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Abstract

Background:

Anterior cruciate ligament (ACL) injuries are among the most common sports-related injuries, accounting for about half of all knee injuries, and most athletes opt to undergo ACL reconstruction (ACLR). The quadriceps tendon (QT) ACLR has only recently been gaining popularity, and therefore few studies exist for evaluating the healing, or ligamentization, timeline of the QT autograft.

Purpose:

To evaluate the signal intensity (SI) ratio of the tendon QT autograft after ACLR during the first postoperative year.

Study Design:

Case series; Level of evidence, 4.

Methods:

A prospective case series of 19 athletes (mean age = 15.63 years) with ACL rupture who underwent ACLR with a QT autograft underwent knee magnetic resonance imaging (MRI) at 4 time points: presurgery (PRE), 3 months postoperative (3M), 6 months postoperative (6M), and 12 months postoperative (12M). SI ratio was calculated across different anatomic landmarks, specifically the intra-articular tissue of the native ACL of the contralateral knee from the PRE time point; the QT graft in the intra-articular space at 3M, 6M, and 12M time points; and the posterior cruciate ligament at each visit, to be used as a reference value and account for visit-to-visit variations in the MRI signal. Means of the SI ratio were calculated at the full graft level, as well as segmented into either 4 or 24 segments for analysis.

Results:

At the full-graft level, there was a significantly higher SI ratio in the QT graft at 3M and 6M compared with the native contralateral ACL. By 12M, the full graft was most like the native contralateral ACL. Similarly at the 4 subsegment level, all regions except the distal segment had significantly higher SI ratios at 3M and 6M as compared with the native contralateral ACL. By 12M, all subsections of the graft were not significantly different from the native contralateral ACL.

Conclusion:

The SI ratio of QT graft was increased at 3M and 6M after surgery and then returned close to that of the native ACL by 12M after surgery, which is largely consistent with the published maturational timeline of patellar tendon and hamstring tendon autografts.

Keywords

anterior cruciate ligament ACLR ligamentization QT autograft MRI SI ratio

Anterior cruciate ligament (ACL) injuries are among the most common sports-related injuries, accounting for about half of all knee injuries, and the vast majority of athletes opt to undergo ACL reconstruction (ACLR).^19,24 However, around 5% to 10% of those who undergo ACLR subsequently undergo a revision after graft failure or rupture,¹³ often due to a variety of factors, such as premature return to sport, traumatic reinjury, tunnel malposition, failure of graft incorporation, or improper graft selection. While traumatic reinjury and graft incorporation failure are not completely avoidable, clinicians have more control over a patient's return-to-sport timing, surgical technique, and graft selection. Currently, the most common autografts for ACL reconstruction include ipsilateral bone–patellar tendon–bone autograft (BPTB)¹² or hamstring tendon (HT).^11,23 While the quadriceps tendon (QT) autograft is historically the least commonly used, making up about 10% of grafts used in ACLR,⁴ use of the QT autograft has become more common over the past 10 to 15 years.^8,9 Regardless of the graft used, early failures often occur in athletes returning to sport too quickly and not allowing the graft to properly and fully mature, a process known as ligamentization.

Ligamentization consists of 3 sequential phases over a 12-month (12M) period after ACLR: early healing, proliferation, and maturation. The early healing phase, occurring within the first month after surgery involves avascular necrosis, digestion, and disorganization of the collagen making up the graft, resulting in weakened tissue.^3,15,27 The subsequent proliferation phase, occurring 2 to 4 months after surgery, begins the process of rebuilding the tissue through vascularization of the graft and introduction of fibroblasts and osteoblasts from the surrounding tissues including the leftover stump of the native ACL.³³ During the proliferation phase, the graft remains in a weakened state, as the new collagen matrix begins remodeling.²¹ The final phase, maturation, generally culminates at 12M after surgery, as collagen density continues to increase along with collagen organization that more closely resembles native ACL than the harvested graft.⁷ Understanding the timeline of these biological changes to the graft may therefore help prevent future graft failure, while also informing return-to-play protocols postoperatively to minimize risk of reinjury.

