Metal backed glenoids: the problem of poly failure

Medium-term rates of radiolucency after primary total shoulder arthroplasty using a cementless metal-backed pegged polyethylene glenoid

Shoulder & Elbow  2020 DOI: 10.1177/1758573219901122

These authors investigated periprosthetic radiolucency rates associated with an uncemented,metal-backed polyethylene glenoid component with medium-term results.

Methods: A single centre retrospective study examining radiological outcomes of the Epoca metal-backed glenoid component

Forty-one patients were followed up with a mean follow-up time of 5.5 years (3–8 years). At follow-up six patients had undergone revision (14.6%). Three patients required revision for cuff failure and were revised to reverse total shoulders. Three patients required revision for failure secondary to polyethylene wear and lysis
Six patients had  radiolucency on follow-up radiographs. 

Comment: The high failure rate of metal backed glenoids from polyethylene failure has been previously described. See the additional references below:

Metal-Backed Glenoid Components Have a Higher Rate of Failure and Fail by Different Modes in Comparison with All-Polyethylene Components: A Systematic Review.

To review, the abstract is reproduced here

BACKGROUND: Glenoid component failure is a common and serious complication of total shoulder arthroplasty. The purpose of this study was to evaluate published evidence on whether metal backing lessens the rate of glenoid component failure.

METHODS: A comprehensive systematic review yielded twenty-one studies on radiolucency, radiographic failure, and revision after arthroplasty with metal-backed glenoid components and twenty-three studies with all-polyethylene components. Our analysis included data on 1571 metal-backed and 3035 all-polyethylene components. The mean duration of follow-up was 5.8 years in the studies with metal-backed components and 7.3 years with all-polyethylene components.

RESULTS: All-polyethylene components had a 42.5% rate of radiolucency compared with 34.9% for metal-backed components (p = 0.0026) and a 21.1% rate of radiographic loosening or failure compared with 16.8% for metal-backed components (p = 0.0005). However, the rate of revision was more than three times higher with metal-backed components (14.0%) than with all-polyethylene components (3.8%, p < 0.0001). Although 77% of the revisions of all-polyethylene components were for loosening, 62% of the revisions of metal-backed components were for other reasons, such as component fracture, screw breakage, component dissociation, polyethylene wear, metal wear, and rotator cuff tear (p < 0.0001).

CONCLUSIONS: The published evidence indicates that metal-backed glenoid components require revision at a significantly higher rate and for different reasons in comparison with all-polyethylene components.

Metal-backed glenoid implant with polyethylene insert is not a viable long-term therapeutic option

These authors reviewed 165 cases of primary osteoarthritis treated with an anatomic total shoulder arthroplasty using an uncemented metal-backed (MB) glenoid components with a polyethylene (PE) insert; the mean patient age was 68 years.

Outcomes were assessed both clinically and radiologically with a minimum of 2 years of follow-up. The survival endpoint was either partial or complete revision.

Survival rate free of revision was 46% at 12 years. At a mean follow-up of 8.5 years (range, 2-16 years), revision was required in 61 patients (37%); 80% of shoulders undergoing revision (49 of 61) had evidence of PE wear. Glenoid loosening (because of osteolysis secondary to wear debris), soft tissue deficiency, and prosthetic instability were the most common modes of failure. Younger patients and biconcave glenoids (with posterior humeral subluxation) had a negative effect on implant survival. Proximal humerus osteolysis was significantly more frequent in shoulders with PE wear as shown in their figure below.

Exchange of the PE insert (with conservation of the MB tray) was possible in only 3% of the revised shoulders.

This study indicates that polyethylene wear with secondary osteolysis from polyethylene and metal debris are the primary failure mechanisms for metal backed glenoids in anatomic total shoulders. 

One of the most important figures in this paper is shown below. It indicates that the often applied standard of a minimum two year followup for arthroplasties does not come close to indicating the failure rate that occurs later on.

