Taper angle influence on liner positioning
The angle of the inner taper of the acetabular shell seems to have a significant influence on the placement of a ceramic liner in the cup and the dissociation force when removing it. For the study reported by Lee et al. three experienced high-volume arthroplasty surgeons participated in a ceramic-liner insertion test with two types of acetabular shells: 1) standard 18° taper, 2) multi-bearing option shell with 10° taper angle.
Each surgeon used three metal shells of each design and 30 ceramic liners (BIOLOX®delta). The metal shells were press-fit into sawbones blocks. For the push-out experiment the components were assembled manually and impacted with a 2 kN compression force. A load was applied through the hole at the apex of the shell and the maximum force to disassembly was recorded.
There was no malseating with the 18° taper, but all the participating surgeons experienced several malseated liners with the 10° taper. The dissociation force from the 10° taper (1,148.8 N) was almost three times higher than with 18° shells (389.7 N). The authors conclude that multibearing acetabular shells have several advantages. However, the authors caution about malplacement of ceramic liners when using a multi-bearing metal shell with a 10° taper, which may negatively affect the outcome of using all-ceramic bearings.
Removal of ceramic liner
For the rare event of revision of a ceramic liner, Pitto describes a simple procedure: “Using a metallic impactor placed along the external rim of the cup, the surgeon can hit the metal cup’s rim with a robust hammer blow. The ceramic liner will dissociate from the internal metal surface. A sharp tool will be necessary to elevate the loosened ceramic liner from the cup margin.” The author states that there may be a few acetabular designs that require more than one hammer blow to achieve liner dissociation.
According to the author, elasto-dynamic compressive stress waves, which are initiated by the impact on the rim of the metal shell cause the dissociation. The stress waves are reflected at physical boundaries and transformed into tensile stress waves. These, if they exceed the local stresses resulting from friction and adhesion at the taper junction, will lead to a separation at the taper interface. The author stresses the importance of choosing compatible components, because “using a wrong liner can result in a disaster. Never mix and match!”
Economic impact of PJI in THA
In a presentation at the last ISTA Congress, Akindolire et al. evaluated the economic burden of PJI on Canadian healthcare. They matched primary and two-stage PJI revision THA cases for age and BMI, recording all costs associated with each procedure. They concluded that septic revision is a significant economic burden to the healthcare system. Compared to primary THA the data show a fivefold increase in healthcare costs.
ista.online/ISTA2016-abstract-book/ISTA2016BostonAbstracts.pdf (Abstract #4646)
Consequences of tribocorrosion
Also at ISTA 2016, McGrory and Hussey gave an update about their ongoing study on the consequences of adverse local tissue reaction (ALTR) for 1,356 consecutive patients with metal-on-polyethylene THA. They found symptomatic mechanically assisted crevice corrosion (MACC) present in 2.9% of their patients. Of these, 56% have already undergone revision surgery, while the remaining 44% opted for ongoing surveillance. Over time, these patients show increasing metal ion levels. The authors consider introducing baseline metal ion serum estimation also in asymptomatic patients.
ista.online/ISTA2016-abstract-book/ISTA2016BostonAbstracts.pdf (Abstract #4774)
Reasons for post-operative fracture of BIOLOX®delta inserts
Ceramic vs. revision burden
Regarding the general revision burden, using a ceramic head is cost-effective in many cases because of the reduction of revisions of THA with ceramic heads. Carnes et al. focused on the factors of implant price difference in the USA and patient age. At a cost differential of US$ 325, ceramic-on-polyethylene bearings are more cost-effective than metal-on-polyethylene for patients under the age of 85. At US$ 600, cost-effectiveness is reached for patients under 65 years. If the difference is US$ 1,003 or above, cost-effectiveness cannot be reached.
The authors used Markov modeling to estimate the national cost burden of choosing CoP vs. MoP implants for primary total hip arthroplasty. The Premier Research Database was used to identify 20,398 patients with a minimum age of 45 years who underwent total hip arthroplasty in 2012.