The purpose of our study is to systematically review the literature for the mechanism of injury, outcomes, and complications of anterior cruciate ligament (ACL) reconstruction in patients who are overweight or obese.
Anterior cruciate ligament tears are one of the most common orthopedic injuries, with an estimated 175 000 tears per year and a cost greater than $2 billion.1 In the United States, the incidence of ACL injuries has been shown to be increasing.2 Surgical reconstruction of ACL tears is the standard of care, especially in young patients and athletes who hope to return to competitive sports.3 Although ACL reconstruction is a successful procedure that restores stability for most patients, post-traumatic osteoarthritis is still common even after stability is restored.3 Among patients with body mass index (BMI) >25 kg/m2, the risk for arthritis is greater because of increased loads across the knee joint.4–6 Furthermore, an elevated BMI has also been associated with an increased risk of concomitant medial meniscus tears and articular cartilage lesions identified at the time of ACL reconstruction.7–10
Obesity is a growing disease in the United States (US). The World Health Organization classifies overweight as a BMI 25 to 30 kg/m2, and obese as a BMI >30 kg/m2.11 A 2011 study found that 34% of adults were classified as obese, marking a 20% increase in the incidence of obesity over the past 3 decades.12 Regression modeling estimates that the United States will experience another 33% increase in obesity prevalence over the next 2 decades.13 Consequently, orthopaedic surgeons should anticipate an increasing number of overweight or obese patients requiring ACL reconstructions. Therefore, it is important to understand the unique risk factors for injury, treatment outcomes, and complications in this patient population.
We identified all articles in the MEDLINE, EMBASE, and OVID electronic libraries that reported the mechanism of injury, outcomes, and complications of ACL reconstruction in the overweight and obese population. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed from the inception of the study.14 Our review was registered with PROSPERO as CRD42017055594.
Studies that were included for review were required to meet the following criteria: (1) the study population underwent primary ACL reconstruction; (2) patients were skeletally mature; (3) patients were classified as overweight (BMI 25-30 kg/m2) or obese (BMI >30 kg/m2) for the entire patient population or as a subgroup in their analysis; (4) data included a minimum follow-up of 1 year postoperatively; (5) the study was an original article; and (6) published in English. Articles were excluded according to the following criteria: (1) patients underwent revision or bilateral ACL reconstructions; (2) data used BMI as a continuous variable for analysis; and (3) publication was a case report, review, abstract, or technical notes. Qualitative assessment of included studies was performed using the Methodological Index for Nonrandomized Studies (MINORS) checklist.15
Two authors (K.J.D. and J.J.J.) independently and systematically searched the MEDLINE, EMBASE, and OVID electronic libraries from inception to December, 2016 for any eligible articles using a combination of the phrases “anterior cruciate ligament,” “ACL,” “overweight,” “obese,” and “BMI.” Abstracts from all search results were reviewed for possible inclusion. After eliminating duplicate records, articles that would potentially meet the inclusion criteria were downloaded and reviewed. The references from all downloaded articles were cross-referenced to identify any other potential articles to include in our analysis. Any discrepancies were resolved by the senior author (R.F.H.).
The following data were extracted: study design, level of evidence, sample size, patient age and demographics, BMI classification, length of follow-up, inclusion criteria, injury mechanism, postoperative outcome measures, results of outcome measures, and complications. The mechanism, outcomes, and complications were extracted for each category of BMI included in the article. Two authors (K.J.D. and J.J.J.) independently extracted all of the data into an electronic spreadsheet (Microsoft Excel; Microsoft Office, Redmond, WA). Any discrepancies were again resolved by the senior author (R.F.H.).
An initial literature search yielded 453 articles (Figure 1). Studies were excluded because 355 were not related to ACL and BMI, 23 were published in a language other than English, and 48 were duplicates. After a complete review of the remaining 27 relevant articles, 3 did not include BMI classification, 8 did not include outcome data, 1 included patients who did not have ACL reconstruction, 1 did not have sufficient outcome data, 4 were review articles, and 1 included skeletally immature patients. The remaining 9 articles were included in our review. Although 2 of the included studies described the same cohort, different outcomes were reported in each publication.16,17 All noncomparative studies had a MINORS score of 13 points; all comparative studies had a score of 21 points with the exception of 1 study that was randomized and therefore, the MINORS score was not applicable. Articles were most frequently deducted points as a result of patients being lost to follow up and study size not being prospectively calculated.
