The main finding of this study is that the patients with cross-pin protrusion after treatment with the reverse Rigidfix femoral fixation device to prepare the femoral tunnel through the AM portal and fix the femur with the cross-pins (Rigidfix) in ACLR were mainly women with small femoral condyles. According to Figs. 4 and 5, the cross-pin penetration rate was 0% among patients with a mediolateral femoral condyle diameter ≥ 76 mm, and the cross-pin penetration rate was 100% among patients with a mediolateral femoral condyle diameter ≤ 70 mm. The cross-pin penetration rate was 0% among patients with an anteroposterior lateral femoral condyle diameter ≥ 66 mm, and the cross-pin penetration rate was 100% among patients with an anteroposterior lateral femoral condyle diameter ≤ 59 mm. The cross-pin penetration rate was 81.25% among patients with a mediolateral femoral condyle diameter of 70–76 mm, and the cross-pin penetration rate was 23.44% among patients with an anteroposterior lateral femoral condyle diameter of 59 mm-66 mm.
The fixed length of the cross-pin (Rigidfix) is 42 mm, and the author believes that the main reasons for these differences are as follows: (1) Although the mediolateral diameter of the femoral condyle and the anteroposterior diameter of the lateral femoral condyle were measured, each patient had obvious individual differences in the shape of the femoral condyle, so these measurements could not fully summarize the morphological characteristics of each patient's femoral condyle. (2) Cross-pin protrusion is closely related to the direction of femoral tunnel insertion and cross-pin insertion in each patient. (3) Measurement error: When the difference between the mediolateral diameter of the femoral condyle and the anteroposterior diameter of the lateral femoral condyle is small, the shape of the femoral condyle and the direction of femoral tunnel insertion and cross-pin insertion will influence the results. In the repeated measurement of the imaging data for patients in the case group, when the entrance to the femoral bone tunnel was determined for patients with relatively small femoral condyles (mediolateral femoral condyle diameter ≤ 70 mm, anteroposterior lateral femoral condyle diameter ≤ 59 mm), the distance between the centre point of the femoral tunnel (the midpoint of the cross-pins) and the cortex of the lateral femoral condyle was less than 21 mm (the length of the Rigidfix cross-pin is 42 mm) and the cross-pin inevitably penetrated the cortex when the entrance to the femoral bone tunnel was determined, regardless of how the angle changed.
In surgery for ACLR using a hamstring tendon graft, the best method of femoral graft fixation remains a matter of debate . Enlargement of the femoral bone tunnel is particularly common in ACLR using hamstring grafts. Past biomechanical studies and clinical studies have found that the "bungee effect" and "wiper effect" are the main reasons for this outcome [12,13,14,15]. As the fixed point is closer to the tunnel entrance in the cross-pin method, this method can effectively reduce the "bungee effect" and "wiper effect". Compared with interference screw fixation and extracortical fixation, cross-pin (Rigidfix) fixation can effectively reduce the incidence of enlargement of the femoral bone tunnel [16,17,18,19]. Although cross-pins (Rigidfix) have obvious advantages in this respect, the Rigidfix femoral fixation device also has an obvious disadvantage in positioning the femoral tunnel. Positioning the femoral tunnel has always been the key to ACLR. Correct positioning of the tunnel is essential for restoration of the original physiological function of the ACL after ACLR . The Rigidfix femoral fixation device is designed to use the TT technique to prepare the femoral tunnel, but due to the limitation of the tibial tunnel, it is difficult to obtain better femoral tunnel positioning. Shin et al.  proved that compared with the AM technique, femoral tunnels located by the TT technique tend to be high and anterior and have a certain distance from the commonly used femoral positioning points ("isometric" points or "anatomical" points). The reconstructed ACL is obviously different from the normal ACL, and it has obvious deficiencies in controlling tibial rotation. Moreover, other studies have shown that the knee joint after ACLR exhibits better rotational stability by femoral tunnel positioning by the AM technique than by the TT technique [22,23,24,25,26]. Therefore, some scholars have tried to use the cross-pin femoral fixation device to prepare the femoral tunnel through the AM portal [3, 23, 27,28,29,30], but iatrogenic injury caused by cross-pin tunnels prepared from the outside to the