The physical demands of riding in National Hunt races.

Heart rate (fc) and post-competition blood lactate concentration ([La+]) were studied in seven male professional National Hunt jockeys over 30 races. The fc response for individual races followed a similar pattern for all subjects. The mean peak fc recorded during competition was 184 beats.min-1 (range 162-198 beats.min-1) with average fc during the races ranging from 136 to 188 beats.min-1. During consecutive races the recovery fc did not return to resting values. The mean [La+] was 7.1 mmol.l-1 (range 3.5-15.0 mmol.l-1). The conclusions of this study suggest that riding in National Hunt races is a physically demanding occupation. The muscular activity in this profession requires a high metabolic drive and produces a significant cardiorespiratory response.

Femoral neck fracture fixation. Comparison of a sliding screw with lag screws.

The rigidity of a sliding compression screw and three cannulated lag screws in the treatment of subcapital fractures was compared in five pairs of female cadaver femora. There were no significant differences between the compressive strength, bone density, cortical thickness or Singh index of the bones in each pair. A subcapital fracture was standardised using a perpendicular saw cut across the femoral neck. A uniaxial 'load test system' with force and length measurement facilities was used to mimic cyclical stressing applied in vivo at a frequency of 0.5 Hz from 0 to 3 times body-weight. There was no significant difference between the fixation afforded by the sliding compression screw and three lag screws. Bone quality was the single most important factor in the stability of the bone implant unit.

An introduction to the 'Mennen plate' and its use in treatment of fractures of the edentulous mandible.

A paraskeletal clamp plate (Mennen) offers a novel method of fixation for mandibular fractures. Its application is relatively atraumatic and this together with the minimal disruption to the local blood supply may make this clamp particularly suited to fixation of fractures of the atrophic edentulous jaw. Its use in six such cases is reported.

A comparative in vitro study of fixation of mandibular fractures with paraskeletal clamps or screw plates.

The use of porcine rib pairs as an in vitro analog for the edentulous mandible is described. Using this model, the relative degree of fixation achieved with a screw plate (Champy) and a paraskeletal clamp plate (Mennen) has been evaluated. The fractured plated ribs failed at significantly lower bending forces than the non-sectioned controls. However, no statistically significant difference in force at failure between the two different methods of plate fixation could be demonstrated.

Electrical impedance assessment of muscle changes following exercise.

Electrical impedance measurements have been assessed as a method of detecting changes in striated muscle following vigorous exercise. Transverse and longitudinal resistivities of the calf and thigh have been measured before and after four subjects ran a half marathon (21 km). No changes were observed in longitudinal resistivity but transverse resistivity rose by an average of 7% following the race. These results are consistent with changes in the muscle fibre membranes or interstitial fluid content.

Pericardial heterografts: a comparative study of suture pull-out and tissue strength.

The force required to pull sutures out of glutaraldehyde fixed bovine pericardium, for four different suture bites: 0.5 mm, 1 mm, 1.5 mm and 2.0 mm, was compared with the tissue strength. The mean suture pull-out force was significantly lower than the tissue strength for all bites, with a minimum value of 2.86 +/- 1.02 N for the 0.5 mm bite and a maximum value of 6.32 +/- 0.77 N for the 2.0 mm bite. The mean force which produced failure of the chemically modified pericardium was 15.49 +/- 8.48 N. The mean force at pull-out of the sutures lay on a regression line: force at failure = 1.68 + 2.25 x Bite. A video film of the experiments showed that the suture does not cut through the pericardium. It pulls a V-shaped band of collagen fibre bundles through the stationary pericardium. Eventually this band breaks away from the free edge of the tissue specimen. The specimens under uniaxial load failed by laminate debonding of two layers of tissue, rupture of the serosal surface layer followed by shear and fibre slippage. These results indicate that any suture which bears load, during the normal functioning of a heart valve substitute, will be a source of weakness, compared to the overall tissue strength. As a consequence the alignment/holding suture of the Standard Ionescu-Shiley valve and the modified stitch of the low profile valve are likely to be potential sites of fatigue failure.

Local variation in the tearing strength of chemically modified pericardium.

Forty squares of tissue, 40 mm X 40 mm, were harvested from bovine pericardium fixed in glutaraldehyde free of tension. Two parallel dumb-bell specimens were excised from each square. In 20 pairs of adjacent samples, two 1 mm cuts were introduced into one of the samples at the centre of the dumb-bell. The other 20 pairs were used as controls. There was no significant difference in tensile strength of the paired samples tested without cuts, but there was marked variation in the strength of the tissue in different squares. There was a significant positive correlation between the strength of the paired samples. Introduction of artificial tears produced a significant reduction in the strength of the tissue locally. However, a significant positive correlation between the sample with tears and the sample without tears was still maintained. Failure of the tissue under uniaxial load was observed dynamically by using a Panasonic video camera with zoom and magnification facilities illuminated by an oblique low-power light source placed behind the specimen. All samples, with or without tears, failed by rupture of a layer of tissue, laminate debonding and shear, followed by fibre slippage through the viscoelastic embedding matrix.

