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Acromioclavicular Joint Injuries
in the National Football League
Epidemiology and Management
T. Sean Lynch,
*
y
MD, Matthew D. Saltzman,
y
MD, Jason H. Ghodasra,
y
MD,
Karl Y. Bilimoria,
y
MD, Mark K. Bowen,
z
MD, and Gordon W. Nuber,
z
MD
Investigation performed at Northwestern University, Chicago, Illinois
Background: Previous studies investigating acromioclavicular (AC) joint injuries in professional American football players have
only been reported on quarterbacks during the 1980s and 1990s. These injuries have not been evaluated across all position play-
ers in the National Football League (NFL).
Purpose: The purpose of this study was 4-fold: (1) to determine the incidence of AC joint injuries among all NFL position players;
(2) to investigate whether player position, competition setting, type of play, and playing surface put an athlete at an increased risk
for this type of injury; (3) to determine the incidence of operative and nonoperative management of these injuries; and (4) to com-
pare the time missed for injuries treated nonoperatively to the time missed for injuries requiring surgical intervention.
Study Design: Descriptive epidemiological study.
Methods: All documented injuries of the AC joint were retrospectively analyzed using the NFL Injury Surveillance System (NFLISS)
over a 12-season period from 2000 through 2011. The data were analyzed by the anatomic location, player position, field con-
ditions, type of play, requirement of surgical management, days missed per injury, and injury incidence.
Results: Over 12 NFL seasons, there were a total of 2486 shoulder injuries, with 727 (29.2%) of these injuries involving the AC
joint. The overall rate of AC joint injuries in these athletes was 26.1 injuries per 10,000 athlete exposures, with the majority of these
injuries occurring during game activity on natural grass surfaces (incidence density ratio, 0.79) and most often during passing
plays. These injuries occurred most frequently in defensive backs, wide receivers, and special teams players; however, the
incidence of these injuries was greatest in quarterbacks (20.9 injuries per 100 players), followed by special teams players
(20.7/100) and wide receivers (16.5/100). Overall, these athletes lost a mean of 9.8 days per injury, with quarterbacks losing
the most time to injury (mean, 17.3 days). The majority of these injuries were low-grade AC joint sprains that were treated with
nonoperative measures; only 13 (1.7%) required surgical management. Players who underwent surgical management lost
a mean of 56.2 days.
Conclusion: Shoulder injuries, particularly those of the AC joint, occur frequently in the NFL. These injuries can result in time lost
but rarely require operative management. Quarterbacks had the highest incidence of injury; however, this incidence is lower than
in previous investigations that evaluated these injuries during the 1980s and 1990s.
Keywords: National Football League (NFL); professional football; football (American); shoulder; AC joint; shoulder separations
In the sport of American football, shoulder injuries account
for 10% to 20% of all musculoskeletal injuries and are
only less common than injuries of the hand, ankle, and
knee.
1-3,5,6,9,12,13,16
Acromioclavicular (AC) joint injuries
commonly occur in contact sports via a direct blow to the
shoulder or indirectly when the player is hit or tackled
and lands on the affected shoulder. Kaplan et al
8
found
that AC joint injuries accounted for 41% of all shoulder
injuries in a cohort of intercollegiate football players. The
AC joint is a diarthrodial joint stabilized by the AC and cor-
acoclavicular ligaments. The superior and posterior AC lig-
aments prevent motion in the anteroposterior plane, while
the more robust coracoclavicular ligaments provide static
checkreins in the vertical plane. Despite the frequency of
shoulder injuries, few studies have reported the incidence
and treatment of these injuries in high-level football
players.
4,8,10,15
*
Address correspondence to T. Sean Lynch, Northwestern University,
676 North St Clair Street, Suite 1350, Chicago, IL 60611 (e-mail: tsean
lynch@gmail.com).
y
Northwestern University, Chicago, Illinois.
z
NorthShore University Health System, Evanston, Illinois.
The authors declared that they have no conflicts of interest in the
authorship and publication of this contribution.
