What causes hip laxity?

By Carol Beuchat PhD

The hip is a ball-and-socket joint, with the round head of the femur cupped in a well-fitting socket (acetabulum) in the normal dog. As long as there is a good fit, with no looseness of the ball in the socket, the weight of the dog is distributed over a large surface. 

​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. 

When this happens, the abnormal force distribution can result in deformation of the acetabulum by flattening the rim. With the socket no longer conforming in shape to the ball,  this sets up a vicious cycle of deformation and remodeling in which the body tries to compensate for the abnormal forces by resorbing bone in some areas and depositing new bone in others. This causes the socket to become shallower and the rim to continue to be flattened until the acetabulum is badly deformed. At this point, there is no longer even a socket to hold the head of the femur, and the joint is completely luxated (dislocated).

If the head of the femur conforms to the shape of the socket with no looseness, the hip will develop normally and hip dysplasia will not occur. But any looseness in the hip joint triggers the vicious cycle of bone deformation and remodeling that results in hip dysplasia. Laxity in the hip joint is the basis of the "distraction index" (DI), one of the statistics used by PennHip to evaluate hip phenotype (xxxx).

​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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An association between hip dysplasia in infants and the way they were swaddled was highlighted by the results of an educational campaign to reduce dysplasia in Japan (Yamamuro & Ishida 1983). The incidence of hip dysplasia in infants in Japan prior to 1965 was high, estimated at 1.1%-3.5%. An educational campaign begun in 1975 encouraging parents to reduce prolonged extension of the legs while swaddled achieved a marked reduction in the incidence of hip dysplasia, to about 0.2%.

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POTENTIAL CONSEQUENCES OF TERES LIGAMENT DAMAGE
Orthopedic
  Hip dysplasia
  Legg-Calves-Perthes
  Luxating patella
  Pes varus
  Cruciate ligament rupture
  Flat chest / Swimmers
  Elbow dysplasia?

Neurological
  Wobblers
  Spinal compression (e.g., pug myelopathy, degeneratiove myelopathy)
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​NEUROLOGICAL
Pug myelopathy
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OTHER ANIMALS

Cats
Rabbits

Captive wildlife
 - Snow leopard
 - Panda

Domestic animals
 - pigs ("spraddle leg")
 - birds

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REFERENCES

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. 
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