Emergency Medicine Hub / Trauma & Orthopedics • 3 min read • 4 sections

Knee & Lower Leg Dislocations & Extensor Injuries

Comprehensive emergency evaluation and protocolized management of acute knee, patellofemoral, and proximal lower leg emergencies: clinical recognition of high-energy tibiofemoral dislocations and occult spontaneous reductions presenting as multi-ligamentously unstable knees; the high hazard of popliteal artery disruption (20-40%) and common peroneal nerve palsy; protocolized Ankle-Brachial Index (ABI) screening and lower extremity CT Angiography (CTA) pathways; closed reduction techniques; differentiation of quadriceps versus patellar tendon extensor mechanism ruptures; and Schatzker classification of tibial plateau fractures with acute compartment syndrome surveillance.

Resuscitation Quick Actions • First 2 Minutes

High-Acuity

The 6-to-8 Hour Window

Popliteal artery disruption requires surgical revascularization within 6-8 hours of injury to prevent permanent ischemic muscle necrosis and limb amputation (> 85% amputation rate after 8 hours)

Ankle-Brachial Index (ABI) Protocol

Measure systolic Doppler pressure at dorsalis pedis/posterior tibial and divide by ipsilateral brachial systolic pressure: ABI < 0.9 = Stat CT Angiography; ABI >= 0.9 = Serial exams q2-4h for 24 hours

Spontaneous Reduction Trap

Never be reassured by a normal radiograph: if the knee exhibits gross multi-ligamentous laxity (ACL + PCL tear), treat presumptively as a reduced dislocation with ABI screening

Common Peroneal Nerve Exam

Document sensation at 1st dorsal webbed space and motor function of great toe extension (EHL)/ankle dorsiflexion (tibialis anterior); peroneal palsy occurs in 20-30%

Extensor Mechanism Rupture

Inability to perform active straight leg raise: Age > 40 -> Quadriceps tendon tear (patella baja); Age < 40 -> Patellar tendon tear (patella alta) -> Knee immobilizer in full extension and urgent Ortho

Tibial Plateau Fracture Warning

Schatzker type IV-VI high-energy fractures have a 30% risk of acute compartment syndrome -> Measure delta-P and repeat neurovascular exams

Bottom-Line Clinical Pearl

A tibiofemoral knee dislocation is a limb-threatening surgical emergency with an associated popliteal artery injury rate of 20% to 40%; up to 50% of knee dislocations reduce spontaneously in the prehospital setting, presenting to the ED only as a 'grossly unstable multi-ligamentous knee' (simultaneous tear of ACL, PCL, and collateral ligaments). Immediately measure an Ankle-Brachial Index (ABI): an ABI < 0.9 (or asymmetric distal pulses) mandates emergent CT Angiography (CTA) and stat vascular surgery consultation. An ischemic warm time exceeding 6 to 8 hours results in an amputation rate approaching 85%.

1. Tibiofemoral Knee Dislocation: Classification & Vascular Anatomy

True knee dislocations are classified by the position of the tibia relative to the femur. Because the popliteal artery is tethered firmly in the popliteal fossa both proximally at the adductor hiatus (Hunter's canal) and distally at the fibrous arch of the soleus muscle, any significant translational displacement of the knee joint produces severe traction, contusion, intimal tearing, or complete transection of the popliteal artery.

Dislocation Type & Mechanism	Anatomical Ligamentous Injury	Vascular & Neurological Hazard
Anterior Dislocation (30–40%) Mechanism: Severe hyperextension (dashboard impact, stepping in hole).	Tear of Posterior Cruciate Ligament (PCL) and Anterior Cruciate Ligament (ACL). Posterior capsule rupture.	Popliteal artery is stretched over femoral condyles: high risk of intimal traction tear and secondary delayed thrombosis (may have normal pulses initially!).
Posterior Dislocation (25–30%) Mechanism: Direct dashboard impact to anterior proximal tibia with flexed knee.	Complete disruption of PCL and ACL. Posterior capsule avulsion.	Highest rate of complete popliteal artery transection (condyles directly shear the artery). High amputation risk.
Lateral/Medial Dislocation (10–15%) Mechanism: Severe varus or valgus blunt stress.	Disruption of both cruciate ligaments plus lateral collateral ligament (LCL) or medial collateral ligament (MCL).	Lateral dislocations have the highest incidence of Common Peroneal (Fibular) Nerve traction injury (producing foot drop and numbness over first dorsal webspace).
Rotatory/Posterolateral (Irreducible) Mechanism: Rotational shear force.	Buttonholing of the medial femoral condyle through the anteromedial capsule ('dimple sign' or 'pucker sign').	IRREDUCIBLE BY CLOSED MANEUVERS! Attempted closed reduction will fail and can cause skin necrosis. Requires emergent open reduction in the operating room.

