The Effect of Autologous Hamstring Graft Diameter on the Likelihood for Revision of Anterior Cruciate Ligament Reconstruction Lindsey Spragg,*y MD, Jason Chen,z MA, Raffy Mirzayan,§ MD, Rebecca Love,z MPH, RN, and Gregory Maletis,§ MD Investigation performed at Kaiser Permanente, Baldwin Park and San Diego, California, USA Background: Hamstring autografts for anterior cruciate ligament (ACL) reconstruction (ACLR) have become popular in the past 2 decades; however, it is difficult to predict the diameter of the harvested tendons before surgery. Previous biomechanical studies have suggested that a smaller graft diameter leads to a lower load to failure, but clinical studies looking at various predictors for failure, including graft size, have been inconclusive. Purpose: To evaluate the relationship of hamstring graft diameter to ACL revision within a large cohort of patients, while controlling for sex, age, body mass index (BMI), and femoral and tibial fixation type. Study Design: Case-control study; Level of evidence, 3. Methods: A case-control study using patients registered in an ACLR registry was conducted. Revision was used as a marker for graft failure. A case was defined as a patient who underwent primary ACLR with a hamstring autograft that was revised during the study period (April 2006 to September 2012). Three controls, defined as patients who underwent primary ACLR with a hamstring autograft that was not revised, were matched to each of the cases according to age, sex, BMI, and femoral and tibial fixation type. Descriptive characteristics were employed, and conditional logistic regression was conducted to produce estimates of odds ratios and 95% CIs. Results: A total of 124 cases and 367 controls were identified. There were no significant differences between cases and controls in the distribution of sex (52.4% male vs 52.9% male, respectively; P = .932), median age (17.6 years [interquartile range (IQR), 15.9-20.4] vs 17.6 years [IQR, 15.9-20.4], respectively; P = .999), median BMI (23.4 kg/m2 [IQR, 21.5-26.4] vs 23.4 kg/m2 [IQR, 21.6-25.8], respectively; P = .954), femoral fixation (P = .459), and tibial fixation (P = .766). The mean (6SD) graft diameter was 7.9 6 0.75 mm in the cases and 8.1 6 0.73 mm in the controls. The likelihood of a patient needing revision ACLR in the study cohort was 0.82 times lower (95% CI, 0.68-0.98) for every 0.5-mm increase in the graft diameter from 7.0 to 9.0 mm. Conclusion: In this study, within the range of 7.0 to 9.0 mm, there was a 0.82 times lower likelihood of being a revision case with every 0.5-mm incremental increase in graft diameter. Keywords: anterior cruciate ligament; failure; graft diameter; hamstring autograft

restore functional knee stability.7,9,24,35 There are multiple graft options that exist with a very diverse usage pattern across the United States; many factors influence the choice of graft for a particular patient, from patient preference to surgeon training and experience.5,15,41 The most commonly used grafts in the United States, as well as other countries, include bone–patellar tendon–bone (BPTB) autografts, hamstring (HS) tendon autografts, and allografts.20,27,30 BPTB autografts have long been considered the standard for ACL reconstruction (ACLR) because of the tissue strength, relative ease of harvest, and bone-to-bone healing.35 HS autografts, however, have increased in popularity over the past few decades, with large prospective registries in the United States and Scandinavia finding HS autografts to account for 30% to 96% of all primary ACL grafts.1,11,20 Several randomized studies, as well as systematic reviews, have found no significant differences in functional or clinical outcomes between BPTB and HS autografts,13,19,22,37,38

The anterior cruciate ligament (ACL) is one of the most reconstructed ligaments, with the goal of reconstruction to

*Address correspondence to Lindsey Spragg, MD, Los Angeles County 1 USC Medical Center, 1200 N State Street, GNH3900, Los Angeles, CA 90033, USA (email: [email protected]). y Los Angeles County 1 USC Medical Center, Los Angeles, California, USA. z Kaiser Permanente San Diego, San Diego, California, USA. § Kaiser Permanente Baldwin Park, Baldwin Park, California, USA. One or more of the authors has declared the following potential conflict of interest or source of funding: R.M. has stock options in Alignmed, Cayenne Medical, and ITS Implants; has received royalties from Springer and Wolters Kluwer Health–Lippincott Williams & Wilkins; and has received research and education support from Arthrex, BioD LLC, and Joint Restoration Foundation. The American Journal of Sports Medicine, Vol. 44, No. 6 DOI: 10.1177/0363546516634011 Ó 2016 The Author(s)

