Remnant preserved anterior cruciate ligament (ACL) reconstruction, which is designed to preserve the ACL remnant, has become increasingly popular over the last decade [19, 30–36]. Since Schultz et al. reported the first detailed description of mechanoreceptors in the ACL [37], the ACL remnant has been recognized in terms of its proprioceptive functions and its vascularity, which may induce more rapid vascularization from the ACL remnant to the grafts [27, 38, 39]. Theoretically, there is a strong possibility that preservation of the ACL remnant tissue may be able to restore proprioceptive function of the graft after ACL reconstruction. Preservation of the ACL remnant tissue may also enhance the revascularization and cellular proliferation of the graft after ACL reconstruction since the ACL remnant tissue has good subsynovial and intrafascicular vascularity [40]. Based on these concepts, several investigators have developed single-bundle ACL reconstruction techniques with preservation of the ACL remnant tissue [31, 32, 34, 35]. Recently, Ahn et al. introduced a remnant preserving technique for double-bundle ACL reconstruction, in which two femoral tunnels and one tibial tunnel are created [33]. Subsequently, Ochi et al. described a double-bundle ACL reconstruction with remnant preserving technique using a hamstring autograft [30]. They utilize the far anteromedial portal to create the femoral tunnels and the central anteromedial portal to make a longitudinal slit in the ACL remnant to allow visualization of the tips of the guide pins during anatomic creation of the tibial tunnels within the native ACL tibial footprint. Yasuda et al. reported the clinical outcomes of the anatomic double-bundle ACL reconstruction with ligament remnant tissue preservation by the use of the transtibial technique [19]. They concluded that the results of this technique were comparable to their previously reported results of anatomic double-bundle reconstruction without remnant tissue preservation. The most important factor of this technique is the evaluation of ACL remnant tissue. To accurately evaluate the morphology of the remnants, arthroscopic examination should be performed at various knee flexion angles to consider the different tension patterns of the two bundles (anteromedial bundle (AMB) and posterolateral bundle (PLB)). The status of the PLB femoral insertion can be evaluated with the knee in a figure-of-4 position for good visualization of the femoral attachment. Most of the ACL remnants do not play a role for biomechanical function. In addition, no long-term studies have shown clinical evidence regarding the utility of ACL remnant tissue preservation, and this technique demands a high level of surgical skills. Anatomic placement of femoral and tibial tunnels should be the basis of ACL reconstruction in order to obtain optimal clinical and biomechanical results.

On the other hand, some of the ACL remnants are identified as an isolated AMB or PLB tear. To identify these types of rupture pattern, careful arthroscopic inspection is the key. First, the locations of the femoral and tibial attachment are evaluated carefully. Second, the directions of the remaining fibers are assessed. Both of these procedures are performed at 90° of knee flexion. If the AMB is intact, the remnant bundle, which connects the AMB insertion site between the femur and the tibia, is tightened with probing at this angle. The remnant bundles are also subsequently evaluated in a figure-of-4 position. When the PLB is intact, the tightened remnant bundle is observed. Nakamae et al. evaluated the biomechanical function of ACL remnants in anterior-posterior and rotational knee stability in patients with a complete ACL injury [41]. They classified the ACL remnants into five morphological patterns by arthroscopic inspection. In these five groups, they demonstrated that patients with partial AMB or PLB rupture had a significantly lesser amount of anterior-posterior translation by KT-2000 measurement. Our previous report by Araki et al. also evaluated the partial rupture of AMB or PLB using the electromagnetic measurement system (EMS) [42]. The quantitative assessments of knees with partial ACL ruptures during the Lachman and the pivot shift tests using the EMS showed less laxity than knees with complete ACL tears, whereas their laxity was greater than the contralateral knees with intact ACLs. Therefore, ACL remnants that can be confirmed by careful inspection as almost completely preserving the AMB or PLB should be considered as partial ACL tears. It is suggested that these ACL remnants contribute to the knee joint stability seen in our study. Based on these findings, ACL augmentation technique, which reproduces selective AMB or PLB reconstruction, has been performed and resulted in the good clinical score. Ochi et al. examined the 2-year follow-up clinical results of 45 patients who had undergone ACL augmentation procedure using an autogenous semitendinosus tendon [43]. When the ACL remnant was regarded as a PLB, only the AMB was reconstructed. In contrast, when the ACL remnant was regarded as an AMB, only the PLB was reconstructed. This procedure showed improved postoperative joint stability, joint position sense, and Lysholm scores. They concluded that this procedure can be a treatment option for patients whose ACL remnants are left in certain conditions. A recent study by Ohsawa et al. also evaluated the clinical results and morphology of the preserved bundle remnants by second-look arthroscopy postoperatively 1 year after selective AMB or PLB ACL reconstruction [44]. They reported that the preserved ACL remnants possessed acceptable morphology and the functions of anterior-posterior and rotational stability after surgery. To successfully perform this technique, it is important to first identify the partial ACL tear, which is almost completely preserved as the AMB or PLB. The augmentation reconstruction technique can be performed less invasively by preservation of these bundles. In addition, the biomechanical function and biological healings are enhanced by preservation of these residual bundles. Therefore, although this technique is also technically demanding, this procedure can be a treatment option for patients whose ACL remnants are left in certain conditions.

We reviewed ACL instability and its treatment in the eastern experience. Chronic ACL-deficient knees had significantly larger anterior tibial subluxation, and it was difficult to reduce anterior tibial subluxation after ACL reconstruction in chronic ACL-deficient knees and large tibial subluxation cases. Regarding the combined intra- and extra-articular reconstruction, it is still unknown whether these surgical procedures could restore normal knee kinematics or not in the literature, and we could not find any advantages so far. From the previous clinical studies in Japan, anatomic reconstruction is crucial to restore normal knee kinematics. In addition, the biomechanical function and biological healings are enhanced by preservation of ACL remnant. Therefore, we believe that although this procedure is technically demanding, anatomic double-bundle ACL reconstruction with ACL remnant preservation has been a better treatment option to restore normal knee kinematics.