INITIAL STABILITY OF UNCEMENTED HIP STEMS - AN IN-VITRO PROTOCOL TO MEASURE TORSIONAL INTERFACE MOTION

The difficulty in quantitatively assessing the inherent variables of s urgical stem insertion and interfemur differences continues to be a pr oblem in experimental methodologies which assess hip stem stability. A n in-vitro torsional stability protocol was developed which limited th e mechanical testing variability and provided a reproducible micromoti on measurement of an uncemented stem in synthetic composite femurs. Us ing a controlled mechanical stem insertion resulted in less interfemur variability within each group with the coefficient of variation being reduced from 35% overall to less than 20%. Femurs with shallow stem i nsertion depths had significantly larger micromotion than femurs havin g deep stem insertion depths. The sensitivity of the experimental prot ocol and the synthetic composite femurs to the varied functional behav iour of three different stem designs was demonstrated. The stem with a hollowed anterior-to-posterior proximal section experienced significa ntly more motion than the two stems with full proximal sections, reinf orcing the need for proximal contact to ensure minimal micromotion in torsional loading.