The high rate of failure of metal-backed glenoid components - especially with eccentric loading - is explained in large part by examining the Young's modulus of the materials involved. This quantity, also known as the tensile modulus or elastic is a measure of the stiffness of a material.

Here are some representative values of the materials of interest in total shoulder arthroplasty. Young's modulus is expressed in Giga Pascals (GPa).

Cancellous Bone 0.4

Ultra high molecular weight polyethylene 0.5

PMMA bone cement 2

Cortical Bone 8

Titanium 112

Cobalt chrome 200

Thus with a metal backed glenoid there are two critical mismatches of Young's modulus: (1) that of metal (100-200) to bone (0.4-8) and (2) that of metal (100-200) to polyethylene (0.5). The first may account for stress shielding of the glenoid bone and the second may account for the increased rate of polyethylene wear and polyethylene-metal dissociation seen with metal backed components. By contrast, all polyethylene components have better modulus matches with bone. PMMA bone cement also has a similar modulus to that of bone and polyethylene.

What this means is that when the component is loaded, the materials of similar Young's modulus deform similarly, whereas those with mismatched Young's modulus do not, resulting in relative displacement at the interface.

See also the post "The challenges of metal backed glenoids - why do they fail more often?" as well as the information posted here and here and here.

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Glenoid failure in total shoulder - do cysts play a role?

Total shoulder arthroplasty with minimum 5-year follow-up: does the presence of subchondral cysts in the glenoid increase risk of failure?

These authors state "In our patient population, we found a high prevalence of radiographic loosening (41%) after 5 years".

They evaluated the effect of preoperative cystic changes in the glenoid  (determined by CT) on postoperative outcomes and implant survival after total shoulder arthroplasty (TSA) for 75 patients having TSA for primary osteoarthritis with minimum 5-year follow-up. 

8 shoulders had no cysts, 27 shoulders had small cyst formation, 19 had medium cysts, and 21 had large cysts. 

The overall revision rate was 7% (5 of 75). All revised patients were in the groups with medium or large cysts. There were no statistical differences in American Shoulder and Elbow Surgeons (ASES) Standardized Shoulder Assessment scores or presence of radiographic loosening among the study groups. 

The authors did not note an increased failure rate for the type B2 gleonoids.

Comment: The authors do not suggest how the cystic glenoid might be managed to avoid the apparent increased risk of glenoid component failure. In our practice, we use an all polyethylene glenoid component with a fluted central peg. 

If glenoid cysts are present, these are autografted using bone from the humeral head.

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Metal backed glenoid components - high failure rate

Outcomes of Trabecular Metal–backed glenoid components in anatomic total shoulder arthroplasty

These authors state that "the current design of the Trabecular Metal–backed glenoid component (Zimmer) was released in 2009. Although over 10,000 of these glenoid components have been implanted worldwide, evidence on either the intermediate- or long-term survival of Trabecular Metal–backed glenoid components in anatomic TSA is very limited."

After a Class 2 recall in 2005 because of concern for fracture at the junction of the base and the trabecular metal keel (see this link), this component was cleared by the FDA (see this link) based on its being substantially equivalent to the predicate recalled devices.

The authors investigated implant survival and two year (average 41 month) clinical outcomes for 36 of 47 patients who received a Trabecular Metal–backed glenoid component (see this link) during primary anatomic total shoulder arthroplasty performed by an individual surgeon.

Three patients showed signs of osteolysis, 4 had radiographic evidence of metal debris, and 1 patient had a catastrophic failure after a fall. Of the 47 TSAs, 5 (11%) were revised to a reverse TSA for subscapularis failure and pain. The authors concluded that Trabecular Metal–backed glenoids had a 25% rate of radiographic metal debris and osteolysis at a minimum 2-year follow-up in this series with one catastrophic failure.

This publication on failures of metal backed glenoids is consistent with a prior post, reproduced below

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 as discussed below.

Radiographic and clinical outcomes of total shoulder arthroplasty with an all-polyethylene pegged bone ingrowth glenoid component: prospective short- to medium-term follow-up.