To study ligamentization, in vivo imaging techniques have been used to provide quantitative evaluations of the different stages of graft remodeling. The most common in vivo technique for evaluating the graft maturation process relies on magnetic resonance imaging (MRI) to provide a quantitative evaluation of the different stages of ligamentization. The primary MRI technique for this evaluation, generally known as signal intensity (SI) ratio, estimates a T2 signal of the ACL relative to an in-image normalization point, where studies have used skeletal muscle (gastrocnemius) or the posterior cruciate ligament (PCL).¹⁸ These changes in SI reflect known changes in gross, histological, and biochemical changes to the graft in models of ACLR.² Multiple studies, including Lutz et al¹⁸ and Chen et al⁶ have shown significant decreases in SI between the 6-month (6M) and 12M time point, and Pauvert et al²⁵ also showed that this decreasing signal extends out to 24 months after ACLR. Additionally, Lutz et al and Pauvert et al showed distinct differences, particularly graft hyperintensities, between the distal aspect of the graft and the rest of the graft. Furthermore, variations in the process of ligamentization have not only been shown between individuals, but also between different regions of the graft. Changes in ligamentization have been demonstrated in the intra-articular component of the graft as compared with the intraossoeus portions.^18,25 Alteration of grafts also occur from proximal to distal aspects of the graft within the intra-articular space.³¹ While these studies have focused on the ligamentization of BPTB and HT autografts, there is limited investigation regarding the SI timeline of the QT autograft, which has gained popularity in recent years.

The purpose of this study was to longitudinally evaluate the graft maturation process of the QT autograft after ACLR. To accomplish this, MRI was used to quantify changes in the SI ratio of the graft at 3 months (3M), 6M, and 12M after ACLR, relative to the SI ratio of the native ACL in the contralateral knee. To further quantify the healing profile and investigate exploratory hypotheses, the graft was further segmented into 4 sections along the length of the graft (distal, distal middle [DM], proximal middle [PM], proximal), as well as 24 equally spaced sections to localize the differences in graft maturation. Similar to other ACL graft types, we hypothesized that the SI of the QT autograft would be significantly increased as compared with the contralateral native ACL at 3M and 6M after ACLR, but the graft's SI ratio values would not differ from the contralateral native ACL at 12M after ACLR.

Methods

Participants and Procedures

Participant data from an ongoing prospective longitudinal ACL registry at the Emory Sports Performance And Research Center were used in the present study. Inclusion criteria for the larger ACL registry included patients aged 8 to 70 years who were scheduled to undergo ACL reconstruction or ACL revision or had previously undergone ACLR. Exclusion criteria were inability to consent for oneself, parent/guardian unwilling to consent if <18 years of age, currently pregnant, prisoners, non-English speaking, contraindications to MRI, or were a member of other vulnerable populations. A total of 20 participants who enrolled between August 2022 and June 2023 and consented to participate in this institutional review board–approved study at Emory University were selected for evaluation in these prospective case series analyses. These participants were referred to our center from a surgeon with clinical excellence in ACL reconstruction in young amateur athletes^26,28 (J.W.X.). Participants that were selected from the larger ACL registry data set and used in the present analyses had no history of knee injury or surgery and primarily consisted of the first participants evaluated with the same MRI sequences (ie, after optimization) at 4 time points when data were queried (larger ACL registry is ongoing). The 4 time points included before surgery (PRE), 3M postoperative, 6M postoperative, and 12M postoperative. At each of the 4 visits, participants underwent a series of MRI scans, with scans collected on both ipsilateral (ie, injured) and contralateral (ie, uninjured) knees during the PRE visit and only on the ipsilateral knee at 3M, 6M, and 12M time points. One participant was excluded for not attending the 12M postoperative visit, which resulted in N = 19 for the final study size. Participants consisted of 19 young amateur athletes (n = 17 female) ages 13 to 19 years (mean age, 15.63 ± 1.64 years at time of PRE visit). Each participant experienced an ACL tear (7/12 right/left knee) and underwent a primary, minimally invasive, all-side QT ACLR performed by a single surgeon (J.W.X.) according to an established technique.²⁹ Of note, we did not purposefully target male or female participants for enrollment, but considered the higher number of female participants in the present analyses to reflect the greater rates of ACL injuries in female than male patients. However, we did purposefully target young active athletes given their risk for ACL injury and interest in evaluating maturational changes to the graft during formative years.

MRI collection was performed on a 3.0-Tesla General Electric (GE) SIGNA Premier whole-body scanner. All knee MRI sequences were collected using an 18-channel transmit/receive knee coil (Quality Electrodynamics) and included a 3D T2-weighted, fat-saturated fast spin-echo sequence (CUBE) with the following parameters: acquired matrix = 256 × 256, interpolated to 512 mm × 512 mm; spacing = 0.7 mm; slices = 202; reconstructed in-slice resolution = 0.3125 mm × 0.3125 mm × 0.69 mm; echo time/repetition time = 1000 ms/39.8 ms, in-plane acceleration factor = 2. An additional vendor-specific, recommended surface coil intensity correction was also applied (GE SCENIC). This sequence was selected based on our previously published work demonstrating its sensitivity for detecting structural, signal-related changes to the knee joint in patients with ACL injury.⁵ In addition, this sequence was optimized via preliminary testing conducted by members of our team (A.A.C., T.M.Z., and J.A.D.) and with support from the MRI vendor (GE HealthCare).

