Stefano Brianza

Biomechanical evaluation of a new fixation technique for internal fixation of three-part proximal humerus fractures in a novel cadaveric model.

BACKGROUND The optimal surgical treatment for displaced proximal humeral fractures is still controversial. A new implant for the treatment of three-part fractures has been recently designed. It supplements the existing Expert Humeral Nail with a locking plate. We developed a novel humeral cadaveric model and the existing implant and the prototype were biomechanically compared to determine their ability in maintaining interfragmentary stability. METHODS The bone mineral density of eight pairs of cadaveric humeri was assessed and a three-part proximal humeral fracture was simulated with a Greater Tuberosity osteotomy and a surgical neck wedge ostectomy. The specimens were randomly assigned to either treatment. A bone anchor simulated part of a rotator cuff tendon pulling on the Greater Tuberosity. Specimens were initially tested in axial compression and afterward with a compound cyclic load to failure. An optical 3D motion tracking system continuously monitored the relative interfragmentary movements. FINDINGS The specimen stabilized with the prototype demonstrated higher stiffness (P=0.036) and better interfragmentary stability (P values<0.028) than the contralateral treated with the existing implant. There was no correlation between the bone mineral density and any of the investigated variables. INTERPRETATION The convenience of this new IM-nail and locking plate assembly must be confirmed in vivo but the current study provides a biomechanical rationale for its use in the treatment of three-part proximal humeral fractures. The improved stability could be advantageous in particular when medial buttress is missing, even in osteoporotic bone.

Biomechanical evaluation of two-part surgical neck fractures of the humerus fixed by an angular stable locked intramedullary nail.

Objective: The aim of the current study was to see how different interlocking mechanisms would affect construct stability and overall failure in the treatment of two-part surgical neck fractures in the proximal humerus in vitro. Methods: Left and right bones of eight pairs of fresh-frozen human cadaveric humeri were assigned to either a group with conventional or a group with angular stable distal interlocking. The different experimental interlocking mechanisms were used in a surgical neck fracture model of the humerus (Orthopaedic Trauma Association 11- A3) stabilized by a proximal humeral nail. The following variables were evaluated by biomechanical tests: hysteresis width in bending and torsion, stiffness, and fracture gap movement during cyclic axial loading until failure and the overall failure mechanism of the construct. Results: The angular stable group showed significantly less motion in initial bending and torsion and higher bending stiffness throughout the complete deformation cycle compared with the conventional interlocked group. Fracture gap movement was significantly less in the angular stable group. Higher stability was mainly observed in the early phase of the applied loading pattern; however, ultimate failure was not related to distal interlocking but occurred in the proximal fragment in both groups. Conclusions: An experimental angular stable distal interlocking system of proximal humeral nails shows higher construct stability in the early phase of fracture fixation in vitro. In terms of overall failure, loss of fixation in the proximal fragment was crucial and not different between groups.

Mechanical Assessment of Local Bone Quality to Predict Failure of Locked Plating in a Proximal Humerus Fracture Model

The importance of osteoporosis in proximal humerus fractures is well recognized. However, the local distribution of bone quality in the humeral head may also have a significant effect because it remains unclear in what quality of bone screws of standard implants purchase. The goal of this study was to investigate whether the failure of proximal humerus locked plating can be predicted by the DensiProbe (ARI, Davos, Switzerland). A 2-part fracture with metaphyseal impaction was simulated in 12 fresh-frozen human cadaveric humeri. Using the DensiProbe, local bone quality was determined in the humeral head in the course of 6 proximal screws of a standard locking plate (Philos; Synthes GmbH, Solothurn, Switzerland). Cyclic mechanical testing with increasing axial loading until failure was performed. Bone mineral density (BMD) significantly correlated with cycles until failure. Head migration significantly increased between 1000 and 2000 loading cycles and significantly correlated with BMD after 3000 cycles. DensiProbe peak torque in all screw positions and their respective mean torque correlated significantly with the BMD values. In 3 positions, the peak torque significantly correlated with cycles to failure; here BMD significantly influenced mechanical stability. The validity of the DensiProbe was proven by the correlation between its peak torque measurements and BMD. The correlation between the peak torque and cycles to failure revealed the potential of the DensiProbe to predict the failure of locked plating in vitro. This method provides information about local bone quality, potentially making it suitable for intraoperative use by allowing the surgeon to take measures to improve stability.

