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Relative Strength of Pylon-to-Socket Attachment Systems Used in Transtibial Composite Sockets

Graebner, Richard H. CP; Current, Thomas A. CPO

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JPO Journal of Prosthetics and Orthotics: July 2007 - Volume 19 - Issue 3 - p 67-74
doi: 10.1097/JPO.0b013e3180cfe8da
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Within the field of prosthetics in the United States today, there is great variation in fabrication techniques, composite lay-ups, and choices of pylon- to-socket attachment systems. When one considers all the possible combinations of these criteria, the variations at the distal end of the transtibial socket are almost endless. The prosthetist, ultimately responsible for patient care and outcome, chooses a combination of materials and components that he or she assumes will provide sufficient strength and function while minimizing weight. When presented with fabrication and componentry options, the practitioner prefers to be guided by empirical, objective data. Because of the lack of such data and because the prosthetist cannot afford to experiment with patient safety, the prosthetic device is often overbuilt. The result may be the extra weight of unnecessary composite materials and resin or a pylon attachment system that one assumes is safer but may be only heavier.

Very little published research has been conducted on the correct amounts of fiber materials to use in the creation of prosthetic sockets. In a published article by Berry1 in 1987 the author studied coupon samples of several different lay-ups and made recommendations based on this information. However, the author never load tested a completed socket.

In an article by Klasson2 in 1995 the author used his extensive knowledge to publish information about how fibers react under stress and strain. However, the author stopped short of recommending specific socket lay-ups.

Current et al3 in 1999 published results from static structural loading of five various socket lay-ups. This was one of the first published articles to look at the complete socket.

In an earlier study, Coombes et al4 user a similar method to Current to test and develop a thermoplastic transtibial prosthesis. This is the earliest reported study of structural testing of a complete prosthesis the authors could locate.

Since the introduction of endoskeletal componentry by Otto Bock (Duderstadt, Germany) in the late 1970s, many distinct styles of pylon-to-socket attachment hardware have been developed and are generally in use in contemporary transtibial prosthesis composite socket construction. Various forces have driven this diversity, including suspension type, ease of fabrication, ease of alignment in the coronal and sagittal planes, weight, strength, durability, availability of new materials, and cost. These attachment systems may be categorized into two main groups according to the fabrication process: single-stage lamination and two-stage lamination. The first group, characterized by the attachment plate, is located internally and, along with a standard four-hole pyramid component, forms a “sandwich” or “bolt-through” of the entire composite. The second group, characterized by the four-prong pyramid socket adapter, is incorporated within the composite lay-up. Within each of these two distinct fabrication types, manufacturers have provided the prosthetist with many choices of geometric shape, configuration, and materials.

An on-line survey was conducted to determine which attachment systems are most popular. This unpublished survey of 67 responses indicated that in transtibial composite sockets, the five most popular attachments systems are a four-prong transtibial lamination pyramid; a socket lamination block; a round socket attachment plate; a square socket attachment plate; and a three-prong transfemoral lamination pyramid (Table 1). Interestingly, these five attachment systems also fit quite well into the possible categories of geometry and type of fabrication (single-stage or two-stage lamination) and seem to cover the spectrum of options in general use today. This variation of hardware design and resultant distal composite fabrication leads to the following hypothesis: some methods of attaching the pylon to the socket are inherently stronger than others. This study demonstrates that the choice of attachment hardware is consequential, particularly in the case of the heavy or highly active patient.

