Can the rasp be used to predict intra-operatively the primary stability that can be achieved by press-fitting the stem in cementless hip arthroplasty?

Background Success of cementless hip stems implantation depends mainly on primary stability which, in turn, depends on the degree of press-fitting achieved, and optimal medullary canal filling by the selected stem size. Stem size, based on templating and pre-operative planning software, only partially avoids stem mis-sizing. Mis-sizing can produce implant instability and/or wasting the wrong-sized stem. Methods An intra-operative device was designed that enabled real-time measurement of rasp–bone micromotions by applying torque manually. Rasp stability was assessed in vitro in seven femurs after femoral canal preparation when the rasp was fully seated. Then, primary implant stability was assessed with the stem press-fitted in the position indicated as optimal by the surgeon. Torque–micromotion curves were compared for the rasp and the stem. Finally, the protocol was applied to two hip patients. Findings Rasp stability micromotions were successfully measured in all in vitro specimens and all patients. The slope of the initial part of the rasp torque–micromotion curve was correlated with the slope of the stem curve (correlation coefficient = 0.798, P-value = 0.001). The entire procedure for measuring rasp stability added less than two minutes to the 75–80-min operating time. Interpretation This study indicates that the rasp stability can be used to predict the maximal primary stability that can eventually be achieved by the selected stem size. Early prediction of stem stability enables correcting the stem size, if necessary, before the press-fitting procedure is initiated, thus saving operating time and the cost of a new stem.