J.T. Bryant

An in vitro investigation of the acetabular labral seal in hip joint mechanics

Labrum pathology may contribute to early joint degeneration through the alteration of load transfer between, and the stresses within, the cartilage layers of the hip. We hypothesize that the labrum seals the hip joint, creating a hydrostatic fluid pressure in the intra-articular space, and limiting the rate of cartilage layer consolidation. The overall cartilage creep consolidation of six human hip joints was measured during the application of a constant load of 0.75 times bodyweight, or a cyclic sinusoidal load of 0.75+/-0.25 times bodyweight, before and after total labrum resection. The fluid pressure within the acetabular was measured. Following labrum resection, the initial consolidation rate was 22% greater (p=0.02) and the final consolidation displacement was 21% greater (p=0.02). There was no significant difference in the final consolidation rate. Loading type (constant vs. cyclic) had no significant effect on the measured consolidation behaviour. Fluid pressurisation was observed in three of the six hips. The average pressures measured were: for constant loading, 541+/-61kPa in the intact joint and 216+/-165kPa following labrum resection, for cyclic loading, 550+/-56kPa in the intact joint and 195+/-145kPa following labrum resection. The trends observed in this experiment support the predictions of previous finite element analyses. Hydrostatic fluid pressurisation within the intra-articular space is greater with the labrum than without, which may enhance joint lubrication. Cartilage consolidation is quicker without the labrum than with, as the labrum adds an extra resistance to the flow path for interstitial fluid expression. However, both sealing mechanisms are dependent on the fit of the labrum against the femoral head.

The influence of the acetabular labrum on hip joint cartilage consolidation : a poroelastic finite element model

The goal of this study was to investigate the influence of the acetabular labrum on the consolidation, and hence the solid matrix strains and stresses, of the cartilage layers of the hip joint. A plane-strain finite element model was developed, which represented a coronal slice through the acetabular and femoral cartilage layers and the acetabular labrum. Elements with poroelastic properties were used to account for the biphasic solid/fluid nature of the cartilage and labrum. The response of the joint over an extended period of loading (10,000s) was examined to simulate the nominal compressive load that the joint is subjected to throughout the day. The model demonstrated that the labrum adds an important resistance in the flow path of the fluid being expressed from the cartilage layers of the joint. Cartilage layer consolidation was up to 40% quicker in the absence of the labrum. Following removal of the labrum from the model, the solid-on-solid contact stresses between the femoral and acetabular cartilage layers were greatly increased (up to 92% higher), which would increase the friction between the joint surfaces. In the absence of the labrum, the centre of contact shifted towards the acetabular rim. Subsurface strains and stresses were much higher without the labrum, which could contribute to fatigue damage of the cartilage layers. Finally, the labrum provided some structural resistance to lateral motion of the femoral head within the acetabulum, enhancing joint stability and preserving joint congruity.

The material properties of the bovine acetabular labrum

The compressive and tensile material properties of the bovine acetabular labrum were measured. Confined compression testing was used to determine the aggregate compressive modulus and the permeability of the labrum. The compressive modulus of the labrum (0.157 ± 0.057 MPa) is comparable to that of the morphologically similar meniscus, and approximately one‐quarter to one‐half that of the adjoining acetabular cartilage. The permeability of the labrum (4.98 ± 3.43 × 10−16 m4/N s) was lower than that of the meniscus and cartilage, with a significantly higher resistance to interstitial fluid flow across the acetabular rim than along the rim. Specimens from the posterior and superior regions of the labrum were tested to failure in uniaxial tension. The maximum stress at failure (11.9 ± 6.1 MPa), maximum strain at failure (26.5 ± 7.6%) and tangent modulus (74.7 ± 44.3 MPa) were similar to those reported for the bovine meniscus, and to other tissues composed of highly oriented collagen fiber bundles. In tension, the labrum is much stiffer (10–15 × ) than the adjoining articular cartilage, and the posterior region of the labrum is significantly stiffer (45%) than the superior region. The labrums low permeability may contribute to sealing of the hip joint. The high circumferential tensile stiffness of the labrum, together with its ring structure, reinforce the acetabular rim and may contribute to joint stability.

A quantitative approach to radiography of the lower limb. Principles and applications

A method is described which provides standardised reproducible radiographic images of the lower limb. Anteroposterior and lateral radiographs are digitised and processed by computer to provide graphic/numeric displays of angles and linear measurements, relating the centre points of the hip, knee, and ankle. Two cases illustrate how surgical planning is facilitated when standardised data are available. These data confirm the close relationship between postoperative limb alignment and positioning of prosthetic elements.

