THE ELASTIC BEHAVIOUR OF MORSELLISED CORTICO-CANCELLOUS BONE: THE IMPORTANCE OF IMPACTION TECHNIQUE.

Abstract

Morsellised bone graft is used extensively in revision arthroplasty surgery. The impaction technique at the time of surgery has a significant effect on the subsequent elastic and inelastic properties of the bone graft bed. Differences in values reported in the literature for the mechanical properties of morsellised cortico-cancellous bone (MCB) can be attributed to the different loading histories used during testing. We performed serial confined compaction tests to assess the optimum compaction strategy. Compaction of the samples was carried out using repeated standardised loading cycles. Optimal preparation of MCB is dependant on the force and frequency of compaction. The maximum compactive pressure the samples were subjected to was 3 N/mm² based on the clinical experience of Ullmark & Nilsson¹ in MCB preparation at the time of surgery. This paper presents the Young’s Modulus, E, vs. number of compaction cycles and inelastic strain, ^ie, vs. number of compaction cycles curves for MCB. Qualitative and quantitative descriptions of the material behaviour of MCB are developed. The importance of frequent percussive episodes prior to implant insertion is illustrated.

MCB was also found to exhibit significant visco-elastic response, with stress relaxation under displacement controlled loading continuing for several hours following initial load application. Bone graft substitutes do not at present exhibit a similar beneficial shock absorbing visco-elastic response.

Our experiments indicate that the material properties of MCB are dependent on the force of impaction and the number of impactions applied with a hammer at the time of surgery. A minimum of 10 to 20 compaction episodes, or hammer blows are required for MCB to achieve 60 to 70% of its long term predicted stiffness.