Movement between the residual limb and the prosthetic socket (pistoning) is important to understand when evaluating control of the prosthesis during activities of daily living. Excessive movement, for example, has been correlated with a decrease in prosthetic user comfort.1 Traditional kinetic models to formally evaluate human movement, for example, the use of inverse dynamics to calculate joint moments, do not take into account movement between the limb and the socket making results from theses models that suspect in obtaining valid laboratory data. Thus, measurement of pistoning is important for clinical outcomes and studies in the biomechanics of prosthetic users.
The methods used to evaluate pistoning are generally limited to a radiographic approach.1–8 Radiographic methods have several limitations including 1) subjects are exposed to radiation, 2) equipment is expensive, and 3) tasks performed are either static or ones where motion is limited. In an effort to measure motion of the limb within a prosthetic socket without radiation, Convery and Murray9 reported a method using ultrasound in a transfemoral amputee during gait. Use of ultrasound systems to capture movement between the limb and the socket is also cost prohibitive and requires tedious data reduction practices. Sanders et al.10 presented a compact, light-weight, low-cost, and noncontact photoelectric sensor that measured movement about the vertical axis during gait. Their results suggest that pistoning could be as high as 40 mm during normal walking.10 A major limitation to this method was that the sensor could only measure movement about one axis. The distal end of the residual limb is curved, and if the limb translated about the anterior/posterior direction, the curvature would be read as additional movement about the SI axis. As a result, Sanders et al.10 recommended that measurement systems should measure about multiple axes to improve accuracy.
The purpose of this investigation was to develop and test a system to measure pistoning in the sagittal plane (2D) without exposing the subject to radiation, used during a dynamic task, and built at low cost. Cycling was chosen as the dynamic task in the evaluation of this system because the mechanical conditions can be better controlled and monitored than during activities such as gait, postural control is easier to monitor, and task demands are presented more consistently. Two types of suspension design (cuff strap and pin) were tested to evaluate whether the limb movement would be altered by changes in location of prosthesis suspension on the limb. The pin suspension method would constrain movement of the distal end of the residual limb in the prosthesis, while the cuff strap would constrain the proximal end of the residual limb.
Our general hypotheses include 1) movement of the distal portion of the residual limb relative to the prosthetic socket will have a minimal effect on joint moments calculated in the sagittal plane and 2) the cuff strap suspension design will have more distal end motion than the pin suspension design.
DEVELOPMENT OF THE LIMB/SOCKET MEASUREMENT DEVICE
The limb/socket measurement (LSM) device consists of an aluminum frame attached to the lateral side of the prosthetic socket (Figure 1). Two linear variable differential transformers (LVDTs) with a measurement range of 100 mm were aligned 90° apart and attached to the frame. The LVDTs were wired into a common 5V DC source with signal output sent using Bayonet Neill-Concelman connectors (Figure 2).
The relationship between output voltage and length of each LVDT was determined by securing one end onto a table and the other to the spindle of a vertical milling machine with digital readout accurate to within 0.001 mm. The table was moved in 25-mm increments, and the voltage was recorded. The relationship between output voltage and length of the LVDT was determined using linear regression.
The LVDTs were then mounted into the LSM frame. The distance between the two endpoints of the LVDTs on the frame was measured with a digital caliper. The opposite ends of the LVDTs were attached using a machine screw to the LSM frame. The location of the floating endpoint could then be calculated relative to the frame by calculating the intersection point of two radii (those radii being the length of the LVDTs based on the output voltage).
The LSM device was calibrated by clamping it to the table of the milling machine and attaching the two floating endpoints of the LVDTs to the spindle. The table of the milling machine was moved in known amounts in both X and Y directions and was checked against the calculated coordinates from the LSM device.
EXPERIMENTAL PROTOCOL TO TEST THE LIMB/SOCKET MOTION DEVICE
Two subjects with unilateral transtibial amputation secondary to trauma (31 ± 11 years, 82 ± 15 kg) volunteered to participate in the study. Both subjects were experienced cyclists. Each subject provided separate written consent to participate in the experimental protocol approved by the Georgia Institute of Technology's Institutional Review Board.
