Anterior cruciate ligament (ACL) injuries


The anterior cruciate ligament is a critical ligament in the center of the knee that allows the knee to work like a hinge. It is very commonly injured in sports injuries rendering the victim unstable and incapable of continuing high level contact and direction changing or cutting  sport. Thankfully, ACL injuries are reliably reconstructed by modern arthroscopic techniques with an expectation of return of high level of function and return to contact sport.


The anterior cruciate ligament (ACL) inserts in the from of the tibial plateau (hence anterior, Figure 1) and is named after the cross in makes with the posterior cruciate ligament in the center of the knee (Figure 2). It is a critical ligament in the stability of the knee. Without it the knee articulates poorly resulting in secondary damage in the knee to other structures like the menisci.

Figure 1. The anterior cruciate (cross-like) ligament is a ligament in the centre of the knee that though technically originates from the rear of the femur gets its name from its attachment to the front or anterior of the tibia (arrowed in left panel). It is commonly reconstructed with hamstring tendons in the event of injury (arrowed in right panel). Note the proximity of minor sensory nerves to this tendon. The rate of minor injury to these nerves resulting in sensory disturbance is high (up to 60%). It does not however result in paralysis.

Figure 2. The anterior cruciate ligament is a broad ligament that functionally has three components (a). That is to say anatomically and certainly in younger individuals it is impossible to dissect out "three bundles". In reality it exists in nature as a broad ribbon-like structure about 1 cm wide and 3 cm long and crosses the posterior cruciate ligament (b).  The three functional bundles are the anteromedial (A), intermediate (I) and posterolateral (P) bundles. As the knee goes into flexion (c), the ligament folds or twists on itself such that the whole complex tightens in flexion. The anteromedial bundle works best in flexion whereas the postrolateral bundle works best in extension. The isometric point of the ACL is anterior and superior to the ACL origin (blue circle in c). This points represents the point where the ACL would originate if it is composed of one band. It is the basis of most techniques of reconstruction advocated by the majority of surgeons who perform this surgery on a regular basis.


Apart from minor variation, ACL surgery through arthroscopic techniques have become very routine and are a part of basic orthopaedic training. Modern variations like double bundle repairs have failed to show clearly superior benefit. The graft is obtained from the patients own tissue (see graft materials below) and prepared. A tunnel is measured and secured in the tibia and then in the femur. The graft is advanced into the tunnel and secured and both ends (Figure 3).

Figure 3. Arthroscopically and certainly in young patients, it is impossible to determine three bands of the ACL. In reality they exist as functional components. There can be associated meniscal and patello-femoral disorders which should be repaired at the same time. In general all modern techniques of athroscopic repair involve harvesting a graft (in this case the hamstrings), drilling a tunnel in the tibia and femur and passing the graft through. The graft is then held in place with screws, loops or pins. There is no evidence of any superiority between different fixation techniques.

Graft materials

Bone patella tendon bone

In this procedure the middle slip of bone from the patella, tendon from the patella tendon (more correctly ligament) and bone from tibia is used. It is considered the gold standard in care. The composite is about 2 times as strong as the patient's ACL. The biggest advantage is it heals from bone to bone on either end. The defect also often heals over and can be used for another donation later in recurrent cases. The biggest problem is that pain in the knee cap may result. Personally, howver, in an experience f over a hundred of these cases over more than 10 years I have not seen this to be a big problem. Patient selection may be an issue as we would not offer this to a patient with anterior knee pain.

Hamstring tendons

The gracilis and semitendinosis is harvested. These are very close to minor nerves and can cause sensory disturbance in up to 60% of patients (Figure 1) although our personal experience of this is much lower. A double loop of both tendons (ie. four slips) three times stronger than the patient's ACL. There is some evidence that the donor site defect will fill in and the tendon will "regenerate". This is called the lizard tail sign. It is improbable that the graft can be re-harvested after. Weakness after harvest is negligible.


These are chiefly used in revision cases when the previous graft repair has ruptured. Studies suggest they are less resilient than the patient's own tissue but in the given circumstance of reconstruction when a previous repair has failed and there is no more donor tissue this is a reasonable alternative. Disease transmission remains the most important (if over-rated) concern

Synthetic materials

Everything from carbon fibre, to metal meshes to plastics have been devised for ACL reconstructions. Initially these were thought to give good results in the knee but long term use has shown that they will not last as they are subject to erosion of material property (wear) without repair. Nevertheless, in the highly competitive sporting world where the ability of a team to win or lose is dependent on a star player, these materials may give just enough resilience for say a season.


