Concussion Incidence and Return-to-Play Time in National Basketball Association Players

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Medicine

The American Journal of Sports

http://ajs.sagepub.com/content/44/9/2263

The online version of this article can be found at:

DOI: 10.1177/0363546516634679

2016 44: 2263 originally published online March 28, 2016Am J Sports Med

Ajay S. Padaki, Brian J. Cole and Christopher S. Ahmad

From 2006 to 2014

Concussion Incidence and Return-to-Play Time in National Basketball Association Players: Results

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Concussion Incidence and

Return-to-Play Time in National

Basketball Association Players

Results From 2006 to 2014

Ajay S. Padaki,

BS, Brian J. Cole,

MD, MBA, and Christopher S. Ahmad,

Investigation performed at Columbia University, New York, New York, USA

Background: Various research efforts have studied concussions in the National Football League, Major League Baseball, and the

National Hockey League. However, no study has investigated the incidence and return-to-play trends in the National Basketball

Association (NBA), which this study aims to do.

Hypothesis: Increased media scrutiny and public awareness, in addition to the institution of a league-wide concussion protocol,

may have resulted in more conservative return-to-play practices.

Study Design: Descriptive epidemiology study.

Methods: All concussions to NBA players that were publicly reported in the media from the beginning of the 2006 NBA season to

the end of the 2014 season were included. The incidence and return-to-play statistics were generated by synthesizing information

from publicly available records.

Results: There were 134 publicly reported concussions to NBA players from the beginning of the 2006 season to the conclusion

of the 2014 season, resulting in an average of 14.9 concussions per season. The incidence has not changed significantly during

this time span. The average games missed after a concussion from 2006 to 2010 was 1.6, significantly less than the 5.0 games

missed from 2011 to 2014, following the institution of the NBA concussion protocol (P = .023).

Conclusion: Although the incidence of publicly reported concussions in the NBA has not changed appreciably over the past 9

seasons, the time missed after a concussion has. While players often returned in the same game in the 2006 season, the com-

bination of implemented policy, national coverage, medical staff awareness, and player education may have contributed to play-

ers now missing an average of 4 to 6 games after a concussion. A multitude of factors has resulted in more conservative return-to-

play practices for NBA players after concussions.

Keywords: concussions; basketball; epidemiology

Estimates of the annual incidence of sports-related concus-

sions in the United States range from 300,000 to more than

3 million.

Concern persists, however, that unreported and

undiagnosed concussions result in these reported ranges

underestimating the true incidence. Concussions have

been brought to the forefront of the media as those that

are suffered in sports have become an area of growing con-

cern. Studies have shown sports-related concussions to be

an issue in every major sport, including at the high school

and collegiate levels.

Furthermore, recent investigations

using national databases have demonstrated increasing

incidences of sports-related concussions at the high school

and collegiate levels for both sexes.

15,18,22,24

A national

study revealed that the high school sports-related concus-

sion incidence more than doubled from 2006 to 2012, includ-

ing a statistically significant increase in basketball.

Research and public awareness for concussion preven-

tion and treatment have expanded dramatically in profes-

sional sports over the past decade. Several investigations

have demonstrated the magnitude of the neuronal damage

incurred after mild traumatic brain injury.

17,19-21

Specifi-

cally, metabolic cascades and ionic balance within neurons

Address correspondence to Christopher S. Ahmad, MD, Columbia

University Medical Center, 622 West 168th Street, PH-1130, New York,

NY 10032, USA (email: [email protected]).

Columbia University Medical Center, New York, New York, USA.

Department of Orthopaedics, Rush University Medical Center, Chi-

cago, Illinois, USA.

One or more of the authors has declared the following potential con-

flict of interest or source of funding: B.J.C. is a team physician for the

National Basketball Association and serves as the head team physician

for the Chicago Bulls. C.S.A. is a team physician for Major League Base-

ball and serves as head team physician for the New York Yankees.

The American Journal of Sports Medicine, Vol. 44, No. 9

DOI: 10.1177/0363546516634679

Ó 2016 The Author(s)

2263

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are disrupted, leading to increased energy demand to

repair the cell.