T1-4
Table 1:
Pylon-to-socket attachment systems evaluated

Two characteristics of carbon fiber composites have particular relevance to this study. First, the addition of holes through the composite may be a source of matrix failure. Holes interrupt the continuity of fibers, not only where the hole is located but also at a distance from the hole.2 Even if the holes are made without actually cutting any materials previous to resin impregnation, once a load is applied and the fibers tend to straighten, stresses in the matrix result. By definition, any attachment system that “sandwiches” the distal lamination requires the addition of holes through the composite to enable the connection to the pyramid adapter. Second, studies of composites using carbon have indicated that because of the high modulus (stiffness) of carbon fiber, high stresses are developed when bending the fibers. To avoid fracturing carbon fiber, the bend radius should be as large as practical. If the fibers are forced to conform over a sharp edge, breakage is likely to occur.2 Similarly, it has been suggested that the anterior edge of the pyramid attachment plate appears to act as a focal point for a stress raiser.3 It was further suggested that if the focal point were reduced by spreading the stress over a larger area, such as a round pyramid attachment plate system rather than a square one, premature socket failure might be reduced. The square and round attachment plates both require holes through the lamination, and both create a very sharp bend of the carbon fibers as they flow over the edge of the component. One might expect failure because of these characteristics.

Using a comparable amount of carbon, the two-stage “integrated” systems to be tested are held in place by essentially one half of the composite materials of those that “sandwich” the entire composite. Although the single-stage lamination types require holes through the lamination, the pyramids of the three-prong and four-prong designs necessarily create a relatively larger hole through the distal end of the composite. In addition, the authors theorize that the two-stage lamination creates a decrease in interlaminate sheer strength between the two separate laminations. A mechanical bond is created between the two laminations, as opposed to a chemical bond within each separate lamination. It is theorized that this mechanical bond is inherently much weaker and prone to interlaminate sheer delamination type failures. Note that the Ohio Willow Wood (OWW) (Mount Sterling, OH) laminating block, although a two-stage lamination system, integrates an extra carbon layer wrapped over the component in both planes.

METHODS

TRANSTIBIAL STRUCTURAL TEST MODEL

To produce identical test samples for each attachment type, a transtibial model was developed using a prosthetic CAD/CAM software package (Shape Maker, MIND Corp., Seattle, WA). The model was created by averaging the measurements of 25 definitive transtibial limbs that contain customary modifications performed by an experienced prosthetist. The model was milled by conventional means with additional modifications completed by hand to remove any undercuts. The final result was a cylindrical model viewed frontally with a slightly triangular shape viewed coronally. The final dimensions of the model were 22 cm from medial tibial plateau to distal end and 32 cm circumference at the midpoint of that length. A thermoplastic socket was blister formed over the test model, and three identical plaster models were poured from this thermoplastic socket. The three plaster models were used for all the test composite sockets.

SOCKET FABRICATION

The sockets were all constructed using identical plaster models and consistent amounts of Nyglass and carbon fiber braid. The five pylon-to-socket attachment systems used were a four prong transtibial, a three prong transfemoral, a lamination block, a round attachment plate, and a square attachment plate (Table 1). Variation of fabrication naturally occurred at the distal attachment. This is because these attachments are distinctly different, requiring distinctly different distal fabrication. Variation also occurred because some attachment hardware is generally used with a two-stage lay-up, whereas other hardware is generally used with a single-stage lay-up. However, every attempt was made to not vary the amount of reinforcement being used. Wherever possible, the hardware manufacturer was consulted as to construction methods. As a result of that consultation, only the OWW Laminated Socket Attachment Block required additional carbon cloth reinforcement, as per its standard installation instruction.

Fifteen sockets were fabricated. Three sockets were made for each of five different attachment systems (Table 1). Two of the three included carbon braid reinforcement, whereas one of the three was composed of Nyglass stockinette only. Test sockets reinforced with carbon were composed of four layers of Nyglass stockinette (Rx Textiles, Monroe, NC), followed by two layers of carbon braid (Foresee Orthopedic Products, Oakdale, CA), and finished with four more layers of Nyglass stockinette. The two layers of carbon braid were each reflected individually over the distal end so that consistency was maintained between the single- and two-stage lay-ups. Consequently, the distal 4 inches of each socket was actually composed of four layers of carbon braid. The Nyglass stockinette-only sockets were composed of 12 layers. In all cases the resin used was Foresee Epoxacryl.

