Ligament injury of the knee

Knee injuries in general, including ligament and meniscal injuries and patellar fractures, are common injuries.

The anterior cruciate ligament (ACL) prevents anterior translation of the tibia, tibial rotation, and valgus/varus displacement. The ACL is the most commonly injured knee ligament, accounting for approximately half of all knee injuries. It’s more common among females.

The posterior cruciate ligament (PCL) is the largest and strongest intra-articular ligament of the knee. It prevents posterior translation of the tibia. The medial collateral ligament (MCL) prevents valgus stress and tibial rotation. The lateral collateral ligament (LCL) prevents varus stress and tibial rotation. Isolated LCL injury is rare.

The “unhappy triad” refers to simultaneous injury of the ACL, MCL, and medial meniscus.

Etiology

ACL injury occurs mostly due to external rotation (twisting) of the knee during sports. It may also occur due to high energy trauma due to forced hyperextension of the knee.

PCL injury occurs due to forced hyperflexion with plantarflexed foot, often occurring during sports. It may also occur due to high energy trauma due to blow to a flexed knee.

MCL injury occurs due to valgus stress to the knee.

Osteophytes from osteoarthritis may predispose to ligament injury as the osteophytes can damage the ligament.

Clinical features

Symptoms include knee instability when walking, pain, and swelling (may be due to haemarthrosis from bleeding from the ligament). Swelling occurs immediately in ACL injuries. The patient reporting hearing a “pop”, “cracking” or “crunching” sound is suspicious for a ligament injury.

PCL, MCL, and LCL injury causes less symptoms than ACL injury.

Diagnosis and evaluation

Many physical examination manoeuvres are used in the evaluation of knee ligament injuries.

The valgus stress test involves applying valgus stress to the knee (by pushing medially) while the knee is in extension and 20 – 30° flexion. Widening of the joint space, felt as medial laxity, indicates MCL, ACL, or posteromedial capsule injury.

The varus stress test involves applying varus stress to the knee (by pushing lateral) while the knee is in extension and 20 – 30° flexion. Widening of the joint space, felt as lateral laxity, indicates LCL, ACL, or posterolateral capsule injury.

Lachman test involves having the patient’s knee in light flexion (15 – 20°), gripping the upper part of the tibia with one hand and the lower part of the femur with the other, and attempting to “pull” the tibia anteriorly. If the tibia can be pulled 3 – 4 mm anteriorly (anterior tibial translation, compare with the other side), the test is positive and indicates ACL injury. Lachman test is superior to the drawer test for diagnosing ACL injuries.

Anterior-posterior drawer test is similar to Lachman test. The patient’s knee is in 90° flexion, and the examiner fixes the patient’s foot on the table and attempts to pull the proximal tibia anteriorly and push it posteriorly. Anterior laxity (translation) indicates ACL injury, while posterior laxity indicates PCL injury.

The patella dip test may be used to demonstrate intraarticular fluid (not only in ligament injuries), including haemarthrosis. The suprapatellar recess is compressed with one hand and try to push the patella into the knee with the other hand. In case intraarticular fluid is present, pushing the patella feels like it’s floating on fluid and you can “dip” it into the fluid.

MRI is the best modality to diagnose soft tissue injuries like ligament and meniscal injuries. Ultrasound may also be used. Radiography cannot diagnose ligament injury. MRI is not needed for diagnosis of MCL and LCL injury.

Treatment

ACL may be treated conservatively or surgically. Conservative therapy is indicated for chronic injuries, patients with minimal activity, if there is no instability, and if there is no associated injury. It involves immobilisation, brace, and physiotherapy.

In most cases, surgery is performed, as it allows return to activity sooner. Surgery is usually arthroscopic and involves reconstruction of the ACL with a tendon graft. Native (autograft), cadaver (allograft), or synthetic grafts may be used. Non-synthetic grafts are usually taken from patellar, hamstring, or quadriceps tendons.

Treatment of the unhappy triad requires surgical treatment with meniscal repair and ACL reconstruction. Management of PCL and LCL injury is similar as for ACL. Isolated MCL injury is managed conservatively, but multi-ligament injury is an indication for surgery.

