Early periprosthetic bone remodelling around cemented and uncemented custom-made femoral components and their uncemented acetabular cups.

INTRODUCTION: Periprosthetic bone remodelling after total hip replacement may contribute to aseptic loosening of the prosthesis. The selection between cemented and uncemented fixation of the stem is mainly determined by patient's age, general constitution and CT scan-estimated bone quality; intra-operative observation may ultimately influence the choice of the fixation method. The influence of cemented versus uncemented stem fixation on periprosthetic bone remodelling around the uncemented cup has, to our knowledge, never been studied until now. METHODS: A total of 75 patients received intra-operatively manufactured stem prostheses and a standard hydroxy apatite-coated pinnacle cup. The pre-operative CT scans provides guidance for the bone quality and hence the type of stem fixation: cemented or uncemented. The influence of either type of stem fixation on periprosthetic bone remodelling around the cup and the stem was measured by bone mineral density at 6 weeks, and 3, 6 and 12 months after surgery. RESULTS: Early changes in bone mineral density were noted. The type of stem fixation had an influence on the bone remodelling of the femur and also of the pelvis. The caudal part of the acetabulum was subject to a greater loss in BMD at 12 months in the group with cemented stem fixation. Changes at 12 months correlated with the changes measured at any time point. CONCLUSIONS: The selection of the stem implant and its type of fixation in the femoral cavity (cemented or uncemented fixation) seems to have an impact on the bone mineral density of the acetabulum. Long-term clinical follow-up is required to draw conclusions regarding the influence on prosthesis survival.

Aberrant pelvis and hip kinematics impair hip loading before and after total hip replacement.

Musculoskeletal loading is an important factor affecting the development of osteoarthritis, bone remodelling and primary fixation of total hip replacement (THR). In this study, we analyzed the relation between muscular force, gait kinematics and kinetics and hip loading in 20 patients before and six weeks after THR. Hip contact forces were calculated from gait analysis data using musculoskeletal modelling, inverse dynamics and static optimization. We found aberrant pelvis and hip kinematics and kinetics before and six weeks after surgery, confirming previous findings in literature. Furthermore, we found a decrease in the total contact force and its vertical component. These changes result in a decrease of the associated inclination angles of the total hip contact force in the sagittal and transverse planes, changing the orientation towards more vertical implant loading after THR. These changes in hip loading were related to observed gait kinematics and kinetics. Most importantly, excessive pelvic obliquity and associated hip adduction related to impaired implant loading. We concluded, therefore, that physical therapy in the early post-operative phase should primarily focus on stretching of anterior and medial structures and strengthening of hip flexors and abductors to achieve normalization of the hip and pelvis kinematics and consequently normalize hip loading.

Subject-specific hip geometry and hip joint centre location affects calculated contact forces at the hip during gait.

Hip loading affects the development of hip osteoarthritis, bone remodelling and osseointegration of implants. In this study, we analyzed the effect of subject-specific modelling of hip geometry and hip joint centre (HJC) location on the quantification of hip joint moments, muscle moments and hip contact forces during gait, using musculoskeletal modelling, inverse dynamic analysis and static optimization. For 10 subjects, hip joint moments, muscle moments and hip loading in terms of magnitude and orientation were quantified using three different model types, each including a different amount of subject-specific detail: (1) a generic scaled musculoskeletal model, (2) a generic scaled musculoskeletal model with subject-specific hip geometry (femoral anteversion, neck-length and neck-shaft angle) and (3) a generic scaled musculoskeletal model with subject-specific hip geometry including HJC location. Subject-specific geometry and HJC location were derived from CT. Significant differences were found between the three model types in HJC location, hip flexion-extension moment and inclination angle of the total contact force in the frontal plane. No model agreement was found between the three model types for the calculation of contact forces in terms of magnitude and orientations, and muscle moments. Therefore, we suggest that personalized models with individualized hip joint geometry and HJC location should be used for the quantification of hip loading. For biomechanical analyses aiming to understand modified hip joint loading, and planning hip surgery in patients with osteoarthritis, the amount of subject-specific detail, related to bone geometry and joint centre location in the musculoskeletal models used, needs to be considered.