All sockets were fabricated from the transtibial test model a minimum of 30 days before testing using Foresee resin and carbon braid. Lamination was completed using the vacuum bagging method in the vertical position at room temperature (66°F). All resin was catalyzed between the range of 2.9% and 3.8% by weight (average 3.27%), and no pigment was used. All laminations were completed under a minimum of 20 mmHg of vacuum and left under vacuum a minimum of one half hour from the time the resin was catalyzed. Gel times were all within normal limits.

SINGLE- AND TWO-STAGE LAY-UPS

In the case of the three two-stage lay-ups (Table 1), the four- and three-prong and lamination block, all three of the identical plaster models were used for each attachment hardware type. Two-stage lay-ups were constructed with the attachment hardware affixed with Siegelharz to the distal end of the first stage composite held in normal bench alignment. The two-stage hardware was affixed after the first reflected carbon layer or, in the case of the Nyglass-only sockets, after the first six layers of material.

In the case of the two single-stage lay-ups (Table 1), the square attachment plate and the Prosthetic Design, Inc. (PDI, Clayton, OH) lamination dummy were affixed with plaster to the distal end of a plaster model held in normal bench alignment. During socket construction, it was not necessary to reattach the square attachment plate following each socket construction; the plate remained fixed to the distal end of the plaster model. While the PDI lamination dummy came off the model when each socket was removed, the residual plaster impression made by the dummy allowed the dummy to be reaffixed in the exact same location and alignment. Therefore, the same plaster model was used for each of the three sockets made for both single-stage attachment hardware types.

ALIGNMENT

The sockets were finished to near identical trim lines and attached to an endoskeletal system. The alignment of the lever arms in relationship to the prosthesis equated to the parameters for structural testing strength of lower limb prostheses (ISO 10328 Standards for Load Level A100, Loading Condition II). This standard specifies the top load application point be offset from the vertical axis 55 mm anterior and 40 mm lateral, the bottom load application point is offset 129 mm anterior and 19 mm lateral. The total distance between the load application points is 650 mm. These offsets have been predetermined by the ISO to relate to the instant of maximum loading occurring in the late stance phase of gait for a 220-pound individual. Because of the specific offsets required, a method was needed to align the lever arms attached to the prosthesis quickly, accurately, and consistently. Furthermore, the technique had to affix the proximal lever arm to the socket without affecting the performance of the device. To achieve these goals, a socket loading fixture (SLF) was fabricated of polyurethane elastomer that held the proximal lever arm’s force reaction point at the specified height and offsets. The reusable SLF extended approximately 10 cm into the socket distal to the knee center, and the remainder of the socket was left hollow. The SLF has been used successfully in previous studies to load transtibial sockets.3 The SLF is a component of the entire test fixture setup (Figure 1), which includes proximal and distal lever arms, socket, pylon, and the SLF. The alignment screws were adjusted on the endoskeletal system for each socket to ensure the loading surface of the proximal lever arm was parallel to the distal lever arm. This configuration is not in accordance with ISO 10328, which requires the alignment to be set to the manufacturer’s guidelines and then set to a “worst condition.” A schematic of the entire test fixture setup can be seen in Figure 1.

F1-4
Figure 1.:
A schematic of the entire test fixture setup.

TEST PROCEDURE

Testing was conducted on a closed-loop computer controlled servohydraulic test system (Material Test System, Eden Prairie, MN). Forces were measured with a load cell set to a full range of ±8.9 kN. Displacements were measured with ±127 mm LVDT. Both instruments are calibrated to standards traceable to National Institute of Standards and Technology. All samples were loaded at specified offsets and loading rates until failure of the system was achieved. The offsets used relate to the instant of maximum loading occurring in late stance phase of the gait cycle. The load was transmitted to the lever arms through a ball-and-socket joint design. Two 47.6-mm diameter automotive trailer hitch balls rated to 8.9 kN (2000 lbs) were attached to the testing apparatus. These pieces mated with a matching concavity on the lever arms attached to the socket and pylon to provide a reaction point in which pure vertical force could be applied to the prosthesis as it deflected. A set force of 80 N was applied to stabilize the specimen in the test apparatus. The test device was then loaded to failure. Ultimate strength was designated as the point at which the prosthetic system lost the ability to support an increasing load. To determine the relative strength of the tested systems, failure of the systems was planned, and loads were expected to exceed the ISO standard. Similar methods were proven to be reliable in a previous study by Current et al.2

