Sam Oussedik and Fares Haddad recap this classic sports injury and report on promising research developments.
The anterior cruciate ligament (ACL) is an intra-articular knee ligament. It is made up of two bundles of fibrous tissue, each 3.5cm long, which act to maintain anatomical relations between the femur and tibia throughout the range of knee motion.
It is a primary restraint to anterior tibial translation, that is, it stops the tibia from sliding forwards against the femur; and a secondary restraint to varus-valgus (side to side) forces and tibial rotation. While its specialised fibres have a relatively good blood supply, complete ruptures of the ligament have virtually no capacity to heal in their normal anatomical position.
The ACL runs from the back of the femur to the front of the tibia, from lateral to medial. It therefore opposes forces that would otherwise displace the tibia anteriorly, those that cause sideways displacement and/or rotation.
Serious injury to the ACL is common: 0.38 ruptures per 1,000 population in north America (1). Despite this relatively high prevalence, primary clinicians do not always detect rupture, and sports support professionals should be alert to the possibility in patients with a relevant history of knee trauma.
In sporting contexts, ACL rupture will typically occur in athletes who suffer a sudden twisting injury, such as the basketball player who suddenly changes direction with the tibia fixed in internal rotation; the football or rugby player who gets their boot stuck in the turf and suffers a flexion, valgus or external rotation injury; the skier whose binding does not release; and the athlete who lands awkwardly, hyperextending the knee (2).
The patient suffering a traumatic disruption of his or her ACL will present soon after the initial trauma, complaining of a swollen, painful knee. Tears to the relatively vascular ACL cause an accumulation of blood within the knee joint, (haemarthrosis). If patients fail to present at this early stage, they may present later complaining of the knee ‘giving way’ or feeling unstable.
Clinical examination focuses on diagnosing the cause of instability, and assessing the knee for other associated injuries. In the early phase the knee will be tight and swollen. It may be necessary to aspirate the knee to draw off excessive build-up of fluid, and severe swelling at this stage may preclude further examination. Once the swelling is down, it will be possible to do specific tests for ACL integrity, including:
These tests are deemed positive if the anterior tibial translation is excessive (considerable laxity) compared with the uninjured knee. The quality of the endpoint of translation is also important to assess: a ruptured ligament lacks normal firmness.
X-ray will help to rule out any associated bony injury, such as a fracture through the tibial insertion of the ACL. Magnetic resonance imaging is particularly useful in assessing a knee with multiple injuries and knees for which a clinical diagnosis is not clear.
People can get by without surgery on a ruptured ACL if they do not make heavy demands on their legs, such as strenuous sporting activity. Their management should involve strengthening local muscle groups and regaining range of motion to provide adequate knee stability.
However, there is evidence that being ACL deficient in the long term may predispose people to further meniscal damage and early degenerative changes within the knee (3). Physiotherapy plays a vital role in both prehabilitation prior to reconstruction, regaining range of motion and quadriceps control, and rehabilitation after surgery.
Operative treatment is offered to those patients who continue to be symptomatic, or those who have a high functional demand. ACL reconstruction is now an accepted treatment with a high success rate. Surgery involves placing a graft of ligament or tendon through tunnels drilled in the tibia and femur, reconstructing the ruptured ligament.
A number of different graft types exist, ranging from the patient’s own hamstring tendons, a section of their patellar or quadriceps tendon or fascia lata, to cadaveric grafts. Artificial materials have been used, but these are usually reserved for repeat surgery where other sources have already been exhausted.
Although this is a highly successful procedure, it is technically challenging. The bony tunnels must be placed in precisely the correct positions to allow the reconstructed ligament to fulfil all the functions of the original ACL.
Bracing after the operation is not usually necessary unless there are associated injuries, such as a medial collateral ligament rupture.
It is vital that prior to reconstruction the patient is made aware of the importance of adhering to post-operative physiotherapy regimes. This optimises a successful return to full function and activity – usually within nine months.
Recent research has focused on different ways of improving patient outcomes after ACL injury. As the native ACL has a double bundle structure, some surgeons have attempted to recreate this by using two-bundle reconstructions, using two sets of distinct tibial and femoral tunnels. The two bundles of the native ACL respond differently to stresses at different degrees of flexion, combining to confer stability. In theory, therefore, the two- bundle reconstruction should improve stability through the full range of motion (4). So far, little evidence exists to confirm this, but several ongoing clinical studies will clarify the situation.
Other research has looked at the way in which the graft is fixed in position. There are two main methods for this, using either screws or a suspensory device to fix the graft in its tunnel. The method of fixation should be sufficiently reliable to hold the graft securely while it heals into its bony tunnel. There remains little evidence to support the use of one method over another, so it remains up to the surgeon’s preference (5).
The use of operative robotic guidance systems has been shown to improve the reliability of tunnel placement and therefore gives better results (6).
New work is also being done on tissue engineering. Pascher et al (7) have shown that introducing genes that help ligament cells to migrate across a gap of scar tissue can lead to ligament regeneration. This work is at the laboratory testing stage. Lu et al (8) have used stem cells seeded on to a bioabsorbable scaffold to reconstruct the ACL, causing the graft to be generated from the patient’s own progenitor cells rather than having to be harvested from elsewhere. This technique is showing encouraging results at the in vitro stage. Stem cells have also been used to improve the way in which a traditional graft heals into its bony tunnels. Lim et (9) have shown that coating the graft in stem cells prior to positioning it helps it to grow more securely into the surrounding bone.
Other researchers are studying the long-term health of both cruciate- deficient and cruciate-reconstructed knees. Long-term cruciate deficiency is associated with an increased chance of developing osteoarthritis (OA). While little evidence exists to support a direct link, it has been shown that in athletes, cruciate deficiency leads to an increased likelihood of meniscal injury, which in turn is associated with the onset of OA (3).
Recent evidence supports the idea that the trauma which leads to ACL injury may be the trigger for developing OA. Lohmander et al (10) studied 84 female football players who had sustained an ACL injury 12 years previously. Of this group 51% had signs and symptoms of OA, and reconstruction appeared not to confer any protection. However, this study was limited in both the number of subjects and the way they were assessed. Patients who undergo ACL reconstruction tend to be the more active and hence may well damage their knees more than people who limit their activities and do not undergo reconstruction.
Sam Oussedik is clinical and research fellow in orthopaedics at university college London. His primary interest is football
Fares Haddad BSc MCh (Orth) FRCS (Orth) is a consultant orthopaedic surgeon at University College London Hospital and editorial consultant to Sports Injury Bulletin
Illustrations by Viv Mullett