A fundamental model of quasi-static wheelchair biomechanics.

The performance of a wheelchair system is a function of user anatomy, including arm segment lengths and muscle parameters, and wheelchair geometry, in particular, seat position relative to the wheel hub. To quantify performance, researchers have proposed a number of predictive models. In particular, the model proposed by Richter is extremely useful for providing initial analysis as it is simple to apply and provides insight into the peak and transient joint torques required to achieve a given angular velocity. The work presented in this paper identifies and corrects a critical error; specifically that the Richter model incorrectly predicts that shoulder torque is due to an anteflexing muscle moment. This identified error was confirmed analytically, graphically and numerically. The authors have developed a corrected, fundamental model which identifies that the shoulder anteflexes only in the first half of the push phase and retroflexes in the second half. The fundamental model has been extended by the authors to obtain novel data on joint and net power as a function of push progress. These outcomes indicate that shoulder power is positive in the first half of the push phase (concentrically contracting anteflexors) and negative in the second half (eccentrically contracting retroflexors). As the eccentric contraction introduces adverse negative power, these considerations are essential when optimising wheelchair design in terms of the user's musculoskeletal system. The proposed fundamental model was applied to assess the effect of vertical seat position on joint torques and power. Increasing the seat height increases the peak positive (concentric) shoulder and elbow torques while reducing the associated (eccentric) peak negative torque. Furthermore, the transition from positive to negative shoulder torque (as well as from positive to negative power) occurs later in the push phase with increasing seat height. These outcomes will aid in the optimisation of manual wheelchair propulsion biomechanics by minimising adverse negative muscle power, and allow joint torques to be manipulated as required to minimise injury or aid in rehabilitation.

The elephant knee joint: morphological and biomechanical considerations.

Elephant limbs display unique morphological features which are related mainly to supporting the enormous body weight of the animal. In elephants, the knee joint plays important roles in weight bearing and locomotion, but anatomical data are sparse and lacking in functional analyses. In addition, the knee joint is affected frequently by arthrosis. Here we examined structures of the knee joint by means of standard anatomical techniques in eight African (Loxodonta africana) and three Asian elephants (Elephas maximus). Furthermore, we performed radiography in five African and two Asian elephants and magnetic resonance imaging (MRI) in one African elephant. Macerated bones of 11 individuals (four African, seven Asian elephants) were measured with a pair of callipers to give standardized measurements of the articular parts. In one Asian and three African elephants, kinematic and functional analyses were carried out using a digitizer and according to the helical axis concept. Some peculiarities of healthy and arthrotic knee joints of elephants were compared with human knees. In contrast to those of other quadruped mammals, the knee joint of elephants displays an extended resting position. The femorotibial joint of elephants shows a high grade of congruency and the menisci are extremely narrow and thin. The four-bar mechanism of the cruciate ligaments exists also in the elephant. The main motion of the knee joint is extension-flexion with a range of motion of 142 degrees . In elephants, arthrotic alterations of the knee joint can lead to injury or loss of the cranial (anterior) cruciate ligament.

Finite-element analysis for lumbar interbody fusion under axial loading.

A parametric study was conducted to evaluate axial stiffness of the interbody fusion, compressive stress, and bulging in the endplate due to changes in the spacer position with/without fusion bone using an anatomically accurate and validated L2-L3 finite-element model exercised under physiological axial compression. The results show that the spacer plays an important role in initial stability for fusion, and high compressive force is predicted at the ventral endplate for the models with the spacer and fusion bone together. By varying the positioning of the spacer anteriorly along anteroposterior axis, no significant change in terms of axial stiffness, compressive stress, and bulging of the endplate are predicted for the implant model. The findings suggest that varying the spacer position in surgical situations does not affect the mechanical behavior of the lumbar spine after interbody fusion.

[In vitro stability study of an improved implant system for minimally invasive transforaminal approach]. / In-vitro-Stabilitätsuntersuchung eines neuartigen Implantatsystems für den minimal-invasiven transforaminalen Zugang.

Calf specimens from the L3-4 and L5-6 levels were used for in vitro stability testing of a new transforaminal lumbar interbody fusion (TLIF) implant. Results were compared with those of the conventional posterior lumbar interbody fusion (PLIF) technique using two cuboid spacers. The specimens were loaded with pure moments of 10 Nm in flexion, extension, lateral bending, and axial rotation without any axial preload. They were allowed to move freely and unconstrained in all directions. In extension the PLIF implants showed slightly higher degrees of stiffness than the TLIF implant. While the conventional PLIF technique results in an increased range of motion by a factor of 2.5 after implant insertion, the TLIF approach prevents segmental destabilization in axial rotation. The facet joint arthrodesis using resorbable pins reveals biomechanically interesting results and will therefore be investigated in further studies.

