The knee joint is an anatomically and biomechanically very complex joint that plays a major role in all physical activity and sports. The knee joint is the joint that suffers the most injuries in sport, even if reliable incident numbers are lacking. At an ordinary Sports Medicine clinic, of all patients, patients with injured knees represent between 30 and 50%. The most commonly injured structures in the knee that athletes seek a physicians’s advice for are: anterior cruciate ligament (ACL),
the medial (inner) meniscus and the medial (inner) collateral ligament (MCL), which together account for over two-thirds of all traumatic knee injuries in sport.
Other structures which may be injured include lateral (outer) meniscus, lateral (outer) collateral ligament (LCL) and posterior cruciate ligament (PCL). Articular cartilage is often damaged simultaneously with ligament and meniscus injuries, but can also be injured in isolation.
Rupture of the ACL is serious and causes the loss of many training and competition sessions. An ACL injury remains the single biggest problem in Sports Medicine traumatology,i.e.orthopedic Sports Medicine.The annual incidence of ACL injuries is high: 81–85 per 100,000 people aged 10–64 years old.
On top of this is a number of overload injuries, of which the most common are patellofemoral pain syndrome (PFPS) and chronic changes (degeneration) of the patellar tendon (patellar tendinopathy.)
Most of these injuries are caused by extreme stress due to twisting and turning motions in sports such as skiing and football/soccer, which in northern Europe together account for half of all knee injuries. Other contact sports such as team handball, basketball, American football, etc. also have a high number of knee injuries, especially ACL injuries .
Female athletes at a young age have about 2–3 times more ACL injuries than men in sports such as football/soccer, basketball and handball. The greatest incidence of ACL injuries is in recreational downhill skiers, while professional downhill skiers have among the lowest, and no gender difference exists. MCL and meniscus injuries occur in sports such as judo, gymnastics and the like.
Functional anatomy and biomechanics
The knee joint connects the thigh bone (femur) and shin bone (tibia).The knee cap (patella) is located in the patellar tendon and glides during knee movements in a groove (sulcus of the trochlea) on the femur.The patella provides a biomechanical advantage for the four-headed thigh muscle (quadriceps) to extend the knee joint.
The contact surfaces of the three bones are lined by articular cartilage, and the joint is surrounded by a fibrous joint capsule with a synovial membrane towards the joint cavity. The stability of the joint is maintained by four strong ligaments: the MCL, the LCL, the ACL and the PCL.The MCL and LCL prevent side-to-side motion, while the ACL and PCL restrict abnormal front and back movements.Together they prevent the knee from hyperextension and hyperflexion, as well as rotation.
The fibrous joint capsule also contributes to stability. Excessive torsion forces on these ligaments can cause them to rupture. The MCL and ACL are often injured together.The surgical methods for repair of these injuries are constantly improving, allowing the athletes often to resume their sport after treatment. Injuries to the PCL and LCL are more difficult to treat, especially if they include the fibrous joint capsule and other structures to the back of the knee.
Medical history and examination
Grading
Ligament injuries to the knee can be graded according to the severity of the injury, into three grades:
●● Grade I: there is tearing within the microstructure but no obvious elongation of the ligament.
●● Grade II: the ligament is stretched and there is a partial tear.
●● Grade III: a complete tear causing the ligament to separate.
A thorough examination of a knee injury is essential for a correct diagnosis.
Medical history
The medical history should include an analysis of the injury mechanism. A perception of the magnitude of force and the direction of impact at the moment of injury are important factors in the severity and type of injury.
Inspection of the injured area
There may be swelling around as well as within the joint. Discoloration over the ligament or along its path indicates bleeding and a ligament injury. In cases of effusion the swelling usually extends above the patella. The examiner can establish whether such an effusion is present by pressing the area above and below the patella with both hands while at the same time pressing the patella toward the femur with the thumb of one hand .
When there is an effusion the patella meets a spongy resistance that ceases when the articular surface of the patella is compressed against the femur. When the pressure is released the patella can be seen to rise again because of the underlying fluid.
Palpation
The examining physician should palpate the joint lines of the knee, checking for tenderness that may indicate a meniscus injury.The palpation should continue over the course of the collateral ligaments noting the location of tenderness, since this may be the injury site. Swelling caused by an effusion can be felt.
Testing the range of motion
The examining physician should look for restriction of extension and flexion of the knee; however, the patient rather than the examiner should move the knee in order to control the pain. Pain on movement or a decreased range of motion (ROM) can be a sign of meniscus injury as well as ligament injury.