Data Analysis and Statistical Analysis

Regional signal means were calculated from the intra-articular tissue of the native ACL of the contralateral knee from the PRE time point; the QT graft in the intra-articular space at 3M, 6M, and 12M timepoints; and the PCL at each visit, to be used as a reference value and account for visit-to-visit variations in the MRI signal. Manually segmented masks were generated by a single evaluator for the native ACL and QT grafts were calculated at the full mask level, as well as sectioned versions of the mask, separated into either 4 (distal, DM, PM, proximal) or 24 equally spaced sections, along the length of the native ACL and QT graft (Figure 1). Previous approaches have used a point approach localized to the proximal, mid-, and distal portions of the graft to extract signal intensities (Lutz et al¹⁸) but resulted in graft portions of unequal size. As a result, we elected to segment the entire graft into 4 and 24 equivalently sized sections to ensure that comparisons would be more consistent, and a more detailed characterization of the graft could be explored. For the PCL, small regions of interest were selected as a single point and then expanded to a small sphere with a radius of 3 voxels around the selected point, in a similar method to that described in Lutz et al.¹⁸ The SI ratios were calculated using the following equation: SI ratio = S_Region/S_PCL, where S_Region is the mean SI from the defined region of the ACL (full mask or segment from either set of segmented masks), QT graft, and S_PCL is the mean SI from the defined region of the PCL.

Figure 1.

Representative slices of the segmentation of the (A) native anterior cruciate ligament and intra-articular section of the quadriceps tendon graft at (B) 3 months, (C) 6 months, and (D) 12 months. The segmented regions of the graft in 3-dimensional representation of the knee: (E) full graft, (F) 4-section graft (1-4), and (G) 24-section graft (1-24), where segments were generated along the length of the graft from distal (D) to proximal (P). DM, distal middle; PM, proximal middle.

All data were first evaluated for outliers in the dependent variable of SI ratio. An outlier was defined by the 3-sigma rule (>3 SD from the mean), which resulted in exclusion of no participants from the study. Assumptions of normality were assessed using the Shapiro-Wilk test. Several dependent variables violated the assumption of normality (Shapiro-Wilk test, P < .05), which resulted in log10 transformation of the SI ratio data before parametric testing. For statistical analysis, we used separate 1-way repeated measures of analysis of variance (factors: PRE contralateral ACL, 3M QT, 6M QT, 12M QT), with Bonferroni-corrected pairwise comparisons and Cohen d were used to determine effect size for SI ratio of the full and 4 subsegments of the QT autograft as compared with the PRE contralateral ACL using IBM SPSS Statistics (Version 31.0; IBM Corp.).

Results

Full Graft

At the full graft level (Figure 2), there was a significant time effect, Greenhouse-Geisser adjusted F (2.12, 38.13) = 8.12; P < .001; partial η² = 0.311. t tests indicated that the SI ratio of the QT graft was significantly greater than the PRE contralateral ACL at the 3M (P = .007; d = 0.884) and 6M time points (P = .026; d = 0.750)

Figure 2.

A comparison of signal intensity (SI) ratio values across the 3 graft time points (3, 6, and 12 months [M]) and the presurgery (PRE) contralateral native anterior cruciate ligament (ACL) (as represented by the dotted line). The asterisk indicates significance (P < .05) between the full quadriceps tendon autograft and the full PRE contralateral ACL at the respective time point. The individual dots represent the value for each of the 19 participants at each time point. The horizontal line and error bars represent the mean and SD, respectively.

Segmented Graft

At the 4-subsegment level (Figure 3), there were significant differences in the DM (F[2.282, 41.069] = 5.859; P = .004; partial η² = 0.246), PM (F[1.846, 33.234] = 15.95; P < .001; partial η² = 0.470), and proximal segments (F[1.685, 30.321] = 16.261; P < .001; partial η² = 0.475), while there were no differences in the distal segment (F[3, 54] = 0.793; P = .503; partial η² = 0.042). t tests indicated that the QT graft was significantly greater than the PRE contralateral ACL at the 3M (DM, P = .023; d = 0.759; PM, P < .001; d = 1.210; Proximal, P = .002; d = 1.017) and 6M time points (DM, P = .039; d = 0.707; PM, P = .003; d = 0.975; Proximal, P = .002; d = 1.017). By 12M, all graft subsegments were not significantly different than the PRE contralateral ACL (all Ps > .05). Further definition in these differences in SI can also be seen in Figure 4, where the regions are segmented into 24 total regions.

