Reverse total shoulder - unwanted contact

Humeral version in reverse shoulder arthroplasty affects impingement in activities of daily living

These authors used a computer model of a specific reverse implant

to predict the relationship between bone-prosthesis contact and humeral component version in activities of daily living. 

In their model they found that for this particular prosthesis, 0° version showed the least amount of unwanted contact. The site of contact changed with version: retroversion increased the risk of extra-articular contact and contact of the humeral cup with the anterior glenoid border in hygienic and feeding activities. Anteversion increased unwanted contact between the inferior scapula border and the humeral cup.

Comment: The reverse total shoulder achieves stability because the socket captures the ball to a much greater extent than in an anatomic shoulder. The price for this stability is the risk of unwanted bone-prosthesis contact in different functional positions. When unwanted contact takes place the results can include polyethylene damage, instability and loss of range of motion (see this link).

Because x-rays do not show the humeral polyethylene cup, bone-prosthesis contact may not be evident; compare the x-ray below to the image of the same component shown above.

A common site of bone-prothesis contact is at the inferior-medial aspect of the joint when the arm is adducted.

but as the authors point out, unwanted bone-bone contact can occur in abduction

The occurrence of unwanted contact depends not only on the version in which the humeral component is inserted, but also the relationship of the humeral cup to the tuberosity (shown above) and other key variables: (a) the geometry (Grammont vs offset) and positioning (low vs high; inclined vs not) of the glenoid component, (b) the geometry of the humeral cup (inferiorly extended vs not, retentive vs standard) and (c) the often altered local bone anatomy.

In performing a reverse total shoulder, we take time with trial components in place to examine the stability and the range of the shoulder in all functional directions. We attempt to identify and resolve unwanted contact that may limit the range of motion, the stability or the durability of the reconstruction. Sometimes the unwanted contact can be resolved by removing bone; other times modification of the prothesis geometry or positioning is required.

As we say, "It is easier to resolve unwanted contact while the wound is still open."

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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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