Total Hip Replacement in the Dog

Total hip replacement (THR or THA – total hip arthroplasty) surgery represents a major breakthrough in the treatment of disabling arthritis and injury in large breed dogs, and with new innovations has become available for dogs of all sizes, and cats too! It is indicated as a salvage procedure when more conservative management measures have failed, and the quality of the animal’s life has declined below the level acceptable to both the pet and its owner. Unlike other salvage operations, total hip replacement provides a biomechanically sound, pain-free joint for the patient, allowing resumption of full athletic lifestyle after just a few short months of convalescence.

INDICATIONS

Total hip replacement surgery is used most often to treat osteoarthritis of the hip joint. Osteoarthritis commonly represents the end-stage of chronic instability or injury of the hip. Such instability might result from canine hip dysplasia, a developmental disorder recognized predominantly in large dogs, or from traumatic injury to the hip, such as dislocation or fracture. The most common signs are stiffness, lameness, and reluctance to exercise; all signs of discomfort and problems with locomotion.

Not all dogs with hip dysplasia or osteoarthritis require surgical treatment. The decision for performing a THR and the timing for joint replacement surgery depends on a number of factors, including the degree of clinical disability and discomfort, intended use of the dog, and the presence of other diseases or injuries. The decision to perform THR (or any treatment for that matter) is never based on radiographs alone, no matter how severe the changes appear. There can be a poor correlation between radiographic severity and clinical severity, and some dogs with terrible looking hips are yet functioning at a high athletic level with no apparent pain*. So the decision to treat arthritic hips, with medicine or surgery, is always based on whatever clinical disability the patient is having.

Candidates for THR must be carefully evaluated by physical examination, orthopaedic examination, neurological testing, blood work and x-rays before they are admitted for the operation. The surgeon will discuss his or her findings with the owner and a decision will be made as to whether total hip replacement would be of value to the patient, and whether the patient is suitable for the operation. It is not uncommon for dogs with hip osteoarthritis to have in addition, other orthopedic injuries. And it is often these other problems (such as knee ligament injuries) that are the source of lameness that prompted the visit to the veterinarian in the first place.

Thus, in looking at the compiled list of indications and contraindications, and from years of experience, we have learned of two critical errors to be avoided in deciding how to treat our patients with hip arthritis:

1. Since dogs can have profound radiographic changes with arthritis and yet have few if any symptoms, any new lameness exhibited by such a dog, or sudden worsening of what had heretofore been a mild lameness, should prompt skepticism that it is due to the hips until proven to be so. Only when other causes have been excluded or treated should attention be focused on the hips, and treatment such as THR contemplated.
   
2. *Symptoms associated with hip osteoarthritis are usually insidious in their onset and progression. Lacking the drama of the dog suddenly crying out or holding up its leg, these symptoms may go unnoticed for a long time, or be attributed to something else. Not infrequently dogs with progressive arthritis have signs such as getting up slowly, showing some hesitation in ascending stairs or jumping into a vehicle, having less endurance for or “interest” in playing or other activity, and these signs, in turn, are misinterpreted as some vague indication that the pet is “just getting older,” and “slowing down.” Age alone does not cause dogs to become less active! So the observation of these types of signs over months or years may be an indication that the dog’s hips really do merit treatment.
   

GENERAL REQUIREMENTS

Age
The prospective patient must be skeletally mature. Most dogs’ bones stop growing between 9 and 12 months of age. X-rays (radiographs) are used to evaluate whether the growth plates are still active or not. There is no upper age limit for THR as long as other medical criteria are met and the client is willing to afford the operation.

Size
THR surgery used to be limited to patients weighing about 35-40 lbs or more, since smaller implants were not available. Nowadays, the size range of available implants has been expanded so it may be possible to perform THR in small dogs or even cats. Again, x-rays are used to determine if the pet is of suitable size for an artificial hip. Patients deemed too small for THR are treated with alternative surgical procedures. There is no limitation due to patients being too large, however, and THR has been used successfully for giant dogs (>200 lbs), zoo cats (mountain lions, etc.), and even larger animals like Alpacas.