Subjects pedaled on a stationary electromagnetically braked cycle ergometer (Excaliber Sport, Lode BV, Groningen, NL), adjusted to the subjects preferred position and adapted with piezoelectric element force pedals.11 Crank angle was determined using a gear driven continuous turn potentiometer. Crank vertical or top dead center was defined as the beginning of the pedal stroke. The force pedals were adapted with an interface (Shimano SPD road, Shimano Inc., Osaka, Japan) allowing for a total of 8° of foot axial rotation relative to the pedal (±4° from neutral) but would not allow translation in any other direction between the foot and the pedal. Subjects received feedback on their cycling cadence through a tachometer mounted on the cycle ergometer.
The mechanical properties of the prosthesis were controlled for both subjects by having a thermoplastic prosthetic socket fabricated similar to the prosthesis used for cycling by each individual. The prosthetic socket had a hole cut into the anterior/distal portion to allow for attachment of the LSM device. The prosthetic foot was a stiff, 10-mm thick plate of aluminum with the cycling cleat mounted in the approximate location of the first metatarsal head in the sagittal plane and the approximate center of the foot in the frontal plane. The socket alignment relative to the foot was transferred from the subject's personal prosthesis by an Otto-Bock laser posture device (Otto Bock Healthcare, Duderstadt, Germany). This prosthetic design is similar to the STIFF foot condition shown to minimize pedaling asymmetries.12
Two prosthetic suspension systems consisting of a mechanical pin/lock system (X-PSH-PLUS, PDI, Dayton OH) and a cuff strap were used in this testing (PTB cuff suspension strap, Trulife USA, Poulsbo, WA; Figure 3). The pin suspension system would constrain movement of the distal end of the residual limb in the prosthesis, while the cuff strap system would constrain the proximal end of the residual limb. All prosthetic modifications were performed by a licensed prosthetist, including the alignment of the cuff strap.
The LSM device was aligned with the longitudinal and orthogonal axes of the prosthetic socket. The longitudinal axis of the socket was a line made through the center of the socket in the transverse plane at the level of the patella tendon and through the center of the prosthetic lock. The frame was attached to the prosthetic socket by three countersunk machine screws recessed into the lateral wall of the socket. The recessed heads of the machine screws were covered with Teflon tape to smooth the inner wall of the prosthetic socket.
The subject wore a small metal bracket adhered to the skin over the distal tibia that protruded through a slit cut into a prosthetic liner (Figure 4). The liner around the slit was then wrapped in sealer tape to prevent movement of the liner relative to the bracket. The opposite ends of each LVDT were connected to the bracket. Pistoning of the residual limb was calculated through the intersection of two radii. Movement was calculated relative to the minimum detected within the pedal stroke over the five pedal strokes averaged together.
The mass, center of mass, and moment of inertia for the prosthesis (with and without the LSM device) and the subject's residual limb were calculated using methods outlined by Goldberg et al.13 The mass, center of mass, and moment of inertia for the subject's lower limb were calculated using published regression equations.14
The order of suspension type was randomized. Subjects pedaled for 2 minutes at 150 W with data collected at 300 Hz for 10 seconds within the final 30 seconds of the 2-minute time of cycling at a constant cadence. Data were selected for five consecutive crank cycles, each cycle was isolated and normalized to 100 data points (% cycle), with a mean of the five cycles calculated for each variable, and finally the data were reduced into crank forces and limb/socket motion.
The system as calibrated using vertical milling machine that had an accuracy of ±0.2 mm within an area of 25 × 25 mm. The mass and moment of inertia was greater for the prosthesis, residual limb, and LSM device combination compared with the combination of just the prosthesis and residual limb (Table 1). These values were still smaller compared with the sound limb (Table 1).
