AAOS Comprehensive Orthopaedic Review

Section 6 - Trauma

Chapter 57. Knee Dislocations and Patella Fractures

I. Knee Dislocations

A. Epidemiology


1. Traumatic knee dislocations are rare, with an incidence of < 0.2% of orthopaedic injuries.


2. Actual incidence is likely higher due to underre-porting because 20% to 50% of knee dislocations spontaneously reduce.


B. Anatomy


1. The knee is a hinge joint with three articulations: patellofemoral, tibiofemoral, and tibiofibular.


2. Normal range of motion of the knee ranges from up to 10° of extension to 140° of flexion with 8° to 12° of rotation through the flexion-extension arc.


3. Dynamic and static knee stability is conferred by the soft tissues (ligaments, muscles, tendons, and menisci) and the bony articulations.


4. The popliteal vascular bundle courses through a fibrous tunnel at the level of the adductor hiatus.


a. Within the popliteal fossa, the popliteal artery gives off five branches: superior medial and lateral geniculate arteries, inferior medial and lateral geniculate arteries, and the middle geniculate artery.


b. The popliteal artery runs deep to the soleus muscle through another fibrous tunnel. It is the tethering of the popliteal artery at the adductor hiatus and soleus that makes it prone to injury during dislocation (

Figure 1).


5. The common peroneal nerve runs behind the fibular head and is also prone to injury (typically a stretch injury) with knee dislocation.


6. The posterolateral corner consists of the lateral collateral ligament (LCL), the popliteus tendon, the popliteofibular ligament, and the iliotibial band. It is often injured with knee dislocation and if not addressed can be a source of instability.


C. Mechanism of injury


1. High-energy mechanism typically is a motor vehicle accident in which the knee strikes the dashboard, causing an axial load to a flexed knee, or a fall from a height.


2. Lower energy mechanism typically is an athletic injury resulting in knee dislocation. A rotatory component to the dislocation is common. Morbid obesity (body mass index [BMI] >35) also is a risk factor for low-energy knee dislocations.


3. Hyperextension of the knee, with or without a varus or valgus stress, results in an anterior dislocation, whereas knee flexion with a posterior-directed force results in a posterior dislocation (dashboard injury).


4. Substantial soft-tissue injury is needed for knee dislocation. Typically at least three of the four ligaments are torn.


[Figure 1. Posterior anatomy of the knee showing the relationship between the popliteal artery and the tibial and common peroneal nerves.]

D. Clinical evaluation


1. Gross knee distortion is common on presentation unless the knee spontaneously reduces. Immediate reduction is indicated, without waiting for radiographs in the dislocated position.


2. Patients who sustain a knee dislocation that spontaneously reduces may have a relatively normal-appearing knee. Subtle signs of injury such as mild abrasions, a minimal effusion, or reports of knee pain may be the only abnormalities.


3. The extent of ligamentous injury is related to the degree of bony displacement. Ligamentous injury occurs with displacement of more than 10% to 25% of the resting length of the ligament. Certain tests can be used to assess ligamentous stability after joint reduction.


a. Anterior cruciate ligament (ACL): Lachman test at 30° knee flexion


b. Posterior cruciate ligament (PCL): Posterior drawer test at 90° knee flexion


c. LCL/posterolateral corner (PLC): Varus stress at 30° and full extension, increased tibial external rotation at 30° flexion, increased posterior tibial translation at 30° flexion


d. Medial collateral ligament (MCL): Valgus stress at 30° knee flexion


e. LCL/PLC and cruciate (at least one cruciate ACL or PCL): Varus in full extension and at 30° flexion


f. MCL and PCL: Increased valgus in full extension and at 30°


g. PLC and PCL: Increased tibial external rotation at 30° and 90°, increased posterior tibial translation at 30° and 90°


E. Vascular injury


1. Careful neurovascular examination is critical, both before and after reduction, and serially thereafter because thrombosis due to an unsuspected intimal tear may cause delayed ischemia hours or even days after reduction.


a. The popliteal artery is at risk during traumatic knee dislocations (in up to 60% of patients) because of the bowstring effect across the popliteal fossa, secondary to proximal and distal tethering.


b. Although distal pulses and capillary refill may be detected as a result of collateral circulation, these are inadequate to maintain limb viability.