A finite element analysis of the vibrational behaviour of the intra-operatively manufactured prosthesis-femur system.

In total hip replacement (THR) a good initial stability of the prosthetic stem in the femur, which corresponds to a good overall initial contact, will help assure a good long-term result. During the insertion the implant stability increases and, as a consequence, the resonance frequencies increase, allowing the assessment of the implant fixation by vibration analysis. The influence of changing contact conditions on the resonance frequencies was however not yet quantitatively understood and therefore a finite element analysis (FEA) was set up. Modal analyses on the hip stem-femur system were performed in various contact situations. By modelling the contact changes by means of the contact tolerance options in the finite element software, contact could be varied over the entire hip stem surface or only in specific zones (proximal, central, distal) while keeping other system parameters constant. The results are in agreement with previous observations: contact increase causes positive resonance frequency shifts and the dynamic behaviour is most influenced by contact changes in the proximal zone. Although the finite element analysis did not establish a monotonous relationship between the vibrational mode number and the magnitude of the resonance frequency shift, in general the higher modes are more sensitive to the contact change.

Subject-specific hip geometry affects predicted hip joint contact forces during gait.

Hip loading affects bone remodeling and implant fixation. In this study, we have analyzed the effect of subject-specific modeling of hip geometry on muscle activation patterns and hip contact forces during gait, using musculoskeletal modeling, inverse dynamic analysis and static optimization. We first used sensitivity analysis to analyze the effect of isolated changes in femoral neck-length (NL) and neck-shaft angle (NSA) on calculated muscle activations and hip contact force during the stance phase of gait. A deformable generic musculoskeletal model was adjusted incrementally to adopt a physiological range of NL and NSA. In a second similar analysis, we adjusted hip geometry to the measurements from digitized radiographs of 20 subjects with primary hip osteoarthrosis. Finally, we studied the effect of hip abductor weakness on muscle activation patterns and hip contact force. This analysis showed that differences in NL (41-74 mm) and NSA (113-140 degrees ) affect the muscle activation of the hip abductors during stance phase and hence hip contact force by up to three times body weight. In conclusion, the results from both the sensitivity and subject-specific analysis showed that at the moment of peak contact force, altered NSA has only a minor effect on the loading configuration of the hip. Increased NL, however, results in an increase of the three hip contact-force components and a reduced vertical loading. The results of these analyses are essential to understand modified hip joint loading, and for planning hip surgery for patients with osteoarthrosis.

Detection of the insertion end point of cementless hip prostheses using the comparison between successive frequency response functions.

Vibration analysis is a non-destructive testing technique, which has a potential to assess the mechanical properties of the stem/femur system in total hip replacement (THR). Different methods based on vibration analysis have already been successfully used to determine bone mechanical properties, to monitor fracture healing, and to quantify the fixation of dental implants. This paper describes an in vitro study of the change in the frequency response function (FRF) of the hip stem/femur structure during implant insertion. At successive insertion stages, the FRF of the system was measured by impulse excitation on the prosthesis neck, in the range 0-5000 Hz. To quantify the difference between two successive FRF spectra, the Pearson's correlation coefficient and the cross correlation function were used. The stiffness of the implant/bone system varies during insertion, which results in a change in FRF, especially in the range of higher frequencies. If the FRF spectrum shifts to the right, then the stiffness of the implant/bone connection increases and, consequently, the stability of the implant increases as well. If the FRF does not change between two successive insertion stages, then the mechanical properties of the prosthesis-femur structure does not change; therefore, the stem-bone connection is stable and the insertion should stop to avoid intra-operative fractures. Based on the obtained results, a per-operative protocol based on FRF analysis can be designed to assess the stability of a cementless hip prosthesis, and to detect the insertion end point.

A new test set-up for skull fracture characterisation.

