The anterior cruciate ligament (ACL) is one of a pair of
cruciate ligament
Cruciate ligaments (also cruciform ligaments) are pairs of ligaments arranged like a letter X. They occur in several joints of the body, such as the knee joint and the atlanto-axial joint. In a fashion similar to the cords in a toy Jacob's la ...
s (the other being the
posterior cruciate ligament
The posterior cruciate ligament (PCL) is a ligament in each knee of humans and various other animals. It works as a counterpart to the anterior cruciate ligament (ACL). It connects the posterior intercondylar area of the tibia to the medial cond ...
) in the
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
knee
In humans and other primates, the knee joins the thigh with the leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest joint in the hu ...
. The two ligaments are also called "cruciform" ligaments, as they are arranged in a crossed formation. In the
quadruped
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
stifle joint
The stifle joint (often simply stifle) is a complex joint in the hind limbs of quadruped mammals such as the sheep, horse or dog. It is the equivalent of the human knee and is often the largest synovial joint in the animal's body. The stifle joint ...
(analogous to the knee), based on its
anatomical position
The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with r ...
, it is also referred to as the cranial cruciate ligament.
The term cruciate translates to cross. This name is fitting because the ACL crosses the posterior cruciate ligament to form an “X”. It is composed of strong, fibrous material and assists in controlling excessive motion. This is done by limiting mobility of the joint. The anterior cruciate ligament is one of the four main
ligament
A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the:
* Peritoneal li ...
s of the knee, providing 85% of the restraining force to
anterior tibial displacement at 30 and 90° of knee flexion. The ACL is the most injured ligament of the four located in the knee.
Structure
The ACL originates from deep within the notch of the
distal femur. Its proximal fibers fan out along the medial wall of the lateral
femoral condyle
The lower extremity of femur (or distal extremity) is the lower end of the femur (thigh bone) in human and other animals, closer to the knee. It is larger than the upper extremity of femur, is somewhat cuboid in form, but its transverse diameter i ...
The two bundles of the ACLare the anteromedial and the posterolateral, named according to where the bundles insert into the tibial plateau. The tibial plateau is a critical weight-bearing region on the
upper extremity of the tibia. The ACL attaches in front of the
intercondyloid eminence
The intercondylar area is the separation between the medial and lateral condyle on the upper extremity of the tibia. The anterior and posterior cruciate ligaments and the menisci attach to the intercondylar area.
The intercondyloid eminence i ...
of the tibia, where it blends with the anterior horn of the
medial meniscus
The medial meniscus is a fibrocartilage semicircular band that spans the knee joint medially, located between the medial condyle of the femur and the medial condyle of the tibia.Platzer (2004), p 208 It is also referred to as the internal semi ...
.
Purpose
The purpose of the ACL is to resist the motions of anterior tibial translation and internal tibial rotation; this is important to have rotational stability.
This function prevents anterior tibial
subluxation
A subluxation is an incomplete or partial dislocation of a joint or organ.
According to the World Health Organization (WHO), a subluxation is a "significant structural displacement", and is therefore always visible on static imaging studies, suc ...
of the lateral and medial tibiofemoral joints, which is important for the
pivot-shift phenomenon.
The ACL has
mechanoreceptors that detect changes in direction of movement, position of the knee joint, and changes in acceleration, speed, and tension.
A key factor in instability after ACL injuries is having altered neuromuscular function secondary to diminished somatosensory information.
For athletes who participate in sports involving cutting, jumping, and rapid deceleration, the knee must be stable in terminal extension, which is the
screw-home mechanism.
Clinical significance
Injury
![VKB-Riss MRT T1 PDW sag](https://upload.wikimedia.org/wikipedia/commons/b/bd/VKB-Riss_MRT_T1_PDW_sag.jpg)
An ACL tear is one of the most common knee injuries, with over 100,000 tears occurring annually in the US. Most ACL tears are a result of a non-contact mechanism such as a sudden change in a direction causing the knee to rotate inward. As the knee rotates inward, additional strain is placed on the ACL, since the femur and tibia, which are the two bones that articulate together forming the knee joint, move in opposite directions, causing the ACL to tear. Most athletes require reconstructive surgery on the ACL, in which the torn or ruptured ACL is completely removed and replaced with a piece of tendon or ligament tissue from the patient (
autograft
Autotransplantation is the transplantation of organs, tissues, or even particular proteins from one part of the body to another in the same person ('' auto-'' meaning "self" in Greek).
The autologous tissue (also called autogenous, autogen ...
) or from a donor (
allograft
Allotransplant (''allo-'' meaning "other" in Greek) is the transplantation of cells, tissues, or organs to a recipient from a genetically non-identical donor of the same species. The transplant is called an allograft, allogeneic transplant, o ...
).
