Anterior Cruciate Ligament


Two functional bundles of the anterior cruciate ligament (ACL), the anteromedial (AM) and posterolateral (PL), were first described in 1836 ( Fig. 6.1 ). On the femoral insertion site, two osseous landmarks can be identified. The lateral intercondylar ridge (i.e., ‘resident’s ridge’) is the anterior border of the ACL and runs from proximal to distal. The lateral bifurcate ridge is located between the AM and PL bundles. For the tibial insertion site, different shapes are described in the literature (e.g., elliptical, triangular or C shaped). , There is a close relationship between the anterior horn of the lateral meniscus and the ACL, with more than 50% overlap of both insertions on the tibial plateau.

Fig. 6.1

Anatomy of native anterior cruciate ligament (ACL).

(A) The complex morphology of the native ACL with focus on the (B) tibial footprint and (C) femoral footprint.

The intraarticular length of the ACL varies from 22 to 41 mm, with a mean of 32 mm. , Moreover, a large variation in length of the femoral and tibial insertions sites has been demonstrated. In 137 patients the arthroscopically measured insertion site length varied between 12 to 22 mm. The ACL midsubstance cross-sectional area (CSA) was reported as 46.9 ± 18.3 mm. , In terms of graft choice and graft size it is important to know that the tibial insertion site (123.5 ± 12.5 mm 2 ) is typically larger than the femoral insertion site (60.1 ± 16.9 mm 2 ).


For successful ACL reconstruction it is crucial to understand the biomechanical function of the ACL and the properties of the native ACL. Structural properties describe the complex containing different tissues (e.g., femoral bone–native ACL–tibial bone) and mechanical properties define the properties of an individual tissue (e.g., ACL without insertion site). The structural properties of the femur–ACL–tibia complex (FATC) are significantly affected by age, with an ultimate load to failure in younger specimens (22 to 35 years) of 2160 ± 157 N and a linear stiffness of 242 ± 28 N/mm. For comparison, the ultimate load of a patellar tendon (PT) graft is 1784 ± 580 N and the stiffness is 210 ± 65 N/mm. The quadruple hamstring graft has an ultimate load of 2422 ± 538 N and a stiffness of 238 ± 71 N/mm. The quadriceps tendon (QT) graft has an ultimate load of 2186 ± 759 N graft and a stiffness of 466 ± 133 N/mm.

In the fully extended knee, the PL bundle is tight and the AM bundle is moderately lax. In contrast, with increasing knee flexion angle, the femoral insertion is oriented more horizontally, making the AM bundle tighter and the PL bundle more lax. As a result, the distribution of load changes within the ACL based on the knee flexion angle. Whereas the amount of in situ forces experienced by the PL bundle is significantly affected by the knee flexion angle, the forces in the AM bundle remain relatively constant.

The ACL is the primary stabiliser for anterior tibial translation (ATT) with regard to the femur and is a secondary restraint to internal tibial rotation. Transection of the AM bundle results in increased ATT, especially at higher knee flexion angles (60 degrees and 90 degrees), and isolated transection of the PL bundle increases ATT at 30 degrees of knee flexion. Moreover, an isolated transection of the PL bundle also leads to increased ATT in response to a combined valgus and internal torque, which is effectively a simulated pivot shift test.

In addition to the ACL, the anterolateral complex (ALC), consisting of the superficial and deep iliotibial band (ITB), capsulo-osseous layer of the ITB, and anterolateral capsule, contributes to rotatory knee stability, especially with increasing knee flexion. Compared with the ITB, the anterolateral capsule is substantially weaker and less stiff. Furthermore, the anterolateral capsule demonstrates a nonuniform strain distribution when subjected to different loading conditions, with a maximum principal strain ranging from 22% to 52% that is not aligned in the direction of a proposed ligament. The anterolateral capsule of the knee behaves like a sheet of fibrous tissue rather than a discreet ligament.


