Hip replacement (animal)
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Hip replacement is a proven surgery in animals, notably dogs. It is usually performed, as in humans, to replace a painful or damaged hip joint, where no better alternative exists. Because animals under about 40 lb (20 kg) carry their own weight with little strain on each leg, hip modification surgeries are often sufficient to restore hip function in many cases. As a result, while hip replacement on animals can be seen in any animal of any size, from cats upwards, it is most often performed in the context of medium-large breeds of dogs.
It is especially seen in the context of a possible treatment in the case of canine hip dysplasia, a congenital deformity of the hip joint which can lead to arthritis and crippling pain, as well as in hip arthritis itself.
It is important to note that an animal suffering from hip problems may well have been in some degree of chronic pain all its life from a very young age. Such animals often show no overt sign of pain, they do not cry out or howl. Rather, they have adapted to it over an extended period, and learned to live with it. As a result in many animals, successful hip replacement is reported to return them to a level of playfulness and happiness not previously seen.
However, as with all surgeries, results vary, and there is both risk, and uncertainty involved. Each case should be measured on its own merits.
Overview of hip replacement 
- Background reading: see hip replacement (human)
Animal hip replacements are usually made from the same material as human replacements were, historically, made—a metal (chrome steel) femoral component, and an ultra high molecular weight polyethylene acetabular component.
Since animals have a shorter life than humans, their hip replacements can safely be assumed to be lifelong and will not wear out or need replacing. By contrast, human hip replacements are sometimes deferred until older, to avoid the possible need for replacement later on in life.
Considerations and aftercare 
It is important, before contemplating hip replacement, to consider whether a lesser treatment might give good quality of life with fewer attendant risks. Surgery for dogs, as with humans, should usually be considered a final alternative when non-invasive methods are unable to treat a condition, since they are irreversible and carry risk. Conditions such as dysplasia and arthritis can often be helped by appropriate medications that help the body handle pain, inflammation, or joint wear and tear. See Hip dysplasia (canine).
It is important before contemplating hip replacement in an animal, to check for any other spinal, neural, or rear leg abnormalities (spine and leg X-rays typically), in order to be sure that the animal will in fact be significantly helped by a hip joint procedure.
Since the reduced joint mobility seen in conditions such as dysplasia may result in loss of muscle mass and quality as a dog ages, there is often an advantage in having hip replacement whilst the dog is at an early age, while muscle is more likely to re-develop, rather than in old age when convalescence is longer and more difficult. However this is a major surgery taking several months to fully recuperate, involving the large muscle groups of the hips, and is irreversible. Whilst it has a high success rate (circa 95%) in the hands of a good surgical team, even in older dogs, it is therefore often recommended to avoid it until quality of life is seriously affected beyond the capability of medication to control.
Thus the benefits of hip replacement at a younger age must be set against the risks and the existing quality of life attainable with medication, lifestyle change, or non-surgical handling. Usually a course of medications is tried in any event, to assess how the animal responds to them. Only if there is continuing evidence of pain and/or significantly reduced life quality, is a dog likely to be ready for surgical intervention's. There is a various range of tolerance that animals can endure before cardiac arrest.
An animal will usually need a minimum of 2 months convalescence to recover from hip replacement surgery. This is crucial, as the new artificial acetabular cup (the caput, or hip socket) bonds properly to the pelvis (hip bone). During this period, the animal must be restricted to carefully limited mobility and exercise, as the joint is still bonding and new bone is being laid down. So animals must be prevented from over exercise, or from climbing, jumping or putting any strain whatsoever beyond gentle use, on the joint. The animal must also be kept away from slippery or smooth flooring such as tiles, marble or polished wood since these put considerable lateral (sideways) strain on the hips. This can be a difficult process for a dog as many usual behaviors must be strictly prevented for this period. A pet crate may be a sensible precaution, if in doubt.
Usually, in the case of a double replacement (both hips), whenever possible one hip is operated on at a time and allowed to heal before the other is replaced. This ensures that there is always the maximum natural support during the healing process, although bilateral (double) hip replacements are possible and can be performed if appropriate.
Variations in vet's views 
Some vets will recommend hip replacement at any age over puberty, if suitable conditions are met. Others view it as a surgery to be avoided at all costs unless there is no alternative. The issue here appears to be threefold: consideration of cost, avoidance of surgery where possible, and historical reasons that this has usually in the past been a treatment afforded older dogs and not usually considered for young and middle aged animals or less advanced conditions.
There is now a significant amount of clinical experience of hip replacement in younger and middle aged animals that it cannot be considered "unusual" to perform hip replacement in younger animals or milder cases. These days, it is not uncommon for vets to perform hip replacements upon a younger animal, or with a less advanced case of hip problems. The main criteria seems to be, is the hip condition suitable for surgery, have non-surgical alternatives been fully tried, and is the owner willing to spend the cost involved. So in the case of a hip condition suitable for hip replacement, the issue revolves more around affordability and balancing the possible benefits with the risk of major surgery.
Aftercare recommendations vary also. Some vets recommend as little as 1–2 days in hospital after replacement, others as much as 4–7 days. All vets agree that movement must be restricted for a significant time after surgery. Some suggest that normal activity can be resumed after 5 weeks, others state it is unsafe until after 8 weeks. In both cases, a cautious approach will probably do no harm, if there is doubt.