2. The Ankle-Brachial Index (ABI) Screening Protocol & Imaging Algorithm

Physical examination alone is notoriously unreliable for detecting vascular injury: up to 10% of patients with complete popliteal artery transection or flow-limiting intimal flaps have palpable distal pulses due to extensive collateral genicular blood flow. Systematic objective screening with the Ankle-Brachial Index (ABI) is mandatory:

Clinical Scenario/ABI Finding	Diagnostic Interpretation	Immediate Emergency Action Protocol
Hard Signs of Ischemia - Absent distal pulses (Doppler silent) - Pallor, cold extremity, poikilothermia - Expanding or pulsatile popliteal hematoma - Distal motor/sensory paralysis	Complete vascular occlusion or transection with critical limb ischemia.	IMMEDIATE OPERATING ROOM EXPEDITION: Do NOT obtain CTA! Proceed directly to the OR for emergent surgical bypass/shunting with temporary intravascular shunt and four-compartment fasciotomy.
ABI < 0.90 (Systolic ankle Doppler/Brachial Doppler < 0.90 in affected limb, or > 0.10 difference compared to uninjured limb)	High probability (> 95% specificity) of significant arterial injury (intimal flap, pseudoaneurysm, dissection, or partial thrombosis).	EMERGENT CT ANGIOGRAPHY (CTA) of the lower extremity with IV contrast. Stat Vascular Surgery consultation.
ABI >= 0.90 (Normal symmetrical distal perfusion without hard signs)	Extremely high negative predictive value (> 99%) for flow-limiting arterial injury requiring intervention.	CTA is NOT routinely mandatory. Admit patient for serial neurovascular examinations every 2 hours for at least 24 hours (intimal flaps can thrombose progressively over 12-24 hours).

3. Closed Reduction Technique for Knee Dislocation

Step/Phase	Procedural Action	Technique Pearl & Safety Check
1. Procedural Sedation & Analgesia (PSA)	Administer IV Propofol (0.5-1 mg/kg) or Ketamine (1-2 mg/kg) to achieve complete muscle relaxation.	Overcoming intense quadriceps and hamstring spasm is required to minimize shear forces during reduction.
2. Longitudinal Traction Maneuver	Assistant applies steady, continuous axial in-line longitudinal traction on the leg while a second provider applies counter-traction on the femur.	For Anterior Dislocation: Lift the proximal tibia anteriorly while maintaining traction; gently flex knee to 15-20 degrees. For Posterior Dislocation: Apply anteriorly directed force on the posterior proximal tibia while flexing the knee slightly.
3. Post-Reduction Stabilization & Splinting	Place leg in a long-leg posterior splint or knee immobilizer locked in 15 to 20 degrees of flexion (do NOT immobilize in full extension or hyperflexion, which pinches the popliteal artery).	Immediately repeat distal neurovascular exam, check Doppler signals, and recalculate the post-reduction Ankle-Brachial Index (ABI)!

4. Extensor Mechanism Ruptures: Quadriceps vs. Patellar Tendon

Rupture of the extensor mechanism results from violent, eccentric contraction of the quadriceps muscle on a flexed knee (e.g., stumbling on a curb or landing from a jump). Patients present with an inability to perform an active straight leg raise against gravity or maintain passive extension.

Feature	Quadriceps Tendon Rupture	Patellar Tendon Rupture
Typical Patient Demographics	Older patients (Age > 40 years); underlying systemic conditions (chronic renal failure, diabetes, gout, chronic steroid use, fluoroquinolones).	Younger, athletic patients (Age < 40 years); forceful jumping or deceleration injury (basketball, volleyball).
Palpable Defect	Palpable sulcus/gap superior to the patella (suprapatellar groove).	Palpable sulcus/gap inferior to the patella (infrapatellar gap).
Lateral Knee Radiograph Findings	Patella Baja (patella sits abnormally low on the femoral condyles; Insall-Salvati ratio < 0.8).	Patella Alta (patella displaced abnormally superiorly into suprapatellar pouch; Insall-Salvati ratio > 1.2).
Emergency Management	Knee immobilizer in full extension; strict non-weight-bearing with crutches; urgent outpatient Orthopedic consultation for operative repair within 7 to 14 days.	Knee immobilizer in full extension; non-weight-bearing; emergent/urgent Orthopedic repair (delayed repair leads to severe quadriceps retraction and poor outcomes).

The Spontaneously Reduced Knee & The Normal Pulse Fallacy

Never assume that a normal-appearing knee radiograph excludes a knee dislocation! Up to 50% of tibiofemoral dislocations reduce spontaneously before paramedics or emergency physicians evaluate the patient. If an examination demonstrates a 'grossly unstable multi-ligamentous knee' (such as simultaneous laxity of the anterior and posterior cruciate ligaments (grossly positive Lachman, anterior drawer, and posterior drawer), the patient suffered a knee dislocation until proven otherwise. Even more treacherous: the presence of palpable pedal pulses DOES NOT exclude a limb-threatening popliteal artery injury. Intimal tears and partial transections frequently maintain distal pulses and normal capillary refill initially due to collateral flow, only to thrombose completely 6 to 12 hours later. An ischemic warm time exceeding 8 hours leads to irreversible ischemic muscle necrosis and an amputation rate of 85%. Every patient with multi-ligamentous knee laxity mandates an immediate Ankle-Brachial Index (ABI) measurement: an ABI < 0.9 mandates emergent CT Angiography, while an ABI >= 0.9 mandates strict inpatient observation with serial neurovascular checks.

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