Glenoid bone density in A2 and B2 glenoids, implications for pathogenesis and treatment

Regional bone density variations in osteoarthritic glenoids: a comparison of symmetric to asymmetric (type B2) erosion patterns.

These authors compared the erosion patterns of 25 symmetric and 25 asymmetric glenoids using computed tomography-based imaging software.

For the symmetric cohort, there were no significant differences in bone density between the four quadrants at depths of 0 to 2.5 mm and 2.5 to 5 mm. For the asymmetric cohort, bone density was significantly higher in the posterior quadrants compared with the anterior quadrants, especially posteroinferiorly at both depths. The bone beneath the pathological posterior concavity also had lower void fraction compared with the bone beneath the normal anterior concavity.

This study demonstrates that osteoarthritic glenoids with symmetric erosion have uniform subarticular bone density. However, asymmetric (B2) erosion patterns have potentially important regional variations in bone density and porosity, with the densest bone with the least porosity found posteroinferiorly beneath the pathological concavity.

Comment: The condensation of bone beneath the pathological posterior glenoid is a response to posterior loading that occurs with functional decentering, that is the posterior subluxation that occurs when the arm is placed in a functional position, a decentering that is not seen on CT scans taken with the arm at the side as further explained here and here. This is one of the many reasons we do not get preoperative CT scans, preferring instead the 'truth view'.

The recognition of the variance in bone quality has lead some to question the advisability of correcting glenoid version. Here is a related post. Taken together, these findings also bring into question the advisability of sacrificing some of the dense posterior bone to accommodate glenoid components with a posterior step off.

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Glenoid bone density in arthritic shoulders - effect of functional decentering

Glenoid subchondral bone density distribution in male total shoulder arthroplasty subjects with eccentric and concentric wear.

These authors state that "glenoid component loosening in total shoulder arthroplasty may be prevented by component placement on a congruent and adequate bony surface".

They measured bone density in three dimensions for the glenoids of 42 men (21 with eccentric and 21 with concentric wear patterns) with glenohumeral arthritis.

They divided the glenoid subarticular layers into three regions: calcified cartilage (≤1.5 mm), subchondral plate (2-4.5 mm) and cancellous bone (≥5 mm).

In concentric glenoids, the subchondral bone density distribution was homogeneous, with greater mineralization in the central zone, compared with the posterior, anterior, and superior zones. In the eccentric group, the subchondral bone density distribution was inhomogeneous. Mineralization was greatest in the posterior zone, 1,739.0 ± 172.6 HU (at 2.5 mm), followed by the inferior zone, 1,722.1 ± 186.6 HU (at 3 mm).

Comment: This study shows how thin the subchondral bone of the glenoid is. Thus attempts to 'normalize' glenoid version - by reaming the anterior aspect of the glenoid - may transgress the subchondral bone there, leaving the glenoid component supported only by less dense cancellous bone.

The increased density of the posterior bone in eccentric glenoids reflects the increased loading from a posteriorly uncentered head. As pointed about by a previous paper*, CT scans and MRIs taken with the arm at the side fail to reflect the degree of posterior displacement of the humeral contact point on the glenoid when the arm is in functional positions - they refer to this as 'functional malcentering'.
As an example, see the image below, taken with the arm at the side showing only a small amount of posterior displacement of the head on the glenoid, but severe posterior glenoid erosion. One can be certain that when the arm is elevated to a functional position, the humeral head would fall into the posterior concavity = functional malcentering.

For this reason, we prefer to judge the degree of posterior subluxation on axillary views taken with the arm in a functional position of elevation in the plane of the scapula.

*"Functional malcentering of the humeral head and asymmetric long-term stress on the glenoid: potential reasons for glenoid loosening in total shoulder arthroplasty.

We tested the hypothesis that functional malcentering of the humeral head during arm elevation exists in patients with glenohumeral osteoarthritis and influences long-term glenoid loading. Twenty-eight healthy volunteers and 10 patients with primary osteoarthritis, 10 with cuff-arthropathy, and 1 with dysplastic glenoid were examined. Open magnetic resonance imaging and 3-dimensional (3D) digital postprocessing techniques were applied in various arm positions. Osteoabsorptiometry was used to determine 3D subchondral mineralization patterns of the glenoid as an indicator of integral long-term stress distribution. At 30 degrees of abduction, 5 patients demonstrated malcentering of the humeral head posteriorly; all patients with cuff arthropathy had malcentering superiorly. At 90 degrees, most patients displayed significant (P < .001) malcentering in the superior and posterior direction. The shoulders showed maximal subchondral mineralization patterns in the direction of malcentering. Most patients with glenohumeral osteoarthritis displayed functional malcentering, which might be responsible for postoperative glenoid loosening in shoulder arthroplasty if not corrected intraoperatively."

Figure 3 from this article is particularly illuminating.

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