Knee and Lower Leg Injuries

Background and Importance

The critical responsibility of the emergency physician in evaluating patients with knee pain includes identifying neurovascular injuries, reducing dislocations, stabilizing fractures, and administering antibiotics when indicated. Definitive treatment of less urgent problems can be referred to the outpatient setting.

Anatomy, Physiology, and Pathophysiology

The knee is a modified hinge, diarthrodial synovial joint that consists of the tibiofemoral and patellofemoral joints. The head of the fibula, although not part of this articulation, is closely approximated laterally and provides a site for the attachment of muscles and ligaments. Joint stability is provided primarily by ligaments, although surrounding muscles and the joint capsule contribute as well ( Fig. 48.1 ). The capsule of the knee joint is reinforced at multiple sites—anteriorly, by the ligamentum patellae; medially and laterally, by the medial and lateral patellar retinacula; and posterolaterally, by a combination of structures termed the posterolateral corner.

Extensor Mechanism

The quadriceps muscles, quadriceps tendon, medial and lateral retinacula, patella, patellar tendon, and tibial tubercle comprise the extensor mechanism of the knee ( Fig. 48.2 ). The patella is the largest sesamoid bone in the body. It is held in place by the quadriceps tendon, patellar tendon, and medial and lateral retinacula. As an integral part of the extensor mechanism, the patella increases the effective lever mechanism of the quadriceps by providing anterior displacement of the quadriceps tendon. The quadriceps tendon is a continuation of the quadriceps femoris muscle, which consists of the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius, which extend the knee.

Physical Examination

Proper examination of the knee requires the patient to be supine, with both legs exposed. Examination of the knee begins with visual inspection ( Box 48.1 ), followed by palpation. On initial inspection, localized swelling should be distinguished from the presence of a joint effusion. If a large joint effusion is present, the patella is elevated from the femur by fluid. Loss of the medial peripatellar concavity may be the only sign of a small knee effusion. Swelling in the prepatellar bursa or infrapatellar bursa, termed bursitis, is found just beneath the skin, superficial to or just inferior to the patella, respectively. Bursitis should not be confused with a knee effusion, because they have different etiologies and treatments.

Inspection should also include assessing for erythema of the joint, which may suggest an inflammatory or infectious process. Obvious deformities or open wounds should also be noted during visual assessment. The posterior aspect of the knee should be examined for fullness.

After inspection, palpation of the knee should be performed ( Fig. 48.3 ). The bony prominences of the knee (patella, tibia, fibula) should be palpated to assess for fracture or overuse injuries. The quadriceps tendon and the patellar tendon should be palpated for palpable defects that might suggest tendon rupture. The joint line should be palpated for tenderness. The posterior aspect of the knee should be palpated for tenderness or palpable defects. The vascular integrity of the knee should be assessed with the dorsalis pedis (DP) and posterior tibial (PT) pulses in the foot.

Accurate diagnosis of knee injuries based on examination alone is challenging in the acute phase because of pain and swelling. Thus the main goals are to relieve pain, ensure proper location and stability of the joint, assess for neurovascular injury, determine the need for radiography, and identify infections and inflammatory conditions.

A number of maneuvers have been developed to aid the emergency clinician in diagnosing ligamentous and meniscal injuries using only the physical examination (see later). However, interpretation of these maneuvers is limited by pain, splinting, or effusion and has been found to be variably accurate when compared with magnetic resonance imaging (MRI) or arthroscopy findings. Accuracy of these maneuvers is improved if they are done after pain and swelling have resolved. The primary goals in emergency care are to identify whether fracture, dislocation, or vascular injury is present, ensure weight-bearing status, and refer for reevaluation by a primary care provider or orthopedist in a timely manner.

Dislocation

A patient presenting with a history of self-reduced knee “dislocation” may have suffered an actual knee dislocation or a patellar dislocation. It is imperative to clarify both the mechanism of injury and what the patient observed to ascertain whether the patella or knee was dislocated, because these two diagnoses have very different management but may self-reduce prior to arrival.

