Concerning Suspension Alignment, and Control
Charles H. Pritham, C.P.O.
In the prescription of any prostheses consideration is naturally given to the proper means of suspending the prosthesis and maintaining it in place. In contrast, not as much concern seems to be given to this crucial matter in the prescription of an orthosis.
Paradoxically, this relative state of neglect is undoubtedly due to the very success with which suspension has been incorporated in most conventional orthosos. To cite but one example, the shoe that inevitably must be used with any ambulatory AFO, KAFO, or HKAFO provides for suspension of the device as well as providing support to the ground.
In recent years with the expansion of new technology in the area of prosthetics and orthotics there has developed a corresponding interest in new techniques to overcome shortcomings in conventional devices. In the process, however, new problems can arise as a result of the intertwining roles played by various components of the device under consideration, and it would therefore appear worthwhile to attempt to sort out these various roles with special emphasis on suspension in order to clarify the picture, and possibly, as a result, to suggest new and unique applications for the various suspension systems available.
For clarity a brief glossary has been prepared, and is included at the conclusion of this article.
Maintenance of Alignment
For any prosthesis or orthosis to provide the maximum benefit possible, it must be held in proper position relative to the body segments concerned. The prevention of inappropriate motion can be classified broadly as maintenance of alignment by either suspension or stabilization depending upon the direction of the motion. As it is defined, suspension is concerned with the prevention of linear displacement along the longitudinal axis, and it will be seen that no discrimination is made as to whether the direction is distal or proximal. Thus, the perineal straps that may be attached to a spinal orthosis to prevent proximal displacement ("riding-up") are just as much a suspension aid as is a suprapatellar cuff suspension strap on a below-knee prosthesis.
Considered in this light, the weightbearing component of any given device naturally prevents proximal displacement, and, thus, may be confused as a suspensory component. The distinction must be made on the basis of intended function.
Weightbearing is a primary characteristic of a lower-limb prosthesis or a weightbearing orthosis without which it cannot function. Suspension is a secondary characteristic inasmuch as it is but one of a number of different components intended to ensure proper weight bearing and thus function of the device. It can be seen, therefore, that the intended role of a weightbearing component is quite a bit different than suspension. However, the use of this same component as a non-weightbearing device for purposes other than weightbearing is not inconceivable. It is possible, if not practical, to use PTB brims about the knees of a patient to prevent proximal displacement of a corset, and the use of quadrilateral sockets as anchor points for the powering of upper-limb prostheses comes to mind.
Stabilization, as it is defined, is concerned with the prevention of displacement about the various rotatory axes of the body rather than along the linear axes. Motion does take place undoubtedly includes some linear motion, either laterally or anterioposteriorly, but in the author's opinion the rotary displacement is inevitably the predominant component. How then is stabilization to be differentiated from control which, as it is defined, is also involved, in part, with the prevention of motion?
Two separate but interrelated definitions of the word control are given. In both instances control is to be considered as a primary characteristic. In the first definition control refers to the regulation of motion in one portion of the body segment relative to another portion, while stabilization (a secondary characteristic) refers to the regulation of the device relative to the body segment. In the second definition control refers to volitional regulation of motion in the device by the patient; while stabilization holds the device in firm contact with the body segment in order to maximize the efficiency of this volitional regulation.
In any event, it can be appreciated that any given component of a prosthetic or orthotic device may play multiple roles in the function of that device. A hip joint and pelvic band fitted to an above-knee prosthesis while providing suspension also provides stabilization against lateral and rotary motion. The same component is likely to be fitted to an HKAFO to control motion about the patient's hip, and is unlikely to be used for suspension or rotary stabilization of the HKAFO since both of these functions are provided effectively by the fit of the foot in the shoe. Supracondylar wedge suspension in a below-knee prosthesis also provides effective stabilization against lateral thrust, while a cuff suspension strap fitted to a below-knee prosthesis does not. A figure-8 harness (Fig. 1) fitted to an above-elbow prosthesis not only provides suspension, but also stabilization against lateral or rotary motion of the socket and control of the elbow and terminal device, while a butterfly harness and Bowden cable (Fig. 2) fitted to a shoulder-driven WHO provides only control of motion in the metacarpal-phalangeal joints of the index and ring finger and neither suspension nor stabilization.
These are but a few of the many examples that could be cited in designing or prescribing a device for a given situation. Consideration must be given to the many intertwining roles played by the many available design elements and selection be made of those elements that perform the intended function with maximum benefits and a minimum of adverse side effects.
A particularly troublesome example of this dilemma is to be found in the design of an orthosis to control knee motion without involving the ankle-foot complex, the traditional source of suspension and rotary stabilization of devices to regulate the knee. If supracondylar suspension is used as with the IRM supracondylar knee orthosis (Fig. 3) or Iowa knee orthosis (to name but two examples of this class of orthosis) adequate suspension and stabilization may be gained initially from the critical fit about the knee, but the patient may not be able to tolerate it, and with compression of the soft tissues fit and, thus, suspension may be lost. The CARS-UBC knee orthosis (Fig. 4) avoids these problems by using a waist belt and suspension strap. Waist belts, however, are not well tolerated by many patients, and considerable effort must be taken in fitting the device to achieve adequate rotary stabilization.
In any given instance it is necessary to weigh the pros and cons of the applicable suspension components available, and select the one that best fits the needs of the patient.
Classification of Suspension Types
In most instances, suspension is secured by obtaining a purchase above a flaring bony prominence (epicondyle, adductor tubercle) or other body segment (buttocks, shoulder). This general principle is the same regardless of type of suspension. Suspension may be classified into two major groups and a third miscellaneous one (Fig. 5).
