Laboratory Studies
For patients with significant knee injuries or individuals who may require surgery, appropriate preoperative laboratory investigations might include blood typing and screening and determination of the CBC count and electrolyte, serum glucose, BUN, and creatinine levels.
If warranted by the initial history and physical findings, laboratory analysis of joint aspiration may assist in confirming a diagnosis. Aspirated synovial fluid should be analyzed for WBC count and differential and for glucose and protein levels. Gram staining, cultures, and sensitivity testing should be performed.
A CBC count, erythrocyte sedimentation rate (ESR), and serum glucose and uric acid levels should also be considered at the time of arthrocentesis if infectious or gouty arthritis is a consideration.
Imaging Studies
Radiographs
Plain images of the knee generally include anteroposterior (AP) and lateral views at minimum. Some centers add 2 oblique projections that are helpful in detecting tibial-plateau fractures. Special views include tunnel view to aid in evaluating the tibial and femoral articular surfaces and a sunrise view to assist in assessing the articular surface and the body of the patella.
Each year, there are over 500,000 visits to the ED for acute knee trauma. Fracture is identified in only 5% of ED knee radiographs. Radiographs are considered the first imaging study for a fall or twisting injury of the knee with focal tenderness, effusion, or inability to bear weight. For a suspected meniscus or ligament tear or injury from a reduced patellar dislocation, MRI is considered the best imaging study. In cases of knee dislocation, patients should undergo radiography, MRI, and angiography. [2]
In most patients with severe ligamentous or meniscal damage, plain radiographic findings are of limited value, and images often appear normal. [6] Fewer than 15% of knee radiographs reveal clinically significant findings.
Various clinical prediction rules exist (Ottawa, Pittsburgh, Weber, Fagan-Davies), with the Ottawa rules being the most validated.
Plain images are recommended for the following scenarios:
-
Patients older than 55 years (because of an increased risk of pathologic fracture associated with osteoporosis)
-
Patients with tenderness over the fibular head
-
Patients with discomfort confined to the patella upon palpation
-
Patients unable to flex the knee to 90°
-
Patients incapable of bearing weight, immediately and in the ED, for at least 4 steps
Although plain radiography tends to be unproductive in diagnosing soft tissue injuries, certain radiographic findings are strongly suggestive of ligamentous, meniscal, or tendon damage. Particularly review the lateral radiograph for fluid within the suprapatellar pouch. The extensor tendon mechanism is normally well outlined. As an effusion accumulates in the suprapatellar pouch, the posterior margin of the quadriceps is apt to be obliterated, and, with additional fluid collecting, the space between the prefemoral and the anterior superior suprapatellar fat pads widens. A cross-table lateral radiograph may expose a fat-fluid level, also known as a lipohemarthrosis, a pathognomonic sign of an intra-articular fracture.
Avulsion fractures of the tibial spine or the femoral condyles imply ligamentous rupture. Occult fractures that are commonly missed on plain radiographs include those of the patella, tibial plateau, or fibular head, as well as Segond fractures (small vertical avulsion crack of the proximal lateral tibia, also called the lateral capsular sign).
Three radiographic findings associated with anterior cruciate ligament (ACL) injuries are avulsion of the intercondylar tubercle, anterior displacement of the tibia with respect to the femur (labeled the radiographic drawer sign), and a Segond fracture. In a study of 160 patients who had an ACL reconstruction, only 14.4% (23 out of 160) had the correct diagnosis of ACL injury diagnosed on initial presentation at the emergency department or by a general practitioner. The median delay from injury to diagnosis was 13 weeks (0 to 926), and the median total time from injury to surgery was 42 weeks. [3]
Avulsion of the fibular head aligns with lateral collateral ligament (LCL) or biceps femoris injury.
The most common radiographic finding with a posterior cruciate ligament (PCL) injury is avulsion at the site of the ligamentous origin on the posterior tibia.
Unilateral widening of the joint space may denote ligamentous instability.
Chronic medial collateral ligament (MCL) damage, usually lasting longer than 6 weeks, may heal with calcification, presenting as Pellegrini-Stieda syndrome.
Patellar abnormalities can be misleading for an acute fracture. A bipartite or multipartite patella is a normal variant that may be difficult to distinguish from a patellar fracture. An unfused secondary ossification center forming the bipartite patella typically appears in the upper lateral quadrant, tends to occur bilaterally, and reveals well-defined margin lines.
CT and MRI
CT scans have proven effective for corroborating areas questionable for fracture in the knee region, particularly tibial-plateau fractures in elderly patients.
The relative sensitivity and specificity of clinical examination and MRI findings have been shown to be comparable (96.5% sensitivity of clinical examination versus 98% of MRI; 87% specificity of clinical examination versus 85.5% of MRI).
