Patellar Instability (Pediatric)

Medial Patellofemoral Ligament (MPFL)

• Origin: Schottle's Point (Femur).
  • Saddle between medial epicondyle and adductor tubercle.
  • Radiographic landmark: Between posterior cortical line and posterior condylar line.
• Insertion: Upper 2/3 of medial patellar border.
• Function: Provides 50-60% of restraint to lateral translation in early flexion (0-30 degrees).
• Biomechanics: Isometric behavior (length changes minimal during flexion).

Trochlear Dysplasia

• Dejour Classification:
  • Type A: Shallow sulcus (Crossing Sign).
  • Type B: Flat trochlea (Supratrochlear Spur).
  • Type C: Convex lateral facet (Double Contour).
  • Type D: Cliff pattern (Supratrochlear Spur + Double Contour).

A flat (dysplastic) trochlea provides no bony containment, relying entirely on the MPFL, which eventually fails.

Patella Alta

• Definition: High riding patella.
• Effect: The patella engages the trochlea LATE in flexion (e.g., at 45 degrees instead of 30 degrees).
• Result: A window of instability exists between 0-45 degrees where neither the MPFL (lax in extension?) nor the bone is effective.
• Measurement: Caton-Deschamps Index (Articular length / Tibia-to-Patella length) greater than 1.2.

MPFL Injury Location

Based on MRI findings in acute dislocation:

• Femoral Avulsion: 60-70% (Most common).
  • Implication: Good target for primary repair if acute (controversial).
• Mid-substance: 20-30%.
  • Implication: Poor healing potential, reconstruction preferred.
• Patellar Avulsion: 10-20%.
  • Implication: May look like medial rim fracture.

Note: In recurrent cases, the ligament is often attenuated/absent rather than discretely torn.

Physeal Status

• Open Physis:
  • Constraint: Cannot drill large tunnels across distal femoral physis.
  • Technique: Soft tissue fixation or epiphysiolysis-sparing drilling.
• Closed Physis:
  • Constraint: None.
  • Technique: Standard anatomic reconstruction (tunnels allowed).

Respecting the growth plate is the primary surgical principle in the pediatric population.

Standard Series:

1. AP View: Generally normal, may show osteochondral fracture loose body.
2. Lateral View (Most useful screening tool):
   • Patella Alta: Calculate Caton-Deschamps Index (Articular surface length / Distance to Tibia).
     • Normal: 0.8 - 1.2.
     • Alta: Greater than 1.2.
   • Trochlear Dysplasia:
     • Crossing Sign: Trochlear floor crosses anterior femoral condyles. Represents a flat groove.
     • Supratrochlear Spur: Prominence of proximal trochlea.
     • Double Contour Sign: Medial condyle hypoplasia seen as a double line on the posterior aspect of the condyles.
3. Skyline (Merchant) View:
   • Taken at 30 degrees flexion.
   • Patellar Tilt: Angle between posterior condylar line and patellar axis.
   • Subluxation: Congruence angle.
   • Avulsion Fractures: Look closely at the medial patellar margin (MPFL avulsion fleck).

Gold Standard for Acute Injury Assessment:

• Soft Tissue:
  • MPFL Tear: define location (Femoral vs Patellar vs Midsubstance).
  • VMO: look for elevation or atrophy.
• Bone/Cartilage:
  • Osteochondral Lesions (OCD): Critical to diagnose. Usually Lateral Femoral Condyle or Medial Patellar Facet.
  • Pearl: Look for loose bodies in the lateral gutter or popliteal hiatus.
  • Bone Bruise Pattern: Classic "Kissing Contusion" on lateral condyle and medial patella confirms recent dislocation mechanism.
• Measurements:
  • Trochlear Sulcus Angle: Normal is less than 145 degrees. Greater than 145 indicates dysplasia.
  • Lateral Trochlear Inclination: Normal greater than 11 degrees. Less than 11 degrees indicates dysplasia.

Gold Standard for Alignment & Bony Measurements:

• TT-TG Distance (Tibial Tubercle - Trochlear Groove):
  • Superimpose axial cut of femoral trochlea and axial cut of tibial tubercle.
  • Measure mediolateral distance.
  • Normal: Less than 15mm.
  • Borderline: 15-20mm.
  • Pathologic: Greater than 20mm. (Indicates Tibial Tubercle Osteotomy).
  • Rotational Profile:
  • Femoral Anteversion: Measure relationship of femoral neck to posterior condyles.
  • Tibial Torsion: Measure relationship of posterior condyles to ankle mortise.

Conservative Management

• Indication:

  • First-time acute dislocation (without large loose body).
  • Patients with low demands or minimal risk factors.

• Phase 1 (0-2 Weeks):

  • Reduction: Extend knee, gentle medial pressure.
  • Immobilization: Extension splint or hinged brace locked in extension. Weight bearing as tolerated (WBAT).
  • Goal: Allow MPFL to heal in reduced position (not stretched).

