Comparison of femoral tunnel position and angle with functional outcomes in arthroscopic anterior cruciate ligament reconstruction: Freehand drilling versus offset aimer technique

Background

The anterior cruciate ligament (ACL) is a key intra-articular structure of the knee and serves as the primary restraint to anterior translation of the tibia. In India, ACL injuries comprise approximately 86.5% of sport-related knee injuries.^[1] ACL injuries most often occur due to non-contact mechanisms, typically during sudden deceleration followed by a change in direction or an improper landing.^[2]

Although the exact incidence is not clearly documented, it is estimated that approximately 400,000 ACL tears and 200,000 ACL reconstruction (ACLR) procedures occur annually in the United States of America.^[3] International registries show that ACL tears predominantly affect young, active individuals, with peak incidences in the late teens to mid-twenties. In the South Asian series, most patients also fall below 30 years of age. For example, Joshi et al. from Nepal reported that around half their ACLRs belonged to the 15– 30 years of age group, with sports injuries being the leading mechanism, whereas road traffic accidents predominated in those older than 30 years.^[4,5] Some studies have suggested an increased risk of ACL injury in females, with estimates ranging from 1.4 to 9.5 times higher, although the underlying reasons remain uncertain.^[6]

The first known documentation of the ACL was in 170 AD by Claudius Galenus of Pergamon, who referred to the ACL and posterior cruciate ligament as ligamenta genu cruciata. By the 19^th century, clinical descriptions and diagnostic tests for ACL injuries had been introduced.^[7] Currently, diagnosis is established through clinical examination supported by magnetic resonance imaging (MRI). Patients often describe a popping sound at the time of injury, followed by deep knee pain, swelling, and a sensation of instability. Clinical findings may include a restricted range of motion, an antalgic gait, and a semi-flexed knee posture. In acute presentations, swelling and a positive Lachman test are common, while chronic cases may present with minimal swelling and positive anterior drawer or pivot shift tests.^[8]

The management of ACL tears has evolved significantly, from conservative treatment with immobilization to open surgery and now arthroscopic reconstruction. There is a broad consensus that ACL rupture substantially increases the long-term risk of knee osteoarthritis, irrespective of treatment.^[9] In young, active patients, ACLR provides greater mechanical stability and superior sport-specific functional outcomes compared with non-operative management.^[10] Surgical options include repair and reconstruction using various graft types, with current focus on achieving anatomical placement of femoral and tibial tunnels to restore native knee kinematics.^[11]

Various femoral tunnel placement techniques have been described. These include the traditional transtibial method and newer tibia-independent approaches such as the transportal and outside-in techniques. Even within the transtibial method, different tools may be used, such as freehand drilling based on bony landmarks, offset aimers, or arthroscopic rulers.^[12] Accurate femoral tunnel placement is essential for successful outcomes, as malposition of the femoral tunnel is consistently identified as the leading technical cause of graft failure. Revision-surgery cohorts have shown that femoral tunnel malposition accounts for up to 80% of technical failures requiring revision ACLR.^[13,14]

This study was undertaken to evaluate the accuracy of femoral tunnel position and angle achieved using an offset aimer compared to freehand drilling. It also aims to assess the graft angle at the femoral tunnel opening and correlate these findings with short-term functional outcomes following arthroscopic ACLR.

Objective

1. To evaluate and compare the position, length, and angle of the femoral tunnel placed using the freehand method and using an offset aimer

2. To evaluate and compare the graft angle of the tunnel placed using the freehand method and using an offset aimer

3. To compare the functional outcome using the Tegner– Lysholm Knee score of the patients with tunnels placed using the freehand method and using an offset aimer.

Study design

A prospective cohort design was selected to allow standardized data collection and uniform surgical and rehabilitation protocols. Patients were alternately assigned to each group to minimize selection bias while maintaining clinical feasibility. This design enabled planned postoperative computed tomography (CT) assessment of tunnel position and angle, allowing accurate correlation with functional outcomes. Randomization was not chosen to mitigate potential logistical problems with patient enrolment, and alternate allocation was carried out.

Setting

This prospective cohort study was conducted at a tertiary care hospital in South India, between August 2022 and January 2024, following approval from the institutional ethics committee. A total of 62 patients, aged above 18 years, diagnosed with ACL tear were included.

Patients were divided into two groups (A and B) based on the method of femoral tunnel drilling. Allocation was done in an alternating manner. Group A underwent femoral tunnel placement using an offset aimer, and Group B underwent tunnel placement by the freehand technique.

