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

Richard H. Graebner
Prosthetics Resident, ACTRA Rehabilitation
Thomas A. Current, CPO
Certified Prosthetist Orthotist, ACTRA Rehabilitation

Abstract

The purpose of this investigation was to quantify the relative structural strength of various pylon to socket attachment systems incorporated into trans-tibial prosthetic composite sockets. To conduct the study, loading parameters and methods were identical to those established by Current et al. The methods imitate the International Standards Organization (ISO) standards for structural testing of lower limb prostheses. The experimental set-up simulated the instant of maximum loading during the late stance phase of the gait cycle. Fifteen trans-tibial sockets were evaluated, three each of five different pylon to socket attachment systems. Two of the three sockets of each attachment type incorporated carbon reinforcement and one of the three was constructed of Nyglass stockinette only to establish a baseline performance. In all cases a standard four-hole pyramid attachment plate was used as the final connection of the socket to the pylon. Each sample was loaded to failure in a hydraulic driven materials test machine at 100 mm/s. The ultimate strength and strength to weight ratio was determined for each attachment system.

All of the prosthetic systems were loaded to failure. Comparisons of the socket attachment systems were made on the basis of ultimate strength, measurement of deflection, testing weight of the socket, and failure mode.

Introduction

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 the above three criteria, the variations at the distal end of the trans-tibial socket are almost endless. The prosthetist, ultimately responsible for patient care and outcome, chooses a combination of materials and components which he or she assumes will provide sufficient strength and function while minimizing weight. When presented with options the prosthetist would like to be guided by empirical, objective data. Since the prosthetist cannot afford to experiment with patient safety, the prosthetic device is often overbuilt.

Since the introduction of endoskeletal componentry by Otto Bock in the late 1970's, many distinct styles of pylon to socket attachment hardware have been developed and are generally in use in contemporary trans-tibial prosthesis composite socket construction. Various forces have driven this diversity. These forces include 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 an attachment plate, is located internally and along with a standard four-hole pyramid attachment component forms a "sandwich" of the entire composite. The second group, characterized by the four prong pyramid 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 unpublished survey of 67 responses indicated that in trans-tibial composite sockets the five most popular attachments systems were; 1) a four prong BK lamination pyramid; 2) a socket attachment block; 3) a round socket attachment plate; 4) a square socket attachment plate; and 5) a three prong AK lamination pyramid. See 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. Consequently, this study will demonstrate that the choice of attachment hardware is consequential, particularly in the case of the heavy or highly active patient.

Two characteristics of carbon fiber composites have particular relevance to this study. Firstly, 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 but also at a long distance from the hole (Klasson, 1995). 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 which "sandwiches" the distal lamination requires the addition of holes through the composite to enable the connection to the pyramid adapter. Secondly, studies of composites using carbon have indicated that due to 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 (Klasson, 1995). 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 (Current, et al, 1999). 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 composite. And while the single-stage lamination types require holes through the lamination, the pyramids of the 3 Prong and 4 Prong necessarily create a relatively larger hole through the distal end of the composite. Also, 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. This mechanical bond is inherently much weaker and prone to interlaminate sheer delamination type failures. The OWW laminating block, while a two-stage lamination system, integrates an extra carbon layer wrapped over the component in both planes.

Methods

Transtibial structural test model

In order to produce identical test samples for each attachment type, a trans-tibial model was developed using a prosthetic CAD/CAM software package (Shape Maker, MIND Corp., Seattle, WA, USA). The model was created by averaging the measurements of 25 definitive trans-tibial limbs which 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. 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 utilized for all the test composite sockets with very little repair required on the proximal trim borders only.

Socket fabrication

All sockets were fabricated from the trans-tibial test model a minimum of 30 days prior to 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 20mm HG 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. A total of 15 sockets were fabricated, three each of five different attachment systems. Two of the three included carbon braid reinforcement, while one of the three was composed of Nyglass stockinette only to establish a baseline for testing performance.

The sockets were all constructed using identical plaster models and equal amounts of Nyglass and carbon fiber braid. 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 as some attachment hardware is generally used with a two-stage lay-up while 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.

