Wolfgang Pichl

Hip fractures and the contribution of cortical versus trabecular bone to femoral neck strength.

Osteoporotic fractures are caused by both cortical thinning and trabecular bone loss. Both are seen to be important for bone fragility. The relative contributions of cortical versus trabecular bone have not been established. The aim of this study was to test the contribution of cortical versus trabecular bone to femoral neck stability in bone strength. In one femur from each pair of 18 human cadaver femurs (5 female; 4 male), trabecular bone was completely removed from the femoral neck, providing one bone with intact and the other without any trabecular structure in the femoral neck. Geometrical, X‐ray, and DXA measurements were carried out before biomechanical testing (forces to fracture). Femoral necks were osteotomized, slices were analyzed for cross‐sectional area (CSA) and cross‐sectional moment of inertia (CSMI), and results were compared with biomechanical testing data. Differences between forces needed to fracture excavated and intact femurs (ΔF/F mean) was 7.0% on the average (range, 4.6–17.3%). CSA of removed spongiosa did not correlate with difference of fracture load (ΔF/F mean), nor did BMD. The relative contribution of trabecular versus cortical bone in respect to bone strength in the femoral neck seems to be marginal and seems to explain the subordinate role of trabecular bone and its changes in fracture risk and the effects of treatment options in preventing fractures.

Femoral neck fracture after removal of the standard gamma interlocking nail: a cadaveric study to determine factors influencing the biomechanical properties of the proximal femur

We retrospectively analyzed 1334 patients who were implanted standard gamma interlocking nails (SGN) to stabilize trochanteric femoral fractures over the years 1992-1998. Reoperation to remove the nails was performed in 37 patients, in 9 of them purely because of pain. Three out of these 9 patients with removed SGN suffered femoral neck fractures in the early postoperative course after having been mobilized to full weight-bearing capacity. This complication was not observed with other implant systems and, considering the notoriously high complication rate of femoral neck fractures, severely reduces the value of the SGN concept per se. These findings in combination with other known shortcomings of SGNs prompted us to conduct an experimental study on the fracture force of excavated femurs addressing the hypothesis that the specific design of the SGN is responsible for the occurrence of fatigue fractures of the femoral neck. Eighteen matched pairs of fresh human cadaveric proximal femurs, which were treated by insertion and removal of (i) SGNs or (ii) dynamic hip screws (DHS) or (iii) by excavation in the absence of an implant, were subjected to incremental loading cycles and compared to the untreated contralateral femurs. Overall, the fracture force was found to be significantly lower among the treated than among the untreated bones. However, the fracture force required after removal of the DHS system was still significantly higher than for SGN or excavation alone. In this way, our findings demonstrate that removing relatively big implants such as SGN can cause serious complications such as femoral neck fractures. We therefore recommend to leave this type of device in place even after fracture healing except in cases of deep and chronic infection.

Metallography of alkali metal single crystals

The paper discusses methods for metallographic preparation of lithium, sodium and potassium single crystals. Because mechanical techniques and electropolishing cannot be applied, chemical polishing methods in organic solvents are considered in detail. A few methods suggested earlier in the literature were re-examined. Improved techniques were developed that yield an excellent metallographic surface quality for optical microscopy and scanning electron microscopy. First results of in situ optical microscopy investigations of the low-temperature phase transformation of lithium and of in situ slip line investigations in all three metals are shown.

The plasticity of potassium

Abstract It has been known for a long time that the low-temperature deformation behaviour of potassium is very similar to that of the refractory b.c.c. metals. Therefore it is generally assumed that the same mechanism, namely kink-pair formation in screw dislocations, determines the deformation rate below a certain knee temperature T k . A detailed investigation of the flow-stress of high-purity potassium single crystals showed that the knee-temperature is about 80 K, in contrast to values of 25–30 K reported in the earlier work. Between 80 and 4.2 K there exist three clearly distinct deformation regimes separated by bends or ‘humps’ in the curve of flow-stress versus temperature. A similar behaviour has been observed in the refractory metals. The close correspondence of these bends in b.c.c metals of radically different electronic structure strongly indicates that they are intrinsic properies of screw dislocation motion in the b.c.c. lattice and do not have their origin in details of the interatomic potential.

Load sharing in augmented anterior cruciate ligament repair: a mathematical analysis based on in vitro measurements

OBJECTIVE: To quantify the stress on a reinserted anterior cruciate ligament (ACL) we studied the load sharing between the ACL and a 3.7 mm polyethylene terephthalate (PET) band in 10 knees of fresh human cadavers. DESIGN: The load sharing between the Marshall sutures-ACL complex and the PET band and between the ACL and the PET band was calculated by means of a mathematical model. BACKGROUND: Augmentation of a reinserted ACL with a synthetic band is an accepted treatment for a ruptured ACL. METHODS: After transsecting the ACL at its femoral origin and reinserting it with four sutures using the Marshall technique we augmented it with the PET band. The augmentation device was inserted by the through-the-condyle (TTC) procedure and attached without preload to the femoral and the tibial condyle with 4 mm staples. Then the length of the ACL, the lengths of the Marshall sutures, the partial lengths and the angles between the different directions, and the adherence-friction force of the PET band were measured. RESULTS: In the beginning the Marshall sutures-ACL complex takes over 40% of an externally applied load and the PET band 60%. After the hypothetical ACL healing phase the PET band takes over 27% and the ACL 73% of the load. CONCLUSIONS: An external force of 40 N acting on the knee will therefore lead to an elongation of the Marshall sutures-ACL complex and result in movement of the proximal end of the ACL away from the femoral condyle of at least 0.5 mm, which is highly undesirable in the early postoperative healing phase.