The American Journal of Sports Medicine, Vol. XX, No. X
DOI: 10.1177/0363546513504284
Ó2013 The Author(s)
1
AJSM PreView, published on September 20, 2013 as doi:10.1177/0363546513504284
In one of the first epidemiological investigations of
shoulder injuries in the National Football League (NFL),
Kelly et al
10
found that AC joint injuries were the most
common shoulder injuries and accounted for 40% of all
shoulder injuries sustained by quarterbacks over a 22-
season period (1980-2001). Because of the nature of the
position, quarterbacks were believed to be particularly
prone to these injuries, with the majority of injuries occur-
ring during game competition rather than in the practice
setting. The most common mechanism of injury was from
either being tackled onto the playing surface or colliding
with another player. Utilizing the Rockwood classification
system for AC joint injuries, Kelly et al reported that 44%
of these injuries were type I injuries, 24% were type II, 20%
were type III, and 12% were not specified.
10
The mean time
lost from play after an AC joint injury was 22 days with
a median of 12.5 days.
10
The majority of these injuries
were treated without surgery.
The purpose of this study was 4-fold: (1) to determine
the incidence of AC joint injuries among all NFL position
players; (2) to investigate whether player position, compe-
tition setting, type of play, and playing surface were asso-
ciated with an increased risk for this type of injury; (3) to
determine the incidence of operative and nonoperative
management of these injuries; and (4) to compare the
time missed for those injuries treated nonoperatively ver-
sus the time missed for those injuries requiring surgical
intervention.
MATERIALS AND METHODS
After approval by the NFL Physician’s Society, a retrospec-
tive epidemiological review of all AC joint injuries between
2000 and 2011 (12 seasons) was performed using the NFL
Injury Surveillance System (NFLISS). This database was
established in 1980 and includes data such as the date of
injury and return to play, type of injury, type of field sur-
face, and type of injury management. These data were col-
lected and managed by Outcome Sciences Inc (Cambridge,
Massachusetts). The NFL records all injuries through the
NFLISS, with an injury being documented if (1) it resulted
in the player being removed from practice or a game or (2)
if it caused a loss of at least 1 practice, game, or training
session. Injuries that met inclusion for this study were
the following: distal clavicle contusions, AC joint sprains/
separations, and AC joint inflammation.
The AC joint separations were classified by the team
physician based on the Rockwood classification scheme
(see Appendix 1, available in the online version of this
article at http://ajsm.sagepub.com/supplemental). We
evaluated each injury for player position, playing condi-
tions (game vs practice, grass vs artificial surface), type
of play (passing play vs running play), type of manage-
ment, and total days lost due to the injury. Data were
available for whether the patient had nonoperative or sur-
gical treatment of the injury; however, specific details on
the type of surgery or surgical findings were not docu-
mented. For those athletes treated without surgery, the
database did not give specific information about what
measures were utilized (eg, corticosteroid or local anes-
thetic injections).
Information regarding each injury was based on the pri-
mary clinical diagnosis made by the team physician. The
trainer for each individual NFL team was responsible for
collecting and submitting these data to the NFLISS. The
collected information includes the diagnosis and evalua-
tion of all injuries that occurred during preseason training
camp, 4 preseason games, the regular 17-week regular sea-
son (16 games and 1 ‘‘bye week’’), and postseason-related
activities (practices, games, and scrimmages). Injuries
were either listed as occurring during the preseason or reg-
ular season (which included the playoffs). Injuries that
took place during off-season workouts were not collected
by the database. There were no follow-up examinations
available for this review. Additionally, data were analyzed
for treatment and return to play for each injury. Finally,
the mechanism of injury, type of play (running vs passing),
and playing conditions (games vs practice, natural vs arti-
ficial surface) were documented.
It should be noted that the special teams’ data required
special analysis because the only players officially listed in
this unit are punters, placekickers, and long snappers. The
NFL rosters do not delineate players from other positions
that might also be playing on the special teams’ unit to
get additional playing time. An analysis of NFL team ros-
ters from 2000 to 2011 revealed that there were 202
punters, placekickers, and long snappers designated on
official NFL team rosters. During any given special teams
situation, there are 9 other athletes on the field with this
unit. As a result, it was determined that 9 ‘‘special teams’’
players should be added to each of the NFL teams during
these 12 seasons.
Categorical variables were compared with the x
2
test.
Positional exposure risk (PER) was calculated to correct
the total injury count for the number of exposures
(player-games) for a particular player position. Incidence
density ratio (IDR) was also calculated to compare the
rates of AC joint injuries during passing versus rush
plays, game play compared with practice competition,
and natural surface compared with artificial surface.