17,19

This increased demand is met with

decreased cerebral blood flow, however, resulting from

overall homeostatic dysregulation after traumatic brain

injury.

1,20,21

This dysregulation results in patients being

even more vulnerable to secondary concussive events as

they recover.

17,21

If sustained, a secondary concussion is

more damaging both intracellularly and with regard to

gross cognitive defect than the initial concussion.

12,17,19

Therefore, player protection after a concussive event is

paramount to allowing proper neuronal recovery.

Although the media have relayed this message from the

medical community, much of this awareness has been

directed toward the National Football League (NFL) and

National Hockey League (NHL) due to the high incidences

of concussions in these sports.

4,5,23

Little, if any, research

has investigated concussions in the National Basketball

Association (NBA), and the total number of studies in the

NBA lags well behind its counterparts in the NHL, NFL,

and Major League Baseball (MLB).

Notably, a recent

study of concussions in the MLB revealed approximately

20 concussions per season with an average return-to-play

time of 9 days.

The growing awareness and attention given to concus-

sions has resulted in longer absences and fewer same-

game returns for professional athletes suffering concus-

sions in both the NHL and NFL.

4,5,23

However, to our

knowledge, no such study has been performed with respect

to the NBA. Prior epidemiological analysis of NBA injuries

shows that approximately 5 concussions were reported in

the NBA per year from 1988 to 2005.

However, as of

2011, the NBA and MLB remained the only major sports

that had not implemented a formal, unified concussion pol-

icy for the entire league to follow. The NBA instituted

a compulsory concussion policy establishing player base-

line testing and return-to-play procedures preceding the

2011-2012 NBA season. While return-to-play guidelines

vary with a multitude of factors, including severity, timing,

and type of recovery testing, the establishment of a formal

concussion policy likely represented a response to the

surge in awareness.

2,6

As NBA players sustain high-

impact injuries and falls without wearing any protective

equipment, studying their concussion incidence and

return-to-play trends is critical to furthering player protec-

tion. No analysis to date has inspected the effect the NBA’s

2011 policy has had on concussion incidence or time missed

after a concussion. We hypothesized that increased aware-

ness and publicity in addition to the institution of a formal,

league-wide concussion policy have resulted in more con-

servative return-to-play timing and more publicly reported

player concussions in the NBA.

METHODS

Concussions sustained by NBA athletes from the start of

the 2006 season to the conclusion of the 2014 season

were tabulated by reviewing public media injury reports.

The list of players having concussions was formulated

from publicly available data, including media-generated

team injury reports and game reports. The length of time

missed was found by synthesizing the information from

team injury reports, national and local media releases,

game reports, and media guides. The information was com-

piled by using national sports media, including the NBA’s

official website (www.nba.com), ESPN, The Associated

Press, CBS Sports, and RotoWire. Team-specific websites

and local game reports and media guides were used to fur-

ther verify each concussion. The concussion data preceding

the institution of the concussion policy (2006-2010) were

contrasted with the data after the policy (2011-2014).

Injuries incurred during the preseason, regular season,

postseason, and All-Star Game were included in calculating

the seasonal incidence of concussions. However, to standard-

ize the data, concussions that were incurred in the preseason,

during the All-Star Game, and in the postseason were not

included in calculating the average games missed after a con-

cussion as external factors such as the weeklong All-Star

break and postseason elimination may have affected the

data. Preseason concussions were excluded from r eturn-to-

play data because of concerns regarding the involvement of

international teams in exhibitions games, the short duration

(8 games), and a potentially different level of in-game inten-

sity. In addition, concussions experienced by players who

were waived before fully recovering, players whose recovery

coincided with the conclusion of the regular season, and play-

ers who had concurrent injuries (orbital fracture, hematoma,

etc) were also excluded because of the potential that these

players’ recovery times could have been influenced by exter-

nal factors.

Each concussion was tabulated independently of whether

a player had incurred a previous concussion in either the

same season or previous season. In addition, for the category

of same-season repeat concussion and concussive symptoms,

a player must have been formally diagnosed with a concus-

sion in the same season, passed return-to-play guidelines,

and played in 1 or more regular season games before missing

more time due to either a second concussive event or a return

of symptoms that resulted in 1 or more missed games. This

definition of same-season repeat concussions was selected

due to the literature consensus regarding the dangers of sus-

taining a second concussion before full recovery

17,19

and that

more than 90% of athletes experience symptom resolution

and return to play within a month.

