Canine Total Hip Replacement (THR)
has come a long way since its first successful canine application in 1974. The procedure evolved from a "big deal" treatment that dogs rarely received even in teaching institutions, to become the gold standard for treating canine hip dysplasia. Now, total hip arthroplasty is considered a highly successful procedure that produces excellent results. Ninety-five percent of dogs receiving THR are expected to regain normal function and maintain it for the rest of their lives. The success of THR has caused other commonly used surgical options such as Triple Pelvic Osteotomies (TPO), Double Pelvic Osteotomies (DPO), and Femoral Head Osteotomies (FMO), to be phased out. THR is now the standard of care in today's veterinary orthopedics, primarily for the following two reasons:
The available implants are the result of an ongoing process of improving the design based on meticulous clinical research and advances in bioengineering and technology. In earlier years, only cemented implants were available for implantation. Indeed, cemented total hip replacement represents one of the major methods of fixation. It has the advantage of providing superior stability almost immediately. However, it has the disadvantage of the cement bond between the implant and the living bone loosening over time. Although, the current generation of cementing techniques makes the cemented implants much more stable such that they can last for several years, we still see occasional premature loosening. In fact, we used to be reluctant to implant these prostheses in juvenile dogs. We wanted to avoid subjecting them to implant loosening at an older age. This fear excluded a large population of dogs with severe hip dysplasia in desperate need of help.
The first significant advancement about cementing techniques in veterinary medicine came with the introduction of uncemented total hip arthroplasty. The uncemented implant relies on a primary fixation method that may be press-fit (implants gently hammered into a meticulously prepared bone) or may require screw or bolt fixation. If the primary fixation is successful, bony ingrowth will occur into the metal implant and will make the construct stable, possibly forever. For bony ingrowth to occur, the implant's surface must be specially prepared with a porous finish.
Here at Veterinary Surgical Specialists, we have had significant experience with different manufacturers' uncemented implants, and it has been our clinical experience that BioMedtrix's Universal Hip System provides the highest success rate. The Universal Hip System's uncemented total hip arthroplasty (BFX or biological fixation) is a press-fit system. The implants were first introduced in 2003. They were initially made from cobalt chrome alloy with a porous coat applied as a layer onto its surface. First-generation implants were somewhat bulky, and the porous coating provided only minimal friction, causing an increase in the complication of femoral stem subsidence (the stem slowly slides into the femoral canal during weight-bearing) leading to catastrophic fractures. The system has gone through several modifications since its introduction.
Radiographs of a recent VSS clinical case with a BioMedtrix BFX total hip replacement.
The current implants are manufactured from titanium alloy, a material that has a better match to the modulus and elasticity of bone than cobalt chrome or stainless steel. The softer metal causes less stress shielding and results in less bone loss through the years, making the implants last longer. Furthermore, the current technology, Electron Beam Melting (EBM), allows us to produce the porous coat with superior friction, therefore reducing the risk of subsidence.EBM is an FDA-approved manufacturing process capable of creating one-piece implants (no additional coating or ingrowth) with optimum mechanical properties. Further modifications involved improving the primary fixation method.
The new generation of titanium EBM femoral stems are now available with a collared or locking-bolt version that allows the surgeon to handle cases with very thin femoral cortices and large medullary cavities (the stovepipe femoral conformation) that were previously considered difficult. Achieving adequate primary fixation relying solely on press-fit fixation in this subset of patients used to be very difficult. Dogs with this type of femoral conformation were considered high risk for subsidence and femoral fracture. The new collared implants combined with the increased frictional coefficient produced by the EBM technology allows us to successfully perform uncemented hip arthroplasty in these patients.
Implants are now available in much wider range of sizes, and they are made to be interchangeable (Universal Hip System). Not only do dogs come in a wide range of sizes, but their femoral morphology and geometry can vary significantly as well. The interchangeable components of the Universal Hip System allow the surgeon to achieve the most perfect fit based on the individual patient's size and femoral or pelvic morphology.
On a final note,
although we prefer uncemented total hip arthroplasty today, there are occasional patients that are simply better candidates for the cemented procedure. In these dogs, we may perform a hybrid THR with an uncemented cup and a cemented femoral stem or vice versa. The current range of universal hip implants allows us to perform this procedure safely and successfully for canines of all sizes.