Clinical investigation of the pressure and shear stress on the trans-tibial stump with a prosthesis.

A system for measuring pressures and bi-axial shear stresses at the body support interfaces has been developed. This system has been used, in five unilateral trans-tibial amputees, to investigate the stresses at multiple points on the residual limb and prosthetic socket interface during standing and walking. The subjects investigated regularly used a patellar-tendon-bearing socket. The maximum peak pressure at the measured points was 320 kPa over the popliteal area during walking. The maximum shear stress was 61 kPa over the medial tibia area. Variable wave-forms of stress during walking were observed at the different measured points. The influence of the angular alignment on the stresses was investigated on one subject. It was found that a miss-alignment of +/- 8 degrees produced a change in peak longitudinal shear stress of between 8% and 11.5%.

Early detection of progression in adolescent idiopathic scoliosis by measurement of changes in back shape with the Integrated Shape Imaging System scanner.

STUDY DESIGN: A retrospective study of 78 patients with right thoracic idiopathic scoliosis was done. OBJECTIVES: To evaluate the reliability of the integrated Shape Imaging System scan (Oxford Metrics Ltd, Oxford, UK) in detecting progression of scoliosis and the use of back shape data in predicting scoliosis progression. SUMMARY OF BACKGROUND DATA: At first presentation and every 3-6 months during the follow-up period, all patients underwent integrated Shape Imaging System scans and radiographic examinations, from which the Cobb angle was measured. The follow-up period was 18-49 months (mean = 31.4 months). METHODS: Patients were divided into three groups according to the severity and progression of the Cobb angle. The spinal fusion, brace, and observation groups were compared using analysis of variance and the student's t test to detect significant differences among groups in the progression of deformity as measured by the integrated Shape Imaging System parameters and the Cobb angle. RESULTS: Three of the Integrated Shape Imaging System parameters detected significant progression in the spinal fusion group 1 year earlier than the Cobb angle. Only one of the Integrated Shape Imaging System parameters detected a significant difference in progression between the brace and observation groups. CONCLUSIONS: The Integrated Shape Imaging System technique demonstrated significant changes in this group of patients with progressive scoliosis. Serial measurements of back surface shape, particularly the size of the rib hump, may be predictive of progression. Serial Integrated Shape Imaging System scanning has advantages over serial radiography in the management of idiopathic scoliosis in addition to the avoidance of exposure to ionizing radiation.

Migration of cemented femoral components after THR. Roentgen stereophotogrammetric analysis.

We studied the migration of 58 cemented Hinek femoral components for total hip replacement, using roentgen stereophotogrammetric analysis over four years. The implants migrated faster during the first year than subsequently, and the pattern of migration in the second period was very different. During the first year they subsided, tilted into varus and internally rotated. After this there was slow distal migration with no change in orientation. None of the prostheses has yet failed. The early migration is probably caused by resorption of bone damaged by surgical trauma or the heat generated by the polymerisation of bone cement. Later migration may be due to creep in the bone cement or the surrounding fibrous membrane. The prosthesis which we studied allows the preservation of some of the femoral neck, and comparison with published migration studies of the Charnley stem suggests that this decreases rotation and may help to prevent loosening.

Frictional action at lower limb/prosthetic socket interface.

The frictional action at stump/socket interface is discussed by a simplified model and finite element model analyses and clinical pressure measurements. The friction applied to the stump skin produces stresses within tissue and these stresses may damage the tissues and affect their normal functions. The combination of normal and shear stresses is considered to be a critical factor leading to amputee's discomfort and tissue damage. However, friction at the stump/socket interface has a beneficial action. A simplified residual limb model and a finite element model using real geometry have been developed to analyse the support action of friction. Both results show that the friction plays a critical role both in supporting the load of the amputee's body during the support phase of the gait cycle and in preventing the prosthesis from slipping off the limb during swing phase. Pressure at the below-knee socket during walking were measured with conditions of different friction. The results reveal that a larger pressures was produced at the lubricated interface than at the normal interface. A proper choice of coefficient of friction will balance the requirements of relief of load stress and reduction of slip with the general ability to support loads.

