A COMPARISON OF HYDROXYAPATITE-COATED, TITANIUM-COATED, AND UNCOATED TAPERED EXTERNAL-FIXATION PINS - AN IN-VIVO STUDY IN SHEEP

Three types of surfaces for external fixation pins were compared. One hundred and eight stainless-steel tapered 5/6-millimeter pins were div ided into three groups: thirty-six pins remained uncoated (Group A), t hirty-six were plasma-sprayed with hydroxyapatite (Group B), and thirt y-six were plasma-sprayed with titanium (Group C), The pins were impla nted in the left tibia of eighteen sheep, with each sheep receiving si x pins from the same group, A unilateral fixator then was assembled on the pins. The medial aspect of the mid-part of the tibial diaphysis w as exposed, and a five-millimeter-long cylinder of bone was removed so that load would be borne by the bone-pin interfaces. Six weeks after the procedure, radiographs demonstrated rarefaction of twenty-nine pin tracks in Group A, fifteen in Group B, and thirty in Group C (p = 0.0 21 for Group A compared with Group B and p = 0.016 for Group B compare d with Group C), The mean final insertion torque land standard deviati on) was 4360 +/- 1050 newton-millimeters in Group A, 3420 +/- 676 newt on-millimeters in Group B, and 3740 +/- 643 newton-millimeters in Grou p C, With the numbers available, no significant differences could be d etected among these values. The mean extraction torque was 253 +/- 175 newton-millimeters in Group A, 3360 +/- 1260 newton-millimeters in Gr oup B, and 1720 +/- 1030 newton-millimeters in Group C (p = 0.002 for Group A compared with Group B, p = 0.017 for Group A compared with Gro up C, and p = 0.03 for Group B compared with Group C), The extraction torque was significantly lower than the corresponding insertion torque in both Group A (p < 0.001) and Group C (p = 0.003); no significant d ifference could be found, with the numbers available, in Group B (hydr oxyapatite-coated pins), At sixty times magnification, direct contact was seen along a mean of 16 +/- 9 per cent of the bone-pin interface i n Group A, 30 +/- 12 per cent of the interface in Group B, and 28 +/- 15 per cent of the interface in Group C (p = 0.042 for Group A compare d with Group C), However, at 10,000 times magnification, direct bone-p in contact was found only in Group B. CLINICAL RELEVANCE: The better f ixation and greater interface strength of the hydroxyapatite-coated ta pered pins should lead to a substantial decrease in the frequency of l oosening and infection and consequently to a remarkable improvement in the clinical results of external fixation.