However, if there is a poor fit between ball and socket, the load-bearing forces will be concentrated on smallerl areas of the acetabulum that might not be designed to support this weight. This displacement of the head of the femur away from the base of the socket is called subluxation. If the femoral head is pulled away from the base of the socket, the forces will be shifted towards the weaker rim of the socket.
The key to preventing hip dysplasia, then, is to prevent the development of laxity in the joint.
In newborn puppies, the head of the emur is held firmly in the base of the socket by the teres ligament (ligamentum teres), which connects to the head of the femur and the face of the hip socket.
The teres ligament stabilizes the joint, preventing the head of the femur from being displaced too far from the base of the acetabulum.
This ligament is very short and tight at birth, restricting the range of motion of the femur by keeping the head of the femur well-seated in the cup of the hip socket. This is why a newborn puppy on its back will hold the legs a bit to the sides with the stifles bent. As the puppy grows, this ligament gradually lengthens and the range of motion of the legs increases until the femur can be extended parallel to the axis of the body. (The same ligament is present in humans, which is why a baby on its back also holds its legs at rest in the same flexed position.)
To solve the joint laxity problem, we need to understand what is causing the joint to be loose. This would probably still be a mystery if not for some observations about the occurrence of hip dysplasia in different populations of humans.
Hip dysplasia is more prevalent in some cultures than others. It is rare in African cultures (0.06 per 1000 live births; Loder & Skopelja 2011), while the recorded incidence in Navajo Indians is among the highest, ranging from 10.9 per 1,000 population (Rabin et al. 1965) to 76.1 per 1,000 (Blatt 2014). It would be tempting to attribute these population differences in the incidence of hip dysplasia to genetics. However, some animal studies revealed that there could also be non-genetic (environmental) causes. In particular, animal studies showed that hip dysplasia could be induced simply by prolonged (days or weeks) extension of the back legs (Wang et al. 2012).
In fact, there were cultural differences in child care practices among human populations that might account for patterns in the incidence of dysplastic hips. In some cultures, infants were carried on the back with the knees straddling the waist of the adult (e.g., Africa), while in others (Japan, Saudi America, Turkey, and Navajo Indians; Mahan & Kasser 2008), the arms and legs were held straight and extended, with movement restricted. Indeed, infants that were swaddled with legs extended had higher incidence of dysplasia
Loder & Shafer 2014 seasonal variation
In the 1950s, hip dysplasia in infants was 1.3 per 1,000 live births in New York City, and
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POTENTIAL CONSEQUENCES OF TERES LIGAMENT DAMAGE
Cruciate ligament rupture
Flat chest / Swimmers
Spinal compression (e.g., pug myelopathy, degeneratiove myelopathy)
- Snow leopard
- pigs ("spraddle leg")
Loder RT & EN Skopelja. 2011. The epidemiology and demographics of hip dysplasia. ISRN Orthopedics, Vol 2011, Article ID 238607. doi:10.5402/2011/238607.
Mahan ST & JR Kasser. 2008. Does swaddling influence developmental dysplasia of the hip? Pediatrics 121:177-178. DOI: 10.1542/peds.2007-1618.
Rabin DL, CR Barnett, WD Arnold, RH Freiberger, & G Brooks. 1965. Untreated congenital hip disease: a study of the epidemiology, natural history, and social aspects of the disease in a Navajo population. Am. J. Public Health Nations Health 55 (Suppl 2): SUPPL: 1-44.
Blatt SH. 2015. To swaddle, or not to swaddle? Paleoepidemiology of developmental dysplasia of the hip and the swaddling dilemma among the indigenous populations of North America. Am. J. Human Biology 27: 116-128.
Wang e, T Liu, J Li, EW Edmonds, Q Zhao, L Zhang, X Zhao, & K Wang. 2012. Does swaddling influence developmental dysplasia of the hip? J. Bone & Joint Surgery 94:1071-1077.
Yamamuro T & K Ishida. 1984. Recent advances in the prevention, early diagnosis, and treatment of congenital dislocation of the hip in Japan. Clinical Orthopaedics and Related Research 184 4:40.