2. The mechanism of arterial injury varies with the type of dislocation.


a. With anterior dislocations, the artery usually is injured as a result of traction, resulting in an intimal tear.


b. With posterior dislocations, the artery frequently is completely torn.


3. Vascular examination includes evaluation of the dorsalis pedis and posterior tibial artery pulses.


a. If the pulses are absent, immediate closed reduction should be considered. If still absent after reduction, emergent surgical exploration is indicated.


b. If pulse returns after closed reduction, angiography should be considered versus observation. The maximum time for ischemia in the limb is 8 hours.


c. If there are palpable pulses, the ankle brachial index (ABI) should be assessed. An ABI > 0.9 warrants observation, but if greater, angiography and/or surgical exploration is indicated (

Figure 2).


[Figure 2. Intraoperative angiogram showing complete disruption of the popliteal artery following posterior knee dislocation.]

F. Principles of treatment of vascular injuries


1. Vascular status (dorsalis pedis and posterior tibial artery pulses and capillary refill) must be evaluated and documented in any patient with a proven or suspected knee dislocation. The presence of arterial insufficiency or abnormality confirms a vascular injury.


2. Revascularization should be performed within 8 hours.


3. Spasm is an unacceptable explanation as a cause for decreased or absent pulses in an attempt to justify observation.


4. Consultation with a vascular surgeon is recommended to verify clinical findings and interpret studies.


G. Treatment of vascular injury


1. Following reduction, circulation should be reassessed, with immediate surgical exploration indicated if the limb is ischemic.


2. Urgent arteriography is indicated for patients with abnormal vascular status (diminished pulses, decreased capillary refill, ABI < 0.9) and a viable limb. Waiting for arteriography, however, should not delay surgical re-anastomosis.


3. Careful observation with serial examinations is indicated for patients with a normal vascular status (normal dorsalis pedis and posterior tibial artery pulses, normal capillary refill, ABI > 0.9).


4. MR arthrography/MRI is useful to evaluate nonocclusive (intimal) injury; however, their sensitivity and specificity are uncertain.


5. Arterial injury is treated with excision of the damaged segment and re-anastomosis with reverse saphenous vein graft.


H. Neurologic injury


1. Injury to the peroneal nerve is commonly associated with posterolateral dislocations, with injury varying from neurapraxia (usual) to complete transection (rare).


2. Primary exploration with grafting or repair is not effective; secondary exploration at 3 months also has been associated with poor results.


3. Bracing and/or tendon transfer may be necessary for treatment of muscular deficiencies.


I. Imaging


1. Radiographic evaluation


a. Because of the high incidence of neurovascular compromise associated with knee dislocation, immediate reduction is recommended before radiographic evaluation.


b. Following reduction, AP and lateral views of the knee should be obtained to assess the reduction and identify associated injuries. Dislocation is suggested by several findings (

Figure 3).


[Figure 3. Pre- and post-reduction radiographs of an anterolateral knee dislocation.]

i. Obvious dislocation


ii. Irregular or asymmetric joint space


iii. Lateral capsular (Segond) sign


iv. Ligamentous avulsions and osteochondral defects


2. Angiography


a. The use of angiography after knee dislocation is controversial.


b. Vascular compromise is an indication for emergent surgical intervention. Identifying intimal tears in a limb with an intact neurovascular status may be unnecessary because most do not result in thrombosis and vascular occlusion.



Figure 4. MRIs of the left knee of a patient who sustained a twisting injury from stepping in a hole during a softball game. T1-weighted (A) and T2-weighted (B) sagittal scans show disruption of the ACL (arrow) and PCL (arrowhead). C, T1-weighted sagittal image shows an injury to the lateral meniscus (asterisk), which appears to be elevated off the lateral tibial plateau. D, T2-weighted coronal image shows injuries to the lateral meniscus (asterisk) and PLC (black dot).]

c. Some authors advocate selective arteriography only if the ABI < 0.9. Regardless, the patient should be closely observed for evidence of vascular insufficiency.


