Glenoid component loosening - relationship to surface geometry

Radiographic performance depends on the radial glenohumeral mismatch in total shoulder arthroplasty

These authors evaluated the radiographic and clinical performance of an anatomic total shoulder arthroplasty (TSA) with respect to radial mismatch between the curvatures of the glenoid and humeral components. The study group was divided in two groups, one with mismatch < 4.5mm (n:52) the others with mismatch ≥4.5mm (n:23) and analyzed for confounding variables as indication, primary or revision surgery, age, gender, glenoidmorphology and implant characteristics.

The mean glenohumeral radial mismatch was 3.4mm (range 0.5–6.9).

At median follow-up of 41 months radiographic loosening was present in 7 cases (9.3%). 

Lucencies around the glenoid pegs were present in 34 cases (45%). 

Radiolucencies were significantly associated with a radial mismatch < 4.5mm.

The pre- to postoperative improvements in Subjective Shoulder Value and absolute Constant Score were significantly better in the group with a mismatch ≥4.5mm.

The authors concluded that lower conformity between the radii of humerus and glenoid seems to improve the loosening performance in TSA.

Comment: It is well recognized that a high degree of conformity between the curvature of the humeral head component and the glenoid component can lead to rocking horse loosening of the glenoid.

A slight degree of mismatch between the curvatures allows for some translation without rim loading.

However, high degrees of mismatch reduce the joint contact area and increase the local joint pressure. 

This can lead, over time, to increased polyethylene wear as shown below.

In our practice, we prefer a mismatch of 6mm in diameter, which is equivalent to a mismatch of 3 mm in radius.

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What about Glenohumeral Mismatch in Anatomic Total Shoulder Arthroplasty?

Glenohumeral Mismatch in Anatomic Total Shoulder Arthroplasty

"Mismatch" refers to the difference in curvature between the humeral head articular surface and the prosthetic glenoid articular surface.

In describing this difference, one must be careful to state if the difference is described in terms of the radius of curvature or the diameter of curvature.

It is reported that "The optimal amount of glenohumeral radial mismatch in anatomicshoulder arthroplasty remains undefined, but the consensus among multiple studies suggests between 4 and 8 mm. Current implant designs offer mismatch of the radii of curvature between the humeral head and the glenoid ranging from 1 to 38 mm"

However, implant companies commonly characterize their component combinations not in terms of radial mismatch, but in terms of diametral mismatch, with a range from 1-24 mm, as shown in the examples below.

This review suggests that conforming components (i.e. those with no or minimal diametral mismatch) may be more prone to loosening because the translations that occur with glenohumeral motion exert greater rocking moments when the humerus is constrained by conformity. These observations echo those made in the 1994 book, Practical Evaluation and Management of the Shoulder (available at this link). Illustrations from that book drawn by Steve Lippitt are reproduced below.

Translation can also result in rim loading of the glenoid component, leading to deformation through cold flow of the polyethylene.

 The trade off for increasing amounts of diametral mismatch is increased joint pressure due to decreased contact area. This increased pressure may lead to increased glenoid surface wear.

Perhaps the key point here is that in total shoulder arthroplasty, glenohumeral stability should be achieved primarily on balancing the forces across the joint so that the humeral head remains centered, without depending on constraint from conforming joint surfaces.  

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Rocking horse loosening in total shoulder arthroplasty

Glenohumeral mismatch affects micromotion of cemented glenoid components in total shoulder arthroplasty 

Back in 1988, we described the phenomenon of 'rocking horse loosening' in which eccentric loading of the glenoid component causes failure of glenoid component fixation to the underlying glenoid bone.

These authors suggest that this “rocking horse” phenomenon is the main cause of glenoid component loosening as a result of eccentric loading of the glenoid rim. They aimed to investigate the influence of increasing glenohumeral implant mismatch on bone-implant interface micromotion in a cemented all-polyethylene pegged glenoid biomechanical model.

Five glenoid sizes, 40 mm, 44 mm, 48 mm, 52 mm, and 56 mm were cyclically loaded with a 44 mm humeral head. They state that these combinations represent  +2 mm, +6 mm, +10 mm, +14 mm, and +18 mm glenohumeral mismatches, respectively. There is an apparent math problem here that can only be resolved if the actual surface diameter of curvature of each glenoid is 6 mm greater than its name, e.g. the 40 mm glenoid actually has a diameter of curvature of 46 mm so the mismatch with a 44 mm head is 2 mm. There is another problem in that they state that the authors refer to  "The radius of curvature mismatch for each glenoid size was +2 mm, +6 mm, +10 mm, +14 mm, and +18 mm". If these were actually radial mismatches, the respective diametral mismatches would extend up to 36 mm, i.e. a 44 mm humeral head on a 80 mm diameter of curvature glenoid. 