• Phase 2 (2-6 Weeks):

  • Motion: Unlocked brace 0-90 degrees.
  • Strengthening:
    • VMO Activation: Straight leg raises with external rotation.
    • Gluteal Control: Clamshells, bridging. Correct dynamic valgus.
    • Core Stability: Plank progressions.
  • Taping: McConnell taping to unload tissues.

• Phase 3 (6+ Weeks):

  • Return to Sport: When strength greater than 90% of contralateral side and functional hopping test passed.
  • Bracing: "J"-buttress brace (e.g., Tru-Pull) for sports.
  
  

Soft Tissue Procedures

• MPFL Repair:

  • Indication: Acute bony avulsion (femoral or patellar).
  • Technique: Suture anchor repair of the avulsed fragment.
  • Outcome: Higher failure rate than reconstruction for midsubstance tears.

• MPFL Reconstruction (Gold Standard):

  • Indication: Recurrent instability, failed non-op, or high-risk first dislocator (athlete).
  • Graft: Autograft (Gracilis, Semitendinosus) or Allograft.
  • Principle: Re-create the check-rein. Do not over-constrain the joint.

• Lateral Release:

  • Indication: ONLY if tight lateral retinaculum causes excessive tilt (negative passive tilt test).
  • Warning: NEVER do an isolated lateral release for instability. It increases instability.
  
  

Bony Procedures

• Tibial Tubercle Osteotomy (TTO):

  • Indication: Skeletally mature (closed physis) + Bony Malalignment.
  • Types:
    • Medialization: For TT-TG greater than 20mm. Moves vector medially.
    • Distalization: For Patella Alta (CDI greater than 1.2). Engages patella earlier.
    • Anteromedialization (Fulkerson): Unloads lateral facet arthrosis + stabilizes.

• Distal Femoral Osteotomy (DFO):

  • Indication: Severe Genu Valgum (greater than 10 degrees anatomic valgus).
  • Technique: Lateral opening wedge or medial closing wedge.

• Trochleoplasty:

  • Indication: Severe Dejour B/D dysplasia (Bump/Cliff) where simple MPFL/TTO will fail.
  • Technique: Reshaping the distal femoral cartilage/bone to create a groove.
  • Risk: Chondral necrosis, stiffness. Specialist procedure.
  
  

Anatomic MPFL Reconstruction steps

1. Graft Harvest: Gracilis tendon (single or double strand). Whip-stitched ends.
2. Patellar Attachment:
   • Exposure: small incision medial patella border.
   • Location: Upper 1/3 of the medial border (junction of proximal and middle thirds).
   • Fixation: Two suture anchors or parallel tunnel technique.
   • Pearl: Ensure fixation is not intra-articular.
3. Femoral Attachment (Critical Step):
   • Exposure: Incision over medial epicondyle.
   • Schottle's Point Identification:
     • Radiographic landmarks (Lateral Fluoro):
       1. Extension of posterior cortical line.
       2. Proximal to posterior condylar line (2.5mm).
       3. Just posterior to Blumensaat's extended line.
   • Pin Placement: Drill guidewire at Schottle's point.
4. Isometry Check:
   • Loop graft around pin. Range knee 0-90.
   • Graft should be tightest at 0-30 degrees and slightly relax in deeper flexion.
   • If gets tight in flexion implies Pin is too Proximal/High.
   • If gets tight in extension implies Pin is too Distal/Low or Anterior.
5. Tunnel & Fixation:
   • Drill 6-7mm tunnel (careful of Notch/ACL).
   • Fix with interference screw with knee at 30 degrees flexion.
   • Tension: Zero tension. Just remove slack. Check lateral glide (10mm).

Fulkerson / Elmslie-Trillat TTO

1. Exposure: Anterior midline incision, lateral to tubercle.
2. Osteotomy:
   • Oblique cut if anteromedialization (Fulkerson).
   • Flat cut if pure medialization (Elmslie-Trillat).
   • Use oscillating saw, complete distal cut with osteotome (tapered).
3. Displacement:
   • Medialize 10-15mm (calculate based on preoperative TT-TG).
   • Distalize if Alta confirmed.
4. Fixation:
   • Two 4.5mm fully threaded cortical screws (bicortical).
   • Countersink heads.
5. Rehab Note:
   • Protected weight bearing for 6 weeks for bone healing.

Hemiepiphysiodesis (Guided Growth)

• Indication: Skeletally immature + Significant Valgus.
• Technique:
  1. Identify medial distal femoral physis with fluoro.
  2. Place 8-plate (tension band plate) spanning the physis.
  3. Extra-periosteal placement.
  4. Secure with screws (epiphyseal and metaphyseal).
• Mechanism: Tethers medial growth, lateral growth continues implies Varus correction over time.
• Follow-up: Essential to check every 3-4 months to prevent over-correction into varus.