Figure 3.

A comparison of the signal intensity (SI) ratio values of 4 sections of the graft: distal (Dis), distal middle (DM), proximal middle (PM), and proximal (Prox) across the 3 graft time points (3, 6, and 12 months [M]), and the segmented presurgery (PRE) contralateral native anterior cruciate ligament (ACL) (as represented by the dotted line for each of the four sections). The asterisk indicates significance (P < .05) between the QT autograft and the PRE contralateral ACL for a given segment at the respective time point. The individual dots represent the value for each of the 19 participants at each time point. The horizontal line and error bars represent the mean and SD, respectively.

Figure 4.

(A) The mean graft signal intensity (SI) ratio changes across 24 regions along the length of the graft, which shows variations from the distal to proximal end of the graft, with error bars at each point showing SE. (B) A visual representation of the graft SI ratio differences between time points across the graft using a heat map on a single slice of representative patient's graft mask. PRE, presurgery; M, month.

Discussion

This study determined that the QT autograft had an increased SI ratio relative to the native contralateral ACL that was most pronounced early after surgery and in the most proximal aspects of the graft, consistent with our hypothesis. Overall, relative to the native contralateral ACL, the graft increased in SI ratio at 3M and 6M before returning to, on average, near native ACL levels at 12M. These findings were also present at the sectioned graft level, where predominately the proximal section showed significant differences between the graft and native contralateral ACL at 3M and 6M, while the distal segment of the graft did not show differences across the graft time points. These maturational changes of the QT graft in young amateur athletes after ACLR using MRI-based SI ratio may help future work to connect graft healing, return-to-sport considerations, and prevention of graft failures.

The primary finding of this study was the significant change in SI ratio over the course of the QT maturation timeline, particularly in relation to the SI ratio of contralateral native ACL. In previous studies of ACL autograft maturation,^16,17,20 graft SI has been shown to correlate with several aspects of the graft remodeling process, particularly graft vascularization as the graft begins to rebuild after the breakdown occurring during the early healing phase. Previous studies of BPTB,^14,22 HT,^6,18 and QT¹ autografts have reported early postoperative increases in graft SI ratio that generally decrease and begin to resemble the contralateral native ACL by 12 months after surgery. This finding was supported in the present study, as elevated SI ratio was observed in the full graft and the segmented graft at 3M and 6M, but was not different from the PRE contralateral ACL by 12M after surgery. These graft maturational changes were consistent with the expected levels of tendon atrophy, remodeling, and maturation at both the full graft and the regional levels. These findings further support MRI SI ratio measures as an effective tool for tracking graft maturation, and indicate a potential comparison for identifying differences in the graft healing timeline between the QT autograft and other autografts, such as BPTB and HT. It is important to note that there were outliers within the cohort that had QT graft SI well above the cohort mean at 12M. In fact, these outliers had a mean graft SI at 12M that was above the cohort mean at 3M. It is possible that these outliers may represent a patient population that was at higher risk of graft failure upon returning to sports due to incomplete ligamentization. It is possible that ligamentization may occur at longer time points after surgery, such as with Pauvert et al,²⁵ or potentially never fully occur in these patients. Future studies will combine SI as assessed by MRI with longitudinal clinical outcomes including patient-reported outcome measures (PROMs), graft failure, and revision surgery. These data will help inform clinical decision-making after ACLR.