12,22,23

These injuries

were tabulated in an identical manner to the concussion inci-

dence. We reviewed each season’s database to assess whether

the concussion was primary or repeat according to the afore-

mentioned distinctions.

Statistical analysis was conducted using R Statistical

Programming Language (version 3.1.0; The R Foundation

for Statistical Computing). The P values reported were cal-

culated using a parametric t test, with P \ .05 indicating

statistical significance.

RESULTS

From the beginning of the 2006 NBA season to the end of

the 2014 season, 134 concussions were incurred, leading

to an average of 14.9 concussions per season. The highest

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number of concussions in a season came in 2012, when there

were 20, while the lowest number of concussions was in 2014,

when there were 8. The tabulation for the 2006-2014 seasons

is shown in Figure 1. A full summary of the data, which

includes both first-time and repeat concussions, is displayed

in Table 1. No significant change in the incidence of concus-

sions was found during this interval (P =.53).

The average number of games missed after a first-time

concussion was also tabulated for this time period. The first

season studied, 2006, represented the year with the lowest

average number of games missed after a concussion at

0.85. The 2006 and 2007 seasons represented the only years

in which less than 1 game, on average, was missed after

a concussion. The highest number of average games missed

was 6.0, in the 2011 season. The next 3 highest values were

consecutive seasons from 2012 to 2014, and the full range is

displayed in a box-and-whisker plot in Figure 2. Each of the

4 seasons after the institution of the NBA concussion policy

led to players missing an average of 4 or more games after

a concussion, while not one of the previous 5 seasons placed

within that range. From 2006 to 2010, an average of 1.6

games were missed after a first-time concussion. However,

after the establishment of the NBA concussion protocol pre-

ceding the 2011 season, an average of 5.0 games were

missed after a concussion from 2011 to 2014. The increase

in games missed after a concussion in the 2011 to 2014

NBA seasons compared with the 2006 to 2010 seasons was

statistically significant (P =.023).

In addition to calculating the incidence of concussions

and number of games missed after a concussion, the number

of players returning either in the same game or in the game

immediately after a concussion was tabulated. The highest

number of players who returned after missing zero games

was 8, in 2007. This season also represented the highest

proportion of players missing zero games, at 50%. In the

2014 season, the last season studied, zero players returned

without missing at least 1 game, as shown in Figure 3.

Notably, this season was the first and only season when

zero players returned without missing at least 1 game.

The number of players returning to play without miss-

ing a game after a concussion was contrasted with the

number of same-season repeat concussions. From 2006 to

2010, 7 publicly reported same-season repeat concussions

were reported. Of note, however, only 1 player had a pub-

licly reported same-season repeat concussion since the con-

cussion policy was instituted preceding the 2011 season.

The decrease in repeat concussion and concussive symp-

tom incidence from 2006 to 2010 compared with 2011 to

2014 was not statistically significant in the small sample

size accrued thus far (P = .11).

Last, a team-specific analysis was conducted to identify

if organizational trends were present. The 134 NBA con-

cussions publicly reported between 2006 and 2014 were

sustained by players from 28 different NBA teams. From

2006 to 2010, 80% of teams reported that at least 1 player

returned from a concussion without missing a game. Of the

21 teams that reported a player concussion from 2011 to

2014, 8 (38.1%) teams reported having a player return

without missing a game. Of this subset, 1 team did not

report a concussion during the 2006 to 2010 NBA seasons.

For the remaining 7 teams, 6 (86%) also reported that

a player returned to play without missing a game before

the institution of the NBA concussion policy (2006-2010).

