Ulrich Witzel

Biology of the sauropod dinosaurs: the evolution of gigantism

The herbivorous sauropod dinosaurs of the Jurassic and Cretaceous periods were the largest terrestrial animals ever, surpassing the largest herbivorous mammals by an order of magnitude in body mass. Several evolutionary lineages among Sauropoda produced giants with body masses in excess of 50 metric tonnes by conservative estimates. With body mass increase driven by the selective advantages of large body size, animal lineages will increase in body size until they reach the limit determined by the interplay of bauplan, biology, and resource availability. There is no evidence, however, that resource availability and global physicochemical parameters were different enough in the Mesozoic to have led to sauropod gigantism.

Posterolateral aspect and stability of the knee joint. I. Anatomy and function of the popliteus muscle-tendon unit: an anatomical and biomechanical study

Abstract. This study examined ten human adult cadaveric knees to dissect the popliteus muscle-tendon unit (PMTU), including its numerous attachments to other posterior and posterolateral structures, and to determine the effect of tensioning the PMTU on the internal and external rotation, total rotational arcs, and neutral tibial rotation in full extension and 30°, 60°, and 90° of knee flexion. The junction between the popliteus tendon and the fibular head commonly described as the popliteofibular ligament became lax in internal and tense in external tibial rotation. The internal and external rotational arcs increased gradually between extension and 90° of flexion. Tensioning the PMTU resulted in a statistically highly significant internal tibial rotation with decreased internal and increased rotational arcs. The anatomical findings and functional data indicate that the PMTU is an important structure maintaining dorsolateral stability, stabilizing the lateral meniscus, and balancing the neutral tibial rotation.

Principles of determination and verification of muscle forces in the human musculoskeletal system: Muscle forces to minimise bending stress

While there are a growing number of increasingly complex methodologies available to model geometry and material properties of bones, these models still cannot accurately describe physical behaviour of the skeletal system unless the boundary conditions, especially muscular loading, are correct. Available in vivo measurements of muscle forces are mostly highly invasive and offer no practical way to validate the outcome of any computational model that predicts muscle forces. However, muscle forces can be verified indirectly using the fundamental property of living tissue to functional adaptation and finite element (FE) analysis. Even though the mechanisms of the functional adaptation are not fully understood, its result is clearly seen in the shape and inner structure of bones. The FE method provides a precise tool for analysis of the stress/strain distribution in the bone under given loading conditions. The present work sets principles for the determination of the muscle forces on the basis of the widely accepted view that biological systems are optimized light-weight structures with minimised amount of unloaded/underloaded material and hence evenly distributed loading throughout the structure. Bending loading of bones is avoided/compensated in bones under physiological loading. Thus, bending minimisation provides the basis for the determination of the musculoskeletal system loading. As a result of our approach, the muscle forces for a human femur during normal gait and sitting down (peak hip joint force) are obtained such that the bone is loaded predominantly in compression and the stress distribution in proximal and diaphyseal femur corresponds to the material distribution in bone.

Biomechanical investigations on the skulls of reptiles and mammals

The skulls of reptiles and mammals can be loaded mechanically in three ways: the weight of the head acting downward, perhaps reinforced by a prey or bunch of food lifted from the ground or water surface; by forces acting in the plane of the tooth row, created by movements of the prey in relation to the head or by a movement of the head in relation to a fixed food object; and by the adduction of the mandible, which leads to reaction forces in the skull. While the former two evoke stress patterns comparable to that in a beam which is supported at its rear end (by the occipital condyle(s) and the neck muscles), the latter evoke stress patterns comparable to a beam supported at both ends. Its anterior bearing are the teeth which transmit a reaction force from the seized prey, the adductor muscles of the mandible move the intermediate part of the skull downward, and the posterior bearing is provided by the mandibular joint.Three-dimensional FEM-analysis of the flow of stresses within solid, homogeneous bodies under loads like those described above have been made. As a result, the stress flows have been found to correspond closely to the arrangements of bony material in the akinetic skulls of Crocodiles, Lacertilia, Sphenodon. Except crocodiles and chelonians, reptilian skulls often show large gaps between the load-bearing plates and rods. These gaps correspond to little stressed areas between the stress-bearing parts. One of the stress-bearing rods is the small braincase. In long, slender jaws like those in crocodiles the stresses are concentrated on the periphery, with more or less stress-free areas in the center of the cross sections.In many mammals (shrews, primates includingHomo), however, the very large bony nasal capsule and braincase lead to a distribution of the forces over large areas like in thin-walled shell structures, which are strong enough to sustain the existing forces, without reinforcing superstructures. Even the zygomatic arch can be dispensible.The decisive a priori factors which determine the development of either a rod- or a shell-like structure in a FEM model are1st the relative shape and length of the toothrow and its position in relation to the posterior part of the skull, especially the braincase, and 2nd the size of the nasal capsule and the braincase.We conclude that the exact form of the skull in both classes of animals is determined by 1st the shape and length of the jaws and 2nd by the space requirements of the olfactory and the optical sense organs, and the braincase. The second factor is an expression of the overall evolutionary level. The literature contains plausible biological arguments to explain the high selective influence of lifestyle characteristics on the first factor. These arguments usually cover also the position of the eye openings, the nasal opening and the relative height and length of the whole skull. If these factors are given, the exact morphology of the bony structure turns out to correspond completely to the pattern of stresses, and no other reasons behind skull shape must be searched.The arrangement of the muscles seems to follow in all cases the principle to distribute the force created at the origines on a large surface or on many individual bony elements.

