Static Load Bearing in Transfemoral Amputees fitted with Osseointegrated Fixation: Pilot Study

Abstract

Background
A direct skeletal fixation method has been developed whereby a titanium implant is screwed
into the residual femur of transfemoral amputees to which their prosthesis is attached(1). It is
necessary that the amputees undergo a lengthy rehabilitation program to return to ambulation.
The program involves incremental loading of the fixation until full weight bearing is
achieved. The rate of loading to be applied on the long axis of the fixation is determined by
the quality of the residual skeleton and the level of pain experienced by the amputee.
Rotational loading of the implant is to be avoided at this stage(1). The amputee uses a
domestic scale to provide feedback of the load applied. A comparison between the prescribed
load and the true loading applied to the implant would help validate the program(2).
Method
A six channel transducer was fitted to the amputee’s usual rehabilitation prostheses and the
true forces and moments were measured while the amputee performed the loading. The
amputee was asked to apply a load of 40 Kg to the residuum for a time period of five seconds.
Results
The results showed the force applied to the long axis of the fixation and the resultant force
were 9% and 15% higher than the prescribed load. The moment applied around the mediolateral
axis was six times higher than around the antero-posterior and long axes.
Conclusion
The resultant force and the force applied on the long axis of the fixation were larger than the
prescribed load indicating the inaccuracy of the measurement using the scale. The magnitude
of the component of force applied to the medio-lateral and antero-posterior axes and the
moment around the medio-lateral axis were larger than anticipated by the clinical team
indicating that rotational loading occurs during the exercise program.
There are several consequential studies arising from these results such as to continue to
investigate the correlation between the applied load and the prescribed load, to collect
kinematic data to investigate the effect of body weight shift, in particular trunk and head
position, on the forces and moments applied to the abutment and compare the loading patterns
between activities of daily living and the static exercises.