Effusion

Differential diagnoses for patients presenting with a large effusion in the setting of trauma include a distal femur fracture, an ACL or a PCL injury, dislocation, and a tibial plateau or spine fracture. Examination in these patients is often difficult, so the mechanism of injury as well as the location of tenderness and x-ray findings may help the clinician to differentiate between these entities. An atraumatic knee effusion commonly results from osteoarthritis. A focused history and physical examination are important as the differential diagnoses includes insufficiency fracture, septic arthritis, inflammatory arthritis (e.g., gout), hemarthrosis or lipohemarthrosis from occult fracture, avascular necrosis, a ruptured Baker cyst, or possible malignancy. Arthrocentesis may be used for diagnosis.

Anterior Knee Pain

Patellofemoral pain is one of the most common causes of anterior knee pain in children and adults but is usually not associated with bony tenderness. Tenderness over the tibial tubercle in a pediatric patient may indicate Osgood-Schlatter disease, but underlying fracture should be considered in the setting of acute trauma. In an adolescent, pain along the femoral or tibial epiphysis after trauma may represent a Salter-Harris type I fracture, a fracture through the physis. Tibial tenderness in an adult may suggest a tibial plateau fracture.

Gradual-onset pain over the anterior femoral condyle in pediatric patients should raise suspicion for osteochondritis dissecans (OD). The anterior knee should also be inspected for any sign of septic or aseptic bursitis over the prepatellar bursa. Acute onset, traumatic pain over the anterior aspect of the knee should raise suspicion for an extensor mechanism injury, such as patellar tendon rupture, quadriceps tendon rupture, or patellar fracture. A straight leg raise is useful in assessing the integrity of the extensor mechanism. More insidious onset of pain over the patellar or quadriceps tendon may be more suggestive of tendinitis.

Medial and Lateral Pain

Acute-onset medial or lateral joint line pain after a twisting mechanism should raise suspicion for a meniscus injury. Differential diagnoses for meniscal tears are extensive and include loose bodies, osteochondrotic lesions, and tibial fractures. Medial knee pain is also seen with medial tibial stress fractures, proximal medial tibial stress syndrome (MTSS), pes anserine bursitis/tendinopathy, or MCL strain. Lateral knee pain is typically associated with LCL strain, IT band dysfunction, popliteal tendinitis, or proximal fibular stress fracture.

Posterior Knee Pain

Baker cysts are a common cause of posterior knee pain. However, differential diagnoses should include a deep vein thrombosis (DVT), hamstring injury, or popliteal artery pseudoaneurysm.

Anterior Drawer/Lachman Test

The anterior drawer test seeks to identify tears of the ACL. The test is performed with the patient in a supine position, hip flexed at 45 degrees, and knee flexed at 90 degrees. While stabilizing the patient’s foot, the examiner places his or her thumbs over the joint line while pulling the tibia forward. The thumbs are used to palpate for any translation of the tibia relative to the femur. A positive test result is defined as greater anterior translation of the tibia relative to the femur as compared with the other knee. The test is poorly sensitive but fairly specific. The Lachman test is a more accurate test for ACL injuries, with a high sensitivity and specificity. The Lachman test is done with the knee flexed 20 to 30 degrees while the examiner uses one hand to grasp and stabilize the femur. The tibia is then pulled anteriorly, and the examiner notes tibial excursion. The examiner records “firmness” or a “soft end point.” The end point can be graded as 1+ (0 to 5 mm more displacement than on the normal side), 2+ (5 to 10 mm), or 3+ (>10 mm). The PCL must be intact for the test results to be valid.

Radiologic Evaluation

X-rays can be used to assess for radiopaque foreign bodies, subluxation, dislocation, fracture, and joint space narrowing. Note that on a straight anteroposterior (AP) view, the anterior and posterior portions of the normal tibial plateau may not appear to be at the same level.

Some plain film findings may be suggestive of ligamentous injuries as well. An effusion can be seen as a radiolucent area (with density similar to that of fat) distending the joint capsule. The presence of a linear interface between two different densities within an effusion suggests lipohemarthrosis ( Fig. 48.4 ), in which the effusion contains not only blood but also fat. This feature results from the entry of marrow fat into the joint cavity and is suggestive of fracture.

Clinical decision rules help to decrease unnecessary radiography. The Ottawa Knee Rules and the Pittsburgh Knee Rules are the most commonly used (see Table 48.2 ). Patients meeting any listed criteria should have knee x-rays performed. The Ottawa Knee Rules are meant to be applied only to acute injuries (<7 days old).

