Serial Magnetic Resonance Imaging Graft Signal Intensity and its Correlation with Functional Recovery Following Primary Anterior Cruciate Ligament Reconstruction

Introduction

Anterior cruciate ligament (ACL) injury is a type of ligamentous injury in the knee that is very frequent among young, physically active people and results in functional instability, performance deterioration, and premature degenerative alterations in the case of untreated injury ^[1]. Primary ACL reconstruction (ACLR) is the standard treatment of the symptomatic rupture of the ACL, with the intended aim of restoring knee stability, biomechanics and functional capacity through the use of autografts or allografts. However, even with the improvement of surgical and rehabilitation protocols, the inconsistency of the overall outcome of patients is a significant clinical issue, and some patients recover fully, whereas others show chronic instability, graft failure, or slow recovery. As such, the biological recovery of the graft and its connection with functional recovery has become a hot discipline of study ^[2].

After ACLR, the graft is subjected to a complex bio-remodeling process termed as ligamentization that involves necrosis, revascularization, cellular proliferation, and collagen reorganization. In the process, the structural and biomechanical characteristics of the graft over time become similar to the native “ACL ^[3]. The most popular parameter that has been assessed is the graft signal intensity using magnetic resonance imaging (MRI) as a non-invasive method to analyze graft maturation. In general, the higher the signal intensity on MRI, the higher the water content, incomplete collagen structure, and the continued remodeling, the lower the signal intensity, the more mature and organized the graft structure ^[4].

Serial MRI evaluation enables the clinician to determine temporal variations in graft signal intensity that occur over time and give input into the healing process. The early post-operative grafts tend to show intermediate to high signal intensity owing to the vascular ingrowth and tissue remodeling that may continue up to 2 years ^[5]. Nonetheless, continued high signal intensity later than anticipated can be a sign of graft degeneration, impingement, or partial failure. In this way, MRI does not just aid in the evaluation of the integrity of the grafts; it also contributes to the detection of complications that can undermine the functional outcomes ^[6].

A number of studies have tried to relate MRI-based graft signal intensity with clinical and functional outcomes after ACLR. The functional recovery can be evaluated by use of parameters like knee stability (instrumented laxity testing), patient-reported outcome measures (e.g., international knee documentation committee [IKDC], Lysholm scores), and return-to-sport criteria ^[7]. There is some evidence that greater graft signal intensity, indicating a biological immaturity, correlates with a worse functional stability and knee laxity. MRI signal intensity and functional outcomes have been reported to have a negative relationship with each other, and patients who have more mature grafts (lower signal intensity) are more likely to have better clinical recovery ^[8].

Moreover, quantitative MRI parameters such as signal-to-noise quotient (SNQ) and graft volume have been identified to forecast the long-term outcome. Indicatively, it has been shown that simultaneous measurement of graft volume and signal intensity can predict clinical and patient outcomes several years post-surgery, indicating that MRI is prognostic in ACLR follow-up ^[9].

Nevertheless, the topic of correlating MRI results with functional recovery is not completely uniform throughout the literature. Although there are studies that provide substantial correlation between graft signal strength and clinical outcome, other studies have shown no dependable association, especially when it comes to return-to-sport outcome. Recent systematic review has concluded that despite the usefulness of MRI to provide information about graft maturation, presently, no single radiologic parameter can be reliably used to predict return to sport after ACLR ^[10].

Moreover, it is also possible that an overabundance of graft signal intensity at later phases of grafts, including 12 months after surgery, can be linked to such undesirable consequences as graft retear, which is a sign of poor biological healing. This emphasizes the significance of serial MRI assessment as compared to single-time-point assessment because longitudinal assessments give a better picture of graft maturation and its clinical consequences.

Although serial MRI has emerged as a useful non-invasive tool for evaluating graft maturation after ACLR, the relationship between MRI graft signal intensity and functional recovery remains inconsistent across the literature. Furthermore, most previous studies have focused on isolated imaging parameters or long-term outcomes, with limited prospective evidence evaluating serial MRI changes alongside validated functional outcome measures during the early post-operative period. Therefore, the present study was undertaken to prospectively evaluate serial changes in MRI graft signal intensity and determine their correlation with functional recovery, as assessed by the IKDC and Lysholm scores, following primary ACLR. We hypothesized that progressive reduction in graft signal intensity would be associated with superior functional outcomes, thereby supporting the clinical utility of serial MRI in monitoring graft maturation and guiding post-operative rehabilitation.

