View Options - Click to expand
Print Options - Click to expand
E-Mail Options - Click to expand

Conventional and Hydrostatic Transtibial Interface Comparison

Jason T. Kahle, CPO

ABSTRACT

Prosthetists have successfully been using specific-area weight-bearing design in prostheses. Hydrostatic design incorporates sound fluid mechanics principles in conjunction with superior material technology. In this 25-patient study, conventional and hydrostatic transtibial interface designs were compared. The preferred design and reasons for patient preference were determined.

Key Words: hydrostatic interface, transtibial interface, total surface bearing interface

With the advent of silicone suction suspension, gel liners, and variations of these two, it has become necessary to reevaluate interface design and biomechanics. For various reasons, patellar tendon-bearing design does not provide the optimum fit when silicone or gel liners are used. In fact, the use of patellar tendon design interfaces can create specific problems in conjunction with these liners. Traditionally, prosthetists have successfully used patellar tendon-bearing designs in conjunction with a pelite liner and a suspension sleeve or a cuff strap. However, silicone sleeves and gel liners can provide not only superior suspension but also better interface fit.1 With good reason, the inventor of silicone suction suspension recommends a hydrostatic interface design.2 In addition to this, most companies that are manufacturing gel or silicone sleeves recommend either a totalsurface bearing or a hydrostatic interface design.3,4

Interface Classifications

From the perspective of weight-bearing characteristics, interface designs can be put into three basic categories. The first category is specific-area weight bearing (PTB). This involves using specific anatomy like the patella tendon, pop-liteal fossa, and the medial flair for weight bearing. Ibis also involves a specific relationship between the anterior and posterior wall (Figure 1 ). The second type of socket design is total-surface bearing (TSB). This involves using tension values to uniformly distribute the weight over the entire residual limb. The goal is to achieve an interface design that uniformly delivers a minimum amount of skin pressure. This usually involves the use of a gel sleeve that helps redistribute notorious pressure areas in the residual limb (Figure 2 ). The last type of socket design is hydrostatic (HST). The hydrostatic design uses specific principles of fluid mechanics and a compression chamber to achieve a uniform fit (Figure 3 ). It also uses a silicone suction suspension sleeve and is considered to be a Total Surface Bearing (TSB) design.

Hydrostatic Mechanics

The first mechanical principle involved in achieving a hydrostatic fit is Pascal's law of fluids. This states that a confined fluid transmits externally applied pressure uniformly in all directions. It further states that the resultant forces act perpendicular to the container's surface.5 These principles would be advantageous if applied to interface fit because the forces are distributed over the larger surface area of the entire residual limb rather than the smaller surface area of a specific anatomic part (Figure 4 ).

Another important principle applied to achieve a hydrostatic fit is that of Rogers and Wilson's curve.6 This curve was determined in a study that was designed to establish tissue-tolerance guidelines. The study, involving 2,000 observations in three years, determined precisely how much pressure the skin could be subjected to before breaking down. It used temperature elevation at the areas of continuous pressure to determine precisely when the skin was at risk (Figure 5 ). This study determined that 60 turn Hg of continuous pressure produced persistent temperature elevations. The clinical appearance of the traumatized areas suggested impending skin necrosis.

A hydrostatic interface provides uniform and skin-safe redistribution of pressure because its design takes advantage of Pascal's law of fluids, Rogers and Wilson's curve, and a pressure chamber. Application of these principles contributes to even pressure distribution. The hydrostatic interface design promotes tissue elongation, which increases the padding at the distal end, as well as produces a residual limb that has a firmer tissue consistency.7 The pressure chamber, the silicone suction suspension sleeve, and the anatomic knee unit create a proximal seal for fluid containment. The posterior proximal trim lines in a hydrostatic interface design are determined by the insertion of the flexor tendons and are not proximal to the mid-patellar tendon (MPT). When compared to a specific weight-bearing interface, this design produces an increase in range of motion as a result of the relationship between the anterior and posterior walls.

A study was recently completed on the relationship between specific weight-bearing interfaces and interfaces formed by using a hydrostatic casting method. Skin pressure was measured during weight bearing. The study determined that the hydrostatic interface produced significantly less pressure peaks during ambulation than did a specific weight-bearing interface (Figure 6 ).8

Methods

Applying Hydrostatic Mechanics to Interface Design

Applying these principles to interface design involved several components. The first is the use of a silicone suction suspension sleeve. A pull sock or a pin mechanism is a prerequisite to achieving a hydrostatic fit. The reason for this is to ensure tissue elongation and a proximal seal of the condyles for fluid containment. Without the proximal seal of the condyles, a hydrostatic fit cannot be achieved (Figure 7 ).

