DISTAL LOCKING (STATIC/DYNAMIC) IN MODULAR HIP PROSTHESES ENHANCES PRIMARY STABILITY. WHAT SOLUTIONS ARE AVAILABLE TO ACHIEVE PROXIMAL BONE CONSOLIDATION?

Abstract

Introduction and purpose: Primary instability and subsidence of hip implants is a very common problem. What is the load on a locking screw?

Materials and methods: Helios® modular prosthesis (IQL / Biomet España) with distal holes (static/dynamic). Fifty patients (46%) with locking, total n=109 (26 of 67 with FX of proximal femur, 14 of 25 aseptic loosening cases and 13 of 17 with periprosthetic fractures). Experimental study: stress on the distal locking screws. Loads applied in 9 cadavers with 3 types of simulated fractures: group I, femoral neck; group II, intertro-chanteric; and group III, subtrochanteric. Imitating slow, normal and fast walking, sitting down and standing up, going up an down stairs (Zwick Z010 and screw capacity meter).

Results: Follow-up 6–18 months. 50 with distal locking (20% dynamic, 80% static): 13 screws removed between 6 and 12 weeks (7 in revisions, 3 in FX, 3 others).

Subsidence: 80% 1–2 mm, 14% 3–5 mm, 6% 6 mm or more.

Experimental study: Weight-bearing: group I, 40.89% (max. 78.61%); group II, 43.15% (max 90.84%); group III, 64.49% (max. 136.74%). No torsional stress. Maximum stress when walking fast and climbing stairs.

Conclusions: When bone consolidation occurs in 6 to 12 weeks, distal locking ensures sufficient reinforcement and prevents movements of over 30 μm. If more time is needed for proximal bone integration, shaft fixation is preferable. The Helios® system provides both solutions satisfactorily.