By Josep Font Grau, Doctor of Veterinary Medicine
Hip dysplasia (HD). dys=abnormal, plasia=form, means an abnormal formation of the hip joint. Although it was described about dogs as early as 1935, today still constitutes a challenge to veterinaries as well as to breeders.
Hip structure relates to the femur, to the coxal bone, and to muscles and ligaments surrounding these structures. The coxal bone consists of three parts, the ilium, ischium, and pubis. The union of these three bones form the acetabulum which is situated near the middle of the outer surface of the bone. The head of the femur meets the pelvis at the acetabulum, forming the hip joint. Contact with articular surface must be steady and smooth. The round ligament and the hip-joint capsule keep the head of the femur in close contact with the acetabulum.
From 1966 we know that unlike man’s, hip dysplasia in dogs is a hereditary condition, not congenital: dogs are not born with HD but environmental factors, diet, excess of exercise, etc. together with a strong hereditary component are the main responsible for developing HD in the animal. The result is a lack of relationship between the acetabulum and head of the femur. This disconcordance may be due to a lack of development of the acetabulum or to its wrong orientation so that it does not conveniently cover the head of the femur (Acetabular Dysplasia) or to a wrong orientation of the head of the femur (Femoral Dysplasia), or to a combination of both. Lack of proper contact between the head of the femur and the acetabulum develops into coxo-femoral slackness and articular instability which is the origin of later osteoartrosis development.
It has been described in over 70 dog breeds, and even in Persian cats. Big and giant breeds are greatly affected, although it can also be found in small sized dogs such as Shi-Tzu, Cavalier King Charles… Some breeds such as San Bernardo and Bullmastiff are specially affected. In 90% of the cases both hips are damaged and is the most frecuent cause of hip arthritis. The incidence varies considerably depending on the breed.
THE HEREDITARY FACTOR
The HD is a multifactorial ailment, and both hereditary and environmental factors are involved. Some of these factors are known but others are not and this complicates its control. It is known that HD has a strong hereditary component. Displasia’s heritability index is established in 0.4-0.6 for the German Shepherd and in 0.8 for the Samoyedo. That is to say, it varies according to breeds as well as broods studied. This is due to many intervening genes (polygenic illness). The index varies from 0 to 1. The bigger its value the less environmental factors are involved in the anatomic expression of the condition. In other words, the higher the hereditary index, the more we will succeed in genetic selection since, in this case, the relationship between fenotype (what is evident in the animal) and the genotype (what the animal carries without being evident is closer and more meaningful).
A displasyc dog necessarily has displasyc genes. On the contrary, a dog without these genes is not displasyc. Such evidence gets complicated if we consider that animals with normal x-rays, free from displasia (fenotype), do not exclude that they are genotypical carriers of displasyc traits without evidence. And therefore they can transmit them to their descendents, which makes it very difficult to eradicate.
An excess of food, usually linked to habits of ad libitum intake, is related to the animal’s growth rate. A young animal, with a skeleton not yet compact enough, who has to withstand excessive weight will be more likely to develop HD. A high protein, calorie, calcium and anabolic steroids diet should be avoided, particularly at the age of maximum growth: between 3 and 8 months. A recent study on a Labrador Retriever fed with 25% less than another group developed dysplasia at a significantly lower rate, which leads us to conclude that dysplasia rate can be lowered by reducing growth rate. There is also evidence that puppies of two months which show high growth rates for their age, show higher grades of dysplasia when adult as well.
Violent exercise in young animals increases slackness. If contact of the head of the femur with the acetabulum is not steady and even enough, the excessive functional slackness when leaning on will lead to cartilage injury and therefore to arthrosis. Moderate exercise that allows an appropriate muscle development, will increase articular stability thus preventing dysplasia.
There are theories that relate HD with an anomalous development between pelvic muscular mass and bone development, in the sense that pelvic muscles develop too slowly to keep joint stability. This would explain HD high tendency in animals with scarce pelvic mass such as Labrador. Though this contradicts high levels of HD in animals with great muscular development such as Rottweiler. Probably, little muscular mass would be the result of dysplasia and not its cause.
Male and female are equally affected by HD. Oestrus females should not undergo dysplasia’s radio diagnose since the estrogenic phase weakens ligament unions and increases articular slackness which could lead to a false diagnose.
HD clinical symptoms can be very diverse; they range from slight limpness, abnormal stepping, encountering hocks, difficulty in getting up or in going upstairs, etc. to completely disabled animals.
