Sunday, May 31, 2015

Shoulder joint replacement - selecting the optimal humeral component

The ideal humeral component is one that allows secure and durable placement of an ample articular surface in the position that optimizes glenohumeral motion and stability. It should also allow for complete component removal - should revision become necessary - without risking damage to the bone of the humerus by attempting to remove a cemented or bony ingrowth implant. The humeral implant is modular so that the head component and the body component can be selected independently. In selecting the humeral component we prioritize the optimization of glenohumeral mechanics over attempting to ‘restore normal anatomy’ Adjustments in humeral diameter of curvature, head thickness, and head offset with respect to the stem give the surgeon the opportunity to modify the capsular tension as well as the fit of the component to the glenoid articular surface. There are two aspects of the humeral component that deserve consideration: the articular surface and the way the articular surface is connected to the humerus.

Humeral articular surface. There are three types of prosthetic humeral articular surfaces: partial resurfacing, complete resurfacing,  and head replacement.

Partial resurfacing prostheses may seem to be appealing for covering partial articular surface defects. However, there is an inevitable factor of 200,000 difference between the modulus of elasticity of the metal prosthesis (about 200,000 MPa) and the surrounding intact articular cartilage (0.5 to 0.9 MPa); this creates a major discontinuity in the deformation of the joint surface under load at the margin of the prosthesis.

Complete resurfacing prostheses are intended to cover the arthritic humeral head. The rationale put forth for these implants is that they (a) preserve humeral bone stock in the event that a subsequent arthrodesis may be required, (b) enable the surgeon to perform an arthroplasty in the face of humeral deformity, (c ) facilitate revision in comparison to prostheses that use cemented or bone ingrowth humeral stems and (d) facilitate restoration of normal anatomy (although the restoration is not always anatomic as shown below).

While attempting to re-establish normal anatomy might be a goal of arthroplasty, a more important goal is to restore functional mechanics for the arthritic shoulder, which often requires adjustments in head diameter of curvature, head thickness, head orientation, and glenoid arthroplasty - adjustments that do not attempt to exactly duplicate the normal glenohumeral anatomy. As a result, we have not found a place for resurfacing prostheses in our practice because (a) the need for arthrodesis after prior arthroplasty is extremely rare, (b) almost all cases of arthritic and post traumatic deformity can be managed with a conventional stemmed prosthesis (as shown below);

(c) the difficulties associated with removal of cemented or ingrowth components can be avoided by humeral stem fixation with impaction grafting; (d) by retaining the humeral head, resurfacing implants limit the surgeon’s ability to modify the orientation and the thickness of the component; and (e) the retention of the anatomic humeral head compromises access to the glenoid, making it difficult to address the glenoid pathology encountered in glenohumeral osteoarthritis as shown below on the axillary 'truth view'.


Finally, is worthwhile noting that registry data indicate a 2.5 year revision rate for resurfacing prostheses that is over three times higher than that for stemmed hemiarthroplasty.

Head replacement prostheses allow extensive versatility in the selection of the humeral head component; the humeral head cut for these prostheses allows excellent access to the glenoid bone so that an appropriate glenoid arthroplasty can be carried out. While some prostheses are nonspherical, these do not appear to be superior to nature’s spherical design (see below).

In our practice, the selection of the diameter of curvature of the spherical humeral articular surface depends upon the management of the socket side of the articulation. With a hemiarthroplasty in the absence of any form of glenoid arthroplasty, the shape of the socket is not changed so that the humeral component articular surface should have the same diameter of curvature as the resected humeral head. 

In a cuff tear arthropathy (CTA) humeral arthroplasty for cuff tear arthropathy without pseudoparalysis or anterosuperior escape, a humeral head with a lateral extension can be used to resurface the articulation of the tuberosity with an intact coracoacromial arch. In this case the humeral diameter of curvature is selected to match that of the sculpted curvature of the coracoacromial arch and upper glenoid – this is usually the same as that of the resected humeral head 



In selecting the height of the CTA prosthesis, we adjust the deltoid tension so that the passively adducted arm springs about 20 degrees away from the side when the applied adduction force is removed.

In a ream and run arthroplasty, the diameter of curvature of the humeral component is two millimeters less than that of the reamed glenoid (in most cases the diameter of curvature of the humeral head prosthesis is 56 mm and that of the reamed glenoid is 58 mm). 

In a total shoulder the diameter of curvature of the humeral head prosthesis is 6 mm less than that of the prosthetic glenoid socket. 

For all types of arthroplasty, the thickness of the humeral head prosthesis is selected so that the glenohumeral joint meets the 40, 50, 60 guidelines – avoiding overstuffing on one hand and excessive laxity on the other.

To maximize the articular area, the prosthesis should not sacrifice its articular surface to a chamfer and should minimize the gap between the articular area and the humeral neck cut. 

The humeral head prosthesis should have eccentric options to help control excessive translation posteriorly or anteriorly. 




The humeral head should attach to the body with a Morse taper that has the male part on the head rather than on the body of the prosthesis so that this taper does not interfere with access to the glenoid. 

The next post will consider the options for fixation of the head component to the humeral bone

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