The typical injury mechanism for the ACL involves a noncontact twisting movement, sometimes associated with a ‘pop’ or ‘tear’ sensation. Afterwards almost every patient presents with an effusion. For the clinical diagnosis of an ACL injury, several physical examination manoeuvres are described. For the anterior drawer test, the patient is positioned supine with the knee flexed to 90 degrees. The examiner translates the tibia forward with respect to the femur. An excessive anterior translation of the tibia indicates a positive test. For the Lachman test (sensitivity 0.87; specificity 0.97), the patient is again in a supine position with the knee in 15 degrees of flexion. The examiner places one hand behind the tibia with the thumb at the tibial tuberosity and the thigh is held with the other hand. The tibia is then translated anteriorly, and the amount of anterior translation, as well as the quality of the endpoint, is evaluated. The pivot shift test (sensitivity 0.49; specificity 0.98) is a dynamic test of rotatory knee stability that produces a subluxation and successive reduction, felt as a glide or clunk, of the lateral tibial plateau. , Video-based imaging analysis technologies were demonstrated as reliable options to assess the translation of the lateral plateau or tibial acceleration and to quantify the pivot shift test. ,

The next step in the diagnostic algorithm, after the physical examination, is plain radiography to rule out a fracture or dislocation. If there is a concern for ACL injury, magnetic resonance imaging (MRI) is recommended to evaluate the ACL and concomitant meniscal, articular cartilage and collateral ligament pathological conditions ( Fig. 6.2 ). Based on the clinical examination and the radiological findings, the optimal treatment is chosen.

Fig. 6.2

Magnetic resonance imaging (MRI) of anterior cruciate ligament (ACL).

Sagittal proton-dense (PD) MR image of (A) intact and (B) torn ACL in same patient.

Treatment Algorithm (Operative Versus Nonoperative)

Once the diagnosis of ACL injury is confirmed, operative versus nonoperative management must be considered. There is limited prospective, comparative high-level evidence to guide this treatment decision. A return to high-level pivoting sports is one of the most common indications for ACL reconstruction (ACLR). However, it is known that there is a group of patients termed copers who are able to return to pivoting sports without undergoing ACLR. , This group includes a minority of patients, and the subsequent recurrent instability in most patients drives the decision to reconstruct the ACL. Multiple cohort studies have shown nonoperative management often results in a lower return to the previous level of sport compared with operative management. Preoperative level of sport and patient expectations certainly play a role in the return-to-sport rate. Data from a randomised controlled trial (RCT) reported that motives for sports participation were a significant factor in predicting outcomes. Similarly, a prospective cohort study of 143 patients showed that preoperative high-level pivoting sport participation was more common in those electing surgical management and also associated with return to pivoting sports. Although selection bias likely plays a role in return to sport, the current literature supports ACLR for athletes attempting return to pivoting sports.

Limiting additional injury to the meniscus and articular cartilage may potentially be the most important benefit favouring ACLR. A prospective cohort study of 209 patients comparing ACLR for high-risk patients and nonoperative management for low-risk patients showed that initial nonoperative management resulted in increased rates of secondary meniscectomy in the low-risk patients compared with early ACLR (21/146 versus 0/63, respectively, P < .001), which was also consistent across all risk stratification groups. Three further cohort studies, including one study of 6576 active-duty military patients, supported the principle that ACLR decreases the risk of subsequent meniscus surgeries. , , Meniscal deficiency is known to increase the rates of posttraumatic osteoarthritis (OA), and two systematic reviews of the development of OA after ACL injury reported that meniscal injury and meniscectomy are significant risk factors. , Even though these findings may lead one to conclude ACLR could decrease rates of posttraumatic OA, a systematic review of the literature has not shown that to be true. A systematic review reported that rates of OA after ACL injury vary in the literature from 0% to 100%. Therefore the evidence supports operative management for meniscal protection, but ultimately it has not been proven to decrease posttraumatic OA after ACLR.

