262 from the National Institute of Arthritis and Musculoskeletal and Skin Diseases/National Institutes of Health. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Institute of Arthritis and Musculoskeletal and Skin Diseases or National Institutes of Health.
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Practice Game Total
Normal 124 8 4,270 1,916 6,186 1.29
Subtle 69 3 2,294 953 3,247 0.92
Obvious 53 1 1,714 862 2,576 0.39
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