A rotational limit for screw insertion may improve screw purchase and plate compression by reducing stripping, as compared to a torque based limit. Over-tightening screws results in inadvertent stripping of 20% of cortical bone screws. The current method of “two-fingers tight” to insert screws relies on the surgeon receiving torque feedback. Torque, however, can be affected by screw pitch, bone density and bone-thread friction. An alternative method of tightening screws is the “turn-of-the-nut” model, commonly used in engineering applications. In the “turn-of-the-nut” method, nuts used to fasten a joint are rotated a specific amount in order to achieve a pre-specified bolt tension. When applied to orthopaedics, bone assumes the role of the nut and the screw is the bolt. The screw is turned a set angular rotation that is independent of torque feedback. Potentially the “turn-of-the-nut” method provides an easier way of screw insertion that might lessen inadvertent screw stripping. The purpose of the current study was to use the “turn-of-the-nut” method to determine the angular rotation that results in peak plate compression and peak screw pullout force.Summary
Introduction
Avulsion fractures of the tip of the olecranon are a common traumatic injury. Kirshner-wire fixation (1.6mm) with a figure of eight tension band wire (1.25mm) remains the most popular technique. Hardware removal mat be required in up to 80% of cases. Modern suture materials have very high tensile strength coupled with excellent usability. In this study we compare a repair using 1.6mm k-wires with a 1.25mm surgical steel, against a repair that uses two strands of 2 fibrewire. Twelve Pairs of cadaveric arms were harvested. A standard olecranon osteotomy was performed to mimic an avulsion fracture. In each pair one was fixed using standard technique, 2 × 1.6mm transcortical ?-wire plus figure of 8 loop of 1.25mm wire. The other fixed with the same ?-wires with a tension band suture of 2.0 fibrewire (two loops, one figure of 8 and one simple loop). The triceps tendon was cyclically loaded (10-120 Newtons) to simulate full active motion 2200 cycles. Fracture gap was measured with the ‘Smart Capture’ motion analysis system. The arm was fixed at 90 degrees and triceps tendon was loaded until fixation failure, ultimate load to failure and mode of failure was noted. The average gap formation at the fracture site for the suture group was 0.91mm, in the wire group 0.96mm, no specimen in either group produced a significant gap after cyclical loading. Mean load to failure for the suture group was 1069 Newtons (SD=120N) and in the wire group 820 Newtons (SD=235N). Both types of fixation allow full early mobilisation without gap formation. The Suture group has a significantly higher load to failure (p=0.002, t-test). Tension Band suture allows a lower profile fixation, potentially reducing the frequency of wound complications and hardware removal.
Ten paired specimens were tested in simulated extension and the remaining ten were tested axially. Fragment motion relative to the humeral shaft was measured using kinematic analysis at the fracture gap. Differences in resultant fragment translations and rotations between fixation groups were checked for significance (p<
0.05) using a one-tailed paired t-test. Differences in cycles to failure were checked for significance using a Wilcoxon signed rank test.
On axial testing, the humeri with locking plates on average survived more loading cycles (4072) than those with non-locking plate fixation (2115), but the difference was not significant. Mean translation for locking plate fixation (3.6 mm) was significantly less than for non-locking plate fixation (5.7 mm) and mean fragment rotation was significantly less for locking plate fixation (13.3 degrees) than for non-locking plate fixation (17.8 degrees).
The distal part of the interosseous membrane (IM) may contribute to ankle joint stability and therefore partly explain the results of a study that reported no difference in outcome in patients with low Weber C fractures treated with or without a syndesmotic screw. The aim of the current study was to compare the strength of the IM to the interosseous ligament (IL).
Six paired cadaveric lower extremities were stripped, leaving only the IM and the IL intact. The tibia was fixed and a load was applied via a steel plate to the lateral surface of the fibula to displace it with respect to the tibia along the line of the fibers of the IM and IL. In group one the interosseous ligament was sectioned and the interosseous membrane was mechanically tested until failure. In group two, the interosseous membrane was sectioned and the interosseous ligament was tested.
The interosseous membrane was 30% stronger than the interosseous ligament (1040 ± 183 N versus 798 ± 322 N, respectively; mean ± SD).
The current biomechanical study found that the IM was 30% stronger than the IL. The interosseous membrane has considerable strength and may play a role in ankle stability.
To compare the mechanical stability of an intramedullary (IM) screw with two crossed interfragmentary compression screws for fixation of the 1st MTPJ in ten pairs of cadaveric feet. One foot underwent fixation with two crossed 4.0-mm cannulated cancellous screws. The contralateral foot was fixed with an IM 1.6-mm Kirschner wire and an IM 6.5-mm partially threaded cancellous lag screw. A plantar-to-dorsal load was applied to the distal end of the proximal phalanx at a rate of 1 mm/sec. Failure was defined as gross actuator displacement of 5 mm. Stiffness was defined as the slope of the force versus deformation curve between 10 and 60 N. Strength was defined as the load at failure. The differences in stiffness and strength parameters between the two fixation techniques were checked for significance (P <
0.05) with a paired t-test.
The intramedullary MTP joint fixation was significantly stiffer (18.7 ± 10.1 N/mm) than control group fixation (10.2 ± 6.1 N/mm). Similarly MTP joint fixation in the IM group was stronger (149.2 ± 88.2 N) than that of the control group (100.2 ± 70.8 N), but this was not significant (P = 0.07).
The IM technique resulted in a stronger stiffer fixation when compared with the standard crossed lag screw technique.
The strength of the Scarf osteotomy has been compared to that of other metatarsal osteotomies, but the effect of increasing the amount of displacement is unknown. The purpose of this study was to determine whether increasing offset adversly affects the strength of the Scarf osteotomy.
Seven pairs of freah frozen cadaveric feet were tested. Specimens in Group 1 underwent Scarf osteotomy with displacement of one third the mid shaft diameter. Specimens in Group 2 were offset two thirds the midshaft diameter. All osteotomies were fixed using two Barouk screws. Each specimen was tested in cantilever bending using a servohydraulic testing machine.
There was no statistically significant difference in strength or stiffness between the two groups. Mean strength was 75.2 N ± 16.8 for Group 1 and 64.8 N ± 28.7 for Group 2 (p>
0.05). Mean stiffness was 12.9 N/mm ± 5.1 for Group I and 10.2 N/mm ± 5.9 for Group 2 (p>
0.05).
All specimens failed at the proximal extent of the osteotomy. Failure did not occur by screw pullout in either Group. The proximal part of the cut is therefore the weakest part of the construct irrespective of the degree of osteotomy displacement.