In vivo distraction forces in extendible endoprosthetic replacements--a study of 34 patients

Using an instrumented distraction tool 34 extendible endoprosthetic replacements were lengthened to measure total resistance to extension. The study consisted of 19 distal femoral, 2 proximal femoral, 1 total femoral, 8 proximal tibial, 1 distal humeral, 2 proximal humeral and 1 total humeral replacements. Among these prostheses a total of 76 measurements were made. The initial load on the prosthesis due to soft tissues and muscles varied between 0 and 578 N with an overall mean of 128 N. There was a linear relationship between extension and load. At 6 mm extension the load varied between 42 and 1513 N with a mean of 476 N. There was very little difference between the means of femoral and tibial measurements. There was very little resistance to extension in the total humeral replacement due to the absence of glenoid and muscle attachment. In the distal humeral replacement all three measurements were comparable with the forces in proximal tibial replacements. Two cases showed very high resistance to extension, which was mainly attributed to the presence of large amounts of scar tissues observed at surgery. During one lengthening operation a 30 s pause at 1 mm increments from 0 to 9 mm showed load recovery of approximately 10 per cent for up to 3 mm, beyond which there appeared to be stiffening of tissues.