How Prosthetic Limbs Stay Attached – Suspension
18 Mar 2017
Creating a prosthetic socket involves two major decisions: (1) the assembly and shape of the socket itself and (2) the method by which the prosthesis is held onto the residual limb, i.e. the suspension. More than any other aspect of prosthetics, socket assembly and suspension strategy usually spell the difference between prosthetic success and the closet.
For appropriate patients, using an atmospheric vacuum to hold the residual limb in the socket can provide a superior outcome. Suction options — whether provided by “pure” suction or a roll-on liner, possibly with some type of vacuum assist — limit limb movement within the socket and provide the best level of proprioception and greatest range of motion among current suspension methods.
With pure suction, precise socket fit enables residual limb skin to remain in full contact with the socket wall and thus preserve the vacuum created at donning. Donning typically involves actuating an expulsion valve at the distal end of the socket to evacuate air as the residual limb enters. A vacuum pump may be employed to enhance the suction once the residual limb is established in the socket.
Contraindications to suction include bony or irregular residual limb contours, often encountered with transtibial amputation levels; significant residual limb volume fluctuation; residual limb skin challenges; and physical or mental impairment that interferes with donning or removing the socket.
Roll-on suspension liners 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 pure suction suspension and improve amputee comfort; more common are liners incorporating some type of locking device —locking pin and shuttle lock, lanyard or locking strap — to securely attach the liner to the socket.
Prefabricated 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.
Other suction suspension aids include elastic suspension sleeves, which cover the proximal end of a transtibial socket and extend over the knee to the thigh; and vacuum-assist, which sustains the negative pressure in the socket and helps compensate for residual limb volume fluctuations.
When suction methods prove unfeasible, suspension utilizing anatomic structures frequently provides a viable alternative, particularly in transtibial and knee disarticulation limbs.
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.
Other anatomic suspension options include constructions that take advantage of congenital protuberances, often involving a cutaway section of the socket and a “door” panel to ease donning and doffing 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.
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 transtibial amputees with a delicate residual limb unable to withstand full weight-bearing loads.
For transfemoral amputees who cannot use suction, a silesian belt, total elastic suspension (TES) belt, or hip joint and pelvic belt may be used for suspending the prosthesis.
Suspension methods for upper-extremity prostheses may utilize suction, intimate fit around anatomic structures, a liner, one of several 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.
Gel liners can be used for both above- and below-elbow systems and function mostly like lower-limb liners. 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.