One important and novel finding of this work was capturing the regional differences of the QT graft during maturation. Overall, the more proximal sections of the graft showed significant changes in SI ratio when compared with the native contralateral ACL SI ratio. The increased signal in the proximal aspects of an ACL autograft, relative to the distal aspects, was previously shown in Tashiro et al³⁰ and Lutz et al¹⁸ indicating this is likely consistent between graft types and may be the result of differences in biomechanical forces across the length of the graft or an inherent aspect of graft maturation. These differences in SI ratio could potentially be indicative of a slower or more complex maturational process in the proximal aspect of the graft in the intra-articular space, as the autograft may be exhibiting higher collagen disorganization in the proximal and middle aspects during the remodeling and ligamentization phases than the distal aspects. This potential suggests that the proximal aspect of the graft is particularly susceptible to reinjury with a premature return to sport or other high-risk activities. Along these lines, a previous study³² showed that higher SI of the graft was correlated with risk of rupturing the graft, indicating these areas may be more vulnerable to retearing due to slower tissue healing. This idea may be particularly important after ACLR, as the proximal and middle aspects of the ACL are also where most native ACL tears occur, indicating potential structural stressors in this intra-articular region around the proximal aspect, exacerbating the reinjury risk in this area of the graft. Furthermore, most graft ruptures have been shown to occur within the proximal aspect of the graft.³⁴ It is possible that patients with elevated SI within the proximal aspect of the graft, such as the outliers within the present study that are outside of the 95% CI at 12M, may be at highest risk for graft rupture after return to sport.

Limitations

There are several limitations to the present study. First, the lack of histological or structural components limited our capacity to directly evaluate graft compositional changes as they related to the measured SI ratio. Additionally, the study population, derived from a sample of convenience from an ongoing prospective longitudinal registry, resulted in a relatively small, predominately female cohort and was limited to the QT autograft in a young athletic population, so the results from this study cannot be generalized to patients outside this age group nor to other autografts. Also, because this study used a 3.0-Telsa MRI, lower field strength scanners may not produce similar results. Likewise, we emphasize that SI values were sequence and scanner dependent, with raw values not directly comparable with other studies. Manual segmentations of the ACL were performed as well, where there may be some error in the generated masks. Future studies should also use a formal control for the patients, measuring changes in both the ipsilateral and the contralateral knees, investigating patient-specific characteristics that alter the maturational timeline, and increasing the age range to include young adults, as the outcomes of this study cannot be applied past the young population that was utilized. Finally, the current imaging metrics and potential outliers should be monitored relative to PROMs or clinical graft failure (grade ≥2 Lachman, overall International Knee Documentation Committee grade C or D, graft rupture, and/or instrumented laxity with a side-to-side difference >5 mm)¹⁰ to determine if it is possible that SI may be used to inform return-to-play decision making.

Conclusion

The SI ratio of QT autograft was increased at both 3M and 6M after surgery before approaching the native contralateral ACL at 12M, with the most pronounced differences observed within the proximal aspect of the graft. These findings largely corroborate previously published studies on other autograft types and suggest that QT graft maturation may take up to 12M, and the proximal aspect of the graft may be particularly susceptible to reinjury with early return to sport or high-risk activities.

Footnotes

Final revision submitted July 25, 2025; accepted August 26, 2025.

One or more of the authors has declared the following potential conflict of interest or source of funding: This work was partially funded by internal support from the Department of Orthopaedics at Emory University. G.D.M. has no direct conflicts of interest related to the current investigation. His institutions have received past and ongoing grant funding from the National Institutes of Health/National Institute of Arthritis and Musculoskeletal and Skin Diseases, the Department of Defense, the Department of Veterans Affairs (CReATE Motion Center), and the Arthritis Foundation Osteoarthritis Clinical Trial Network. He has also received industry-sponsored research support to his institution’s research related to injury prevention, sport performance, rehabilitation and surgical interventions including current funding from Arthrex Inc. for evaluation of ACL surgical techniques, GE Healthcare for MRI sequence development, and the National Basketball Association for validation of biomechanical models. His research program further benefits from philanthropic support from the Arthur M. Blank Family Foundation to advance dissemination and community-based translation of research discoveries. He also receives author royalties from Human Kinetics and Wolters Kluwer and is an inventor of biofeedback technologies (U.S. Patent 11350854B2; “Augmented and Virtual Reality for Sport Performance and Injury Prevention Application,” approved 6/7/2022, with associated software copyright), which have generated licensing royalties. J.A.D. receives royalties from Kendall Hunt Publishing Company. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from Emory University (No. STUDY00002682).

ORCID iDs

Kyle D. Anderson

Shayla M. Warren

Hung-I Hsiao

Gregory D. Myer

Jed A. Diekfuss

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Twelve-Month Maturation Timeline of Quadriceps Tendon Autograft Signal Intensity Ratio After ACL Reconstruction Surgery Using Magnetic Resonance Imaging

Abstract

Background:

Purpose:

Study Design:

Methods:

Results:

Conclusion:

Keywords

Methods

Participants and Procedures

Data Analysis and Statistical Analysis

Results

Full Graft

Segmented Graft

Discussion

Limitations

Conclusion

Footnotes

ORCID iDs

References