Biomechanical change in exterior and interior medullary cavity at fracture end during axial compressing of external fixator in vivo

Jianghua MING; Panghu ZHOU; Jianlin ZHOU

ABSTRACT

BACKGROUND:

Biomechanics of bone in vivo is concerned by the scholars who work for orthopaedic and medical bioengineering all the time. The application of strainmeter in this measurement has been studied for many years, but the long-term and successive measuring problems have not been solved. A new installing method for analyzing biomechanics of bone in vivoshould be considered to be explored.

OBJECTIVE:

To investigate the biochemical change of external fixator at both external and internal medullary cavity of anxial compressing fracture end invivo when external fixator is used

DESIGN:

Randomized and controlled animal experiment

SETTING:

Laboratory of Orthopaedic Department, Renmin Hospital, Wuhan University MATERIALS Totally 18 healthy white rabbits, of rather gender, with the body mass of 3.6 to 4.2 kg , were chosen.

METHODS:

This experiment was conducted at the laboratory of Or thopaedic Department, Renmin Hospital, Wuhan University in November 2004. The chosen 18 healthy white rabbits were randomly divided into 2groups. External measuring group (Group A) the strain guage was affixed to the tibia external cortex by 502 gauge. Internal measuring group (Group B) strain guage coated with bone cement was installed on the tibia internal cortex. Group A and Group B were divided into two subgroups A1, A2 and B1, B2, respectively according to the pressure 0.5 time body mass and 1 time body mass. Strain voltage change and attenuation coefficient after compressing were measured with scaler. Statistical comparison was performed among the groups. MAIN OUTCOME

MEASURES:

① The data of scaler curve. ② Attenuation coefficient of different compressions.

RESULTS:

① The scaler curves of Group A changed greatly during early stage. After reaching stable stage, trendline of internal and external cortex went consistent but the former value was higher than that of the latter . The time of compression reaching stable was shorter in the Group B than in the Group A, and the absolute value of strain was smaller in the Group B than in the Group A. ② Before reaching stable after compression, the attenuation coefficient was low in the Group A1 as compared with Group A2. There was the same result between Group B1 and Group B2 group. When comparison was conducted between Group A and Group B, the curve of Group A decreased firstly, then ascended, but there was no ascending tendency in the Group B. After reaching stable, there were descending tendency in the Group A1 group and Group A2. It descended fast in the Group A2, while it kept at normal level in the Group B1 group. Fluctuation appeared in the Group A2.

CONCLUSION:

Strain of external medullary cavity is significantly larger than that of internal medullary cavity. It is easier to reach stable in the inter nal medullary cavity than in the external medullary cavity. 0.5 time of body mass is suitable at the initial period in treating fracture with external fixator.