Magnetic resonance imaging-based observations of ligamentization and tunnel widening in relation to functional outcomes after all-inside anterior cruciate ligament reconstruction: A pilot case series

INTRODUCTION

The rupture of the anterior cruciate ligament (ACL) is one of the most studied injuries in orthopedic research, with an estimated incidence of 30–78 cases/100,000 person-years.^[1] ACL injuries commonly result in joint laxity, instability, decreased physical activity, and a higher risk of subsequent knee injuries if not properly treated.^[2-6] Surgical intervention remains the standard approach for young, active individuals eager to return to their previous activity levels. Among surgical options, the “all-inside” ACL reconstruction technique has gained popularity for its minimally invasive characteristics. It utilizes a single autograft tendon rather than the double-tendon autografts used in traditional methods.^[7]

The all-inside ACL reconstruction technique utilizes a single semitendinosus autograft with dual suspensory fixation, offering an alternative to traditional methods that commonly employ both the gracilis and semitendinosus tendons.^[8] This approach offers several benefits, including reduced donor site morbidity, preservation of bone stock, decreased postoperative pain, and smaller surgical incisions.^[9] Compared to conventional techniques, the all-inside method may lead to improved surgical outcomes.^[10] In addition, Lysholm and Tegner scales are widely recognized for their validity and reliability in assessing post-operative knee function and physical activity levels.^[11]

Despite positive clinical outcomes, the biological processes following ACL reconstruction, such as graft ligamentization and tunnel widening, remain active research topics. The remodeling of the graft and its integration into the bone tunnels involve complex biological changes that can be indirectly assessed through magnetic resonance imaging (MRI) signal intensities.^[12,13] However, most existing studies have focused separately on functional outcomes and MRI findings, and the relationship between patient-reported functional recovery and imaging assessments is largely unclear. Ligamentization, the final stage of graft healing involving avascular necrosis, remodeling, and maturation, was assessed using the Figueroa score.^[14] By linking clinical outcomes with radiological healing, this study seeks to inform surgical and rehabilitation strategies for ACL injuries.

This gap is especially significant in the context of all-inside ACL reconstruction, where the unique biomechanical conditions created by dual suspensory fixation may affect tunnel enlargement differently than traditional techniques.^[15] Nevertheless, few studies have systematically investigated whether functional improvements, measured by patient-reported outcomes, align with structural changes observed on MRI following this newer surgical technique. This pilot case series aimed to describe functional outcomes (Lysholm and Tegner scores) and MRI findings (graft ligamentization and tunnel widening) after all-inside ACL reconstruction. By reporting these observations, we seek to provide insights into the clinical relevance of MRI findings and inform postoperative care and rehabilitation strategies.

CASE SERIES

RESULTS

A total of 258 patients who underwent ACL reconstruction using the all-inside technique between 2019 and 2023 were initially considered for this study [Table 1]. After recruiting patients through telephone invitations and direct clinic contacts, 18 patients consented to participate. On applying the exclusion criteria, 12 patients remained eligible for analysis. Among these participants, 91.7% were males, with the majority (66.7%) under 30 years of age and 33.3% between 31 and 40 years; no participants were older than 40. Eleven patients (91.7%) underwent ACL reconstruction alone, and 1 (8.3%) had ACL reconstruction with meniscus repair. Follow-up periods were 2 years for 10 patients (83.3%), 3 years for 1 (8.3%), and 4 years for 1 (8.3%). One patient developed knee stiffness, which was treated with range of motion manipulation under general anesthesia. No infections or graft failures were observed.

