Sports Training, Medicine and Rehabilitation, 1990, Vol. 1, pp. 287-296.

A TWO-YEAR PROSPECTIVE STUDY OF PROPHYLACTIC KNEE BRACES IN A NATIONAL SAMPLE OF COLLEGE FOOTBALL PLAYERS

ERIC D. ZEMPER, Ph.D.

International Institute for Sport and Human Performance, University of Oregon,

Eugene, Oregon, US.A.

Current address: Exercise Research Associates of Oregon, P.O. Box 10123

Eugene, OR 97440

(Received October 15, 1989)

 

ABSTRACT

Data were collected during the 1986 and 1987 seasons from a representative stratified national sample of college football teams to assess the efficacy of prophylactic knee braces in reducing the incidence and/or severity of knee injuries in football players. Over the two seasons there were a combined total of 6,229 players involved, of whom 28.0% wore prophylactic knee braces, with a total of 445,856 athlete-exposures to the possibility of being injured in a game or a practice. The results indicate that the total incidence of knee injuries was slightly higher in braced players. Further analyses indicate that braces have no impact in reducing the incidence of medial collateral ligament (MCL) injuries, and there is no apparent difference in incidence among various brands of braces. There is no difference in the severity of total knee injuries or MCL injuries in braced and unbraced players when measured in days lost from participation or degree of MCL injury. When injuries in games were isolated to control for the "intensity" of exposure to the possibility of being injured, the analyses provided the same results as seen for games and practices combined.

KEYWORDS: Prophylactic knee brace, football, prospective study

 

INTRODUCTION

For many years it has been known that the knee is the most frequent site of injury in the sport of football (Alles et al., 1979; Zemper, 1984). Recently, the most prominent method of attempting to reduce knee injuries has been the use of some variation of the prophylactic knee brace first developed by Anderson (Anderson et al., 1979). Following the initial appearance of a few small-scale studies using individual college teams, and a large number of anecdotal testimonials, all supporting the ability of the braces to reduce the number of knee injuries, the use of these braces has become fairly common. In some cases schools are requiring all of their players to wear them. More recently, the results of a few studies have begun to question the protective ability of these knee braces (Baker et al., 1987; France et al., 1987; Hewson et al., 1986; Paulos et al., 1987; Revere et al., 1987; Teitz et al., 1987), raising one of the major current questions in sports medicine: Does the prophylactic knee brace reduce the number and/or severity of football knee injuries?

Some studies on individual college teams (Hansen et al., 1985; Randall et al., 1984) have indicated fewer knee injuries in players using the braces, while similar studies at other schools have demonstrated no difference in the number of injuries between braced and unbraced players (Hewson et al., 1986), or even more injuries in braced players (Revere et al., 1987; Teitz et al., 1987). Two recently reported biomechanical studies indicated that little or no significant protective effect could be documented for the braces (Baker et al., 1987; Paulos et al., 1987). A third biomechanical study (France et al., 1987) indicated that a protective capability for the braces could be shown only in certain very limited conditions, and that better designs for these braces were needed.

The only large-scale national study reported thus far, by Teitz et al. (1987), stated that the severity of knee injuries was no different in braced and unbraced players, and braced players sustained significantly more knee injuries. Garrick and Requa (1987) reviewed several of these recent studies and concluded that no definitive conclusions can be reached yet regarding the efficacy of prophylactic knee braces. They recommended that more national-scale studies be completed using larger study populations.

This study is a preliminary report of an ongoing national study of injury rates in prophylactic knee braces that is part of a continuing sports injury data collection effort. Several common questions regarding the use of prophylactic knee braces in football are addressed here:

1. Are there fewer total knee injuries among players wearing knee braces?

2. Are there fewer medial collateral ligament (MCL) injuries among players wearing knee braces?

3. Does wearing a knee brace reduce the severity of knee injuries?

4. Since there often is a major difference in "intensity" of exposure between games and practices, does wearing a knee brace reduce the incidence of total knee injuries or MCL injuries, or the severity of injuries, when considering game injuries only?

SUBJECTS AND METHODS

The data used in this study were collected over the first two years of operation of the National Sports Injury Surveillance System (NSISS)* at the International Institute for Sport and Human Performance of the University of Oregon. NSISS began collecting football injury data during the 1986 season, and meets the major criteria for reliable studies of sports injury rates outlined in 1987 by the American Orthopaedic Society for Sports Medicine (Thompson et al., 1987). A stratified sample of National Collegiate Athletic Association and National Association of Intercollegiate Athletics members sponsoring intercollegiate football was used, with stratification based on four geographic regions and three NCAA/NAIA divisions (Table I). This sample was designed to be approximately a five percent proportionally representative sample of all intercollegiate football teams, both NCAA and NAIA, and the results therefore should be generalizable to the total population of intercollegiate football players. The study population included all intercollegiate football players at these institutions. During the 1986 season 32 teams with a total of 3,431 players participated, and during the 1987 season 27 teams with a total of 2,798 players participated in the injury surveillance system.

