Physical activity, an important component of health

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1 ORIGINAL RESEARCH CONTRIBUTION Epidemiology of 6.6 Million Knee Injuries Presenting to United States Emergency Departments From 1999 Through 2008 Brett E. Gage, Natalie M. McIlvain, Christy L. Collins, MA, Sarah K. Fields, JD, PhD, and R. Dawn Comstock, PhD Abstract Background: While knee injuries are treated by a wide range of clinicians, patients with knee injuries frequently present to emergency departments (EDs). The knee is the most commonly injured joint by adolescent athletes with an estimated 2.5 million sports-related injuries presenting to EDs annually. Objectives: The objective was to examine the epidemiology of knee injuries presenting to EDs in the United States from 1999 through Methods: The Consumer Product Safety Commission National Electronic Injury Surveillance System database (NEISS) was used to examine causes of knee injuries treated in U.S. EDs from 1999 through Results: An estimated 6,664,324 knee injuries presented to U.S. EDs from 1999 through 2008, for a rate of 2.29 knee injuries per 1,000 population. Those 15 to 24 years of age had the highest injury rate (3.83), while children younger than 5 years had the lowest rate (0.55). The most common diagnoses were strains and sprains (42.1%), contusions and abrasions (27.1%), and lacerations and punctures (10.5%). The most common general product categories causing injury were sports and recreation (49.3%), home structures (30.2%), and home furnishings (13.6%). Several sex and age group differences were identified. For example, males sustained a higher proportion of basketball-related injuries (11.1%) than females (3.6%; injury proportion ratio [IPR] = 3.11, 95% confidence interval [CI] = 2.79 to 3.46, p < 0.001). Additionally, individuals 65 years and older sustained a higher proportion of injury due to stairs, ramps, landings, and floors (42.0%), compared to all other ages (20.1%; IPR = 2.09, 95% CI = 1.95 to 2.23, p < 0.001). Conclusions: Rates and patterns of knee injuries vary by sex and age. Although knee injuries will likely continue to occur most frequently among youth and young adult athletes, anticipating and responding to trends such as an increase in the incidence of knee injuries among adult and senior patients will enable clinicians to better anticipate caseloads, allocate resources, and determine best practices for diagnosis and treatment of knee injuries in different age groups. ACADEMIC EMERGENCY MEDICINE 2012; 19: ª 2012 by the Society for Academic Emergency Medicine From The Research Institute at Nationwide Children s Hospital, Center for Injury Research and Policy (BEG, NMM, CLC, RDC), Columbus, OH; the College of Medicine, Department of Pediatrics and College of Public Health, Division of Epidemiology (RDC), and the School of Physical Activity and Educational Services (SKF), The Ohio State University, Columbus, OH. Received March 22, 2011; revisions received June 21, August 23, and October 2, 2011; accepted October 3, We thank the U.S. Consumer Product Safety Commission for providing National Electronic Injury Surveillance System (NEI- SS) data. This study was not supported by any funding agency. The authors have no disclosures or conflicts of interest to report. Supervising Editor: Rochelle Fu, MD. Address for correspondence and reprints: Natalie McIlvain; natalie.mcilvain@nationwidechildrens.org. Physical activity, an important component of health and well-being for people of all ages, 1,2 has been directly linked to increased physical competency, decreased health risks, decreased body mass index, and increased social interaction. 3 Unfortunately, physical activity also carries a risk of injury, 4,5 which can exact psychological, emotional, physical, social, and economic tolls. 4,6 More specifically, participation in physical activities such as sports and recreation has been identified as a strong risk factor for knee injury As life expectancy continues to increase, the span over which adults participate in such physical activities is also increasing. Therefore, the incidence of knee injuries among older adults can also be expected to increase. Although knee injuries will likely continue to occur most frequently among adolescent and young adult athletes, anticipating and responding to trends such as an increase in the incidence ISSN ª 2012 by the Society for Academic Emergency Medicine 378 PII ISSN doi: /j x