Group A (aimer method)

• Graft diameter (g) was halved, and an aimer 2 mm larger than g was selected (e.g., for a 10 mm graft, g = 5, aimer = 7 mm)

• The knee was hyperflexed

• The aimer was introduced through the anteromedial portal and placed against the posterior border of the lateral femoral condyle [Figure 1]

• The guidewire was passed through the aimer, followed by serial reaming to the required graft diameter.

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Figure 1:

Intraoperative image of the aimer position.

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Participants

Variables

Post-operative assessment

All patients followed the same rehabilitation protocol and underwent a post-operative CT scan on day 1. The following measurements were taken:

1. Femoral tunnel position using Bernard–Hertel grid (X and Y axis)^[15] [Figure 2]

2. Femoral tunnel angle relative to the intercondylar line (coronal) [Figure 3]

3. Femoral tunnel angle relative to intercondylar line (axial) and length [Figure 4]

4. Graft-femoral tunnel aperture angle (coronal section) [Figure 5].

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Figure 2:

Bernard - Hertel grid for calculation of X and Y axis position of femoral tunnel.

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Figure 3:

Angle between femoral tunnel and bicondylar axis in coronal plane.

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Figure 4:

Angle between femoral tunnel and bicondylar axis in axial plane.

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Figure 5:

Angle between the femoral tunnel and the graft.

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Functional outcome was assessed using the Tegner–Lysholm Knee Score (TLS) at 6 months postoperatively.

Data sources/management

For each patient, demographic details, injury history, clinical findings, and MRI reports were recorded in a case pro forma. Post-operative CT scans were performed on day 1 to assess femoral tunnel characteristics. Measurements included tunnel position using the Bernhard–Hertel grid, tunnel length in the axial and coronal planes, tunnel angle in axial and coronal planes relative to a line joining both femoral condyles, and graft angle at the tunnel aperture in the coronal plane. All measurements were performed by the primary investigator using the same imaging protocol and measurement techniques across both groups to ensure uniformity. Functional outcome was assessed at 6 months using the TLS scale and clinical examination. All patients followed an identical rehabilitation protocol and follow-up schedule to maintain comparability between groups.

Bias

Despite efforts to standardize the methodology, this study is subject to certain limitations. Selection bias is possible due to the non-randomized, alternating allocation of patients into study groups. Performance bias cannot be ruled out since the surgeon was aware of the group assignment during the procedure. In addition, the lack of blinding during outcome assessment may have influenced subjective interpretation. However, several measures were taken to minimize bias. Surgical techniques for both femoral tunnel drilling methods were strictly standardized to ensure intra-group consistency. All patients underwent post-operative CT scans on day 1, allowing uniform and objective imaging-based assessment of tunnel placement and graft angle. A single surgeon performed all procedures, and the measurements were done by a single observer, thereby reducing inter-observer variability. Furthermore, both groups followed an identical post-operative rehabilitation protocol. These steps helped to reduce confounding variables and improve the internal validity of the study.

Study size

The sample size was estimated using the standard formula for comparing two means:

• n = (2 × [standard deviation squared] × [Za/2+Zb]²) ÷ (mean difference)²

Where:

• Za/2 is 1.96 for a 5% significance level (two-sided)

• Zb is 0.84 for 80% power

• The standard deviation and mean difference were derived from a previous study.

Based on the study by Thapa et al., the femoral tunnel position along Blumensaat’s line had a standard deviation of 8.02%.^[16] Assuming that a 6% difference in tunnel position between groups would be clinically meaningful, the calculated sample size was 28 participants per group. To allow for potential data loss or attrition, 2 additional participants were added per group, resulting in a final sample size of 30 per group (total n = 60).

Quantitative variables

Quantitative variables in this study, including femoral tunnel position (X and Y coordinates on the Bernhard–Hertel grid), graft–tunnel angle, tunnel angles in the axial and coronal planes, and femoral tunnel length, were treated as continuous variables. All linear measurements were recorded in millimeters, and angular measurements in degrees, as obtained from standardized post-operative CT scans. The TLS at 6 months postoperatively was also treated as a continuous variable to allow for precise assessment of functional outcomes. No arbitrary grouping or categorization of these variables was done to maintain the integrity and resolution of the data. Comparisons between the two groups – offset aimer and freehand drilling – were performed using sample t-tests, based on the assumption of normal distribution. Identical methods of measurement and analysis were used across both groups to ensure comparability and reduce assessment bias.