Our initial recommendation for the amount of diametral mismatch was 6 mm and that many systems now 'name' the glenoid component for the humeral head that is recommended to go with it rather than the actual diameter of curvature of the glenoid (which is 6 mm greater). However, it is important that surgeons understand these relationships for the particular systems they are using in that nomenclature varies. Thus it is important note that some descriptions use radial rather than diametral mismatch, so again it is important that the surgeon know the system.

The recommendation of 6 mm diametral mismatch was based on our original work on the principles of total shoulder arthroplasty which found that range of motion was enhanced if some translation of the humeral head was allowed before it rode up on the lip of the glenoid component - in shoulder arthroplasty this is allowed by a diametral mismatch with the head being of slightly less curvature than the glenoid. On the other hand we recognized that large amounts of mismatch reduced the glenohumeral contact area and increased the stress in the glenoid component to a degree that challenged the yield stress of the polyethylene. More detail can be found on page 192 of the PEMS book, available for free here.

The authors' biomechanical study found, predictably, that greater degrees of mismatch allowed higher degrees of translation and more eccentric loading in a mechanical model. They did not study the effect of the degree mismatch on range of motion or the relation of mismatch to stresses in the polyethylene.

Comment: To see a YouTube of our technique for total shoulder arthroplasty, click on this link.

Our approach to minimizing rocking horse loosening is shown here. We continue to use a 6 mm diametral mismatch, in that this surface anatomy seems to (1) provide sufficient stability, (2) allow some translation before rim loading, and (3) avoids subjecting the polyethylene to excessive stresses. The actual degree of translation in vivo depends, of course, on factors other than the surface anatomy relationships, such as the orientation of the glenoid component and the ability of the shoulder musculature to balance the ball in the socket.

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Glenohumeral radial mismatch and shoulder stability - a computer model.

Nonconforming glenoid increases posterior glenohumeral translation after a total shoulder replacement.

These authors state that the major complication in nonconforming total shoulder replacement (TSR) is glenoid loosening attributed to posteriorly directed humeral head translations.

They used a 6-degrees-of-freedom computational model of the glenohumeral joint to estimate the muscle forces, joint contact force, and glenohumeral translation for radial mismatches  - the difference between the radius of curvature of the humeral articular surface and the radius of curvature of the glenod articular surface - ranging from 1 mm to 20 mm with the shoulder positioned from 20° to 60° of elevation in the plane of the scapula. Their model suggested that as the radial mismatch increased, the contact location of the humeral head moved posteriorly and inferiorly.

Comment: The title of this paper suggests that this was a study of total shoulders, but rather it was a computer modeling study. It is obvious that less joint surface conformity allows more translation - we don't need a computer model to tell us that. However, in a normal shoulder the humeral head does translate on the glenoid - a completely conforming set of humeral and glenoid components will only allow translation if the prosthetic rim is loaded, risking rim wear of the polyethylene, cold flow of the polyethylene, and rocking horse loosening. 

For truly clinical information on the effect of mismatch, see this post and here.

In our practice we use a 3 mm radial mismatch - the glenoid radius of curvature is 3 mm larger than that of the humeral head. 

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Total shoulder arthroplasty - the effect of glenohumeral non-conformity

Nonconforming glenoid increases posterior glenohumeral translation after a total shoulder replacement.

These authors used a 6-degrees-of-freedom computational model of the glenohumeral joint in a single specimen to estimate the muscle forces, joint contact force, and glenohumeral translation for radial mismatches from 1 mm to 20 mm with the shoulder positioned from 20° to 60° of scaption. Posterior glenohumeral translation increased linearly with radial mismatch.

Comment: The title of this paper should indicate that this study was done with a computer model and does not examine 'glenohumeral translation after a total shoulder replacement'.  The authors examined only one plane of motion, elevation in the plane of the scapula, whereas the greatest amount of posterior loading comes with flexion. They did not model the material properties of the polyethylene glenoid nor the effects of repeated poly loading in a conforming joint configuration. The study included a simulation of only one shoulder and did not include scapular kinematics which can affect the direction of the net humeral joint reaction force.