Stability examination
A stability examination is essential for the physician to decide if a possible ligament injury is of such severity that the knee is unstable. It is important that the muscles are relaxed when testing. If the athlete’s pain is too severe, in order to examine it may be necessary to wait a day or two or perform the examination under general anesthesia.The following tests are warranted for:
●● The ACL: Lachman test, anterior drawer test, pivot shift test
●● The MCL: valgus stress test (a stress placed on the outside of the knee forcing the lower leg outward in relation to the femur)
●● The PCL: posterior drawer test and quadriceps active test
●● The LCL: varus stress test (a stress placed on the inside of the knee forcing the lower leg inward in relation to the femur) and reverse pivot shift test
●● The PCL and LCL for posterior lateral instability: reverse pivot shift test.
Aspiration
Withdrawal of fluid by way of a needle aspiration of the knee joint can be performed in cases of extensive swelling in order to verify if there is blood present in the fluid
Radiology
Plain X-rays are essential in any serious knee injury, to exclude fractures, avulsions or to show defects of the bone
lying beneath the articular cartilage.An X-ray shows the a) bones of the knee joint and joint space height. Soft tissue,
such as ligament,cannot be seen on an X-ray.X-rays taken
while the athlete is supporting weight can show changes
in the thickness of the articular cartilage.Thinning of the cartilage, which indicates early osteoarthritis, will cause
the femur and tibia to appear closer together.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) is useful for evaluating the skeleton 
and soft tissues (ligaments, tendons, muscles, capsule and meniscus). The images can show swelling in the bone that accompanies fractures and it shows PCL ruptures very well. For the ACL, the sensitivity for detecting tears is good, although it is sometimes difficult to decide whether the injury is complete or partial. In most situations MRI is unnecessary because the diagnosis can be made by clinical examination. In unusual or difficult cases an MRI with high sensitivity will show an ACL or meniscal tear. MRI using contrast agents (dGEMRIC technique, i.e. gadolinium MRI of cartilage) has been developed relatively recently. This technology has increased the sensitivity for detecting articular cartilage injuries, early osteoarthritis, bone marrow edema, and damage to the trabeculae in the bone under the cartilage, known as a ‘bone bruise’.
A bone scan (scintigraphy) demonstrates areas of increased (or decreased) bone metabolism.The method is sensitive for stress fracture and osteonecrosis (a dead part of a bone). It is now seldom used.
Ligament injuries
Anterior cruciate ligament injuries
The ACL is the most common of the ligament injuries to the knee joint.The loss of the ACL function does not only result in abnormal kinematics (motion, movement patterns) but often in severe degenerative changes in the knee joint over time, such as osteoarthritis . A rupture of the ACL is the most common serious injury occurring in sports traumatology.
Tip
The ACL injury is sport’s biggest problem and is still an injury with many difficult and unresolved issues, such as best surgery, graft choice and tension, criteria for return to sport etc., despite extensive research. This injury requires a lot of experience, reflection, commitment and choice of optimal treatment technology to get
a successful outcome and allow a return to previous activity level.
Anatomy
The human ACL is a complex structure at every level.The ligament is designed to act as a stabilizer while allowing normal joint motion throughout the functional ROM. The ACL is a band of strategically oriented connective tissue, that connects the femur and tibia . It has an average weight of 20 g (0.7 oz) and an average length of 35 mm (1.4 in.). It is narrow in the middle, fanning out inferiorly and to a lesser extent superiorly. The ACL attaches to the posterior aspect of the medial surface of the lateral femoral condyle. There is a rim of bone of 2.5 mm between the posterior ACL fibers and the posterior articular cartilage margin. Distally the ACL is attached to a fossa in front and lateral of the tibial spine (intercondyloid eminence of the tibia).The tibial attachment is somewhat broader than the femoral attachment.
The ACL consists of an anteromedial band that is taut with the knee in flexion and relaxed when the knee is in extension, and a posterolateral bundle that is tight in extension and relaxed in flexion. An interconnecting intermediate band may be identifiable, which is tight through the whole ROM from extension to flexion.
Biomechanics and function
The ACL is the second strongest ligament in the knee with a maximum load of around 2200 N (500 lb). In extreme extension, the anteromedial band is slack and the posterolateral band is tight; with increasing flexion, there is tightening of the anteromedial band and increased laxity in the posterolateral band .
The ACL prevents the anterior movement of the tibia in relation to the femur.The ligament takes up 75% of the anterior force in full extension, 87% at 30° of flexion, and 85% at 90° of flexion. Other stabilizing structures are the iliotibial band, the medial and lateral capsule, plus the medial and lateral collateral ligaments. In ACL- deficient knees, the medial extra-articular structures resist anterior (forward) translation and outward rotation throughout the ROM, while the lateral collateral ligament and the posterolateral structures resist anterior translation in a straight knee only.The medial meniscus also resists anterior translation through the ROM.
The ACL is the main stabilizer for anterior translation of the tibia in relation to the femur.Together with the PCL, the ACL resists and limits hyperextension, hyperflexion and internal rotation
