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BK Suspension Systems 



Prosthetic socks can belong to one of three very general design categories.  In Patellar Tendon Bearing (PTB) sockets, very specific areas of the residual limb bear the majority of the weight bearing pressure.  Pressure here is usually focused on the patellar tendon and medial tibial flare.  In Total-Surface Bearing (TSB) sockets, the weight bearing pressure is evenly distributed throughout the residual limb.  The theory here is that if pressure and stress is evenly distributed, all areas experience less pressure.  Hydrostatic sockets involve distraction and Pascals’ theory of fluids.

PTB Sockets (Patellar Tendon Bearing)

In PTB sockets, a soft custom insert, usually composed of pelite foam material, aids in cushioning the residual limb.  The sides of the socket extend about 3” above the mid-patellar tendon and provide higher, more secure support around the sides of the amputee's knee.   

The pressures of a PTB socket on the residual limb are directed to the tendon just below the knee cap as well as the back of the calf and on either side of the tibia (shin bone).  The PTB sockets also have custom built in reliefs for the fibula head as well as the distal tibia.

To further increase stability in this type of socket, a cuff strap can be added.  This will assist in holding the residual limb more firmly into the socket and also prevents hyperextension of the knee.  However, a cuff strap can sometimes restrict the patient at the knee and does not necessarily improve medial lateral (side-to-side) stability.  Furthermore, if the strap does not fit correctly, the amputee may experience pistoning in the socket.  It may also cause atrophy to the lower quadriceps due to the cuff being snug around the leg.

In lieu of a cuff strap, as seen in this photograph, a suspension sleeve can be added to this socket.  A suspension sleeve is a knee sleeve that is usually composed of neoprene, latex, or silicone, and slips over the outside of the prosthesis, covers the amputee's knee, and comes up towards the thigh.  Prosthetic suspension sleeves are excellent at providing auxiliary support and concealing prosthetic trimlines. 

PTB-SC (Patellar Tendon Bearing – A PTB socket with supracondylar suspension comes up the sides of the amputee's knee, approximately 3.5 inches above the mid-patellar tendon.  This provides increased stability due to a tighter fit over the medial and lateral condyles.  This helps reduce side-to-side movement of the residual limb during ambulation

PTB-SCSP (Supracondylar/Suprapatellar) socket trimlines come up even higher than those in PTB sockets, enclosing the patella and encompassing the femoral condyles.  The advantage of this type of socket design is anterior stability and overall added security.  However, the encompassing nature of this prosthetic socket design can be perceived as restrictive especially when kneeling. 

TSB Sockets (Total-Surface Bearing)

The most important feature of a TSB socket is total contact.  Total contact is a casting and fabrication technique that ensures there is no gapping between the residual limb and prosthetic socket.   All areas of the residual limb must be in sufficient contact with the prosthetic socket in order to achieve total-surface bearing.   

Methods used with TSB

Locking Gel Liner Suspension involves using a locking gel liner as the main form of suspension.  The silicone liner is rolled onto the residual limb and is secured via suction.   The silicone gel liner includes a pin that protrudes straight out from the distal end.  The pin locks into a mechanism inside the plastic or laminated socket.   This is an incredibly secure form of prosthetic suspension as the amputee is 'locked' into the prosthesis

The silicone liner also cushions the residual limb and can vary in thickness, material, and size.   The amputee 'unlocks' the suspension with the simple push of a release button on the side of the socket

This also provides for a very cosmetically appealing and low profile prosthetic socket design.  The image below shows a lock gel liner inside a below knee prosthetic socket.

Gel Liner Suspension with Sleeve suspension uses a cushion liner to protect the residual limb.  The silicone cushion gel liner is rolled onto the residual limb and is secured via suction.   Sometimes the socket includes a suction valve, to expel air.  To supplement a secure suspension, an outer suspension sleeve is used.   A suspension sleeve is a knee sleeve and is usually composed of neoprene, latex, or silicone, and slips over the outside of the prosthesis, covers the amputee's knee, and comes up towards the thigh.   Prosthetic suspension sleeves are excellent at providing auxiliary support and concealing prosthetic trimlines.  One type of gel liner used, which does not require an additional suspension sleeve over the prosthesis, is a seal-in liner.   This liner is designed at the distal end to plug in securely to the socket using a unique umbrella design.  This type of liner can also include an expulsion valve that promotes suction. 

Suction sockets , for below knee prostheses, refer to two types of designs:  suction via suction valve and prosthetic suspension sleeve (see 'gel liner with sleeve' suspension description above), and suction via a Harmony VAS system (Vacuum Assisted Suspension).   A VAS system includes a pump that constantly expels air from the socket at every step, ensuring a secure and reliable suspension.   

Hydrostatic suspension relies on the theory of Pascal's law of fluids, which states that any confined custom fluid will react to external forces by evenly distributed the pressure.  A suction suspension sleeves serves as a pressure chamber that confines the fluids in the residual limb

Hydrostatic casting, casting the residual limb within a pressure chamber, also helps distribute fluids in the residual limb, often elongating the tissue in the residual limb and providing an anatomically natural cushion at the distal end. This is a technique that works well with a mature residual limb that does not experience much volume fluctuation.



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