avulsed fragment The visible ossified fragment may be small because the tibial spine is mostly radiolucent cartilage Dislocations In a child, the knee joint itself rarely dislocates; usually, the distal femoral or proximal tibial epiphysis separates first Dislocation occurs more frequently after the growth plates have closed and usually with trauma that involves significant force, such as a motor vehicle collision or contact sports The knee will appear obviously deformed with the tibia or femoral condyles abnormally prominent in an anterior or posterior dislocation, respectively Disruption of the popliteal artery may occur with the dislocation, and the resulting hypoperfusion may be limb threatening Posterior tibial and dorsalis pedis pulses and peroneal nerve function (sensation between the great and second toe and ankle dorsiflexion) must be documented Radiographs will confirm the diagnosis TABLE 42.1 DIFFERENTIAL DIAGNOSIS OF THE INJURED KNEE Acute injuries Fractures Distal femoral epiphysis a Proximal tibial epiphysis a Tibial tubercle avulsion Patella Tibial spine avulsion Osteochondral fractures Soft tissue injuries Collateral ligament sprain b or tear Anterior cruciate ligament sprain or tear Posterior cruciate ligament sprain or tear Meniscal tears Quadriceps tendon rupture Patellar tendon rupture Hamstring strain b Posttraumatic infections a Dislocations and subluxations Patellar b Knee joint a Subacute injuries Osgood–Schlatter disease b Patellofemoral dysfunction b Patellar tendonitis (jumper knee) Prepatellar bursitis Osteochondritis dissecans (OCD) Baker cyst Iliotibial (IT) band syndrome Other Pathologic fractures a Referred pain from hip disease Slipped capital femoral epiphysis Avascular necrosis of the femoral head a Life- or limb-threatening causes of the injured knee b Common causes of the injured knee Patellar dislocation occurs as the quadriceps muscles pull the patellar tendon to extend the knee, especially when the knee has a valgus stress or the extremity is suddenly abducted when the leg slips laterally If the vastus medialis fibers not keep the patella in the intercondylar groove, the patella may dislocate laterally This injury rarely occurs from direct force but more commonly from a noncontact twisting injury The patient may feel a popping sensation with intense pain and present with the knee flexed The patella will be displaced laterally, and the diagnosis is usually made based on history and examination The dislocation may be reduced before radiographs are taken Postreduction radiographs should be obtained to rule out an associated avulsion or osteochondral fracture of the patella Patellar dislocations are often recurrent FIGURE 42.1 Anatomy of the knee—anterior view (patella removed)