Double bundle versus single bundle repairs

The idea here is that the ACL is a broad ligament with two functional components and if the graft could be secured in such a way to reproduce this (Figure 2) the result would be a better repair. Unfortunately, this premise ignores the fact that there is a third , intermediate bundle that bridges the 2 and makes it a broad ribbon like structure. Therefore as the knee flexes, the ACL through the intermediate bundle turns in or twists on itself. Most authors conclude that this is a functional division at the edges of the ACL and not "two bundles" per se. In fact in most healthy young patients there is no such thing as two bundles (Figure 2). The Cochrane review has recently determined that most studies that prove this are biased and  "there is insufficient evidence to determine the relative effectiveness of double-bundle and single-bundle reconstruction for anterior cruciate ligament rupture in adults, although there is limited evidence that double-bundle ACL reconstruction has some superior results in objective measurements of knee stability (ie. rotation) and protection against repeat ACL rupture or a new meniscal injury". In the long term, properly designed trials are not likely to show a clear benefit of the procedure. We had evaluated the procedure between 2005 and 2008 and did not find any difference. As a result until better evidence becomes available we do not feel that the added operative time and complications posed by the procedure is justifiable. Unlike single bundle (4 strand) repairs, theoretically there is only one bundle of 2 strands resisting anterior strain in flexion or extension as opposed to all 4 strands in flexion and extension. A recent study of 60 cases with 30 of single and 30 of double bundle reconstruction showed no difference between the two.

Figure 4. Double-bundle reconstructions are typically impractical to perform in Asian patients with ACL ruptures (a). Recommendations are for the the AM graft to be generally made slightly larger than the PL graft. The AM bundle uses the doubled or tripled semitendinosus graft, resulting in a 7–9 mm graft diameter. The PL bundle uses the doubled or tripled gracilis graft,resulting in a 5–7 mm graft diameter.A minimal graft length of 26 cm for the gracilis and 28 cm for the semitendinosus tendon are recommended for double or triple preparation. In practical terms, the entire four strands of 2 grafts doubled (gracilis and semitendinosus) usually measures 8 mm in diameter and 18cm in length in the local setting (!). To circumvent these size issues in Asians, we create 2 tibial tunnels (b) and one screw-based femoral fixation point (c) which allows a double-bundle construct in a smaller footprint (d). This compares favorably to anatomical drawings (inset d).

Fixation techniques

On the femoral side, endo-button loops, screws and cross pins have all been used. There is no evidence of any superiority of one technique over the other. Studies have shown that femoral side graft fixations are not at high risk of pulling out and this is the likely reason for the fixation there to be immaterial. On the tibial side secure fixation is however critical. Screws are used at the minimum but supplemental fixation with sutures around a post or staples have been used.

Prevention of arthitis

There is no good evidence to show that ACL reconstructions prevent subsequent arthritis in the knee.

Secondary damage

ACL reconstruction does prevent secondary injury to the menisci.


Return to pre-injury levels are the norm. In fact many patients will actually do better as they are now made to retrain the knees as they return to sport. It is critical that some form of physical therapy regime is used to guide the patient through the first year after surgery. We would usually issue this to the patient who would work with the therapist to achieve these goals (Figure 4). Most of our patients return to contact sport after one year though professional athletes may do it in 6 months.

Figure 5. Typical example of ACL protocol that should be given to patients undergoing post-ACL surgery rehabilitation. It is key that the therapist works in conjunction with the patient and surgeon in achieving the patient's required milestones before going to the next step.


The results of ACL surgery is very satisfying for patient and doctor alike. Few procedures can give the sort of reproducible results with return to high levels of activities that this surgery brings. 

Figure 6 (video). ACL reconstruction can be done using single bundle and double bundle techniques (provided the donor tendon is big enough). In this case the patient had a medial meniscus tear together with an ACL rupture. A double bundle reconstruction and repair of meniscus was planned for but given the small size of the tendon we elected to do a single bundle repair and meniscus repair (not shown but an example of meniscus repair is seen here).