RESULTS

Both of the single-stage lamination systems resulted in pylon failure (Tables 1 and 2). These two systems were tested with various pylons or reinforced pylons in an attempt to fail the prosthesis at the attachment system. However, in all cases the pylon failed (Figure 2). The remaining three systems (the two-stage lamination systems) resulted in the failure of the socket attachment system (Tables 1 and 2). All three tests of the lamination block resulted in the bolt anchors pulling out of the attachment component (Figure 3). All the tests of both the four-prong and three-prong adapters resulted in those adapters being pulled out of lamination (Figure 4).

T2-4
Table 2:
Ultimate strength and strength-to-weight ratio
F2-4
Figure 2.:
Typical failed pylon.
F3-4
Figure 3.:
Failed Ohio Willow Wood lamination block anchor bolts.
F4-4
Figure 4.:
Failed four prong attachment.

The load point deflection curves show each individual socket and loading profile. It should be noted that deflection includes deflection of the entire prosthetic system. Three series of curves are provided in two figures; two series (series 1 and 2) of carbon-reinforced sockets (Figure 5), and one series (series 3) of Nyglass-only sockets (Figure 6). In general, carbon braid-reinforced sockets tended toward a more elastic failure than did the Nyglass-only sockets. The addition of carbon braid also afforded an increase in strength, with the notable exception of the lamination block.

F5-4
Figure 5.:
Load point deflection curves for series one and two.
F6-4
Figure 6.:
Load point deflection curves for Nyglass-only sockets.

The system failure points (ultimate strength) and deflection at failure for both the carbon-reinforced sockets and Nyglass-only sockets can be seen in Figures 7 and 8. For ISO testing load level A100, loading condition II, the failure test force standard for static testing is 4025 N. All of the carbon braid-reinforced attachment systems exceeded ISO standards.

F7-4
Figure 7.:
Ultimate strength at failure for each socket.
F8-4
Figure 8.:
Maximum deflection at failure for each sample.

The strength-to-weight ratio was calculated by dividing the ultimate strength in Newtons by the weight of each socket system tested. The weight was the socket composite plus the weight of the attachment component. Then a percentage comparing each system to the strongest was calculated to determine a ranking. Note that the results of the tests for the sockets reinforced with carbon braid were averaged for each system. Because the stated purpose of the study was to determine the relative strength of the socket attachment systems, a ranking of strength was devised as a percentage of the highest in rank, namely the square attachment plate with carbon reinforcement (Table 2). For example, the carbon-reinforced square attachment plate had the best strength-to-weight ratio (8.97), and the Nyglass-only four-prong had the worst (5.38). By calculation, 5.38 is 60% of 8.97. In other words, the four-prong Nyglass only system is 60% as strong as the square attachment plate with carbon system. Note that the system with the second best strength-to-weight ratio was the OWW without carbon braid reinforcement. Failure mode results are also listed (Table 2).

DISCUSSION

This study compared the relative strength of various pylon-to-socket attachment systems used in transtibial prosthesis construction. A technique developed by Current et al.2 was used for testing the sockets that incorporated the loading parameters and methods established by the International Standards Organization (ISO) for structural testing of lower limb prosthetic components.

Clinical experience indicates that when failure of a transtibial prosthesis occurs, that failure is frequently at the site of the pylon-to-socket attachment system. Knowledge of the relative strength of types of systems can be critical when the practitioner is considering patient safety, the weight of the prosthesis, versatility of attachment, ease of fabrication, and cost. It was suggested here that prostheses may be “overbuilt.” The practitioner may be aware of the published weight limit and activity limit of various prosthetic components but is not sure of the actual performance of components relative to each other; how the type of fabrication at the distal socket may affect performance; and what the proper and necessary material composite at the distal socket should be. Thus, it is logical to assume that because relatively few failures of pylon-to-socket attachment systems actually occur, prostheses are often overbuilt with a choice of component or material composite that may be heavier than necessary. The results of this study shed empirical light on the first two points. More research is needed to determine with any specificity proper distal composite socket lay-up.