Test sockets were composed of four layers of Nyglass stockinette (R_x Textiles) followed by two layers of carbon braid (Foresee), 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 four inches of each socket was actually composed of four layers of carbon braid. The Nyglass stockinette only "base-line" sockets were composed of twelve layers. In all cases the resin used was Epoxacryl (Foresee).

Single and two-stage lay-ups

In the case of the three two-stage lay-ups (see Table 1), 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 with the Nyglass only sockets after the first six layers of material.

In the case of the two single-stage lay-ups, the square attachment plate and the PDI 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 socket. 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 cut 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). Due to 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 out of polyurethane elastomer which held the proximal lever arm's force reaction point at the specified height and offsets. The SLF extended approximately 10cm 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 trans-tibial sockets (Current et al.). 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."

Test procedure

Testing was conducted on a closed-loop computer controlled servo-hydraulic test system (Material Test System, Eden Prairie, MN). Forces were measured with a load cell set to a full range of +/- 8.9kN. Displacements were measured with a +/- 127mm LVDT. Both instruments are calibrated to standards traceable to NIST (National Institute of Standards and Technology). All samples were loaded at specified offsets and loading rates until failure 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 (2) 47.6mm (1- 7/8in) diameter automotive trailer hitch balls rated to 8.9kN (2000lbs) were attached to the testing apparatus. These pieces mated with 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 80N was applied to stabilize the specimen in the test apparatus. The test device was then loaded to failure. Ultimate failure was designated as the point at which the prosthetic socket lost the ability to support an increasing load. Methods for loading were identical to those established by Current et al.

Results

The single-stage lamination systems resulted in pylon failure. (Table 1) These two systems were tested multiple times 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. (Photo 1) The remaining three systems (two-stage lamination systems) resulted in the failure of the socket attachment system. (Table 1) In all three tests of the OWW attachment block, the bolt anchors pulled out of the attachment component. (Photo 2) In all tests of both the 4 Prong and 3 Prong adapters, the adapters pulled out of lamination. (Photo 3)

Typical Failed Pylon	OWW bolt anchors	Four Prong
Photo 1	Photo 2	Photo 3

The Load Point Deflection Curves show each individual socket and loading profile. (Graphs 1-4) It should be noted that deflection includes deflection of entire prosthetic system. Four series of curves are provided; two series of carbon reinforced sockets, one series of Nyglass only sockets, and one series combining the three OWW curves. The OWW sample is included to highlight the remarkable consistency with and without carbon braid reinforcement. In general, carbon braid reinforced sockets tended toward a more elastic failure as compared to the Nyglass only sockets. The addition of carbon braid also afforded an increase in strength, with the notable exception of the OWW laminating block.

Graph 1. Load Point Deflection Curve Series 1.

Graph 2. Load Point Deflection Curve Series 2.

Graph 3. Load Point Deflection Curve Series 3.

Graph 4. Load Point Deflection Curve OWW test only.

The ultimate strengths and deflection at failure for the carbon braid reinforced sockets were averaged for each distal attachment system. (Graphs 5-6)

Graph 5. Ultimate Strength Carbon Braid Reinforced Sockets.

Graph 6. Ultimate Strength Nyglass Only Sockets

A strength to weight ratio was calculated by dividing the ultimate strength in Newtons by the testing weight of each socket system. The two carbon reinforced sockets were averaged for each system. A ranking was then devised as a percentage of the highest in rank, namely the SPS plate with carbon reinforcement. (Table 2) The carbon reinforced SPS plate had the best strength to weight ratio with the Nyglass only 4 Prong the worst. 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 in Table 2.

Discussion

This study compared the relative strength of various pylon to socket attachment systems used in trans-tibial prosthesis construction. A technique (developed by Current, et al) 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 trans-tibial 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 where the practitioner is considering patient safety, the weight of the prosthesis, versatility of attachment, ease of fabrication, and cost. In the study, none of the sockets, including the Nyglass only sockets, failed proximally to the socket/pylon attachment system. In a similar conclusion of Current et al, the type and configuration of the attachment system appears to be an important component to the structural integrity of the socket. Not all systems are equally strong or offer the same strength to weight ratio. Table 2.