When the two rules are compared, both are more than 95% sensitive, but the Pittsburgh rule may be more specific, allowing fewer radiographs to be done without sacrificing sensitivity. Compliance with knee rules is poor in academic emergency departments (EDs) for various reasons. We recommend considering the use of either decision rule as support in evaluating patients with knee pain in the ED. However, neither rule is meant to supersede clinical judgment, because neither rule is 100% sensitive or specific for fracture.

In the evaluation of knee trauma, knee x-rays should include a standard AP, lateral, and “sunrise” view. Tunnel views, which image the intercondylar notch, are used to detect tibial spine fractures and loose bodies within the notch. Oblique views can be helpful in identifying tibial plateau fractures. Sunrise views are essential in evaluating for patellar fractures.

Because the sensitivity of radiographs for acute knee injuries is low, we recommend computed tomography (CT) or knee immobilization and urgent orthopedic referral reevaluation when fracture is suspected and plain films are negative. MRI may also be used to detect fractures and is the imaging gold standard (comparable with arthroscopy findings) for soft tissue injury but is rarely indicated in the evaluation of knee and leg trauma in the ED.

Vascular Imaging

CTA has largely replaced catheter-based angiography in assessing for popliteal artery injury after knee dislocations because it carries less risk without sacrificing a high sensitivity and specificity in detecting arterial injury. MR angiography and duplex ultrasonography can also be used for evaluation of the popliteal artery after tibiofemoral dislocation, but their roles in the ED setting are less well defined.

Arthroscopy

Arthroscopy is a nonemergent but useful modality in the diagnosis and treatment of knee injuries, including injuries of the meniscus, cruciate ligaments, articular cartilage, capsule, and synovium. Arthroscopy is superior to MRI for the diagnosis of meniscal tears and other soft tissue injuries, and the diagnosed problem can be repaired immediately. The need for arthroscopy need not be determined in the emergent setting as long as appropriate and timely referral to an orthopedist is made.

Joint Injection

An open joint is considered a surgical emergency. When violation of the joint capsule is suspected but not obvious, studies suggest that CT imaging performs better than the sterile saline load test in identifying traumatic knee arthrotomies. The sterile saline load test involves the injection of 200 mL of sterile saline into the joint capsule while observing the laceration to see if the saline emerges from the laceration. Methylene blue has not been shown to be more accurate than saline, can interfere with arthroscopy, and may cause an inflammatory reaction; therefore it is not advised. For open joints, we recommend orthopedic consultation, emergent antibiotics, and CT imaging to confirm diagnosis. Of note, there is mounting literature that CT imaging is sensitive for an open joint and less painful than infusing a saline load into the involved joint.

Arthrocentesis

Aspiration of fluid from the knee joint can be diagnostic as well as therapeutic by reducing pressure from an effusion. Arthrocentesis may be performed if the injured knee is greatly distended with a tight effusion and can be useful to perform if the cause of the joint effusion is unclear. Analysis of the aspirate can differentiate simple effusion, hemarthrosis, lipohemarthrosis, rheumatologic conditions, and septic arthritis and often provides significant relief of pain for the patient. Arthrocentesis should be avoided in the setting of overlying cellulitis .

Knee Dislocations—Foundations

Knee dislocation refers to tibiofemoral dislocation and should not be confused with patellofemoral dislocation, a relatively minor injury. Knee dislocation is a limb-threatening emergency due to the risk of popliteal artery injury. Knee dislocations are always associated with significant ligamentous injury. The joint capsule is disrupted, with accompanying trauma to the muscles and tendons. Injury to the popliteal artery is the most severe complication and is the primary cause of morbidity and limb loss.

Knee dislocation is relatively uncommon but should be considered in the setting of an appropriate injury mechanism because many dislocations spontaneously reduce before the patient arrives in the ED. Importantly, reduction before ED arrival does not lessen the likelihood of vascular injury, and these patients should be evaluated for vascular injury, particularly if the patient is obese, has an associated distracting injury, or if the injury involved in a high-energy mechanism.

The neurovascular bundle, which is composed of the popliteal artery, popliteal vein, and common peroneal nerve, runs posteriorly behind all bony and ligamentous structures in the popliteal fossa. The popliteal artery is fixed in the fibrous tunnel of the adductor magnus hiatus proximally and traverses the fibrous arch of the soleus and interosseous membrane distally. In essence, it is tethered to the femur and tibia, and its inherent immobility renders it susceptible to injury during dislocation. Because of the parallel course of the popliteal vein and peroneal nerve, they are often injured simultaneously.