Strength data were produced for Nyglass-only composite systems. The original thought was to produce a set of “baseline” data to compare with the carbon braid-reinforced sockets. This may not have been entirely useful because Nyglass-only sockets are not routinely fabricated. However, in the end, test results demonstrated that Nyglass-only sockets are not strong enough to meet the base ISO standard of 4025 N, with the notable exception of the OWW lamination block. Of course, this attachment block incorporates carbon strips at the distal attachment point in both the coronal and sagittal planes per standard fabrication instructions, which were followed in all cases.

The sockets integrating the OWW lamination block require special notice. This was the only pylon-to-socket attachment component that failed. The lamination or the lamination/ component interface did not fail, the component itself failed as the bolt anchors pulled out of the blocks in all three tested samples. All the lamination blocks failed at similar ultimate strengths with or without the additional four distal layers of carbon braid reinforcement. In other words, the lamination block system tests as Nyglass-only were as strong as the lamination block system tests with the carbon braid reinforcement. In addition, the lamination block without additional carbon (Nyglass only) essentially tested as strong as the other systems with carbon braid reinforcement (Figure 7). Note that the Nyglass-only socket labeled as series 3 shows a break occurring at about 4200 N (Figure 6). At that point, a bolt head sheared, but the load increase continued until the ultimate failure of the bolt anchors. (According to OWW, and subsequent to this investigation, the bolt anchors were re-engineered with new production beginning in November 2004.)

Although the ultimate strength of a system is without doubt important, the goal of the practitioner is to maximize strength while minimizing weight. The frail 90-pound elderly woman requires a prosthesis that is as light as possible. The 250-pound weightlifter requires a very strong prosthesis. Neither can afford a failure. Therefore, the strength-to-weight ratio of any prosthetic attachment system is critical. Not all systems are equally strong or offer the same strength-to-weight ratio, as can be clearly seen in Table 2. Incorporating carbon braid can achieve a greater strength-to-weight ratio than Nyglass only. Although Nyglass-only systems weighed appreciably less than those with carbon braid reinforcement, Nyglass-only systems were also appreciably less strong. One must conclude that the weight saved by not including carbon braid does not make up for the lack of strength. (The OWW lamination block system is a notable and interesting exception.)

Considering that four layers of carbon braid were used distally in the fabrication, it should be noted that the four-prong attachment just minimally exceeded the standard, whereas the other types greatly surpassed the ISO standard. The two “single-stage” lamination systems were the strongest of those tested. Again, considering the amount of carbon braid, we could not find any commercially available pylon components to result in socket/attachment system failure. These two systems were tested with various pylons and/or reinforced pylons in an attempt to fail the prosthesis at the attachment system. However, in all cases the pylon failed. We have reported only the data from the original tests because varying the pylons made no difference in the results. The remaining three systems, the two-stage lamination systems, resulted in the failure of the socket attachment system. In all three tests of the OWW system, the bolt anchors pulled out of the adapter, starting with the posterior anchors. In all tests of both the four-prong and three-prong components, the components pulled out of lamination with the failure initiating posterior.

The load point deflection curves show each individual socket’s loading profile (Figure 5 and 6). These curves are a plot of the force placed on the system measured in Newtons resulting in displacement of the system measured in centimeters. It should be noted that deflection includes deflection of the entire prosthetic system. Carbon braid-reinforced sockets tended toward a more elastic failure than did the Nyglass-only sockets. The addition of carbon braid also afforded an increase in strength, with the notable exception of the aforementioned laminating block. When present, the carbon braid allowed for a more elastic and less catastrophic failure of the attachment system. The exception is the lamination block system, whose failure was the most brittle, complete, and abrupt.

