Tuesday, July 5, 2016

Metal backed porous tantalum glenoid components - debris formation, more on trabeculated metal

Early Debris Formation with a Porous Tantalum Glenoid Component: Radiographic Analysis with 2-Year Minimum Follow-up.

These authors point out that a first-generation porous tantalum glenoid component previously demonstrated failure, usually preceded by the appearance of intra-articular metallic debris. An example of component dissociation with this first-generation component is shown here.

After redesign, the component was reintroduced in 2009.

The authors reviewed 68 patients receiving a Trabecular Metal porous tantalum glenoid component (73 components; 5 patients underwent staged bilateral procedures) inserted without polymethylmethacrylate cement (representing off-label usage in the U.S.).

Sixty-six (90%) of the 73 components were evaluated at a minimum of 2 years of follow-up (mean radiographic follow-up of 50.8 months; range, 24 to 68 months). Of these, 92.4% demonstrated minimal or no glenoid radiolucency. Overall, the prevalence of metallic tantalum debris formation was 44% (29 of 66). Sequential radiograph review demonstrated that the incidence of metallic debris formation increased for each year of follow-up, with radiographs from 2, 3, 4, and ≥5 years of follow-up demonstrating a metallic debris incidence of 23%, 36%, 49%, and 52%, respectively.

The severity of metallic debris formation also increased with follow-up duration.

Here's an example of Grade 1, debris noted at the bone-metal interface;

of Grade 2, debris visible in soft tissues intra-articularly;

of Grade 3, visible but incomplete cracking or fracturing of the metal component;


They concluded that the development of metallic debris, increasing in both overall incidence and degree of severity over time, raises concern for potential failure of this glenoid component.

Comment: Metal backed glenoid components continue to manifest problems not present with all-polyethylene components. They demonstrate an increased rate of revision because of loosening, front side and back side polyethylene wear, component dissociation, fracture, instability, and cuff failure (possibility related to the increased thickness of the components) - see this link and the figure below.

 If an arthroplasty with bone ingrowth components requires revision because of infection, cuff failure or instability, removal of the components can result in substantial problems with bone integrity. Such bone damage may compromise secure fixation of a reverse total shoulder glenoid component.

This article presents another issue with metal-backed glenoid components, that of metallic debris, that appears to increase in rate and severity with time after implantation. The mechanism for this debris formation is unclear, but it could be that micromotion of the component pulls the porous trabeculated metal apart.

It can be seen from the list of Young's moduli below (in GPa), that the elastic modulus of polyethylene is closest to that of cortical and cancellous bone:
Cancellous Bone 0.4
Ultra high molecular weight polyethylene 0.5
PMMA bone cement 2
Cortical Bone 8
Titanium 112
Tantalum 186
Cobalt chrome 200.

The Young's modulus of a porous material can be modified by changing the degree of porosity. This is demonstrated in a recent article regarding porous tanalum (see this link). Here is the abstract:

"Relatively high cost of manufacturing and inability to produce modular all tantalum implants has limited its widespread acceptance, in spite of its excellent in vitro and in vivo biocompatibility. In this article, we report how to process Ta to create net shape porous structures with varying porosity using Laser Engineered Net Shaping (LENS™) for the first time. Porous Ta samples with relative densities between 45 to 73% have been successfully fabricated and characterized for their mechanical properties. In vitro cell materials interactions, using human osteoblast cell line hFOB, have been accessed on these porous Ta structures and compared with porous Ti control samples. The results show that the Young’s modulus of porous Ta can be tailored between 1.5 to 20 GPa by changing the pore volume fraction between 27 and 55%. In vitro biocompatibility in terms of MTT assay and immunochemistry study showed excellent cellular adherence, growth and differentitation with abundant extracellular matrix formation on porous Ta structures compared to porous Ti control. These results indicate that porous Ta structures can promote enhanced/early biological fixation. The enhanced in vitro cell-materials interactions on porous Ta surface are attributed to chemistry and its high wettability and surface energy relative to porous Ti. Our results show that these laser processed porous Ta structures can find numerous applications, particularly among older patients, for metallic implants because of their excellent bioactivity."

Nevertheless, based on the evidence available, metal backed glenoids may not offer to the patient advantages over an all polyethylene component.


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