Skull fracture is a frequently observed type of severe head injury. Historically, a variety of impact test set-ups and techniques have been used for investigating skull fracture. The most frequently used are the free-fall technique, the guided fall or drop tower set-up and the piston-driven impactor set-up. This document proposes a new type of set-up for cadaver head impact testing which combines the strengths of the most frequently used techniques and devices. The set-up consists of two pendulums, which allow for a 1 degree of freedom rotational motion. The first pendulum is the impactor and is used to strike the blow. The head is attached to the second pendulum using a polyester resin. Local skull deformation and impact force are measured with a sample frequency of 65 kHz. From these data, absorbed energy until skull fracture is calculated. A set-up evaluation consisting of 14 frontal skull and head impact tests shows an accurate measurement of both force and local skull deformation until fracture of the skull. Simplified mechanical models are used to analyse the different impacting techniques from literature as well as the new proposed set-up. It is concluded that the proposed test set-up is able to accurately calculate the energy absorbed by the skull until fracture with an uncertainty interval of 10%. Second, it is concluded that skull fracture caused by blunt impact occurs before any significant motion of the head. The two-pendulum set-up is the first head impact device to allow a well-controlled measurement environment without altering the skull stress distribution.

Resonance frequency analysis of implants in the guinea pig model: influence of boundary conditions and orientation of the transducer.

The goal of this study was to identify the parameters that must be controlled during in vivo resonance frequency measurements with a custom Osstell transducer for custom implants in the guinea pig animal model. A numerical study and in vitro measurements were performed to determine the influence of the boundary conditions as well as the transducer orientation on the resonance frequency measured by the custom Osstell transducer. In the reported guinea pig model, the type of boundary condition, the orientation of the transducer (parallel or perpendicular to the long axis of the bone) and the length of the modelled bone have a large influence on the resonance frequency values. This implies that a follow-up in time of the stability of an implant requires the boundary conditions applied to the bone in which the implant is installed as well as the orientation of the transducer to be highly repeatable. Applying controlled boundary conditions during in vivo measurements had a highly positive influence on the repeatability of the Osstell measurements. This improves the possibility of the technique to measure changes in the implant-bone interface during healing of the implant.

The resonance frequencies and mode shapes of dental implants: Rigid body behaviour versus bending behaviour. A numerical approach.

The purpose of this study was to evaluate the modal behaviour of the bone-implant-transducer (Osstell) system by means of finite element analyses. The influence of different parameters was determined: (1) the type of implant anchorage being trabecular, cortical, uni-cortical, or bi-cortical, (2) the implant diameter, (3) the length of the implant embedded in the bone, and (4) the bone stiffness. The type of anchorage determines the resulting modal behaviour of the implant-transducer system. A rigid body behaviour was found for a uni-cortical anchoring and for a homogeneous anchoring with low bone stiffness (< or =1000 MPa), whereas a bending behaviour was found for a homogeneous anchoring with a high bone stiffness (> or =5000 MPa) and for a bi-cortical anchorage. The implant dimensions influence the values for the resonance frequencies. Generally, an increase in implant diameter or implant length (in bone) results in higher resonance frequencies. This study also showed that resonance frequencies in case of rigid body behaviour of the implant-transducer system are more sensitive to changes in bone stiffness than resonance frequencies in case of bending behaviour. In conclusion, it seems that the Osstell transducer is suited for the follow-up in time of the stability of an implant, but not for the quantitative comparison of the stability of implants.

A 3D active shape model for the evaluation of the alignment of the spine during sleeping.

This paper explains how the shape of the spine can be evaluated from back surface measurements in a recumbent position, by using point distribution models (PDM) and typical shape variability of the spine in a lateral sleeping position. CT-scans of 12 volunteers were taken in this posture on a firm and a soft sleeping system to provide a training set for the PDM. Active shape models (ASM) were used to enhance the accuracy of the spinal reconstruction from measurements by limiting the shape of the spine to characteristic shapes from a biomechanical and/or clinical point of view. A comparison was made between calculated shapes, obtained from surface measurements, and those measured vertebral body centres (from CT-scans). An RMS accuracy of 2.6mm was obtained in 3D, and 1.8mm in frontal view, which was sufficient to compare spinal deformations of a subject on different sleeping systems.