The tearing strength of glutaraldehyde fixed bovine pericardium.

Destructive uniaxial load tests were performed on bovine pericardium that had been chemically modified by glutaraldehyde fixation. To investigate the tearing strength of this tissue, an artificial tear, in the form of a 2mm notch, was introduced into the test specimens. Specimens without a notch, harvested from the same sites in the pericardial sac but from different sacs, were used as controls. A video system with zoom and lens magnification facilities combined with low power oblique illumination was used to observe the events leading to tissue failure. Analysis of variance demonstrated that there was no overall significant difference in tensile strength or percentage strain at fracture between the test specimens with a tear and the control specimens without a tear. Rupture of the tissue occurred following laminate debonding by shear and fibre slippage through the viscous ground matrix.

Pericardial heterografts: why do these valves fail?

Eighteen explanted pericardial heterografts were studied (16 standard Ionescu-Shiley, one Hancock, and one Mitroflow). Regurgitation was the reason given for explantation of all the Ionescu-Shiley valves. The other two valves were removed for technical reasons. All the Ionescu-Shiley valves had commissural tears and there was concomitant gross calcification in 10 of the 16 valves. In addition, an apparent increase in cusp area had caused "leaflet sagging". The explanted leaflets were thicker and stiffer than leaflets from an unimplanted valve. These features were confirmed directly with an animal model of subcutaneous implantation. Examination with an electron microscope revealed that these changes in mechanical properties seemed to be linked to fiber separation and infiltration by an amorphous proteinlike matrix. The durability of the glutaraldehyde-fixed pericardium depended on a number of factors. Early and midterm failure appeared to be stress induced. Predisposition to high mechanical stresses near the stent was exacerbated by the changes induced by the host environment. This problem was aggravated further in the Ionescu-Shiley valves by stress concentrations around the hole associated with the holding suture. In the long term, collagen disruption associated with leaflet flexure was followed by secondary calcification at the boundary between the intact and disrupted material.

The tensile strength of natural and chemically modified bovine pericardium.

Nondestructive and destructive uniaxial load tests were performed on natural and chemically modified bovine pericardium. Five specimens were selected from the same sites in different pericardial sacs by using a template. The mean maximum extension of one particular site in both the natural and chemically modified material was significantly greater than that of the other positions at a stress level of 0.6 Nmm.-2 The maximum extensibility of the fixed tissue was significantly greater (p less than 0.01) than that of the natural tissue. There was also an anatomical variation in tensile strength of the natural material which was retained after chemical modification. However, the overall tensile strength of the pericardium was not increased by this procedure. In contrast, glutaraldehyde fixation did increase the percentage strain at which fracture of this biomaterial occurred.

Pericardial heterograft valves: an assessment of leaflet stresses and their implications for heart valve design.

Bovine pericardium, stabilized with glutaraldehyde, is used widely in the construction of heart valve substitutes, but the design and construction of valve substitutes from this material are empirically based. Collagenous tissue can support tension, but experimental evidence indicates that flexure-induced compressive stresses can lead to fatigue failure. This study uses experimental results obtained from cyclic uniaxial load tests to predict the type and magnitude of operational stresses which occur in pericardial heterograft leaflets. Both Young's modulus and Poisson's ratio varied with uniaxial loading in pericardium, chemically modified free of tension. Leaflet stresses were analysed in using effective incremental representations of these parameters. In leaflets with unrestricted rotation at the point of attachment to the stent, the mid-plane tensions always exceeded the bending stresses, and no zones of leaflet compression were predicted. In contrast, with totally restricted leaflet rotation induced by clamping (possibly between a male and female frame) the bending stresses were greater than the mid-plane tensions at the hinge line and significant compressive stresses were predicted at this site. If elastic boundary conditions were introduced at the stent (possibly by wrapping the stent in pericardium) then the compressive stresses were reduced as the degree of elasticity was increased. Glutaraldehyde fixation of the pericardium under load produced a stiffer material; higher compressive stresses at the stent and significant increases in total stress were predicted for this tissue. The application of elevated pressure loading also increased the compressive and total stresses in the leaflet. Finally, it was shown that bicuspid leaflets were likely to experience higher stresses than tricuspid leaflets. This simple stress analysis should help valve designers of pericardial heterografts to identify those conditions which lead to tissue compression, high total stress, and ultimately material fatigue.

