CANINE-Pathophysiology,Diagnosis and Treatment of Canine Hip Dysplasia

Vol.18, No. 8

August 1996

Continuing Education Article

FOCAL POINT ★ Accurate diagnosis, treatment, and prognosis of patients with canine hip dysplasia depends on comprehensive evaluation of affected dogs.

KEY FACTS ■ Dogs that are affected with hip dysplasia are genetically predisposed to developing a biomechanical imbalance between muscle mass and skeletal stresses on the hip joint. ■ The disparity between soft tissue strength and biomechanical forces during skeletal growth causes a loss of congruity between the articular surfaces of the acetabulum and the femoral head. ■ A comprehensive evaluation includes signalment, history, general physical examination, orthopedic examination, neurologic examination, and radiography. ■ The goals of treatment are to alleviate pain, to arrest secondary degenerative changes, and to maximize joint function.

Pathophysiology, Diagnosis, and Treatment of Canine Hip Dysplasia University of Missouri

James L. Cook, DVM James L. Tomlinson, DVM, MVSc Gheorghe M. Constantinescu, DVM, PhD, Drhc

C

anine hip dysplasia (CHD) is a common developmental disease of the coxofemoral joint of dogs.1–6 The disease affects 1.8% to 48.1% of dogs of a given breed.4 Heritability indexes (which measure the percentage of phenotypic variation attributable to genetics) also vary greatly among breeds. Canine hip dysplasia is most frequently reported in large and giant canine breeds but can affect any breed. The disease most commonly occurs bilaterally; unilateral disease reportedly ranges from 3% to 30%, depending on the breed.4 Although many causes have been proposed, a definitive cause has not been established. A genetic predisposition involving a polygenic mode of inheritance has been reported.1–5 Affected dogs are predisposed to developing a biomechanical imbalance between muscle mass and skeletal stresses on the hip joint.2,5 The imbalance leads to laxity, subluxation, and degenerative joint disease (DJD). Nutrition and rapid growth rate are important factors that contribute to the disparity between muscle mass and skeletal stresses.1–3 Other proposed causal factors include hormonal influences; collagen, muscle fiber, ligament and nerve abnormalities; and abnormal synoviocyte ratios.1–3,6 These factors are probable components of the polygenic basis of the disease. The multifactorial nature of CHD can confuse client education and management of the disease. The basic concept involved is the biomechanical imbalance between the forces on the coxofemoral joint and the associated muscle mass; the result is joint laxity in young, growing dogs. This laxity leads to incongruity; the eventual result is DJD. This article considers the pathophysiology and treatment options for patients with CHD and provides a framework for decision making and prognosis for various categories of patients with the disease.

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PATHOPHYSIOLOGY The coxofemoral joint is normal at birth and begins as a congruent unit.2,5 Genetically susceptible dogs become dysplastic when the primary muscle mass that supports the joint fails to mature at the same rate as the skeletal structures.2,5,7,8 The resultant disparity between soft tissue strength and biomechanical forces during skeletal growth is manifested as a loss of congruity between the articular surfaces of the acetabulum and the femoral head.2,5 This incongruity results in the instability that leads to DJD. The most important time in the development of the canine coxofemoral joint is the first 60 days of life, when the periarticular soft tissues are immature.2 During this period, the muscles and nerves are functionally limited. If the stresses on the hip from weight bearing and activity exceed the strength of the supporting soft tissue, the articular surfaces are forced apart. If the elastic limit of the soft tissue is exceeded, the laxity results in irreversible incongruity.2 The incongruity in the joint results in abnormal joint motion (instability). The abnormal motion produces remodeling of the joint and joint effusion, both of which result in further incongruity and laxity due to articular cartilage changes and loss of hydrostatic pressure.2,9–17 Overloading of the hip can begin as soon as a puppy begins to push itself to nurse and can continue into the later developmental stages, when additional stresses exacerbate the incongruity and lead to further remodeling.2 The dysplastic changes evident in CHD apparently correlate with the severity and duration of the biomechanical overloading.2,7,8 If congruity is maintained until the soft tissue is mature or is restored before irreversible change, nerve function and muscle mass are sufficient to maintain congruity of the joint.2 When the patient is approximately 6 months of age (i.e.,

Figure 1—Schematic illustration of normal, increased, and decreased angles (from left to right) of inclination. An increased angle results in greater stress on the coxofemoral joint.

