Hip and Knee Pain

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

The hip and knee represent the most highly stressed weight-bearing joints in the human body, frequently presenting to the ED following acute mechanical overload or infectious seeding. The hip is a highly stable enarthrosis (ball-and-socket) joint reinforced by a strong fibrocartilaginous labrum and dense capsule, making it highly resistant to dislocation. Conversely, the knee is a modified hinge diarthrodial joint highly reliant on static (capsule and ligaments) and dynamic (rotator cuff equivalent/musculature) stabilizers, rendering it vulnerable to rotational and sheer forces. Cellular breakdown often involves the avascular zones of the intra-articular structures; for example, the medial meniscus lacks intrinsic blood supply along its inner margins, preventing cellular repair after a mechanical tear. In infectious pathologies such as septic arthritis, bacterial infiltration (most commonly Staphylococcus aureus) triggers a massive exudative inflammatory response within the closed synovial space, leading to purulent effusions that rapidly destroy articular cartilage via enzymatic degradation if not surgically decompressed .

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Resuscitation & Reductions: For any patient presenting with a suspected knee dislocation and "hard signs" of vascular injury (absent pedal pulses, expanding popliteal hematoma, cool/mottled foot), immediate reduction is required, followed by emergent surgical exploration to salvage the limb.
• Analgesia: Uncontrolled pain must be treated as a medical emergency. Initiate rapid IV non-opioid and opioid analgesia in small, titrated increments until pain is controlled.
• Regional Anesthesia: For proximal femur, hip, or patellar injuries, prioritize ultrasound-guided femoral nerve or fascia iliaca compartment blocks. This provides superior analgesia while avoiding the systemic hypotension and respiratory depression risks of opiates, and prevents dangerous tension on the sciatic or femoral nerves.
• Hip Dislocation Reduction: If a posterior hip dislocation is identified, utilize the Allis maneuver (steady in-line traction with the knee flexed, bringing the hip to 90 degrees while applying upward traction and gentle rotation) to reduce the joint.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

The "Can't-Miss" Differentials:

1. Septic Arthritis (Destroys the joint rapidly; cannot be ruled out by isolated clinical findings).
2. Knee Dislocation with Popliteal Artery Injury (High risk of amputation).
3. Slipped Capital Femoral Epiphysis (SCFE) (Must be ruled out in pediatric patients with knee/thigh pain).
4. Ruptured Abdominal Aortic Aneurysm (Can masquerade catastrophically as isolated, severe hip or knee/flank pain).

The Prioritized Workup:

• Arthrocentesis & Synovial Fluid Analysis: The gold standard for a hot, swollen joint. Send fluid for cell count; a WBC > 50,000 with > 75% PMNs is definitively diagnostic for septic arthritis .
• Vascular Screening: In suspected knee trauma/dislocation, assess the Ankle-Brachial Index (ABI). An ABI < 0.9 or "soft signs" of vascular injury (asymmetric pulses, paresthesias) mandate a CT angiography or duplex ultrasound to rule out popliteal artery disruption.
• Radiography: Plain films are first-line for traumatic bony injuries but are notoriously poor for acute osteomyelitis, taking approximately two weeks and 50% bone mineral loss to show lucent lytic changes .

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• Bedside POCUS: Utilize ultrasound to rapidly detect hip or knee joint effusions in patients with non-specific pain, which directs the need for diagnostic arthrocentesis. In an elderly patient with acute, severe hip/leg pain and hypotension, visualize the abdominal aorta immediately to rule out a ruptured AAA.
• Plain Radiography (Pediatrics): In boys aged 10-17 presenting with atraumatic knee, thigh, or groin pain, you must obtain anteroposterior, lateral, and frog-leg lateral radiographs of the hip to visualize the displaced epiphysis indicative of SCFE.
• Visual Exam Findings:
• Posterior Hip Dislocation: The patient's leg will visibly rest in a flexed, adducted, and internally rotated position.
• The Locked Knee: The patient will be visually and physically unable to fully extend the knee, most commonly caused by a torn meniscus.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

The Ottawa Knee Rules:
Obtain knee radiographs only if the patient meets any of the following criteria (98.5% sensitivity):

• Age > 55 years
• Isolated patellar tenderness
• Isolated fibular head tenderness
• Inability to flex the knee to 90 degrees
• Inability to bear weight for 4 steps both immediately after the injury and in the ED.

The Pittsburgh Knee Rules:
Obtain knee radiographs if there is a fall or blunt trauma mechanism PLUS either of the following (100% sensitivity):

• Age < 12 years OR > 50 years
• Inability to walk 4 weight-bearing steps in the ED.

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• The Spontaneous Reduction Trap: Fifty percent of knee dislocations spontaneously reduce before ED arrival. Do not assume a normally aligned knee on X-ray lacks a lethal vascular injury; the popliteal artery may already be sheared.
• The Referred Pain Trap: A deadly cognitive error is focusing exclusively on the knee in a pediatric or adolescent patient complaining of knee pain, thereby missing a Slipped Capital Femoral Epiphysis (SCFE) located at the hip.
• The Crystal Exclusion Trap: Finding uric acid or CPPD crystals in a joint aspirate does not exclude infection. Crystal arthropathies and septic arthritis frequently coexist.
• The Gram Stain Trap: A negative Gram stain of synovial fluid occurs in 50% of true septic arthritis cases.
• CRITICAL ACTION: If septic arthritis is suspected clinically, do not withhold antibiotics pending cultures. Administer IV Vancomycin and Ceftriaxone empirically and arrange emergent surgical washout .

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzword: "Popping sensation" at the exact onset of knee trauma = Ligamentous tear (commonly ACL).
• Buzzword: "Locking" of the knee = Meniscal injury.
• Buzzword: "Giving out" or "buckling" of the knee = Typically represents reflex muscle inhibition due to pain, not an acute neurological catastrophe.
• Buzzword: Sexually active young adult with a "pustular rash and tenosynovitis" = Neisseria gonorrhoeae Septic Arthritis .
• Distractor Trap: An exam question presents an elderly woman who fell, cannot bear weight on her leg, but has entirely normal hip and pelvic X-rays. The distractor option will be "Discharge home with analgesia and a walker." The correct answer is to admit and obtain an MRI of the hip to assess for an occult fracture.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

"This is a 65-year-old male presenting with an acutely painful, swollen right knee. My immediate life- and limb-threatening concerns include septic arthritis, occult fracture, and a spontaneously reduced knee dislocation with vascular compromise. On examination, the patient's distal pulses are symmetric and capillary refill is intact, reassuring against a popliteal artery injury. He cannot bear weight, meeting the Ottawa Knee Rules, so I will order a plain radiograph to evaluate for fracture. Given the acute monoarticular effusion, I will use bedside ultrasound to confirm the fluid collection and guide an immediate diagnostic arthrocentesis. I will send the synovial fluid for cell count, Gram stain, and cultures to rule out a WBC count > 50,000 indicative of infection . Given the severe pain, I will perform an ultrasound-guided fascia iliaca compartment block to provide targeted, opiate-sparing analgesia. If the fluid analysis is concerning for septic arthritis, I will emergently consult orthopedics for joint washout and initiate empiric IV Vancomycin and Ceftriaxone ."