DISCUSSION

The incidence and return-to-play data regarding publicly

reported concussions in the NBA demonstrate trends that

support increased player protection in the years following

TABLE 1

Summary of National Basketball Association Concussion Statistics (First-Time and Repeat), 2006-2014

Season Concussions, n

Games Missed,

Mean 6 SD

Zero Games

Missed, n

Range of

Games Missed, n

Repeat Concussions

and Symptoms, n

2006 16 0.846 6 0.99 6 0-18 2

2007 16 0.929 6 1.33 8 0-4 1

2008 14 2.92 6 3.66 3 0-12 1

2009 16 1.15 6 1.07 5 0-27 3

2010 12 2.83 6 2.14 2 0-5 0

2011 15 6.0 6 10.5 2 0-32 0

2012 20 4.38 6 4.94 1 0-20 0

2013 17 4.93 6 4.88 1 0-20 1

2014 8 5.67 6 5.51 0 2-12 0

Indicates players returning either later in the same game or in the game immediately after a concussion.

Figure 1. Concussions per season in the National Basketball

Association (NBA) from 2006 to 2014.

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the institution of a formal concussion protocol. While the

data accrued represent publicly reported information and

not official data from the NBA, the trends are important

to analyze and comprehend to further player protection.

During the 9 seasons analyzed, the incidence of publicly

reported concussions in the NBA has not changed signifi-

cantly (P = .53), with an average of 15 concussions per sea-

son. At least 12 concussions were reported per season from

the 2006 through 2013 seasons. However, only 8 concus-

sions were recorded in the 2014 season, likely representing

an anomalous value as no significant rule or diagnostic

changes were implemented preceding the season. The inci-

dence in this interval does appear to reflect a distinct

increase compared with the incidence from the previous 2

decades, however. The average concussion incidence over

the time period analyzed, 2006 to 2014, is more than 3

times greater than the value reported from 1988 to 2005.

Notably, the number of games a player missed after

a concussion following the institution of the NBA concussion

policy has increased in a statistically significant fashion as

players missed 3 times as many games from 2011 to 2014

as they did from 2006 to 2010 (P = .023). Strikingly, while

an average of 0.85 games were missed after a concussion

in the 2006 season, 5.67 games were missed after a concus-

sion in the 2014 season. In addition, while 31 players (42%)

from 2006 to 2010 returned either in the same game or the

game immediately after a concussion, that number

decreased to 14 players (23.3%) from 2011 to 2014. Most

important, these trends coincided with a decrease in repeat

concussions and concussive symptoms. While 7 players had

repeat concussions in the same season or missed additional

games due to a return of concussive symptoms from the

2006 through the 2010 seasons, only 1 player has sustained

a same-season repeat concussion since the concussion policy

was instituted. The lack of statistical significance in demon-

strating this trend (P = .11) is likely indicative of the small

sample size, since, at the time the study was conducted, only

4 NBA seasons had been completed after the concussion pol-

icy was instituted.

Concussions in the NBA have received significantly less

media coverage and national publicity than concussions in

other major sports. This deficit, in addition to the relatively

low incidence of concussions with respect to the NFL and

NHL, may have resulted in the NBA being the last major

American sport to institute a league-wide formal concussion

policy. A succinct summary of the concussion policy can be

found on the NBA’s official website (http://www.nba.com/

official/concussion_policy_summary.html).

A multitude of factors, including this concussion policy,

likely influenced the trends displayed over the past 9 sea-

sons. National awareness regarding the deleterious effects

of concussions has likely made players more willing to

report concussions and team physicians more conservative

in holding out players for longer periods of time. In addi-

tion, the concussion policy itself explicitly bans concussed

players from returning in the same game and has imple-

mented baseline neurocognitive testing to help guide

a safe return to play. The concussion policy also mandates

coach and player education regarding concussions and

involves multiple physicians in the care of the players, as

the team physician now collaborates with the NBA concus-

sion program director before officially clearing a player to

return to exhibition games. The increase in games missed

in the 2008 and 2010 seasons, before the institution of the

concussion policy, likely indicates the importance of aware-

ness and education in addition to the influence of concus-

sion coverage nationally. These factors, compounded with

the formal concussion policy, help to explain the dramatic

increase in the number of games players miss after concus-

sions in the time span studied.

In addition, team-specific analysis indicates the further

potential that awareness and formal policy can have in

protecting players. Most players who returned from a con-

cussion without missing a game after 2011 played for

organizations that also had players return without missing

a game before 2011. These organizations and their players

Figure 2. A box-and-whisker plot of games missed following

a first-time concussion spanning the 2006 to 2014 National

Basketball Association (NBA) seasons.

