Werner Sternstein

Influence of formalin fixation on the biomechanical properties of human diaphyseal bone

Abstract Owing to the lack of fresh human bones, formalin-fixed specimens are frequently used in biomechanical testing. However, formalin fixation is assumed to affect the biomechanical properties of bone. The aim of this study was to compare axial and torsional stiffness and bone mineral density in fresh and embalmed human bones. The subtrochanteric regions of 12 pairs of fresh human femora were randomised into two groups for paired comparison. After bone mineral density measurement, one group was preserved in 4% formalin. After 6 weeks, bone mineral density was remeasured and each specimen underwent axial and torsional loading. The formalin group showed significant higher stiffness values for torsional and axial loads than the fresh group. Axial stiffness increased by 14.1%, whereas torsional stiffness increased by 14.3%. These differences were not reflected in bone mineral density values. Formalin fixation significantly influences the stiffness of human cadaveric bones. Fresh bones represent the in vivo conditions better than formalin fixed bones.

New intramedullary locking nail for olecranon fracture fixation--an in vitro biomechanical comparison with tension band wiring.

BACKGROUND The aim of this study was to determine the difference in displacement of a newly designed intramedullary olecranon fracture fixation device compared with multifilament tension band wiring after 4 cycles and 300 cycles of dynamic continuous loading. METHODS In eight pairs of fresh-frozen cadaver ulnae, oblique olecranon fractures were created and stabilized using either newly designed intramedullary olecranon nail or multifilament tension band wiring. The specimens were then subjected to continuous dynamic loading (from 25 N to 200 N) using matched pairs of cadaveric upper extremities. The Wilcoxon test was used to determine statistical differences of the displacement in the fracture gap. RESULTS After 4 cycles and 300 cycles, the displacement in the fracture model was significantly higher in the tension band wiring group than in the intramedullary nailing group. CONCLUSIONS The newly designed interlocking nailing system showed higher stability in comparison with multifilament tension band wiring after continuous dynamic loading.

Bending and Torsional stiffness in cadaver humeri fixed with a self-locking Expandable or interlocking nail system a mechanical study

Objectives: This study was designed to gain data about a new expandable, noninterlocked intramedullary nails capacity to stabilize unstable transverse humeral shaft fractures without the need for interlocking, thus making nail implantation simpler and to prove our goal hypothesis: that in a midshaft osteotomy of the humeral shaft the expandable humeral nail will show the same bending and torsional stiffness as an interlocked humeral nail, when implanted correctly according to the manufacturers instructions. Design: Pair randomization. Setting: Mechanical laboratory testing. Participants: Eight pairs of freshly harvested cadaveric humeri. Interventions: Fracture model was a midshaft transverse osteotomy, gapped to 3 mm. Each humerus pair received an expandable humeral nail (Fixion®) or an interlocked humerus nail (Synthes) through a retrograde approach. The humeri were fixed in polymethylmethacrylate cylinders and tested in a servo-pneumatic material-testing machine. Main Outcome Measurements: Torsional stiffness and bending stiffness of the nail-bone-construction. Results: Expandable nails (interlocked nails) showed a lateral bending stiffness of 0.73 ± 0.14 (0.63 ± 0.1) KN/mm (P = 0.026) and a frontal bending stiffness of 0.67 ± 0.18 (0.58 ± 0.09) KN/mm (P = 0.084). Torsional stiffness values were 0.13 ± 0.19 (0.43 ± 0.09 Nm/°) (P = 0.012). Lower torsional stiffness in the expandable nail group was observed in humeri with a funnel shaped proximal intramedullary canal. Conclusions: The nail systems showed similar characteristics for frontal bending (P = 0.084), but not for lateral bending (P = 0.026). For lateral bending, the Fixion® nail showed significantly more stiffness than the UHN® nail (P = 0.026). There was significantly lower torsional stiffness with expandable nails compared with interlocked nails. Clinical correlation would suggest that in rotationally unstable fractures (A2 and A3 diaphyseal fractures), interlocked nails would provide increased stability over expandable nails.

Screw fixation of radial head fractures: Compression screw versus lag screw—A biomechanical comparison