Effect of muscular activity on valgus/varus laxity and stiffness of the knee

Quantitative changes in valgus/varus knee stability with different levels of muscular activity were determined for five subjects. A specially designed machine was used to measure resistance to angulation in the frontal plane. This device held the thigh stationary, the knee straight, an cycled the leg from side to side at a constant rate between present moment limits. Resistance to this forced valgus/varus motion was measured simultaneously with torque about the knee in the sagittal plane. Muscle activity was monitored by electromyography (EMG). Direct comparison of moment-rotation characteristics allowed changes in stability to be quantified as a function of extension and flexion torque. Extension torques less than 20% of the maximum increased varus stability more than valgus stability. Flexion torques of the same relative magnitude increased valgus stability more than varus stability. Comparison with the literature suggested that prevention of opening of the lateral side of the joint under varus loading was responsible for increased varus stability with increasing torque, both with extension and flexion torques.

Short-term load bearing capacity of osteochondral autografts implanted by the mosaicplasty technique: an in vitro porcine model

Articular surface congruency and graft stability are considered essential factors in the success of osteochondral grafting; however, quantitative measures of short-term load bearing capacity of grafts implanted by the mosaicplasty technique have not been reported. The purpose of this study was to develop a live tissue in vitro model to examine short-term fixation strength of mosaicplasty autografts immediately after and 1 week following graft implantation. Cylindrical osteochondral autografts were implanted in vitro by the mosaicplasty technique on five pairs of porcine femoral condyles within one and a half hours of animal sacrifice. Immediately following the surgical procedure, graft push-in and pull-out strength tests as well as indentation tests to determine modulus of the surrounding cancellous bone were performed on half of the specimens from the distal femurs of each animal. The remaining specimens, matched for location in the contralateral leg, were incubated in culture medium for 7 days prior to performing the same set of mechanical tests. Averaged push-in and pull-out graft fixation strength decreased 44% from 135.7 to 75.5N over the 7-day period, while no change in modulus was detected in the surrounding cancellous bone. These in vitro results demonstrate a substantial deterioration of short-term fixation strength of mosaicplasty grafts from the immediate post-operative state. Such a reduction in short-term graft load bearing capacity may pose a threat to the surgically established articular surface congruency and blood vessels formed during the early stages of the healing response.

Effects of grid dimensions on finite element models of an articular surface

Recent activity in finite element analysis of articular joints has emphasized refinements in geometry and material properties. In the implementation of such models, it is necessary to ensure that grid dimensions are optimal for suitable solutions of displacements, strains and stresses. A method of grid optimization was developed to ensure that for typical material properties, finite element models of an articular surface agree with known analytical solutions. The layered axisymmetric model presented by Askew and Mow (J. biomech. Engng 100, 105-115, 1978) was used as a reference. From this reference, an STZ of 0.2 mm, middle and deep zones of 0.8 mm and tidemark region of 0.2 mm were chosen. Cancellous bone was an infinite elastic half space under these layers. Loading was a parabolic distribution over a 10 mm radius having a peak of 1 MPa. Agreement was obtained between analytical solutions and finite element solutions when the finite element model had a radial boundary of 30 mm radius and a bone depth of 32 mm. These results suggested that in models of real joints, care must be taken to ensure the boundaries are reasonably represented and that sufficient bone is modelled for adequate solutions.

Stereoradiogrammetric technique for estimating alignment of the joints in the hand and wrist.

A method and apparatus for quantitative measurement of the alignment and motion of the joints of the hand in three dimensions has been developed using stereoradiogrammetric principles. Alignment in planes of flexion-extension and radial-ulnar deviation can be determined to within 2.5 degrees; rotation about the long axis of the metacarpals or phalanges is more difficult to determine, and can be measured to within 7 degrees. Stereo views subtending angles in the range of 40 degrees were found to optimize the total system accuracy.

Single camera photogrammetric technique for restricted 3D motion analysis

The purpose of this study was to use a single camera to determine the true elbow flexion angle with the arm plane oriented arbitrarily with respect to the camera. A mathematical theory was developed and a mechanical arm model was constructed to validate the theory. A static weightlifting skill was analysed to investigate the viability of the technique on the human subject. The validation of the theory showed the error associated with the elbow flexion angle was reduced from as high as 108 degrees when uncorrected to within 3 degrees when corrected by the technique. The elbow flexion angle of the human arm can be calculated to within 6 degrees of error for static weightlifting skill analysis.

Alignment of the first metatarsal-phalangeal joint: important criteria for a new joint replacement

Joint replacement is one of many options for the treatment of the first metatarsophalangeal (MTP) joint. Studies of the geometry of that joint have shown that it consists of two distinct articulations, the metatarsophalangeal and the MT-sesamoidal. Both are important, but the MT-sesamoidal tolerates only small deviations from an ideal alignment. The aim of this study was to investigate the alignment of the first MTP joint of a potential patient population, in order to design an optimal surface replacement. One measurement, the extension angle between the MT and the proximal phalanx was found to be the most important alignment criterion for the successful design of an implant and the necessary instruments. This angle controls the delicate interplay between the metatarsal head and the sesamoids and tolerates only small deviations from the normal range before the chance of sesamoidal subluxation increases significantly. The pre-operative knowledge of this and other alignment criteria is important for the ideal placement of an anatomical implant.