Sunday, May 31, 2015

Shoulder joint replacement - the glenoid component

Total shoulder arthroplasty involves insertion of a prosthetic glenoid component. Without question this glenoid component is the weakest link in total shoulder arthroplasty. The reasons for its high rare of failure are apparent: (1) a prosthetic component cannot duplicate the basic structure of the normal glenoid with its firm center and compliant periphery that allows small amounts of translation without damage to the rim, (2) a prosthetic component cannot duplicate the wear resistance of normal articular cartilage and (3) the prosthetic joint component cannot duplicate the natural secure fixation of hyaline cartilage to the underlying bone, especially in its ability to withstand eccentric loading.

The goal of glenoid component insertion is to optimize its support by excellent carpentry - rather than by using cement ‘putty’ as exemplified in the three cases below.


The optimal glenoid insertion technique (1) conservatively reams the glenoid bone surface so that it has congruent contact with the back of the prosthesis without interposed cement that can crack and fail (as shown in the examples above), (2) selects a prosthesis that covers the maximal amount of prepared glenoid bone surface and that is relatively thin (3-4 mm) so as to avoid contributing to overstuffing and to minimize the loosening moment, (3) precisely aligns the prosthesis with the prepared bone minimizing the need for large amounts of cement that can kill bone, (4) achieves firm fixation, and (5) preserves glenoid bone stock, the most precious commodity in total shoulder arthroplasty.

The purpose of glenoid reaming is to conservatively conform the bone to the back of the glenoid component for maximal component stability and load transfer without needlessly sacrificing glenoid bone stock to accommodate complex back side geometry or in an attempt to ‘normalize’ glenoid version by reaming down the high side . Sharp reamers are important to avoid thermal damage in preparing the glenoid bone. In 1991 we started using a spherical reamer stabilized by a nub placed in a hole centered on the glenoid face [Fig 51 Photo First Reamer].

The modern version of the reamer is bladed and collects bone that can be used as fine bone graft to enhance the fixation of the glenoid central peg. The nubbed reamer allows the surgeon to make small changes in the superior/inferior and anterior/posterior inclination of the reamer to assure that the entire glenoid surface is reamed. Because conservative reaming requires this fine-tuning of the reamer orientation to achieve a single glenoid concavity with minimal bone removal we have not been attracted to approaches that rigidly define the orientation reamer with a guide pin based on a theoretical preoperative plan. A guide pin not only prevents fine-tuning of the reamer orientation, but is also at risk for bending, breakage or inadvertent advancement into the chest. 

Because of their precise geometry and spread of points of fixation across the glenoid face, fixation systems using pegs provide more secure fixation and require less cement in comparison to keeled components. Interference-fit pegs enhance initial fixation and allow for subsequent bone ingrowth.
Note the absence of radiolucent lines in the postoperative x-rays below.

While a number of metal backed components have been designed, metal backed components are reported to have a three times higher rate of revision than all-polyethylene components

One of the intrinsic difficulties with metal-backed glenoid components relates to the difference in elastic (Young’s) modulus for metal in comparison to the other materials involved: 

Cobalt chrome     200.0 GPa
Titanium              112.0 GPa
Cortical bone           8.0 GPa
PMMA cement        2.0 GPa
Polyethylene            0.5 GPa
Cancellous bone      0.4 GPa

The wide differences in these values indicate that the different materials will deform differently under load leading to shear stresses at the interfaces, especially that between metal and polyethylene. These challenges cannot be avoided when glenoids are made of substantially dissimilar materials. Note that the closest modulus match is between polyethylene and bone, a fact that may contribute to the lower revision rate of all polyethylene components. The issues with metal backed glenoid components also include their increased thickness, which may limit the range of motion and predispose the reconstruction to instability, increased contact stresses and polyethylene cold flow.

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