Posterolateral aspect and stability of the knee joint. II. Posterolateral instability and effect of isolated and combined posterolateral reconstruction on knee stability: a biomechanical study

Abstract. This study evaluated the correlation between the number of transected posterolateral structures (PLS) and the grade of posterolateral rotational instability, determined the effect of the popliteus muscle-tendon unit on the tibial rotation, and examined the effect of an isolated posterior cruciate ligament (PCL) and combined PCL-PLS reconstruction on knee stability. Sectioning the popliteofibular and lateral collateral ligaments both caused an increase in tibial external rotation. Cutting the PT resulted in a statistically highly significant excessive external rotation and externally shifted neutral position of the tibia over the full range of motion. Tensioning the popliteus muscle-tendon unit led to a statistically highly significant internally shifted neutral tibial rotation and a decreased internal and an increased external rotation without affecting the total rotational arcs. The isolated PCL reconstruction did not affect the external rotation, whereas the combined PCL-PLS reconstruction reset the knee to nearly physiological laxity patterns.

The Mechanical Significance of the Temporal Fasciae in Macaca fascicularis: An Investigation Using Finite Element Analysis

Computational finite element analyses (FEAs) of the skull predict structural deformations under user specified loads and constraints, with results normally presented as stress and strain distributions over the skulls surface. The applied loads are generally a representation of the major adductor musculature, with the skull constrained at bite positions and at the articulating joints. However, virtually all analyses ignore potentially important anatomical structures, such as the fasciae that cover the temporalis muscle and attach onto the zygomatic arch. In vivo experimental studies have shown that removal of the temporal fasciae attachment onto the zygomatic arch in Cebus monkeys results in significant bone adaptation and remodeling in this region, suggesting the fasciae play an important role in stabilising the arch during biting. Here we investigate this potential stabilising role by carrying out FEAs of a macaque skull with and without temporal fasciae included. We explore the extent to which the zygomatic arch might be stabilized during biting by a synchronized tensioning of the temporal fasciae, acting to oppose masseteric contraction forces. According to our models, during temporalis muscle bulging the forces generated within the tensioned temporal fasciae are large enough to oppose the pull of the masseter. Further, a near bending‐free state of equilibrium within the arch can be reached, even under forceful biting. We show that it is possible to eliminate the high strain gradients in and around the zygomatic arch that are present in past computational studies, with strains being more uniform in magnitude than previously thought. Anat Rec, 2011.