Development of a non-linear finite element modelling of the below-knee prosthetic socket interface.

A non-linear finite element model has been established to predict the pressure and shear stress distribution at the limb-socket interface in below-knee amputees with consideration of the skin-liner interface friction and slip. In this model, the limb tissue and socket liner were respectively meshed into 954 and 450 three-dimensional eight-node isoparametric brick elements, based on measurements of an individual's amputated limb surface; the bone was meshed into three-dimensional six-node triangular prism elements, based on radiographic measurements of the individual's residual limb. The socket shell was assumed to be a rigid boundary. An important feature of this model is the use of 450 interface elements (ABAQUS INTER4) which mimic the interface friction condition. The results indicate that a maximum pressure of 226 kPa, shear stress of 53 kPa and less than 4 mm slip exist at the skin-liner interface when the full body weight of 800 N is applied to the limb. The results also show that the coefficient of friction is a very sensitive parameter in determining the interface pressures, shear stresses and slip. With the growth of coefficient of friction, the shear stresses will increase, while the pressure and slip will decrease.

Roentgen stereophotogrammetric analysis for assessing migration of total hip replacement femoral components.

A new Roentgen stereophotogrammetric analysis system, using a biplane technique, has been developed to determine the migration and rotation of total hip replacement (THR) femoral components in three dimensions. Stainless steel marker balls were injected into the femur during the operation. The patients stood within a calibration frame during the X-ray. The two exposures were taken consecutively allowing radio-opaque shutters to be moved in front of the films to prevent fogging. Studies with a model demonstrated that the system was capable of measuring the position of an implant to better than 0.11 mm (2 SD). In vivo measurements demonstrated that the migration rate of the different parts of the femoral component could be determined with an accuracy of 0.25 to 0.50 mm/year. By considering the accuracy determined in different ways, methods for improving the system have been identified. The migration and rotation rate of 58 Hinek cemented femoral components was studied for four years. Migration was three to five times greater (p < 0.001) during the first year than subsequently. The prosthesis head moved the most during the first year (0.94 mm). A better understanding of the cause of implant failure could be obtained by studying the early migration of different types of prosthesis and comparing this with their clinical results and design features.

Three-dimensional movement at externally fixated tibial fractures and osteotomies during normal patient function.

Three-dimensional movements at four externally fixated tibial fractures and four leg-lengthening osteotomies were measured using the Oxford micromovement transducer. By measuring movements in 6 degrees of freedom during normal patient activity, both shear and angulatory displacements have been found to be of the same order as axial displacements, and are often more dominant. The magnitude of such movements may be as high as 2 mm and 2 degrees in early healing to a small fraction of this in later stages, regardless of whether the fractures are initially reduced and whether they are oblique or transverse. Also, the main bars of certain fixators have torsional 'backlash' looseness, causing significant torsional movement at the fracture and shear perpendicular to the plane of the bone screws. Further interesting findings concern the movement of fractures during walking, 'dynamization' (telescopic movement of the fixator bar) and muscle activity. Axial, angular, and shear movements reverse direction in phase with the 'heel-strike' and 'toe-off' sequence of walking. During dynamization (where the fixator bar is 'loose'), fracture movements at a given load are often less than when not dynamizing (where the fixator bar is 'locked'). Active muscle response can cause movements of 5-6 times greater than movements arising from the transfer of force through the tibia during weight bearing. Also, the tensile stiffness during the above activities has been shown to be more than an order of magnitude lower than the apparent compressive stiffness.

Quantifying the cosmetic defect of adolescent idiopathic scoliosis.

Cosmesis is important in the treatment of adolescent idiopathic scoliosis patients. The aim of this study was to quantify the cosmetic defect using parameters of the ISIS (Oxford Metrics Ltd., Oxford, England) scan. Ten nonmedical judges scored photographs of 100 adolescent idiopathic scoliosis patients based on cosmetic criteria. This Cosmetic Spinal Score proved to be a reliable figure. The Cosmetic Spinal Score was then compared with ISIS parameters and the Cobb angle. An equation based on ISIS parameters was developed, which could predict Cosmetic Spinal Score with sufficient reproducibility to have a useful clinical application. Bracing reduced the rib hump but not enough to improve the cosmetic appearance. Spinal fusion and Harrington instrumentation improved all measured parameters influencing physical appearance.