A semi-active milling procedure in view of preparing implantation beds in robot-assisted orthopaedic surgery.

Bone cutting in total joint reconstructions requires a high accuracy to obtain a well-functioning and long-lasting prosthesis. Hence robot assistance can be useful to increase the precision of the surgical actions. A drawback of current robot systems is that they autonomously machine the bone, in that way ignoring the surgeon's experience and introducing a safety risk. This paper presents a semi-active milling procedure to overcome that drawback. In this procedure the surgeon controls robot motion by exerting forces on a force-controlled lever that is attached to the robot end effector. Meanwhile the robot constrains tool motion to the planned motion and generates a tool feed determined by the feed force that the surgeon executes. As a case study the presented milling procedure has been implemented on a laboratory set-up for robot-assisted preparation of the acetabulum in total hip arthroplasty. Two machining methods have been considered. In the first method the surgeon determines both milling trajectory and feed by the forces that he/she executes on the force-controlled lever. In the second method the cavity is machined contour by contour, and the surgeon only provides the feed. Machining experiments have shown that the first method results in large surface irregularities and is not useful. The second method, however, results in accurate cavity preparation and has therefore potential to be implemented in future robot systems.

A three-dimensional active shape model for the detection of anatomical landmarks on the back surface.

In this study relations between anatomical landmarks on the dorsal surface of the human torso corresponding to underlying skeletal structures are established. By examining the statistics of the positions of the landmarks in a training set of subjects a point distribution model is derived. Rotations of the pelvis are simulated in order to show that the main mode shapes of variation are consistent with rotations of the pelvis relative to the trunk. The parameters of these mode shapes can therefore be used as independent measures of clinical parameters such as pelvic inclination, pelvic tilt, etc. The point distribution model is further applied to improve reliability and robustness for an automatic and objective detection of the anatomical landmarks on the back surface (active shape model). The results show that it is possible to replace radiographs by surface measurements in order to measure position and orientation of the pelvis, which is particularly valuable in the case of functional examinations that normally involve a large number of radiographs (e.g. to measure the position of the pelvis in a scoliosis).

Three-dimensional mathematical reconstruction of the spinal shape, based on active contours.

To reduce the amount of radiographs needed for patients with a scoliosis, a radiation-free method based on topographic images of the back was developed. An active contour model simulating spinal stiffness has been applied to video rasterstereographic (VRS) data. The aim of the present study is (a) to evaluate the applicability of active contours to improve the accuracy and the reliability of the three-dimensional (3D) spinal midline reconstruction from back surface data and (b) to design a more robust method to detect the spinal midline. To evaluate the reliability and accuracy, the active contour-based method is compared to a conventional procedure, which has been specifically developed for scoliosis; both methods produce a 3D curve of the spinal midline. The frontal projections and surface rotations of these spinal midlines are compared; r.m.s. deviations of 0.9 mm between the frontal curves and 0.4 degrees between the surface rotations were obtained. Applying the active contour-based method does therefore not result in a substantial difference in accuracy to the conventional procedure. As a conclusion the active contour method is a valuable mathematical method that can accurately reconstruct the spinal midline based on back surface data. In addition, the method can be applied to various postures.

Skull biomechanics: The energy absorbability of the human skull frontal bone during fracture under quasi-static loading.

Four human skulls were studied in order to determine the energy absorbed corresponding to a fracture due to quasi-static loading on the frontal bone. Using a dedicated experimental set up, the force-deformation characteristics of the specimens were recorded, calculating energy absorption. Mean values of 1975 +/- 703 N, 4.17 +/- 0.81 mm and 3.95 +/- 1.18 J were found for the peak force, maximum deformation and absorbed energy to fracture, respectively. Linear fractures were always seen in the frontal bone, with a similar pattern in three of the four skulls. These results can be used to develop bench-mark skull models to validate analytic models. (Journal of Applied Biomaterials & Biomechanics 2003; 1: 194-9).