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with up to 10-year follow-up in 1 study.13 In addition, in studies looking at patient-reported outcomes, the incidence of anterior knee pain was lower in the HS tendon group (7%-14%) compared with the BPTB group (25%-36%).14,36,43 A cumulative meta-analysis further strengthens the evidence for reduced morbidity using HS autografts.33 Despite the fact that the previously cited studies have suggested equivalent outcomes between BPTB and HS autografts with less associated morbidity in the HS tendon group, recent literature has suggested higher rates of graft failure or revision with HS autografts.10,24,31,34,35 Several recent studies have attempted to determine predictors of failure in ACLR using HS autografts.17,21,26,29 These studies focus on graft diameter at least in part as a possible predictor, given early biomechanical evidence suggesting a positive correlation between graft size and ultimate failure loads12; however, the results have been inconclusive to date. Some studies have shown increased failure rates with HS autograft diameters of less than 8 mm,21,26,29 whereas others have found no correlation between graft size and the failure rate.17 A recent systematic review concluded that ACLR with a quadrupled HS autograft with a diameter of 8 mm decreases failure rates.6 Most of these current studies looking at predictors of failure are limited by the low total number of revisions in their study groups, leading to a possible inability to detect differences among variables. In addition, failure is often defined with clinical outcomes, and there is no benefit of matched controls. HS autograft size is of particular interest, given the large variability in diameter among patients28 and its potential to affect outcomes. While it has been found previously that a quadrupled HS tendon is biomechanically stronger than a matched patellar tendon and is sufficiently strong for use in ACLR,12,42 there is recent evidence to suggest that tensile strength is highly dependent on graft diameter.4 Previous studies have attempted to find patient characteristics that might be analyzed preoperatively to predict graft size, including height, weight, age, sex, and body mass index (BMI).32,39,40 In addition, preoperative calculations of the cross-sectional area of HS tendons on magnetic resonance imaging and ultrasound have been shown to reliably predict HS graft diameter.6,8 The surgeon, however, might only need to worry about these preoperative correlations and calculations and patient characteristics if graft diameter truly plays a role in ACL failure using an HS autograft, a question yet to be definitively answered. The purpose of this large-volume case-control study was to determine the relationship of HS graft diameter to revision ACLR, with other variables being equal. Our hypothesis was that a smaller diameter HS autograft would be associated with a higher likelihood for revision ACLR.

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Figure 1. Flowchart of cases and controls. underwent primary ACLR with an HS autograft that was revised during the study period. Revision, therefore, was used as a marker for graft failure in our study. A control was defined as a patient who underwent primary ACLR with an HS autograft that was not revised before the end of the study. The controls were matched to each of the (revised) cases by potential confounders at a 1:3 ratio (1 case matched to 3 controls) using a greedy algorithm with statistical software that matched the first controls for all cases and then performed additional matches to select the second and third controls during the second and then third iterations.18 A total of 132 cases with revised HS autografts were identified within the study period, with 396 matched controls. The total 528 patients were then chart reviewed by 1 author (L.S.) to search the value for the exposure of interest, HS graft diameter, which was not prospectively captured by the registry. The procedures included in this case-control cohort were performed by 102 surgeons in 33 hospitals across 6 distinct geographic regions (Hawaii, Northwest, Southern and Northern California, Colorado, and MidAtlantic). The surgical technique and rehabilitation protocol are at the discretion of the operating surgeon and vary across the health system. There were 34 patients excluded because of the graft diameter not being reported and 3 patients excluded from the control sample with a graft diameter of 11 mm, treating diameter as a continuous variable. There were therefore 491 patients left in the sample for analysis (Figure 1).

Data Source The study data were provided by the Kaiser Permanente ACLR Registry, which was established in 2005 to monitor surgical outcomes and to improve quality of care. Data collection methods have been previously published.23,30 In brief, surgical data are collected by the surgeons after each surgery and sent to the registry. Clinical research associates prospectively monitor and validate revisions. More than 20,000 ACLRs have been registered, with a surgeon participation rate of over 93% as of September 2012.