Statistical method

Statistical analysis was conducted using IBM Statistical Package for the Social Sciences Statistics for Windows, Version 29.0 (IBM Corp., Armonk, NY). Descriptive statistics were used to summarize the data: Categorical variables were expressed as frequencies and percentages, and continuous variables as means with standard deviations. Between-group comparisons for continuous variables (e.g., tunnel position, tunnel angle, graft angle, tunnel length, and functional outcome scores) were performed using the independent samples t-test. Categorical variables were compared using the Chi-square test or Fisher’s exact test when cell counts were <5. No subgroup or interaction analyses were performed. As the study had no loss to follow-up at the 6-month endpoint, attrition bias was not a concern. Normal distribution was confirmed with the Shapiro–Wilk test. Missing data were minimal and excluded from analysis on a per-variable basis without imputation. Confounding was minimized through a uniform surgical protocol, a single operating surgeon, and standardized rehabilitation and imaging protocols. Sensitivity analyses were not applicable.

Participants

A total of 72 patients with ACL injuries were assessed for eligibility between August 2022 and January 2024. Of these, 10 patients were excluded:

• 3 due to multiligamentous knee injuries,

• 1 due to ACL avulsion injury, and

• 6 declined to participate.

The remaining 62 patients who met the inclusion criteria and provided informed consent were enrolled in the study. All 62 participants completed the study and were included in the final analysis.

Descriptive data

A total of 62 patients were included in the study, with a mean age of 28.3 years in the aimer group and 27.7 years in the freehand group. The majority of the patients (58.1%) were aged between 21 and 30 years, followed by 16.1% below 20 years, and 12.9% each in the 31–40 years and above 40 years of age groups. Males constituted the predominant demographic, comprising 88.7% of the total cohort, while females accounted for 11.3%. Within the aimer group, 3% were female, compared to 20% in the freehand group. Twisting injury was the most common mode of injury across both groups, followed by road traffic accidents and falls from height. The right knee was more frequently affected than the left in both groups. The mean time since injury was 5.4 ± 4.1 months in the aimer group and 7.0 ± 3.7 months in the freehand group; however, this difference was not statistically significant (t = 1.560, P = 0.124).

Outcome data

In this study, key tunnel parameters and functional outcomes were compared between the aimer and freehand groups. As shown in Table 1, the mean femoral tunnel position along the X-axis was significantly more posterior in the aimer group (25.8 ± 4.3) compared to the freehand group (30.4 ± 10.1), (P = 0.027). Although the Y-position of the tunnel showed a trend toward being lower (more inferior) in the aimer group (29.8 ± 6.1 vs. 34.1 ± 11.6), as shown in Graph 1, this difference was not statistically significant (P = 0.076). Graph 2 demonstrates the variability in both the X and Y axes of the tunnel in the freehand group. Table 2 and Graph 3 show that the mean femoral tunnel graft angle was significantly more acute in the aimer group (137.0° ± 3.7) compared to the freehand group (142.1° ± 12.1), (P = 0.035). The variability in the absolute values of the graft angle in the freehand group is seen in Graph 4 with a single outlier in the aimer group. No significant differences were observed between the groups in coronal (P = 0.916) or axial tunnel angulation (P = 0.762) with respect to the bicondylar axis [Table 3 and Graph 5].

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Graph 1:

Position of tunnel between groups.

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Graph 2:

Variability of tunnel positions. The blue dots outside the box is to show outlier values.

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Graph 3:

Graft femoral tunnel angle.

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Graph 4:

Variability of graft femoral tunnel angle. The blue dot outside the box is to show outlier values.

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Graph 5:

Angle to bicondylar axis.

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The femoral tunnel length was slightly longer in the aimer group (35.9 ± 3.9 mm) compared to the freehand group (34.4 ± 3.2 mm), though this was not statistically significant (P = 0.095). Preoperatively, the mean TLS was 66.3 ± 5.4 in the aimer group and 67.3 ± 6.0 in the freehand group, with no statistically significant difference (P = 0.499). At 6 months, the scores were comparable between groups, with mean scores of 94.9 ± 4.3 in the aimer group and 93.6 ± 5.7 in the freehand group (P = 0.287) [Table 4 and Graph 6].

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Graph 6:

Tegner-Lysholm knee score (TLS) at 6 months.

