Total hip replacement (THR) is far from a novel procedure. An ivory ball and socket joint was fixed, in a man, in 1891 with nickel plated steel screws. Various unipolar (acetabular or femoral head) procedures were reported in the early part of the twentieth century and a metal bipolar (femoral head and acetabulum) (THR) procedure in 1938. Polymethylmethacrylate (PMMA) (bone cement) was introduced in 1951 by Haboush.

Charnley, considered to be the 'father' of modern THR, developed the use of PMMA grout, a high-density polyethylene acetabular cup and a metal femoral component. The use of a small metal femoral head and the polyethylene acetabulum resulted in considerably less friction than was encountered with previous systems.

Brown reported the first unipolar arthroplasty in veterinary medicine in which the femoral head was replaced with a stainless steel prosthesis. In 1957 Gorman reported THR in 53 military dogs. Various designs and materials were used but success tended to be variable and unacceptable until the introduction of the Richards Canine II Total Hip Prosthesis (Richards Medical Company, Smith & Nephew) in 1974. Though other implants were developed and marketed, some quite successfully, this became the standard hip implant in the USA and the UK. Olmstead and colleagues reported the 5-year follow up on 221 hips in 190 dogs in 1983.

Amongst the limitations of the Richards, and similar, implants was a restriction in compatibility of sizes between acetabular and femoral components. This was addressed by the development of modular femoral head and neck components (Biomechanique, Biomedtrix and others).

Indications for both cemented and uncemented THR are similar (degenerative joint disease of the hip, chronic traumatic hip luxation, failure of reduction of hip luxation and irreparable fractures of the femoral head). Cemented THR may also be considered as a means of managing a failed cementless THR or in some cases where anatomy or secondary changes render a cementless procedure inadvisable.

The surgery for both cementless and cemented THR procedures is similar. Preparation of both acetabulum and femur is important to both procedures but whilst preparation is critical in cementless arthroplasty (as it will totally dictate the positioning of implants and have a bearing on bone ingrowth) it may be of slightly lesser importance in cemented procedures. In cemented THR a gap between bone and implant is required for the cement mantle and therefore implant positioning can be altered, to a limited degree, independent of bone preparation.

Cemented THR does, however, require the use of PMMA, and the technique of cement preparation and application is important in ensuring secure fixation and long-term success. Bone cement acts as a cohesive (grout) rather than an adhesive, maximum strength of the bone-cement interface relies on the ability of the cement to penetrate and interdigitate with the irregularities of the bone surface. For this reason it is not desirable to remove all cancellous bone. Pulsatile lavage for removal of debris including fragments of loose bone, blood clot and fat is desirable and can nearly double the push-out strength of low viscosity cement from the femoral canal. Bleeding from the bone surfaces should be reduced to improve interdigitation. This can be assisted with iced saline lavage, or the use of sterile hydrogen peroxide or adrenaline-soaked swabs. In addition, the use of swabs in the femoral canal and acetabulum for tamponade will assist haemostasis.

Method of cement mixing and application is important. Originally (first-generation technique) manual mixing and finger packing of cement was used. Second-generation cementing (use of intra-medullary plug and cement gun) was introduced in the 1970s to improve penetration and distribution of the cement. Third-generation cementing (centrifugation and vacuum mixing) minimises air inclusion and reduces mean pore size and thereby increases PMMA fatigue strength by as much as 136% and fatigue life by a factor of five. True centrifugation is not practical in most surgical facilities but the use of a commercially available system for slow mix under vacuum (improved strength, evacuation of PMMA fumes) may be justified. Although there are clear in vitro mechanical advantages of third-generation cementing techniques, these advantages are not fully supported in veterinary clinical practice.

Pressurisation of the cement is desirable to improve cement penetration into bone. In the femur, this is generally achieved with the use of a cement restrictor, injection of cement from distal to proximal within the femur and slow insertion of the femoral component whilst manually restricting escape of cement proximally (it is imperative also to ensure there are no defects in the cortical bone within the region of cement fill). Rapid insertion of the femoral component or manipulation of it once inserted is likely to introduce voids between cement and implant and reduce security.

