Abstract
The knee is the most frequently injured joint in the body, and an acutely swollen knee is a common presentation of knee pathology in the emergency department and primary care setting. Common causes of an effusion include inflammation, infection, and structural abnormalities in the knee. Most often, the underlying etiology can be treated conservatively until seen by orthopedics or another specialist. There are, however, a few diagnoses that need immediate treatment and close observation. Therefore, it is imperative that you formulate a comprehensive differential based on your history and physical. Diagnostic studies and laboratory tests can then be used to narrow and confirm the diagnosis. This chapter will focus on common causes and management of an acutely swollen knee.
Keywords
acutely swollen knee, septic joint
1
Why is it important to be competent in evaluating an acutely swollen knee?
The knee is the most frequently injured joint in the body, and an acutely swollen knee is a common presentation of knee pathology in the emergency department and primary care setting. Common causes of an effusion include inflammation, infection, and structural abnormalities in the knee. Most often, the underlying etiology can be treated conservatively until seen by orthopedics or another specialist. There are, however, a few diagnoses that need immediate treatment and close observation. Therefore, it is imperative that you formulate a comprehensive differential based on your history and physical. Diagnostic studies and laboratory tests can then be used to narrow and confirm the diagnosis ( Fig. 32.1 ). This chapter will focus on common causes and management of an acutely swollen knee.
2
What is the most useful question to ask in your history to determine the etiology of an acutely swollen knee?
Is the effusion traumatic or nontraumatic? Mechanism of injury is important when evaluating a traumatic knee injury. Other key components of the history include timing of swelling after an injury, localization of pain, locking, giving way, and exacerbating factors. For a nontraumatic effusion, it is imperative to ask about fevers, night sweats, night pain, weight loss, other joints involved, and social history. You will start to formulate a solid differential based on a detailed history of present illness. Refer to Table 32.1 for a broad differential diagnosis of an acutely swollen knee.
| Traumatic | Nontraumatic |
|---|---|
| Ligamentous injury | Osteoarthritis |
| Meniscal injury | Infection (Lyme, bacterial, fungal, tuberculosis) |
| Knee dislocation | Crystal deposition |
| Patellar dislocation | Rheumatic disease |
| Intraarticular fracture | Tumor |
| Tendon rupture (quad/patellar) | Idiopathic synovitis/capsulitis |
| Prepatellar bursitis | Baker cyst |
| Baker cyst |
3
In the presence of trauma, what is the most important part of the physical examination of an acutely swollen knee?
Neurovascular exam! Get into the habit of always starting your exam by checking distal pulses, sensation, and strength at the ankle. The popliteal artery is especially at risk in the setting of knee dislocations. If knee dislocations are not quickly identified and there is vascular compromise, a high percentage of these patients will eventually require amputation.
4
A 17-year-old female basketball player presents to your clinic complaining of an acutely swollen knee after hearing a loud “pop” when cutting to change direction through the paint. What is the most likely diagnosis?
Anterior cruciate ligament (ACL) rupture. ACL ruptures will typically occur after noncontact pivoting or with hyperextension of the knee. Oftentimes, a loud “pop” is heard or felt, and the patient is usually unable to continue in sport participation due to associated instability. Physical examination will reveal a positive Lachman, anterior drawer, and potentially pivot shift test. X-rays are generally unremarkable; magnetic resonance imaging (MRI) will confirm the diagnosis. Patients should be placed in a hinged knee brace until seen by orthopedics.
5
What is a pathognomonic sign for ACL rupture on radiographs of the knee?
Segond fracture: an avulsion fracture of the lateral tibial condyle (bony attachment of soft tissue structures) as a result of abnormal varus stress to the knee, combined with internal rotation of the tibia. The fracture is best seen on the anteroposterior (AP) view above the level of the fibular head ( Fig. 32.2 ).
6
A 45-year-old female presents to your clinic with a complaint of a swollen knee that started 2 days ago after accidentally stepping into a pothole when running a 5K. She notes pain along the posteromedial joint line, especially with getting into and out of the car. She admits to locking of the knee upon standing from a seated position. X-rays are normal. What is the diagnosis?
Acute medial meniscus tear. Locking of the joint, intermittent swelling, and pain with weight-bearing twisting motions are typical of a meniscal injury. On exam, the patient is generally tender along the joint line with occasional inability to fully extend the knee. Thessaly test has a higher sensitivity and specificity in detecting a meniscal tear on exam compared to McMurray test. X-rays typically show no acute abnormality, and MRI will confirm the diagnosis. Inability to fully extend the knee is concerning for displaced “bucket handle” meniscal tear, and the patient should remain non–weight-bearing until seen by orthopedics. Treatment of other meniscal injuries will depend on the patient’s symptoms, age, comorbidities, and imaging studies.
7
What is the most common mechanism for isolated posterior cruciate ligament (PCL) rupture?
A direct blow to the anterior proximal tibia with a flexed knee or a fall onto a flexed knee with the foot plantarflexed. Most of these injuries will occur as a result of a dashboard injury in a motor vehicle accident or a high-energy collision in sport. Examination will be positive for posterior drawer test and possibly tibial sag test.
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