________________

* NOTE: NSISS is the predecessor of the Athletic Injury Monitoring System (AIMS).

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TABLE I

Distribution of 1986 NSISS sample

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East

South

Midwest

West

Totals

NCAA Division I

1

4

1

2

8

NCAA Division II and NAIA Division I

2

3

4

3

12

NCAA Division III and NAIA Division II

4

1

5

2

12

Totals

7

8

10

7

32

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East = Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, New York, Pennsylvania, New Jersey, Delaware, Maryland, West Virginia. South = Virginia, Kentucky, Tennessee, North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Arkansas, Louisiana, Oklahoma, Texas. Midwest = Ohio, Michigan, Indiana, Illinois, Wisconsin, Minnesota, Iowa, Missouri, North Dakota, South Dakota, Nebraska, Kansas. West = Montana, Wyoming, Colorado, New Mexico, Arizona, Utah, Idaho, Nevada, Washington, Oregon, California, Alaska, Hawaii.

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Prior to the start of each season the head athletic trainer of each participating team was sent copies of forms for reporting exposure and injury data, along with detailed instructions on how to use the forms. (Copies of the data collection forms used in this study are available from the author.) On a weekly basis throughout the season, from the first preseason practice until the final regular season or post-season game, the trainers returned a form listing the number of practices and any games played during the week, and the number of players participating in each, as well as separate forms detailing each football-related injury that kept a player from full participation for one day or more. Upon arrival at the Institute, each form was screened for completeness and consistency before being entered into a computer file for later analysis. In the event of incomplete or inconsistent data, individual trainers were contacted for clarification. Over the first two years of the data collection effort reported here, 98.8% of the weekly reports were submitted by the involved institutions.

NSISS is designed to be a longitudinal sports injury data gathering system that provides analyses of national injury rates and patterns for the purpose of providing data to assist in developing ways to prevent or reduce the severity of sports injuries through rule changes, equipment modifications or changes in coaching techniques. It also provides a vehicle for doing studies of special questions, such as currently ongoing studies on injury rates on artificial turf and natural turf, cerebral concussions in various brands of football helmets, and the present study of the efficacy of prophylactic knee braces.

For this study trainers completed an additional form at the beginning of the season indicating the number of players at each position, how many wore prophylactic knee braces and what brands they wore. Since this is an observational study, the decision as to who wore braces and who did not was under local control, based on the policies at each participating institution. With a relatively large sample such as this, it is assumed that a variety of policies are involved, representing the situation in the total national population. On the individual injury forms trainers provided information on details such as the type of injury, the circumstances, type of surface being used, number of days away from participation, whether or not it required surgery and, if it was a knee injury or ankle injury, whether or not a prophylactic knee brace was worn and what brand. In 1987 information regarding the specific type of knee injury (MCL, ACL, meniscus, etc.) was also provided.

The data for this study were analyzed using a chi-square test a = .05). Unless otherwise indicated, the results presented are for the 1986 and 1987 seasons combined. The results of statistical tests of significance on the data for the individual seasons did not differ from the results for the combined data. Analysis of the data was completed with the use of the SAS statistical package (SAS Institute, Inc., Cary, NC).

RESULTS

Background Data

For the 1986 and 1987 football seasons combined there were a total of 6,229 players in the study, with a total of 445,856 athlete-exposures. An athlete-exposure (A-E) is one player taking part in one practice or one game where he is exposed to the possibility of being injured. If a football team has 100 players that all take part in 5 practices during a given week, that team has 500 A-E for the week in practices. If 40 of them play in the game on Saturday, there are 40 game exposures and a total of 540 A-E for that team for the week. During the two football seasons covered in this study there were 410,165 A-E in practices and 35,691 A-E in games (varsity and sub-varsity); or 92.0% of the exposures were in practices and 8.0% in games. There were a total of 2,820 reported injuries over the two seasons, for an overall injury rate of 6.32 injuries/1,000 A-E, or 45.27 injuries/100 players. The injury rate in practices was 3.93/1,000 A-E and in games it was 33.90/1,000 A-E. While 57.1% of the total injuries occurred in practice, the game injury rate was 8.6 times higher than the practice rate. The knee was the most frequently injured body part with 521 reported injuries, for a rate of 1.17 injuries/1,000 A-E.

