The aim of this study was to evaluate the influence of graft choice (autograft vs allograft) on the risk of tearing an ACLR graft and to identify other patient or surgical variables associated with subsequent failure of ACL grafts. A logistic regression model was developed using a single surgeon’s data that included a similar amount of autografts and allografts for primary ACLRs. Within the single-surgeon cohort, the same operative technique, graft fixation, postoperative rehabilitation, and return-to-play guidelines were used regardless of graft source, thus eliminating the potential that these surgical and rehabilitation variables could influence the primary outcome of ACL graft rupture. According to multivariable regression for the single-surgeon data, age and graft type (allograft) were both associated with an increased risk of ACL graft rupture. Patient characteristics (sex, body mass index) and surgical characteristics (revision ACL surgery and meniscal pathology) were not associated with ACL graft rupture.
Data for the entire group of surgeons were analyzed using the logistic regression model created from the single surgeon’s data. Age and graft type remained significant risk factors or predictors of ACL graft rupture. Marx activity score at index injury was found to be a significant predictor of ACL graft rupture but was also found to be closely related to age and therefore not included in the model. Unlike the single-surgeon cohort, the multicenter cohort used varied graft types and fixation techniques, but the single-surgeon model was found to be generalizable to the larger group.
The consortium chose the Marx activity scale20
because it was designed to measure patient activity level instead of health status or knee function. The Marx score has been shown to positively correlate with patient activity and returning to pivoting sports and inversely with age.12,20
Even though the Marx score is a validated measure of activity, it was not chosen to be included in the model. Marx score can change over time. The Marx score chosen for this study was that at the time of index injury. We do not know if the patients who tore their ACL graft returned to the same level of activity at a different rate compared to those who did not tear their ACL graft, but an analysis of the MOON data by Dunn et al8
showed that the strongest predictor of return to activity after ACLR was the Marx score at time of index injury.
A statistically significant increase in ACL graft ruptures was found in younger patients. The highest percentage (37.5%) of ACLR graft failures in the consortium cohort was in the 10- to 19-year age group. The risk of ACL graft tear for this youngest age group was 2.33 times higher than their peers who were 10 years older, controlling for graft type. Our analysis showed that the risk of graft retear fell approximately in half for each 10-year increase in age. Shelbourne et al showed a similar relationship between age and risk of graft retear.33,34
Patient age is likely a proxy for activity level in patients with ACL injuries and graft failures. The inverse relationship between the Marx activity score and age has been demonstrated20
Female patients have been shown to have a higher likelihood of sustaining a primary ACL injury.2,3,13
Salmon et al31
reported no sex difference in the risk of ACL graft failure or contralateral ACL injury. In a prospective cohort of 1820 patients, Shelbourne showed that female patients had an increased risk of tearing the contralateral ACL but no increased risk of ACL graft failure.33
Others have also found no sex differences in the risk of graft failure after ACLR with bone–patellar tendon–bone autograft.4,10,33
In this study, sex was not a predictor of ACL graft failure. To the contrary, Stevenson et al, in a survey of Alpine skiers with a 40% response rate, showed increased ACL graft retears in female patients that did not reach statistical significance, and Noojin et al, in a series of 65 ACLR patients, showed female patients to have a higher retear rate.22,38
That sex is a risk factor for native ACL tears but possibly not for ACL graft tears warrants further investigation.
Patients in this study who had an allograft reconstruction were 4 times more likely to tear their ACL graft than those who underwent autograft reconstruction. The data used in the model to generate this increased risk of failure came from all consortium surgeons and included tibialis anterior, tibialis posterior, Achilles tendon, and bone–patellar tendon–bone allografts. The choice of which allograft and fixation method to use was made by each surgeon and patient. Several individual studies have reported a similar risk of failure for bone–patellar tendon–bone autografts and allografts, but all were underpowered.7,14,16,17,19,26,39
One meta-analysis found a significantly higher failure rate (P
< .0.04) between allograft versus autograft, but the limited number of failures within the allograft sample precluded risk factor identification.11
In a retrospective case series review of primary ACLRs using tibialis anterior allograft, Singhal et al36
reported a 23.1% risk of graft failure. Two groups of patients were created, younger and older than 25 years, 35% and 13% of whom required revision ACLR, respectively. Given the higher risk of ACL graft failure in the younger group, these authors did not recommend the use of tibialis anterior allograft in patients younger than 25 years. In this study, the use of allograft in younger patients increased the absolute rate difference of graft failure between autograft and allograft. can be used to make clinical decisions by placing a mark at the patient’s age and then taking the absolute difference between the autograft and allograft as the expected difference in failures. For example, the absolute differences by age are as follows: 14 years, 15%; 18 years, 13%; 22 years, 9%; 30 years, 4%; and 40 years, 2%. If you now choose autograft ACLR rather than allograft, the expected number of ACLR graft failures prevented can be predicted by dividing 100 by the absolute rate difference from (also called the number needed to treat). For each age, the number of autograft ACLRs performed to prevent one failure is as follows: 14 years, 7 ACLRs; 18 years, 8 ACLRs; 22 years, 11 ACLRs; 30 years, 25 ACLRs; 40 years, 50 ACLRs. Thus, either the absolute difference or the number needed to treat can be discussed with the patient and factored into the overall risks and benefits of ACLR graft choice.
