Casting a residual limb is crucial to producing a well-fitting prosthetic socket. In my experience, the most common casting techniques used by prosthetists to create the negative mold and capture the residual limb volume are hand casting, vacuum, Icex,® or computer-assisted shape capture. Literature review found no scientific evidence supporting the effectiveness of these techniques. Prosthetists base their selection and application of a particular casting technique on their foundational education and anecdotal experience.
Johannesson and Larsson,2 from the Kristianstad-Hassleholm Hospital in Sweden, presented their results of using the standard Icex casting technique during a 10-year period at the Academy’s Annual Meeting and Scientific Symposium held in San Francisco in 2007. The results were successful fittings reported by 140 of 153 patients. No scientific outcomes were presented; the results were based on the fittings being subjectively deemed successful by the judgment of the authors. The authors provided no explanation for the 13 patients who were not treated with this casting technique. In a second study, Quigley and Wilson3 evaluated three casting techniques in 1975. The investigators compared three versions of hand casting methods: the original UC-BL, two-part procedure, and premodification casting techniques. The results were inconclusive, and, according to the authors, no correlations could be made on the basis of any measurements or volume differences. Clearly, on the basis of the literature, little to no scientific evidence exists to guide a prosthetist’s choice of casting technique.
MODIFIED ICEX CASTING TECHNIQUE
The standard Icex casting technique is outlined in all of Ossur’s4–6 training manuals. The modified technique uses a few significant variations that differentiate it from the standard technique. The step-by-step process used to apply the modified Icex casting technique is described in depth.
First, the residual limb is prepared by selecting and properly applying the locking gel liner. Ossur’s casting system requires a locking liner to attach to the lock inside the bladder chamber. Even if the patient will use a cushion liner in the actual socket, the equivalent locking version of that liner needs to be selected. The liner selected should be the same gel thickness and design (i.e., tapered or uniform) to be used by the patient within the prosthetic socket being built. Then, two of the appropriately sized silicone disks are selected and applied over the special connecting pin included in Ossur’s system (Figure 1).
Next, cellophane is wrapped over the liner to protect the gel liner from the casting process. Then, a sheath is applied over the cellophane-covered liner. Subsequently, the surface anatomy and the reference marks for the circumferential measurement locations at 1-in increments down the residual limb are denoted with an indelible pencil on the nylon (Figure 2). All measurements are completed, and the information is recorded on the work order.
A 2-in or half of a 4-in roll of plaster is used to cap the distal end of the residual limb to prevent the air bladder from making the limb too conical. The custom plaster distal cap helps maintain a more natural cylindrical shape of the residuum. The end cap should include the entire distal taper of the residual limb and, optimally, the cut ends of both the tibia and the fibula, typically 5–8 cm of the distal residuum (Figure 3). The plaster cap must be allowed to harden before moving to the next step, typically less than 5 minutes. The puck-stabilizing tool included in Ossur’s system is applied while the plaster cap is curing (Figure 4). The air bladder and pump are also connected, and the bladder is pressurized to approximately 20 mm Hg while the plaster cap is curing.
A 6-in rigid plaster is used to apply three layers over the residual limb, minimizing any overlap through a shift technique. The plaster is wrapped circumferentially around the residual limb starting with a 1-in overlap of the proximal aspect of the distal plaster cap. Three layers of plaster wrap are applied directly on top of each other. After the third layer is completed, the roll of plaster is shifted opposite the starting point, usually on the posterior of the residual limb (Figure 5). The next three layers are applied more proximally, maintaining approximately a half-inch overlap with the first three layers applied (Figure 6). Experience has taught me that large areas of overlap and using too many layers of plaster reduce the effectiveness of the pressure bladder.
The air bladder chamber is reflected and applied over the cast, keeping the system in line with the residual limb. The amount of pressure in the bladder after it has been fully rolled over the cast residuum is noted, and the bladder is inflated to 120 mm Hg. The patient must be educated that this is going to feel similar to having their blood pressure taken and that they might feel their heartbeat in their residual limb while the pressure is being maintained. The cast is allowed to cure, typically in 5 minutes or less, while maintaining 120 mm Hg through the pump as needed (Figure 7).
