IN-VIVO DISTRACTION FORCES IN EXTENDIBLE ENDOPROSTHETIC REPLACEMENTS - A STUDY OF 34 PATIENTS

Using an instrumented distraction tool 34 extendible endoprosthetic re placements were lengthened to measure total resistance to extension. T he study consisted of 19 distal femoral, 2 proximal femoral, 1 total f emoral, 8 proximal tibial, 1 distal humeral, 2 proximal humeral and 1 total humeral replacements. Among these prostheses a total of 76 measu rements were made. The initial load on the prosthesis due to soft tiss ues 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 m m 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 ti bial measurements. There was very little resistance to extension in th e total humeral replacement due to the absence of glenoid and muscle a ttachment. In the distal humeral replacement all three measurements we re comparable with the forces in proximal tibial replacements. Two cas es showed very high resistance to extension, which was mainly attribut ed to the presence of large amounts of scar tissues observed at surger y. During one lengthening operation a 30 s pause at 1 mm increments fr om 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.