BACKGROUND

In previous years, the study of osteoporosis has focused on post-menopausal women and the elderly. However, with the recent incorporation of passive standing in the therapeutic programs of immobilized children, the prevalence of osteoporosis and associated diseases in these children has begun to draw more attention. With daily, prolonged immobilization and absence of mechanical loading on the body, specifically in the long bones of the legs, bone resorption rates are abnormally high and place these children at significant risk of non-traumatic fractures and osteoporosis early in life. Prolonged immobilization is further believed to lead to complications in the cardiopulmonary, digestive and respiratory systems as well as increasing the incidences of joint contractures, hip dislocations and spasticity (1).

Within the last decade, updates in bone physiology and its mechanostat have suggested that the strains, stresses and pressures sensed by bone cells during mechanical loading are responsible for bone cell differentiation into osteoblasts (cells responsible for bone formation) or osteoclasts (cells responsible for bone resorption). Specifically, it has been found that one major aspect of bone signaling potentially responsible for determining the differentiation of osteoprogenitor cells (bone cell precursors) into osteoblasts or osteoclasts is the oscillating flow caused during reciprocal mechanical loading and unloading (2,3).

Recent work with ambulatory populations further supports the updates in the bone mechanostat. Low-magnitude, high-frequency vibration has become highly investigated in postmenopausal women and children. Studies conducted by Rubin and Ward have shown promise for vibrational therapeutic interventions in maintaining bone mineral density in postmenopausal women and immobilized children. However, the studies with postmenopausal women have further determined that women who walk more throughout the day (in addition to the vibration therapy) experienced correlating increases in their bone mineral density compared to the women who participated in the vibration therapy but experienced minimal ambulation throughout the course of the day (4-6). Based upon this latter finding and Chad’s determination of the benefit of increasing the daily physical activity and loading in healthy children (7), the design of this study has been created to further investigate the feasibility and impact of a dynamic stander which mimics the reciprocal loading normally experienced during walking and daily activities on immobilized children.

  While the impact of ‘dynamic’ standing is the investigation of this research, ‘dynamic’ standing is not a new concept. Previous work by Gudjonsdottir has found that the use of a motorized ‘dynamic’ stander which provides intermittent loading of the lower extremities may increase bone mineral density of children and adolescents and warranted further investigation (8). However, neither Gudjonsdottir’s or other ‘dynamic’ standing devices available on the market are appropriate for the immobilized children in this study, who possess additional complications such as joint contractures and severe risk of hip dislocations. Therefore a new dynamic stander which mimics the reciprocal loading experienced during walking was designed in collaboration between New Jersey Institute of Technology in Newark, NJ and Children's Specialized Hospital in Mountainside, NJ. This stander was incorporated into a 13-week pilot study to determine the feasibility of its use in the clinical and educational settings.

METHODOLOGY

Study approval was obtained through the New Jersey Institute of Technology Institutional Review Board in Newark, NJ. A full chart review was done prior to beginning the study. Exclusions from the study include: not previously participating in a standing program, not obtaining approval from the primary physician to participate in a dynamic or static standing program, a diagnosis of severe osteoporosis, undergoing treatment for severe osteoporosis with medications, or younger than the age of 2 or older than the age of 9 years. Subjects in the study included two children recruited from the Long Term Care (LTC) unit of Children's Specialized Hospital (CSH) in Mountainside, NJ.

The first child was using a Rifton supine stander in the school program at the Fanwood Preschool, an affiliate of the Children’s Specialized Program located in Fanwood, NJ. This child was 8-years-old and had a diagnosis of traumatic brain injury.  The child stood passively in the supine stander 5 days per week for 30 minutes each session. This protocol followed the routine protocol used at Fanwood Preschool and was chosen so as not to disrupt the educational program or current standing routine utilized at the school. The staff recorded time spent in the stander and the angle of standing and the braces worn daily. During extended breaks and absences from school, the child’s passive standing program was carried out in the LTC unit for continuity, with the only alteration being the subject stood for a duration of 60 minutes as opposed to 30 minutes. 