inside is major, especially chondral injury. Castoldi et al.  found through a cadaver study using the Rigidfix system to prepare a femoral tunnel through the AM portal that as the insertion angle of the pin increases, the risk of chondral injury increases from 80 to 100%. Inácio et al. also found that in postoperative CT three-dimensional reconstruction after Rigidfix femoral fixation through the AM portal for ACLR, the chondral injury rate was as high as 49.99%. However, what is more dangerous is that with the increase in the insertion angle of the femoral tunnel, the cross-pins may penetrate from the posterior cortex to the popliteal fossa, causing damage to important vessels and nerves in the popliteal fossa . Therefore, most scholars do not recommend using the Rigidfix femoral fixation device to prepare femoral tunnels through the AM portal. The reverse placement of the Rigidfix femoral fixation device to prepare the femoral tunnel through the AM portal effectively reduces iatrogenic injury while providing better femoral tunnel positioning than the traditional TT technique . Our department has found through clinical practice that although this technique is effective in ACLR, a small number of patients show cross-pin penetration of the lateral femoral condyle cortex, with some even penetrating the cartilage, resulting in chondral injury. In severe cases, this penetration can cause injury to the popliteal tendon or posterolateral joint capsule, which can lead to discomfort during movement of the knee joint and reduced range of motion in the early postoperative period. Therefore, this study was conducted. To the best of our knowledge, this is the first clinical study that has identified and analysed cross-pin protrusion after the use of this technique for ACLR. This technique poses a safety hazard in patients with relatively small femoral condyles. Based on the analysis of a large amount of clinical imaging data, the safety range of the femoral condyle for the use of this technology is proposed; using this range could effectively improve the clinical safety and practicality of this technology and provide clinicians with a safer and more effective ACLR technology.
Although the cross-pin protrusion rate of this technique was 21.19%, the position of cross-pin penetration was mostly behind the popliteal tendon and the lateral collateral ligament femoral insertion point. Only 16 patients (5.79%) had chondral damage at the edge of the posterolateral femoral condyle. Moreover, a large proportion of patients had a cross-pin penetration length of approximately 1–2 mm, and 271 patients (98.19%) had no symptoms after surgery; additionally, there was no impact on postoperative recovery. Only 5 patients (1.81%) experienced discomfort during flexion and extension around the femoral insertion point of the popliteal tendon after surgery, but there was no obvious pain, and the range of knee joint motion was reduced in the early postoperative period. The range of knee joint flexion just reached 70–90° within 8 weeks after surgery. Through postoperative MRI and CT examination, approximately 4–5 mm of the cross-pins was exposed from the femoral condylar cortex, which directly injured the popliteal tendon or posterolateral joint capsule (Fig. 1d). During the follow-up period, the symptoms of patients with discomfort caused by cross-pin protrusion completely disappeared within 3 months after the operation, with no impact on the stability of the ACL or the recovery of knee function after reconstruction. However, further scientific research is needed. None of the 276 patients experienced any adverse events, such as vascular or nerve injury, rupture or cross-pin failure. Compared with traditional Rigidfix femoral fixation to prepare the femoral tunnel, whether with the TT technique or the AM technique, the present approach significantly reduces the frequency of iatrogenic injury and increases safety. Before surgery, the mediolateral diameter of the femoral condyle and the anteroposterior diameter of the lateral femoral condyle can be measured with MRI to assess the risk of cross-pin protrusion and the suitability of this ACLR technique for the patient. After preoperative evaluation, there may be a risk of cross-pin protrusion, but this technology can still be applied if desired. According to clinical experience, the hub is kept 5 mm from the guide when drilling the Rigidfix sleeves, and the same distance is reserved for asymmetric cross-pin fixation when the cross-pins are placed. For patients with relatively small femoral condyles (mediolateral femoral condyle diameter ≤ 70 mm, anteroposterior lateral femoral condyle diameter ≤ 59 mm), this technique is not recommended.