Injury to either the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) can cause instability in the knee. Damage to the ACL is common, particularly amongst athletes. PCL injuries are less common.
Basic Anatomy
At the top of the tibia, there are the medial and lateral condyles, which are slightly convex surfaces that correspond to the condyles of the femur. Between the condyles, there is an intercondylar area. The cruciate ligaments are named after where they attach to the tibia:
- The ACL attaches at the anterior intercondylar area on the tibia
- The PCL attaches at the posterior intercondylar area on the tibia
There are rounded areas of bone at the end of the femur, which are also called condyles. At the back of the distal end of the femur is an intercondylar notch, which is a groove between the two condyles. Both cruciate ligaments originate from the intercondylar notch, the ACL on the lateral aspect and the PCL on the medial aspect.
The ACL stops the tibia from sliding forward in relation to the femur. The PCL tops the tibia sliding backwards in relation to the femur.
Presentation
The ACL is typically damaged during a twisting injury to the knee. The injury causes:
- Pain
- Swelling
- “Pop” sound or sensation
Patients with ACL injuries will have instability of the knee joint. The tibia can move anteriorly below the femur. The knee can buckle, and patients often feel a lack of confidence that the knee is stable. Over time, muscle weakness develops, and there is an increased risk of other knee injuries (e.g., meniscal tears).
The anterior drawer test can be used to assess for anterior cruciate ligament damage. The patient is supine with the hip flexed to 45 degrees and the knee flexed to 90 degrees, with the foot flat on the couch. The examiner sits on the patient’s toes to stabilise the foot. The examiner holds the leg just below the knee and pulls the proximal tibia anteriorly, sliding it forward from the femur at the knee. In a normal knee, there will be slightly anterior movement of the proximal tibia but a definite end-point to movement, as the ACL holds the joint securely. With ACL damage, the tibia can move an excessive distance anteriorly, and the examiner will not be able to feel a clear end-point to the movement.
The Lachman test is similar to the anterior drawer test, but the knee is tested while flexed at around 20-30 degrees.
Examination can be difficult to interpret shortly after the injury due to pain and swelling. Examination after the acute pain and swelling has settled is more likely to be accurate.
Investigations
MRI scan is usually the first-line imaging investigation for establishing the diagnosis.
Arthroscopy can be used to visualise the cruciate ligament and is the gold-standard investigation for diagnosing a cruciate ligament tear.
Management
The NICE clinical knowledge summaries on knee pain (updated 2017) recommend urgent referral in patients with an acute onset of knee pain associated with symptoms suggestive of an acute anterior cruciate ligament tear. Local pathways vary, and this may involve sending the patient to A&E or the fracture clinic. Symptom suggestive of an acute anterior cruciate ligament tear include:
- A “pop”
- Rapid onset swelling
- Instability or giving way
Conservative management involves RICE:
- R – Rest
- I – Ice
- C – Compression
- E – Elevation
NSAIDs are usually used first-line for analgesia in MSK injuries.
Crutches and knee braces may be required to help protect the knee while mobilising.
Physiotherapy can be used before and after surgery for rehabilitation.
Arthroscopic surgery to reconstruct the ligament is often required, particularly in active and young patients. The type and timing of surgery will be based on individual factors, such as the extent of the ACL injury and the patient’s activities (e.g., are they a young competitive athlete or a sedentary older patient). A new ligament is formed using a graft of tendon from another location. Options for graft tendons used to reconstruct the ACL include:
- Hamstring tendon
- Quadriceps tendon
- Bone-patellar tendon-bone (taking part of the patella tendon as well as the bone it inserts into)
Last updated August 2021
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