A. Extrinsic Suspension: The means of suspension are not contained within the proper borders of a device, and must be gained by the addition of extraneous elements that pass beyond the borders of the device and may not be otherwise absolutely necessary for the function of the device. However, the extrinsic elements may also serve as means of stabilization or control.
Examples of extrinsic suspension are:
PTB cuff suspension strap
Knee joints and thigh corset
Rubber suspension sleeve
Hip joint and pelvic band
Silesian belt f. Suspenders
Perineal straps on a spinal orthosis
Various harnesses used in upper-limb orthotics and prosthetics
B. Intrinsic Suspension: Suspension is gained by means of some elements) contained within the proper borders of the device. The elements) may also serve as a means of stabilization.
Examples of intrinsic suspension are:
All self-suspending prostheses
All orthoses with few exceptions
A shoe is necessary for the proper function of lower-limb orthoses while a waist belt used on a KO is not absolutely necessary for the function of the KO as suspension can be accomplished by other means. Therefore, an AFO is a case of intrinsic suspension while a KO is not necessarily an example.
Types of intrinsic suspension can be broken down as follows:
Supracondylar: purchase is obtained above any of the various condyles or epicondyles of the body.
Flaring body segments other than bony prominences: purchase is obtained above any of the flaring body segments not covered in Item 1, such as the buttocks or shoulders.
Suction, or negative atmospheric pressure: In general, suction suspension is used with amputation stumps that exhibit a high soft-tissue-to-bone ratio with few prominent subcutaneous bony prominences such as above-knee or above-elbow stumps; however, suction suspension has been used with below-knee prostheses in Europe and there is a current resurgence of interest in it in America.
Muscular grasp: This is the often greatly overlooked ancillary of suction suspension and other suspension types. Rudolf Poets has described briefly the principle of an "undercut socket" he attributes to Dr. Oskar Hepp, and every clinician is familiar with the admonition to the patient that he should use his stump muscles to hold the above-knee prosthesis on. Many below-knee amputees have reported being able to hold their prosthesis on with muscular contractions, and Dr. Ernest Burgess is currently studying how to capitalize on this phenomenon.
Compression of soft tissue and friction: This means of suspension serves for such lightweight, elastic, and readily conformable devices as a spinal corset or knee support and may be used in conjunction with other means of suspension.
C. Other Miscellaneous: This serves as a catch-all division to contain those means that do not readily fit in the other divisions and are rarely used in prosthetics and orthotics.
Examples of the miscellaneous category are:
Medical grade adhesive used with rigid dressings, some cosmetic finger prostheses, facial restoration, and stoma appliances.
Skeletal attachment. While under active consideration by some, this means of suspension is not currently in use.
As can be seen in Fig. 6 and Fig. 7, selection of an appropriate means of suspension for a specific device can often pose problems. A variety of factors must be considered, a few of which are listed here.
Clinic team preferences
Fitting difficulties and problems maintaining proper fit.
Necessary related functions (stabilization or control) provided by a specific suspension system.
In any event the essential matter is to balance the pros and cons of the various suspension systems available and select the one that offers the most advantages with the fewest disadvantages. The matter becomes even more important when the emphasis is shifted from routine clinical prescription to the design of one-of-a-kind applications for a
specific patient's unique problems or in research and development of a new style device.
Conclusion and Summary
Suspension is inevitably related closely to a wide variety of interrelated factors, all of which are involved in the determination of proper fit. An attempt has been made to logically sort out the various factors and concentrate on suspension. Further, suspension has been broken down into various categories and some of the inherent difficulties in selecting between a number of suspension techniques relevant to a specific patient or prosthetic or orthotic device have been suggested.
Orthosis: An externally applied device for the control of motion about the joints of a body segment.
Prosthesis: (Artificial Limb)-an externally applied device to substitute for a missing body segment.
Suspension: The method of maintaining a prosthesis or orthosis in proper place relative to the affected body segment and resisting linear displacement along the longitudinal axis.
Displacement due to:
"oozing" "creeping" (movement due to compression of a conical section)
Weight-Bearing: The transmission of a person's mass (or weight) to the ground from a relatively distant body segment by means of a prosthesis or orthosis.
Stabilization: The method of maintaining a prosthesis or orthosis in proper placement relative to the affected body segment and resisting angular or rotary displacement about one of the three axes.
Moments created by the eccentric application of forces about the various axes or centers of rotation.
Orthotic: The maintenance of a body segment in a desired position or positions by an orthosis (also called correction or corrective control).
Orthotic or Prosthetic: The voluntary activation of a prosthesis or orthosis (or of an artificial articulation thereof) by means of the body segment enclosed in the device or by a signal generated by a remote body segment and transmitted to the device or articulation by means of a mechanical, hydraulic, pneumatic, or electric linkage (also called volitional control).
Alignment: The relationships that exist or are to be created between the components of a device or between the device as a whole and the affected body segment.
Pistoning: The cyclical linear displacement that takes place along a body segment with the cyclic application and removal of a load and due to inadequate suspension.
"Bell-Clappering" : Cyclical angular displacement in the A-P or M-L planes due to inadequate angular stabilization.
Whipping: A specific form of rotary instability that occurs in AK Prostheses.
Primary Characteristic: An absolutely essential property of a device if it is to carry out its intended function.
Secondary Characteristic: A property of a device necessary to facilitate one of its primary characteristics but not itself absolutely necessary to achieve the intended function of the device.
- Blakeslee, Burton (ed.), The limb-deficient child, University of California Press, 1963.
- Poets, Rudolfe, The fitting of the above-knee stump, Orth. and Pros., 28:1, March 1974.