P. F. Leyvraz

Spatio-temporal parameters of gait measured by an ambulatory system using miniature gyroscopes

In this study we describe an ambulatory system for estimation of spatio-temporal parameters during long periods of walking. This original method based on wavelet analysis is proposed to compute the values of temporal gait parameters from the angular velocity of lower limbs. Based on a mechanical model, the medio-lateral rotation of the lower limbs during stance and swing, the stride length and velocity are estimated by integration of the angular velocity. Measurements accuracy was assessed using as a criterion standard the information provided by foot pressure sensors. To assess the accuracy of the method on a broad range of performance for each gait parameter, we gathered data from young and elderly subjects. No significant error was observed for toe-off detection, while a slight systematic delay (10 ms on average) existed between heelstrike obtained from gyroscopes and footswitch. There was no significant difference between actual spatial parameters (stride length and velocity) and their estimated values. Errors for velocity and stride length estimations were 0.06 m/s and 0.07 m, respectively. This system is light, portable, inexpensive and does not provoke any discomfort to subjects. It can be carried for long periods of time, thus providing new longitudinal information such as stride-to-stride variability of gait. Several clinical applications can be proposed such as outcome evaluation after total knee or hip replacement, external prosthesis adjustment for amputees, monitoring of rehabilitation progress, gait analysis in neurological diseases, and fall risk estimation in elderly.

Viscoelastic constitutive law in large deformations: application to human knee ligaments and tendons

Traction tests on soft tissues show that the shape of the stress strain curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to properly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and viscous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics. Three parameters -- two for the elasticity and one for the viscosity -- were enough to precisely fit the non-linear stress strain curves obtained at different strain rates with human cruciate ligaments and patellar tendons. The identification results then in a realistic, three-dimensional viscoelastic constitutive law. The developed constitutive law can be used regardless of the strain or rotation values. It can be incorporated into a finite element program to model the viscoelastic behavior of ligaments and tendons under dynamic situations.

Temporal feature estimation during walking using miniature accelerometers: an analysis of gait improvement after hip arthroplasty

A new method for the detection of gait cycle phases using only two miniature accelerometers together with a light, portable digital recorder is proposed. Each subject is asked to walk on a walkway at his/her own preferred speed. Gait analysis was performed using an original method of computing the values of temporal parameters from accelerometer signals. First, to validate the accelerometric method, measurements are taken on a group of healthy subjects. No significant differences are observed between the results thus obtained and those from pressure sensors attached under the foot. Then, measurements using only accelerometers are performed on a group of 12 patients with unilateral hip osteo-arthritis. The gait analysis is carried out just before hip arthroplasty and again, three, six and nine months afterwards. A mean decrease of 88% of asymmetry of stance time and especially a mean decrease of 250% of asymmetry of double support time are observed, nine months after the operation. These results confirm the validity of the proposed method for healthy subjects and its efficiency for functional evaluation of gait improvement after arthroplasty.

Technical NoteViscoelastic constitutive law in large deformations: application to human knee ligaments and tendons

Traction tests on soft tissues show that the shape of the stress strain curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to properly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and viscous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics. Three parameters -- two for the elasticity and one for the viscosity -- were enough to precisely fit the non-linear stress strain curves obtained at different strain rates with human cruciate ligaments and patellar tendons. The identification results then in a realistic, three-dimensional viscoelastic constitutive law. The developed constitutive law can be used regardless of the strain or rotation values. It can be incorporated into a finite element program to model the viscoelastic behavior of ligaments and tendons under dynamic situations.

