Long-term biological fixation and stability of uncemented acetabular implant are influenced by peri-prosthetic bone ingrowth which is known to follow the principle of mechanoregulatory tissue differentiation algorithm. A tissue differentiation is a complex set of cellular events which are largely influenced by various mechanical stimuli. Over the last decade, a number of cell-phenotype specific algorithms have been developed in order to simulate these complex cellular events during bone ingrowth. Higher bone ingrowth results in better implant fixation. It is hypothesized that these cellular events might influence the peri-prosthetic bone ingrowth and thereby implant fixation. Using a three-dimensional (3D) microscale FE model representing an implant-bone interface and a cell-phenotype specific algorithm, the objective of the study is to evaluate the influences of various cellular activities on peri-prosthetic tissue differentiation. Consequently the study aims at identifying those cellular activities that may enhance implant fixation.

The 3D microscale implant-bone interface model, comprising of Porocast Bead of BHR implant, granulation tissue and bone, was developed and meshed in ANSYS (Fig. 1b). Frictional contact (µ=0.5) was simulated at all interfaces. The displacement fields were transferred and prescribed at the top and bottom boundaries of the microscale model from a previously investigated macroscale implanted pelvis model (Fig. 1a) [4]. Periodic boundary conditions were imposed on the lateral surfaces. Linear elastic, isotropic material properties were assumed for all materials. Young's modulus and Poisson's ratios of bone and implant were mapped from the macroscale implanted pelvis [4]. A cell-phenotype specific mechanoregulatory algorithm was developed where various cellular activities and tissue formation were modeled with seven coupled differential equations [1, 2]. In order to evaluate the influence of various cellular activities, a Plackett-Burman DOE scheme was adopted. In the present study each of the cellular activity was assumed to be an independent factor. A total of 20 independent two-level factors were considered in this study which resulted in altogether 24 different combinations to be investigated. All these cellular activities were in turn assumed to be regulated by local mechanical stimulus [3]. The mechano-biological simulation was run until a convergence in tissue formation was attained.

The cell-phenotype specific algorithm predicted a progressive transformation of granulation tissue into bone, cartilage and fibrous tissue (Fig. 1c). Various cellular activities were found to influence the time to reach equilibrium in tissue differentiation and, thereby, attainment of sufficient implant fixation (Fig. 2, Table 1). Negative regression coefficients were predicted for the significant factors, differentiation rate of MSCs and bone matrix formation rate, indicating that these cellular activities favor peri-prosthetic bone ingrowth by facilitating rapid peri-prosthetic bone ingrowth. Osteoblast differentiation rate, on the contrary, was found to have the highest positive regression coefficient among the other cellular activities, indicating that an increase in this cellular activity delays the attainment of equilibrium in bone ingrowth prohibiting rapid implant fixation.

To view tables/figures, please contact authors directly.

Despite the generally inferior clinical performance of acetabular prostheses as compared to the femoral implants, the causes of acetabular component loosening and the extent to which mechanical factors play a role in the failure mechanism are not clearly understood yet. The study was aimed at investigating the load transfer and bone remodelling around the uncemented acetabular prosthesis.

The 3-D FE model of a natural right hemi-pelvis was developed using CT-scan data. The same bone was implanted with two uncemented hemispherical acetabular components, one metallic (CoCrMo alloy) and the other ceramic (Biolox delta), with 54 mm outer diameter and 48 mm bearing diameter. The FE models of the implanted pelvis (containing ∼116000 quadratic tetrahedrals) were generated using a submodelling approach, which were based on an overall full model of implanted pelvis (containing ∼217600 quadratic tetrahedrals) acted upon by hip joint force and twenty one muscle forces. The apparent density (ρ in g cm⁻³) of each cancellous bone element was calculated using linear calibration of CT numbers of bone, from which the Young's modulus (E in MPa) was determined using the relationship, E = 2017.3 ρ^2.46 [1]. Implant-bone interface conditions, fully bonded and debonded with friction coefficient μ = 0.5, were simulated using contact elements. Applied loading conditions consist of two load cases during a gait cycle, corresponding to 13% and 52% of the walking cycle. Fixed constraints were prescribed at the pubis and at the sacroiliac joint. The bone remodelling algorithm was based on strain energy based site-specific formulation [2]. The FE analysis, in combination with the bone remodelling simulation, was performed using ANSYS FE software.