These authors reviewed 80 total shoulders using an all polyethylene glenoid component.  The glenoid was reamed minimally to preserve subchondral bone, which was given priority above correcting retroversion.

1 of 80 shoulders was revised for aseptic glenoid loosening.  81.6% had a radiolucency grade of 0 or 1. Nearly 90% had a glenoid seating grade of A or B. Grade 2 or 3 bone around the central peg was seen in 88.2%. 

No statistical association existed between Walch glenoid types and radiolucency grades, bone grades around the central peg, perfect radiolucency grade, seating grade, and grade 3 bone around the central peg. 

Comment: We previously posted on this article here, emphasizing the importance of register (see link).

We'd like to add a few additional thoughts. These authors use a technique similar to the one used in our practice (see link).  No patient-specific instrumentation or complex 3D reconstructions were used. The glenoid was reamed conservatively to a single concavity without emphasizing a change in version. 

Using the all poly glenoid after careful glenoid preparation with minimal reaming yields consistently better results than those achieved with metal backed components (see link). The DePuy and the Tornier versions of the modern all-poly glenoids are shown below. Other relevant links are here and here. 

The quality of fixation and the bone ingrowth into the pegs is substantial as shown in some of our cases below.

Against this background, we wonder whether more complex components, such as the one shown below and discussed here, provide additional value.

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Failed resurfacing with mini glenoid component

We had the opportunity to see a patient whose glenohumeral arthritis

was treated in another state with a humeral head resurfacing with a mini glenoid component.

Eight months later, the shoulder was increasingly painful; x-rays showed evidence of loosening of the glenoid component.

Two months later the glenoid was removed by the original surgeon.

The shoulder continued to be stiff and painful. The patient presented to our service. After discussion of the alternatives and the possibility of low grade infection, we performed a revision with multiple tissue and explant cultures, all of which were fortunately negative.

Three months after the revision the patient reports that the discomfort has yet to completely resolve.

The patient's therapist reported
AAROM flexion 165* (able to consistently achieve)
AAROM abduction 160* (limited at times to 90*)
AAROM Internal Rotation in Abduction 40*
AAROM External Rotation in Abduction 60*

We'll keep you posted!

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The B2 glenoid

Management of the Biconcave (B2) Glenoid in Shoulder Arthroplasty: Technical Considerations.

These authors point out that severely biconcave arthritic glenoid pathoanatomy can create major technical challenges in shoulder arthroplasty and jeopardize the longevity of prosthetic glenoid components.  They review different strategies for managing this anatomy, ranging from eccentric reaming and total shoulder arthroplasty, to posterior glenoid bone grafting, to posteriorly augmented implants and to  reverse shoulder arthroplasty.


Comment: As we consider the management of various types of glenoid pathoanatomy it is important to recognize that there is a continuum in "B" glenoids - between those that have minimal biconcavity to those that have severe biconcavity as shown in the diagram below. In fact as our recent posts show, there are many types and many shades of glenoid pathoanatomy.

In our experience, every shoulder with posterior humeral decentering has some degree of biconcavity, so maybe there is no such thing as a pure B1.

One of the interesting things that can be observed from this paper (Figure 3) is that the position of the humeral head relative to the glenoid articular surface depends on the position of the arm when the image is obtained. Note that in the CT scan below taken with the arm at the side, the head is relatively centered in the glenoid and not resting in the posterior pathologic concavity.

 However, when the arm was abducted to obtain the axillary view, the humeral head falls in to the posterior concavity as shown below.

For that reason we make a point of taking the axillary view when the arm in the functional position of elevation in the plane of the scapula, what we refer to ask the 'truth view'. See this link.

Another interesting observation in this paper (Figure 5) is the possibility of overcorrecting posterior subluxation. The preoperative view shows posterior subluxation into a pathologic posterior concavity.

 The post operative view shows the humeral head to be anteriorly decentered on the glenoid.