Loss of correction after lateral closing wedge high tibial osteotomy--a human cadaver study.

In 12 human cadaver tibiae, osteotomies were carried out at two levels (2 and 3 cm from the distal joint line) with three different wedges (5 degrees, 10 degrees, 15 degrees) to evaluate the influence of displacement of the osteotomy fragments on areas of cortical contact. In undisplaced osteotomies (medical cortical edges superposed) cortical contact areas formed 28% (level 2 cm) and 40.5% (level 3 cm) of the cortical circumference of the proximal fragments (NS). Wedge angles and levels of osteotomy displayed no statistical differences. In displaced osteotomies cortical contact decreased significantly (P < 0.05). Displacing the distal fragment laterally, medial cortical contact is lost, and weight-bearing leads to revarisation as cancellous bone sustains only 3 MPa, and the measured compressive stresses at the medial edge amounted to 6 MPa on average. Displacing the distal fragment medially leads to a decrease of total cortical contact, too, but at the medial edge of the osteotomy cortical contact areas are still present. As a result of the study, postoperative weight-bearing without additional plaster cast fixation is recommended only in cases with undisplaced fragments.

Biomechanical alterations in the carpal arch and hand muscles after carpal tunnel release: a further approach toward understanding the function of the flexor retinaculum and the cause of postoperative grip weakness.

The difference between maximal and minimal distance covered (the distance between the trapezium ridge and hamate hook; moment exerted on structures: 1 Nm) by an intact flexor retinaculum (FR; minimum, 3.3 +/- 0.1 cm; maximum, 3.7 +/- 0.2 cm) and the increase in the maximal distance on carpal tunnel release (CTR; increase, 1.6 +/- 0.2 mm) were significant. Under an external supination moment, the distance between the attachments of the trapeziopisiform band increased after CTR. Under external pronation and ulnar abduction moments, the distance between the attachments of the scaphoideohamate band increased after CTR. The CTR resulted in an anatomic attachment loss for the following muscles: the superficial head of the flexor pollicis brevis (shortening by approximately 25%, relative to rest length), the ulnar part of the abductor pollicis brevis (with opposition and adductory functions, approximately 20%), the opponens pollicis (approximately 20%), the middle part of the abductor pollicis brevis (approximately 7%), and the opponens digiti minimi (approximately 10%). Preoperative and postoperative (2-7 weeks after surgery) measurements of the reaction force of the distal phalanx (under isometric thumb opposition and finger II-IV flexion with extended carpal joint) led to differentiation of three groups: (1) significant strength loss--the patients showed difficulties with grasping, lifting, twisting off lids and caps, screwing, pulling ropes, and pinching; (2) no significant change in force values; and (3) a significant increase in strength (patients who could grip more firmly).

Morphology and function of the cruciate ligaments in diapsids.

The extant diapsids (amniotes with two temporal fenestrate) consist of archosaurs (birds and crocodiles) and of lepidosaurs (sphenodon and squamates). The dichotomy of these two lineages occurred in the upper Permian. Originally, the intraarticular knee ligaments were three cruciates. The intermediate cruciate (a posterior one with guiding function) is present in all diapsids, whilst the medial one (an anterior with guiding function) is absent in birds. The lateral cruciate ligament is subdivided into a lateral anterior and a medial posterior shank. The posterior shank disappeared in birds and crocodiles. The anterior shank disposes of a guiding function in archosaurs, and is in the course of reduction in lepidosaurs. The reduction process has progressed least in sphenodons, whereas the lateral cruciate of chameleons is totally absent. The anterior cruciates of birds (the lateral one) and of chameleons (the medial one) are hence not homologous. The morphology and function of the cruciate ligaments can be related with gait evolution.

Fibrous tissue on the tibia plateau of the kangaroo. A theory on the pressure absorption of joint surfaces.

