In its limb prosthetics application, the term suspension derives from a primary definition of its root word suspend … “to hang from somewhere.”
In fact, that definition reveals the primary challenge of attaching a replacement limb to the human anatomy: Hanging from a residual limb, the prosthesis is subject to gravity, which in most postures applies a distal force that seeks to pull the replacement limb away from the body. In lower-limb applications, this displacement occurs primarily during gait swing phase; it is reversed during stance phase as the patient’s weight and momentum push the residual limb down into the socket. The resultant distal-proximal movement of the prosthesis on the residuum during ambulation is known as pistoning, a process that significantly hampers prosthetic performance through suboptimal gait, increased energy expenditure, discomfort, fatigue, substantial skin breakdown, and safety risks.
Prosthetic science has made great strides in recent years at minimizing pistoning through improved suspension methods and socket designs. Finding the right suspension option for any given patient is frequently the pre-eminent factor in achieving prosthetic success.
For appropriate patients, suspension methods utilizing an atmospheric vacuum to hold the residual limb in the socket usually provide the best outcomes. Suction options – whether provided by “pure” suction, a roll-on liner, hypobaric sock, and/or some type of vacuum assist – limit pistoning and provide the best level of proprioception and greatest range of motion of all current suspension methods.
With pure suction, precise socket fit enables residual limb skin to remain in total contact with the socket wall, thereby maintaining the vacuum created at donning. Success with pure suction requires that the wearer consistently be able to don and doff the prosthesis properly. Donning typically involves actuating an expulsion valve at the distal end of the socket to evacuate air as the residual limb enters. A “pull sock” or lubricating agent may be used to assist the donning process, and a vacuum pump may be employed to enhance the suction once the residual limb is established in the socket.
Contraindications to successful suction use include bony or irregular residual limb contours, as are often encountered with transtibial amputation levels; significant residual limb volume fluctuation; residual limb skin challenges; and physical or mental impairment that will interfere with donning and creating the needed suction or with removing the socket.
A roll-on suspension liner can overcome most of these obstacles, protecting the residual limb from shear forces, providing an easier donning
method, and compensating for irregular skin contours.
“Cushion” liners can enhance a suction suspension and improve amputee comfort. More common are liners incorporating some type of locking device – pin and shuttle lock, lanyard or locking strap – to securely attach the liner to the socket.
Pre-fabricated liners, available in a variety of materials, designs, and sizes, will work for many patients, while custom liners can be created for patients with major residual limb issues or special needs. Innovation continues to produce more functional and enduring prosthetic gel liners.
Other suction suspension aids include hypobaric socks, which incorporate a gel band to maintain the seal between the skin and socket wall; elastic suspension sleeves, which cover the proximal end of a transtibial socket and extend over the knee to the thigh; and the Harmony Vacuum Assist Socket System, which incorporates a small weight-actuated suction pump that sustains the negative pressure in the socket and helps compensate for residual limb volume loss.
When suction methods prove unfeasible, suspension utilizing anatomic structures frequently provides a viable alternative, particularly in transtibial and knee disarticulation prostheses.
Supracondylar suspension is accomplished by extended medial and lateral socket walls that fully encompass the femoral condyles and a compressible, contoured wedge that fits snugly above and against the medial condyle. The wedge can either be fabricated into the proximal socket wall or molded into a soft socket insert donned before the prosthesis. This method can provide excellent suspension as well as a degree of rotational control.
Other anatomic suspension options include constructions that take advantage of congenital protuberances, often involving a cutaway section of the socket and a “door” panel that is applied after the residual limb is in place.
Straps, Belts and Hinges
When suction or anatomical suspension is unavailable for various reasons, some older suspension methods may be employed. Strap suspension schemes, often used in combination with a waist belt, are relatively easy for the wearer to adjust and therefore are sometimes a good choice for individuals likely to encounter substantial changes in residual limb volume, as in the weeks after amputation surgery. However, strap methods allow significant pistoning and are generally not as comfortable as other suspension types.
A suprapatellar cuff, which encircles the thigh over the femoral condyles and attaches to the socket with straps, may be a good choice for transtibial patients who have good knee stability. It is normally used with a waist belt. A thigh corset with metal side joints may be prescribed for a delicate residual limb unable to withstand weight-bearing loads.
For transfemoral amputees who cannot use suction, a silesian belt, total elastic suspension (TES) belt, or pelvic joint and belt may be used for suspending the prosthesis.
Suspension methods for upper-extremity prostheses, both body-powered and externally-powered (myoelectric), are in many ways similar to those for a lower-limb system: They may utilize suction intimate fit around anatomic structures, a gel liner, one of several tried-and-true harness suspension methods, or a combination of these.
Harness systems are generally easy to don and remove but can significantly restrict range of motion, and strap chafing is common.
Pure suction, where applicable, can provide excellent suspension and is a desirable choice for an externally powered system
that does not require a harness for body control. Upper-limb suction sockets require some skill to don and generally stable residual limb volume.
Gel liners can be used for both above- and below elbow systems and function mostly like lower-limb liners; in fact, it was not until recently that liners designed specifically for upper-limb applications have been offered. A pin and shuttle lock may be used for short-to-medium transhumeral and transradial limbs, while a lanyard system is generally indicated for long transradial and wrist disarticulation levels.
Various transradial and wrist disarticulation socket designs incorporate anatomic suspension utilizing the humeral epicondyles. The best known of these is the Muenster socket for short transradial deficiencies.
Keeping Suspension ‘Simple’
In their on-going search for more precise and functional ways of replicating the function of the human limb, prosthetic practitioners regularly evaluate new ideas and technology that may provide improved functional outcomes for their patients.
The Keep It Simple Suspension (KISS) system provides a potentially better method of suspension for patients with a transfemoral deficiency, notably those who encounter difficulty donning their prosthesis. KISS components include a roll-on locking liner configured with distal and proximal attachment straps (photo 1) and socket fabricated with openings in the distal end and lateral side. The accompanying series of photos demonstrates how the system works.
In donning the prosthesis (either standing or sitting), the patient rolls the locking liner onto the residual limb in the usual manner (photo 2), threads the distal liner strap through the opening in the end of the socket (photo 3), feeds the lateral liner strap through the lateral socket opening (photo 4) while sliding the residual limb into the socket opening, inserts the distal strap into a D-ring sewn into the lateral strap (photo 5) and through pulley action seats the residual limb fully in the socket by pulling down on the distal strap and affixes the strap to a Velcro attachment on the socket’s distal exterior (photo 6).
The KISS attachment method has been demonstrated to virtually eliminate pistoning and distal draw and minimize rotation, even in the presence of volume loss.
Determining, designing, and creating the most effective suspension method for any prosthetic patient is a role properly reserved for the well-experienced, board-certified prosthetist.
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