title: Unconstrained tripolar hip implants: effect on hip stability.
abstract: Tripolar implants were developed to treat unstable total hip arthroplasties. However, there is limited confirmation that they achieve this purpose despite their increasing use. Because they have a larger effective head size, these implants are expected to increase range of motion to impingement and improve stability in situations at risk for impingement compared with conventional implants. We assessed the range of motion to impingement using a tripolar implant mounted to an automated hip simulator using 22.2-mm and 28-mm femoral head sizes. The 22 and 28-mm tripolar implants provided increases of 30.5 degrees in flexion, 15.4 degrees in adduction, and 22.4 degrees in external rotation compared with the conventional 22.2-mm femoral head diameter implant. At the critical position of 90 degrees hip flexion, there was an increase of 45.2 degrees in internal rotation. At 0 degrees and 30 degrees external rotation, extension increases were 18.8 degrees and 7.8 degrees, respectively. Bony impingement was the limiting factor. Tripolar implants increased the arc of motion before impingement in positions at risk for dislocation and are expected to provide greater stability.
authors.affiliation: Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic, Rochester, MN 45905, USA.
authors.forename: Olivier
authors.initials: O
authors.lastname: Guyen
authors.forename: Qing Shan
authors.initials: QS
authors.lastname: Chen
authors.forename: Jacques
authors.initials: J
authors.lastname: Bejui-Hugues
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Berry
authors.forename: Kai-Nan
authors.initials: KN
authors.lastname: An
mesh_terms: D019644:Arthroplasty, Replacement, Hip; D003198:Computer Simulation; D006617:Hip Dislocation / Q000517:prevention & control; D006621:Hip Joint / Q000503:physiopathology; D006622:Hip Prosthesis; D006801:Humans; D007593:Joint Instability / Q000517:prevention & control; D011183:Postoperative Complications / Q000601:surgery*; D011474:Prosthesis Design; D016059:Range of Motion, Articular