Biomechanical properties of new mini compression screws

Fracture Gap Closure and Reduction Are Affected by the Orientation of the Headless Compression Screw

BACKGROUND

We evaluated the impact of a variable-pitch headless screw's angle of insertion relative to the fracture plane on fracture gap closure and reduction.

METHODS

Variable-pitch, fully threaded headless screws were inserted into polyurethane blocks of "normal" bone model density using a custom jig. Separate trials were completed with a 28-mm screw placed perpendicular and oblique/longitudinal to varying fracture planes (0°, 15°, 30°, 45°, and 60°). Fluoroscopic images were taken after each turn during screw insertion and analyzed. Initial screw push-off, residual fracture gap at optimal fracture gap reduction, and malreduction were determined in each trial. Statistical analysis was performed via a 1-way analysis of variance followed by Student t tests.

RESULTS

Malreduction was found to be significantly different between the perpendicular (1.88 mm ± 1.38) and the oblique/longitudinal (0.58 mm ± 0.23) screws. The malreduction increased for the perpendicular screw as the fracture angle increased (60° > 45°=30° > 15° > 0°). Residual fracture gap at optimal fracture gap reduction was also found to be significantly different between the perpendicular (0.97 ± 0.42) and oblique/longitudinal (1.43 ± 1.14) screws. The residual fracture gap increased for the oblique/longitudinal screw as the fracture angle increased, although the oblique/longitudinal screw with a 60° fracture angle was the only configuration significantly larger than all the other configurations. Screw push-off was not found to be significantly different between the oblique/longitudinal screw and perpendicular screw trials.

CONCLUSIONS

The perpendicular screw had a larger malreduction that increased with fracture angle, whereas the oblique/longitudinal screw had a larger residual fracture gap that increased with fracture angle.

The Effect of Derotational Kirschner Wires on Fracture Gap Reduction With Variable-Pitch Headless Screws

PURPOSE

We evaluated the impact of angled derotational Kirschner wires (K-wires) on fracture gap reduction with variable-pitch headless screws.

METHODS

Fully threaded variable-pitch headless screws (20 and 28 mm) were inserted into "normal" bone models of polyurethane blocks. In separate trials, derotational K-wires were inserted at predetermined angles of 0°, 15°, 30°, and 40° and compared with each other, with no K-wire as a control. Fluoroscopic images taken after each screw turn were analyzed. The optimal fracture gap closure, initial screw push-off, and screw back-out gap creation were determined and compared at various derotational K-wire angles.

RESULTS

Initial screw push-off due to screw insertion and screw back-out gap creation were not significantly affected by the angle of the derotational K-wire. With a 20-mm screw, only a 40° derotational K-wire led to significantly less gap closure compared with control and with 0°, 15°, and 30° derotational K-wires. It led to an approximately 60% decrease in gap closure compared with no K-wire. With the 28-mm screw, compared with no K-wire, 15° and 30° derotational K-wires led to statistically significant decreases in gap closure (approximately 25%), whereas a 40° derotational K-wire led to an approximately 60% decrease. With the 28-mm screw, the 40° derotational K-wire also led to a statistically significant smaller gap closure when compared with 0°, 15°, and 30° derotational K-wires.

CONCLUSIONS

A derotational K-wire placed in parallel to the planned trajectory of a headless compression screw does not affect fracture gap closure. With greater angulation of the derotational K-wire, the fracture gap is still closed, but less tightly.

CLINICAL RELEVANCE

Derotational K-wires can help prevent fracture fragment rotation during headless compression screw insertion. At small deviations from parallel (≤30°), fracture gap closure achieved by the screw is minimally affected. At greater angles (ie, 40°), fracture gap closure may be substantially reduced, preventing fracture compression.

Bone Coverage around Hydroxyapatite/Poly(L-Lactide) Composite Is Determined According to Depth from Cortical Bone Surface in Rabbits

Composites of unsintered hydroxyapatite (HA) and poly(_L-lactide) (PLLA) reinforced by compression forging are biodegradable, bioactive, and have ultrahigh strength. However, foreign body reactions to PLLA and physical irritation can occur when not covered by bone. We aimed to confirm the relationships between the depth of the implanted HA-PLLA threaded pins and the new bone formation. We inserted HA-PLLA composite threaded pins (diameter: 2.0 or 4.5 mm) into the femoral and tibial bones of 32 mature male Japanese white rabbits (weight 3.0–3.5 kg) with the pin head 1 or 0 mm below or protruding 1 or 2 mm above surrounding cortical bone. Eight euthanized rabbits were radiologically and histologically assessed at various intervals after implantation. Bone bridging was complete over pins of both diameters at ~12 weeks, when inserted 1 mm below the surface, but the coverage of the pins inserted at 0 mm varied. Bone was not formed when the pins protruded >1 mm from the bone surface. No inflammation developed around the pins by 25 weeks. However, foreign body reactions might develop if composites are fixed above the bone surface, and intraosseous fixation would be desirable using double-threaded screws or a countersink to avoid screw head protrusion.

Fracture Gap Reduction With Variable-Pitch Headless Screws

PURPOSE

Fully threaded, variable-pitch, headless screws are used in many settings in surgery and have been extensively studied in this context, especially in regard to scaphoid fractures. However, it is not well understood how screw parameters such as diameter, length, and pitch variation, as well as technique parameters such as depth of drilling, affect gap closure.

METHODS

Acutrak 2 fully threaded variable-pitch headless screws of various diameters (Standard, Mini, and Micro) and lengths (16-28 mm) were inserted into polyurethane blocks of "normal" and "osteoporotic" bone model densities using a custom jig. Three drilling techniques (drill only through first block, 4 mm into second block, or completely through both blocks) were used. During screw insertion, fluoroscopic images were taken and later analyzed to measure gap reduction. The effect of backing the screw out after compression was evaluated.

RESULTS

Drilling at least 4 mm past the fracture site reduces distal fragment push-off compared with drilling only through the proximal fragment. There were no significant differences in gap closure in the normal versus the osteoporotic model. The Micro screw had a smaller gap closure than both the Standard and the Mini screws. After block contact and compression with 2 subsequent full forward turns, backing the screw out by only 1 full turn resulted in gapping between the blocks.

CONCLUSIONS

Intuitively, fully threaded headless variable-pitch screws can obtain compression between bone fragments only if the initial gap is less than the gap closed. Gap closure may be affected by drilling technique, screw size, and screw length. Fragment compression may be immediately lost if the screw is reversed.

CLINICAL RELEVANCE

We describe characteristics of variable-pitch headless screws that may assist the surgeon in screw choice and method of use.