Materials and Methods

Results

A total of 50 patients undergoing primary ACLR were included in the study. The majority of patients were aged 26–35 years (40%), followed by 36–45 years (32%) and 18–25 years (28%). Males constituted 76% of the study population. Sports-related injuries were the most common mechanism of injury (60%), followed by road traffic accidents (24%) and falls (16%) (Table 1).

Serial MRI evaluation demonstrated progressive graft maturation over time. At 3 months, high graft signal intensity was observed in 52% of patients, decreasing to 28% at 6 months and 12% at 12 months. Conversely, the proportion of patients with low signal intensity increased from 12% at 3 months to 56% at 12 months, indicating progressive ligamentization and graft remodeling (Table 2 and Fig. 1).

Functional assessment revealed favorable outcomes, with 40% of patients achieving excellent and 36% good IKDC scores. Similarly, Lysholm scores were excellent in 44% and good in 32% of patients, indicating satisfactory recovery in the majority of cases. At 12 months, patients with low graft signal intensity demonstrated significantly higher mean IKDC scores (88.5 ± 4.2) compared with those exhibiting intermediate (76.3 ± 5.1) and high signal intensity (65.8 ± 6.4). Correlation analysis demonstrated a significant negative association between MRI graft signal intensity and functional outcomes, with signal intensity showing inverse correlations with both IKDC scores (r = −0.62, P < 0.001) and Lysholm scores (r = −0.58, P < 0.001) (Table 3 and Fig. 2).

Discussion

The current prospective observational trial compared serial graft signal intensity at MRI and its relationship with primary ACLR functional recovery in 50 cases, and the results showed that there is a definite pattern of graft signal maturation with time and a statistically significant correlation between MRI signal characteristics and clinical outcomes. The demographic data of the study revealed that most patients were aged between 26 and 35 years (40%), 36–45 years (32%), and 18–25 years (28), which is indicative of the fact that ACL injuries are high among young and active people. This was also facilitated by the fact that the majority of them were males (76%) as opposed to females (24%), probably due to higher exposure to sports and high-impact activities. The mode of injury analysis showed that the most common (60%) were sports-related injuries compared to road traffic injuries (24%), and falls (16), which supports the already known fact that ACL injuries are mostly related to sporting activities.

The current study demonstrated serial MRI results of progressive reduction in graft signal intensity, which signified biological graft maturation with time. Most of the patients (52%), at 3 months post-operative, exhibited high signal intensity, and 36% had intermediate and 12% low signal intensity, which were indicators of early graft remodeling with high vascularity and water content. The transition to maturation was seen by the 6 months, with 44% of patients having intermediate signal intensities, as well as 50% (28% each) in low and high signal groups. By 12 months, this change to graft maturity was pronounced, with 56% of the patients having low signal intensity, 32% intermediate, and only 12% high signal intensity. This is in line with the biological mechanism of ligamentization, wherein the graft slowly becomes more like the native ACL. The same temporal variation in graft signal intensity was reported in by with the mean signal intensity ratio (SIR) recorded at 1 year of 2.7, and in most patients, the graft signal did not vary significantly after 1 year, indicating the stabilization of graft healing. Nonetheless, the patients with high initial signal intensity (SIR >4) showed a significant signal reduction over time and an increase in sporting level, which showed a further graft maturation later than the initial year. This result confirms the observation made in the current study that patients with a strong signal at earlier stages can still progress through with maturation, as determined by the decreasing number of high signal cases at 52% at 3 months and 12% at 12 months.

The present study demonstrated that a functional outcome assessment indicates that 40% of patients experienced excellent IKDC scores, 36% good scores, 16% fair scores, and 8% poor scores, whereas the Lysholm scoring indicates that 44% had excellent results, 32% good results, and 16% fair results, though a few had poor results. The results show that about 76–80% of patients recovered well, with some having excellent functional recovery, which implies the effectiveness of ACLR. Notably, MRI graft signal intensity was significantly negatively correlated with functional outcomes, with the group of patients with low signal intensity at 12 months showing the highest mean of the IKDC score (88.5 ± 4.2), intermediate (76.3 ± 5.1), and high signal intensity groups (65.8 ± 6.4). Correlation analysis also indicated that signal intensity and IKDC score (r = −0.62, P < 0.001) and Lysholm score (r = −0.58, P < 0.001) are statistically significantly negatively correlated and thus higher signal intensity is associated with worse functional outcomes. These results are consistent with the study by Biercevicz et al and Miyawaki M., ^[3,11] also showed that MRI-based graft parameters (signal intensity and graft volume) predicted functional outcomes (hop test performance and patient-reported outcome measures at 3 and 5 years) and were significantly predictive (R = 0.62, P =0.003 of hop test at 5 years) of the outcome. It is helpful in arguing that MRI signal strength is an indicator of the biomechanical integrity of the graft, and it can be used to predict the functional recovery.