2 ACADEMIC EMERGENCY MEDICINE April 2012, Vol. 19, No of knee injuries among older patients will enable clinicians to better anticipate caseloads, allocate resources, and determine best practices for diagnosis and treatment of knee injuries in different age groups. While knee injuries are treated by a wide range of clinicians, patients with knee injuries frequently present to emergency departments (EDs). The knee is the most commonly injured joint by adolescent athletes, with an estimated 2.5 million sports-related injuries presenting to EDs annually. 7,8 The multidirectional forces imposed on the complex knee joint during physical activity explain the types of severe knee injuries, including ruptures of the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL), commonly seen in EDs. 8,10 Such injuries frequently require expensive surgery and extensive rehabilitation. 11 Prevention is important because a knee injury can end an athlete s competitive career and can permanently affect physical activity capabilities among individuals of all ages. Additionally, while knee injuries can have economic effects on the injured individuals, they also pose a burden on the health care system The current literature on knee injuries, although extensive, focuses primarily on young athletes, particularly female populations. 4,5,15 24 For example, knee injuries are reported to account for 60% of high school sports-related surgeries, and female athletes have been reported to be four to six times more likely to sustain a major knee injury. 17,25,26 Additionally, researchers have attributed the vulnerability of female knees to numerous factors, including anatomical, environmental, hormonal, neuromuscular, and biomechanical differences. 21,22,27,28 While a great deal of effort continues to be devoted to the study of knee injuries among young female athletes, the peer-reviewed literature lacks a comprehensive epidemiologic analysis of current knee injury trends and risk factors across the age continuum in both sexes. The objective of this study was to provide clinicians with a description of the current epidemiology of knee injuries by evaluating all knee injuries presenting to a nationally representative sample of U.S. EDs from 1999 through Our specific aims were to compare trends in knee injury rates over time by sex and age groups and to describe patterns of knee injury by sex and age groups. Thus, this study will fill the current gap in the knee injury literature, providing clinicians with current data to drive evidence-based decisions regarding clinical care and preventive efforts. products or activities were involved with the injury (e.g., if an individual standing on a ladder fell hitting his or her knee on the floor, ladder and floor would be primary and secondary product codes). Study Protocol All knee injuries reported to CPSC NEISS from 1999 through 2008 were studied. Variables of interest were age, sex, date of injury, injury diagnosis, injury disposition, and primary product code. Age was categorized into six groups based on U.S. census data 5-year groupings: younger than 5, 5 to 14, 15 to 24, 25 to 44, 45 to 64, and 65 years and older. 30 Additionally, age was further dichotomized into youth young adult and adult senior categories (i.e., under 25 and 25 years and older) to evaluate broader age-related differences. Injury diagnoses were classified as strain sprain, contusion abrasion, laceration puncture, dislocation, fracture, burn, or other. Injury dispositions were classified as released, admitted, transferred, held for observation, left without being seen, and not recorded. There were a total of 681 product codes associated with knee injuries in the study data set. Product codes were analyzed both individually and by three CPSC NEISS-designated general categories: sports and recreation, home furnishings, and home structures. Data Analysis Data analyses were conducted using SPSS software, version 17.0 (SPSS Inc., Chicago, IL). Statistical weights provided by CPSC were applied to knee injury case counts to calculate national estimates of knee injuries. Additionally, as recommended by CPSC, due to the complex survey design of NEISS, the Complex Samples Module of SPSS was used to adjust for the NEISS sampling strata and clustering variables (provided by the CPSC) as well as survey changes during the study period. 29 All data represent national estimates unless otherwise stated. Population-based injury rates were calculated using the 1999 through 2008 population estimates for 5-year age groups obtained from the U.S. Census Bureau as the denominators. 30 Statistical analyses included the calculation of injury proportion ratios (IPRs) with 95% confidence intervals (CIs) to assess magnitude and direction of associations. Trends of knee injuries over time were analyzed using linear regression while controlling for sex or age. An example of IPR calculation is METHODS Study Design This was a retrospective review using data from the U.S. Consumer Product Safety Commission (CPSC) s National Electronic Injury Surveillance System (NEISS), a nationally representative, stratified probability sample of 100 U.S. hospital EDs, including eight children s hospitals. 29 IPR ¼ (nationally estimated number of male strain or sprain injuries/total nationally estimated number of make knee injuriesþ (nationally estimated number of female strain or sprain injuries/ total nationally estimated number of female knee injuriesþ Study Setting and Population At NEISS hospitals, trained coders review all ED records daily, abstracting information on each injury case into the NEISS database. Coders assign each case one or two CPSC-specific codes that designate what The 95% CIs not containing 1.0 and p < 0.05 were considered statistically significant. This study was approved by the institutional review board at Nationwide Children s Hospital.

