Anatomy

The knee is the largest and most complex joint in the body. Injuries to this joint are common, so a clear understanding of the anatomy and pathophysiology of the knee is essential for appropriate evaluation, diagnosis, and treatment of disorders in this area.

The knee has a wide range of motion, including flexion, extension, abduction, adduction, and internal and external rotation. Three different articulations are present in the knee: the patellofemoral articulation (anterior) and articulations between the lateral and medial tibial and femoral condyles. In full extension, the stabilizing ligaments of the knee are tight and prevent rotary motion of the knee. Beyond 20 degrees of flexion, the ligaments are relaxed and allow axial rotation of the joint.

Knee stability is provided solely by ligaments and tendons in and around the joint (Fig. 84.1). The knee joint is encapsulated by fibrous connective tissue lined by a synovial membrane. The knee capsule is continuous with the suprapatellar bursa, which expands when a joint effusion is present.

Fig. 84.1 Knee anatomy.

ACL, Anterior cruciate ligament; LCL, lateral collateral ligament; MCL, medial collateral ligament; PCL, posterior cruciate ligament.

(From Brown JR, Trojian TH. Anterior and posterior cruciate ligament injuries. Prim Care 2004;31:925–56.)

The popliteal fossa contains the popliteal artery and vein and the peroneal and tibial nerves. The popliteal fossa is delineated laterally by the biceps femoris muscle, medially by the semimembranosus and semitendinosus muscles, and inferiorly by the gastrocnemius muscle.

The popliteal artery is a continuation of the femoral artery after it leaves the adductor hiatus. It gives rise to the geniculate arteries, which form a rich vascular anastomosis around the knee, and divides to form the anterior and posterior tibial arteries at the level of the tibial tubercle. The popliteal artery is immobilized proximally and distally within the popliteal fossa, which predisposes it to vascular injury in the setting of traumatic knee injuries.

The tibial nerve and common peroneal nerve (a branch of the tibial nerve) innervate the knee. Because the tibial nerve is not immobilized proximally, it is less likely than the popliteal artery to be injured in the setting of joint disruption. The common peroneal nerve travels around the head of the fibula and divides into the deep and superficial peroneal nerves.

Evaluation

Key aspects of evaluation of knee are listed in Box 84.1. In general, evaluation of knee complaints should also include an examination of the hip and back to prevent overlooking a source of pain referred to the lower extremity.

During evaluation, the point of maximal tenderness should be assessed last. Specific tests for evaluating ligamentous and meniscal injuries are detailed in Table 84.1. Comparison with the uninjured or normal knee is helpful, especially for assessment of ligamentous laxity.

Table 84.1 Stability Testing

Joint pain or swelling may limit full evaluation of the knee in the acute setting. Patients with limited evaluations should undergo immobilization and follow-up examination within 7 days. Key physical findings are listed in discussions of specific disorders later in the chapter.

Diagnostic Testing

Because acute injuries to the knee commonly involve soft tissue, plain radiographic examination is not always indicated. The Ottawa Knee Rules¹ (Box 84.2) and the Pittsburgh Knee Rules² (Box 84.3) are useful guides to aid in the decision of whether to order plain radiographs. Both criteria are sensitive for fractures, but the Pittsburgh criteria are more specific and can be applied to both children and adults.

If plain radiographs are indicated, a minimum of an anteroposterior (AP) and a lateral view should be obtained. Oblique radiographs are helpful for detecting subtle tibial plateau fractures. The intercondylar or tunnel view is helpful in evaluating for tibial spine fractures and osteochondral defects (Fig. 84.2). Assessment of the patellofemoral joint and evaluation for the presence of patellar tilt (increased propensity for patellar subluxation or dislocation) can be done with the Merchant or sunrise view (Fig. 84.3). Comparison radiographs of the unaffected extremity are helpful in discerning problems in skeletally immature patients.

Fig. 84.2 Knee radiograph, tunnel or intercondylar view, showing an osteochondritis dissecans lesion (arrows).

Fig. 84.3 Merchant view of the knee showing patellar tilt and subluxation.

When describing the knee radiograph, the examiner should note the alignment and joint spacing of the femoral condyles in relation to the tibial plateau. Narrowing of the joint space (particularly in weight-bearing views) indicates articular cartilaginous and meniscal degeneration. The patella should be examined for possible fractures (in the event of a direct blow to the anterior aspect of the knee) and the presence of a bipartite or tripartite patella. Significant joint effusions are evident as a water-density radiolucency on the lateral view, anterior to the distal end of the femur.³ Effusions seen shortly after injury are suggestive of anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) tears, tibial plateau fractures, femoral condyle fractures, or patellar fractures.⁴

In the setting of acute injuries, radiographs should be examined for the presence of fractures involving the tibial plateau (depression fracture) or tibial spine (suggesting rupture of the ACL). Segond fractures are avulsion fractures of the lateral tibial plateau at the site of attachment of the lateral capsular ligament. These fractures are associated with ACL and meniscal injuries. The presence of posterior opaque bodies should also be noted. These may be fabellae (congenital sesamoid) or loose bodies. More than 75% of loose bodies originate from osteochondral lesions.³ Occult fractures that are commonly missed on plain radiographs include patellar, tibial plateau, fibular head, and Segond fractures.⁵

Musculoskeletal ultrasound techniques can be used to diagnose ACL and PCL tears and may be helpful in the diagnosis of meniscal injuries.⁴ Magnetic resonance imaging (MRI) can be used to confirm suspected meniscal injuries, ligamentous disruptions, osteochondral lesions, and occult fractures; however, it is rarely indicated in the acute setting. Computed tomography (CT) is helpful for establishing the extent of certain fractures (e.g., tibial plateau fractures) and is often more readily available than MRI.