3. Magnetic Resonance Imaging


a. MRI is indicated for all knee dislocations and equivalents.


b. MRI has value for preoperative planning, identifying avulsions and other ligamentous injuries, and identifying meniscal pathology and articular cartilage lesions (Figure 4).


J. Dislocation classification


1. Descriptive terms are based on displacement of the proximal tibia in relation to the distal femur.


a. Anterior: Forceful hyperextension of the knee beyond -30°. This is the most common type of dislocation, affecting between 30% and 50% of patients. Associated injuries include PCL (and possibly ACL) tears, with high incidence of popliteal artery disruption with increasing degree of hyperextension.


b. Posterior: Posteriorly directed force against proximal tibia of flexed knee (25%), also called a "dashboard" injury. Associated injuries include anterior and posterior ligament disruption and popliteal artery compromise with increasing proximal tibial displacement.


c. Lateral: Valgus force (13%) that disrupts the medial supporting structures, often with associated tears of both cruciate ligaments.


d. Medial: Varus force (3%) in which both lateral and posterolateral structures are disrupted.


e. Rotational: Varus/valgus with rotatory component (4%) that usually results in buttonholing of the femoral condyle through the articular capsule.


2. Other descriptive terms include open versus closed, reducible versus irreducible, and "occult" fractures, which indicate a knee dislocation with spontaneous reduction.


K. Closed reduction


1. Immediate closed reduction is essential, even in the field and especially in the compromised limb. Direct pressure on the popliteal space should be avoided during or after reduction.


2. Reduction maneuvers for specific dislocations


a. Anterior: Axial limb traction combined with lifting of the distal femur


b. Posterior: Axial limb traction combined with extension and lifting of the proximal tibia


c. Medial/lateral: Axial limb traction combined with lateral/medial translation of the tibia


d. Rotatory: Axial limb traction combined with derotation of the tibia


3. Posterolateral dislocation is believed to be "irreducible" because of buttonholing of the medial femoral condyle through the medial capsule, resulting in a dimple sign over the medial aspect of the limb. This dislocation requires open reduction.


4. Arthroscopy can be used to assess residual laxity.


5. Following reduction, the knee is splinted at 20° to 30° of flexion. It is essential to maintain this reduction.


6. Initial stabilization can be done with either a knee immobilizer (in extension for 6 weeks) or external fixation.


a. External fixation is better for grossly unstable knees that may subluxate in the brace.


b. It also protects vascular repair and fasciotomy and allows skin care for open injuries.


c. External fixation is also indicated for obese or multiple trauma patients.


L. Surgical repair


1. Repair or reconstruction of the torn structures generally is recommended because nonsurgical treatment in active individuals often leads to poor results. Shorter periods of immobilization result in improved knee motion and residual laxity whereas longer periods may improve stability but limit motion. Recent clinical series have reported better results with surgical treatment. However, no prospective, controlled, randomized trials of comparable injuries have been performed.


2. Complete posterolateral corner disruption is best treated with early open repair. Reconstitution of the PLC is important; the PLC should always be repaired before the ACL.


3. Immediate surgical repair


a. Unsuccessful closed reduction


b. Residual soft-issue interposition


c. Open injuries


d. Vascular injuries


i. These require external fixation and vascular repair typically with a reverse saphenous vein graft from the contralateral leg.


ii. Amputation rates as high as 86% have been reported when there is a delay beyond 8 hours with documented vascular compromise to limb.


iii. A fasciotomy should be performed at time of vascular repair for limb ischemia of longer than 6 hours.


4. Ligamentous repair is controversial: The current literature favors acute repair of lateral ligaments followed by early motion and functional bracing.


5. Timing of surgical repair depends on the condition of both the patient and the limb.


6. Meniscal injuries also should be addressed at the time of surgery.


M. Complications


1. Limited range of motion, most commonly related to scar formation and capsular tightness. This reflects the balance between sufficient immobilization to achieve stability versus mobilization to restore motion. If severely limiting, lysis of adhesions may be indicated to restore range of motion.


