gms | German Medical Science

Objectives: Model-based RSA (mbRSA) using EGS models to detect migration of femoral stems has only been validated using straight stem designs in neutral position [1], [2], [3]. Aim of this study was to investigate if mbRSA-EGS could be affected by femoral stem design and leg positioning.

Methods: Totally n=7 femoral stems were investigated: n=2 straight stems (medial-/lateral-located taper); n=3 curved stems (round, spiky or semi-round tip) and n=2 modular revision stems (long straight or curved stem). A phantom model and a femoral stem-bone-model (FSBM) were used to mimic the leg positioning in two scenarios: (I) leg flexion of 0, 5, 10 deg around medial-lateral axis (FSBM-F-0, FSBM-F-5, FSBM-F-10); (II) internal/external leg rotation of -10, 0, 10 deg around superior-inferior axis (FSBM-R-neg-10, FSBM-R-0, FSBM-R-10). This protocol was repeated n=10 for each implant in each position.

Results and Conclusion: Migration results of mbRSA-EGS were affected by femoral stem design: the worst case for translation along anterior-posterior axis (Ta.p.) (mean±SD: -0.044±0.183 mm) and rotation around superior-inferior axis (Rs.i.) (mean±SD: -0.323±0.785 deg) were observed (Table 1 [Tab. 1]). Meanwhile, patients leg positioning affected mbRSA-EGS migration results as well: the worst case in Ta.p. (mean±SD: -0.016±0.118 mm) and Rs.i. (mean±SD: 0.025±0.690 deg) for scenario (I), as well as in Ta.p. (mean±SD: 0.040±0.163 mm) and Rs.i. (mean±SD: 0.203±0.780 deg) for scenario (II) were measured (Table 2 [Tab. 2]).

The result of mbRSA-EGS is affected by femoral stem design and leg positioning: Femoral stem components with a long stem, a lateral-located taper and a spiky tip, as well as leg in non-neutral position present less precision. Using mbRSA-EGS for out-of-plane migration detection has to be therefore carefully considered. Patient should always be neutrally positioned during the RSA examination.