Anatomically, dislocations are described according to the displacement of the tibia relative to the femur. They are classified into five types—anterior, posterior, medial, lateral, and rotary. Most knee dislocations are anterior and result from hyperextension. Posterior dislocations are the second most common type and usually result from high-velocity direct trauma to the flexed knee, often in association with vehicular trauma (e.g., automobile dashboard impact).

Knee Dislocations—Clinical Features

The diagnosis of knee dislocation is based on the mechanism of injury and clinical and radiographic findings. When a dislocation is present and has not reduced spontaneously, it is usually clinically obvious. However, there may be no effusion after reduction because the ruptured capsule allows blood and joint fluid to escape into the thigh and leg.

Popliteal artery injury most commonly occurs with posterior dislocations. The collateral geniculate arteries around the knee also may be damaged directly or may be secondarily compressed by hematoma formation after the dislocation. Direct arterial injury, decreased collateral circulation, and elevated compartment pressures all may compromise limb perfusion.

Findings associated with peripheral vascular injury and management approaches are described in Chapter 40 . The PT and DP pulses should be evaluated, but popliteal artery injury may still be present in some patients with palpable pulses.

Isolated intimal tears are not detectable on physical examination and are seen only angiographically or with duplex ultrasound. Although these tears are usually managed nonoperatively, a vascular surgeon should be consulted when identified. Injuries to small branches of the popliteal artery can be managed by observation and serial examinations.

As with all limb injuries, neurologic integrity should be assessed and documented. The peroneal nerve is at risk for injury, especially in posterolateral dislocations. Peroneal nerve function is evaluated by determining sensation of the dorsum of the foot and by having the patient dorsiflex the ankle. Less commonly, the posterior tibial nerve may be injured, which causes diminished plantar sensation and inability to flex the foot. Complete nerve palsy in the acute setting is associated with a poor prognosis for recovery.

Delayed complications associated with traumatic knee dislocations include DVT, compartment syndrome, pseudoaneurysm, and arterial thrombosis. Compartment syndrome may develop within 24 to 48 hours of the initial injury. Pseudoaneurysms are rare but may form several hours to months after popliteal artery injury. Heterotopic ossification is a poorly understood syndrome of calcification of the soft tissues of the knee. It has been observed in uninjured knees of patients who have sustained major trauma. In its most severe form, heterotopic ossification can cause severe decrease in knee mobility. Almost half of dislocated knees are found to have subsequent heterotopic ossification.

Knee Dislocations—Diagnostic Testing

The diagnostic evaluation begins with an understanding of two crucial facts. First, half of all tibiofemoral dislocations are reduced before presentation, and injury to the popliteal artery should be assumed with tibiofemoral dislocation regardless of spontaneous reduction. Therefore the diagnostic strategy is applied to all patients with known knee dislocation, multiligament knee injury, or high-force trauma to the knee. The intubated or unresponsive multitrauma patient with ecchymosis around the knee may harbor an occult popliteal injury.

Popliteal artery injury can be assessed by measurement of the ankle-brachial index (ABI) with serial physical examinations, CTA, and duplex ultrasonography. Formal angiography is rarely used. Of note, increased body mass index (BMI) is associated with a greater risk of vascular injury after knee dislocations. Serial palpation of the pedal pulses is not sufficiently sensitive for an injury with such potentially devastating consequences. Adding measurement of the ABI improves sensitivity. The ABI is the ratio of the systolic blood pressure measured in the standard humeral location relative to the systolic pressure measured at the ankle. An ABI more than 0.9 has a negative predictive value for popliteal artery injury, approaching 100% in knee dislocation. There is recent evidence that palpable dorsalis pedis and posterior tibial pulses combined with an ABI of 0.9 or greater is 100% sensitive in detecting clinically relevant popliteal artery injuries after knee dislocations ; however, the diagnostic strategy depends on the patient’s presentation. In a patient with hard signs of vascular injury (see Table 48.3 ), emergent surgical exploration is warranted. If the patient has asymmetric pulses or distal pulses are present but ABI is less than 0.9, then CTA is warranted. If the distal pulses are both present and ABI is greater than 0.9, we advise admission for serial neurovascular examinations every 3 to 4 hours for at least 24 hours. An algorithm for management is depicted in Fig. 48.5 .