Pedal reaction forces in the normal direction were qualitatively similar to the movement of the distal residual limb (Figure 5). The pin suspension produced less displacement in the SI direction (Table 2). Prosthetic suspension had no effect on the magnitude of displacement in the anterior/posterior direction (Table 2).
A device to measure motion between the distal end of a residual limb and a prosthetic socket was developed and evaluated. The resolution of the LSM device was greater than the measured displacements indicating that the system could provide accurate measurements of the motion between the limb and the socket. In addition, the device was inexpensive, simple to manufacture, and the data could be reduced using readily available software.
Movement of the distal portion of the residual limb relative to the prosthetic socket was less than 5 mm in either direction regardless of suspension design. This motion is very small compared with ∼40 mm of SI motion recorded during gait.10 In addition, the motion between the residuum and the prosthesis generally increased and decreased with pedal forces, suggesting that movement of the distal limb within the socket was related to load, for example, task mechanics. The smaller displacements measured with the LSM device during cycling compared with walking is likely due to the differences in the task. Forces parallel with the tibia during cycling are ∼30% body weight15 versus ∼104% during normal gait.16 The pin suspension displayed a trend toward less SI motion that seems to support, in part, our second hypothesis, but more data are needed to more fully explain these results.
The effect of motion between the distal end of the residual limb and the prosthetic socket will have a minimal effect on the calculation of joint moments during cycling. The relatively small amount of movement recorded is similar to the accuracy of a typical multicamera motion capture system.17 In addition, an uncertainty and sensitivity analysis that demonstrated the uncertainty in the calculation of joint moments is related mostly to pedal forces as long as error in the calculation of joint centers is less than 13 mm.18
The added mass of the LSM device did increase the moment of inertia but did not appreciably change the center of mass compared with a prosthesis without an LSM device. The mechanical properties of the prosthesis and LSM device were still less than the subject's intact limb. The potential interaction effect of inertia on the measured motion seems to be minimal, but this cannot be determined with the limited data collected.
This device was designed to measure gross movement of the residual limb within the prosthetic socket, while radiographic-based methods are able to measure the displacement of the tibia relative to the socket.7,8 The movement of the skin/liner relative to the tibia was shown to be ∼2.5 mm.8 This movement of the skin relative to the tibia would, in effect, increase the displacements measured by the LSM device to ∼6 mm. However, this increase in displacement would still have a minimal influence on the calculation of joint moments.18
The position of the knee joint relative to the prosthetic socket was not measured with the LSM device. Anterior/posterior translational movement of the knee relative to the prosthetic socket was noticed visually during data collection and is a common compliant among cyclists with amputation.19 Therefore, it may be more appropriate to concentrate on knee motion relative to the prosthetic socket for future work and treat the intersection of the distal end of the residual limb and the prosthetic socket as a pseudojoint. This would allow for the calculation of angular movement between the residuum and the prosthetic socket using a motion capture system by placing a marker at this point on the socket and another marker at the knee center. However, the lateral superior wall of the prosthetic socket may interfere with placement of the marker on the knee center. This portion of the prosthetic socket may be removed to allow space for a knee center marker during cycling research, because this portion of the prosthetic socket is not necessary for cycling. The lateral superior border of a prosthetic socket is designed to resistance movement in the frontal plane during gait,20 yet the forces and movement in the frontal plane during cycling are much lower than gait.21
The LSM device could measure limb pistoning with high resolution in two dimensions and produced consistent results during a dynamic task. The device could measure differences between two different prosthetic suspension systems, but these differences were very small. The relatively small amount of movement measured during cycling is within the typical error allowed for motion capture systems, thus would not increase error associated with joint moment calculation. Therefore, it seems reasonable to assume that the intersection of the distal residuum and the inferior portion of the prosthetic socket could be treated as a pseudojoint. Future research may then address angular movement between the residuum and the prosthesis using motion capture and reflective markers on the prosthesis and the knee center. The LSM device may also be used to measure the displacement of the distal end of the residuum during gait.
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