Dislocation can be anterior or posterior; each having its own peculiar etiologies, anatomic and pathologic features. The ratio of anterior to posterior dislocation in primary THA is around 1:4.6. Thus, it is imperative to identify the differences in these two types for prevention, early diagnosis and treatment of these complications.
Anterior dislocation following THA is a rare occurrence, irrespective of the surgical exposure used. Patient factors such as neuromuscular disorders (Parkinson’s disease, epilepsy), obesity, alcohol consumption, scar tissues from previous surgeries can also contribute to hip instability (1). Of all the risk factors, component malposition is the most important and strongly associated with anterior hip dislocation. During THA, surgeons rely on bony landmarks such as anterior pelvic plane (APP) as reference markers to decide the cup position. The transverse acetabular ligament also serves as an intraoperative landmark for assessment of inclination and cup version. Changes in pelvic tilt, improper patient positioning, loss of bony landmarks due to fibrosis or prior surgeries or inadequate surgical experience may lead to excessive anteversion of the cup and/or the stem leading to impingement and subsequent dislocation. Many surgeons who regularly practice the posterior approach tend to place the cup in excess anteversion to minimize the risk of posterior instability. This when combined with excess anteverted stem or abnormal pelvic tilt can also predispose to anterior dislocation. Small femoral head sizes (22,28) tend to dislocate more likely due to reduced jump height (2).
Closed reduction and immobilization in a ‘desk-chair position’ is a successful mode of treatment in primary and recurrent anterior dislocations as well. Schino et al. reviewed 19 cases of anterior dislocations treated conservatively with good outcome (3). They proposed an excess femoral and acetabulum anteversion of 10ᵒ compared to the healthy side as an important risk factor. Elderly patients are more prone to progressive posterior pelvic tilt due to thoracolumbar kyphosis which can lead to late onset anterior hip instability even in cases wherein the placement of the cup was initially appropriate (4).
Even though anterior dislocation following THA is a rare occurrence, surgeons should be vigilant regarding this untoward complication. Prevention of excessive anteversion of cup and maintaining the combined anteversion in the ‘safe zone’ is the key surgical step. In our case, proper component position even precluded the use of constrained liner during the revision surgery. Removal of the constrained liner may pose a challenge due to the presence of the metal ring within the polyliner. Intra-operative assessment of stability is crucial and use of lipped or constrained liners should be encouraged for added stability. Patients who are elderly should be advised regular follow-up to look for progressive pelvic tilt to prevent late onset instability.