Aims and methods

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.

Results

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).

Conclusions

The IM technique resulted in a stronger stiffer fixation when compared with the standard crossed lag screw technique.

Introduction

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).

Method

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.

Results

The interosseous membrane was 30% stronger than the interosseous ligament (1040 ± 183 N versus 798 ± 322 N, respectively; mean ± SD).

Conclusion

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.

Introduction

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.

Methods

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.

Results

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).

Discussion

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.