Family stature in idiopathic scoliosis.

The standing heights of 40 children with scoliosis and 110 persons from their immediate families were compared with those of a group of 349 contemporary control subjects. Skeletal bone ages and puberty ratings did not differ from normal, but the children with scoliosis were found to be significantly taller than the controls. They were also taller than their unaffected relatives, suggesting that the increased stature is not due to genetic factors alone. Scoliotic children with an affected immediate-family member were of normal height, which suggests that growth is of less etiological importance in children with a strong genetic susceptibility.

An analysis of normal back shape measured by ISIS scanning.

Two hundred seventy-one school children aged 10-16 years and 72 normal adults aged 21-59 years were scanned by ISIS. Subjects with any evidence of scoliosis were excluded from the study. Characteristic ISIS parameters of back shape showed no statistically significant differences related to age or sex for the children. Back shape in both men and women was not significantly different from that of the children, except for kyphosis angles which were greater.

Familial back shape in adolescent scoliosis. A photogrammetric population study.

A stereophotogrammetric technique (ISIS scanning) was used to assess the back shape of children with adolescent idiopathic scoliosis (index cases), their unaffected relatives, normal adults and children, and children with small idiopathic scoliotic curves (Cobb's angle less than 30 degrees) to detect any familial trends that may predispose to scoliosis. The analysis revealed a characteristic three-dimensional scoliotic back shape; no differences in unaffected relatives as regards sex or a positive family history of scoliosis; similar back shapes in unaffected parents and normal adults, both of whom differ from index cases and children with small curves. Unaffected siblings had a sagittal profile between those of normal children and children with small curves. This may represent an inherited tendency to scoliosis.

Surface stereophotogrammetry of thoracic kyphosis.

The thoracic kyphosis angles of 16 normal individuals, 10 patients with Scheuermann's disease and 11 with adolescent idiopathic scoliosis were measured both radiographically and from Integrated Shape Imaging System (ISIS) scans obtained by surface stereophotogrammetry. There was a high correlation between the two measures. The method of kyphosis measurement from ISIS scans was then used for 30 patients with adolescent idiopathic scoliosis who underwent corrective surgery. A significant reduction in thoracic kyphosis was observed postoperatively. In another group of 28 patients managed conservatively by bracing, some hypokyphosis developed after treatment. However, we found no association between hypokyphosis and curve progression.

Back shape in brace treatment of idiopathic scoliosis.

This preliminary study has shown that bracing may improve the cosmetic appearance of a scoliotic back although the underlying curve remains unchanged. Thirty-two patients treated with the Boston brace for adolescent idiopathic scoliosis were assessed both roentgenographically and by integrated shape imaging system scanning before and after treatment. The mean follow-up time was 16 months. Surface shape improved in 41% of the patients, whereas roentgenographic improvement occurred in only 9%. Roentgenographic measurements should, therefore, be considered in conjunction with corresponding surface shape measurements in the assessment of brace treatment of scoliosis.

Correction of body height in scoliotic patients using ISIS scanning.

Measurement of height in scoliotic patients is complicated by changes in spinal shape in both the coronal and sagittal planes. A method is described by which these changes can be measured and compared with the shape of the normal spine, using an automated stereophotogrammetric technique (ISIS). The resultant height change may be of use in the prediction of respiratory function.

The differentiation of loose and secure femoral implants in total hip replacement using a vibrational technique: an anatomical and pilot clinical study.

