Primary stability of short stem prostheses for the shoulder-a biomechanical comparative study of two short stem designs.

Introduction: The use of short stems has been a trend in shoulder arthroplasty within the last 15 years with excellent short- to mid-term clinical outcomes. Short stems anchor in the cancellous metaphysis and a high level of postoperative stability with absence of micro-movements at the bone-implant interface can be crucial for the following healing. The study aimed to assess the primary rotational stability of two uncemented short stems. The hypothesis was that the rotational stability is independent of implant design.

Materials And Methods: A biomechanical in-vitro study was conducted on 12 fresh-frozen (six matched pairs) human cadaveric shoulders. Two short stems, one with a cylindrical and one with a rectangular stem shape design, were implanted in a cement-free technique. A sinusoidal torque force was applied, starting from 0.5 Nm (load level one) to 3.0 Nm (load level six) with increasing load levels (0.5 Nm per 500 cycles). The relative rotation between stem and bone was assessed with an optical measurement system.

Results: Mean donor age was 80 years (range 67-89 years), mean bone density was 0.41 g/cm (range 0.21-0.64 g/cm) with no significant difference between stem design groups (p = 0.155). The cylindrical stem design demonstrated a significantly higher relative rotation than the rectangular stem design at 2.0 Nm (p = 0.047), 2.5 Nm (p = 0.034) and 3.0 Nm (p = 0.016). The metaphyseal and diaphyseal filling ratio was significantly higher in the rectangular stem design group (p = 0.002, p = 0.001).

Conclusions: The cylindrical stem design shows a higher relative rotation in vitro imitating the immediate postoperative situation, indicating that rotational stability might depend on the implant design. However, the mid- to first long-term rates of aseptic stem loosening for the cylindrical stem design are generally low. It is important to consider the sensible postoperative healing phase during postoperative rehabilitation, especially for cylindrical stem designs, to promote secondary osseointegration.

Level Of Evidence: Experimental study.