Introduction

In the veterinary field, total hip replacement (THR) was first used clinically in the mid-1970s in several clinics in the US, and its use has been slowly expanding, at a faster pace in the US than in Europe. Many factors have limited the rate of expansion, not the least of which is the financial burden of the procedure on the owners. However, it appears now that a broad public awareness of the benefits which THR has brought to human patients, and a constant increase in financial commitments which owners seem to be making for the sake of their pets (including nutrition and medical care), have significantly reduced this particular obstacle to performing THR in dogs. The clinical need is certainly not a limiting factor with many popular breeds having over 50% incidence of hip arthrosis at mid life (most frequently due to hip dysplasia). In some countries, strict control and selection by breeders has helped reduce the incidence of dysplasia, but the limits of these interventions have probably been reached and milder forms, which still lead to arthrosis later in life, have reached a plateau.

Zurich Cementless THR

The femoral and acetabular components of Zurich Cementless reflect different, novel, approaches to address the problems observed in conventional THR, both cemented and cementless. The Zurich Cementless concept has been built around a practical solution to the problem of micromotion at the interface to the cortical bone of the femur. This was achieved in part by reducing the difficulty of the task by anchoring the stem to the medial cortex alone (one femoral stem size can fit a wide range of femur sizes), and then by using bone screws locked into the stem as well as into the bone. This system also has the advantage of anchoring the stem to the medial cortex of the femur, which is the primary load-bearing cortex during weightbearing.

The acetabular component is also unique and has successfully addressed the challenges of interfacing to cancellous bone, by first providing a highly compliant titanium shell to suspend the plastic insert of the cup, and then by promoting bone ingrowth into this shell by leaving it hydraulically open, thus allowing convective mechanisms of mass transport to play their important role in bone formation. The adaptation of bone to the implants allows the application of the prosthesis to young growing dogs too, as soon as the femoral and acetabular bone sizes are compatible with the implant sizes, usually from 6 month of age. The fixation of the stem inside the femur makes it easier to implant the prosthesis even in deformed femurs caused by unsatisfactory excision arthroplasties. To minimise wear of the artificial joint the geometry of the contact area between the head of the prosthesis and the polyethylene liner has been modified, thus reducing the contact stresses several fold and by providing for an improved, hydrodynamic lubrication from within an artificial fossa.

The personal experience of the author in over 500 cementless Zurich THRs performed over the last 10 years will be presented concerning results and complications. Complications are distinguished between manageable complications solved with or without surgical revision and failure of the procedure requiring explantation of the implants. With the improved new-generation prosthesis, available since 2003, in a series of 464 consecutive cases performed in 5 years the solved complication rate was 7.8%, including luxation, femoral fracture, aseptic and septic cup loosening, aseptic stem loosening and breakage of the implant. The unsolved complication rate was limited to 0.8%. Long-term evaluations, over 3 years after implantation, including prosthesis implanted in growing dogs under 8 months of age, will be presented.

With about 6,500 dogs treated with Kyon THR by over 120 surgeons in the world, and with over 14 years of clinical experience, eight of those in widespread clinical use, it seems that the goal of providing an immediately and indefinitely stable THR, with a clinically acceptable surgical procedure, is within reach. This may help establish a role for THR in veterinary surgery comparable to that in human surgery.

References

1.  Montavon PM, Tepic S. Joint surgery in canine hind limb--recent contributions from the University of Zurich. European Companion Animal Health 2006, www.touchbriefings.com

2.  Tepic S, Montavon PM. Concepts of Zurich cementless prosthesis, ESVOT Proceedings, September 2004, Munich.

3.  Vezzoni A, Oropallo S, et al. Cementless total hip replacement in the dog: clinical experience in 140 cases with 'Zurich' prosthesis. Veterinaria 2006; Anno 20:21-36.