All of the carbon braid reinforced attachment systems exceeded the ISO standards. Considering the reinforcement method, ie, four layers of carbon braid distally, it should be noted that the four prong attachment minimally exceeded the standard while the other types greatly surpassed the ISO standard. For ISO testing load level A100, loading condition II, the failure test force standard for static testing is 4025N. The two single-stage lamination systems were the strongest of those tested. Using the type and amount of carbon braid, we could not find any commercially available components to result in socket failure. These two systems were tested multiple times 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 as 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 4 Prong and 3 Prong components, the components pulled out of lamination, again the failure initiating posteriorly.

The sockets integrating the OWW attachment 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. As stated previously, these sockets were fabricated using the small carbon strips included from the manufacturer. All the OWW attachment blocks failed at similar ultimate strengths with or without the additional carbon braid reinforcement. In other words, the OWW attachment block socket attachment system tested without additional carbon braid was as strong as the OWW with carbon braid reinforcement. Not only that, but the OWW without additional carbon is essentially as strong as the other systems with carbon braid reinforcement. A Load Point Deflection Curve for just the OWW tests is included to illustrate. (Graph 4) Note that the Nyglass only socket labeled as Series 3 shows a break occurring at about 4200N. At that point a bolt head sheared but the load increase continued until the ultimate failure of the bolt anchors.

The Load Point Deflection Curves show each individual socket and loading profile. (Graphs 1-3) It should be noted that deflection includes deflection of entire prosthetic system. Carbon braid reinforced sockets tended toward a more elastic failure as compared to the Nyglass only sockets. The addition of carbon braid also afforded an increase in strength, with the notable exception of the aforementioned OWW laminating block. When present, the carbon braid allowed for a more elastic and less catastrophic failure of the attachment system. The exception is the OWW system whose failure is 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 the fact that these systems did not fail while the single-stage systems did fail. A case could be made that the OWW and the 3 Prong two-stage lamination systems are as strong as the single-stage systems tested since the ultimate strength of those systems was similar. However, the OWW component did fail, while both the PDI and square attachment plate systems did not fail. Additionally, the strength to weight ratio of the carbon reinforced OWW system was low as compared to the single-stage systems. (To be fair, the strength to weight ration of the un-reinforced OWW system was amazingly high). Similarly, the 3 Prong carbon reinforced system had an ultimate strength comparable to the "bolt-through" systems, but again, its strength to weight ratio was low.

All of the prosthetic systems, but not all of the sockets, in the study were tested to failure. There seems to be limit of strength where some component of the prosthesis will fail at about 5400N. In this study failure occurred at the site of the pylon to socket attachment system (3 Prong and 4 Prong) and with the attachment component itself (OWW). However, failure of the prosthetic systems constructed with single-stage laminations 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.

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

Conclusion

A study was conducted to determine the relative strength of various pylon to socket attachment systems. All the carbon braid reinforced socket systems met the minimum ISO standard for ultimate strength. In general, single-stage lamination systems which "sandwich" the entire composite performed with greater ultimate strength and strength to weight ratio than did their two-stage lamination counterparts. However, the ultimate strength of the OWW component without carbon 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. 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. To produce consistency, all of the socket systems tested were made from the exact same plaster model. Future studies are needed to determine if the results would vary incorporating sockets of different lengths.

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.

Acknowledgements

The authors gratefully acknowledge the assistance of both 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, Ohio Willow Wood.

References

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• Current TA, Kogler GF, Barth DG (1999). Static structural testing of transtibial composite sockets. Prosthet Orthot Int 23, 113-122.

• Klasson BL (1995). Carbon fibre and fibre lamination in prosthetics and orthotics; some basic theory and practical advice for the practitioner. Prosthet Orthot Int 19, 74-91.

• Roff WJ (1956). Fibres, plastics and rubbers. – London: Butterworths Scientific Publications.

• Taig IC (1972). General considerations in the applications of advanced composites. In: AGAR D Lecture Series #55 – Composite Materials./ Lecture Series Director Rosen BW. – London: Hartford House. P61-69.

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