Importantly, this study found that there is a safe distance between the trajectory of the LARAI portal and PA.

Gupta et al. reported that the distance of 2 mm from the popliteal neurovascular bundle (PNVB) is empirically a safe distance. They statistically explained that a mean −3SD −2 value of more than zero is safe in 99.87% of cases [10]. We used their method to assess vascular safety during the establishment of the LARAI portal. Our 3D-CT results showed that more than 99.8% of cases were at no risk of PA injury when the puncture needle was inserted 10–20 mm lateral to the edge of the patellar tendon at 0 mm from the PCL. The cadaveric study showed that all specimens had a safe distance between the PA and the puncture needle. Our final analysis demonstrates consistency between the results obtained from both complementary methods. Compared with Gupta’s study [10], we only assessed the safety of PA. Some studies reported that the PA is the most anterior and medial structure in the PNVB [2, 11]. Therefore, the safety of the PA during the establishment of the LARAI portal theoretically ensures the safety of the whole PNVB.

Herein, we used 3D-CT instead of magnetic resonance imaging (MRI) to determine the safe distance. Compared with MRI, 3D-CT may be more accurate for measuring the safe distance because the Blender software can provide more visualized and real 3D images to simulate the LARAI portal for analyzing the risk of PA injury.

In the present study, we also found that 20 mm lateral to the edge of the patellar tendon at 0 mm from the PCL was the safest trajectory. The trajectory might remain safe at longer distances, for example, 25 mm or more lateral to the edge of the patellar tendon. However, the trajectories with great distances from the edge of the patellar tendon may damage the lateral condyle of the femur. Thus, we recommend 10–20 mm lateral to the edge of the patellar tendon at the level of the knee joint line as the optimal insertion point for the LARAI portal. Consequently, the exit point of the needle would be the medial skin of the distal thigh, approximately 70–90 mm proximal to the medial epicondyle of the femur and 15 mm behind the surface projection of the posterior femoral margin (Fig. 7).

Clinical illustration shows the position of the exit point of the needle (left knee). Distances between the exit point and medial epicondyle of the femur is approximately 85 mm, and distances between the exit point and the surface projection of the posterior femoral margin is approximately 15 mm. A Measurement in a patient. B Measurement in three-dimensional computed tomography

The resection of the inferior leaf of the anterior half of the lateral meniscus (AHLM) through routine arthroscopic portals is challenging for arthroscopic surgeons. Particularly, tears involving the anterior horn of the lateral discoid meniscus are the main problem [12,13,14]. Several accessory portals, including inframeniscal, lateral patellofemoral auxiliary, high anteromedial, or far anteromedial portal, have been proposed to improve the visualization and repair of AHLM [15,16,17,18,19]. However, these were still insufficient to observe the deep-seated inferior leaf of the anterior horn meniscus and potentially increased the risk of meniscus injury. In addition, due to their improper insertion angle, surgeons rarely used these accessory portals to perform all-inside device repairs of tears extending to the AHLM. To date, the outside-in method remains the standard strategy for repairing the anterior horn of the meniscus [20]. However, it requires a skin incision to bury the suture thread, and if the stitch is made over the lateral fascia with a non-absorbable monofilament wire, it may cause knots protrusion or local pain due to tissue strangulation and knots irritation [21,22,23,24]. The LARAI portal is an alternative method for repairing AHLM by all-inside devices under direct arthroscopic visualization (Fig. 3A, B).

Using an all-inside device for repairing lateral meniscus tears is still controversial. Ouanezar et al. found that the popliteus tendon provides a safe and reliable location for all-inside meniscal repair device placement with a low failure rate and no complications [25]. However, some authors have reported that the thin capsule around the PH makes the sutures less efficient. Additionally, sutures of the popliteus tendon with an all-inside device may become loose or cause pain and irritation during knee motion [14, 26,27,28]. Furthermore, Chuaychoosakoon et al. demonstrated that repairing the lateral meniscus using the all-inside meniscal device through either the anterolateral portal or the anteromedial portal can endanger the peroneal nerve or popliteal artery [29]. The LARAI portal helps to vertically stitch the meniscus in the PH zone and posterior horn without capturing the popliteus tendon or capsule (Fig. 3C, D). Therefore, it can theoretically prevent over-constraining the meniscus and surrounding normal tissues, and avoid iatrogenic neurovascular injury (including peroneal nerve) caused by the tip of the all-inside device.

There were several limitations to this study. First, the presence of anatomic variances, popliteal lumps, such as Baker’s cysts and posterior osteophytes, may alter the anatomy of the PA; therefore, careful preoperative evaluation of PA by MRI is necessary. Second, patients’ characteristics such as age, sex, body mass index (BMI), and the side of the knee should be taken into account. Some studies indicated that the anatomy of the PA does not significantly change by height, weight, BMI, sex, or side of the knee [2, 5]. However, Gilat et al. reported that young and female patients may be at greater risk of PNVB injury [30]. Lastly, we did not assess the anatomical relationship between the trajectory and the saphenous nerve (SN). Sharp knives should be only used to reduce the risk of SN injury during the establishment of the LARAI portal. The trajectory can then be broadened by a blunt switching stick or slotted cannula.

In conclusion, the establishment of the LARAI portal in the “figure of four” position is safe. The optimal insertion point is 10–20 mm lateral to the edge of the patellar tendon and closely adjoined to the posterolateral margin of the PCL at knee joint line level.