METHODS

Exposure of Interest

Study Design

HS graft diameter was evaluated as the exposure of interest in this study. Graft diameters were measured at 0.5-mm intervals and recorded in the operative notes by the surgeons at the point of care. Because of the multiple surgeons involved in the registry, graft diameter was not uniformly

A case-control study was conducted using patients registered with the Kaiser Permanente ACLR Registry from April 2006 to September 2012. A case was defined as a patient who

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dictated. It was either recorded as the graft diameter in general, or femoral and tibial end diameters were recorded separately. The minimum value of the recorded graft diameters was used if multiples were recorded. In some cases, graft diameter itself was not available, in which case femoral or tibial tunnel diameter was used as a proxy. Again, in these instances, the minimum tunnel diameter recorded was used for the graft diameter value. Cases or controls without a value for graft diameter or tunnel diameter were excluded.

Outcome of Interest The outcome of interest was defined as any aseptic failure of a primary ACLR procedure that required revision surgery.

Covariates Sex, age at the time of surgery, BMI, and primary femoral and tibial fixations were considered as potential confounders and were used to match the cases and controls as described previously. Primary femoral fixation was categorized into cross-pin, interference screw, or suspensory fixation. Primary tibial fixation was categorized into interference screw or suspensory fixation.

Statistical Analysis Descriptive characteristics (frequency, proportion, median and interquartile range [IQR], and mean 6 SD) were employed, and survival analysis was conducted to produce estimates for plotting the cumulative failure plot (defined as 1 – Kaplan-Meier estimate) (Figure 2). The x2 test was used to compare proportions with P values provided. Because of insufficient events in the 6.0, 6.5, 9.5, and 10.0 mm–diameter groups, conditional logistic regression stratified by matching groups from 7.0 to 9.0 mm was used to produce odds ratios (ORs) for incremental changes in the graft diameter with 95% CIs. Because all covariates were used during matching to obtain the controls, no covariates were added into the model. There were no missing values or outliers in the covariates. Data were analyzed using SAS (version 9.2; SAS Institute). Statistical significance was set to a = .05. This study was approved by the institutional review board at Kaiser Permanente (approval No. 5691) before its commencement. No outside funding was obtained.

RESULTS There were 124 cases and 367 controls in this cohort. All covariates such as sex, age, BMI, and primary fixation on the femoral side and on the tibial side were properly matched between the cases and controls (all P . .05) (Table 1). The median follow-up time was 1.2 years in the cases and 2.4 years in the controls. Seventy-one (14.5%) of the controls terminated their membership before the end of the study, with a mean follow-up time of 1.82 years. The mean graft diameter was 7.9 6 0.75 mm in the cases and 8.1 6 0.73 mm in the controls. When comparing

Figure 2. Cumulative revision probability estimates with 95% CIs by hamstring graft diameter category in a 1-to-3 matched case-control sample after primary anterior cruciate ligament reconstruction (ACLR). the proportions of HS graft diameter in 0.5-mm increments (Table 2), the proportion of grafts in categories between 6.0 and 7.5 mm was consistently higher in cases than in controls, and the proportion of grafts in categories between 8.0 and 10.0 mm was consistently higher in controls than in cases. After stratifying by matched case-control groups, the likelihood of a patient being a case (ie, requiring revision) in our cohort was 0.82 times lower (95% CI, 0.68-0.98) for every 0.5-mm increase in the graft diameter from 7.0 to 9.0 mm after primary ACLR with an HS autograft (P = .026). Using the HS autograft crude revision rate of 2.37% (132 events) reported by Maletis et al25 in a previous work, and the OR of revision for a 7-mm graft using an 8-mm graft as the reference, the number needed to treat to harm of 86.64 was derived.3 This number can be interpreted as the following: for approximately every 87 cases with 7-mm grafts instead of 8-mm grafts, 1 additional revision is yielded.

DISCUSSION This study shows that there is a significant relationship between HS autograft diameter and the likelihood for revision ACLR. In our cohort, when looking at graft size alone, a patient with a 9 mm–diameter graft is 55% less likely to be a case than a patient with a 7 mm–diameter graft. Our findings support recent biomechanical and clinical studies suggesting that HS graft size does have an effect on potential failure. Magnussen et al21 found, at an average of 14 months, a 1.7% revision rate in grafts .8 mm, a 6.5% revision rate for grafts 7.5 to 8 mm, and a 13.6% rate for grafts \7 mm. Age was also found to be a significant factor, with a revision rate of less than 1% for patients older than 20 years. They concluded that a decreased graft size and patient age are predictors of ACL revision and that the

