The paper by Wang et al. is timely because of the rising popularity of infant swaddling in Western societies1,2. Swaddling involves wrapping or bundling babies in cloth blankets with the arms restrained to dampen the startle response and provide a sense of comfort. Studies have demonstrated that swaddling promotes sleep consistency and duration while encouraging infants to remain in the recommended supine position during sleep2,3. Approximately 90% of infants in the United States are swaddled during the first few months of life3. Traditional swaddling involves wrapping infants with the legs straight and the hips and knees extended. This practice should be discouraged because it increases the risk of developmental dysplasia of the hip and hip dislocation1. However, many parents learn to swaddle their infant by viewing videos on the Internet, where some videos promote incorrect swaddling3. Proper swaddling methods restrain the upper extremities but allow hip flexion-abduction and knee flexion2.
The paper by Wang et al. adds new information regarding the potential effects of swaddling on immature hips while further defining the pathomechanics of hip dysplasia. Previous studies in experimental animals have identified prolonged hip and knee extension as a cause of hip dislocation4-6. However, the previous studies used a cast or a pin crossing the joint of a single limb. Wang et al. attempted to reproduce traditional swaddling by wrapping both hindlimbs together with a soft wrap. Thus, both hindlimbs were held in adduction as well as hip and knee extension. In addition, the authors studied three different time periods of swaddling: the first five days after birth, the second five days after birth, and the first ten days after birth.
The authors report that hip dislocation occurred predominantly in the two groups that began swaddling at the time of birth. A longer duration of swaddling beginning at birth produced a greater rate of dislocation and more severe dysplasia than the shorter period of swaddling. The animals that were swaddled beginning six days after birth developed dysplasia but rarely dislocation. The control animals were not swaddled and had normal hips at the final evaluation. The authors concluded that earlier swaddling and prolonged swaddling were more detrimental to hip development in this experimental model.
Although genetics and hormonal factors influence the development of hip dysplasia, this study confirms that abnormal mechanical factors are detrimental during early hip development. Similar findings have been noted in dogs, in which the critical period for hip dysplasia is from birth to sixty days of age7. Canine hip dysplasia has been called a “biomechanical disease” because dogs are born with normal hips and dysplasia occurs when puppies begin to walk.
The clinical relevance of the paper by Wang et al. is that it calls attention to the potential harmful effects of traditional infant swaddling with the lower extremities extended and adducted. The authors recommend that a less harmful method of swaddling should be substituted for the traditional straight-leg method.