Given the literature published to date, the recommended treatment algorithm is as follows. ACLR is performed for those patients that desire a return to pivoting sports. ACLR may be considered for lower demand patients with less strenuous physical activity, for the potential benefit of meniscal protection. This is especially considered in adolescents and young adults because of the long-term risk of meniscal injury. Concomitant injuries may also shift the decision to operative management. Certainly, multiligamentous injury requires careful operative consideration, as discussed in a separate chapter. Meniscal injury, including bucket-handle or root tears, may affect surgical decision making. Given that both the meniscus and ACL contribute to rotatory knee stability, ACLR with concomitant meniscus repair is the desired surgical treatment when the situation presents. , Nonoperative management may be considered for patients with lower activity levels and/or low-grade knee instability.

Conservative Approach

For patients with lower activity levels, the initial management is a period of activity limitation, crutches until gait normalises, control of swelling and regaining range of motion. Early studies simply used a home exercise program, and patients clinically improved with reasonable return to light activity. , Supervised physical therapy programs have been proposed to start early after injury to aid in improving knee function before either operative or nonoperative management, with progression of activities over the following months for those who continue with nonoperative management. Traditional nonoperative ACL rehabilitation programs have emphasised lower extremity muscular strength and endurance, restoring joint range of motion, activity modification, agility training and bracing. Perturbation training techniques were subsequently added to the rehabilitation regimen. This technique involves perturbations of support surfaces, such as tilt or roller boards, to induce compensatory muscle activity that may improve rotatory knee stability. Perturbation training has been shown to normalise joint loading, improve knee kinematics and reduce quadriceps–hamstrings cocontraction, improve gait and reduce giving-way episodes.

Functional knee braces are common after both operative and nonoperative management. Nonoperative studies have shown that bracing can provide benefit in the absence of surgery. A prospective cohort study of professional skiers reported that functional brace wear resulted in fewer medial collateral ligament, meniscal and osteochondral injuries. A randomised trial of brace use in nonoperative management found that brace use was associated with less subjective instability but no difference in Knee Injury and Osteoarthritis Outcome Scale (KOOS) or Cincinnati Knee scores and no change in quadriceps and hamstrings torque.

Surgical Techniques

Discussion of the surgical technique requires an understanding of the goal of the individualised, anatomical ACLR. Nonanatomical ACLR is associated with worse outcomes compared with anatomical reconstruction, and tunnel positioning has been cited multiple times as the most common reason for ACLR graft failure. , Additionally, impingement and loss of range of motion are known complications with nonanatomical tunnel placement. Anatomical reconstruction provides improved kinematics of the graft and improved relationship with the adjacent native stabilisers.

Patients’ individual anatomy varies, and therefore graft size should match the native anatomy. Studies have shown higher failure rate with graft diameters less than 8 mm, , but the goal of the individualised approach is to restore the native femoral and tibial insertion sites. Given that the native ACL and potential autograft options vary in size and are not predictable based on patient characteristics, such as height and weight, preoperative and intraoperative ACL and graft measurements are made. , Preoperatively, quadriceps and patellar tendon thickness measurements can be made on MRI and hamstring tendon diameter can be measured on ultrasound. , Intraoperative measurements of the tibial and femoral footprints can provide guidance on individualising the ACL graft size to each patient ( Fig. 6.3A–F ). The ACL midsubstance is 50% ± 15% of the tibial insertion area, and the goal of individualised reconstruction is to restore 50% to 80% of the measured tibial insertion size.

Fig. 6.3

Intraoperative photographs of individualised, anatomical anterior cruciate ligament reconstruction.

(A) Tibial insertion site after debridement of torn remnant; dimensions measured using arthroscopic ruler for (B) sagittal length and (C) coronal width. (D) Femoral insertion site with small residual remnant; dimensions measured using arthroscopic ruler for (E) proximal–distal length and (F) anteroposterior height. Single bundle quadriceps tendon autograft in anatomical position shown through (G) anterolateral, (H) anteromedial, and (I) accessory anteromedial portals.

