Matthias Lerch

Bone remodeling after total hip arthroplasty with a short stemmed metaphyseal loading implant: Finite element analysis validated by a prospective DEXA investigation

In total hip arthroplasty (THA), short stemmed cementless implants are used because they are thought to stimulate physiological bone remodeling and reduce stress shielding. We performed a numerical investigation on bone remodeling after implantation of a specific short stemmed implant using finite element analysis (FEA). Overall bone mass loss was 2.8% in the entire femur. Bone mass decrease was mostly found in the proximal part of the calcar and in the greater trochanter due to the vast cross section of the implant, probably leading to stress shielding. In the diaphysis, no change in the apparent bone density was proven. The assumptions made agreed well with bone remodeling data from THA recipients who underwent dual‐energy X‐ray absorptiometry. However, the clinical investigation revealed a bone mass increase in the minor trochanter region that was less pronounced in the FEA. Further comparisons to other stem designs must be done to verify if the relative advantages of the investigated implant can be accepted.

Clinical results of the metha short hip stem: a perspective for younger patients?

In recent years, various uncemented proximal metaphyseal hip stems were introduced for younger patients as a bone preserving strategy. Initial osteodensitometric analyses of the surrounding bone of short stems indicate an increase of bone mass with secondary bone ingrowth fixation as a predictor of long-term survival of these types of implants. We report the outcome of 151 modular Metha short hip stem implants in 148 patients between March 2005 and October 2007. The mean follow-up was 5.8±0.7 years and the mean age of the patients was 55.7±9.8 years. Along with demographic data and co-morbidities, the Harris Hip Score (HHS), the Hip dysfunction and Osteoarthritis Outcome Score (HOOS), and also the results of a patient-administered questionnaire were recorded pre-operatively and at follow-up. The mean HHS increased from 46±17 pre-operatively to 90±5 the HOOS improved from 55±16 pre-operatively to 89±10 at the final follow-up. A total of three patients have been revised, two for subsidence with femoral revision and one for infection without femoral revision (Kaplan Meier survival estimate 98%). The radiological findings showed no radiolucent lines in any of the patients. The modular Metha short hip stem was implanted in younger patients, who reported an overall high level of satisfaction. The clinical and radiographic results give support to the principle of using short stems with metaphyseal anchorage. However, long-term results are necessary to confirm the success of this concept in the years to come.

Bone remodelling around a cementless straight THA stem: a prospective dual-energy X-Ray absorptiometry study

The design of the Bicontact® stem (BBraun, Aesculap, Tuttlingen, Germany) and the implantation technique have undergone no major alterations in the last 20 years leading, and good clinical results have been reported. The aim of our study was to investigate whether the implant encourages beneficial bone remodelling. Twenty-four patients were included in a prospective dual-energy X-ray absorptiometry (DEXA) study of this stem, after appropriate statistical power analysis. Preoperative and postoperative (1 week, 6 months, and 12 months) clinical and DEXA examinations were performed. The Harris Hip Score increased significantly by 39 points. The strongest decreases in BMD were observed in the greater trochanter region (–11%) and the calcar (–12%). In the second half of the study period the bone mineral density recovered slightly and even returned to baseline values in the lesser trochanter region. Therefore, proximal load transfer and physiological bone remodelling around the Bicontact® stem appeared to be achieved.

Numerical Investigations of the Strain-Adaptive Bone Remodeling in the Prosthetic Pelvis

Bone remodeling due to stress shielding is a major cause of hip implants aseptic loosening. Previously, investigations on this phenomenon were based mainly on clinical observations. Currently, the finite element method (FEM) has been established as a reliable and efficient computing method to examine stress shielding after total hip arthroplasty and the related bone remodeling in the prosthetic Femur.

