Verzögerte Bruchheilung und Pseudarthrose nach Kahnbeinfraktur der Hand : Versuchsaufbau und -durchführung zur Messung von Torsionskräften am Os scaphoideum

Zielsetzung Ziel dieser Arbeit ist es, einen Versuchsaufbau zu evaluieren, der geeignet ist, an einem Kadaverpräparat Biege- und Torsionskräfte innerhalb des Kahnbeines zu messen, die durch Bewegung und Belastung des Handgelenkes auftreten. Methode Im Rahmen einer an der Medizinischen Universität Wien (MUW) durchgeführten Studie wurde bei 10 Kadaverpräparaten nach einer Osteotomie des Os scaphoideum ein Messsensor implantiert und in einem eigens dafür entwickelten Teststand fixiert. Anschließend wurden aktive und passive Flexions- und Extensionsbewegungen sowie Abduktionsbewegungen nach radial und ulnar durchgeführt und die dabei auftretenden Biege- und Torsionskräfte gemessen. Die passive Führung erfolgte händisch unter Zuhilfenahme eines Goniometers zur Ablesung bzw. Aufzeichnung des Bewegungswinkels. Zusätzlich wurden diese Messungen nach einer Bohrdrahtfixierung zwischen distalem Kahnbeinfragment und Os capitatum sowie zwischen den Ossa lunatum und capitatum wiederholt. Im Laufe dieser Versuche wurden verschiedene Modifikationen am Testaufbau durchgeführt, um eine gute Reproduzierbarkeit der Messungen zu erreichen. Ergebnis Aktive Bewegungen konnten aufgrund technischer Probleme nicht durchgeführt werden. Aus diesem Grund wurden in weiterer Folge passive Bewegungen in unbelastetem Zustand sowie nach einer Gewichtsbelastung ausgewählter Muskelsehnen des Unterarms durchgeführt. Bei den Messwerten zeigten sich große Abweichungen des Ausmaßes und der Richtung der einwirkenden Kräfte. Durch die Anwendung eines im Zuge der vorliegenden Arbeit standardisierten Testverfahrens mit einer hohen Anzahl an Messwiederholungen wurden diese Varianzen großteils auf die individuellen biomechanischen Eigenschaften des Karpus zurückgeführt. ^Schlussfolgerungen Während der Durchführung der Forschungsarbeit konnte die Versuchsanordnung so weit optimiert werden, dass eine verlässliche Aussage reproduzierbarer Ergebnisse möglich ist. Es konnten einige Vorschläge zu zukünftigen Versuchen gemacht werden, um die Auswertbarkeit der Messdaten zu verbessern. So wurde für die Zukunft als wesentliche Voraussetzung für eine valide Auswertung empfohlen, gepaarte Unterarmpräpate zu verwenden, bildgebend bzw. computerassistiert ein Koordinatensystem des Os scaphoideum zu schaffen und zur Vermeidung der Übertragung von Kräften vom Untersucher auf das Präparat Datenwerte zu betrachten, die nicht im Bereich der maximalen Bewegungsauslenkung liegen.Objectives The objective of this dissertation is to evaluate a test setup, which is capable of measuring flexural and torsional forces in the scaphoid bone of a human cadaveric specimen during movement of the wrist. Methods Within the frame of a scientific study at the Medical University of Vienna 10 fresh frozen human cadaveric forearm specimens got a measuring sensor implanted into the scaphoid bone after osteotomy of the scaphoid waist. Subsequently the specimen was attached to a customized test stand. Thereafter flexion and extension movements as well as radial and ulnar abduction movements were performed actively and passively with intend to measure flexural and torsional forces transmitted through the scaphoid bone. Passive motion was carried out manually using a goniometer for reading respectively recording the range of motion. This method of measurement was rerun after Kirschner wire fixation of the carpal bones between the distal scaphoid fragment and the capitate bone and between the lunate and capitate bone. During the conducted research the experimental design was adapted and modified to illustrate forces transmitted through the scaphoid bone and to achieve good reproducibility. Results The simulation of active movement was abandoned due to technical difficulties. It was substituted with non-weight-bearing passive motion and passive motion under load of selected muscle tendons. The results showed significant deviations in magnitude and in the effective directions of the acting forces. During the research project a standardized method of measurement was developed. The use of this setup in combination with a large number of measurements could illustrate that the variance is due to specific biomechanical properties of the carpus. ^Conclusions In the course of the research the test setup could be optimized, resulting in reliable and reproducible data. Some suggestions were given to improve the evaluation of these measurements. The main requirements for reliable results include the use of paired cadaveric specimens, the application of a coordinate system of the scaphoid bone derived from imaging or computer-assisted reconstruction and the prevention of human influence on the specimen by using data values other than the maximal possible excursion of the movements.eingereicht von Philip SchefzigAbweichender Titel laut Übersetzung der Verfasserin/des VerfassersZsfassung in engl. SpracheWien, Med. Univ., Diss., 2013OeBB(VLID)294165

Wrist movements induce torque and lever force in the scaphoid: an ex vivo study

PURPOSE We hypothesised that intercarpal K-wire fixation of adjacent carpal bones would reduce torque and lever force within a fractured scaphoid bone. METHODS In eight cadaver wrists, a scaphoid osteotomy was stabilised using a locking nail, which also functioned as a sensor to measure isometric torque and lever forces between the fragments. The wrist was moved through 80% of full range of motion (ROM) to generate torque and force within the scaphoid. Testing was performed with and without loading of the wrist and K-wire stabilisation of the adjacent carpal bones. RESULTS Average torque and lever force values were 49.6 ± 25.1 Nmm and 3.5 ± 0.9 N during extension and 41 ± 26.7 Nmm and 8.1 ± 2.8 N during flexion. Torque and lever force did not depend on scaphoid size, individual wrist ROM, or deviations of the sensor versus the anatomic axis. K-wire fixation did not produce significant changes in average torque and lever force values except with wrist radial abduction (P = 0.0485). Other than wrist extension, torque direction was not predictable. CONCLUSION In unstable scaphoid fractures, we suggest securing rotational stability with selected implants for functional postoperative care. Wrist ROM within 20% extension and radial abduction to 50% flexion limit torque and lever force exacerbation between scaphoid fragments

