Modular hip stems: determination of disassembly force of a neck-stem coupling

An increasing number of hip prostheses feature double head-neck and neck-stem modularities. One possible complication is loosening of the neck-stem coupling. Current surgical technique includes application of hammer blows to the proximal end of the neck-stem coupling. This could compromise the cleanness of the head-neck modularity and damage the bearing surfaces. The goal of this work was to assess whether such hammer blows are really necessary to avoid neck-stem disassembly during in vivo service. Commercially available hip stems featuring neck-stem modularity were tested. Extraction force was measured in vitro when different levels of press fitting were simulated (manual assembly followed by various combinations of simulated loads) to assess critical conditions in relation to surgical technique and applied loads. The disassembly force after manual insertion followed by the first small postoperative loads imposed by the patient during walking was as high as that obtained with hammer blows. Thus, application of hammer blows to fix the neck-stem coupling seemed unnecessary.