Production of Patient-Individual Hip Cups by Sheet Metal Forming: Simulation-Based Planning and Metal Forming Adapted Design Method

Total hip arthroplasty (THA) is a routine procedure for the treatment of advanced hip joint damage. The long-term result of the prosthesis is mainly determined by migration or aseptic loosening caused by bone remodelling. Especially the migration of the artificial hip cup as a consequence of the remodelling process is a major problem. Patient-specific hip cups can be used to counteract this. However, individual hip cups are currently only implanted for the treatment of great deformations or tumours in the hip joint due to the cost-intensive manufacturing. The aim of this project is the development and establishment of a concept for the economical production of patient-individual prosthetic hip cups out of titanium sheets. This process consists of two steps. First, undersized cups of a universal acetabulum geometry are produced. In the second step a true-to-size enlargement of the produced universal cup prothesis is carried out by means of a modified adaptive rubber-die forming process. The development of this process is accompanied by a simulationbased planning of the production process as well as by a realization of a metal forming adapted design method. For the examination of the feasibility of the concept, CT-data of canine pelvis geoemtries are used because of the large number of CT data, which were aviable for the project. Furthermore it is planned, that the first manufactured prototypes will be tested using canine cadaver. In this study the planning of the manufacturing of the standardized titanium sheet metal components is carried out. For this two methods of producing the standardized hip cup were compared. The first method is a hydraulic forming; the second is a normal pressing process with a bunch die and a binder. Pure titanium was introduced in the simulation, which shows the same mechnical properties like the in prosthetics normally used titanium alloy TiAl6V5. The results of the process simulation of both methods showed that the reducing of the blank thickness is a problem of the manufacturing of the prosthesis. Because of that an adaption of the tool geometry was executed and the influence of the increase of the forming temperature at 200 C was examined. These simulations indicated, that the hydraulic forming seems to be a convenient method to produce the prosthetic acetabulum. The first part of the metal forming adapted design method is the deduction of a universal acetabulum geometry, which has to be designed for the production of the standardized component. This deduction shall be realized by means of a superposition of 3D models of pelvis geometries. For this, two different superposition methods were compared and the Best Fit method was determined as the suitable method. By means of the Best fit method a first universal geometry was created.

Finite element model of a novel short stemmed total hip arthroplasty implant developed from cross sectional CT scans

BACKGROUND Numerous short stemmed total hip arthroplasty (THA) implants have been introduced over the last decades. It is questionable if little differences between the implant designs affect stress shielding and bone remodeling. The finite element analysis allows an evaluation of the design rationale of the implant without negative side effects for the patient. OBJECTIVE We investigated a relatively new short stemmed implant designed from clustered CT datasets of proximal femurs. How does the implant affect femoral bone remodeling? Can we see a positive effect on bone remodeling from the CT based design? METHODS We used a Finite Element Model that was validated by a prospective dual-energy-x-ray-absorptiometry study to calculate apparent bone density. RESULTS Apparent bone density (ABD) decreased by 2.3% in the entire femur. Bone mass loss was pronounced in the proximal calcar region. Little ABD increase was seen in the lateral aspect of the cortical ring, in the minor trochanter area and at the lateral aspect of the stem. CONCLUSIONS ABD reduction occurs in the proximal regions of the femur. The overall bone mass loss was little after THA with the investigated implant. The specific design seems to have no major effect on stress shielding or load distribution.

Patient-individual hip cups: simulation-based design and sheet metal forming manufacturing

The revision of an hip prosthesis can have diffrent reasons. One frequent cause, especialley after implantation of a conventional cup, is the so called stress-shielding ef- fect which can lead to a migration or loosening. Patient- specific hip cups can be used to counteract this. However, individual hip cups are only implanted for the treatment of great deformations or tumours because of the cost-intensive manufacturing. Within this project a patient-specific hip cup prosthesis has to be developed and manufactured. Be- sides the numerical design by means of a coupling between multi-body simulation (MBS) and finite element method (FEM), an inovative concept for the production of patient- individual hip prosthesis out of titanium sheets is introduced in this study. Patient-individual hip cups, bone remodelling, reverse en- gineering, process planning, sheet metal forming

The impact of seating forces from a cementless femoral component in hip resurfacing arthroplasty on the femoral head - A cadaver study using μ-CT analysis