To best use hydrostatic principles, the ICEX interface technique was used.9 It is a true hydrostatic interface design that provides an optimal, consistent, and reproducible fit. The technique incorporates Pascal's law and Rogers and Wilson's curve. The ICEX interface design presupposes the use of a silicone suction suspension sleeve The silicone sleeve is donned and silicone pads are applied over the bony prominence (Figure 8 ), rather than a relief, as in a specific weight-bearing in terface. This insures that total contact as well as total surface bearing is achieved. Graphite, impregnated with resin, is rolled on and a pressure chamber is applied. The pressure chamber, inflated to 80 mm Hg, applies even pressure to the entire layup and to the residual limb (Figure 9 ). Pressures have been measured on the skin of the residual limb during this pressure procedure. The pressure on the skin is approximately 40 mm Hg. After five minutes, the interface is formed. The medial and lateral proximal trim lines are established by the proximal border of the condyles, anteriorly by mid-patella, and posteriorly by the insertions of the flex or tendons.

Specific Weight-Bearing Interface Design

The Northwestern patellar tendon-bearing design was used for the specific weight-bearing models. It is a widely accepted method and had a history of successful fittings. In the casting technique, Tubegauz is used under tension, and pulled in a proximal direction with an elastic strap. This is to simulate the forces applied to the soft tissues during weight-bearing (Figure 10 ). A two-stage plaster application is used. An anterior plaster splint defines the anterior residual limb anatomy; an elastic plaster wrap defines the remaining residual limb. Modifications were made to the plaster model (Figure 11 )10: either a suspension sleeve or a cuff strap were given to the patient, depending on which was preferred or which best eliminated pistoning. Six millimeters of pistoning or less is considered acceptable under the guidelines of the Northwestern manual.10 Twenty-five patients participated in the study (Table 1 ).

Methods of Comparison

Measurements. Measurements were taken to establish the linear differences and relationships between hydrostatic interfaces and specific weight-bearing interfaces and the patient's anatomy. Traditionally, specific-area weight-bearing interfaces have relied heavily on the linear relationship between the anterior and posterior walls at the MPT level. The sagittal measurement was taken at the tibial tubercle level because in most hydrostatic interfaces the posterior trim line is well distal to MPT level.

The coronal measurement has traditionally also been an important measurement in relationship to the patient's anatomy. This measurement oftentimes is associated with coronal stability in a prosthesis. The final measurement taken was length. Length is important in determining how much tissue elongation or shortening has occurred in the prosthesis, and subsequently in the patient's residual limb.

Radiographs. Radiographs help confirm the measurements. They clearly established the coronal, sagittal, and length changes that take place in a patient's residual limb during prosthesis wear. Radiographs can document the tissue elongation and shortening that takes place during the wearing of each type of prosthesis. Sagittal and coronal radiographic views were taken during full weight bearing on the patient's residual limb (Figure 12 ).

Computer-aided Design (CAD). CAD helped determine volume differences, shapes, and tissue elongation or shortening. To accurately depict the true socket environments, a cast was made over the specific weight-bearing model after modifications were completed. For the hydrostatic interface, the thickness of the silicone suction sleeve was subtracted from the equation. These two casts were then compared. The only quantitative data consistent and able to be collected from CAD and compared were the length measurements. The shape and volume differences were interesting, but difficult to quantitatively compare (Figure 13 ).

Patient Preferences. Patients were asked to wear both an HST and a PTB interface design. They were not given any specific information or bias. They were simply told it was two different interface designs, and they needed to wear, and comment, on each design's characteristics. Ideally, the patients were able to wear each prosthesis for one week; however, some patients rejected a particular design immediately. Some patients rejected both of the prostheses after trying them. In those cases, an alternative interface design (TSB) was chosen to accommodate the patient's needs.

Results

Measurements

The following mean values were obtained (percentage is relative to the patient's residual limb): sagittal, HST +19.4%; PTB +4.4%; coronal, HST +6.3%; PTB +3.7%; length, HST +20.1%; PTB -3.8%

Mid-Patellar Tendon to Posterior Wall Height

In a PTB interface, the posterior wall is 12mm proximal to MPT. The difference between the PTB interface and HST was more than 30mm at all locations along the posterior wall (Figure 14 ).

Patient Preferences

Most patients, 68%, preferred the hydrostatic interface design (Figure 15 ). The reason for their preference included increased range of motion, uniform pressure, and a perception of decreased weight. This is explained by the impression technique, which provides uniform pressure and an even distribution of weight over the residual limb. More than half of the patients who rejected the PTB prosthesis complained of pressure in the popliteal fossa and over the patella tendon. Almost all of the patients commented on the increase in range in motion during sitting. Some even noted that it was easier to go from a sitting to a standing position because of an increase in flexion. Many patients also noted a perception of decreased weight as a result of superior suspension. Most patients who preferred the HST interfaces had medium to long in residual limbs with a medium or firm tissue consistency.

