Volume 6, Case 7
Annabelle R. Mateo, MD
Kapiolani Medical Center For Women And Children
University of Hawaii John A. Burns School of Medicine
This is an 18 year old male with a history of moderate psychomotor retardation and cerebral palsy, who is brought to the ED by ambulance with a painful and deformed left knee. As he attempted to stand up from his chair, he turned his body and his knee buckled. He heard a crack and fell onto the tile floor landing on the lateral aspect of his left knee. The patient kept his left knee in a flexed position and a splint was applied by paramedics. There was no breakage of the skin or prior history of any knee trauma. He denies any numbness or tingling sensation.
Exam: VS T37, P86, R26, BP120/80. He is a slender adolescent male in severe discomfort refusing to move his left lower extremity. His left knee is visibly deformed but no break in the skin or ecchymosis is noted. His patella is displaced laterally. His left leg and foot are warm with good distal pulses and intact sensation. He is given morphine prior to obtaining AP, lateral and sunrise views of his left knee.
View knee radiographs: Oblique, lateral and sunrise views.
View oblique view.
View lateral view.
View sunrise view.
An oblique view is taken instead of an AP because of the patient's discomfort. The oblique and lateral views are not very revealing, but the sunrise view shows a laterally displaced patella.
The arrow points to where the patella should be.
The infrapatellar or "sunrise" view of the patella with the knee at 20 degrees flexion isolates the patella. This view is helpful in evaluating an older child or adolescent since the patella ossifies after 5 years of age. The patella is a sesamoid bone that articulates in the groove between the femoral condyles [Arrow]. The patella is stabilized by the quadriceps tendon (superiorly), the patellar tendon (inferiorly), the vastus medialis oblique (medially), and the iliotibial band and vastus lateralis (laterally). The patella is rarely fractured in childhood because the overlying cartilage during early growth cushions it.
Patellar Dislocation
Patellar dislocations commonly occur between 16 and 20 years of age and tend to occur in females. Acute patellar dislocations have occurred in simple falls and sports injuries. Acute patellar dislocations almost always occur laterally. These typically result from a force displacing the patella while the foot is planted. Commonly, patients feel like the knee cap is going to pop out. This condition may spontaneously reduce or remain dislocated.
Clinically, the swollen knee is kept in a flexed position revealing a prominent medial femoral condyle. Chronic recurrent subluxation or dislocation tends to occur in patients with inherent mechanical patellofemoral instability such as genu valgum (knock-knees), shallow lateral femoral condyles, elongated patellar tendon, deficient vastus medialis, lateral insertion of the patellar tendon, shallow patellar groove, ligamenous laxity and/or deformed patella.
Reduction of a Dislocated Patella
Nonoperative treatment is usually recommended. Reduction of a laterally displaced patella requires extension (preferably slight hyperextension) of the knee while applying pressure directed medially to the displaced patella which helps to guide the patella back into position. Forceful manipulation is not needed and should be avoided. Post reduction films should be obtained to confirm the position of the patella and to exclude the presence of an osteochondral fracture. A posterior knee splint or knee immobilizer should be placed for 4 to 6 weeks and an orthopedic referral is recommended. Rehabilitation should be initiated as soon as possible, starting with straight leg raises to minimize quadriceps atrophy. Surgical repair is considered in patients at risk for recurrent dislocation.
Approach to Acute Knee Pain
1) Medical history. Describe the quality of acute knee pain and the location. Any previous knee injury or pain should be documented.
2) Examination begins with inspection for asymmetry, knee effusion, atrophy and congenital anomalies. Palpate the knee to assess areas of tenderness and crepitance. Muscle strength, joint range of motion and gait should also be evaluated. Palpation of the knee to ascertain tenderness and crepitance should be done.
There are several tests for patellar instability. a) The patella tilt test involves tilting the patella, which normally should be less than 20 degrees. b) With the knee flexed at 30 degrees, passive lateral and medial movement of the patella evaluates for possible dislocation or subluxation. This is referred to as the patellar glide test. c) Similarly, the patellar apprehension test is performed with the knee flexed at 30 degrees while the examiner attempts to sublux the patella laterally. It is positive if pain, involuntary quadriceps contraction or sense of "apprehension" is observed. This is indicative of a previous patellar dislocation or subluxation.
3) Plain radiographs - AP, lateral at 30 to 45 degrees of flexion, and infrapatellar (sunrise) views should be done in patients with recent knee trauma, open physes, high energy contact sports, pain that impairs activities of daily living, 3 or more months of anterior knee pain and poor compliance. The AP view identifies accessory ossification centers, osteochondral fractures, and varus/valgus alignments. Patella alta (high riding patella due to quadriceps contracture or an avulsed patellar ligament) or patella baja (low riding patella which may be congenital or due to ACL reconstruction) may be diagnosed on lateral views. Infrapatellar view (also called horizon view, Merchant's view, tangential or sunrise view) assesses patellar subluxation. The lateral patellar tilt, the sulcus angle, and congruence angle are measurements that can be made on the sunrise view which help to assess patellar dislocation.
