Abstract
Introduction
A new conservative hip stem has been designed to address the complex problem of total hip arthroplasty in the younger population.
Objectives
To assess the stability and strain distribution of a new conservative hip stem.
Materials and Methods
The prosthesis is tapered and collared and made from titanium (Ti6Al4V) with a titanium porous plasma spray to encourage bony ingrowth (Figure.1). It is circular-trapezoidal in cross-section to provide optimal ‘fit and fill’ in the femoral neck.
(i) Finite Element Analysis (FEA)
Computed tomography scans of an intact femur were modelled using MARC software and consisted of 161390 elements and 174881 nodes. The implant was modelled (Unigraphics) as a titanium alloy stem with a cobalt-chrome alloy head and consisted of 93440 hexahedral elements and 101133 nodes.
This study compared the strains in the femoral calcar of an intact femur with a stem ‘implanted’ in neck shaft angles of 125°, 135°, and 145°. The head of all models received a load of 2.3KN at 7 degrees medially.
(ii) Photoelastic Coating
A photoelastic coating was moulded around the medial cortices of ten third generation femora Sawbones. Strain before and after prosthesis insertion was measured at one-centimetre intervals down the medial cortex of the bones using a polariscope. The bones were positioned in a simplified single leg stance (7° physiological alignment), and loaded at 2.3 KN with strain recorded.
(iii) Linear Variable Differential Transducers (LVDT's)
Micromotion and migration of the prosthesis was measured using LVDT's. The femoral heads were cyclically loaded with 2.3KN at 1Hz for 2,500 cycles and held in a single leg stance. The bones were then repositioned at 70° of flexion to produce torsional (stair climbing) forces and loaded with 0.5KN for 2,500 cycles.
Statistical analysis of non-parametric data was performed using a two-tailed Wilcox signed rank test (p<0.05).
Results
The FEA analysis revealed strains in the neutral position most closely resembled that of an intact femur (Figure.2). Photoelastic strain readings for intact bone and following insertion were paired and statistically analysed using the Wilcox signed rank test (two tailed). The composite bones with prostheses inserted at 125° and 145° demonstrated a significant difference to the intact bones, whereas those at 135° showed no significant difference in the surface strain pattern of the femur following prosthetic insertion (Figure.3).
Under single leg stance loading all prostheses produced axial micromotion of less than 200 µm and 50 µm in the varus-valgus direction. Implants inserted at 135° and 125° produced the least micromotion, the implants inserted at 145° had the greatest magnitude of motion and may be more susceptible to loosening. Under torsional load the same was true with the 135° and 125° producing the least micromotion while with the angulation of 145° micromotion increased over the test period – again suggesting loosening.
Conclusion
This design transfers load in a physiological manner and the prosthesis is most stable in the neutral position. The findings from this study have been translated into clinical practice with the prosthesis implanted into two patients with promising results.
