Bone Mineral Density It’s widely recognized that non-ambulatory children with physical disabilities such as cerebral palsy are at risk for decreased bone mineral density (BMD). (1-3) Children with CP can also present with altered skeletal maturation and thinner, smaller bones. (4-6) As a consequence, these children are at an increased risk for fractures. (7-10) Research indicates that fracture risk, at any age, is dependent on the degree of immobilization and bone quality. (11)
What’s so important about BMD? Let’s take a look at current knowledge about bone health.
Bone is a living tissue: there is continual bone resorption, balanced by bone formation and remodeling, which is triggered by the mechanical forces of dynamic loading and strain on bone tissue. (12)
When bones are immobile, resorption will continue to occur; however, the osteoblasts are not stimulated to lay down new bone. This results in decreased bone mass and thus weaker bones. (13)
The good news is that research has long shown that active weight bearing, or dynamic loading, effectively initiates an osteogenic response in bone. (14) Load-bearing exerts dynamic strains in the bone tissue, resulting in this anabolic stimulus. However, to maintain a given level of bone mass requires that this loading-related stimulus is repeated over time. (15) If the mechanical stimulus is discontinued, the bone mineral density will decrease due to ongoing resorption.
Animal studies suggest that even relatively few cycles of loading stimulus can have an osteogenic effect on bone tissue. (12) In other words, an increase in BMD can be evident even without a prolonged period of loading. In fact, shorter and more frequent loading may even be superior to less frequent and longer duration loading. (16)
Additionally, research suggests that these dynamic loading forces do not need to be at a high level. The low level, high frequency strains that are present continuously during subtle activities such as normal active standing are anabolic. These subtle strains can be considered as important for bone tissue strength as the larger strains typically associated with vigorous activity. We now know that even low-level dynamic stresses will result in the physiologic building response of bone tissue, and thus effectively contribute toward the bone remodeling process. (17) Animal studies have shown that low magnitude mechanical signals applied at a high frequency (such as with vibration) can result in anabolic changes that enhance bone strength, even when used “passively” as a non-invasive intervention. (18-20)
It is also interesting to note that bone cells are less responsive to routine signals, and more responsive to variations of stress. (15)(21)
So what does this mean for children with disabilities who do not stand and walk typically during development?
Lack of Standing and Walking
Clearly, to ensure optimal skeletal development, mechanical loading is crucially important. When this mechanical loading is absent, due to a lack of normal loading from the musculature, then “physiologic osteopenia” results. (2)(22) In effect, the bones of children with physical disabilities such as cerebral palsy can become incapable of withstanding daily activities. Fractures can occur during routine activities such as dressing and handling. These same children are often challenged in obtaining adequate nutrition due to feeding difficulties, and may require anticonvulsant medications, or have a history of previous fractures, all of which also contribute toward reduced BMD and fracture risk. (23)
Research has assessed the bone mineral density (BMD) in children and adolescents with moderate to severe cerebral palsy. Findings showed lower BMD for those with greater severity of CP. Findings also showed that declining BMD z scores correlated with increasing age. (23, 24) So without intervention, this suggests a natural change towards a further decrease of BMD as the child becomes chronologically older.
It is important to understand that this decreased BMD is not due to losses in bone mineral such as is seen in osteoporosis of elderly adults, but rather is due to a rate of growth in bone mineral that is diminished relative to the rate of growth in active, typically-developing children. (24)
Specifically, research studies have reflected an association between the ability or inability to walk, and the child’s bone mineral density. Measurements and scores for BMD showed that those children with CP who were independent ambulators had higher scores than those children who were non-mobile, or non-independent ambulators, such as children who were bed-ridden, rolling, or crawling.
These findings suggest that it is lack of ambulation (which is a dynamic load-bearing and osteogenic activity for typical children during normal development) that accounts for the low BMD observed in children with CP. (1)(25-27)
Use of Standers to Improve Bone Mineral Density
So now the question is whether standing with adaptive devices helps reverse decreased BMD in children with CP. What do we know about the types of standers used and the length of time the child is in the stander? What is likely to be most beneficial?
