First Time Patella Dislocation

Acute patella dislocations form a 2-3% of acute knee injuries. The patient gives history of a fixed foot twisting injury and giving way with the patella lying over the lateral side. Sometimes as the patient extends the knee the patella relocates spontaneously. More often there is history of relocation by a physiotherapist or Paramedic. Occasionally the pivoting episode during ACL injury may be mistaken as patella dislocation. Haemarthrosis is extremely common in both Patella dislocations and ACL injury.

Facts

Most of the literature is retrospective. In such studies diagnosing the first episode can be non-specific as most patients don’t present to A&E with a dislocated patella The natural history after patellar dislocation including recurrence dislocation rate is not clear Hence there is confusion about who would benefit from surgery The incidence of osteochondral injuries is as high as 71% and only 1/3rd are picked up on radiographs. There is no clear indication as to which group of patients would benefit from surgery

Applied Anatomy

There are two restraints to lateral patellar dislocation

  1. bony constraint due to patella-trochlear congruity
  2. soft tissue restraint.

Three layered arrangement of medial retinaculum described by Warren and Marshall Layer 1- superficial medial retinaculum and Medial Patellotibial ligament Layer 2- Medial patellofemoral ligament (MPFL) and Superficial MCL Layer 3- Medial patellomeniscal ligament

In full extension the soft tissue restraints are the only stabilisers and there is no constraint offered by the trochlea. The guiding role of the femoral groove prevailed over soft-tissue action through most of the range of motion (Heegaard, CORR 1994)

The MPFL courses from supero-posterior to the medial femoral epicondyle to the super medial two thirds of the patella. Its fibres fuse with the under surface of the Vastus Medialis tendon. The MPFL is variable in cadaveric specimens. Total length and width of the MPFL was 58.8 +/- 4.7 mm and 12.0 +/- 3.1 mm (Nomura) The MPFL contributes 41-80% of the restraining force to lateral patellar dislocation.

The influence of Vastus Medialis Obliquus on lateral patellar dislocation is less clearly understood. VMO does offer passive restraint but varies in angle of insertion. Contraction of VMO can increase joint reaction force but it is questionable if it can counter the deficiency of passive restraints.

Epidemiology

The risk is highest amongst females 10-17 years of age. A lot of patients have a prior history of instability. The cause of dislocation is usually multifactorial. The recurrent dislocation rate varies from 15% (Macnab), Fithian 17%, Buchner 26% to Cofield 50%. Fithian showed that in patients with previous instability symptoms, the risk of recurrent dislocation is 49%. Patients with hyperlaxity, dysplasia of trochlea and altered patella height are at high risk of dislocations.

Diagnosis

History is important, as most patellae have been reduced by the time they present to A & E.

  1. Clear history of dislocation
  2. Haemarthrosis
  3. Tenderness medial retinaculum and or femoral attachment of MPFL
  4. Apprehension to lateral patella displacement
  5. Bruise medial edge of patella

Previous History- previous subluxations, patello femoral pain or arthritis Previous activity level Hyperlaxity- Beighton and Horan score (JBJS 1969) Score of 4/9 or more is significant

Investigations
Xray AP, True Lateral with knee flexed 30 degrees and Skyline patella at 30 degrees. Routine MRI is not indicated but one should have a high suspicion for osteochondral injuries and/or significant medial stabiliser disruption and obtain MRI in these situations.

Segond fracture

MRI

Findings on MRI after first time patella dislocation are

  1. MPFL rupture in one or more sites in more than 80%.
  2. Tear of inferior fibres of VMO
  3. Impaction injury to inferomedial patella
  4. Contusions of lateral femoral condyle
  5. Intra-articular loose bodies
  6. MCL injury not uncommon

Management of the first time patella dislocation
Non operative treatment

Non-operative treatment is the primary method of treatment for most first time patella dislocations. Different methods and duration of immobilisation have been reported. There is general agreement that 3 weeks of immobilisation with the knee in extension is sufficient. The author recommends a removable splint worn full time for 3 weeks but it is important to perform intermittent static quadriceps exercises during this period.

Role of surgery
Presence of Osteochondral fractures

Osteochondral fractures are seen in up to 71% of MRIs after first time dislocation and less than 1/3rd are picked up on plain radiographs. Nomura in their arthroscopic study shoed that 95% of their 39 consecutive knees had articular cartilage injuries to the central and medial aspect of patella and 12 knees had damage to the lateral femoral condyle. Stanitski reported that only a third of the osteochondral injuries seen at arthroscopy are seen on xrays. It does not still mean that a majority of patella dislocations need surgery as we don’t know the natural history of such articular damage. Fractures usually occur at the inferomedial aspect of patella or of the lateral femoral condyle. Lateral femoral condyle bruising is seen in 80% of MRI scans after patella dislocation. Picture shown shows classical location of osteochondral fracture off patella that has been fixed with bio-absorbable pins.

Significant disruption of medial stabilisers

Role of MPFL repair Presence of bruising over the antero-medial aspect of the knee would suggest substantial disruption of the medial stabilisers and warrant investigations and potential MPFL repair. Patella avulsion fractures can be seen on skyline xrays and similarly lateral subuluxation that may indicate surgery. MRI scans suggest that single or multiple location MPFL injury can be identified in more than 50% of patients. There is controversy as to MPFL rutures more commonly at the patella or femoral end but it appears that Patellar end rupture is more common (Guerrero, Elias). At arthroscopy one may identify a bare medial edge of patella (Picture). Haemorrhage and oedema at the inferior edge of VMO can be seen on MRI and also at surgery. Christiansen showed that delayed primary repair at the adductor tubercle may not be beneficial though it may improve subjective patella stability score. Silanpaa has reported that the incidence of recurrent dislocation is significantly lower in the surgically treated group.

Reference:

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