Same offset, different stability: how head length affects taper junction micromotions in total hip arthroplasty

Modular neck systems allow intraoperative adjustment to adapt the implant length to patient’s anatomy or to replace only the damaged element of a hip prosthesis. Morse taper principle is frequently adopted to combine prosthetic components: after the application of the assembly force, the trunnion (male portion) locks the artificial joint through the compression of the taper (female portion). Nevertheless, modular prostheses present issues related to the establishment of relative micromotions between coupled elements, which can lead to implant failure due to cytotoxic metal debris onset. Micromotions can be influenced by surgeon-related factors including the selection of prosthetic components. Although equal implant offsets can be achieved by assembling tapers and trunnions of different sizes, intraoperative adjustments are often limited to the head length. However, different studies reported that implant failure can be correlated with to the head length variation due to presence of a lever arm between the head centre and the taper engagement area. In this work, the head length impact on the head-neck junction stability was investigated through an in silico comparison between couplings with the same offset.