Materials and Methods

Case selection—Dogs selected for use in the study were included in an in-house data registry. Dogs admitted for THR because of osteoarthritis secondary to hip dysplasia between January 1, 1992, and December 31, 2009, were eligible for inclusion. Dogs that underwent THR for other reasons were excluded. A cemented or cementless hip prosthesisa was implanted in each dog. In part 1, dogs were assigned to 1 of 2 groups depending on the date of THR. Dogs that underwent THR between January 1, 1992, and December 31, 2000, were assigned to group 1, whereas dogs that underwent THR between January 1, 2001, and December 31, 2009, were assigned to group 2. In dogs that underwent bilateral THR, only data from the first THR were included. In part 2, only dogs undergoing bilateral THR were included.

Data collection—Relevant data (including age, sex, BCS, body weight, duration of clinical signs, and severity of clinical signs) was prospectively entered into the in-house THR data registry at the time of the initial examination. The BCS for all dogs was assigned by 1 surgeon (WDL) on the day of surgery; BCS was scored on a scale of 1 to 9.^b A thorough medical history was obtained from each owner, and the duration of clinical signs was defined as the number of months prior to THR, starting at the time when the owner first noticed clinical signs (eg, pelvic limb lameness, abnormal gait, difficulty standing, or decreased exercise tolerance) consistent with hip joint osteoarthritis.

Severity of clinical signs was assigned by 1 surgeon (WDL) on the basis of information from the medical history and results of the physical examination at the initial consultation. Severity of clinical signs was assessed on the basis of various criteria and classified as mild (lameness detected 1 time/wk, inconsistent pain response on extension of the hip joint, and no asymmetry of thigh girth), moderate (lameness detected 1 to 6 times/wk, inconsistent signs of pain on extension of the hip joint, and asymmetry of thigh girth detected only by measurement), or severe (lameness detected daily, consistent pain response on extension of the hip joint, and visible asymmetry of thigh girth [ie, atrophy of the affected limb]).

Statistical analysis—In part 1, an unpaired t test was used to detect significant differences between the 2 groups for duration of clinical signs prior to THR. In part 2, an unpaired t test was used to determine whether there was a significant difference between duration of clinical signs prior to the first THR and the interval between the first THR and the second THR on the contralateral hip joint in dogs that underwent bilateral THR. For all statistical analyses, significance was set at values of P< 0.05.

Results

Part 1—A total of 833 dogs met the inclusion criteria. Group 1 comprised 334 dogs, with 168 females (17 sexually intact and 151 spayed) and 166 males (60 sexually intact and 106 neutered). The most common breed was mixed (n = 82 [25%]), which was followed by Labrador Retriever (56 [17%]) and German Shepherd Dog (50 [15%]). Body weight ranged from 13.6 to 65.5 kg (29.9 to 144.1 lb; mean, 33.5 kg [73.7 lb]; median, 31.8 kg [70.0 lb]), and BCS ranged from 3 to 9 (mean, 6.2; median, 6). Age at the time of THR ranged from 8 to 186 months (mean, 58.5 months; median, 50 months). Severity of disease was classified as severe in 293 (88%) dogs, moderate in 35 (10%) dogs, and mild in 6 (2%) dogs. The duration of clinical signs prior to THR ranged from 0.2 to 120 months (mean, 10.64 months; median, 4 months).

Group 2 comprised 499 dogs, with 270 females (23 sexually intact and 247 spayed) and 229 males (56 sexually intact and 173 neutered). The most common breed was Labrador Retriever (n = 111 [22%]), which was followed by mixed (91 [18%]) and German Shepherd Dog (62 [12%]). Body weight ranged from 4.5 to 64.5 kg (9.9 to 141.9 lb; mean, 31.5 kg [69.3 lb]; median, 31.8 kg), and BCS ranged from 3 to 9 (mean, 6.2; median, 6). Age at the time of THR ranged from 6 to 178 months (mean, 60.1 months; median, 54 months). Severity of disease was classified as severe in 432 (87%) dogs, moderate in 58 (12%) dogs, and mild in 9 (2%) dogs (percentages do not sum to 100% because of rounding). Duration of clinical signs prior to THR ranged from 0.1 to 120 months (mean, 14.56 months; median, 6 months).

The groups were compared with regard to the duration of clinical signs. Analysis revealed that the mean duration of clinical signs prior to THR was significantly (P = 0.005) longer for group 2 dogs (14.56 months) than for group 1 dogs (10.64 months).

Part 2—The evaluation involved 272 dogs that underwent bilateral THR. Duration of clinical signs prior to the first THR in this population ranged from 0.1 to 96 months (mean, 12.64 months; median, 6 months). The interval between the first THR and the second THR ranged from 1 to 103 months (mean, 9.59 months; median, 5 months). The duration of clinical signs prior to the first THR was significantly (P = 0.03) longer than the interval between surgeries.

Discussion

In the study reported here, 833 dogs underwent THR during the 18-year period. For this population of dogs, there was a significant increase in the amount of time between the onset of clinical signs associated with osteoarthritis of the hip joint and examination for THR. On average, dogs evaluated for THR between 2001 and 2009 had pelvic limb lameness for 4 months more than did those evaluated between 1992 and 2000. There are several factors that could have contributed to an increase or decrease in the time interval of these 2 periods. The increase in availability of surgeons trained to perform a THR, client financial affluence in the early 2000s, and increased public awareness of THR for dogs through marketing, continuing education, and Internet searches would have been expected to cause a decrease in the duration of clinical signs from group 1 to group 2. Our finding that the duration of signs increased suggests that other factors, such as a greater variety of medical treatment options and owner reluctance to pursue surgery, had a greater influence.

