Abstract
Introduction
Acetabular component position is considered a major factor affecting the etiology of hip dislocation. The ‘Lewinnek safe zone’ has been the most widely accepted range for component position to avoid hip dislocation, but recent studies suggest that this safe zone is outdated. We used a large prospective institutional registry to ask: 1) is there a ‘safe zone’ for acetabular component position, as measured on an anteroposterior radiograph, within which the risk of hip dislocation is low?, and 2) do other patient and implant factors affect the risk of hip dislocation?
Materials and Methods
From 2007 to 2012, 19,449 patients (22,097 hip procedures) were recorded in an IRB approved prospective total joint replacement registry. All patients who underwent primary THA were prospectively enrolled, of which 9,107 patients consented to participate in the registry. An adverse event survey (80% compliance) was used to identify patients who reported a dislocation event in the six months after hip replacement surgery. Postoperative AP radiographs of hips that dislocated were matched with AP radiographs of stable hips, and acetabular position was measured using Ein Bild Röntgen Analyse software. Dislocators in radiographic zones (± 5°, ± 10°, ± 15° boundaries) were counted for every 1° of anteversion and inclination angles.
Results
The frequency of dislocation was 2.1% (147 of 7040 patients) over the six months following primary THA. No significant difference was found in the number of dislocated hips among the zones (Figure 1). Dislocators were significantly older (p=0.012) and less active (p=0.001) compared to nondislocators (Figure 2). Patients <50 years old (p=0.007) and patients ≥70 years old (p=0.019) had a two-fold higher risk of dislocation. Dislocators <50 years old were less active preoperatively than nondislocators (p=0.006). A trend suggested larger head sizes had a lower frequency of dislocation (Figure 3).
Conclusions
Acetabular component position alone is not protective against instability. Age and activity level significantly affect the occurrence of dislocation after total hip replacement.
