Rick P Csintalan, MD; Maria C S Inacio, MS; Tadashi T Funahashi, MD

Abstract

Context: Anterior cruciate ligament (ACL) reconstructions are among the most common sports medicine procedures performed in the US each year. Differences have been reported in the incidence rates (IRs) of ACL tears among male and female national elite athletes. However, there is little information in the published literature that assesses IRs for ACL reconstructions done in the Health Maintenance Organization (HMO) setting specifically. Different populations may show variation in ACL reconstruction IRs.

Objective: This study reports on the IR of ACL reconstructions in a predefined population and compares the differences in age and sex over time.

Design: A retrospective analysis of 4485 ACL reconstructions performed within Kaiser Permanente Southern California between 2001 and 2005 was completed by a query of an administrative database. Trends in IRs per 100,000 members were calculated and compared across age, sex, and the five-year study period.

Main Outcome Measures: Linear regression was used to test trends in IR. Sex distribution was compared using the χ2 test. Analysis of variance was used to compare the mean age from year to year in males and females. The independent sample t-test was used to compare mean age between males and females for each independent year.

Results: The IR of ACL reconstructions in females rose significantly (p = 0.010) from 14.4 in 2001 (95% confidence interval [CI], 12.6–16.3) to 19.3 in 2005 (95% CI, 17.2–21.5). Within specific age groups, IR increased significantly for females age 14 to 17 (p = 0.013), 18 to 21 (p = 0.017), and 45 to 49 years (p = 0.014). The most dramatic change was seen in the female age category of 14 to 17 years, which increased at a rate of 8.14 cases/100,000 members per year.

Conclusion: Identifying the sex and age groups with most rapidly increasing rates of ACL reconstructions is important in implementing ACL injury-prevention programs.

Introduction

Differences have been reported in the incidence rates (IRs) of anterior cruciate ligament (ACL) tears among male and female national elite athletes.1 However, IRs have not been assessed for Kaiser Permanente (KP) patients. In fact, there is little information in the published literature that assesses IRs for ACL reconstructions done in the Health Maintenance Organization (HMO) setting specifically. HMO populations are larger and more diverse than elite athlete populations. The elite athlete population also lacks the ethnic and socioeconomic diversity of the KP membership population. Different populations may show variation in ACL reconstruction IRs. Therefore, it is necessary to evaluate the IRs of ACL reconstructions in our HMO population.

ACL reconstructions are among the most common sports medicine procedures performed in the United States, numbering about 100,000 each year.2 Currently there is no evidence that ACL reconstructions prevent the development of arthritis.3 Therefore it is not enough to just diagnose and treat ACL tears. The focus of many orthopedic surgeons and of ACL-related research is on the prevention of ACL tears and the development of prevention programs.4 KP, where our social mission includes improving the overall health of the population we serve, is particularly interested in identifying specific populations at risk for injury. Prevention programs may then be implemented for the highest-risk group. Particularly with respect to complicated and costly procedures such as ACL reconstructions, where much may be gained but much is risked, it is important to identify the IRs of various populations over time.

Our study objectives were to determine the IR of ACL reconstructions in the KP Southern California (KPSC) population and compare the differences in age and sex over time.

Materials and Methods

We conducted a retrospective review of the number of ACL reconstructions performed in KPSC between 2001 and 2005. The KPSC institutional review board approved this project. IRs per 100,000 members were calculated and compared across years, sex, and age categories.

The number of ACL reconstruction procedures between 2001 and 2005 was obtained using the KP Anesthesia and Surgery Information System, a surgery-scheduling and case-tracking database. ICD-9-CM codes (International Classification of Diseases, Ninth Revision, Clinical Modification) were used to query the administrative data. Patients who underwent ACL reconstruction were identified by procedure code 81.45. Sex, age, and admitting diagnosis were noted.

We calculated ACL reconstruction IRs by dividing the number of procedures performed each year from 2001 through 2005 by the total number of KP members reported at the end of that calendar year. IRs are reported here per 100,000 members. Sex-specific and age category-specific IRs were also calculated. Linear regression was used to test trends in IRs where IRs were the dependent variable and surgery year was the independent variable.

Sex distribution over the years was compared using the χ2 test. Analysis of variance was used to compare the mean age from year to year in males and females. Finally, the independent sample t-test was used to compare mean age between males and females for each independent year.

Results

Between 2001 and 2005, 4485 ACL reconstructions were performed. Table 1 shows demographics for the KP ACL reconstruction population between 2001 and 2005. The number of males is higher than the number of females consistently throughout the years (overall, 68.4% vs 31.0%), but the overall sex distribution did not change over time (p = 0.282). The overall male mean age was 29.8 years (±10.0 years; range, 12–85 years) and did not change from 2001 to 2005 (p = 0.226). The overall female mean age was 26.8 years (±11.6 years; range, 13–68 years), and it also did not change over the years studied (p = 0.335). Males were consistently older than females in every year studied; see Table 1 for p values.

Detailed IRs for ACL reconstruction can be seen in Figure 1. The trend in IR was tested using linear regression, where the IR was the dependent variable and the calendar year was the independent variable. The IR of ACL reconstruction in females increased significantly (p = 0.010) from 2001 to 2005, going from 14.4 in 2001 (95% confidence interval [CI], 12.6–16.3) to 19.3 in 2005 (95% CI, 17.2–21.5) at a rate of 1.26 cases/100,000 members per year (95% CI, 0.57–1.96). We also calculated IR by sex and by age categories (Tables 2 and 3) and found that in females age 14 to 17 (p = 0.013), 18 to 21 (p = 0.017), and 45 to 49 years (p = 0.014), IR increased significantly, with the IR for the age category of 14 to 17 years increasing fastest, at the rate of 8.14 cases/100,000 members per year. Overall IR and IR for males did not change significantly from year to year.

