Sunday, February 26, 2017

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."