The central part of the articular surface on the tibia plateau of Macropus agilis consists of fibrous cartilage of soft consistency and the fiber arrangement is macroscopically visible. The peripheral portions of the plateau are covered by hyaline cartilage but do not communicate with the hyaline articular surfaces of the femur, as they are covered by the menisci. The fibrous cartilage covering of the tibia plateau is a compliant or readily deformed pad that could serve the function of deforming enough under high joint loads to allow surrounding regions of the articular cartilage to share in carrying those loads, thereby magnifying the articular contact surface and decreasing the magnitude of the peak unit loads in the region of the fibrous tissue pad. This pressure-absorbing mechanism represents the evolutionary response to the higher articular stress resulting from kangaroo locomotion.

The knee joint of the flying fox (Pteropus rufus).

As the flying fox (Pteropus rufus) knee does not exhibit any sign of allowing rotation, the aim of this study was to evaluate the factors preventing rotation. Menisci are not present. Yet, the femoral and tibial articular surfaces are not anatomically congruent as the kinematic mechanism of the knee is not based on a pure hinge but on a "four-bar linkage" construction, which consists of femur, tibia and the guiding bundles of the cruciates. Therefore, the cruciate ligaments have the same function as in man, i.e., they contain extension and flexion restricting fibers, fibers taut in intermediate positions and a guiding bundle which is constantly taut. Also in the collateral ligaments these fiber groups can be identified and hence parts of them are taut throughout the entire range of motion (5 degrees hyperextension, 170 degrees flexion). These parts prevent rotation together with the femoral and tibial articular surfaces, the latter of which are concave.

Contribution to the trigone musculature in the case of ureteral duplication.

A case of bilateral ureteral duplication with asymmetric orifices is reported. This case served as an example for the evaluation of the arrangement of the trigone musculature in such patients. The muscle fibers radiating from the craniomedial of the 2 orifices constitute the upper portion of the trigonum musculature (Mercier's bar), while the fibers radiating from the caudolateral orifice constitute the lower portions (Bell's muscles). Never did a "muscle fiber fan" of 1 orifice contribute to the upper and lower areas. The farther from the median an orifice is situated, the longer and, possibly, the weaker are its related muscle fibers.

Principles and mechanisms of automatic rotation during terminal extension in the human knee joint.

The mechanisms solely responsible for automatic rotation during terminal extension were investigated by means of destruction experiments on knee joint structures. These are the posterior cruciate ligament (PCL), the anterior cruciate ligament (ACL) and the curvature of the medial femoral condyle. The first mechanism is based on the extraordinary obliquity of the constantly taut PCL guiding bundle, which produces torque in the final extension stage. The second mechanism depends on the fact that the ACL becomes 'too short' towards the final extension stage, which in turn is due to the shape of the articular surfaces. The third mechanism is caused by the deflection of the medial femoral condyle by the intercondylar eminence of the tibia. Each of the collateral ligaments can, in their own right, prevent excessive automatic rotation. On the one hand, the mechanism of the automatic rotation is reversed during initial flexion by the medial collateral ligament (this would be 'too short' in the case of nonreversal). On the other hand, it is reversed by the popliteus muscle. It is suggested that automatic rotation is not related to securing an amuscular stance (locking mechanism).

Voluntary rotation in the human knee joint.

The structures responsible for the restriction of knee joint rotation were established by means of sequential destruction of the menisci, cruciate ligaments, capsular and collateral ligaments; the capsular and collateral ligaments restrict both medial and lateral rotation. The anterior cruciate ligament was also found to restrict medial rotation, thus acting as a synergist to the capsular and collateral ligaments. The posterior cruciate ligament was found to have no effect on rotation. Under no circumstances do the cruciates as a pair restrict medial rotation as this is drastically increased as a consequence of the removal of the capsular and collateral ligaments, and the lateral meniscus. The axis of medial rotation ran through the medial tubercle of the intercondylar eminence while that of lateral rotation was located slightly lateral to it.

Anatomy and function of the cruciate ligaments of the domestic pig (Sus scrofa domestica): a comparison with human cruciates.

