Advances in Joint Replacement for Hip and Elbow Dysplasia
World Small Animal Veterinary Association World Congress Proceedings, 2007
Chris Preston

Quality research and an improved understanding of failure mechanisms have allowed progress to be made in both implant design as well as in the techniques of application. For many years, cemented THR was the only arthroplasty option in dogs. Today we have a choice of cementless hip implants, an unconstrained elbow system and a recently released knee prosthesis. My experience has been with cemented and uncemented THRs as well as the TER system more recently.

Cemented THR

The most significant implant improvements have been:

1.  Modularity to control neck length and soft tissue tension

2.  Elimination of the cobalt chrome coating associated with wear products

3.  Elimination of the 'shoulder' of the stem to allow centralization

4.  Provision of grooves on the stem for rotation resistance

5.  Expanded range of implants for all size patients including mini-implants

Third generation cementing techniques have reduced the incidence of aseptic loosening:

1.  Use of distal cement restriction plugs

2.  Use of centralizing devices on the distal tip of the femoral stem

3.  Use of vacuum mixing of cement

4.  Injection using a cement gun to pressurize the cement

5.  Injection from distal to proximal to avoid air voids in the cement mantle

Identification of operative factors that correlate with luxation:

1.  Requirement of dorsal bone shelf for cup

2.  Relatively 'open' cup orientation predisposes to dorsal luxation

3.  Poor soft tissue tension may lead to luxation

4.  Undersizing of cup contributes to laxity too

5.  Quality joint capsule closure important

Biomedtrix Cementless THR

The initial experience in my hospital has been favourable and encouraging however anecdotal experience at other institutions suggests that intra- and perioperative catastrophic femoral fractures are a concern. Fissuring of the medial and cranial femoral cortex occurs frequently and should be anticipated. Use of a prophylactic cerclage wire should be considered.

It seems that success with the cementless system is heavily dependant on surgical technique and that the instrumentation is not the main obstacle. With caseload experience, few intraoperative complications are encountered in my early experience.

The pelvic positioning guide is difficult to apply in obese patients and serves as a 'guide' only. I like to observe the cranial and caudal 'pillars' when preparing the acetabular bone bed and aligning the cup between these structures.

The acetabular reaming instrumentation seems extremely accurate and evaluation of cup 'seating' on V-D hip-extended, lateral and open-leg lateral views consistently reveals contact of the circumference of the back of the cup with the bone. In my first ten cases, I had two acetabular failures due to loosening. I suspect one case was due to insufficient reaming and possibly insufficient cup impaction. This case was successfully revised using further reaming and reinsertion of the same cup. The second case was revised using a cemented cup which went on to get infected. Other than that, I have had no cup failures and feel that the cementless cup is easy to insert and offers speed with few complications. I do not think reaming through the medial wall is of major concern (personal communication Jackson San Diego) and it allows the cup to be placed with improved dorsal bone coverage. I have not experienced or heard of surgeons causing an acetabular fracture secondary to excessive impaction and feel that the tolerance for 'excessive' impaction is high. Certainly, conservative impaction can lead to insufficient implant-bone contact and risks instability, loosening and poor osseo-integration.

The technique of first drilling an alignment pin from the intertrochanteric fossa distally before femoral neck osteotomy is very useful in achieving a lateral preparation as well as improved longitudinal alignment. If the femoral osteotomy is performed without first predrilling an alignment hole, there is a tendency for the broaches to migrate medially and into a varus position when reaming is initiated. The pin can only enter the proximal femur caudal to the 'bridge' of bone between the femoral head and greater trochanter and this ensures that the initial hole is centered caudally. The trocar of the pin tends to follow the path of least resistance and not to enter the femoral neck or trochanter. In addition, the operator can concentrate on alignment of the pin with the femoral axis and ensure that the reaming starts parallel to the shaft.

The concerns when broaching are that the broach will contact endosteal bone before it is completely seated and the femoral cortex will fracture with ongoing impaction. Minor fissures are easily managed with a double loop cerclage to limit further propagation distally. If the broaching is started caudal and lateral, there should be a 'cuff' of cancellous bone medial and cranial to the broach and this will significantly reduce the incidence of fissures. I have encountered fissuring during stem impaction and I feel that this was due to insufficient broaching and overzealous stem impaction.

It is critical to properly template the femoral prosthesis to ensure that you are confident of the goal during surgery. The ideal implant will be in contact with the medial and lateral endosteal surfaces with the base of the neck level with the osteotomy. If you oversize the implant without knowing it during templating you may not be able to implant the femoral broach/stem completely, leaving it proud. This means less contact of the porous coating with cancellous bone and difficulties in reduction.

