Screw thread

Abstract: The Locking Compression Plate (LCP), in combination with the LISS and the PHILOS, is part of a new plate generation requiring an adapted surgical technique and new thinking about commonly used concepts of internal fixation using plates. The following guidelines are needed to avoid failures and possible complications in the hands of surgeons not yet confident with the new implant philosophy. The importance of the reduction technique and minimal-invasive plate insertion and fixation is addressed to keep bone viability undisturbed. Understanding of mechanical background for choosing the proper implant length and the type and number of screws is essential to obtain a sound fixation with a high plate span ratio and a low plate screw density. A high plate span ration decreases the load onto the plate. A high working length of the plate in turn reduces the screw loading, thus fewer screws need to be inserted and the plate screw density can be kept low. Knowledge of the working length of the screw is helpful for the proper choice of monocortical or bicortical screws. Selection is done according to the quality of the bone structure and is important to avoid problems at the screw thread bone interface with potential pullout of screws and secondary displacement. Conclusive rules are given at the end of this chapter.

Abstract: A bone plate for stabilizing the anterior column of the spine has a lower curved surface and an upper surface having two intersecting planes in each of which is a screw hole, the axes of the holes being skewed relative to one another. A bone screw used to secure the plate, has a single start buttress thread near the tip and a two start machine thread near the head, the threads being continuous and of constant pitch.

Abstract: A self-tapping screw with improved cutting point includes a head, a screw body, a double cutting point, and a washer. A first thread and a second thread are formed on the screw body. The second thread has the same pitch as the first thread and is formed between the first thread. The second thread has a major diameter which is less than the major diameter of the first thread. The first thread starts at substantially a shoulder and continues to a point of the screw body The second thread starts at substantially the shoulder and continues to the point of the screw body. The shoulder is formed on a bottom of the head. A washer preferably metal with rubber vulcanized thereon is placed under the shoulder of the head. The rubber seals the hole created by the self-tapping screw with improved cutting point. A notch may be included to improve the deburring and drilling ability thereof. The major diameter of the second thread may be increased to equal the major diameter of the first thread near the head to facilitate the automated application of the washer.

Abstract: Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.