David D. Aronsson

STUDY DESIGN: The authors developed a rat-tail model to investigate the hypothesis that vertebral wedging during growth in progressive spinal deformities results from asymmetric loading in a "vicious cycle." OBJECTIVES: To document growth curves with axial compression or distraction applied to tail vertebrae to determine whether compression load slows growth and distraction accelerates it. SUMMARY OF BACKGROUND DATA: Progression of skeletal deformity during growth is believed to be governed by the Hueter-Volkmann law, but there is conflicting evidence to support this idea. METHODS: Twenty-eight 6-week-old Sprague-Dawley rats were assigned to one of three groups: compression loading, distraction loading, or sham (apparatus applied without loading). Under general anesthesia, two 0.7-mm diameter stainless steel percutaneous pins were used to transfix each of two vertebrae. The pins were glued to 25-mm diameter external ring fixators. Springs (load rate, 35 g/mm) were installed on three stainless steel threaded rods that were passed through holes in each ring and compressed with nuts to apply compression or distraction forces between 25-75% of bodyweight. Vertebral growth rates in microns/day were measured by digitizing the length of the vertebrae images in radiographs taken 0, 1, 3, 5, 7, and 9 weeks later. RESULTS: The loaded vertebrae grew at 68% of control rate for compressed vertebrae and at 114% for distracted vertebrae. (Differences statistically significant, P < 0.01 by analysis of variance.) For the compressed vertebrae, the pinned vertebrae, which were loaded at one of their two growth cartilages, grew at a reduced rate (85%), although this effect was not apparent for the distraction animals. CONCLUSIONS: The findings confirm that vertebral growth is modulated by loading, according to the Hueter-Volkmann principle. The quantification of this relationship will permit more rational design of conservative treatment of spinal deformity during the adolescent growth spurt.

STUDY DESIGN: An Ilizarov-type apparatus was applied to the tails of rats to assess the influence of immobilization, chronically applied compression, and sham intervention on intervertebral discs of mature rats. OBJECTIVES: To test the hypothesis that chronically applied compressive forces and immobilization cause changes in the biomechanical behavior and biochemical composition of rat tail intervertebral discs. SUMMARY OF BACKGROUND DATA: Mechanical factors are associated with degenerative disc disease and low back pain, yet there have been few controlled studies in which the effects of compressive forces on the structure and function of the disc have been isolated. METHODS: The tails of 16 Sprague-Dawley rats were instrumented with an Ilizarov-type apparatus. Animals were separated into sham, immobilization, and compression groups based on the mechanical conditions imposed. In vivo biomechanical measurements of disc thickness, angular laxity, and axial and angular compliance were made at 14-day intervals during the course of the 56-day experiment, after which discs were harvested for measurement of water, proteoglycan, and collagen contents. RESULTS: Application of pins and rings alone (sham group) resulted in relatively small changes of in vivo biomechanical behavior. Immobilization resulted in decreased disc thickness, axial compliance, and angular laxity. Chronically applied compression had effects similar to those of immobilization alone but induced those changes earlier and in larger magnitudes. Application of external compressive forces also caused an increase in proteoglycan content of the intervertebral discs. CONCLUSIONS: The well-controlled loading environment applied to the discs in this model provides a means of isolating the influence of joint-loading conditions on the response of the intervertebral disc. Results indicate that chronically applied compressive forces, in the absence of any disease process, caused changes in mechanical properties and composition of tail discs. These changes have similarities and differences in comparison with human spinal disc degeneration.

Slipped capital femoral epiphysis is a common hip disorder in adolescents, with an incidence of 0.2 (Japan) to 10 (United States) per 100,000. The etiology is unknown, but biomechanical and biochemical factors play an important role. Symptoms at presentation include pain in the groin, thigh, or knee. Ambulatory patients also may present with a limp. Nonambulatory patients present with excruciating pain. The slipped capital femoral epiphysis is classified as stable when the patient can walk and unstable when the patient cannot walk, even with the aid of crutches. Because the epiphysis slips posteriorly, it is best seen on lateral radiographs. The treatment of choice for stable slipped capital femoral epiphysis is single-screw fixation in situ. This method has a high probability of long-term success, with minimal risk of complications. In the patient with unstable slipped capital femoral epiphysis, urgent hip joint aspiration followed by closed reduction and single- or double-screw fixation provides the best environment for a satisfactory result, while minimizing the risk of complications.

OBJECTIVE: The definition and early treatment of congenital dysplasia of the hip are controversial. The purpose of this study was to discuss the reasons for changing the acronym to developmental dysplasia of the hip (DDH) and to address its early detection and treatment. DESIGN: This multicenter study was designed to provide an updated assessment of the definition, pathologic anatomy, prevalence, etiology, natural history, early detection, and treatment of DDH. RESULTS: DDH more accurately describes the condition previously termed congenital dysplasia of the hip. The disorder is not always present at birth (congenital) and an infant may have a normal neonatal hip screening examination and subsequently develop a dysplastic or dislocated hip. Developmental dysplasia encompasses the wide spectrum of hip problems seen in infants and children. Physicians should understand that a normal neonatal screening examination does not assure normal hip development. The diagnosis of developmental dysplasia is made by physical examination. The Ortolani and Barlow maneuvers were designed to detect a subluxatable, dislocatable, or dislocated hip in the neonatal period. In the older child, limited abduction becomes a more reliable sign. The examination is variable depending on the type of dysplasia and changes with growth. The ultrasound is proving to be a sensitive tool in confirming the diagnosis in newborns and infants from birth to 4 months of age. The ultrasound is also valuable in older infants in terms of documenting that the dysplasia is responding to treatment. However, the ultrasound depends on an experienced sonographer and, in some cases, may be too sensitive, resulting in overtreatment. After 3 to 4 months of age, an anteroposterior pelvis radiograph can confirm the diagnosis. CONCLUSIONS: All newborns should have a neonatal hip screening physical examination. After screening, the hips should be re-examined during health examination visits at 2 weeks, 2 months, 4 months, 6 months, 9 months, and 1 year of age. If any question arises during these visits or if there are associated risk factors, we recommend an ultrasound if the infant is < 4 months of age or an anteroposterior pelvis radiograph if > 4 months of age.

Sustained mechanical loading alters longitudinal growth of bones, and this growth sensitivity to load has been implicated in progression of skeletal deformities during growth. The objective of this study was to quantify the relationship between altered growth and different magnitudes of sustained altered stress in a diverse set of nonhuman growth plates. The sensitivity of endochondral growth to differing magnitudes of sustained compression or distraction stress was measured in growth plates of three species of immature animals (rats, rabbits, calves) at two anatomical locations (caudal vertebra and proximal tibia) with two different ages of rats and rabbits. An external loading apparatus was applied for 8 days, and growth was measured as the distance between fluorescent markers administered 24 and 48 h prior to euthanasia. An apparently linear relationship between stress and percentage growth modulation (percent difference between loaded and control growth plates) was found, with distraction accelerating growth and compression slowing growth. The growth-rate sensitivity to stress was between 9.2 and 23.9% per 0.1 MPa for different growth plates and averaged 17.1% per 0.1 MPa. The growth-rate sensitivity to stress differed between vertebrae and the proximal tibia (15 and 18.6% per 0.1 MPa, respectively). The range of control growth rates of different growth plates was large (30 microns/day for rat vertebrae to 366 microns/day for rabbit proximal tibia). The relatively small differences in growth-rate sensitivity to stress for a diverse set of growth plates suggest that these results might be generalized to other growth plates, including human. These data may be applicable to planning the management of progressive deformities in patients having residual growth.