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TABLE 1 Patient and Implant Characteristics for Primary ACLR With Hamstring Autograftsa Total (N = 491) Sex (matching variable) Female Male Age, y (matching variable) Median (IQR) \14 14-17 18-21 22-25 26-29 30-34 35-39 40-49 BMI, kg/m2 (matching variable) Median (IQR) \25 25-29 30 Primary femoral fixation (matching variable) Cross-pin Interference screw Suspensory Primary tibial fixation (matching variable) Interference screw Suspensory Follow-up time, median (IQR), y

Cases (n = 124)

Controls (n = 367)

P Value .932

232 (47.3) 259 (52.8)

59 (47.6) 65 (52.4)

173 (47.1) 194 (52.9) .999

17.6 10 265 123 30 21 20 13 9

(15.9-20.4) (2.0) (54.0) (25.1) (6.1) (4.3) (4.1) (2.7) (1.8)

17.6 3 66 31 7 6 5 3 3

(15.9-20.4) (2.4) (53.2) (25.0) (5.7) (4.8) (4.0) (2.4) (2.4)

17.6 7 199 92 23 15 15 10 6

(15.9-20.4) (1.9) (54.2) (25.1) (6.3) (4.1) (4.1) (2.7) (1.6)

23.4 326 118 47

(21.6-25.9) (66.4) (24.0) (9.6)

23.4 83 30 11

(21.5-26.4) (66.9) (24.2) (8.9)

23.4 243 88 36

(21.6-25.8) (66.2) (24.0) (9.8)

.954

.459 85 (17.3) 124 (25.3) 282 (57.4)

20 (16.1) 27 (21.8) 77 (62.1)

65 (17.7) 97 (26.4) 205 (55.9) .766

462 (94.1) 29 (5.9) 1.9 (1.2-3.3)

116 (93.6) 8 (6.5) 1.2 (0.9-1.9)

346 (94.3) 21 (5.7) 2.4 (1.5-3.7)

a Data are reported as n (%) unless otherwise indicated. ACLR, anterior cruciate ligament reconstruction; BMI, body mass index; IQR, interquartile range.

TABLE 2 Hamstring Diameter Characteristics Comparing Cases and Controls After Primary ACLR With Hamstring Autograftsa Hamstring Autograft Diameter, mm Mean 6 SD 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

Cases (n = 124), n (%)

Cases vs Controlsb

Controls (n = 367), n (%)

7.9 6 0.75 2 (1.6) 2 (1.6) 27 (21.8) 15 (12.1) 53 (42.7) 7 (5.7) 15 (12.1) 0 (0.0) 3 (2.4)

\ . . . . \ \ \ \ \

8.1 6 0.73 1 (0.3) 4 (1.1) 55 (15.0) 38 (10.4) 165 (45.0) 22 (6.0) 70 (19.1) 2 (0.5) 10 (2.7)

a

ACLR, anterior cruciate ligament reconstruction. Comparison of percentage values for each diameter size.

b

use of a 8-mm HS autograft in patients younger than 20 years is associated with higher revision rates. In a study by Park et al,29 there was a significant difference in the failure rate between cases with a graft size \8 mm compared with a graft size 8 mm, with no revisions in the 8-mm group. The investigators concluded that a small graft diameter did not necessarily increase instability or the chance of failure but that a diameter of 8 mm would likely lead to better results. While these studies were able to show

a difference in failure rates based on graft size, they were limited by the low total number of revisions in their cohorts, with 18 of 256 patients undergoing revision in the former and 12 of 296 in the latter. A recent Multicenter Orthopaedic Outcomes Network (MOON) cohort study found a 0% revision rate with grafts .8 mm in diameter and a 7% revision rate with grafts 8 mm in diameter.26 While it was the first study to relate graft size to patient-reported outcome scores, concluding