Knee Dislocations—Management and Disposition

The dislocated knee should be reduced at the earliest opportunity during the secondary survey. The neurovascular status documented before and after reduction. For patients being transferred from a nontrauma center to a trauma center, reduction should be attempted prior to transfer. Reductions usually can be accomplished with simple traction-countertraction, almost always requiring procedural sedation in the alert patient. Lateral pressure may be required. For an anterior dislocation, the femur can be pushed posteriorly while the tibia is pulled anteriorly, with special care taken not to apply undue pressure to the popliteal fossa. Posterolateral dislocations may not be reducible because the medial femoral condyle and MCL may secure the dislocated joint in place, in which case emergent open reduction in the operating room is indicated. Because many reductions are unstable, the limb should be immobilized in a long leg posterior splint with the knee in 15 to 20 degrees of flexion after reduction; serial neurovascular assessments will be necessary, warranting admission.

Disability is minimized by expedient revascularization and primary arterial repair, and thorough wound débridement. Open joints require prophylactic antibiotics, such as intravenous (IV) cefazolin 2 g. If the neurovascular structures remain intact after dislocation, the knee joint is reduced, splinted, and allowed to rest for 2 to 3 days before reconstruction of the torn ligaments.

Distal Femur Fractures—Foundations

Distal femur fractures are uncommon and usually result from a high-energy mechanism. An isolated fracture of the femoral condyle may occur, or the fracture may extend in a T or Y pattern to include the intercondylar or supracondylar region of the femur. Condylar fractures are intraarticular and may result in disruption of the articular surface, with subsequent arthritis. See Chapter 47 for more detailed discussion of femoral shaft and proximal femur fractures.

Distal Femur Fractures—Clinical Features

Patients with condylar or intercondylar fractures have pain and swelling in the distal femur and suprapatellar region and often are unable to bear weight. Examination may reveal shortening, rotation, and angulation of the extremity and tenderness to palpation along the medial or lateral joint line. Acute hemarthrosis is common and may be caused by intra-articular extension of the fracture or associated ligamentous injury. Distal neurovascular status should be documented. Any laceration in the region of the fracture represents an open fracture until proven otherwise. Distal femur fractures may be associated with thrombophlebitis, fat embolus syndrome, delayed union or malunion if reduction is incomplete or not maintained. Intra-articular or quadriceps adhesions if the fracture is intraarticular, angulation deformities, and osteoarthritis, particularly in the patellofemoral articulation are some potential long-term complications.

Distal Femur Fracture—Diagnostic Testing

Routine AP and lateral views should be obtained and usually show the fracture pattern and any significant displacement of fragments. In high-energy injuries, radiographs of the ipsilateral hip and tibia are recommended to exclude associated fractures. Occasionally, CT imaging or MRI may be required to diagnose a nondisplaced fracture. If signs of vascular impairment are present, consultation for angiography or surgical exploration should be obtained emergently.

Distal Femur Fractures—Management and Disposition

Distal femur fractures are extremely painful. Ultrasound-guided femoral nerve blocks can be considered to reduce the need for opiates. After the initial examination, the leg should be splinted to prevent excessive motion and reduce pain at the fracture site. Emergent orthopedic consultation is advised. In a stable patient with an uncomplicated fracture dislocation, reduction may be done with skeletal traction, followed by immobilization. Intraarticular fractures generally are treated with open reduction and internal fixation. Virtually all patients with distal femur fracture require admission.

Tibial Plateau Fractures—Foundations

Tibial plateau fractures often are intra-articular. The most common mechanism of injury is a strong valgus force with axial loading. Severe high-energy tibial plateau fractures occur primarily in younger patients often after motor vehicle collisions (MVCs) or falls from heights. These fractures may be open and occur in concert with other associated injuries. Fatigue stress fractures of the tibial plateau occur mostly in older and obese adults. These low-energy fractures are the result of compression forces in osteoporotic bones.

The Segond fracture represents a bone avulsion of the lateral tibial plateau ( Fig. 48.6 ). The avulsion occurs at the site of attachment of the lateral capsular ligament. On radiographs, an oval-shaped fragment can be seen adjacent to the lateral tibial plateau. Segond fractures are usually accompanied by ACL disruption. Most Segond fractures are caused by sports injuries; the mechanism is almost always knee flexion with excessive internal rotation and varus stress.