INTRODUCTION Secondary loss of reduction and pseudarthrosis due to unstable fixation methods remain challenging problems of surgical stabilisation of radial head fractures. The purpose of our study was to determine whether the 3.0mm Headless Compression Screw (HCS) provides superior stability to the standard 2.0 mm cortical screw (COS). MATERIALS AND METHODS Eight pairs of fresh frozen human cadaveric proximal radii were used for this paired comparison. A standardised Mason II-Fracture was created with a fragment size of 1/3 of the radial heads articular surface that was then stabilised either with two 3.0 mm HCS (Synthes) or two 2.0 mm COS (Synthes) according to a randomisation protocol. The specimens were then loaded axially and transversely with 100 N each for 4 cycles. Cyclic loading with 1000 cycles as well as failure load tests were performed. The Wilcoxon test was used to assess statistically significant differences between the two groups. RESULTS No statistical differences could be detected between the two fixation methods. Under axial loads the COS showed a displacement of 0.32 mm vs. 0.49 mm for the HCS. Under transverse loads the displacement was 0.25 mm for the COS vs. 0.58 mm for the HCS group. After 1000 cycles of axial loading there were still no significant differences. The failure load for the COS group was 291 N and 282 N for the HCS group. CONCLUSION No significant differences concerning the stability achieved by 3.0 mm HCS and the 2.0 mm COS could be detected in the experimental setup presented.

Volar fixed-angle plating of distal radius fractures: screws versus pegs--a biomechanical study in a cadaveric model.

Objectives: The purpose of this biomechanical study was to determine whether a multidirectional fixed-angle plate with locking screws or with locking pegs in the distal fragment would optimize fixation of Orthopaedic Trauma Association (OTA) type A3 distal radius fractures. Methods: Eight pairs of fresh–frozen human distal radii were used. Extra-articular distal radius fractures were created and stabilized with a multidirectional volar fixed-angle plate. The radii were randomized into 2 matched-paired groups. The distal fragment in Group I was stabilized with 7 locking screws. The distal fragment in Group II was fixed with 7 locking pegs. The proximal fragment in both groups was fixed with 3 screws. The specimens were tested under torsion and axial compression during static and cyclic tests. Finally, load-to-failure tests were performed under torsion. Results: After 1000 cycles, 99% of the median torsional stiffness remained in the group using screws, whereas only 76% of the median stiffness under torsion remained in the group using pegs (P = 0.018). Under axial compression, median stiffness remained at 93% in the group using screws after 1000 cycles compared with a median of 0% in the group using pegs (P = 0.018). Conclusions: This biomechanical study showed a statistically significant difference between the locking screw and locking smooth peg configuration with regard to stiffness of the constructs after 1000 cycles. The use of locking screws as opposed to smooth locking pegs for OTA type A3 extra-articular distal radius fractures optimizes construct stability.

Sind winkelstabile Plattensysteme mit elastischen Eigenschaften für die Stabilisierung der Oberarmkopffraktur vorteilhaft? / Eine in-vitro biomechanische Untersuchung

Abstract Winkelstabile Platten werden mit zunehmender Häufigkeit zur Stabilisierung von Oberarmkopfbrüchen verwendet. Rigide Implantate führen bei osteoporotischen Knochenverhältnissen u. U. aber häufiger zum Versagen. Um den Effekt der Implantatelastizität zu beurteilen wurden in einer in-vitro Studie die biomechanischen Eigenschaften eines rigiden und eines elastischen winkelstabilen Plattensystems vergleichend analysiert. An 8 Humeruspaaren wurde eine instabile subkapitale Fraktur simuliert. Die Proben wurden axial- und torsions-belastet. Analysiert wurden die Steifigkeit, das Setzverhalten unter Dauerlast und die Versagensbelastung. Das elastische Plattensystem kennzeichnete sich durch eine geringere Torsionssteifigkeit und eine größere Verformung unter Axial- und Torsionsbelastung. Die Proben versagten früher als beim rigiden Plattenystem. Unter biomechanischen Gesichtspunkten bieten Implantate mit elastischen Eigenschaften am Oberarmkopf somit keine Vorteile. Are angle-fixed implants with elastic properties advantageous for the internal fixation of proximal humerus fractures? There is a recent interest for the use of angle-fixed plates in the management of proximal humerus fractures. Rigid implants might be associated with an increased risk of cutting-out. In order to analyse the potential beneficial effects of the implant elasticity on fracture fixation, the biomechanical properties of a rigid and an elastic angle-fixed plating system were assessed in an experimental study. An unstable fracture of the surgical neck was created in 8 pairs of human humeri. Specimens were subjected to axial loading and torque. Stiffness, subsidence and load to failure were assessed. The implant with elastic properties was characterized by a lower torsional stiffness and a higher subsidence during axial loading and torque. This implant failed at lower loads than the rigid implant did. Elastic implant properties of angle-fixed plates have shown not to be advantageous for the management of fractures of the proximal humerus.

Locking reconstruction double plating of distal humeral fractures: how many screws in the distal ulnar column segment in A3 fracture provide superior stability? A comparative biomechanical in vitro study.