Discussion

In our study, the IR of ACL reconstructions was clearly increasing in the female population. The largest increase was in female high school students between ages 14 and 17 years, followed by the 18-to-21-year-old and 45-to-49-year-old female age groups. The overall IR of ACL reconstructions was 29.6/100,000 members by the end of the study period. In a population similar to ours, an overall IR of 60 knee ligament injuries per 100,000 persons was seen in a knee injury clinic. About 50% of these ligament injuries were ACL tears, for 30 ACL tears/100,000 persons. Seventy-two percent of the patients were male and 28% were female. Sixty-five percent of the injuries were sustained during sports.5 The sex distribution in that study was very similar to that in ours. In another study,6 the sex distribution was also comparable, at 68% male and 32% female. Of the knee injuries, 45% were ACL tears. For both men and women, the highest number of knee injuries was seen in the 20-to-29-year-old age group.

IRs for ACL reconstructions in adolescent soccer and basketball athletes were analyzed according to sex.7 Girls had a higher adjusted IR of ACL reconstruction in both sports than did boys. The number of ACL surgeries in soccer players increased significantly during the five years studied for both boys and girls; however, the rate of increase was faster among the girls. The frequency of ACL reconstructions among the basketball players also increased for both sexes, although the rate of increase was similar for both sexes. Participation on soccer teams increased by 30% for girls and 10% for boys during the study period. Even after adjustment for participation rates, girls had a significantly higher rate of ACL reconstructions. In another study, the IRs of ACL reconstructions was 1.39/1000 aviators per year among females and 0.50/1000 aviators per year among males. Females, when compared with males, had a significantly increased risk of ACL reconstructions, with the highest risk occurring in the 18-to-29-year-old group.8 Among competitive alpine ski racers, females were 3.1 times more likely to have sustained an ACL tear than were their male counterparts.9 In an athletic population of patients undergoing ACL reconstruction, females athletes outnumbered male athletes in high school basketball (9:1) and soccer (1.7:1), whereas there were more male than female participants in amateur basketball (5:1), soccer (2.7:1), and skiing (1.6:1).10

One of the potential limitations of our study was that the administrative database was not developed for the sole purpose of research and may be subject to coding errors. Another limitation is the IR calculated was of ACL reconstructions, not ACL tears, and might have been influenced by surgeons’ changing perceptions of surgical indications over time. This may be reflected in the 45-to-49-year-old female group. Our younger female populations’ increasing IR with ACL reconstructions could reflect the increasing participation of these groups in sports, as indicated in earlier studies by other researchers.7–10 However, the database did not contain information about whether the population was athletic or about the mechanism of injury.

Conclusion

Our study identified certain female age groups with increasing rates of ACL reconstructions. Future directions may include initiating preventive programs aimed at these high-risk groups, additional study to identify risk factors, and improved methods of data collection.

Disclosure Statement

The author(s) have no conflicts of interest to disclose.

Acknowledgment

Katharine O’Moore-Klopf of KOK Edit provided editorial assistance.

References

1. Agel J, Arendt EA, Bershadsky B. Anterior cruciate ligament injury in national collegiate athletic association basketball and soccer. Am J Sports Med 2005 Apr;33(4):524–30.

2. Centers for Disease Control and Prevention, National Center for Health Statistics. National hospital discharge survey: annual summary, 1996 [monograph on the Internet]. Atlanta, GA: Centers for Disease Control and Prevention; 1996 [cited 2008 Mar 11]. Available from: www.cdc.gov/nchs/data/series/sr_13/sr13_140.pdf.

3. Fithian DC, Paxton EW, Stone ML, et al. Prospective trial or a treatment algorithm for the management of the anterior cruciate ligament-injured knee. Am J Sports Med 2005 Mar;33(3):335–46.

4. Griffin LY, Albohm MJ, Arendt EA, et al. Understanding and preventing noncontact anterior cruciate ligament injuries: a review of the Hunt Valley II meeting, January 2005. Am J Sports Med 2006 Sep;34(9):1512–32.

5. Miyasaka KC, Daniel DM, Stone ML, Hirschman P. The incidence of knee ligament injuries in the general population. Am J Knee Surg 1991;4:43–8.

6. Majewski M, Habelt S, Steinbrück K. Epidemiology of athletic knee injuries: a 10-year study. Knee 2006;13:184–8.

7. Micheli LJ, Metzl JD, Di Canzio J, Zurakowski D. Anterior cruciate ligament reconstructive surgery in adolescent soccer and basketball players. Clin J Sports Med 1999 Jul;9(3):138–41.

8. Belmont PJ Jr, Shawen SB, Mason MT, Sladicka SJ. Incidence and outcomes of anterior cruciate ligament reconstruction among US Army aviators. Aviat Space Environ Med 1999 Apr;70(4):316–20.

9. Stevenson H, Webster J, Johnson R, Beynnon B. Gender differences in knee epidemiology among competitive alpine ski racers. Iowa Orthop J 1998;(18):64–6.

10. Piasecki DP, Spindler KP, Warren TA, Andrish JT, Parker RD. Intraarticular injuries associated with anterior cruciate ligament tear: findings at ligament reconstruction in high school and recreational athletes. An analysis of sex-based differences. Am J Sports Med 2003 Jul–Aug;31(4):601–5.