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that a smaller HS autograft size is a predictor of poorer Knee injury and Osteoarthritis Outcome Score sport/recreation function 2 years after ACLR, the authors were unable to conclude a relationship between graft size and the risk of revision because of the small sample size. The study of Kamien et al17 was the only other recent study specifically looking at the relationship between ACLR and graft diameter to not find a correlation between the two. This retrospective study of 98 primary ACLR procedures did find statistically higher failure rates in the younger patients; however, this was not the case for smaller grafts. This study may have been unable to detect such a difference, however, given the small sample size. A recent systematic review of the previous studies concluded that a diameter of 8 mm would decrease failure rates in ACLR using a quadrupled HS autograft.6 In keeping with our findings and the results of these previous clinical studies, a recent biomechanical study has similarly found a significant increase in tensile strength of quadrupled HS grafts as the diameter increases.4 The authors found significant differences between the 6- and 7-mm, 6- and 8-mm, 6- and 9-mm, and 7- and 9-mm groups, suggesting that an increase in diameter of just 1 to 2 mm can have a real effect on strength of the graft. The difference in strength seen between these groups substantiates our clinical findings that a 0.5-mm incremental increase in graft diameter can affect the likelihood of revision. These findings put into question whether we should be looking at a single threshold for graft diameter or if it really should be thought of as more of a continuous variable. The median age in our cohort was 17.6 years. This is significant because it allows our findings to be especially applicable to a group of patients deemed to be at an increased risk for ACL revision. While several studies have found a significant association between HS autograft failure and age,16,17,21,24,26,31 Magnussen et al21 found patients younger than 20 years to be at particular risk. Patients older than 20 years had a less than 1% rate of revision, while those younger than 20 years had a 14.3% rate of revision. While a recent MOON cohort study found no difference in retear rates between HS autografts and BPTB autografts,16 4 recent large registry studies from the United States and multiple Scandinavian countries have found higher revision rates among HS autografts.10,24,31,34 It should be noted, however, that while these registry studies had very large cohorts, with between 9000 and 45,000 patients, there was very little information on graft size, therefore precluding its analysis as a potential risk factor for the increased revision rates seen in the HS tendon group. Understanding the factors associated with ACL graft failure, of which autograft size may be an important component, will help guide preoperative discussions with patients, assist in intraoperative decision making, and ultimately improve patient outcomes. Limitations of this study include using only revision surgery as the endpoint for graft failure. Although revision surgery as the definition for graft failure has been used in previous studies,21,26 this might underestimate the total number of failures, as patients with a retear and clinical signs of laxity who did not undergo revision surgery were not captured in our analysis. We expect that this would

have a similar influence across all graft diameters studied. This is a study utilizing a community-based registry, and therefore, the surgical technique and rehabilitation, including time to return to sports, were not standardized and were based on the discretion of the surgeon and patient. Surgeon experience and volume were also variable and not investigated. The surgical technique including tunnel drilling methods and graft composition was not standardized and left up to the operating surgeon. There was no postoperative imaging or a way to assess the original technique and tunnel position, which if not anatomic, cannot be compensated for by any size of graft.2 Additionally, notch width was not prospectively collected within our registry database and may have had an effect on our findings. Activity levels after primary ACLR were similarly not standardized, nor recorded, and may affect the likelihood of a new injury. In the recent study by Kamien et al,17 however, there was no difference in revision rates based on preoperative Tegner activity levels. This was a case-control study design, not a randomized controlled study, so while we did find a lower likelihood of being a revision case with an increased graft size, we are inherently unable to determine the causation of revision surgery. Our study was based on matching across several variables to isolate the effect of graft size alone. While our matching was very good and there were no significant differences across groups, this was not perfect. Also, while we were able to match across multiple variables, we were likely unable to account for all confounding variables. Ethnicity is one such variable shown to be of some significance in the previous study by Maletis et al24; however, we were unable to include this as a matched variable with our sample size. Finally, the reporting of graft size was not standardized; however, we thought the use of tibial or femoral tunnel diameter to be an adequate proxy. This study is unique in that a matched case-control design was utilized, taking into account sex, age, BMI, and femoral and tibial fixations with matched controls. Age, BMI, and sex have been found to affect revision rates in several studies.16,17,21,24,26,32 The type of graft fixation was also controlled for, which while not previously studied, may have confounded the results. By controlling for most of the other significant variables, the effect of graft diameter could be isolated. Another strength of this study was the very large sample size, including large numbers of total revisions. This allowed us to evaluate small differences in graft size, which has been a limitation with some of the previous work.17,21,26,29 Finally, this study stems from a large community-based registry with a very diverse patient, surgeon, and hospital population across the United States. The surgeons have various levels of training and caseloads, making these findings generalizable to a greater population of patients undergoing ACLR and providers. Also, while the data were retrospectively reviewed, graft size was prospectively collected, ensuring a high level of internal validity.

CONCLUSION Surgeons performing ACLR should be aware of factors that may influence the likelihood of success. HS graft diameter

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is one such factor. In this study, we have found that within the range of 7.0 to 9.0 mm, there is a 0.82 times lower likelihood of being a revision case with every 0.5-mm incremental increase in the graft diameter. This information may help to explain the reason for some of the failures seen with HS autografts and can help guide preoperative discussions with patients and surgeons.

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17.

18.

19.

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