The future of treatment for avascular necrosis of the femoral head: hip resurfacing arthroplasty health economics and surgical technology

Introduction Avascular necrosis of the hip (AVN) is a disease which causes a lack of blood supply in the femoral head, resulting in the bone death, and a subsequent biomechanical failure of the hip joint. Current treatment is mainly to seek for total hip replacement. However, the majority of these affected patients < 60 years of age, having total hip replacement will lose the ability to engage in massive physical work, or lower their life qualities. On the contrary, hip resurfacing arthroplasty (HRA), which is a femoral head preserving surgery, seems to be an ideal intervention for AVN patients because HRA does not change native hip anatomy and helps restoring hip joint. The present question when performing HRA on AVN patients is, surgeons do not know to what lesion extent can they perform such surgery. This thesis aims to review all aspects of AVN, to find out whether the HRA is more cost-effective than THA, to determine the maximum lesion extent to perform HRA, and to plan the surgery better. Method First, we did a comprehensive review on AVN’s mechanism, treatments and staging systems. Second, we use a health economic model to simulate the benefits of performing HRA over THA. Third, we simulate a series of lesions with bone graft HRA in composite bone mechanical tests under healthy human walking load in the hip joint, and compare these lesions data to non-lesion data. From these data, we are able to summarise an indication table for AVN-HRA classification. Last, we develop a pre-operative planner to optimse HRA on AVN, and validate the planner’s reliability and reproducibility. All these steps have never been fully studied before. Results First, the AVN prevalence rates in easter Asian countries were quite high, and the non-surgical treatment cannot cure AVN, as well as the lack of hip resurfacing AVN staging system. Second, the health economic model (Markov model), specifically its Monte Carlo simulation showed a 60% probability that HRA was more cost-effective than THA during 10 years post-operatively. Third, the simulated lesions on HRA mechanical tests demonstrated that 10mm depth of lesion down from the tip of the prepared femoral head with autograft, were able to achieve initial stability. Any lesion depth deeper than 10mm is not recommended for hip resurfacing. Four, the inter-observer and intra-observer reliability and repeatability were all higher than 80% using our pre-operative planner, meaning that this planner is reliable enough to be a workhorse for AVN hip resurfacing pre-operative planning. Conclusion Overall, our results showed that HRA is more cost-effective than THA, and we found out that 10mm of lesion depth was on the margin of the safe HRA, with autograft to fill the defect cause by lesion. Finally, we created a reliable planner that helps planning AVN HRA preoperatively. Our results well matched the hypothesis mentioned above.Open Acces

Image Based Fracture Prediction Diagnostic Tool for Avascular Necrosis of the Femoral Head

Current methods to diagnose bone diseases like avascular necrosis (AVN) are subjective and a reliable assessment of the fracture risk is not available. A diagnostic fracture prediction tool would aid clinical diagnosis, anticipate disease progression and help with the planning of subsequent interventions. The strength of bones, including the femur, can be calculated using structural mechanics with a view to ascertaining fracture risk. The aim of this thesis was to develop and validate a fracture prediction method based tomographic imaging and beam theory. In-vitro disease models were created from additive manufacturing, explanted porcine and human femoral heads. The disease models contained a simulated lesion that was either lateral or medial to the fovea to analyse the effects of different lesion positions and to verify the ability of the developed fracture prediction tool. Current classification methods rely on the identification of the lesion volume and location to quantify the fracture risk, an approach that is purely based on geometrical information. The fracture prediction method based on structural stiffness also considered material properties which potentially added predictive capability. The tool was subsequently validated by predicting the fracture risk of femoral heads from AVN patients to demonstrate the ability to identify necrotic lesions that were likely to progress to fracture. The predicted fracture risk was compared to the current diagnostic gold standard to diagnose AVN. The beam tool was also compared against another novel fracture prediction tool based on FEA to identify possible advantages of beam theory. The verification tests confirmed that samples with a lesion in the weight bearing area were statistically more likely to fracture at a low load. A low fracture load meant a high fracture risk. However the experimental fracture load of porcine and human femoral heads, even among samples with similar lesions, showed variations indicating that lesion volume and location were not good predictors of fracture risk alone. There was a good correlation between the predicted fracture risk and in-vitro fracture loads of the human femoral head disease model indicating that the developed tool was able to objectively predict the fracture risk. The beam tool had similar good predictive capabilities as current diagnostic methods and fracture prediction methods based on FEA. An objective in-vivo analysis of the mechanical fracture risk helps identifying patients whose disease is at risk of progressing, as well as stratifying surgical interventions