Martensitic transformation and mechanical deformation of high-purity lithium

Abstract The martensitic transformation of Li is characterized by a large temperature hysteresis and a complex low-temperature phase diagram. The phase transition is very sensitive to small mechanical strains. On the other hand, within the hysteresis region peculiar mechanical properties are observed. The critical resolved shear stress exhibits a pronounced anisotropy and asymmetry between tension and compression while the strong temperature dependence typical for other b.c.c. metals is absent. Investigations of the plasticity of Li single crystals above the M s temperature indicated that stress-induced nuclei of the low temperature phase interact with the glide dislocations. This was partially confirmed by in situ deformation experiments during neutron scattering and seems to be in agreement with recent theoretical approaches which predict a stabilization of martensite clusters in the nucleation state above the M s temperature.

Dynamics of Tibiofibular Syndesmosis in Correlation with the Level of the Syndesmotic Scress in Maisonneuvre Fractures of the Ankle – a Cadaver Study

Background: The objective of this study was to determine the level of the syndesmotic screw, the number of screws needed, and whether one or both cortices of tibia should be engaged to closely reproduce the physiological stability and elastic of the ankle mortise in syndesmotic rupture after Maisonneuve fractures. Material and Methods: Twelve fresh frozen cadaveric pairs of lower extremities were used for this study. The knee was exarticulated and soft tisue removed leaving the lateral and medial ankle ligaments intact. The tibiofibular syndesmosis was then sharply sectioned. All specimens were mounted in an Instron 2000 material testing machine. An axial loading of 500 N was applied. Syndesmotic diastasis was measured by a linear transducer system. The specimens were divided into three groups. In group I, the specimens were instrumented with one syndesmotic screw 2.5 cm above tibia plafond. In group II, the same procedure was performed as in group I with the syndesmotic screw positioned 4 cm above tibia plafond. In group III, the syndesmosis was fixed in each pair with two screws 2.5 and 4 cm above tibia plafond. In all groups, the screws were positioned randomly through three cortices on the one side and through four cortices on the contralateral side. Results: Significant difference was observed between the application of one or two screws through three or four cortical shells on one side and the level of the screw on the other. Conclusions: According to the results of this study, we recommend to use one screw through four cortices 2.5 cm above the tibia plafond.

Structure and Lattice Expansion of Pure Sodium Confined inside Nanoporous Vycor Glass at Low Temperatures

Behavior of sodium metal subject to geometrical constraints has been studied at variable temperatures. A porous medium, Vycor glass of an average pore size of 9.2 nm, was loaded with Na by vacuum vaporization. Synchrotron X-ray powder diffraction of the specimen revealed the existence of close-packed phases such as FCC, HCP, 9R and 4H already at room temperature whereas the bulk BCC phase of Na was completely absent. In contrast, close-packed phases in bulk sodium are only observed at temperatures below 35 K upon cooling. This result can be qualitatively understood in view of earlier zero Kelvin calculations from which it is well known that the differences of structural energies between BCC and close-packed phases are very small. As a consequence, also various close-packed structures can be formed at ambient temperature under certain conditions and environments. The lattice expansion in the restricted geometry has been determined in the range of 160 K to 300 K and compared with the bulk metal. Confined Na exhibits smaller lattice expansion as compared to that of the bulk which likewise can be explained as being due to confinement.

Neutron scattering investigation of pressure-induced phase transitions in Li and Ba

The elemental alkali metals as well as the earth-alkaline metal barium undergo phase transitions from an ambient bcc structure to close-packed structures (fcc or hcp, respectively) in a largely varying pressure range from 4.5 GPa in Cs up to more than 50 GPa in Na. Little is known about the mechanisms of these pressure-induced phase transitions. In particular, it is an open question whether the and transformations are related to similar phase transitions at ambient pressure and low temperatures which occur in Li and Na but are absent in the heavy alkali metals and in Ba. In the present work, we have adapted the Paris–Edinburgh pressure cell for neutron scattering studies on single crystals of Li and Ba. It was found that under carefully chosen experimental conditions almost hydrostatic loading of Li and Ba up to about 7 GPa is possible. We describe the experimental methods which have allowed us to overcome the technical difficulties and report the first results of elastic neutron scattering on Li and Ba and inelastic neutron scattering on Li.

