Introduction
No single canine disease has been the focus of more controversy, debate and scientific publications than hip dysplasia (HD). There are few points of general agreement regarding this disease apart from the fact that it has a multifactorial etiology which involves a genetic component and that it is associated with joint laxity.
Any disease associated with joint laxity has the potential to be a more significant clinical problem in athletic or working dogs than in non-working breeds. Joints with abnormal laxity which are subjected to high levels of activity are likely to suffer very high levels of physical stress on certain areas of articular cartilage and reduced stress on adjacent areas. The resultant shear stress forces acting on the cartilage precipitate loss of proteoglycan and damage to the collagen framework of the articular cartilage. HD occurs in most breeds, with a higher prevalence in large working breeds such as German Shepherds and Labradors. In contrast, the prevalence in greyhounds is very low.
Presenting signs
Dogs with HD may be presented at a young age (6-12 months) due to hip laxity causing gait abnormality or pain associated with joint injury. Alternatively they may be presented after maturity and into middle age due to secondary osteoarthritis (OA) and its associated pain and lameness. However, many dogs with severe radiographic signs of HD will show minimal if any clinical signs during the stage of skeletal growth and indeed well into maturity. Similarly, the mature dog with severe OA secondary to HD may never experience significant disability or clinical signs of lameness, stiffness or joint pain. This is not just a feature of OA associated with HD, as the poor correlation between the degree of joint pathology of OA of any joint and clinical disability in that joint is well recognized. In such cases, the patient has apparently been able to successfully adjust to the alterations in joint lubrication, and range of movement of both joints and muscles, thus maintaining a normal or near normal level of activity and function. Such compensations presumably occur in some young dogs which can exhibit signs of difficulty rising, gait abnormality, poor exercise tolerance and hip pain for a short period followed by a spontaneous remission of these signs and a return to normal function without veterinary intervention.
This poor correlation between radiographic signs, joint pathology and clinical function, and the fact that spontaneous improvement in clinical function does occur in some case of HD must be considered when making decisions regarding the most appropriate treatment for any clinical case of HD.
Factors affecting the Stability/Laxity of the Canine Hip Joint
The normal canine hip joint is a deep ball and socket joint, the stability of which is governed by the following factors:
Conformation of the bony elements, with the dorsal acetabular rim providing critical support during weight bearing
The balance of muscle forces acting across the hip joint
The joint capsule
Ligamentum teres
Synovial fluid
The importance of the bony conformation is emphasized by the presence of only one ligament supporting the hip joint, in contract to the stifle joint which has many important ligamentous supports. Poor development of this bony structure as occurs in hip dysplasia obviously has important consequences for joint function and maintenance.
Hip joint laxity has long been associated with hip dysplasia. The more recent development of methods for objectively evaluating hip laxity (such as the PennHIP method) has allowed us to accurately measure what is referred to as "passive hip laxity." Passive laxity is what is assessed by the commonly used clinical palpation tests such as the Bardens Hip lift test and the Ortolani Sign. The relationship between passive laxity and functional laxity (i.e., the laxity occurring during normal activity) remains unclear but hip assessment schemes such as Penn HIP have increased our ability to predict in the young dog the probability that significant hip dysplasia will develop in a particular individual.
Assessment of the Dysplastic Dog
Palpation for hip laxity-The Ortolani Sign
In the dog that is presented for hindlimb lameness or gait abnormality at 6-12 months of age, determination of the degree of hip laxity is very important. The first test I use is to put one hand lightly over each of the greater trochanters while the dog is walking and in some cases hip subluxation can be readily detected as clicking or clunking as the dog walks. The significant factor here is that this test is performed in the most normal physiological position and while the dog is walking so any laxity is likely to be highly significant as it is functional laxity, not passive laxity..
The hip lift test is performed with the dog under general anesthesia and in lateral recumbency. One hand is placed on the medial side of the femoral shaft with the fingers extended as far as possible up into the groin area. While the other hand is placed lightly on the greater trochanter, the first hand attempts to luxate the head of the femur laterally out of the acetabulum. This test has also been referred to as the Bardens test, with a positive result recorded if the trochanter can be elevated more than 5-6mm.