HEALTH STATUS

The patient must be in good general health and free of co-existing infection. Conditions such as dermatitis or severe dental disease may cause surgery to be postponed until these infections are cleared up. Other causes of hind limb disability, such as neurologic disorders or orthopaedic injuries to other joints, may preclude or postpone the use of THR, even in patients who also have end-stage osteoarthritis of the hips. If THR is to be employed we want to be as sure as possible to (a) minimize the risk of complications, and (b) ensure we are treating the problem that is causing the patient’s major symptoms.

SURGERY

Dogs are usually admitted the day before surgery is scheduled. This allows time for acclimation to the hospital environment, complete physical examination, and clipping of hair from the surgical site. If not done already, the patient may be sedated so that appropriate and specific radiographs can be taken. These radiographs are used to determine the size implants needed and to identify any other issues that could impact on the surgical procedure.

Surgery is done on one hip at a time in dogs. In humans it has become somewhat more common to operate on both hips concurrently, but it is far easier to control activity and weight bearing in people than it is in pets! Many dogs, even though afflicted with bilateral hip arthritis, do not require the second hip be operated, or at least not very soon after the first side. For those dogs that do require a second side to be done, a delay of at least 8 to 10 weeks between surgeries is employed. There is no upper limit in time delay between the first and second sides. Of the dogs that do go on to have a second THR, the time span between operations is often 3 to 4 years, or more (bearing in mind, however, the subtlety of signs as alluded to above, which may cause us to delay operating the second hip longer than perhaps is ideal).

The operation itself is done under the strictest sterile conditions. All of the drapes, gowns, gloves, etc., are disposable and are only used once. Usually three to four doctors and nurses are scrubbed in for the procedure. The patient is placed under general anesthesia and receives intravenous fluids and antibiotics. As with all surgery done at VSES, multiple parameters are monitored throughout the operation to ensure that patients are handling anesthesia without problems. Most dogs will have an epidural injection of pain medication and anesthetics in addition to the general anesthetic and IV analgesics. This optimizes pre-emptive pain control (resulting in smoother, more comfortable recoveries), and lessens the dose of general anesthetic required.

The hip is a ball and socket type joint. During the operation, the arthritic ball (femoral head) and socket (acetabulum) are removed using specially designed saws and reamers. After preparation of the respective bones, careful test fitting and adjustment of trial prostheses, the permanent implants are secured in place using either special surgical cement, or are “press fit” into place using a cementless technique. In the first instance the special bone cement bonds the prostheses to the bones, whereas in the second instance stability is achieved via friction (the bone and prosthesis are more exactly matched) and ultimately by actual ingrowth of bone from the patient into the prosthesis. The surgeon will discuss with you the availability of cemented vs. uncemented (also called, press-fit, or porous-ingrowth) prostheses for your pet, and the rationale for choosing one vs. the other (in some cases we will place “hybrid” hip replacements, where one component is cemented and the other is press fit. The goal is to tailor the operation to the specific needs for each individual patient to try and achieve the best possible results). The prostheses themselves are made to exacting standards (meeting, or in some cases exceeding the requirement for human artificial hips) of very high-grade cobalt-chromium alloy stainless steel, and high molecular weight polyethylene plastic. They are specifically designed for the dog. Before closure, cultures are obtained to ensure that no bacteria gained entrance to the surgical site. Multiple layers of sutures are used to re-establish the joint capsule, muscles, tendons, subcutaneous tissues. The outer layer of skin is closed either with sutures or specially-designed skin staples. After surgery a bandage is applied over the incision and x-rays are obtained. When the patient is awake it is returned to the ward for continued monitoring.

