HYDROXYAPATITE COATING OF THREADED PINS ENHANCES FIXATION

We measured the insertion and extraction torque forces in a randomised study of 76 external fixation screws in 19 patients treated by hemica llotasis for osteoarthritis of the medial side of the knee, The patien ts were randomised to have either standard tapered screws (Orthofix 6/ 5 mm) or the same screws with hydroxyapatite (HA) coating, One patient had two standard and two HA-coated screws, All patients had an anteri or external fixator (Orthofix T-garche), with two screws in the proxim al tibial metaphysis parallel to and about 2 cm below the joint surfac e and two in the tibial diaphysis. The mean torque forces for insertio n of the standard screws were 260 Ncm for the proximal to medial screw , 208 for the proximal to lateral screw and 498 and 546 Ncm for the di aphyseal pins, The corresponding forces for the HA-coated pins were no t significantly different The torque forces for the extraction of the standard pins were 2 Ncm for the proximal pins, 277 and 249 Ncm for th e distal pins and 482, 478, 585 and 620 Ncm, respectively (p < 0.005) for the HA-coated pins. All 18 of the metaphyseal standard screws were loose at extraction (extraction force < 20 Ncm), but only one of the HA screws in the metaphysis was loose, In the diaphysis the standard s crews lost about 40% of their fixation in contrast to the HA-coated sc rews which retained full fixation strength.