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Dislocation of the Patella

  • 5% are associated with an osteochondral fracture
  • 4 types:
    1. Congenital dislocation
    2. Habitual dislocation
    3. Traumatic dislocation
    4. Recurrent dislocation

Congenital

  • Rare
  • Associated with Downs syndrome and arthrogryposis
  • An irreducible lateral patellar dislocation
  • Pathology : small patella, laterally positioned over a hypoplastic condyle, with fixation of patella to lateral condyle and iliotibial band
  • Treatment : operative correction
    • Mobilisation of quadriceps
    • Medial plication
    • Medial hemitransfer of the patellar tendon
    • Done in infancy

Habitual

  • Characterised by recurrent dislocation of patella each time the knee flexes
  • Due to abnormal contracture of vastus lateralis and iliotibial band
  • Treatment: operative release to centralise patella

Traumatic

  • Usually due to sudden twisting, more rarely due to direct blow
  • Patient usually falls to ground, may tell of lateral displaced patella at the time
  • Differential diagnosis
    • ACL rupture
    • Other disruption of extensor mechanism
    • Meniscal tear
  • Treatment
    • Conservative
      • First time dislocation : immobilise in extension splint for 4 weeks, quadriceps rehabilitation immediately, can do WB prn in splint
      • Approximately 1/3 will re-dislocate after conservative treatment
    • Operative
      • For osteochondral fracture

Recurrent

  • Repeated dislocation
  • 55% have some degree of subluxation

Aetiology

  • Soft tissue insufficiency (Dynamic)
    • Generalised joint laxity
    • Weak medial restraints
      • Hypoplasia or atrophy of VMO
      • Attenuated medial retinaculum
    • Iliotibial tract attachments to the patella
  • Malalignment
    • Genu valgum
    • High 'Q' angle
      • Greater than 15o in a male and greater than 20o in a female
      • Should be measured with the knee slightly flexed
      • Test during quadriceps contraction, which results in a lateralising force
    • Rotational malalignment
      • Femoral antetorsion
      • External tibial torsion
  • Bone problems (Static)
    • Small high-riding patella
      • Assessed by Blumensaat line and Insall-Salvati ratio
      • Insall-Salvati ratio normal range is 1.0 +/- 0.2
    • Lateral condylar hypoplasia; i.e. flat trochlea
      • Assessed by Merchant view sulcus angle
      • Normal = 140 Abnormal greater than 150o
      • CT scan
    • Incongruence of the PF joint
      • Assessed by the Merchant view congruence angle
      • Normal - 6o, Abnormal = greater than + 16o
    • Abnormal patella shape

Clinically

  • History
    • Pain is usually antero-medial and aggravated by bent knee activities
    • Patients often report clicking, catching and giving way
  • Patient standing
    • Varus / valgus
    • Patellar squinting
    • Signs of hypermobility
  • Patient sitting
    • 'Grass hopper eye' appearance : high and lateral patellas
    • Lateral patellar tilt
    • Patellar tracking: pain and crepitation, 'J' SIGN
    • Position of tibial tubercle with respect to trochlear midline < 20 mm lateral to the midline
  • Patient supine
    • VMO wasting
    • Hamstring tightness (popliteal angle)
    • Size and position of patella
    • Measure Q angle
    • Local tenderness; palpate articular surfaces, retinaculae, insertions
    • Patellar tracking : Maltracking begins in the first 10 - 20o of flexion, so if examined in greater flexion, maltracking may be missed as the patella may be caught in the trochlea
    • Patellofemoral crepitus
    • Patella apprehension test of Fairbank or Apley : displacement of the patella laterally in slight flexion causes pain and expectation of pending dislocation
    • Patellar shift (glide) test : knee flexed 20° to 30°; 1 quadrant or less medial glide is indicative of tight lateral
    • Patellar tilt test : evaluates tension of the lateral restraint
    • Clarke snatch test : pain on contraction of the quadriceps with the patella fixed
  • Patient prone
    • Staheli test

X-Rays

  • Plain AP + lateral + Merchant view
  • Lateral subluxation and loss of articular cartilage may be evident as well as the depth of the femoral trochlea
  • CT may show more accurate assessment of congruence
  • Patella Baja more likely to cause anterior knee pain