Some patients, 16%, preferred the PTB interface design. The primary reason for their preference was past use of PTB design or preference for a pelite liner or the suspension technique. Many of the patients rejected the HST interface because they experienced a pulling sensation on the distal end. Some of the patients who rejected the HST interfaces also did not like the feeling of uniform pressure. Occasionally, a throbbing or cramping sensation was experienced. Most patients who preferred the PTB design had short residual limbs that were conical in shape.

The remaining 16% rejected both interface designs. Their reasons for rejection included a variety of the aforementioned. In these cases alternative methods were used. They fell into the category of total surface-bearing interface design. Most of these patients had a residual limb with a soft tissue consistency.

Discussion

In a hydrostatic interface, the relationship between the posterior and anterior wall is not as crucial as in the PTB interface. There is no reduction or matching of anatomy as in the PTB prosthesis. The residual limb and interface takes its natural shape under pressure, rather than as a manipulation of the soft tissue. This is the reason for a larger AP, or sagittal diameter, in an HST interface. The coronal measurements were relatively the same. This is due to the area of the condyles having a relatively small amount of soft tissue. The length measurement was the most interesting. The hydrostatic interface produced an elongated residual limb. However, the PTB interface produced a shortened residual limb. This is due to the pressure chamber and the silicone suction suspension sleeve elongating the residual limb in the HST interface impression technique. The shortening of the soft tissue and the residual limb of the PTB interface were due to the impression technique, and more specifically to the Tubegauz.

The posterior wall measurement in relationship to MPT also produced interesting results. This measurement helped establish the relationship between the height of the poster wall and mid-patellar tendon. In a specific area weight-bearing prosthesis, this relationship is crucial. It is imperative that the posterior wall is, at the very least, at mid-patellar tendon. This increases pressure in sensitive areas (popliteal fossa and the patellar tendon) (Figure 16 ), it creates a longer lever arm that promotes distal anterior tibia pressure during sitting, and it limits the range of motion. This inhibits the patient from achieving full flexion at the knee. As previously stated, in an HST interface, the posterior wall is established by the flexor tendons. In the hydrostatic interface, the posterior wall is formed by the insertions of the flexor tendons. Therefore, greater range of motion can be achieved in this type of interface design.

Conclusion

The study clearly demonstrates the significant physical, volumetric, and mechanical differences between the two interface designs. The specific area weight-bearing prosthesis is a traditional design that has incorporated mechanical principles that can be improved. The hydrostatic design applies sound principles of physics that produce positive biomechanical results. This can benefit a significant percentage of our patient populations. With the overwhelming positive response of the amputees to the hydrostatic design, it would be more beneficial to our patients if we incorporated these principles as an alternative to traditional methods.

There are situations in which alternative designs are more appropriate. Through the regular use of hydrostatic design, we can more clearly recognize this smaller population of patients.


References:

  1. Narita H, Yokogushi K, Shii S, Kakizawa M, Nosaka T Suspension effect and dynamic evaluation of the total surface bearing trans-tibial prosthesis: a comparison with the patellar tendon bearing trans-tibial prosthesis. Prosthe Orthot Int. 1997;21:175-178.
  2. Kristinsson 0. The ICEROSS concept: a discussion of a philosophy. Prosth Orthot Int. 1993;17:49-55.
  3. Ohio Willow Wood Company. Alpha Liner Technical Manual. Mt. Sterling, Ohio:
  4. TEC Interface Systems. TEC Liner Instructional Manual. Waite Park, Minnesota:
  5. Streeter V, Wylie EB. Advanced Mechanics of Fluids. 1959, reprint 1976; fluid mechanics 7 ed., 1979.
  6. Rogers J, Wilson L. Preventing recurrent tissue breakdowns after "pressure sore" closures. Plast Reconstr Surg. October 1975; 419-422.
  7. Long, Ivan A. Normal shape- normal alignment (NSNA) above-knee prosthesis. Clin Prosthet Orthotics. 1985;9 (4):9-14.
  8. Buis AWP, Dynamic Interface Pressure Measurement: Comparing Two Trans-tibial Socket Concepts, 1997. Glasgow, Scotland, United Kingdom: National Center for Training and Education and Prosthetics and Orthotics, University of Strathclyde; 1997.
  9. Ossur -HE ICEX Technical Manual. Reykjavic, Iceland; 1996.
  10. Northwestern Technical Manual. Chicago, Illinois: Rehabilitation Institute of Chicago, Northwestern University, 1993, section 5.