What we’ve discussed here so far strongly suggests that active weight bearing will be the most beneficial in terms of altering bone mineral density and bone strength. Research reflects this, showing that weight bearing physical activity and dynamic weight bearing positively impact bone mineral density. (28-30)
Devices that give the child the opportunity to actively and dynamically bear weight in a safe and supported manner are therefore ideal. Considering that typically developing children begin to actively weight bear in supported standing and walking at an early age, it would seem best to introduce these devices for children with disabilities as early as would be age-appropriate. (31)
A gait trainer supports both walking and active standing for dynamic load bearing. The Rifton Pacer is available in five sizes, from the Mini Pacer, suitable for children as young as 10 - 18 months of age, up to the Pacer XL with a maximum user height of 6’ 4”. Suspension harness systems from other manufacturers also provide for upright positioning with weight bearing through the legs and will promote muscle activity, resulting in the strains that are anabolic for bone tissue.
Specific standers can also enable active weight bearing. For example, the Rifton Mobile Stander can have prompt accessories positioned to encourage active extension of the knees within a controlled range. This enables the active muscle activity that promotes bone-building (rather than locking the knees into passive, static knee extension). This positioning can be accomplished by adjusting the strap lengths on the seat pad to allow for a repeated small sit-to-stand movement from the near-standing position up to fully standing. Another way to position the dynamic stander to encourage weight bearing through the long bones is to lower the trunk support as much as possible, promoting the postural stabilization and weight shift that occurs with active standing. When a child is able to actively extend hips and knees, the dynamic stander can be used in the reverse position. With the child’s back against the body support, and with the knee straps providing security and the seat pad placed across the child’s anterior, the child will actively weight bear in a functional position that allows free arm movement and access to the environment.
Under direct medical professional supervision, the small Rifton Prone Stander may be used with the knee board removed, yet with adequate and safe positioning assist at the pelvis and trunk, to similarly promote active leg use.
One article suggests that 45-90 minutes of weight bearing activity a day is sufficient to assure some improvement of BMD. (32) Of course, more is better. This weight bearing activity does not have to occur in one session. All weight-bearing activities (such as using a gait trainer) count toward this daily total.
Use of static standers can help to maintain BMD, and, if used intensively, can even increase BMD. (33-35) It is important to be aware that the actual weight borne by the lower extremities while in a stander is quite variable, and in some instances may be only a fraction of the child’s total body weight. (36) With a traditional supine or prone stander set at 80 degrees, only about 20% of the body weight is translated through the femur. (32)
Placing a vibration plate under the child’s feet may be considered as well, as research indicates a positive impact on BMD with low frequency mechanical signals. (37) This may require some careful planning for compatibility with available adaptive standing devices.
Recent systematic literature reviews acknowledge that important effects on BMD have been observed, but that there remains a need for further empirical evidence to guide clinical supported standing programs. (38,39)
Bone Mineral Density Research
The table below summarizes five research studies using standing devices.
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||Type of device
||Amount of time in device
||BMD measurement outcomes
|Stuberg, 1991 (33)
||20 non-ambulatory school-age children with CP
||Various static standers during school day
||Group 1: 30 min, 3-4 days/week Group 2: 60 min, 4-5 days/week
||BMD did not increase. BMD was maintained in those that stood for 60 min/day. BMD decreased in those that stood 30 min/day. BMD decreased in both groups when standing stopped over summer school vacation.
|Gudjonsdottir, 2002 (30)
||4 preschool age children with severe cerebral palsy
||Prone stander compared to dynamic stander (oscillating: rocks side-to-side for intermittent weight bearing)
||30 minutes/day 5 days/week over 8 weeks
||Increased BMD in lumbar spine, and in proximal and distal femur (thigh bone), in children who used dynamic stander and in one child who used static stander.
|Ward, 2004 (37)
||20 ambulant, disabled children (ages 4 – 19 years)
||Group 1: Vibration (placed under feet while in stander) Group 2: Static stander
||10 min/day 5 days/week over 6 months
||Vibration group had increase in BMD at proximal tibia (lower leg). Both groups had increase in BMD of vertebrae (spine).
|Caulton, 2004 (40)
||26 non-ambulant children with cerebral palsy (ages 4.3 – 10.8 years)
||Upright or semi-prone standing frames
||6¼ hours/week over 9 months (This averages to 48 min/day for 5 days/week)
||Increased BMD of vertebrae (spine). Did not increase BMD of tibia (lower leg).
|Katz, 2006 (35)
||12 non-ambulatory children with quadriplegic CP (ages 12 – 21 years)
||Upright (rigid support)
||2 hours/day 5 days/week over 6 months
||BMD of femur (thigh bone) and calcaneous (heel bone) increased when subjects were compliant. When subjects decreased their standing, bone density in the calcaneous (heel bone) decreased.
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