THE ASSESSMENT OF THE MINIMAL FORCE PROVIDING ROTATIONAL STABILITY FOR A FEMORAL PROSTHETIC STEM DURING CABLE TENSIONER USE

Abstract

Introduction: While providing an easier compression, cable tensioners also bring the risk of an iatrogenic fracture when they are unnecessarily over-tightened. In this pilot study we have designed a split femur fracture/osteotomy model to assess the minimal force providing rotational stability for a femoral prosthetic stem during tightening with a cable tensioner.

Methods: Twelve volunteer residents of orthopedics were asked to tighten gradually a cerclage wiring of steel cable with a cable tensioner on a longitudinally split bone encircling a prosthetic stem. Each resident repeated the test 10 times and they aimed to tighten until to a point to provide rotational stability for the stem, that they decided with manual control. The fracture model was reproduced on the distal diaphysis of a 12 mm diameter femur bone of a one-year old sheep cadaver. The femur bone was longitudinally split with an oscillating saw and a semi-cylindrical 5 cm long bone window was split. A 13 mm diameter femoral stem (Restoration HA, Styker) was then inserted into the open segment of the diaphysis and the bone window was closed on it and gently hold in place with a cerclage of steel cables. A special aluminum cable tensioner, integrated with a special digital strain measurement device (Vishay MM, NJ, USA), was used for tightening of the cable. The minimal tension loads that the residents found enough to provide a rotational stability were recorded. The descriptive modules and Student t-test were used in statistical analysis. The p values < 0.05 were considered statistically significant.

Results: Mean tension loads provided by 12 residents were between 176±32N and 876±211N. The mean tension loads of the total 120 tigtening trials was 540 N. Significant difference was found between the highest and the lowest (p< 0.0001). Plateau of the rotational stability was 6N/m. First tension load for reaching this level was found to be 550±45N. The force to break the bone was found to be around 2000N.

Discussion: For secure use, the force interval that will ensure a secure fixation without causing a fracture should be known and the tensioner should be tightened in these ranges. In this in-vitro experimental pilot study we have compared the individual assessment of secure fixation during cable tensioner use. Our results suggest that the individual decision for rotational stability depends largely on the person who evaluates it. A torque-meter can be implemented to our experiment model to obtain more objective assessment of the optimal tightening of the cable tensioner for secure fixation with rotational stability. After determining the force interval for secure fixation a torquesensitive crank can be designed and implemented to the cable tensioner. Such a device should provide a more safe and secure fixation during tightening of cerclage wirings.