7
A
95% confidence interval (CI) was calculated for each
IDR. When the IDR value is .1, the observed injury
rate is greater during ‘‘exposed’’ events (passing plays,
game play, or artificial surface); when the IDR value is
\1, the observed injury rate is greater during ‘‘unex-
posed’’ events (rush plays, practice competition, or natu-
ral surface). If the 95% CI for the IDR does not include
1, this reflects a statistically significant difference
between the 2 observed injury rates. A Pvalue of \.05
was considered significant.
RESULTS
Over 12 consecutive NFL seasons from 2000 to 2011, there
were a total of 30,304 injuries reported to the NFLISS,
with 2486 (8.2%) shoulder injuries. Of these shoulder inju-
ries, 727 (29.2%) involved the AC joint during 278,758 ath-
lete exposures. The overall rate of AC joint injuries in NFL
2Lynch et al The American Journal of Sports Medicine
football players was 26.1 per 10,000 athlete exposures. The
most common injuries were 319 unspecified AC joint
sprains (43.9%), 210 type I AC joint sprains (28.9%), 79
type II AC joint sprains (10.9%), and 40 type III AC joint
sprains (5.5%). Over the period of the study, no type IV,
V, or VI injuries were reported (Table 1).
Of those players reported to have sustained an AC joint
injury, 360 were offensive players, 268 were defensive
players, and 99 were special teams players (P\.001 for
all comparisons). The position players that most regularly
sustained the largest number of AC joint injuries were the
defensive backs (117 injuries, 16.1%), followed by wide
receivers (114 injuries, 15.7%). The total number of AC
joint injuries in defensive backs were significantly greater
than the total number of injuries in quarterbacks (P\
.001) (Table 2).
In terms of incidence, quarterbacks were most suscepti-
ble to an AC joint injury (20.9 injuries per 100 players), fol-
lowed by special teams players (20.7/100) and wide
receivers (16.5/100) (P\.001 for quarterbacks and special
teams players vs other positions) (Table 3). The calcula-
tions for special teams players were performed while con-
sidering the total number of players on the field for
special teams’ play and not just the athletes who were
only designated special teams players (punters, kickers,
and long snappers), as discussed in the Materials and
Methods section. With PER, special teams players had
the highest risk of injury with 7.6 injuries per 1000
player-games. Quarterbacks had the next highest PER of
7.4 injuries per 1000 player-games (Table 4).
A review of the NFL data from 2000 to 2011 showed that
there were 3952 games (7904 team games). Specifically,
4750 team games were played on grass (60.1%), and 3154
team games were played on an artificial surface (39.9%).
Four hundred seventy-eight AC joint injuries occurred dur-
ing plays on natural surfaces (64.7%) versus 249 AC joint
injuries occurring during plays on artificial surfaces
(33.8%) (P\.001). Eleven AC joint injuries occurred on
unknown surfaces (1.5%). The IDR for injuries occurring
on artificial surfaces was calculated to be 0.79 (95% CI,
0.67-0.92), indicating that the observed injury rate is
greater on grass than on artificial surfaces (P\.001)
(see Appendix 2, available online).
Finally, 602 AC joint injuries were sustained during
game play (81.5%), while 125 injuries occurred during
practice (18.5%) (P\.001). During game play over the
12-season study period, there were 259,666 passing plays
(54.2%) and 219,634 rush plays (45.8%). Passing plays
accounted for 199 AC joint injuries (33.1%), while running
plays made up 170 AC joint injuries (28.2%) (P= .0926).
Other/unknown plays accounted for 233 AC joint injuries
(38.7%). The IDR for injuries occurring on passing plays
was calculated to be 0.99 (95% CI, 0.80-1.20) (see Appendix
3, available online). Meanwhile, the IDR for injuries
TABLE 1
Distribution of AC Joint Injuries, 2000-2011
a
Injury
Total AC Joint and
Distal Clavicle Injuries
Clavicle AC sprain, unspecified 319
Clavicle AC sprain, type I 210
Clavicle AC sprain, type II 79
Clavicle AC contusion 66
Clavicle AC sprain, type III 40
Clavicle AC inflammation 9
Clavicle AC dislocation 3
Clavicle AC dislocation/recurrent 1
a
AC, acromioclavicular.
TABLE 2
AC Injuries by Player Position, 2000-2011
a
Position Total (N = 727)
Defensive back 117
Wide receiver 114
Special teams player 99
Defensive lineman 82
Offensive lineman 80
Linebacker 69
Running back 67
Quarterback 57
Tight end 42
a
AC, acromioclavicular.
