Histological and biomechanical analysis of bone and interface reactions around hydroxyapatite-coated intramedullary implants of different stiffness: a pilot study on the goat

High incidence of migration with poor initial fixation of the Accolade stem

BACKGROUND

Numerous cementless femoral stem design variations are in clinical use. Because initial implant instability and micromotion are associated with aseptic loosening of the femoral component, migration analysis provides an early assessment of implant survivorship.

QUESTIONS/PURPOSES

We determined the (1) migration pattern of the Accolade cementless femoral stem; (2) clinical factors predisposing to stem migration; (3) self-reported patient outcomes; and (4) our current rate of aseptic stem loosening.

METHODS

We retrospectively analyzed 81 femoral stems for aseptic migration using Ein-Bild-Roentgen-Analyse-femoral component analysis. Postoperatively, patients completed the WOMAC and SF-12 questionnaires. We assessed radiographic factors potentially associated with subsidence: indices of bone shape and quality, canal fill of the implant, and radiographic signs of loosening. Minimum followup was 24 months (mean, 29 months; range, 24-48 months).

RESULTS

The average subsidence at 24 months was 1.3 mm (range, 0-1.5 mm). In the first 2 years, 36% of stems subsided more than 1.5 mm. Large stem size was associated with subsidence. Radiolucent lines (> 1.5 mm in three zones) were present in 10% of stems and associated with lower questionnaire scores. The 5-year survivorship for aseptic loosening of the 367 stems was 97% with revision as end point and 95% for radiographic failure.

CONCLUSIONS

The high incidence of migration and stems with radiographic failure raises concerns about patient clinical function and long-term survivorship of this stem design. This migration pattern may be due to poor initial stability with a subsequent lack of osseointegration. Our results differ from radiographic findings and clinical durability of other similar cementless stem designs.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Fixation and bone remodeling around a low-modulus stem seven-year follow-up of a randomized study with use of radiostereometry and dual-energy x-ray absorptiometer

Thirty-eight patients (40 hips) randomly received either an uncemented fully porous-coated composite stem (Epoch; Zimmer, Warsaw, Ind) or an uncemented proximally porous-coated solid stem (Anatomic; Zimmer). Patients were followed up for 7 years using radiostereometry, dual-energy x-ray absorptiometry, conventional radiography, the Harris Hip Score, and a pain questionnaire. Both stem designs achieved excellent outcome for fixation (stem subsidence and stem rotations close to zero) and clinical outcome, without any difference between the 2 groups (P > .12). Median wear rates were low despite use of conventionally gamma-sterilized polyethylene. No stem was radiographically loose on the postoperative radiographs. The low-modulus composite stem had positive effects on early proximal bone remodeling in Gruen regions 1, 2, 6, and 7 (P < .04). However, at 7 years, this bone-sparing effect persisted in only the calcar region (Gruen region 7).

Initial biocompatibility and enhanced osteoblast response of Si doping in a porous BCP bone graft substitute

Granular shape biphasic calcium phosphate (BCP) bone grafts with and without doping of silicon cations were evaluated in regards to biocompatibility and MG-63 cellular response. To do this we studied Cellular cytotoxicity, cellular adhesion and spreading behavior and cellular differentiation with alizarin red S staining. Gene expression in MG-63 cells on the implanted bone substitutes was also examined at different time points using RT-PCR. In comparison, the Si-doped BCP granule showed more cellular viability than the BCP granule without doping in MTT assay. Moreover, cell proliferation was much higher when Si doping was employed. The cells grown on the silicon-doped BCP substitutes had more active filopodial growth with cytoplasmic webbing that proceeded to the flattening stage, which was indicative of well cellular adhesion. When these cells were exposed to Si-doped BCP granules for 14 days, well differentiated MG-63 cells were observed. Osteonectin and osteopontin genes were highly expressed in the late stage of differentiation (14 days), whereas collagen type I mRNA were found to be highly expressed during the early stage (day 3). These combined results of this study demonstrate that silicon-doped BCP enhanced osteoblast attachment/spreading, proliferation, differentiation and gene expression.

Basic science considerations in primary total hip replacement arthroplasty

Total Hip Replacement is one of the most common operations performed in the developed world today. An increasingly ageing population means that the numbers of people undergoing this operation is set to rise. There are a numerous number of prosthesis on the market and it is often difficult to choose between them. It is therefore necessary to have a good understanding of the basic scientific principles in Total Hip Replacement and the evidence base underpinning them. This paper reviews the relevant anatomical and biomechanical principles in THA. It goes on to elaborate on the structural properties of materials used in modern implants and looks at the evidence base for different types of fixation including cemented and uncemented components. Modern bearing surfaces are discussed in addition to the scientific basis of various surface engineering modifications in THA prostheses. The basic science considerations in component alignment and abductor tension are also discussed. A brief discussion on modular and custom designs of THR is also included. This article reviews basic science concepts and the rationale underpinning the use of the femoral and acetabular component in total hip replacement.