3 380 Gage et al. KNEE INJURIES ACROSS THE AGE CONTINUUM RESULTS Demographics and Injury Rates From 1999 through 2008, an estimated 6,664,324 knee injuries presented to U.S. EDs (based on 172,982 individual reported cases), for an injury rate of 2.29 knee injuries per 1,000 population. The overall knee injury rate (Figure 1A), which peaked in 2001 and again in 2008, did not change significantly (p = 0.97). Knee injuries ranged from a low of 628,306 in 1999 to a high of 723,773 in The mean (±SD) age of injured individuals was 32.7 (±21.5) years; range = 1 month to 101 years) and 52.2% were male. Although males had a slightly higher injury rate than females (2.43 vs. 2.15; Figure 1A), it is unlikely that this difference is clinically significant given the small magnitude of this subgroup difference. There were no significant sex trends over time. Individuals aged 15 to 24 years had the highest injury rate (3.83), while children younger than 5 years had the lowest (0.55). The age group that sustained the greatest proportion of knee injuries was 25 to 44 years of age (27.2%; Table 1). Knee injury rates for individuals 5 to 14 years decreased significantly from 1999 to 2008 (p = 0.02) while the injury rates among adults 45 to 64 and 65 years and older significantly increased (p = and p = 0.01, respectively; Figure 1B). Sex Differences Females sustained a higher proportion of contusions abrasions (31.8%) and fractures (4.3%) than males (22.8% and 3.5%; contusions abrasions IPR = 1.40, 95% CI = 1.35 to 1.45, p < 0.001; fractures IPR = 1.23, 95% CI = 1.14 to 1.34, p < 0.001), but males sustained a higher proportion of lacerations punctures (13.5%) than females (7.3%; IPR = 1.86, 95% CI = 1.76 to 1.97, p < 0.001; Table 1). Males sustained a higher proportion of knee injuries from sports and recreation products (61.6%) than females (35.9%; IPR = 1.719, 95% CI = to 1.722, p < 0.001; Table 1). More specifically, males sustained a higher proportion of football-related injuries (10.5%) than females (0.7%; IPR = 15.85, 95% CI = to Knee Injury per 1,000 US Population A Year Overall Male Female Knee Injury per 1,000 US Population B Year < 5 years 5-14 years* years years years* 65 years* Figure 1. Nationally estimated number of knee injuries presenting to US EDs from 1999 through 2008 by (A) sex and (B) age. *Age categories with significant knee injury trends over time: 5 to 14 years (p = 0.02), 45 to 64 years (p = 0.002), and 65 years (p = 0.01).