Characterisation and Development of an Experimental Mechanical Model of Avascular Necrosis of the Femoral Head

Avascular necrosis (AVN) of the femoral head is a debilitating disease of the bone that may result from a variety of aetiologies, and progresses to the death of the bone and collapse of the femoral head, and ultimately to deformation of the hip joint. In vitro models of AVN have been previously developed in live animals; however none of these models produce the mechanical failures that are observed in AVN femoral heads. The aim of this study was to produce a mechanical simulation model of AVN in vitro. To achieve this, a series of preliminary studies were carried out. First, femoral heads from AVN patients undergoing total hip arthroplasty were studied to understand the effects of the disease on the mechanical and structural properties of the bone. The findings were compared with mechanical and structural properties of bone from nonpathological control femoral heads, and the results demonstrated mean reductions in the mechanical properties of bone with AVN. Methods of reducing mechanical properties of bone were analysed to develop a mechanical simulation model of AVN in vitro. For this, chemical methods were analysed which either demineralised or dissolved the collagen matrix in the bone. The effects of treatment time were analysed on mechanical properties of bone, and it was found that demineralising bone plugs in hydrochloric acid was the most effective method of reducing the mechanical properties of bone. In order to develop a model of AVN, porcine bone sections were treated with hydrochloric acid and returned to the femoral heads to simulate lesions in AVN. The resultant change in the structural mechanical properties of the femoral heads were analysed to determine the most suitable method of simulating AVN in porcine bone, and to provide recommendations for achieving a mechanical bone model representing the structural and mechanical properties of AVN femoral heads

In vitro and in silico simulations of femoral heads with avascular necrosis

Avascular necrosis of the femoral head (AVN) is a complex disease that is linked to multiple aetiologies including steroid use and alcohol abuse. Its pathology is characterised by localised ischemia leading to cell death followed by a period of partial repair during which a sclerotic boundary forms around the necrotic lesion. In many patients the pathology progresses to include involvement of the articular surface leading to arthritic degeneration of the joint. Current classification systems for AVN evaluate lesion size and location and cannot accurately predict whether fracture will occur. There is a need for a prognostic tool to differentiate patients who would benefit from conservative therapies from those for whom arthroplasty is indicated. Previous research has indicated that lesion size and location affects disease progression and it was hypothesised that a morphology-based assessment would better quantify risk of progression. In vitro experimental disease models were constructed by substituting a plug of bone from the central portion of porcine femoral heads with less stiff, weaker, bone from bovine lateral epicondyles. These models and control femoral heads were compressed to incrementally increasing displacements between flat platens. A parametric study using finite element analysis was used to demonstrate the sensitivity of the disease model to geometry and material properties and to develop a risk score that quantified stress discontinuities at the lesion boundary. A more physiologically representative method of load application through a compliant and conforming surface was developed in silico. This was used to evaluate cone-shaped simulated lesions with varying size and orientation and a series of 47 organic lesion geometries derived directly from the proximal femoral anatomy of eight patients suffering from AVN, two of whom had subchondral fractures. The experimental disease models were significantly less stiff than the control femoral heads and both were shown to behave linear-elastically at displacements below 1mm. This was beneficial as it allowed linear-elastic material properties to be used in the in silico simulations. These simulations confirmed that lesion properties and morphology significantly affected the stress distribution and highlighted that a physiologically representative method of load application is essential for future in vitro studies. The risk score allowed patients to be ranked according to their fracture risk. The rank matched that obtained using the current gold standard grading system but with improved granularity as well as the opportunity to apply a threshold value for categorizing risk. This proved the potential for using a morphology-based risk analysis to better differentiate patients for whom early surgical intervention may be beneficial from patients who would benefit more from total hip arthroplasty