Transradial Interface Variations

Gerald Stark

Although transradial self-suspension techniques have been known since the 1950's, they still present a variety of technical challenges for the prosthetist.2 Originally intended for use with cosmetic arms, self-suspension found its greatest application with stand-alone external power transradial systems. More recently many prosthetists have opted for suspension or gel liners which promise to achieve suspension more easily than with anatomic modifications. But because of unique limb shapes, added thickness, rotation, accommodation of the distal attachment, and greater difficulty with myocontact, many return to the direct loading of selfsuspension techniques.

This is not to say that self-suspension does not have its own set of disadvantages. The objective of securing the prosthesis is often counteracted by the need for comfort and maximized range of motion. It must be remembered that since the interface extends above the epicondyles it effectively blocks all pronation or supination which is compensated by using glenohumeral rotation or wrist rotation components. The modification process remains fairly involved, requiring a balance of compression between the triceps bar and cubital trimline. Another relationship exists between the range of motion and the amount of suspension security largely dependent on the trimline heights. Frequently the trimlines are lowered for comfort or range of motion only to lose suspension. Still the advantages of direct contact for loading and myo control, minimized interface thickness, harness elimination, and donning ease makes it still an attractive option.

The original self-suspension method was developed by Dr. Oscar Hepp and Dr. G.G. Kuhn in the mid 1950's for use with cosmetic passive arms at the University of Munster, Germany which was later introduced to the United States in 1958.2 New York University published a fabrication manual for Munster method in 1965 in which it attempted to quantify the technique into a teachable method.2 Originally used for body powered cable systems, the NYU-Muenster design was well suited to short to very short unilateral transradial amputees (ranging from 1½" to 5½".2 The interface first introduced epicondylar suspension and encapsulation of the olecranon. To fit the shorter limb lengths the NYU design used relatively high anterior and posterior wall combined with a tight A-P through the cubital fold with relief for the biceps tendon. This limited range of motion to 70° or from 35°-105° and required pull-in donning.2 This limited range of motion was deemed acceptable at the time for short to very short unilateral amputees since the prosthesis was used at the extreme limits and then only as an assistive device. To accommodate this loss of motion the forearm was typically set in preflexion to 35° in relation to the humerus.2

The impression technique was fairly involved with the limb held at 90°, the olecranon was cupped in the ball of the hand, the second, third, and forth finger applying triceps bar pressure, and the fifth finger and thumb curling in to make contact but not pressure with the epicondyles. The opposite hand used the second and third fingers to load the cubital fold and were split to create a biceps tendon relief. As with other classic German fittings of the time, the impression was used for an evaluation interface. The impression was reinforced, the proximal edge was trimmed, the inside was smoothed with a slurry of plaster, a pull hole was cut, and the interface was redonned. To maximize range of motion from 35° to 70° the anterior and posterior trimline as cut to increase flexion and extension respectively.2 If any impingement was detected a relief was made or a simple cross-hatch incision was made over the epicondyles and/or olecranon. While this method produced relatively acceptable results for shorter limb lengths, the loss of range of motion continued to be problematic for longer limb lengths who expected more functionality especially when combined with myoelectric systems.