The single-stage lamination systems tested offered the highest ultimate strength and highest strength-to-weight ratios. These bolt-through systems are without a doubt stronger than the two-stage lamination systems tested, as evidenced by these systems not failing while the two-stage systems did fail. A case could be made that the lamination block and the three-prong, two-stage lamination systems are as strong as the single-stage systems tested because the ultimate strength of those systems was similar. However, these systems did fail, while both the round attachment plate and the square attachment plate systems did not fail. In addition, the strength-to-weight ratio of the carbon-reinforced lamination block system was low compared with that of the single-stage systems. Yet to be fair, the strength-to-weigh ratio of the unreinforced lamination block system was amazingly high. Similarly, the three-prong, carbon-reinforced system had an ultimate strength comparable to the bolt-through systems, but again, its strength-to-weight ratio was low.

None of the sockets, including the Nyglass-only sockets, failed proximally to the socket/pylon attachment system. All of the prosthetic systems were tested to failure. There seems to be a limit of strength at which some component of the prosthesis will fail at about 5400 N. In this study, failure occurred at the site of the pylon-to-socket attachment system (three prong and four prong) and with the attachment component itself (lamination block). However, failure of the prosthetic systems constructed with single-stage laminations (bolt-through types) did not occur at either of these sites. Instead, carbon pylons split, titanium pylons bent, bolt heads sheared off, and titanium female receivers cracked. Because a prosthetic system is made up of many components of various ultimate strengths, in a sense, a choice of failure can be made. It is less expensive to replace a failed pylon then to replace a failed pylon-to-socket attachment system. Single-stage systems can be built with not only high ultimate strengths, but more usefully, with high strength-to-weight ratios.

Because neither of the single-stage lamination carbon-reinforced attachment systems failed, it could not be determined which was the stronger of the two. However, in the Nyglass-only construction, the round attachment plate had a higher ultimate strength than did the square attachment plate but a lower strength-to-weight ratio.

Additional studies are needed to determine if the results would vary if sockets of different lengths were incorporated. Because the bolt-through sockets proved stronger than other components in those particular systems, a future study using only two layers of carbon braid distally for each system may result in the failure of all pylon-to-socket attachment systems, producing definitive relative strength results for all systems tested.

CONCLUSION

A study was conducted to determine the relative strength of various pylon-to-socket attachment systems. To produce consistency, all of the socket systems tested were made from the exact same plaster model. All the carbon braid-reinforced socket systems met the minimum ISO standard for ultimate strength. In general, single-stage lamination or bolt-through systems that “sandwich” the entire socket composite performed with greater ultimate strength and strength-to-weight ratio than did their two-stage lamination counterparts. However, the ultimate strength of the lamination block component without carbon braid was as high as the other systems with carbon braid reinforcement. The amount of carbon braid reinforcement used was more than what was required for the single-stage lamination systems. Those sockets were stronger than any commercially available pylon. Finally, it has been demonstrated that all pylon-to-socket attachment systems do not have equal ultimate strength or equal strength-to-weight ratios. The practitioner may choose which system best fulfills the clinical situation.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of Dr. Barbara Silver-Thorn and Linda McGrady of Marquette University. Several manufacturers generously contributed fabrication materials and components used in this study: Ossur, Foresee, PDI, and Ohio Willow Wood. Jamie Sisson provided the artistic rendering of the test fixture set-up.

REFERENCES

1. Berry DA. Composite materials for orthotics and prosthetics. Orthot Prosthet 1987;40:35–43.
2. Klasson BL. Carbon fibre and fibre lamination in prosthetics and orthotics; some basic theory and practical advice for the practitioner. Prosthet Orthot Int 1995;19:74–91.
3. Current TA, Kogler GF, Barth DG. Static structural testing of transtibial composite sockets. Prosthet Orthot Int 1999;23:113–122.
4. Coombes AGA, MacCaughlan J. Development and testing of thermoplastic structural components for modular prostheses. Prosthet Orthot Int 1988;12:19–40.
Keywords:

attachment; prosthesis; pylon; strength; transtibial

© 2007 American Academy of Orthotists & Prosthetists