Unconstrained tripolar hip implants: Effect on hip stability

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° in flexion, 15.4° in adduction, and 22.4° in external rotation compared with the conventional 22.2-mm femoral head diameter implant. At the critical position of 90° hip flexion, there was an increase of 45.2° in internal rotation. At 0° and 30° external rotation, extension increases were 18.8° and 7.8°, 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.