Objectives: Two 90-degree configurations of locking reconstruction plates with different numbers of screws in the distal ulnar column segment of distal extra-articular humeral fractures with metaphyseal comminution (A3) were biomechanically investigated. Methods: Eight pairs of fresh-frozen human humeri were used. For paired comparison, the humeri were divided into 2 randomized groups. In both groups, double-plate osteosyntheses with locking reconstruction plates were performed in 90-degree configurations. In group 1, the posteriorly placed radial column plate exceeded the capitellum and the ulnar column plate extended into the ulnar sulcus. The ulnar plate was molded around the medial epicondyle and fixed with 3 short angular stable screws distally. In group 2, the posteriorly placed radial column plate was applied analogous to group 1. The locking reconstruction plate placed on the ulnar column was used reaching to the ulnar epicondyle, fixed with 1 long, angular, stable screw in the distal fragment. Stiffness testing for axial load and bending in static and cyclic tests were performed. In static test mode, a load was applied with a frequency of 0.1 Hz. For cyclic loading conditions, a load was applied at 1 Hz for 5000 cycles. Results: All tested specimens adequately resisted simulated physiologic loading conditions with no failure. Comparable stiffness values for axial load (P = 0.161) and significant lower stiffness values for bending (P = 0.017) in group 2 under static bending conditions were found. Considering cyclic loading conditions, no significant alterations in stiffness in each group under axial load occurred. In bending conditions, stiffness values for group 2 were significantly lower than that for group 1 (P = 0.036). Conclusions: Under static and cyclic bending conditions, stiffness in group 2 was significantly lower than that in group 1. Nevertheless, both implant configurations showed no failure of the constructs. Based on these data, when applying locked plates in the clinical setting, more than 1 locked screw applied into the distal ulnar column of the articular segment is recommended.

Osteosynthese am proximalen Humerus mittels winkelstabiler Platte oder Doppelplatte: eine vergleichende biomechanische Untersuchung / Angle-fixed plate fixation or double-plate osteosynthesis in fractures of the proximal humerus: a biomechanical study

Zusammenfassung Für die Osteosynthese einer proximalen Humerusfraktur wird insbesondere bei osteoporotischen Knochenverhältnissen eine hohe Stabilität der Osteosynthese gefordert, um sekundäre Repositionsverluste zu vermeiden. In einer experimentellen Untersuchung wurden die biomechanischen Eigenschaften der winkelstabilen Philos-Platte und der Doppelplattenosteosynthese mit zwei Drittelrohrplatten miteinander verglichen. Frakturmodell war eine instabile Dreifragmentfraktur (AO Typ B2). Acht Probenpaare wurden unter Axiallast und in Torsion über 200 Zyklen belastet. Gemessen wurde, in welchem Ausmaß die ursprüngliche Probensteifigkeit nach der Osteotomie durch die Osteosynthese wiederhergestellt werden konnte. Weiterhin wurde das Setzverhalten der Proben ermittelt. Unter Axiallast zeigte sich sowohl bezüglich Steifigkeit und Setzverhalten eine signifikante Überlegenheit der Philos-Proben. Zwei Doppelplatten-Osteosynthesen versagten unter der Dauerbelastung. Unter Torsion zeigte sich dagegen kein signifikanter Unterschied zwischen den beiden Osteosynthesen. Unter biomechanischen Gesichtspunkten weist die winkelstabile Philos-Platte im Vergleich zur Doppelplattenosteosynthese Vorteile hinsichtlich einer höheren Stabilität auf, so dass sie insbesondere bei hochgradig instabilen Frakturen des proximalen Humerus gegenüber der Doppelplatte zu bevorzugen ist. Abstract Internal fixation of fractures of the proximal humerus needs a high stability of fixation to avoid secondary loss of fixation. This is especially important in osteoporotic bone. In an experimental study, the biomechanical properties of the angle-fixed Philos plate (internal fixator) and a double-plate osteosynthesis using two one-third tubular plates were assessed. The fracture model was an unstable three-part fracture (AO type B2). Eight pairs of human cadaveric humeri were submitted to axial load and torque. In the first part of the study, it was assessed to which degree the original stiffness of the humeri could be restored after the osteotomy by the osteosynthesis procedure. Subsequently, subsidence during 200 cycles of axial loading and torque was analysed. During axial loading, the Philos plate was significantly stiffer and showed less irreversible deformation. Two double-plate fixations, but none of the Philos plate osteosynthesis, failed. During torsion, there were no significant differences between the two implants. From the biomechanical point of view, the angle-fixed Philos plate represents the implant of choice for the surgical fixation of highly unstable three-part fractures of the proximal humerus, as the internal fixator system is characterised by superior biomechanical properties.