An alternative approach to the description of glenohumeral pathoanatomy can be based on three parametric measurements:

(1) The percent of the glenoid surface that has a pathologic biconcavity (33% posterior in the example below).

 (2) The angle of retroversion of the glenoid face (G) in relation to the scapular body (S)

(3) The centering of the humeral head with respect to the glenoid  (the distance between the anterior glenoid lip and the center of glenohumeral contact (C) divided by the distance between the anterior and posterior glenoid lips (G)).  0.5 indicates a centered humeral head.

Using this system, 

the glenoid below would be 50% anterior biconcavity, 10 degree retroversion, and decentering of .25 (subluxation).

the example glenoid below would be 0 biconcavity, 40 degree retroversion, and centering of .5. 

the example  glenoid below would be 0 biconcavity, 15 degree retroversion, and centering of .5. 

Such a system can provide the information necessary for characterizing the pathology and for planning treatment.

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Glenoid components - do polyethylene and metal mix? - Young's modulus - of princesses and peas

We know that in hip surgery, the combination of a polyethylene articulating surface within a metal cup has proven long-term clinical results, in spite of the dissimilarity in the mechanical properties of the two materials. We suggest that this is because the increased deformability of the poly is contained within the less compliant metal shell and because the loading is generally concentric.

A similar situation exists for the contained poly of a reverse total shoulder

However in anatomic total shoulders were the poly is not contained and the loading is characteristically eccentric, the results are different,  as shown in Metal-Backed Glenoid Components Have a Higher Rate of Failure and Fail by Different Modes in Comparison with All-Polyethylene Components: A Systematic Review, and in Metal-backed glenoid implant with polyethylene insert is not a viable long-term therapeutic option and in this figure below from the Australian Orthopaedic Association annual report metal+poly glenoid components appear to have a higher revision rate than all poly components.

One of the most important figures in the Metal-backed glenoid implant with polyethylene insert is not a viable long-term therapeutic option paper is shown below. It indicates that the often applied standard of a minimum two year followup for arthroplasties may not come close to indicating the failure rate that occurs later on.

In looking at some of these implants we can identify areas where loading of the component may produce different amounts of deformation in the two materials with a possible increase in risk of failure.

The high rate of failure of metal-backed glenoid or hybrid components - especially with eccentric loading - may be explained in large part by examining the Young's modulus of the materials involved. This quantity reflects the relationship between stress (force per unit area) and strain (proportional deformation) in a material - in other words, how much the material changes shape when a load is applied to it.

Here are some representative values of the materials of interest in total shoulder arthroplasty. Young's modulus is expressed in Giga Pascals (GPa). It can be seen that in spite of claims to the contrary, the three metals do not have " biomechanical properties similar to native 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

Thus with a metal backed or hybrid glenoid there are two critical mismatches of Young's modulus: (1) that of metal (100-200) to bone (0.4-8) and (2) that of metal (100-200) to polyethylene (0.5). The first may account for stress shielding of the glenoid bone and the second may account for the increased rate of polyethylene wear and polyethylene-metal dissociation seen with metal backed components. By contrast, all polyethylene components have better modulus matches with bone.

What this means is that when a component is loaded, the materials of similar Young's modulus deform similarly, whereas those with mismatched Young's modulus do not, resulting in relative sheer at the interface.

See also the post "The challenges of metal backed glenoids - why do they fail more often?" as well as the information posted here and here and here.

In summary, metal-backed glenoid components do not seem to solve -but rather exacerbate -  the problem of glenoid failure in total shoulder arthroplasty. Our preference remains the all polyethylene component shown here inserted with no cement between the back of the component and the well prepared face of the glenoid bone. The Young's modulus of poly matches that of bone, without having to use materials that are over 100 times stiffer.

Fixation is achieved with bone ingrowth between the flanges of the central peg as nicely shown in this two year x-ray.

Readers with children can explain Young's modulus using the story of the Princess and the Pea, by Hans Christian Andersen.

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