Methods. 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.
Findings. 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.
Interpretation. 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.