Once the cast has cured completely, it is time to remove the bladder from the patient. The pressure is reduced to the noted amount of pressure after the bladder was rolled on the patient’s residual limb. The air bladder is reflected, and then the release button is pushed on the lock inside the bladder to remove it from the connecting pin. Next, the puck stabilizer and the connecting pin are unscrewed from the gel liner. Now, the hardened cast is removed from the patient’s residual limb. Personal experience has shown that having the patient relax his/her residual limb muscles makes cast removal easier.
The cast is examined for imperfections or weak spots and corrected on the outside of the impression before creating the positive model. Sometimes, a weakened area can occur if the roll of plaster is wrung out too aggressively, reducing the amount of plaster in the material of the first layer. After the positive model has been poured and the negative cast is removed, circumferential measurements are repeated on the mold, at the same 1-in increments marked during the casting, and recorded on the work order. This is the technique that was used for all of the subjects included in this study.
STUDY DESIGN AND METHODS
This study is a retrospective chart review approved by the University of Oklahoma Health Sciences Center for protection of human subjects. The patients who underwent a unilateral or a bilateral transtibial amputation and were cast using the Icex casting procedure outlined above were included in this study. Male and female patients of all ethnicities and all types of transtibial amputation with an age range of 18–75 years were included in this study. The patients were initially identified through a billing code search of all patients treated from January 2001 through December 2008. The patients receiving their initial preparatory prosthetic socket after amputation were included in this study. Within this sample, additional measurement data were collected for a second casting procedure. The patients who maintained continuous treatment at the facility during their residual limb maturation were included in this secondary sample. Patients with mature residual limbs who were treated for socket replacements or definitive prostheses to begin their prosthetic care were excluded. This patient population was excluded because the data collected were going to be used to compare immature and mature residual limb measurement data within the same population.
Demographic data including age, sex, height, weight, cause of amputation, type of amputation, side of amputation, type of prosthetic socket being provided, functional level (K-level), and relevant comorbidities were collected for each patient. Residual limb measurement data from the casting procedure, including residual limb length and all circumferences, were collected. Positive model measurement data were collected for comparison. The variation in residual limb length required the circumferential measurement data to be normalized for statistical analysis. The residual limb and mold circumferential measurement data were normalized to three levels: proximal, middle, and distal. The two most proximal circumferential measurements, the midpatellar tendon and 1 in below, were averaged to create the “proximal” value. The “distal” value was created by averaging the two most distal measurements. All remaining circumferential measurements were averaged to create the “middle” value. This normalized the data for analysis in the Statistical Package for the Social Sciences (SPSS) version 18 (SPSS Inc, 2010).
Seventy-eight patients, 4 with bilateral transtibial amputations, who met the inclusion criteria with sufficient measurement information documented in the chart to complete data collection were included in this study. Eighty-two residual limbs were identified for inclusion in this study. The subjects included 56 male and 22 female patients with a mean age of 50.6 (20–72) years. The studied patients’ mean height was 69.2 in (60–79 in), and their mean weight was 191 (75–380) lbs. The causes of the 82 amputations included diabetes, 31; trauma, 28; vascular complications, 12; infection, 5; cancer, 2; and others, 4. The studied population included 44 right and 38 left transtibial residual limbs, with the breakdown of the type of surgical amputation being 33 traditional and 49 osteomyoplastic amputations. All of the subjects were functional level K3 or K4, with a mean residual limb length of 20.03 cm. (10.4–34.3 cm; Table 1). One patient’s residual limb was very short, having only three circumferential measurements distal to the midpatellar tendon. This patient’s data could not be normalized for inclusion in statistical analysis. Thus, 81 (n = 81) sets of normalized residual limb measurement data were analyzed using a two-way repeated analysis of variance (ANOVA) in the SPSS, comparing the precast residual limb measurements with the postcast positive model measurements. The mean volume reduction produced by the pressurized casting technique was statistically significant (p < 0.001) at all three levels. The modified Icex casting technique reduced the proximal level by 0.776 cm, the middle by 1.121 cm, and the distal by 0.438 cm (Figure 8).
The original patient sample included 24 subjects who were cast using the Icex system for their second prosthetic socket. All four sets of measurement data were analyzed using a two-way repeated ANOVA for these 24 subjects. The mean volume changes over time and the mean volume reduction produced by both castings were statistically significant (p < 0.001). The second casting procedure created a volume reduction at all three levels. The mean volume reduction of the residual limb was 1.03 cm proximally, 1.34 cm in the middle, and 0.379 cm distally (Figure 9).
