Evaluation of a Short Femoral Stem in Total Hip Arthroplasty

Total hip arthroplasty (THA) is a successful treatment of osteoarthritis and other destructive diseases of the hip joint, relieving pain and restoring the function of the joint. In Norway more than 7000 patients undergo primary hip replacement every year, and the incidence is increasing.

The femoral component of a hip prosthesis is traditionally anchored to the femoral bone with a stem introduced into the femoral canal and fixated with cement, or through bony ingrowth. Over the recent years, short femoral stems have been introduced, among other reasons to meet the increasing popularity of mini-invasive surgical procedures. The short stems are meant to be easier to introduce through a small incision like used in muscle conserving anterior and posterior approaches. Secondly, and biomechanically relevant, an important design related benefit of the short femoral stems is thought to prevent periprosthetic bone resorption. Bone remodels as a response to mechanical loading. When a stiff implant is inserted into the femoral canal, the load will "bypass" the proximal femur through the implant, and is transferred to bone distally. This results in a negative bone remodelling in the proximal femur, which leads to bone resorption, often termed as "stress shielding". Stress shielding is observed around most uncemented implants, and mainly occurs within the first 6 - 24 months postoperatively. The extent of implant coating, the material stiffness, design and the size of the stem are found to influence the degree of stress shielding. The short stems are thus designed based on the theory that proximal load transfer preserves metaphyseal bone.

Loss of proximal bone stock due to stress shielding is a controversial subject, but has several possible consequences. The exact clinical implications are not entirely determined, but periprosthetic fractures and more challenging revision surgery are recognized problems. In a revision setting it is favourable to have good proximal bone stock to achieve primary stability of the implant. Peroperative fracture and compromised stability of the implant may be a potential problem in stress-shielded bone. Furthermore, already stress-shielded bone in the proximal femur may give wear debris easier access to the interface between implant and bone, resulting in further osteolysis. Short-term results for short-stemmed implants are encouraging in some clinical papers, but most of the available implants lack long-time clinical documentation. Most papers on stress shielding are not randomized trials between long and short stems. There are a few reports of increased revision rate and challenging surgical procedures.

Uncemented prosthetic hip implants are dependent on excellent primary stability to achieve osseointegration and long-term stability. It is recognized that excessive micromotion at the bone-implant interface is associated with formation of a soft tissue and loosening of implants. Micromotion above 40μm leads to partial bone ingrowth, while values above 150μm completely inhibit bone ingrowth. The main concern in short femoral stems is that stability might be compromised. This might be due to the relatively less contact surface. There are no long term results on stability of short femoral stems available, but in vitro studies shows no significant reduction in stability of prostheses with comparable stem length to the Furlong Evolution.

Malpositioning may result in a discrepancy between the reconstructed and the native biomechanical anatomy of the hip. Altered biomechanical properties may influence the clinical outcome and survival of implants. It has been shown that horizontal femoral offset, increases significantly more using a short femoral stem compared to conventional femoral stems. Compared to the contralateral hip, horizontal femoral offset was significantly increased only when using short stems. Increased femoral offset increases the torsion forces along the stem, which in turn raises the need for primary stability to avoid micromotion. It is also shown that a short stem has a wider range of varus-valgus in the stem-shaft axis, On the other there were found no significant differences in biomechanical reconstruction of the hip when using a "broach only" short stem, or a "ream and broach" standard stem.

If there are significant differences in the biomechanical hip reconstruction, will this affect patient reported outcome?