Anatomy

The lower part of the leg contains the tibia and fibula. The tibia is the only weight-bearing bone in this part of the leg and is the most commonly fractured long bone in the body. Its superficial course predisposes it to a higher incidence of open fractures. The two bones are bound together by the superior and inferior tibiofibular joint along with an interosseous membrane. The interosseous membrane aids in stability of the ankle mortise.

The thigh muscles attach to the upper part of the tibia and lend stability to the knee joint. The muscles of the lower part of the leg, which are enclosed by fascia, can be divided into four compartments: anterior, posterior, deep posterior, and lateral (Fig. 84.4). Specific motor and sensory nerve distributions are listed by compartment in Table 84.2.

Fig. 84.4 Cross section of the lower extremity at the level of the calf.

(From Khatri VP, Asensio JA. Operative surgery manual. Philadelphia: Saunders; 2003.)

Table 84.2 Peripheral Nerve Assessment of the Leg

Evaluation

A directed history of the traumatic events, duration of symptoms, and exacerbating events or activities is necessary to assess lower leg injuries. Examination of lower leg complaints should include an evaluation of the back, hips, knee, and ankle to assess for pain referred to the lower part of the leg or associated injuries.

Thorough assessment of skin integrity, neurovascular status, and the leg compartments is essential in the evaluation of any traumatic lower leg injury. The point of maximal tenderness should be evaluated last.

Diagnostic Testing

Imaging of the lower part of the leg should include the joint above and the joint below the injury. Oblique views are useful for detecting tibial plateau fractures, which may not be seen on routine views.

Knee Dislocations

Knee dislocations are relatively uncommon and represent, at most, 0.2% of all orthopedic injuries. They require disruption of at least three of the four major ligaments of the knee. Common mechanisms include traffic accidents, sporting injuries, or simple mechanical falls. Dislocations are classified on the basis of the direction of tibial movement in relation to the femur. Anterior and posterior dislocations account for approximately 70% of all knee dislocations. Knee dislocations may also have associated intraarticular fractures involving the tibial plateau or femoral condyles.⁶

Knee dislocations are associated with a high risk for neurovascular injury and should be considered an orthopedic emergency. The neurovascular bundle runs posterior to the bony and ligamentous structures in the popliteal fossa. The popliteal artery and nerve are fixed in the fibrous tunnel of the adductor magnus muscle proximally and traverse the fibrous arch of the soleus muscle and interosseous membrane distally. The relative immobility of the neurovascular bundle makes it susceptible to injury. The popliteal artery may be injured in up to 14% of all knee dislocations (Fig. 84.5). Traction injuries to the common peroneal and, less commonly, to the tibial nerve may also be present.⁶

Fig. 84.5 A, Radiograph demonstrating posterior knee dislocation from a dashboard injury. B, Arteriogram showing an injury to the popliteal artery.

(From Browner BD, Jupiter JB, Levine AM, et al. Skeletal trauma; basic science, management, and reconstruction. 3rd ed. Philadelphia: Saunders; 2003.)

A grossly unstable knee after a traumatic injury should be assumed to be a reduced dislocation until proved otherwise. Any patient suspected of having sustained a knee dislocation should undergo a careful neurovascular examination. Anterior and posterior dislocations have a higher incidence of vascular injury. Vascular compromise in a dislocated knee requires immediate reduction.

Neurovascular status should be documented before and after reduction. All patients with a suspected or confirmed knee dislocation should have the ankle-brachial index (ABI) calculated. An ABI of less than 0.9 has high predictive value for a vascular injury, and such patients should undergo either CT or traditional angiography. Patients with normal findings on vascular examination and an ABI lower than 0.9 should be observed for at least 24 hours with neurovascular checks every 2 to 3 hours.⁴ Prompt diagnosis of vascular injury is essential given the chance of development of progressive distal ischemia. When injury to the popliteal artery has occurred, patient outcome is directly related to the duration of ischemia.

Standard AP and lateral radiographs are adequate for initial evaluation of knee dislocations. After appropriate analgesia and sedation, emergency reduction of the dislocated knee should be attempted. Longitudinal traction on the tibia (to free it from the femur) should be followed with a force in the opposite direction of the dislocation. The rotary components should also be corrected to restore normal leg alignment. After reduction, the knee should be immobilized in 15 to 20 degrees of flexion.⁴ Orthopedic consultation in the ED is mandatory for all suspected and confirmed knee dislocations.

Patellar Dislocation

The patella normally articulates in the groove between the femoral condyles. The vastus medialis, medial and lateral patellofemoral, and patellotibial ligaments and the medial retinaculum all stabilize the patella (Fig. 84.6).

Fig. 84.6 Patellofemoral and patellotibial ligaments.

These structures act as static stabilizers of the patella.

(From DeLee JC, Drez Jr D, Miller MD. DeLee & Drez’s orthopaedic sports medicine: principles and practice. 2nd ed. Philadelphia: Saunders; 2003.)

The overall incidence of patellar dislocations is estimated to be 7 per 100,000 per year and as high as 31 per 100,000 per year in patients between the age of 10 and 19 years.⁷ Patellar dislocations most commonly occur when a varus force is applied to a flexed knee or after forced contraction of a flexed quadriceps. Dislocations may be associated with meniscal tears, disruption of the medial collateral ligament, and osteochondral fractures.⁴

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