Diagnosing hip dysplasia (Proceedings)

Hip dysplasia is the most common developmental orthopedic disease in dogs. First described in the 1930's, it continues to affect millions of dogs worldwide. Large breed dogs are most commonly diagnosed, however small breed dogs and cats also are affected. The etiology is multifactorial, having both genetic and environmental components. Screening for hip dysplasia in young dogs is important for treating affected individuals as well as for making breeding recommendations for owners.

Pathophysiology

Simplistically, the pathophysiology of hip dysplasia can be described as joint laxity leading to degenerative joint disease. While various etiologies have been proposed, it is generally accepted that coxofemoral joint laxity is a principle component of hip dysplasia at an early stage. Polygenic and environmental factors contribute in a complex manner to cause laxity via morphologic changes such as joint incongruity or abnormal pelvic musculature, joint capsule, or round ligament.

In turn, laxity leads simultaneously to subluxation, inflammation, and periarticular new bone formation. Subluxation results in joint capsule stretching, which causes the initial pain and lameness associated with hip dysplasia in young animals. Repeated subluxation causes cartilage fibrillation and destruction, remodeling of the femoral head and acetabulum, and periarticular new bone formation. Concurrently, inflammation results in an increase in joint fluid volume, contributing further to incongruity of the joint. It also causes the production of cytokines that contribute to cartilage degradation. Inflammation, cartilage degradation, and joint remodeling are hallmarks of degenerative joint disease. It is these changes that are responsible for the chronic pain and lameness seen with progressive hip dysplasia. Virtually all animals progress from laxity to degenerative joint disease. The individual variability is in the rate of progression and the degree of clinical signs exhibited by the patient.

Presentation and signalment

Hip dysplasia most commonly affects large breed dogs. However, it is increasingly being diagnosed in small dogs and cats. Affected animals may present with a variety of clinical signs, including: a bunny hopping gait, difficulty rising, jumping or stair climbing, exercise intolerance, behavior change, or unilateral lameness. In particular, older dogs with progressive disease may be more likely to exhibit stiffness in the pelvic limbs, lameness that worsens with exercise, or muscle atrophy.

A bimodal age distribution for affected animals has been described. Young dogs less than one year of age present with pain and lameness that is most likely due to joint laxity, subluxation, and synovitis. Older dogs present with dysfunction that is more often due to progressive degenerative joint disease. For the veterinarian discussing treatment options, the important distinction to make is between young animals with joint laxity but no degenerative changes, and animals of any age with degenerative joint disease. These two groups have distinctly different options for surgical management.

Physical examination

Gait evaluation is useful for assessing changes in locomotion in patients compensating for hind limb discomfort or dysfunction. Initially, it is important to confirm that the lameness is of orthopedic origin and not neurologic. Patients may exhibit a variety of gait abnormalities, including unilateral lameness, bunny hopping, or pelvic/spinal swaying, and a shortened stride. All of these adaptations are an attempt to limit hip joint excursion, especially in extension. When standing, affected dogs may appear to shift weight off of their hind limbs and onto their front limbs. To accomplish this, some animals may stand with their stifles and hocks more extended than normal, creating a somewhat 'straight-legged' appearance.

Palpation of the hip joint is usually best accomplished with the patient in lateral recumbency. Examination should begin with the foot of the limb in question, working proximally toward the hip joint. To assess for coxofemoral luxation, the pelvic landmarks of the ischial tuberosity, iliac wing, and greater trochanter are palpated. These points should form a triangle, with the point of the greater trochanter lying below the line connecting the ischium and iliac wing. The hip should be put through a limited range of motion and palpated for crepitus before assessing full range of motion. The normal hip should allow the femur to be parallel with the spine during full flexion and should allow 90 of external rotation, 45 of internal rotation, and ~180 of extension. Dogs with hip dysplasia are most often limited in their extension of the hip joint. Extension/abduction is a particularly uncomfortable position for these dogs, and the one most likely to elicit a response. Other than discomfort in extension/abduction, dogs with hip dysplasia may palpate as having a normal coxofemoral joint.

Coxofemoral laxity is the hallmark of hip dysplasia in young dogs. This laxity can be assessed clinically by several exams-the Bardens, Barlow, and Ortolani tests. Of these, the Ortolani test is the most widely used, and is relatively easy to perform. It can be done with the dog in lateral or dorsal recumbency. With the stifle at 90, force is applied along the long axis of the femur. This will cause subluxation in a dog with joint laxity. While maintaining this force, the limb is abducted until the femoral head is felt to reduce into the acetabulum. The veterinarian can evaluate the subjective quality of the 'crispness' of the reduction. An abrupt 'clunk' is thought to indicate a relatively normal acetabular rim and acetabular depth. In contrast, a more gradual sliding reduction may indicate wear of the acetabular rim or a shallow acetabulum. The Ortolani sign may be difficult to appreciate in fully awake dogs, as muscle tone can inhibit subluxation of the joint or abduction of the limb. In order to be fully confident that the test is negative, it should be performed under sedation.