The predicted changes in peri-prosthetic bone density were similar for the metallic and the ceramic implant. For debonded implant-bone interface, stress shielding led to ∼20% reductions in bone density at supero-anterior, infero-anterior and posterior part of the acetabulum (Fig. 1). However, bone apposition was observed at the supero-posterior part of the acetabulum, where implantation led to ∼60% increase in bone density (Fig. 1). The effect of bone resorption was higher for the fully bonded implant-bone interface, wherein bone density reductions of 20–50% were observed in the cancellous bone underlying the implant (Fig. 1), which is indicative of implant loosening over time. However, implantation led to an increase in bone density around the acetabular rim for both the interface conditions (Fig. 1). These results are well corroborated by the earlier studies [3, 4]. Implantation with a ceramic component resulted in 2–7% increase in bone density at supero-posterior part of the acetabulum as compared to the metallic component, for the debonded interface condition. Considering better wear resistant properties and absence of metal ion release, results of this study suggest that the ceramic component might be a viable alternative to the metallic prosthesis.

The effects of metal ion release and wear particle debris in metal-on-metal articulation warrants an investigation of alternative material, like ceramics, as a low-wear bearing couple [1]. Short-stem resurfacing femoral implant, with a stem-tip located at the centre of the femoral head, appears to provide a better physiological load transfer within the femoral head and therefore seems to be a promising alternative to the long-stem design [2]. The objective of this study was to investigate the effect of evolutionary bone adaptation on load transfer and interfacial failure in cemented metallic and ceramic resurfacing implant.

Bone geometry and material properties of 3D finite element (FE) models (intact, short-stem metallic and ceramic resurfaced femurs of 44 mm head diameter) were derived from the CT scan data. The FE models consisted of 170352 quadratic tetrahedral elements and 238111 nodes with frictional contact at the implant-cement (μ = 0.3) and stem-bone interfaces (μ = 0.4) and fully bonded cement-bone interface. Normal walking and stair climbing were considered as two different loading conditions. A time-dependant “site specific” bone remodelling simulation was based on the strain energy density and internal free surface area of bone [3]. The variable time-step was determined after each remodelling iteration. The Hoffman failure criterion was used to assess cement-bone interfacial failure.

Predicted change in bone density due to bone remodelling was very much similar in both the metallic and ceramic resurfaced femurs (Fig. 1). Both the metallic and ceramic implant resulted in strain reduction in the proximal regions (Region of interest, ROI 2 and 4) and subsequent bone resorption, average bone density reduction by 72% (Fig. 1). Higher strains were generated in ROI 5 and 7, which caused bone apposition, an average increase in bone density of 145% (Fig. 1). The tensile stresses in the resurfacing implants increased with change in bone density; a maximum stress of 83 MPa and 63 MPa were observed in the ceramic and the metallic implants, respectively. The tensile stress in the cement mantle also increased with bone remodelling. Although the cement-bone interface was secure against interface debonding in the post-operative situation, calculations of Hoffman number indicated that risk of cement-bone interfacial failure was increased with peri-prosthetic bone adaptation.

During the remodelling simulation, maximum tensile stress in the implant and the cement was far below its strength. However, with bone adaptation greater volume of cement mantle was exposed to higher stresses which, in-turn, resulted in greater risk of interfacial failure around the periphery of the cement mantle. Both the short-stem ceramic and metallic resurfacing component, under debonded stem-bone interface, resulted in more physiological stress distribution across the femoral head. Based on these results, short-stem ceramic resurfacing component appears to be a viable alternative to the metallic design.

Incidence of pars defect associated with idiopathic scoliosis has been reported as 6% based on roentgenographic evaluation in previous studies. (Fisk et al, 1978). We aim to present our results in an MRI based study.