MRI has supplanted the arthrography as the procedure of choice for evaluating soft tissue injuries of the knee. In patients with a locked knee, MRI findings are helpful in deciding between conservative treatment and surgical intervention. Although MRI plays an important role in future surgical management of acute knee disorders, it is rarely an essential part of the ED workup. [6, 11, 12]
Studies report variable 78-100% sensitivity and 68-100% specificity of MRI for the diagnosis of ACL tears. Sensitivity is significantly decreased if other major ligamentous injuries are present in the knee. MRI also may identify bone bruising, which is present in approximately 90% of ACL injuries. An MRI scan allows the physician to confirm an ACL tear, but it should not be used as a substitute for a good history and physical examination. [8, 9, 19, 20]
Ultrasonography
Ultrasonography simplifies differentiation of a Baker cyst, popliteal artery aneurysm, and thrombophlebitis, and it has also been used to diagnose tendon ruptures. Additionally, sonographic examination can accurately detect effusion of the knee. Ultrasound may prove particularly helpful in aspiration and injection of the knee joint in morbidly obese patients in whom landmark identification may prove difficult. [13] The detection of knee effusion in patients with traumatic knee injury by sonographic examination is highly indicative of internal knee derangement. As experience improves, ultrasonography is proving to be a useful and inexpensive method of detecting the presence of rupture of the anterior cruciate ligament in the clinical setting of a traumatic hemarthrosis. [21, 15]
Arteriography
An arteriogram is required when a knee dislocation is strongly suspected, even in the absence of reliable signs or symptoms of vascular impairment. Palpation of a regular pulse or Doppler confirmation of pedal pulses does not exclude vascular injury, as intimal tears may be undetectable. The incidence of a concomitant popliteal artery injury is 20-50%. Do not delay emergency surgery for arteriography; imaging can be performed intraoperatively.
Although not 100% sensitive, an injection of methylene blue into the affected knee until the joint is fully distended and observing for extravasations can be done to test for open joint injury.
Procedures
Knee joint aspiration
Because the knee is the largest synovial joint in the body and given its relatively accessible location, the knee joint provides one of the easiest sites in which to perform arthrocentesis. Indications for knee joint aspiration include confirmation of a diagnosis (The knee is a common site for septic and inflammatory arthritis.) and pain arising from a tense effusion.
Place the patient supine with the knee joint extended, trying to ease any contraction of the quadriceps muscles.
After properly cleansing the skin, infiltrate the skin and underlying dermis with local anesthetic at the point where aspiration will take place.
Approach the joint for aspiration from the medial aspect of the knee, with the site of puncture being 1 cm medial to the anteromedial border of the patella.
Insert an 18-gauge or 20-gauge needle or catheter through the same tract used to inject local anesthetic, at the midpoint or superior position of the patella, aiming for a point between the posterior surface of the patella and the femoral intercondylar notch. The author has found the use of a spinal needle (ie, 21-gauge) to be useful, particularly in obese patients.
Approach the anterolateral border in a similar manner, mirroring the access sites.
Occasionally, the examiner perceives a slight give as the needle perforates the joint capsule.
Aspirate as much fluid as possible, recalling that the knee may contain 50 mL or more of fluid. When the fluid stops flowing freely, compress the suprapatellar pouch and attempt to push additional fluid into the adjacent pouch. If the bore appears obstructed during the arthrocentesis, try rotating the needle or injecting some aspirate, attempting to clear the needle. If this fails, try reinserting the needle one quarter of an inch deeper.
Aspiration of blood indicates a ligamentous tear (eg, ACL, PCL), osteochondral fracture, peripheral meniscus tear, capsular tear, or patellar dislocation.
Presence of fat globules in the aspirant is pathognomonic of an intra-articular fracture.
Hypertrophied synovial tissue or clot formation may hinder aspiration.
Instilling 5-10 mL of 1% lidocaine into the joint and then reexamining the knee may facilitate knee testing.
After withdrawing the needle, dress the iatrogenic puncture wound with an antibiotic ointment and appropriate adhesive sterile dressing.
In the presence of a hot and swollen knee joint, consider the possibility of septic or acute inflammatory arthritis. Joint aspirate should be analyzed for a CBC count with differential, glucose levels, and protein measurements. Also order polarized light microscopy for crystals, as well as culture and sensitivity testing of the joint fluid. Although rare, infection and hemarthrosis may complicate arthrocentesis.
-
Anatomy of the knee.
-
Knee ballottement.
-
Lachman test.
-
Anterior drawer sign.
-
Pivot test.
-
McMurray test.
-
Apley compression test.