Each of the potential graft options have their role in the individualised, anatomical ACLR. Autografts of bone–patellar tendon–bone (BPTB), hamstring tendon (HT) and QT with or without bone block are all options with unique advantages and disadvantages ( Table 6.1 ). Allograft overcomes donor site morbidity but is avoided in young, active patients because of higher failure rates. Graft choice is individualised for each patient based on a multitude of factors, including age, preoperative imaging, functional level and sport. Given the large potential graft size available for harvest, QT autograft has the benefit of potentially being able to completely individualise the graft size to any footprint size.


Advantages and Disadvantages of Autograft Sources

Bone–Patellar Tendon–Bone (BPTB) Hamstring Tendon (HT) Quadriceps Tendon (QT)

  • Gold standard in pivoting sport athlete

  • Ease of harvesting

  • Consistency of graft size

  • Rectangular/flat morphology

  • Good cosmesis

  • Minimal anterior knee pain

  • Most common autograft for double bundle ACLR

  • Large volume, permits individualisation

  • Superior functional outcomes (vs. HT)

  • Minimal harvest site morbidity


  • Increased anterior knee pain

  • Increased patellar tendon rupture risk

  • Increased OA risk (vs. HT) a

  • Decreased knee flexion strength

  • Tunnel widening

  • Lower knee stability (vs. BPTB) a

  • Higher rerupture rate (vs. BPTB) a

  • No long-term outcomes (e.g., rates of failure, OA)

  • Delayed recovery of quadriceps strength a

ACLR, Anterior cruciate ligament reconstruction; OA, osteoarthritis.

a Inconsistent finding but of historical concern.

The individualised, anatomical ACLR begins with adequate visualisation, which can be performed well with a three-portal approach. The anterolateral (AL) portal is made at the inferior pole of the patella and the lateral border of the patellar tendon. The anteromedial (AM) and accessory anteromedial (AAM) portals are made under direct arthroscopic visualisation with spinal needle localisation. The AM portal is transpatellar tendon, or just medial to the tendon, aiming toward the central and inferior third of the intercondylar notch. The AAM portal is placed 2 cm medial to the patellar tendon and just above the medial tibial plateau. To avoid iatrogenic chondral injury, it is important to confirm there is at least 2 mm between the spinal needle and the medial femoral condyle. All three portals are used to comprehensively visualise the ACL insertion sites, but for the majority of the procedure, the AL and AM portals are used for visualisation, and the AAM portal is used for instrumentation.

Anatomical tunnel placement is the next key step. Traditional transtibial femoral tunnel drilling has been questioned more recently because multiple studies have shown that it consistently fails to achieve anatomical tunnel placement. , Anteromedial portal drilling has improved anatomical femoral tunnel placement. This has resulted in improved knee kinematics and improved kinematic relationship with the adjacent native stabilising structures. Compared with a more vertical femoral tunnel, typical of a transtibial approach, anatomically positioning the femoral and tibial tunnels produces graft forces and laxity that more closely resemble the native ACL in cadaveric testing.

The centres of the femoral and tibial footprints are next identified with direct arthroscopic visualisation. The femoral footprint is viewed best from the AM and AAM portals, but all portals are used. The centres of the anteromedial and posterolateral ACL bundles are identified by noting the remnant ACL tissue and the lateral intercondylar and lateral bifurcate ridges that mark the borders. , The femoral tunnel is drilled through the AAM portal with arthroscopic visualisation from the AM portal. On the tibial side, the centre of the anteromedial and posterolateral bundles is visualised from the AL portal, and the tibial tunnel guide is placed through the AAM portal. In the majority of cases a single bundle (SB) reconstruction is able to reproduce the native anatomy, but the individualised approach also takes into consideration double bundle (DB) reconstruction and single bundle augmentation.