Scheffler et al32
evaluated the biological incorporation and mechanical properties of allografts and autografts in a sheep model. They found that allografts incorporated more slowly than autografts at 6 and 12 weeks, with this difference decreasing at 52 weeks. That allograft ACLR patients have a quicker postoperative rehabilitation course and diminished pain compared to patients with autografts may lead the former to return to high level of activity earlier than the latter and before sufficient biological healing of the graft,27
which may place allograft ACLR at a higher risk of retear. In this cohort, it is possible that although the return-to-play guidelines were identical, the allograft ACLRs may have achieved a higher activity level sooner than the autograft ACLRs. Shino et al35
reported a 3% risk of ACL graft failure for soft tissue allografts after allowing patients to return to sports at 12 months. Singhal et al36
commented that their postoperative protocol of release to sport at 4 months could have been a driver of increased risk of graft failure given the increased time needed for tendon graft incorporation.25
In this study, all patients followed a similar postoperative protocol and were released to sports at 4 to 6 months postreconstruction if they had achieved the following criteria: no functional complaints; confidence when running, cutting, jumping at full speed; and at least 85% contralateral values on hop tests. Patients who underwent allograft reconstruction were 4 times more likely to tear their ACL graft, holding age constant.
Allograft type and processing may also influence risk of retear. Rappe et al29
reported a statistically higher risk of failure for irradiated Achilles allograft (33.0%) versus nonirradiated Achilles allograft (2.4%). Krych et al18
performed a meta-analysis demonstrating that irradiation and chemical processing may place allografts at increased risk of failure. The focus of this study was to evaluate the risk of graft failure between allograft and autograft tissue; thus, we did not evaluate the effect of allograft processing. Due to the large number of potential variables regarding allografts, such as type, donor characteristics, and processing techniques, a much larger number of allograft failures would be needed for a logistic regression. The allografts in this study were predominantly fresh frozen, without proprietary processing, and were nonirradiated or irradiated less than 2.5 mRad. In an ad hoc analysis, we could not identify tissue bank, allograft type, or processing (eg, irradiation status) as a significant variable for allograft retear. Future studies will be completed to determine the contributions of allograft tissue–processing details and age or sex of donor on the risk of failure.
The choice of graft type was driven by patient choice and not randomly assigned. This possible source of bias with respect to age, sex, and activity level is addressed by the multivariate regression analysis, which controlled for these variables. This lack of randomization does not account for any other unknown confounders.
Graft failure in this study was defined as that which went on to a revision ACLR within 2 years of primary ACLR. This definition does not include patients who may have torn their grafts but chose to modify their activities and not undergo a revision ACLR, those who tore greater than 2 years after their primary ACLR, and those with stretching of the graft with laxity on exam but personal satisfaction with functional result. Thus, this definition underestimates the number of failures that occurred, but it does represent the most clinically meaningful failures from the patient’s perspective.
The single-surgeon cohort had identical postoperative rehabilitation and return-to-activity guidelines. The activity level analyzed in this study was the Marx score at time of index injury. This measure has been shown to be the strongest preoperative predictor of the level of activity to which patients returned after ACLR.8
Thus, by controlling for the Marx score at index injury, the postoperative activity as a confounder has been evaluated. The guidelines for release to full activity were the same. Unfortunately, what was not evaluated was when the patients actually returned to high activity. This study did not evaluate whether a longer time before return to activity influences failure rates, thereby warranting further investigation. Future efforts to analyze the time interval of graft rupture after ACLR, the graft rupture in relation to allograft tissue type and processing, the extent and time frame of the biological incorporation of grafts, and the activity level at the time of graft failure would provide additional insight into risk factors for ACL graft ruptures. A more complete identification and quantification of the predictors of ACLR graft tear would be beneficial in counseling patients who are contemplating ACLR; it would aid surgeons in preoperative planning; and it would facilitate efforts to minimize repeat ACL injuries.
This prospective longitudinal cohort study demonstrates that the risk of ACL graft rupture is higher for patients who undergo allograft reconstruction. The risk of graft failure is also higher for younger patients. The clinical relevance for graft choice by age between allograft versus autograft is best shown in . The absolute rate difference between the 2 grafts at a given age can be discussed with patients, and for surgeons, the number needed to treat is relevant to avoid the graft failure experienced in their practice.
Given that further investigation into allograft type and processing, as well as the time interval to return to full activity, may influence outcomes, allograft ACLRs should be performed with caution in the younger population, with the decision being made with an understanding of the available evidence. Analysis of number needed to treat shows that in older patients, allograft use is less of a concern.