Figure 3. Averages games missed after a concussion, num-

ber of players missing zero games, and number of players

having repeat concussions from the 2006 to 2014 National

Basketball Association (NBA) seasons.

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will likely benefit from the continuing publicity and

increased education regarding concussions.

The challenges in continuing to improve care in the

treatment of concussions in the NBA and other profes-

sional sports remain substantial. Beginning with diagno-

sis, previous studies have shown that less than half of

players will report a concussion at the time it occurs,

and as only a minority of concussions result in loss of con-

sciousness, team physicians must be vigilant in monitoring

for signs and symptoms. While public awareness continues

to grow, both the medical staff and the sports culture must

adapt to encourage players to report their symptoms

promptly. Failure to do so increases the chances of further

concussive events in the same season and can result in

a variety of serious complications.

After diagnosis, a comprehensive examination and lon-

gitudinal monitoring must be performed to best guide

return to play. The policy the NBA has instituted helps

to protect players by banning a return to play in the

same game that a concussion was diagnosed, a practice

that was not always upheld before this rule. While the

underlying goal of the medical staff should be to protect

the player with a conservative approach,

external pres-

sures, including the player’s desire to return, must be care-

fully managed to optimize safety.

Therefore, methodical

and thorough testing must be performed. Comparing post-

concussive cognitive test scores to baseline levels, prevent-

ing physical activity before symptom resolution, and astute

neurological examination all assist the medical staff in

ensuring an optimal return-to-play date.

2,6

While the data reported in this study resulted from

a review of various injury reports and databases, a major

limitation of this study is the inability to include concussed

players who were not formally diagnosed with a concussion.

In addition, as no official, central database exists to dis-

close NBA concussions, those that were not publicly

reported could not be included in this investigation. Also,

despite the methodical approach to compiling the concus-

sion data, human error could also affect the data due to

the decentralized compilation. As with studies performed

in other sports,

4,9,15,23

the incidence reported likely signif-

icantly underrepresents the true incidence of concussions

in the NBA. This phenomenon consequently alters other

data that were reported, including the average games

missed after a concussion and the incidence of players

returning after having missed zero games. This being

said, significant efforts aimed toward accurate injury

reporting have been instituted, including the use of an

electronic medical record in the NBA in 2011.

Regardless of this limitation, however, this study shows

that the concussion trends reported in other American pro-

fessional sports also are present in the NBA. The number

of games missed after a concussion has risen significantly

after the institution of a formal concussion protocol and

the number of players returning to play the game immedi-

ately after a concussion has dropped to almost zero. Fur-

ther studies analyzing these trends in both the NBA and

other professional sports are needed to help protect players

and raise awareness regarding the importance of reporting

symptoms and sitting out until the players are fully

recovered. Greater transparency and public disclosure of

concussion data in professional sports could help research-

ers and physicians protect players of all ages.

ACKNOWLEDGMENT

The authors thank Jeff Stotts, founder of InStreetClothes,

for his dedicated work in tabulating NBA injury incidence

and for his assistance with this study.

REFERENCES

1. Barkhoudarian G, Hovda DA, Giza CC. The molecular pathophysiology

of concussive brain injury. Clin Sports Med. 2011;30(1):33-48, vii-iii.

2. Broglio SP, Guskiewicz KM. Concussion in sports: the sideline

assessment. Sports Health. 2009;1(5):361-369.

3. Cantu RC. Athletic concussion: current understanding as of 2007.

Neurosurgery. 2007;60(6):963-964.

4. Casson IR, Viano DC, Powell JW, Pellman EJ. Twelve years of National

Football League concussion data. Sports Health. 2010;2(6):471-483.

5. Casson IR, Viano DC, Powell JW, Pellman EJ. Repeat concussions in

the National Football League. Sports Health. 2011;3(1):11-24.

6. Covassin T, Elbin R III, Stiller-Ostrowski JL. Current sport-related

concussion teaching and clinical practices of sports medicine profes-

sionals. J Athl Train. 2009;44(4):400-404.

7. Dick R, Agel J, Marshall SW. National Collegiate Athletic Association

Injury Surveillance System commentaries: introduction and methods.