Figure 2—Schematic illustration of normal (left) and increased (right) angles of an-

teversion. Increased anteversion results in greater stress on the coxofemoral joint.

Figure 3—Schematic illustration of femoral head subluxation (arrows) as the result

of a shallow acetabulum.

GENETIC SUSCEPTIBILITY ■ COXOFEMORAL JOINT ■ DYSPLASTIC CHANGES

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when 90% of the ossification process is complete), changes in joint shape can only be accomplished by the production or resorption of bone.2

BIOMECHANICS Normal weight bearing through the coxofemoral joint is transmitted through the shaft of the femur, the femoral neck and head, the acetabulum, the ilium, and the sacrum. The load placed on the coxofemoral joint is determined by body weight, conformation, and activity.6 When a dog is standing, 30% to 40% of the body weight is distributed to the pelvic limbs.6,9 During activity, however, the coxofemoral joint load may approach three times body weight.6 The amount of force placed on the joint is influenced by the femoral neck–shaft angle of inclination, the length of the femoral neck, the position of the greater trochanter, and the distance of the femoral head from the body’s center of gravity.6 Increased stress on the joint occurs with an increased angle of inclination (Figure 1), an increased angle of anteversion (Figure 2), a relatively short femoral neck, and distal or medial displacement of the greater trochanter.6 Increased stress on the joint potentiates laxity, incongruity, and DJD. In addition, if the craniodorsal aspect of the acetabulum does not meet the force vector of the femoral head perpendicularly (because of slanting or incomplete coverage), subluxation results6 (Figure 3).

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nized.2,10 Dogs 4 to 12 months of age often present with a sudden onset of clinical signs.10 These dogs exhibit a sudden decrease in activity in conjunction with pain and/or lameness in the pelvic limbs.10 The cause of the pain and lameness has been attributed to joint effusion, tearing or stretching of the round ligament, synovitis, acetabular microfractures, and loss of articular cartilage.10,18–49 Most of these patients have a positive Ortolani sign in the affected joints and atrophy of the associated pelvic muscle mass.7,10 In older dogs, clinical signs result from degenerative changes in the joint.10 Signs are usually insidious but may present suddenly as the result of acute trauma to the abnormal tissue.10 Older dogs with CHD are often bilaterally lame, especially after exercise; joint crepitus, restricted range of motion (especially extension), and muscle atrophy of the hindlimbs may be present.10 A positive Ortolani sign is rare.7,10 In both groups of dogs, clinical signs may include varying degrees of lameness, so-called bunny hopping, difficult rising, and abnormal stance or gait.

Palpation Techniques Bardens’ method11 of coxofemoral joint palpation in puppies at 6 to 8 weeks of age is reportedly accurate in predicting CHD in commonly affected breeds.12 The

DIAGNOSIS Dogs that have clinical signs attributable to CHD are commonly presented to veterinary practitioners for evaluation. Dysplastic dogs without clinically evident abnormalities may be evaluated during routine physical examinations. A comprehensive evaluation is essential for accurate diagnosis, treatment, and prognosis. Other causes of hindlimb lameness (e.g., cruciate ligament rupture, patellar luxation, osteochondrosis, and trauma) must be ruled out. Evaluation should include signalment, history, complete general physical examination, complete orthopedic examination, neurologic examination, and radiography.1,3,7 Signalment and History Figure 4—Schematic illustration of hand placement for eliciting of the Bardens’ Two distinct groups of dogs with sign. CHD have been clinically recogNORMAL WEIGHT BEARING ■ INCREASED JOINT STRESS ■ CREPITUS