4 ACADEMIC EMERGENCY MEDICINE April 2012, Vol. 19, No Table 1 National Estimates of Knee Injuries Presenting to U.S. EDs From 1999 Through 2008 Characteristic Male (n =3,478,749) Female (n =3,184,490) Total* (N =6,664,324) Age (yr) <5 63,713 (1.8) 46,157 (1.4) 109,980 (1.7) ,256 (24.5) 538,978 (16.9) 1,391,579 (20.9) ,082 (28.0) 593,927 (18.7) 1,568,217 (23.5) ,720 (27.8) 845,015 (26.5) 1,812,881 (27.2) ,527 (12.1) 638,397 (20.0) 1,058,056 (15.9) ,452 (5.8) 522,015 (16.4) 723,610 (10.9) Diagnosis Sprain strain 1,504,895 (43.3) 1,301,926 (40.9) 2,807,395 (42.1) Contusion abrasion 791,827 (22.8) 1,011,384 (31.8) 1,803,471 (27.1) Laceration puncture 470,081 (13.5) 231,025 (7.3) 701,137 (10.5) Dislocation 141,590 (4.1) 119,195 (3.7) 260,784 (3.9) Fracture 121,163 (3.5) 136,905 (4.3) 258,158 (3.9) Burns 6,150 (0.2) 4,477 (0.1) 10,627 (0.2) Other 443,043 (12.7) 379,579 (11.9) 822,751 (12.3) Disposition Released 3,399,597 (97.7) 3,090,882 (97.1) 6,491,564 (97.4) Admitted 46,780 (1.3) 64,334 (2.0) 111,114 (1.7) Otherà 32,371 (1.0) 29,274 (0.9) 61,646 (0.9) General product codes Sports and recreation 2,144,598 (61.6) 1,142,088 (35.9) 3,287,416 (49.3) Home structures 729,545 (21.0) 1,284,166 (40.3) 2,013,928 (30.2) Home furnishings 345,362 (9.9) 559,611 (17.6) 907,973 (13.6) Other 260,906 (7.5) 197,438 (6.2) 459,838 (6.9) Data are reported as n (%). *Due to a low level of nonresponse, these totals are always similar, but are not always equal to the total number of injuries. Other diagnoses included effusion, avulsion, swelling etc. à Other disposition category (combined due to totals of <1.0%) included transferred, held for observation, left without being seen, and not recorded. Other general product code included child nursery equipment and supplies, toys, yard and garden, general household appliances, etc. Table 2 Male and Female Knee Injuries Seen in U.S. EDs From 1999 Through 2008 by Injury Diagnosis and Age Age (yr) Diagnosis < Male n=63,713 n=852,256 n=974,082 n=967,720 n=419,527 n=201,452 Sprain 13,467 (21.1) 259,176 (30.4) 499,553 (51.3) 495,986 (51.3) 180,042 (42.9) 56,672 (28.1) strain Contusion 21,326 (33.5) 254,130 (29.8) 182,623 (18.7) 166,339 (17.2) 90,919 (21.7) 76,490 (38.0) abrasion Laceration 18,194 (28.6) 205,543 (24.1) 88,197 (9.1) 91,368 (9.4) 48,989 (11.7) 17,790 (8.8) puncture Dislocation 523 (0.8) 23,207 (2.7) 61,452 (6.3) 40,942 (4.2) 11,724 (2.8) 3,741 (1.9) Fracture 1,943 (3.0) 25,196 (3.0) 24,026 (2.5) 31,117 (3.2) 22,402 (5.3) 16,479 (8.2) Burns 878 (1.4) 606 (0.1) 810 (0.1) 1,878 (0.2) 1,402 (0.3) 575 (0.3) Other* 7,382 (11.6) 84,398 (9.9) 117,421 (12.1) 140,089 (14.5) 64,048 (15.3) 29,705 (14.7) Female n=46,157 n=538,978 n=593,927 n=845,015 n=638,397 n=522,015 Sprain 7,974 (17.3) 188,469 (35.0) 292,811 (49.3) 404,347 (47.9) 264,347 (41.4) 143,979 (27.6) strain Contusion 15,962 (34.6) 172,456 (32.0) 152,820 (25.7) 242,255 (28.7) 209,033 (32.7) 218,858 (41.9) abrasion Laceration 14,123 (30.6) 93,220 (17.3) 34,362 (5.8) 42,249 (5.0) 25,141 (3.9) 21,930 (4.2) puncture Dislocation 120 (0.3) 22,295 (4.1) 38,711 (6.5) 33,600 (4.0) 17,107 (2.7) 7,361 (1.4) Fracture 1,888 (4.1) 10,777 (2.0) 8,313 (1.4) 19,662 (2.3) 37,277 (5.8) 58,988 (11.3) Burns 977 (2.1) 987 (0.2) 374 (0.1) 907 (0.1) 714 (0.1) 518 (0.1) Other* 5,113 (11.1) 50,774 (9.4) 66,537 (11.2) 101,994 (12.1) 84,779 (13.3) 70,381 (13.5) All values are reported as n (%) *Other diagnoses included effusion, avulsion, swelling, etc.