Acute Knee Injuries
1) A knee dislocation is considered an orthopedic emergency since neurovascular compromise is common. Lower extremity infarction is the complication if vascular integrity is not restored. The popliteal artery is involved in 30% to 40% of dislocated knee injuries while the peroneal and tibial nerves are frequently involved in traction injuries. Knee dislocations are classified as anterior, posterior, medial, lateral or rotary (anterolateral, posteromedial and posterolateral).
Anterior dislocation results from hyperextension and is the most common type of knee dislocation. This occurs when a person walking briskly, steps into a hole hyperextending the lower extremity. This usually results in a posterior capsule tear followed by a rupture of the anterior cruciate and a partial tear of the posterior cruciate. Popliteal arterial injuries occur secondary to traction or laceration.
Posterior dislocation results when there is a direct posterior force applied to the anterior tibia when the knee is slightly flexed. The posterior capsule and cruciates rupture as the tibia is displaced posteriorly. Arterial injuries are not commonly seen.
Lateral dislocations typically involves damage to the medial collateral, both cruciates and the medial posterior capsule. This injury is occurs when there is a violent abduction force on the tibia against the femur. Arterial injuries are not common.
Medial dislocation occurs when there is a significant adduction force on the tibia against the femur resulting in damage to the lateral collateral ligaments, both cruciate ligaments, the posterior capsule and commonly, the peroneal nerve.
Rotary dislocations result from anteromedial or anterolateral force on the anterior tibia.
2) Patellar fractures commonly occur in adolescents and present as avulsion fractures from dislocations, osteochondritis dessicans due to overuse, symptomatic bipartite conditions, avulsion or "sleeve" fractures and transverse displaced fractures. Clinically, patients with direct tenderness over the superolateral patella should be suspicious for bipartite patellar stress fracture and fibrous nonunion. In nondisplaced fractures, a cast should be placed for 4 to 6 weeks. Open reduction and internal fixation (ORIF) is used to treat fractures that are displaced more than 3 to 4 mm. Knee stiffness, quadriceps atrophy, extensor lag and persistent pain are possible complications of patellar fractures.
3) Most ligamentous injuries are commonly due to direct knee trauma as seen in motor vehicle accidents or during vigorous sports activities when the knee is subjected to marked valgus and/or varus stresses. The most frequently injured are the medial collateral and anterior cruciate ligaments. Avulsion of the tibial spine is associated with anterior cruciate ligament injury. Radiographs should be routinely ordered to rule out fractures since knee injuries in children less than 14 years of age are more commonly seen than ligamentous injuries.
Avulsion of the tibial spine results from a hyperflexion of the knee (as would occur during a fall from a bicycle). Clinically, there is significant pain and the patient refuses to bear weight. Hemarthrosis may be evident.
Avulsion fracture of the tibial tuberosity is uncommon but is seen in adolescence. Males between 12 to 17 year old who actively engage in sporting activities such as basketball or high jumping are at risk. This injury usually occurs during jumping when the quadriceps is strongly contracted that the tibial tuberosity can be torn.
4) Patellar tendonitis is commonly seen in running or jumping athletes who train or play on hard surfaces. Clinically, pain and tenderness localizes at the patellar tendon, inferior or superior aspects of the patella or tibial tuberosity. Tendon thickening and dystrophic calcification may be detected on lateral view radiographs. Initial treatment modalities include rest, ice, NSAIDS, local modalities and rehabilitation programs to strengthen the muscles. Surgery is reserved for complete tendon rupture and considered in patients with refractory pain.
Key Points:
1) An unstable knee after traumatic injury is a reduced dislocation until proven otherwise.
2) An acutely traumatized unstable knee with absent distal pulses, an ability to move the toes or absent sensation to touch and pinch requies emergent surgical intervention.
3) PE: inspection, palpation and distal neurovascular examination. Consider obtaining radiographs before a more stressful examination in patients with extreme pain, swelling or inability to bear weight. An effusion may not be evident because a tear in the joint capsule will allow the blood to dissect into the surrounding tissues. Hyperextension places an unnecessary traction on the peroneal nerve and should be avoided. Swelling in the popliteal fossa may be indicative of popliteal artery injury.
4) AP and Lateral views are adequate. Infrapatellar or "sunrise" view is recommended if patellar dislocation or fracture is suspected.
5) Treatment : Reduction, immobilization, assessment of vascular injuries and emergent referral.
6) All reductions should be confirmed radiographically.
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