The biomechanics of the human patella during passive knee flexion

The fundamental objectives of patello-femoral joint biomechanics include the determination of its kinematics and of its dynamics, as a function of given control parameters like knee flexion or applied muscle forces. On the one hand, patellar tracking provides quantitative information about the joints stability under given loading conditions, whereas patellar force analyses can typically indicate pathological stress distributions associated for instance with abnormal tracking. The determination of this information becomes especially relevant when facing the problem of evaluating surgical procedures in terms of standard (i.e. non-pathological) knee functionality. Classical examples of such procedures include total knee replacement (TKR) and elevation of the tibial tubercle (Maquets procedure). Following this perspective, the current study was oriented toward an accurate and reliable determination of the human patella biomechanics during passive knee flexion. To this end, a comprehensive three-dimensional computer model, based on the finite element method, was developed for analyzing articular biomechanics. Unlike previously published studies on patello-femoral biomechanics, this model simultaneously computed the joints kinematics, associated tendinous and ligamentous forces, articular contact pressures and stresses occurring in the joint during its motion. The components constituting the joint (i.e. bone, cartilage, tendons) were modeled using objective forms of non-linear elastic materials laws. A unilateral contact law allowing for large slip between the patella and the femur was implemented using an augmented Lagrangian formulation. Patellar kinematics computed for two knee specimens were close to equivalent experimental ones (average deviations below 0.5 degrees for the rotations and below 0.5 mm for the translations) and provided validation of the model on a specimen by specimen basis. The ratio between the quadriceps pulling force and the patellar tendon force was less than unity throughout the considered knee flexion range (30-150 degrees), with a minimum near 90 degrees of flexion for both specimens. The contact patterns evolved from the distal part of the retropatellar articular surface to the proximal pole during progressive flexion. The lateral facet bore more pressure than the medial one, with corresponding higher stresses (hydrostatic) in the lateral compartment of the patella. The forces acting on the patella were part of the problem unknowns, thus leading to more realistic loadings for the stress analysis, which was especially important when considering the wide range of variations of the contact pressure acting on the patella during knee flexion.

The morphology of the proximal femur. A three-dimensional radiographic analysis

Biological fixation of cementless femoral implants requires primary stability by optimal fit in the proximal femur. The anatomy of the bone must then be known precisely. We analysed in vitro the accuracy of bone measurements of 32 femurs and compared the dimensions obtained from radiographs and CT scans with the true anatomical dimensions. Standard radiographs gave only a rough approximation of femoral geometry (mean difference: 2.4 +/- 1.4 mm) insufficiently accurate to allow selection of the best fitting prosthesis from a range of sizes and altogether inadequate to design a custom-made prosthesis. CT scans give greater accuracy (mean difference: 0.8 +/- 0.7 mm) in our experimental conditions, but in clinical practice additional sources of error exist.

Three-dimensional morphology of the proximal femur

In the field of uncemented hip arthroplasties, secondary biologic fixation of femoral implants depends directly on the quality of the primary stability. Metaphyseal filling and a good fit between the implant and the proximal femur improve initial stabilization and optimize the transmission of forces to the bone. Precise knowledge of the three-dimensional femoral shape is essential to the design and selection of adapted implants. Three hundred ten femurs in 300 patients suffering from primary hip osteoarthritis were analyzed by computed tomography scanning. After three-dimensional reconstruction, several measurements were extracted, and the parameters essential to the characterization of the diverse femoral morphologies encountered were identified. A new classification of the proximal femur is proposed. The consequences on the design and the preoperative selection of femoral implants are discussed.

The fixation of the cemented femoral component EFFECTS OF STEM STIFFNESS, CEMENT THICKNESS AND ROUGHNESS OF THE CEMENT-BONE SURFACE