The results of the current study, however, are not consistent with Saupe et al. ^[12] have stated that there is no significant relation between graft signal intensity and IKDC scores (P = 0.667), arthrometric laxity, or time since surgery at long-term follow-up of 412 years. In their research, a large proportion of patients (70%) had increased intrasubstance signal intensity that was not associated with functional impairment or instability. This can possibly be explained by the differences in the follow-up period since the current study was based on early to mid-term outcomes (up to 12 months), whereas Saupe et al. considered long-term outcomes where the graft remodeling can have reached a plateau, and the compensatory mechanisms can affect the functional performance.

Equally, Miyawaki et al. ^[11,13] have found that signal intensity in graft bundles differed (posterolateral bundle exhibited higher signal intensity than anteromedial bundle, 2.5 ± 1.7 versus 1.7 ± 1.5 on proton density-weighted imaging, P < 0.05), and that signal intensity was only weak to moderately correlated with time since surgery (r = 0.38 – This implies that although the MRI signal intensity is an indicator of graft remodeling, its dependence on time and functional outcomes might not be linear or even consistent across all graft components. Conversely, the current research showed a more reliable temporal attenuation of signal intensity and a closer relationship with functional outcomes, perhaps due to homogeneous graft type and homogeneous rehabilitation guidelines.

Moreover, Flannery et al. ^[14] found that the MRI-based measures of cross-sectional area, volume, and estimated failure load were predictive of functional improvement (r = 0.44–0.48, P = 0.01); however, signal intensity alone was not significantly correlated with IKDC” scores or knee laxity when covariate with demographic variables. The result of this finding is that, although structural parameters are more likely to be predictive, the intensity of signals alone would not be a complete predictor of graft functionality. The current study, however, showed a significant association between signal intensity and functional scores, and this could be attributed to variations in study design, sample size, imaging methods, and outcome measures.

On the whole, the current study demonstrates that serial MRI graft signal intensity is an effective non-invasive biomarker to evaluate graft maturation and predict functional recovery during the initial post-operative period. The gradual reduction of the level of high signal intensity at 52% at 3 months–12% at 12 months, and the corresponding increases observed in the IKDC and Lysholm scores confirm the importance of MRI in the process of graft healing monitoring ^[14]. The correlation is very negative (r = −0.62 and −0.58), which underlines that reduced signal strength correlates with improved clinical results. Although there are some studies that show that there is limited correlation between MRI signal and long-term functionality, the current results show that during the early stages of recovery, the signal intensity is a key indicator of graft biology and functional performance. Thus, serial MRI assessment could be integrated with clinical and functional assessment to assist in shaping the rehabilitation plans and enhancing patient outcomes after ACLR.

The strengths of the present study include its prospective design, standardized serial MRI evaluation at multiple post-operative intervals, and the use of validated functional outcome measures such as the IKDC and Lysholm scores. However, certain limitations should be acknowledged. The study was conducted at a single center with a relatively small sample size, which may limit the generalizability of the findings. The follow-up period was limited to 12 months and therefore may not fully reflect long-term graft maturation and clinical outcomes. In addition, MRI graft signal intensity was assessed primarily using qualitative grading rather than advanced quantitative imaging parameters. Future multicenter studies with larger sample sizes, longer follow-up durations, and quantitative MRI assessment are warranted to further validate these findings.

Conclusion

Serial MRI graft signal intensity is a useful indicator of graft maturation following primary ACLR and demonstrates a significant correlation with functional recovery. Progressive reduction in graft signal intensity over 12 months reflected ongoing ligamentization, while lower signal intensity was associated with superior IKDC and Lysholm scores. These findings suggest that serial MRI assessment may serve as a valuable non-invasive tool for monitoring graft healing and guiding post-operative rehabilitation.

Clinical Message

Non-invasive graft signal intensity MRI can be utilized to assess graft healing in the post-operative period and patients at risk of slower recovery. It can assist clinicians to customize rehabilitation plans and make effective decision-making on return to activity or sports.

References

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About the Authors

• 

  Dr. Siddharth Malik

• 

  Dr. Piyush Suresh Vasaikar

• 

  Dr. Prajwal Ganesh Senthilnathan

• 

  Dr. Ayush Gupta

• 

  Dr. Manvir Singh

• 

  Dr. Shubham Gupta