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Emergency and Acute Medicine – Knee Injuries: ACL, PCL, MCL, and Meniscus
Basic Description
Knee ligament and meniscal injuries are common causes of acute knee pain and instability, particularly in athletes and patients involved in high-energy trauma. Cruciate ligament injuries involve the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), which provide primary anteroposterior and rotational stability to the knee. The ACL runs from the posteromedial aspect of the lateral femoral condyle to the interspinous area of the tibia and prevents excessive anterior tibial translation, internal rotation, and hyperextension. The PCL is thicker and stronger than the ACL and extends from the anterolateral aspect of the medial femoral condyle to the posterior tibia, limiting posterior tibial translation.
Meniscal injuries most commonly involve the medial meniscus, which is less mobile and more firmly attached to the joint capsule than the lateral meniscus. Tears occur due to compressive or rotational forces between the femur and tibia. Displaced meniscal fragments can cause a true locked knee with inability to extend. The medial collateral ligament (MCL) extends from the medial femoral condyle to the tibia distal to the joint and is frequently injured in association with other ligamentous injuries.
Epidemiology
ACL injuries are the most common ligamentous knee injury, with approximately 200,000 cases annually in the United States. Nearly two-thirds are noncontact injuries, and females have a threefold increased risk compared with males. About half of ACL injuries are associated with meniscal tears, and chondral or subchondral damage is common. Medial meniscus injuries are significantly more frequent than lateral meniscus injuries, although true mechanical locking occurs in only about 30% of cases.
Etiology
ACL injuries typically result from deceleration combined with rotation or hyperextension, often during sports such as football or skiing, using a plant-and-pivot or stop-and-jump mechanism. PCL injuries are classically caused by a “dashboard injury,” where a posterior force is applied to the proximal tibia with the knee flexed, or from a fall onto a flexed knee. Meniscal injuries usually occur with sudden twisting motions during squatting, pivoting, or kneeling, and are common in both athletes and certain occupations. MCL injuries most often result from valgus stress applied to a flexed knee, such as from a lateral blow or ski injury.
In children, the ACL is the most frequently injured knee ligament, while isolated MCL injuries are uncommon before closure of the growth plates.
Diagnosis: Signs and Symptoms
Patients with cruciate ligament injuries often report a popping sensation, tearing feeling, or the knee “giving way,” followed by immediate dysfunction. A large effusion developing within 2–3 hours suggests hemarthrosis, most commonly from ACL injury. MCL injuries cause medial knee pain and tenderness, sometimes more pronounced in partial tears. Meniscal injuries are characterized by joint-line pain, intermittent swelling, mechanical symptoms, and difficulty fully extending the knee; patients may still complete activity at the time of injury. Degenerative meniscal tears present more insidiously and are often associated with osteoarthritis.
Physical Examination
Examination should include assessment of weight-bearing ability, palpation of bony landmarks and joint lines, evaluation of range of motion, and identification of effusion or locking. Immediate effusion suggests significant intra-articular injury, whereas delayed swelling is more typical of MCL, PCL, or meniscal injuries. A thorough neurovascular exam is essential. Stress testing should always compare the injured knee to the uninjured side.
The Lachman test is the most reliable maneuver for ACL injury in the acute setting. Anterior and posterior drawer tests assess ACL and PCL integrity, respectively, while the quadriceps active test helps confirm PCL injury. Valgus and varus stress testing evaluate MCL and LCL stability. Meniscal tests such as McMurray and Apley are best performed once acute pain is controlled.
Diagnostic Testing and Imaging
Laboratory testing is rarely required unless infection or inflammatory disease is suspected. Plain radiographs are recommended in most acute knee injuries to exclude fractures, especially in suspected ACL injuries and in children, where growth plate or tibial spine fractures may occur. MRI is highly sensitive and specific for ligamentous and meniscal injuries but is rarely needed emergently. Ottawa knee rules guide imaging decisions in adults but do not apply to children.
Essential Workup
Key priorities include careful neurovascular assessment, exclusion of fracture or infection, evaluation for multidirectional instability, and focused ligament testing, particularly the Lachman test for suspected ACL injury.
Differential Diagnosis
Consider growth plate injuries, tibial plateau fractures, transient knee or patellar dislocation, referred hip pathology, septic arthritis, gout, osteoarthritis, and inflammatory arthritides.
Treatment and Initial Management
Initial management includes immobilization, ice, elevation, and analgesia, along with documentation of neurovascular status. In the emergency department, locked knees due to meniscal injury may sometimes be reduced with gentle traction and rotation. Arthrocentesis can relieve pain from large effusions. In the absence of fracture, treatment consists of rest, ice, compression, elevation, and weight bearing as tolerated, often with crutches. Knee immobilizers may be used for comfort, but early gentle motion is encouraged.
Disposition and Follow-Up
Most isolated ligament or meniscal injuries can be managed as outpatients. Admission is rarely required unless a knee dislocation or vascular injury is suspected. Orthopedic follow-up within 1–2 weeks is recommended for confirmed or suspected significant ligamentous injury, and re-examination within 48 hours is helpful when initial assessment is limited by pain or swelling.
Pearls and Pitfalls
Always perform and document a thorough neurovascular exam. Maintain a high index of suspicion for occult knee dislocation in patients with multidirectional instability or multiple ligament injuries. Do not miss fractures, septic joints, referred hip pathology, or vascular compromise.
