Introduction
1. Background
The use of endoscopes in joints began in 1920, when Kenji Takagi of Japan first observed a cadaver knee joint with a cystoscope. Progress continued, and in 1959 Masaki Watanabe performed the world’s first arthroscopic partial meniscectomy [1]. In Korea, arthroscopic knee surgery was introduced in the 1970s by Ahn et al. [2]. Knee arthroscopy became the foundation of orthopedic arthroscopy, profoundly influencing the treatment of other joints. Today, it is indispensable for managing most knee joint pathologies, including anterior cruciate ligament (ACL) injuries, meniscal tears, and articular cartilage lesions.
Treatment of ACL Injuries
Rising sports participation and the occurrence of various accidents have contributed to an increasing incidence of ACL tears, leading to more reconstructive procedures. In the United States, approximately 200,000 ACL injuries occur annually, with 100,000 ACL reconstructions performed each year [3]. In Korea, ACL reconstruction ranks as the second most common arthroscopic knee operation after partial meniscectomy [4].
ACL reconstruction is often recommended because untreated injuries can limit daily and athletic activities and are associated with early-onset osteoarthritis [5]. However, for less active patients without significant instability, some reports document favorable results with non-surgical treatment [6–8]. Despite advances in arthroscopy and surgical techniques, a notable proportion of patients remain dissatisfied with post-reconstruction outcomes. Long-term follow-up studies exceeding 20 years have reported a high incidence of advanced osteoarthritis, ranging from 29% to 54% [9,10].
In Korea, ACL reconstruction began in the 1970s with open surgery, transitioning to arthroscopic methods in the 1980s. The single-incision arthroscopic approach remained the dominant method until the 2000s. Concerns about residual rotational instability and failure rates prompted the introduction of anatomic and double-bundle reconstruction techniques. Although double-bundle reconstruction gained popularity in the 2000s, its use has since declined due to a lack of significant clinical advantage and potential complications during revision surgery.
From 2010 onward, anatomic ACL reconstruction has become the prevailing approach. Current practice emphasizes anatomic reconstruction using a single-bundle graft, maintaining key principles from the double-bundle era, such as oblique femoral tunnel placement [11–13]. Interest in the anterolateral ligament (ALL) has also grown, particularly regarding its role in controlling rotational instability. Numerous studies have examined its anatomy, biomechanics, and function [14–16].
Regarding graft selection, the autologous bone–patellar tendon–bone (BPTB) graft was considered the gold standard for ACL reconstruction in the 1990s. However, due to high donor-site morbidity—such as kneeling pain—and the advent of fixation devices for soft-tissue grafts, autologous hamstring tendon grafts have become increasingly favored [17–19]. Currently, hamstring tendon autografts are the most frequently used worldwide, although some surgeons prefer BPTB or quadriceps tendon grafts.
Several recent studies have shown that combined ACL and ALL reconstruction yields better clinical outcomes than isolated ACL reconstruction [20,21]. A systematic review also reported that lateral extra-articular tenodesis can reduce internal rotation and anterior instability in ACL-deficient knees [22]. Nevertheless, both ALL reconstruction and lateral extra-articular tenodesis lack standardized surgical protocols. This variability may result in graft elongation with functional loss or excessive restriction of normal internal rotation. Consequently, there is limited consensus regarding their use in primary ACL reconstruction. These procedures are generally reserved for cases with high-grade pivot shift, elevated risk of failure (e.g., patients under 25 years of age, participation in pivoting sports, severe instability, high tibial slope, hyperlaxity), persistent pivot shift after reconstruction, or the need for revision surgery (Figure 1) [23–25].
Treatment of Meniscus Injuries
The meniscus, located between the tibial and femoral articular surfaces, plays an essential role in maintaining normal knee function. It increases the contact area between the articular surfaces, disperses stress to protect the articular cartilage, and contributes to weight-bearing, joint stability, and lubrication [26,27]. The importance of its function is highlighted by evidence showing that total meniscectomy accelerates osteoarthritis progression, which has driven efforts toward meniscal preservation [28].
Although the meniscus is largely avascular, its peripheral 3 to 5 mm (approximately 10% to 30% of the medial meniscus width and 10% to 25% of the lateral meniscus width) receives blood supply from the parameniscal capillary plexus in the joint capsule and synovium. This vascular supply diminishes with age. Based on vascular distribution, the meniscus is divided into three zones: red-red, red-white, and white-white [29–31]. Tears in the red-red zone, which has the richest blood supply, are most suitable for repair. Recent studies have shown that the lateral meniscus has a more extensive vascular network than the medial meniscus, and that the popliteal hiatus area is also vascularized via branches of the inferior genicular artery, making it another repairable region [32]. Indications for meniscal repair depend on multiple factors, including the patient’s age, tear location and size, chronicity, and tear surface condition. Age and vascular zone location are especially critical. When a meniscal tear occurs in conjunction with an ACL tear, repair should be strongly considered, as healing rates are higher in these cases [33,34].
Tears in the red-white and white-white zones often fail to heal despite repair attempts. Therefore, the decision to repair should consider both tear location and the quality of remaining tissue. Although some reports describe favorable clinical outcomes for horizontal tear repair, objective healing rates confirmed by magnetic resonance imaging or second-look arthroscopy are generally low, and failures after repair with meniscal fixators have been reported. Repair of degenerative horizontal tears should be approached cautiously and is typically reserved for younger patients (under 40 years) with healthy tissue [35,36].