A variety of graft fixation devices are available, and there is no proven ideal technique. Suspensory devices and metal or biocomposite interference screws are options, with no reported difference in clinical outcomes. For femoral fixation, suspensory fixation for soft tissue grafts and biocomposite interference screws for BPTB grafts are increasingly common. Tibial-sided fixation with interference screws remains common because of the ease of insertion and minimal graft slippage, but suspensory fixation remains an option. There is no definitive consensus on preconditioning, tensioning or knee flexion angle for fixation. In general, SB-ACLR is fixated in 0 to 20 degrees of flexion, in neutral rotation and with maximal manual tension. Fixation in greater flexion risks a loss of full extension postoperatively. An anatomical SB-ACLR with QT autograft is shown in Fig. 6.3G–I .

Outcomes After ACLR

Relevant outcomes for ACLR include both objective measures, such as physical examination grading and quantifiable measures of knee laxity, and subjective measures by which the patient evaluates knee stability and its effect on limb function and quality of life. Graft failure rates ranging from 3% to 25% have been reported after primary ACLR, and return to sport (especially at preinjury level) may not be possible in 10% to 40% of patients, with even lower rates after revision or contralateral ACLR. As mentioned, ACL injury entails a fivefold increase in the relative risk of developing osteoarthritis after 10 years, a risk that may be potentially reduced, but not eliminated, with ACLR. These persistent shortcomings motivate ongoing research to improve treatment strategies for ACL injuries, with numerous studies investigating different surgical approaches and techniques. Of particular interest, numerous studies have investigated the effects of autograft source (e.g., BPTB, HT, QT), drilling technique (i.e., transtibial versus independent drilling) and number of tunnels (i.e., single bundle versus double bundle ACLR), as briefly summarised next.

Graft source

As noted, allograft use is discouraged in young, active patients, given a consistent association with increased retear rates, but is a viable option in older patients, with similar outcomes to autograft use. Depending on surgeon preference, patient characteristics and geographical location, either BPTB or HT autograft is the most commonly used graft for primary ACL reconstruction. The bone blocks of the BPTB graft have been found to integrate with the bone tunnel faster than HT, and BPTB is accordingly associated with decreased tunnel widening. , On the other hand, BPTB use has been reported to increase postoperative complications such as anterior knee pain, kneeling discomfort and extension deficit, as well as osteoarthritis prevalence at long-term follow-up, compared with HT autografts. , However, a meta-analysis of RCTs comparing long-term OA prevalence after ACLR with BPTB versus HT autograft found no significant difference. Uncertainty also exists regarding the superiority of the BPTB autograft in mitigating residual laxity (as commonly measured by KT-1000 arthrometer), with inferior instrumented laxity after ACLR with HT autograft shown in an RCT, but not in some systematic reviews. , Although BPTB has been suggested to result in lower graft retear rates as evidenced by one meta-analysis, multiple other high-level studies have consistently reported no difference in graft retear rates.

The respective limitations of BPTB and HT autografts, real or perceived, have promoted the exploration for alternative graft sources to further reduce the risk of retear and postoperative complications while facilitating return to preinjury activity levels. The QT is the thickest autograft option in the ipsilateral knee, can be reliably harvested and mitigates the possibility of insufficient graft size, which would in turn necessitate augmentation with allograft. Although use of QT autograft has increased, meta-analyses of studies comparing outcomes after ACLR with QT versus BPTB or HT autografts have reported promising preliminary findings. Namely, QT use resulted in lower anterior knee pain than BPTB and better functional outcomes (i.e., Lysholm scores) compared with HT autograft. , There are concerns of delayed quadriceps strength recovery and residual strength deficits after ACLR with QT autograft compared with BPTB or HT autograft, yet a cohort study found no differences between QT and BPTB groups in terms of quadriceps recovery and postoperative outcomes at a mean time from surgery of 8 months. It has also been hypothesised that the use of full-thickness QT autograft, compared with partial-thickness graft, may contribute to delayed quadriceps recovery and poorer outcomes, but a systematic review found no effect of QT graft thickness on outcomes. Given the relatively recent implementation of QT autograft use, long-term studies on OA prevalence are not available. Additionally, because studies comparing outcomes with different graft types have been performed concomitantly with an evolution in drilling techniques and number of tunnels (i.e., SB versus DB-ACLR), conclusions regarding the superiority of a particular graft should be made with these potential confounders in mind.

