Eric D. Zemper1

A FOUR-YEAR PROSPECTIVE STUDY OF PREVENTIVE KNEE BRACES AND MCL INJURIES IN A NATIONAL SAMPLE OF COLLEGE FOOTBALL PLAYERS

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REFERENCE: Zemper, E.D., "A Four-Year Study of Preventive Knee Braces and MCL Injuries in a National Sample of College Football Players," Safety in American Football, ASTM STP XXX, Earl F. Hoerner, Ed., American Society for Testing and Materials, Philadelphia (In Press).

ABSTRACT: Data were collected during four seasons (1987-1990) from a representative stratified national sample of college football teams to assess the ability of preventive knee braces to reduce the number and severity of knee injuries, specifically medial collateral ligament (MCL) injuries. There were 95 team-seasons covered, with a total of 9 557 player-seasons accumulating 737 041 exposures to the possibility of being injured in a game or practice. All players on each team were a part of the study, and 2 176 players (22.77%) wore braces. Analyses of the data indicate that the number of MCL injuries in braced players was significantly higher. When analyzed by severity of MCL injury (grade, days lost or surgical injury), there were no significant differences between braced and unbraced players. When game injuries were isolated to control for the "intensity" of exposure to the possibility of being injured, the analyses again showed no demonstrable ability of the braces to prevent or reduce the severity of MCL injuries.

KEYWORDS: preventive knee brace, football, medial collateral ligament injuries, epidemiology

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1President and Director of Research, Exercise Research Associates of Oregon, P.O. Box 10123, Eugene, OR 97440.

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The most frequent site of injury in football is the knee [1,2], and the MCL is the most frequent type of knee injury in this sport. Over the past fifteen years the most common means of attempting to prevent the number and severity of knee injuries in football, specifically of the MCL, has been the use of preventive knee braces. While there have been a large number of anecdotal testimonials to the efficacy of these braces, the few small-scale studies published have shown equivocal results. Until recently, large national-scale studies have been practically non-existent.

In an effort to assess the value of preventive knee braces in reducing the number and severity of MCL injuries in football, data were collected from a representative national sample of college football teams over a four year period by the Athletic Injury Monitoring System (AIMS). AIMS is designed to be a longitudinal sports injury data collection system that provides analyses of national injury rates and patterns for the purpose of assisting in developing ways to prevent or reduce the severity of sports injuries through rule changes, equipment modifications or changes in coaching techniques. AIMS also is designed to collect data for specialized studies of specific issues such as the knee brace study presented here.

 

METHODS

The data for this study were collected during four seasons (1987-1990) by AIMS. This data collection system meets the major criteria for reliable studies of sports injury rates outlined in 1987 by the American Orthopaedic Society for Sports Medicine [3]. The AIMS sample was a stratified, proportionally representative sample, based on geographic region and size of athletic program, of all NCAA and NAIA intercollegiate football teams, and the results of this study are generalizable to the total population of intercollegiate football teams. The study population included all intercollegiate football players at the institutions involved in this study. Over the four year period of this study, data were collected for 95 team-seasons, with a total of 9 557 player-seasons accumulating 737 041 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.

Prior to the start of each football season the head athletic trainer for each participating team received copies of forms for reporting exposure and injury data, along with detailed instructions on using the forms. On a weekly basis, from the first preseason practice to the final regular season or postseason game, the athletic trainers returned a form listing the number of practices and any games played during the week, and the number of players participating in each. A separate report form was completed for each injury that kept a player from participating for one day or more, detailing the nature and circumstances of the injury. Upon receipt at the AIMS office, each form was logged in and screened for completeness and consistency before being entered into a computer file for later analysis. In the case of missing or inconsistent data on any form, the athletic trainer was contacted for clarification. During the 4 years of this study, 98 percent of the weekly forms were returned.

For this study the athletic trainers also completed a form at the beginning of each season indicating the number of players at each position, how many wore preventive 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 school. Since this study involves a relatively large sample, it is assumed that a variety of policies were in use, representing the situation in the total national population. Using the individual injury forms, the athletic trainers provided details on each injury such as the type of injury, circumstances, type of surface being used, number of days away from participation, whether or not surgery was required and, if it was a knee or ankle injury, whether or not a preventive knee brace was being worn at the time of injury, and what brand. In the case of knee injuries, information on the specific type of knee injury (MCL, ACL, meniscus, etc.) were provided. The grade of any ligamentous knee injury also was provided using the criteria of Fetto and Marshall [4], which were included in the instructions for completing the injury forms. Early in this study it was found that at least 95% of those who wore preventive knee braces wore them in both games and practices, so the simplifying assumption was made that all players who wore braces were wearing them in both games and practices. In another national-scale study by Teitz et al. [5], it was found that the distribution of braces across strings (i.e., ability levels) were similar, so it was assumed that this was not a factor affecting results in this study.