In the early 70's John Billock, C.P.O. at Northwestern University began applying different techniques to achieve self-suspension, but with increased range of motion desired for selfcontained externally controlled systems. In 1974 he published a description that outlined the principle of a tighter M-L dimension fit over the anterior epicondyles as well as a drastically lower anterior trimline approximately 45% of the overall length to 2" proximal of the distal end.3 By creating a counterpressure between the triceps bar modification and the cubital fold loading near the trimline, the range of motion was greatly increased. It was now possible to expect full range of motion for longer limb lengths. Another difference was that the impression taking was done at 45° to capture the anatomic contours superior to the humeral epicondyles which were frequently distorted when the limb was held at 90° with the Muenster design. The slight M-L modification was done anterior and posterior to the epicondyles with a slight channel to allow for easy push-in rather than pull-in type donning and was not as dramatic as the Muenster. No modifications were made to distort the mold which would "disrupt the general contours of the amputation limb and displace the outlined areas of the olecranon process and humeral condyles."3 The modifications were intended to rotate about the epicondyle to maintain suspension at 90°, full flexion, and full extension. The posterior triceps bar modifications, approximately ½" proximal to the olecranon was not as high as the Muenster so extension range of motion was not inhibited while providing counterpressure anteriorly. The radius was still abrupt enough to create tension over the olecranon to also provide counterpressure to the anterior trim line. Though accurately described by Billock, this feature is often obviated by excessive relief or flaring. Similar modifications have been described and reintroduced by later self-suspension methods to provide this necessary suspensatory function. Overall the Northwester design greatly simplified the impression technique and modification, making easy push-in donning possible, especially beneficial to those with dexterity issues or bilateral patients. The M-L rather than A-P pressure with lower trimlines let the amputee expect almost free range of motion. Many prosthetists use slight modifications to this procedure primarily greater pre-molding of the epicondyles and anterior trimline pressure for fleshy patients, but invariably risk deformation the anatomy as originally expressed.

In the later 70's Otto Bock began to explore interface options for their growing commercial myoelectric systems that combined the suspension security of the Muenster with the increased range of motion of the Northwestern design. They, in effect, modified the Muenster by applying the Northwestern principles of an anterior ML pressure over the anterior epicondyles, and ranged the impression from full flexion to full extension. The impression employed a proximal splint pattern that cupped well over the epicondyles, but dropped over the cubital fold and posterior trimline to allow a much greater range of motion than the Muenster. Another narrow rope splint, narrowed at the cubital fold, was pulled tightly over the apex of the olecranon also described as a "String Method" by Staats in 1982.10 This created an extremely secure fit and created a relief for the cubital tissue by pushing into the cubital fold but required pull-in type donning. Anteriorly a biceps tendon cubital tissue relief was borrowed from the Muenster, but this was molded as the patient ranged the impression throughout their range of motion. This highly successful design continues to be used today with slight modifications to the impression procedure. Other designs have been presented that accentuate the anatomic contours even more to achieve, what is hoped to be, better suspension, rotation control, range of motion and optimized myoelectric function. Most recently the Anatomic Contoured Control Interface (ACCI) from Randall Alley, CP8 and the Transradial Anatomic Contour Interface (TRAC) from John Miguelez, CP9 accentuate the anterior pressure, cubital loading, biceps tendon relieve, and olecranon cupping found in the original Otto Bock Muenster.

Although the preceding interfaces have distinct characteristics, many prosthetists employ the various principles depending on the patient's limb length, presentation, and functional goals. The longer limb lengths, 55% or more, can benefit from the dropped anterior trimline and increased range of motion of the NU design. Shorter limb lengths 50-30% benefit from the greater A-P cubital loading and suspension characteristics of the Otto Bock Muenster with out sacrificing too much motion. Very short limbs, 30% or less, may require a more classic Muenster type interface with higher anterior and posterior trimlines to insure greater suspension security. The delineation between the different limb lengths is not a definitive one, but rather, indicate the degree to which the different features are added or lessened.

New materials including bioelastomers have made prosthetic use much more comfortable, but result in different suspension characteristics. This may require adjustments at the trimline to account for a greater amount of tension and account for elastic bending. It must be remembered that the original Northwestern design was intended for rigid laminated interfaces and the Otto Bock interface for flexible lamination which were stiffer in nature. Because of the flexibility of the newer bioelastomers, more aggressive contours are possible and may explain the modifications presented in the ACCI and TRAC.

Regardless of the interface design chosen, the prosthetic goals remain the same. The prosthesis should be secure, yet comfortable, throughout range of motion. Range of motion should be maximized, but not at the expense of secure suspension. Relief should be provided for comfortable loading axially, at 90°, and full flexion. The patient should be able to easily don and doff the prosthesis independently without excessive impingement on the epicondyles or olecranon. The patient should be able to maintain suspension passively without inadvertently losing suspension by flexing or extending the arm.