Suction, especially in the acetabulum (hole drilled craniodorsal to acetabulum and suction applied during application of low-viscosity cement) can be used to improve cement penetration. This, however, may not be practical and pressurisation, to some extent, can be achieved by hand packing cement that has achieved dough consistency.

To limit the incidence of infection, the use of antibiotic-impregnated cement is to be advised, though the rate and duration of various antibiotics differ. Cement already prepared with antibiotics is to be preferred to addition at the time of mixing. Addition of materials to the cement has deleterious effects on its mechanical properties, liquid antibiotic having a greater adverse effect than antibiotic powder.

A cement mantle of about 2 mm is desirable circumferentially around the femoral prosthesis. Correct centralisation of the femoral component within the femoral canal with normoversion (or slight anteversion) of the femoral neck and an elimination of varus (avoid contact between tip of prosthesis and lateral cortex) is advised. Correct centralisation is more easily achieved with the femoral osteotomy at the level of the lesser trochanter rather than mid cervical, and with the use of a centralising device.

The cement mantle for the acetabular component is achieved by use of the appropriate reamer, or perhaps even the next size up. Correct positioning with use of commercially available implanters is often difficult and unreliable, possibly due in part to movement of the patient during surgery and failure of the introducer to allow adequate control of the prosthesis whilst attempting to alter orientation. Frequently the prosthesis is introduced and then orientation altered digitally by direct observation. Empirically, positioning the acetabulum in slight retroversion and slightly open with the line of the bevel aligned between the long axis of the pelvis and that of the spine appears desirable/effective.

Cement curing is exothermic. As far as is possible, the PMMA should be restricted to within the prepared femoral shaft and acetabulum. Large volumes should not be allowed to escape into the soft tissues and particular care should be taken to protect the sciatic nerve. All free fragments of cement and any cement that may fracture/separate should be removed to limit the development of third body wear between acetabulum and femoral head.

Complications

THR is still the most commonly performed joint replacement in the dog and an increasingly utilised procedure for the management of coxofemoral degenerative joint disease. Success rates of between 92% and 95% are reported but, when present, complications can be significant, at times resulting in excision arthroplasty, amputation, death or euthanasia. The main complications of THR are luxation, infection, femoral fracture, aseptic loosening and sciatic neuropraxia. Additional concerns include pulmonary thromboembolism and incision granuloma.

Combining the results of several retrospective series the complications for 1361 THRs was:

 Luxation 57 (4.2%)

 Loose implants 30 (2.2% (acet:femur (2:1))

 Infection 14 (1%)

 Fracture 17 (1.25%)

 Sciatic neuropraxia 15 (1.1%)

In humans, the NICE guidelines circa 2000 recommended the use of cemented THR with metal (cobalt chrome) and high-density polyethylene bearing surfaces and commented that the best results in the literature had been with cemented prostheses. The Wrightington Group have reported excellent long-term results with cemented prostheses, and the NICE guidelines commented that any prosthesis being used should have a lower than 10% failure rate at 10 years.

In spite of the trend to uncemented hips and re-surfacing prostheses, the best long-term results are still those of cemented hip replacements with metal on polyethylene surfaces.

References

1.  Abercromby RH, Clayton-Jones DG. Canine total hip replacement: results in 127 cases. Proceedings: British Veterinary Orthopaedic Association, 1991; Liverpool.

2.  Liska WD. Canine total hip replacement complications: an overview. Proceedings: Contemporary Issues in Total Hip Replacement, 2000; San Diego: 30-36.

3.  Massat BJ, Vasseur PB. Clinical and radiographic results of total hip replacement in dogs: 96 cases. Journal of American Veterinary Medical Association 1994; 205: 448-454.

4.  Olmstead ML, Hohn RB, Turner TM. A five-year study of 221 total hip replacements in the dog. Journal of American Veterinary Medical Association 1983; 183: 191-195.

5.  Olmstead ML. Complications: removal and revision techniques. Proceedings: Contemporary Issues in Canine Hip Replacement, 2001; Orlando: 116-119.