Does the cement mantle thickness influence the glenoid loosening in anatomic total shoulder arthroplasty? An experimental study.

BACKGROUND: Glenoid component loosening is the most frequent failure mode. Few data are available on the effect of thickness of cement on glenoid loosening. The purpose of this study is to determine if the cement mantle thickness influences the mode and localization of loosening. Our hypotheses are: 1) failure is caused by traction stresses generated within the cement mantle and 2) a thicker cement mantle amplifies the rocking horse effect.

METHODS: Using bone substitute, an experimental protocol was designed to compare loosening of a keeled glenoid prosthesis in axial traction and off-centered-load, to recreate the rocking-horse effect (1.000.000 cycles). Different standardized mantle of cement between the back of the glenoid and the foam were tested (0-1 - 2-3 mm). The displacement of the polyethylene was assessed with an LVDT (Linear Variable Differential Transformer) gauge when the prosthetic humeral head loaded the opposite part of the implant.

RESULTS: The loosening took place within the keel of the implant, and at the polyethylene-cement interface in traction if there was cement at the back of the polyethylene. For cycling loading, we observed a loosening at this interface, with associated fracture of the cement, only for cement 2 and 3 mm thick.

CONCLUSION: This experimental study suggests that the cement mantle should be as thin as possible between the back of the implant and the sub-chondral bone but should be optimized around the keel of the implant.

LEVEL OF EVIDENCE: Basic Science Study.