Finite element based estimation of contact areas and pressures of the human scaphoid in various functional positions of the hand

The scaphoid is the most frequently fractured carpal bone. When investigating fixation stability, which may influence healing, knowledge of forces and moments acting on the scaphoid is essential. The aim of this study was to evaluate cartilage contact forces acting on the intact scaphoid in various functional wrist positions using finite element modeling. A novel methodology was utilized as an attempt to overcome some limitations of earlier studies, namely, relatively coarse imaging resolution to assess geometry, assumption of idealized cartilage thicknesses and neglected cartilage pre-stresses in the unloaded joint. Carpal bone positions and articular cartilage geometry were obtained independently by means of high resolution CT imaging and incorporated into finite element (FE) models of the human wrist in eight functional positions. Displacement driven FE analyses were used to resolve inter-penetration of cartilage layers, and provided contact areas, forces and pressure distribution for the scaphoid bone. The results were in the range reported by previous studies. Novel findings of this study were: (i) cartilage thickness was found to be heterogeneous for each bone and vary considerably between carpal bones; (ii) this heterogeneity largely influenced the FE results and (iii) the forces acting on the scaphoid in the unloaded wrist were found to be significant. As major limitations, accuracy of the method was found to be relatively low, and the results could not be compared to independent experiments. The obtained results will be used in a following study to evaluate existing and recently developed screws used to fix scaphoid fractures

Rotational Stability of Scaphoid Osteosyntheses: An In Vitro Comparison of Small Fragment Cannulated Screws to Novel Bone Screw Sets

<div><p>Background</p><p>The current standard of care for operative repair of scaphoid fractures involves reduction and internal fixation with a single headless compression screw. However, a compression screw in isolation does not necessarily control rotational stability at a fracture or nonunion site. The single screw provides rotational control through friction and bone interdigitation from compression at the fracture site. We hypothesize that osteosyntheses with novel bone screw sets (BSS) equipped with anti-rotational elements provide improved rotational stability.</p><p>Methods</p><p>Stability of osteosynthesis under increasing cyclic torsional loading was investigated on osteotomized cadaveric scaphoids. Two novel prototype BSS, oblique type (BSS-obl.) and longitudinal type (BSS-long.) were compared to three conventional screws: Acutrak2^®mini, HCS^®3.0 and Twinfix^®. Biomechanical tests were performed on scaphoids from single donors in paired comparison and analyzed by balanced incomplete random block design. Loading was increased by 50 mNm increments with 1,000 cycles per torque level and repeated until a rotational clearance of 10°. Primary outcome measure was the number of cycles to 10° clearance, secondary outcome measure was the maximum rotational clearance for each torque level.</p><p>Findings</p><p>BSS-obl. performed significantly better than Acutrak2^®mini and HCS^® (p = 0.015, p<0.0001). BSS-long. performed significantly better than HCS^® (p = 0.010). No significant difference in performance between BSS-obl. and BSS-long. (p = 0.361), between BSS obl. and Twinfix^® (p = 0.50) and BSS long. and Twinfix^® (p = 0.667) was detected. Within the torque range up to 200 mNm, four of 21 (19%) BSS-long. and four of 21 (19%) BSS-obl. preparations showed early failure. The same loading led to early failure in four (29%) Twinfix^®, seven (50%) Acutrak2^®mini and 10 (71%) HCS^® of 14 screw samples, respectively.</p><p>Conclusions</p><p>For both BSS and to a lesser extent for Twinfix^® (as dual-component screw), higher rotational stabilities were identified in comparison to single component headless compression screws.</p></div

Statistics for secondary outcome by implant.

<p>Adjustment for Multiplicity: Holm- Simulated.</p

a-d. Sample of a force/angle diagram.

<p>Force/angle diagrams of the cyclic load of a scaphoid bone with a torque load from 150 to 300 mNm and resulting loss of stability. In Fig 4d loosening of the screw fastening is clearly visible by the curves which run out to the right and the line up to 10°.</p

Primary outcome.

<p>Observed Cycles until failure by Implant. Median of maximum attained cycles of the respective screw models with average parameter and standard deviation. Box plot of maximum cycles per screw type. Median, 25% and 75% percentiles, the points correspond to the individual results per screw.</p

primary outcome (Least squares means estimates for screw comparisons): dataset on cycles until failure with reference to torque loads (restricted to load range, where majority of failures occurred); N stands for number of screws which didn’t fail at a previous torque level and a further tested in the indicated load range.

<p>primary outcome (Least squares means estimates for screw comparisons): dataset on cycles until failure with reference to torque loads (restricted to load range, where majority of failures occurred); N stands for number of screws which didn’t fail at a previous torque level and a further tested in the indicated load range.</p

a-e. Screw types.

<p>The shown screws were used for the test series: Acutrak^®2mini (Acumed) (Fig 1a), Twinfix^® (Stryker) (Fig 1b), HCS^® 3.0 (Synthes) (Fig 1c), and prototypes of the two new BSS sets with an additional crosswise drill hole with an inner thread in oblique type (BSS-obl.) (Fig 1d) and with a longitudinal groove with an inner thread and longitudinal screw as a longitudinal type (BSS-long.) (Fig 1e).</p