Knee (Tibiofemoral Joint) Dislocation

Overview

Definition

A dislocation of the knee (tibiofemoral joint) is a rare injury that is important to recognize due to the frequent association with vascular injury and associated risk of limb amputation. These are emergent cases that require immediate reduction and evaluation of the neurovascular system.  

Epidemiology

• 0.02% of all musculoskeletal injuries (very uncommon)
• Incidence may be higher, as the actual percentage of cases that dislocate and spontaneously reduce is unknown.
• Obesity is an independent risk factor.
• High-velocity dislocations are more commonly associated with vascular injury.
• Vascular injuries reported in approximately 5%–50%  of patients
• Peroneal nerve injuries reported in approximately 20% of patients

Etiology

• High-velocity or high-energy trauma:
  • Motor vehicle accidents, industrial injuries, sports-related injuries 
  • Potentially limb-threatening secondary to vascular injury
• Low-velocity trauma: associated with morbid obesity

Classification

Knee dislocations (KDs) can be classified based on the position of the tibia in relation to the femur, etiology, or on the pattern of ligament tears (Schenck classification).

Classification based on the position of the tibia in relation to the femur:

• Types:
  • Anterior dislocation
  • Posterior dislocation
  • Medial dislocation
  • Lateral dislocation
  • Rotatory dislocation
• Posterior and anterior KDs: most common, highest risk of associated popliteal artery injury
• Posterior dislocations: primarily occur secondary to direct trauma to the anterior tibia of a flexed knee (dashboard injury)

Classification by etiology:

• High velocity: 
  • Most common mechanism
  • High-velocity impact or trauma
• Low velocity:
  • Lower-velocity injuries/falls
  • Associated with morbid obesity

Schenck classification of KDs: based on pattern of ligament tears

Usually a high-energy mechanism of injury is required for multiple ligaments to fail in order for dislocation to occur.

• KD I: involvement of the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL)
• KD II: injury to both ACL/PCL with both collaterals intact (rare)
• KD III: injury to ACL and PCL and either medial collateral ligament (MCL) or lateral collateral ligament (LCL) (not both)
  • KD IIIM: MCL torn
  • KD IIIL: LCL torn
• KD IV: all 4 ligaments torn (ACL, PCL, MCL, LCL), highest rate of vascular injury
• KD V: multiple ligamentous injuries with periarticular fracture (knee fracture-dislocation)

Pathophysiology

Trauma mechanism

• Posterior: 
  • Direct force to the anterior tibia, with the knee flexed at 90 degrees
  • Seen in motor vehicle accidents: tibia striking the dashboard
• Anterior: 
  • Hyperextension injury
  • Disruption of the posterior structures of the knee
• Lateral: 
  • Varus/valgus stress
  • Commonly associated with a tibial plateau fracture
  • Associated with peroneal nerve injury
• Medial:
  • Varus/valgus stress
  • Commonly associated with a tibial plateau fracture
• Rotatory: 
  • Rotational forces
  • Posterolateral dislocations are unable to be reduced by closed reduction and require surgical intervention.

Pathological anatomy

• 2 or more ligaments (cruciate or collateral) must be compromised to allow the knee to dislocate.
• Most commonly, both cruciate ligaments and at least 1 collateral ligament are disrupted.
• The popliteal artery:
  • Traverses the posterior portion of the knee
  • Attaches proximally at the adductor hiatus
  • Attaches distally to the proximal arch of the soleus muscle
  • This tethering makes it vulnerable to injury with a KD.
• The peroneal nerve:
  • Winds laterally around the proximal fibula
  • Tethered above and below the femoral head my fascial/ligamentous structures
  • This tethering makes it vulnerable to injury with a KD.
• KDs can be open or closed.
• Posterolateral dislocations are generally irreducible and require surgical management.

Clinical Presentation

Knee dislocations are an uncommon injury but require emergent evaluation and management secondary to the possibility of limb ischemia, which may require surgical intervention to avoid  amputation.

Important considerations

• If dislocation occurred in the context of high-energy trauma: Patients may require simultaneous evaluation and management following the advanced trauma life support (ATLS) method.
• Consider the possibility of spontaneous reductions: In the absence of obvious deformity, the clinician may overlook the high risk of an associated vascular injury.

History

• In isolated trauma, the patient may be able to describe the mechanism. 
• Patients or first responders will usually report a high-energy blunt trauma (e.g., motor vehicle accident, industrial accident, sports injury).
• As these injuries are often high-energy trauma, evaluation for other life-threatening injuries is essential.

Physical examination

• Obvious deformity of the knee:
  • Associated swelling
  • Limited range of motion
  • Appearance less dramatic in morbidly obese patients
  • Up to 50% of KDs may have spontaneously reduced prior to presentation (exact percentage is unknown).
• Meticulous vascular examination:
  • Popliteal artery injury:
    • Decreased distal pulses (always check distal pulses and perfusion)
    • Ankle-brachial index < 0.9
    • Compare to contralateral limb.
  • Signs of severe vascular compromise: 
    • Absent pulses
    • Pale or cool extremity
    • Paresthesias
    • Paralysis 
    • Palpable thrill or audible bruit
    • Visible expanding hematoma
• Neurological examination:
  • Peroneal nerve injury common with KD
  • Peroneal nerve exam: 
    • Evaluate sensation: 1st web space 
    • Evaluate motor involvement: eversion of the foot and foot/toe dorsiflexion
    • No associated deep tendon reflex
• Ligamentous examination:
  • Evaluate ligamentous integrity of the knee after neurovascular integrity.
  • Rule out associated fracture in high-energy trauma prior to ligament evaluation.
  • Evidence of multiple ligamentous disruption in the appropriate clinical scenario may indicate a KD with a spontaneous reduction.