Conservative treatment has poor outcomes in ACL injury, since the ACL is unable to form a fibrous clot, as it receives most of its nutrients from synovial fluid; this washes away the reparative cells, making the formation of fibrous tissue difficult. The two most common sources for tissue are the patellar ligament and the hamstrings tendon. The patellar ligament is often used, since bone plugs on each end of the graft are extracted, which helps integrate the graft into the bone tunnels during reconstruction. The surgery is arthroscopic, meaning that a tiny camera is inserted through a small surgical cut.
The camera sends video to a large monitor so the surgeon can see any damage to the ligaments. In the event of an autograft, the surgeon makes a larger cut to get the needed tissue. In the event of an allograft, in which material is donated, this is not necessary, since no tissue is taken directly from the patient's own body.
The surgeon drills a hole forming the tibial bone tunnel and femoral bone tunnel, allowing for the patient's new ACL graft to be guided through.
Once the graft is pulled through the bone tunnels, two screws are placed into the tibial and femoral bone tunnel.
Recovery time usually ranges between one and two years, but is sometimes longer, depending if the patient chose an autograft or allograft. A week or so after the occurrence of the injury, the athlete is usually deceived by the fact that he/she is walking normally and not feeling much pain.
This is dangerous, as some athletes start resuming some of their activities such as jogging, which with a wrong move or twist, could damage the bones, as the graft has not completely become integrated into the bone tunnels. Injured athletes must understand the significance of each step of an ACL injury to avoid complications and ensure a proper recovery.
Nonoperative treatment of the ACL
ACL reconstruction is the most common treatment for an ACL tear, but it is not the only treatment available for individuals. Some may find it more beneficial to complete a nonoperative rehabilitation program. Individuals who are going to continue with physical activity that involves cutting and pivoting, and individuals who are no longer participating in those specific activities both are candidates for the nonoperative route.
In comparing operative and nonoperative approaches to ACL tears, few differences were noted between surgical and nonsurgical groups, with no significant differences in regard to knee function or muscle strength reported by the patients.
The main goals to achieve during rehabilitation (rehab) of an ACL tear is to regain sufficient functional stability, maximize full muscle strength, and decrease risk of reinjury. Typically, three phases are involved in nonoperative treatment - the acute phase, the neuromuscular training phase, and the return to sport phase. During the acute phase, the rehab is focusing on the acute symptoms that occur right after the injury and are causing an impairment. The use of therapeutic exercises and appropriate therapeutic modalities is crucial during this phase to assist in repairing the impairments from the injury. The neuromuscular training phase is used to focus on the patient regaining full strength in both the lower extremity and the core muscles. This phase begins when the patient regains full range of motion, no effusion, and adequate lower extremity strength. During this phase, the patient completes advanced balance,
proprioception
Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons ...
, cardiovascular conditioning, and neuromuscular interventions.
In the final, return to sport phase, the patient focuses on sport-specific activities and agility. A functional performance brace is suggested to be used during the phase to assist with stability during pivoting and cutting activities.
Operative treatment of the ACL
Anterior cruciate ligament surgery is a complex operation that requires expertise in the field of orthopedic and
sports medicine
Sports medicine is a branch of medicine that deals with physical fitness and the treatment and prevention of injuries related to sports and exercise. Although most sports teams have employed team physicians for many years, it is only since the ...
. Many factors should be considered when discussing surgery, including the athlete's level of competition, age, previous knee injury, other injuries sustained, leg alignment, and graft choice. Typically, four graft types are possible, the bone-patella tendon-bone graft, the semitendinosus and gracilis tendons (quadrupled hamstring tendon), quadriceps tendon, and an allograft.
Although extensive research has been conducted on which grafts are the best, the surgeon typically chooses the type of graft with which he or she is most comfortable. If rehabilitated correctly, the reconstruction should last. In fact, 92.9% of patients are happy with graft choice.
Prehabilitation
Preoperative rehabilitation, prehabilitation or prehab, is a form of healthcare intervention that takes place before a medical or surgical intervention with the aim to reduce side effects and complications, and enhance recovery. Multidisciplinary ...
has become an integral part of the ACL reconstruction process. This means that the patient exercises before getting surgery to maintain factors such as range of motion and strength. Based on a single leg hop test and self-reported assessment, prehab improved function; these effects were sustained 12 weeks postoperatively.
Postsurgical rehabilitation is essential in the recovery from the reconstruction. This typically takes a patient 6 to 12 months to return to life as it was prior to the injury.
The rehab can be divided into protection of the graft, improving range of motion, decrease swelling, and regaining muscle control.