The Ortolani Sign can be performed in the conscious dog but as with the hip lift it is best performed under anesthesia in the clinically affected dog. The Ortolani Sign can be elicited in either dorsal or lateral recumbency by holding the femur in the neutral position for adduction/abduction and compressing the hip joint by applying pressure to the flexed stifle joint. When performed in lateral recumbency, the other hand is placed over the dorsal aspect of the pelvis to provide a counter force. The table provides this force when the test is performed in dorsal recumbency. If joint laxity is present, the head of the femur will ride up onto the dorsal rim of the acetabulum and then as the limb is abducted it will fall back into the acetabular fossa with a palpable and sometimes audible "clunk." The test can be extended further to provide a more objective measurement of joint laxity by measuring the angle of reduction and the angle of subluxation as described by Slocum. With the dog in dorsal recumbency, the angle of reduction is the angle of the femur from the vertical plane at the point where the femoral head relocates into the acetabulum during abduction. The angle of subluxation is now calculated by reversing the procedure. As the limb is adducted a point is reached at which the femoral head subluxates again and the angle between the femur and the vertical plane is again measured. The angle of subluxation is a measure of the minimal angle necessary for joint stability. The angle of reduction is the maximal angle for rotation of the pelvis if a pelvic osteotomy is planned.
The nature of the "clunk" associated with subluxation and reduction of the femoral head can give additional information about the hip joint conformation. A "soft clunk" on reduction suggests that the dorsal acetabular rim may have been damaged by repeated subluxation during weight bearing, involving microfractures and flattening of the dorsal rim. Damage to the dorsal acetabular rim is highly significant as weight bearing is concentrated in this area of the acetabulum.
It must be remembered that although hip joint laxity is an integral part of hip dysplasia, not all dysplastic dogs will have palpable hip laxity. This is particularly true in the older dog as secondary changes of OA including joint capsule thickening and osteophytosis will act to reduce joint instability. Therefore a negative hip lift test or Ortolani Sign does not exclude the diagnosis of hip dysplasia.
The role of radiological examination in decision making for treatment of hip dysplasia
The standard ventrodorsal extended position is utilized by most hip scoring schemes which aim to control hip dysplasia by genetic selection. This position has recently been criticized because it is nonphysiological-in other words a dog never uses its legs in the position that has been adopted to radiographically examine the hip joints. This view is the best for assessment of the extent of OA in the older HD patient.
The dorsal acetabular rim (DAR) radiograph is an alternative radiographic position for use in young dogs and is performed by positioning the dog in sternal recumbency with the hindlimbs extended forwards parallel to the trunk. The pelvis is aligned vertically so that the x-ray beam passes through the shaft of the ilium. In a normal dog, the acetabulum is angled so that its dorsal rim is seen as a projecting rim and the edge is silhouetted against the soft tissues. In many dysplastic dogs the acetabulum is not angled correctly and is more parallel to the plane of the x-ray beam so that the dorsal acetabular rim does not project out from the line of the iliac shaft. I find the DAR view particularly useful if I am considering a triple pelvic osteotomy, particularly when coupled with the result of palpation for hip laxity such as the Ortolani sign.
Radiographic techniques which focus on the demonstration and assessment of hip laxity have been growing in popularity over the past decade. The most widely used and recognized technique is the PennHIP scheme (University of Pennsylvania Hip Improvement Program). This technique requires the operator to be trained on the use of the program and a numerically identified distractor device is used to assess passive hip laxity. Three radiographs are taken, one being the standard ventrodorsal view as described above and the other two are taken with the hips and stifles flexed in a near standing position. One of these views is taken with the femoral heads forced deeply into the acetabulum and the other with the distracting device forcing the femoral heads as laterally as possible to maximize laxity. Assessment of films is performed centrally in the USA and a measure of hip laxity called the Distraction Index is calculated. The DI correlates closely with the chance of developing OA in the hip joints. One significant advantage of the PennHIP scheme is that pups can be assessed at 4-5 months of age instead of delaying evaluation for hip dysplasia until 12-24 months.
Treatment of Hip Dysplasia in Young Dogs
Non-surgical or conservative treatment for hip dysplasia has been reviewed by Johnston (Vet Clinics Nth America 22:595-606, 1992). Both active physiotherapy with controlled exercise as well as strict confinement have been advocated for young dogs designated as at risk for HD, albeit for very different reasons. Controlled exercise is aimed at increasing the muscle mass and therefore the soft tissue support for the joint. Confinement to a 1 m3 cage which encouraged the young pup to sit with its hindlimbs spread was shown to reduce the incidence of HD in genetically susceptible pups. Problems with socialization complicate this form of therapy.