5 382 Gage et al. KNEE INJURIES ACROSS THE AGE CONTINUUM Table 3 Top Five Most Common Specific Products Associated With Knee Injuries Presenting to US EDs From by Age and Sex Most Common Specific Products* Male Female Total <5 yr Stairs, ramps, landings, floors Beds, mattresses, pillows Trampolines Chairs, sofas, and sofa beds Playground and equipment yr Bicycles and accessories Football Basketball Stairs, ramps, landings, floors Soccer yr Basketball Stairs, ramps, landings, floors Football Soccer Miscellaneous sports yr Stairs, ramps, landings, floors Basketball Exercise and equipment Baseball softball Bicycles and accessories yr Stairs, ramps, landings, floors Exercise and equipment Ladders, stools Chairs, sofas, and sofa beds Bicycles and accessories yr Stairs, ramps, landings, floors Beds, mattresses, pillows Misc personal use items Chairs, sofas, and sofa beds Bathtub and shower structures Overall Stairs, ramps, landings, floors Basketball Bicycles and accessories Football Exercise and equipment All values are percentages. *Only primary specific product codes associated with knee injury were analyzed , p < 0.001), basketball-related injuries (11.1%) than females (3.6%; IPR = 3.11, 95% CI = 2.79 to 3.46, p < 0.001), and bicycles- and accessories-related injuries (8.2%) than females (3.8%; IPR = 2.16, 95% CI = 2.14 to 2.17, p < 0.001; Table 3). Females sustained a higher proportion of injuries due to home structures (40.3%) and home furnishings (17.6%) than males (21.0% and 9.9%, respectively; IPR = 1.92, 95% CI = 1.86 to 1.99, p < 0.001; IPR = 1.77, 95% CI = 1.68 to 1.86, p < 0.001; Table 1). More specifically, females sustained a higher proportion of injuries related to stairs, ramps, landings, and floors (32.1%) than males (13.7%; IPR = 2.34, 95% CI = 2.24 to 2.46, p < 0.001; Table 3). Age Differences Youth Young Adult. Children younger than 5 years had a higher proportion of lacerations punctures (29.4%) than all other ages (10.2%; IPR = 2.88, 95% CI = 2.67 to 3.11, p < 0.001) and burns (1.7%) than all other ages (0.1%; IPR = 12.61, 95% CI = 8.58 to 18.52, p < 0.001). Children younger than 5 years sustained a higher proportion of injury due to trampolines (7.6%) than all other ages (0.9%; IPR = 8.51, 95% CI = 6.86 to 10.57, p < 0.001) and beds, mattresses, and pillows (10.4%) than other all other ages (2.6%; IPR = 3.96, 95% CI = 3.44 to 4.55, p < 0.001). Children 5 to 14 years sustained a higher proportion of bicycles- and accessories-related injuries (14.4%) than all other ages (3.9%; IPR = 3.69, 95% CI = 3.66 to 3.71, p < 0.001). Those 15 to 24 years sustained a higher proportion of strains and sprains (50.5%) compared with all other ages (39.5%; IPR = 1.28, 95% CI = 1.24 to 1.32, p < 0.001) and dislocations (6.4%) compared to all other ages (3.2%; IPR = 2.03, 95% CI = 1.88 to 2.19, p < 0.001). Individuals 15 to 24 years sustained a higher proportion of basketball injuries (14.5%) than all other ages (5.4%; IPR = 2.71, 95% CI = 2.69 to 2.72, p < 0.001) and a higher proportion of football injuries (11.3%) than all other ages (4.1%; IPR = 2.77, 95% CI = 2.75 to 2.78, p < 0.001). Adult Senior. Those 25 to 44 years sustained a higher proportion of injury due to exercise and equipment (4.9%) than all other ages (4.0%; IPR = 1.226, 95% CI = to 1.227, p < 0.001). Those 45 to 64 years sustained a higher proportion of injury due to ladders and stools (4.2%) compared to all other ages (1.4%; IPR = 3.06, 95% CI = 3.02 to 3.09, p < 0.001). Individuals 65 years and older had a higher proportion of contusions abrasions (40.8%) compared to all other ages (25.4%; IPR = 1.61, 95% CI = 1.52 to 1.70, p < 0.001) and fractures (10.4%) than all other ages (3.1%; IPR = 3.40, 95% CI = 3.14 to 3.69, p < 0.001). Adults 65 years and older sustained a higher proportion of injury due to stairs, ramps, landings, and floors (42.0%) compared to all other ages (20.1%; IPR = 2.09, 95% CI = 1.95 to 2.23, p < 0.001) and bathtub- and shower structure related injuries (4.1%) compared to all other ages (1.5%; IPR = 2.74, 95% CI = 2.45 to 3.06, p < 0.001). Youth Young Adult Versus Adult Senior. Additional patterns were evident when larger age groups were compared. Individuals younger than 25 years sustained a