Mechanism of Injury
Ballal et al reported the different mechanisms of ACL injury comparing normal BMI (18.5-24.9 kg/m2) to elevated BMI (≥25.0 kg/m2).18 The group of patients with a normal BMI had the following mechanisms of injury: 18 (37%) contact sport, 24 (49%) noncontact sport, 4 (8%) sport nonspecific, 0 trauma, and 3 (6%) at work. The patients with an elevated BMI had 17 (40%) contact sport, 17 (40%) noncontact sport, 2 (5%) sport nonspecific, 3 (7%) trauma, and 4 (9%) at work. There was no significant difference between these 2 groups for mechanism of injury (P = 0.32).18
All 9 articles (8 patient cohorts) included in this review reported outcomes of ACL reconstruction in overweight and obese patients compared with patients with a normal BMI (Tables 1 and 2).4–6,16–21 Almeida et al (2010) and Ballal et al reported Lysholm scores and both found no significant difference in scores between patients with a BMI below or above 25 kg/m2.17,18 Knee injury and Osteoarthritis Outcome Scores (KOOS) were also not significantly different between the overweight (BMI >25 kg/m2) and normal BMI populations.18 Almeida et al (2014) analyzed the same cohort of patients and found that patients with a BMI <25 kg/m2 had a 72.8% rate of return to sports compared with 65.3% for patients with a BMI >25 kg/m2, but there was no significant difference between these groups (P = 0.28).16 Kowalchuk et al used the International Knee Documentation Committee (IKDC) Subjective Knee Forms to measure patient-reported outcomes. An outcome was considered successful if the patient's score was above the subject-specific age- and gender-matched population mean and unsuccessful if the score was less than the subject-specific mean. They reported that patients with a BMI >30 kg/m2 had 0.35 the odds of success compared with patients with a normal BMI [P < 0.01, 95% confidence interval (CI), 0.17-0.71], whereas overweight patients did not have significantly different odds of success compared with patients with a normal BMI (P = 0.82).19
Three articles reported a relationship between BMI and osteoarthritis after ACL reconstruction.4–6 Li et al evaluated radiographs from 249 patients using the Kellgren–Lawrence scale to determine the degree of osteoarthritis associated with BMI classification.4 They concluded that overweight patients had an odds ratio of 2.04 (P = 0.045; 95% CI, 1.08-3.84) and obese patients (BMI >30 kg/m2) had an odds ratio of 3.24 (P = 0.012; 95% CI, 1.34-7.80) for developing osteoarthritis, when compared with patients with BMI lower than 25 kg/m2.4 Barenius et al analyzed radiographs from 135 patients and reported that being overweight (BMI ≥25 kg/m2) was associated with an odds ratio of 3.3 (P = 0.004; 95% CI, 1.48-7.28) for medial compartment osteoarthritis and an odds ratio of 3.5 (P < 0.05; 95% CI, 1.53-7.84) for patellofemoral compartment osteoarthritis using the Kellgren–Lawrence scale to evaluate radiographs.5 Culvenor et al evaluated 3.0T magnetic resonance imaging (MRI) scans from 111 patients with the MRI OA Knee Score and reported that having a BMI ≥25 kg/m2 was associated with an odds ratio of 3.0 (95% CI, 1.3-6.9) for tibiofemoral osteophytes and an odds ratio of 3.9 (95% CI, 1.2-12.4) for a patellofemoral bone marrow lesion.6
Two studies examined the risk for revision surgery in overweight patients compared with patients with a lower BMI.20,21 Maletis et al (2015) reported a significantly decreased risk for revision ACL reconstruction surgery in overweight patients compared with patients with BMI < 25 kg/m2 (0.70 hazard ratio, P = 0.002; 95% CI, 0.56-0.88) and for obese patients compared with patients with a BMI <25 kg/m2 (0.67 hazard ratio, P = 0.004; 95% CI, 0.51-0.88).20 Similarly, Maletis et al (2016) compared patients with BMI 30 to 35 kg/m2 with patients with BMI <30 kg/m2 and reported that the hazard ratio for risk of revision ACL reconstruction surgery was 0.77 (P < 0.05; 95% CI, 0.65-0.92). They also compared patients with BMI >35 kg/m2 to patients with BMI <30 kg/m2 and reported that the hazard ratio for risk of revision was 0.68 (P < 0.05; 95% CI, 0.54-0.87).21
Contralateral Anterior Cruciate Ligament Tear
Maletis et al (2015) found a decreased risk of contralateral ACL tears in overweight and obese patients. The hazard ratio for contralateral ACL reconstruction was 0.70 (P = 0.004; 95% CI, 0.55-0.89) for overweight patients compared with patients with BMI <25 kg/m2 and 0.63 (P = 0.004; 95% CI, 0.46-0.86) for obese patients compared with patients with BMI <25 kg/m2.20
Ballal et al reported the complication rates for patients based on BMI classification.18 Three of 49 (6.1%) patients with a BMI <25 kg/m2 had complications from ACL reconstruction compared with 8 of 43 (18.6%) in the patients with BMI ≥25 kg/m2 (P = 0.77).18 In the normal BMI group, there were 2 graft failures and 1 infection. In the elevated BMI group, there were 4 graft failures, 3 infections, and another patient required removal of the tibial screw secondary to persistent pain.18
The incidence of obesity is increasing in the United States, as the number of Americans classified as obese has surpassed one-third of the population and continues to rise.12 Given the burden of disease, orthopaedic surgeons can expect to evaluate and treat increasing numbers of overweight and obese patients with ACL tears. This systematic review of the existing literature regarding overweight and obese patients undergoing ACL reconstruction yielded several important findings. Furthermore, this review also highlights the limited number of published reports that exists for this patient population. This study is the first systematic review of articles that present data on patients undergoing ACL reconstruction that are classified as overweight or obese.