It is important to remember that the periarticular fibrosis and acetabular remodeling seen with advancing hip dysplasia will eliminate the ability to palpate an Ortolani sign in dogs with joint laxity. In addition, while a positive Ortolani test is abnormal and is an indicator of hip dysplasia, it does not correlate with clinical signs, nor can it be used to predict development of degenerative joint disease. It should be used in combination with other physical exam findings to recommend treatment options.

Diagnostic imaging

Radiographs are an essential tool for diagnosing and planning treatment for hip dysplasia. While the physical exam may raise index of suspicion or even confirm joint laxity, radiographs can be used to objectively evaluate hip conformation, degenerative changes, and even joint laxity. Radiographs are also important to rule out other causes of hind limb lameness. Several different radiographic examinations are used for this purpose-the ventrodorsal extended hip view, the PennHIP distraction view, the dorsal acetabular rim view, and the dorsolateral subluxation score.

The ventrodorsal extended hip view is the oldest of the currently used radiographic exams, and it remains the most widespread in its use. The positioning is repeatable, it does not require specialized equipment, and it is inexpensive to perform. However, this exam does require precise patient positioning, so sedation of the patient and technical expertise are necessary. Once sedated, the patient should be positioned in dorsal recumbency, with the pelvic limbs retracted caudally in maximal extension. The femora should be internally rotated so that the patellae are centered over their corresponding femoral condyles. The X-ray beam is directed ventrodorsally to include the region from the caudal lumbar spine to the distal femur.

In the young dog with hip dysplasia, the earliest abnormality visible on ventrodorsal extended hip view is subluxation of the femoral head. Subluxation is defined as less than 50% coverage of the femoral head by the dorsal acetabular rim. With persistent instability and joint capsule stretching, a mineralized insertion of the joint capsule may become apparent lateral to the femoral head. This is referred to as the "Morgan's line". As laxity progresses to degenerative joint disease, remodeling of the joint is apparent. Osteophytosis, flattening of the femoral head, and shallowing of the acetabulum may be evident.

For animals that exhibit degenerative joint disease, the V-D extended hip view is sufficient to diagnose hip dysplasia. Diagnosing patients who only have joint laxity without degenerative changes can be difficult. First, the positioning for this view minimizes radiographically apparent laxity. By extending the femur and internally rotating the femur, the joint capsule is tightened, and the femoral head appears to have better coverage. Some animals with palpable Ortolani on physical exam will lack the appearance of laxity on a V-D hip radiograph. Second, the V-D extended view does not provide a quantitative assessment of joint laxity and cannot be used to predict outcome in patients diagnosed with hip dysplasia. A large number of dogs with radiographic signs of hip dysplasia exhibit minimal or no clinical signs.

The V-D extended hip view is used by the Orthopedic Foundation of America (OFA) to certify hip conformation in dogs greater than 2 yrs of age. While this has process has eliminated some affected individuals from the breeding pool of dogs, it probably has not significantly affected the overall prevalence of canine hip dysplasia. This is most likely due to the relatively poor predictive value of this test, the late age at which the test can be performed (2yrs), and the underestimation of disease by the V-D extended hip positioning.

The Penn Hip Improvement Program (PennHIP) technique is a more quantitative assessment of hip joint laxity. This technique uses a combination of three radiographs to evaluate laxity: V-D extended hip view, compression view, and distraction view. The distraction view is performed with the legs at 80 to the horizontal table, using a fulcrum device which forces the femoral heads laterally when the stifles are adducted. With this view, a distraction index (DI) is calculated. The DI ranges from 0-1, with 0 being no subluxation and 1 being complete luxation. The PennHIP technique has been validated as a predictive test for dogs as young as four months. Dogs with a DI of less than 0.3 have a lower likelihood of developing degenerative joint disease later in life, while dogs with a DI higher than 0.7 have a higher likelihood. The advantages of the PennHIP technique are that it can be performed at an early age (4months), it provides a quantitative assessment of hip laxity, and it has validated predictive value for development of degenerative joint disease.

The dorsal acetabular rim (DAR) view was developed to allow accurate assessment of the weight-bearing portion of the acetabulum, and to plan the required rotation for a triple pelvic osteotomy. The view is performed with the patient in sternal recumbency, with the hindlimbs drawn forward to the thorax. The X-ray beam is positioned parallel to the long axis of the pelvis. A DAR angle is calculated for the right and left sides. A normal dog is thought to have a combined DAR angle of <15, with >20 consistent with hip dysplasia. This technique has not been validated, and is challenging to perform correctly.

The dorsolateral subluxation score and a modified PennHIP technique have also been described. Neither of these views requires special equipment, and both measure laxity in a similar manner to the PennHIP technique. They also may be useful for assessing joint laxity in young dogs.