The Effect of Femoral Head Size, Neck Length, and Offset on Dislocation Rates of Constrained Acetabular Liners

Constrained acetabular liners (CALs) are used in both primary and revision total hip arthroplasty in cases where stability and abductor deficiency are of concern. The efficacy of CALs has been shown to be design dependent. There is clear evidence that the use of small head sizes and shorter offset in unconstrained total hip arthroplasty is associated with higher rates of dislocation. To our knowledge, no such study has assessed the effect of femoral head size, neck length, and offset for CALs.

We performed a retrospective study assessing the outcomes of CALs with minimum 2-year follow-up. A Kaplan-Meier survivorship analysis was conducted for all patients and for patients revised for instability. A binomial regression analysis was performed to assess for variables significantly associated with CAL failure.

A total of 285 CALs in 281 patients were identified with a mean follow-up of 5.7 years. Ten-year Kaplan-Meier survival analyses were as follows: all indication 91.9% vs instability 85.5% ( P = .15). Increasing neck length was associated with lower rates of failure (odds ratio, 0.81; P = .042). Femoral head size, offset, and abductor reconstruction were not significantly associated with CAL failure.

Larger head size has not been demonstrated to lead to lower failure in CALs. Increasing neck length was associated with lower failure rate. Surgeons should be cautious when attempting to ream to larger acetabular shell sizes for the purpose of using larger heads with CALs. Increasing neck length may instead be targeted intraoperatively.