The data for this study were analyzed using a chi-square test (alpha = .05).

 

 

RESULTS

A summary of the numbers and rates of injuries observed in this study is presented in Table 1. Nearly one out of every five time-loss injuries reported was to the knee. Nearly one-half the total number of knee injuries and over two-thirds of the ligamentous knee injuries were to the MCL. Table 2 presents the chi-square analysis of the MCL injuries in braced and unbraced players. Chi-square values in this study were calculated based on the null hypothesis that the braces have no effect on knee injuries and therefore the expected distribution of MCL injuries between braced and unbraced players should match the distribution of players wearing (22.77%) and not wearing (77.23%) preventive knee braces. If the braces are having a positive effect in reducing the number of MCL

 

TABLE 1--Injury Summary for 1987-1990 Seasons

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Total # players (Player-seasons)

9 557

Total # players wearing preventive knee braces

2 176

Total Athlete-Exposures (A-E)

737 041

Total # injuries reported

4 559

Total # knee injuries reported

884

Total # ligamentous knee injuries reported

604

Total # MCL injuries reported

425

Injury Rates

 

Total injuries per 1 000 A-E

6.19

Total injuries per 100 players

47.7

Total knee injuries per 1 000 A-E

1.20

Ligamentous injuries per 1 000 A-E

0.82

MCL injuries per 1 000 A-E

0.58

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TABLE 2--MCL injuries in braced and unbraced football players

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MCL Injuries

 

 

Observed

(Expected)

Standardized Rediduals

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No Brace

303

(327.2)

-1.34

Braced

122

( 96.8)

2.46

 

_________

___________

 

Totals

425

(425.0)

 

 

Chi-square = 7.840

Critical Value = 3.841

alpha=.05, df=1

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injuries, it would be expected that the chi-square value would be significant (rejecting the null hypothesis of no effect) and there would be fewer than the expected number of MCL injuries in braced players. As can be seen in Table 2, the chi-square value is significant, but the number of MCL injuries in braced players is higher than the expected number.

A chi-square test will tell whether or not a set of results fall within an expected distribution range, but it will not indicate which categories, in this case braced or unbraced players, are contributing most to the results. To identify which category was contributing the most to this significant chi-square value, standardized residuals were calculated and are presented in the right-hand column of Table 2. Major contributors to a significant chi-square value are those categories with a standardized residual having an absolute value of 2.00 or more [6,7]. As indicated in Table 2, the major factor in this significant chi-square value is the higher than expected number of MCL injuries in the braced players.

To investigate the possibility that preventive knee braces reduce the severity of MCL injuries, allowing players to return sooner, data were analyzed by grade of MCL injury (Table 3). The chi-square analysis shows that there is no significant difference in the distribution of the three grades of MCL injuries between braced and unbraced players, indicating that the preventive knee braces have no significant effect in reducing the severity of MCL injuries. Another way of looking at severity of injury is by days of participation lost for each injury. Table 4 shows the distribution of MCL injuries in braced players by number of days lost, blocked as indicated in the table. To avoid skewing the data, surgical injuries and season-ending injuries were eliminated from this analysis. The distribution of days lost for braced players is not significantly different from unbraced players. In addition, the average number of days lost per injury was the same for both braced (11.5 days) and unbraced players (11.4 days). Again, surgical and season-ending injuries were excluded from this analysis.

