Tag Archive | acetabular cup

Durability of Constrained Liners in Revision THA

The indications for treating total hip arthroplasty (THA) dislocations by cementing a constrained polyethylene liner into a well-fixed, retained acetabular component at the time of revision are narrow. That’s largely due to concerns about the durability of the resulting acetabular construct. Now, thanks to a study by Brown et al. in the April 3, 2019 issue of JBJS, hip surgeons have some hard data about the long-term outcomes of this approach.

After reviewing 125 cases in which a constrained liner was cemented into a retained, osseointegrated acetabular component during revision THA, with a mean follow-up of 7 years, the authors found that:

  • Survivorship free from revision for instability was 86% at 5 years and 81% at 10 years. The cumulative incidence of instability at 7 years was 18%.
  • Survivorship free from aseptic acetabular component revision was 78% at 5 years and 65% at 10 years. The most common failure mechanism was dissociation of the constrained liner from the retained component.
  • Harris hip scores (HHS) did not improve significantly after revision. This finding is consistent with prior research that shows better post-revision HHS scores in patients whose revisions include the entire acetabular component.
  • Position of the retained cup did not affect implant survivorship or risk of dislocation.

The authors mention alternative strategies for reducing the risk of dislocation after revision THA, such as the use of large-diameter heads and dual-mobility constructs. Still, they conclude that this constrained-liner approach, in the setting of a relatively well-positioned acetabular component, is a viable and durable THA revision option, especially for those “with a compromised abductor mechanism, recurrent instability, [and] a well-fixed and well-positioned acetabular component, for whom an acetabular revision would not be tolerated.”

Cup-Placement Precision Declines as Obesity Increases

Obesity can negatively affect outcomes after total hip arthroplasty (THA), and an inadvertent reduction in cup anteversion may be one reason why, according to findings from Brodt et al. in the May 4, 2016 edition of The Journal of Bone & Joint Surgery.

Cup Anteversion Obesity.gifThe authors retrospectively analyzed postoperative radiographs from 790 THA patients (all of whom were operated on via a direct lateral approach) within three BMI ranges: normal weight (BMI <25 kg/m2), moderately obese (BMI between 25 and 34 kg/m2), and morbidly obese (BMI of ≥35 kg/m2). Reduced cup anteversion significantly correlated with increasing BMI and younger patient age, with the morbidly obese group demonstrating a 3.4° anteversion reduction compared with the normal-weight group. The authors attribute the reduced anteversion to increased pressure applied to dorsal and ventral acetabular rim retractors to ensure adequate visualization during THA surgery in obese patients.

When the authors applied their findings to the Lewinnek “safe zone” for acetabular positioning, only 59% of the morbidly obese patients were in that zone.  While this study was not designed to track subsequent dislocations (a common consequence of incorrect cup positioning), the authors claim that these findings are nevertheless clinically important. “Knowledge of a systemic error in obese patients should raise surgeons’ awareness of the need to perform cup implantation with greater attention,” they conclude.

3-D Planning Enhances Cup-Placement Accuracy in THA

Screen Shot 2016-02-02 at 8.47.07 AMWhen it comes to acetabular cup positioning during total hip arthroplasty (THA), precision really matters. Malpositioned cups increase the risk of dislocation, early wear, and loosening, among other unwanted outcomes.

In the January 20, 2016 issue of The Journal of Bone & Joint Surgery, Sariali et al. report on results of a randomized trial that compared cup positioning guided by three-dimensional (3-D) visualization tools used intraoperatively (28 patients) with freehand cup placement (28 patients). Cup anteversion was more accurate in the 3-D planning group, and the percentage of anteversion outliers according to the Lewinnek safe zone was lower in the 3-D planning group. Although cup abduction was restored with greater accuracy in the 3-D planning group, the percentage of abduction outliers was comparable between groups.

Interestingly, operative times did not differ between the two groups. The authors note that CT-based navigation, a more expensive technology used to improve acetabular-cup positioning, does increase operative times, although its reported accuracy is higher than that of the 3-D planning technique used in this trial. That apparent tradeoff leads the authors to conclude that “3-D planning may be a good compromise between accuracy on the one hand and extra cost and duration of surgery on the other hand.”

It should also be noted that Sariali et al. did not measure clinical outcomes in this study, so there’s no evidence here that the accuracy enhancements arising from 3-D planning translate into meaningful clinical improvements.