The extension rate independence of the hysteresis in glutaraldehyde-fixed bovine pericardium.

Glutaraldehyde-fixed bovine pericardium demonstrates both relaxation of stress and hysteresis during uniaxial loading and unloading. These phenomena suggest viscoelastic behaviour of the material. This study uses analysis of variance to test statistically the hypothesis that the chemically modified pericardium possesses a 'fading memory' for the history of previous load procedures and that the hysteresis is extension rate independent.

Pericardial heterografts. Toward quality control of the mechanical properties of glutaraldehyde-fixed leaflets.

The mechanical properties of five different, equally distributed, and randomly oriented sites in natural and glutaraldehyde-fixed calf pericardial sacs were investigated. Identical sites in different sacs were obtained by placing a template over each sac and using the ligaments still attached to the pericardium as a reference frame. Mean maximum extension of the fixed tissue was statistically significantly greater than that of natural tissue at the same degree of stress. There was no significant difference in thickness from position to position within a sac, from sac to sac, or from the natural to fixed pericardium. However, the extensibility of one particular site was significantly greater than that of other positions in the pericardial sac. This position lay entirely in a well-defined sector that emanated radially from the pericardial ligaments. Light and electron microscopy showed no difference in collagen structure in the five positions studied. In contrast, both the content and ultrastructure of the tissue elastin differed in the region of high extensibility compared with those sites studied in the remainder of the sac. This region contained coarse condensed elastin whereas the other sites had fine elastin dispersed diffusely throughout the tissue. These results suggest that pericardial heterografts of uniform thickness may be harvested from any position in calf pericardium. However, unless the region of coarse condensed elastin is excluded, leaflets with significantly different mechanical properties may be produced.

Evidence that deformations which occur during mechanical conditioning of bovine pericardium are not permanent.

Natural and glutaraldehyde fixed bovine pericardium samples were mechanically conditioned by a cyclic uniaxial load procedure. The samples tested were controlled for both position and direction in the pericardial sac. The natural tissue demonstrated a significant increase in length and a significant decrease in width after mechanical conditioning. The deformations were not permanent. The test specimens had returned to their original length by 10.5 h after the mechanical test. The control samples of natural tissue showed no significant changes in length during this time. The chemically modified tissue showed a significant increase in length but no significant changes in width or thickness after mechanical conditioning. As in the natural tissue, the length changes were not permanent. Twenty four hours after returning the fixed tissue to its normal buffered glutaraldehyde storage medium the test samples had returned to their original length. A subsequent mechanical retest 8 d after the initial test procedure suggested that the history of the original test had been removed. These observations may be important in the interpretation of 'in vitro' hydrodynamic tests for heart valve substitutes.

The standardisation of gauge length: its influence on the relative extensibility of natural and chemically modified pericardium.

Cyclic uniaxial load tests were performed on natural and chemically modified bovine pericardium which is used in the construction of heart valve substitutes. A template was employed to select specimens from the same sites in different pericardial sacs. When the pericardium was chemically modified by glutaraldehyde fixation as an entire sac the tissue showed increased extensibility after modification compared with the natural tissue. The undeformed stress-free length (gauge length) in both the natural and modified tissue was determined by a highly reproducible experimental method giving a coefficient of variation of less than 0.5%. Specimens excised from a natural pericardial sac demonstrated a significant increase (p less than 0.03) in length, 4.97 +/- 3.49%, after a single load cycle, compared with controls placed in isotonic saline but not mechanically tested. The test pieces had returned to their original length by 8.5 h after the mechanical test. After fixation the same specimens decreased significantly in length (p less than 0.001) by 11.18 +/- 4.28%. This shrinkage was not significantly different to that of the control specimens (11.09 +/- 2.47%) which had not been tested. Uniaxial loading of these chemically modified strips demonstrated a similar increase in tissue extensibility compared to the natural tissue if the undeformed length of the test specimen after shrinkage was used as the gauge length. After mechanical conditioning the chemically modified tissue also demonstrated a significant increase (p less than 0.001) in stress-free length (5.35 +/- 0.59% after 36 cycles, 8.92 +/- 1.50% after 2085 cycles). These deformations were not permanent. The tissue had returned to its original length after 38 h in its normal buffered glutaraldehyde storage medium. On the basis of these observations, recommendations for the clarification and standardisation of gauge length definitions were made in natural, chemically modified and mechanically conditioned tissue.