TABLE 3
Incidence of AC Injuries by Player Position, 2000-2011
a
Player Position Incidence (per 100 Players)
Quarterback 20.9
Special teams player 20.7
Wide receiver 16.5
Tight end 12.1
Running back 11.5
Defensive back 11.0
Linebacker 9.8
Offensive lineman 9.4
Defensive lineman 8.7
a
AC, acromioclavicular.
TABLE 4
Positional Exposure Risk of AC Injuries by
Player Position, 2000-2011
a
Player Position
Positional Exposure Risk
(Injuries per 1000 Player-Games)
Special teams player 7.6
Quarterback 7.4
Wide receiver 4.1
Running back 2.7
Tight end 2.5
Defensive back 2.3
Linebacker 2.0
Offensive lineman 1.8
Defensive lineman 1.8
a
AC, acromioclavicular.
Vol. XX, No. X, XXXX AC Joint Injuries in the NFL 3
occurring during game competition was calculated to be
17.7 (95% CI, 14.7-21.5), indicating a significantly greater
rate of injury during game play compared with practice
competition (P\.001).
The overall mean time loss for these injuries was 9.8
days (range, 0-205 days). The injuries that required the
most time lost were AC joint dislocations (mean days lost,
77.7) and AC sprains, type III (mean days lost, 26.4) (Table
5). Quarterbacks (mean days lost, 17.3) and defensive backs
(mean days lost, 14.8) lost the most days to injury (Table 6).
Surgical management was performed for 13 of these AC
joint injuries (1.7%). Details of the procedures were not
available; however, the most common diagnosis requiring
surgery was a type I AC joint sprain (n = 4). The other
diagnoses that required surgery were an unspecified AC
joint sprain (n = 3), type II AC joint sprain (n = 2), type
III AC joint sprain (n = 2), and AC joint inflammation (n
= 2). Offensive linemen underwent the most surgical proce-
dures for this injury (n = 3), followed by quarterbacks, spe-
cial teams players, wide receivers, defensive backs, and
defensive linemen (all with n = 2).
DISCUSSION
Injuries frequently occur in contact sports such as American
football. Shoulder injuries are the fourth most common injury
sustained by football players,
1-3,5,6,9,12,13,16
and AC joint
injuries are the most common shoulder injury sustained by
these athletes.
8,10
We observed that injuries to the AC joint
accounted for 30% of all shoulder injuries that occurred
over 12 consecutive NFL seasons from 2000 to 2011. Previous
studies have reported that AC joint injuries account for
37.2% to 41% of all shoulder injuries in football players.
4,8,10
These injuries typically result from contact plays in which
a direct blow to the shoulder is sustained or when the player
is tackled to the ground onto his shoulder.
One possible explanation for the slightly lower rate of AC
joint injuries in the present study is changes to practice regi-
mens that have been implemented over the past decade. For
example, quarterbacks wear red jerseys during all practices
and scrimmages, and direct contact with the quarterback is
prohibited during these activities. In the senior authors’
(M.K.B., G.W.N.) experience as NFL team physicians, the
number of full contact practices and the number of 2-a-day
workouts during preseason training have also decreased con-
siderably since the initial investigation by Kelly et al,
10
thus
resulting in less opportunities for these injuries to occur.
Kelly et al
10
also reported that shoulder injuries were
overall the second most common injury sustained by quar-
terbacks, with AC joint injuries being the most common of
these shoulder injuries. Our study differs from Kelly et al
in that we evaluated players across all playing positions
and not just quarterbacks. We found that AC joint injuries
were most commonly sustained by defensive backs, fol-
lowed by wide receivers and special teams players.
TABLE 5
Days Lost to AC Joint Injuries, 2000-2011
a
Injury Mean Days Lost Median Days Lost Range of Days Lost
Clavicle AC dislocation (n = 3) 77.7 35.0 29.0-169.0
Clavicle AC sprain, type III (n = 40) 26.4 16.0 1.0-130.0
Clavicle AC inflammation (n = 9) 12.4 4.0 1.0-81.0
Clavicle AC sprain, type II (n = 79) 10.3 6.0 1.0-50.0
Clavicle AC sprain (n = 319) 9.3 4.0 1.0-163.0
Clavicle AC dislocation/recurrent (n = 1) 8.0 8.0 8.0-8.0
Clavicle AC sprain, type I (n = 210) 7.3 3.0 1.0-158.0
Clavicle AC contusion (n = 66) 6.2 3.0 0.0-57.0
All AC joint injuries (N = 727) 9.8 4.0 0.0-205.0
a
AC, acromioclavicular.
