Simultaneous anterior and posterior compression of the pelvic ring with external fixation using a pre-tensed curved bar: A biomechanical study.

INTRODUCTION: External fixators continue to be essential tools in the urgent treatment of pelvic fractures for compression and stabilization of the pelvic ring. Current systems fail to produce simultaneous anterior and posterior compression. A modified application of an existing curved bar fixator is proposed using a specifically designed tensioner to pre-tense the bar prior to its connection to Schanz screws. Subsequent pre-tension release and elastic recovery of the bar could potentially compress the pelvis. The aim of this work was to determine if the modified application could produce greater simultaneous compression across the sacroiliac joint and the symphysis of an unstable fractured pelvis than the standard application without pretension. MATERIALS AND METHODS: Six synthetic pelvis models with symphyseal and unilateral sacroiliac joints disruptions, simulating a Tile type C pelvic ring fracture, were used. Each specimen was stabilized using two 5mm×250mm supra-acetabular Schanz pins, a couple of open adjustable clamps and a semicircular carbon fibre rod applied without and with pre-tension. Two distances from bar to bone and three levels of pretension were compared. Each pelvis was tested with the six possible parameter combinations. Compressive forces at the disrupted joints were measured using pressure sensitive film sensors. RESULTS: The modified application produced forces significantly higher than the minimal compression achieved with standard application. At the sacroiliac joint, after pre-tension release, mean compressive forces measured ranged from 28.7 to 85.6N. The closest bar-to-bone distance always produced a significantly higher force; similarly, a significant increase in compression was found as the pre-tension level rose. At the symphysis, mean compressive forces between 35.3N and 49.0N were determined. No significant variations were seen with changes of any of the two factors analyzed CONCLUSIONS: To pre-tense a semi-circular bar before its use for external fixation of the fractured pelvis, is an effective means of applying compression simultaneously through the sacroiliac joint and the symphysis. The proposed method generates the highest compressive forces at the sacroiliac joint when the rod is subject to the highest pre-tension level not producing subluxation and is subsequently positioned as close as possible to the bone depending on patient's condition.

Close-looped graft suturing improves mechanical properties of interference screw fixation in ACL reconstruction.

PURPOSE: In anterior cruciate ligament reconstruction with looped soft-tissue grafts, an interference screw is frequently used for tibial fixation. This study compared three alternatives thought to improve the initial mechanical properties of direct bioabsorbable interference screw fixation: suturing the graft to close the loop, adding a supplementary staple, or increasing the oversize of the screw diameter relative to the bone tunnel from 1 to 2 mm. METHODS: Twenty-eight porcine tibiae and porcine flexor digitorum profundus tendons were randomized into four testing groups: a base fixation using 10-mm-diameter screw with open-looped graft, base fixation supplemented by an extracortical staple, base fixation but closing the looped graft by suturing its ends, and base fixation but using an 11-mm screw. Graft and bone tunnel diameters were 9 mm in all specimens. Constructs were subjected to cyclic tensile load and finally pulled to failure to determine their structural properties. RESULTS: The main mode of failure in all groups was pull-out of tendon strands after slippage past the screw. The sutured graft group displayed significantly lower residual displacement (mean value reduction: 47-67 %) and higher yield load (mean value increase: 38-54 %) than any alternative tested. No other statistical differences were found. CONCLUSIONS: Suturing a soft-tissue graft to form a closed loop enhanced the initial mechanical properties of tibial fixation with a bioabsorbable interference screw in anterior cruciate ligament reconstructions using a porcine model, and thus, this may be an efficient means to help in reducing post-operative laxity and early clinical failure. No mechanical improvement was observed for an open-looped tendon graft by adding an extracortical staple to supplement the screw fixation or by increasing the oversize of the screw to tunnel diameter from 1 to 2 mm.

Comparison of initial mechanical properties of 4 hamstring graft femoral fixation systems using nonpermanent hardware for anterior cruciate ligament reconstruction: an in vitro animal study.

PURPOSE: To compare the initial mechanical characteristics of 4 systems used to fix tendons to the femur during anterior cruciate ligament reconstruction. METHODS: A total of 32 porcine femurs were used to study the following fixation systems: Bioabsorbable interference screw (Stryker, Kalamazoo, MI), Bio-Transfix Cross-pin (Arthrex, Naples, FL), Biosteon Cross-pin (Stryker), and a fixation technique based on wrapping the graft around the femoral condyle itself, thus allowing it to be fixed in place without the use of any hardware. The mechanical characteristics of each system were obtained by a preconditioned failure tensile test. RESULTS: The yield load values (990.9 +/- 242.6 N for Bio-Transfix, 905.1 +/- 158.8 N for Biosteon Cross-pin, 684.4 +/- 119.7 N for the without-hardware system (WHS), and 369.4 +/- 120.1 N for the interference screw) revealed significant differences between the techniques that used cross-pins and the other 2 techniques (P < .006) on the one hand, and between the without hardware technique and the interference screw (P < .004) on the other. The stiffness of the 2 cross-pin fixation systems (117.6 +/- 22.5 N for Bio-Transfix and 112.6 +/- 22.5 N for Biosteon) was greater (P < .01) than those of the other systems (79.4 +/- 15.2 N for the WHS and 68.5 +/- 13 N for the interference screw). CONCLUSIONS: The initial biomechanical properties of the 2 cross-pin fixation systems proved to be superior to those of the other 2 systems studied. The WHS fixation system exhibited better mechanical properties than its interference screw counterpart. CLINICAL RELEVANCE: The better initial mechanical characteristics encountered using the Bio-Transfix and Biosteon Cross-pin systems indicate that these systems are better equipped to bear the loads generated by aggressive rehabilitation. The WHS fixation system provides an alternative to interference screw fixation.

Combination of finite element modeling and optimization for the study of lumbar spine biomechanics considering the 3D thorax-pelvis orientation.

A model of the lumbar spine capable of taking into account realistic loads derived from human activity would be of great benefit in studying its normal biomechanical functioning as well as its in vivo behavior in injured and surgically altered states. This paper proposes a method to analyze the mechanical response of the lumbar spine subjected to loads derived from human activity, combining a non-linear finite element model (FEM) and an optimization-based force predicting algorithm. Loads borne by the lumbar spine at the T12-L1 level (joint loads) are first predicted with the optimization algorithm and then applied to the FEM, while a boundary condition prescribing the relative L1-sacrum rotation is imposed onto the FEM to account for three-dimensional physiological thorax-pelvis orientation. The prescribed rotation is achieved through the application of moments on L1. To account for the effect of these moments on lumbar joint loads, an iteration between the optimization technique and the FEM computation has been carried out. This method provides two main benefits over previous studies: first, it allows for the application of any 3D loading condition while considering the real 3D rotation measured between the thorax and the pelvis, and second, it makes it possible to estimate the moments that must be applied on L1 in order to maintain this rotation, taking them into account when predicting joint loads. As an example application of the method, results are presented for the lumbar spine mechanical response at the time of peak T12-L1 joint force during walking.