Optimum predrilled hole size for bone screws used in osteochondral fixation: in vitro biomechanical study and clinical case.
BACKGROUND
Bone screws have a mechanical advantage over bone pegs for fixation of osteochondral fractures. The purpose of this study was to investigate predrilling methods to facilitate bone screw insertion.


METHODS
The 162 bone screws (major diameter 3.0 mm; minor diameter 2.5 mm; length 10 mm) used were made from porcine tibial cortical bone. Metal screws of the same design were used as a control. The screws were inserted, without tapping, into predrilled holes of six different sizes (2.4-2.9 mm diameter; 80%-97% of the screw) made in cancellous bone harvested from porcine femoral condyle, and pullout strength was determined. The volumetric bone mineral densities of porcine bones were compared to those of human bones. In a clinical case, an osteochondral fracture of the patella was repaired using three autologous bone screws.


RESULTS
Bone screws occasionally broke due to high insertion torque (108-158 N.mm); the incidences were 30% and 19% with the 80% and 83% sized holes, respectively. None of the screws inserted into holes of size >/= 87% broke, and their insertion torques never exceeded 100 N.mm. Metal screws were inserted without breakage regardless of hole size. Bone screws inserted into 80%-93% holes had the greatest pullout strengths (range 154-165 N), whereas 97% holes had a mean strength of 84 N. Metal screws had large pullout strengths when inserted into 80% and 83% holes (164 and 169 N, respectively). The mineral density of porcine bone was lower than that of human bone (1167 and 1357 mg/cm(3) for cortical bone and 193 and 269 mg/cm(3) for cancellous bone, respectively). In the clinical case, bone screws were inserted smoothly into 90% holes.


CONCLUSIONS
The optimal size of predrilled holes for bone screws (87%-93%) differs from that for metal screws.
Nagatani
Toru
T
Department of Orthopaedic Surgery, Faculty of Medicine, Shimane University, 89-1 Enya, Izumo, Shimane, 693-8501, Japan.
Mori
Ryuji
R
Wang
Yingxu
Y
Nakai
Takahisa
T
Ozoe
Nobuaki
N
Uchio
Yuji
Y
D000293:Adolescent; D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002358:Cartilage, Articular / Q000293:injuries; D004867:Equipment Design; D050723:Fractures, Bone / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D008297:Male; D010329:Patella / Q000293:injuries* / Q000601:surgery*; D013552:Swine; D014057:Tomography, X-Ray Computed