We performed an audit of 71 children with consecutive displaced, extension-type supracondylar fractures of the humerus over a period of 30 months. The fractures were classified according to the Wilkins modification of the Gartland system. There were 29 type IIA, 22 type IIB and 20 type III. We assessed the effectiveness of guidelines proposed after a previous four-year review of 83 supracondylar fractures. These recommended that: 1) an experienced surgeon should be responsible for the initial management; 2) closed or open reduction of type-IIB and type-III fractures must be supplemented by stabilisation with Kirschner (K-) wires; and 3) K-wires of adequate thickness (1.6 mm) must be used in a crossed configuration. The guidelines were followed in 52 of the 71 cases. When they were observed there were no reoperations and no malunion. In 19 children in whom they had not been observed more than one-third required further operation and six had a varus deformity. Failure to institute treatment according to the guidelines led to an unsatisfactory result in 11 patients. When they were followed the result of treatment was much better. We have devised a protocol for the management of these difficult injuries.Accurate quantitative measurements of micromovement immediately after operation would be a reliable indicator of the stability of an individual component. We have therefore developed a system for measuring micromovement of the tibial component using three non-contact displacement transducers attached to the tibial cortex during total knee arthroplasty (TKA). Using this system we measured the initial stability in 31 uncemented TKAs. All the tibial components were fixed by a stem and four screws. The initial stability was defined as the amount of displacement when a load of 20 kg was applied. The mean subsidence was 60.7 μm and the mean lift-off was 103.3 μm. We also studied the migration of the tibial component using roentgen stereophotogrammetric analysis (RSA) for up to two years after operation. Most migration occurred during the first six months, after which all prostheses remained stable. We defined migration as the maximum total point motion (MTPM) at two years after operation. The mean migration was 1.29 mm at two years. Our results show that there was a significant correlation between the initial stability and migration (pWe enrolled 34 normal volunteers to test the hypothesis that there were two types of movement of the wrist. On lateral radiographs two distinct patterns of movement emerged. Some volunteers showed extensive rotation of the lunate with a mean range of dorsiflexion of 65°, while others had a mean range of 50°. The extensive rotators were associated with a greater excursion of the centre of articulation of the wrist. It is suggested that dynamic external fixation of a fracture of the distal radius carries with it the risk of stretching the ligaments or causing volar displacement at the site of the fracture.We have reviewed 202 consecutive primary hip replacements using a Freeman cemented femoral component after a mean period of follow-up of 64 months (23 to 113). There was only one revision for aseptic loosening. Retention of the femoral neck may act to reduce the torsional and shear forces at the implant-cement interface and may provide a seal against the migration of polyethylene-laden joint fluid in the potential joint space. The cemented Freeman femoral component with retention of the femoral neck was successful for up to nine years.: Acute compartment syndrome is a serious complication of injury. It occurs when raised pressure within a closed osteofascial compartment compromises the circulation and function of tissues within the compartment. Most cases are caused by fracture. The largest group are tibial diaphyseal fractures, followed by soft tissue injury, crush syndrome, distal radial fractures and forearm diaphyseal fractures. Those at risk of developing acute compartment syndrome are young males, patients with bleeding disorders or on anticoagulant therapy and in the upper limb, high energy injury. The clinical symptoms and signs of acute compartment syndrome are pain, stretch pain and neurological abnormality, none of which are constantly present. Since early diagnosis of this condition is of paramount importance compartment monitoring is recommended. The recommended tissue pressure threshold for decompression has been variable through the years but should be related to the patients blood pressure. A difference of less than 30 mmHg between the diastolic and tissue pressures has been validated clinically and it is recommended that at this level serious consideration should be given to decompression of the affected compartments. Use of this pressure threshold with compartment monitoring has been shown to half the delay to fasciotomy and significantly reduces the late complications of acute compartment syndrome.After cemented total hip arthroplasty (THA) there may be failure at either the cement-stem or the cement-bone interface. This results from the occurrence of abnormally high shear and compressive stresses within the cement and excessive relative micromovement. We therefore evaluated micromovement and stress at the cement-bone and cement-stem interfaces for a titanium and a chromium-cobalt stem. The behaviour of both implants was similar and no substantial differences were found in the size and distribution of micromovement on either interface with respect to the stiffness of the stem. Micromovement was minimal with a cement mantle 3 to 4 mm thick but then increased with greater thickness of the cement. Abnormally high micromovement occurred when the cement was thinner than 2 mm and the stem was made of titanium. The relative decrease in surface roughness augmented slipping but decreased debonding at the cement-bone interface. Shear stress at this site did not vary significantly for the different coefficients of cement-bone friction while compressive and hoop stresses within the cement increased slightly.We reviewed 120 consecutive primary total hip replacements in 109 patients in whom a Freeman uncemented metal-backed component had been used. Acetabular components were used with a Freeman neck-retaining stem in one of three configurations: cemented smooth stem, uncemented smooth stem or uncemented ridged stem. After a mean follow up of 72 months (62 to 113) there were nine cases of aseptic loosening; a survivorship at eight years of 83%. In addition, more than one-third of the remaining surviving cups showed loosening radiologically. The Freeman acetabular component demonstrated a characteristic pattern of loosening on radiographs. The high incidence of aseptic loosening can be explained by poor design and material failure.Although the technique of autogenous acetabular bone grafting has been widely used to augment containment of the acetabulum in total hip arthroplasty (THA) for developmental dysplasia, the role of this technique in improving long-term results remains controversial. We present the long-term results of cemented THA with acetabular bone grafting in 112 patients (133 hips) in order to clarify the factors which affect the outcome. The mean follow-up was for 12.3 years (8 to 24). Kaplan-Meier survivorship analysis predicted a rate of survival of the acetabular component at 15 years of 96% (95% confidence interval (CI) 92 to 99) with revision for aseptic loosening as the endpoint, and of 75% (95% CI 65 to 85) when radiological loosening was used. Parametric survivorship analysis using the Cox proportional-hazards model indicated that trochanteric nonunion, lateral placement of the socket, and delayed trabecular reorientation of the bone graft were risk factors for loosening of the acetabular component. Our findings have shown that autologous acetabular bone grafting is of value for long-term success provided that the risk factors are reduced.

