PI-82 Determination of Screw Tightening Force

The “PI-82 Screw Compression Force Determination” standard, developed by PI Medical Implants , describes a test method for determining the compressive force generated when double-threaded screws with different thread pitches, known as Herbert screws, are inserted into a standard material.

Herbert Screws have a headless design. This allows the screw to be completely embedded in the bone without any protrusions that could cause tissue irritation, even in intra-articular placement. Cannulation helps ensure precise screw placement. Herbert cannulated guide pins hold the fragment and act as a guide for drilling, tapping, and screw placement. The screw's thread design creates compression and provides fracture stability. As the proximal threads penetrate the bone, the fracture is pulled together, helping to create and maintain stability of the fracture site. These screws are manufactured in both titanium and stainless steel.

Herbert screws are intended for the management of fractures where minimal tissue coverage makes the use of standard screws impractical or where extreme precision in fragment alignment is imperative. Such locations may include: the olecranon and malleolar region, distal femur, proximal tibia, and distal tibia.

Herbert screws mainly come in the following sizes:

• The 2,5 mm and 3 mm Herbert screws are indicated for fixation of small bone fractures and nonunions and small bone arthrodeses, including scaphoid fractures, intra-articular fractures of the tarsal, metatarsal, carpal and metacarpal bones, bunionectomies and osteotomies, arthrodeses of small joints (e.g., phalanges), and fractures of the patella, ulna and radial styloid.
• The 3,5 mm and 4,5 mm Herbert screws are intended for fixation of intra- and extra-articular fractures and nonunions of small bones and small bone fragments, small joint arthrodeses, bunionectomies and osteotomies including the scaphoid and other carpal bones, metacarpals, tarsals, metatarsals, patella, ulnar styloid, capitellum, radial head and radial styloid.
• The 5,5 mm and 6,5 Herbert screws are indicated for fracture fixation, reconstruction, osteotomy, and arthrodesis of a variety of bones and bone fragments, including joint fusions in the foot and fixation of intra-articular fractures of the humerus, femur, and tibia.

Bone screws are the most commonly used orthopedic implants. There are many different types and sizes of screws for different types of bones. Most bone screws are made of stainless steel or titanium alloys. The outer diameter, root diameter, and thread pitch and angle are important in determining screw mechanics.

In orthopaedics, screws are usually described by their outside diameter. The pitch of a screw is the linear distance the screw travels for one complete rotation. The screw advances a distance equal to the distance between the threads for each complete rotation. Cortical screws have a lower pitch and therefore a greater number of threads. Cancellous bone screws have a greater screw depth to increase surface area and therefore improve uptake since the bone is weaker.

Screws function by converting tightening torque into internal stress in the screw and elastic reactions in the surrounding bone. This creates compression between the fracture fragments held together by the screw. The screw is usually placed in drilled holes equal to the root diameter and is inserted into self-tapping or tapped (threaded) holes. The torque required to insert cortical bone screws can be high, so screws must be properly inserted into the correct size hole and designed to withstand the expected levels of insertion torque in the cortical bone.

Among the numerous testing, measurement, analysis and evaluation studies carried out for businesses in various sectors, our organization also provides testing services defined in the "PI-82 Determination of Screw Compression Force" standard with its trained and expert staff and advanced technological equipment.