1 Introduction

The anterior cruciate ligament (ACL) courses between the lateral femoral notch and the medial tibial eminence and provides important knee stability by resisting anterior translation of the tibia. With an overall ACL tear and reconstruction rate of 68.6 per 100,000 person-years, ACL rupture represents a common orthopedic injury.¹ Patients who require a complete ACL reconstruction procedure have a few options including hamstring, quadriceps, or patellar tendon autografts as well as various allografts. Research has demonstrated that bone-patellar tendon-bone (BTB) grafts tend to experience fewer instances of failure compared to hamstring grafts, but are associated with increased chronic anterior knee pain.² When implanting BTB grafts surgeons must navigate the bone blocks through narrow tibial and femoral tunnels and place them in correct anatomical positioning using an arthroscope for visualization. A wide variety of fixation options for the BTB graft exist including but not limited to interference screws and various cortical or cortical-cancellous suspensory systems. Adequate fixation of the screw in the femoral tunnel is vital to minimize future complications.³

When using interference screws, the degree of divergence between the screw trajectory and the tunnel trajectory has been shown to be associated with graft failure. Previous research has compared the degrees of divergence created via screw insertion through various portals.^4,5 For example, Miller et al. compared the anteromedial and tibial tunnel as insertion sites for the screwdriver, while Pandey et al. compared insertion of interference screws via the anteromedial portal versus through the patellar tendon defect.

One novel surgical tool, the FlipCutter drill (Arthrex, Naples, FL), allows for insertion of the drill through the lateral femoral cortex and then deployment of the drill inside the joint to be used in a reverse fashion (Fig. 1). The FlipCutter allows for femoral tunnel drilling completely independently of the tibial tunnel or anteromedial portal, which may make it easier or more difficult for the surgeon to match the angle of the femoral tunnel and interference screw. However, the effects of the FlipCutter on the divergence angle of the screw are yet to be explored. The goal of this study is to investigate whether interference screw fixation through an anteromedial portal into an outside-in drilled femoral tunnel via a flip cutter results in acceptable hardware position.

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Fig. 1 Images of flipcutter and cannulated ReamerData Acquisition and Analysis.

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2 Methods

2.1

2.1 Study design

10 cadaveric knee joints successfully underwent ACL reconstruction using the FlipCutter, and 8 cadaveric knee joints had the cannulated reamer used for reconstruction surgery as performed in the study by Miller et al.⁴ The sections below discuss specifics of specimen preparation, dissection, and surgical intervention.

2.1.1

2.1.1 Specimen preparation

Eighteen femoral BTB autografts were evaluated from the cadaveric ACL reconstruction surgeries. Specimens were harvested from fresh frozen cadavers acquired by the American Association of Tissue Banks (AATB), a certified tissue bank. Male to female ratio was 1:1. Age at time of death ranged from 23 to 68 years (mean 53 years). BMI ranged from 20 to 24 (mean 21) (Table 1).

Table 1 Specimen summary.

Specimen Label	Gender	Age	Height (inches)	Weight (lbs)	BMI
A	M	48	68	160	24
B	M	23	66	130	21
C	M	23	66	130	21
D	F	65	62	108	20
E	F	68	67	140	22
F	F	68	67	140	22
G	F	57	62	122	22
H	F	57	62	122	22
I	M	59	72	148	20
J	M	59	72	148	20

2.1.2

2.1.2 Specimen surgery & dissection

After thawing and removal of overlying integument and adipose tissue, eight cadaveric knees were rigidly fixed on the operating table and flexed to 90° for proper surgical approach. BTB grafts were harvested from the mid-point of the patellar tendon with a width of 10 mm. All bone blocks were also sized to a length of 20 mm and a width of 10 mm. A #2 FiberWire was passed through each bone block to allow tensioning upon insertion. Grafts were wrapped in gauze soaked with saline before implantation to keep from drying out. The actual cross sectional area of the whole graft was measured before implantation according to Goodship et al.⁶ ImageJ was used to determine the mean cross-sectional area of the grafts.

ACL reconstruction surgery and interference screw fixation were simulated according to the instructions of the Arthrotek Instrument Kit. Employing standard technique, arthroscopic portals were carefully created in the knees. The ACL was cut with an arthroscopic shaver, then removed with basket forceps. A burr was then introduced in order to create a notchplasty to prevent the risk of graft impingement. Next, a guide pin was inserted 2 cm medial to the distal-most aspect of the tibial tuberosity at a 50° angle, with the pin exiting through the ACL attachment point. A standard tibial tunnel 2 mm smaller than the BTB graft was drilled with a cannulated reamer in similar fashion to those created by Miller et al.⁴ Utilizing another guide pin introduced through the tibial tunnel to the medial aspect of lateral condyle 5 mm anterior to the posterior femoral cortex, a cannulated reamer was employed to drill the femoral tunnel 2 mm smaller than the BTB graft. Subsequently, tunnel dilators were employed to expand the tibial and femoral tunnels, achieving a diameter of 10 mm. The BTB graft was threaded via the tibial tunnel and into the femoral tunnel via sutures attached to a guide pin. Finally, a screwdriver along with a 9 × 25 mm interference screw was passed via the anteromedial portal over a guidewire with the interference screw being seated into the femoral tunnel, all while maintaining the knee at 110° of flexion.