Pig cruciate ligaments were dissected and examined radiologically to evaluate their function. The variations in distance between the origin and insertion of the fibre bundles were measured in different joint positions; the maximal decrease in distance amounted to 50% in the anterior (ACL) and 30% in the posterior cruciate ligament (PCL). The ACL consists of 2 morphologically distinct portions. Most of its fibres are found to be taut in extension. Its posterolateral portion contains a 'guiding bundle' taut in all joint positions as well as fibres that are taut either in extreme positions (in extension or in flexion or in both) or in intermediate positions. Its anteromedial portion is mainly composed of fibres taut in extension with some additional fibres taut in intermediate positions. The PCL is not subdivided. Most of its fibres are taut in flexion and some in intermediate positions. Its guiding bundle is consistently taut in all joint positions. The guiding bundle of the ACL and the one of the PCL form a 4-bar link. The functions of the cruciates basically are the same in humans and in pigs, namely in guiding the joint and in restricting motion. It must, however, be borne in mind that the maximal extension of the pig corresponds to a human flexion of 30 degrees. It is not possible to correlate the differing gaits of the pig and human with respect to the function of the cruciates, as the knee has to perform flexion and extension (with secondary rotational) movements in both species, irrespective of unguligrady, digitigrady or plantigrady.

New aspects of the morphology and function of the human hip joint ligaments.

The capsular ligaments of the human hip joint were submitted to exact morphological analysis, and they proved to be multiple and numerous. We have described various ligamentous systems and their interconnections, and have suggested new terminologies and systematics. The ligaments were subjected to functional analysis by means of measuring strips to determine the positions in which the ligaments are taut. The ligament systems were all found to serve a restrictive function, and various parts of the apparatus restricted all possible movements in the hip joint. Extension is restricted by the medial iliofemoral complex, abduction by the pubofemoral ligament, and adduction by the posterior coxal ligaments and by the superior ischiofemoral ligament. Flexion is restricted by the inferior ischiofemoral ligaments, inward rotation by the superior ischiofemoral ligament, and outward rotation by the lateral iliofemoral complex. Only the ligament of the femoral head is unable to exert a restricting function, despite reaching a state of tension in extreme adduction.

Radial nerve entrapment at the elbow: surgical anatomy.

The surgical anatomy of interest in the posterior interosseous nerve syndrome was studied to shed light on the ramifying pattern of the radial nerve, the number of its muscular branches and their branching levels, and to pinpoint the location of the fibrous bands that may cause radial nerve entrapment. The fibrous arch of the supinator muscle (arcade of Frohse) was found to lie 3 cm to 5 cm below Hueter's line, that of the extensor carpi radialis brevis muscle lay 0.5 cm to 1 cm proximal to the arcade of Frohse. Symptom patterns in terms of muscle weakness caused by radial nerve entrapment at different levels were also evaluated.

The restraining function of the cruciate ligaments on hyperextension and hyperflexion of the human knee joint.

Each of the cruciate ligaments contains functionally different fiber groups; one fiber bundle is always taut; numerous others are taut in intermediate or extreme positions. The bulk of the fibers of the anterior cruciate ligament (ACL) is taut in maximal extension, while that of the posterior cruciate ligament (PCL) is taut in the intermediate positions and in maximal flexion. Fibers taut in extreme positions serve as restraints: during hyperextension, the ACL restrains forward migration of its tibial attachment, while the PCL interacts with other structures to prevent posterior opening of the joint. The inverse situation occurs in hyperflexion. Cruciate fibers are dissimilar in length and angular arrangement so that, when movements are restrained, they lengthen to different extents. To define this phenomenon in quantitative terms, the term isokolyons was coined for lines from which fibers showing identical elongation in percentage on exposure to a force take their origin.

The ulnar collateral ligament of the human elbow joint. Anatomy, function and biomechanics.

The posterior portion of the ulnar collateral ligament, which arises from the posterior surface of the medial epicondyle, is taut in maximal flexion. The anterior portion, which takes its origin from the anterior and inferior surfaces of the epicondyle, contains three functional fibre bundles. One of these is taut in maximal extension, another in intermediate positions between middle position and full flexion while the third bundle is always taut and serves as a guiding bundle. Movements of the elbow joint are checked by the ligaments well before the bony processes forming the jaws of the trochlear notch lock into the corresponding fossae on the humerus.

Course of the arteria vertebralis in its segment V1 from the origin to its entry into the foramen processus transversi.

The segment V1 of the arteria vertebralis (pathway from its origin from the a. subclavia to the entry into the respective foramen processus transversi) has a special significance in vascular surgery. Contrary to indications in the literature, we found 47.15% of the specimens to have a contorted course in this segment. The tortuosities carried by the respective vessels were found to be horizontal in 42.5%, in a sagittal direction in 30% and in the frontal plane in 27% of the cases. However, no significant difference was found between vessels carrying a tortuosity and those without, regarding the average lengths of the arteries concerned in the segment V1.