A comparison of parallel and diverging screw angles in the stability of locked plate constructs

We investigated the static and cyclical strength of parallel and angulated locking plate screws using rigid polyurethane foam (0.32 g/cm(3)) and bovine cancellous bone blocks. Custom-made stainless steel plates with two conically threaded screw holes with different angulations (parallel, 10° and 20° divergent) and 5 mm self-tapping locking screws underwent pull-out and cyclical pull and bending tests. The bovine cancellous blocks were only subjected to static pull-out testing. We also performed finite element analysis for the static pull-out test of the parallel and 20° configurations. In both the foam model and the bovine cancellous bone we found the significantly highest pull-out force for the parallel constructs. In the finite element analysis there was a 47% more damage in the 20° divergent constructs than in the parallel configuration. Under cyclical loading, the mean number of cycles to failure was significantly higher for the parallel group, followed by the 10° and 20° divergent configurations. In our laboratory setting we clearly showed the biomechanical disadvantage of a diverging locking screw angle under static and cyclical loading.

Finite element analysis of a novel pin-sleeve system for external fixation of distal limb fractures in horses

The transfixation pin cast (TPC) is an external skeletal fixation technique used to treat horses with distal limb fractures, but its use is often associated with pin-loosening and an increased risk of treatment failure. To address implant loosening, the pin sleeve cast system (PSC) was recently designed and consists of a pin-sleeve unit inserted into the bone. Each pin runs through a sleeve placed in the bone, making contact at two fixed points only within the sleeve. Each pin is attached to a ring embedded in a resin cast. In this report, the mechanical performance of a traditional TPC pin arrangement was compared with that of the PSC using validated finite element models of bone substitutes previously tested in vitro. The PSC resulted in a marked reduction in peak strain magnitude around the pins and a more even distribution of strain across the bone cortex. The two systems resulted in comparable proximal fragment displacement and had a similar stress concentration around bone defects during implant removal. The findings suggest that the PSC load transfer mechanism is effective even in geometrically complex structures like equine bones.

In Vitro Mechanical Evaluation of a Novel Pin–Sleeve System for External Fixation of Distal Limb Fractures in Horses: A Proof of Concept Study

OBJECTIVE To evaluate the efficacy of a novel pin-sleeve cast (PSC) system for external fixation of distal limb fractures in horses and to compare it with the transfixation pin cast (TPC) system. STUDY DESIGN Experimental. SAMPLE POPULATION One bone substitute each was used for the TPC and PSC systems. The PSC was tested in 4 configurations characterized by different pin preloads. METHODS Specimens were loaded in axial compression in the elastic range. Variables compared statistically were: bone substitute axial displacement and axial strain measured above implants with strain gauges. Pin preload was correlated with the variables investigated. Load to failure and a fatigue tests supplemented the investigation. RESULTS The PSC configuration with the highest pin preload showed a significantly lower axial displacement compared with the TPC. No significant differences were observed between all other PSC configurations and the TPC. All PSC systems had a significant decrease in recorded strain compared with the TPC system. Pin axial preload inversely correlated with axial displacement but had no effect on axial strain. In the failure test, the PSC encountered plastic deformation earlier than the TPC. In the fatigue test, the PSC ran >200,000 cycles. CONCLUSIONS Preliminary in vitro tests showed that the PSC system significantly reduced peri-implant strain while concurrently having comparable axial displacement to the TPC system. CLINICAL RELEVANCE The PSC system has the potential to reduce the risk of pin loosening in horses.