Femoral tunnel creation in the remaining ten cadaveric knees was performed utilizing the FlipCutter. After ACL removal, a femoral guide was placed between the PCL and lateral femoral condyle and a FlipCutter, 10 mm in diameter, was used to drill outside-in 20 mm from the medial wall of the lateral condyle in ten cadaveric knees. The BTB graft and tibial tunnel were prepared identical to the method mentioned previously. Next, the BTB graft was subsequently guided through the tibial tunnel using sutures, traversing the knee joint, and entering the femoral tunnel. Using a screwdriver, a 9 × 25 mm interference screw was introduced into the femoral tunnel through the anteromedial portal all while maintaining the knee at 110° of flexion. Finally, lateral (sagittal plane) and AP (coronal plane) x-rays were taken of the knee to assess the divergence of the screw.

Using x-ray images from each specimen, the screw divergence angles in the AP and lateral views were measured using the angle between the femoral tunnel and interference screw in ImageJ. Divergence angles were determined by measuring the angle formed between lines drawn along the long axis of the interference screw and the femoral tunnel on lateral and anterior-posterior plain films (Figs. 2 and 3). The intersecting angle is referred to as divergence, which indicates the degree to which the screw inserted through the anteromedial portal diverges from the tunnel. Total angle divergence was calculated as the sum of the coronal and sagittal divergence. A two-tailed T-test was utilized to compare the means of the AP and lateral divergence angles derived from this study and the study by Miller et al.⁴ If the p-value was below 0.05, it was regarded as statistically significant.

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Fig. 2 Example of Lateral Divergence Angle Measure in ImageJ (mean° ± standard deviation°).

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Fig. 3 Example of Anterior-Posterior Divergence Angle Measure in ImageJ (mean° ± standard deviation°).

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3 RESULTS

Using the FlipCutter, the AP and lateral divergence angles were 7.3° ± 4.5° and 9.3° ± 9.3° respectively, while the total angle divergence was 16.6° ± 11.8. Using the cannulated reamer, the angles were 14.4° ± 2.5° and 6.8° ± 2.8° for AP and lateral, respectively, and 21.1° ± 5.2° for the total divergence (Table 2). The AP divergence angles were significantly different between each group (p = 0.001). The lateral divergence angles were not significantly different (p = 0.435). The total divergence angles were also not significantly different (p = 0.269) (Fig. 4).

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Fig. 4 Comparing means for screw divergence angles.

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4 Dicussion

The FlipCutter method yielded significantly lower mean AP divergence angles of the interference screw compared to results using a cannulated reamer. The mean AP divergence with the FlipCutter was 7.3°, compared to 14.4° with the reamer, representing a 49% reduction that was statistically significant (p = 0.001). This indicates the FlipCutter's potential to enable more precise screw placement in the coronal plane. Previous studies that attempted to decrease femoral tunnel interference screw divergence via screwdriver placement in the tibial tunnel came at the cost of potential graft damage and inappropriate tunnel widening.⁴ This shows that the FlipCutter could be a suitable instrument for decreasing divergence angle without the risk of iatrogenic damage. Both the Miller et al. study and the current study are similar in that they both utilized a cannulated reamer to create the tibial tunnel and utilized the anteromedial portal for interference screw fixation in the femoral tunnel; both had satisfactory screw trajectory. Moreover, both studies evaluated degree of sagittal and coronal divergence of the interference screw. Although not the same, the current study methodology is similar to the study by Miller et al.

The use of a femoral interference screw when performing BTB autograft ACL reconstruction has been the standard of care for several decades.⁷ Historically, the femoral tunnel for a BTB autograft has been prepared via a transtibial or anterior-medial drilling technique.⁸ In recent years, outside-in femoral drilling has become more popular.⁹ This study clearly establishes the outside-in technique using a retrograde reamer as a viable independent femoral drilling solution for ACL reconstruction when using a BTB autograft with a femoral interference screw. The femoral interference screw divergence angle when using our technique compares favorably with historical controls, and it should translate to excellent clinical outcomes.

However, there was no significant difference found in lateral divergence angles between the FlipCutter (9.3°) and reamer (6.8°). Additionally, high standard deviations were seen with the FlipCutter technique, suggesting more variability in screw placement compared to the reamer. Both drilling methods resulted in mean total divergence angles under 30°, which some biomechanical studies have deemed acceptable to avoid compromising fixation strength.^10,11 Of note, previous biomechanical studies examining load to failure have revealed a higher incidence of graft pullout when the angles of divergence surpass 15°–30°.^12,13 In our study, the mean AP, lateral, and total divergence angles from both drilling methods fall within that range, indicating acceptable positioning.