Total Knee Injuries

A total of 1,744 players in the sample (28.0%) wore prophylactic knee braces. During the 1986 season it was found that 95% of the players who wore braces wore them in both practices and games, so to simplify data collection and analysis procedures it is assumed that all players who wear braces wear them in both games and practices. Teitz et al. (1987), in a national-scale study, found that the proportion of players wearing braces at all levels of ability (starters through fourth string) were essentially the same, therefore it is reasonable to assume that the proportion of exposures in games and practices for braced players is approximately equal to the proportion wearing braces. The distribution of the brands worn is shown in Table II.

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TABLE II

Distribution of knee braces by brand (1986-1987 combined)

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_N_

_%_

Anderson-Omni (Omni Scientific, Lafayette, CA)

653

37.44

Don-Joy (Don Joy, Carlsbad, CA)

358

20.53

McDavid (McDavid Knee Guard, Clarendon Hills, IL)

360

20.64

Others

373

21.39

Totals

1,744

100.00

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The total number of knee injuries in braced and unbraced players is shown in Table III. Chi-square values were calculated based on the null hypothesis that the braces have no effect on knee injuries and therefore the expected distribution of knee injuries between braced and unbraced players should match the distribution of players wearing (28.0%) and not wearing (72.0%) prophylactic knee braces. The result for total knee injuries was statistically significant (X2 = 14.56, df = 1, p < 0.05). Because the observed distribution of injuries statistically is significantly different from the expected distribution, the null hypothesis must be rejected, and since the number of injuries in braced knees is greater than the expected value, it must be concluded that players wearing prophylactic knee braces are experiencing a significantly greater incidence of knee injuries than players not wearing braces.

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TABLE III

Total knee injuries in braced and unbraced players

(1986-1987 combined)

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__________Knee Injuries__________

 

Observed

(Expected)

No Brace

336

(375.1)

Braced

185

(145.9)

Total

521

(521.0)

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Medial Collateral Ligament Injuries

The prophylactic knee brace is designed specifically to protect against damage to the medial collateral ligament caused by lateral blows. Therefore, an analysis of MCL injuries was done on the 1987 data, which was the first season that NSISS collected information on the specific types of knee injuries sustained by the players. The results are presented in Table IV. While there were more than the expected number of MCL injuries in braced players, there was no statistically significant difference in the proportion of MCL injuries in braced and unbraced players (X2 = 1.21,df = 1, p > 0.05). Therefore, these data indicate that prophylactic knee braces do not reduce the number of MCL injuries.

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TABLE IV

Medial collateral ligament injuries

in braced and unbraced players (1987 only)

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_________MCL Injuries_________

 

Observed

(Expected)

No Brace

69

(73.9)

Braced

32

(27.1)

Total

101

(101.0)

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The 1987 NSISS individual injury forms also requested the degree of injury for all MCL injuries, based on the criteria outlined by Fetto and Marshall (1978), which were presented in the instructions for completing the forms. It is presumed that the grading was done by the team physician. The distribution of these injuries is presented in Table V. A chi-square analysis of this distribution, compared with an expected distribution based on the proportion of braced and unbraced players in the sample, shows no significant difference in the distribution of the three grades of MCL injuries between braced and unbraced players (X2 = 1.72, df = 1, p > 0.05).

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TABLE V

Medial collateral ligament injuries by degree

in braced and unbraced players (1987 only)

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____MCL-I____

____MCL-II____

___MCL-III___

____Totals____

 

Observed

(Expected)

Observed

(Expected)

Observed

(Expected)

Observed

(Expected)

No Brace

41

(45.3)

22

(21.9)

6

(6.6)

69

(73.9)

Braced

21

(16.7)

8

(8.1)

3

(2.4)

32

(27.1)

Totals

62

(62.0)

30

(30.0)

9

(9.0)

101

(101.0)

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Severity of Injuries

To investigate the possibility that prophylactic knee braces reduce the severity of knee injuries, data on total knee injuries from 1986 and 1987 were analyzed, as well as data on MCL injuries from 1987. In both cases surgical injuries and season-ending injuries that were not treated surgically were eliminated from the analyses. Analyses were done based on the number of days away from participation, blocked as shown in Table VI. The distribution of days lost for braced players is not significantly different from unbraced players for either total knee injuries (X2 = 1.50, df = 3, p > 0.05) or for MCL injuries (X2 = 3.77, df = 3, p > 0.05). In addition, the average number of days lost per injury was calculated from data on actual days lost due to each injury. For non-surgical, non-season-ending injuries the average number of days lost per injury for total knee injuries was 8.8 days in unbraced players and 9.1 days in braced players; for MCL injuries the average was 10.3 days in unbraced players and 11.6 days in braced players. The results for both total knee injuries and specifically for MCL injuries indicate that the number of days lost from participation for those wearing prophylactic knee braces is slightly higher, but is not significantly different from those not wearing braces.