In vitro Evaluation of the Torsional Strength Reduction of Neonate Calf Metatarsal Bones with Bicortical Defects Resulting from the Removal of External Fixation Implants

OBJECTIVE To compare the torsional strength of calf metatarsal bones with defects produced by removal of 2 different implants. STUDY DESIGN In vitro mechanical comparison of paired bones with bicortical defects resulting from the implantation of 2 different external fixation systems: the transfixation pin (TP) and the pin sleeve system (PS). SAMPLE POPULATION Neonatal calf metatarsal bones (n = 6 pairs). METHODS From each pair, 1 bone was surgically instrumented with 2 PS implants and the contralateral bone with 2 TP implants. Implants were removed immediately leaving bicortical defects at identical locations between paired metatarsi. Each bone was tested in torque until failure. The mechanical variables statistically compared were the torsional stiffness, the torque and angle at failure, and work to failure. RESULTS For TP and PS constructs, respectively, there were no significant differences between construct types for any of the variables tested. Mean ± SD torsional stiffness: 5.50 ± 2.68 and 5.35 ± 1.79 (Nm/°), P = .75; torque: 57.42 ± 14.84 and 53.43 ± 10.16 (Nm); P = .34; angle at failure: 14.76 ± 4.33 and 15.45 ± 4.84 (°), P = .69; and work to failure 7.45 ± 3.19 and 8.89 ± 3.79 (J), P = .17). CONCLUSIONS Bicortical defects resulting from the removal of PS and TP implants equally affect the investigated mechanical properties of neonate calf metatarsal bones.Objective To compare the torsional strength of calf metatarsal bones with defects produced by removal of 2 different implants. Study Design In vitro mechanical comparison of paired bones with bicortical defects resulting from the implantation of 2 different external fixation systems: the transfixation pin (TP) and the pin sleeve system (PS). Sample Population Neonatal calf metatarsal bones (n = 6 pairs). Methods From each pair, 1 bone was surgically instrumented with 2 PS implants and the contralateral bone with 2 TP implants. Implants were removed immediately leaving bicortical defects at identical locations between paired metatarsi. Each bone was tested in torque until failure. The mechanical variables statistically compared were the torsional stiffness, the torque and angle at failure, and work to failure. Results For TP and PS constructs, respectively, there were no significant differences between construct types for any of the variables tested. Mean ± SD torsional stiffness: 5.50 ± 2.68 and 5.35 ± 1.79 (Nm/°), P = .75; torque: 57.42 ± 14.84 and 53.43 ± 10.16 (Nm); P = .34; angle at failure: 14.76 ± 4.33 and 15.45 ± 4.84 (°), P = .69; and work to failure 7.45 ± 3.19 and 8.89 ± 3.79 (J), P = .17). Conclusions Bicortical defects resulting from the removal of PS and TP implants equally affect the investigated mechanical properties of neonate calf metatarsal bones.

Biomechanical and computational evaluation of two loading transfer concepts for pancarpal arthrodesis in dogs

OBJECTIVE To evaluate 2 plate designs for pancarpal arthrodesis and their effects on load transfer to the respective bones as well as to develop a computational model with directed input from the biomechanical testing of the 2 constructs. SAMPLE Both forelimbs from the cadaver of an adult castrated male Golden Retriever. PROCEDURES CT imaging was performed on the forelimb pair. Each forelimb was subsequently instrumented with a hybrid dynamic compression plate or a castless pancarpal arthrodesis plate. Biomechanical testing was performed. The forelimbs were statically loaded in the elastic range and then cyclically loaded to failure. Finite element (FE) modeling was used to compare the 2 plate designs with respect to bone and implant stress distribution and magnitude when loaded. RESULTS Cyclic loading to failure elicited failure patterns similar to those observed clinically. The mean ± SD error between computational and experimental strain was < 15% ± 13% at the maximum loads applied during static elastic loading. The highest bone stresses were at the distal extent of the metacarpal bones at the level of the screw holes with both plates; however, the compression plate resulted in slightly greater stresses than did the arthrodesis plate. Both models also revealed an increase in bone stress at the proximal screw position in the radius. The highest plate stress was identified at the level of the radiocarpal bone, and an increased screw stress (junction of screw head with shaft) was identified at both the most proximal and distal ends of the plates. CONCLUSIONS AND CLINICAL RELEVANCE The FE model successfully approximated the biomechanical characteristics of an ex vivo pancarpal plate construct for comparison of the effects of application of different plate designs.