Anterior cruciate ligament (ACL) tears are a common injury worldwide. The incidence of ACL is estimated to be 35/100,000, with female athletes having 2-8 times the incidence of male athletes. This injury not only leads to knee instability, joint laxity, and limited mobility, but also leads to long-term osteoarthritis of the knee. surgical reconstruction of ACL is designed to help patients return to their daily activities, including sports, especially in younger, more active patients.
It is estimated that as much as $3 billion is spent on ACL reconstructive surgery in the United States alone each year, making the achievement of satisfactory results in ACL reconstructive surgery a topic of high interest to clinicians and research. This article reviews the literature on surgical treatment of initial ACL tears in adult patients over the age of 18 years and focuses on principles of clinical decision making, clinical outcomes, and guidelines for motor recovery.
Anatomy and Function
The ACL is divided into an anteromedial bundle and a posterior lateral bundle based on its location at the tibial stop. The lateral tibial stop of the ACL is fan-shaped, whereas the lateral femoral stop is oval-shaped and can be seen as two bony prominences on the medial wall of the lateral femoral condyle. The lateral intercondylar ridge, also known as the resident physician’s ridge, is located at the anterior edge of the femoral stop, and the lateral bifurcation ridge is perpendicular to the lateral intercondylar ridge, which is located between the anteromedial and posterior lateral bundle femoral stops.
During knee flexion and extension, the anteromedial and posterolateral bundles of the ACL function simultaneously to provide stability in the anterior-posterior and rotational directions of the knee joint. During knee flexion and extension, the length of the anteromedial bundle remains constant and gains its maximum tension at 45-60 degrees of flexion. However, the posterior lateral bundle is tense in extension and relaxed in flexion, thus allowing axial rotation of the knee. Numerous studies have been reported on the biomechanical behavior of the two functional bundles of the ACL.
A comprehensive understanding of the anatomy and function of the ACL is a prerequisite for the treatment of ACL injuries and will benefit the operator when developing the best strategy for cases of partial or total ACL tears.
Treatment of ACL injuries
ACL can be treated with both non-surgical and surgical treatment. The surgical decision for an acute ACL tear must consider multiple influencing factors to determine the final surgical plan based on the patient’s age, surgical expectations, and comorbid injuries. In general, younger, more active patients can potentially require surgery to restore preoperative mobility. In a later article, we focus on a review of the surgical treatment of ACL injuries. postoperative rehabilitation of ACL reconstruction is also important to the final outcome, but will not be a focus in this article.
Surgical Treatment
Once the decision to surgically treat an ACL tear is made, the timing of surgery becomes the first issue to consider. Preoperative range of motion, swelling, and quadriceps muscle strength are all important factors in determining the success of the surgery. Preoperative joint swelling and limited motion may lead to postoperative fibrous joint adhesions.
When preoperative quadriceps muscle strength decreases by more than 20%, function is significantly compromised at 2 years postoperatively in cases of ACL reconstruction using autologous bone-patellar tendon-bone. It has also been reported that when the preoperative quadriceps muscle strength of the affected limb reaches more than 90% of the healthy side, the muscle strength at 2 years after surgery is significantly better than that of cases in which the preoperative muscle strength is less than 75% of the healthy side. Therefore, the preoperative treatment should focus on restoring the range of motion, reducing swelling, and strengthening the quadriceps muscle strength.
The type of ACL tear should first be evident intraoperatively. If a significant single bundle partial tear is present, a strengthening procedure should be considered. The incidence of single bundle ACL tears has been reported to be 5%-35%. The theoretical advantage of single bundle strengthening surgery is the preservation of proprioception, biomechanics, and bioprosthetic capabilities. Careful debridement and preservation of the original ligamentous stop facilitates further identification of a suitable bone tract.
Today, most surgeons performing ACL reconstruction typically use a single-beam reconstruction. In contrast to the United States, double-bundle reconstruction is more commonly used in Europe and Asia. Regardless of the type of reconstruction used, it is important to understand the anatomy of the double bundle in order for the surgeon to perform the anatomical reconstruction of the ACL pair. Because of the relative complexity of the dual-beam reconstruction technique, the decision to use single- or dual-beam reconstruction depends on many factors in addition to the physician’s familiarity with the dual-beam reconstruction technique.
