Second study assesses ‘creep’ risk
By Larry Hand
Sometimes, even when new treatment materials are available, it may be better to rely on traditional options. Such may be the case when it comes to choosing a molding material to form casts for children with clubfoot or fractures.
Researchers from the University of Vermont College of Medicine (UVM) in Burlington reached that conclusion after comparing three cast-molding materials.
“Molding plays a pivotal role in the nonoperative treatment of many conditions like clubfoot casting and children’s fractures,” said first author Steven B. Daines, MD, clinical instructor in the departments of orthopedics and rehabilitation at UVM. “A mater- ial’s failure to hold a mold could mean a poor correction of a deformity or a loss of reduction. This could result in the prolongation of treatment, the failure of treatment causing a need for surgical intervention, or a poor outcome.”
The researchers compared 12.7 cm-wide casts of plaster, fiberglass, and soft cast. They prepared the casts in 40° C water, five layers thick, and placed over them two layers of cotton padding on 5.1 cm- and 15.2 cm-diameter foam cylinders. They used loading devices to simulate the thumb loads physicians apply when molding casts for clubfoot and other conditions that included developmental dysplasia of the hip (DDH) and femoral fracture.
For clubfoot, the loading device applied a thumb-shaped 50-N load on the 5.1-cm model for seven minutes. For DDH, the device applied a 100-N load on the 15.2-cm model, as did the device for femur fracture.
The researchers made five casts of each material and, when the molded casts were removed, they photographed them and compared maximal deformation areas of molded and unmolded casts. A large maximal deformation area meant the molding was less precise.
They found plaster was more precise than fiberglass and soft cast for clubfoot, DDH, and femur fracture because it had the best moldability, or ability to retain the desired shape.
“Our study has further emphasized the importance of considering the benefits of various materials when using them in the clinical setting,” Daines said. “At UVM, we have utilized various casting materials based on the advantages of their material properties. For example, many of our clubfoot casts are made of plaster, which helps us carefully mold casts to correct the clubfoot deformity. Some of our children’s fractures, like buckle fractures of the distal radius, do not require precise molding. We have used removable soft casts with waterproof liners to treat these fractures, maximizing patient comfort and minimizing follow-up appointments as parents can remove the casts at home.”
The UVM study, published online in February by the Journal of Pediatric Orthopedics, comes on the heels of an August 2013 study looking into the mechanical performance and displacement (or “creep”) of three materials used for casting clubfoot patients. Using a cast-testing device built to model clubfoot correction, including the internal force on the cast coming from the foot, researchers analyzed the rotational displacement and linearity of the limb-cast composite during three 10-minute intervals after the cast had set.
The results suggested that at least 65% of cast creep occurs during the first 10 minutes after setting, according to study coauthor Tamara Cohen, a PhD candidate in biomedical engineering at Marquette University in Milwaukee, WI. The amounts of displacement were small for all of the materials, indicating that any could be used with the Ponseti method of clubfoot treatment. However, displacement occurred to the greatest extent with plaster (2°), followed by semirigid fiberglass (1°) and rigid fiberglass (.4°), and those differences were statistically significant.
The two studies don’t conflict with each other, they just look at mechanical properties in different ways, according to Cohen and coauthor Peter Smith, MD, a pediatric orthopedic surgeon at Shriners Hospital for Children in Chicago.
The earlier study addresses changes over time, while the new study addresses how perfectly a cast conforms to initial molding. Both studies, Cohen and Smith said, add information on a topic that, previously, was basically ignored in the literature.
“We’ve changed over the years to using more of the soft casts for clubfoot, because it can be removed more easily. But, for the ones that are difficult to mold, the ones where we think that it would change the outcome, then we definitely use plaster. It’s what works,” Smith said.
Larry Hand is a writer in Massachusetts.
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Daines SB, Aronsson DD, Behnnon BD, et al. What is the best material for molding casts in children. J Pediatr Orthop 2014 Feb 28. [Epub ahead of print]
Cohen TL, Altiok H, Wang M, et al. Evaluation of cast creep occurring during simulated clubfoot correction. Proc Inst Mech Eng H 2013;227(8):919-927.