Drilling technique

As an increased focus on native ACL anatomy and function has developed, surgical techniques have evolved in an effort to achieve anatomical ACL reconstruction ; that is, the restoration of the ACL to its native dimensions, collagen orientation and insertion sites according to individual anatomy. Because transtibial drilling inconsistently placed the femoral tunnels within the native femoral ACL footprint, , independent drilling of the tibial and femoral tunnels has been increasingly employed. Despite promising results in in vitro cadaveric studies, few studies have compared in vivo kinematics of knees after ACLR with transtibial versus independent drilling, but the available literature suggests enhanced kinematic restoration with independent drilling, in turn preventing cartilage loss at short-term follow-up. , Similarly, two meta-analyses comparing studies examining outcomes after ACLR with transtibial versus anteromedial drilling technique found that the latter promoted superior stability, as evaluated by objective International Knee Documentation Committee (IKDC) grading, Lachman test and pivot shift test. , However, patient-reported outcomes were largely equivalent regardless of the technique.

In contrast to the noted improvements in functional recovery promoted by independent tunnel drilling, several studies, including a summary of outcomes in the Danish Knee Ligament Reconstruction Registry, found an increased graft failure rate with transportal (i.e., independent) drilling. On the other hand, the MOON group reported increased odds of repeat ipsilateral knee surgery with transtibial drilling. Interestingly, when the relative risk of revision ACLR in Danish registry patients was compared between the years of 2007–10 and 2012–15, the increased failure rate in patients undergoing ACLR with transportal drilling noted in the earlier period was not found in the later period, suggesting a surgical learning curve.

Further confounding the studies comparing ACLR techniques is the consistent underreporting of specific surgical details that are necessary for valid comparisons. In an effort to standardise reporting criteria for ACLR, in turn facilitating comparison across surgical techniques, van Eck et al. developed and validated the Anterior Cruciate Ligament Reconstruction Checklist (AARSC) ( Table 6.2 ). Unfortunately, a subsequent systematic review of studies investigating DB or SB-ACLR found consistent underreporting of surgical details when scored with the AARSC. As before, this study concluded that underreporting creates difficulties when analysing, comparing and pooling results of scientific studies on ACLR; consequently, standardised reporting with the AARSC is highly encouraged.


Anatomical ACL Reconstruction Scoring Checklist (AARSC)

AARSC Item Points

  • 1.

    Individualisation of surgery for each patient


  • 2.

    Use of a 30-degree arthroscope


  • 3.

    Use of an accessory medial portal


  • 4.

    Direct visualisation of the femoral ACL insertion site


  • 5.

    Measuring the femoral ACL insertion site dimensions


  • 6.

    Visualising the lateral intercondylar ridge


  • 7.

    Visualising the lateral bifurcate ridge


  • 8.

    Placing the femoral tunnel(s) in the femoral ACL insertion site


  • 9.

    Transportal drilling of the femoral ACL tunnel(s)


  • 10.

    Direct visualisation of the tibial ACL insertion site


  • 11.

    Measuring the tibial ACL insertion site dimensions


  • 12.

    Placing the tibial tunnel(s) in the tibial ACL insertion site


  • 13.

    Documenting the femoral fixation method


  • 14.

    Documenting the tibial fixation method


  • 15.

    Documenting knee flexion angle during femoral tunnel drilling


  • 16.

    Documenting graft type


  • 17.

    Documenting knee flexion angle during femoral tunnel drilling


  • 18.

    Highest level of documentation used for ACL tunnel position

a. Drawing, diagram, operative note, dictation, or clock face reference 0
b. Arthroscopic pictures, radiographs, 2D MRI or 2D CT 1
c. 3D MRI, 3D CT or navigation 2

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May 3, 2021 | Posted by in ORTHOPEDIC | Comments Off on Anterior Cruciate Ligament
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