Cement penetration and primary stability of the femoral component after impaction allografting

The effect of the addition of hydroxyapatite graft substitutes upon the hoop strain and subsequent subsidence of a femoral model during impaction bone grafting

Impaction bone grafting using morcellised allograft can successfully restore bone stock in revision surgery. However, concerns exist regarding supply of bone and transmission of infection. Bone-graft extenders, such as tricalcium phosphate (TCP) and hydroxyapatite (HA), are used to minimise the use of donor bone. However, concerns exist around a reported increased risk of femoral fracture during impaction bone grafting with a 1:1 mixture of TCP/HA and morcellised bone graft (MBG) during impaction grafting in human cadaveric femora. Using a sawbones model, it was evaluated whether there was increased femoral cortical strain with a HA:MBG mixture during impaction grafting compared to MBG impacted at the same and a greater force. Subsequently the subsidence behaviour of the different graft mixes was compared by using a loaded femoral stem in an endurance test. It was demonstrated that the femora with the MBG:HA graft had greater cortical hoop stresses but improved subsidence behaviour compared to a graft composed of pure MBG impacted at the same force.

Biomechanical analysis of a synthetic, biodegradable impaction graft substitute

Impaction bone grafting in revision arthroplasty is a common and successful procedure to restore primary bone stock. Reducing the amount of bone needed to fill large grafts has been a driving force for the use of synthetic materials that can act as extenders or substitutes. To this end, we evaluated the mechanical properties of a new class of biodegradable polymer beads with and without donor bone to determine its suitability for use in impaction grafting. Biodegradable methacrylated anhydride beads were synthesized using thermal polymerization techniques. The mechanical properties of the beads were then tested in an impaction grafting test chamber and compared with morsellised porcine allograft. The beads, porcine allograft and a 50/50 combination all had similar mechanical properties, both in compression and relaxation. Pure polymer beads compacted significantly less than pure allograft and retained macroporosity after impaction. Our results suggest that the biodegradable beads have sufficient mechanical properties to be considered as an impaction grafting substitute or extender. Their ability to fill space whilst retaining macroporosity may be advantageous for tissue ingrowth and remodeling.

Impaction bone grafting in revision hip surgery: past, present and future

Joint replacement surgery can have excellent clinical results. However, as the number of patients undergoing surgery increases, the number of failed joint replacements is set to rise. One of the greatest challenges for the revision surgeon is the restoration of bone stock. This article focuses upon revision hip surgery, with particular reference to the scope of the problem; historical and current solutions to bone loss in the femur and acetabulum; the clinical results following revision surgery; and the basic science behind impaction bone grafting, before ending with possible future directions for improving the restoration of bone stock.

Influence of cement penetration and graft density on stem stability in impaction allografting: a finite element study

BACKGROUND

Excessive stem migration is often problematic after impaction allografting. The mechanisms responsible for migration are not known, but achieving a dense graft bed has traditionally been believed to be essential for stem stability. When the stem is cemented into the allograft bed, however, the graft becomes infiltrated with bone cement. Extensive cement penetration into the graft has been observed in previous studies, resulting in regions of cement-endosteum contact.

METHODS

This study explored the effects of graft density and cement penetration on stem motion using a finite element model that was validated against experimental data.

FINDINGS

Cement penetration has a considerable stabilizing effect on stem motion, whereas graft density is important only when there is no cement-endosteum contact. Stem migration can be attributed primarily to slippage at the endosteum and stem-cement interfaces rather than to shear failure within the graft.

INTERPRETATION

Partial cement penetration to the endosteum increases the likelihood of meeting clinical requirements of early implant stability, particularly when a dense graft bed cannot be achieved.