Introduction

Computer navigation is a highly sophisticated tool in orthopedic surgery for component placement in total hip arthroplasty (THA). A number of recommendations have been published. Although Lewinnek's safe-zone is the best-known among these its significance is questioned in recent years since it addresses the acetabular socket only ignoring the femoral stem. Modern target definitions consider both socket and stem and provide well-defined recommendations for complementary component positioning. We present a new small-sized hand-held imageless navigation system that implies these targets and supports the surgeon in realizing the concept of combined anteversion and combined Target-Zone (cTarget- Zone) in THA and to control leg length and offset without altering the standard surgical work-flow and we report initial results.

Only someone with good common sense paired with a grain of fortune-telling might be able to foresee further improvements of THR implants adequately. After carefully reviewing the studies of the past, we have evolved our personal belief of what future improvements could look like. A new, improved stem should be made of titanium alloy, have a sandblasted surface structure with a tapered proximal fit stem design. Further advantages would be to allow a certain amount of bony restitution in the metaphyseal region and minimise the stress shielding effects on the femur. As an example of the often-difficult way from the idea on the drawing board to a commercially exploitable implant, we demonstrate the development of our newly designed “hollow-stem” prosthesis.

In an animal study a tapered cementless hollow-stem prosthesis was implanted in 10 foxhounds and subsequently analysed after 12 and 24 weeks. As a result, mineralisation occurred between the titanium stays of the prosthesis, as well as in the central hollow area as early as two to four weeks after implantation. After 12 weeks, density of cancellous bone between the titanium stays and in the hollow centre was considerably higher than on the contralateral untreated side. Load bearing within the centre of the stem was obvious due to the regular orientation of the trabecular bone.

After these positive results we began the conversion to a production line human stem prosthesis. With the help of a finite-element analysis, the stem was then modified in critical areas, resulting in an improved prototype which was subjected to a fatigue test according to ISO 7206-3 of 10.000.000 cycles without any damage. After careful deliberation with our ethics committee, we were finally able to commence with our pilot study consisting of the implantation of 20 hollow-stems and 20 Spotorno stems as a matched pairs study design.

The clinical and radiological evaluation after a minimum 12-month follow-up shows comparably good clinical results in both groups. Radiologically, some degree of subsidence was seen in three hollow stems compared to two Spotorno stems without being clinically relevant. The next step will be a larger randomized study using the hollow-stem as well as a solid stem of the same design. We will consider a more widespread utilisation of the hollow-stem only if this randomised study proves the hollow-stem to be superior.