Fact-sheet: Total hip arthroplasty dislocation
Updated on 10/06/2017 at 4:27 PM
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Definition
When the prosthetic femoral head dislocates from the acetabular cup. Estimated complication rate of 5%. Occurs more frequently in the first few months. Etiology should be investigated in case of recurrent dislocation! - Implant malposition (cf. acetabular anteversion angle, prosthetic neck anteversion, acetabular version in the frontal plane) - Non union of the greater trochanter - Implant sizing - Muscle atrophy - Polyethylene wear. Immediate postoperative imaging is required to look for dislocation, most often posterior, related to muscular hypotonia and the surgical approach. Subsequently, an initial radiologic workup is required to assess proper positioning of the prosthetic components: - assessment of symmetric limb length: weight-bearing pelvic radiograph: use the bi-ischial line, the line connecting the lesser trochanters, or the line connecting the acetabular teardrops relative to the horizontal to assess discrepancies. - Femoral offset: distance between the axis of the femoral shaft and the center of the head (normal between 41 and 44 mm). Risk of dislocation if > 45 mm: tight muscle -> pain. - Cup orientation: 1. Horizontal acetabular version (AP radiograph): angle between the bi-ischial line and the acetabular line. Normal between 40 and 50°. If > 50°: risk of dislocation. 2. Acetabular anteversion (CT++, axial slice): normal anteversion of 10-20°. If > 20°: risk of anterior dislocation. - Prosthetic femoral neck anteversion angle: angle between the bicondylar plane and the neck axis. Normal between 10 and 20°. If > 20°: risk of anterior dislocation. On routine annual follow-up radiographs, look for signs of polyethylene wear (femoral head decentered within the acetabular liner)
Clinical features
Functional impairment, pain, and limb shortening are the main manifestations of hip prosthesis dislocation.
Radiography
AP and lateral hip radiographs:
Posterior dislocations: iliac (superior/head projecting over the iliac bone), ischiatic (inferior/head projecting over the ischium).
Anterior dislocations: pubic (superior/head projecting over the pubis), obturator (inferior/head projecting over the obturator foramen).
Search for associated complications:
fractures associated with THA.
Search for predisposing causes
CT
Positive diagnosis (prosthetic head dislocated within the soft tissues, outside the acetabulum).
Search for complications (associated fractures, nerve or vascular compression...).
Search for predisposing causes.
Rare dislocation: dislocation of the polyethylene acetabular liner
Management
Emergency reduction under general anesthesia.