POSTOPERATIVE MANAGEMENT AND CONVALESCENCE

Dogs usually remain in the hospital for 2 to 5 days following surgery. This enables us to monitor the incision, oversee the animal’s first steps with its new hip, and await the results of the surgical cultures. All bandages are removed when your pet goes home. Antibiotics and analgesics are administered orally for several days, and many animals as well will go home with an analgesic patch adhered to the skin. Skin sutures or staples are removed 10 to 14 days after surgery, either at our facility or at your regular veterinarian’s office.

During the first month at home, the dog must have very limited activity. During this crucial period the joint capsule, muscles and tendons are healing, and helping to stabilize the hip. This means that dogs are allowed short walks only, and only on a leash. Otherwise the dog should be kept in a crate or be similarly confined. Dogs should not run, play, jump, be allowed to slip and fall, or climb flights of stairs. Going up and down one or two steps to get outside is acceptable. We recommend a towel or specially designed sling be used to support the rear end whenever the dog is out of his or her crate.

During the second month dogs are still limited in their activity, but can start increasing the length of their walks outside. Between the second and third month activity is gradually increased so that by the end of the third month the dog is nearly back to normal function and activity. In some instances we will also recommend consultation with canine physical therapists so that a rehabilitation program can be formally designed and implemented.

Dogs with total hip replacements usually begin using their operated leg shortly after surgery; in fact, we expect them to at least touch the toes down by the first or second postoperative day. Many dogs will be nearly fully weight bearing on their operated leg the day of surgery or the next day. This is rewarding to see, but at the same time indicates how we must not trust our patients to be careful and limit their own use of the leg! It is the client’s responsibility to police the activity of their dog and prevent the kind of over-use that could lead to dislocation or other complications. Dogs are wonderful, but they don’t know how to use crutches, and cannot think abstractly along the lines of, “I just had major surgery and better not do ‘X’ for fear of causing complications.”

A set of x-rays are usually obtained 2 months after surgery. They can be taken here, or done at your regular veterinarian’s and sent here for review. Another set is recommended at 1 year after surgery, and then annually thereafter. This allows us to closely monitor each individual dog, as well as to compile data used in the long-term evaluation of canine THR.

COMPLICATIONS

The overall complication rate for THR in most joint replacement centers is about 10%. This includes everything from the very minor (eg. a superficial wound reaction) to the most severe. The major complications we worry about include dislocation, loosening, infection, tumor development, and fracture.

Dislocation (luxation) occurs when the ball comes out of the socket. In some instances this can be due to technical problems with the orientation of the artificial socket or the length of the femoral neck chosen at the operating table. More commonly, it is the result of trauma associated with inappropriate types or amounts of activity in the first 4-6 weeks after surgery. In people it can occur with something as innocuous-seeming as crossing one’s legs when sitting in a chair. Prevention, by the strict confinement and control described above, is our best treatment for this complication in dogs. If luxation does occur, it can sometimes be treated non-operatively by manipulating the ball back into the socket, and then applying a sling device afterwards. However, most cases of luxation require a trip back to the operating room. In some instances the treatment may be as simple as repairing a tear in the joint capsule. In other instances more elaborate treatment may be required, such as replacement of the some of the implants, or performing pelvic ostetotomy – cutting the bones that surround and anchor the acetabulum so that the socket can be repositioned, and then fixing the bones in place with additional hardware (bone plates, screws, wires, etc.).

Implant loosening is a frustrating complication in both humans and dogs. In many cases it can be traced to “wear debris”: microscopic particles of plastic or metal that are worn away in the hip over time, and then work their way between the implants and the bone. This is generally a less frequent problem in dogs since it can take tens of years to develop significant wear debris, and this of course exceeds the lifespan of our patients, even those receiving THR at a year of age. Another cause of loosening is breakdown of the cement bond that holds the implants in place (cemented THR only). This can be due to wear debris, but in some instances (especially when seen just a few months after implantation) there are other, often unknown factors at play. This has been one of the chief motivators for the development and refinement of uncemented prostheses in humans. It is assumed that the use of uncemented prostheses in dogs will reduce the incidence of these so-called “aseptic loosenings” in our patients as well. Treatment for non-infected (aseptic) loosening could involve revision with new implants (and, for example, replacing a cemented hip with an uncemented one), but in many instances may require explantation (complete removal) of the artificial hip components. In this instance the dog is left with an older form of salvage procedure (femoral head and neck excision arthroplasty, or Girdlestone procedure). This is less ideal of course than a functioning THR, but still will provide an acceptable degree of function and quality of life.