On the assets of CAD planning for craniosynostosis surgery.

SkullWiz is a computer-aided design program that transforms computer tomographic data of the neurocranium into a mathematical model that can be interactively manipulated to plan craniosynostosis surgery. Proper planning of this type of surgery involves reference to the underlying viscerocranium and to normal neurocranial dimensions, simulation of all basic surgical actions (closed and open osteotomy, translation, rotation, bending, removal, burring), and reference to the mechanical properties of calvarial bone at a given age. With SkullWiz, infinite trials are possible to develop a surgical plan that combines minimal action with maximum morphologic result. In contrast, physical models, e.g., foam milled or stereolitographic, provide just a single (or double, after gluing) opportunity to visualize three-dimensional morphology and simulate a treatment plan, without reference support. Validation of SkullWiz is difficult due to parameter variability. Its assets are therefore graphically exemplified in two common types of nonsyndromatic single-suture craniosynostosis-trigonocephaly and anterior plagiocephaly. SkullWiz is one of the most accurate planning tools currently available for craniosynostosis surgery. Accurate transfer of the planning by aluminium templates results in efficient and precise surgery by avoiding per-operative "chipping and fitting."

Quantitative ultrasound and trabecular architecture in the human calcaneus.

Relationships between quantitative ultrasound (QUS), density (bone volume density [BV/TV]), and trabecular architecture were investigated in 69 calcaneal cancellous bone cubes. Ultrasound signal velocity, phase velocity, attenuation, and broadband ultrasonic attenuation (BUA) measurements were made along the mediolateral axis. Density and architectural parameters were measured using microcomputed tomography (microCT). Density yielded the best correlations with QUS (r2 = 73-77%). Of the individual architectural parameters, correlations with QUS were highest for the Structure Model Index (SMI), a parameter quantifying the relative proportion of rods and plates (r2 = 57-63%). After adjustment for density, significant associations with QUS remained for SMI, trabecular spacing (Tb.Sp), and trabecular number (Tb.N), although the variance in QUS attributable uniquely to individual architectural parameters was at best 4%. In multivariate regression models, combinations of density and architectural parameters explained 76-82% of the variance in QUS, representing an r2 increase of, at most, 8% compared with using density alone. However, multivariate models using combinations of architectural parameters alone (i.e., density excluded) also had a good predictive ability for QUS (r2 = 73-81%). Thus, although these data show modest but significant density-independent relationships between QUS and trabecular architecture in the human calcaneus for the first time, the causal relationships behind the variation in acoustic properties remain obscure. Given the relative weakness and complexity of the emerging associations between QUS and architecture, it is prudent to regard QUS measurements in calcaneal bone primarily as an indicator of bone density.

Structural and radiological parameters for the nondestructive characterization of trabecular bone.

Trabecular bone is characterized by compositional and organizational factors. The former include porosity at microlevel and mineralization. The latter refer to the trabecular architecture. Both determine the mechanical properties of the trabecular bone. The aim of this study is to investigate the relationship between the mechanical properties and the local HU value, the bone mineral density, the in vitro histomorphometric properties assessed by means of microcomputed tomography, and the Young's modulus determined by ultrasound measurement. Also the correlation between local HU values based on CT data of the full bone and HU values based on CT data of excised trabecular bone cylinders is investigated. Therefore density and strength related parameters of 22 trabecular bone cylinders retrieved from a fresh cadaver femur were measured by using different techniques. The mean HU value of the excised bone samples is very highly correlated with the pQCT density (R2=0.95) and the microCT-based morphometric parameter BV/TV (R2=0.95). The mean HU values, determined from the CT images of the planned and excised bone samples, are less highly correlated (R2=0.75). The Young's modulus E(US) determined from the ultrasound measurement is highly correlated with the maximal stress sigmamax (R2 = 0.88) but not with the mechanically determined Young's modulus Emech (R2 = 0.67). The maximal stress sigmamax correlates well with the density parameters (R2 varies between 0.76 and 0.86). On the contrary the mechanically determined Young's modulus Emech does not correlate well with the density parameters (R2 varies between 0.52 and 0.56). The absorbed energy Eabs during the deformation is only highly correlated with the maximal stress sigmamax (R2 = 0.83). The inclusion of structural parameters besides a density related parameter did improve the prediction of the Young's modulus and the maximal stress. In conclusion, it seems that the HU value from clinical CT scanning is a good predictor of the local bone density and volume fraction. A combination of local density and a measure of the structural anisotropy is clearly needed to achieve good predictions of bone mechanics.