TABLE 6
Days Lost to AC Joint Injuries by Player Position, 2000-2011
a
Position Mean Days Lost Median Days Lost Range of Days Lost
Quarterback (n = 57) 17.3 9.0 1.0-145.0
Defensive back (n = 117) 14.8 4.0 0.0-169.0
Running back (n = 67) 11.0 4.0 1.0-158.0
Offensive lineman (n = 80) 10.0 4.0 1.0-117.0
Wide receiver (n = 114) 9.7 3.0 1.0-205.0
Special teams player (n = 99) 9.5 4.0 1.0-94.0
Tight end (n = 42) 8.6 7.0 1.0-30.0
Defensive lineman (n = 82) 7.7 3.0 1.0-81.0
Linebacker (n = 69) 6.9 4.0 1.0-40.0
a
AC, acromioclavicular.
4Lynch et al The American Journal of Sports Medicine
However, the incidence of these injuries was greatest in
quarterbacks, followed by special teams players and wide
receivers. In addition, the mean time lost to injury in this
study was 17.3 days for quarterbacks compared with 22
days in the study by Kelly et al. This difference may be
because the Kelly et al study defined an injury as one in
which the athlete was restricted for at least 2 days, while
our study defined an injury as one in which the athlete
left the game or practice or if the injury resulted in a loss
of at least 1 practice, game, or training session. The more
inclusive definition of injury in this study likely captured
more occurrences of shoulder injuries and may provide
a truer sense of the number and distribution of the type of
AC joint injury over the course of the football season.
Of the AC joint injuries analyzed in the current study, all
were unexpectedly type III or less according to the Rockwood
classification system. One explanation for this finding is that
severe AC joint injuries (types IV-VI) rarely occur in NFL
athletes in part because of the protective effect of shoulder
pads. Moreover, grade V injuries may have been misclassi-
fied as type III injuries because of the low interobserver
and intraobserver reliability of this classification system.
11,14
Finally, injuries classified as ‘‘clavicle AC dislocations’’ may
include higher (type IV-VI) AC joint injuries.
Interestingly, the majority of AC joint injuries included
in this study were treated nonoperatively, with only 1.7%
of these injuries requiring surgery. This is substantially
less than a prior report by Kaplan et al
8
in which 12% of
prospective NFL players underwent surgical treatment
for these injuries. However, Dragoo et al
4
reported a similar
operative rate of 2.4% for these injuries. Although the
NFLISS database lacks specific surgical information such
as the type of surgical intervention or acuity of the proce-
dure, in the experience of the senior authors, the operative
intervention for these types of injuries is typically a distal
clavicle excision for chronic symptoms.
The strengths of this study are that we investigated AC
joint injuries over 12 seasons. We also evaluated these
injuries in all NFL players during all facets of play includ-
ing all games, practices, and scrimmages, and as a result,
our data may provide a more accurate incidence of AC joint
injuries in the NFL than prior studies. Despite an increase
in player size over this time, we did not see an increase in
the number or severity of AC joint injuries.
There are several important limitations to our study.
The NFLISS is a well-established and highly regarded
reporting system, but as with any large database, there
is a possibility of inclusion of incomplete or inaccurate
information. For example, the database allows the trainer
to select ‘‘unspecified AC joint sprain.’’ This relatively
vague category was the most commonly entered diagnosis,
and it would have been preferable to have these cases clas-
sified as a more specific diagnosis. Additionally, clinical
follow-up and specific details on the surgical procedures
were not available for review. Clearly, these variables
would have allowed for better insight into how these inju-
ries are managed and how players perform after surgical
treatment. Finally, it is possible that injuries were unre-
ported or underreported by the medical team. Despite
these limitations, we were able to show that AC joint inju-
ries account for approximately one third of all shoulder
injuries in NFL players and that the vast majority of these
injuries are treated without surgery. The data in the pres-
ent study represent the most complete and current data on
AC joint injuries in NFL players.
ACKNOWLEDGMENT
The authors acknowledge the assistance of Dr Vandana
Menon, Kristina Franke, and the rest of the team with
Outcome Sciences Inc in the preparation of the NFLISS
data that were used in this article.
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