A comprehensive flow chart has been reported to assist the surgeon in preoperative decision making. Anatomic variation of the tibial stop is one of the factors that must be considered (Figure 1), and if the tibial stop of the ACL is less than 14 mm when measured microscopically, it is difficult to perform a double-bundle reconstruction. In addition to this, arthritic changes, multiple ligament injuries, severe bone contusions, unclosed epiphyseal plates, and narrow intercondylar fossa width are all considered indications for single-bundle reconstruction. Variations in the shape of the intercondylar fossa itself can also have an impact on the safety of drilling a double femoral tunnel during double-bundle reconstruction.
Figure 1
The size of the tibial stop in the sagittal position is measured using an arthroscopic scale, the tibial impression of the ACL is carefully separated, and the anteromedial (AM) bundle and the posterolateral (PL) bundle are marked using a standard arthroscopic radiofrequency ablation device.
Commonly used grafts for ACL reconstruction include autologous bone-patellar tendon-bone graft, autologous N-cord muscle graft, autologous quadriceps tendon graft, and allograft (Table I). Of these, bone-patellar tendon-bone grafts are not suitable for double-bundle reconstruction and the sagittal thickness of the patella and quadriceps tendon should be measured on magnetic resonance during preoperative planning to facilitate the operator’s understanding of the possible thickness of the graft. One study measured the size of the N cord muscle by magnetic resonance and found that the cross-sectional area of the N cord muscle on magnetic resonance correlated positively with the size of the graft obtained intraoperatively, whereas the diameter of the graft did not. magnussen et al. concluded that the early postoperative revision rate was significantly higher in cases with autologous N cord grafts less than or equal to 8 mm in diameter than in cases larger than 8 mm. The use of allografts may be considered in patients who are concerned about the development of donor-area symptoms and require aesthetics for initial surgery. Freshly frozen allografts usually require radiological and chemical treatment before preservation, and it can yield the same results as autografts. However, some recent studies have suggested that ACL reconstruction with allografts may imply a higher failure rate in young patients who wish to resume physical activity early.
Table I Advantages and disadvantages of the currently used grafts for ACL reconstruction
Finally, the patient’s daily activities and lifestyle may also influence the individual choice when reconstructing an ACL. For example, the use of an autologous bone-patellar tendon-bone graft is likely to cause anterior knee pain, making this inappropriate for patients who need to kneel in their daily lives due to wrestling sports or religious activities.
Accurate tunnel positioning is important for anatomic reconstruction of the ACL. Previous studies have demonstrated that non-anatomically located bone tunnels can cause limited knee motion and allow abnormal rotation of the knee during dynamic loading. A recent study evaluating the location of the ACL bone tunnel selected by 12 surgeons found significant differences in the ideal location of the ACL single-beam reconstructed bone tunnel. There are various methods for evaluating the intraoperative and postoperative bone tunnel position. One of the postoperative methods requires the evaluation of the angle of the bone tunnel and the position of the endophyseal plant by taking an ortho-lateral radiograph, and Illingworth et al. described a method to measure the angle of the femoral tunnel based on the long axis of the femur on an ortho-slice, which is likely to be non-anatomic if the angle is less than 32.7 degrees (Figure 2). The ligament stop position, bone tunnel angle, and ACL length can also be evaluated by comparison of pre- and post-operative MRI (Figure 3). The current gold standard for tract position evaluation remains 3D CT scans (Figures 4 and 5), and Meuffel et al. demonstrated that 3D CT scans are most reliable in evaluating the femoral and tibial tracts, in addition to being particularly useful for knees that will eventually require revision surgery.
Figure 2 Standard weight-bearing orthopantomogram of the knee in 45 degrees of flexion at 1 year of ACL single-beam reconstruction, showing the femoral tunnel at a 45-degree angle to the long axis of the femur, suggesting an anatomic position of the osseous tract.
Figure 3 A-C Magnetic resonance images of the ACL reconstruction using autologous bone-patellar tendon-bone anatomy. Figure 3A Preoperative images for initial measurements of metrics including ACL length; Figure 3B
Sagittal scan at 3 months postoperatively showing the size of the tibial stop and the angle of inclination of the ligament; Figure 3C
Coronal oblique view at 3 months postoperatively, showing the long axis of the ACL starting at the Blumensaat line located at the highest point between the condyles. This image sequence can be used for imaging evaluation after ACL reconstruction.