Functional Analysis of the Primate Shoulder

Studies of the shoulder girdle are in most cases restricted to morphological comparisons and rarely aim at elucidating function in a strictly biomechanical sense. To fill this gap, we investigated the basic functional conditions that occur in the shoulder joint and shoulder girdle of primates by means of mechanics. Because most of nonhuman primate locomotion is essentially quadrupedal walking—although on very variable substrates—our analysis started with quadrupedal postures. We identified the mechanical situation at the beginning, middle, and end of the load-bearing stance phase by constructing force parallelograms in the shoulder joint and the scapulo-thoracal connection. The resulting postulates concerning muscle activities are in agreement with electromyographical data in the literature. We determined the magnitude and directions of the internal forces and explored mechanically optimal shapes of proximal humerus, scapula, and clavicula using the Finite Element Method. Next we considered mechanical functions other than quadrupedal walking, such as suspension and brachiation. Quadrupedal walking entails muscle activities and joint forces that require a long scapula, the cranial margin of which has about the same length as the axillary margin. Loading of the hand in positions above the head and suspensory behaviors lead to force flows along the axillary margin and so necessitate a scapula with an extended axillary and a shorter cranial margin. In all cases, the facies glenoidalis is nearly normal to the calculated joint forces. In anterior view, terrestrial monkeys chose a direction of the ground reaction force requiring (moderate) activity of the abductors of the shoulder joint, whereas more arboreal monkeys prefer postures that necessitate activity of the adductors of the forelimb even when walking along branches. The same adducting and retracting muscles are recruited in various forms of suspension. As a mechanical consequence, the scapula is in a more frontal, rather than parasagittal, position on the thorax. In both forms of locomotion—quadrupedal walking and suspension—the compression-resistant clavicula contributes to keeping the shoulder complex distant from the rib cage. Future studies should consider the consequences for thorax shape. The morphological specializations of all Hominoidea match the functional requirements of suspensory behavior. The knowledge of mechanical functions allows an improved interpretation of fossils beyond morphological similarity.

Die native Reißfestigkeit der Sehne des M. supraspinatus beim Menschen

SummaryThe traumatic tear of the rotator cuff has been discussed very intensively for a long time despite the fact that there do not exist representative objective data about the native tensile strength of these tendons. The aim of this study was to evaluate the age related native strength of the supraspinatus tendon. 25 fresh frozen cadaver specimen (age: 23–94, 24 h post mortem, 18 male, 7 female) were tested using so called cryo-jaws for soft tissue fixation. The results showed the major part of the tensile forces to be transmitted through the anterior thicker part of the tendon (e. g. 14 bony avulsions in this area). We found significant correlations between age and maximum strength (p < 0.001), age and stiffness of the tendon (p < 0.005) and stiffness and maximum strength (p < 0.001). These results show that tensile strength and stiffness of the supraspinatus tendon decrease with age. However, a 65 year old specimen still demonstrates a weight bearing structure (about 900 N maximum tensile strength) and is not necessarily ruptured or degeneratively altered.ZusammenfassungDie Reißfestigkeit der Rotatorenmanschette wird im Zusammenhang mit Unfallschäden seit Jahren intensiv diskutiert. Trotzdem finden sich bisher in der Literatur keine repräsentativen quantitativen biomechanischen Untersuchungen. Die native Reißfestigkeit der Supraspinatussehne wurde anhand von 25 frischen Leichenschultern (23–94 Jahre, 24 h post mortem, 18 männlich, 7 weiblich) untersucht. Die Supraspinatussehnen wurden in physiologischer Richtung bis zum Versagen beansprucht. Die Problematik der Fixierung der Weichteile wurde mit modifizierten Kältebacken gelöst.Der Hauptanteil dieser Zugkräfte wurde vom stärker ausgeprägten ventralen Anteil der Sehne getragen (u. a. 14 knöcherne Ausrisse in diesem Anteil). Zwischen dem Alter und maximaler Kraft (F max.) (p < 0,001), dem Alter und der Steifigkeit der Sehne (p = 0,005), sowie der Steifigkeit der Sehne und F max. (p < 0,001) ließ sich eine signifikante umgekehrte Proportionalität nachweisen. Die Ergebnisse belegen, daß zwar Reißfestigkeit und Steifigkeit kontinuierlich mit zunehmendem Alter abnehmen, bei einem 65 jährigen aber immerhin noch ca. 900 N erforderlich sind, um die Sehne zu zerreißen. Damit wird klar, daß die Ruptur in diesem Alter keineswegs immer nur schicksalsmäßiger degenerativer Natur sein muß.

The Biomechanical Function of Periodontal Ligament Fibres in Orthodontic Tooth Movement

Orthodontic tooth movement occurs as a result of resorption and formation of the alveolar bone due to an applied load, but the stimulus responsible for triggering orthodontic tooth movement remains the subject of debate. It has been suggested that the periodontal ligament (PDL) plays a key role. However, the mechanical function of the PDL in orthodontic tooth movement is not well understood as most mechanical models of the PDL to date have ignored the fibrous structure of the PDL. In this study we use finite element (FE) analysis to investigate the strains in the alveolar bone due to occlusal and orthodontic loads when PDL is modelled as a fibrous structure as compared to modelling PDL as a layer of solid material. The results show that the tension-only nature of the fibres essentially suspends the tooth in the tooth socket and their inclusion in FE models makes a significant difference to both the magnitude and distribution of strains produced in the surrounding bone. The results indicate that the PDL fibres have a very important role in load transfer between the teeth and alveolar bone and should be considered in FE studies investigating the biomechanics of orthodontic tooth movement.