higher proportion of knee injuries resulting from sports and recreation products (73.1%) than those 25 (30.3%; IPR = 2.41, 95% CI = 2.21 to 2.63, p < 0.001). More specifically, individuals younger than 25 years sustained a higher proportion of football injuries (11.2%) than those 25 (1.4%; IPR = 7.88, 95% CI = 6.88 to 9.04, p < 0.001). Individuals 25 years or older sustained a higher proportion of injuries associated with home structures (41.9%) than individuals under 25 years (16.6%; IPR = 2.52, 95% CI = 2.37 to 2.69, p < 0.001) and injuries associated with home furnishings (18.6%) than individuals younger than 25 years (7.6%; IPR = 2.44, 95% CI = 2.24 to 2.65, p < 0.001). DISCUSSION The magnitude of the public health and clinical burden posed by knee injuries is clearly evident by the incidence

6 ACADEMIC EMERGENCY MEDICINE April 2012, Vol. 19, No < 5 years 5-14 years years years years 65 years 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Strain/sprain Contusion/abrasion Laceration/puncture Dislocation Fracture Burns Other Figure 2. Nationally estimated number of knee injuries presenting to U.S. EDs from 1999 through 2008 by diagnoses and age. of knee injuries reported in this study (an estimated 6,664,324 knee injuries presenting to U.S. EDs during the study period), as well as the fact that 1.7% of these knee injuries were so severe the injured individual required hospitalization. To the best of our knowledge, this report is the first comprehensive look at the epidemiology of knee injuries across the entire age continuum in both sexes. Previous studies primarily focused on sports-related knee injuries, particularly among young female athletes, specifically ACL injuries. 4,7,10,11,15 21,23,24,26,27,31,32 Even prior studies reporting ED cases have focused on sports-related injuries. 7,33 Thus, our study fills an important gap in the current body of knowledge. A full understanding of sex and age group differences in knee injury patterns and risk factors is required, both to assist clinicians in providing the best clinical care as the demographics of individuals sustaining knee injuries changes and to drive effective targeted prevention efforts. Clinicians and researchers should be encouraged to focus attention on the prevention of knee injuries, given the high incidence of these injuries, the physical and economic effect to the injured individuals, and the burden they pose on the health care system Because the NEISS data set includes only those knee injuries presenting to an ED, our study undoubtedly underestimates true knee injury rates. 34 In reality, the clinical and public health burden of knee injuries is likely much higher than reported here. Individuals with knee injuries may seek care from a wide variety of clinicians (athletic trainers, primary care physicians, urgent care centers, sports medicine clinics, etc). Unfortunately no current surveillance system is capable of capturing data from this wide array of settings. However, our results are similar to those of previous studies with respect to general epidemiologic patterns. For example, similar to prior studies, we found the most common mechanism of knee injury, particularly among those younger than 25 years, was sports and recreation activities. Similarly, consistent with prior research we found the most common diagnoses were strains sprains. 16,33 As Burt and Overpeck 33 reported, strains sprains are more prevalent among sports-related injury visits treated in the ED than non sports-related injuries. Such diagnoses are associated with physical movements common to sports and recreational activities (i.e., rotation around a planted foot, jumping landing). These findings are not surprising, as youth are frequently exposed to physical activity in gym class and youth and young adults to sports and recreation programs both in school and in community settings. Thus sports-related knee injury prevention efforts have largely been targeted at the younger athletic population. Our findings support such sports-related knee injury prevention efforts, which have included a variety of approaches such as sport-specific skills training (e.g., planting and changing direction, jumping landing); strengthening and conditioning programs aimed to address potential deficits in the neuromuscular strength and coordination of stabilizing muscles about the knee joint; and combined programs of stretching, plyometrics, and strength training drills. 