2. Ligamentous laxity and instability


3. Vascular compromise, which may result in atrophic skin changes, hyperalgesia, claudication, and muscle contracture


4. Nerve traction injury resulting in sensory and motor disturbances portends a poor prognosis, as surgical exploration in the acute (<24 hours), subacute (1 to 2 weeks), and long-term settings (3 months) has yielded poor results. Bracing or muscle tendon transfers may be necessary to improve function.

II. Patella Fractures

A. Epidemiology


1. Patella fractures represent 1% of all skeletal injuries.


2. The male to female ratio is 2:1.


3. The most common age group is 20 to 50 years old.


4. Bilateral injuries are uncommon.


B. Anatomy


1. The patella is the largest sesamoid bone in the body. The quadriceps tendon inserts on the superior pole, and the patellar ligament originates from the inferior pole of the patella.


2. There are seven articular facets. Of these, the lateral facet is the largest (50% of the articular surface). The articular cartilage may be up to 1 cm thick.


3. The medial and lateral extensor retinaculae are strong longitudinal expansions of the quadriceps and insert directly onto the tibia. If these remain intact in the presence of a patella fracture, then active extension will be preserved (

Figure 5).


4. The function of the patella is to increase the mechanical advantage and leverage of the quadriceps tendon, help nourish the femoral articular surface, and protect the femoral condyles from direct trauma. The blood supply arises from the genicu-late arteries, which form an anastomosis circumferentially around the patella.


C. Mechanism of injury


1. Direct: Trauma to the patella may produce incomplete, simple, stellate, or comminuted fracture patterns.


a. Displacement may be minimal due to preservation of the medial and lateral retinacular expansions.


b. Abrasions over the area or open injuries are common. Active knee extension may be preserved.


[Figure 5. Anatomy of the extensor mechanism of the knee.]


Figure 6. AP (A) and lateral (B) radiographs of a displaced patella fracture.]

2. Indirect (most common): This is the most common mechanism, occurring secondary to forcible quadriceps contraction while the knee is in a semiflexed position (eg, in a stumble or fall).


a. The intrinsic strength of the patella is exceeded by the pull of the musculotendinous and ligamentous structures.


b. A transverse fracture pattern is most commonly seen with this mechanism, with variable inferior pole comminution.


c. The degree of fragment displacement suggests the degree of retinacular disruption. Active knee extension is usually lost.


3. Combined direct/indirect: The patient experiences direct and indirect trauma to the knee, such as in a fall from a height.


D. Clinical evaluation


1. Patients typically present with limited or no ambulatory capacity with pain, swelling, and tenderness of the involved knee. A defect at the patella may be palpable.


2. Open fracture must be ruled out because these constitute a surgical emergency; assessment may require instillation of 50 to 70 mL of saline solution into the knee to determine communication with overlying lacerations.


3. Active knee extension should be evaluated to assess injury to the retinacular expansions. This examination may be aided by decompression of hemarthrosis.


4. Associated lower extremity injuries may be present following high-energy trauma. The ipsilateral hip, femur, tibia, and ankle must be carefully examined, with appropriate radiographic evaluation, if indicated.


E. Radiographic evaluation—AP, lateral, and axial (sunrise) views of the knee should be obtained.


1. On the AP view, a bipartite patella (8% of population) may be mistaken for a fracture; these usually occur in the superolateral position and have smooth margins and are bilateral in 50% of individuals.


2. On the lateral view, displaced fractures usually are obvious (Figure 6).


3. The axial view (sunrise) may help identify osteochondral or vertical marginal fractures.


F. Fracture classification


1. Patella fractures are classified descriptively in one of the following ways: open or closed, nondisplaced or displaced.


2. They also are described based on the fracture pattern: transverse, vertical, marginal, comminuted, osteochondral, or sleeve (

Figure 7).


G. Treatment


1. Nonsurgical treatment


a. Indications include nondisplaced or minimally displaced (2 to 3 mm) fractures with minimal articular disruption (1 to 2 mm) and an intact extensor mechanism.


b. Treatment involves a cylinder cast or knee immobilizer for 4 to 6 weeks. Early weight bearing is encouraged, advancing to full weight bearing with crutches as tolerated. Early straight leg raises and isometric quadriceps strengthening exercises should be started within a few days.


c. After radiographic evidence of healing, progressive active flexion and extension strengthening exercises are begun with a hinged knee brace initially locked in extension for ambulation.


