The effect of shoulder arthroplasty on humeral strength: An in vitro biomechanical investigation

Abstract

Periprosthetic humeral fractures are a serious complication of shoulder arthroplasty. While adequate reaming of the canal and insertion of an oversized implant optimizes fit, such maneuvers also weaken the bone and predispose it to fracture. The impact of the humeral arthroplasty was assessed in vitro on human cadaveric specimens. Strain gauges were attached to the distal diaphyses and the specimens were mounted in a torsion-loading fixture throughout the tests. An initial series examined the effect of reaming of the canal to its clinically appropriate diameter using uniaxial strain gauges. A second series utilized strain rosettes to evaluate the cumulative effects of reaming, broaching, and implant insertion. Reaming of the canal to its clinically appropriate diameter significantly increased (P=0.007) uniaxial strain measurements by a mean of 30% with five of eight specimens showing increases of over 49% on at least one of four diaphyseal locations. In the second series, the surface strain was significantly affected by arthroplasty (P<0.008). Post-hoc analysis showed that the maximum in-plane shear strain following implant insertion was significantly increased relative to strain levels following reaming and broaching (P<0.009). The direction of the principal strain axes did not significantly change (P>0.46). Unexpected decreases in some strain measurements were observed as the arthroplasty procedure progressed perhaps reflecting overt mechanical failure within the humeral shaft. The strain increase following reaming suggests a reduction in torsional strength by over 33% which is further reduced following broaching and implant insertion. For the practicing surgeon, post-operative strength can be adversely affected by both canal preparation and implant insertion.