The amount of limb volume change that occurs through the residuum maturation process was not specifically evaluated by this study. However, these unanticipated results are provided on the 24 subjects who were followed through the provision of their first definitive socket. The residual limb changed during the maturation process, reducing in mean volume by 2.46 cm, 4.02 cm, and 3.72 cm distally. This statistically significant (p < 0.001) volume reduction justifies multiple sockets for patients during the maturation of their residual limbs to maintain a properly fitting prosthesis. The volume reduction changed the shape of the residuum but did not impact the effectiveness of the casting technique.
Hand casting has many variables that can influence the quality of the impression of the residual limb. In the author’s experience, the strength and the size of the hands of the practitioner applying the cast and fatigue can impact the overall quality of the impression. The impact of vacuum-assisted casting on the residual limb’s volume is unknown. No literature could be found to scientifically explain the impact of vacuum-assisted casting on the residual limb. Anecdotally, vacuum casting should be more consistent and reproducible compared with hand casting because it reduces the human variables. Computer-assisted residual limb shape capture is consistent but requires more consideration to optimize the socket’s fit during the modification stage. The Icex bladder system eliminates these confounding variables through the consistent application of pressure during the casting procedure and reduces the volume of the positive model, making modification less time consuming when compared with using a Surform® to shave down the positive model to achieve the same volume reduction.
Casting the residual limb with a bladder pressurized to 120 mm Hg allows the tissue to yield according to its density and bony proportions. The casting bladder inflated to 120 mm Hg for all patients was determined through consideration of the reference range of systolic blood pressure, less than 120 mm Hg, and clinical experience.7,8 Keeping the pressure at the high end of the reference range for systolic blood pressure has yielded consistent results for many years. This reduces the amount of material that has to be removed during positive model modification and increases the prosthetist’s consistency for achieving an optimally fitting socket. The prosthetist’s ability to consistently achieve reproducible results using all casting techniques and knowledge on how to modify the model, based on the cast, to optimize the fit of the socket are a testament to the medical professionals in the field, not the science. Appropriate volume reduction of the positive model is also critical to optimizing the overall fit of the prosthetic socket. Current volume reduction recommendations are applied to the entire remnant limb without consideration of the boney anatomy or tissue density.9 Otto Bock recommends a 4% global reduction in their Harmony® training course.9 The volume reduction of the proximal aspect of the residual limbs studied did not yield as much as that of the middle. The difference in volume reduction is attributed to the amount of bone present and limited soft tissue in the proximal region of the residuum. The pressure bladder automatically contours the limb on the basis of the yield rate of the tissue. The pressure bladder achieved a 3.01% (0.07–8.69) reduction in the middle level of the immature residual limb. The pressure bladder achieved a 3.45% (0.48–6.54) reduction in the middle level of the mature residuum. The mean reduction percentages of 3.01% and 3.45% correlate well with Otto Bock’s recommendation, but the range is concerning. The prosthetist does not know when applying a 3% to 4% reduction will be accurate, too much, or too little. The pressure bladder eliminates the need for the prosthetist to guess, allowing the individual’s tissue characteristics of their residuum to determine the reduction.
Casting a residual limb is arguably the most important step in the process of providing a well-fitting prosthetic socket. The modified Icex casting technique provides consistency and reproducibility while eliminating some of the human variables that can complicate the casting process. The volume reduction created through the use of a pressurized bladder is statistically and clinically significant. Consistently providing patients with an optimally fitting prosthetic socket to maximize their comfort and function is the ultimate goal of the casting and modification procedures.
The author thanks the following individuals for their contributions to the research and manuscript: Derek A. Crawford, MS, MEd, a doctoral research assistant in the Department of Rehabilitation Sciences at OUHSC, for providing assistance with statistical analysis of the data; Carol P. Dionne, PT, DPT, PhD, OCS, Cert MDT, the interim co-director of Post-Professional Programs, associate professor of Rehabilitation Sciences, adjunct assistant professor of Allied Health Sciences, and director of Mechanical Therapy Research Lab for the College of Allied Health, for providing editorial guidance for the manuscript; Robert Edmondson, PhD, faculty member of the College of Liberal Studies at the University of Oklahoma, for providing editorial guidance regarding style of the manuscript; and Michael J. Varro, CP, a prosthetist for Hanger in Minnesota, for providing assistance with development of the research protocol, institutional review board submission, and data collection during his prosthetic residency.
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