J Athl Train. 2007;42(2):173-182.

8. Drakos MC, Domb B, Starkey C, Callahan L, Allen AA. Injury in the

National Basketball Association: a 17-year overview. Sports Health.

2010;2(4):284-290.

9. Green GA, Pollack KM, D’Angelo J, et al. Mild traumatic brain injury in

Major and Minor League Baseball players. Am J Sports Med.

2015;43(5):1118-1126.

10. Guskiewicz KM, McCrea M, Marshall SW, et al. Cumulative effects

associated with recurrent concussion in collegiate football players:

the NCAA Concussion Study. JAMA. 2003;290(19):2549-2555.

11. Halstead ME, Walter KD; Council on Sports Medicine and Fitness.

Clinical report—sport-related concussion in children and adoles-

cents. Pediatrics. 2010;126(3):597-615.

12. Harmon KG, Drezner JA, Gammons M, et al. American Medical Soci-

ety for Sports Medicine position statement: concussion in sport. Br J

Sports Med. 2013;47(1):15-26.

13. Kroshus E, Baugh CM, Daneshvar DH, Stamm JM, Laursen RM, Aus-

tin SB. Pressure on sports medicine clinicians to prematurely return

collegiate athletes to play after concussion. J Athl Train. 2015;

50(9):944-951.

14. Makhni EC, Buza JA, Byram I, Ahmad CS. Sports reporting: a com-

prehensive review of the medical literature regarding North American

professional sports. Phys Sportsmed. 2014;42(2):154-162.

15. Marar M, McIlvain NM, Fields SK, Comstock RD. Epidemiology of

concussions among United States high school athletes in 20 sports.

Am J Sports Med. 2012;40(4):747-755.

16. McCrea M, Hammeke T, Olsen G, Leo P, Guskiewicz K. Unreported

concussion in high school football players: implications for preven-

tion. Clin J Sport Med. 2004;14(1):13-17.

17. Prins ML, Hales A, Reger M, Giza CC, Hovda DA. Repeat traumatic

brain injury in the juvenile rat is associated with increased axonal injury

and cognitive impairments. Dev Neurosci. 2010;32(5-6):510-518.

18. Rosenthal JA, Foraker RE, Collins CL, Comstock RD. National high

school athlete concussion rates from 2005-2006 to 2011-2012. Am

J Sports Med. 2014;42(7):1710-1715.

19. Shrey DW, Griesbach GS, Giza CC. The pathophysiology of concus-

sions in youth. Phys Med Rehabil Clin North Am. 2011;22(4):577-

602, vii.

AJSM Vol. 44, No. 9, 2016 NBA Concussion Incidence and Time Missed, 2006-2014 2267

at NORTHWESTERN UNIV LIBRARY on September 21, 2016ajs.sagepub.comDownloaded from

20. Vagnozzi R, Signoretti S, Cristofori L, et al. Assessment of metabolic

brain damage and recovery following mild traumatic brain injury:

a multicentre, proton magnetic resonance spectroscopic study in

concussed patients. Brain. 2010;133(11):3232-3242.

21. Vagnozzi R, Tavazzi B, Signoretti S, et al. Temporal window of

metabolic brain vulnerability to concussions: mitochondrial-related

impairment—part I. Neurosurgery. 2007;61(2):379-389.

22. Wasserman EB, Kerr ZY, Zuckerman SL, Covassin T. Epidemiology

of sports-related concussions in National Collegiate Athletic Associ-

ation athletes from 2009-2010 to 2013-2014: symptom prevalence,

symptom resolution time, and return-to-play time. Am J Sports

Med. 2016;44(1):226-233.

23. Wennberg RA, Tator CH. Concussion incidence and time lost from

play in the NHL during the past ten years. Can J Neurol Sci.

2008;35(5):647-651.

24. Zuckerman SL, Kerr ZY, Yengo-Kahn A, Wasserman E, Covassin

T, Solomon G S. Epidemiology of sports-related concussion in

NCAA athletes fr om 2009-2010 to 2013-2014: incidence , recur-

rence, and mechanisms. Am J Sports Med. 2015;43(11):2 654-

2662.

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