Total hip replacement (THR) is now a well established procedure in veterinary orthopaedics. Although there are some variations in technique and types of implant, most involve the replacement of the acetabulum with an ultra-high molecular weight polyethylene cup and the femoral head with a cobalt chrome ball and stem which are secured in position using polymethylmethacrylate. The use of uncemented prostheses has also been reported (De Young et al, 1992) and more recently the Zurich Cementless (Montavon & Tepic, 2002).

Olmstead et al. (1981) described the technique for THR in the dog using a Richards type 11 Canine Total Hip Prosthesis and reported satisfactory results in over 90 % of cases. More recently a modular prosthesis has been developed by a number of manufacturers in collusion with canine orthopaedic surgeons. These include:

 BioMedtrix Inc., Ohio USA

 La Biomechanique Integree, Bretigny-Sur-Orge, France

 Veterinary Instrumentation, Sheffield, U K.

Each of these systems presents the surgeon with a choice of several sizes of acetabular cup and femoral stem which can be matched to the radiographs of the individual dog and finally checked during surgery. The femoral head forms the third component and for each femoral stem there will be a choice of two or three femoral heads (depending on the system used and the size of femoral stem in question). The femoral head forms a tapercut junction with the femoral neck and the depth of the hole in the femoral head varies between the choices, thus providing options in effective length of femoral neck (Figure 1). The systems come complete with specialist instrumentation required for the procedure. The results of using these modular systems are similar to those using the original Richards prosthesis and their main advantages are ease of use and range of sizes of patients in which they can be used. The use of cemented THRs was reviewed by Olmstead (1995)

The procedure is technically demanding and should only be carried out by a surgeon who is regularly performing surgical procedures on canine hip joints (preferably THR itself) in an environment where the highest standards of asepsis can be achieved and where appropriate instrumentation and surgical assistance is available.

INDICATIONS FOR THR

 Osteoarthritis secondary to hip dysplasia

 Osteoarthritis secondary to healed fractures of the acetabulum or proximal femur

 Primary treatment of non-reconstructable fractures of the femoral head

 Recurrent coxofemoral luxation

Conversion of an excision arthroplasty to a THR should not be recommended, calcar support for the prosthesis is removed by excision arthroplasty, there is more risk of infection and there may be inadequate muscular support for the prosthesis.

Other criteria which must be met for an animal to be considered for THR:

 The patient should be clinically lame with hip pain which is non-responsive to conservative management, (i.e., the decision to perform THR is not based on the radiographic appearance of the hip alone).

 The animal should be of sufficient size to accommodate at least the smallest prosthesis (this varies with systems in use but the lower body weight is generally between 20-30 kgs).

 Ideally THR should be delayed until the animal is at least 14 months of age however THR have been successfully carried out on dogs as young as 11 months.

 There must not be evidence of other disease that might compromise the value of THR (e.g., progressive neurological problems such as CDRM).

 There must be no established source of bacteraemia which might promote infection around the prosthesis (e.g., periodontal disease, anal furunculosis).

 The ideal case for THR has osteoarthritis in one hip and a normal contralateral hip , in the majority of dogs both hips are affected , one is often worse clinically, if this hip is replaced making the dog comfortable on that side then the animal can compensate better for the unoperated hip . In our series only some 32% of cases require bilateral THR and there is usually a 12-4 month interval between surgeries.

Surgical technique for THR is illustrated in Figures a-h

Figures 2a-2h

Results and complications of THR

Success rates of 85-95% have been reported with less complications being seen as surgeons gain more experience. However, in those cases that are unsuccessful the reason usually relates to a serious complication which may develop soon after surgery or several months/years later and usually involves:

 Luxation of the prosthesis

 Loosening of the acetabular cup

 Infection

 Fatigue fracture of the femoral prosthesis

 Fracture of the femur

 'Implant-related' osteosarcoma

There are revision techniques available for most of the complications and if removal of the prosthesis should become necessary then there is plenty of thickened joint capsule available to provide support for the resultant excision arthroplasty.

References

1.  DeYoung , DJ.,DeYoung, B.A., Aberman, H.A., Kenna, R.V., & Hungerford, D.S., ( 1992) Implantation of an uncemented total hip prosthesis. Technique and initial results of 100 arthroplasties. Veterinary Surgery, 21, 168-77.

2.  Olmstead , M.L., ( 1995) Canine cemented total hip replacement : state of the art. Journal of Small Animal Practice, 36, 395-9.

3.  Olmstead, M.L., Hohn, R.B., &Turner, T.M. ( 1981) Technique for total hip replacement. Veterinary Surgery, 10, 44-50

4.  Olmstead, M.L., Hohn, R.B., &Turner, T.M. ( 1983) A five year study of 221 total hip replacements in the dog. Journal of the American Veterinary Medical Association, 183, 191-4.

5.  Montavon P.M., & Tepic S., (2002 ) Clinical application of Zurich Cementless-canine total hip prosthesis. In Abstracts , ESVOT-VOS, in Veterinary and Comparative Orthopaedics and Traumatology 15, A26

6.  Matis U., Holz., ( 2003 ) Clinical experience and long-term results of the cemented Biomechanique hip. In Abstracts , ESVOT-VOS, in Veterinary and Comparative Orthopaedics and Traumatology 16, A3