CORR Insights®: Are Hooded, Crosslinked Polyethylene Liners Associated with a Reduced Risk of Revision After THA?

Where Are We Now?

Dislocation is one of the most-common early complications following primary THA [2, 6], and the risk of its occurrence has been related both to factors associated with the surgeon as well as with the patient. Patient-related risk factors have included age, sex, BMI, diagnostic indication for surgery, and comorbid conditions, while surgeon-associated risk factors have included surgical approach, orientation of components, and femoral head size [2, 4-6, 8, 11, 12].

But there is a third potential source of risk: Implant type. Hooded acetabular polyethylene components were introduced early in the history of hip arthroplasty to reduce the risk of dislocation. Hooded liners are available both for cemented fixation as well as for use with cementless shells. The asymmetrically augmented rim increases the jump distance in the direction the hood is oriented [10] and is associated with a reduced risk of dislocation and with risk of revision to treat recurrent dislocation [7].

However, some research suggests that paradoxical impingement of the prosthetic femoral neck on the elevated polyethylene rim can result in dislocations in unexpected directions, as well as increased polyethylene wear and osteolysis [3]. And while a study from the New Zealand Joint Registry found a reduced risk of revision due to dislocation for hooded polyethylene liners [7], I am aware of no large studies with long-term followup to investigate the revision results of hooded highly crosslinked polyethylene (HXLPE) liners.

In the current study, Bauze and colleagues [1] use the Australian Orthopaedic Association National Joint Replacement Registry to compare implant survival for hooded and nonhooded HXLPE liners. Their implant-survival analysis showed higher revision rates related to dislocation in patients who received nonhooded liners compared to those who received hooded liners. More importantly, the reduced risk of revision for dislocation was not offset by any increased in the risk of revision for aseptic loosening and osteolysis. On the contrary, hooded liners had better implant survival with respect to revision from aseptic loosening or osteolysis as the endpoint.

Where Do We Need To Go?

Although Bauze and colleagues [1] found that hooded liners were associated with reduced revision risks, we still need to determine whether this result is implant-specific or generalizable to all hooded designs. The study included all uncemented cup designs but did not consider whether the analyzed cups were available in both hooded and nonhooded versions. Theoretically, the results could depend on inferior results for specific cup designs for which hooded liners were not available or that were used in few patients.

While the study indicated that influence of hooded liner was particularly strong for head sizes of 32 mm or larger, it is not clear—and future studies should assess—whether surgical approach has a differential impact on dislocation between patients receiving hooded and nonhooded liners. Future research also should investigate design features like the degree of rim elevation of hooded liners, as this difference might also have an important effect on the risk of dislocation, polyethylene wear, or osteolysis. Furthermore, our current knowledge is limited to hooded liners for cementless acetabular components, so future studies need to explore whether hooded, cemented cups also provide superior implant survival.

Finally, the mechanism behind reduced risk of revision due to loosening/osteolysis is unclear. The authors speculate that reduced wear and edge loading may be the cause, as the hooded liners provide additional coverage of the femoral head. One could also speculate that component position has a role to play; the hooded liner may allow the surgeon to find a cup position that provides stability with more bone contact, which could contribute to lower risk of loosening. However, findings like these that are hard to explain—and require so much speculation to arrive at an explanation—should also be viewed with some suspicion, as they suggest that unmeasured confounding variables might also be at work here. For that reason, even the primary study findings in the study by Bauze and colleagues [1] really should be independently confirmed if we are to have confidence in it.

How Do We Get There?

It seems to me that the questions of whether the findings of Bauze and colleagues [1] are generalizable or implant-specific, whether they will hold true for cemented designs as well as uncemented ones, and whether there are differences in dislocation risk across different degrees of polyethylene rim elevation, can be answered only in the setting of large, national registries. Given the high number of arthroplasties needed to arrive at robust analyses, these questions might be approached through international register collaborations, by pooling individual data or aggregate data using a meta-analysis approach [9].

By contrast, the question of whether there will be differences in terms of polyethylene wear associated with different polyethylene rim configurations, cup designs, and cup positions can be approached by in vitro wear simulator tests and in vivo studies using CT-based implant micromotion analysis, which is a promising alternative to marker-based radiostereometric analysis.