Techniques for meniscal repair include the inside-out, outside-in, and all-inside methods, each with distinct indications and advantages depending on tear type and location [37–39]. Longitudinal tears of the posterior horn are particularly good candidates for repair. Available options include the arthroscopic inside-out technique, Morgan’s all-inside repair, and various meniscal fixators. Technique selection is based on tear location, shape, and size.
Recent advancements in meniscal fixator technology have simplified repairs, making them faster and less invasive, as they avoid additional incisions while providing relatively strong fixation [40–42]. The Fast-Fix system (Smith & Nephew Endoscopy) has undergone continuous improvements to enhance fixation strength and accessibility at multiple angles, contributing to favorable clinical results. However, meniscal fixators may not offer sufficient stability for tears at the meniscocapsular junction and generally have lower fixation strength compared to all-inside suturing. Potential complications include foreign body reactions causing synovitis, cartilage injury, and fixator migration, necessitating long-term follow-up (Figure 2) [42,43].
Treatment of Cartilage Injuries
Articular cartilage has minimal intrinsic healing capacity; thus, cartilage defects rarely heal spontaneously and can progress to osteoarthritis if untreated. However, symptom severity can vary widely, and there is often no direct correlation between defect size and clinical presentation. This variability complicates treatment decisions. Planning should consider patient age, symptom severity, defect size and location, and patient expectations.
Surgical options include lavage, debridement, abrasion, chondroplasty, and marrow stimulation techniques such as microfracture and multiple drilling. Other strategies involve grafting (transfer or transplantation), cell-based therapies, and the use of growth factors or other biologic agents [44]. Selection of the optimal method depends on lesion size and the patient’s activity level. For small lesions (≤2 cm2) with mild symptoms, debridement alone can provide short-term symptom relief. Lavage and debridement primarily reduce inflammation and mechanical irritation rather than repair cartilage. Abrasion chondroplasty and microfracture are typically used for lesions ≤2 cm2 in low-activity patients. These techniques stimulate cartilage repair by penetrating the vascularized subchondral bone; however, the resulting fibrocartilage lacks the biomechanical properties and long-term durability of native hyaline cartilage [45].
To improve microfracture outcomes, a category of “enhanced microfracture techniques” has been developed. These approaches combine microfracture with cellular augmentation or the use of biomaterials as scaffolds to promote more robust cartilage matrix formation. Examples include autologous matrix-induced chondrogenesis, bone marrow aspirate stem cell concentrate, and matrix-induced autologous chondrocyte implantation (ACI) [46]. These techniques utilize bone marrow aspirate concentrate and collagen scaffolds, though their long-term efficacy requires further investigation.
For high-activity patients, options include osteochondral autograft transfer systems (OATS), osteochondral allograft transplantation, and ACI. OATS is indicated for lesions ≤2 cm2, in which osteochondral plugs harvested from a low-load area of the femoral condyle are transplanted into the defect. For larger lesions (2–3.5 cm2), osteochondral allografts are preferred [47]. ACI is a suitable option for young patients with extensive defects (≤10 cm2) [48]. This two-stage procedure involves arthroscopic harvest of a small cartilage sample, followed by laboratory cell culture. After 3 to 6 weeks, when sufficient cells have proliferated, an open procedure is performed to implant them into the defect (Figure 3).
Future of Knee Arthroscopy
Since the 1990s, arthroscopic surgery has primarily relied on 4-mm lenses, with procedures viewed on a monitor. More recently, needle arthroscopy has emerged, utilizing a thinner lens under local anesthesia in an outpatient setting to perform simpler procedures [49]. This approach enables more accurate diagnosis than MRI in some cases and can be used for both diagnosis and treatment of meniscal and ACL injuries. While the technology shows considerable promise, its current application is limited to procedures feasible under local anesthesia; therefore, its indications and scope of use must be carefully evaluated.
Knee arthroscopy is technically more demanding than arthroscopy of other joints due to the restricted intra-articular space, particularly during meniscus surgery. Consequently, the application of robotic assistance and navigation systems, which have been increasingly adopted in other orthopedic fields, remains limited in knee arthroscopy. However, considering the current pace of technological advancement, it is expected that innovative technologies such as robot-assisted surgery, advanced imaging modalities, and patient-specific instrumentation will enhance surgical accuracy, improve patient outcomes, shorten recovery times, and reduce postoperative complications. Furthermore, with the integration of patient-specific data, personalized treatment strategies—including individualized ligament reconstruction, meniscal repair, and cartilage restoration—are anticipated to become feasible through these emerging technologies [50].
Conclusion
Knee arthroscopy was the first and remains the most advanced subspecialty within orthopedic arthroscopy, serving as a foundation for the development of arthroscopic techniques in other joints. Although surgical methods continue to evolve, certain limitations persist. Future advancements are expected to enhance surgical precision, shorten recovery periods, and reduce complication rates. Additionally, the integration of personalized treatment strategies based on individual patient characteristics is likely to further improve clinical outcomes.
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