Tibial Plateau Fractures—Clinical Features

Tibial plateau fractures cause pain, tenderness, ecchymosis, soft tissue swelling, and hemarthrosis when intra-articular. A valgus or varus limb deformity may be present and usually indicates a depressed fracture or concomitant leg fracture. The most important aspect of the initial examination is assessment of neurovascular status. Many tibial plateau fractures cause vascular complications. The popliteal artery may be injured by fragments from bicondylar or comminuted fractures involving the subcondylar area. Displaced fractures of the lateral condyle may cause peroneal nerve paralysis or anterior tibial artery injury. Stretch of the peroneal nerve is the usual cause of injury. Ligamentous injuries frequently accompany tibial plateau fractures, most often involving the ACL and MCL. Of note, these patients are also at high risk for compartment syndromes.

Tibial Plateau Fractures—Diagnostic Testing

Lipohemarthrosis, seen as a fat-fluid level on a plain film, suggests an occult fracture and is caused by entry of marrow fat into the joint space (see Fig. 48.4 ). Lipohemarthrosis may also be detected on aspiration of a joint effusion. All knee radiographs should be examined closely for avulsion fragments from the fibular head, femoral condyles, and intercondylar eminence because these may indicate ligamentous injury. Widened joint spaces associated with a fracture of the opposite condyle also may indicate concomitant ligamentous injury.

CT imaging and MRI are more sensitive than plain radiography and quantify the amount of depression in displaced fractures and extent of articular surface involvement in comminuted fractures. In a patient with acute traumatic knee pain characterized by tibial tenderness to palpation and inability to bear weight, a CT scan should be done to rule out fracture if x-rays are nondiagnostic.

Tibial Plateau Fractures—Management and Disposition

All patients with a tibial plateau fracture should be referred for evaluation by an orthopedist, which often occurs with direct consultation in the ED. Absolute indications for emergent orthopedic consultation would include joint instability, open fracture, neurovascular compromise, and compartment syndrome. If orthopedic consultation is not obtained in the ED, acute fractures should be immobilized in a noncircumferential splint such as a knee immobilizer and the patient should not bear weight on the limb until evaluated by an orthopedist, preferably within the week. Patients should be given strict return precautions for compartment syndrome. Stable nondisplaced fractures may be treated with immobilization alone, but instability or significant depression or disruption of the joint surface requires surgical management. CT scanning is often required for surgical planning, even when indications for surgery are evident based on the clinical examination and x-ray.

Prolonged immobilization can result in DVT. Prophylactic treatment with low-molecular-weight heparin can reduce the risk of DVT but remains controversial. According to a Cochrane review, there is moderate-quality evidence that low-molecular-weight heparin could reduce the incidence of DVT following immobilization of the leg. However, both the heparin and placebo groups exhibited a wide range of venous thromboembolism (VTE) across studies. A recent randomized controlled trial failed to show evidence that heparin reduced the incidence of VTE after casting of the lower leg. Given the controversy surrounding this topic, we suggest that the decision to anticoagulate patients requiring lower limb immobilization be made on an individual basis after engaging in shared decision making with the patient and orthopedic specialist. Scoring systems have recently been developed to assist the physician in deciding whether to anticoagulate when immobilizing the lower extremity but have not been externally validated.

Fractures of the Intercondylar Eminence (Tibial Spine)—Foundations

A fracture of the anterior tibial spine usually is associated with an ACL rupture. Tibial spine fractures are more common in children than in adults because the ligaments are stronger than the adjacent physeal plates in the immature skeleton. This fracture may be an isolated injury in the presence of open physes.

Most tibial spine fractures occur as a result of abrupt knee twisting, hyperflexion, hyperextension, or valgus-varus forces generated during MVCs or athletic activities. Tibial spine fractures occur by twisting knee movements, whereas hyperextension or hyperflexion forces may cause avulsion of the intercondylar eminence or cruciate ligaments from their tibial attachments.

Tibial Spine Fractures—Clinical Features

After a tibial spine fracture, the patient reports pain and swelling of the knee and may be unable to bear weight on the affected extremity. The examination confirms an acute hemarthrosis which may impair full knee extension. Tense effusion will limit ROM, hinder physical examination, and mask ligament disruption. ACL laxity is expected.