The hip joint is the only joint in the dog that can be replaced in its entirety. The main indication for performing a total hip replacement is hip dysplasia and the associated secondary osteoarthritis that ensues. However not all dogs with hip dysplasia require hip replacement. When assessing dogs for total hip replacement the following criteria should be met

• An obvious hind limb lameness
• Pain on the manipulation of the hip joint
• Failure to respond to conservative modes of therapy
• No other orthopaedic disease e.g. elbow osteoarthritis
• No evidence of neurological disease
• No skin disease
• No evidence of a septic focus anywhere in the dog
• Skeletally mature (i.e. fully grown)
• Owners who are willing to accept the inherent risks associated with hip replacement
• Owners who are willing to accept the financial burden of this procedure and the possible complications.

Hip dysplasia is a common condition in the large dog population. This developmental disorder may have no adverse effects on individual dogs. However there are two reasons why dogs may suffer from problems with this condition.

1. Dogs are not born with hip dysplasia it occurs as the puppy grows. Initial problems with regard to hip development may be picked up as young as 4-5 months of age. The initial changes that occur are in the soft tissue attachments that hold the hip together. These soft tissues stretch and become lax allowing the femoral head to partially escape from the acetabulum (subluxation) . As a consequence of this the acetabulum will become shallower and the femoral head will change in shape, which compounds the whole problem, thus young dogs may suffer pain, discomfort and lameness as a result of this pathology. However if these young dogs are managed conservatively 75% of them will have dramatic improvements in their hip stability at 18-24 months of age  due to thickening of the joint capsule. These hips are not normal but functionally dogs will be able to exercise to an acceptable "pet level" of activity. The remaining 25% will either require ongoing conservative/ medical therapy or may require surgery to be performed.
2. As already mentioned dogs with hip dysplasia will develop a degree of osteoarthritis. The main reason why dogs may become lame as a result of osteoarthritis is because of progress loss of the cartilage surface of the joint. This may be managed conservatively or surgically.

There are other indications for hip replacement but these are relatively rare e.g. treatment of fractures of the femoral head and neck, healed fractures of the acetabulum, hip dislocation where the original hip conformation was poor and tumors of the femoral neck.

Currently there are several "hip systems" available for use in the dog (Richards 2, Biomedtrix, Biomechanique, Perot, Howmedica cement less system and the Zurich hip system). In the UK the most commonly used hip system is the Biomedtrix system; this is a modular system where the implants are held in place with bone cement. Essentially there are 3 components to this system: a high-density polyethylene cup, a metallic femoral stem and femoral head. This system is modular which means that the size of the implants can be tailored to the individual needs of the patient.

If implantation and the subsequent convalesces from the hip replacement goes well then we can expect the "new" hip to function to 95% of a total normal hip. However there are several major complications that can occur:

• Bacterial infection (acute surgically induced infection and delayed infection as a result of a distant focus of infection which has seeded into the hip replacement via the blood stream)
• Hip dislocation (this can occur in the initial recovery phase of 4-6 weeks)
• Femoral shaft fracture
• Aseptic loosening of the implants

These complications can result in revision surgery being necessary and potentially removal of the hip replacement. One or more of these complications will affect 10-15% of dogs that undergo hip replacement.

Hip replacement in the dog can be a very rewarding surgery for everyone concerned however it can also be extremely frustrating and costly exercise.

Here Mr D G Thomson BVMS, MRCVS, CertVR, CertSAS takes us through the hip replacement procedure.