Canine hip dysplasia (CHO) is a highly prevalent multifactorial disorder. Although the etiology is not completely understood, increased laxity of the hip joint is the most frequent cause reported and usually results in secondary degenerative joint disease (DJD).
While CHO can be suspected based on clinical symptoms, the actual diagnosis is confirmed on radiographs. The most popular radiographic technique is the standard ventrodorsal hip extended radiographic (StVD) projection. Aside from its clinical use, this VD is also used as a screening tool by the Federation Cynologique Internationale (FCI), the Orthopedic Foundation for Animals (OFA) and the British Veterinary Association/Kennel Club (BVA/KC). Although the scoring system and assessment protocol is not identical, all three organizations base their evaluation on one radiograph per animal and often impose breeding restrictions based on their evaluation. An evaluation based on one radiograph implies an assessment of both the primary laxity and the secondary degenerative changes on the same radiographic view.
Despite its widespread use, the StVD projection has many shortcomings. Firstly, basic issues such as the effect of sedation/anesthesia, evaluation of a correct positioning or the effect of slight malpositioning on the evaluation of hip dysplasia are still topics of controversy. Secondly, the Norberg angle (NA), mainly used by screening organizations to demonstrate laxity, is not an accurate measurement to score normal or abnormal hips. Thirdly, the repeatability and reproducibility of the presence of osteoarthritic changes (a circumferential femoral head osteophyte, a curvilinear caudolateral osteophyte, osteosclerosis of the cranial acetabular edge) in different groups of experienced observers is only fairly reliable within and between experienced observers. For these reasons, the StVD is a useful tool for use in the daily clinic but care must be taken when apply this technique in official screening and surgical decision making.
Alternative radiographic evaluation methods have been developed that mainly focus on the detection and quantification (using an index) of joint laxity. Examples of these so-called laxity-based diagnostic techniques are the dorsolateral subluxation index (1998), the subluxation index (1999), the half-axial position (1977) and its improvement, called the ‘Vezzoni’ modified Badertscher distention device (VMBDD) (2008) and, the most popular of these techniques, PennHIP (Pennsylvania Hip Improvement Program) (1990).
The position of the dogs in the dorsolateral subluxation approach system is described as weight bearing. Using the dorsolateral subluxation index, dogs with scores >55% are unlikely to develop hip dysplasia in later life, whereas dogs with a score <45% have a greater chance of developing the disorder. It has been demonstrated that the scores did not change after 8 months of agenda reasonably predicted cartilage lesions at a later age. Although not in use in mass selection screening programs, the dorsolateral subluxation score is one of the traits used in genetic studies.
The PennHIP is a three-radiographs based evaluation method that uses the StVD projection to evaluate degenerative changes, a compression projection to evaluate congruency and to determine landmarks for measurements and a distraction projection to evaluate hip joint laxity. The PennHIP method mainly focuses on passive hip joint laxity with the objective of detecting it as young as 16 weeks to create a breeding pool of dogs with tighter hip joints in successive generations (unfortunately, most puppies are sold at 10–12 weeks). The PennHIP report contains a distraction index (DI) that ranges from 0 to 1 (with 0 representing full congruency of the hip joint and 1 representing complete luxation) and relates the performance of the assessed animal to the laxity scores of that breed. Only PennHIP-certified veterinarians can officially perform the PennHIP procedure and enroll the radiographs into the evaluation procedure at the PennHIP Analysis Center. After the radiographs are reviewed, a hip evaluation report is mailed to both the veterinarian and the owner, who are informed of the DI of each hip and the grade of DJD (based on the looser of the 2 hips) relative to the other members of its breed. It is not a pass or fail system. Dogs with a DI <0.3 are considered not to develop OJD in later life, whereas dogs with a DI of 0.7 are very likely to develop the disease. While PennHIP is rather popular in the USA, this is less the case in Europe. This might be due to the evaluation fee and the waiting time for the official PennHIP report.
Recently, the VMBDD was compared to PennHIP. It was first assessed whether the DI from the PennHIP evaluation center could be reproduced by two individual observers. In the next two steps, the DI measurements from the individual observers and the PennHIP evaluation center were compared to the laxity index (LI) measured on the VMBDD view. The results were similar in every comparison: no evidence for bias, a linear relationship and a small variability. Based on these comparisons, it was concluded that the LI (VMBDD) is similar to the DI (PennHIP). Finally, the interobserver agreement of the DI, LI and NA was assessed and compared to classification criteria. For the NA, the measurement variability was substantial, consequently a clear measurement protocol is necessary. The DI and LI were equally reproducible and scored much better. Overall, the VMBDD seems a promising alternative for a complete and correct in-house evaluation of the hip.
To conclude, it’s time to change our approach. The methods based on the detection/quantification of hip joint laxity have demonstrated their superiority compared to the StVD projection and should be used as well in clinical patients as in screening programs. PennHIP offers a strong scientific basis but the constraints related to its use prevent a more extended use, mainly in Europe. Alternative techniques, such as the VMBDD, that allow a complete and correct in-house evaluation of the hip joint by trained clinicians might increase the popularity of laxity-based radiographic techniques.
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