Pathology

  • Dislocation causes medial retinacular injury, which if does not heal and the VMO is not rehabilitated causes recurrent dislocation
  • Recurrent dislocation can damage to the lateral femoral condyle and undersurface of the patella
  • Osteochondral fractures following patella dislocation occur in between 5 - 25%

Treatment

  • Depends on a precise diagnosis of the pathology
  • Identifiable biomechanical abnormalities should be corrected surgically
  • Splint and begin quadrices exercises (especially VMO) immediately 
  • When able to SLR, mobilise in the splint
  • McConnell proprioceptive program has questionable benefit over simple mobilisation and strength exercises
  • Rehabilitation may reduce frequency of dislocations to a level where surgery is not required
  • Remove the splint for ROM exercises
  • Recurrent dislocation seldom needs surgery
    • Usually not until at least 6 months of conservative therapy tried
    • Before skeletal maturity proximal realignment procedures are performed :
      1. lateral release - commencing on the lateral side of patella tendon from 4 cm below the patella to the commencement of fibres of vastus lateralus (coagulate the superior lateral geniculate vessels) and should be able to tilt the patella 90o on itself after the release
      2. VMO advancement / medial plication
      3. Hemitransfer of the patellar tendon (Goldthwaite)
      4. Semitendinosus tenodesis (Galeazzi procedure)
    • After skeletal maturity, distal realignment procedures can be added to above :
      1. Hauser procedure (historical)
        • Medialization of the tibial tubercle in order to decrease Q angle
        • Due to the anatomy of the proximal tibia, translating the tibial tubercle medially, will also translate the tubercle
          posteriorly
        • Posterior translation of the tibial tubercle will have the effect of increasing patellofemoral contact pressures, which leads to pain and OA
        • Can produce a low patella (baja)
      2. Elmslie-Trillat type procedure (medial tibial tubercle transfer)
        • Aim for 'Q' angle of 10o
        • Never move tubercle posteriorly
      3. Maquet procedure (elevation of tibial tubercle)
        • Maquet recommended elevation of 2 - 2.5 cm, but this much elevation may cause skin necrosis; so elevation of 1 cm usually performed (use of a tissue expander may enable considerable elevation without risk of skin necrosis)
        • It has no effect on the Q angle
        • May help symptoms of patellofemoral OA or improve the mechanical effect of patellectomised quadriceps
      4. Goldthaite-Roux procedure : The patellar tendon is split and the lateral half is passed under the medial half and attached to the periosteum of the tibia in a medial position
      5. Fulkerson procedure
        • Anteromedial tibial tubercle osteotomy
        • Slope of cut dependent on amount of subluxation and DJD
        • Not as successful with medial facet chondromalacia
    • Patellectomy is the final option, but causes weakness of quadriceps mechanism, extensor lag and decreased flexion range (Kaufer, 1979). May relieve pain of patellofemoral OA, but if subluxation / dislocation is the problem, this may continue post patellectomy, unless anatomical problems addressed

Prognosis

  • Tends to stabilise with increasing age of the patient
  • After conservative treatment of acute dislocations :
    • Cofield and Bryan "Acute dislocation of the patella -results of conservative treatment" J Trauma 17: 526, 1977
      • 52% unsatisfactory results
      • 27% required further surgery
    • Cash and Hughston " Treatment of acute patellar dislocation" Am J Sports Med 16: 244, 1988
      • 43% recurrence rate, if signs of patellofemoral dysplasia
      • 20% recurrence rate, if no signs of dysplasia
  • After surgical Treatment of acute dislocations:
    • Cash and Hughston ( see above) medial reefing in 16 knees
      • Satisfactory results in 87%
  • After surgical treatment of recurrent dislocations:
    • Chrisman etal " A long term prospective study of the Hauser and Roux- Goldthwaite procedures for recurrent patellar dislocation " CORR 144:27, 1979
      • Hauser
        • 17% redislocation
        • 8.5% PF OA (due to overcorrection)
      • Roux- Goldthwaite
        • 5% redislocation
        • 5% PF OA
    • Cerullo etal " Evaluation of the results of Extensor mechanism reconstruction"Am J Sports Med 16:93, 1988
      • Results of proximal and distal realignment 100% satisfactory in dislocating patellae
      • 80% satisfactory in subluxing patellae

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