Figure 4 3D CT reconstruction of the femur and tibia showing the position of the bone tracts in the single bundle anatomical reconstruction of the ACL
Figure 5 Osseous tract position at intercondylar ACL double bundle anatomical reconstruction in 3D CT reconstruction of femur and tibia
Clinical results of ACL reconstruction
Frobell et al. conducted a Level I clinical trial in 121 active adult patients comparing the rehabilitation outcomes of early ACL reconstruction with those of delayed reconstruction. At the 2-year postoperative follow-up, the mean knee injury and osteoarthritis scores (KOOS4) were 39.2 in the ACL early reconstruction group and 39.4 in the ACL delayed reconstruction group, respectively (P=0.96). The proportion of meniscal surgery performed in the delayed reconstructive surgery group was significantly higher than in the early reconstructive surgery group. The latest reported 5-year results from this study also showed the same trend. A total of 30 (51%) patients in the delayed reconstructive group underwent ACL surgery. Thus, non-surgical treatment may become a viable option for acute ACL tears.
The clinical outcomes of single- and double-bundle reconstruction have been more frequently reported previously (Figures 6 and 7).Tiamklang et al. performed a Cochrane systematic review of 17 randomized and semi-randomized controlled trials comparing the outcomes of single- or double-bundle reconstruction surgery in adult patients. The authors concluded that there were no significant differences in patient self-assessment outcomes between the two groups at 5 years postoperatively.
Between 2 and 5 years after surgery, the double-bundle reconstruction group had better results in terms of knee laxity as measured by the International Knee Documentation Committee (IKDC) knee examination, the axial shift test, and the KT-1000 joint kinematic examiner. Also, the proportion of fresh meniscal injuries was higher for single-beam reconstruction. However, it is worth noting that all clinical trials included in this systematic review had methodological shortcomings, so we need to view the above findings with caution.
Figure 6A-B Intraoperative microscopic ACL single bundle anatomical reconstruction of the femoral tunnel and tibial tunnel location. Figure 6A Enlargement of the tibial tunnel using dilators; Figure 6B Autologous N cord tendon stretched and fixed in anatomic position.
Figure 7A-B Intraoperative microscopic ACL double-beam anatomical reconstruction of the femoral tunnel and tibial tunnel locations. Figure 7A Enlargement of the tibial tunnel using dilators; Figure 7B
Reconstruction of the anteromedial (AM) bundle and the posterolateral (PL) bundle using allografts, stretched and fixed in anatomic position.
Hussein et al. recently published a Class I randomized controlled trial comparing the results of anatomic double bundle reconstruction with anatomic single bundle reconstruction and conventional single bundle reconstruction of the ACL using an autologous N cord tendon in a total of 281 patients prospectively followed up for a mean of 51.5 months. Compared with anatomic single-bundle reconstruction, anatomic double-bundle reconstruction significantly improved anterior-posterior laxity (KT-1000 arthrokinetic test) and rotational laxity (axial shift test), and anatomic single-bundle reconstruction was also superior to conventional single-bundle reconstruction in these two aspects.
Only the Lysholm score was higher in the anatomic double-bundle reconstruction group than in the conventional single-bundle reconstruction group, while there was no significant difference in the self-assessment of patients in the anatomic double-bundle reconstruction group compared to the anatomic single-bundle reconstruction group. In another prospective comparative study comparing the results of autologous N-tendon anatomic single-bundle reconstruction with anatomic double-bundle reconstruction, the surgical approach was determined intraoperatively based on the measured size of the ACL tibial stop. At a mean postoperative follow-up of 30 months, there were no differences between groups in either Lysholm score, IKDC subjective knee score, or KT-1000 measurements and axial shift tests.
Most published studies to date have concluded that there is no difference between ACL anatomic unibundle reconstruction and bifascicular reconstruction in terms of patient self-rated outcomes, whereas there may be some difference in knee laxity measurements between the two surgical approaches, with bifascicular reconstruction having superior outcomes. There is also some clinical evidence that detailed clinical outcomes can be obtained after both procedures, regardless of whether single- or double-bundle reconstruction is used, if the choice is individualized to the patient’s condition.