Femoral broaching is a skill that is learned. You should keep impacting if the broach continues to advance. I do not feel that a change in the 'feel' or 'sound' is obvious during impaction and have created fissures whilst waiting for one of these two warnings to surface. The size difference between femoral broaches means that you will need to commit to accepting the size that is fully seated or risk going to the next size up knowing that it needs to be fully inserted. It is not acceptable to get the next reamer partly inserted and then decide it will not fit and implant the previous sized stem. Once you have started the next broach, you will need to completely impact it and this might be a good time to place a prophylactic cerclage wire (or two). I have had three dramatic subsidence cases within the first postoperative month. Two were incidental findings noted during routine pelvic radiology. In both cases the femoral prosthesis subsided until the prosthetic neck was in contact with the calcar bone. In one case the calcar fractured. Both cases did well with strict rest and had no evidence of problems. The third case involved implantation into a severe femoral valgus malunion and the implant became grossly loose.

There have been a number of cases that have anecdotally failed with catastrophic femoral fractures distal to the femoral implant. I have had one patient develop a long oblique femoral fracture two weeks postoperatively that was able to be repaired with a lateral femoral plate.

I have noticed that the neck length of the current BFX stems are very long and often reduction is difficult even with a +0mm head. It does worry me if I am implanting a #9, #10 or #11 stem.

Selection of Cases for Cementless

Juvenile patients where there is concern for chronic aseptic loosening. Based on my medical records of over 150 cemented and cementless THRs, I have found a reduction in the rate of infection with the BFX system.

Avoid 'stove pipe' conformation and giant breeds as the femoral canal seems to be tubular rather than wedge shaped and this may predispose to subsidence.

Total Elbow Replacement

I have had a limited experience with the BioMedtrix TER system over the last 18 months. We have implanted ten dogs and had to extract two systems. I feel that the procedure is technically challenging and relatively unforgiving however favourable outcomes can be achieved. My early impression is that physical therapy seems to be far more important in the outcome following TER than it does after THR in dogs.

The system is limited in is application with the smallest implants not being able to be used in a Cocker Spaniel or Kelpie sized elbow. The instrumentation has been updated and changed on a regular basis so that the kit I purchased in Jan 2006 is out of date already.

The surgical approach is difficult due to the marked periarticular fibrosis in most surgical candidates. An experienced assistant is ideal to position the limb appropriately. The humeral cutting guides allow a wedge shaped section of humeral condyle to be resected. The key here is to preserve sufficient supracondylar bone so that a fracture does not develop through the supratrochlear foramen. One of the first five cases developed a medial epicondylar fracture that was repaired using two miniplates with monocortical screws only. This patient went on to do well.

It is difficult to use the humeral cutting guide to make perfect cuts so that the porous coating of the sides of the humeral implant are in contact with the remaining bone. I am unsure if placing 'chips' of cancellous bone between the sides of the implant and the cut bone surface would help achieve osseo-integration.

Cementing of the stem of the humeral component is not similar to femoral cementing in THR. The isthmus of the distal humeral diaphysis limits canal preparation and use of a cement restriction plug. The composite fixation of the humeral component is designed to have initial stability from the cementing process and later osseous stability will develop where bone grows onto the porous surfaces.

Preparation of the radioulnar surfaces is somewhat 'fiddly' and a high speed drill is used to 'finish' the preparation. I have noticed that placement of the radioulnar component in varus or valgus predisposes to subluxation and uneven wear of the polyethylene. In my opinion it is critical to identify the medial coronoid process and ensure that the radial head/ulnar cut removes this region of bone. Drill and burrs are used to open up the medullary canals of the radius and ulna. It is not difficult to snap a drill bit or to erroneously drill out through the cortical bone rather than down the medullary canal. Again cementing of the radial and ulnar medullary canals is complicated as bleeding can be uncontrolled.

Trial components are available to assess the ability to achieve a stable reduction and assess soft tissue tension. Downsizing of implants is preferred over an excessively difficult reduction.

Closure is extremely important in my opinion. A technique of using a tissue anchor located between the proximal radial and ulnar metaphyses has been used with success. A second anchor can be placed at the level of the lateral humeral epicondyle. Pronation during closure helps maintain reduction.

Application and use of external coaptation has been recommended to limit carpal contracture and minimize the risks of wound problems.

TER patients can be very painful in the immediate postoperative period and aggressive analgesia protocols should be instituted.

Speaker Information
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Chris Preston
Animal Surgery Centre
Victoria, Australia

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