To control for 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 in situations that can result in knee injury, which in some way may skew the results in favor of braced or unbraced players), separate analyses were completed for game exposures only. Presumably every player participating in a game receives the same "quality" or "intensity" of exposure to the possibility of being injured. The results of these analyses are shown in Table 5, which indicates that again there is no significant difference in incidence of MCL injuries for braced and unbraced players. With regard to severity of MCL injuries in

 

 

TABLE 3--MCL injuries in braced and unbraced players by severity

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# MCL Injuries

 

Observed

(Expected)

MCL - I

 

 

No Brace

177

(194.6)

Braced

75

( 57.4)

 

_______

_______

Totals

252

(252.0)

 

 

 

MCL - II

 

 

No Brace

89

( 95.8)

Braced

35

( 28.2)

 

_______

__________

Totals

124

(124.0)

 

 

 

MCL - III

 

 

No Brace

37

( 37.8)

Braced

12

( 11.2)

 

________

__________

Totals

49

( 49.0)

Chi-square = 0.595 (NS)

Critical Value = 5.991

alpha=.05, df=2

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games, the average number of days lost per non-surgical, non-season-ending injury for braced players was 12.4 days and for unbraced players 11.7 days.

Several other observations made during these analyses are worth mentioning. While the decision whether or not to surgically repair a knee may vary from one physician to another, depending on philosophy and training, it still is interesting to note that there was no significant difference in the incidence of surgical knee injuries between braced and unbraced players. Analysis of the brands of braces worn showed no difference between brands. The data showed no difference in the ankle injury rate between braced and unbraced players, therefore the preventive knee brace does not appear to have any impact on the occurrence of ankle

 

 

TABLE 4--MCL injuries by days lost in braced players *

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# MCL Injuries

 

Days of Participation Lost

Observed

(Expected)

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1 - 2

12

( 15.8)

3 - 6

37

( 30.8)

7 - 9

10

( 10.0)

10 +

42

( 44.3)

 

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_________

Totals

101

(100.9)

 

Chi-square = 2.281 (NS)

Critical Value = 7.815

alpha=.05, df=3

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

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TABLE 5--MCL injuries in braced and unbraced players

(Game only - 1989-1990 seasons)

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MCL Injuries

 

Observed

(Expected)

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No Brace

57

(59.5)

Braced

20

(17.5)

 

_______

___________

Totals

77

(77.0)

 

Chi-square = 0.462 (NS)

Critical Value = 3.841

alpha=.05, df=1

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injuries. The proportion of non-contact knee injuries in braced players almost exactly matches the proportion of braced players, so wearing a knee brace does not appear to affect the occurrence of non-contact knee injuries. When occurrence of MCL injuries was analyzed by player position, the only statistically significant differences found were that defensive linemen wearing braces had higher than the expected number of injuries and defensive safeties wearing braces had fewer than the expected number of MCL injuries.

 

DISCUSSION

In the years since preventive knee braces were first introduced, there have been a number of studies attempting to demonstrate the ability of these braces to prevent or reduce the severity of knee injuries. Most of these studies have been done on single teams, often over a short time span [e.g., 8-10]. The results of these studies have been equivocal at best, with a fairly even balance between the number showing a positive effect and those showing no effect. Biomechanical studies have not been able to document a significant protective effect for the braces [12-14]. Only two national-scale studies have been reported, by Teitz et al. [5] and earlier reports from this study [15-16], and neither has been able to show a significant protective effect for these braces. One of the few well-designed and well-controlled small-scale studies, by Stitler et al. [17] at the U.S. Military Academy, did show some positive effect in reducing the number MCL injuries by wearing preventive knee braces, but only in defensive players. There was no effect on the severity of knee injuries. However, the subjects were cadets playing intramural football rather than larger and heavier intercollegiate players, which may indicate a possible size/weight and, therefore, a force threshold involvement.

The results presented here, based on four years' data from a large representative national sample of college football teams, does not support the ability of preventive knee braces to reduce the number or severity of MCL injuries. The question of the efficacy of preventive 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 conditions of surfaces used in practices and games, and policies on the use of braces (required or voluntary, all players or selected players). According to epidemiological theory, the impact of these factors can be most effectively minimized by using large numbers of players from a national sampling of schools [18], as done in this study and as recommended by Garrick and Requa [19] in their review of the research on preventive knee braces. It appears that most of the recent biomechanical studies and the statistical studies using larger, more representative samples over more than one or two seasons, 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. The concept of preventive knee braces is intuitively logical, but the results from this and a number of other recent studies appear to indicate that the device needs more development work before it will have any consistent and demonstrable impact on reducing the number and severity of MCL injuries in football.

 

ACKNOWLEDGEMENTS

This study was funded by Exercise Research Associates of Oregon, Inc. in Eugene OR, and the International Institute for Sport and Human Performance at the University of Oregon.

