Thursday, September 25, 2014

Humeral head subluxation in osteoarthritis

Importance of a three-dimensional measure of humeral head subluxation in osteoarthritic shoulders.

As the authors point out, the management of posterior humeral subluxation can be a challenge in shoulder arthroplasty. Persistent posterior instability can contribute to cold-flow and wear of the posterior lip of the glenoid component as well as to rocking horse loosening of the glenoid component in total shoulder arthroplasty.

These authors analyzed 112 computed tomography scans of osteoarthritic shoulders. They compared the 2D and 3D assessments of humeral head subluxation and glenoid version. They defined the 3D glenohumeral subluxation as the relative distance between the humeral head center and the glenoid center projected onto a plane perpendicular to the glenoid centerline. They defined the 3D scapulohumeral subluxation with the same distance but projected onto a plane perpendicular to the scapular axis. They found that scapulohumeral subluxation correlated with glenoid version, but that glenohumeral subluxation was not correlated to glenoid version.

The authors concluded that the direction of humeral subluxation was rarely within the usual computed tomography plane and should therefore be measured in 3D to detect out-of-plane subluxation. 

Comment: Several points need to be made. (1) A CT scan looks at the shoulder with the arm at the side, yet this is not the position in which posterior humeral subluxation occurs. Rather posterior humeral subluxation occurs when the arm is elevated to a position of function, say 60 degrees of elevation in the scapular plane where the posteriorly directed deltoid force challenges the posterior stability. CT scan is not useful with the arm in this position, but a standardized axillary view works very well as shown here. (2) The relationship of the center of the humeral head to the glenoid surface or to the plane of the scapula is not the primary concern in posterior subluxation. Rather the factor that produces posterior wear and rocking horse loosening is the posterior point of contact of the humeral head on the glenoid as shown here.  Fortunately this point of contact is simple to measure on a standardized axillary view and does not require spending money or radiation on CT scans, 3D reconstructions, or debate on whether the scapula or the glenoid is the proper reference.

We routinely use the 'truth' view rather that CT scans to understand the glenohumeral pathoanatomy prior to shoulder arthroplasty.