A finite element model for evaluation of tibial prosthesis-bone interface in total knee replacement

A numerical model based on the finite element method was developed for the load transfer analysis at the tibial bone-implant interfaces in total knee replacement. A transverse isotropic material model, based on a quadratic elastic potential and on Hills quadratic yield criterion, was next developed for bone constitutive laws. The bone-cement and bone-prosthesis interfaces were both assumed to be discontinuous. A dry friction model based on Coulombs criterion was adopted for the interfaces friction. The model was shown to be able to give compressive and shear stresses distributions and distractive and relative shear micromotions at these interfaces. A preliminary application was conducted for cemented metal tray total condylar (MTTC) and for cemented and uncemented porous coated anatomic (PCA) tibial plateaus. The PCA plateaus were found to be more deformable and had greater global displacements than the MTTC one. Debonding of the bone-peg interface was observed for the uncemented PCA. Correspondingly, the stress peaks at the interface beneath the tray were lower for the uncemented PCA. Correspondingly, the stress peaks at the interface beneath the tray were lower for the PCA than for the MTTC. Shear micromotions appeared under the tray for both the two prostheses. We observed that bone anisotropy and interface discontinuity affected the results sensibly.

Comparison between magnetic resonance imaging and arthroscopy in the diagnosis of patellar cartilage lesions

A blind and prospective study was conducted to assess the accuracy of magnetic resonance imaging (MRI) for diagnosing patellar cartilage lesions. Thirty-three consecutive patients undergoing knee arthroscopy were examined by MRI prior to surgery. Imaging was performed in the axial plane on a 1.5-Tesla unit with spinecho and gradient-echo T1 and T2 sequences. The MRI and arthroscopic data were compared using a four-grade classification and a patellar map which divided the patellar surface into four quadrants. The overall sensitivity of MRI was 84.7% and the specificity 97.2%. The same pit-fall led to two MRI false positives. A perfect correlation of grading was obtained in 76.5%. When discordance was found, the tendency with MRI was commonly to underestimate the grade of the lesions. The MRI accuracy was high in this study in spite of a high rate of low-grade lesions which are difficult to assess. Related criteria for cartilage assessment with MRI and arthroscopy are suggested for further studies.