Meniscal injury of the knee

Meniscal injuries (tears) are, like knee ligament injuries, also common sports injuries. Typically, these injuries cause “clicking” or “popping” and the sensation of the knee not moving properly during rotation of the knee joint. Injury of the medial meniscus is more common than the lateral, impairing external rotation.

The function of the menisci is to provide a deeper joint surface, equalise the weight load on the joints, reduce stress, and stabilise the joint. The blood supply of the menisci is best at the outer (lateral) edge (red zone) and poorest at the inner (medial) edge (white zone).

Etiology

Meniscal injuries occur due to sudden twisting of the knee with the lower leg in a fixed position with an axial load on the knee (usually the person's weight). In elderly it may also be due to degenerative disease.

Classification

Tears are classified according to their morphology:

Localisation of tear
- Medial meniscus
- Lateral meniscus
- White zone
  - The inner third
  - Avascular part of meniscus
- Red-white zone
  - Middle third
  - Poorly vascularized
- Red zone
  - Outer third
  - Well vascularized
Dignity of tear
- Simple
- Complex (combination of multiple types)
Type of tear
- Vertical tear
- Radial tear
- Horizontal tear
- Displaced tear
  - Bucket handle tear
  - Parrot beak tear

Clinical features

Symptoms include pain on the lateral or medial side, swelling, and joint lock-up or instability. Swelling may be due to hydrops or haemarthrosis due to a meniscal tear. Swelling develops over the subsequent 24 hours (and not immediately as in ligament injuries).

The sensation of something “slipping” or “tearing” is suspicious for meniscal injury. There may be pain upon palpation of the relevant joint space.

Diagnosis and evaluation

Many physical examination manoeuvres are used in the evaluation of meniscal injuries. The sensitivity of the McMurray and Apley tests are in the 50-60s percentages. The Thessaly test has much higher sensitivity (80-90%) but is not as widely used.

Apley test involves having the patient prone with the hip extended and knee flexed. Then, the patient’s upper leg is fixated (with the examiner’s leg), the knee is brought to 90° flexion. Then, the lower leg is internally and externally rotated both with and without applying downward pressure on the foot. External rotation tests the medial meniscus while internal rotation tests the lateral. Positive findings include increased pain during rotation when downward pressure is applied.

McMurray test involves having the patient supine with the hip and knee flexed. Then, varus stress and internal rotation is applied to the knee, after which the knee is extended. This tests the lateral meniscus, and positive findings include pain in the lateral joint space or “clicking” or “locking” in the knee. The test is then repeated while applying valgus stress and external rotation, which tests the medial meniscus.

Thessaly test involves having the patient and examiner facing each other (holding hands for support), and having the patient stand on one knee, flexing the knee to 20°, and rotating their body back and forth. Positive findings include pain or a locking or catching sensation.

MRI is the best modality to diagnose soft tissue injuries like ligament and meniscal injuries. Ultrasound may also be used.

Treatment

Treatment is almost always surgical and involves arthroscopy with suture, partial or total resection, or meniscus transplantation. Tears in the red zone of the meniscus have good prognosis due to good blood flow, while tears in the white zone rarely heal and instead require resection.

Patellar fractures

Patellar fractures are the result of blunt trauma to the patella or due to a contracting quadriceps. These fractures are rare, accounting for approximately 1% of all fractures.

Etiology

Fall onto flexed knee or the knee hitting the dashboard during motor vehicle accident are the most frequent causes of patellar fracture.

Classification

Patellar fractures are classified according to their morphology:

Undisplaced
Transverse
Lower or upper pole
Comminuted undisplaced
Comminuted displaced
Vertical
Osteochondral

Clinical features

Symptoms include swollen knee and patellar pain and tenderness.

Diagnosis and evaluation

During physical examination it’s important to evaluate the extensor function, as disrupted extensor mechanism is an indication for surgery. Patient should be challenged to extend their knee fully against gravity. However, pain may prevent full extension, and it may be difficult to discern whether abnormal extension is due to pain or a disrupted mechanism. Physical examination will likely also reveal intraarticular fluid.

X-ray is usually sufficient for diagnosis. Ultrasound or CT/MRI may be used to exclude other injuries.

Treatment

For undisplaced, closed patella fractures with intact extensor mechanism, conservative treatment is sufficient.

For displaced or avulsed patella fractures, surgery is necessary. Surgery involves tension band wiring or partial patellectomy with anchoring the patella.