Diagnosis

The majority of complete KDs are clinically obvious with a history of significant trauma and gross deformity of the knee. Diagnosis is made clinically, although imaging studies may be indicated to confirm suspected vascular injury or fracture and/or for surgical planning.

Steps

• Determine the direction of the KD.
• Consider potential dislocation with spontaneous reduction: 
  • Gross instability of the knee in the setting of trauma
  • Evidence of significant hyperextension of the knee in the setting of trauma
• Imaging studies are indicated for evaluation of associated fracture, ligamentous injury, and/or arterial injury.

X-ray

• Plain X-rays: indicated post-reduction or prior to any ligamentous evaluation
• Anteroposterior (AP) and lateral projections of the knee: ideal to visualize the dislocation

Vascular evaluation

• Ankle-brachial indices evaluation:
  • Required for patients without obvious severe vascular compromise (all reduced KDs without obvious signs of arterial injury)
  • Ankle-brachial index < 0.9:
    • High incidence of vascular injury
    • Vascular imaging indicated
• Duplex ultrasound:
  • Noninvasive alternative to direct angiography for vascular assessment
  • High sensitivity and specificity for clinically significant arterial injury
  • Can be performed at bedside; does not require transport to CT scanner or angiography suite
  • Lowest cost
• CT angiogram:
  • Indicated in cases of asymmetric pulses, decreased ankle-brachial index, or abnormal duplex ultrasound
  • Noninvasive alternative to direct arteriography 
  • Requires transport to CT scanner; does not require an angiography suite
  • Intermediate cost
• Direct arteriography:
  • Criterion standard method of assessing vascular integrity
  • Formerly indicated for all KDs after reduction:
    • There is debate over the appropriate application of imaging options.
    • Some experts utilize advanced imaging in all instances of KD since vascular damage can go undetected. 
  • Risk of injury related to procedure (requires arterial puncture)
  • Requires interventional radiologist or vascular surgeon
  • Highest cost

Management

Closed reduction of a KD should not be delayed, especially in a limb with obvious vascular impairment. As these injuries are often high-energy trauma, evaluation for other life-threatening injuries is essential.

Prompt reduction

• Closed reduction of the knee with procedural sedation
• Reduction should not be delayed if the patient has any evidence for vascular compromise.
• Procedure:
  • Longitudinal or axial traction followed by translation of the tibia
  • Anterior and posterior dislocations usually reduce easily.
• Immediate orthopedic consultation for dislocations that are not easily reduced
• Posterolateral dislocations are generally irreducible; this may be indicated by a skin dimple in the anteromedial aspect of the knee. 
• Immediate post-reduction evaluation of pulses and perfusion
• Bedside X-rays post-reduction to confirm reduction and evaluate for fracture 
• Advanced imaging indicated if suspected vascular compromise 
• Immediate vascular consultation if any concerns for arterial injury

Post reduction

• Pulses and perfusion:
  • Immediate post-reduction evaluation of pulses and perfusion
  • Serial perfusion evaluations:
    • Presence of normal distal pulses alone does not rule out popliteal artery injury.
    • Thrombus related to arterial injury may present with delayed findings of poor perfusion.
    • Arterial-brachial index assessments
    • Bedside duplex ultrasonography
    • Patients with any abnormalities on vascular exam, ankle-brachial index, or duplex ultrasound: CT angiography or direct arteriography
• Post-reduction imaging:
  • Bedside X-rays post-reduction to confirm reduction and evaluate for fracture
  • Advanced imaging indicated if suspected vascular compromise
• Consultations:
  • Immediate vascular surgery consultation for injuries with any signs or concerns of vascular compromise
  • Immediate orthopedic consultation:
    • Indicated for complicated reduction or irreducible reductions
    • Indicated for management of the fractures and/or ligamentous injury

Further management

• Long leg splint at 20–30 degrees of flexion
• After initial management of acute injury, orthopedic management is related to the other specific structural injuries to the knee.

Complications

• Injury to the popliteal artery or vein
• Delayed arterial or venous thrombus formation
• Peroneal nerve injury
• Residual instability
• Arthrofibrosis (stiffness): most common complication
• Compartment syndrome
• Amputation

Prognosis

• Depends on the velocity of the injury, neurovascular damage, and associated injuries
• Athletes with a high-velocity injury are unlikely to return to pre-injury levels of sport.
• Low-velocity dislocations have a relatively better prognosis.
• Prognosis improves with timely repair of vascular injuries.

Clinical Relevance

• Anterior cruciate ligament (ACL) injury: frequently injured, important stabilizing ligament of the knee. The ACL is most commonly injured in sporting endeavors and frequently torn when the knee is dislocated. 
• Meniscus tear: injury to the meniscus occurs due to rotational or shearing forces placed across the knee joint. Meniscus tears may be associated with an ACL tear and/or dislocation. 
• Patellar instability and dislocation: a spectrum of conditions affecting the patella secondary to trauma or activity. The conditions are characterized by peripatellar pain and knee instability. With dislocation of the patella, there may be obvious deformity and an inability to extend the knee.
• Amputation: amputation may be required after prolonged ischemia associated with vascular compromise during dislocation.
• Injury to the popliteal artery or vein: the vessels of the popliteal fossa may be injured with KD. Delay in diagnosis or recognition, treatment with reduction, and/or surgical repair may lead to arterial injury and amputation. 
• Compartment syndrome: a surgical emergency usually occurring secondary to trauma. Compartment syndrome is marked by increased pressure within a fascial compartment, which compromises the circulation and function of the tissues within that space.