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tion or general anesthesia may be required to facilitate palpation, we do not routinely recommend sedation or anesthesia of puppies in order to perform this procedure. The Ortolani sign was first reported in human infants in 193713 and is used in veterinary medicine to diagnose coxofemoral laxity. Although the technique can be performed on an awake patient or after sedation, accurate interpretation of joint laxity can be diminished. For the best results, the animal should be anesthetized deeply enough to cause loss of the palpebral reflex. 7 The technique can be performed with the paFigure 5— Schematic illustration of proper positioning for palpation of the tient in lateral or dorsal recumbency. Ortolani sign. With the dog in dorsal recumbency, the veterinarian stands behind the animal and grasps each stifle firmly (with the femurs perpendicular to the surface of the examination table). Pressure is applied down the shaft of the femur toward the acetabulum. Each femur is individually abducted to its limit. In patients with CHD, downward pressure on the femur causes dorsal subluxation of the femoral head. When the limb is abducted, an audible or palpable click is elicited as the subluxated femoral head is reduced.7 With the patient in right lateral recumbency, the examiner stands caudal to the dog and grasps the left stifle with the left hand. The open palm of the right hand is placed against the dorsum of the pelvis, with the thumb resting on the Figure 6— Schematic illustration of the angles of reduction and subluxation greater trochanter. The left femur is (arrows) of a dysplastic coxofemoral joint. positioned so that it is perpendicular to the long axis of the pelvis and parpuppy is placed in lateral recumbency. Using one hand, allel to the table surface. Pressure is applied to the shaft the thumb is placed on the ischiatic tuberosity, the of the femur proximally toward the acetabulum while middle finger on the dorsal iliac spine, and the index the pelvis is supported with the right hand. The left finger on the greater trochanter. The opposite hand is femur is then abducted (Figure 5). As in the previous used to attempt to pull the femoral head out of the actechnique, hips that are subluxated elicit a click during etabulum by lifting the femur laterally (Figure 4). The abduction.7 A palpable or audible click during this technique is amount of laxity is estimated by the amount of moveknown as a positive Ortolani sign.7 False-negative rement of the index finger away from the acetabulum. A sults may result from positioning or technique errors, more objective measurement can be made using this inadequate depth of anesthesia, excessive patient size technique and a simple lever device.12 Although sedaANGLES OF REDUCTION AND SUBLUXATION ■ ORTOLANI SIGN

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(prohibiting effective palpation), gross destruction of the dorsal acetabular rim, a thickened fibrotic joint capsule, proliferation of osteophytes, limited range of motion, or fixed luxation of the femoral head.7 Using the Ortolani technique, the point of femoral head subluxation can be distinguished as proximal pressure is applied to the femur. The angle of the femur from the sagittal plane at the point of initial femoral head subluxation is the angle of subluxation. 10 The angle of the femur from the sagittal plane at the point of reduction of the femoral head in the acetabulum is the angle of reduction10 (Figure 6). These angles are useful if triple pelvic osteotomy is considered as a treatment option.7,10,14

Radiography The most commonly used radiographic grading system for CHD was established by the Orthopedic Foundation for Animals (OFA). One function of the OFA is to provide a service for the diagnosis and registry of hip status for all canine breeds.15 Sedation or anesthesia is recommended. With the patient in dorsal recumbency, the femurs are extended parallel to each other and to the film cassette. The patellae are centered over the trochlear grooves of the femurs.16 A single ventrodorsal radiograph of the pelvis and femurs is obtained for evaluation.16 Proper positioning and technique are essential to correct interpretation of the radiographic severity of the disease.3,10,16 The coxofemoral joints are evaluated radiographically for the following factors10,16: ■ Congruity of the femoral head and the acetabular margin ■ Amount of coverage of the femoral head by the acetabular rim, as defined by the intersection of the femoral head physeal scar with the dorsal acetabular rim (At least 50% of the femoral head should be covered by the acetabulum.) ■ Remodeling and flattening of the femoral head (The femoral head should approximate a hemicircle.)

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Grading Criteria for Radiography of Canine Coxofemoral Joints ■ Excellent—nearly perfect conformation ■ Good—normal conformation for age and breed ■ Fair—less than ideal, but within normal radiographic limits ■ Near normal— borderline; minor hip abnormalities often cannot be clearly assessed because of poor positioning during radiographic procedures ■ Mild dysplasia— minimal deviation from normal with slight flattening of the femoral head and minor subluxation ■ Moderate dysplasia— obvious deviation from normal with evidence of shallow acetabulum, flattened femoral head, poor joint congruity, and (in some cases) subluxation with marked changes of the femoral head and neck ■ Severe dysplasia— complete dislocation of the hip and severe flattening of the acetabulum and femoral head