Table 2 presents individual functional outcomes and graft ligamentization scores for the 12 patients. Lysholm scores ranged widely from 34 to 100, indicating varied patient-reported knee function, with some patients (Subject 8, score: 100) achieving excellent outcomes and others (Subject 4, score: 34) reporting poorer function. Tegner activity scores ranged from 2 to 7, reflecting diverse activity levels post-surgery, with several patients (Subjects 2, 3, 4, 8, and 11) returning to high activity levels (score: 7). Figueroa scores, assessing graft ligamentization through MRI signal intensity, ranged from 2 to 5, showing variability in graft maturation. For instance, subject 7 had a high Figueroa score (5) and strong functional outcomes (Lysholm: 95, Tegner: 6), while subject 4 had a lower Figueroa score (2) and poor Lysholm score (34), though others with similar Figueroa scores (Subject 3, Figueroa: 2, Lysholm: 86) reported better function. This variability suggests that graft ligamentization, as seen on MRI, may not consistently align with functional recovery.

Functional outcomes were evaluated using the Lysholm Knee score and the Tegner activity scale. The average Lysholm score was 80.67 (SD: 17.77), with a median of 85.5, ranging from a minimum score of 34 to a maximum of 100. The average Tegner activity score was 5.33 (SD: 1.72), with a median of 5.5; the scores ranged from 2 to 7.

MRI evaluations were conducted to assess graft ligamentization using the Figueroa score and to measure tunnel widening in both femoral and tibial tunnels [Figure 1]. The average Figueroa score was 3.33 (SD: 0.98), with scores ranging from 2 to 5. The tibial tunnel showed a mean diameter increase of 2.25 mm (SD: 1.49 mm), with a range of 0–5 mm [Figure 2]. The femoral tunnel displayed a mean widening of 1.81 mm (SD: 1.66 mm), ranging from 0 to 5.67 mm. Individual MRI assessments indicated variability in structural changes, with some cases showing tibial tunnel widening as pronounced as 5 mm and femoral tunnel widening reaching up to 5.67 mm. Despite these radiological findings, most patients maintained relatively high functional scores.

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Figure 1:

Femoral tunnel widening of 12 patients.

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Figure 2:

Tibial tunnel widening of 12 patients.

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Normality assessments using the Shapiro-Wilk test indicated that the Lysholm score and Tegner activity scale did not follow a normal distribution (P < 0.05), whereas the Figueroa Score and measurements of tunnel widening did not conform to normality (P > 0.05).

DISCUSSION

The mean Lysholm score was 80.67 (SD: 17.77, median: 85.50), indicating generally good knee function [Table 3], though the wide range (34–100) highlights heterogeneity among patients. The mean Tegner activity score was 5.33 (SD: 1.72, median: 5.50), suggesting moderate activity levels, with a range (2–7) reflecting varied return-to-activity outcomes. The mean Figueroa score was 3.33 (SD: 0.98, range: 2–5), indicating moderate graft maturation on average, with some patients showing more advanced ligamentization. The tibial tunnel widening averaged 2.25 mm (SD: 1.49, range: 0–5 mm), and femoral tunnel widening averaged 1.81 mm (SD: 1.66, range: 0–5.67 mm), with some patients exhibiting pronounced widening (up to 5 mm tibial, 5.67 mm femoral). Despite these radiological changes, many patients (58.3%) maintained high functional scores, suggesting that tunnel widening may not have a direct impact on patient-reported outcomes.

The all-inside technique in ACL reconstruction procedures shows potential as a minimally invasive approach, evidenced by low graft failure rates and short-term improvements in knee function, stability, pain, and patient-centered outcomes. Primary anatomic all-inside ACL reconstruction demonstrates significant improvements in functional outcomes (Lysholm, International Knee Documentation Committee, visual analog scale, and Tegner activity scale scores) between baseline and clinical evaluation at 24 months post-surgery. In addition, no differences in knee stability (KT-2000 test) were found between the reconstructed knee and the contralateral normal knee after 24 months. This study also observed functional improvements following all-inside ACL reconstruction, reflected in increased Lysholm and Tegner scores. A study involving 108 patients also found significant improvements in clinical outcomes (Lysholm and Tegner activity scale scores) at 24 months post-surgery.