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TABLE VI

Total knee injuries and medial collateral ligament injuries by days lost in braced players

_______________________________________________________________________________________

Days of

Total Knee Injuries (1986-1987 Combined)

MCL Injuries (1987 Only)

Participation Lost

Observed

(Expected)

Observed

(Expected)

1-2

26

(31.4)

1

(3.7)

3-6

45

(45.0)

11

(7.4)

7-9

18

(16.0)

5

(5.3)

10+

37

(33.7)

11

(11.6)

Totals

126

(126.1)

28

(28.0)

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Surgical injuries and season-ending non-surgical injuries have been eliminated from this analysis.

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Injuries in Games

To assess the possibility that there is a difference in results between game situations and practices (e.g., not all players "exposed" in practice are necessarily participating fully in situations that can result in knee injury, which in some way might skew the results in favor of braced or unbraced players), separate analyses were completed on the data from game exposures only, where every player participating in a game receives the same "quality" or "intensity" of exposure to the possibility of being injured. While eight percent of the total athlete-exposures over the two years reported here were in games, approximately half (50.3%) of the recorded knee injuries occurred in games. The results of these analyses exactly mirror those for games and practices combined, as presented above. The only statistically significant result was found in total knee injuries, with braced players receiving more injuries than expected. There was no significant difference in incidence of MCL injuries between braced and unbraced players. Time away from participating was not significantly different for braced and unbraced players for either total knee injuries or MCL injuries. The average days lost for non-surgical, non-season-ending game injuries for total knee injuries was 8.4 days in unbraced players and 9.6 days in braced players; for MCL injuries the average was 10.8 days in unbraced players and 14.0 days in braced players.

Other Observations

During the analyses done for this study, a number of other observations were made that deserve some mention. While the decision as to whether or not an injured knee should be surgically repaired varies depending on the philosophy of the team physician, it is interesting to note that these data showed no significant difference in incidence of surgical knee injuries between braced and unbraced players. Analyses of the brand of brace worn showed no differences between brands for total knee injuries, surgical knee injuries or MCL injuries. Therefore, no brand appears to be significantly better or worse at protecting the knee.

A question frequently raised is whether or not wearing a prophylactic knee brace increases the chance of sustaining an ankle injury. To assess this possibility, for every ankle injury reported trainers also provided information on whether or not knee braces were worn by the injured player. Analyses of the data indicated that wearing a brace has no apparent impact on occurrence of ankle injuries. However, the data did show an apparent interaction between wearing the knee brace and playing on artificial turf, which resulted in a greater incidence of MCL injuries. This observation needs further exploration as more data is collected in the future.

In looking at the activity at the time of injury, blocking causes the greatest proportion of total knee injuries and MCL injuries, followed by being blocked. The proportion of non-contact knee injuries in braced players almost exactly matches the proportion of braced players, so wearing braces does not appear to affect the occurrence of non-contact knee injuries.

Since the risk of football injury varies by position played (Shields and Zomar, 1982; Zemper, 1984), an analysis of knee injuries by position was attempted on the data collected thus far. At this time the numbers of total knee injuries and MCL injuries when broken down by position are too small to provide an adequate basis for statistical analysis. As further data is collected by NSISS in the future, this factor will be analyzed and reported in detail.

DISCUSSION

The results of analysis of the data collected during the first two years of operation of NSISS support the findings of several recent studies of prophylactic knee braces (Hewson et al., 1986; Revere et al., 1987; Teitz et al., 1987) showing no apparent protective effect of this type of brace. The question of the efficacy of prophylactic knee braces is not likely to be settled by any one definitive study because of the presence of a number of uncontrollable variables inherent in the nature of the sport and the differing conditions under which it is practiced and played from one school to the next. The differing results found in many of the earlier studies of knee braces done at individual schools may be partially attributed to such factors as different practice methods, types and condition of surfaces used in practices and games, and policies on the use of braces (required or voluntary, all players or selected players). According to epidemiologic theory, the impact of these factors can be most effectively minimized by using large numbers of players from a national sampling of schools, as recommended by Garrick and Requa (1987) and as done in this study. It appears that the recent biomechanical studies or statistical studies using larger, more representative samples over more than one season, and utilizing better research designs from an epidemiological perspective, are finding no protective effect in using these braces. In epidemiological research, short of doing a completely controlled experimental study, definitive causal relationships are difficult to prove in a single study. One must depend on building up a consistent body of evidence across a number of studies utilizing different methodologies and different samples of subjects over a period of time.