Recent studies have assumed micro-fractures of the femoral head during hip resurfacing arthroplasty as a possible reason for fractures at the implant/neck junction. The purpose of this study was to analyze whether implantation of a cementless femoral hip resurfacing component which requires high seating forces, causes micro-fractures of the femoral head. A cementless hip resurfacing femoral component was installed on 20 human, cadaveric femoral heads with an impaction device that generated 4.5 kilonewton force in one group and by hand in the other. Before and after impaction, the specimens were scanned with a μ-CT-System. The CT datasets were segmented and registered for detection of small trabecular fractures. The average percentage of shared voxels was 80.29% (standard deviation 3.24%). Change in bone structure after impaction (19.71%) was found on the surfaces of all samples. No formation that was ascribed to fracture was found. No difference between the specimens that were impacted by hand or by impaction device was noted. No fractures of the cancellous femoral head during installation of a cementless femoral THR component occurred. Cementless hip resurfacing might not increase the risk of fracturing the cancellous femoral head during implantation in a cadaveric μ-CT study.

Ex-vivo-µCT-Untersuchungen zu möglichen Mikrofrakturierungen des Hüftkopfs bei der Implantation eines zementfreien Oberflächenersatzes am Hüftgelenk

AIM Microfractures of the femoral head during implantation of the femoral components are suspected to be a cause of fractures at the implant/neck junction which represent a common failure mode in hip resurfacing arthroplasty. Callus formation observed in femoral head retrievals suggests the occurrence of microfractures inside the femoral head, which might be inadvertently caused by the surgeon during implantation. The aim of this biomechanical study was to analyse whether or not the implantation of a cementless femoral component hip resurfacing system causes microfractures in the femoral head. METHOD After the preparation of 20 paired human cadaveric femoral heads, the cementless femoral component ESKA Typ BS (ESKA Implants GmbH & Co., Lübeck) was implanted on 9 specimens with an impaction device that generates 4.5 kN impaction force. On 9 specimens the femoral component was implanted by hand. One head was used as a fracture model, 1 specimen served as control without manipulation. The femoral component used for impaction was equipped with hinges to enable its removal without further interfering with the bone stock. Specimens were scanned with a microCT device before and after impaction and the microCT datasets before and after impaction were compared to identify possible microfractures. RESULTS Twenty strikes per hand or with the impaction device provided sufficient implant seating. Neither the macroscopic examination nor the 2-dimensional microCT analysis revealed any fractures of the femoral heads after impaction. CONCLUSION At least macroscopically and in the 2-dimensional microCT analysis, implantation of the cementless hip resurfacing femoral component ESKA Typ BS with 4.5 kN or by hand does not seem to cause fractures of the femoral head.

Comparative investigation of bone mineral density using CT and DEXA in a canine femoral model

Bone density measurements using computed tomography (CT) instead of dual‐energy X‐ray absorptiometry (DEXA) are currently of great interest in human and veterinary medical research as it would be beneficial to use CT scans obtained for other indications also for determining bone density. For Hounsfield units (HU) measured with CT in specific regions of interests (ROIs) in one or several slice/s a correlation with bone mineral density (BMD) measured by DEXA in humans and dogs of between 0.44 and 0.77 is reported in the literature. In the present study, instead certain volumes of interest (VOIs) obtained by CT scan and the corresponding HU to the respective VOIs were compared with the bone mineral density of the corresponding areas measured by DEXA. The aim of the study was to investigate whether this procedure gives more accurate information about bone density of the bones as three‐dimensional objects of the respective patient. Correlation between measured HU in the respective VOI and BMD measured with DEXA in the corresponding ROI showed a very good correlation of 0.93. Linear regression with R2 = 0.85 (p = 0.0262) was calculated. Except for VOI5, similar distribution of values and significant differences (p < 0.0001–0.0087) between ROIs/VOIs were detected. Determining HU for assessing bone mineral density in a certain volume provides more accurate results than those previously reported from two‐dimensional (2D) CT measurements.