Based on these criteria, the coxofemoral joints are given one of seven grades15 (see the box). In German shepherds, evaluation of CHD by the OFA radiographic method reportedly has a reliability for correct diagnosis of 69.9% at 12 months, 82.7% at 18 months, and 95.4% at 24 months.15 After radiographic evaluation by three boardcertified OFA radiologists, a dog can be registered with the OFA if it is at least 2 years of age and the radiographs are appropriately identified.15 Another radiographic evaluation technique for CHD is the University of Pennsylvania Hip Improvement Program (PennHIP). The PennHIP technique is a stress-radiographic method intended to provide a quantitative means of determining laxity before the dog is 24 months of age.17 By correlating joint laxity with subsequent incidence and severity of CHD, this method can provide optimum predictive value.17–19 The stress-radiographic method requires deep sedation or general anesthesia.17 The patient is positioned in dorsal recumbency, with the coxofemoral joints in a neutral flexion–extension angle to allow maximum lateral displacement of the femoral head.17 The neutral positioning avoids spiral tensioning of the fibrous elements of the joint capsule and hydrostatic influences that decrease hip laxity.17 A compression view and a distraction view are obtained via the PennHIP compression–distraction device. The distance between the center of the acetabulum and the center of the femoral head is measured on both views via templates or gauges. This distance is divided by the radius of the femoral head; a numerical value between 0 and 1 (the distraction index) is determined.17 This index quantitates the relative displacement of the femoral head center from the acetabular center.17 In addition, the compression view delineates the articular surfaces and provides information concerning acetabular depth, thickness of articular cartilage, and the center of rotation of the joint.17 The distraction index quantitates passive joint laxity. Dogs with an index less than 0.3 rarely develop radiographic evi-

DORSAL ACETABULAR RIM DESTRUCTION ■ DISTRACTION INDEX

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dence of DJD.18 Hips with a distraction index greater than 0.3 are considered to be susceptible to DJD.18 In some breeds, an increase in the distraction index has been correlated with an increased incidence of DJD associated with CHD.18,19 Not all dogs with passive laxity (an index greater than 0.3) eventually develop DJD, however, and results should be interpreted with caution. The PennHIP method shows promise for detecting susceptibility to CHD in dogs as early as 16 weeks of age.17 The method is designed to minimize radiographic artifacts caused by positioning and to allow dogto-dog comparison of hip joint laxity. 17 To use the PennHIP system in practice, a veterinarian must be certified at a 1-day training session and must have a compression–distraction device, a 300-mA radiographic unit, and an automatic film processor. The dorsal acetabular rim (DAR) radiographic view was developed to image the rim using standard radiographic equipment and technique.20 The DAR technique was designed to address the failure of standard ventrodorsal and lateral radiographic techniques to isolate the weight-bearing portion of the acetabulum.20 The value of DAR radiographs is in evaluating the rim for damage and secondary osteoarthritic changes, correlating palpable joint laxity with observable radiographic findings, and displaying acetabular filling. This technique has been proposed to provide an objective, reproducible method for determining whether a coxofemoral joint is normal, dysplastic, or injured by trauma.20 The DAR view is obtained with an anesthetized patient in sternal recumbency. The pelvic limbs are pulled cranially so that the femurs are parallel with the long axis of the torso.20 A restraining device is placed around the thighs and back to align the femurs close to the body.20 The tibias are angled 120˚ to the femurs, and the hips are internally rotated 45˚ to prevent the greater trochanter from interfering with visualization of the dorsal acetabular rim.20 A 2-inch spacer is placed between the calcaneal tuber and the examination table to increase the tension in the hamstring muscles in order to pull the ischiatic tuberosity cranially with respect to the tuber sacrale.20 This causes the pelvis to be aligned vertically so that the radiographic beam passes through the shaft of the ilium.20 After the DAR view is obtained, lines can be drawn tangential to the point of lateral contact between the femoral head and the dorsal acetabular rim on both hips. In dogs with normal hips, the angle at the point of intersection of the lines is 165˚ to 180˚.20 In dogs with hip dysplasia, the angle is reportedly less than 165˚20; however, there is no reported correlation between this angle and the development of DJD.

The DAR radiographic view accurately confirms palpation findings of damage to the weight-bearing portion of the acetabulum.20 The view is used to identify dorsal acetabular rim osteophytes, acetabular filling, and the lateral extent of the acetabulum and resultant dorsal coverage of the femoral head.20,21 The DAR radiographic view has been recommended as part of the evaluation process for patients with CHD and as an aid in determining disease, treatment options, and prognosis.20,21