224 patients of adolescent idiopathic scoliosis (AIS) who had an MRI scan over a period of three years (2006-2008), performed either as a preoperative investigation or due to other symptoms were reviewed. All MRI scans were reviewed by two experienced musculoskeletal radiology consultants independently.

Among 224 patients 13 (5.8%) patients were found to have pars defect. Mean age-group of patients were (19 years, range-11-40). 84.6% (n=11) patients were female and 15.3% patients were male. There was varying severity of curve patterns. We have noted two lumbar/thoraco-lumbar curves (Lenke 5), King Type I-6, King Type II-2 and three King Type III curves. All scoliotic deformities were non-structural. Bilateral pars defect was noted in nine (69%) patients.

Previous studies (Fisk et al, 1978; Mau H 1981) have described the incidence of pars defect as approximately 6.2%. Recent studies have emphasized use of MRI to diagnose pars defect based on signal changes in the pedicle (Sairyo et al.). Our study reveals the incidence of pars defect in AIS to be 5.8 % based on MRI diagnosis which does not seem to be different to previous roentgenographic studies.

Incidence of pars defect associated with idiopathic scoliosis has been reported as 6% based on roentgenographic evaluation in previous studies. (Fisk et al, 1978). We aim to present our results in an MRI based study.

Methods: 229 patients of adolescent idiopathic scoliosis (AIS) who had an MRI scan over a period of three years (2006–2008), performed either as a preoperative investigation or due to other symptoms were reviewed. All MRI scans were reviewed by two experienced musculoskeletal radiology consultants independently.

Results: Among 229 patients 18 (7.86%) patients were found to have pars defect. Mean age-group of patients were (19 years, range-11–40). Male -5 (27.7%), female- 13(72.2%). There was varying severity of curve patterns, major thoracic curve-10, major lumbar curve-4, mild thoracic curve-3 and mild lumbar curve in one. Bilateral pars defect was noted in 14 (77.8%) patients.

Discussion: Previous studies (Fisk et al, 1978; Mau H 1977) have described the incidence of pars defect as approximately 6.2%. Recent studies have emphasized use of MRI to diagnose pars defect based on signal changes in the pedicle (Sairyo et al, 2009). Our study reveals the incidence of pars defect in AIS to be 7.86 % based on MRI diagnosis which does not seem to be different to previous roentgenographic studies.

Conflicts of Interest: none

Source of Funding: none

Background: Guidelines for the management of Low Back Pain (LBP) consistently recommend that initial assessment should focus on the detection of serious spinal pathologies. In 1994 the UK Clinical Standards Advisory Group introduced the concept of “red flags”. One flag is the presentation of back pain in people under the age of 20. LBP in children is common, with an annual and lifetime prevalence of around 30%. Because many cases of benign and malignant spinal tumours and spinal infection have been documented in this age group, young people with LBP who are referred to secondary care are investigated by MRI.

Purpose: The purpose of this study is to review the incidence of tumour and infection in subjects under age 20, who present to secondary care with LBP, but do not have concerning objective findings such as neurology, fever, acute deformity or scoliosis.

Method/Results: A retrospective analysis of the MRI database, at a specialist orthopaedic hospital, from 1994 until 2005 identified 403 limited MRI’s taken in LBP patients under the age of 20. Analysis revealed 2 radiological reports of spinal tumour, 1 ependymoma and 1 osteoid osteoma and no cases of spinal infection. Histology confirmed pathology in the ependymoma but excluded serious pathology in the osteoid oste-oma. Other MRI findings, included minor degenerative change (17%), pars defect (9%) and disc prolapse (4%). Therefore, over an 11 year period only 0.25% of young people with LBP who underwent limited MRI were found to have serious pathology.

Conclusion: In conclusion, LBP is relatively common in people under the age of 20 and can be severe enough to warrant secondary care referral. However, in the absence of other objective findings, serious pathologies such as cancer and infection remain rare. These results support the use of limited MRI for screening. However, further research into the clinical value of “under 20” as an independent red flag is recommended.