Über die Primärfestigkeit konventioneller und alternativer Nahttechniken der RotatorenmanschetteEine biomechanische Untersuchung

ZusammenfassungNeben den transossären Standardverfahren werden bei Rotatorenmanschettenrupturen vermehrt alternative Fadenankertechniken angewandt. Ziel der vorliegenden Studie war es, 2 konventionelle Nahttechniken biomechanisch in ihrer primären Zugbelastbarkeit mit 3 verschiedenen Fadenankern zu vergleichen: Acufex-Wedge-TAG, Acufex-Rod-TAG und Mitek-GII.Hierzu wurde an 50 Leichenpräparaten, bestehend aus M. supraspinatus, Supraspinatussehne und proximalem Humerus, ein Defekt im Bereich der Supraspinatussehne gesetzt. Anschließend wurde die Sehne in den oben genannten Techniken refixiert. Die Zugfestigkeit wurde bis zum maximalen Versagen geprüft (Gruppengröße n=10, Sterbealter in Durchschnitt 56 Jahre). Die gemessenen Zugfestigkeiten wurden mir der hypothetischen Nativreißfestigkeit der intakten Supraspinatussehne verglichen.Die maximale Zugfestigkeit der klassischen transossären Rotatorenmanschettennaht lag im Median bei 410 N. Eine Modifikation dieser Technik erzielte 552 N. Die Fadenankertechniken erreichten im Median folgende Werte: Wedge-TAG 207 N, Rod-TAG 217 N, Mitek-GII 186 N. Die Unterschiede zwischen Fadenanker- und transossären Techniken waren hochsignifikant (p<0,001).Die Mitek-Anker zeigten deutliche Vorteile bei der Implantation am Tuberculum majus sowie bezüglich der Luxationsrate. Diese betrug beim Mitek-Anker 3% (n=1), beim Wedge-TAG 27% (n=8) und beim Rod-TAG 43% (n=13). Die Nähte in Fadenankertechnik erreichten ca. 20%, die klassische Naht 34% und die Modifikation dieser 60% der hypothetisch errechneten Nativstärke der Supraspinatussehne.Die Verwendung von Fadenankern an der Rotatorenmanschette (RM) kann die Primärfestigkeit der Refixation nicht erhöhen; unter maximaler Zugbelastung besitzen sie eine geringere Festigkeit als die transsossären Techniken. Gemessen an der geringen Luxationsrate im spongiösen Knochenbett des Tuberculum majus ist ein metallisches Implantat mit Widerhaken von Vorteil. Alle untersuchten Nahttechniken, einschließlich der klassischen transossären Sehnenrefixation, erreichten deutlich geringere Zugfestigkeiten als die hypothetische Zugbelastbarkeit der intakten Supaspinatussehne. Vor allem bei der Verwendung von Fadenankern ist daher eine vorsichtige und schonende postoperative Mobilisation indiziert.AbstractThe aim of this biomechanical study was to evaluate rotator cuff repair strength using different suture anchor techniques compared to conventional repair, taking into consideration the native strength of the supraspinatus tendon.Therefore, a defined defect of the supraspinatus was created in 50 freshly frozen cadaver specimen (group size n=10; median age at death: 56 years). Five methods were employed for cuff repair: standard transosseous suture, modified transosseous suture with patch augmentation and three suture anchors (Acufex Wedge TAG, Acufex Rod TAG und Mitek GII).The maximum tensile load of the five techniques was: standard transosseous suture, 410 N; modified transosseous suture, 552 N; Wedge TAG, 207 N; Rod TAG, 217 N; Mitek GII, 186 N. The differences between the suture anchor and standard techniques were highly significant (P<0.001).In this series, the Mitek GII anchor showed the lowest anchor dislocation rate at 3% (n=1). The Wedge TAG system had a dislocation rate of 27% (n=8) and the Rod TAG system 43% (n=13).Suture anchor techniques revealed about 20%, the standard technique 34% and its modification 60% of the hypothetically calculated native tendon strength.Compared to conventional transosseous suture techniques, the use of the suture anchors tested in this series does not significantly increase the primary fixation strength of rotator cuff repair. The metallic implant with two barbs (Mitek G II) seems to be superior to the polyacetal anchors when inserted into the spongiform bone of the greater tubercle.The considerably weaker repair strength needs to be taken into consideration in postoperative patient rehabilitation, especially after the use of suture anchors.