Porous-coated cementless stems were introduced in the late 1970s, in response to the high incidence of aseptic loosening associated with cemented stems (Judet et al 1978, Kavanagh et al 1989, Lord 1982, Stauffer 1982). The stem's porous surface achieves fixation via bony ingrowth at the endosteum (Engh et al 1987). In an effort to mimic the femur's natural stress distribution, many designs preferentially load the metaphysis (Joshi et al 2000). One such design is the proximally coated titanium tapered wedge, which is widely used today (Pitto et al 2010). Because loading is primarily dictated by bony ingrowth, the proximal porous coating avoids fixation at the diaphysis. The flat, tapered geometry reduces stiffness compared to cylindrical stems, thereby propagating stress to the proximal femur, rather than down the stem's axis (Engh et al 1987, Harvey et al 1999), (Boehm 1998). Further contributing to decreased stiffness is titanium's low elastic modulus, relative to that of cobalt-chromium (Harvey et al 1999, Mulliken et al 1996). Some contend the need for distal fixation in Dorr type C hips due to thin cortical bone at the metaphysis; however, proximally engaging stems have been shown to perform well in these patients (Kelly et al 2007, Reitman et al 2003).
If a stem's objective is to load proximally, then perhaps it need not extend into the diaphyseal canal. The emergence of short stems has initiated an alternative means of proximal loading, without the disruption of diaphyseal bone stock. The smaller incisions involved with short stem total hip arthroplasty (THA) reduce damage to muscle and soft tissue (Molli et al 2012). This enables a faster, more complete recovery for the patient, as well as a cosmetically superior result (Sherry et al 2003). Additionally, the preservation of bioavailable bone can be advantageous if a revision surgery is required (Toth et al 2010). Because short stems do not extend into the diaphysis, issues regarding proximal-distal mismatch of the femur are avoided (Patel et al 2013). This eases implantation and reduces the risk of intraoperative fracture (Azzam et al 2010, Cooper \& Rodriguez 2010). The risk of intraoperative fracture is further mitigated because short stem THA does not require the use of reamers (Molli et al 2012, Scott et al 1975, Taylor et al 1978).
Although the standard-length proximally coated titanium tapered wedge has a successful long-term track record, its design leaves room for improvement (Mallory et al 2001, Marshall et al 2004). Standard length stems cause stress-shielding in Gruen zones 1 and 7, suggesting that the diaphyseal portion of a stem may interfere with proximal loading (Gibbons et al 2001, Schmidt et al 2004). Even proximally coated flat tapered stems are subject to diaphyseal loading (Cooper et al 2011). The Accolade stem has been associated with significant early subsidence, especially in males with Dorr type A hips (Jacobs \& Christensen 2009). This suggests that the stem may wedge distally, thereby interfering with osseointegration (White et al 2012). By achieving a purely metaphyseal fit, short stems can prevent excessive bone loss secondary to stress shielding (Gustke 2012). The investigators believe short stems are an equally effective alternative to traditional tapered stems in THA; however, long-term studies are essential to proving their efficacy.
Long-term studies of short stems are lacking in current literature. With respect to initial stability and bony ingrowth, short-term data is promising; however, there is still potential for late aseptic failure (Capello et al 2009, Kroell et al 2009, Morales de Cano et al 2013, Schmidutz et al 2012). By conducting a single blinded, prospective, randomized investigation of two stems of varying length with equal metallurgy, coating, and proximal geometry, the investigators can identify the effects of stem length on long-term outcomes.