Only 1 study compared the mechanism of injury between normal BMI and overweight/obese patients. Ballal et al did not find a significant difference between mechanisms for patients with a BMI above or below 25.18 However, it is possible their study was underpowered to detect a difference. In addition, using a BMI of 30 or 40 kg/m2 as a cutoff might yield different results because a cutoff of 25 kg/m2 may be erroneous in the athletic population secondary to increased muscle bulk.22 A fit mesomorph athlete with low body fat may have the same BMI as a patient with a high body fat percentage.
Numerous studies demonstrated an increased incidence of postoperative radiographic osteoarthritis in patients with an elevated BMI after ACL reconstruction; the risk was found to be higher for obese patients (BMI >30 kg/m2) than overweight patients (BMI >25 kg/m2).4–6 However, only 1 study found that the patients with a BMI >30 kg/m2 who undergo ACL reconstruction have lower patient-reported outcome scores than normal weight patients.19 Further investigation is warranted to determine the clinical significance of early arthritic radiographic changes. This patient population is already at an increased risk for developing arthritis, even without an ACL injury. Men and women in the heaviest quintile of BMI have relative risks for osteoarthritis of 1.51 and 2.07, respectively, compared with patients in the 3 lightest quintiles.23
Interestingly, having an elevated BMI was associated with a decreased risk of revision ACL reconstruction and contralateral ACL tear.20,21 Maletis et al suggested that patients with an elevated BMI may be less active after having an ACL reconstruction and therefore less likely to need a revision or tear their contralateral side. However, the authors did not measure activity levels to support this theory.20 Alternatively, previously unconditioned and high-risk patients may gain muscle control in postoperative rehabilitation that could reduce the risk of contralateral injury (similar to an ACL prevention program).
Only 1 article presented complication rates by BMI classification.18 Although the rate of complications was 3 times higher (6.1% vs 18.6%) in the group with an elevated BMI, the difference was not significant.18 The reasons for this increase in complication rate could include increased operative times, increased stress on the graft, and poor muscle control. All patients in this study underwent primary autologous ACL reconstruction using hamstring grafts only. This study only had 92 patients and may have been underpowered to detect a significant difference. More research is needed to analyze the complications specific to the patient population that is overweight and obese.
This study has several limitations. As a systematic review, this study is limited to the strength of the existing literature and there is a paucity of reports on ACL reconstructions in overweight and obese patients. Eight of the 9 included studies had a Level of Evidence III. Only 1 article addressed mechanism of injury and complications.18 In addition, several studies combined overweight and obese patients with a BMI of 25 kg/m2 as a cutoff for comparison, and these studies did not identify a difference in outcomes. It is certainly possible that more significant differences would be found if the study populations were BMI >30 or >40. Another important limitation is the lack of control for factors such as age, activity level, ACL reconstruction technique, and concomitant procedures such as meniscus surgery. Four of the included studies did not control for type of graft across different BMI classifications.4,19–21 Finally, within the obese population, the fat distribution and lower limb girth may play an important role in terms of the surgical challenges, none of which are clearly described in the evaluated literature.
The current literature does not demonstrate any significant differences in mechanism of injury or complications for patients with a BMI >25 kg/m2 who undergo ACL reconstruction. Patient-reported outcome measures were similar for patients with a BMI above and below 25 kg/m2, but there is evidence that obese patients have lower postoperative IKDC scores. There is a consistent association between being overweight and developing arthritis in patients undergoing ACL reconstruction. Overweight and obese patients have lower risk of revision ACL reconstruction and contralateral ACL tear. More research is needed to confirm these findings and to better understand how to counsel and treat overweight and obese patients with ACL tears.
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Keywords:Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.
ACL; obese; overweight; BMI