Compression-locked nailing of the humerus: A mechanical analysis

Background In the treatment of humeral fractures, reamed nailing and compression have been reported to give higher stability. In this cadaver study, we compared the Unreamed Humeral Nail (UHN) with the (reamed) Telescopic Locking Nail (TLN) to find out whether any differences exist concerning bending and rotational stability, both with and without compression. Methods Nails were tested in a paired set-up with 8 pairs of fresh frozen cadaveric humeri. The nail-bone constructs were submitted to axial distraction to test compression, four-point bending and torsion. After creating a bone defect simulating an unstable fracture, bending and torsional tests were run again Results After cyclic loading, distraction under compression with the TLN was significantly less than with the UHN: 0.10 (SD 0.06) vs. 0.31 (SD 0.18) mm (difference = –67%, 95% CI = –84% to –37%; p = 0.01). In bending, the constructs with TLN under compression were stiffer than those with the UHN: 0.96 (SD 0.25) vs. 0.80 (SD 0.25) kN/mm (difference = 0.16, 95% CI = 0.07 to 0.25; p = 0.01). In torsion and with a bone defect, no significant differences were found. Interpretation Both nails are capable of resisting physiological forces acting on the humerus. The constructs with the TLN under compression are more stable in bending. Compression with an axial set screw is the more stable option.

Dynamische Analyse von Osteosynthesen am Olecranon: ein in-vitro Vergleich zweier Osteosynthesesysteme / Dynamic analysis of olecranon osteosyntheses – an in vitro comparison of two osteosynthesis systems

Zusammenfassung Hintergrund: Ziel dieser Studie war die Entwicklung eines Test-Setups mit kontinuierlicher Winkeländerung für die Imitation der Gelenkbewegung des Ellenbogens zum mechanischen Vergleich von Zuggurtungosteosynthese und einem neu entwickelten intramedullären Nagelsystem. Material und Methode: Der Rotationsmotor des servopneumatischen Testapparates arbeitet winkelgesteuert für die Ellenbogenbewegung, der Linearmotor kraftgesteuert für den M. triceps brachii. Das Frakturmodell wurde dynamisch unter zyklischer Belastung getestet. An 14 frischen Leichenulnae wurde eine schräge Osteotomie des Olecranon durchgeführt und zwei verschiedene Osteosynthesesysteme angelegt. Eine Gruppe erhielt die Zuggurtungsosteosynthese, die andere Gruppe den intramedullären Nagel implantiert. Das Bewegungsausmaß des Ellenbogengelenkes änderte sich zwischen 0° Extension und 100° Flexion unter einer dynamischen Zugkraftvariation zwischen 25 und 150 N. Anhand zweier Paare von Markerpins, die jeweils am proximalen und am distalen Fragment frakturnah angebracht waren, wurde die Relativbewegung zwischen den Pins gemessen. Die Bewegungsanalyse wurde nach 4 und nach 300 Zyklen kontinuierlicher Belastung durchgeführt. Ergebnisse: Nach 300 Zyklen war die Lockerung der Zuggurtungsosteosynthese signifikant höher als die der intramedullären Nagelosteosynthese. Diskussion: Andere Untersuchungen von Osteosynthesen mit Gelenkbeteiligung veränderten die Krafteinwirkung nicht dynamisch. Das Test-Setup der vorliegenden Studie imitiert die Gelenkbewegung mit einer kontinuierlichen Änderung des Gelenkwinkels. Das ist eine wichtige Voraussetzung für eine akkurate biomechanische Untersuchung von unterschiedlichen Osteosyntheseverfahren zur Fixierung von Olecranonfrakturen. Das getestete intramedulläre Nagelsystem zeigte im Vergleich zur Zuggurtungsosteosynthese eine signifikant geringere Lockerung unter Belastung. Abstract Introduction: The aim of the present study was to develop a test setup with continuous angle alteration to imitate elbow joint motion for the mechanical evaluation of tension band wiring and a newly designed intramedullary nail. Materials and methods: The servo-pneumatical test stand worked with a rotational angle-adjusted and a linear force-adjusted engine. The fracture model was dynamically tested under cyclic loading imitating elbow joint motion. In total, 14 fresh cadaver upper extremities underwent olecranon fracture by means of transverse osteotomy and were assigned to two groups: tension band wiring and intramedullary nailing. There was a continuous angle alteration between 0 and 100° of flexion, with continuous changing pull force between 25 N and 150 N. Two steel pins were placed in the proximal, two in the distal olecranon fragment for video analysis of the motion between the two pairs of pins. Displacement in the fracture gap was determined after 4 and 300 cycles. Results: After 300 cycles, the displacement in the fracture fixation model was significantly higher in the tension band wiring group than in the intramedullary nailing group. Discussion: Other studies evaluating biomechanical properties of olecranon osteosyntheses with joint involvement did not change the force direction dynamically. We introduced a test setup with continuous angle alteration to imitate joint motion. This is an important step for accurate biomechanical evaluation of the treatment of different fixation methods in olecranon fractures. The tested nailing system showed significant advantages in loosening under cyclic loading compared to tension band wiring.