Infection is a devastating complication since it causes loosening, and makes the environment unsafe for re-implanting new components in most instances. So explantation is the usual treatment. In a few instances it may be possible to eradicate the infection and then place new (uncemented) prostheses in, but this is difficult and prone to further complications. The key to infection is prevention! We use very sterile operating technique when we do THR, and screen patients beforehand to rule out concurrent infection elsewhere in the body. Intravenous and then oral antibiotics are used to control any inadvertent contamination that might occur during surgery (we even mix antibiotics directly into the cement used for fixation of the prostheses in cemented THR), and these will be continued for a longer period if our surgical cultures reveal any germs. Long-term, the most effective prevention involves use of prophylactic antibiotics before and during any procedures that might cause bacteria to temporarily enter the blood stream (most late infections in fact come from this blood-borne or hematogenous route). The most common of these would be dental cleanings. Any other infections (ears, skin, urinary tract, etc.) should be aggressively treated to prevent entry of bacteria into the blood.

Tumors are very rare complications after THR. Benign tumors termed granulomas have been reported associated with bone cement that has extruded beyond the confines of the bone, especially within the pelvic canal. Rarer still are the development of malignant tumors. Bone cancers have been known to develop rarely in bones with any injury, such as a fracture. They have been seen in bones repaired with bone plates, screws, pins and wires. We have no reason at present to believe that bone cancers that occur after THR are in any way different from these other so-called fracture-associated sarcomas, and there is no specific indication that the metal implants or the bone cement involved with THR are particular triggers. The risk is exceedingly low, based on both the human and canine experience with THR over decades of research.

Fractures of the femur can develop in association with THR. These can occur during surgery as well as long-term. Intra-operative fractures are associated with the process required to prepare the bone for THR implantation. The medullary cavity (central hollow area of the bone) must be drilled and reamed (widened) to accept the implant. This thins out the strong outer wall (cortex) and could weaken it enough to permit the bone to break during manipulation. Dogs are particularly susceptible since they start out with relatively thinner cortices compared with humans. That said, the rate of fracture during surgery is exceedingly low. Moreover, it can be treated readily during the operation. It entails extending the incision to a longer one than usually required, and then applying appropriate fixation hardware (wires or a bone plate) in addition to the THR. It usually does not preclude the completion of the THR.

Late fractures are almost always the result of acute external trauma, and vary little from the types of injuries seen in dogs without pre-existing THR. The nature of the fracture dictates the type of repair required, and whether the THR can be preserved. The worst case scenario is a combination of fracture and tumor (termed pathologic fracture), whereby the tumor weakens the bone to the point of spontaneous failure with minimal trauma. Given the rarity of secondary bone tumors associated with THR, pathologic fractures of this type are equally rare.

CONCLUSIONS

In large studies done at several major institutions, the success rate (dogs with good to excellent function by very strict criteria) after THR is in excess of 95%. Most dogs have demonstrable increases in muscle mass, improved hip motion, and increased activity levels. Many owners report that their pet can now do things they have not done since they were a puppy. The quality of these dogs’ performances and the percentage achieving normal function is far greater than with any other salvage procedures commonly available to treat hip disorders. In fact, dogs with total hip replacements have gone on to win field-trial championships!

Although the state-of the-art equipment, implants, training, and experience, which go into the THR surgery, are expensive, few other procedures are capable of so dramatically changing the quality of a beloved pet’s life.

If your pet is a potential candidate for total hip replacement, we will be happy to discuss the procedure with you in greater detail, and help guide you in the selection of the most appropriate treatment for your pet’s condition.