Canine hip dysplasia is an inherited, developmental orthopaedic disorder of the hip joint, in large and giant breeds of dog. A tentative diagnosis of hip dysplasia can be made on the basis of history, clinical signs and by hip joint palpation (<xref ref-type="bibr" rid="bib009">Smith, 1997</xref>). However, a definitive diagnosis is made only if characteristic signs of canine hip dysplasia are evident on the radiographic evaluation of pelvis (<xref ref-type="bibr" rid="bib004">Kapatkin <italic>et al.</italic>, 2004</xref>). Since hip dysplasia has a polygenetic mode of inheritance, control of the disease is only possible through selective breeding. A systematic hip scoring system is needed for evaluation of hip joint, which may help select animals for breeding to control hip dysplasia in dogs.The study was carried out in 50 dogs of recognized breeds presented to Department of Surgery and Radiology, Veterinary College Hospital, Hebbal, Bangalore with the history of hindquarter lameness or disability. The dogs were fasted by withholding food for 12 h, and were premedicated with triflupromazine hydrochloride 1 mg/kg i.v. Subsequently they were anaesthetized by administering thiopentone sodium given “to effect”. Anaesthetized dogs were radiographed in three views. Viz. ventrodorsal hip extended view, distraction view and compression view on the day of presentation and six weeks and 12 weeks later.<sec id="sec002"><title>Ventrodorsal hip extended view

The Hip Extended Index (HEI) was calculated from the ventrodorsal hip extended view by the following formula:

HEI = d/r; where ‘d’ is the separation distance from the femoral head centre and acetabular centre of each hip joint measured in centimeters and ‘r’ is the radius of the femoral head of that particular hip joint measured in centimeters (Fig. 1). Figures 1–3Calculation of Hip Extended Index (HEI) from the ventrodorsal hip extended view (1); measurement of Norberg Angle from the ventrodorsal hip extended view (2); calculation of Distraction Index (DI) from Distraction view (3).

The standard hip extended view was also used for measurement of Norberg angle. The femoral head centers were identified and joined by a straight line. Each femoral head center was then connected to the tangential line of the ipsilateral cranial dorsal acetabular edges. The angle between these two lines is the Norberg angle (Fig. 2).

Distraction and compression views

For the distraction view, dogs were placed on the dorsal recumbency with the hip in neutral orientation (femurs perpendicular to the long axis of the table) prior to applying stress on the hip. Stifles were abducted and firmly pushed craniodorsally during the exposure with the tibia acting as lever. This manipulation would result in the cranial, dorsal and lateral displacement of the femoral head in an unstable hip.

For compression view, dogs were placed in the same position as above and a medially directed compression force was applied to the trochanteric area, firmly seating the hip in the most congruent position. The compression view is useful in determining the overall joint congruity and to identify landmarks for laxity measurement from the distraction view. The Distraction Index (DI) was calculated from the distraction view by the following formula: DI = d/r; where ‘d’ is the separation distance from the femoral head centre and acetabular centre of each hip joint measured in centimeters, and ‘r’ is the radius of the femoral head of that particular hip joint measured in centimeters (Fig. 3).

The radiographs obtained from the standard ventrodorsal hip extended projection were evaluated for hip joint abnormalities according to the British Veterinary Association (BVA) hip scoring scheme based on the nine radiographic features such as Norberg angle, Subluxation, Cranial Acetabular Edge, Dorsal Acetabular Edge, Cranial Effective Acetabular Rim, Caudal Acetabular Edge, Acetabular Fossa, Femoral Head and Neck Exostosis and Femoral Head Recontouring for each joint. All the nine features were scored on a scale from ‘0’ (ideal) to ‘6’ (worst) except for caudal acetabular edge, which was scored on a scale from ‘0’ to ‘5’. A dog could thus score from 0’ to ‘53’ for each hip or from ‘0’ to ‘106’ for both the hips. Hip scores obtained were compared with the Breed Mean Score for each breed in order to provide a definitive diagnosis of hip dysplasia.

The overall occurrence of hip dysplasia in the study was 60% (30 among the 50 dogs evaluated). Fluckiger et al. (1995) reported an overall prevalence of 42% hip dysplasia in purebred dogs in Switzerland. In the present study the occurrence of 60% appears to be high and might be due to the fact that all the dogs evaluated were showing clinical signs of hindquarter weakness or disability. The dogs in the age group of 6–12 months had the highest occurrence of hip dysplasia. Smith et al. (1998) suggested that an increase in the synovial fluid volume from 2 to 4 months of age in dogs may cause increased joint laxity and may predispose the dogs for hip dysplasia at the age of 4–6 months. The highest occurrence of hip dysplasia was seen in Labrador Retrievers followed by German shepherd dogs. A similar observation was made by Mason (1976) who stated an higher incidence of hip dysplasia in Labrador Retrievers. All the dogs diagnosed with hip dysplasia were of large or giant breeds. This finding is in concurrence with Priester and Mulivihill (1972) who reported a 30 to 50 times more risk for development of hip dysplasia in large and giant breed of dogs due to low pelvic muscle mass index, which may be because of inherent poor thigh muscle development that did not reinforce and provide strength to the hip joint, thus allowing the joint stresses to act over the hip joint causing hip dysplasia (Riser and Shirer, 1969).

Male dogs had a higher occurrence of hip dysplasia as compared to females. These findings were contradictory to that of Priester and Mulivihill (1972), who observed an equal distribution of hip dysplasia in males and females. A small population size might be the reason for the unequal distribution of occurrence of hip dysplasia between sexes in the present study.

Based on Norberg Angle measurement, 90% of the right and left hips were found to be dysplastic. The increased subluxation of femoral head leading to lesser Norberg Angle in dysplastic dogs was due to the combined effect of increased synovial volume and delayed development of muscle, which could have retained the femoral head in the acetabulum as reported by Cardinet et al. (1997).

Based on the HEI, more than 20% of right hips and 10% of the left hips in the dogs, which were having clinical signs of hip dysplasia were found normal. Higher proportion of normal hips may be due to the fact that HEI was calculated from standard hip extended ventrodorsal view in which there will be masking of actual joint laxity as a result of the winding up of tensile elements in the joint capsule (Kapatkin et al., 2004).

Based on the DI, more than 90% right and left hips revealed mild to moderate degree of hip dysplasia. In most of the dogs DI was much greater than the HEI. These findings were in accordance with that of Smith (1997) who reported that the distraction view was more effective in quantifying hip joint laxity when compared to the standard ventrodorsal hip extended view.

Hip scoring scheme based on the 9- point anatomical scale as employed by BVA was found accurate for diagnosing hip dysplasia. Similar observations were made by Gibbs (1997), who opined that the distinct advantage with hip scoring system was the requirement to assess a number of specific anatomical landmarks ensuring a relative objective evaluation of each joint. Furthermore, the wide range of score points available permitted smaller differences between hips to be recorded (Willis, 1997).

Hip scoring in dogs