In vitro biomechanical properties of 2 compression fixation methods for midbody proximal sesamoid bone fractures in horses

Biomechanical testing of a hybrid locking plate fixation of equine sesamoid osteotomies

OBJECTIVE

To compare the biomechanical properties of a hybrid locking compression plate (LCP) construct with the compression screw technique as a treatment for transverse mid-body proximal sesamoid bone fractures.

METHODS

Ten paired forelimbs from abattoir horses were used. The medial proximal sesamoid bone of each limb was osteotomized transversely and randomly assigned, to either repair with a two-hole 3.5 mm LCP or a 4.5 mm cortical screw placed in lag fashion. Each limb was tested biomechanically by axial loading in single cycle until failure. The point of failure was evaluated from the load-displacement curves. Then a gross evaluation and radiographs were performed to identify the mode of failure.

RESULTS

The loads to failure of limbs repaired with the hybrid LCP construct (4968 N ± 2167) and the limbs repaired with the screw technique (3009 N ± 1091) were significantly different (p <0.01). The most common mode of failure was through a comminuted fracture of the apical fragment of the proximal sesamoid bone.

CLINICAL SIGNIFICANCE

The LCP technique has potential to achieve a better fracture stability and healing when applied to mid-body fractures of the proximal sesamoid bone. Further testing, particularly fatigue resistance is required to corroborate its potential as a treatment option for mid-body fractures of the proximal sesamoid bone.

A Biomechanical Comparison of Headless Tapered Variable Pitch Compression and AO Cortical Bone Screws for Fixation of a Simulated Midbody Transverse Fracture of the Proximal Sesamoid Bone in Horses

OBJECTIVE

To compare mechanical properties and failure characteristics of 2 methods of fixation for repair of a transverse, midbody fracture of the proximal sesamoid bone (PSB): 4.5-mm AO cortical bone screw (AO) placed in lag fashion and 4/5-mm Acutrak (AT) self-compressing screw.

STUDY DESIGN

An in vitro biomechanical evaluation of intact forelimb preparations and forelimb preparations with a simulated midbody PSB fracture stabilized by a bone screw.

SAMPLE POPULATION

Sixteen paired and 8 unilateral cadaveric equine forelimbs.

METHODS

A midbody transverse osteotomy was created in the medial PSB of bilateral forelimbs of 8 equine cadavers. The osteotomized PSB in 1 forelimb from each cadaver was repaired with an AO screw. The osteotomized PSB in each contralateral limb was repaired with an AT screw. Eight unilateral intact control limbs were also studied. Mechanical properties were determined from axial compression, single cycle to failure, load-deformation curves. Failure characteristics were determined by evaluation of video images and radiographs.

RESULTS

No statistically significant differences were found between repair groups. Both AO and AT groups had significantly lower mechanical properties than intact limbs except for stiffness.

CONCLUSION

AO and AT constructs were mechanically comparable when used to stabilize a simulated midbody fracture of the medial PSB. Both constructs were mechanically inferior to intact limbs. Clinical Relevance- The AT screw should be considered for clinical use because of the potential for less soft tissue impingement and superior biocompatibility compared with the stainless-steel AO screw. However, postoperative external coaptation is necessary to augment initial fracture stability for either fixation method, and to maintain a standing metacarpophalangeal joint dorsiflexion angle between 150 degrees and 155 degrees.

A comparison of ultra-high-molecular weight polyethylene cable and stainless steel wire using two fixation techniques for repair of equine midbody sesamoid fractures: an in vitro biomechanical study

OBJECTIVE

To compare the monotonic tensile and fatigue strength of 16-gauge stainless steel wire (SSW) to ultra-high-molecular weight polyethylene (UHMWPE) cable using a transfixed cerclage technique in an in vitro midbody sesamoid osteotomy model. Endoscopic modifications to Martins transfixed cerclage technique were developed. A new suture technique of fixation was compared with the transfixed cerclage technique by measuring gap formation after cyclic testing.

STUDY DESIGN

An in vitro biomechanical paired equine cadaver limb study.

SAMPLE POPULATION

Twenty-one paired cadaveric adult equine forelimbs.

METHODS

Uniaxial medial midbody sesamoid osteotomies were created in paired adult equine forelimbs. Monotonic tensile strength was measured on 10 forelimbs repaired by a transfixed cerclage technique using wire or cable. Fatigue testing to failure was performed on 4 forelimbs repaired using the transfixed cerclage technique by cycling the limbs between 500 N and 2,000 N. The limbs were initially repaired with wire, cycled until the wire broke, then repaired with cable and cycled again to failure. Fatigue testing for gap displacement was performed on 8 limbs repaired with either the transfixed cerclage technique or the suture technique. Limbs were cycled between 500 N and 2,000 N for 10,000 cycles. The limbs were repaired with wire initially, tested, and then repaired with cable and tested again. Twenty-two limbs were used for mechanical testing. The remaining limbs (20) were used to develop and practice the endoscopic transfixed cerclage (10 limbs) and suture (10 limbs) techniques.

RESULTS

Ultimate tensile strength (UTS) of UHMWPE cable constructs was 34% greater than the UTS of SSW constructs. Fatigue strength was 2 to 20 times greater for UHMWPE cable constructs than SSW constructs. Separation of fragments was 153% less for limbs repaired by the suture technique compared with those repaired by the transfixed cerclage technique.

CONCLUSIONS

UHMWPE cable shows promise for this clinical application because of its greater tensile and fatigue strength. The newly described suture technique significantly reduced gap formation compared with the transfixed cerclage technique. Osteotomy gap formation occurred early in cycling, suggesting that rigid support in the form of a cast may be needed during the early postoperative period for wiring techniques.

CLINICAL RELEVANCE

Clinical testing of UHMWPE cable should eliminate problems of wire breakage seen with SSW. The endoscopic transfixed cerclage technique can be used by surgeons familiar with arthroscopic surgery. However, the suture technique needs to be tested in vivo to determine whether there is a clinical advantage compared with the transfixed cerclage technique.