Pandey et al. found a significantly decreased divergence angle in the sagittal plane with BTB graft portal (defect from graft harvesting) than with interference screw insertion via the anteromedial portal (p < 0.0001). Nevertheless, when patients were assessed at the 2 year mark, there were no statistically significant variations in clinical outcomes, as indicated by the International Knee Documentation Committee (IKDC) and Lysholm score.⁵ This suggests that within an acceptable range of divergence angles, there is no clinical difference at long term follow up.

Ensuring minimal divergence is crucial for achieving rigid fixation of the graft. One method to achieve this involves drilling the femoral tunnel using a cannulated reamer through the tibial tunnel. Consequently, inserting the interference screw through the tibial tunnel may offer a dependable method to align the screw trajectory with that of the tunnel. However, this has a risk of damaging the graft and widening the tibial tunnel thereby limiting adequate fixation and is thus avoided. Therefore, screws are more often placed through the anteromedial portal, which portends an inherent risk of divergence. The reduced coronal plane divergence with the FlipCutter may be attributable to its ability to create femoral tunnels independently of the tibial tunnel, in turn allowing for better matching to the trajectory of the screw placed through the anteromedial portal. The sagittal plane variability remains a concern, indicating less precision that surgeons and future researchers will need to address. Previously, Dworsky et al. showed that femoral divergence occurred more frequently in the sagittal plane than the coronal plane, and no early graft failures were noted.¹³ The true clinical relevance of sagittal plane divergence is yet to be determined, given the lack of current literature specifically looking at the impact of lateral divergence angles on patient outcomes. However, sagittal plane divergence is a component of overall divergence and therefore likely contributes to risk of pullout.

Fineberg et al. stressed the importance of limiting screw and tunnel divergence with the objective of maintaining graft integrity and resistance to pullout in biomechanical studies.¹⁴ However, postoperative ACL rehabilitation likely avoids pullout failure due to manageable physiological loads even when there is a small degree of divergence between the screw and tunnels. Nonetheless, ACL reconstruction surgery should still prioritize limiting divergence in order to enhance pullout strength.

Limitations to this study necessitate discussion. First, the operations in this study were performed on cadaveric models instead of live patients. Related to this, our study was not able to assess bone quality, which has been shown to affect interference fixation clinically.¹⁵ Additionally, our study had a small sample size, decreasing the power of findings.

Future research should analyze this technique clinically and correlate divergence angles with longer-term graft stability and outcomes. Additionally, further modifications like custom guides to control sagittal alignment could be explored to standardize lateral divergence. Ultimately, the strength of interference fixation also depends on other factors, such as bone quality, compression of the bone plug in the tunnels, length of screw-thread-bone contact, and direction of ligamentous forces, of which surgeons should remain cognizant.¹⁶ In addition, the Miller et al. study was used as a control for this study, which differed in its choice of graft (i.e. hamstring allograft) from the current study (i.e. patellar BTB autograft), but most importantly the method of interference screw insertion was identical.

5 Conclusion

Overall, this study demonstrates promising results from the FlipCutter drill in minimizing AP screw divergence through the anteromedial portal in ACL reconstruction. Surgeons can utilize this tool to achieve better fixation accuracy; with further refinement, this approach could become an integral part of improving techniques and outcomes in BTB ACL repairs, divergence, and ultimately pullout and failure of the graft.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Guardian/patient's consent

This is a biomechanical study and did not involve patient subjects nor animal subjects.

Ethical statement

It is imperative to note that this research is a biomechanical study, and as such, did not involve human or animal patients. The investigations were conducted strictly within in-vitro parameters, eliminating the need for ethical approval related to human or animal experimentation. Advanced Orthopaedics and Sports Medicine ensures the highest ethical standards were maintained throughout the study, and the research design and implementation adhered to established guidelines.

We appreciate your consideration of our manuscript and look forward to the opportunity to contribute to the ethical and scholarly discourse within the Journal of Orthopaedics.

Funding statement

Furthermore, we wish to clarify that this study did not receive any financial support or funding from any public or private sources that could potentially influence the outcomes or interpretation of the study. This absence of external funding ensures the impartiality and integrity of the research findings presented in this manuscript.

CRediT authorship contribution statement

Carson Gardner: Writing – original draft, Writing – review & editing, Investigation. Samuel Wu: Investigation. Jamal Zahir: Writing – original draft, Writing – review & editing. Oliver Dong: Writing – original draft, Writing – review & editing. Oluwatodimu Raji: Methodology, Formal analysis, Writing – review & editing. Jeremi Leasure: Methodology, Formal analysis. Patrick McGahan: Conceptualization, Methodology, Writing – review & editing.