Basic Description
Knee ligament and meniscal injuries are common causes of acute knee pain and instability, particularly in athletes and patients involved in high-energy trauma. Cruciate ligament injuries involve the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), which provide primary anteroposterior and rotational stability to the knee. The ACL runs from the posteromedial aspect of the lateral femoral condyle to the interspinous area of the tibia and prevents excessive anterior tibial translation, internal rotation, and hyperextension. The PCL is thicker and stronger than the ACL and extends from the anterolateral aspect of the medial femoral condyle to the posterior tibia, limiting posterior tibial translation.
Meniscal injuries most commonly involve the medial meniscus, which is less mobile and more firmly attached to the joint capsule than the lateral meniscus. Tears occur due to compressive or rotational forces between the femur and tibia. Displaced meniscal fragments can cause a true locked knee with inability to extend. The medial collateral ligament (MCL) extends from the medial femoral condyle to the tibia distal to the joint and is frequently injured in association with other ligamentous injuries.
Epidemiology
ACL injuries are the most common ligamentous knee injury, with approximately 200,000 cases annually in the United States. Nearly two-thirds are noncontact injuries, and females have a threefold increased risk compared with males. About half of ACL injuries are associated with meniscal tears, and chondral or subchondral damage is common. Medial meniscus injuries are significantly more frequent than lateral meniscus injuries, although true mechanical locking occurs in only about 30% of cases.
Etiology
ACL injuries typically result from deceleration combined with rotation or hyperextension, often during sports such as football or skiing, using a plant-and-pivot or stop-and-jump mechanism. PCL injuries are classically caused by a “dashboard injury,” where a posterior force is applied to the proximal tibia with the knee flexed, or from a fall onto a flexed knee. Meniscal injuries usually occur with sudden twisting motions during squatting, pivoting, or kneeling, and are common in both athletes and certain occupations. MCL injuries most often result from valgus stress applied to a flexed knee, such as from a lateral blow or ski injury.
In children, the ACL is the most frequently injured knee ligament, while isolated MCL injuries are uncommon before closure of the growth plates.
Diagnosis: Signs and Symptoms
Patients with cruciate ligament injuries often report a popping sensation, tearing feeling, or the knee “giving way,” followed by immediate dysfunction. A large effusion developing within 2–3 hours suggests hemarthrosis, most commonly from ACL injury. MCL injuries cause medial knee pain and tenderness, sometimes more pronounced in partial tears. Meniscal injuries are characterized by joint-line pain, intermittent swelling, mechanical symptoms, and difficulty fully extending the knee; patients may still complete activity at the time of injury. Degenerative meniscal tears present more insidiously and are often associated with osteoarthritis.
Physical Examination
Examination should include assessment of weight-bearing ability, palpation of bony landmarks and joint lines, evaluation of range of motion, and identification of effusion or locking. Immediate effusion suggests significant intra-articular injury, whereas delayed swelling is more typical of MCL, PCL, or meniscal injuries. A thorough neurovascular exam is essential. Stress testing should always compare the injured knee to the uninjured side.
The Lachman test is the most reliable maneuver for ACL injury in the acute setting. Anterior and posterior drawer tests assess ACL and PCL integrity, respectively, while the quadriceps active test helps confirm PCL injury. Valgus and varus stress testing evaluate MCL and LCL stability. Meniscal tests such as McMurray and Apley are best performed once acute pain is controlled.
Diagnostic Testing and Imaging
Laboratory testing is rarely required unless infection or inflammatory disease is suspected. Plain radiographs are recommended in most acute knee injuries to exclude fractures, especially in suspected ACL injuries and in children, where growth plate or tibial spine fractures may occur. MRI is highly sensitive and specific for ligamentous and meniscal injuries but is rarely needed emergently. Ottawa knee rules guide imaging decisions in adults but do not apply to children.
Essential Workup
Key priorities include careful neurovascular assessment, exclusion of fracture or infection, evaluation for multidirectional instability, and focused ligament testing, particularly the Lachman test for suspected ACL injury.
Differential Diagnosis
Consider growth plate injuries, tibial plateau fractures, transient knee or patellar dislocation, referred hip pathology, septic arthritis, gout, osteoarthritis, and inflammatory arthritides.
Treatment and Initial Management
Initial management includes immobilization, ice, elevation, and analgesia, along with documentation of neurovascular status. In the emergency department, locked knees due to meniscal injury may sometimes be reduced with gentle traction and rotation. Arthrocentesis can relieve pain from large effusions. In the absence of fracture, treatment consists of rest, ice, compression, elevation, and weight bearing as tolerated, often with crutches. Knee immobilizers may be used for comfort, but early gentle motion is encouraged.
Disposition and Follow-Up
Most isolated ligament or meniscal injuries can be managed as outpatients. Admission is rarely required unless a knee dislocation or vascular injury is suspected. Orthopedic follow-up within 1–2 weeks is recommended for confirmed or suspected significant ligamentous injury, and re-examination within 48 hours is helpful when initial assessment is limited by pain or swelling.
Pearls and Pitfalls
Always perform and document a thorough neurovascular exam. Maintain a high index of suspicion for occult knee dislocation in patients with multidirectional instability or multiple ligament injuries. Do not miss fractures, septic joints, referred hip pathology, or vascular compromise.
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