Anglia Ruskin University

PhD research student: Steeve Lamvohee
Supervisory team: Prof Paul Ingle, Dr Rajshree Mootanah, Prof Kevin Cheah, Prof John Dowell,
Funding body: Anglia Ruskin University research and development fund

Severe disorders of the hip can be very painful, reduce mobility and interfere seriously with the patient's capacity for working. Successful replacement of the damaged hip has improved the quality of life and enabled independent living for numerous people who would otherwise be disabled. With the current advances in hip replacements, there is a great demand among people suffering from hip pain to undergo hip surgery. Approximately 55,000 hip replacements are carried out each year in the UK and this number is expected to increase with the ageing population. Currently, in the UK, 91% of the hip surgeries that are carried out are cemented hip replacements (Wirz et al., 2005). There appears to be immediate and substantial improvement in the patient's pain, functional status, and overall health-related quality of life.

To our knowledge, no study has looked specifically at how stress development in the cement mantles of reconstructed hips can be influenced by factors such as cement mantle thickness, femoral implants of different head sizes, patient's bone quality, acetabulae sizes and body mass index (BMI). Too high stresses developed in the cement mantle can lead to premature failure of the fixation (Kuehn et al., 2005). The aim of this study is to investigate the effects of femoral implants with head diameters of 22 mm and 28 mm on the stress distribution in the components of the reconstructed hip with different bone quality when different cement mantle thicknesses are produced.

A three dimensional model of a reconstructed hip was built from CT-Scan data from the Visible Human Data set (Figure 1). 200 axial CT-Scan images at 1 mm intervals were downloaded to commercially-available Materialise software, which acts as an interface between medical images and FE packages, where the contours of the cortical bone and cancellous bone were created by means of polylines drawn based on the Hounsfield unit for both types of bones (-37 to 1027 HU). The contours were then exported into I-Deas 11.0 commercially-available finite element pre-processing and post-processing package. The inner contours and outer contours of the cortical bone were lofted to produce anatomically accurate volumetric bodies of the respective bones. The volumes representing the cortical and cancellous bones were joined together to ensure that the interfaces between the cortical bone and the cancellous bone were completely merged. The hemi-pelvis was scaled up and down to represent different sizes of patients. Reconstructions of the acetabulum were simulated in I-Deas FE package to simulate different surgical fixation techniques.

Two sets of FE models were used in this study, one simulating patients with reconstructed hemi pelvis with prosthetic head size of 22 mm and the other simulating total hip replacement patients with prosthetic head size of 28 mm. For both sets, finite element analyses were carried out to investigate the effect of cement mantle thicknesses on the stress distribution for the four sizes of acetabulae (56 mm, 58 mm, 60 mm and 62 mm). When increasing the head implant size from 22 mm to 28 mm, the outside diameter of the acetabular component remained the same while the wall thickness of the acetabular component was reduced.

The overall findings show that a 28 mm head implant results in an increase in the tensile stresses developed in the cement mantle and a reduction in the Von Mises stresses developed in the acetabular cups. Our study shows that, when a 28 mm femoral head implant is used, a 4 mm thick cement mantle is recommended, even on large acetabulae, in order to avoid premature failure of the bone cement. This FE study also shows that different methods of fixation are recommended on patients with different bone qualities. Results of our FE analyses show an increase as high as 50% in the tensile stresses of the cement mantle when a 28 mm head implant is used on acetabulae with poor bone quality. Our results for 22 mm femoral head implants show that lower tensile stresses are generated in the cement mantle than when 28 mm femoral head implants are used, leading to a lower probability of failure. These results indicate that a 22 mm head implant is recommended for patients with poor bone quality. Increasing the cement mantle thickness can help improve the mechanical stability of the fixation by reducing the tensile stresses in the cement mantle, especially when a 28 mm femoral implant is used. Too high stresses in the cement mantle can lead to premature failure of the reconstructed acetabulum. The results of this study indicate that, in order to keep the stress level in the cement below the threshold value of 8.25 MPa, a specific thickness of cement mantle can be used. For instance, patients with acetabulae sizes of 56 mm or higher need a 1 mm thick cement mantle when a 22 mm diameter femoral head implant is used and a 4 mm thick cement mantle when a 28 mm diameter femoral head implant is used.

Publications:

• Lamvohee, J.M.S., Mootanah, R., Dowell, J.K., Cheah, K., Ingle, P., 2009. Stresses in cement mantles of hip replacements: effect of femoral implant sizes, body mass index and bone quality, Computer Methods in Biomechanics and Biomedical Engineering 2009;12(5):501-10
• Lamvohee, J.M.S., Dowell, J, Ingle, P., Cheah, K. Mootanah, R., 2010, "Total Hip Replacement: Effect of Cement Mantle Thickness - An Integrated Computational Simulation and In Vitro Study", 18th European Orthopaedic Research Society Conference, Davos.
• Lamvohee J.M.S., Mootanah R., Ingle P., Dowell J.K., Cheah K. 2009, "Total hip replacement: effects of body mass index, acetabular morphology, and bone quality on stresses developed in cement mantles", International Society of Technology in Arthroplasty, Hawaii.
• Lamvohee, J.M.S., Mootanah, R., Dowell, Ingle, P. 2007, ?Patients' bone morphology and bone quality affect the performance of fixation techniques in cemented total hip replacements?, Journal of Biomechanics, 40( 2): 228
• Lamvohee, J., Mootanah, R., Cheah, K., Dowell, J., & Ingle, P. 2006, "Optimum hip replacement fixation technique on patients with different acetabulae morphology, bone quality and body mass index ", J. Middleton, M. Jones, & N. Shrive, eds., First Numerics, University of Wales, College of Medicine, UK.
• Lamvohee, J., Mootanah, R., Ingle, P., Cheah, K., & Dowell, J. 2005, "Effect of different sizes of acetabulae and anchorage holes on the stability of total hip replacement, using finite element analysis", Proceedings of the II International Conference on Computational Bioengineering, IST Press, Portugal.
• Lamvohee, J., Mootanah, R., Ingle, P., & Cheah, K. 2004, "Total hip replacement: Effect of bone quality on the long-term fixation of the acetabular component", J. Middleton, M. Jones, & N. Shrive, eds., Computer Methods in Biomechanics & Biomedical Engineering, ISBN: 0-9549670-0-3, First Numerics, UK.