 

REFERENCES

[1] Alles, W.F., Powell, J.W., Buckley, W., and Hunt, E.E., "The National Athletic Injury/Illness Reporting System 3-Year Findings of High School and College Football Injuries," Journal of Orthopaedic and Sports Physical Therapy, Vol. 1, No. 2, 1979, pp 103-108.

[2] Zemper, E.D., "Injury Rates in a National Sample of College Football Teams: A 2-Year Prospective Study," The Physician and Sportsmedicine, Vol. 17, No. 11, 1989, pp 100-113.

[3] Thompson, N., Halpern, B., Curl, W.W., Andrews, J.R., and McCleod, W.D., "High School Football Injuries: Evaluation," American Journal of Sports Medicine, Vol. 15, 1987, pp 117- 124.

[4] Fetto, J.F., and Marshall, J.L., "Medial Collateral Ligament Injuries of the Knee: A Rationale for Treatment," Clinical Orthopaedics, Vol. 132, 1978, pp 206-218.

[5] Teitz, C.C., Hermanson, B.K., Kronmal, R.A., and Diehr, P.H., "Evaluation of the Use of Braces to Prevent Injury to the Knee in Collegiate Football Players," Journal of Bone and Joint Surgery, Vol. 69A, No. 1, 1987, pp 2-9.

[6] Haberman, S.J., "Residuals in Cross-Classified Data," Biometrics, Vol. 29, 1973, pp 205-220.

[7] Hinkle, D., Wiersma, W., and Jurs, S., Applied Statistics for the Behavioral Sciences, 2nd Ed., Houghton-Mifflin Co., Boston, 1988, p 556.

[8] Hansen, B.L., Ward, J.C., and Diehl, R.C., "The Preventive Use of the Anderson Knee Stabler in Football," The Physician and Sportsmedicine, Vol. 13, No. 9, 1985, pp 75-81.

[9] Randall, F., Miller, H,, and Shurr, D., "The Use of Prophylactic Knee Orthoses at Iowa State University," Orthotics and Prosthetics, Vol. 37, No. 4, 1984, pp 54-57.

[10] Hewson, G.F., Mendini, R.A., and Wang, J.B., "Prophylactic Knee Bracing in College Football," American Journal of Sports Medicine, Vol. 14, 1986, pp 262-266.

[11] Rovere, G.D., Haupt, H.A., and Yates, C.S., "Prophylactic Knee Bracing in College Football," American Journal of Sports Medicine, Vol. 15, 1987, pp 111-116.

[12] Baker, B.E., Van Hanswyk, E., Bogosian, S., et al., "A Biomechanical Study of the Static Stabilizing Effect of Knee Braces on Medial Stability," American Journal of Sports Medicine, Vol. 15, 1987, pp 566-570.

[13] Paulos, L.E., France, E.P., Rosenberg, T.D., et al., "The Biomechanics of Lateral Knee Bracing. Part I. Response of the Valgus Restraints to Loading," American Journal of Sports Medicine, Vol. 15, 1987, pp 419-429.

[14] France, E.P., Paulos, L.E., and Jayaraman, G., "The Biomechanics of Lateral Knee Bracing. Part II. Impact Response of the Braced Knee," American Journal of Sports Medicine, Vol. 15, 1987, pp 430-438.

[15] Zemper, E.D., "A Two-Year Prospective Study of Prophylactic Knee Braces in a National Sample of College Football Players," Sports Training, Medicine and Rehabilitation, Vol. 1, 1990, pp 287-296.

[16] Zemper, E.D., "A Prospective Study of Prophylactic Knee Braces in a National Sample of American College Football Players,' in: Proceedings of the First International Olympic Committee World Congress on Sport Sciences, US Olympic Committee, Colorado Springs, 1989, pp 202-203.

[17] Stitler, M., Ryan, J., Hopkinson, W., Wheeler, J., Santomier, J., Kolb, R., and Polley, D., "The Efficacy of a Prophylactic Knee Brace to Reduce Knee Injuries in Football," American Journal of Sports Medicine, Vol. 18, No. 3, 1990, pp 310-315.

[18] Zemper, E.D., "Epidemiology of Athletic Injuries," in: McKeag, D.B., Hough, D.O., and Zemper, E.D., Primary Care Sports Medicine, Brown & Benchmark, Dubuque IA, 1994, pp 63-73.