In full flexion the distal radial area and proximal ulnar area are loaded respectively. If triceps bar is placed incorrectly or it is too narrow the olecranon may pop out of its modification and the interface will migrate distally. This may also happen if there is excessive cubital fold pressure because the trimline is too high or the cubital tissue has not been accommodated. Typically a heavier pressure in the cubital fold will protrude proximally to the trimline and effectively block flexion. With different selfsuspending methods the anterior trimline can be lowered or the cubital bulge can be accommodated by pulling it into the interface. Because of this pull-in donning is often chosen in varying degree for more proximal amputations where cubital bunching can be an issue. The biceps tendon does move anterior and should be accommodated in the anterior trimline.

In full extension the proximal ulna area has the tendency to pull away from the interface wall. A certain amount of gapping is acceptable since a well loaded triceps bar especially with a firm triceps tendon, causes this gapping naturally. Too much gapping may be a result of a posterior trimline that is too high or an anterior opening at the cubital fold that is too wide. Also reliefs should be directed more proximally at the posterior epicondyle with some loading at the trimline to provide comfortable loading at full extension.

At 90° the anterior epicondylar modification combined with the anterior trimline loading provides the suspension. Obviously it is important to locate the epicondylar modifications accurately for comfort and relief. During elbow flexion the load areas must rotate concentrically about the epicondyle and not impinge during flexion or extension. Often patients indicate that the olecranon is impinged at 90° because of the counter-rotation that occurs when the hand is loaded or on a table top. For this reason a relief hole may be cut for the olecranon described as the "Three-Quarter Socket Type" by William Sauter of Toronto.9 In the Netherlands this concept is taken a step farther and the triceps bar is simply a padded rod or strap and the entire proximal brim is left open.

Gerald Stark, CP, FAAOP Fillauer Companies Chattanooga, Tennessee


  1. Hepp, O., Kuhn, G., Upper Extremity Prostheses, Proceedings of the Second International Prosthetics Course, pp. 133-180. Copenhagen, Denmark, July 30-August 8, 1959, Copenhagen, Denmark, 1960.
  2. Kay, H., Cody, K., Hartmann, G., Casella, D., A Fabrication Manual The "Muenster-Type" Below-Elbow Prosthesis., New York University, School of Engineering and Science Research Division, Prosthetic and Orthotic Studies, New York, New York, April 1965,
  3. Billock, J., The Northwestern University Supracondylar Suspension Technique for Below-Elbow Amputation., Selected Readings: A review of Orthotics and Prosthetics, pp. 229-235. American Orthotic and Prosthetic Association, Washington, D.C. 1980.
  4. Northwestern University Upper Extremity Prosthetic Manual, Northwestern University Prosthetic-Orthotic Center, Chicago, Illinois, 2004.
  5. Nader, M. Otto Bock Prosthetic Compendium: Upper Extremity Prostheses. Schiele & Schön, Berlin, Germany, 1990.
  6. Daly, W., Clinical Application of Roll-on Sleeves for Myoelectrically Controlled Transradial and Transhumeral Prostheses . Journal of Prosthetics and Orthotics, 2000, Vol. 12, Num. 3, pp 88-91.
  7. Miguelez, J., Lake, C., Conyers, D., Zenie, J., The Transradial Anatomically Contoured (TRAC) Interface: Design Principles and Methodology . Journal of Prosthetics and Orthotics, 2003, Vol. 15, Num. 4, pp 148-157.
  8. Alley, R., Advancements in Interface Design, 2003 American Academy Annual Meeting and Scientific Symposium, Journal of Proceedings, American Academy of Orthotists and Prosthetists.
  9. Sauter, WF, Nauman, S., Milner, M. A Three-Quarter Type Below Elbow Socket for Myoelectric Prostheses . Prosthetics Orthotics International, 1986, Vol. 10, Num. 2, pp. 79-82.
  10. Staats, T. The String Casting Technique for Below Elbow Amputations. Orthotics and Prosthetics Journal of the American Orthotic and Prosthetic Association. Spring 1982, Vol. 36, Num. 1, pp. 35-40.