Ceramic-on-ceramic bearing decreases the cumulative long-term risk of dislocation

Abstract

Background: It is unclear whether late THA dislocations are related to mechanical impingement or to a biological mechanism that decreases the stability provided by the capsule (eg, inflammation secondary to osteolysis). It is also unknown if alumina-on-alumina bearing couples decrease the risk of late dislocation as a result of the absence of wear and osteolysis.

Question/purposes: We asked (1) whether the cumulative number of dislocations differed with alumina-on-alumina (AL/AL) or alumina-on-polyethylene bearings (AL/PE); (2) whether patient factors (age, sex, and diseases) affect risk of late dislocation; (3) whether mechanical factors (component malposition, penetration resulting from creep and wear) or (4) biologic hip factors at revision (thickness of the capsule, volume of joint fluid removed at surgery, histology) differed with the two bearing couples.

Methods: One hundred twenty-six patients (252 hips) with bilateral THA (one AL/AL and the contralateral AL/PE) received the same cemented implants except for the cup PE cup or an AL cup. The cumulative risk of dislocation (first-time and recurrent dislocation) was calculated at a minimum of 27 years. We measured cup position, creep and wear, and capsular thickness in the hips that had revision.

Results: AL/PE and AL/AL hips differed by the cumulative number of dislocation (31 with AL/PE versus four with AL/AL) and by the number of late dislocations (none with AL/AL, 28 with AL/PE). Cause of osteonecrosis, age, and sex affected the number of dislocations. The frequency of component malposition did not differ between the two bearing couples. The risk of late dislocation appeared less in AL/AL hips with increased capsular thickness (mean, 4.5 mm; range, 3-7 mm) compared with the thinnest (mean, 1.2 mm; range, 0.2-2 mm) capsule of AL/PE hips.

Conclusions: AL/AL bearing couples decreased the cumulative risk of dislocation as compared with AL/PE bearing couples.

Fig. 1

For hips with PE cups, the percentage of first-time dislocations was 1% at 1 month and 2% at 1 year, rose with an increasing rate of 2.4% from Year 2 to 10 to reach 4.5% at 10 years, rose with an increasing rate of 3.6% from Year 10 to 20 to reach 8% at 20 years, and then rose with an increasing rate of 5% from Year 20 to 30 to elevate to 13% (95% confidence interval, 9%–17%) at 30 years for patients who were alive and had not had a revision by that time. For hips with AL cups, the percentage of first-time dislocations was 1% at 1 month, 2% at 1 year, and then did not change at 20 years and at 30 years (2%; 95% confidence interval, 1%–3%) for patients who were alive and had not had a revision by that time.