An increasing number of owners may have chosen to pursue medical management, rather than surgical treatment, because there are more options for conservative treatment, including use of cyclooxygenase-2–selective NSAIDs. Prior to the late 1990s, this drug class was not available for use in veterinary medicine, and the availability of drugs such as carprofen^c has provided more options for the treatment of osteoarthritis that were not available for most of the 1990s.¹⁰ The use of nutraceuticals and prescription diets formulated to promote joint health and function has also gained popularity over the past few years, which has given veterinarians and owners more methods for conservative treatment of osteoarthritis. We believe these developments in the medical management of osteoarthritis were the most important factors for the increased time interval before THR in part 1 of our study.

Another possible reason for a greater time interval before THR may have been owner or veterinarian concerns about potential complications. However, we have no reason to believe that there has been a change in public concern regarding complications from the 1990s to the 2000s; therefore, we do not believe that this factor was involved in the increase in time between the 2 groups. Continuing education programs on surgical treatment of osteoarthritis have expanded at veterinary conferences, and awareness of THR has increased as a result. Despite this, primary-care veterinarians are possibly exposed to information regarding medical pain management via methods other than THR because most publications and seminars about THR primarily target veterinary orthopedic surgeons.

Many clients may not have pursued surgery because of the cost of the procedure, although when compared with the cost of medical management (including analgesics, laboratory monitoring for toxicosis, nutraceuticals, and prescription food for joint health), the cost of surgery may be comparable or even less expensive, depending on life span, age at onset of disease, and the products used. A cost analysis of medical management and surgical treatment of hip joint osteoarthritis in humans found that the cost of medical management was approximately twice the cost of surgical treatment.¹¹ Additional studies are needed to determine whether there is a similar pattern in veterinary medicine.

The cost of surgery (regardless of the exact amount) will likely always be a factor for some dog owners. However, the best option to alleviate pain and to restore biomechanical function for each patient should always be offered to the owner as an option. When there are financial constraints, other default treatment options fortunately exist. Veterinary expenditures by dog-owning households in the United States have increased from $7 billion in 1996 to $16.1 billion in 2006.¹² This suggests that the public is willing to spend disposable income for pet health when the need arises, and treatment options should not be denied on the basis of prejudgment of the financial resources of pet owners.

We found that in dogs that underwent bilateral hip joint replacement, the duration of clinical signs prior to the first THR was significantly longer than the interval between the first and second procedures. Our interpretation of this finding is that the benefits the owners observed following the first THR were sufficiently rewarding that they elected to pursue a second THR on the contralateral hip joint. Owner awareness of a surgical option may also have factored into this finding. However, we did not question owners with regard to the amount of time that they were aware of a surgical option prior to the initial consultation; therefore, we cannot conclude that owner awareness was the reason for the difference.

The duration of clinical signs prior to THR is of prognostic importance in humans for postoperative function following THR.^13–19 Authors of several studies in the human literature have described the benefit of early THR. It was found in 1 study that people who elected to have surgery early in the progression of osteoarthritis had significantly better postoperative outcomes than did people who delayed surgery and had more severe disability prior to surgery. It was found in another study¹³ that THR performed before the failure of medical treatment resulted in a shorter postoperative hospitalization period and better postsurgical function. There is also evidence that better preoperative function results in more substantial improvement after surgery.¹⁴

To the authors' knowledge, comparable studies have yet to be performed in veterinary medicine; however, there is evidence that at least 1 complication, sciatic neurapraxia, may be less common in affected dogs that undergo surgical treatment earlier in life and earlier in the course of disease.²⁰ In addition, a higher degree of sclerotic bone and dorsal acetabular rim wear is evident later in the course of osteoarthritis, which makes THR more technically challenging and therefore increases the anesthetic time. Dorsal acetabular rim wear can also progress to a point at which it is not possible to perform THR because of inadequate bone stock in which to seat the prosthesis. When surgery is delayed, muscle atrophy progresses, which may contribute to poorer postoperative function and a greater need for physical therapy.

Total hip replacement earlier in the course of disease is of prognostic importance in humans.^13–19 In the study reported here, we detected that during the 18-year period, there has been a significant increase in the amount of time a dog has clinical signs of hip joint osteoarthritis before THR. A greater variety of options for medical treatment of osteoarthritis may have contributed to this finding. Although the reason for this increase is unknown, it is possible that this delay may adversely affect postoperative outcome, as determined on the basis of studies reported in the human literature. In veterinary medicine, studies to determine the importance of THR early in the course of hip dysplasia have not yet been performed. Additional studies are needed to evaluate the outcome of THR performed early in the course of disease, rather than late in the disease course, and to compare the long-term costs and outcomes for young dogs treated surgically with those for young dogs treated nonsurgically.

A limitation to the retrospective nature of this study is that details regarding the medical treatment administered prior to THR were not available for every dog. Therefore, we were unable to determine the extent and variability of conservative treatment that was attempted in each dog prior to surgery. Also, we recognize that the ability to recognize lameness varies among owners. However, the duration of clinical signs was based on the owners' perception of lameness at the time of the initial consultation, and this information was collected in a consistent manner from each owner.

The findings for the study reported here indicated that dogs that underwent THR during the latter 9-year period had a longer duration of lameness than those that underwent THR during the preceding 9-year period. Additional studies are needed to determine whether such a delay may be detrimental to the patients or costly for owners. It can be difficult to make therapeutic decisions with regard to hip joint osteoarthritis, but it is important to be aware of and to discuss all options, both medical and surgical, with pet owners when the disease is initially diagnosed.