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Comprehensive knee injury assessment using Ottawa Knee Rules, ACL injury criteria, and meniscus tear indicators. Determine imaging needs and suspected injury patterns for evidence-based management.
X-ray needed if ANY of the following:
Knee injuries are among the most common musculoskeletal injuries, particularly in athletes. Proper assessment is crucial to identify fractures requiring immediate imaging and soft tissue injuries that may need advanced imaging like MRI. This calculator combines validated clinical decision rules with common injury patterns.
Developed in 1996, the Ottawa Knee Rules help clinicians determine when knee X-rays are necessary. The rules have high sensitivity (97-99%) for detecting fractures and can reduce unnecessary radiography by 20-30%. They apply to acute knee injuries in patients aged 5 years and older, excluding those with superficial injuries, injuries more than 7 days old, or reassessment visits.
Anterior Cruciate Ligament (ACL) tears are serious injuries requiring orthopedic evaluation. Classic presentation includes an audible pop at the time of injury, rapid swelling within 2 hours (hemarthrosis), immediate knee instability, and a positive Lachman test. The Lachman test has approximately 85% sensitivity and 94% specificity for ACL tears. Early recognition is important as delayed treatment can lead to meniscal tears and cartilage damage.
Meniscus tears often occur with twisting injuries and can present with mechanical symptoms like true locking (inability to fully extend the knee), joint line tenderness, and pain with specific maneuvers. The McMurray test, which involves rotating the tibia while extending the knee, can help identify meniscal tears. MRI is the gold standard for diagnosis, with sensitivity and specificity exceeding 90% for most tear types.
MRI should be considered when there's clinical suspicion for internal derangement (ligament or meniscus injury), persistent symptoms despite conservative management, mechanical symptoms like locking or catching, or when surgical intervention is being considered. MRI is not typically needed acutely unless there's suspicion of a multi-ligament injury or locked knee requiring urgent arthroscopy.
Mechanical locking is a true inability to fully extend the knee due to a physical block (like a displaced meniscus fragment). Pseudo-locking is when pain limits extension but the knee can be extended with force or manipulation. Only true mechanical locking suggests meniscus tear.
Not usually. Most knee injuries can be managed conservatively initially with RICE protocol. MRI is typically considered after 4-6 weeks if symptoms persist, if there's mechanical locking that doesn't resolve, or if there's high clinical suspicion for ACL tear requiring surgical planning.
An audible or felt pop at the time of injury is highly suggestive of ACL tear, especially when combined with rapid swelling and instability. About 85% of patients with ACL tears report hearing or feeling a pop. However, pops can occur with other injuries too, so clinical examination is important.
Rapid swelling within 2 hours usually indicates hemarthrosis (blood in the joint), which suggests significant structural damage like ACL tear, patellar dislocation, or osteochondral fracture. Slower, gradual swelling is more typical of sprains and less severe injuries.
Some people can walk and even return to limited activity with a torn ACL, especially after the initial swelling subsides. However, the knee may feel unstable, particularly with pivoting or cutting movements. Walking ability doesn't rule out ACL tear.
The Lachman test is a physical examination maneuver where the examiner holds the thigh stable and pulls forward on the lower leg with the knee bent at 20-30 degrees. Excessive forward movement (laxity) or a soft endpoint suggests ACL tear. It's more accurate than the anterior drawer test.
No. Many meniscus tears, especially degenerative tears in older patients, can be managed non-surgically with physical therapy. Surgery is typically considered for younger patients with acute tears, tears causing mechanical symptoms like locking, or tears that don't improve with conservative treatment.
Mild knee sprains typically heal in 2-4 weeks, moderate sprains in 4-8 weeks, and severe sprains may take 8-12 weeks or longer. ACL tears don't heal without surgery. Meniscus tears may heal if small and in the vascular zone, but many require surgical treatment.
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