Development of hip joint prostheses with modular stems

Minimally invasive surgery for THR (Total Hip joint Replacement) is attractive for both surgeons and patients. Since such surgery needs an incision of only 3–4 inches around the hip joint for THR instead of the traditional, large incision of 10–12 inches, it causes less pain and enables early recovery for patients, besides facilitating THR for the operating surgeons. In this research, for minimally invasive THR, a unique type of a cementless stem, named a modular stem, is devised. It consists of two different parts in a stem that can be joined to and separated from each other. For actualizing the modular stem, Bio-CAD modeling technique is applied. The bony structure around the hip joint is three dimensionally reconstructed and its geometric solid model is fabricated. The geometric solid models of modular stems are designed and their prototypical models are fabricated using an acryl-based polymer. The geometric suitability of the prototypical modular stems is manually examined. The strength of the stem to sustain the applied load is evaluated using the finite element method. Finally, various sizes of actual modular stems with circular or rectangular cross-sections are fabricated using a biocompatible Ti alloy (Ti–6Al–4V). The developed modular stems will be used in the near future for minimally invasive THR surgery.