Each phase has different exercises based on the patients' needs. For example, while the ligament is healing, a patient's joint should not be used for full weight-bearing, but the patient should strengthen the quadriceps and hamstrings by doing quad sets and weight shifting drills. Phase two would require full weight-bearing and correcting gait patterns, so exercises such as core strengthening and balance exercises would be appropriate. In phase three, the patient begins running, and can do aquatic workouts to help with reducing joint stresses and cardiorespiratory endurance. Phase four includes multiplanar movements, thus enhancing a running program and beginning agility and
plyometric
Plyometrics, also known as jump training or plyos, are exercises in which muscles exert maximum force in short intervals of time, with the goal of increasing power (speed-strength). This training focuses on learning to move from a muscle extensi ...
drills. Lastly, phase five focuses on sport- or life-specific motions, depending on the patient.
A 2010 ''Los Angeles Times'' review of two medical studies discussed whether ACL reconstruction was advisable. One study found that children under 14 who had ACL reconstruction fared better after early surgery than those who underwent a delayed surgery. For adults 18 to 35, though, patients who underwent early surgery followed by rehabilitation fared no better than those who had rehabilitative therapy and a later surgery.
The first report focused on children and the timing of an ACL reconstruction. ACL injuries in children are a challenge because children have open growth plates in the bottom of the
femur
The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...
or thigh bone and on the top of the
tibia
The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
or shin. An ACL reconstruction typically crosses the growth plates, posing a theoretical risk of injury to the growth plate, stunting leg growth, or causing the leg to grow at an unusual angle.
The second study noted focused on adults. It found no significant statistical difference in performance and pain outcomes for patients who receive early ACL reconstruction vs. those who receive physical therapy with an option for later surgery. This would suggest that many patients without instability, buckling, or giving way after a course of rehabilitation can be managed nonoperatively, but was limited to outcomes after two years and did not involve patients who were serious athletes.
Patients involved in sports requiring significant cutting, pivoting, twisting, or rapid acceleration or deceleration may not be able to participate in these activities without ACL reconstruction.
ACL injuries in women
Risk differences between outcomes in men and women can be attributed to a combination of multiple factors, including anatomical, hormonal, genetic, positional, neuromuscular, and environmental factors. The size of the anterior cruciate ligament is often the most reported difference. Studies look at the length, cross-sectional area, and volume of ACLs. Researchers use cadavers, and ''in vivo'' placement to study these factors, and most studies confirm that women have smaller anterior cruciate ligaments. Other factors that could contribute to higher risks of ACL tears in women include patient weight and height, the size and depth of the intercondylar notch, the diameter of the ACL, the magnitude of the tibial slope, the volume of the tibial spines, the convexity of the lateral tibiofemoral articular surfaces, and the concavity of the medial tibial plateau. While anatomical factors are most talked about, extrinsic factors, including dynamic movement patterns, might be the most important risk factor when it comes to ACL injury.
Environmental factors also play a big role. Extrinsic factors are controlled by the individual. These could be strength, conditioning, shoes, and motivation.
Gallery
Image:Gray347.png, Right knee joint, from the front, showing interior ligaments
Image:Gray348.png, Left knee joint from behind, showing interior ligaments
Image:Gray349.png, Head of right tibia seen from above, showing menisci and attachments of ligaments
Image:Gray352.png, Capsule of right knee-joint (distended), posterior aspect
File:MRT ACL PCL 01.jpg, MRI shows normal signal of both cruciate ligaments (arrows)
File:Slide1wewe.JPG, Knee joint, deep dissection, anteromedial view
See also
*
Posterior cruciate ligament
The posterior cruciate ligament (PCL) is a ligament in each knee of humans and various other animals. It works as a counterpart to the anterior cruciate ligament (ACL). It connects the posterior intercondylar area of the tibia to the medial cond ...
*
Anterior cruciate ligament reconstruction
*
Anterior drawer test
*
Anterolateral ligament
The anterolateral ligament (ALL) is a ligament on the lateral aspect of the human knee, anterior to the fibular collateral ligament.
Perhaps the earliest account of the ALL was written by French surgeon Paul Segond in 1879, in which he describe ...
*
Lateral collateral ligament
*
Medial collateral ligament
The medial collateral ligament (MCL), or tibial collateral ligament (TCL), is one of the four major ligaments of the knee. It is on the medial (inner) side of the knee joint in humans and other primates. Its primary function is to resist out ...
*
Unhappy triad
The unhappy triad, also known as a blown knee among other names, is an injury to the anterior cruciate ligament, medial collateral ligament, and meniscus. Analysis during the 1990s indicated that this 'classic' O'Donoghue triad is actually an un ...
References
External links
* - "Extremity: Knee joint"
* - "Superior view of the tibia."
* - "Medial and lateral views of the knee joint and cruciate ligaments."
* ()
{{DEFAULTSORT:Anterior Cruciate Ligament
Knee ligaments