In the analyses done in this study, if prophylactic knee braces were reducing the incidence of injuries, it would be expected that the chi-square test would show a statistically significant result with the observed incidences in braced players lower than the expected values. As can be seen, the only statistically significant result appears in Table III regarding total knee injuries; however, the observed incidence of knee injuries was higher than the expected incidence in braced players, indicating that braced players have a higher rate of total knee injuries.

The data presented here indicate that the total knee injury incidence in braced players is statistically significantly higher in braced players, but the incidence of MCL injuries is not significantly different. In an effort to find what type of injury was causing the difference in total knee injuries between braced and unbraced players, analyses were done of the incidence of type of injury (i.e., MCL grades I-II-III, ACL, meniscus, combination injuries and "other" injuries), which was available from the 1987 season data. In each case the chi-square test was not significant for each individual type of injury, yet the test for the combined total did reach significance. This appears to be a case where the individual components of a set of data are not significantly different, but the combined data do result in a significant difference. As more data is added to the data base in future seasons, this anomaly will likely be resolved.

Even though the prophylactic knee braces do not appear to reduce the incidence of total knee injuries or of MCL injuries, it is still possible that use of the braces could reduce the severity of injuries to the knee. However, the results presented here indicate that, whether measured by incidence of surgery, by degree of MCL injury or by days lost from participation, the use of braces has no significant effect in reducing the severity of all knee injuries and specifically of MCL injuries.

The introduction of prophylactic knee braces, along with an impressive array of anecdotal testimonials and a few early studies at individual schools attesting to the protective ability of these braces, posed a liability dilemma for football programs. If the braces do greatly reduce the incidence and severity of knee injuries, the school could be held liable for a career-ending knee injury if the school did not provide braces for its players While the cost of supplying braces to all football players on the team is much less than the cost of a lawsuit, it is still a considerable ongoing expense. Some schools reacted by requiring all players to wear braces, some by requiring them on players in positions at high risk for knee injury, and others by making braces available on a voluntary basis. Recently, as newer studies on larger samples over longer periods of time and with better epidemiological designs are beginning to show little reduction in knee injury incidence with braces, and some even showing a higher incidence of knee injuries in players wearing braces, schools are again faced with a decision, this time whether or not to continue requiring the use of braces. Many schools that previously had required use of the braces are now making their use voluntary.

As suggested by Teitz et al. (1987), one important explanation for some of the positive results of the early small-scale, local studies of injury rates in prophylactic knee braces may be found in the statistical phenomenon known as "regression to the mean. " If a football team just completed a season with a particularly high number of knee injuries, this would understandably provide the incentive to use prophylactic knee braces. When the number of knee injuries during the following season is considerably reduced, it is taken as "proof" that the knee braces are effective. However, the principle of regression to the mean states that whenever the results of an observation (in this case the number of knee injuries in a team for a season) differs greatly from the long-term average or mean, the results of the next observation will most likely return to a level closer to the mean. In other words, the number of knee injuries in a season following a high number of injuries will most likely be lower (closer to the long-term average) whether braces are used or not. This phenomenon emphasizes the need for national-scale, long-term data collection such as this study represents.

The data presented here represent results from the first two years of operation of the National Sports Injury Surveillance System, and this effort is continuing. Further reports from this data collection system will be forthcoming. During the 1988 football season data will be collected on whether a player with a reported knee injury has a previous history of significant knee injury, in order to assess the impact of that factor on the incidence of injuries in braced and unbraced players. It should be noted, however, that Teitz et al. (1987) removed from their analyses all players with a history of knee injury and still found results similar to those presented here.

The concept of the prophylactic knee brace is intuitively logical, but the results from a growing number of large-scale, long-term studies and biomechanical studies indicate that they are not accomplishing the protection from knee injuries they are designed to provide. It is beginning to appear that the prophylactic knee brace needs more development work before it will have any significant impact on reducing the number and severity of knee injuries in football .

Acknowledgments

Funding for this study was provided by the International Institute for Sport and Human Performance, of the College of Human Development and Performance, University of Oregon. The author wishes to thank David Wetter and Brian Fendrich for their extensive efforts in developing and maintaining the NSISS computer programming, Doug Spencer and Willy Pieter for their help in handling the considerable volume of data, and especially the many athletic trainers across the country whose efforts in providing the injury and exposure data to NSISS are greatly appreciated.

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