Bone strains and anterior lift-off, measured with three alternative designs of tibial components of TKA.

Total knee replacement is a successful procedure with high clinical success rates. Problems are mostly initiated on the tibial side, and may be due to - amongst others - improper mechanical design of the tibial base plate. In this paper some new design concepts for the tibial component of a total knee prosthesis are presented. They are evaluated experimentally using a model for a proximal tibia, and strain gauge measurements and displacement measurements as experimental techniques. The designs are meant to yield a physiological load sharing between the trabecular and the cortical bone in the proximal tibia, and to minimize anterior lift-off of the tibial base plate. The optimal design required a metal backing of the plastic part and a thin continuous metallic rim in contact with the proximal tibial cortex. An optimal macro-composite structure within the plastic part was obtained by using thin steel wires in the transversal direction, connected to the metallic rim. With this optimal design, it was shown that the force required to close the anterior gap at simulated knee bending was smaller than 250 N, which can easily be applied clinically by an anteriorly placed clamp or bone screw.

Use of microfocus computerized tomography as a new technique for characterizing bone tissue around oral implants.

Qualitative and quantitative analysis of peri-implant tissues around retrieved oral implants is typically done by means of light microscopy on thin histological sections containing the metal surface and the undecalcified bone. It remains, however, a labor-intensive and thus time-consuming job. Moreover, it is a destructive technique that allows tissue quantification in only a limited number of two-dimensional sections. As an alternative, we evaluated the bone structure around screw-shaped titanium implants by means of microfocus computerized tomography (micro-CT) because it presents a number of advantages compared to conventional sectioning techniques: micro-CT is nondestructive, fast, and allows a fully three-dimensional characterization of the bone structure around the implant. Images can be reconstructed in an arbitrary plane, and three-dimensional reconstructions are also possible. Because of its high resolution, individual trabeculae can be visualized. The accuracy of micro-CT was qualitatively evaluated by comparing histological sections with the corresponding CT slices for the same specimen. The overall trabecular structure is very similar according to both techniques. Even very close to the interface, the titanium implant does not seem to produce significant artifacts. Furthermore, because the complete digital data on the trabecular bone structure around the implant is available, it is possible to create finite-element models of the bone-implant system that model the trabeculae in detail so that mechanical stress transfer at the interface can be studied at the level of individual trabeculae. Therefore, micro-CT seems to be very promising for the in vitro assessment of the three-dimensional bone structure around oral implants. Further research will be needed to evaluate its accuracy in a more quantitative way.

Computer-aided craniofacial surgical planning implemented in CAD software.

Accurate presurgical planning is imperative for successful cranial surgery. This article introduces a simulation program developed in a computer-aided design environment. The neurocranium is introduced as a mathematical surface, since this is the part on which the actual operation will be performed. The viscerocranium, which serves as reference, is visualized using small triangular surfaces. The development of the program commenced with a classification of the different surgical techniques mentioned in the literature into six basic actions. The use of mathematically described surfaces has the advantage that the program can simulate actions which change the shape of a surface and perform an on-line estimation of the fracture risk during bending. Three-point bending tests were carried out to provide the necessary data to perform the mathematical check, as these data are not available in the literature. A database with reference distances was introduced to guide the surgeon to obtain the best possible results. During one clinical trial, the computer was taken into the operating room so that the surgical plan developed with the simulation program could be applied to the actual operation.