2. Surgical treatment


a. Open reduction and internal fixation


i. Indications for open reduction and internal fixation include >2 mm articular incongruity, >3 mm fragment displacement, or open fracture.


ii. There are multiple methods of surgical fixation, including tension banding (using parallel longitudinal Kirschner wires or cannulated screws), circumferential cerclage wiring, and interfragmentary screw compression supplemented by cerclage wiring (

Figure 8). Retinacular disruption should be repaired at the time of surgery.


iii. Postoperatively, the patient should be placed in a splint for 3 to 6 days until skin healing, with early initiation of knee motion. Active-assist range-of-motion exercises should be started, progressing to partial and full weight bearing by 6 weeks. Severely comminuted or marginally repaired fractures, particularly in older patients, may necessitate immobilization for 3 to 6 weeks.


b. Patellectomy


i. Partial patellectomy—Indications include the presence of a large, salvageable fragment in the presence of smaller comminuted polar fragments in which restoring the articular surface or achieving stable fixation is considered impossible (

Figure 9). The quadriceps or patellar tendons should be reattached without creating a patella baja or alta. Reattachment of the patellar tendon close to the articular surface will help to prevent patellar tilt.


ii. Total patellectomy—Total patellectomy is reserved for patients with severe, extensive comminution and is rarely indicated. Peak torque of the quadriceps is reduced by 50%. Repair of medial and lateral retinacular injuries at the time of patellectomy is essential. Postoperatively, the knee should be immobilized in a long leg cast at 10° of flexion for 3 to 6 weeks.


[Figure 7. Classification of patella fractures on the basis of the configuration of fracture lines.]

[Figure 8. AP (A) and lateral (B) radiographs showing interfragmentary screw fixation and wiring of a patella fracture.]

[Figure 9. Technique of partial patellectomy.]

H. Complications


1. Postoperative infection related to open injuries may necessitate serial debridement. Relentless infection may require excision of nonviable fragments and repair of the extensor mechanism.


2. Fixation failure has an increased incidence in osteoporotic bone or failure to achieve compression at fracture site.


3. Refracture can occur in up to 5% of patients secondary to decreased inherent strength at the fracture site.


4. Nonunion occurs in up to 2% of patients. Most patients retain good function, although partial patellectomy can be considered for painful non-union. Revision osteosynthesis should be considered in active, younger individuals.


5. Osteonecrosis (proximal fragment) can occur in association with greater degrees of initial fracture displacement. Treatment consists of observation only, with spontaneous revascularization occurring by 2 years.


6. Posttraumatic osteoarthritis develops in more than 50% of patients in long-term studies. Intractable patellofemoral pain may require Maquet tibial tubercle advancement.


7. Loss of knee motion secondary to prolonged immobilization or postoperative scarring can also occur.


8. Painful retained hardware may necessitate removal for adequate pain relief.


9. Loss of extensor strength and extensor lag is possible, and most patients will experience a loss of knee extension of approximately 5°, although this is rarely clinically significant.

Top Testing Facts

1. The actual incidence of knee dislocations is likely higher than reported because of spontaneous reduction.


2. Typically, at least three of the four main knee ligaments (ACL, PCL, LCL, MCL) are disrupted with knee dislocation.


3. Vascular injury is common with knee dislocation (reported range 20% to 60%).


4. Arteriography should be performed if the ABI is <0.9.


5. Timing and extent of ligament repair/reconstruction is controversial, but there is general agreement that repair of the lateral ligaments (including the PLC) should be performed in the acute period.


6. The patella is the largest sesamoid bone in the body.


7. The patella functions to increase the mechanical advantage of the quadriceps tendon.


8. Active knee extension should be assessed to determine the status of the retinacular extension around the patella.


9. Bipartite patella (present in 8% of the population; most often found in the superolateral region) may be mistaken for a fracture.


10. Nonsurgical treatment of patella fractures assumes an intact extensor mechanism.


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