Cortical and interfacial bone changes around a non-cemented hip implant: simulations using a combined strain/damage remodelling algorithm

Prostheses alter the loading pattern in bones. For example femoral hip implants cause the cortical bone to remodel and the implant/bone interface to react. To date, simulations of peri-prosthetic bone adaptations have succeeded for the bulk bone remodelling only. In this study we use an approach based on a combined strain/damage algorithm to simultaneously predict both bulk and interfacial peri-prosthetic bone adaptation around a non-cemented hip prosthesis. The influence of stem stiffness is investigated; a flexible iso-elastic stem (20GPa), an titanium alloy stem (110GPa), and a stiff cobalt chrome (CoCrMo) stem (210GPa). The results predict that an iso-elastic stem reduces proximal bone loss because stress shielding is prevented but it increases proximal interface resorption due to damage-stimulated resorption. On the other hand, a stiff cobalt chrome stem increases proximal strain-stimulated resorption but does not induce proximal interfacial damage-stimulated resorption; however damage-stimulated resorption surrounding the distal tip is predicted to increase with stiffer stems. Simulations for the titanium stem were predicted to minimise both strain and damage related remodelling. We propose that this combined strain/damage remodelling algorithm can provide realistic simulations of the response of bone around load-bearing orthopaedic implants.

Improved design of cementless hip stems using two-dimensional functionally graded materials

Increasingly, it is acknowledged that bone resorption around cementless hip implants may cause future problems. The solution is frequently sought in reducing implant stiffness. However, this confronts the designer with a true design conflict: how to reduce the stiffness without excessively loading the proximal bone/prosthesis interface? The aim of this work is to improve the design of cementless hip stem material, using two-dimensional (2D) functionally graded material (FGM) concept in order to solve the above problems. Two models were used in this analysis, using three materials with different elastic moduli, E(1), E(2), and E(3). In model I, the elastic moduli E(1) and E(2) gradually change along the upper stem surface, while E(3) is maintained constant along all the lower surface of the stem. However, in model II, the elastic moduli E(1) and E(2) gradually change along the lower stem surface, while E(3) is maintained constant all along the upper stem surface. It is found that the recommended model is model I, which has three distinct materials of hydroxyapatite, Bioglass, and collagen. The recommended design of 2D FGM is expected to reduce the stress shielding by 91% and 12%, respectively, compared with titanium stem and model II of FGM. It is found that this new design reduces the maximum interface shear stress at the lateral and medial sides of the femur by about 50%, compared with titanium stem. Furthermore, the maximum interface shear stress is reduced by about 17% and 11% at the lateral and medial sides of the femur, respectively, compared with that of model II of FGM.

Design optimization of cementless metal-backed cup prostheses using the concept of functionally graded material

Metal backing has been widely used in acetabular cup design. A stiff backing for a polyethylene liner was initially believed to be mechanically favourable. Yet, recent studies of the load transfer around acetabular cups have shown that a stiff backing causes two problems. It generates higher stress peaks around the acetabular rim than those caused by full polyethylene cups and reduces the stresses transferred to the dome of the acetabulum causing stress shielding. The aim of this study is to overcome these two problems by improving the design of cementless metal-backed acetabular cups using the two-dimensional functionally graded material (FGM) concept through finite-element analysis and optimization techniques. It is found that the optimal 2D FGM model must have three bioactive materials of hydroxyapatite, Bioglass and collagen. This optimal material reduces the stress shielding at the dome of the acetabulum by 40% and 37% compared with stainless steel and titanium metal backing shells, respectively. In addition, using the 2D FGM model reduces the maximum interface shear stress in the bone by 31% compared to the titanium metal backing shell.

Microstructural analysis of implant-bone interface of hydroxyapatite-coated and uncoated Schanz screws

The aim of the study was to compare the pin-bone interface microstructural characteristics of hydroxyapatite-coated (HAC) and stainless steel Schanz screws after 2, 4 and 6 months of implantation in a sheep model. The microstructure and composition of the hydroxyapatite coating were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Twelve coated and 12 uncoated screws were implanted into both femora of three sheep, each sheep receiving eight screws. Specimens of polished bone with screws were examined with SEM and light microscope for morphometric analyses. The HAC was approx. 40 microm thick, the grain size ranged from 5 to 40 microm, with pores less than 20 microm. The atomic ratio of Ca/P was 1.62. SEM showed that the bone-implant contact was better with HAC than with uncoated implants. The ingrowth of the bone in the HAC was clearly seen. Morphometric analysis showed good bone-implant contact in 65.1 (+/-24.6)% in the HAC and 32.0 (+/-23.3)% in the uncoated group (p<0.001). Although the percentage of good contact increased with time for both groups, it was significantly higher for HAC screws. Our investigation demonstrated a time dependent improvement of implant-bone contact of the HAC compared to standard stainless steel implants in the chosen experimental conditions.

Methods for quantitative analysis of the primary stability in uncemented hip prostheses

Torsional loads of daily activities contribute to the failure of the primary fixation of hip prostheses. Implant torsional stability must be evaluated prior to in vivo clinical trials. Whereas previous work has investigated this phenomenon, descriptions of physiologically accurate and reproducible in vitro methodologies are rare. The present study aimed to detect and control the sources of error and variability that influence in vitro methods. A typical set-up for the analysis of primary stability of hip stems was studied. Load cycles included proximal-to-distal axial force, torque, and bending moment. The effects of loading frequency and strain distribution across the cortical bone were investigated in order to optimize testing conditions and measurement set-up. The relative shear motion at the bone-stem interface was measured transcortically using linear variable displacement transducers. The procedures developed for mounting specimens on the testing machine and for positioning sensors on the specimen were standardized and tested for reproducibility. The protocol was finally tested for repeatability and accuracy. Measurement errors were 2.3 microm between load cycles and 4.9 microm for repeated set-ups, comparing favorably with the literature.