The biological remodeling process following ACL reconstruction can lead to changes in tunnel width without directly affecting knee function. The healing and tissue adaptation around the graft may result in tunnel widening without causing a decline in clinical function. In addition, the timing of evaluation plays a significant role; many studies indicate that early post-operative measurements may not reflect long-term outcomes, thus obscuring the relationship between tunnel size and clinical results. A study by Eichinger et al. (2023) showed that, despite significant changes in tunnel volume within the first 2 years post-surgery, these changes did not always correlate with functional scores such as Lysholm after longer follow-up periods.^[20] These findings suggest that, although tunnel widening may occur, it does not always impact patients’ perception of their knee function, thus showing no association with patients’ Lysholm scores. Other studies also support these findings by demonstrating that patients with greater tunnel widening can still report good functional outcomes, indicating that factors such as muscle strength and joint stability may have a greater influence on Lysholm scores than tunnel size itself.^[20]

Prior studies by Lubowitz et al. (2011) highlighted that graft maturation on MRI follows biological remodeling phases, such as avascular necrosis, revascularization, and cellular repopulation, which do not always align with clinical improvement.^[8,21] Several biomechanical and biological factors have been proposed to explain tunnel widening. Mechanisms such as stress shielding, the “bungee cord effect,” and the “windshield wiper effect” contribute to microinstability at the graft-tunnel interface, encouraging tunnel expansion.^[3-6,15,22] Despite these factors, the mechanical stability provided by suspensory fixation in the all-inside technique appears sufficient to maintain functional outcomes.^[3-7,22] This supports the idea that advances in surgical techniques may help reduce the clinical impact of radiological tunnel changes. These observations align with findings that tunnel enlargement, common in the early post-operative period, does not necessarily indicate clinical failure.^[18]

The favorable functional outcomes in this series suggest that recovery may be driven more by rehabilitation, neuromuscular adaptation, and psychological readiness than by MRI findings. For orthopedic practitioners, this highlights the importance of prioritizing functional goals, such as strength and proprioception training, in post-operative care. Clinicians can counsel patients that tunnel widening or variable ligamentization on MRI does not necessarily indicate poor recovery, provided that clinical assessments are positive.^[21] This supports a holistic approach to ACL reconstruction evaluation, integrating patient-reported outcomes with imaging data.

From a practical perspective, these findings inform postoperative care and patient counseling. Rehabilitation should focus on functional recovery and return-to-sport criteria rather than solely on imaging milestones. Patients with suboptimal MRI findings can be reassured that good functional outcomes are achievable with effective rehabilitation. These observations contribute to understanding the clinical reliability of the all-inside technique, which appears to yield stable functional outcomes comparable to traditional methods.^[23]

This pilot case series has several limitations. The small sample size of 12 patients limits the ability to generalize findings to a broader population. The retrospective design may introduce selection bias, as only patients available for follow-up were included, potentially skewing the cohort toward those with better access to care or fewer complications. Single-time-point MRI assessments may miss dynamic changes in graft maturation and tunnel remodeling, which evolve over time. The minimum 12-month follow-up for some patients may not capture complete ligamentization, which can take up to 24 months to occur. While MRI was chosen for its soft-tissue visualization and lack of radiation, computed tomography may offer greater precision for measuring tunnel widening. Larger, prospective case series with longitudinal imaging at multiple intervals (6, 12, and 24 months) are needed to further describe these outcomes and inform optimal imaging and rehabilitation strategies.

CONCLUSION

This pilot case series described functional outcomes and MRI findings in 12 patients following all-inside ACL reconstruction. Patients generally achieved favorable functional outcomes, with mean Lysholm and Tegner scores indicating good knee function and moderate activity levels, despite variable MRI findings of graft ligamentization and tunnel widening. Some patients exhibited pronounced tunnel widening, yet maintained high functional scores, suggesting that radiological changes may not fully reflect patient-reported recovery. These observations suggest that functional recovery is likely to depend more on rehabilitation, neuromuscular adaptation, and psychological readiness than on MRI findings alone. Larger, prospective case series with longitudinal imaging are needed to further describe these outcomes and refine post-operative care strategies.