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Correlation analysis revealed a statistically significant negative correlation between the X-position of the tunnel and the TLS in the aimer group (r = −0.370, P = 0.037), indicating that more anterior tunnel placement may be associated with slightly lower functional outcomes [Table 5 and Graph 7]. No significant correlations were found between functional scores and tunnel Y-position or graft angle in either group (P > 0.05). Regression analysis was considered but not performed due to the limited sample size.

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Graph 7:

Correlation analysis of tunnel position and Tegner– Lysholm knee score (TLS).

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Other analyses

A post hoc sensitivity analysis was performed using the Mann–Whitney U test for two primary outcome measures (TLS and Tunnel X position). The results were consistent with the original t-test findings — TLS remained statistically non-significant between groups (P = 0.301), and Tunnel X position remained significantly different (P = 0.021), confirming the robustness of these results under non-parametric conditions.

Key results

ACLR is a commonly performed orthopedic procedure, with ongoing evolution in surgical technique. Among the variables in ACLR, femoral tunnel positioning remains a critical determinant of graft orientation, angle, and long-term stability.^[17] Our study aimed to compare two techniques of femoral tunnel drilling – offset aimer versus freehand technique – with respect to tunnel placement, tunnel angle, and early functional outcome.

Consistent with existing epidemiological trends, the majority of our study population was between 21 and 30 years of age, with a male preponderance and sport-related mechanisms being the most common cause of injury. These demographics align with findings from prior studies on ACL injury patterns.

Post-operative CT-based analysis revealed that the mean femoral tunnel position on the X-axis (Bernard–Hertel grid) was 25.8% in the aimer group and 29.8% in the freehand group, a statistically significant difference (P = 0.027). Given that a lower X value corresponds to a more posterior tunnel position (closer to the anatomic footprint), our findings suggest that the aimer technique achieves more anatomical femoral tunnel placement. In addition, variability in tunnel placement – in both X and Y axes – was higher in the freehand group, indicating that the aimer technique provides more consistent tunnel positioning [Graph 2].

Similarly, the graft tunnel angle was significantly different between groups, with the aimer group showing a mean of 137° versus 142.1° in the freehand group (P = 0.035). This angle plays a role in graft isometry and tensioning; hence, a lower and more consistent angle in the aimer group could imply more optimal graft biomechanics. These findings echo earlier studies that emphasized the role of femoral tunnel orientation in preventing graft laxity and failure.

Regarding functional outcomes, the TLS at 6 months showed no statistically significant difference between groups (mean TLS: 94.9 in aimer vs. 93.6 in freehand, P = 0.287). Although short-term functional outcomes were comparable, it remains to be seen whether anatomical tunnel placement will lead to superior long-term results, especially in light of previous studies linking tunnel malposition to graft failure. While this difference is significantly lower than the MCID of the Tegner–Lysholm Score, which is about 10, it highlights the need for long-term randomized controlled trials to study the effect tunnel placement has.^[18] Since the focus of our research was the tunnel morphology, a single functional outcome score was used.

Importantly, Pearson correlation analysis showed a significant negative correlation between the X-position of the femoral tunnel and TLS in the aimer group (r = −0.370, P = 0.037), suggesting that more anterior tunnel positions (i.e., higher X values) were associated with poorer functional outcomes. This highlights the importance of achieving posterior femoral tunnel positioning within the anatomic footprint.

Compared to previous literature that has primarily focused on tunnel positioning techniques in cadaveric or radiological models, our study provides additional clinical correlation by associating tunnel position with patient-reported outcomes. Although our 6-month follow-up limits conclusions about long-term graft integrity or osteoarthritis progression, the study reinforces the relevance of surgical precision in femoral tunnel placement, especially when aiming to optimize early patient-reported outcomes.

Limitations and strength

A key strength of this study is the use of post-operative 3D CT scans, which allowed for accurate assessment of femoral tunnel position and angle. All surgeries were performed by a single experienced surgeon, thereby minimizing inter-operator variability. In addition, functional outcomes were systematically evaluated using standardized scoring systems, providing a comprehensive comparison between the two techniques.

However, the study has certain limitations. The relatively lower proportion of females may limit the generalizability of this study’s results. The lack of randomization introduces potential selection bias, and the relatively short duration of follow-up limits the ability to conclude long-term outcomes. This necessitates long-term follow-up studies of 2–5 years to assess graft function and tunnel-related complications. Although a single surgeon performed all procedures, potential intra-operator variability still exists – particularly in the aimer group – due to variations in guide orientation during tunnel drilling. The absence of blinding and reliance on a single subjective functional score may also introduce measurement bias.