Factors Affecting the Pullout Strength of Cancellous Bone Screws

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.

Biomechanical efficacy of monoaxial or polyaxial pedicle screw and additional screw insertion at the level of fracture, in lumbar burst fracture: An experimental study

“…Although not human, calf spine has been used before as a valid biomechanical model 25–27. Calf is a tetrapod; its anatomical characteristics and common fracture site are different from those of the human, and because the calf lumbar spines (L1–L5) can be easily dissected from the other vertebrae of calf, we chose the L1–L5 calf lumbar spines and made the L3 vertebral fracture instead of the T12 or L1 which are the most common fracture sites in humans.…”

Additive manufactured push‐fit implant fixation with screw‐strength pull out

“…of 773 ± 61 N and 1124 ± 146 N, respectively) is of the same magnitude as that measured for a variety of bone screws in synthetic (Fig. 11)5, 7, 17, 40 and cadaveric bone 6, 17. It greatly exceeds current additive manufactured knee replacement designs which have peg fixation of only 100 N;16 applying the technology to such designs, or equivalent components in the shoulder, could offer clinical benefit through overcoming lift‐off42 or rocking43, 44 loosening mechanisms.…”

“…Screws are often used for ACL surgery, dental implants, fracture fixation, and early intervention chondral repair implants. They offer high levels of implant stability, with high pull‐out loads (around 1,000 N),5, 6 and by changing the thread, can be optimized for different bone densities 7. However, because screws must be rotated about their own axis to achieve fixation, they are of no use for non‐axisymmetric/multiple fixation features.…”

“…Press‐fit fixations enable non‐axisymmetric/multiple fixation features and are frequently used for arthroplasty components 12, 13, 14, 15. However, they provide lower initial fixation strength than screws (only around 50–150 N of pull‐out force, an order of magnitude less than a screw equivalent) 5, 6, 9, 16, 17. A technology that is able to provide screw‐strength fixation, while allowing non‐axisymmetric/multiple fixation features could therefore enable new/improved orthopaedic treatments.…”

Pull-out strength of the suprapedicle claw construct: a biomechanical study

“…Several variables can affect the strength of fixation, such as screw design, screw tapping, orientation and bone quality [3][4][5]. The use of a cancellous thread configuration, bicortical purchase in the vertebral bodies, and an upper purchase of the screw below the end plate may provide some solutions for a stronger purchase [3][4][5][6]. A further method to prevent pull-out is to add a pull-out stop in the form of a specially designed nut on the opposite side of the vertebral body [8].…”