The second child participated in the dynamic standing program and was 2-years-old with a diagnosis of hydrocephaly and seizure disorder. The child was using a Prospect Designs supine stander, which was easily adapted to accept the dynamic component designed for this study (9).  The second child stood for 5 days a week. To ensure tolerance for the dynamic standing and reduce the risk of discomfort and pain, the child initially stood for 30 minutes per day for the first week, progressing to 45 minutes during week two and reaching a plateau of 60 minutes during weeks 3 through 13. Once again, the duration of the standing program was chosen to complement the current protocol used in the LTC unit of CSH. Research staff were present at all times to ensure proper functioning of the equipment and closely monitor the child for discomfort, redness and any indication of shearing forces from movement against the straps. The duration of standing and the forces applied to the feet were recorded daily through the computer programming.

Both standing programs ran for 13 weeks with the child standing in the static stander for approximately 34 hours and the child in the dynamic stander standing approximately 54 hours. 

Dual-energy x-ray absorptiometery (DXA) were completed at 0- and 13-weeks to determine the pre- and post-bone mineral density for each child. A Pediatric GE Lunar Prodigy DXA machine was used for all scans and the same technician performed each scan. The same staff placed each child in the supine position and a scan of the right and left distal femurs were obtained. Once obtained, the scan was analyzed using three regions of interest (ROI), modeled after the ROIs determined by Henderson (10). The bottommost ROI (L3) began at the proximal edge of the growth plate and extended upwards a distance of two times the width of the femur, encompassing primarily trabecular bone. The second ROI (L2) began at the proximal edge of L3 and extended the same height, encompassing a combination of trabecular and cortical bone. The uppermost ROI (L1) encompassed primarily cortical bone and extended from the proximal edge of L2 with an equal height. The Lunar software package was used to determine the bone mineral density of each region in grams per centimeter2.

EVALUATION/OUTCOMES/DISCUSSION

As described above in the methods, bone mineral density was obtained at 0- and 13-weeks in the distal femur for three distinct regions (L1-L3). The tables below illustrate the density found in (grams/centimeter2).

No measure could be derived from the post-scans of the right distal femur in the dynamic standing child, as movement during the scan disrupted the image. Rotation of the right femur in the passive standing child at 13-weeks also yielded no measure of bone density. The preliminary results obtained from the left leg of each child support more investigation on the impact of both passive and dynamic standing in immobilized children before any conclusions may be drawn. Due to the inconsistency of the scans, drastic changes to the DXA protocol have been made for future studies. Subsequent studies will use the lateral distal femoral DXA procedure to obtain future bone mineral densities (10). This method will be incorporated due to its wide acceptance in the populations being studied. Training in this new method has already been completed.     

Throughout the 13-weeks, the applied loads under the dynamic standing subject were recorded daily and were found to reciprocate under each foot for an average applied load between 25% (in swing phase) and 75% (in stance phase) of total body weight at a consistent interval, verifying the programming and mechanical function of the dynamic stander. The use of the current stander in the clinical and educational settings was found to be feasible and showed promise for further investigation with a few modifications. During the pilot study, a desktop computer on a cart placed next to the stander was utilized to run the necessary programming and circuitry. However, the free standing cart led to the air lines and load cell wires extending between the computer consol and stander, posing an undue hazard of tripping and equipment damage and malfunction. For future studies, a laptop will be employed and located on the stander so all wires and associated connections are able to be run under the stander. The programming shut down button was also found to malfunction during the study, so a physical emergency stop button will be located on the stander in future models as an additional option for termination of the session.

At the present time, a six-month study with twenty children is being coordinated at Fanwood Pre-school in Fanwood, NJ and Passaic County Elks Cerebral Palsy Center in Clifton, NJ. Dual energy x-ray absorptiometry scans will be performed and compared for pre- and post bone mineral densities.