DIFFERENTIAL DIMENSIONAL ANALYSIS OF A TRIAL VERSUS HIP MONOBLOC – IMPLICATIONSFOR NARROW FEMORAL CANALS

Abstract

Introduction: Following the introduction of a hip hemiarthroplasty monobloc a number of unusual problems occurred in our unit. These included the need for narrow stem prostheses, on table revision for excessive anteversion and intra-operative fracture, revision for irreducible dislocation and excision arthroplasty. It was noted that the trial component fitted easily whereas there were problems with insertion of the actual prostheses.

Methods: To investigate the issue of differences in the trial component versus the actual prosthesis the following methods were employed. Digital micrometer measurements were taken from the trial and real components. Differences in the antero-posterior and medial-lateral dimensions of the components were analysed. Cross-sectional area analysis was also performed. The effect of femoral canal diameter on the insertion depth of trial and component was investigated using a canal model with digital image processing. The changes in cross-sectional geometry were investigated with respect to canal width, insertion depth and the consequential effect of component version.

Results: It was shown that the trial was smaller than the actual prosthesis, particularly in the medial-lateral plane. No significant difference was observed between real components. A critical region of the component was identified which corresponded to a canal width of 10mm. Components not fully seated at this point drifted in to excessive anteversion with impaction.

Discussion: This study identified that an unacceptable difference existed between the trial and real prosthesis. The study explained the effect of a narrow canal in terms of increased complications which resulted in increased morbidity for patients. Consequently the use of this implant was abandoned in narrow femoral canal cases and an alternative implant has been selected for use in fracture patients requiring hemiarthroplasty in our unit.