Apart from the above, non-surgical treatment in the young HD patient is aimed primarily at pain relief by the use of NSAIDs. The chondroprotective agents such as pentosan polysulphate (Cartrophen Vet) and polysulphated glycosaminoglycan (Adequan), may have a place in reducing the clinical signs of HD in young dogs. One report claimed that pups injected intramuscularly once per week with PSGAG from 6 weeks to 8 months of age had better hip conformation and closer coxofemoral congruity than a similar group injected with saline (Lust G et al Am J Vet Res 53: 1836-1843). This study did not involve long term follow up of the dogs to see if there was any difference in the severity of hip dysplasia.
Triple pelvic osteotomy (TPO)
In the young dog, a strong argument can be made for performing a TPO because it offers the best chance for a well-supported femoral head, minimal subluxation during vigorous exercise and is likely to slow the progression of OA, resulting in a longer satisfactory working life. There are few reports of long term follow up of TPO cases. Early function appears to be good, and many references state that radiographic evidence of OA is greatly reduced or minimal. However, concrete evidence based on large numbers of cases with long term follow up (>5 years) is lacking.
Complications and results of TPO will be addressed by others at this symposium.
Non-surgical treatment of Hip Dysplasia in Mature Dogs with OA
In these cases, non-surgical treatment is aimed primarily at pain relief, usually by the use of NSAIDs. The place of chondroprotective or "disease modifying" drugs such as pentosan polysulphate (Cartophen Vet) and the various food additive compounds (Cartiflex Dog and Cosequin)as treatments for end stage OA in HD cases is a topic of significant current interest. A scientifically constructed double-blind dose response study was conducted in Western Australia using dogs presenting with hindlimb stiffness/weakness due to chronic hip dysplasia and other osteoarthritic conditions of the hindlimbs (Read et al JSAP 37: 108-114, 1996). Dogs were treated once weekly for 4 weeks by subcutaneous injection at dose rates of 0, 1, 3 or 5 mg/kg depending on treatment group. Clinical responses were assessed weekly and again 4 weeks after treatment ceased. The 3 mg/kg group showed significant improvement in the dogs ability to exercise, willingness to "get up and go" and levels of joint pain and stiffness. However, these dogs were predominantly pets, not working or athletic dogs.
NSAIDs remain an important tool in the treatment of many dogs with OA associated with HD. The recent discovery that there are two forms of cyclo-oxygenase (COX) produced in the body has helped our understanding of the variation in incidence of side effects with the various NSAIDs. Carprofen (Rimadyl-Pfizer) and meloxicam (Metacam-Boerhinger Ingelheim) both offer excellent long term medical management of OA, without significant side effects in most cases. This form of therapy may even be sufficient to permit some working and athletic dogs with HD and associated OA to return to relatively normal active work.
Results of Treatment of Hip dysplasia
The results of conservative treatment are often anecdotal reports of individual cases. Barr et al (JSAP 28: 243-252, 1987) reviewed 68 cases in immature dogs and claimed 76% of the dogs had a satisfactory recovery and were adjudged to have acceptable long term function. However, the majority of dogs in this study were not working dogs. Nineteen dogs were presented for repeat physical and radiographic examination and 84% showed moderate to severe subluxation with 79% showing moderate to severe periarticular new bone formation. Despite this, 17 of the 19 showed minimal or no gait abnormality and only 7 dogs showed pain on forced extension of the hips. These results emphasize again the poor correlation between the radiographic changes of OA and joint function. The results of the study by Planté (see above) support the view that conservative management offers satisfactory functional results in a significant number of patients.
In a separate study, the progression of HD and its effect on the length and quality of working life was examined in 116 military working dogs. No difference was found in the length or quality of effective working life for dysplastic and non-dysplastic dogs (Banfield C et al, J Amer Anim Hosp Assoc 32:423-430, 1996). A small number of these dogs were treated by FHO and function was not as good in these dogs.
Conclusions
Although dogs with HD may require intermittent or constant therapy to control the signs associated with hip OA, a reasonable prognosis can be given for a useful active life, with or without surgical intervention.