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Firm Fixation, Complete Degradation
 


In the successful reconstruction of a cruciate ligament, a tendon from the patient’s own body is fixed with two interference screws directly in the bone.

 

When cruciate ligaments are torn in an accident, they are usually replaced by tendons from the patient’s own body. Sysorb® is a bioresorbable interference screw that fixes the “new” cruciate ligament in the knee. This fixation is as stable as with a metal screw. If bioresorbable material is used instead of metal, a second operation to remove the metal implants is no longer necessary.

 


Info direct

Sulzer Orthopedics Ltd
Monica Fucentese
Erlenauweg 17
CH-3110 Münsingen
Switzerland
Telephone
+41 (0)41-768 36 80
Fax
+41 (0)41-768 16 80
E-mail
monica.fucentese
@sulzer.ch

 


Monica Fucentese
Sulzer Orthopedics

Whether playing soccer in summertime or skiing in wintertime, the knee is the joint most subject to sports injuries. One of the possible injuries to the knee is a torn of the anterior cruciate ligament. As generally is the case with sports injuries, most patients with torn anterior cruciate ligaments are active young persons. A complete reconstruction of the ligament is very important for them, so that they can participate as unhindered as possible in every athletic as well as professional activity.
.....
Removing a tendon surgically from the patient’s own body and implanting it in place of the injured ligament is becoming the treatment of choice. Either the middle third of the patellar tendon or tendons taken from the inner thigh (semitendinosus and gracilis) can be used. The so-called interference screws (Fig. 1n) are normally used to secure the “new” cruciate ligament into place in the bone. These screws settle the graft in the immediate vicinity of the articulation, in the holes previously drilled into the femur and into the tibia (Fig. 2n). In highly industrialized countries like Switzerland or the United States, this operation is carried out annually about 800 times per million residents.

Completely degradable
The latest developments in biotechnology combined with modern manufacturing technologies have enabled the production of bioresorbable screws which can be successfully implanted in humans. The Sysorb interference screw is a product of this technical evolution. It was designed in 1995 with the help of the orthopaedist Dr. Andreas Stähelin, and is made of polylactide, a lactic acid polymer. Lactic acid polymers are metabolized in the human body to water and CO2 and then eliminated. The speed of this degradation depends on the polymer structure and on the proportion of L(evorotary)- and D(extrorotary)-lactides. Pure poly-L-lactide for example, which is likewise used for interference screws, is partially crystalline and is degraded only after 2–6 years. Sysorb, on the other hand, consisting of 50% L- and 50% D-lactides (poly-D,L-lactide), is amorphous and is completely degraded within only one year (Fig. 3n). This time frame of degradation is long enough to stabilize the tendon transplant during the one-to-two-months healing period, but markedly shorter than that of the partially crystalline screw.

Secure screwing-in guaranteed
The design and manufacture of the Sysorb screw are adapted to the specific properties of the material. The screws meet the mechanical requirements made and are easy to implant. The design of the screw compensates completely for the lower biomechanical strength of the amorphous, as compared with a crystalline, material. A special screwdriver has been developed, with which the screw can be placed securely into the bone (Fig. 4n). The screwdriver is made in such a way that the torque is applied along the entire length of the screw as it is screwed in. There are no centrifugal or torsion forces that could cause the screw to break. Biomechanical studies by Weiler et al. have shown that of all the bioresorbable screws tested, Sysorb can stand the greatest torque at failure while being screwed in.
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The primary fixation of the transplanted tendon achieved with the bioresorbable Sysorb is as stable as primary fixation supplied by conventional metallic interference screws. Several biomechanical studies by Weiler et al. have shown that the resistance to being pulled out is comparable to that of metallic interference screws; about 600 N. A cruciate ligament has to be able to stand forces of about 400 N as a rule.

No second surgery necessary
In addition to possessing certain biomechanical properties and degradation capability, bioresorbable implants have to meet certain biocompatibility requirements. Among other difficulties, rejection by the body can lead to severe inflammation, associated with pain. The good biocompatibility of the Sysorb has been confirmed clearly by extensive clinical experience acquired over many years. Histological tests show that the bone grows into the tendon within a short time (Fig. 5n), and that during the degradation of the Sysorb screw, there is no evidence of inflammation in the bone tissue (Fig. 6n).
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A great advantage of bioresorbable implants, as opposed to metal implants, is the fact that a second surgery to remove the screws is no longer necessary. The whole procedure is therefore a lesser strain for the patient, and costs are lower. In addition, postoperative radiologic examinations such as MRI (magnetic resonance imaging) or CT (computer tomography) can be carried out without artefacts. Metal parts, on the other hand, distort the pictures created by MRI or CT, making their interpretation more difficult.



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