LOWER EXTREMITY

PREPARATORY PROSTHESIS

This is the first prosthesis a new amputee receives.  The goal is to provide a light stable design, which acts as your training leg.  This device is typically worn for the first 4-6 months until the time when your residual limb has become stable in volume.  This prosthesis typically does not have a cosmetic cover since your prosthetist must make adjustments.

DEFINITIVE PROSTHESIS

This device is typically used for 2-4 years and is ultimately finished with a cosmetic cover if the user so desires.  By this time, you have successfully completed your physical therapy and have become a proficient user.  You will be introduced to additional types of feet, knees and other various componentry that is available.

Ertl

To understand what an Ertl bone bridge is, and why it is important, it helps to first understand the typical person’s lower leg boney anatomy. In the lower leg, there are two bones connected just below the knee and they are called the tibia and the fibula.  The tibia is the larger of the two and bears the majority of the weight, while the fibula is thinner and designed to act as a stabilizing bone particularly for the ankle.  They run vertically and parallel down the leg then stabilized again just above the ankle. By design, the relationship between these two bones is fairly solid and there tends to be very little to no motion between the two. 

When a person undergoes a traditional below the knee amputation, the lower leg is generally cut directly across both the tibia and the fibula, with the fibula ideally cut about 2 cm shorter than the tibia.  In most cases, the cut ends of the bones are left open allowing intramedullary pressure and the bone’s blood supply to flow out. Once transected, the underlying musculature retracts as they have lost most of the length-tension relationship from their opposing muscles. The muscles will atrophy in time, loosing strength and integrity due to lack of the original terminal connections. The nerves are transected and rarely given any special treatment.  

The previously mentioned motionless relationship between the tibia and fibula diminishes from an extremely stable design to something that may allow motion to occur. Generally, when someone wears a below the knee prosthesis, their residual limb is held up off the bottom of the socket by proximal and circumferential tension that is built into the socket.  This is due to the sensitivity of the cut ends of the bones; they typically will not tolerate much end pressure in the prosthetic socket.   

As the day progresses, most amputees have a tissue volume change and their prosthetic fit may become loose.  The tension that was incorporated in the socket design is no longer a tight enough fit to hold their limb up off the bottom of the socket; so proper sock management (adding additional socks) is required.  The worst case scenario is the amputee’s limb will sink down into the socket squeezing the fibula and tibia together particularly if the socket is not designed to accommodate the limb.   Over time, a poor fitting prosthesis, or improper sock ply management, can cause the fibula to become mobile, which may or may not become an issue and cause discomfort.  

This is one source of discomfort amputees may encounter. Another issue may occur as the limb sinks too far in the socket and the end of the tibia is now forced to bear too much weight due to the volume change and not managing the sock ply fit properly. Now that there is a clear understanding of a typical conventional method for a below the knee amputation, it should be much easier to understand what makes the Ertl bone bridge so different, and why it is such beneficial reconstructive alternative to consider.