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 an ACL is designed to help patients return to daily activities, including sports, especially in younger, more active patients.
With the cost of ACL reconstructive surgery in the United States alone estimated at $3 billion per year, achieving satisfactory results is a topic of great interest to clinicians and research. This article reviews the literature on surgical treatment of primary 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, the anteromedial and posterolateral bundles of the ACL function simultaneously to provide stability in the anterior-posterior and rotational directions of the knee. During knee flexion and extension, the length of the anteromedial bundle remains constant and gains its maximum tension at 45-60 degrees of flexion. The posterior lateral bundle, however, is tense in extension and relaxed in flexion, thus allowing axial rotation of the knee. A number of studies have been reported on the biomechanical behavior of the two functional bundles of the ACL.
A thorough understanding of the anatomy and function of the ACL is a prerequisite for the treatment of ACL injuries and will help inform the surgeon when developing the best strategy for cases of partial or total ACL tears.
Treatment of ACL Injuries
The treatment of ACL can be both non-surgical and surgical. The surgical decision for an acute ACL tear must consider a variety of influencing factors to determine the final surgical option 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 surgical treatment of ACL injuries. postoperative rehabilitation of ACL reconstruction is also important to the final outcome, but will not be the focus of this article.
Surgical Treatment
Once the decision to surgically treat an ACL tear has been made, the timing of surgery is the first consideration. Preoperative range of motion, swelling, and quadriceps muscle strength are all important factors in the success of the surgery. Preoperative joint swelling and limited motion may lead to postoperative fibrous joint adhesions.
When preoperative quadriceps strength decreases by more than 20%, function is significantly impaired in cases of ACL reconstruction with autologous bone-patellar tendon-bone at 2 years postoperatively. 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 is significantly better 2 years after surgery than in cases where the preoperative muscle strength is less than 75% of the healthy side. Therefore, preoperative treatment should focus on restoring range of motion, reducing swelling, and strengthening quadriceps muscle strength.
The type of ACL tear should first be identified intraoperatively. If there is a significant single bundle partial tear, strengthening surgery 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 taken into account; 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 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.
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 likely 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 8mm in diameter than in cases greater than 8mm. The use of allografts may be considered for first-time patients concerned about donor-area symptoms and aesthetic requirements. Freshly frozen allografts, which usually require radiological and chemical treatment before preservation, can yield the same results as autografts. However, some recent studies have concluded that ACL reconstruction using allografts may imply a higher failure rate in young patients who wish to return to sports activity sooner.
Advantages and Disadvantages of Current Use of Reconstructive ACL Grafts
Finally, the patient’s daily activities and lifestyle may also influence the individual choice when it comes to ACL reconstruction. For example, the use of an autologous bone-patellar tendon-bone graft is likely to cause anterior knee pain, making it inappropriate for patients who need to kneel during their daily lives for 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 lead to limited knee motion and allow abnormal knee rotation during dynamic loading. A recent study evaluating 12 surgeons’ choice of ACL bone tunnel locations found significant differences in the ideal location of the ACL single-beam reconstructed bone tunnel. There are various methods for evaluating the position of the bone tunnel intraoperatively and postoperatively. Illingworth et al. described a method to measure the angle of the femoral tunnel based on the long axis of the femur on an orthopantomogram, and if the angle is less than 32.7 degrees, it is likely to be non-anatomic. Ligamentous stop position, tunnel angle, and ACL length can also be evaluated by comparison of pre- and post-operative MRI. The gold standard for tract position evaluation is still the 3-D CT scan, which Meuffel et al. demonstrated to be the most reliable for evaluating the femoral and tibial tracts, but is also particularly useful for knees that will eventually require revision surgery.
Clinical results of ACL reconstruction
In a Level I clinical trial conducted by Frobell et al. in 121 active adult patients comparing the rehabilitation outcomes of early and delayed ACL reconstruction, the mean knee injury and osteoarthritis scores (KOOS4) at 2-year postoperative follow-up were 39.2 for the ACL early reconstruction group and 39.4 for the ACL delayed reconstruction group (P=0.96). A significantly higher proportion of meniscal surgery was performed in the delayed reconstructive surgery group than in the early reconstructive surgery group. The latest 5-year results reported in this study also showed the same trend. A total of 30 (51%) patients in the delayed reconstructive group underwent ACL surgery. Therefore, non-surgical treatment may be a viable option for acute ACL tears.
The clinical outcomes of single- and double-bundle reconstruction have been well reported, and Tiamklang et al. conducted a Cochrane systematic review of 17 randomized and semi-randomized controlled trials comparing the outcomes of single- or double-bundle reconstruction 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.
At 2-5 years postoperatively, 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 arthrodynamics checker. Also, the percentage of fresh meniscal injuries was higher for the single-beam reconstruction. It is worth noting, however, that all clinical trials included in this systematic review had methodological shortcomings, so we need to view these findings with caution.
Hussein et al. recently published a Class I randomized controlled trial comparing the results of ACL anatomic double-bundle reconstruction with anatomic single-bundle reconstruction and conventional single-bundle reconstruction using an autologous N-tendon, with 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 superior to conventional single-bundle reconstruction in both areas.