The results of single-bundle reinforced reconstructive surgery for partial ACL tears are frequently reported on the results page.Sonnery-Cottet et al. performed anteromedial bundle reconstruction in cases where the posterior lateral bundle remained, which resulted in a significant reduction in anterior-posterior laxity (Telos stress radiographic method) with a significant increase in IKDC subjective knee scale scores and Lysholm Adachi et al. compared strengthening surgery for partial ACL tears to reconstructive surgery for complete ACL tears with a mean follow-up of 2.6 years and found that strengthening surgery had better knee stability and position awareness. A recent systematic review concluded that although the current clinical evidence supporting strengthening surgery is slightly weak, it is still encouraging.
In vivo biomechanics after ACL reconstruction
Conducting knee biomechanics in vivo, without the time-zero limitations of in vitro, also allows for serial studies of knee functional recovery outcomes after ACL reconstruction, and includes actual weight-bearing activities such as running, jumping, and stair climbing.
Georgoulis et al. compared the reconstructed ACL with the healthy knee using surface markers using traditional video motion analysis, and found no significant difference between the knee during walking bilaterally, with more internal tibial rotation visible only in the reconstructed knee. tashman et al. evaluated the kinematic characteristics of the starting phase of the patient during downhill running using a dynamic stereoradiographic approach The results showed more pronounced external rotation and internal rotation in the ACL reconstructed knee compared to the healthy limb.
Abebe et al. used biplane fluoroscopy and magnetic resonance to evaluate the knee function in different static positions and found that the single bundle reconstruction of the femoral tunnel in anatomic position was closer to the knee kinematics than the non-anatomic reconstruction. The kinematic characteristics of the knee were closer to those of a normal knee joint.
Some studies have used a biplane radiographic method to compare the rotation and displacement between the ACL anatomical two-beam reconstructed knee and the tibiofemoral joint on the healthy side during early running and mid-standing, and others have used a model-based tracking technique to evaluate the kinematic characteristics of the tibiofemoral joint. Regardless of the method used, there were no significant differences in the kinematic indices after ACL anatomic double-bundle reconstruction compared with the healthy side. This result suggests that anatomic double-bundle reconstruction of the knee may restore knee function to the level of the healthy side, but it is unclear whether anatomic single-bundle reconstruction yields the same results as anatomic double-bundle reconstruction for near-normal knee function.
Motor recovery after ACL reconstruction
The timing of motor recovery after ACL reconstruction is influenced by a number of factors. A systematic review by Ardern et al. analyzed 48 studies including a total of 5,770 patients with a mean follow-up of 41.5 months after surgery, resulting in 82% of patients having improved motion levels, 63% of patients having returned to pre-injury levels, and only 44% of patients having returned to pre-injury levels. Only 44% of the patients were able to participate in competitive sports activities. The main reason for the failure to return to exercise level was the patient’s fear of re-injury.
Brophy et al. studied soccer players who returned to sports and found that younger, male athletes were more likely to return to sports than older, female athletes. smith et al. evaluated the return of athletic performance in 77 athletes with an average age of 21 years and found that 71% (55) of the athletes returned to their pre-injury athletic level 12 months after surgery. Future studies should also be conducted to address the percentage of sports recovery by type, frequency, intensity, and duration of exercise.
Graft failure after ACL reconstruction
One study has analyzed graft failure after ACL reconstruction and contralateral knee ACL injury. Data from the Danish Knee Ligament Registry compared the use of anteromedial drilling of the femoral tunnel with transtibial drilling of the femoral tunnel in ACL reconstruction, with a higher rate of postoperative revision in the former (5.16%) than in the latter (3.20%) and a relative risk of 2.04 (95% confidence interval 1.39-2.99). ACL anatomic reconstruction may carry a higher risk of graft failure, and the closer the graft is to the anatomic position, the higher the risk of failure.