22,28,35 However, the long-term clinical effects of such prevention efforts on reducing the incidence or severity of sports-related knee injuries is not yet fully clear. Our study also identified a largely unexamined emerging issue of increasing knee injury rates among adults and seniors as well as several significant differences in patterns of and risk factors for knee injury among this group compared to youth and young adults. For example, the injury rate among the 45 to 64 and 65 years and older age groups increased significantly during the study period, individuals 25 years and older were nearly three times more likely to sustain an injury associated with home structures and home furnishings than those younger than 25, and fractures were more common among seniors than other age groups. These are important findings given the ageing U.S. population, coupled with the relative paucity of research in the prevention of household-associated knee injuries. 5 Although a large body of work has focused on preventing household falls among the elderly, prevention efforts specific to knee injuries are largely lacking. Based on our findings, we believe that continued

7 384 Gage et al. KNEE INJURIES ACROSS THE AGE CONTINUUM research is needed to drive the development, implementation, and evaluation of targeted efforts to prevent knee injuries among adults and seniors. LIMITATIONS The limitations of this study are primarily associated with the NEISS data set, which provides limited information about cause of injury and surrounding circumstances. For example, no distinction is made between organized and unorganized sports. Additionally, the NEISS data set only contains the most severe injury for each patient. Thus, the number of knee injuries presenting to EDs for medical care is undoubtedly underreported here. However, to the best of our knowledge, this study is the first to use national estimates gleaned from a large, nationally representative sample to evaluate patterns and risk factors for injury across the entire age continuum in both sexes. While the important information that can be learned from this very large, longstanding data set outweigh these limitations, the sheer size of the data set does introduce the possibility of misinterpretation of the clinical importance of subgroup differences. Although large data sets like NEISS provide the power to identify statistical significance of subgroup differences with very small measures of magnitude, readers should be cautioned when interpreting IPRs lower than 2.0, as these may have little clinical significance. CONCLUSIONS Knee injuries present one of the highest clinical and public health injury related burdens, given the high incidence of knee injuries across the age continuum and the frequent need for surgical repair and long-term rehabilitation. Driven by the large quantity of literature on anterior cruciate ligament injuries, knee injuries are often thought to be primarily a problem of young female athletes. However, as demonstrated by our results, knee injuries occur among both sexes and across all age groups. Although a certain endemic level of knee injury will always be associated with physical activity, and efforts to increase physical activity levels among the U.S. population must continue to be supported, an increased commitment should be made to reduce the number and severity of knee injuries in the general population. Injury rates, diagnoses, and mechanisms of injury vary by age and sex. Although knee injuries will likely continue to occur most frequently among adolescent and young adult athletes, anticipating and responding to trends such as an increase in the incidence of knee injuries among adult and senior patients will enable clinicians to better anticipate caseloads, allocate resources, and determine best practices for diagnosis and treatment of knee injuries in different age groups. References 1. Prasad DS, Das BC. Physical inactivity: a cardiovascular risk factor. 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