The Cementless, Triple-Tapered, Collared Stem Has Minimal Proximal Femoral Stress Shielding at One Year

The triple-tapered, collared (TTC) femoral stems are increasingly being used in primary total hip arthroplasty (THA) and have demonstrated favorable clinical outcomes. However, their effect on proximal femoral bone mineral density (BMD) and stress shielding remains unknown. The purpose of this study was to assess changes in proximal femoral BMD at one year following implantation of a cementless TTC stem. The secondary aims were to evaluate the influence of surgical approach and collar seating on postoperative BMD and to compare BMD changes associated with the TTC stem to those of a cementless, single-tapered, wedge-type (STW) stem.

This single-institution prospective cohort study included 94 cementless primary THAs (45 direct anterior approach, 49 posterior approach) performed with a TTC stem. Dual-energy X-ray absorptiometry was obtained at six weeks and one year postoperatively. Ratios of one-year to six-week BMD values were calculated for each Gruen zone (R1 to R7). Results were compared to a previously published cohort of 31 STW stems from our institution. A mixed-effects model with repeated measures and Student's t-test were used for analyses.

Across all Gruen zones, the TTC stem demonstrated minimal BMD changes between six weeks and one year postoperatively (range, 0.91 to 1.03, P > 0.05). Collar seating on the calcar (77% of cases) was not associated with significant differences in BMD change in any Gruen zone. The direct anterior approach was associated with greater BMD change in Gruen zone 1 compared with the posterior approach (0.90 versus 0.96, P = 0.025), with no differences in the other zones. There were no significant differences in BMD ratios between the TTC and STW stems at one year.

The TTC stem demonstrated minimal proximal femoral BMD changes between six weeks and one year following primary THA. Collar seating was not associated with differences in BMD, whereas a modest difference in Gruen zone 1 was observed between surgical approaches. Longer-term follow-up is warranted to further characterize proximal femoral bone remodeling with the TTC stem design.