There is no correlation between radiological signs and clinic symptoms. Some animals with serious dysplasia may be asymptomatic whilst others with less severe dysplasia may show important symptoms that can be disabling.
Nowadays radiological exams are the only way to diagnose HD. Minimum age for adequate assessment is two years in large breeds, and three years in giant breeds. In German Shepherds, for example, x-ray detection reliability is of 16% at the age of 6 months, 70% at one year old, 82% at one year and a half, and of 95% at 2 years old.
The standard and worldwide accepted radiological technique requires anaesthesia or animal’s sedation, placing it in a supine position with parallel, extended extremities and undergoing internal rotation, so that kneecaps are placed over the femoral troclea. Symmetry must be perfect; otherwise the radiological image can lead us to error. Normal radiological anatomy of a hip means:
- Even articular space at 1/3 dorsal length of the articulation.
- Head of the femur has to be round and smooth, and must fit the acetabulum properly.
- Well defined femur neck.
- Absence of degenerative changes.
Dysplasia classification varies from country to country. Accepted classification in our country (Spain¹) is the one proposed by the Dysplasia’s International Committee of the ICF (International Canine Federation):
- Free from dysplasia. Suitable for breeding.
- Transition form. Suitable for breeding.
- Slight dysplasia. It is recommended to carry out another x-ray control 8 months later.
- Medium dysplasia. Not fit for breeding.
- Severe dysplasia. Not fit for breeding.
The O.F.A., for example, uses a scale with 7 classifications: normal (excellent, good, slight), transition form and dysplasic (slight, medium, severe)
It pursues articular cartilage injury prevention in young animals, and secondary pain relief in osteoarthrosis in adults. Traditionally, moderate exercise has been recommended, as well as weight control. These are the guidelines to reduce dysplasia’s incidence. Violent exercise should be avoided in growing animals, since if there is any articular slackness, the repetitive shock between the head of the femur and the acetabulum will result in erosion and cartilage degeneration. Equally, in adult dogs affected by dysplasia, a reduction of physical exercise will be positive in pain crisis. Swimming is an excellent alternative since it helps maintaining muscular tone in the animal by freeing the joint of its biomechanical burden. As in humans, coldness and humidity aggravate oesteoarthrosis articular pain.
Overfeeding animals cause disproportionate growth and predisposes to HD due to the unbalance between muscular mass and bone structure. In adult animals, being overweight implies a heavier burden on joints. If these are affected by degenerative changes, pain will be greater. Implementing a low calorie diet is a good resource for weight control in adults. The owner’s awareness and cooperation are essential.
Medical treatment is based on the use of chondroprotector substances and anti-inflammatory painkillers. The former with the objective of delaying osteoarthrosis, and the latter with the aim of relieving pain.
Chondroprotectors use is controversial, since there are many factors intervening in osteoarthritis appearance. Clinical response is difficult to evaluate and varies greatly from one individual to another.
GUIDELINES TO REDUCE DYSPLASIA’S INCIDENCE
- Use only A or B dysplasia classification for breeding.
- Do not use animals with early articular slackness (Ortolani’s sign) for breeding.
- Take x-ray every six moths, until the age of two, for animals intended for breeding so that hip’s evolution can be properly evaluated.
- Regarding animals that are going to be used for reproduction, information about parents and grandparents should be obtained.
- Also get information about dysplasia classification on descendants.
- Choose breeds that come from a family with a dysplasic rate lower than 75%. An animal with perfect hips but with a dysplasic brother’s background is less reliable than another one with perfect hips but with brothers free from dysplasia.
- For reproduction, use only animals with a dysplasia rate below the breeding average.
- Use breeds with better hips than their parents and better hips than average breeds.
- Avoid overfeeding in the maximum growth period, from 3 to 8 months. If the animal’s weight is above average it would be better to restrict the puppy’s diet instead of supplying a lower protein one, as it would be the case in adults.
- Avoid over-exercising animals in their growing period in order to avoid articular slackness.
Due to HD’s polygenic and multifactorial nature, this protocol may help to reduce its incidence but will not eradicate it.
CURRENT STATE OF DYSPLASIA’S LECTURE IN SPAIN
HD concerns all dog related professionals, from breeders to vets and, or course, owners who often see how their pet is seriously affected by dysplasia. Nowadays it is well known dysplasia’s high hereditary component, thus the need for breeding animals to undergo x-ray procedures in a standardised position, with anaesthesia. Reproduction of animals affected by displasia should not be allowed.
Countries and Breeding Clubs that have applied such practices for several years have reduced considerably dysplasia’s incidence.