Only Lysholm scores were 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, the results of anatomic single-bundle reconstruction and anatomic double-bundle reconstruction of the autologous N-tendon were compared intraoperatively based on the measured size of the ACL tibial stop. At a mean follow-up of 30 months after surgery, there were no differences between groups in either Lysholm scores, IKDC subjective knee scores, or KT-1000 measurements and axial shift tests.
Most published studies to date have concluded that there is no difference in patient self-rated outcomes between ACL anatomic unibundle and bifundle reconstruction, while there may be some difference in knee laxity measurements between the two surgical approaches, with bifundle reconstruction yielding 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 reconstruction for partial ACL tears have been frequently reported on this page. Adachi et al. compared strengthening surgery for partial ACL tears with reconstructive surgery for complete ACL tears at a mean follow-up of 2.6 years and found better knee stability and position awareness with strengthening surgery. 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
Biomechanics of the knee in vivo, without the time-zero limitations of in vitro, also allows for serial studies of functional recovery outcomes of the knee 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 and traditional video motion analysis. The results showed that the ACL reconstruction showed more pronounced external rotation and internal rotation of the knee compared to the healthy limb.
Abebe et al. used biplane fluoroscopy and MRI to evaluate knee function in different static positions and found that single bundle reconstruction using anatomically positioned femoral tunnels resulted in knee kinematic characteristics that were closer to those of the anatomically positioned femur than to those of the non-anatomical 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 double-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 as close to normal knee function as anatomic double-bundle reconstruction.
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 5,770 patients with an average 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 recover 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, and Smith et al. evaluated the recovery of athletic performance in 77 athletes with an average age of 21 years and found that 71% (55) of the athletes returned to pre-injury levels 12 months after surgery. Future studies should also be conducted on the percentage of athletic recovery by type, frequency, intensity, and duration of exercise.
Graft Failure After ACL Reconstruction
Studies have analyzed graft failure after ACL reconstruction and contralateral knee ACL injuries. Data from the Danish Knee Ligament Registry compared anteromedial drilling of the femoral tunnel with transtibial drilling of the femoral tunnel during 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 ACL reconstruction failure rates 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. Shelbourne et al. followed 1415 patients with autologous bone-patellar tendon-bone ACL reconstruction for more than 5 years and found that younger age and higher activity levels resulted in increased bilateral knee injuries.
Return to activity up to 6 months after surgery did not increase the risk of injury, and patients under 18 years of age took an average of 4.6 months to return to activity after surgery. In a prospective study by van Eck et al. on the failure rate of allogeneic ACL anatomic reconstruction, 17% (13/27) of patients had a retear at 9 months postoperatively. Further analysis of the factors influencing ACL graft failure is needed in the future. Based on the current available evidence, younger age and higher activity levels may be predictors of re-injury, independent of the time to return to sport.
Osteoarthritis after ACL reconstruction
The development of osteoarthritis after ACL reconstruction is of great clinical interest. in a retrospective analysis of predictors of osteoarthritis after single-beam ACL non-anatomic reconstruction, Li et al. defined radiographic presentation as Kellgren and Lawrence grade 2 in at least one compartment or Kellgren and Lawrence grade 1 in at least two compartments as Osteoarthritis, with a mean follow-up of 7.86 years and an overall incidence of 39% (96/249).
Ideal predictors of osteoarthritis included BMI, length of follow-up, history of previous meniscectomy, and grade 2 or greater medial cartilage formation. At 7 years of follow-up, the rate of osteoarthritis in the bone-patellar tendon-bone graft group was 45% (24/53) compared to 14% (7/51) in the N-cord muscle graft group (p=0.002).
Oiestad et al. prospectively studied the function of the knee in patients with ACL reconstruction alone versus those with combined meniscal and/or cartilage pathology over a 10-15 year period, using Kellgren and Lawrence grading for radiological evaluation, and found that 80% of patients in the combined lesion group Grade 2 joint space narrowing was found in 80% of patients in the combined lesion group, which was significantly higher than the 62% in the reconstructed group alone (p = 0.008), but there was no significant difference in osteoarthritic symptoms between the two groups. The proportion of these patients with postoperative patellofemoral arthritis was 26.5% (48/181) and correlated with older age, progression of symptoms, severity of tibiofemoral arthritis, and degree of functional limitation of the knee.
Salmon et al. also reported on the relationship between degenerative joint changes and meniscectomy, with a significant increase in knee laxity and limitation of joint motion 13 years after ACL reconstruction with an autologous bone-patellar tendon-bone graft. 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 cause and progression of osteoarthritis after ACL reconstruction 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 in young, active patients with reliable results. There is no significant difference in patient self-assessed outcomes between using double- and single-beam reconstruction. The patient’s age and activity level are valid predictors of their return to sport and re-injury. Based on the current available data, time to return to sport may not be related to reinjury of the reconstructed ACL. meniscal and/or cartilage pathological changes detected at the time of ACL reconstruction, postoperative knee motion limitation, and future osteoarticular progression are associated. More convincing studies on surgical treatment of ACL injuries using patient-related sensitive measures are needed in the future.