ANATOMICAL STUDY: The common pathway to failure in total hip replacements is loosening of the acetabular and femoral components. The reliable diagnosis of this loosening is difficult. A non-invasive technique has been developed which analyses a vibration signal transmitted through the femur. This can distinguish between a femoral component which is loose and one which is firmly fixed. Charnley femoral components were cemented securely into ten cadaver femurs and the transmitted vibration signal recorded. The prostheses were then loosened first at the cement-prosthesis and then at the cement-bone interface. The tests were then repeated. Consistent and distinct differences between output vibration signals were observed between the firmly implanted and the loss prostheses. PILOT CLINICAL STUDY: Seven patients admitted for revision surgery because of clinical and radiological evidence of femoral implant loosening were tested using this technique. A further four patients with secure femoral components were tested approximately two weeks after total hip replacement. The differences in output signal, which had been observed between firmly implanted and loose prostheses during the anatomical study, were again demonstrated.

ISIS scanning: a useful assessment technique in the management of scoliosis.

The value of surface topographical measurements in the assessment of curve progression in adolescent idiopathic scoliosis is demonstrated in a group of 51 patients. Cobb angles at the commencement of the study ranged from 10 degrees -55 degrees (mean 34.5 degrees), and the mean follow-up period was greater than 2 years. The surface shape method correctly identified curve evolution in 84% of the patient group. Of these, the eight patients who underwent spinal instrumentation were correctly predicted as candidates for surgery.

Scoliosis surgery and its effect on back shape.

Thirty-four patients with adolescent idiopathic scoliosis were assessed by radiography and the integrated shape imaging system (ISIS) both before and after spinal surgery. Twenty-seven patients underwent Harrington instrumentation, after which lateral indices of curvature were significantly improved, but changes in the transverse plane were less pronounced. Sublaminar wiring was carried out in two patients whose thoracic lordosis was corrected by the surgery. Five patients whose severe deformity had persisted after previous spinal surgery underwent costoplasty, which resulted in a significant improvement in back shape measurements. We conclude that the cosmetic deformity of the back in scoliosis is only partially corrected by operations on the spine itself, whilst costoplasty addresses the problem directly, and improves the surface shape.

A method for analysis of back shape in scoliosis.

The shape of the back is an important factor in the clinical assessment of various spinal disorders, in particular scoliosis. A method of analysis of back surface shape is described which was designed to present most of the numerical parameters needed to assess the progress of the disease as it affects body shape. Measurements of back surface shape and manually marked anatomical landmarks were taken from a television/computer surface measurement system in which a plane of light was scanned over the back and from moiré topographs. The anatomical landmarks were used to define reference planes from which successive analyses were matched. Asymmetry in the transverse plane was illustrated by horizontal cross-sections and skin surface angles. The lateral deformity was shown by an estimate of the line of the vertebral bodies beneath the skin, derived by adding an extra lateral displacement to the palpated positions of the spinous processes, proportional to the rotation of the skin in the transverse plane. This model was used to estimate vertebral end-plate angles and Cobb angles. Lateral sections showed kyphosis and lordosis. Correlations of Lateral Asymmetry from the surface shape analysis with Cobb angle from X-ray measurements in three groups of patients (totalling 119 subjects) were in the range r = 0.77 to r = 0.94, p less than 0.0001. The analysis has reduced follow-up X-ray examinations at the Nuffield Orthopaedic Centre because it indicates quantitatively and with complete safety both lateral asymmetry and deformity in the transverse plane.

A television/computer three-dimensional surface shape measurement system.

An optical scanner is described which has been designed primarily for the measurement of human back shape. A projector and television camera were mounted together in a box which could rotate about a horizontal axis. The projector shone a horizontal plane of light, which was viewed at an angle from below by the television camera, linked directly to a minicomputer. The shape of the line of light formed by the plane as it fell on an object, together with a knowledge of the geometry of the system, enabled three-dimensional coordinates of points on the line to be calculated. A record of a surface shape was built up by scanning the object in about 2 s. Calibration of the system was achieved by scanning an object of known dimensions. Sets of algorithms are described which derive geometric parameters from the calibration scan and which sort surface shape coordinates, outline them and detect special markers from the surface shape scan. The accuracy of measurement exceeded the design aim of +/- 3 mm in each axis within a volume of 400 mm x 500 mm x 300 mm.