A recent study by Boourke et al. found that failure rates after ACL reconstruction could reach 11% with either bone-patellar tendon-bone or autologous N-cord grafts, and 13% with secondary ACL tears in the contralateral knee, while the type of graft had no effect on the failure rate. Other authors have suggested that the risk of tearing of the contralateral knee ACL is much higher than the failure rate after ipsilateral knee ACL grafting. Shelbourne et al. followed 1415 patients who underwent autologous bone-patellar tendon-bone ACL reconstruction for more than 5 years and found that both younger age and greater activity resulted in increased bilateral knee injury.
Resumption of activity up to 6 months after surgery did not increase the risk of injury, and the average time to return to activity after surgery for patients younger than 18 years was 4.6 months. In van
Eck et al.’s prospective study on the failure rate of anatomic reconstruction of homogeneous ACL, 17% (13/27) of patients had a re-tear at 9 months postoperatively. Further analysis of the factors influencing the failure of ACL grafts is still needed in the future. Based on the currently available evidence, younger age and higher activity levels may be predictors for the occurrence of re-injury, independent of the time to return to sport.
Osteoarthritis after ACL reconstruction
Osteoarthritis after ACL reconstruction is of great clinical interest. li et al. performed a retrospective analysis of predictors of osteoarthritis after non-anatomic reconstruction of single-beam ACL, and they classified radiological manifestations of Kellgren and Lawrence in at least one interventricular compartment
grade 2 or Kellgren and Lawrence grade 1 in at least two interventricular compartments was defined as osteoarthritis with a mean follow-up of 7.86 years and an overall incidence of 39% (96/249).
Ideal predictors of developing osteoarthritis included BMI, length of follow-up, history of previous meniscectomy, and grade 2 or greater medial cartilage formation. roe et al. also studied the rate of developing osteoarthritis in consecutive nonrandomized patients with ACL reconstruction with N cord muscle or bone-patellar tendon-bone autografts. at 7 years of follow-up, the rate of developing osteoarthritis in the bone-patellar tendon-bone graft group was 45% (24/ 53), while only 14% (7/51) of the N-cord muscle graft group developed osteoarthritis (p=0.002).
Many authors have also performed some long-term follow-up studies. oiestad et al. prospectively studied the function of the knee joint in patients with ACL reconstruction alone versus those with combined meniscal and/or cartilage pathology for up to 10-15 years, using Kellgren and Lawrence grading for radiological evaluation, and found that 80% of patients in the combined lesion group 80% of patients in the combined lesion group developed grade 2 joint space stenosis, significantly higher than the 62% rate in the reconstruction alone group (p
= 0.008).
0.008), but there was no significant difference in the symptoms of osteoarthritis between the two groups. The proportion of these patients who developed postoperative patellofemoral arthritis was 26.5% (48/181) and correlated with older age, degree of symptom progression, severity of tibiofemoral arthritis, and degree of functional limitation of the knee.
The relationship between degenerative joint changes and meniscectomy was also reported by Salmon et al. 13 years after ACL reconstruction with autologous bone-patellar tendon-bone graft, which resulted in a significant increase in knee laxity with limited joint motion.A similar study was conducted by Shelbourne et al. in 780 patients who underwent ACL reconstruction with bone-patellar tendon-bone autograft for at least 5 years of Shelbourne and Gray followed patients without other knee pathology at the time of surgery for more than 10 years and found a 2% incidence of osteoarthritis, while a similar study by Lebel et al. put the rate at 8%.
It is now generally accepted, based on the available evidence, that postoperative cases with meniscal and/or cartilage damage and limited knee motion lead to progression of osteoarthritis, whereas cases without other joint pathology at the time of ACL reconstruction have a low incidence of osteoarthritis, even after prolonged follow-up. Further studies on the causes and progression of osteoarthritis after ACL reconstruction surgery are needed in the future, including early diagnosis by advanced imaging methods or relevant biomarkers.
In summary, surgical treatment of acute ACL tears is very common and reliable in young, active patients (Table II). There was no significant difference in patient self-rated outcomes between reconstruction using a double- and single-beam reconstruction. The patient’s age and activity level were valid predictors of his or her return to sport and re-injury. Based on the currently available data, time to return to sport may not be related to reinjury in reconstructed ACL. meniscal and/or cartilage pathological changes detected at the time of ACL reconstruction, postoperative knee motion limitation, and future osteoarticular progression were correlated. More convincing studies on surgical treatment of ACL injuries using patient-related sensitive measures are needed in the future.