Eccentric loading is a recognized feature of normal and post arthroplasty mechanics. We've shown previously that eccentric loading causes not only rocking of the glenoid but also deformation of the poly.  Rocking horse loosening appears to be a principal mode of glenoid failure. In a completely conforming glenoid, any translational forces exert a rocking moment on the component as well as increased pressure on the posterior glenoid polyethylene that can lead to cold flow. On the other hand, as this study shows, a high degree of mismatch diminishes the effectiveness of the concavity compression mechanism of glenohumeral stability.  

In a clinical study published in 2002, the authors found significant inverse linear relationship between mismatch and the glenoid radiolucency score (p < 0.0001), with significantly lower (better) radiolucency scores associated with radial mismatches of >5.5 mm.

The stability of a total shoulder arthroplasty depends on many factors, including prosthetic design, glenoid prosthetic orientation, soft tissue balance, and muscle coordination. It is not as simple as it may seem.

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Interface micromotions increase with less-conforming cementless glenoid components - JSES

Interface micromotions increase with less-conforming cementless glenoid components - JSES.

In this in vitro model using 'bone substitute', less conforming glenoid components allowed greater translation, greater rim loading and greater interface micromotion that more conforming glenoid components. 

The clinical relevance of this study is unclear, in that there are a number of pros and cons for different surface geometries. Totally conforming glenohumeral relationships offer greater initial stability through the concavity compression mechanism, but this comes at the expense of limited motion and increase loads on the polyethylene at the constraining rim and increased loads on the component fixation with even minor degrees of translation. Increased degrees of mismatch provide less stability, the possibility of greater translation with eccentric loading of the component, but also may allow greater glenohumeral motion and less interface loads with minor translations.

The interested reader may wish to revisit previous posts on surface mismatch, concavity compression, rocking horse, and glenoid component (see labels at right).

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Total Shoulder Arthroplasty: Relationship of glenohumeral curvature mismatch to stability and interface motion.

JSES recently published "Interface micromotions increase with less-conforming cementless glenoid components."
Readers my want to view this study in view of the concept of concavity compression. You should surely treat yourself to the video on this topic prepared by my late partner, Doug Harryman. as well as his article "The effect of articular conformity and the size of the humeral head component on laxity and motion after glenohumeral arthroplasty. " It is important to recognize that normal glenohumeral kinematics include translation of the humeral head on the glenoid surface. Also, you should be aware of the work of David Collins, who did a very similar study many years ago.

A note of caution about nomenclature. Different prosthesis systems use different ways of describing the components. In this study, the authors describe the surface in terms of the RADIUS of curvature. They used a consistent head radius of 20 mm and examined glenoid surfaces ranging from 20 to 26. Some systems, including the one we use here, describe the head curvature in terms of the DIAMETER, 40 mm to 56 mm for example. In the system we use, the glenoid components are all 6 mm in DIAMETER greater than the humeral head: a 6 mm diametric mismatch or a 3 mm radial mismatch. In contrast, Neer's original total shoulder system had no mismatch in diameter between the glenoid and humeral components.

The premise of this article seems to be that cementless bone ingrowth fixation is desirable for glenoid components and that, for that reason, motion at the prosthesis bone-prosthesis interface is to be avoided. We can agree with the second part of this assertion, although evidence for the first part is lacking.

This study shows that the greater the mismatch between the curvature of the humeral head and the glenoid, the more translation allowed when a displacing force is applied to it. Once one understands the concept of concavity compression, it is intuitive that a greater degree of mismatch between the curvatures would allow for more translation per unit applied displacing force. This is exactly what the authors of the JSES paper found.

Much of the remainder of this article attempts to relate glenohumeral diameter mismatch to glenoid-bone micromotions. The problem is that so many variables we encounter clinically - direction of load, magnitude of load, quality of bone, type of fixation, seating of the component, compliance of the glenoid component, etc -  are not emulated in this in vitro study using 'bone substitute'. To appreciate the complexity of the clinical situation, you may like to read "The influence of glenohumeral prosthetic mismatch on glenoid radiolucent lines: results of a multicenter study."

As we pointed out in our book, The Shoulder is A Balance of Mobility and Stability.  In shoulder arthroplasty, many factors need to be considered in achieving this balance. While glenoid component loosening is the major complication of total shoulder arthroplasty, the geometry of the glenoid and humeral joint surfaces are only one of the many variables that need to be considered.

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