Patellofemoral Instability

Definition

  • Symptomatic dislocation or subluxation of the patella, usually laterally.

Epidemiology

  • Can be congenital or acquired (usually with mechanical or anatomical predisposition).
  • Women > Men (due to laxity).
  • Recurrence:
    • 20% risk after first dislocation.
    • 50% risk after second dislocation.
    • Younger age at first dislocation increases long-term risk.

Anatomy

  • Patella: Has the thickest cartilage in the body.
    • Facets: Lateral, medial, and odd (lateral facet is typically larger).
    • Types (Wrisberg classification):
      1. Type 1: Central ridge, equal size of medial and lateral facets.
      2. Type 2: Medialized ridge, small medial facet (common).
      3. Type 3: Nearly absent medial facet or ridge.
  • Trochlea: V-shaped, with a higher lateral wall (bony buttress), may be dysplastic (classified by Dejour).

Soft Tissue Restraints

  • Medial:
    • MPFL (Medial Patellofemoral Ligament): 50% of the medial pull (primary static restraint).
    • VMO (Vastus Medialis Obliquus): Primary dynamic restraint.
    • Patellomeniscal ligament, medial retinaculum.
  • Lateral:
    • ITB (Iliotibial Band), Vastus Lateralis, LPFL (Lateral Patellofemoral Ligament).

Biomechanics

  • Joint Reaction Forces:
    • Level walking: 2x body weight.
    • Stairs: 4x body weight.
    • Squatting: 8x body weight.
    • Jumping: 20x body weight.
  • Q Angle:
    • Angle between ASIS, the center of the patella, and tibial tuberosity.
    • Indicates the lateral force vector on the patella.
    • Decreased Q angle stabilizes the patella by reducing quadriceps pull in flexion.
    • Alternative to Q angle: Trochlea Groove to Tibial Tuberosity (via MRI/CT).

Anatomical Causes of Instability

  1. High Q angle (lateralized or externally rotated tuberosity, lateralized ASIS).
  2. Trochlea dysplasia.
  3. Anteverted femoral neck.
  4. External tibial torsion.
  5. Genu valgum.
  6. Soft tissue laxity/incompetence.
  7. Hyper-pronated feet.

Clinical Assessment

  • History:
    • Dislocation or subluxation?
    • Associated pain and its location.
    • Global laxity, trauma history, family history.
    • Age of onset and functional impact.
  • Examination of Patellofemoral Joint:
    • Look: Coronal alignment, Q angle, foot pronation, scars, quads bulk, swelling, Beighton score.
    • Gait: Foot progression angle, patella orientation.
    • Sit: Extensor mechanism competence, crepitus, J sign, patella tracking, patella alta.
    • Lie Down: Effusion, Clarke’s test, patella excursion and apprehension test, patella tilt, screen knee ROM.
    • Prone: Thigh-foot axis, tibial and femoral versions, Gage’s test for anteversion.
    • Finishing: Knee ligaments, Beighton score, hip and foot examination, pulses & neurology, weight-bearing plain X-rays, patella merchant view.

Imaging

  • Plain X-ray:
    • AP, lateral, and merchant views (knee semi-flexed to engage patella).
    • May show tilt, genu valgum, patella alta/baja.
  • CT:
    • Most useful modality for assessing patella height, tilt, and trochlea groove.
    • Protocol: Mid-axial cuts in 10° increments (0-60°).
    • Measurements:
      • Patella height.
      • Patella tilt (<7° = tilt, >11° = normal).
      • Trochlea Groove-Tibial Tuberosity Offset (<10mm = normal, >20mm = abnormal).
  • MRI:
    • Used for soft tissue evaluation if considering soft tissue reconstruction.
    • Often used as an alternative to CT scans.

Management

Acute Dislocations

  • Non-operative: In almost all cases, non-operative management is effective.
    • Indications for surgery: Osteochondral injury, irreducible lateral subluxation.

Chronic Instability

  • Recurrent symptomatic instability with imaging evidence of the cause.

Non-Operative Treatment

  1. Extension splint for 2-4 weeks, followed by physiotherapy.
  2. VMO strengthening program.
  3. McConnell taping.

Operative Options

  • Proximal Realignment:
    • MPFL repair/reconstruction (+/- VMO advancement/re-attachment).
    • Indications: No mechanical/anatomical abnormality, evidence of tilt/maltracking.
  • Lateral Release:
    • Can be combined with other procedures but controversial when done alone.
    • Only if lateral structures are truly tight with isolated tilt and no mechanical abnormality (rare).
    • Avoid in case of arthrosis.
  • Distal Realignment:
    1. Elsie Tremellat:
      • Medial and internal rotation tibial tubercle realignment.
      • Must have proven lateralization of tibial tuberosity.
      • Consider distal unloading procedure if arthrosis present.
    2. Fulkerson:
      • Medial and anterior realignment.
      • Reduces patellofemoral joint reaction forces by unloading the joint.
    3. Tibial Tubercle Distalization:
      • For patella alta (late engagement of patella).
    4. Trochleoplasty:
      • If dysplastic trochlea needs addressing.
    5. Address Other Underlying Deformities:
      • E.g., tibial torsion, if symptoms are severe.
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