Abstract
Prosthetic joint instability is a challenging concern for a minority of total hip
arthroplasty (THA) patients. Placement of the acetabular component within the traditional
safe zone does not eliminate dislocation, and the relative contribution of femoral
length and offset to instability risk has not been well defined. The authors compared
53 dislocated primary THAs treated against an age- and gender-matched cohort of 134
stable primary THAs. Anteroposterior and cross-table lateral radiographs were used
to determine whether reconstructions met targets for acetabular inclination (30–50 degrees),
acetabular anteversion (5–30 degrees), femoral length (0–9.9 mm) and femoral offset
(0–9.9 mm). Statistical analysis was performed to assess univariate and multivariate
relationships with an instability event; statistical significance was set using a
two-sided p-value < 0.05. Forty-seven (88.7%) of the dislocating hips had nonoptimal acetabular
or femoral reconstructions. While a similar proportion of patients in the study and
control groups had acetabular reconstruction within the safe zone (51.5 vs. 47.2%,
p = 0.63) patients with unstable hips were more likely to have acetabular component
inclination outside of the target zone (30.2 vs. 7.5%, p < 0.01), acetabular anteversion < 15 degrees (30.2 vs. 3.7%, p < 0.0001), reduced femoral length (35.9 vs. 3.7%, p < 0.0001), and reduced femoral offset (41.5 vs. 7.46%, p < 0.0001). Stepwise multivariate logistic regression was performed and identified
femoral head size less than 32 mm (OR 2.9, 95% CI 1.4–6.2) and higher inclination
angle (OR 1.1, 95% CI 1.04–1.2) as significant independent risk factors for hip instability.
The authors' study findings suggest that insufficient acetabular anteversion, femoral
length, and femoral offset reconstruction contribute significantly to instability
risk following THA. Using a larger femoral head is protective, but should be balanced
against long-term volumetric wear risk.
Keywords
total hip arthroplasty - prosthetic joint instability - dislocation - component malposition
- femoral length
