A study explains that “hip dysplasia is characterized by an abnormal formation of the hip joint, causing incongruity and/or laxity of the joint, which can lead to osteoarthritis. It has been observed in several mammals, including humans, where it is referred to as developmental dysplasia of the hip (DDH), and in dogs where the term canine hip dysplasia (CHD) is used.
In both species delayed femoral capital ossification, hip joint laxity and subluxation are observed in dysplastic hips,” via Plos One.
The study adds that “The etiology of CHD is not understood, although two broad etiological categories have been proposed i.e., laxity of the periarticular soft tissues (ligaments, muscles, joint capsule), and an abnormal progression of the endochondral ossification in the hip joint, or a combination of both processes. A majority of 88% of the affected dogs in our sample was graded before the age of 20 months.
We were not concerned with the possibility of including phenocopies by adding dogs that were diagnosed at a later age, because their number was small and these could not have led to falsely positive results. The combination of the candidate genes implicates bone and soft tissue development in CHD.
First, disturbances during the process of endochondral bone formation attributing to the abnormal formation of the hip joint may well be related to disturbances in the hypertrophic differentiation of the chondrocytes. Second, there are indications for soft tissue involvement (cartilage, muscles, and ligaments).”
The Institute of Canine Biology adds that “Hip dysplasia is a so-called "developmental disease" because the first signs of abnormality appear in young puppies. A key sign of early risk is "coxofemoral laxity", a loose fit of the head of the femur in the hip socket.
This looseness means the head of the femur is not held snugly in the socket but can move around, putting abnormal pressures on the sides and rim of the socket.
This poor fit results in deformation of the socket and ultimately leads to osteoarthritis and the condition of hip dysplasia.” The study adds thatgenetics plays a part in canine hip dysplasia, and that “the condition is complex and clearly polygenic andspecific genes that are predictive of hip dysplasia across breeds have not been found.”
An important note to keep in mind is that“Heritability is the fraction of the variation in hip phenotype that is accounted for by variation in genotype. The higher the heritability of a trait, the more it will respond to selection. Heritability of hip dysplasia varies widely depending on the breed, the sample population, and the criterion used to assess phenotype.
Values for heritability are usually about 0.2-0.3 using the OFA (Orthopedic Foundation for Animals) hip scoring method, while much higher heritability (0.8) has been reported in some dogs evaluated using PennHIP (Pennsylvania Hip Improvement Program).