Alloplastisch augmentiertes femoral reinseriertes vorderes Kreuzband

The rupture of the anterior cruciate ligament (ACL) near its femoral origin is a common injury of the knee and can lead to lesions of the meniscus due to instability and to early gonarthrosis. One procedure applied in current orthopaedic practice to prevent such impairment of knee joint function is ACL repair reinforced with a synthetic intraarticular ligament. In this study we used twelve knees of cadavers and after sectioning the ACL in each repaired it according to the Marshall technique with USP 1 PDS II sutures. We augmented the repair in each case with a 3-mm PET (Trevira hochfest) band inserted by the through-the-condyle (TTC) procedure and attached without preload to the femoral and tibial condyle with a 4-mm staple. We then measured the length of the ACL, the length of the Marshall sutures-ACL complex, the partial lengths, and the deviation angles and adherence-friction force of the 3-mm PET augmentation device, and applied the law of Hooke to calculate the load-sharing between the USP 1 PDS II sutures-ACL complex and the 3-mm PET band and between the ACL and the 3-mm PET band, respectively. We also evaluated the load on the femoral and the tibial fixation of the augmentation device. The results showed that the 3-mm PET band took over 60% of an externally applied load on the knee during the hypothetic period of ACL healing and 27% of the force acting on the knee thereafter. It was calculated that a maximum of 75% of the load taken over by the augmentation device was at the tibial staple and only up to 45% of the force at the femoral one. With due consideration for the requirement for absolute protection (“stress shielding”) of the healing ACL but also for the aim of early postoperative accelerated functional rehabilitation without casts, splints or other restrictions of joint movement, we believe that a pretensioned 3-mm PET band is the best choice, since augmentation without preload cannot fulfil these requirements.ZusammenfassungDie femorale ursprungsnahe Ruptur des vorderen Kreuzbands ist eine häufige Kniebandverletzung. Um bei diesem Verletzungsmuster instabilitätsbedingte Meniskusläsionen und eine vorzeitige Gonarthose mit einer konsekutiven Verschlechterung der Kniegelenkfunktion zu verhindern, besteht die Mögichkeit der alloplastisch augmentierten femoralen Reinsertion des vorderen Kreuzbands. Als Beweis der Notwendigkeit eines vorgespannten synthetischen Augmentationsbands hinsichtlich einer verbesserten postoperativen Rehabilitation untersuchten wir in einer Leichen-experimentell-mathematischen Studie nach femoraler Durchtrennung und anschließender Reinsertion des vorderen Kreuzbands mit 4 USP1-PDS®-II-Marshall-Nähten die potentielle Kraftübernahme eines nach der Zweikanal-(TTC)-Technik parallel zum vorderen Kreuzband eingebrachten, femoral und tibial mit je einer 4mm-Knochenklammer ohne Vorspannung fixierten 3 mm-PET-(Trevira® hochfest)-Augmentationsbands. Aus diesem Anlaß bestimmte man an 12 unfixierten anatomischen Kniegelenkoperationspräparaten die Längen der Marshall-Reinsertionsnähte, die Teillängen, die Umlenkwinkel sowie die Haftreibungskraft des 3 mm-PET-Bands und die L↭ge des vorderen Kreuzbands. Unter Anwendung des Hooke-Gesetzes wurde die Kraftverteilung zwischen dem mit USP1-PDS®-II-Nähten reinserierten vorderen Kreuzbands und dem 3 mm-PET-Band bzw. zwischen dem hypothetisch eingeheilten vorderen Kreuzband und dem 3 mm-PET-Band berechnet. Weiters wurden in diesem Zusammenhang auch die Belastung der femoralen und tibialen Augmentationsbandfixation evaluiert. Die Ergebnisse zeigten in bezug auf das frisch reinserierte vordere Kreuzband eine Teilaufnahme des 3 mm-PET-Bandes von 60% und bezüglich des hypothetisch restituierten vorderen Kreuzbands eine partielle Aufnahme des Kunststoffaugmentationsbands von 27% der auf das Kniegelenk einwirkenden Kraft. Die tibiale Augmentationsbandknochenklammer war mit maximal 75%, die femorale Klammer aber nur mit höchstens 45% der vom 3 mm-PET-Band aufgenommenen Kraft belastet. Unter Berücksichtigung der Forderung nach absoluter Protektion (stress shielding) des einheilenden vorderen Kreuzbands, des potentiellen Augmentations-(Protektions-)Ausmaßes und der materialtechnisch-biologischen Voraussetzungen erweist sich bei Anstreben einer gips- und schienenfreien frühfunktionellen aggressiven postoperativen Nachbehandlung, ohne Einschränkung der Kniegelenkexkursioneu, derzeit das nach der Zweikanal-(TTC)Technik ins Kniegelenk eingebrachte und vorgespannte 3 mm-PET-Band zur Augmentation reinserierter vorderer Kreuzbänder am geeignetsten.