TREATMENT By the time of diagnosis, the pathologic changes of CHD are often irreversible. No treatment is effective in restoring a dysplastic joint to a completely normal coxofemoral joint. The aims of treatment are alleviating pain, arresting secondary degenerative changes, and producing maximum joint function. Decision making in treating a patient with CHD should be based on the age and health of the dog, the clinical severity of the disease, the radiographic appearance of the joint, the intended function of the dog, and the financial constraints of the owner. Medical Therapy Medical therapy for patients with CHD involves controlled exercise, weight control, antiinflammatory agents, and analgesics. Medical therapy is oriented toward alleviating pain and slowing the deterioration of the coxofemoral joint. Medical therapy does not correct the problems associated with CHD or arrest the progression of disease. Such therapy thus should be reserved for patients with mild or intermittent clinical signs. Medical therapy may be effective as palliative treatment for young dogs before total hip replacement and for geriatric patients with minimal activity levels. Monetary considerations may prompt the client to choose medical therapy alone. Activity should be reduced to a level that the patient can tolerate without signs of pain or exercise intolerance.22 Exercise should initially be restricted to short leash walks, then gradually increased to the maximum level of function that does not cause pain or lameness.22 Non–weight-bearing activities (e.g., swimming) are an excellent means of exercise and avoid concussive trauma to the joints. Strict cage rest has been recommended as preventive therapy for dogs predisposed to or affected by CHD.5 Severe exercise restriction can result in increased exercise intolerance, loss of muscle mass and tone, decreased range of motion, exacerbation of cartilage destruction, and social maladjustment.5,22 The patient’s weight must be maintained in the optimum range for the age and breed. Weight loss is vital in

ACETABULAR FILLING ■ PALPATION FINDINGS ■ STRICT CAGE REST

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obese patients. Diet control can be accomplished via foods specifically formulated for this purpose (reducing diets or low-calorie diets) or via reduced portions of a maintenance-type dog food.22 Protocols for weight control have been outlined.22,50 Conservative therapy, consisting of exercise restriction and weight control, is effective in controlling pain and lameness in a significant number of dogs with CHD.51 Antiinflammatory analgesics should be considered for adjunctive therapy in medically managed patients. Many protocols have been suggested.2,3,10,22–26,52,53 Oral aspirin (10 to 25 mg/kg every 8 to 12 hours), 3,10,22 phenylbutazone (1 to 5 mg/kg divided every 8 hours),3,10,22 meclofenamic acid (1.1 mg/kg every 24 hours),3,22 and carprofen (2 mg/kg every 12 hours)52,53 are reportedly effective. Aspirin is effective in most cases and is easily accessible and inexpensive. Buffered or enteric-coated products are recommended to minimize the gastric irritation and ulceration that can be a common side effect of these medications.22 Recent reports suggest that polysulfated glycosaminoglycan (4.4 mg/kg every 3 to 5 days for eight intramuscular injections) has beneficial antiinflammatory effects, but no statistically significant results have been reported in severe cases of CHD.23–26 Corticosteroids can be used, but care is necessary to avoid immunosuppression, adrenal suppression, and exacerbation of cartilage damage. Hyaluronate and glucosamine– chondroitin salts have been promoted for medical management of DJD.54,55 To date, no controlled studies have reported use of these products to treat CHD.

Pectineal Myectomy Pectineal myectomy is a controversial treatment for patients with CHD. The procedure, which does not correct the disease or arrest its progression,10,27,28 was originally proposed to relieve muscle spasm and the associated pain.28 The technique is reportedly effective in relieving pain in some cases.2,3,10,27,28 The pain relief has been attributed to release of muscle tension, reduced stress on the joint capsule, and increased abduction resulting in articular alterations. Myectomy is preferred to myotomy in preventing postoperative fibrosis and contracture.3,10 The procedure is usually performed bilaterally. Postoperative care should include moderate exercise 2 to 3 days after surgery.10 Complications associated with pectineal myectomy include postoperative hematoma formation and the possibility of fibrous bands forming at the excision site.3,10,27,28 Furthermore, pectineal myectomy may be ineffective in controlling the pain associated with CHD.3,10,27,28 The technique is no longer widely advocated for treating patients with CHD.

Triple Pelvic Osteotomy If the biomechanical imbalance in a dysplastic hip is corrected early in the progression of CHD, the coxofemoral joint can return to normal function.3,29,30 Triple pelvic osteotomy (TPO) is a surgical procedure designed for this purpose. Ideally, correction takes place before complete skeletal maturity and before secondary degenerative changes occur.56 The goals of TPO are correction of femoral head subluxation and restoration of the coxofemoral weight-bearing surface area.31–35 In our opinion, the ideal candidate for this procedure is a young dog (